Emacs Lisp

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This is the GNU Emacs Lisp Reference Manual corresponding to Emacs version 25.2.

Copyright © 1990–1996, 1998–2017 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being “GNU General Public License,” with the Front-Cover Texts being “A GNU Manual,” and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled “GNU Free Documentation License.”

(a) The FSF's Back-Cover Text is: “You have the freedom to copy and modify this GNU manual. Buying copies from the FSF supports it in developing GNU and promoting software freedom.”

Introduction	Introduction and conventions used.
Lisp Data Types	Data types of objects in Emacs Lisp.
Numbers	Numbers and arithmetic functions.
Strings and Characters	Strings, and functions that work on them.
Lists	Lists, cons cells, and related functions.
Sequences Arrays Vectors	Lists, strings and vectors are called sequences. Certain functions act on any kind of sequence. The description of vectors is here as well.
Hash Tables	Very fast lookup-tables.
Symbols	Symbols represent names, uniquely.
Evaluation	How Lisp expressions are evaluated.
Control Structures	Conditionals, loops, nonlocal exits.
Variables	Using symbols in programs to stand for values.
Functions	A function is a Lisp program that can be invoked from other functions.
Macros	Macros are a way to extend the Lisp language.
Customization	Making variables and faces customizable.
Loading	Reading files of Lisp code into Lisp.
Byte Compilation	Compilation makes programs run faster.
Debugging	Tools and tips for debugging Lisp programs.
Read and Print	Converting Lisp objects to text and back.
Minibuffers	Using the minibuffer to read input.
Command Loop	How the editor command loop works, and how you can call its subroutines.
Keymaps	Defining the bindings from keys to commands.
Modes	Defining major and minor modes.
Documentation	Writing and using documentation strings.
Files	Accessing files.
Backups and Auto-Saving	Controlling how backups and auto-save files are made.
Buffers	Creating and using buffer objects.
Windows	Manipulating windows and displaying buffers.
Frames	Making multiple system-level windows.
Positions	Buffer positions and motion functions.
Markers	Markers represent positions and update automatically when the text is changed.
Text	Examining and changing text in buffers.
Non-ASCII Characters	Non-ASCII text in buffers and strings.
Searching and Matching	Searching buffers for strings or regexps.
Syntax Tables	The syntax table controls word and list parsing.
Abbrevs	How Abbrev mode works, and its data structures.
Processes	Running and communicating with subprocesses.
Display	Features for controlling the screen display.
System Interface	Getting the user id, system type, environment variables, and other such things.
Packaging	Preparing Lisp code for distribution.
Appendices
Antinews	Info for users downgrading to Emacs 24.
GNU Free Documentation License	The license for this documentation.
GPL	Conditions for copying and changing GNU Emacs.
Tips	Advice and coding conventions for Emacs Lisp.
GNU Emacs Internals	Building and dumping Emacs; internal data structures.
Standard Errors	List of some standard error symbols.
Standard Keymaps	List of some standard keymaps.
Standard Hooks	List of some standard hook variables.
Index	Index including concepts, functions, variables, and other terms.

Detailed Node Listing

Here are other nodes that are subnodes of those already listed, mentioned here so you can get to them in one step: Introduction
Caveats	Flaws and a request for help.
Lisp History	Emacs Lisp is descended from Maclisp.
Conventions	How the manual is formatted.
Version Info	Which Emacs version is running?
Acknowledgments	The authors, editors, and sponsors of this manual.
Conventions
Some Terms	Explanation of terms we use in this manual.
nil and t	How the symbols nil and t are used.
Evaluation Notation	The format we use for examples of evaluation.
Printing Notation	The format we use when examples print text.
Error Messages	The format we use for examples of errors.
Buffer Text Notation	The format we use for buffer contents in examples.
Format of Descriptions	Notation for describing functions, variables, etc.
Format of Descriptions
A Sample Function Description	A description of an imaginary function, foo.
A Sample Variable Description	A description of an imaginary variable, electric-future-map.
Lisp Data Types
Printed Representation	How Lisp objects are represented as text.
Comments	Comments and their formatting conventions.
Programming Types	Types found in all Lisp systems.
Editing Types	Types specific to Emacs.
Circular Objects	Read syntax for circular structure.
Type Predicates	Tests related to types.
Equality Predicates	Tests of equality between any two objects.
Programming Types
Integer Type	Numbers without fractional parts.
Floating-Point Type	Numbers with fractional parts and with a large range.
Character Type	The representation of letters, numbers and control characters.
Symbol Type	A multi-use object that refers to a function, variable, or property list, and has a unique identity.
Sequence Type	Both lists and arrays are classified as sequences.
Cons Cell Type	Cons cells, and lists (which are made from cons cells).
Array Type	Arrays include strings and vectors.
String Type	An (efficient) array of characters.
Vector Type	One-dimensional arrays.
Char-Table Type	One-dimensional sparse arrays indexed by characters.
Bool-Vector Type	One-dimensional arrays of t or nil.
Hash Table Type	Super-fast lookup tables.
Function Type	A piece of executable code you can call from elsewhere.
Macro Type	A method of expanding an expression into another expression, more fundamental but less pretty.
Primitive Function Type	A function written in C, callable from Lisp.
Byte-Code Type	A function written in Lisp, then compiled.
Autoload Type	A type used for automatically loading seldom-used functions.
Finalizer Type	Runs code when no longer reachable.
Character Type
Basic Char Syntax	Syntax for regular characters.
General Escape Syntax	How to specify characters by their codes.
Ctl-Char Syntax	Syntax for control characters.
Meta-Char Syntax	Syntax for meta-characters.
Other Char Bits	Syntax for hyper-, super-, and alt-characters.
Cons Cell and List Types
Box Diagrams	Drawing pictures of lists.
Dotted Pair Notation	A general syntax for cons cells.
Association List Type	A specially constructed list.
String Type
Syntax for Strings	How to specify Lisp strings.
Non-ASCII in Strings	International characters in strings.
Nonprinting Characters	Literal unprintable characters in strings.
Text Props and Strings	Strings with text properties.
Editing Types
Buffer Type	The basic object of editing.
Marker Type	A position in a buffer.
Window Type	Buffers are displayed in windows.
Frame Type	Windows subdivide frames.
Terminal Type	A terminal device displays frames.
Window Configuration Type	Recording the way a frame is subdivided.
Frame Configuration Type	Recording the status of all frames.
Process Type	A subprocess of Emacs running on the underlying OS.
Stream Type	Receive or send characters.
Keymap Type	What function a keystroke invokes.
Overlay Type	How an overlay is represented.
Font Type	Fonts for displaying text.
Numbers
Integer Basics	Representation and range of integers.
Float Basics	Representation and range of floating point.
Predicates on Numbers	Testing for numbers.
Comparison of Numbers	Equality and inequality predicates.
Numeric Conversions	Converting float to integer and vice versa.
Arithmetic Operations	How to add, subtract, multiply and divide.
Rounding Operations	Explicitly rounding floating-point numbers.
Bitwise Operations	Logical and, or, not, shifting.
Math Functions	Trig, exponential and logarithmic functions.
Random Numbers	Obtaining random integers, predictable or not.
Strings and Characters
String Basics	Basic properties of strings and characters.
Predicates for Strings	Testing whether an object is a string or char.
Creating Strings	Functions to allocate new strings.
Modifying Strings	Altering the contents of an existing string.
Text Comparison	Comparing characters or strings.
String Conversion	Converting to and from characters and strings.
Formatting Strings	format: Emacs's analogue of printf.
Case Conversion	Case conversion functions.
Case Tables	Customizing case conversion.
Lists
Cons Cells	How lists are made out of cons cells.
List-related Predicates	Is this object a list? Comparing two lists.
List Elements	Extracting the pieces of a list.
Building Lists	Creating list structure.
List Variables	Modifying lists stored in variables.
Modifying Lists	Storing new pieces into an existing list.
Sets And Lists	A list can represent a finite mathematical set.
Association Lists	A list can represent a finite relation or mapping.
Property Lists	A list of paired elements.
Modifying Existing List Structure
Setcar	Replacing an element in a list.
Setcdr	Replacing part of the list backbone. This can be used to remove or add elements.
Rearrangement	Reordering the elements in a list; combining lists.
Property Lists
Plists and Alists	Comparison of the advantages of property lists and association lists.
Plist Access	Accessing property lists stored elsewhere.
Sequences, Arrays, and Vectors
Sequence Functions	Functions that accept any kind of sequence.
Arrays	Characteristics of arrays in Emacs Lisp.
Array Functions	Functions specifically for arrays.
Vectors	Special characteristics of Emacs Lisp vectors.
Vector Functions	Functions specifically for vectors.
Char-Tables	How to work with char-tables.
Bool-Vectors	How to work with bool-vectors.
Rings	Managing a fixed-size ring of objects.
Hash Tables
Creating Hash	Functions to create hash tables.
Hash Access	Reading and writing the hash table contents.
Defining Hash	Defining new comparison methods.
Other Hash	Miscellaneous.
Symbols
Symbol Components	Symbols have names, values, function definitions and property lists.
Definitions	A definition says how a symbol will be used.
Creating Symbols	How symbols are kept unique.
Symbol Properties	Each symbol has a property list for recording miscellaneous information.
Symbol Properties
Symbol Plists	Accessing symbol properties.
Standard Properties	Standard meanings of symbol properties.
Evaluation
Intro Eval	Evaluation in the scheme of things.
Forms	How various sorts of objects are evaluated.
Quoting	Avoiding evaluation (to put constants in the program).
Backquote	Easier construction of list structure.
Eval	How to invoke the Lisp interpreter explicitly.
Kinds of Forms
Self-Evaluating Forms	Forms that evaluate to themselves.
Symbol Forms	Symbols evaluate as variables.
Classifying Lists	How to distinguish various sorts of list forms.
Function Indirection	When a symbol appears as the car of a list, we find the real function via the symbol.
Function Forms	Forms that call functions.
Macro Forms	Forms that call macros.
Special Forms	Special forms are idiosyncratic primitives, most of them extremely important.
Autoloading	Functions set up to load files containing their real definitions.
Control Structures
Sequencing	Evaluation in textual order.
Conditionals	if, cond, when, unless.
Combining Conditions	and, or, not.
Iteration	while loops.
Generators	Generic sequences and coroutines.
Nonlocal Exits	Jumping out of a sequence.
Conditionals
Pattern matching case statement	How to use pcase.
Nonlocal Exits
Catch and Throw	Nonlocal exits for the program's own purposes.
Examples of Catch	Showing how such nonlocal exits can be written.
Errors	How errors are signaled and handled.
Cleanups	Arranging to run a cleanup form if an error happens.
Errors
Signaling Errors	How to report an error.
Processing of Errors	What Emacs does when you report an error.
Handling Errors	How you can trap errors and continue execution.
Error Symbols	How errors are classified for trapping them.
Variables
Global Variables	Variable values that exist permanently, everywhere.
Constant Variables	Variables that never change.
Local Variables	Variable values that exist only temporarily.
Void Variables	Symbols that lack values.
Defining Variables	A definition says a symbol is used as a variable.
Tips for Defining	Things you should think about when you define a variable.
Accessing Variables	Examining values of variables whose names are known only at run time.
Setting Variables	Storing new values in variables.
Variable Scoping	How Lisp chooses among local and global values.
Buffer-Local Variables	Variable values in effect only in one buffer.
File Local Variables	Handling local variable lists in files.
Directory Local Variables	Local variables common to all files in a directory.
Variable Aliases	Variables that are aliases for other variables.
Variables with Restricted Values	Non-constant variables whose value can not be an arbitrary Lisp object.
Generalized Variables	Extending the concept of variables.
Scoping Rules for Variable Bindings
Dynamic Binding	The default for binding local variables in Emacs.
Dynamic Binding Tips	Avoiding problems with dynamic binding.
Lexical Binding	A different type of local variable binding.
Using Lexical Binding	How to enable lexical binding.
Buffer-Local Variables
Intro to Buffer-Local	Introduction and concepts.
Creating Buffer-Local	Creating and destroying buffer-local bindings.
Default Value	The default value is seen in buffers that don't have their own buffer-local values.
Generalized Variables
Setting Generalized Variables	The setf macro.
Adding Generalized Variables	Defining new setf forms.
Functions
What Is a Function	Lisp functions vs. primitives; terminology.
Lambda Expressions	How functions are expressed as Lisp objects.
Function Names	A symbol can serve as the name of a function.
Defining Functions	Lisp expressions for defining functions.
Calling Functions	How to use an existing function.
Mapping Functions	Applying a function to each element of a list, etc.
Anonymous Functions	Lambda expressions are functions with no names.
Generic Functions	Polymorphism, Emacs-style.
Function Cells	Accessing or setting the function definition of a symbol.
Closures	Functions that enclose a lexical environment.
Advising Functions	Adding to the definition of a function.
Obsolete Functions	Declaring functions obsolete.
Inline Functions	Defining functions that the compiler will expand inline.
Declare Form	Adding additional information about a function.
Declaring Functions	Telling the compiler that a function is defined.
Function Safety	Determining whether a function is safe to call.
Related Topics	Cross-references to specific Lisp primitives that have a special bearing on how functions work.
Lambda Expressions
Lambda Components	The parts of a lambda expression.
Simple Lambda	A simple example.
Argument List	Details and special features of argument lists.
Function Documentation	How to put documentation in a function.
Advising Emacs Lisp Functions
Core Advising Primitives	Primitives to manipulate advice.
Advising Named Functions	Advising named functions.
Advice combinators	Ways to compose advice.
Porting old advice	Adapting code using the old defadvice.
Macros
Simple Macro	A basic example.
Expansion	How, when and why macros are expanded.
Compiling Macros	How macros are expanded by the compiler.
Defining Macros	How to write a macro definition.
Problems with Macros	Don't evaluate the macro arguments too many times. Don't hide the user's variables.
Indenting Macros	Specifying how to indent macro calls.
Common Problems Using Macros
Wrong Time	Do the work in the expansion, not in the macro.
Argument Evaluation	The expansion should evaluate each macro arg once.
Surprising Local Vars	Local variable bindings in the expansion require special care.
Eval During Expansion	Don't evaluate them; put them in the expansion.
Repeated Expansion	Avoid depending on how many times expansion is done.
Customization Settings
Common Keywords	Common keyword arguments for all kinds of customization declarations.
Group Definitions	Writing customization group definitions.
Variable Definitions	Declaring user options.
Customization Types	Specifying the type of a user option.
Applying Customizations	Functions to apply customization settings.
Custom Themes	Writing Custom themes.
Customization Types
Simple Types	Simple customization types: sexp, integer, etc.
Composite Types	Build new types from other types or data.
Splicing into Lists	Splice elements into list with :inline.
Type Keywords	Keyword-argument pairs in a customization type.
Defining New Types	Give your type a name.
Loading
How Programs Do Loading	The load function and others.
Load Suffixes	Details about the suffixes that load tries.
Library Search	Finding a library to load.
Loading Non-ASCII	Non-ASCII characters in Emacs Lisp files.
Autoload	Setting up a function to autoload.
Repeated Loading	Precautions about loading a file twice.
Named Features	Loading a library if it isn't already loaded.
Where Defined	Finding which file defined a certain symbol.
Unloading	How to unload a library that was loaded.
Hooks for Loading	Providing code to be run when particular libraries are loaded.
Dynamic Modules	Modules provide additional Lisp primitives.
Byte Compilation
Speed of Byte-Code	An example of speedup from byte compilation.
Compilation Functions	Byte compilation functions.
Docs and Compilation	Dynamic loading of documentation strings.
Dynamic Loading	Dynamic loading of individual functions.
Eval During Compile	Code to be evaluated when you compile.
Compiler Errors	Handling compiler error messages.
Byte-Code Objects	The data type used for byte-compiled functions.
Disassembly	Disassembling byte-code; how to read byte-code.
Debugging Lisp Programs
Debugger	A debugger for the Emacs Lisp evaluator.
Edebug	A source-level Emacs Lisp debugger.
Syntax Errors	How to find syntax errors.
Test Coverage	Ensuring you have tested all branches in your code.
Profiling	Measuring the resources that your code uses.
The Lisp Debugger
Error Debugging	Entering the debugger when an error happens.
Infinite Loops	Stopping and debugging a program that doesn't exit.
Function Debugging	Entering it when a certain function is called.
Explicit Debug	Entering it at a certain point in the program.
Using Debugger	What the debugger does; what you see while in it.
Debugger Commands	Commands used while in the debugger.
Invoking the Debugger	How to call the function debug.
Internals of Debugger	Subroutines of the debugger, and global variables.
Edebug
Using Edebug	Introduction to use of Edebug.
Instrumenting	You must instrument your code in order to debug it with Edebug.
Edebug Execution Modes	Execution modes, stopping more or less often.
Jumping	Commands to jump to a specified place.
Edebug Misc	Miscellaneous commands.
Breaks	Setting breakpoints to make the program stop.
Trapping Errors	Trapping errors with Edebug.
Edebug Views	Views inside and outside of Edebug.
Edebug Eval	Evaluating expressions within Edebug.
Eval List	Expressions whose values are displayed each time you enter Edebug.
Printing in Edebug	Customization of printing.
Trace Buffer	How to produce trace output in a buffer.
Coverage Testing	How to test evaluation coverage.
The Outside Context	Data that Edebug saves and restores.
Edebug and Macros	Specifying how to handle macro calls.
Edebug Options	Option variables for customizing Edebug.
Breaks
Breakpoints	Breakpoints at stop points.
Global Break Condition	Breaking on an event.
Source Breakpoints	Embedding breakpoints in source code.
The Outside Context
Checking Whether to Stop	When Edebug decides what to do.
Edebug Display Update	When Edebug updates the display.
Edebug Recursive Edit	When Edebug stops execution.
Edebug and Macros
Instrumenting Macro Calls	The basic problem.
Specification List	How to specify complex patterns of evaluation.
Backtracking	What Edebug does when matching fails.
Specification Examples	To help understand specifications.
Debugging Invalid Lisp Syntax
Excess Open	How to find a spurious open paren or missing close.
Excess Close	How to find a spurious close paren or missing open.
Reading and Printing Lisp Objects
Streams Intro	Overview of streams, reading and printing.
Input Streams	Various data types that can be used as input streams.
Input Functions	Functions to read Lisp objects from text.
Output Streams	Various data types that can be used as output streams.
Output Functions	Functions to print Lisp objects as text.
Output Variables	Variables that control what the printing functions do.
Minibuffers
Intro to Minibuffers	Basic information about minibuffers.
Text from Minibuffer	How to read a straight text string.
Object from Minibuffer	How to read a Lisp object or expression.
Minibuffer History	Recording previous minibuffer inputs so the user can reuse them.
Initial Input	Specifying initial contents for the minibuffer.
Completion	How to invoke and customize completion.
Yes-or-No Queries	Asking a question with a simple answer.
Multiple Queries	Asking a series of similar questions.
Reading a Password	Reading a password from the terminal.
Minibuffer Commands	Commands used as key bindings in minibuffers.
Minibuffer Windows	Operating on the special minibuffer windows.
Minibuffer Contents	How such commands access the minibuffer text.
Recursive Mini	Whether recursive entry to minibuffer is allowed.
Minibuffer Misc	Various customization hooks and variables.
Completion
Basic Completion	Low-level functions for completing strings.
Minibuffer Completion	Invoking the minibuffer with completion.
Completion Commands	Minibuffer commands that do completion.
High-Level Completion	Convenient special cases of completion (reading buffer names, variable names, etc.).
Reading File Names	Using completion to read file names and shell commands.
Completion Variables	Variables controlling completion behavior.
Programmed Completion	Writing your own completion function.
Completion in Buffers	Completing text in ordinary buffers.
Command Loop
Command Overview	How the command loop reads commands.
Defining Commands	Specifying how a function should read arguments.
Interactive Call	Calling a command, so that it will read arguments.
Distinguish Interactive	Making a command distinguish interactive calls.
Command Loop Info	Variables set by the command loop for you to examine.
Adjusting Point	Adjustment of point after a command.
Input Events	What input looks like when you read it.
Reading Input	How to read input events from the keyboard or mouse.
Special Events	Events processed immediately and individually.
Waiting	Waiting for user input or elapsed time.
Quitting	How C-g works. How to catch or defer quitting.
Prefix Command Arguments	How the commands to set prefix args work.
Recursive Editing	Entering a recursive edit, and why you usually shouldn't.
Disabling Commands	How the command loop handles disabled commands.
Command History	How the command history is set up, and how accessed.
Keyboard Macros	How keyboard macros are implemented.
Defining Commands
Using Interactive	General rules for interactive.
Interactive Codes	The standard letter-codes for reading arguments in various ways.
Interactive Examples	Examples of how to read interactive arguments.
Generic Commands	Select among command alternatives.
Input Events
Keyboard Events	Ordinary characters -- keys with symbols on them.
Function Keys	Function keys -- keys with names, not symbols.
Mouse Events	Overview of mouse events.
Click Events	Pushing and releasing a mouse button.
Drag Events	Moving the mouse before releasing the button.
Button-Down Events	A button was pushed and not yet released.
Repeat Events	Double and triple click (or drag, or down).
Motion Events	Just moving the mouse, not pushing a button.
Focus Events	Moving the mouse between frames.
Misc Events	Other events the system can generate.
Event Examples	Examples of the lists for mouse events.
Classifying Events	Finding the modifier keys in an event symbol. Event types.
Accessing Mouse	Functions to extract info from mouse events.
Accessing Scroll	Functions to get info from scroll bar events.
Strings of Events	Special considerations for putting keyboard character events in a string.
Reading Input
Key Sequence Input	How to read one key sequence.
Reading One Event	How to read just one event.
Event Mod	How Emacs modifies events as they are read.
Invoking the Input Method	How reading an event uses the input method.
Quoted Character Input	Asking the user to specify a character.
Event Input Misc	How to reread or throw away input events.
Keymaps
Key Sequences	Key sequences as Lisp objects.
Keymap Basics	Basic concepts of keymaps.
Format of Keymaps	What a keymap looks like as a Lisp object.
Creating Keymaps	Functions to create and copy keymaps.
Inheritance and Keymaps	How one keymap can inherit the bindings of another keymap.
Prefix Keys	Defining a key with a keymap as its definition.
Active Keymaps	How Emacs searches the active keymaps for a key binding.
Searching Keymaps	A pseudo-Lisp summary of searching active maps.
Controlling Active Maps	Each buffer has a local keymap to override the standard (global) bindings. A minor mode can also override them.
Key Lookup	Finding a key's binding in one keymap.
Functions for Key Lookup	How to request key lookup.
Changing Key Bindings	Redefining a key in a keymap.
Remapping Commands	A keymap can translate one command to another.
Translation Keymaps	Keymaps for translating sequences of events.
Key Binding Commands	Interactive interfaces for redefining keys.
Scanning Keymaps	Looking through all keymaps, for printing help.
Menu Keymaps	Defining a menu as a keymap.
Menu Keymaps
Defining Menus	How to make a keymap that defines a menu.
Mouse Menus	How users actuate the menu with the mouse.
Keyboard Menus	How users actuate the menu with the keyboard.
Menu Example	Making a simple menu.
Menu Bar	How to customize the menu bar.
Tool Bar	A tool bar is a row of images.
Modifying Menus	How to add new items to a menu.
Easy Menu	A convenience macro for defining menus.
Defining Menus
Simple Menu Items	A simple kind of menu key binding.
Extended Menu Items	More complex menu item definitions.
Menu Separators	Drawing a horizontal line through a menu.
Alias Menu Items	Using command aliases in menu items.
Major and Minor Modes
Hooks	How to use hooks; how to write code that provides hooks.
Major Modes	Defining major modes.
Minor Modes	Defining minor modes.
Mode Line Format	Customizing the text that appears in the mode line.
Imenu	Providing a menu of definitions made in a buffer.
Font Lock Mode	How modes can highlight text according to syntax.
Auto-Indentation	How to teach Emacs to indent for a major mode.
Desktop Save Mode	How modes can have buffer state saved between Emacs sessions.
Hooks
Running Hooks	How to run a hook.
Setting Hooks	How to put functions on a hook, or remove them.
Major Modes
Major Mode Conventions	Coding conventions for keymaps, etc.
Auto Major Mode	How Emacs chooses the major mode automatically.
Mode Help	Finding out how to use a mode.
Derived Modes	Defining a new major mode based on another major mode.
Basic Major Modes	Modes that other modes are often derived from.
Mode Hooks	Hooks run at the end of major mode functions.
Tabulated List Mode	Parent mode for buffers containing tabulated data.
Generic Modes	Defining a simple major mode that supports comment syntax and Font Lock mode.
Example Major Modes	Text mode and Lisp modes.
Minor Modes
Minor Mode Conventions	Tips for writing a minor mode.
Keymaps and Minor Modes	How a minor mode can have its own keymap.
Defining Minor Modes	A convenient facility for defining minor modes.
Mode Line Format
Mode Line Basics	Basic ideas of mode line control.
Mode Line Data	The data structure that controls the mode line.
Mode Line Top	The top level variable, mode-line-format.
Mode Line Variables	Variables used in that data structure.
%-Constructs	Putting information into a mode line.
Properties in Mode	Using text properties in the mode line.
Header Lines	Like a mode line, but at the top.
Emulating Mode Line	Formatting text as the mode line would.
Font Lock Mode
Font Lock Basics	Overview of customizing Font Lock.
Search-based Fontification	Fontification based on regexps.
Customizing Keywords	Customizing search-based fontification.
Other Font Lock Variables	Additional customization facilities.
Levels of Font Lock	Each mode can define alternative levels so that the user can select more or less.
Precalculated Fontification	How Lisp programs that produce the buffer contents can also specify how to fontify it.
Faces for Font Lock	Special faces specifically for Font Lock.
Syntactic Font Lock	Fontification based on syntax tables.
Multiline Font Lock	How to coerce Font Lock into properly highlighting multiline constructs.
Multiline Font Lock Constructs
Font Lock Multiline	Marking multiline chunks with a text property.
Region to Refontify	Controlling which region gets refontified after a buffer change.
Automatic Indentation of code
SMIE	A simple minded indentation engine.
Simple Minded Indentation Engine
SMIE setup	SMIE setup and features.
Operator Precedence Grammars	A very simple parsing technique.
SMIE Grammar	Defining the grammar of a language.
SMIE Lexer	Defining tokens.
SMIE Tricks	Working around the parser's limitations.
SMIE Indentation	Specifying indentation rules.
SMIE Indentation Helpers	Helper functions for indentation rules.
SMIE Indentation Example	Sample indentation rules.
SMIE Customization	Customizing indentation.
Documentation
Documentation Basics	Where doc strings are defined and stored.
Accessing Documentation	How Lisp programs can access doc strings.
Keys in Documentation	Substituting current key bindings.
Describing Characters	Making printable descriptions of non-printing characters and key sequences.
Help Functions	Subroutines used by Emacs help facilities.
Files
Visiting Files	Reading files into Emacs buffers for editing.
Saving Buffers	Writing changed buffers back into files.
Reading from Files	Reading files into buffers without visiting.
Writing to Files	Writing new files from parts of buffers.
File Locks	Locking and unlocking files, to prevent simultaneous editing by two people.
Information about Files	Testing existence, accessibility, size of files.
Changing Files	Renaming files, changing permissions, etc.
File Names	Decomposing and expanding file names.
Contents of Directories	Getting a list of the files in a directory.
Create/Delete Dirs	Creating and Deleting Directories.
Magic File Names	Special handling for certain file names.
Format Conversion	Conversion to and from various file formats.
Visiting Files
Visiting Functions	The usual interface functions for visiting.
Subroutines of Visiting	Lower-level subroutines that they use.
Information about Files
Testing Accessibility	Is a given file readable? Writable?
Kinds of Files	Is it a directory? A symbolic link?
Truenames	Eliminating symbolic links from a file name.
File Attributes	File sizes, modification times, etc.
Extended Attributes	Extended file attributes for access control.
Locating Files	How to find a file in standard places.
File Names
File Name Components	The directory part of a file name, and the rest.
Relative File Names	Some file names are relative to a current directory.
Directory Names	A directory's name as a directory is different from its name as a file.
File Name Expansion	Converting relative file names to absolute ones.
Unique File Names	Generating names for temporary files.
File Name Completion	Finding the completions for a given file name.
Standard File Names	If your package uses a fixed file name, how to handle various operating systems simply.
File Format Conversion
Format Conversion Overview	insert-file-contents and write-region.
Format Conversion Round-Trip	Using format-alist.
Format Conversion Piecemeal	Specifying non-paired conversion.
Backups and Auto-Saving
Backup Files	How backup files are made; how their names are chosen.
Auto-Saving	How auto-save files are made; how their names are chosen.
Reverting	revert-buffer, and how to customize what it does.
Backup Files
Making Backups	How Emacs makes backup files, and when.
Rename or Copy	Two alternatives: renaming the old file or copying it.
Numbered Backups	Keeping multiple backups for each source file.
Backup Names	How backup file names are computed; customization.
Buffers
Buffer Basics	What is a buffer?
Current Buffer	Designating a buffer as current so that primitives will access its contents.
Buffer Names	Accessing and changing buffer names.
Buffer File Name	The buffer file name indicates which file is visited.
Buffer Modification	A buffer is modified if it needs to be saved.
Modification Time	Determining whether the visited file was changed behind Emacs's back.
Read Only Buffers	Modifying text is not allowed in a read-only buffer.
Buffer List	How to look at all the existing buffers.
Creating Buffers	Functions that create buffers.
Killing Buffers	Buffers exist until explicitly killed.
Indirect Buffers	An indirect buffer shares text with some other buffer.
Swapping Text	Swapping text between two buffers.
Buffer Gap	The gap in the buffer.
Windows
Basic Windows	Basic information on using windows.
Windows and Frames	Relating windows to the frame they appear on.
Window Sizes	Accessing a window's size.
Resizing Windows	Changing the sizes of windows.
Preserving Window Sizes	Preserving the size of windows.
Splitting Windows	Splitting one window into two windows.
Deleting Windows	Deleting a window gives its space to other windows.
Recombining Windows	Preserving the frame layout when splitting and deleting windows.
Selecting Windows	The selected window is the one that you edit in.
Cyclic Window Ordering	Moving around the existing windows.
Buffers and Windows	Each window displays the contents of a buffer.
Switching Buffers	Higher-level functions for switching to a buffer.
Choosing Window	How to choose a window for displaying a buffer.
Display Action Functions	Subroutines for display-buffer.
Choosing Window Options	Extra options affecting how buffers are displayed.
Window History	Each window remembers the buffers displayed in it.
Dedicated Windows	How to avoid displaying another buffer in a specific window.
Quitting Windows	How to restore the state prior to displaying a buffer.
Window Point	Each window has its own location of point.
Window Start and End	Buffer positions indicating which text is on-screen in a window.
Textual Scrolling	Moving text up and down through the window.
Vertical Scrolling	Moving the contents up and down on the window.
Horizontal Scrolling	Moving the contents sideways on the window.
Coordinates and Windows	Converting coordinates to windows.
Window Configurations	Saving and restoring the state of the screen.
Window Parameters	Associating additional information with windows.
Window Hooks	Hooks for scrolling, window size changes, redisplay going past a certain point, or window configuration changes.
Frames
Creating Frames	Creating additional frames.
Multiple Terminals	Displaying on several different devices.
Frame Geometry	Geometric properties of frames.
Frame Parameters	Controlling frame size, position, font, etc.
Terminal Parameters	Parameters common for all frames on terminal.
Frame Titles	Automatic updating of frame titles.
Deleting Frames	Frames last until explicitly deleted.
Finding All Frames	How to examine all existing frames.
Minibuffers and Frames	How a frame finds the minibuffer to use.
Input Focus	Specifying the selected frame.
Visibility of Frames	Frames may be visible or invisible, or icons.
Raising and Lowering	Raising a frame makes it hide other windows; lowering it makes the others hide it.
Frame Configurations	Saving the state of all frames.
Mouse Tracking	Getting events that say when the mouse moves.
Mouse Position	Asking where the mouse is, or moving it.
Pop-Up Menus	Displaying a menu for the user to select from.
Dialog Boxes	Displaying a box to ask yes or no.
Pointer Shape	Specifying the shape of the mouse pointer.
Window System Selections	Transferring text to and from other X clients.
Drag and Drop	Internals of Drag-and-Drop implementation.
Color Names	Getting the definitions of color names.
Text Terminal Colors	Defining colors for text terminals.
Resources	Getting resource values from the server.
Display Feature Testing	Determining the features of a terminal.
Frame Geometry
Frame Layout	Basic layout of frames.
Frame Font	The default font of a frame and how to set it.
Size and Position	Changing the size and position of a frame.
Implied Frame Resizing	Implied resizing of frames and how to prevent it.
Frame Parameters
Parameter Access	How to change a frame's parameters.
Initial Parameters	Specifying frame parameters when you make a frame.
Window Frame Parameters	List of frame parameters for window systems.
Geometry	Parsing geometry specifications.
Window Frame Parameters
Basic Parameters	Parameters that are fundamental.
Position Parameters	The position of the frame on the screen.
Size Parameters	Frame's size.
Layout Parameters	Size of parts of the frame, and enabling or disabling some parts.
Buffer Parameters	Which buffers have been or should be shown.
Management Parameters	Communicating with the window manager.
Cursor Parameters	Controlling the cursor appearance.
Font and Color Parameters	Fonts and colors for the frame text.
Positions
Point	The special position where editing takes place.
Motion	Changing point.
Excursions	Temporary motion and buffer changes.
Narrowing	Restricting editing to a portion of the buffer.
Motion
Character Motion	Moving in terms of characters.
Word Motion	Moving in terms of words.
Buffer End Motion	Moving to the beginning or end of the buffer.
Text Lines	Moving in terms of lines of text.
Screen Lines	Moving in terms of lines as displayed.
List Motion	Moving by parsing lists and sexps.
Skipping Characters	Skipping characters belonging to a certain set.
Markers
Overview of Markers	The components of a marker, and how it relocates.
Predicates on Markers	Testing whether an object is a marker.
Creating Markers	Making empty markers or markers at certain places.
Information from Markers	Finding the marker's buffer or character position.
Marker Insertion Types	Two ways a marker can relocate when you insert where it points.
Moving Markers	Moving the marker to a new buffer or position.
The Mark	How the mark is implemented with a marker.
The Region	How to access the region.
Text
Near Point	Examining text in the vicinity of point.
Buffer Contents	Examining text in a general fashion.
Comparing Text	Comparing substrings of buffers.
Insertion	Adding new text to a buffer.
Commands for Insertion	User-level commands to insert text.
Deletion	Removing text from a buffer.
User-Level Deletion	User-level commands to delete text.
The Kill Ring	Where removed text sometimes is saved for later use.
Undo	Undoing changes to the text of a buffer.
Maintaining Undo	How to enable and disable undo information. How to control how much information is kept.
Filling	Functions for explicit filling.
Margins	How to specify margins for filling commands.
Adaptive Fill	Adaptive Fill mode chooses a fill prefix from context.
Auto Filling	How auto-fill mode is implemented to break lines.
Sorting	Functions for sorting parts of the buffer.
Columns	Computing horizontal positions, and using them.
Indentation	Functions to insert or adjust indentation.
Case Changes	Case conversion of parts of the buffer.
Text Properties	Assigning Lisp property lists to text characters.
Substitution	Replacing a given character wherever it appears.
Registers	How registers are implemented. Accessing the text or position stored in a register.
Transposition	Swapping two portions of a buffer.
Decompression	Dealing with compressed data.
Base 64	Conversion to or from base 64 encoding.
Checksum/Hash	Computing cryptographic hashes.
Parsing HTML/XML	Parsing HTML and XML.
Atomic Changes	Installing several buffer changes atomically.
Change Hooks	Supplying functions to be run when text is changed.
The Kill Ring
Kill Ring Concepts	What text looks like in the kill ring.
Kill Functions	Functions that kill text.
Yanking	How yanking is done.
Yank Commands	Commands that access the kill ring.
Low-Level Kill Ring	Functions and variables for kill ring access.
Internals of Kill Ring	Variables that hold kill ring data.
Indentation
Primitive Indent	Functions used to count and insert indentation.
Mode-Specific Indent	Customize indentation for different modes.
Region Indent	Indent all the lines in a region.
Relative Indent	Indent the current line based on previous lines.
Indent Tabs	Adjustable, typewriter-like tab stops.
Motion by Indent	Move to first non-blank character.
Text Properties
Examining Properties	Looking at the properties of one character.
Changing Properties	Setting the properties of a range of text.
Property Search	Searching for where a property changes value.
Special Properties	Particular properties with special meanings.
Format Properties	Properties for representing formatting of text.
Sticky Properties	How inserted text gets properties from neighboring text.
Lazy Properties	Computing text properties in a lazy fashion only when text is examined.
Clickable Text	Using text properties to make regions of text do something when you click on them.
Fields	The field property defines fields within the buffer.
Not Intervals	Why text properties do not use Lisp-visible text intervals.
Parsing HTML and XML
Document Object Model	Access, manipulate and search the DOM.
Non-ASCII Characters
Text Representations	How Emacs represents text.
Disabling Multibyte	Controlling whether to use multibyte characters.
Converting Representations	Converting unibyte to multibyte and vice versa.
Selecting a Representation	Treating a byte sequence as unibyte or multi.
Character Codes	How unibyte and multibyte relate to codes of individual characters.
Character Properties	Character attributes that define their behavior and handling.
Character Sets	The space of possible character codes is divided into various character sets.
Scanning Charsets	Which character sets are used in a buffer?
Translation of Characters	Translation tables are used for conversion.
Coding Systems	Coding systems are conversions for saving files.
Input Methods	Input methods allow users to enter various non-ASCII characters without special keyboards.
Locales	Interacting with the POSIX locale.
Coding Systems
Coding System Basics	Basic concepts.
Encoding and I/O	How file I/O functions handle coding systems.
Lisp and Coding Systems	Functions to operate on coding system names.
User-Chosen Coding Systems	Asking the user to choose a coding system.
Default Coding Systems	Controlling the default choices.
Specifying Coding Systems	Requesting a particular coding system for a single file operation.
Explicit Encoding	Encoding or decoding text without doing I/O.
Terminal I/O Encoding	Use of encoding for terminal I/O.
Searching and Matching
String Search	Search for an exact match.
Searching and Case	Case-independent or case-significant searching.
Regular Expressions	Describing classes of strings.
Regexp Search	Searching for a match for a regexp.
POSIX Regexps	Searching POSIX-style for the longest match.
Match Data	Finding out which part of the text matched, after a string or regexp search.
Search and Replace	Commands that loop, searching and replacing.
Standard Regexps	Useful regexps for finding sentences, pages,...
Regular Expressions
Syntax of Regexps	Rules for writing regular expressions.
Regexp Example	Illustrates regular expression syntax.
Regexp Functions	Functions for operating on regular expressions.
Syntax of Regular Expressions
Regexp Special	Special characters in regular expressions.
Char Classes	Character classes used in regular expressions.
Regexp Backslash	Backslash-sequences in regular expressions.
The Match Data
Replacing Match	Replacing a substring that was matched.
Simple Match Data	Accessing single items of match data, such as where a particular subexpression started.
Entire Match Data	Accessing the entire match data at once, as a list.
Saving Match Data	Saving and restoring the match data.
Syntax Tables
Syntax Basics	Basic concepts of syntax tables.
Syntax Descriptors	How characters are classified.
Syntax Table Functions	How to create, examine and alter syntax tables.
Syntax Properties	Overriding syntax with text properties.
Motion and Syntax	Moving over characters with certain syntaxes.
Parsing Expressions	Parsing balanced expressions using the syntax table.
Syntax Table Internals	How syntax table information is stored.
Categories	Another way of classifying character syntax.
Syntax Descriptors
Syntax Class Table	Table of syntax classes.
Syntax Flags	Additional flags each character can have.
Parsing Expressions
Motion via Parsing	Motion functions that work by parsing.
Position Parse	Determining the syntactic state of a position.
Parser State	How Emacs represents a syntactic state.
Low-Level Parsing	Parsing across a specified region.
Control Parsing	Parameters that affect parsing.
Abbrevs and Abbrev Expansion
Abbrev Tables	Creating and working with abbrev tables.
Defining Abbrevs	Specifying abbreviations and their expansions.
Abbrev Files	Saving abbrevs in files.
Abbrev Expansion	Controlling expansion; expansion subroutines.
Standard Abbrev Tables	Abbrev tables used by various major modes.
Abbrev Properties	How to read and set abbrev properties. Which properties have which effect.
Abbrev Table Properties	How to read and set abbrev table properties. Which properties have which effect.
Processes
Subprocess Creation	Functions that start subprocesses.
Shell Arguments	Quoting an argument to pass it to a shell.
Synchronous Processes	Details of using synchronous subprocesses.
Asynchronous Processes	Starting up an asynchronous subprocess.
Deleting Processes	Eliminating an asynchronous subprocess.
Process Information	Accessing run-status and other attributes.
Input to Processes	Sending input to an asynchronous subprocess.
Signals to Processes	Stopping, continuing or interrupting an asynchronous subprocess.
Output from Processes	Collecting output from an asynchronous subprocess.
Sentinels	Sentinels run when process run-status changes.
Query Before Exit	Whether to query if exiting will kill a process.
System Processes	Accessing other processes running on your system.
Transaction Queues	Transaction-based communication with subprocesses.
Network	Opening network connections.
Network Servers	Network servers let Emacs accept net connections.
Datagrams	UDP network connections.
Low-Level Network	Lower-level but more general function to create connections and servers.
Misc Network	Additional relevant functions for net connections.
Serial Ports	Communicating with serial ports.
Byte Packing	Using bindat to pack and unpack binary data.
Receiving Output from Processes
Process Buffers	By default, output is put in a buffer.
Filter Functions	Filter functions accept output from the process.
Decoding Output	Filters can get unibyte or multibyte strings.
Accepting Output	How to wait until process output arrives.
Low-Level Network Access
Network Processes	Using make-network-process.
Network Options	Further control over network connections.
Network Feature Testing	Determining which network features work on the machine you are using.
Packing and Unpacking Byte Arrays
Bindat Spec	Describing data layout.
Bindat Functions	Doing the unpacking and packing.
Bindat Examples	Samples of what bindat.el can do for you!
Emacs Display
Refresh Screen	Clearing the screen and redrawing everything on it.
Forcing Redisplay	Forcing redisplay.
Truncation	Folding or wrapping long text lines.
The Echo Area	Displaying messages at the bottom of the screen.
Warnings	Displaying warning messages for the user.
Invisible Text	Hiding part of the buffer text.
Selective Display	Hiding part of the buffer text (the old way).
Temporary Displays	Displays that go away automatically.
Overlays	Use overlays to highlight parts of the buffer.
Size of Displayed Text	How large displayed text is.
Line Height	Controlling the height of lines.
Faces	A face defines a graphics style for text characters: font, colors, etc.
Fringes	Controlling window fringes.
Scroll Bars	Controlling scroll bars.
Window Dividers	Separating windows visually.
Display Property	Enabling special display features.
Images	Displaying images in Emacs buffers.
Buttons	Adding clickable buttons to Emacs buffers.
Abstract Display	Emacs's Widget for Object Collections.
Blinking	How Emacs shows the matching open parenthesis.
Character Display	How Emacs displays individual characters.
Beeping	Audible signal to the user.
Window Systems	Which window system is being used.
Tooltips	Tooltip display in Emacs.
Bidirectional Display	Display of bidirectional scripts, such as Arabic and Farsi.
The Echo Area
Displaying Messages	Explicitly displaying text in the echo area.
Progress	Informing user about progress of a long operation.
Logging Messages	Echo area messages are logged for the user.
Echo Area Customization	Controlling the echo area.
Reporting Warnings
Warning Basics	Warnings concepts and functions to report them.
Warning Variables	Variables programs bind to customize their warnings.
Warning Options	Variables users set to control display of warnings.
Delayed Warnings	Deferring a warning until the end of a command.
Overlays
Managing Overlays	Creating and moving overlays.
Overlay Properties	How to read and set properties. What properties do to the screen display.
Finding Overlays	Searching for overlays.
Faces
Face Attributes	What is in a face?
Defining Faces	How to define a face.
Attribute Functions	Functions to examine and set face attributes.
Displaying Faces	How Emacs combines the faces specified for a character.
Face Remapping	Remapping faces to alternative definitions.
Face Functions	How to define and examine faces.
Auto Faces	Hook for automatic face assignment.
Basic Faces	Faces that are defined by default.
Font Selection	Finding the best available font for a face.
Font Lookup	Looking up the names of available fonts and information about them.
Fontsets	A fontset is a collection of fonts that handle a range of character sets.
Low-Level Font	Lisp representation for character display fonts.
Fringes
Fringe Size/Pos	Specifying where to put the window fringes.
Fringe Indicators	Displaying indicator icons in the window fringes.
Fringe Cursors	Displaying cursors in the right fringe.
Fringe Bitmaps	Specifying bitmaps for fringe indicators.
Customizing Bitmaps	Specifying your own bitmaps to use in the fringes.
Overlay Arrow	Display of an arrow to indicate position.
The display Property
Replacing Specs	Display specs that replace the text.
Specified Space	Displaying one space with a specified width.
Pixel Specification	Specifying space width or height in pixels.
Other Display Specs	Displaying an image; adjusting the height, spacing, and other properties of text.
Display Margins	Displaying text or images to the side of the main text.
Images
Image Formats	Supported image formats.
Image Descriptors	How to specify an image for use in :display.
XBM Images	Special features for XBM format.
XPM Images	Special features for XPM format.
PostScript Images	Special features for PostScript format.
ImageMagick Images	Special features available through ImageMagick.
Other Image Types	Various other formats are supported.
Defining Images	Convenient ways to define an image for later use.
Showing Images	Convenient ways to display an image once it is defined.
Multi-Frame Images	Some images contain more than one frame.
Image Cache	Internal mechanisms of image display.
Buttons
Button Properties	Button properties with special meanings.
Button Types	Defining common properties for classes of buttons.
Making Buttons	Adding buttons to Emacs buffers.
Manipulating Buttons	Getting and setting properties of buttons.
Button Buffer Commands	Buffer-wide commands and bindings for buttons.
Abstract Display
Abstract Display Functions	Functions in the Ewoc package.
Abstract Display Example	Example of using Ewoc.
Character Display
Usual Display	The usual conventions for displaying characters.
Display Tables	What a display table consists of.
Active Display Table	How Emacs selects a display table to use.
Glyphs	How to define a glyph, and what glyphs mean.
Glyphless Chars	How glyphless characters are drawn.
Operating System Interface
Starting Up	Customizing Emacs startup processing.
Getting Out	How exiting works (permanent or temporary).
System Environment	Distinguish the name and kind of system.
User Identification	Finding the name and user id of the user.
Time of Day	Getting the current time.
Time Conversion	Converting a time from numeric form to calendrical data and vice versa.
Time Parsing	Converting a time from numeric form to text and vice versa.
Processor Run Time	Getting the run time used by Emacs.
Time Calculations	Adding, subtracting, comparing times, etc.
Timers	Setting a timer to call a function at a certain time.
Idle Timers	Setting a timer to call a function when Emacs has been idle for a certain length of time.
Terminal Input	Accessing and recording terminal input.
Terminal Output	Controlling and recording terminal output.
Sound Output	Playing sounds on the computer's speaker.
X11 Keysyms	Operating on key symbols for X Windows.
Batch Mode	Running Emacs without terminal interaction.
Session Management	Saving and restoring state with X Session Management.
Desktop Notifications	Desktop notifications.
File Notifications	File notifications.
Dynamic Libraries	On-demand loading of support libraries.
Security Considerations	Running Emacs in an unfriendly environment.
Starting Up Emacs
Startup Summary	Sequence of actions Emacs performs at startup.
Init File	Details on reading the init file.
Terminal-Specific	How the terminal-specific Lisp file is read.
Command-Line Arguments	How command-line arguments are processed, and how you can customize them.
Getting Out of Emacs
Killing Emacs	Exiting Emacs irreversibly.
Suspending Emacs	Exiting Emacs reversibly.
Terminal Input
Input Modes	Options for how input is processed.
Recording Input	Saving histories of recent or all input events.
Preparing Lisp code for distribution
Packaging Basics	The basic concepts of Emacs Lisp packages.
Simple Packages	How to package a single .el file.
Multi-file Packages	How to package multiple files.
Package Archives	Maintaining package archives.
Tips and Conventions
Coding Conventions	Conventions for clean and robust programs.
Key Binding Conventions	Which keys should be bound by which programs.
Programming Tips	Making Emacs code fit smoothly in Emacs.
Compilation Tips	Making compiled code run fast.
Warning Tips	Turning off compiler warnings.
Documentation Tips	Writing readable documentation strings.
Comment Tips	Conventions for writing comments.
Library Headers	Standard headers for library packages.
GNU Emacs Internals
Building Emacs	How the dumped Emacs is made.
Pure Storage	Kludge to make preloaded Lisp functions shareable.
Garbage Collection	Reclaiming space for Lisp objects no longer used.
Stack-allocated Objects	Temporary conses and strings on C stack.
Memory Usage	Info about total size of Lisp objects made so far.
C Dialect	What C variant Emacs is written in.
Writing Emacs Primitives	Writing C code for Emacs.
Object Internals	Data formats of buffers, windows, processes.
C Integer Types	How C integer types are used inside Emacs.
Object Internals
Buffer Internals	Components of a buffer structure.
Window Internals	Components of a window structure.
Process Internals	Components of a process structure.

1 Introduction

Most of the GNU Emacs text editor is written in the programming language called Emacs Lisp. You can write new code in Emacs Lisp and install it as an extension to the editor. However, Emacs Lisp is more than a mere extension language; it is a full computer programming language in its own right. You can use it as you would any other programming language.

Because Emacs Lisp is designed for use in an editor, it has special features for scanning and parsing text as well as features for handling files, buffers, displays, subprocesses, and so on. Emacs Lisp is closely integrated with the editing facilities; thus, editing commands are functions that can also conveniently be called from Lisp programs, and parameters for customization are ordinary Lisp variables.

This manual attempts to be a full description of Emacs Lisp. For a beginner's introduction to Emacs Lisp, see An Introduction to Emacs Lisp Programming, by Bob Chassell, also published by the Free Software Foundation. This manual presumes considerable familiarity with the use of Emacs for editing; see The GNU Emacs Manual for this basic information.

Generally speaking, the earlier chapters describe features of Emacs Lisp that have counterparts in many programming languages, and later chapters describe features that are peculiar to Emacs Lisp or relate specifically to editing.

This is the GNU Emacs Lisp Reference Manual, corresponding to Emacs version 25.2.

• Caveats: Flaws and a request for help.
• Lisp History: Emacs Lisp is descended from Maclisp.
• Conventions: How the manual is formatted.
• Version Info: Which Emacs version is running?
• Acknowledgments: The authors, editors, and sponsors of this manual.

1.1 Caveats

This manual has gone through numerous drafts. It is nearly complete but not flawless. There are a few topics that are not covered, either because we consider them secondary (such as most of the individual modes) or because they are yet to be written. Because we are not able to deal with them completely, we have left out several parts intentionally.

The manual should be fully correct in what it does cover, and it is therefore open to criticism on anything it says—from specific examples and descriptive text, to the ordering of chapters and sections. If something is confusing, or you find that you have to look at the sources or experiment to learn something not covered in the manual, then perhaps the manual should be fixed. Please let us know.

As you use this manual, we ask that you send corrections as soon as you find them. If you think of a simple, real life example for a function or group of functions, please make an effort to write it up and send it in. Please reference any comments to the node name and function or variable name, as appropriate. Also state the number of the edition you are criticizing.

Please send comments and corrections using M-x report-emacs-bug.

1.2 Lisp History

Lisp (LISt Processing language) was first developed in the late 1950s at the Massachusetts Institute of Technology for research in artificial intelligence. The great power of the Lisp language makes it ideal for other purposes as well, such as writing editing commands.

Dozens of Lisp implementations have been built over the years, each with its own idiosyncrasies. Many of them were inspired by Maclisp, which was written in the 1960s at MIT's Project MAC. Eventually the implementers of the descendants of Maclisp came together and developed a standard for Lisp systems, called Common Lisp. In the meantime, Gerry Sussman and Guy Steele at MIT developed a simplified but very powerful dialect of Lisp, called Scheme.

GNU Emacs Lisp is largely inspired by Maclisp, and a little by Common Lisp. If you know Common Lisp, you will notice many similarities. However, many features of Common Lisp have been omitted or simplified in order to reduce the memory requirements of GNU Emacs. Sometimes the simplifications are so drastic that a Common Lisp user might be very confused. We will occasionally point out how GNU Emacs Lisp differs from Common Lisp. If you don't know Common Lisp, don't worry about it; this manual is self-contained.

A certain amount of Common Lisp emulation is available via the cl-lib library. See Overview.

Emacs Lisp is not at all influenced by Scheme; but the GNU project has an implementation of Scheme, called Guile. We use it in all new GNU software that calls for extensibility.

1.3 Conventions

This section explains the notational conventions that are used in this manual. You may want to skip this section and refer back to it later.

• Some Terms: Explanation of terms we use in this manual.
• nil and t: How the symbols nil and t are used.
• Evaluation Notation: The format we use for examples of evaluation.
• Printing Notation: The format we use when examples print text.
• Error Messages: The format we use for examples of errors.
• Buffer Text Notation: The format we use for buffer contents in examples.
• Format of Descriptions: Notation for describing functions, variables, etc.

1.3.1 Some Terms

Throughout this manual, the phrases “the Lisp reader” and “the Lisp printer” refer to those routines in Lisp that convert textual representations of Lisp objects into actual Lisp objects, and vice versa. See Printed Representation, for more details. You, the person reading this manual, are thought of as the programmer and are addressed as “you”. The user is the person who uses Lisp programs, including those you write.

Examples of Lisp code are formatted like this: (list 1 2 3). Names that represent metasyntactic variables, or arguments to a function being described, are formatted like this: first-number.

1.3.2 nil and t

In Emacs Lisp, the symbol nil has three separate meanings: it is a symbol with the name ‘nil’; it is the logical truth value false; and it is the empty list—the list of zero elements. When used as a variable, nil always has the value nil.

As far as the Lisp reader is concerned, ‘()’ and ‘nil’ are identical: they stand for the same object, the symbol nil. The different ways of writing the symbol are intended entirely for human readers. After the Lisp reader has read either ‘()’ or ‘nil’, there is no way to determine which representation was actually written by the programmer.

In this manual, we write () when we wish to emphasize that it means the empty list, and we write nil when we wish to emphasize that it means the truth value false. That is a good convention to use in Lisp programs also.

     (cons 'foo ())                ; Emphasize the empty list
     (setq foo-flag nil)           ; Emphasize the truth value false

In contexts where a truth value is expected, any non-nil value is considered to be true. However, t is the preferred way to represent the truth value true. When you need to choose a value that represents true, and there is no other basis for choosing, use t. The symbol t always has the value t.

In Emacs Lisp, nil and t are special symbols that always evaluate to themselves. This is so that you do not need to quote them to use them as constants in a program. An attempt to change their values results in a setting-constant error. See Constant Variables.

1.3.3 Evaluation Notation

A Lisp expression that you can evaluate is called a form. Evaluating a form always produces a result, which is a Lisp object. In the examples in this manual, this is indicated with ‘⇒’:

     (car '(1 2))
          ⇒ 1

You can read this as “(car '(1 2)) evaluates to 1”.

When a form is a macro call, it expands into a new form for Lisp to evaluate. We show the result of the expansion with ‘==>’. We may or may not show the result of the evaluation of the expanded form.

     (third '(a b c))
          ==> (car (cdr (cdr '(a b c))))
          ⇒ c

To help describe one form, we sometimes show another form that produces identical results. The exact equivalence of two forms is indicated with ‘==’.

     (make-sparse-keymap) == (list 'keymap)

1.3.4 Printing Notation

Many of the examples in this manual print text when they are evaluated. If you execute example code in a Lisp Interaction buffer (such as the buffer *scratch*), the printed text is inserted into the buffer. If you execute the example by other means (such as by evaluating the function eval-region), the printed text is displayed in the echo area.

Examples in this manual indicate printed text with ‘-|’, irrespective of where that text goes. The value returned by evaluating the form follows on a separate line with ‘⇒’.

     (progn (prin1 'foo) (princ "\n") (prin1 'bar))
          -| foo
          -| bar
          ⇒ bar

1.3.5 Error Messages

Some examples signal errors. This normally displays an error message in the echo area. We show the error message on a line starting with ‘error-->’. Note that ‘error-->’ itself does not appear in the echo area.

     (+ 23 'x)
     error--> Wrong type argument: number-or-marker-p, x

1.3.6 Buffer Text Notation

Some examples describe modifications to the contents of a buffer, by showing the before and after versions of the text. These examples show the contents of the buffer in question between two lines of dashes containing the buffer name. In addition, ‘-!-’ indicates the location of point. (The symbol for point, of course, is not part of the text in the buffer; it indicates the place between two characters where point is currently located.)

     ---------- Buffer: foo ----------
     This is the -!-contents of foo.
     ---------- Buffer: foo ----------
     
     (insert "changed ")
          ⇒ nil
     ---------- Buffer: foo ----------
     This is the changed -!-contents of foo.
     ---------- Buffer: foo ----------

1.3.7 Format of Descriptions

Functions, variables, macros, commands, user options, and special forms are described in this manual in a uniform format. The first line of a description contains the name of the item followed by its arguments, if any. The category—function, variable, or whatever—appears at the beginning of the line. The description follows on succeeding lines, sometimes with examples.

• A Sample Function Description: A description of an imaginary function, foo.
• A Sample Variable Description: A description of an imaginary variable, electric-future-map.

1.3.7.1 A Sample Function Description

In a function description, the name of the function being described appears first. It is followed on the same line by a list of argument names. These names are also used in the body of the description, to stand for the values of the arguments.

The appearance of the keyword &optional in the argument list indicates that the subsequent arguments may be omitted (omitted arguments default to nil). Do not write &optional when you call the function.

The keyword &rest (which must be followed by a single argument name) indicates that any number of arguments can follow. The single argument name following &rest receives, as its value, a list of all the remaining arguments passed to the function. Do not write &rest when you call the function.

Here is a description of an imaginary function foo:

By convention, any argument whose name contains the name of a type (e.g., integer, integer1 or buffer) is expected to be of that type. A plural of a type (such as buffers) often means a list of objects of that type. An argument named object may be of any type. (For a list of Emacs object types, see Lisp Data Types.) An argument with any other sort of name (e.g., new-file) is specific to the function; if the function has a documentation string, the type of the argument should be described there (see Documentation).

See Lambda Expressions, for a more complete description of arguments modified by &optional and &rest.

Command, macro, and special form descriptions have the same format, but the word ‘Function’ is replaced by ‘Command’, ‘Macro’, or ‘Special Form’, respectively. Commands are simply functions that may be called interactively; macros process their arguments differently from functions (the arguments are not evaluated), but are presented the same way.

The descriptions of macros and special forms use a more complex notation to specify optional and repeated arguments, because they can break the argument list down into separate arguments in more complicated ways. ‘[optional-arg]’ means that optional-arg is optional and ‘repeated-args...’ stands for zero or more arguments. Parentheses are used when several arguments are grouped into additional levels of list structure. Here is an example:

1.3.7.2 A Sample Variable Description

A variable is a name that can be bound (or set) to an object. The object to which a variable is bound is called a value; we say also that variable holds that value. Although nearly all variables can be set by the user, certain variables exist specifically so that users can change them; these are called user options. Ordinary variables and user options are described using a format like that for functions, except that there are no arguments.

Here is a description of the imaginary electric-future-map variable.

User option descriptions have the same format, but ‘Variable’ is replaced by ‘User Option’.

1.4 Version Information

These facilities provide information about which version of Emacs is in use.

1.5 Acknowledgments

This manual was originally written by Robert Krawitz, Bil Lewis, Dan LaLiberte, Richard M. Stallman and Chris Welty, the volunteers of the GNU manual group, in an effort extending over several years. Robert J. Chassell helped to review and edit the manual, with the support of the Defense Advanced Research Projects Agency, ARPA Order 6082, arranged by Warren A. Hunt, Jr. of Computational Logic, Inc. Additional sections have since been written by Miles Bader, Lars Brinkhoff, Chong Yidong, Kenichi Handa, Lute Kamstra, Juri Linkov, Glenn Morris, Thien-Thi Nguyen, Dan Nicolaescu, Martin Rudalics, Kim F. Storm, Luc Teirlinck, and Eli Zaretskii, and others.

Corrections were supplied by Drew Adams, Juanma Barranquero, Karl Berry, Jim Blandy, Bard Bloom, Stephane Boucher, David Boyes, Alan Carroll, Richard Davis, Lawrence R. Dodd, Peter Doornbosch, David A. Duff, Chris Eich, Beverly Erlebacher, David Eckelkamp, Ralf Fassel, Eirik Fuller, Stephen Gildea, Bob Glickstein, Eric Hanchrow, Jesper Harder, George Hartzell, Nathan Hess, Masayuki Ida, Dan Jacobson, Jak Kirman, Bob Knighten, Frederick M. Korz, Joe Lammens, Glenn M. Lewis, K. Richard Magill, Brian Marick, Roland McGrath, Stefan Monnier, Skip Montanaro, John Gardiner Myers, Thomas A. Peterson, Francesco Potortì, Friedrich Pukelsheim, Arnold D. Robbins, Raul Rockwell, Jason Rumney, Per Starbäck, Shinichirou Sugou, Kimmo Suominen, Edward Tharp, Bill Trost, Rickard Westman, Jean White, Eduard Wiebe, Matthew Wilding, Carl Witty, Dale Worley, Rusty Wright, and David D. Zuhn.

For a more complete list of contributors, please see the relevant change log entries in the Emacs source repository.

2 Lisp Data Types

A Lisp object is a piece of data used and manipulated by Lisp programs. For our purposes, a type or data type is a set of possible objects.

Every object belongs to at least one type. Objects of the same type have similar structures and may usually be used in the same contexts. Types can overlap, and objects can belong to two or more types. Consequently, we can ask whether an object belongs to a particular type, but not for the type of an object.

A few fundamental object types are built into Emacs. These, from which all other types are constructed, are called primitive types. Each object belongs to one and only one primitive type. These types include integer, float, cons, symbol, string, vector, hash-table, subr, and byte-code function, plus several special types, such as buffer, that are related to editing. (See Editing Types.)

Each primitive type has a corresponding Lisp function that checks whether an object is a member of that type.

Lisp is unlike many other languages in that its objects are self-typing: the primitive type of each object is implicit in the object itself. For example, if an object is a vector, nothing can treat it as a number; Lisp knows it is a vector, not a number.

In most languages, the programmer must declare the data type of each variable, and the type is known by the compiler but not represented in the data. Such type declarations do not exist in Emacs Lisp. A Lisp variable can have any type of value, and it remembers whatever value you store in it, type and all. (Actually, a small number of Emacs Lisp variables can only take on values of a certain type. See Variables with Restricted Values.)

This chapter describes the purpose, printed representation, and read syntax of each of the standard types in GNU Emacs Lisp. Details on how to use these types can be found in later chapters.

• Printed Representation: How Lisp objects are represented as text.
• Comments: Comments and their formatting conventions.
• Programming Types: Types found in all Lisp systems.
• Editing Types: Types specific to Emacs.
• Circular Objects: Read syntax for circular structure.
• Type Predicates: Tests related to types.
• Equality Predicates: Tests of equality between any two objects.

2.1 Printed Representation and Read Syntax

The printed representation of an object is the format of the output generated by the Lisp printer (the function prin1) for that object. Every data type has a unique printed representation. The read syntax of an object is the format of the input accepted by the Lisp reader (the function read) for that object. This is not necessarily unique; many kinds of object have more than one syntax. See Read and Print.

In most cases, an object's printed representation is also a read syntax for the object. However, some types have no read syntax, since it does not make sense to enter objects of these types as constants in a Lisp program. These objects are printed in hash notation, which consists of the characters ‘#<’, a descriptive string (typically the type name followed by the name of the object), and a closing ‘>’. For example:

     (current-buffer)
          ⇒ #<buffer objects.texi>

Hash notation cannot be read at all, so the Lisp reader signals the error invalid-read-syntax whenever it encounters ‘#<’. In other languages, an expression is text; it has no other form. In Lisp, an expression is primarily a Lisp object and only secondarily the text that is the object's read syntax. Often there is no need to emphasize this distinction, but you must keep it in the back of your mind, or you will occasionally be very confused.

When you evaluate an expression interactively, the Lisp interpreter first reads the textual representation of it, producing a Lisp object, and then evaluates that object (see Evaluation). However, evaluation and reading are separate activities. Reading returns the Lisp object represented by the text that is read; the object may or may not be evaluated later. See Input Functions, for a description of read, the basic function for reading objects.

2.2 Comments

A comment is text that is written in a program only for the sake of humans that read the program, and that has no effect on the meaning of the program. In Lisp, a semicolon (‘;’) starts a comment if it is not within a string or character constant. The comment continues to the end of line. The Lisp reader discards comments; they do not become part of the Lisp objects which represent the program within the Lisp system.

The ‘#@count’ construct, which skips the next count characters, is useful for program-generated comments containing binary data. The Emacs Lisp byte compiler uses this in its output files (see Byte Compilation). It isn't meant for source files, however.

See Comment Tips, for conventions for formatting comments.

2.3 Programming Types

There are two general categories of types in Emacs Lisp: those having to do with Lisp programming, and those having to do with editing. The former exist in many Lisp implementations, in one form or another. The latter are unique to Emacs Lisp.

• Integer Type: Numbers without fractional parts.
• Floating-Point Type: Numbers with fractional parts and with a large range.
• Character Type: The representation of letters, numbers and control characters.
• Symbol Type: A multi-use object that refers to a function, variable, or property list, and has a unique identity.
• Sequence Type: Both lists and arrays are classified as sequences.
• Cons Cell Type: Cons cells, and lists (which are made from cons cells).
• Array Type: Arrays include strings and vectors.
• String Type: An (efficient) array of characters.
• Vector Type: One-dimensional arrays.
• Char-Table Type: One-dimensional sparse arrays indexed by characters.
• Bool-Vector Type: One-dimensional arrays of t or nil.
• Hash Table Type: Super-fast lookup tables.
• Function Type: A piece of executable code you can call from elsewhere.
• Macro Type: A method of expanding an expression into another expression, more fundamental but less pretty.
• Primitive Function Type: A function written in C, callable from Lisp.
• Byte-Code Type: A function written in Lisp, then compiled.
• Autoload Type: A type used for automatically loading seldom-used functions.
• Finalizer Type: Runs code when no longer reachable.

2.3.1 Integer Type

The range of values for an integer depends on the machine. The minimum range is −536,870,912 to 536,870,911 (30 bits; i.e., −2**29 to 2**29 − 1) but many machines provide a wider range. Emacs Lisp arithmetic functions do not check for integer overflow. Thus (1+ 536870911) is −536,870,912 if Emacs integers are 30 bits.

The read syntax for integers is a sequence of (base ten) digits with an optional sign at the beginning and an optional period at the end. The printed representation produced by the Lisp interpreter never has a leading ‘+’ or a final ‘.’.

     -1               ; The integer −1.
     1                ; The integer 1.
     1.               ; Also the integer 1.
     +1               ; Also the integer 1.

As a special exception, if a sequence of digits specifies an integer too large or too small to be a valid integer object, the Lisp reader reads it as a floating-point number (see Floating-Point Type). For instance, if Emacs integers are 30 bits, 536870912 is read as the floating-point number 536870912.0.

See Numbers, for more information.

2.3.2 Floating-Point Type

Floating-point numbers are the computer equivalent of scientific notation; you can think of a floating-point number as a fraction together with a power of ten. The precise number of significant figures and the range of possible exponents is machine-specific; Emacs uses the C data type double to store the value, and internally this records a power of 2 rather than a power of 10.

The printed representation for floating-point numbers requires either a decimal point (with at least one digit following), an exponent, or both. For example, ‘1500.0’, ‘+15e2’, ‘15.0e+2’, ‘+1500000e-3’, and ‘.15e4’ are five ways of writing a floating-point number whose value is 1500. They are all equivalent.

See Numbers, for more information.

2.3.3 Character Type

A character in Emacs Lisp is nothing more than an integer. In other words, characters are represented by their character codes. For example, the character A is represented as the integer 65.

Individual characters are used occasionally in programs, but it is more common to work with strings, which are sequences composed of characters. See String Type.

Characters in strings and buffers are currently limited to the range of 0 to 4194303—twenty two bits (see Character Codes). Codes 0 through 127 are ASCII codes; the rest are non-ASCII (see Non-ASCII Characters). Characters that represent keyboard input have a much wider range, to encode modifier keys such as Control, Meta and Shift.

There are special functions for producing a human-readable textual description of a character for the sake of messages. See Describing Characters.

• Basic Char Syntax: Syntax for regular characters.
• General Escape Syntax: How to specify characters by their codes.
• Ctl-Char Syntax: Syntax for control characters.
• Meta-Char Syntax: Syntax for meta-characters.
• Other Char Bits: Syntax for hyper-, super-, and alt-characters.

2.3.3.1 Basic Char Syntax

Since characters are really integers, the printed representation of a character is a decimal number. This is also a possible read syntax for a character, but writing characters that way in Lisp programs is not clear programming. You should always use the special read syntax formats that Emacs Lisp provides for characters. These syntax formats start with a question mark.

The usual read syntax for alphanumeric characters is a question mark followed by the character; thus, ‘?A’ for the character A, ‘?B’ for the character B, and ‘?a’ for the character a.

For example:

     ?Q ⇒ 81     ?q ⇒ 113

You can use the same syntax for punctuation characters, but it is often a good idea to add a ‘\’ so that the Emacs commands for editing Lisp code don't get confused. For example, ‘?\(’ is the way to write the open-paren character. If the character is ‘\’, you must use a second ‘\’ to quote it: ‘?\\’.

You can express the characters control-g, backspace, tab, newline, vertical tab, formfeed, space, return, del, and escape as ‘?\a’, ‘?\b’, ‘?\t’, ‘?\n’, ‘?\v’, ‘?\f’, ‘?\s’, ‘?\r’, ‘?\d’, and ‘?\e’, respectively. (‘?\s’ followed by a dash has a different meaning—it applies the Super modifier to the following character.) Thus,

     ?\a ⇒ 7                 ; control-g, C-g
     ?\b ⇒ 8                 ; backspace, <BS>, C-h
     ?\t ⇒ 9                 ; tab, <TAB>, C-i
     ?\n ⇒ 10                ; newline, C-j
     ?\v ⇒ 11                ; vertical tab, C-k
     ?\f ⇒ 12                ; formfeed character, C-l
     ?\r ⇒ 13                ; carriage return, <RET>, C-m
     ?\e ⇒ 27                ; escape character, <ESC>, C-[
     ?\s ⇒ 32                ; space character, <SPC>
     ?\\ ⇒ 92                ; backslash character, \
     ?\d ⇒ 127               ; delete character, <DEL>

These sequences which start with backslash are also known as escape sequences, because backslash plays the role of an escape character; this has nothing to do with the character <ESC>. ‘\s’ is meant for use in character constants; in string constants, just write the space.

A backslash is allowed, and harmless, preceding any character without a special escape meaning; thus, ‘?\+’ is equivalent to ‘?+’. There is no reason to add a backslash before most characters. However, you should add a backslash before any of the characters ‘()\|;'`"#.,’ to avoid confusing the Emacs commands for editing Lisp code. You can also add a backslash before whitespace characters such as space, tab, newline and formfeed. However, it is cleaner to use one of the easily readable escape sequences, such as ‘\t’ or ‘\s’, instead of an actual whitespace character such as a tab or a space. (If you do write backslash followed by a space, you should write an extra space after the character constant to separate it from the following text.)

2.3.3.2 General Escape Syntax

In addition to the specific escape sequences for special important control characters, Emacs provides several types of escape syntax that you can use to specify non-ASCII text characters.

Firstly, you can specify characters by their Unicode values. ?\unnnn represents a character with Unicode code point ‘U+nnnn’, where nnnn is (by convention) a hexadecimal number with exactly four digits. The backslash indicates that the subsequent characters form an escape sequence, and the ‘u’ specifies a Unicode escape sequence.

There is a slightly different syntax for specifying Unicode characters with code points higher than U+ffff: ?\U00nnnnnn represents the character with code point ‘U+nnnnnn’, where nnnnnn is a six-digit hexadecimal number. The Unicode Standard only defines code points up to ‘U+10ffff’, so if you specify a code point higher than that, Emacs signals an error.

Secondly, you can specify characters by their hexadecimal character codes. A hexadecimal escape sequence consists of a backslash, ‘x’, and the hexadecimal character code. Thus, ‘?\x41’ is the character A, ‘?\x1’ is the character C-a, and ?\xe0 is the character à (a with grave accent). You can use any number of hex digits, so you can represent any character code in this way.

Thirdly, you can specify characters by their character code in octal. An octal escape sequence consists of a backslash followed by up to three octal digits; thus, ‘?\101’ for the character A, ‘?\001’ for the character C-a, and ?\002 for the character C-b. Only characters up to octal code 777 can be specified this way.

These escape sequences may also be used in strings. See Non-ASCII in Strings.

2.3.3.3 Control-Character Syntax

Control characters can be represented using yet another read syntax. This consists of a question mark followed by a backslash, caret, and the corresponding non-control character, in either upper or lower case. For example, both ‘?\^I’ and ‘?\^i’ are valid read syntax for the character C-i, the character whose value is 9.

Instead of the ‘^’, you can use ‘C-’; thus, ‘?\C-i’ is equivalent to ‘?\^I’ and to ‘?\^i’:

     ?\^I ⇒ 9     ?\C-I ⇒ 9

In strings and buffers, the only control characters allowed are those that exist in ASCII; but for keyboard input purposes, you can turn any character into a control character with ‘C-’. The character codes for these non-ASCII control characters include the 2**26 bit as well as the code for the corresponding non-control character. Ordinary text terminals have no way of generating non-ASCII control characters, but you can generate them straightforwardly using X and other window systems.

For historical reasons, Emacs treats the <DEL> character as the control equivalent of ?:

     ?\^? ⇒ 127     ?\C-? ⇒ 127

As a result, it is currently not possible to represent the character Control-?, which is a meaningful input character under X, using ‘\C-’. It is not easy to change this, as various Lisp files refer to <DEL> in this way.

For representing control characters to be found in files or strings, we recommend the ‘^’ syntax; for control characters in keyboard input, we prefer the ‘C-’ syntax. Which one you use does not affect the meaning of the program, but may guide the understanding of people who read it.

2.3.3.4 Meta-Character Syntax

A meta character is a character typed with the <META> modifier key. The integer that represents such a character has the 2**27 bit set. We use high bits for this and other modifiers to make possible a wide range of basic character codes.

In a string, the 2**7 bit attached to an ASCII character indicates a meta character; thus, the meta characters that can fit in a string have codes in the range from 128 to 255, and are the meta versions of the ordinary ASCII characters. See Strings of Events, for details about <META>-handling in strings.

The read syntax for meta characters uses ‘\M-’. For example, ‘?\M-A’ stands for M-A. You can use ‘\M-’ together with octal character codes (see below), with ‘\C-’, or with any other syntax for a character. Thus, you can write M-A as ‘?\M-A’, or as ‘?\M-\101’. Likewise, you can write C-M-b as ‘?\M-\C-b’, ‘?\C-\M-b’, or ‘?\M-\002’.

2.3.3.5 Other Character Modifier Bits

The case of a graphic character is indicated by its character code; for example, ASCII distinguishes between the characters ‘a’ and ‘A’. But ASCII has no way to represent whether a control character is upper case or lower case. Emacs uses the 2**25 bit to indicate that the shift key was used in typing a control character. This distinction is possible only when you use X terminals or other special terminals; ordinary text terminals do not report the distinction. The Lisp syntax for the shift bit is ‘\S-’; thus, ‘?\C-\S-o’ or ‘?\C-\S-O’ represents the shifted-control-o character.

The X Window System defines three other modifier bits that can be set in a character: hyper, super and alt. The syntaxes for these bits are ‘\H-’, ‘\s-’ and ‘\A-’. (Case is significant in these prefixes.) Thus, ‘?\H-\M-\A-x’ represents Alt-Hyper-Meta-x. (Note that ‘\s’ with no following ‘-’ represents the space character.) Numerically, the bit values are 2**22 for alt, 2**23 for super and 2**24 for hyper.

2.3.4 Symbol Type

A symbol in GNU Emacs Lisp is an object with a name. The symbol name serves as the printed representation of the symbol. In ordinary Lisp use, with one single obarray (see Creating Symbols), a symbol's name is unique—no two symbols have the same name.

A symbol can serve as a variable, as a function name, or to hold a property list. Or it may serve only to be distinct from all other Lisp objects, so that its presence in a data structure may be recognized reliably. In a given context, usually only one of these uses is intended. But you can use one symbol in all of these ways, independently.

A symbol whose name starts with a colon (‘:’) is called a keyword symbol. These symbols automatically act as constants, and are normally used only by comparing an unknown symbol with a few specific alternatives. See Constant Variables.

A symbol name can contain any characters whatever. Most symbol names are written with letters, digits, and the punctuation characters ‘-+=*/’. Such names require no special punctuation; the characters of the name suffice as long as the name does not look like a number. (If it does, write a ‘\’ at the beginning of the name to force interpretation as a symbol.) The characters ‘_~!@$%^&:<>{}?’ are less often used but also require no special punctuation. Any other characters may be included in a symbol's name by escaping them with a backslash. In contrast to its use in strings, however, a backslash in the name of a symbol simply quotes the single character that follows the backslash. For example, in a string, ‘\t’ represents a tab character; in the name of a symbol, however, ‘\t’ merely quotes the letter ‘t’. To have a symbol with a tab character in its name, you must actually use a tab (preceded with a backslash). But it's rare to do such a thing.

Common Lisp note: In Common Lisp, lower case letters are always folded to upper case, unless they are explicitly escaped. In Emacs Lisp, upper case and lower case letters are distinct.

Here are several examples of symbol names. Note that the ‘+’ in the fourth example is escaped to prevent it from being read as a number. This is not necessary in the sixth example because the rest of the name makes it invalid as a number.

     foo                 ; A symbol named ‘foo’.
     FOO                 ; A symbol named ‘FOO’, different from ‘foo’.
     1+                  ; A symbol named ‘1+’
                         ;   (not ‘+1’, which is an integer).
     \+1                 ; A symbol named ‘+1’
                         ;   (not a very readable name).
     \(*\ 1\ 2\)         ; A symbol named ‘(* 1 2)’ (a worse name).
     
     
     +-*/_~!@$%^&=:<>{}  ; A symbol named ‘+-*/_~!@$%^&=:<>{}’.
                         ;   These characters need not be escaped.

As an exception to the rule that a symbol's name serves as its printed representation, ‘##’ is the printed representation for an interned symbol whose name is an empty string. Furthermore, ‘#:foo’ is the printed representation for an uninterned symbol whose name is foo. (Normally, the Lisp reader interns all symbols; see Creating Symbols.)

2.3.5 Sequence Types

A sequence is a Lisp object that represents an ordered set of elements. There are two kinds of sequence in Emacs Lisp: lists and arrays.

Lists are the most commonly-used sequences. A list can hold elements of any type, and its length can be easily changed by adding or removing elements. See the next subsection for more about lists.

Arrays are fixed-length sequences. They are further subdivided into strings, vectors, char-tables and bool-vectors. Vectors can hold elements of any type, whereas string elements must be characters, and bool-vector elements must be t or nil. Char-tables are like vectors except that they are indexed by any valid character code. The characters in a string can have text properties like characters in a buffer (see Text Properties), but vectors do not support text properties, even when their elements happen to be characters.

Lists, strings and the other array types also share important similarities. For example, all have a length l, and all have elements which can be indexed from zero to l minus one. Several functions, called sequence functions, accept any kind of sequence. For example, the function length reports the length of any kind of sequence. See Sequences Arrays Vectors.

It is generally impossible to read the same sequence twice, since sequences are always created anew upon reading. If you read the read syntax for a sequence twice, you get two sequences with equal contents. There is one exception: the empty list () always stands for the same object, nil.

2.3.6 Cons Cell and List Types

A cons cell is an object that consists of two slots, called the car slot and the cdr slot. Each slot can hold any Lisp object. We also say that the car of this cons cell is whatever object its car slot currently holds, and likewise for the cdr.

A list is a series of cons cells, linked together so that the cdr slot of each cons cell holds either the next cons cell or the empty list. The empty list is actually the symbol nil. See Lists, for details. Because most cons cells are used as part of lists, we refer to any structure made out of cons cells as a list structure.

A note to C programmers: a Lisp list thus works as a linked list built up of cons cells. Because pointers in Lisp are implicit, we do not distinguish between a cons cell slot holding a value versus pointing to the value.

Because cons cells are so central to Lisp, we also have a word for an object which is not a cons cell. These objects are called atoms.

The read syntax and printed representation for lists are identical, and consist of a left parenthesis, an arbitrary number of elements, and a right parenthesis. Here are examples of lists:

     (A 2 "A")            ; A list of three elements.
     ()                   ; A list of no elements (the empty list).
     nil                  ; A list of no elements (the empty list).
     ("A ()")             ; A list of one element: the string "A ()".
     (A ())               ; A list of two elements: A and the empty list.
     (A nil)              ; Equivalent to the previous.
     ((A B C))            ; A list of one element
                          ;   (which is a list of three elements).

Upon reading, each object inside the parentheses becomes an element of the list. That is, a cons cell is made for each element. The car slot of the cons cell holds the element, and its cdr slot refers to the next cons cell of the list, which holds the next element in the list. The cdr slot of the last cons cell is set to hold nil.

The names car and cdr derive from the history of Lisp. The original Lisp implementation ran on an IBM 704 computer which divided words into two parts, the address and the decrement; car was an instruction to extract the contents of the address part of a register, and cdr an instruction to extract the contents of the decrement. By contrast, cons cells are named for the function cons that creates them, which in turn was named for its purpose, the construction of cells.

• Box Diagrams: Drawing pictures of lists.
• Dotted Pair Notation: A general syntax for cons cells.
• Association List Type: A specially constructed list.

2.3.6.1 Drawing Lists as Box Diagrams

A list can be illustrated by a diagram in which the cons cells are shown as pairs of boxes, like dominoes. (The Lisp reader cannot read such an illustration; unlike the textual notation, which can be understood by both humans and computers, the box illustrations can be understood only by humans.) This picture represents the three-element list (rose violet buttercup):

         --- ---      --- ---      --- ---
        |   |   |--> |   |   |--> |   |   |--> nil
         --- ---      --- ---      --- ---
          |            |            |
          |            |            |
           --> rose     --> violet   --> buttercup

In this diagram, each box represents a slot that can hold or refer to any Lisp object. Each pair of boxes represents a cons cell. Each arrow represents a reference to a Lisp object, either an atom or another cons cell.

In this example, the first box, which holds the car of the first cons cell, refers to or holds rose (a symbol). The second box, holding the cdr of the first cons cell, refers to the next pair of boxes, the second cons cell. The car of the second cons cell is violet, and its cdr is the third cons cell. The cdr of the third (and last) cons cell is nil.

Here is another diagram of the same list, (rose violet buttercup), sketched in a different manner:

      ---------------       ----------------       -------------------
     | car   | cdr   |     | car    | cdr   |     | car       | cdr   |
     | rose  |   o-------->| violet |   o-------->| buttercup |  nil  |
     |       |       |     |        |       |     |           |       |
      ---------------       ----------------       -------------------

A list with no elements in it is the empty list; it is identical to the symbol nil. In other words, nil is both a symbol and a list.

Here is the list (A ()), or equivalently (A nil), depicted with boxes and arrows:

         --- ---      --- ---
        |   |   |--> |   |   |--> nil
         --- ---      --- ---
          |            |
          |            |
           --> A        --> nil

Here is a more complex illustration, showing the three-element list, ((pine needles) oak maple), the first element of which is a two-element list:

         --- ---      --- ---      --- ---
        |   |   |--> |   |   |--> |   |   |--> nil
         --- ---      --- ---      --- ---
          |            |            |
          |            |            |
          |             --> oak      --> maple
          |
          |     --- ---      --- ---
           --> |   |   |--> |   |   |--> nil
                --- ---      --- ---
                 |            |
                 |            |
                  --> pine     --> needles

The same list represented in the second box notation looks like this:

      --------------       --------------       --------------
     | car   | cdr  |     | car   | cdr  |     | car   | cdr  |
     |   o   |   o------->| oak   |   o------->| maple |  nil |
     |   |   |      |     |       |      |     |       |      |
      -- | ---------       --------------       --------------
         |
         |
         |        --------------       ----------------
         |       | car   | cdr  |     | car     | cdr  |
          ------>| pine  |   o------->| needles |  nil |
                 |       |      |     |         |      |
                  --------------       ----------------

2.3.6.2 Dotted Pair Notation

Dotted pair notation is a general syntax for cons cells that represents the car and cdr explicitly. In this syntax, (a . b) stands for a cons cell whose car is the object a and whose cdr is the object b. Dotted pair notation is more general than list syntax because the cdr does not have to be a list. However, it is more cumbersome in cases where list syntax would work. In dotted pair notation, the list ‘(1 2 3)’ is written as ‘(1 . (2 . (3 . nil)))’. For nil-terminated lists, you can use either notation, but list notation is usually clearer and more convenient. When printing a list, the dotted pair notation is only used if the cdr of a cons cell is not a list.

Here's an example using boxes to illustrate dotted pair notation. This example shows the pair (rose . violet):

         --- ---
        |   |   |--> violet
         --- ---
          |
          |
           --> rose

You can combine dotted pair notation with list notation to represent conveniently a chain of cons cells with a non-nil final cdr. You write a dot after the last element of the list, followed by the cdr of the final cons cell. For example, (rose violet . buttercup) is equivalent to (rose . (violet . buttercup)). The object looks like this:

         --- ---      --- ---
        |   |   |--> |   |   |--> buttercup
         --- ---      --- ---
          |            |
          |            |
           --> rose     --> violet

The syntax (rose . violet . buttercup) is invalid because there is nothing that it could mean. If anything, it would say to put buttercup in the cdr of a cons cell whose cdr is already used for violet.

The list (rose violet) is equivalent to (rose . (violet)), and looks like this:

         --- ---      --- ---
        |   |   |--> |   |   |--> nil
         --- ---      --- ---
          |            |
          |            |
           --> rose     --> violet

Similarly, the three-element list (rose violet buttercup) is equivalent to (rose . (violet . (buttercup))). It looks like this:

         --- ---      --- ---      --- ---
        |   |   |--> |   |   |--> |   |   |--> nil
         --- ---      --- ---      --- ---
          |            |            |
          |            |            |
           --> rose     --> violet   --> buttercup

2.3.6.3 Association List Type

An association list or alist is a specially-constructed list whose elements are cons cells. In each element, the car is considered a key, and the cdr is considered an associated value. (In some cases, the associated value is stored in the car of the cdr.) Association lists are often used as stacks, since it is easy to add or remove associations at the front of the list.

For example,

     (setq alist-of-colors
           '((rose . red) (lily . white) (buttercup . yellow)))

sets the variable alist-of-colors to an alist of three elements. In the first element, rose is the key and red is the value.

See Association Lists, for a further explanation of alists and for functions that work on alists. See Hash Tables, for another kind of lookup table, which is much faster for handling a large number of keys.

2.3.7 Array Type

An array is composed of an arbitrary number of slots for holding or referring to other Lisp objects, arranged in a contiguous block of memory. Accessing any element of an array takes approximately the same amount of time. In contrast, accessing an element of a list requires time proportional to the position of the element in the list. (Elements at the end of a list take longer to access than elements at the beginning of a list.)

Emacs defines four types of array: strings, vectors, bool-vectors, and char-tables.

A string is an array of characters and a vector is an array of arbitrary objects. A bool-vector can hold only t or nil. These kinds of array may have any length up to the largest integer. Char-tables are sparse arrays indexed by any valid character code; they can hold arbitrary objects.

The first element of an array has index zero, the second element has index 1, and so on. This is called zero-origin indexing. For example, an array of four elements has indices 0, 1, 2, and 3. The largest possible index value is one less than the length of the array. Once an array is created, its length is fixed.

All Emacs Lisp arrays are one-dimensional. (Most other programming languages support multidimensional arrays, but they are not essential; you can get the same effect with nested one-dimensional arrays.) Each type of array has its own read syntax; see the following sections for details.

The array type is a subset of the sequence type, and contains the string type, the vector type, the bool-vector type, and the char-table type.

2.3.8 String Type

A string is an array of characters. Strings are used for many purposes in Emacs, as can be expected in a text editor; for example, as the names of Lisp symbols, as messages for the user, and to represent text extracted from buffers. Strings in Lisp are constants: evaluation of a string returns the same string.

See Strings and Characters, for functions that operate on strings.

• Syntax for Strings: How to specify Lisp strings.
• Non-ASCII in Strings: International characters in strings.
• Nonprinting Characters: Literal unprintable characters in strings.
• Text Props and Strings: Strings with text properties.

2.3.8.1 Syntax for Strings

The read syntax for a string is a double-quote, an arbitrary number of characters, and another double-quote, "like this". To include a double-quote in a string, precede it with a backslash; thus, "\"" is a string containing just one double-quote character. Likewise, you can include a backslash by preceding it with another backslash, like this: "this \\ is a single embedded backslash".

The newline character is not special in the read syntax for strings; if you write a new line between the double-quotes, it becomes a character in the string. But an escaped newline—one that is preceded by ‘\’—does not become part of the string; i.e., the Lisp reader ignores an escaped newline while reading a string. An escaped space ‘\ ’ is likewise ignored.

     "It is useful to include newlines
     in documentation strings,
     but the newline is \
     ignored if escaped."
          ⇒ "It is useful to include newlines
     in documentation strings,
     but the newline is ignored if escaped."

2.3.8.2 Non-ASCII Characters in Strings

There are two text representations for non-ASCII characters in Emacs strings: multibyte and unibyte (see Text Representations). Roughly speaking, unibyte strings store raw bytes, while multibyte strings store human-readable text. Each character in a unibyte string is a byte, i.e., its value is between 0 and 255. By contrast, each character in a multibyte string may have a value between 0 to 4194303 (see Character Type). In both cases, characters above 127 are non-ASCII.

You can include a non-ASCII character in a string constant by writing it literally. If the string constant is read from a multibyte source, such as a multibyte buffer or string, or a file that would be visited as multibyte, then Emacs reads each non-ASCII character as a multibyte character and automatically makes the string a multibyte string. If the string constant is read from a unibyte source, then Emacs reads the non-ASCII character as unibyte, and makes the string unibyte.

Instead of writing a character literally into a multibyte string, you can write it as its character code using an escape sequence. See General Escape Syntax, for details about escape sequences.

If you use any Unicode-style escape sequence ‘\uNNNN’ or ‘\U00NNNNNN’ in a string constant (even for an ASCII character), Emacs automatically assumes that it is multibyte.

You can also use hexadecimal escape sequences (‘\xn’) and octal escape sequences (‘\n’) in string constants. But beware: If a string constant contains hexadecimal or octal escape sequences, and these escape sequences all specify unibyte characters (i.e., less than 256), and there are no other literal non-ASCII characters or Unicode-style escape sequences in the string, then Emacs automatically assumes that it is a unibyte string. That is to say, it assumes that all non-ASCII characters occurring in the string are 8-bit raw bytes.

In hexadecimal and octal escape sequences, the escaped character code may contain a variable number of digits, so the first subsequent character which is not a valid hexadecimal or octal digit terminates the escape sequence. If the next character in a string could be interpreted as a hexadecimal or octal digit, write ‘\ ’ (backslash and space) to terminate the escape sequence. For example, ‘\xe0\ ’ represents one character, ‘a’ with grave accent. ‘\ ’ in a string constant is just like backslash-newline; it does not contribute any character to the string, but it does terminate any preceding hex escape.

2.3.8.3 Nonprinting Characters in Strings

You can use the same backslash escape-sequences in a string constant as in character literals (but do not use the question mark that begins a character constant). For example, you can write a string containing the nonprinting characters tab and C-a, with commas and spaces between them, like this: "\t, \C-a". See Character Type, for a description of the read syntax for characters.

However, not all of the characters you can write with backslash escape-sequences are valid in strings. The only control characters that a string can hold are the ASCII control characters. Strings do not distinguish case in ASCII control characters.

Properly speaking, strings cannot hold meta characters; but when a string is to be used as a key sequence, there is a special convention that provides a way to represent meta versions of ASCII characters in a string. If you use the ‘\M-’ syntax to indicate a meta character in a string constant, this sets the 2**7 bit of the character in the string. If the string is used in define-key or lookup-key, this numeric code is translated into the equivalent meta character. See Character Type.

Strings cannot hold characters that have the hyper, super, or alt modifiers.

2.3.8.4 Text Properties in Strings

A string can hold properties for the characters it contains, in addition to the characters themselves. This enables programs that copy text between strings and buffers to copy the text's properties with no special effort. See Text Properties, for an explanation of what text properties mean. Strings with text properties use a special read and print syntax:

     #("characters" property-data...)

where property-data consists of zero or more elements, in groups of three as follows:

     beg end plist

The elements beg and end are integers, and together specify a range of indices in the string; plist is the property list for that range. For example,

     #("foo bar" 0 3 (face bold) 3 4 nil 4 7 (face italic))

represents a string whose textual contents are ‘foo bar’, in which the first three characters have a face property with value bold, and the last three have a face property with value italic. (The fourth character has no text properties, so its property list is nil. It is not actually necessary to mention ranges with nil as the property list, since any characters not mentioned in any range will default to having no properties.)

2.3.9 Vector Type

A vector is a one-dimensional array of elements of any type. It takes a constant amount of time to access any element of a vector. (In a list, the access time of an element is proportional to the distance of the element from the beginning of the list.)

The printed representation of a vector consists of a left square bracket, the elements, and a right square bracket. This is also the read syntax. Like numbers and strings, vectors are considered constants for evaluation.

     [1 "two" (three)]      ; A vector of three elements.
          ⇒ [1 "two" (three)]

See Vectors, for functions that work with vectors.

2.3.10 Char-Table Type

A char-table is a one-dimensional array of elements of any type, indexed by character codes. Char-tables have certain extra features to make them more useful for many jobs that involve assigning information to character codes—for example, a char-table can have a parent to inherit from, a default value, and a small number of extra slots to use for special purposes. A char-table can also specify a single value for a whole character set.

The printed representation of a char-table is like a vector except that there is an extra ‘#^’ at the beginning.¹

See Char-Tables, for special functions to operate on char-tables. Uses of char-tables include:

• Case tables (see Case Tables).
• Character category tables (see Categories).
• Display tables (see Display Tables).
• Syntax tables (see Syntax Tables).

2.3.11 Bool-Vector Type

A bool-vector is a one-dimensional array whose elements must be t or nil.

The printed representation of a bool-vector is like a string, except that it begins with ‘#&’ followed by the length. The string constant that follows actually specifies the contents of the bool-vector as a bitmap—each character in the string contains 8 bits, which specify the next 8 elements of the bool-vector (1 stands for t, and 0 for nil). The least significant bits of the character correspond to the lowest indices in the bool-vector.

     (make-bool-vector 3 t)
          ⇒ #&3"^G"
     (make-bool-vector 3 nil)
          ⇒ #&3"^@"

These results make sense, because the binary code for ‘C-g’ is 111 and ‘C-@’ is the character with code 0.

If the length is not a multiple of 8, the printed representation shows extra elements, but these extras really make no difference. For instance, in the next example, the two bool-vectors are equal, because only the first 3 bits are used:

     (equal #&3"\377" #&3"\007")
          ⇒ t

2.3.12 Hash Table Type

A hash table is a very fast kind of lookup table, somewhat like an alist in that it maps keys to corresponding values, but much faster. The printed representation of a hash table specifies its properties and contents, like this:

     (make-hash-table)
          ⇒ #s(hash-table size 65 test eql rehash-size 1.5
                                  rehash-threshold 0.8 data ())

See Hash Tables, for more information about hash tables.

2.3.13 Function Type

Lisp functions are executable code, just like functions in other programming languages. In Lisp, unlike most languages, functions are also Lisp objects. A non-compiled function in Lisp is a lambda expression: that is, a list whose first element is the symbol lambda (see Lambda Expressions).

In most programming languages, it is impossible to have a function without a name. In Lisp, a function has no intrinsic name. A lambda expression can be called as a function even though it has no name; to emphasize this, we also call it an anonymous function (see Anonymous Functions). A named function in Lisp is just a symbol with a valid function in its function cell (see Defining Functions).

Most of the time, functions are called when their names are written in Lisp expressions in Lisp programs. However, you can construct or obtain a function object at run time and then call it with the primitive functions funcall and apply. See Calling Functions.

2.3.14 Macro Type

A Lisp macro is a user-defined construct that extends the Lisp language. It is represented as an object much like a function, but with different argument-passing semantics. A Lisp macro has the form of a list whose first element is the symbol macro and whose cdr is a Lisp function object, including the lambda symbol.

Lisp macro objects are usually defined with the built-in defmacro macro, but any list that begins with macro is a macro as far as Emacs is concerned. See Macros, for an explanation of how to write a macro.

Warning: Lisp macros and keyboard macros (see Keyboard Macros) are entirely different things. When we use the word “macro” without qualification, we mean a Lisp macro, not a keyboard macro.

2.3.15 Primitive Function Type

A primitive function is a function callable from Lisp but written in the C programming language. Primitive functions are also called subrs or built-in functions. (The word “subr” is derived from “subroutine”.) Most primitive functions evaluate all their arguments when they are called. A primitive function that does not evaluate all its arguments is called a special form (see Special Forms).

It does not matter to the caller of a function whether the function is primitive. However, this does matter if you try to redefine a primitive with a function written in Lisp. The reason is that the primitive function may be called directly from C code. Calls to the redefined function from Lisp will use the new definition, but calls from C code may still use the built-in definition. Therefore, we discourage redefinition of primitive functions.

The term function refers to all Emacs functions, whether written in Lisp or C. See Function Type, for information about the functions written in Lisp.

Primitive functions have no read syntax and print in hash notation with the name of the subroutine.

     (symbol-function 'car)          ; Access the function cell
                                     ;   of the symbol.
          ⇒ #<subr car>
     (subrp (symbol-function 'car))  ; Is this a primitive function?
          ⇒ t                       ; Yes.

2.3.16 Byte-Code Function Type

Byte-code function objects are produced by byte-compiling Lisp code (see Byte Compilation). Internally, a byte-code function object is much like a vector; however, the evaluator handles this data type specially when it appears in a function call. See Byte-Code Objects.

The printed representation and read syntax for a byte-code function object is like that for a vector, with an additional ‘#’ before the opening ‘[’.

2.3.17 Autoload Type

An autoload object is a list whose first element is the symbol autoload. It is stored as the function definition of a symbol, where it serves as a placeholder for the real definition. The autoload object says that the real definition is found in a file of Lisp code that should be loaded when necessary. It contains the name of the file, plus some other information about the real definition.

After the file has been loaded, the symbol should have a new function definition that is not an autoload object. The new definition is then called as if it had been there to begin with. From the user's point of view, the function call works as expected, using the function definition in the loaded file.

An autoload object is usually created with the function autoload, which stores the object in the function cell of a symbol. See Autoload, for more details.

2.3.18 Finalizer Type

A finalizer object helps Lisp code clean up after objects that are no longer needed. A finalizer holds a Lisp function object. When a finalizer object becomes unreachable after a garbage collection pass, Emacs calls the finalizer's associated function object. When deciding whether a finalizer is reachable, Emacs does not count references from finalizer objects themselves, allowing you to use finalizers without having to worry about accidentally capturing references to finalized objects themselves.

Errors in finalizers are printed to *Messages*. Emacs runs a given finalizer object's associated function exactly once, even if that function fails.

2.4 Editing Types

The types in the previous section are used for general programming purposes, and most of them are common to most Lisp dialects. Emacs Lisp provides several additional data types for purposes connected with editing.

• Buffer Type: The basic object of editing.
• Marker Type: A position in a buffer.
• Window Type: Buffers are displayed in windows.
• Frame Type: Windows subdivide frames.
• Terminal Type: A terminal device displays frames.
• Window Configuration Type: Recording the way a frame is subdivided.
• Frame Configuration Type: Recording the status of all frames.
• Process Type: A subprocess of Emacs running on the underlying OS.
• Stream Type: Receive or send characters.
• Keymap Type: What function a keystroke invokes.
• Overlay Type: How an overlay is represented.
• Font Type: Fonts for displaying text.

2.4.1 Buffer Type

A buffer is an object that holds text that can be edited (see Buffers). Most buffers hold the contents of a disk file (see Files) so they can be edited, but some are used for other purposes. Most buffers are also meant to be seen by the user, and therefore displayed, at some time, in a window (see Windows). But a buffer need not be displayed in any window. Each buffer has a designated position called point (see Positions); most editing commands act on the contents of the current buffer in the neighborhood of point. At any time, one buffer is the current buffer.

The contents of a buffer are much like a string, but buffers are not used like strings in Emacs Lisp, and the available operations are different. For example, you can insert text efficiently into an existing buffer, altering the buffer's contents, whereas inserting text into a string requires concatenating substrings, and the result is an entirely new string object.

Many of the standard Emacs functions manipulate or test the characters in the current buffer; a whole chapter in this manual is devoted to describing these functions (see Text).

Several other data structures are associated with each buffer:

• a local syntax table (see Syntax Tables);
• a local keymap (see Keymaps); and,
• a list of buffer-local variable bindings (see Buffer-Local Variables).
• overlays (see Overlays).
• text properties for the text in the buffer (see Text Properties).

The local keymap and variable list contain entries that individually override global bindings or values. These are used to customize the behavior of programs in different buffers, without actually changing the programs.

A buffer may be indirect, which means it shares the text of another buffer, but presents it differently. See Indirect Buffers.

Buffers have no read syntax. They print in hash notation, showing the buffer name.

     (current-buffer)
          ⇒ #<buffer objects.texi>

2.4.2 Marker Type

A marker denotes a position in a specific buffer. Markers therefore have two components: one for the buffer, and one for the position. Changes in the buffer's text automatically relocate the position value as necessary to ensure that the marker always points between the same two characters in the buffer.

Markers have no read syntax. They print in hash notation, giving the current character position and the name of the buffer.

     (point-marker)
          ⇒ #<marker at 10779 in objects.texi>

See Markers, for information on how to test, create, copy, and move markers.

2.4.3 Window Type

A window describes the portion of the terminal screen that Emacs uses to display a buffer. Every window has one associated buffer, whose contents appear in the window. By contrast, a given buffer may appear in one window, no window, or several windows.

Though many windows may exist simultaneously, at any time one window is designated the selected window. This is the window where the cursor is (usually) displayed when Emacs is ready for a command. The selected window usually displays the current buffer (see Current Buffer), but this is not necessarily the case.

Windows are grouped on the screen into frames; each window belongs to one and only one frame. See Frame Type.

Windows have no read syntax. They print in hash notation, giving the window number and the name of the buffer being displayed. The window numbers exist to identify windows uniquely, since the buffer displayed in any given window can change frequently.

     (selected-window)
          ⇒ #<window 1 on objects.texi>

See Windows, for a description of the functions that work on windows.

2.4.4 Frame Type

A frame is a screen area that contains one or more Emacs windows; we also use the term “frame” to refer to the Lisp object that Emacs uses to refer to the screen area.

Frames have no read syntax. They print in hash notation, giving the frame's title, plus its address in core (useful to identify the frame uniquely).

     (selected-frame)
          ⇒ #<frame emacs@psilocin.gnu.org 0xdac80>

See Frames, for a description of the functions that work on frames.

2.4.5 Terminal Type

A terminal is a device capable of displaying one or more Emacs frames (see Frame Type).

Terminals have no read syntax. They print in hash notation giving the terminal's ordinal number and its TTY device file name.

     (get-device-terminal nil)
          ⇒ #<terminal 1 on /dev/tty>

2.4.6 Window Configuration Type

A window configuration stores information about the positions, sizes, and contents of the windows in a frame, so you can recreate the same arrangement of windows later.

Window configurations do not have a read syntax; their print syntax looks like ‘#<window-configuration>’. See Window Configurations, for a description of several functions related to window configurations.

2.4.7 Frame Configuration Type

A frame configuration stores information about the positions, sizes, and contents of the windows in all frames. It is not a primitive type—it is actually a list whose car is frame-configuration and whose cdr is an alist. Each alist element describes one frame, which appears as the car of that element.

See Frame Configurations, for a description of several functions related to frame configurations.

2.4.8 Process Type

The word process usually means a running program. Emacs itself runs in a process of this sort. However, in Emacs Lisp, a process is a Lisp object that designates a subprocess created by the Emacs process. Programs such as shells, GDB, ftp, and compilers, running in subprocesses of Emacs, extend the capabilities of Emacs. An Emacs subprocess takes textual input from Emacs and returns textual output to Emacs for further manipulation. Emacs can also send signals to the subprocess.

Process objects have no read syntax. They print in hash notation, giving the name of the process:

     (process-list)
          ⇒ (#<process shell>)

See Processes, for information about functions that create, delete, return information about, send input or signals to, and receive output from processes.

2.4.9 Stream Type

A stream is an object that can be used as a source or sink for characters—either to supply characters for input or to accept them as output. Many different types can be used this way: markers, buffers, strings, and functions. Most often, input streams (character sources) obtain characters from the keyboard, a buffer, or a file, and output streams (character sinks) send characters to a buffer, such as a *Help* buffer, or to the echo area.

The object nil, in addition to its other meanings, may be used as a stream. It stands for the value of the variable standard-input or standard-output. Also, the object t as a stream specifies input using the minibuffer (see Minibuffers) or output in the echo area (see The Echo Area).

Streams have no special printed representation or read syntax, and print as whatever primitive type they are.

See Read and Print, for a description of functions related to streams, including parsing and printing functions.

2.4.10 Keymap Type

A keymap maps keys typed by the user to commands. This mapping controls how the user's command input is executed. A keymap is actually a list whose car is the symbol keymap.

See Keymaps, for information about creating keymaps, handling prefix keys, local as well as global keymaps, and changing key bindings.

2.4.11 Overlay Type

An overlay specifies properties that apply to a part of a buffer. Each overlay applies to a specified range of the buffer, and contains a property list (a list whose elements are alternating property names and values). Overlay properties are used to present parts of the buffer temporarily in a different display style. Overlays have no read syntax, and print in hash notation, giving the buffer name and range of positions.

See Overlays, for information on how you can create and use overlays.

2.4.12 Font Type

A font specifies how to display text on a graphical terminal. There are actually three separate font types—font objects, font specs, and font entities—each of which has slightly different properties. None of them have a read syntax; their print syntax looks like ‘#<font-object>’, ‘#<font-spec>’, and ‘#<font-entity>’ respectively. See Low-Level Font, for a description of these Lisp objects.

2.5 Read Syntax for Circular Objects

To represent shared or circular structures within a complex of Lisp objects, you can use the reader constructs ‘#n=’ and ‘#n#’.

Use #n= before an object to label it for later reference; subsequently, you can use #n# to refer the same object in another place. Here, n is some integer. For example, here is how to make a list in which the first element recurs as the third element:

     (#1=(a) b #1#)

This differs from ordinary syntax such as this

     ((a) b (a))

which would result in a list whose first and third elements look alike but are not the same Lisp object. This shows the difference:

     (prog1 nil
       (setq x '(#1=(a) b #1#)))
     (eq (nth 0 x) (nth 2 x))
          ⇒ t
     (setq x '((a) b (a)))
     (eq (nth 0 x) (nth 2 x))
          ⇒ nil

You can also use the same syntax to make a circular structure, which appears as an element within itself. Here is an example:

     #1=(a #1#)

This makes a list whose second element is the list itself. Here's how you can see that it really works:

     (prog1 nil
       (setq x '#1=(a #1#)))
     (eq x (cadr x))
          ⇒ t

The Lisp printer can produce this syntax to record circular and shared structure in a Lisp object, if you bind the variable print-circle to a non-nil value. See Output Variables.

2.6 Type Predicates

The Emacs Lisp interpreter itself does not perform type checking on the actual arguments passed to functions when they are called. It could not do so, since function arguments in Lisp do not have declared data types, as they do in other programming languages. It is therefore up to the individual function to test whether each actual argument belongs to a type that the function can use.

All built-in functions do check the types of their actual arguments when appropriate, and signal a wrong-type-argument error if an argument is of the wrong type. For example, here is what happens if you pass an argument to + that it cannot handle:

     (+ 2 'a)
          error--> Wrong type argument: number-or-marker-p, a

If you want your program to handle different types differently, you must do explicit type checking. The most common way to check the type of an object is to call a type predicate function. Emacs has a type predicate for each type, as well as some predicates for combinations of types.

A type predicate function takes one argument; it returns t if the argument belongs to the appropriate type, and nil otherwise. Following a general Lisp convention for predicate functions, most type predicates' names end with ‘p’.

Here is an example which uses the predicates listp to check for a list and symbolp to check for a symbol.

     (defun add-on (x)
       (cond ((symbolp x)
              ;; If X is a symbol, put it on LIST.
              (setq list (cons x list)))
             ((listp x)
              ;; If X is a list, add its elements to LIST.
              (setq list (append x list)))
             (t
              ;; We handle only symbols and lists.
              (error "Invalid argument %s in add-on" x))))

Here is a table of predefined type predicates, in alphabetical order, with references to further information.

atom

See atom.

arrayp

See arrayp.

bool-vector-p

See bool-vector-p.

bufferp

See bufferp.

byte-code-function-p

See byte-code-function-p.

case-table-p

See case-table-p.

char-or-string-p

See char-or-string-p.

char-table-p

See char-table-p.

commandp

See commandp.

consp

See consp.

custom-variable-p

See custom-variable-p.

floatp

See floatp.

fontp

See Low-Level Font.

frame-configuration-p

See frame-configuration-p.

frame-live-p

See frame-live-p.

framep

See framep.

functionp

See functionp.

hash-table-p

See hash-table-p.

integer-or-marker-p

See integer-or-marker-p.

integerp

See integerp.

keymapp

See keymapp.

keywordp

See Constant Variables.

listp

See listp.

markerp

See markerp.

wholenump

See wholenump.

nlistp

See nlistp.

numberp

See numberp.

number-or-marker-p

See number-or-marker-p.

overlayp

See overlayp.

processp

See processp.

sequencep

See sequencep.

stringp

See stringp.

subrp

See subrp.

symbolp

See symbolp.

syntax-table-p

See syntax-table-p.

vectorp

See vectorp.

window-configuration-p

See window-configuration-p.

window-live-p

See window-live-p.

windowp

See windowp.

booleanp

See booleanp.

string-or-null-p

See string-or-null-p.

The most general way to check the type of an object is to call the function type-of. Recall that each object belongs to one and only one primitive type; type-of tells you which one (see Lisp Data Types). But type-of knows nothing about non-primitive types. In most cases, it is more convenient to use type predicates than type-of.

2.7 Equality Predicates

Here we describe functions that test for equality between two objects. Other functions test equality of contents between objects of specific types, e.g., strings. For these predicates, see the appropriate chapter describing the data type.

The test for equality is implemented recursively; for example, given two cons cells x and y, (equal x y) returns t if and only if both the expressions below return t:

     (equal (car x) (car y))
     (equal (cdr x) (cdr y))

Because of this recursive method, circular lists may therefore cause infinite recursion (leading to an error).

3 Numbers

GNU Emacs supports two numeric data types: integers and floating-point numbers. Integers are whole numbers such as −3, 0, 7, 13, and 511. Floating-point numbers are numbers with fractional parts, such as −4.5, 0.0, and 2.71828. They can also be expressed in exponential notation: ‘1.5e2’ is the same as ‘150.0’; here, ‘e2’ stands for ten to the second power, and that is multiplied by 1.5. Integer computations are exact, though they may overflow. Floating-point computations often involve rounding errors, as the numbers have a fixed amount of precision.

• Integer Basics: Representation and range of integers.
• Float Basics: Representation and range of floating point.
• Predicates on Numbers: Testing for numbers.
• Comparison of Numbers: Equality and inequality predicates.
• Numeric Conversions: Converting float to integer and vice versa.
• Arithmetic Operations: How to add, subtract, multiply and divide.
• Rounding Operations: Explicitly rounding floating-point numbers.
• Bitwise Operations: Logical and, or, not, shifting.
• Math Functions: Trig, exponential and logarithmic functions.
• Random Numbers: Obtaining random integers, predictable or not.

3.1 Integer Basics

The range of values for an integer depends on the machine. The minimum range is −536,870,912 to 536,870,911 (30 bits; i.e., −2**29 to 2**29 − 1), but many machines provide a wider range. Many examples in this chapter assume the minimum integer width of 30 bits. The Lisp reader reads an integer as a sequence of digits with optional initial sign and optional final period. An integer that is out of the Emacs range is treated as a floating-point number.

      1               ; The integer 1.
      1.              ; The integer 1.
     +1               ; Also the integer 1.
     -1               ; The integer −1.
      9000000000000000000
                      ; The floating-point number 9e18.
      0               ; The integer 0.
     -0               ; The integer 0.

The syntax for integers in bases other than 10 uses ‘#’ followed by a letter that specifies the radix: ‘b’ for binary, ‘o’ for octal, ‘x’ for hex, or ‘radixr’ to specify radix radix. Case is not significant for the letter that specifies the radix. Thus, ‘#binteger’ reads integer in binary, and ‘#radixrinteger’ reads integer in radix radix. Allowed values of radix run from 2 to 36. For example:

     #b101100 ⇒ 44
     #o54 ⇒ 44
     #x2c ⇒ 44
     #24r1k ⇒ 44

To understand how various functions work on integers, especially the bitwise operators (see Bitwise Operations), it is often helpful to view the numbers in their binary form.

In 30-bit binary, the decimal integer 5 looks like this:

     0000...000101 (30 bits total)

(The ‘...’ stands for enough bits to fill out a 30-bit word; in this case, ‘...’ stands for twenty 0 bits. Later examples also use the ‘...’ notation to make binary integers easier to read.)

The integer −1 looks like this:

     1111...111111 (30 bits total)

−1 is represented as 30 ones. (This is called two's complement notation.)

Subtracting 4 from −1 returns the negative integer −5. In binary, the decimal integer 4 is 100. Consequently, −5 looks like this:

     1111...111011 (30 bits total)

In this implementation, the largest 30-bit binary integer is 536,870,911 in decimal. In binary, it looks like this:

     0111...111111 (30 bits total)

Since the arithmetic functions do not check whether integers go outside their range, when you add 1 to 536,870,911, the value is the negative integer −536,870,912:

     (+ 1 536870911)
          ⇒ -536870912
          ⇒ 1000...000000 (30 bits total)

Many of the functions described in this chapter accept markers for arguments in place of numbers. (See Markers.) Since the actual arguments to such functions may be either numbers or markers, we often give these arguments the name number-or-marker. When the argument value is a marker, its position value is used and its buffer is ignored.

In Emacs Lisp, text characters are represented by integers. Any integer between zero and the value of (max-char), inclusive, is considered to be valid as a character. See Character Codes.

3.2 Floating-Point Basics

Floating-point numbers are useful for representing numbers that are not integral. The range of floating-point numbers is the same as the range of the C data type double on the machine you are using. On all computers currently supported by Emacs, this is double-precision IEEE floating point.

The read syntax for floating-point numbers requires either a decimal point, an exponent, or both. Optional signs (‘+’ or ‘-’) precede the number and its exponent. For example, ‘1500.0’, ‘+15e2’, ‘15.0e+2’, ‘+1500000e-3’, and ‘.15e4’ are five ways of writing a floating-point number whose value is 1500. They are all equivalent. Like Common Lisp, Emacs Lisp requires at least one digit after any decimal point in a floating-point number; ‘1500.’ is an integer, not a floating-point number.

Emacs Lisp treats -0.0 as numerically equal to ordinary zero with respect to equal and =. This follows the IEEE floating-point standard, which says -0.0 and 0.0 are numerically equal even though other operations can distinguish them.

The IEEE floating-point standard supports positive infinity and negative infinity as floating-point values. It also provides for a class of values called NaN, or “not a number”; numerical functions return such values in cases where there is no correct answer. For example, (/ 0.0 0.0) returns a NaN. Although NaN values carry a sign, for practical purposes there is no other significant difference between different NaN values in Emacs Lisp.

Here are read syntaxes for these special floating-point values:

infinity

‘1.0e+INF’ and ‘-1.0e+INF’

not-a-number

‘0.0e+NaN’ and ‘-0.0e+NaN’

The following functions are specialized for handling floating-point numbers:

3.3 Type Predicates for Numbers

The functions in this section test for numbers, or for a specific type of number. The functions integerp and floatp can take any type of Lisp object as argument (they would not be of much use otherwise), but the zerop predicate requires a number as its argument. See also integer-or-marker-p and number-or-marker-p, in Predicates on Markers.

3.4 Comparison of Numbers

To test numbers for numerical equality, you should normally use =, not eq. There can be many distinct floating-point objects with the same numeric value. If you use eq to compare them, then you test whether two values are the same object. By contrast, = compares only the numeric values of the objects.

In Emacs Lisp, each integer is a unique Lisp object. Therefore, eq is equivalent to = where integers are concerned. It is sometimes convenient to use eq for comparing an unknown value with an integer, because eq does not report an error if the unknown value is not a number—it accepts arguments of any type. By contrast, = signals an error if the arguments are not numbers or markers. However, it is better programming practice to use = if you can, even for comparing integers.

Sometimes it is useful to compare numbers with equal, which treats two numbers as equal if they have the same data type (both integers, or both floating point) and the same value. By contrast, = can treat an integer and a floating-point number as equal. See Equality Predicates.

There is another wrinkle: because floating-point arithmetic is not exact, it is often a bad idea to check for equality of floating-point values. Usually it is better to test for approximate equality. Here's a function to do this:

     (defvar fuzz-factor 1.0e-6)
     (defun approx-equal (x y)
       (or (= x y)
           (< (/ (abs (- x y))
                 (max (abs x) (abs y)))
              fuzz-factor)))

Common Lisp note: Comparing numbers in Common Lisp always requires = because Common Lisp implements multi-word integers, and two distinct integer objects can have the same numeric value. Emacs Lisp can have just one integer object for any given value because it has a limited range of integers.

3.5 Numeric Conversions

To convert an integer to floating point, use the function float.

There are four functions to convert floating-point numbers to integers; they differ in how they round. All accept an argument number and an optional argument divisor. Both arguments may be integers or floating-point numbers. divisor may also be nil. If divisor is nil or omitted, these functions convert number to an integer, or return it unchanged if it already is an integer. If divisor is non-nil, they divide number by divisor and convert the result to an integer. If divisor is zero (whether integer or floating point), Emacs signals an arith-error error.

3.6 Arithmetic Operations

Emacs Lisp provides the traditional four arithmetic operations (addition, subtraction, multiplication, and division), as well as remainder and modulus functions, and functions to add or subtract 1. Except for %, each of these functions accepts both integer and floating-point arguments, and returns a floating-point number if any argument is floating point.

Emacs Lisp arithmetic functions do not check for integer overflow. Thus (1+ 536870911) may evaluate to −536870912, depending on your hardware.

3.7 Rounding Operations

The functions ffloor, fceiling, fround, and ftruncate take a floating-point argument and return a floating-point result whose value is a nearby integer. ffloor returns the nearest integer below; fceiling, the nearest integer above; ftruncate, the nearest integer in the direction towards zero; fround, the nearest integer.

3.8 Bitwise Operations on Integers

In a computer, an integer is represented as a binary number, a sequence of bits (digits which are either zero or one). A bitwise operation acts on the individual bits of such a sequence. For example, shifting moves the whole sequence left or right one or more places, reproducing the same pattern moved over.

The bitwise operations in Emacs Lisp apply only to integers.

3.9 Standard Mathematical Functions

These mathematical functions allow integers as well as floating-point numbers as arguments.

In addition, Emacs defines the following common mathematical constants:

3.10 Random Numbers

A deterministic computer program cannot generate true random numbers. For most purposes, pseudo-random numbers suffice. A series of pseudo-random numbers is generated in a deterministic fashion. The numbers are not truly random, but they have certain properties that mimic a random series. For example, all possible values occur equally often in a pseudo-random series.

Pseudo-random numbers are generated from a seed value. Starting from any given seed, the random function always generates the same sequence of numbers. By default, Emacs initializes the random seed at startup, in such a way that the sequence of values of random (with overwhelming likelihood) differs in each Emacs run.

Sometimes you want the random number sequence to be repeatable. For example, when debugging a program whose behavior depends on the random number sequence, it is helpful to get the same behavior in each program run. To make the sequence repeat, execute (random ""). This sets the seed to a constant value for your particular Emacs executable (though it may differ for other Emacs builds). You can use other strings to choose various seed values.

4 Strings and Characters

A string in Emacs Lisp is an array that contains an ordered sequence of characters. Strings are used as names of symbols, buffers, and files; to send messages to users; to hold text being copied between buffers; and for many other purposes. Because strings are so important, Emacs Lisp has many functions expressly for manipulating them. Emacs Lisp programs use strings more often than individual characters.

See Strings of Events, for special considerations for strings of keyboard character events.

• Basics: Basic properties of strings and characters.
• Predicates for Strings: Testing whether an object is a string or char.
• Creating Strings: Functions to allocate new strings.
• Modifying Strings: Altering the contents of an existing string.
• Text Comparison: Comparing characters or strings.
• String Conversion: Converting to and from characters and strings.
• Formatting Strings: format: Emacs's analogue of printf.
• Case Conversion: Case conversion functions.
• Case Tables: Customizing case conversion.

4.1 String and Character Basics

A character is a Lisp object which represents a single character of text. In Emacs Lisp, characters are simply integers; whether an integer is a character or not is determined only by how it is used. See Character Codes, for details about character representation in Emacs.

A string is a fixed sequence of characters. It is a type of sequence called a array, meaning that its length is fixed and cannot be altered once it is created (see Sequences Arrays Vectors). Unlike in C, Emacs Lisp strings are not terminated by a distinguished character code.

Since strings are arrays, and therefore sequences as well, you can operate on them with the general array and sequence functions documented in Sequences Arrays Vectors. For example, you can access or change individual characters in a string using the functions aref and aset (see Array Functions). However, note that length should not be used for computing the width of a string on display; use string-width (see Size of Displayed Text) instead.

There are two text representations for non-ASCII characters in Emacs strings (and in buffers): unibyte and multibyte. For most Lisp programming, you don't need to be concerned with these two representations. See Text Representations, for details.

Sometimes key sequences are represented as unibyte strings. When a unibyte string is a key sequence, string elements in the range 128 to 255 represent meta characters (which are large integers) rather than character codes in the range 128 to 255. Strings cannot hold characters that have the hyper, super or alt modifiers; they can hold ASCII control characters, but no other control characters. They do not distinguish case in ASCII control characters. If you want to store such characters in a sequence, such as a key sequence, you must use a vector instead of a string. See Character Type, for more information about keyboard input characters.

Strings are useful for holding regular expressions. You can also match regular expressions against strings with string-match (see Regexp Search). The functions match-string (see Simple Match Data) and replace-match (see Replacing Match) are useful for decomposing and modifying strings after matching regular expressions against them.

Like a buffer, a string can contain text properties for the characters in it, as well as the characters themselves. See Text Properties. All the Lisp primitives that copy text from strings to buffers or other strings also copy the properties of the characters being copied.

See Text, for information about functions that display strings or copy them into buffers. See Character Type, and String Type, for information about the syntax of characters and strings. See Non-ASCII Characters, for functions to convert between text representations and to encode and decode character codes.

4.2 Predicates for Strings

For more information about general sequence and array predicates, see Sequences Arrays Vectors, and Arrays.

4.3 Creating Strings

The following functions create strings, either from scratch, or by putting strings together, or by taking them apart.

4.4 Modifying Strings

The most basic way to alter the contents of an existing string is with aset (see Array Functions). (aset string idx char) stores char into string at index idx. Each character occupies one or more bytes, and if char needs a different number of bytes from the character already present at that index, aset signals an error.

A more powerful function is store-substring:

To clear out a string that contained a password, use clear-string:

4.5 Comparison of Characters and Strings

See also the function compare-buffer-substrings in Comparing Text, for a way to compare text in buffers. The function string-match, which matches a regular expression against a string, can be used for a kind of string comparison; see Regexp Search.

4.6 Conversion of Characters and Strings

This section describes functions for converting between characters, strings and integers. format (see Formatting Strings) and prin1-to-string (see Output Functions) can also convert Lisp objects into strings. read-from-string (see Input Functions) can convert a string representation of a Lisp object into an object. The functions string-to-multibyte and string-to-unibyte convert the text representation of a string (see Converting Representations).

See Documentation, for functions that produce textual descriptions of text characters and general input events (single-key-description and text-char-description). These are used primarily for making help messages.

Here are some other functions that can convert to or from a string:

concat

This function converts a vector or a list into a string. See Creating Strings.

vconcat

This function converts a string into a vector. See Vector Functions.

append

This function converts a string into a list. See Building Lists.

byte-to-string

This function converts a byte of character data into a unibyte string. See Converting Representations.

4.7 Formatting Strings

Formatting means constructing a string by substituting computed values at various places in a constant string. This constant string controls how the other values are printed, as well as where they appear; it is called a format string.

Formatting is often useful for computing messages to be displayed. In fact, the functions message and error provide the same formatting feature described here; they differ from format-message only in how they use the result of formatting.

A format specification is a sequence of characters beginning with a ‘%’. Thus, if there is a ‘%d’ in string, the format function replaces it with the printed representation of one of the values to be formatted (one of the arguments objects). For example:

     (format "The value of fill-column is %d." fill-column)
          ⇒ "The value of fill-column is 72."

Since format interprets ‘%’ characters as format specifications, you should never pass an arbitrary string as the first argument. This is particularly true when the string is generated by some Lisp code. Unless the string is known to never include any ‘%’ characters, pass "%s", described below, as the first argument, and the string as the second, like this:

       (format "%s" arbitrary-string)

If string contains more than one format specification, the format specifications correspond to successive values from objects. Thus, the first format specification in string uses the first such value, the second format specification uses the second such value, and so on. Any extra format specifications (those for which there are no corresponding values) cause an error. Any extra values to be formatted are ignored.

Certain format specifications require values of particular types. If you supply a value that doesn't fit the requirements, an error is signaled.

Here is a table of valid format specifications:

‘%s’

Replace the specification with the printed representation of the object, made without quoting (that is, using princ, not prin1—see Output Functions). Thus, strings are represented by their contents alone, with no ‘"’ characters, and symbols appear without ‘\’ characters.

If the object is a string, its text properties are copied into the output. The text properties of the ‘%s’ itself are also copied, but those of the object take priority.

‘%S’

Replace the specification with the printed representation of the object, made with quoting (that is, using prin1—see Output Functions). Thus, strings are enclosed in ‘"’ characters, and ‘\’ characters appear where necessary before special characters.

‘%o’

Replace the specification with the base-eight representation of an unsigned integer.

‘%d’

Replace the specification with the base-ten representation of a signed integer.

‘%x’

‘%X’

Replace the specification with the base-sixteen representation of an unsigned integer. ‘%x’ uses lower case and ‘%X’ uses upper case.

‘%c’

Replace the specification with the character which is the value given.

‘%e’

Replace the specification with the exponential notation for a floating-point number.

‘%f’

Replace the specification with the decimal-point notation for a floating-point number.

‘%g’

Replace the specification with notation for a floating-point number, using either exponential notation or decimal-point notation. The exponential notation is used if the exponent would be less than -4 or greater than or equal to the precision (default: 6). By default, trailing zeros are removed from the fractional portion of the result and a decimal-point character appears only if it is followed by a digit.

‘%%’

Replace the specification with a single ‘%’. This format specification is unusual in that it does not use a value. For example, (format "%% %d" 30) returns "% 30".

Any other format character results in an ‘Invalid format operation’ error.

Here are several examples, which assume the typical text-quoting-style settings:

     (format "The octal value of %d is %o,
              and the hex value is %x." 18 18 18)
          ⇒ "The octal value of 18 is 22,
              and the hex value is 12."
     
     (format-message
      "The name of this buffer is ‘%s’." (buffer-name))
          ⇒ "The name of this buffer is ‘strings.texi’."
     
     (format-message
      "The buffer object prints as `%s'." (current-buffer))
          ⇒ "The buffer object prints as ‘strings.texi’."

A specification can have a width, which is a decimal number between the ‘%’ and the specification character. If the printed representation of the object contains fewer characters than this width, format extends it with padding. The width specifier is ignored for the ‘%%’ specification. Any padding introduced by the width specifier normally consists of spaces inserted on the left:

     (format "%5d is padded on the left with spaces" 123)
          ⇒ "  123 is padded on the left with spaces"

If the width is too small, format does not truncate the object's printed representation. Thus, you can use a width to specify a minimum spacing between columns with no risk of losing information. In the following two examples, ‘%7s’ specifies a minimum width of 7. In the first case, the string inserted in place of ‘%7s’ has only 3 letters, and needs 4 blank spaces as padding. In the second case, the string "specification" is 13 letters wide but is not truncated.

     (format "The word '%7s' has %d letters in it."
             "foo" (length "foo"))
          ⇒ "The word '    foo' has 3 letters in it."
     (format "The word '%7s' has %d letters in it."
             "specification" (length "specification"))
          ⇒ "The word 'specification' has 13 letters in it."

Immediately after the ‘%’ and before the optional width specifier, you can also put certain flag characters.

The flag ‘+’ inserts a plus sign before a positive number, so that it always has a sign. A space character as flag inserts a space before a positive number. (Otherwise, positive numbers start with the first digit.) These flags are useful for ensuring that positive numbers and negative numbers use the same number of columns. They are ignored except for ‘%d’, ‘%e’, ‘%f’, ‘%g’, and if both flags are used, ‘+’ takes precedence.

The flag ‘#’ specifies an alternate form which depends on the format in use. For ‘%o’, it ensures that the result begins with a ‘0’. For ‘%x’ and ‘%X’, it prefixes the result with ‘0x’ or ‘0X’. For ‘%e’ and ‘%f’, the ‘#’ flag means include a decimal point even if the precision is zero. For ‘%g’, it always includes a decimal point, and also forces any trailing zeros after the decimal point to be left in place where they would otherwise be removed.

The flag ‘0’ ensures that the padding consists of ‘0’ characters instead of spaces. This flag is ignored for non-numerical specification characters like ‘%s’, ‘%S’ and ‘%c’. These specification characters accept the ‘0’ flag, but still pad with spaces.

The flag ‘-’ causes the padding inserted by the width specifier, if any, to be inserted on the right rather than the left. If both ‘-’ and ‘0’ are present, the ‘0’ flag is ignored.

     (format "%06d is padded on the left with zeros" 123)
          ⇒ "000123 is padded on the left with zeros"
     
     (format "'%-6d' is padded on the right" 123)
          ⇒ "'123   ' is padded on the right"
     
     (format "The word '%-7s' actually has %d letters in it."
             "foo" (length "foo"))
          ⇒ "The word 'foo    ' actually has 3 letters in it."

All the specification characters allow an optional precision before the character (after the width, if present). The precision is a decimal-point ‘.’ followed by a digit-string. For the floating-point specifications (‘%e’ and ‘%f’), the precision specifies how many digits following the decimal point to show; if zero, the decimal-point itself is also omitted. For ‘%g’, the precision specifies how many significant digits to show (significant digits are the first digit before the decimal point and all the digits after it). If the precision of %g is zero or unspecified, it is treated as 1. For ‘%s’ and ‘%S’, the precision truncates the string to the given width, so ‘%.3s’ shows only the first three characters of the representation for object. For other specification characters, the effect of precision is what the local library functions of the printf family produce.

4.8 Case Conversion in Lisp

The character case functions change the case of single characters or of the contents of strings. The functions normally convert only alphabetic characters (the letters ‘A’ through ‘Z’ and ‘a’ through ‘z’, as well as non-ASCII letters); other characters are not altered. You can specify a different case conversion mapping by specifying a case table (see Case Tables).

These functions do not modify the strings that are passed to them as arguments.

The examples below use the characters ‘X’ and ‘x’ which have ASCII codes 88 and 120 respectively.

See Text Comparison, for functions that compare strings; some of them ignore case differences, or can optionally ignore case differences.

4.9 The Case Table

You can customize case conversion by installing a special case table. A case table specifies the mapping between upper case and lower case letters. It affects both the case conversion functions for Lisp objects (see the previous section) and those that apply to text in the buffer (see Case Changes). Each buffer has a case table; there is also a standard case table which is used to initialize the case table of new buffers.

A case table is a char-table (see Char-Tables) whose subtype is case-table. This char-table maps each character into the corresponding lower case character. It has three extra slots, which hold related tables:

upcase

The upcase table maps each character into the corresponding upper case character.

canonicalize

The canonicalize table maps all of a set of case-related characters into a particular member of that set.

equivalences

The equivalences table maps each one of a set of case-related characters into the next character in that set.

In simple cases, all you need to specify is the mapping to lower-case; the three related tables will be calculated automatically from that one.

For some languages, upper and lower case letters are not in one-to-one correspondence. There may be two different lower case letters with the same upper case equivalent. In these cases, you need to specify the maps for both lower case and upper case.

The extra table canonicalize maps each character to a canonical equivalent; any two characters that are related by case-conversion have the same canonical equivalent character. For example, since ‘a’ and ‘A’ are related by case-conversion, they should have the same canonical equivalent character (which should be either ‘a’ for both of them, or ‘A’ for both of them).

The extra table equivalences is a map that cyclically permutes each equivalence class (of characters with the same canonical equivalent). (For ordinary ASCII, this would map ‘a’ into ‘A’ and ‘A’ into ‘a’, and likewise for each set of equivalent characters.)

When constructing a case table, you can provide nil for canonicalize; then Emacs fills in this slot from the lower case and upper case mappings. You can also provide nil for equivalences; then Emacs fills in this slot from canonicalize. In a case table that is actually in use, those components are non-nil. Do not try to specify equivalences without also specifying canonicalize.

Here are the functions for working with case tables:

Some language environments modify the case conversions of ASCII characters; for example, in the Turkish language environment, the ASCII capital I is downcased into a Turkish dotless i (‘ı’). This can interfere with code that requires ordinary ASCII case conversion, such as implementations of ASCII-based network protocols. In that case, use the with-case-table macro with the variable ascii-case-table, which stores the unmodified case table for the ASCII character set.

The following three functions are convenient subroutines for packages that define non-ASCII character sets. They modify the specified case table case-table; they also modify the standard syntax table. See Syntax Tables. Normally you would use these functions to change the standard case table.

5 Lists

A list represents a sequence of zero or more elements (which may be any Lisp objects). The important difference between lists and vectors is that two or more lists can share part of their structure; in addition, you can insert or delete elements in a list without copying the whole list.

• Cons Cells: How lists are made out of cons cells.
• List-related Predicates: Is this object a list? Comparing two lists.
• List Elements: Extracting the pieces of a list.
• Building Lists: Creating list structure.
• List Variables: Modifying lists stored in variables.
• Modifying Lists: Storing new pieces into an existing list.
• Sets And Lists: A list can represent a finite mathematical set.
• Association Lists: A list can represent a finite relation or mapping.
• Property Lists: A list of paired elements.

5.1 Lists and Cons Cells

Lists in Lisp are not a primitive data type; they are built up from cons cells (see Cons Cell Type). A cons cell is a data object that represents an ordered pair. That is, it has two slots, and each slot holds, or refers to, some Lisp object. One slot is known as the car, and the other is known as the cdr. (These names are traditional; see Cons Cell Type.) cdr is pronounced “could-er”.

We say that “the car of this cons cell is” whatever object its car slot currently holds, and likewise for the cdr.

A list is a series of cons cells chained together, so that each cell refers to the next one. There is one cons cell for each element of the list. By convention, the cars of the cons cells hold the elements of the list, and the cdrs are used to chain the list (this asymmetry between car and cdr is entirely a matter of convention; at the level of cons cells, the car and cdr slots have similar properties). Hence, the cdr slot of each cons cell in a list refers to the following cons cell.

Also by convention, the cdr of the last cons cell in a list is nil. We call such a nil-terminated structure a true list. In Emacs Lisp, the symbol nil is both a symbol and a list with no elements. For convenience, the symbol nil is considered to have nil as its cdr (and also as its car).

Hence, the cdr of a true list is always a true list. The cdr of a nonempty true list is a true list containing all the elements except the first.

If the cdr of a list's last cons cell is some value other than nil, we call the structure a dotted list, since its printed representation would use dotted pair notation (see Dotted Pair Notation). There is one other possibility: some cons cell's cdr could point to one of the previous cons cells in the list. We call that structure a circular list.

For some purposes, it does not matter whether a list is true, circular or dotted. If a program doesn't look far enough down the list to see the cdr of the final cons cell, it won't care. However, some functions that operate on lists demand true lists and signal errors if given a dotted list. Most functions that try to find the end of a list enter infinite loops if given a circular list.

Because most cons cells are used as part of lists, we refer to any structure made out of cons cells as a list structure.

5.2 Predicates on Lists

The following predicates test whether a Lisp object is an atom, whether it is a cons cell or is a list, or whether it is the distinguished object nil. (Many of these predicates can be defined in terms of the others, but they are used so often that it is worth having them.)

5.3 Accessing Elements of Lists

The most common way to compute the length of a list, when you are not worried that it may be circular, is with length. See Sequence Functions.

5.4 Building Cons Cells and Lists

Many functions build lists, as lists reside at the very heart of Lisp. cons is the fundamental list-building function; however, it is interesting to note that list is used more times in the source code for Emacs than cons.

Here is an example of using append:

     (setq trees '(pine oak))
          ⇒ (pine oak)
     (setq more-trees (append '(maple birch) trees))
          ⇒ (maple birch pine oak)
     
     trees
          ⇒ (pine oak)
     more-trees
          ⇒ (maple birch pine oak)
     (eq trees (cdr (cdr more-trees)))
          ⇒ t

You can see how append works by looking at a box diagram. The variable trees is set to the list (pine oak) and then the variable more-trees is set to the list (maple birch pine oak). However, the variable trees continues to refer to the original list:

     more-trees                trees
     |                           |
     |     --- ---      --- ---   -> --- ---      --- ---
      --> |   |   |--> |   |   |--> |   |   |--> |   |   |--> nil
           --- ---      --- ---      --- ---      --- ---
            |            |            |            |
            |            |            |            |
             --> maple    -->birch     --> pine     --> oak

An empty sequence contributes nothing to the value returned by append. As a consequence of this, a final nil argument forces a copy of the previous argument:

     trees
          ⇒ (pine oak)
     (setq wood (append trees nil))
          ⇒ (pine oak)
     wood
          ⇒ (pine oak)
     (eq wood trees)
          ⇒ nil

This once was the usual way to copy a list, before the function copy-sequence was invented. See Sequences Arrays Vectors.

Here we show the use of vectors and strings as arguments to append:

     (append [a b] "cd" nil)
          ⇒ (a b 99 100)

With the help of apply (see Calling Functions), we can append all the lists in a list of lists:

     (apply 'append '((a b c) nil (x y z) nil))
          ⇒ (a b c x y z)

If no sequences are given, nil is returned:

     (append)
          ⇒ nil

Here are some examples where the final argument is not a list:

     (append '(x y) 'z)
          ⇒ (x y . z)
     (append '(x y) [z])
          ⇒ (x y . [z])

The second example shows that when the final argument is a sequence but not a list, the sequence's elements do not become elements of the resulting list. Instead, the sequence becomes the final cdr, like any other non-list final argument.

5.5 Modifying List Variables

These functions, and one macro, provide convenient ways to modify a list which is stored in a variable.

Two functions modify lists that are the values of variables.

Here's a scenario showing how to use add-to-list:

     (setq foo '(a b))
          ⇒ (a b)
     
     (add-to-list 'foo 'c)     ;; Add c.
          ⇒ (c a b)
     
     (add-to-list 'foo 'b)     ;; No effect.
          ⇒ (c a b)
     
     foo                       ;; foo was changed.
          ⇒ (c a b)

An equivalent expression for (add-to-list 'var value) is this:

     (or (member value var)
         (setq var (cons value var)))

Here's a scenario showing how to use add-to-ordered-list:

     (setq foo '())
          ⇒ nil
     
     (add-to-ordered-list 'foo 'a 1)     ;; Add a.
          ⇒ (a)
     
     (add-to-ordered-list 'foo 'c 3)     ;; Add c.
          ⇒ (a c)
     
     (add-to-ordered-list 'foo 'b 2)     ;; Add b.
          ⇒ (a b c)
     
     (add-to-ordered-list 'foo 'b 4)     ;; Move b.
          ⇒ (a c b)
     
     (add-to-ordered-list 'foo 'd)       ;; Append d.
          ⇒ (a c b d)
     
     (add-to-ordered-list 'foo 'e)       ;; Add e.
          ⇒ (a c b e d)
     
     foo                       ;; foo was changed.
          ⇒ (a c b e d)

5.6 Modifying Existing List Structure

You can modify the car and cdr contents of a cons cell with the primitives setcar and setcdr. These are destructive operations because they change existing list structure.

Common Lisp note: Common Lisp uses functions rplaca and rplacd to alter list structure; they change structure the same way as setcar and setcdr, but the Common Lisp functions return the cons cell while setcar and setcdr return the new car or cdr.

• Setcar: Replacing an element in a list.
• Setcdr: Replacing part of the list backbone. This can be used to remove or add elements.
• Rearrangement: Reordering the elements in a list; combining lists.

5.6.1 Altering List Elements with setcar

Changing the car of a cons cell is done with setcar. When used on a list, setcar replaces one element of a list with a different element.

When a cons cell is part of the shared structure of several lists, storing a new car into the cons changes one element of each of these lists. Here is an example:

     ;; Create two lists that are partly shared.
     (setq x1 '(a b c))
          ⇒ (a b c)
     (setq x2 (cons 'z (cdr x1)))
          ⇒ (z b c)
     
     ;; Replace the car of a shared link.
     (setcar (cdr x1) 'foo)
          ⇒ foo
     x1                           ; Both lists are changed.
          ⇒ (a foo c)
     x2
          ⇒ (z foo c)
     
     ;; Replace the car of a link that is not shared.
     (setcar x1 'baz)
          ⇒ baz
     x1                           ; Only one list is changed.
          ⇒ (baz foo c)
     x2
          ⇒ (z foo c)

Here is a graphical depiction of the shared structure of the two lists in the variables x1 and x2, showing why replacing b changes them both:

             --- ---        --- ---      --- ---
     x1---> |   |   |----> |   |   |--> |   |   |--> nil
             --- ---        --- ---      --- ---
              |        -->   |            |
              |       |      |            |
               --> a  |       --> b        --> c
                      |
            --- ---   |
     x2--> |   |   |--
            --- ---
             |
             |
              --> z

Here is an alternative form of box diagram, showing the same relationship:

     x1:
      --------------       --------------       --------------
     | car   | cdr  |     | car   | cdr  |     | car   | cdr  |
     |   a   |   o------->|   b   |   o------->|   c   |  nil |
     |       |      |  -->|       |      |     |       |      |
      --------------  |    --------------       --------------
                      |
     x2:              |
      --------------  |
     | car   | cdr  | |
     |   z   |   o----
     |       |      |
      --------------

5.6.2 Altering the CDR of a List

The lowest-level primitive for modifying a cdr is setcdr:

Here is an example of replacing the cdr of a list with a different list. All but the first element of the list are removed in favor of a different sequence of elements. The first element is unchanged, because it resides in the car of the list, and is not reached via the cdr.

     (setq x '(1 2 3))
          ⇒ (1 2 3)
     (setcdr x '(4))
          ⇒ (4)
     x
          ⇒ (1 4)

You can delete elements from the middle of a list by altering the cdrs of the cons cells in the list. For example, here we delete the second element, b, from the list (a b c), by changing the cdr of the first cons cell:

     (setq x1 '(a b c))
          ⇒ (a b c)
     (setcdr x1 (cdr (cdr x1)))
          ⇒ (c)
     x1
          ⇒ (a c)

Here is the result in box notation:

                        --------------------
                       |                    |
      --------------   |   --------------   |    --------------
     | car   | cdr  |  |  | car   | cdr  |   -->| car   | cdr  |
     |   a   |   o-----   |   b   |   o-------->|   c   |  nil |
     |       |      |     |       |      |      |       |      |
      --------------       --------------        --------------

The second cons cell, which previously held the element b, still exists and its car is still b, but it no longer forms part of this list.

It is equally easy to insert a new element by changing cdrs:

     (setq x1 '(a b c))
          ⇒ (a b c)
     (setcdr x1 (cons 'd (cdr x1)))
          ⇒ (d b c)
     x1
          ⇒ (a d b c)

Here is this result in box notation:

      --------------        -------------       -------------
     | car  | cdr   |      | car  | cdr  |     | car  | cdr  |
     |   a  |   o   |   -->|   b  |   o------->|   c  |  nil |
     |      |   |   |  |   |      |      |     |      |      |
      --------- | --   |    -------------       -------------
                |      |
          -----         --------
         |                      |
         |    ---------------   |
         |   | car   | cdr   |  |
          -->|   d   |   o------
             |       |       |
              ---------------

5.6.3 Functions that Rearrange Lists

Here are some functions that rearrange lists destructively by modifying the cdrs of their component cons cells. These functions are destructive because they chew up the original lists passed to them as arguments, relinking their cons cells to form a new list that is the returned value.

See delq, in Sets And Lists, for another function that modifies cons cells.

5.7 Using Lists as Sets

A list can represent an unordered mathematical set—simply consider a value an element of a set if it appears in the list, and ignore the order of the list. To form the union of two sets, use append (as long as you don't mind having duplicate elements). You can remove equal duplicates using delete-dups. Other useful functions for sets include memq and delq, and their equal versions, member and delete.

Common Lisp note: Common Lisp has functions union (which avoids duplicate elements) and intersection for set operations. Although standard GNU Emacs Lisp does not have them, the cl-lib library provides versions. See Lists as Sets.

The delq function deletes elements from the front of the list by simply advancing down the list, and returning a sublist that starts after those elements. For example:

     (delq 'a '(a b c)) == (cdr '(a b c))

When an element to be deleted appears in the middle of the list, removing it involves changing the cdrs (see Setcdr).

     (setq sample-list '(a b c (4)))
          ⇒ (a b c (4))
     (delq 'a sample-list)
          ⇒ (b c (4))
     sample-list
          ⇒ (a b c (4))
     (delq 'c sample-list)
          ⇒ (a b (4))
     sample-list
          ⇒ (a b (4))

Note that (delq 'c sample-list) modifies sample-list to splice out the third element, but (delq 'a sample-list) does not splice anything—it just returns a shorter list. Don't assume that a variable which formerly held the argument list now has fewer elements, or that it still holds the original list! Instead, save the result of delq and use that. Most often we store the result back into the variable that held the original list:

     (setq flowers (delq 'rose flowers))

In the following example, the (4) that delq attempts to match and the (4) in the sample-list are not eq:

     (delq '(4) sample-list)
          ⇒ (a c (4))

If you want to delete elements that are equal to a given value, use delete (see below).

The following three functions are like memq, delq and remq, but use equal rather than eq to compare elements. See Equality Predicates.

Common Lisp note: The functions member, delete and remove in GNU Emacs Lisp are derived from Maclisp, not Common Lisp. The Common Lisp versions do not use equal to compare elements.

See also the function add-to-list, in List Variables, for a way to add an element to a list stored in a variable and used as a set.

5.8 Association Lists

An association list, or alist for short, records a mapping from keys to values. It is a list of cons cells called associations: the car of each cons cell is the key, and the cdr is the associated value.³

Here is an example of an alist. The key pine is associated with the value cones; the key oak is associated with acorns; and the key maple is associated with seeds.

     ((pine . cones)
      (oak . acorns)
      (maple . seeds))

Both the values and the keys in an alist may be any Lisp objects. For example, in the following alist, the symbol a is associated with the number 1, and the string "b" is associated with the list (2 3), which is the cdr of the alist element:

     ((a . 1) ("b" 2 3))

Sometimes it is better to design an alist to store the associated value in the car of the cdr of the element. Here is an example of such an alist:

     ((rose red) (lily white) (buttercup yellow))

Here we regard red as the value associated with rose. One advantage of this kind of alist is that you can store other related information—even a list of other items—in the cdr of the cdr. One disadvantage is that you cannot use rassq (see below) to find the element containing a given value. When neither of these considerations is important, the choice is a matter of taste, as long as you are consistent about it for any given alist.

The same alist shown above could be regarded as having the associated value in the cdr of the element; the value associated with rose would be the list (red).

Association lists are often used to record information that you might otherwise keep on a stack, since new associations may be added easily to the front of the list. When searching an association list for an association with a given key, the first one found is returned, if there is more than one.

In Emacs Lisp, it is not an error if an element of an association list is not a cons cell. The alist search functions simply ignore such elements. Many other versions of Lisp signal errors in such cases.

Note that property lists are similar to association lists in several respects. A property list behaves like an association list in which each key can occur only once. See Property Lists, for a comparison of property lists and association lists.

The function assoc-string is much like assoc except that it ignores certain differences between strings. See Text Comparison.

5.9 Property Lists

A property list (plist for short) is a list of paired elements. Each of the pairs associates a property name (usually a symbol) with a property or value. Here is an example of a property list:

     (pine cones numbers (1 2 3) color "blue")

This property list associates pine with cones, numbers with (1 2 3), and color with "blue". The property names and values can be any Lisp objects, but the names are usually symbols (as they are in this example).

Property lists are used in several contexts. For instance, the function put-text-property takes an argument which is a property list, specifying text properties and associated values which are to be applied to text in a string or buffer. See Text Properties.

Another prominent use of property lists is for storing symbol properties. Every symbol possesses a list of properties, used to record miscellaneous information about the symbol; these properties are stored in the form of a property list. See Symbol Properties.

• Plists and Alists: Comparison of the advantages of property lists and association lists.
• Plist Access: Accessing property lists stored elsewhere.

5.9.1 Property Lists and Association Lists

Association lists (see Association Lists) are very similar to property lists. In contrast to association lists, the order of the pairs in the property list is not significant, since the property names must be distinct.

Property lists are better than association lists for attaching information to various Lisp function names or variables. If your program keeps all such information in one association list, it will typically need to search that entire list each time it checks for an association for a particular Lisp function name or variable, which could be slow. By contrast, if you keep the same information in the property lists of the function names or variables themselves, each search will scan only the length of one property list, which is usually short. This is why the documentation for a variable is recorded in a property named variable-documentation. The byte compiler likewise uses properties to record those functions needing special treatment.

However, association lists have their own advantages. Depending on your application, it may be faster to add an association to the front of an association list than to update a property. All properties for a symbol are stored in the same property list, so there is a possibility of a conflict between different uses of a property name. (For this reason, it is a good idea to choose property names that are probably unique, such as by beginning the property name with the program's usual name-prefix for variables and functions.) An association list may be used like a stack where associations are pushed on the front of the list and later discarded; this is not possible with a property list.

5.9.2 Property Lists Outside Symbols

The following functions can be used to manipulate property lists. They all compare property names using eq.

6 Sequences, Arrays, and Vectors

The sequence type is the union of two other Lisp types: lists and arrays. In other words, any list is a sequence, and any array is a sequence. The common property that all sequences have is that each is an ordered collection of elements.

An array is a fixed-length object with a slot for each of its elements. All the elements are accessible in constant time. The four types of arrays are strings, vectors, char-tables and bool-vectors.

A list is a sequence of elements, but it is not a single primitive object; it is made of cons cells, one cell per element. Finding the nth element requires looking through n cons cells, so elements farther from the beginning of the list take longer to access. But it is possible to add elements to the list, or remove elements.

The following diagram shows the relationship between these types:

               _____________________________________________
              |                                             |
              |          Sequence                           |
              |  ______   ________________________________  |
              | |      | |                                | |
              | | List | |             Array              | |
              | |      | |    ________       ________     | |
              | |______| |   |        |     |        |    | |
              |          |   | Vector |     | String |    | |
              |          |   |________|     |________|    | |
              |          |  ____________   _____________  | |
              |          | |            | |             | | |
              |          | | Char-table | | Bool-vector | | |
              |          | |____________| |_____________| | |
              |          |________________________________| |
              |_____________________________________________|

• Sequence Functions: Functions that accept any kind of sequence.
• Arrays: Characteristics of arrays in Emacs Lisp.
• Array Functions: Functions specifically for arrays.
• Vectors: Special characteristics of Emacs Lisp vectors.
• Vector Functions: Functions specifically for vectors.
• Char-Tables: How to work with char-tables.
• Bool-Vectors: How to work with bool-vectors.
• Rings: Managing a fixed-size ring of objects.

6.1 Sequences

This section describes functions that accept any kind of sequence.

See also string-bytes, in Text Representations.

If you need to compute the width of a string on display, you should use string-width (see Size of Displayed Text), not length, since length only counts the number of characters, but does not account for the display width of each character.

The seq.el library provides the following additional sequence manipulation macros and functions, prefixed with seq-. To use them, you must first load the seq library.

All functions defined in this library are free of side-effects; i.e., they do not modify any sequence (list, vector, or string) that you pass as an argument. Unless otherwise stated, the result is a sequence of the same type as the input. For those functions that take a predicate, this should be a function of one argument.

The seq.el library can be extended to work with additional types of sequential data-structures. For that purpose, all functions are defined using cl-defgeneric. See Generic Functions, for more details about using cl-defgeneric for adding extensions.

6.2 Arrays

An array object has slots that hold a number of other Lisp objects, called the elements of the array. Any element of an array may be accessed in constant time. In contrast, the time to access an element of a list is proportional to the position of that element in the list.

Emacs defines four types of array, all one-dimensional: strings (see String Type), vectors (see Vector Type), bool-vectors (see Bool-Vector Type), and char-tables (see Char-Table Type). Vectors and char-tables can hold elements of any type, but strings can only hold characters, and bool-vectors can only hold t and nil.

All four kinds of array share these characteristics:

• The first element of an array has index zero, the second element has index 1, and so on. This is called zero-origin indexing. For example, an array of four elements has indices 0, 1, 2, and 3.
• The length of the array is fixed once you create it; you cannot change the length of an existing array.
• For purposes of evaluation, the array is a constant—i.e., it evaluates to itself.
• The elements of an array may be referenced or changed with the functions aref and aset, respectively (see Array Functions).

When you create an array, other than a char-table, you must specify its length. You cannot specify the length of a char-table, because that is determined by the range of character codes.

In principle, if you want an array of text characters, you could use either a string or a vector. In practice, we always choose strings for such applications, for four reasons:

• They occupy one-fourth the space of a vector of the same elements.
• Strings are printed in a way that shows the contents more clearly as text.
• Strings can hold text properties. See Text Properties.
• Many of the specialized editing and I/O facilities of Emacs accept only strings. For example, you cannot insert a vector of characters into a buffer the way you can insert a string. See Strings and Characters.

By contrast, for an array of keyboard input characters (such as a key sequence), a vector may be necessary, because many keyboard input characters are outside the range that will fit in a string. See Key Sequence Input.

6.3 Functions that Operate on Arrays

In this section, we describe the functions that accept all types of arrays.

The general sequence functions copy-sequence and length are often useful for objects known to be arrays. See Sequence Functions.

6.4 Vectors

A vector is a general-purpose array whose elements can be any Lisp objects. (By contrast, the elements of a string can only be characters. See Strings and Characters.) Vectors are used in Emacs for many purposes: as key sequences (see Key Sequences), as symbol-lookup tables (see Creating Symbols), as part of the representation of a byte-compiled function (see Byte Compilation), and more.

Like other arrays, vectors use zero-origin indexing: the first element has index 0.

Vectors are printed with square brackets surrounding the elements. Thus, a vector whose elements are the symbols a, b and a is printed as [a b a]. You can write vectors in the same way in Lisp input.

A vector, like a string or a number, is considered a constant for evaluation: the result of evaluating it is the same vector. This does not evaluate or even examine the elements of the vector. See Self-Evaluating Forms.

Here are examples illustrating these principles:

     (setq avector [1 two '(three) "four" [five]])
          ⇒ [1 two (quote (three)) "four" [five]]
     (eval avector)
          ⇒ [1 two (quote (three)) "four" [five]]
     (eq avector (eval avector))
          ⇒ t

6.5 Functions for Vectors

Here are some functions that relate to vectors:

The append function also provides a way to convert a vector into a list with the same elements:

     (setq avector [1 two (quote (three)) "four" [five]])
          ⇒ [1 two (quote (three)) "four" [five]]
     (append avector nil)
          ⇒ (1 two (quote (three)) "four" [five])

6.6 Char-Tables

A char-table is much like a vector, except that it is indexed by character codes. Any valid character code, without modifiers, can be used as an index in a char-table. You can access a char-table's elements with aref and aset, as with any array. In addition, a char-table can have extra slots to hold additional data not associated with particular character codes. Like vectors, char-tables are constants when evaluated, and can hold elements of any type.

Each char-table has a subtype, a symbol, which serves two purposes:

• The subtype provides an easy way to tell what the char-table is for. For instance, display tables are char-tables with display-table as the subtype, and syntax tables are char-tables with syntax-table as the subtype. The subtype can be queried using the function char-table-subtype, described below.
• The subtype controls the number of extra slots in the char-table. This number is specified by the subtype's char-table-extra-slots symbol property (see Symbol Properties), whose value should be an integer between 0 and 10. If the subtype has no such symbol property, the char-table has no extra slots.

A char-table can have a parent, which is another char-table. If it does, then whenever the char-table specifies nil for a particular character c, it inherits the value specified in the parent. In other words, (aref char-table c) returns the value from the parent of char-table if char-table itself specifies nil.

A char-table can also have a default value. If so, then (aref char-table c) returns the default value whenever the char-table does not specify any other non-nil value.

There is no special function to access default values in a char-table. To do that, use char-table-range (see below).

A char-table can specify an element value for a single character code; it can also specify a value for an entire character set.

6.7 Bool-vectors

A bool-vector is much like a vector, except that it stores only the values t and nil. If you try to store any non-nil value into an element of the bool-vector, the effect is to store t there. As with all arrays, bool-vector indices start from 0, and the length cannot be changed once the bool-vector is created. Bool-vectors are constants when evaluated.

Several functions work specifically with bool-vectors; aside from that, you manipulate them with same functions used for other kinds of arrays.

There are also some bool-vector set operation functions, described below:

The printed form represents up to 8 boolean values as a single character:

     (bool-vector t nil t nil)
          ⇒ #&4"^E"
     (bool-vector)
          ⇒ #&0""

You can use vconcat to print a bool-vector like other vectors:

     (vconcat (bool-vector nil t nil t))
          ⇒ [nil t nil t]

Here is another example of creating, examining, and updating a bool-vector:

     (setq bv (make-bool-vector 5 t))
          ⇒ #&5"^_"
     (aref bv 1)
          ⇒ t
     (aset bv 3 nil)
          ⇒ nil
     bv
          ⇒ #&5"^W"

These results make sense because the binary codes for control-_ and control-W are 11111 and 10111, respectively.

6.8 Managing a Fixed-Size Ring of Objects

A ring is a fixed-size data structure that supports insertion, deletion, rotation, and modulo-indexed reference and traversal. An efficient ring data structure is implemented by the ring package. It provides the functions listed in this section.

Note that several rings in Emacs, like the kill ring and the mark ring, are actually implemented as simple lists, not using the ring package; thus the following functions won't work on them.

The newest element in the ring always has index 0. Higher indices correspond to older elements. Indices are computed modulo the ring length. Index −1 corresponds to the oldest element, −2 to the next-oldest, and so forth.

If you are careful not to exceed the ring size, you can use the ring as a first-in-first-out queue. For example:

     (let ((fifo (make-ring 5)))
       (mapc (lambda (obj) (ring-insert fifo obj))
             '(0 one "two"))
       (list (ring-remove fifo) t
             (ring-remove fifo) t
             (ring-remove fifo)))
          ⇒ (0 t one t "two")

7 Hash Tables

A hash table is a very fast kind of lookup table, somewhat like an alist (see Association Lists) in that it maps keys to corresponding values. It differs from an alist in these ways:

• Lookup in a hash table is extremely fast for large tables—in fact, the time required is essentially independent of how many elements are stored in the table. For smaller tables (a few tens of elements) alists may still be faster because hash tables have a more-or-less constant overhead.
• The correspondences in a hash table are in no particular order.
• There is no way to share structure between two hash tables, the way two alists can share a common tail.

Emacs Lisp provides a general-purpose hash table data type, along with a series of functions for operating on them. Hash tables have a special printed representation, which consists of ‘#s’ followed by a list specifying the hash table properties and contents. See Creating Hash. (Hash notation, the initial ‘#’ character used in the printed representations of objects with no read representation, has nothing to do with hash tables. See Printed Representation.)

Obarrays are also a kind of hash table, but they are a different type of object and are used only for recording interned symbols (see Creating Symbols).

• Creating Hash: Functions to create hash tables.
• Hash Access: Reading and writing the hash table contents.
• Defining Hash: Defining new comparison methods.
• Other Hash: Miscellaneous.

7.1 Creating Hash Tables

The principal function for creating a hash table is make-hash-table.

You can also create a new hash table using the printed representation for hash tables. The Lisp reader can read this printed representation, provided each element in the specified hash table has a valid read syntax (see Printed Representation). For instance, the following specifies a new hash table containing the keys key1 and key2 (both symbols) associated with val1 (a symbol) and 300 (a number) respectively.

     #s(hash-table size 30 data (key1 val1 key2 300))

The printed representation for a hash table consists of ‘#s’ followed by a list beginning with ‘hash-table’. The rest of the list should consist of zero or more property-value pairs specifying the hash table's properties and initial contents. The properties and values are read literally. Valid property names are size, test, weakness, rehash-size, rehash-threshold, and data. The data property should be a list of key-value pairs for the initial contents; the other properties have the same meanings as the matching make-hash-table keywords (:size, :test, etc.), described above.

Note that you cannot specify a hash table whose initial contents include objects that have no read syntax, such as buffers and frames. Such objects may be added to the hash table after it is created.

7.2 Hash Table Access

This section describes the functions for accessing and storing associations in a hash table. In general, any Lisp object can be used as a hash key, unless the comparison method imposes limits. Any Lisp object can also be used as the value.

7.3 Defining Hash Comparisons

You can define new methods of key lookup by means of define-hash-table-test. In order to use this feature, you need to understand how hash tables work, and what a hash code means.

You can think of a hash table conceptually as a large array of many slots, each capable of holding one association. To look up a key, gethash first computes an integer, the hash code, from the key. It reduces this integer modulo the length of the array, to produce an index in the array. Then it looks in that slot, and if necessary in other nearby slots, to see if it has found the key being sought.

Thus, to define a new method of key lookup, you need to specify both a function to compute the hash code from a key, and a function to compare two keys directly.

This example creates a hash table whose keys are strings that are compared case-insensitively.

     (defun case-fold-string= (a b)
       (eq t (compare-strings a nil nil b nil nil t)))
     (defun case-fold-string-hash (a)
       (sxhash (upcase a)))
     
     (define-hash-table-test 'case-fold
       'case-fold-string= 'case-fold-string-hash)
     
     (make-hash-table :test 'case-fold)

Here is how you could define a hash table test equivalent to the predefined test value equal. The keys can be any Lisp object, and equal-looking objects are considered the same key.

     (define-hash-table-test 'contents-hash 'equal 'sxhash)
     
     (make-hash-table :test 'contents-hash)

7.4 Other Hash Table Functions

Here are some other functions for working with hash tables.

8 Symbols

A symbol is an object with a unique name. This chapter describes symbols, their components, their property lists, and how they are created and interned. Separate chapters describe the use of symbols as variables and as function names; see Variables, and Functions. For the precise read syntax for symbols, see Symbol Type.

You can test whether an arbitrary Lisp object is a symbol with symbolp:

• Symbol Components: Symbols have names, values, function definitions and property lists.
• Definitions: A definition says how a symbol will be used.
• Creating Symbols: How symbols are kept unique.
• Symbol Properties: Each symbol has a property list for recording miscellaneous information.

8.1 Symbol Components

Each symbol has four components (or “cells”), each of which references another object:

Print name

The symbol's name.

Value

The symbol's current value as a variable.

Function

The symbol's function definition. It can also hold a symbol, a keymap, or a keyboard macro.

Property list

The symbol's property list.

The print name cell always holds a string, and cannot be changed. Each of the other three cells can be set to any Lisp object.

The print name cell holds the string that is the name of a symbol. Since symbols are represented textually by their names, it is important not to have two symbols with the same name. The Lisp reader ensures this: every time it reads a symbol, it looks for an existing symbol with the specified name before it creates a new one. To get a symbol's name, use the function symbol-name (see Creating Symbols).

The value cell holds a symbol's value as a variable, which is what you get if the symbol itself is evaluated as a Lisp expression. See Variables, for details about how values are set and retrieved, including complications such as local bindings and scoping rules. Most symbols can have any Lisp object as a value, but certain special symbols have values that cannot be changed; these include nil and t, and any symbol whose name starts with ‘:’ (those are called keywords). See Constant Variables.

The function cell holds a symbol's function definition. Often, we refer to “the function foo” when we really mean the function stored in the function cell of foo; we make the distinction explicit only when necessary. Typically, the function cell is used to hold a function (see Functions) or a macro (see Macros). However, it can also be used to hold a symbol (see Function Indirection), keyboard macro (see Keyboard Macros), keymap (see Keymaps), or autoload object (see Autoloading). To get the contents of a symbol's function cell, use the function symbol-function (see Function Cells).

The property list cell normally should hold a correctly formatted property list. To get a symbol's property list, use the function symbol-plist. See Symbol Properties.

The function cell or the value cell may be void, which means that the cell does not reference any object. (This is not the same thing as holding the symbol void, nor the same as holding the symbol nil.) Examining a function or value cell that is void results in an error, such as ‘Symbol's value as variable is void’.

Because each symbol has separate value and function cells, variables names and function names do not conflict. For example, the symbol buffer-file-name has a value (the name of the file being visited in the current buffer) as well as a function definition (a primitive function that returns the name of the file):

     buffer-file-name
          ⇒ "/gnu/elisp/symbols.texi"
     (symbol-function 'buffer-file-name)
          ⇒ #<subr buffer-file-name>

8.2 Defining Symbols

A definition is a special kind of Lisp expression that announces your intention to use a symbol in a particular way. It typically specifies a value or meaning for the symbol for one kind of use, plus documentation for its meaning when used in this way. Thus, when you define a symbol as a variable, you can supply an initial value for the variable, plus documentation for the variable.

defvar and defconst are special forms that define a symbol as a global variable—a variable that can be accessed at any point in a Lisp program. See Variables, for details about variables. To define a customizable variable, use the defcustom macro, which also calls defvar as a subroutine (see Customization).

In principle, you can assign a variable value to any symbol with setq, whether not it has first been defined as a variable. However, you ought to write a variable definition for each global variable that you want to use; otherwise, your Lisp program may not act correctly if it is evaluated with lexical scoping enabled (see Variable Scoping).

defun defines a symbol as a function, creating a lambda expression and storing it in the function cell of the symbol. This lambda expression thus becomes the function definition of the symbol. (The term “function definition”, meaning the contents of the function cell, is derived from the idea that defun gives the symbol its definition as a function.) defsubst and defalias are two other ways of defining a function. See Functions.

defmacro defines a symbol as a macro. It creates a macro object and stores it in the function cell of the symbol. Note that a given symbol can be a macro or a function, but not both at once, because both macro and function definitions are kept in the function cell, and that cell can hold only one Lisp object at any given time. See Macros.

As previously noted, Emacs Lisp allows the same symbol to be defined both as a variable (e.g., with defvar) and as a function or macro (e.g., with defun). Such definitions do not conflict.

These definitions also act as guides for programming tools. For example, the C-h f and C-h v commands create help buffers containing links to the relevant variable, function, or macro definitions. See Name Help.

8.3 Creating and Interning Symbols

To understand how symbols are created in GNU Emacs Lisp, you must know how Lisp reads them. Lisp must ensure that it finds the same symbol every time it reads the same set of characters. Failure to do so would cause complete confusion.

When the Lisp reader encounters a symbol, it reads all the characters of the name. Then it hashes those characters to find an index in a table called an obarray. Hashing is an efficient method of looking something up. For example, instead of searching a telephone book cover to cover when looking up Jan Jones, you start with the J's and go from there. That is a simple version of hashing. Each element of the obarray is a bucket which holds all the symbols with a given hash code; to look for a given name, it is sufficient to look through all the symbols in the bucket for that name's hash code. (The same idea is used for general Emacs hash tables, but they are a different data type; see Hash Tables.)

If a symbol with the desired name is found, the reader uses that symbol. If the obarray does not contain a symbol with that name, the reader makes a new symbol and adds it to the obarray. Finding or adding a symbol with a certain name is called interning it, and the symbol is then called an interned symbol.

Interning ensures that each obarray has just one symbol with any particular name. Other like-named symbols may exist, but not in the same obarray. Thus, the reader gets the same symbols for the same names, as long as you keep reading with the same obarray.

Interning usually happens automatically in the reader, but sometimes other programs need to do it. For example, after the M-x command obtains the command name as a string using the minibuffer, it then interns the string, to get the interned symbol with that name.

No obarray contains all symbols; in fact, some symbols are not in any obarray. They are called uninterned symbols. An uninterned symbol has the same four cells as other symbols; however, the only way to gain access to it is by finding it in some other object or as the value of a variable.

Creating an uninterned symbol is useful in generating Lisp code, because an uninterned symbol used as a variable in the code you generate cannot clash with any variables used in other Lisp programs.

In Emacs Lisp, an obarray is actually a vector. Each element of the vector is a bucket; its value is either an interned symbol whose name hashes to that bucket, or 0 if the bucket is empty. Each interned symbol has an internal link (invisible to the user) to the next symbol in the bucket. Because these links are invisible, there is no way to find all the symbols in an obarray except using mapatoms (below). The order of symbols in a bucket is not significant.

In an empty obarray, every element is 0, so you can create an obarray with (make-vector length 0). This is the only valid way to create an obarray. Prime numbers as lengths tend to result in good hashing; lengths one less than a power of two are also good.

Do not try to put symbols in an obarray yourself. This does not work—only intern can enter a symbol in an obarray properly.

Common Lisp note: Unlike Common Lisp, Emacs Lisp does not provide for interning a single symbol in several obarrays.

Most of the functions below take a name and sometimes an obarray as arguments. A wrong-type-argument error is signaled if the name is not a string, or if the obarray is not a vector.

Common Lisp note: In Common Lisp, you can intern an existing symbol in an obarray. In Emacs Lisp, you cannot do this, because the argument to intern must be a string, not a symbol.

8.4 Symbol Properties

A symbol may possess any number of symbol properties, which can be used to record miscellaneous information about the symbol. For example, when a symbol has a risky-local-variable property with a non-nil value, that means the variable which the symbol names is a risky file-local variable (see File Local Variables).

Each symbol's properties and property values are stored in the symbol's property list cell (see Symbol Components), in the form of a property list (see Property Lists).

• Symbol Plists: Accessing symbol properties.
• Standard Properties: Standard meanings of symbol properties.

8.4.1 Accessing Symbol Properties

The following functions can be used to access symbol properties.

8.4.2 Standard Symbol Properties

Here, we list the symbol properties which are used for special purposes in Emacs. In the following table, whenever we say “the named function”, that means the function whose name is the relevant symbol; similarly for “the named variable” etc.

:advertised-binding

This property value specifies the preferred key binding, when showing documentation, for the named function. See Keys in Documentation.

char-table-extra-slots

The value, if non-nil, specifies the number of extra slots in the named char-table type. See Char-Tables.

customized-face

face-defface-spec

saved-face

theme-face

These properties are used to record a face's standard, saved, customized, and themed face specs. Do not set them directly; they are managed by defface and related functions. See Defining Faces.

customized-value

saved-value

standard-value

theme-value

These properties are used to record a customizable variable's standard value, saved value, customized-but-unsaved value, and themed values. Do not set them directly; they are managed by defcustom and related functions. See Variable Definitions.

disabled

If the value is non-nil, the named function is disabled as a command. See Disabling Commands.

face-documentation

The value stores the documentation string of the named face. This is set automatically by defface. See Defining Faces.

history-length

The value, if non-nil, specifies the maximum minibuffer history length for the named history list variable. See Minibuffer History.

interactive-form

The value is an interactive form for the named function. Normally, you should not set this directly; use the interactive special form instead. See Interactive Call.

menu-enable

The value is an expression for determining whether the named menu item should be enabled in menus. See Simple Menu Items.

mode-class

If the value is special, the named major mode is special. See Major Mode Conventions.

permanent-local

If the value is non-nil, the named variable is a buffer-local variable whose value should not be reset when changing major modes. See Creating Buffer-Local.

permanent-local-hook

If the value is non-nil, the named function should not be deleted from the local value of a hook variable when changing major modes. See Setting Hooks.

pure

If the value is non-nil, the named function is considered to be side-effect free. Calls with constant arguments can be evaluated at compile time. This may shift run time errors to compile time.

risky-local-variable

If the value is non-nil, the named variable is considered risky as a file-local variable. See File Local Variables.

safe-function

If the value is non-nil, the named function is considered generally safe for evaluation. See Function Safety.

safe-local-eval-function

If the value is non-nil, the named function is safe to call in file-local evaluation forms. See File Local Variables.

safe-local-variable

The value specifies a function for determining safe file-local values for the named variable. See File Local Variables.

side-effect-free

A non-nil value indicates that the named function is free of side-effects, for determining function safety (see Function Safety) as well as for byte compiler optimizations. Do not set it.

variable-documentation

If non-nil, this specifies the named variable's documentation string. This is set automatically by defvar and related functions. See Defining Faces.

9 Evaluation

The evaluation of expressions in Emacs Lisp is performed by the Lisp interpreter—a program that receives a Lisp object as input and computes its value as an expression. How it does this depends on the data type of the object, according to rules described in this chapter. The interpreter runs automatically to evaluate portions of your program, but can also be called explicitly via the Lisp primitive function eval.

• Intro Eval: Evaluation in the scheme of things.
• Forms: How various sorts of objects are evaluated.
• Quoting: Avoiding evaluation (to put constants in the program).
• Backquote: Easier construction of list structure.
• Eval: How to invoke the Lisp interpreter explicitly.

9.1 Introduction to Evaluation

The Lisp interpreter, or evaluator, is the part of Emacs that computes the value of an expression that is given to it. When a function written in Lisp is called, the evaluator computes the value of the function by evaluating the expressions in the function body. Thus, running any Lisp program really means running the Lisp interpreter.

A Lisp object that is intended for evaluation is called a form or expression⁴. The fact that forms are data objects and not merely text is one of the fundamental differences between Lisp-like languages and typical programming languages. Any object can be evaluated, but in practice only numbers, symbols, lists and strings are evaluated very often.

In subsequent sections, we will describe the details of what evaluation means for each kind of form.

It is very common to read a Lisp form and then evaluate the form, but reading and evaluation are separate activities, and either can be performed alone. Reading per se does not evaluate anything; it converts the printed representation of a Lisp object to the object itself. It is up to the caller of read to specify whether this object is a form to be evaluated, or serves some entirely different purpose. See Input Functions.

Evaluation is a recursive process, and evaluating a form often involves evaluating parts within that form. For instance, when you evaluate a function call form such as (car x), Emacs first evaluates the argument (the subform x). After evaluating the argument, Emacs executes the function (car), and if the function is written in Lisp, execution works by evaluating the body of the function (in this example, however, car is not a Lisp function; it is a primitive function implemented in C). See Functions, for more information about functions and function calls.

Evaluation takes place in a context called the environment, which consists of the current values and bindings of all Lisp variables (see Variables).⁵ Whenever a form refers to a variable without creating a new binding for it, the variable evaluates to the value given by the current environment. Evaluating a form may also temporarily alter the environment by binding variables (see Local Variables).

Evaluating a form may also make changes that persist; these changes are called side effects. An example of a form that produces a side effect is (setq foo 1).

Do not confuse evaluation with command key interpretation. The editor command loop translates keyboard input into a command (an interactively callable function) using the active keymaps, and then uses call-interactively to execute that command. Executing the command usually involves evaluation, if the command is written in Lisp; however, this step is not considered a part of command key interpretation. See Command Loop.

9.2 Kinds of Forms

A Lisp object that is intended to be evaluated is called a form (or an expression). How Emacs evaluates a form depends on its data type. Emacs has three different kinds of form that are evaluated differently: symbols, lists, and all other types. This section describes all three kinds, one by one, starting with the other types, which are self-evaluating forms.

• Self-Evaluating Forms: Forms that evaluate to themselves.
• Symbol Forms: Symbols evaluate as variables.
• Classifying Lists: How to distinguish various sorts of list forms.
• Function Indirection: When a symbol appears as the car of a list, we find the real function via the symbol.
• Function Forms: Forms that call functions.
• Macro Forms: Forms that call macros.
• Special Forms: Special forms are idiosyncratic primitives, most of them extremely important.
• Autoloading: Functions set up to load files containing their real definitions.

9.2.1 Self-Evaluating Forms

A self-evaluating form is any form that is not a list or symbol. Self-evaluating forms evaluate to themselves: the result of evaluation is the same object that was evaluated. Thus, the number 25 evaluates to 25, and the string "foo" evaluates to the string "foo". Likewise, evaluating a vector does not cause evaluation of the elements of the vector—it returns the same vector with its contents unchanged.

     '123               ; A number, shown without evaluation.
          ⇒ 123
     123                ; Evaluated as usual---result is the same.
          ⇒ 123
     (eval '123)        ; Evaluated "by hand"---result is the same.
          ⇒ 123
     (eval (eval '123)) ; Evaluating twice changes nothing.
          ⇒ 123

It is common to write numbers, characters, strings, and even vectors in Lisp code, taking advantage of the fact that they self-evaluate. However, it is quite unusual to do this for types that lack a read syntax, because there's no way to write them textually. It is possible to construct Lisp expressions containing these types by means of a Lisp program. Here is an example:

     ;; Build an expression containing a buffer object.
     (setq print-exp (list 'print (current-buffer)))
          ⇒ (print #<buffer eval.texi>)
     ;; Evaluate it.
     (eval print-exp)
          -| #<buffer eval.texi>
          ⇒ #<buffer eval.texi>

9.2.2 Symbol Forms

When a symbol is evaluated, it is treated as a variable. The result is the variable's value, if it has one. If the symbol has no value as a variable, the Lisp interpreter signals an error. For more information on the use of variables, see Variables.

In the following example, we set the value of a symbol with setq. Then we evaluate the symbol, and get back the value that setq stored.

     (setq a 123)
          ⇒ 123
     (eval 'a)
          ⇒ 123
     a
          ⇒ 123

The symbols nil and t are treated specially, so that the value of nil is always nil, and the value of t is always t; you cannot set or bind them to any other values. Thus, these two symbols act like self-evaluating forms, even though eval treats them like any other symbol. A symbol whose name starts with ‘:’ also self-evaluates in the same way; likewise, its value ordinarily cannot be changed. See Constant Variables.

9.2.3 Classification of List Forms

A form that is a nonempty list is either a function call, a macro call, or a special form, according to its first element. These three kinds of forms are evaluated in different ways, described below. The remaining list elements constitute the arguments for the function, macro, or special form.

The first step in evaluating a nonempty list is to examine its first element. This element alone determines what kind of form the list is and how the rest of the list is to be processed. The first element is not evaluated, as it would be in some Lisp dialects such as Scheme.

9.2.4 Symbol Function Indirection

If the first element of the list is a symbol then evaluation examines the symbol's function cell, and uses its contents instead of the original symbol. If the contents are another symbol, this process, called symbol function indirection, is repeated until it obtains a non-symbol. See Function Names, for more information about symbol function indirection.

One possible consequence of this process is an infinite loop, in the event that a symbol's function cell refers to the same symbol. Otherwise, we eventually obtain a non-symbol, which ought to be a function or other suitable object.

More precisely, we should now have a Lisp function (a lambda expression), a byte-code function, a primitive function, a Lisp macro, a special form, or an autoload object. Each of these types is a case described in one of the following sections. If the object is not one of these types, Emacs signals an invalid-function error.

The following example illustrates the symbol indirection process. We use fset to set the function cell of a symbol and symbol-function to get the function cell contents (see Function Cells). Specifically, we store the symbol car into the function cell of first, and the symbol first into the function cell of erste.

     ;; Build this function cell linkage:
     ;;   -------------       -----        -------        -------
     ;;  | #<subr car> | <-- | car |  <-- | first |  <-- | erste |
     ;;   -------------       -----        -------        -------
     (symbol-function 'car)
          ⇒ #<subr car>
     (fset 'first 'car)
          ⇒ car
     (fset 'erste 'first)
          ⇒ first
     (erste '(1 2 3))   ; Call the function referenced by erste.
          ⇒ 1

By contrast, the following example calls a function without any symbol function indirection, because the first element is an anonymous Lisp function, not a symbol.

     ((lambda (arg) (erste arg))
      '(1 2 3))
          ⇒ 1

Executing the function itself evaluates its body; this does involve symbol function indirection when calling erste.

This form is rarely used and is now deprecated. Instead, you should write it as:

     (funcall (lambda (arg) (erste arg))
              '(1 2 3))

or just

     (let ((arg '(1 2 3))) (erste arg))

The built-in function indirect-function provides an easy way to perform symbol function indirection explicitly.

9.2.5 Evaluation of Function Forms

If the first element of a list being evaluated is a Lisp function object, byte-code object or primitive function object, then that list is a function call. For example, here is a call to the function +:

     (+ 1 x)

The first step in evaluating a function call is to evaluate the remaining elements of the list from left to right. The results are the actual argument values, one value for each list element. The next step is to call the function with this list of arguments, effectively using the function apply (see Calling Functions). If the function is written in Lisp, the arguments are used to bind the argument variables of the function (see Lambda Expressions); then the forms in the function body are evaluated in order, and the value of the last body form becomes the value of the function call.

9.2.6 Lisp Macro Evaluation

If the first element of a list being evaluated is a macro object, then the list is a macro call. When a macro call is evaluated, the elements of the rest of the list are not initially evaluated. Instead, these elements themselves are used as the arguments of the macro. The macro definition computes a replacement form, called the expansion of the macro, to be evaluated in place of the original form. The expansion may be any sort of form: a self-evaluating constant, a symbol, or a list. If the expansion is itself a macro call, this process of expansion repeats until some other sort of form results.

Ordinary evaluation of a macro call finishes by evaluating the expansion. However, the macro expansion is not necessarily evaluated right away, or at all, because other programs also expand macro calls, and they may or may not evaluate the expansions.

Normally, the argument expressions are not evaluated as part of computing the macro expansion, but instead appear as part of the expansion, so they are computed when the expansion is evaluated.

For example, given a macro defined as follows:

     (defmacro cadr (x)
       (list 'car (list 'cdr x)))

an expression such as (cadr (assq 'handler list)) is a macro call, and its expansion is:

     (car (cdr (assq 'handler list)))

Note that the argument (assq 'handler list) appears in the expansion.

See Macros, for a complete description of Emacs Lisp macros.

9.2.7 Special Forms

A special form is a primitive function specially marked so that its arguments are not all evaluated. Most special forms define control structures or perform variable bindings—things which functions cannot do.

Each special form has its own rules for which arguments are evaluated and which are used without evaluation. Whether a particular argument is evaluated may depend on the results of evaluating other arguments.

If an expression's first symbol is that of a special form, the expression should follow the rules of that special form; otherwise, Emacs's behavior is not well-defined (though it will not crash). For example, ((lambda (x) x . 3) 4) contains a subexpression that begins with lambda but is not a well-formed lambda expression, so Emacs may signal an error, or may return 3 or 4 or nil, or may behave in other ways.

Here is a list, in alphabetical order, of all of the special forms in Emacs Lisp with a reference to where each is described.

and

see Combining Conditions

catch

see Catch and Throw

cond

see Conditionals

condition-case

see Handling Errors

defconst

see Defining Variables

defvar

see Defining Variables

function

see Anonymous Functions

if

see Conditionals

interactive

see Interactive Call

lambda

see Lambda Expressions

let

let*

see Local Variables

or

see Combining Conditions

prog1

prog2

progn

see Sequencing

quote

see Quoting

save-current-buffer

see Current Buffer

save-excursion

see Excursions

save-restriction

see Narrowing

setq

see Setting Variables

setq-default

see Creating Buffer-Local

track-mouse

see Mouse Tracking

unwind-protect

see Nonlocal Exits

while

see Iteration

Common Lisp note: Here are some comparisons of special forms in GNU Emacs Lisp and Common Lisp. setq, if, and catch are special forms in both Emacs Lisp and Common Lisp. save-excursion is a special form in Emacs Lisp, but doesn't exist in Common Lisp. throw is a special form in Common Lisp (because it must be able to throw multiple values), but it is a function in Emacs Lisp (which doesn't have multiple values).

9.2.8 Autoloading

The autoload feature allows you to call a function or macro whose function definition has not yet been loaded into Emacs. It specifies which file contains the definition. When an autoload object appears as a symbol's function definition, calling that symbol as a function automatically loads the specified file; then it calls the real definition loaded from that file. The way to arrange for an autoload object to appear as a symbol's function definition is described in Autoload.

9.3 Quoting

The special form quote returns its single argument, as written, without evaluating it. This provides a way to include constant symbols and lists, which are not self-evaluating objects, in a program. (It is not necessary to quote self-evaluating objects such as numbers, strings, and vectors.)

Because quote is used so often in programs, Lisp provides a convenient read syntax for it. An apostrophe character (‘'’) followed by a Lisp object (in read syntax) expands to a list whose first element is quote, and whose second element is the object. Thus, the read syntax 'x is an abbreviation for (quote x).

Here are some examples of expressions that use quote:

     (quote (+ 1 2))
          ⇒ (+ 1 2)
     (quote foo)
          ⇒ foo
     'foo
          ⇒ foo
     ''foo
          ⇒ (quote foo)
     '(quote foo)
          ⇒ (quote foo)
     ['foo]
          ⇒ [(quote foo)]

Other quoting constructs include function (see Anonymous Functions), which causes an anonymous lambda expression written in Lisp to be compiled, and ‘`’ (see Backquote), which is used to quote only part of a list, while computing and substituting other parts.

9.4 Backquote

Backquote constructs allow you to quote a list, but selectively evaluate elements of that list. In the simplest case, it is identical to the special form quote (described in the previous section; see Quoting). For example, these two forms yield identical results:

     `(a list of (+ 2 3) elements)
          ⇒ (a list of (+ 2 3) elements)
     '(a list of (+ 2 3) elements)
          ⇒ (a list of (+ 2 3) elements)

The special marker ‘,’ inside of the argument to backquote indicates a value that isn't constant. The Emacs Lisp evaluator evaluates the argument of ‘,’, and puts the value in the list structure:

     `(a list of ,(+ 2 3) elements)
          ⇒ (a list of 5 elements)

Substitution with ‘,’ is allowed at deeper levels of the list structure also. For example:

     `(1 2 (3 ,(+ 4 5)))
          ⇒ (1 2 (3 9))

You can also splice an evaluated value into the resulting list, using the special marker ‘,@’. The elements of the spliced list become elements at the same level as the other elements of the resulting list. The equivalent code without using ‘`’ is often unreadable. Here are some examples:

     (setq some-list '(2 3))
          ⇒ (2 3)
     (cons 1 (append some-list '(4) some-list))
          ⇒ (1 2 3 4 2 3)
     `(1 ,@some-list 4 ,@some-list)
          ⇒ (1 2 3 4 2 3)
     
     (setq list '(hack foo bar))
          ⇒ (hack foo bar)
     (cons 'use
       (cons 'the
         (cons 'words (append (cdr list) '(as elements)))))
          ⇒ (use the words foo bar as elements)
     `(use the words ,@(cdr list) as elements)
          ⇒ (use the words foo bar as elements)

9.5 Eval

Most often, forms are evaluated automatically, by virtue of their occurrence in a program being run. On rare occasions, you may need to write code that evaluates a form that is computed at run time, such as after reading a form from text being edited or getting one from a property list. On these occasions, use the eval function. Often eval is not needed and something else should be used instead. For example, to get the value of a variable, while eval works, symbol-value is preferable; or rather than store expressions in a property list that then need to go through eval, it is better to store functions instead that are then passed to funcall.

The functions and variables described in this section evaluate forms, specify limits to the evaluation process, or record recently returned values. Loading a file also does evaluation (see Loading).

It is generally cleaner and more flexible to store a function in a data structure, and call it with funcall or apply, than to store an expression in the data structure and evaluate it. Using functions provides the ability to pass information to them as arguments.

10 Control Structures

A Lisp program consists of a set of expressions, or forms (see Forms). We control the order of execution of these forms by enclosing them in control structures. Control structures are special forms which control when, whether, or how many times to execute the forms they contain.

The simplest order of execution is sequential execution: first form a, then form b, and so on. This is what happens when you write several forms in succession in the body of a function, or at top level in a file of Lisp code—the forms are executed in the order written. We call this textual order. For example, if a function body consists of two forms a and b, evaluation of the function evaluates first a and then b. The result of evaluating b becomes the value of the function.

Explicit control structures make possible an order of execution other than sequential.

Emacs Lisp provides several kinds of control structure, including other varieties of sequencing, conditionals, iteration, and (controlled) jumps—all discussed below. The built-in control structures are special forms since their subforms are not necessarily evaluated or not evaluated sequentially. You can use macros to define your own control structure constructs (see Macros).

• Sequencing: Evaluation in textual order.
• Conditionals: if, cond, when, unless.
• Combining Conditions: and, or, not.
• Iteration: while loops.
• Generators: Generic sequences and coroutines.
• Nonlocal Exits: Jumping out of a sequence.

10.1 Sequencing

Evaluating forms in the order they appear is the most common way control passes from one form to another. In some contexts, such as in a function body, this happens automatically. Elsewhere you must use a control structure construct to do this: progn, the simplest control construct of Lisp.

A progn special form looks like this:

     (progn a b c ...)

and it says to execute the forms a, b, c, and so on, in that order. These forms are called the body of the progn form. The value of the last form in the body becomes the value of the entire progn. (progn) returns nil.

In the early days of Lisp, progn was the only way to execute two or more forms in succession and use the value of the last of them. But programmers found they often needed to use a progn in the body of a function, where (at that time) only one form was allowed. So the body of a function was made into an implicit progn: several forms are allowed just as in the body of an actual progn. Many other control structures likewise contain an implicit progn. As a result, progn is not used as much as it was many years ago. It is needed now most often inside an unwind-protect, and, or, or in the then-part of an if.

Two other constructs likewise evaluate a series of forms but return different values:

10.2 Conditionals

Conditional control structures choose among alternatives. Emacs Lisp has four conditional forms: if, which is much the same as in other languages; when and unless, which are variants of if; and cond, which is a generalized case statement.

Any conditional construct can be expressed with cond or with if. Therefore, the choice between them is a matter of style. For example:

     (if a b c)
     ==
     (cond (a b) (t c))

• Pattern matching case statement

10.2.1 Pattern matching case statement

The cond form lets you choose between alternatives using predicate conditions that compare values of expressions against specific values known and written in advance. However, sometimes it is useful to select alternatives based on more general conditions that distinguish between broad classes of values. The pcase macro allows you to choose between alternatives based on matching the value of an expression against a series of patterns. A pattern can be a literal value (for comparisons to literal values you'd use cond), or it can be a more general description of the expected structure of the expression's value.

Here is an example of using pcase to implement a simple interpreter for a little expression language (note that this example requires lexical binding, see Lexical Binding):

     (defun evaluate (exp env)
       (pcase exp
         (`(add ,x ,y)       (+ (evaluate x env) (evaluate y env)))
         (`(call ,fun ,arg)  (funcall (evaluate fun env) (evaluate arg env)))
         (`(fn ,arg ,body)   (lambda (val)
                               (evaluate body (cons (cons arg val) env))))
         ((pred numberp)     exp)
         ((pred symbolp)     (cdr (assq exp env)))
         (_                  (error "Unknown expression %S" exp))))

Here `(add ,x ,y) is a pattern that checks that exp is a three-element list starting with the literal symbol add, then extracts the second and third elements and binds them to the variables x and y. Then it evaluates x and y and adds the results. The call and fn patterns similarly implement two flavors of function calls. (pred numberp) is a pattern that simply checks that exp is a number and if so, evaluates it. (pred symbolp) matches symbols, and returns their association. Finally, _ is the catch-all pattern that matches anything, so it's suitable for reporting syntax errors.

Here are some sample programs in this small language, including their evaluation results:

     (evaluate '(add 1 2) nil)                 ;=> 3
     (evaluate '(add x y) '((x . 1) (y . 2)))  ;=> 3
     (evaluate '(call (fn x (add 1 x)) 2) nil) ;=> 3
     (evaluate '(sub 1 2) nil)                 ;=> error

Additional UPatterns can be defined using the pcase-defmacro macro.

10.3 Constructs for Combining Conditions

This section describes three constructs that are often used together with if and cond to express complicated conditions. The constructs and and or can also be used individually as kinds of multiple conditional constructs.

10.4 Iteration

Iteration means executing part of a program repetitively. For example, you might want to repeat some computation once for each element of a list, or once for each integer from 0 to n. You can do this in Emacs Lisp with the special form while:

The dolist and dotimes macros provide convenient ways to write two common kinds of loops.

10.5 Generators

A generator is a function that produces a potentially-infinite stream of values. Each time the function produces a value, it suspends itself and waits for a caller to request the next value.

To use a generator function, first call it normally, producing a iterator object. An iterator is a specific instance of a generator. Then use iter-next to retrieve values from this iterator. When there are no more values to pull from an iterator, iter-next raises an iter-end-of-sequence condition with the iterator's final value.

It's important to note that generator function bodies only execute inside calls to iter-next. A call to a function defined with iter-defun produces an iterator; you must drive this iterator with iter-next for anything interesting to happen. Each call to a generator function produces a different iterator, each with its own state.

Some convenience functions are provided to make working with iterators easier:

The Common Lisp loop facility also contains features for working with iterators. See See Loop Facility.

The following piece of code demonstrates some important principles of working with iterators.

     (require 'generator)
     (iter-defun my-iter (x)
       (iter-yield (1+ (iter-yield (1+ x))))
        ;; Return normally
       -1)
     
     (let* ((iter (my-iter 5))
            (iter2 (my-iter 0)))
       ;; Prints 6
       (print (iter-next iter))
       ;; Prints 9
       (print (iter-next iter 8))
       ;; Prints 1; iter and iter2 have distinct states
       (print (iter-next iter2 nil))
     
       ;; We expect the iter sequence to end now
       (condition-case x
           (iter-next iter)
         (iter-end-of-sequence
           ;; Prints -1, which my-iter returned normally
           (print (cdr x)))))

10.6 Nonlocal Exits

A nonlocal exit is a transfer of control from one point in a program to another remote point. Nonlocal exits can occur in Emacs Lisp as a result of errors; you can also use them under explicit control. Nonlocal exits unbind all variable bindings made by the constructs being exited.

• Catch and Throw: Nonlocal exits for the program's own purposes.
• Examples of Catch: Showing how such nonlocal exits can be written.
• Errors: How errors are signaled and handled.
• Cleanups: Arranging to run a cleanup form if an error happens.

10.6.1 Explicit Nonlocal Exits: catch and throw

Most control constructs affect only the flow of control within the construct itself. The function throw is the exception to this rule of normal program execution: it performs a nonlocal exit on request. (There are other exceptions, but they are for error handling only.) throw is used inside a catch, and jumps back to that catch. For example:

     (defun foo-outer ()
       (catch 'foo
         (foo-inner)))
     
     (defun foo-inner ()
       ...
       (if x
           (throw 'foo t))
       ...)

The throw form, if executed, transfers control straight back to the corresponding catch, which returns immediately. The code following the throw is not executed. The second argument of throw is used as the return value of the catch.

The function throw finds the matching catch based on the first argument: it searches for a catch whose first argument is eq to the one specified in the throw. If there is more than one applicable catch, the innermost one takes precedence. Thus, in the above example, the throw specifies foo, and the catch in foo-outer specifies the same symbol, so that catch is the applicable one (assuming there is no other matching catch in between).

Executing throw exits all Lisp constructs up to the matching catch, including function calls. When binding constructs such as let or function calls are exited in this way, the bindings are unbound, just as they are when these constructs exit normally (see Local Variables). Likewise, throw restores the buffer and position saved by save-excursion (see Excursions), and the narrowing status saved by save-restriction. It also runs any cleanups established with the unwind-protect special form when it exits that form (see Cleanups).

The throw need not appear lexically within the catch that it jumps to. It can equally well be called from another function called within the catch. As long as the throw takes place chronologically after entry to the catch, and chronologically before exit from it, it has access to that catch. This is why throw can be used in commands such as exit-recursive-edit that throw back to the editor command loop (see Recursive Editing).

Common Lisp note: Most other versions of Lisp, including Common Lisp, have several ways of transferring control nonsequentially: return, return-from, and go, for example. Emacs Lisp has only throw. The cl-lib library provides versions of some of these. See Blocks and Exits.

10.6.2 Examples of catch and throw

One way to use catch and throw is to exit from a doubly nested loop. (In most languages, this would be done with a goto.) Here we compute (foo i j) for i and j varying from 0 to 9:

     (defun search-foo ()
       (catch 'loop
         (let ((i 0))
           (while (< i 10)
             (let ((j 0))
               (while (< j 10)
                 (if (foo i j)
                     (throw 'loop (list i j)))
                 (setq j (1+ j))))
             (setq i (1+ i))))))

If foo ever returns non-nil, we stop immediately and return a list of i and j. If foo always returns nil, the catch returns normally, and the value is nil, since that is the result of the while.

Here are two tricky examples, slightly different, showing two return points at once. First, two return points with the same tag, hack:

     (defun catch2 (tag)
       (catch tag
         (throw 'hack 'yes)))
     ⇒ catch2
     
     (catch 'hack
       (print (catch2 'hack))
       'no)
     -| yes
     ⇒ no

Since both return points have tags that match the throw, it goes to the inner one, the one established in catch2. Therefore, catch2 returns normally with value yes, and this value is printed. Finally the second body form in the outer catch, which is 'no, is evaluated and returned from the outer catch.

Now let's change the argument given to catch2:

     (catch 'hack
       (print (catch2 'quux))
       'no)
     ⇒ yes

We still have two return points, but this time only the outer one has the tag hack; the inner one has the tag quux instead. Therefore, throw makes the outer catch return the value yes. The function print is never called, and the body-form 'no is never evaluated.

10.6.3 Errors

When Emacs Lisp attempts to evaluate a form that, for some reason, cannot be evaluated, it signals an error.

When an error is signaled, Emacs's default reaction is to print an error message and terminate execution of the current command. This is the right thing to do in most cases, such as if you type C-f at the end of the buffer.

In complicated programs, simple termination may not be what you want. For example, the program may have made temporary changes in data structures, or created temporary buffers that should be deleted before the program is finished. In such cases, you would use unwind-protect to establish cleanup expressions to be evaluated in case of error. (See Cleanups.) Occasionally, you may wish the program to continue execution despite an error in a subroutine. In these cases, you would use condition-case to establish error handlers to recover control in case of error.

Resist the temptation to use error handling to transfer control from one part of the program to another; use catch and throw instead. See Catch and Throw.

• Signaling Errors: How to report an error.
• Processing of Errors: What Emacs does when you report an error.
• Handling Errors: How you can trap errors and continue execution.
• Error Symbols: How errors are classified for trapping them.

10.6.3.1 How to Signal an Error

Signaling an error means beginning error processing. Error processing normally aborts all or part of the running program and returns to a point that is set up to handle the error (see Processing of Errors). Here we describe how to signal an error.

Most errors are signaled automatically within Lisp primitives which you call for other purposes, such as if you try to take the car of an integer or move forward a character at the end of the buffer. You can also signal errors explicitly with the functions error and signal.

Quitting, which happens when the user types C-g, is not considered an error, but it is handled almost like an error. See Quitting.

Every error specifies an error message, one way or another. The message should state what is wrong (“File does not exist”), not how things ought to be (“File must exist”). The convention in Emacs Lisp is that error messages should start with a capital letter, but should not end with any sort of punctuation.

Common Lisp note: Emacs Lisp has nothing like the Common Lisp concept of continuable errors.

10.6.3.2 How Emacs Processes Errors

When an error is signaled, signal searches for an active handler for the error. A handler is a sequence of Lisp expressions designated to be executed if an error happens in part of the Lisp program. If the error has an applicable handler, the handler is executed, and control resumes following the handler. The handler executes in the environment of the condition-case that established it; all functions called within that condition-case have already been exited, and the handler cannot return to them.

If there is no applicable handler for the error, it terminates the current command and returns control to the editor command loop. (The command loop has an implicit handler for all kinds of errors.) The command loop's handler uses the error symbol and associated data to print an error message. You can use the variable command-error-function to control how this is done:

An error that has no explicit handler may call the Lisp debugger. The debugger is enabled if the variable debug-on-error (see Error Debugging) is non-nil. Unlike error handlers, the debugger runs in the environment of the error, so that you can examine values of variables precisely as they were at the time of the error.

10.6.3.3 Writing Code to Handle Errors

The usual effect of signaling an error is to terminate the command that is running and return immediately to the Emacs editor command loop. You can arrange to trap errors occurring in a part of your program by establishing an error handler, with the special form condition-case. A simple example looks like this:

     (condition-case nil
         (delete-file filename)
       (error nil))

This deletes the file named filename, catching any error and returning nil if an error occurs. (You can use the macro ignore-errors for a simple case like this; see below.)

The condition-case construct is often used to trap errors that are predictable, such as failure to open a file in a call to insert-file-contents. It is also used to trap errors that are totally unpredictable, such as when the program evaluates an expression read from the user.

The second argument of condition-case is called the protected form. (In the example above, the protected form is a call to delete-file.) The error handlers go into effect when this form begins execution and are deactivated when this form returns. They remain in effect for all the intervening time. In particular, they are in effect during the execution of functions called by this form, in their subroutines, and so on. This is a good thing, since, strictly speaking, errors can be signaled only by Lisp primitives (including signal and error) called by the protected form, not by the protected form itself.

The arguments after the protected form are handlers. Each handler lists one or more condition names (which are symbols) to specify which errors it will handle. The error symbol specified when an error is signaled also defines a list of condition names. A handler applies to an error if they have any condition names in common. In the example above, there is one handler, and it specifies one condition name, error, which covers all errors.

The search for an applicable handler checks all the established handlers starting with the most recently established one. Thus, if two nested condition-case forms offer to handle the same error, the inner of the two gets to handle it.

If an error is handled by some condition-case form, this ordinarily prevents the debugger from being run, even if debug-on-error says this error should invoke the debugger.

If you want to be able to debug errors that are caught by a condition-case, set the variable debug-on-signal to a non-nil value. You can also specify that a particular handler should let the debugger run first, by writing debug among the conditions, like this:

     (condition-case nil
         (delete-file filename)
       ((debug error) nil))

The effect of debug here is only to prevent condition-case from suppressing the call to the debugger. Any given error will invoke the debugger only if debug-on-error and the other usual filtering mechanisms say it should. See Error Debugging.

Once Emacs decides that a certain handler handles the error, it returns control to that handler. To do so, Emacs unbinds all variable bindings made by binding constructs that are being exited, and executes the cleanups of all unwind-protect forms that are being exited. Once control arrives at the handler, the body of the handler executes normally.

After execution of the handler body, execution returns from the condition-case form. Because the protected form is exited completely before execution of the handler, the handler cannot resume execution at the point of the error, nor can it examine variable bindings that were made within the protected form. All it can do is clean up and proceed.

Error signaling and handling have some resemblance to throw and catch (see Catch and Throw), but they are entirely separate facilities. An error cannot be caught by a catch, and a throw cannot be handled by an error handler (though using throw when there is no suitable catch signals an error that can be handled).

Here is an example of using condition-case to handle the error that results from dividing by zero. The handler displays the error message (but without a beep), then returns a very large number.

     (defun safe-divide (dividend divisor)
       (condition-case err
           ;; Protected form.
           (/ dividend divisor)
         ;; The handler.
         (arith-error                        ; Condition.
          ;; Display the usual message for this error.
          (message "%s" (error-message-string err))
          1000000)))
     ⇒ safe-divide
     
     (safe-divide 5 0)
          -| Arithmetic error: (arith-error)
     ⇒ 1000000

The handler specifies condition name arith-error so that it will handle only division-by-zero errors. Other kinds of errors will not be handled (by this condition-case). Thus:

     (safe-divide nil 3)
          error--> Wrong type argument: number-or-marker-p, nil

Here is a condition-case that catches all kinds of errors, including those from error:

     (setq baz 34)
          ⇒ 34
     
     (condition-case err
         (if (eq baz 35)
             t
           ;; This is a call to the function error.
           (error "Rats!  The variable %s was %s, not 35" 'baz baz))
       ;; This is the handler; it is not a form.
       (error (princ (format "The error was: %s" err))
              2))
     -| The error was: (error "Rats!  The variable baz was 34, not 35")
     ⇒ 2

10.6.3.4 Error Symbols and Condition Names

When you signal an error, you specify an error symbol to specify the kind of error you have in mind. Each error has one and only one error symbol to categorize it. This is the finest classification of errors defined by the Emacs Lisp language.

These narrow classifications are grouped into a hierarchy of wider classes called error conditions, identified by condition names. The narrowest such classes belong to the error symbols themselves: each error symbol is also a condition name. There are also condition names for more extensive classes, up to the condition name error which takes in all kinds of errors (but not quit). Thus, each error has one or more condition names: error, the error symbol if that is distinct from error, and perhaps some intermediate classifications.

In addition to its parents, the error symbol has a message which is a string to be printed when that error is signaled but not handled. If that message is not valid, the error message ‘peculiar error’ is used. See Definition of signal.

Internally, the set of parents is stored in the error-conditions property of the error symbol and the message is stored in the error-message property of the error symbol.

Here is how we define a new error symbol, new-error:

     (define-error 'new-error "A new error" 'my-own-errors)

This error has several condition names: new-error, the narrowest classification; my-own-errors, which we imagine is a wider classification; and all the conditions of my-own-errors which should include error, which is the widest of all.

The error string should start with a capital letter but it should not end with a period. This is for consistency with the rest of Emacs.

Naturally, Emacs will never signal new-error on its own; only an explicit call to signal (see Definition of signal) in your code can do this:

     (signal 'new-error '(x y))
          error--> A new error: x, y

This error can be handled through any of its condition names. This example handles new-error and any other errors in the class my-own-errors:

     (condition-case foo
         (bar nil t)
       (my-own-errors nil))

The significant way that errors are classified is by their condition names—the names used to match errors with handlers. An error symbol serves only as a convenient way to specify the intended error message and list of condition names. It would be cumbersome to give signal a list of condition names rather than one error symbol.

By contrast, using only error symbols without condition names would seriously decrease the power of condition-case. Condition names make it possible to categorize errors at various levels of generality when you write an error handler. Using error symbols alone would eliminate all but the narrowest level of classification.

See Standard Errors, for a list of the main error symbols and their conditions.

10.6.4 Cleaning Up from Nonlocal Exits

The unwind-protect construct is essential whenever you temporarily put a data structure in an inconsistent state; it permits you to make the data consistent again in the event of an error or throw. (Another more specific cleanup construct that is used only for changes in buffer contents is the atomic change group; Atomic Changes.)

For example, here we make an invisible buffer for temporary use, and make sure to kill it before finishing:

     (let ((buffer (get-buffer-create " *temp*")))
       (with-current-buffer buffer
         (unwind-protect
             body-form
           (kill-buffer buffer))))

You might think that we could just as well write (kill-buffer (current-buffer)) and dispense with the variable buffer. However, the way shown above is safer, if body-form happens to get an error after switching to a different buffer! (Alternatively, you could write a save-current-buffer around body-form, to ensure that the temporary buffer becomes current again in time to kill it.)

Emacs includes a standard macro called with-temp-buffer which expands into more or less the code shown above (see Current Buffer). Several of the macros defined in this manual use unwind-protect in this way.

Here is an actual example derived from an FTP package. It creates a process (see Processes) to try to establish a connection to a remote machine. As the function ftp-login is highly susceptible to numerous problems that the writer of the function cannot anticipate, it is protected with a form that guarantees deletion of the process in the event of failure. Otherwise, Emacs might fill up with useless subprocesses.

     (let ((win nil))
       (unwind-protect
           (progn
             (setq process (ftp-setup-buffer host file))
             (if (setq win (ftp-login process host user password))
                 (message "Logged in")
               (error "Ftp login failed")))
         (or win (and process (delete-process process)))))

This example has a small bug: if the user types C-g to quit, and the quit happens immediately after the function ftp-setup-buffer returns but before the variable process is set, the process will not be killed. There is no easy way to fix this bug, but at least it is very unlikely.

11 Variables

A variable is a name used in a program to stand for a value. In Lisp, each variable is represented by a Lisp symbol (see Symbols). The variable name is simply the symbol's name, and the variable's value is stored in the symbol's value cell⁶. See Symbol Components. In Emacs Lisp, the use of a symbol as a variable is independent of its use as a function name.

As previously noted in this manual, a Lisp program is represented primarily by Lisp objects, and only secondarily as text. The textual form of a Lisp program is given by the read syntax of the Lisp objects that constitute the program. Hence, the textual form of a variable in a Lisp program is written using the read syntax for the symbol representing the variable.

• Global Variables: Variable values that exist permanently, everywhere.
• Constant Variables: Variables that never change.
• Local Variables: Variable values that exist only temporarily.
• Void Variables: Symbols that lack values.
• Defining Variables: A definition says a symbol is used as a variable.
• Tips for Defining: Things you should think about when you define a variable.
• Accessing Variables: Examining values of variables whose names are known only at run time.
• Setting Variables: Storing new values in variables.
• Variable Scoping: How Lisp chooses among local and global values.
• Buffer-Local Variables: Variable values in effect only in one buffer.
• File Local Variables: Handling local variable lists in files.
• Directory Local Variables: Local variables common to all files in a directory.
• Variable Aliases: Variables that are aliases for other variables.
• Variables with Restricted Values: Non-constant variables whose value can not be an arbitrary Lisp object.
• Generalized Variables: Extending the concept of variables.

11.1 Global Variables

The simplest way to use a variable is globally. This means that the variable has just one value at a time, and this value is in effect (at least for the moment) throughout the Lisp system. The value remains in effect until you specify a new one. When a new value replaces the old one, no trace of the old value remains in the variable.

You specify a value for a symbol with setq. For example,

     (setq x '(a b))

gives the variable x the value (a b). Note that setq is a special form (see Special Forms); it does not evaluate its first argument, the name of the variable, but it does evaluate the second argument, the new value.

Once the variable has a value, you can refer to it by using the symbol itself as an expression. Thus,

     x ⇒ (a b)

assuming the setq form shown above has already been executed.

If you do set the same variable again, the new value replaces the old one:

     x
          ⇒ (a b)
     (setq x 4)
          ⇒ 4
     x
          ⇒ 4

11.2 Variables that Never Change

In Emacs Lisp, certain symbols normally evaluate to themselves. These include nil and t, as well as any symbol whose name starts with ‘:’ (these are called keywords). These symbols cannot be rebound, nor can their values be changed. Any attempt to set or bind nil or t signals a setting-constant error. The same is true for a keyword (a symbol whose name starts with ‘:’), if it is interned in the standard obarray, except that setting such a symbol to itself is not an error.

     nil == 'nil
          ⇒ nil
     (setq nil 500)
     error--> Attempt to set constant symbol: nil

These constants are fundamentally different from the constants defined using the defconst special form (see Defining Variables). A defconst form serves to inform human readers that you do not intend to change the value of a variable, but Emacs does not raise an error if you actually change it.

11.3 Local Variables

Global variables have values that last until explicitly superseded with new values. Sometimes it is useful to give a variable a local value—a value that takes effect only within a certain part of a Lisp program. When a variable has a local value, we say that it is locally bound to that value, and that it is a local variable.

For example, when a function is called, its argument variables receive local values, which are the actual arguments supplied to the function call; these local bindings take effect within the body of the function. To take another example, the let special form explicitly establishes local bindings for specific variables, which take effect within the body of the let form.

We also speak of the global binding, which is where (conceptually) the global value is kept.

Establishing a local binding saves away the variable's previous value (or lack of one). We say that the previous value is shadowed. Both global and local values may be shadowed. If a local binding is in effect, using setq on the local variable stores the specified value in the local binding. When that local binding is no longer in effect, the previously shadowed value (or lack of one) comes back.

A variable can have more than one local binding at a time (e.g., if there are nested let forms that bind the variable). The current binding is the local binding that is actually in effect. It determines the value returned by evaluating the variable symbol, and it is the binding acted on by setq.

For most purposes, you can think of the current binding as the innermost local binding, or the global binding if there is no local binding. To be more precise, a rule called the scoping rule determines where in a program a local binding takes effect. The default scoping rule in Emacs Lisp is called dynamic scoping, which simply states that the current binding at any given point in the execution of a program is the most recently-created binding for that variable that still exists. For details about dynamic scoping, and an alternative scoping rule called lexical scoping, See Variable Scoping.

The special forms let and let* exist to create local bindings:

Here is a complete list of the other facilities that create local bindings:

• Function calls (see Functions).
• Macro calls (see Macros).
• condition-case (see Errors).

Variables can also have buffer-local bindings (see Buffer-Local Variables); a few variables have terminal-local bindings (see Multiple Terminals). These kinds of bindings work somewhat like ordinary local bindings, but they are localized depending on where you are in Emacs.

11.4 When a Variable is Void

We say that a variable is void if its symbol has an unassigned value cell (see Symbol Components).

Under Emacs Lisp's default dynamic scoping rule (see Variable Scoping), the value cell stores the variable's current (local or global) value. Note that an unassigned value cell is not the same as having nil in the value cell. The symbol nil is a Lisp object and can be the value of a variable, just as any other object can be; but it is still a value. If a variable is void, trying to evaluate the variable signals a void-variable error, instead of returning a value.

Under the optional lexical scoping rule, the value cell only holds the variable's global value—the value outside of any lexical binding construct. When a variable is lexically bound, the local value is determined by the lexical environment; hence, variables can have local values even if their symbols' value cells are unassigned.

11.5 Defining Global Variables

A variable definition is a construct that announces your intention to use a symbol as a global variable. It uses the special forms defvar or defconst, which are documented below.

A variable definition serves three purposes. First, it informs people who read the code that the symbol is intended to be used a certain way (as a variable). Second, it informs the Lisp system of this, optionally supplying an initial value and a documentation string. Third, it provides information to programming tools such as etags, allowing them to find where the variable was defined.

The difference between defconst and defvar is mainly a matter of intent, serving to inform human readers of whether the value should ever change. Emacs Lisp does not actually prevent you from changing the value of a variable defined with defconst. One notable difference between the two forms is that defconst unconditionally initializes the variable, whereas defvar initializes it only if it is originally void.

To define a customizable variable, you should use defcustom (which calls defvar as a subroutine). See Variable Definitions.

Warning: If you use a defconst or defvar special form while the variable has a local binding (made with let, or a function argument), it sets the local binding rather than the global binding. This is not what you usually want. To prevent this, use these special forms at top level in a file, where normally no local binding is in effect, and make sure to load the file before making a local binding for the variable.

11.6 Tips for Defining Variables Robustly

When you define a variable whose value is a function, or a list of functions, use a name that ends in ‘-function’ or ‘-functions’, respectively.

There are several other variable name conventions; here is a complete list:

‘...-hook’

The variable is a normal hook (see Hooks).

‘...-function’

The value is a function.

‘...-functions’

The value is a list of functions.

‘...-form’

The value is a form (an expression).

‘...-forms’

The value is a list of forms (expressions).

‘...-predicate’

The value is a predicate—a function of one argument that returns non-nil for success and nil for failure.

‘...-flag’

The value is significant only as to whether it is nil or not. Since such variables often end up acquiring more values over time, this convention is not strongly recommended.

‘...-program’

The value is a program name.

‘...-command’

The value is a whole shell command.

‘...-switches’

The value specifies options for a command.

When you define a variable, always consider whether you should mark it as safe or risky; see File Local Variables.

When defining and initializing a variable that holds a complicated value (such as a keymap with bindings in it), it's best to put the entire computation of the value into the defvar, like this:

     (defvar my-mode-map
       (let ((map (make-sparse-keymap)))
         (define-key map "\C-c\C-a" 'my-command)
         ...
         map)
       docstring)

This method has several benefits. First, if the user quits while loading the file, the variable is either still uninitialized or initialized properly, never in-between. If it is still uninitialized, reloading the file will initialize it properly. Second, reloading the file once the variable is initialized will not alter it; that is important if the user has run hooks to alter part of the contents (such as, to rebind keys). Third, evaluating the defvar form with C-M-x will reinitialize the map completely.

Putting so much code in the defvar form has one disadvantage: it puts the documentation string far away from the line which names the variable. Here's a safe way to avoid that:

     (defvar my-mode-map nil
       docstring)
     (unless my-mode-map
       (let ((map (make-sparse-keymap)))
         (define-key map "\C-c\C-a" 'my-command)
         ...
         (setq my-mode-map map)))

This has all the same advantages as putting the initialization inside the defvar, except that you must type C-M-x twice, once on each form, if you do want to reinitialize the variable.

11.7 Accessing Variable Values

The usual way to reference a variable is to write the symbol which names it. See Symbol Forms.

Occasionally, you may want to reference a variable which is only determined at run time. In that case, you cannot specify the variable name in the text of the program. You can use the symbol-value function to extract the value.

11.8 Setting Variable Values

The usual way to change the value of a variable is with the special form setq. When you need to compute the choice of variable at run time, use the function set.

11.9 Scoping Rules for Variable Bindings

When you create a local binding for a variable, that binding takes effect only within a limited portion of the program (see Local Variables). This section describes exactly what this means.

Each local binding has a certain scope and extent. Scope refers to where in the textual source code the binding can be accessed. Extent refers to when, as the program is executing, the binding exists.

By default, the local bindings that Emacs creates are dynamic bindings. Such a binding has dynamic scope, meaning that any part of the program can potentially access the variable binding. It also has dynamic extent, meaning that the binding lasts only while the binding construct (such as the body of a let form) is being executed.

Emacs can optionally create lexical bindings. A lexical binding has lexical scope, meaning that any reference to the variable must be located textually within the binding construct⁷. It also has indefinite extent, meaning that under some circumstances the binding can live on even after the binding construct has finished executing, by means of special objects called closures.

The following subsections describe dynamic binding and lexical binding in greater detail, and how to enable lexical binding in Emacs Lisp programs.

• Dynamic Binding: The default for binding local variables in Emacs.
• Dynamic Binding Tips: Avoiding problems with dynamic binding.
• Lexical Binding: A different type of local variable binding.
• Using Lexical Binding: How to enable lexical binding.

11.9.1 Dynamic Binding

By default, the local variable bindings made by Emacs are dynamic bindings. When a variable is dynamically bound, its current binding at any point in the execution of the Lisp program is simply the most recently-created dynamic local binding for that symbol, or the global binding if there is no such local binding.

Dynamic bindings have dynamic scope and extent, as shown by the following example:

     (defvar x -99)  ; x receives an initial value of −99.
     
     (defun getx ()
       x)            ; x is used free in this function.
     
     (let ((x 1))    ; x is dynamically bound.
       (getx))
          ⇒ 1
     
     ;; After the let form finishes, x reverts to its
     ;; previous value, which is −99.
     
     (getx)
          ⇒ -99

The function getx refers to x. This is a free reference, in the sense that there is no binding for x within that defun construct itself. When we call getx from within a let form in which x is (dynamically) bound, it retrieves the local value (i.e., 1). But when we call getx outside the let form, it retrieves the global value (i.e., −99).

Here is another example, which illustrates setting a dynamically bound variable using setq:

     (defvar x -99)      ; x receives an initial value of −99.
     
     (defun addx ()
       (setq x (1+ x)))  ; Add 1 to x and return its new value.
     
     (let ((x 1))
       (addx)
       (addx))
          ⇒ 3           ; The two addx calls add to x twice.
     
     ;; After the let form finishes, x reverts to its
     ;; previous value, which is −99.
     
     (addx)
          ⇒ -98

Dynamic binding is implemented in Emacs Lisp in a simple way. Each symbol has a value cell, which specifies its current dynamic value (or absence of value). See Symbol Components. When a symbol is given a dynamic local binding, Emacs records the contents of the value cell (or absence thereof) in a stack, and stores the new local value in the value cell. When the binding construct finishes executing, Emacs pops the old value off the stack, and puts it in the value cell.

11.9.2 Proper Use of Dynamic Binding

Dynamic binding is a powerful feature, as it allows programs to refer to variables that are not defined within their local textual scope. However, if used without restraint, this can also make programs hard to understand. There are two clean ways to use this technique:

• If a variable has no global definition, use it as a local variable only within a binding construct, such as the body of the let form where the variable was bound. If this convention is followed consistently throughout a program, the value of the variable will not affect, nor be affected by, any uses of the same variable symbol elsewhere in the program.
• Otherwise, define the variable with defvar, defconst, or defcustom. See Defining Variables. Usually, the definition should be at top-level in an Emacs Lisp file. As far as possible, it should include a documentation string which explains the meaning and purpose of the variable. You should also choose the variable's name to avoid name conflicts (see Coding Conventions).

  Then you can bind the variable anywhere in a program, knowing reliably what the effect will be. Wherever you encounter the variable, it will be easy to refer back to the definition, e.g., via the C-h v command (provided the variable definition has been loaded into Emacs). See Name Help.

  For example, it is common to use local bindings for customizable variables like case-fold-search:

            (defun search-for-abc ()
              "Search for the string \"abc\", ignoring case differences."
              (let ((case-fold-search nil))
                (re-search-forward "abc")))
  

11.9.3 Lexical Binding

Lexical binding was introduced to Emacs, as an optional feature, in version 24.1. We expect its importance to increase in the future. Lexical binding opens up many more opportunities for optimization, so programs using it are likely to run faster in future Emacs versions. Lexical binding is also more compatible with concurrency, which we want to add to Emacs in the future.

A lexically-bound variable has lexical scope, meaning that any reference to the variable must be located textually within the binding construct. Here is an example (see Using Lexical Binding, for how to actually enable lexical binding):

     (let ((x 1))    ; x is lexically bound.
       (+ x 3))
          ⇒ 4
     
     (defun getx ()
       x)            ; x is used free in this function.
     
     (let ((x 1))    ; x is lexically bound.
       (getx))
     error--> Symbol's value as variable is void: x

Here, the variable x has no global value. When it is lexically bound within a let form, it can be used in the textual confines of that let form. But it can not be used from within a getx function called from the let form, since the function definition of getx occurs outside the let form itself.

Here is how lexical binding works. Each binding construct defines a lexical environment, specifying the variables that are bound within the construct and their local values. When the Lisp evaluator wants the current value of a variable, it looks first in the lexical environment; if the variable is not specified in there, it looks in the symbol's value cell, where the dynamic value is stored.

(Internally, the lexical environment is an alist of symbol-value pairs, with the final element in the alist being the symbol t rather than a cons cell. Such an alist can be passed as the second argument to the eval function, in order to specify a lexical environment in which to evaluate a form. See Eval. Most Emacs Lisp programs, however, should not interact directly with lexical environments in this way; only specialized programs like debuggers.)

Lexical bindings have indefinite extent. Even after a binding construct has finished executing, its lexical environment can be “kept around” in Lisp objects called closures. A closure is created when you define a named or anonymous function with lexical binding enabled. See Closures, for details.

When a closure is called as a function, any lexical variable references within its definition use the retained lexical environment. Here is an example:

     (defvar my-ticker nil)   ; We will use this dynamically bound
                              ; variable to store a closure.
     
     (let ((x 0))             ; x is lexically bound.
       (setq my-ticker (lambda ()
                         (setq x (1+ x)))))
         ⇒ (closure ((x . 0) t) ()
               (setq x (1+ x)))
     
     (funcall my-ticker)
         ⇒ 1
     
     (funcall my-ticker)
         ⇒ 2
     
     (funcall my-ticker)
         ⇒ 3
     
     x                        ; Note that x has no global value.
     error--> Symbol's value as variable is void: x

The let binding defines a lexical environment in which the variable x is locally bound to 0. Within this binding construct, we define a lambda expression which increments x by one and returns the incremented value. This lambda expression is automatically turned into a closure, in which the lexical environment lives on even after the let binding construct has exited. Each time we evaluate the closure, it increments x, using the binding of x in that lexical environment.

Note that unlike dynamic variables which are tied to the symbol object itself, the relationship between lexical variables and symbols is only present in the interpreter (or compiler). Therefore, functions which take a symbol argument (like symbol-value, boundp, and set) can only retrieve or modify a variable's dynamic binding (i.e., the contents of its symbol's value cell).

11.9.4 Using Lexical Binding

When loading an Emacs Lisp file or evaluating a Lisp buffer, lexical binding is enabled if the buffer-local variable lexical-binding is non-nil:

When evaluating Emacs Lisp code directly using an eval call, lexical binding is enabled if the lexical argument to eval is non-nil. See Eval.

Even when lexical binding is enabled, certain variables will continue to be dynamically bound. These are called special variables. Every variable that has been defined with defvar, defcustom or defconst is a special variable (see Defining Variables). All other variables are subject to lexical binding.

The use of a special variable as a formal argument in a function is discouraged. Doing so gives rise to unspecified behavior when lexical binding mode is enabled (it may use lexical binding sometimes, and dynamic binding other times).

Converting an Emacs Lisp program to lexical binding is easy. First, add a file-local variable setting of lexical-binding to t in the header line of the Emacs Lisp source file (see File Local Variables). Second, check that every variable in the program which needs to be dynamically bound has a variable definition, so that it is not inadvertently bound lexically.

A simple way to find out which variables need a variable definition is to byte-compile the source file. See Byte Compilation. If a non-special variable is used outside of a let form, the byte-compiler will warn about reference or assignment to a free variable. If a non-special variable is bound but not used within a let form, the byte-compiler will warn about an unused lexical variable. The byte-compiler will also issue a warning if you use a special variable as a function argument.

(To silence byte-compiler warnings about unused variables, just use a variable name that start with an underscore. The byte-compiler interprets this as an indication that this is a variable known not to be used.)

11.10 Buffer-Local Variables

Global and local variable bindings are found in most programming languages in one form or another. Emacs, however, also supports additional, unusual kinds of variable binding, such as buffer-local bindings, which apply only in one buffer. Having different values for a variable in different buffers is an important customization method. (Variables can also have bindings that are local to each terminal. See Multiple Terminals.)

• Intro to Buffer-Local: Introduction and concepts.
• Creating Buffer-Local: Creating and destroying buffer-local bindings.
• Default Value: The default value is seen in buffers that don't have their own buffer-local values.

11.10.1 Introduction to Buffer-Local Variables

A buffer-local variable has a buffer-local binding associated with a particular buffer. The binding is in effect when that buffer is current; otherwise, it is not in effect. If you set the variable while a buffer-local binding is in effect, the new value goes in that binding, so its other bindings are unchanged. This means that the change is visible only in the buffer where you made it.

The variable's ordinary binding, which is not associated with any specific buffer, is called the default binding. In most cases, this is the global binding.

A variable can have buffer-local bindings in some buffers but not in other buffers. The default binding is shared by all the buffers that don't have their own bindings for the variable. (This includes all newly-created buffers.) If you set the variable in a buffer that does not have a buffer-local binding for it, this sets the default binding, so the new value is visible in all the buffers that see the default binding.

The most common use of buffer-local bindings is for major modes to change variables that control the behavior of commands. For example, C mode and Lisp mode both set the variable paragraph-start to specify that only blank lines separate paragraphs. They do this by making the variable buffer-local in the buffer that is being put into C mode or Lisp mode, and then setting it to the new value for that mode. See Major Modes.

The usual way to make a buffer-local binding is with make-local-variable, which is what major mode commands typically use. This affects just the current buffer; all other buffers (including those yet to be created) will continue to share the default value unless they are explicitly given their own buffer-local bindings.

A more powerful operation is to mark the variable as automatically buffer-local by calling make-variable-buffer-local. You can think of this as making the variable local in all buffers, even those yet to be created. More precisely, the effect is that setting the variable automatically makes the variable local to the current buffer if it is not already so. All buffers start out by sharing the default value of the variable as usual, but setting the variable creates a buffer-local binding for the current buffer. The new value is stored in the buffer-local binding, leaving the default binding untouched. This means that the default value cannot be changed with setq in any buffer; the only way to change it is with setq-default.

Warning: When a variable has buffer-local bindings in one or more buffers, let rebinds the binding that's currently in effect. For instance, if the current buffer has a buffer-local value, let temporarily rebinds that. If no buffer-local bindings are in effect, let rebinds the default value. If inside the let you then change to a different current buffer in which a different binding is in effect, you won't see the let binding any more. And if you exit the let while still in the other buffer, you won't see the unbinding occur (though it will occur properly). Here is an example to illustrate:

     (setq foo 'g)
     (set-buffer "a")
     (make-local-variable 'foo)
     (setq foo 'a)
     (let ((foo 'temp))
       ;; foo ⇒ 'temp  ; let binding in buffer ‘a’
       (set-buffer "b")
       ;; foo ⇒ 'g     ; the global value since foo is not local in ‘b’
       body...)
     foo ⇒ 'g        ; exiting restored the local value in buffer ‘a’,
                      ; but we don't see that in buffer ‘b’
     (set-buffer "a") ; verify the local value was restored
     foo ⇒ 'a

Note that references to foo in body access the buffer-local binding of buffer ‘b’.

When a file specifies local variable values, these become buffer-local values when you visit the file. See File Variables.

A buffer-local variable cannot be made terminal-local (see Multiple Terminals).

11.10.2 Creating and Deleting Buffer-Local Bindings

A buffer-local variable is permanent if the variable name (a symbol) has a permanent-local property that is non-nil. Such variables are unaffected by kill-all-local-variables, and their local bindings are therefore not cleared by changing major modes. Permanent locals are appropriate for data pertaining to where the file came from or how to save it, rather than with how to edit the contents.

11.10.3 The Default Value of a Buffer-Local Variable

The global value of a variable with buffer-local bindings is also called the default value, because it is the value that is in effect whenever neither the current buffer nor the selected frame has its own binding for the variable.

The functions default-value and setq-default access and change a variable's default value regardless of whether the current buffer has a buffer-local binding. For example, you could use setq-default to change the default setting of paragraph-start for most buffers; and this would work even when you are in a C or Lisp mode buffer that has a buffer-local value for this variable.

The special forms defvar and defconst also set the default value (if they set the variable at all), rather than any buffer-local value.

A variable can be let-bound (see Local Variables) to a value. This makes its global value shadowed by the binding; default-value will then return the value from that binding, not the global value, and set-default will be prevented from setting the global value (it will change the let-bound value instead). The following two functions allow to reference the global value even if it's shadowed by a let-binding.

     (defvar variable 'global-value)
         ⇒ variable
     (let ((variable 'let-binding))
       (default-value 'variable))
         ⇒ let-binding
     (let ((variable 'let-binding))
       (default-toplevel-value 'variable))
         ⇒ global-value

11.11 File Local Variables

A file can specify local variable values; Emacs uses these to create buffer-local bindings for those variables in the buffer visiting that file. See Local Variables in Files, for basic information about file-local variables. This section describes the functions and variables that affect how file-local variables are processed.

If a file-local variable could specify an arbitrary function or Lisp expression that would be called later, visiting a file could take over your Emacs. Emacs protects against this by automatically setting only those file-local variables whose specified values are known to be safe. Other file-local variables are set only if the user agrees.

For additional safety, read-circle is temporarily bound to nil when Emacs reads file-local variables (see Input Functions). This prevents the Lisp reader from recognizing circular and shared Lisp structures (see Circular Objects).

You can specify safe values for a variable with a safe-local-variable property. The property has to be a function of one argument; any value is safe if the function returns non-nil given that value. Many commonly-encountered file variables have safe-local-variable properties; these include fill-column, fill-prefix, and indent-tabs-mode. For boolean-valued variables that are safe, use booleanp as the property value.

When defining a user option using defcustom, you can set its safe-local-variable property by adding the arguments :safe function to defcustom (see Variable Definitions).

Some variables are considered risky. If a variable is risky, it is never entered automatically into safe-local-variable-values; Emacs always queries before setting a risky variable, unless the user explicitly allows a value by customizing safe-local-variable-values directly.

Any variable whose name has a non-nil risky-local-variable property is considered risky. When you define a user option using defcustom, you can set its risky-local-variable property by adding the arguments :risky value to defcustom (see Variable Definitions). In addition, any variable whose name ends in any of ‘-command’, ‘-frame-alist’, ‘-function’, ‘-functions’, ‘-hook’, ‘-hooks’, ‘-form’, ‘-forms’, ‘-map’, ‘-map-alist’, ‘-mode-alist’, ‘-program’, or ‘-predicate’ is automatically considered risky. The variables ‘font-lock-keywords’, ‘font-lock-keywords’ followed by a digit, and ‘font-lock-syntactic-keywords’ are also considered risky.

The ‘Eval:’ “variable” is also a potential loophole, so Emacs normally asks for confirmation before handling it.

If the expression is a function call and the function has a safe-local-eval-function property, the property value determines whether the expression is safe to evaluate. The property value can be a predicate to call to test the expression, a list of such predicates (it's safe if any predicate succeeds), or t (always safe provided the arguments are constant).

Text properties are also potential loopholes, since their values could include functions to call. So Emacs discards all text properties from string values specified for file-local variables.

11.12 Directory Local Variables

A directory can specify local variable values common to all files in that directory; Emacs uses these to create buffer-local bindings for those variables in buffers visiting any file in that directory. This is useful when the files in the directory belong to some project and therefore share the same local variables.

There are two different methods for specifying directory local variables: by putting them in a special file, or by defining a project class for that directory.

11.13 Variable Aliases

It is sometimes useful to make two variables synonyms, so that both variables always have the same value, and changing either one also changes the other. Whenever you change the name of a variable—either because you realize its old name was not well chosen, or because its meaning has partly changed—it can be useful to keep the old name as an alias of the new one for compatibility. You can do this with defvaralias.

Variable aliases are convenient for replacing an old name for a variable with a new name. make-obsolete-variable declares that the old name is obsolete and therefore that it may be removed at some stage in the future.

You can make two variables synonyms and declare one obsolete at the same time using the macro define-obsolete-variable-alias.

     (defvaralias 'foo 'bar)
     (indirect-variable 'foo)
          ⇒ bar
     (indirect-variable 'bar)
          ⇒ bar
     (setq bar 2)
     bar
          ⇒ 2
     foo
          ⇒ 2
     (setq foo 0)
     bar
          ⇒ 0
     foo
          ⇒ 0

11.14 Variables with Restricted Values

Ordinary Lisp variables can be assigned any value that is a valid Lisp object. However, certain Lisp variables are not defined in Lisp, but in C. Most of these variables are defined in the C code using DEFVAR_LISP. Like variables defined in Lisp, these can take on any value. However, some variables are defined using DEFVAR_INT or DEFVAR_BOOL. See Writing Emacs Primitives, in particular the description of functions of the type syms_of_filename, for a brief discussion of the C implementation.

Variables of type DEFVAR_BOOL can only take on the values nil or t. Attempting to assign them any other value will set them to t:

     (let ((display-hourglass 5))
       display-hourglass)
          ⇒ t

Variables of type DEFVAR_INT can take on only integer values. Attempting to assign them any other value will result in an error:

     (setq undo-limit 1000.0)
     error--> Wrong type argument: integerp, 1000.0

11.15 Generalized Variables

A generalized variable or place form is one of the many places in Lisp memory where values can be stored. The simplest place form is a regular Lisp variable. But the cars and cdrs of lists, elements of arrays, properties of symbols, and many other locations are also places where Lisp values are stored.

Generalized variables are analogous to lvalues in the C language, where ‘x = a[i]’ gets an element from an array and ‘a[i] = x’ stores an element using the same notation. Just as certain forms like a[i] can be lvalues in C, there is a set of forms that can be generalized variables in Lisp.

• Setting Generalized Variables: The setf macro.
• Adding Generalized Variables: Defining new setf forms.

11.15.1 The setf Macro

The setf macro is the most basic way to operate on generalized variables. The setf form is like setq, except that it accepts arbitrary place forms on the left side rather than just symbols. For example, (setf (car a) b) sets the car of a to b, doing the same operation as (setcar a b), but without having to remember two separate functions for setting and accessing every type of place.

The following Lisp forms will work as generalized variables, and so may appear in the place argument of setf:

• A symbol naming a variable. In other words, (setf x y) is exactly equivalent to (setq x y), and setq itself is strictly speaking redundant given that setf exists. Many programmers continue to prefer setq for setting simple variables, though, purely for stylistic or historical reasons. The macro (setf x y) actually expands to (setq x y), so there is no performance penalty for using it in compiled code.
• A call to any of the following standard Lisp functions:

            aref      cddr      symbol-function
            car       elt       symbol-plist
            caar      get       symbol-value
            cadr      gethash
            cdr       nth
            cdar      nthcdr
  

• A call to any of the following Emacs-specific functions:

            alist-get                     process-get
            frame-parameter               process-sentinel
            terminal-parameter            window-buffer
            keymap-parent                 window-display-table
            match-data                    window-dedicated-p
            overlay-get                   window-hscroll
            overlay-start                 window-parameter
            overlay-end                   window-point
            process-buffer                window-start
            process-filter                default-value
  

setf signals an error if you pass a place form that it does not know how to handle.

Note that for nthcdr, the list argument of the function must itself be a valid place form. For example, (setf (nthcdr 0 foo) 7) will set foo itself to 7.

The macros push (see List Variables) and pop (see List Elements) can manipulate generalized variables, not just lists. (pop place) removes and returns the first element of the list stored in place. It is analogous to (prog1 (car place) (setf place (cdr place))), except that it takes care to evaluate all subforms only once. (push x place) inserts x at the front of the list stored in place. It is analogous to (setf place (cons x place)), except for evaluation of the subforms. Note that push and pop on an nthcdr place can be used to insert or delete at any position in a list.

The cl-lib library defines various extensions for generalized variables, including additional setf places. See Generalized Variables.

11.15.2 Defining new setf forms

This section describes how to define new forms that setf can operate on.

For more control over the expansion, see the macro gv-define-expander. The macro gv-letplace can be useful in defining macros that perform similarly to setf; for example, the incf macro of Common Lisp. Consult the source file gv.el for more details.

Common Lisp note: Common Lisp defines another way to specify the setf behavior of a function, namely setf functions, whose names are lists (setf name) rather than symbols. For example, (defun (setf foo) ...) defines the function that is used when setf is applied to foo. Emacs does not support this. It is a compile-time error to use setf on a form that has not already had an appropriate expansion defined. In Common Lisp, this is not an error since the function (setf func) might be defined later.

12 Functions

A Lisp program is composed mainly of Lisp functions. This chapter explains what functions are, how they accept arguments, and how to define them.

• What Is a Function: Lisp functions vs. primitives; terminology.
• Lambda Expressions: How functions are expressed as Lisp objects.
• Function Names: A symbol can serve as the name of a function.
• Defining Functions: Lisp expressions for defining functions.
• Calling Functions: How to use an existing function.
• Mapping Functions: Applying a function to each element of a list, etc.
• Anonymous Functions: Lambda expressions are functions with no names.
• Generic Functions: Polymorphism, Emacs-style.
• Function Cells: Accessing or setting the function definition of a symbol.
• Closures: Functions that enclose a lexical environment.
• Advising Functions: Adding to the definition of a function.
• Obsolete Functions: Declaring functions obsolete.
• Inline Functions: Functions that the compiler will expand inline.
• Declare Form: Adding additional information about a function.
• Declaring Functions: Telling the compiler that a function is defined.
• Function Safety: Determining whether a function is safe to call.
• Related Topics: Cross-references to specific Lisp primitives that have a special bearing on how functions work.

12.1 What Is a Function?

In a general sense, a function is a rule for carrying out a computation given input values called arguments. The result of the computation is called the value or return value of the function. The computation can also have side effects, such as lasting changes in the values of variables or the contents of data structures.

In most computer languages, every function has a name. But in Lisp, a function in the strictest sense has no name: it is an object which can optionally be associated with a symbol (e.g., car) that serves as the function name. See Function Names. When a function has been given a name, we usually also refer to that symbol as a “function” (e.g., we refer to “the function car”). In this manual, the distinction between a function name and the function object itself is usually unimportant, but we will take note wherever it is relevant.

Certain function-like objects, called special forms and macros, also accept arguments to carry out computations. However, as explained below, these are not considered functions in Emacs Lisp.

Here are important terms for functions and function-like objects:

lambda expression

A function (in the strict sense, i.e., a function object) which is written in Lisp. These are described in the following section. See Lambda Expressions.

primitive

A function which is callable from Lisp but is actually written in C. Primitives are also called built-in functions, or subrs. Examples include functions like car and append. In addition, all special forms (see below) are also considered primitives.

Usually, a function is implemented as a primitive because it is a fundamental part of Lisp (e.g., car), or because it provides a low-level interface to operating system services, or because it needs to run fast. Unlike functions defined in Lisp, primitives can be modified or added only by changing the C sources and recompiling Emacs. See Writing Emacs Primitives.

special form

A primitive that is like a function but does not evaluate all of its arguments in the usual way. It may evaluate only some of the arguments, or may evaluate them in an unusual order, or several times. Examples include if, and, and while. See Special Forms.

macro

A construct defined in Lisp, which differs from a function in that it translates a Lisp expression into another expression which is to be evaluated instead of the original expression. Macros enable Lisp programmers to do the sorts of things that special forms can do. See Macros.

command

An object which can be invoked via the command-execute primitive, usually due to the user typing in a key sequence bound to that command. See Interactive Call. A command is usually a function; if the function is written in Lisp, it is made into a command by an interactive form in the function definition (see Defining Commands). Commands that are functions can also be called from Lisp expressions, just like other functions.

Keyboard macros (strings and vectors) are commands also, even though they are not functions. See Keyboard Macros. We say that a symbol is a command if its function cell contains a command (see Symbol Components); such a named command can be invoked with M-x.

closure

A function object that is much like a lambda expression, except that it also encloses an environment of lexical variable bindings. See Closures.

byte-code function

A function that has been compiled by the byte compiler. See Byte-Code Type.

autoload object

A place-holder for a real function. If the autoload object is called, Emacs loads the file containing the definition of the real function, and then calls the real function. See Autoload.

You can use the function functionp to test if an object is a function:

Unlike functionp, the next three functions do not treat a symbol as its function definition.

12.2 Lambda Expressions

A lambda expression is a function object written in Lisp. Here is an example:

     (lambda (x)
       "Return the hyperbolic cosine of X."
       (* 0.5 (+ (exp x) (exp (- x)))))

In Emacs Lisp, such a list is a valid expression which evaluates to a function object.

A lambda expression, by itself, has no name; it is an anonymous function. Although lambda expressions can be used this way (see Anonymous Functions), they are more commonly associated with symbols to make named functions (see Function Names). Before going into these details, the following subsections describe the components of a lambda expression and what they do.

• Lambda Components: The parts of a lambda expression.
• Simple Lambda: A simple example.
• Argument List: Details and special features of argument lists.
• Function Documentation: How to put documentation in a function.

12.2.1 Components of a Lambda Expression

A lambda expression is a list that looks like this:

     (lambda (arg-variables...)
       [documentation-string]
       [interactive-declaration]
       body-forms...)

The first element of a lambda expression is always the symbol lambda. This indicates that the list represents a function. The reason functions are defined to start with lambda is so that other lists, intended for other uses, will not accidentally be valid as functions.

The second element is a list of symbols—the argument variable names. This is called the lambda list. When a Lisp function is called, the argument values are matched up against the variables in the lambda list, which are given local bindings with the values provided. See Local Variables.

The documentation string is a Lisp string object placed within the function definition to describe the function for the Emacs help facilities. See Function Documentation.

The interactive declaration is a list of the form (interactive code-string). This declares how to provide arguments if the function is used interactively. Functions with this declaration are called commands; they can be called using M-x or bound to a key. Functions not intended to be called in this way should not have interactive declarations. See Defining Commands, for how to write an interactive declaration.

The rest of the elements are the body of the function: the Lisp code to do the work of the function (or, as a Lisp programmer would say, “a list of Lisp forms to evaluate”). The value returned by the function is the value returned by the last element of the body.

12.2.2 A Simple Lambda Expression Example

Consider the following example:

     (lambda (a b c) (+ a b c))

We can call this function by passing it to funcall, like this:

     (funcall (lambda (a b c) (+ a b c))
              1 2 3)

This call evaluates the body of the lambda expression with the variable a bound to 1, b bound to 2, and c bound to 3. Evaluation of the body adds these three numbers, producing the result 6; therefore, this call to the function returns the value 6.

Note that the arguments can be the results of other function calls, as in this example:

     (funcall (lambda (a b c) (+ a b c))
              1 (* 2 3) (- 5 4))

This evaluates the arguments 1, (* 2 3), and (- 5 4) from left to right. Then it applies the lambda expression to the argument values 1, 6 and 1 to produce the value 8.

As these examples show, you can use a form with a lambda expression as its car to make local variables and give them values. In the old days of Lisp, this technique was the only way to bind and initialize local variables. But nowadays, it is clearer to use the special form let for this purpose (see Local Variables). Lambda expressions are mainly used as anonymous functions for passing as arguments to other functions (see Anonymous Functions), or stored as symbol function definitions to produce named functions (see Function Names).

12.2.3 Other Features of Argument Lists

Our simple sample function, (lambda (a b c) (+ a b c)), specifies three argument variables, so it must be called with three arguments: if you try to call it with only two arguments or four arguments, you get a wrong-number-of-arguments error (see Errors).

It is often convenient to write a function that allows certain arguments to be omitted. For example, the function substring accepts three arguments—a string, the start index and the end index—but the third argument defaults to the length of the string if you omit it. It is also convenient for certain functions to accept an indefinite number of arguments, as the functions list and + do.

To specify optional arguments that may be omitted when a function is called, simply include the keyword &optional before the optional arguments. To specify a list of zero or more extra arguments, include the keyword &rest before one final argument.

Thus, the complete syntax for an argument list is as follows:

     (required-vars...
      [&optional optional-vars...]
      [&rest rest-var])

The square brackets indicate that the &optional and &rest clauses, and the variables that follow them, are optional.

A call to the function requires one actual argument for each of the required-vars. There may be actual arguments for zero or more of the optional-vars, and there cannot be any actual arguments beyond that unless the lambda list uses &rest. In that case, there may be any number of extra actual arguments.

If actual arguments for the optional and rest variables are omitted, then they always default to nil. There is no way for the function to distinguish between an explicit argument of nil and an omitted argument. However, the body of the function is free to consider nil an abbreviation for some other meaningful value. This is what substring does; nil as the third argument to substring means to use the length of the string supplied.

Common Lisp note: Common Lisp allows the function to specify what default value to use when an optional argument is omitted; Emacs Lisp always uses nil. Emacs Lisp does not support supplied-p variables that tell you whether an argument was explicitly passed.

For example, an argument list that looks like this:

     (a b &optional c d &rest e)

binds a and b to the first two actual arguments, which are required. If one or two more arguments are provided, c and d are bound to them respectively; any arguments after the first four are collected into a list and e is bound to that list. If there are only two arguments, c is nil; if two or three arguments, d is nil; if four arguments or fewer, e is nil.

There is no way to have required arguments following optional ones—it would not make sense. To see why this must be so, suppose that c in the example were optional and d were required. Suppose three actual arguments are given; which variable would the third argument be for? Would it be used for the c, or for d? One can argue for both possibilities. Similarly, it makes no sense to have any more arguments (either required or optional) after a &rest argument.

Here are some examples of argument lists and proper calls:

     (funcall (lambda (n) (1+ n))        ; One required:
              1)                         ; requires exactly one argument.
          ⇒ 2
     (funcall (lambda (n &optional n1)   ; One required and one optional:
                (if n1 (+ n n1) (1+ n))) ; 1 or 2 arguments.
              1 2)
          ⇒ 3
     (funcall (lambda (n &rest ns)       ; One required and one rest:
                (+ n (apply '+ ns)))     ; 1 or more arguments.
              1 2 3 4 5)
          ⇒ 15

12.2.4 Documentation Strings of Functions

A lambda expression may optionally have a documentation string just after the lambda list. This string does not affect execution of the function; it is a kind of comment, but a systematized comment which actually appears inside the Lisp world and can be used by the Emacs help facilities. See Documentation, for how the documentation string is accessed.

It is a good idea to provide documentation strings for all the functions in your program, even those that are called only from within your program. Documentation strings are like comments, except that they are easier to access.

The first line of the documentation string should stand on its own, because apropos displays just this first line. It should consist of one or two complete sentences that summarize the function's purpose.

The start of the documentation string is usually indented in the source file, but since these spaces come before the starting double-quote, they are not part of the string. Some people make a practice of indenting any additional lines of the string so that the text lines up in the program source. That is a mistake. The indentation of the following lines is inside the string; what looks nice in the source code will look ugly when displayed by the help commands.

You may wonder how the documentation string could be optional, since there are required components of the function that follow it (the body). Since evaluation of a string returns that string, without any side effects, it has no effect if it is not the last form in the body. Thus, in practice, there is no confusion between the first form of the body and the documentation string; if the only body form is a string then it serves both as the return value and as the documentation.

The last line of the documentation string can specify calling conventions different from the actual function arguments. Write text like this:

     \(fn arglist)

following a blank line, at the beginning of the line, with no newline following it inside the documentation string. (The ‘\’ is used to avoid confusing the Emacs motion commands.) The calling convention specified in this way appears in help messages in place of the one derived from the actual arguments of the function.

This feature is particularly useful for macro definitions, since the arguments written in a macro definition often do not correspond to the way users think of the parts of the macro call.

12.3 Naming a Function

A symbol can serve as the name of a function. This happens when the symbol's function cell (see Symbol Components) contains a function object (e.g., a lambda expression). Then the symbol itself becomes a valid, callable function, equivalent to the function object in its function cell.

The contents of the function cell are also called the symbol's function definition. The procedure of using a symbol's function definition in place of the symbol is called symbol function indirection; see Function Indirection. If you have not given a symbol a function definition, its function cell is said to be void, and it cannot be used as a function.

In practice, nearly all functions have names, and are referred to by their names. You can create a named Lisp function by defining a lambda expression and putting it in a function cell (see Function Cells). However, it is more common to use the defun special form, described in the next section. See Defining Functions.

We give functions names because it is convenient to refer to them by their names in Lisp expressions. Also, a named Lisp function can easily refer to itself—it can be recursive. Furthermore, primitives can only be referred to textually by their names, since primitive function objects (see Primitive Function Type) have no read syntax.

A function need not have a unique name. A given function object usually appears in the function cell of only one symbol, but this is just a convention. It is easy to store it in several symbols using fset; then each of the symbols is a valid name for the same function.

Note that a symbol used as a function name may also be used as a variable; these two uses of a symbol are independent and do not conflict. (This is not the case in some dialects of Lisp, like Scheme.)

12.4 Defining Functions

We usually give a name to a function when it is first created. This is called defining a function, and it is done with the defun macro.

You cannot create a new primitive function with defun or defalias, but you can use them to change the function definition of any symbol, even one such as car or x-popup-menu whose normal definition is a primitive. However, this is risky: for instance, it is next to impossible to redefine car without breaking Lisp completely. Redefining an obscure function such as x-popup-menu is less dangerous, but it still may not work as you expect. If there are calls to the primitive from C code, they call the primitive's C definition directly, so changing the symbol's definition will have no effect on them.

See also defsubst, which defines a function like defun and tells the Lisp compiler to perform inline expansion on it. See Inline Functions.

Alternatively, you can define a function by providing the code which will inline it as a compiler macro. The following macros make this possible.

Functions defined via define-inline have several advantages with respect to macros defined by defsubst or defmacro:

• They can be passed to mapcar (see Mapping Functions).
• They are more efficient.
• They can be used as place forms to store values (see Generalized Variables).
• They behave in a more predictable way than cl-defsubst (see Argument Lists).

Like defmacro, a function inlined with define-inline inherits the scoping rules, either dynamic or lexical, from the call site. See Variable Scoping.

The following macros should be used in the body of a function defined by define-inline.

Here's an example of using define-inline:

     (define-inline myaccessor (obj)
       (inline-letevals (obj)
         (inline-quote (if (foo-p ,obj) (aref (cdr ,obj) 3) (aref ,obj 2)))))

This is equivalent to

     (defsubst myaccessor (obj)
       (if (foo-p obj) (aref (cdr obj) 3) (aref obj 2)))

12.5 Calling Functions

Defining functions is only half the battle. Functions don't do anything until you call them, i.e., tell them to run. Calling a function is also known as invocation.

The most common way of invoking a function is by evaluating a list. For example, evaluating the list (concat "a" "b") calls the function concat with arguments "a" and "b". See Evaluation, for a description of evaluation.

When you write a list as an expression in your program, you specify which function to call, and how many arguments to give it, in the text of the program. Usually that's just what you want. Occasionally you need to compute at run time which function to call. To do that, use the function funcall. When you also need to determine at run time how many arguments to pass, use apply.

Sometimes it is useful to fix some of the function's arguments at certain values, and leave the rest of arguments for when the function is actually called. The act of fixing some of the function's arguments is called partial application of the function⁹. The result is a new function that accepts the rest of arguments and calls the original function with all the arguments combined.

Here's how to do partial application in Emacs Lisp:

It is common for Lisp functions to accept functions as arguments or find them in data structures (especially in hook variables and property lists) and call them using funcall or apply. Functions that accept function arguments are often called functionals.

Sometimes, when you call a functional, it is useful to supply a no-op function as the argument. Here are two different kinds of no-op function:

Some functions are user-visible commands, which can be called interactively (usually by a key sequence). It is possible to invoke such a command exactly as though it was called interactively, by using the call-interactively function. See Interactive Call.

12.6 Mapping Functions

A mapping function applies a given function (not a special form or macro) to each element of a list or other collection. Emacs Lisp has several such functions; this section describes mapcar, mapc, and mapconcat, which map over a list. See Definition of mapatoms, for the function mapatoms which maps over the symbols in an obarray. See Definition of maphash, for the function maphash which maps over key/value associations in a hash table.

These mapping functions do not allow char-tables because a char-table is a sparse array whose nominal range of indices is very large. To map over a char-table in a way that deals properly with its sparse nature, use the function map-char-table (see Char-Tables).

12.7 Anonymous Functions

Although functions are usually defined with defun and given names at the same time, it is sometimes convenient to use an explicit lambda expression—an anonymous function. Anonymous functions are valid wherever function names are. They are often assigned as variable values, or as arguments to functions; for instance, you might pass one as the function argument to mapcar, which applies that function to each element of a list (see Mapping Functions). See describe-symbols example, for a realistic example of this.

When defining a lambda expression that is to be used as an anonymous function, you can in principle use any method to construct the list. But typically you should use the lambda macro, or the function special form, or the #' read syntax:

The read syntax #' is a short-hand for using function. The following forms are all equivalent:

     (lambda (x) (* x x))
     (function (lambda (x) (* x x)))
     #'(lambda (x) (* x x))

In the following example, we define a change-property function that takes a function as its third argument, followed by a double-property function that makes use of change-property by passing it an anonymous function:

     (defun change-property (symbol prop function)
       (let ((value (get symbol prop)))
         (put symbol prop (funcall function value))))
     
     (defun double-property (symbol prop)
       (change-property symbol prop (lambda (x) (* 2 x))))

Note that we do not quote the lambda form.

If you compile the above code, the anonymous function is also compiled. This would not happen if, say, you had constructed the anonymous function by quoting it as a list:

     (defun double-property (symbol prop)
       (change-property symbol prop '(lambda (x) (* 2 x))))

In that case, the anonymous function is kept as a lambda expression in the compiled code. The byte-compiler cannot assume this list is a function, even though it looks like one, since it does not know that change-property intends to use it as a function.

12.8 Generic Functions

Functions defined using defun have a hard-coded set of assumptions about the types and expected values of their arguments. For example, a function that was designed to handle values of its argument that are either numbers or lists of numbers will fail or signal an error if called with a value of any other type, such as a vector or a string. This happens because the implementation of the function is not prepared to deal with types other than those assumed during the design.

By contrast, object-oriented programs use polymorphic functions: a set of specialized functions having the same name, each one of which was written for a certain specific set of argument types. Which of the functions is actually called is decided at run time based on the types of the actual arguments.

Emacs provides support for polymorphism. Like other Lisp environments, notably Common Lisp and its Common Lisp Object System (CLOS), this support is based on generic functions. The Emacs generic functions closely follow CLOS, including use of similar names, so if you have experience with CLOS, the rest of this section will sound very familiar.

A generic function specifies an abstract operation, by defining its name and list of arguments, but (usually) no implementation. The actual implementation for several specific classes of arguments is provided by methods, which should be defined separately. Each method that implements a generic function has the same name as the generic function, but the method's definition indicates what kinds of arguments it can handle by specializing the arguments defined by the generic function. These argument specializers can be more or less specific; for example, a string type is more specific than a more general type, such as sequence.

Note that, unlike in message-based OO languages, such as C++ and Simula, methods that implement generic functions don't belong to a class, they belong to the generic function they implement.

When a generic function is invoked, it selects the applicable methods by comparing the actual arguments passed by the caller with the argument specializers of each method. A method is applicable if the actual arguments of the call are compatible with the method's specializers. If more than one method is applicable, they are combined using certain rules, described below, and the combination then handles the call.

Each time a generic function is called, it builds the effective method which will handle this invocation by combining the applicable methods defined for the function. The process of finding the applicable methods and producing the effective method is called dispatch. The applicable methods are those all of whose specializers are compatible with the actual arguments of the call. Since all of the arguments must be compatible with the specializers, they all determine whether a method is applicable. Methods that explicitly specialize more than one argument are called multiple-dispatch methods.

The applicable methods are sorted into the order in which they will be combined. The method whose left-most argument specializer is the most specific one will come first in the order. (Specifying :argument-precedence-order as part of cl-defmethod overrides that, as described above.) If the method body calls cl-call-next-method, the next most-specific method will run. If there are applicable :around methods, the most-specific of them will run first; it should call cl-call-next-method to run any of the less specific :around methods. Next, the :before methods run in the order of their specificity, followed by the primary method, and lastly the :after methods in the reverse order of their specificity.

12.9 Accessing Function Cell Contents

The function definition of a symbol is the object stored in the function cell of the symbol. The functions described here access, test, and set the function cell of symbols.

See also the function indirect-function. See Definition of indirect-function.

If you have never given a symbol any function definition, we say that that symbol's function cell is void. In other words, the function cell does not have any Lisp object in it. If you try to call the symbol as a function, Emacs signals a void-function error.

Note that void is not the same as nil or the symbol void. The symbols nil and void are Lisp objects, and can be stored into a function cell just as any other object can be (and they can be valid functions if you define them in turn with defun). A void function cell contains no object whatsoever.

You can test the voidness of a symbol's function definition with fboundp. After you have given a symbol a function definition, you can make it void once more using fmakunbound.

12.10 Closures

As explained in Variable Scoping, Emacs can optionally enable lexical binding of variables. When lexical binding is enabled, any named function that you create (e.g., with defun), as well as any anonymous function that you create using the lambda macro or the function special form or the #' syntax (see Anonymous Functions), is automatically converted into a closure.

A closure is a function that also carries a record of the lexical environment that existed when the function was defined. When it is invoked, any lexical variable references within its definition use the retained lexical environment. In all other respects, closures behave much like ordinary functions; in particular, they can be called in the same way as ordinary functions.

See Lexical Binding, for an example of using a closure.

Currently, an Emacs Lisp closure object is represented by a list with the symbol closure as the first element, a list representing the lexical environment as the second element, and the argument list and body forms as the remaining elements:

     ;; lexical binding is enabled.
     (lambda (x) (* x x))
          ⇒ (closure (t) (x) (* x x))

However, the fact that the internal structure of a closure is exposed to the rest of the Lisp world is considered an internal implementation detail. For this reason, we recommend against directly examining or altering the structure of closure objects.

12.11 Advising Emacs Lisp Functions

When you need to modify a function defined in another library, or when you need to modify a hook like foo-function, a process filter, or basically any variable or object field which holds a function value, you can use the appropriate setter function, such as fset or defun for named functions, setq for hook variables, or set-process-filter for process filters, but those are often too blunt, completely throwing away the previous value.

The advice feature lets you add to the existing definition of a function, by advising the function. This is a cleaner method than redefining the whole function.

Emacs's advice system provides two sets of primitives for that: the core set, for function values held in variables and object fields (with the corresponding primitives being add-function and remove-function) and another set layered on top of it for named functions (with the main primitives being advice-add and advice-remove).

For example, in order to trace the calls to the process filter of a process proc, you could use:

     (defun my-tracing-function (proc string)
       (message "Proc %S received %S" proc string))
     
     (add-function :before (process-filter proc) #'my-tracing-function)

This will cause the process's output to be passed to my-tracing-function before being passed to the original process filter. my-tracing-function receives the same arguments as the original function. When you're done with it, you can revert to the untraced behavior with:

     (remove-function (process-filter proc) #'my-tracing-function)

Similarly, if you want to trace the execution of the function named display-buffer, you could use:

     (defun his-tracing-function (orig-fun &rest args)
       (message "display-buffer called with args %S" args)
       (let ((res (apply orig-fun args)))
         (message "display-buffer returned %S" res)
         res))
     
     (advice-add 'display-buffer :around #'his-tracing-function)

Here, his-tracing-function is called instead of the original function and receives the original function (additionally to that function's arguments) as argument, so it can call it if and when it needs to. When you're tired of seeing this output, you can revert to the untraced behavior with:

     (advice-remove 'display-buffer #'his-tracing-function)

The arguments :before and :around used in the above examples specify how the two functions are composed, since there are many different ways to do it. The added function is also called a piece of advice.

• Core Advising Primitives: Primitives to manipulate advice.
• Advising Named Functions: Advising named functions.
• Advice combinators: Ways to compose advice.
• Porting old advice: Adapting code using the old defadvice.

12.11.1 Primitives to manipulate advices

12.11.2 Advising Named Functions

A common use of advice is for named functions and macros. You could just use add-function as in:

     (add-function :around (symbol-function 'fun) #'his-tracing-function)

But you should use advice-add and advice-remove for that instead. This separate set of functions to manipulate pieces of advice applied to named functions, offers the following extra features compared to add-function: they know how to deal with macros and autoloaded functions, they let describe-function preserve the original docstring as well as document the added advice, and they let you add and remove advice before a function is even defined.

advice-add can be useful for altering the behavior of existing calls to an existing function without having to redefine the whole function. However, it can be a source of bugs, since existing callers to the function may assume the old behavior, and work incorrectly when the behavior is changed by advice. Advice can also cause confusion in debugging, if the person doing the debugging does not notice or remember that the function has been modified by advice.

For these reasons, advice should be reserved for the cases where you cannot modify a function's behavior in any other way. If it is possible to do the same thing via a hook, that is preferable (see Hooks). If you simply want to change what a particular key does, it may be better to write a new command, and remap the old command's key bindings to the new one (see Remapping Commands). In particular, Emacs's own source files should not put advice on functions in Emacs. (There are currently a few exceptions to this convention, but we aim to correct them.)

Special forms (see Special Forms) cannot be advised, however macros can be advised, in much the same way as functions. Of course, this will not affect code that has already been macro-expanded, so you need to make sure the advice is installed before the macro is expanded.

It is possible to advise a primitive (see What Is a Function), but one should typically not do so, for two reasons. Firstly, some primitives are used by the advice mechanism, and advising them could cause an infinite recursion. Secondly, many primitives are called directly from C, and such calls ignore advice; hence, one ends up in a confusing situation where some calls (occurring from Lisp code) obey the advice and other calls (from C code) do not.

12.11.3 Ways to compose advice

Here are the different possible values for the where argument of add-function and advice-add, specifying how the advice function and the original function should be composed.

:before

Call function before the old function. Both functions receive the same arguments, and the return value of the composition is the return value of the old function. More specifically, the composition of the two functions behaves like:

          (lambda (&rest r) (apply function r) (apply oldfun r))

(add-function :before funvar function) is comparable for single-function hooks to (add-hook 'hookvar function) for normal hooks.

:after

Call function after the old function. Both functions receive the same arguments, and the return value of the composition is the return value of the old function. More specifically, the composition of the two functions behaves like:

          (lambda (&rest r) (prog1 (apply oldfun r) (apply function r)))

(add-function :after funvar function) is comparable for single-function hooks to (add-hook 'hookvar function 'append) for normal hooks.

:override

This completely replaces the old function with the new one. The old function can of course be recovered if you later call remove-function.

:around

Call function instead of the old function, but provide the old function as an extra argument to function. This is the most flexible composition. For example, it lets you call the old function with different arguments, or many times, or within a let-binding, or you can sometimes delegate the work to the old function and sometimes override it completely. More specifically, the composition of the two functions behaves like:

          (lambda (&rest r) (apply function oldfun r))

:before-while

Call function before the old function and don't call the old function if function returns nil. Both functions receive the same arguments, and the return value of the composition is the return value of the old function. More specifically, the composition of the two functions behaves like:

          (lambda (&rest r) (and (apply function r) (apply oldfun r)))

(add-function :before-while funvar function) is comparable for single-function hooks to (add-hook 'hookvar function) when hookvar is run via run-hook-with-args-until-failure.

:before-until

Call function before the old function and only call the old function if function returns nil. More specifically, the composition of the two functions behaves like:

          (lambda (&rest r) (or (apply function r) (apply oldfun r)))

(add-function :before-until funvar function) is comparable for single-function hooks to (add-hook 'hookvar function) when hookvar is run via run-hook-with-args-until-success.

:after-while

Call function after the old function and only if the old function returned non-nil. Both functions receive the same arguments, and the return value of the composition is the return value of function. More specifically, the composition of the two functions behaves like:

          (lambda (&rest r) (and (apply oldfun r) (apply function r)))

(add-function :after-while funvar function) is comparable for single-function hooks to (add-hook 'hookvar function 'append) when hookvar is run via run-hook-with-args-until-failure.

:after-until

Call function after the old function and only if the old function returned nil. More specifically, the composition of the two functions behaves like:

          (lambda (&rest r) (or  (apply oldfun r) (apply function r)))

(add-function :after-until funvar function) is comparable for single-function hooks to (add-hook 'hookvar function 'append) when hookvar is run via run-hook-with-args-until-success.

:filter-args

Call function first and use the result (which should be a list) as the new arguments to pass to the old function. More specifically, the composition of the two functions behaves like:

          (lambda (&rest r) (apply oldfun (funcall function r)))

:filter-return

Call the old function first and pass the result to function. More specifically, the composition of the two functions behaves like:

          (lambda (&rest r) (funcall function (apply oldfun r)))

12.11.4 Adapting code using the old defadvice

A lot of code uses the old defadvice mechanism, which is largely made obsolete by the new advice-add, whose implementation and semantics is significantly simpler.

An old piece of advice such as:

     (defadvice previous-line (before next-line-at-end
                                      (&optional arg try-vscroll))
       "Insert an empty line when moving up from the top line."
       (if (and next-line-add-newlines (= arg 1)
                (save-excursion (beginning-of-line) (bobp)))
           (progn
             (beginning-of-line)
             (newline))))

could be translated in the new advice mechanism into a plain function:

     (defun previous-line--next-line-at-end (&optional arg try-vscroll)
       "Insert an empty line when moving up from the top line."
       (if (and next-line-add-newlines (= arg 1)
                (save-excursion (beginning-of-line) (bobp)))
           (progn
             (beginning-of-line)
             (newline))))

Obviously, this does not actually modify previous-line. For that the old advice needed:

     (ad-activate 'previous-line)

whereas the new advice mechanism needs:

     (advice-add 'previous-line :before #'previous-line--next-line-at-end)

Note that ad-activate had a global effect: it activated all pieces of advice enabled for that specified function. If you wanted to only activate or deactivate a particular piece, you needed to enable or disable it with ad-enable-advice and ad-disable-advice. The new mechanism does away with this distinction.

Around advice such as:

     (defadvice foo (around foo-around)
       "Ignore case in `foo'."
       (let ((case-fold-search t))
         ad-do-it))
     (ad-activate 'foo)

could translate into:

     (defun foo--foo-around (orig-fun &rest args)
       "Ignore case in `foo'."
       (let ((case-fold-search t))
         (apply orig-fun args)))
     (advice-add 'foo :around #'foo--foo-around)

Regarding the advice's class, note that the new :before is not quite equivalent to the old before, because in the old advice you could modify the function's arguments (e.g., with ad-set-arg), and that would affect the argument values seen by the original function, whereas in the new :before, modifying an argument via setq in the advice has no effect on the arguments seen by the original function. When porting before advice which relied on this behavior, you'll need to turn it into new :around or :filter-args advice instead.

Similarly old after advice could modify the returned value by changing ad-return-value, whereas new :after advice cannot, so when porting such old after advice, you'll need to turn it into new :around or :filter-return advice instead.

12.12 Declaring Functions Obsolete

You can mark a named function as obsolete, meaning that it may be removed at some point in the future. This causes Emacs to warn that the function is obsolete whenever it byte-compiles code containing that function, and whenever it displays the documentation for that function. In all other respects, an obsolete function behaves like any other function.

The easiest way to mark a function as obsolete is to put a (declare (obsolete ...)) form in the function's defun definition. See Declare Form. Alternatively, you can use the make-obsolete function, described below.

A macro (see Macros) can also be marked obsolete with make-obsolete; this has the same effects as for a function. An alias for a function or macro can also be marked as obsolete; this makes the alias itself obsolete, not the function or macro which it resolves to.

In addition, you can mark a certain a particular calling convention for a function as obsolete:

12.13 Inline Functions

An inline function is a function that works just like an ordinary function, except for one thing: when you byte-compile a call to the function (see Byte Compilation), the function's definition is expanded into the caller. To define an inline function, use defsubst instead of defun.

Making a function inline often makes its function calls run faster. But it also has disadvantages. For one thing, it reduces flexibility; if you change the definition of the function, calls already inlined still use the old definition until you recompile them.

Another disadvantage is that making a large function inline can increase the size of compiled code both in files and in memory. Since the speed advantage of inline functions is greatest for small functions, you generally should not make large functions inline.

Also, inline functions do not behave well with respect to debugging, tracing, and advising (see Advising Functions). Since ease of debugging and the flexibility of redefining functions are important features of Emacs, you should not make a function inline, even if it's small, unless its speed is really crucial, and you've timed the code to verify that using defun actually has performance problems.

After an inline function is defined, its inline expansion can be performed later on in the same file, just like macros.

It's possible to use defsubst to define a macro to expand into the same code that an inline function would execute (see Macros). But the macro would be limited to direct use in expressions—a macro cannot be called with apply, mapcar and so on. Also, it takes some work to convert an ordinary function into a macro. To convert it into an inline function is easy; just replace defun with defsubst. Since each argument of an inline function is evaluated exactly once, you needn't worry about how many times the body uses the arguments, as you do for macros.

As an alternative to defsubst, you can use define-inline to define functions via their exhaustive compiler macro. See define-inline.

12.14 The declare Form

declare is a special macro which can be used to add meta properties to a function or macro: for example, marking it as obsolete, or giving its forms a special <TAB> indentation convention in Emacs Lisp mode.

12.15 Telling the Compiler that a Function is Defined

Byte-compiling a file often produces warnings about functions that the compiler doesn't know about (see Compiler Errors). Sometimes this indicates a real problem, but usually the functions in question are defined in other files which would be loaded if that code is run. For example, byte-compiling fortran.el used to warn:

     In end of data:
     fortran.el:2152:1:Warning: the function ‘gud-find-c-expr’ is not
         known to be defined.

In fact, gud-find-c-expr is only used in the function that Fortran mode uses for the local value of gud-find-expr-function, which is a callback from GUD; if it is called, the GUD functions will be loaded. When you know that such a warning does not indicate a real problem, it is good to suppress the warning. That makes new warnings which might mean real problems more visible. You do that with declare-function.

All you need to do is add a declare-function statement before the first use of the function in question:

     (declare-function gud-find-c-expr "gud.el" nil)

This says that gud-find-c-expr is defined in gud.el (the ‘.el’ can be omitted). The compiler takes for granted that that file really defines the function, and does not check.

The optional third argument specifies the argument list of gud-find-c-expr. In this case, it takes no arguments (nil is different from not specifying a value). In other cases, this might be something like (file &optional overwrite). You don't have to specify the argument list, but if you do the byte compiler can check that the calls match the declaration.

To verify that these functions really are declared where declare-function says they are, use check-declare-file to check all declare-function calls in one source file, or use check-declare-directory check all the files in and under a certain directory.

These commands find the file that ought to contain a function's definition using locate-library; if that finds no file, they expand the definition file name relative to the directory of the file that contains the declare-function call.

You can also say that a function is a primitive by specifying a file name ending in ‘.c’ or ‘.m’. This is useful only when you call a primitive that is defined only on certain systems. Most primitives are always defined, so they will never give you a warning.

Sometimes a file will optionally use functions from an external package. If you prefix the filename in the declare-function statement with ‘ext:’, then it will be checked if it is found, otherwise skipped without error.

There are some function definitions that ‘check-declare’ does not understand (e.g., defstruct and some other macros). In such cases, you can pass a non-nil fileonly argument to declare-function, meaning to only check that the file exists, not that it actually defines the function. Note that to do this without having to specify an argument list, you should set the arglist argument to t (because nil means an empty argument list, as opposed to an unspecified one).

12.16 Determining whether a Function is Safe to Call

Some major modes, such as SES, call functions that are stored in user files. (see Top, for more information on SES.) User files sometimes have poor pedigrees—you can get a spreadsheet from someone you've just met, or you can get one through email from someone you've never met. So it is risky to call a function whose source code is stored in a user file until you have determined that it is safe.

Being quick and simple, unsafep does a very light analysis and rejects many Lisp expressions that are actually safe. There are no known cases where unsafep returns nil for an unsafe expression. However, a safe Lisp expression can return a string with a display property, containing an associated Lisp expression to be executed after the string is inserted into a buffer. This associated expression can be a virus. In order to be safe, you must delete properties from all strings calculated by user code before inserting them into buffers.

12.17 Other Topics Related to Functions

Here is a table of several functions that do things related to function calling and function definitions. They are documented elsewhere, but we provide cross references here.

apply

See Calling Functions.

autoload

See Autoload.

call-interactively

See Interactive Call.

called-interactively-p

See Distinguish Interactive.

commandp

See Interactive Call.

documentation

See Accessing Documentation.

eval

See Eval.

funcall

See Calling Functions.

function

See Anonymous Functions.

ignore

See Calling Functions.

indirect-function

See Function Indirection.

interactive

See Using Interactive.

interactive-p

See Distinguish Interactive.

mapatoms

See Creating Symbols.

mapcar

See Mapping Functions.

map-char-table

See Char-Tables.

mapconcat

See Mapping Functions.

undefined

See Functions for Key Lookup.

13 Macros

Macros enable you to define new control constructs and other language features. A macro is defined much like a function, but instead of telling how to compute a value, it tells how to compute another Lisp expression which will in turn compute the value. We call this expression the expansion of the macro.

Macros can do this because they operate on the unevaluated expressions for the arguments, not on the argument values as functions do. They can therefore construct an expansion containing these argument expressions or parts of them.

If you are using a macro to do something an ordinary function could do, just for the sake of speed, consider using an inline function instead. See Inline Functions.

• Simple Macro: A basic example.
• Expansion: How, when and why macros are expanded.
• Compiling Macros: How macros are expanded by the compiler.
• Defining Macros: How to write a macro definition.
• Problems with Macros: Don't evaluate the macro arguments too many times. Don't hide the user's variables.
• Indenting Macros: Specifying how to indent macro calls.

13.1 A Simple Example of a Macro

Suppose we would like to define a Lisp construct to increment a variable value, much like the ++ operator in C. We would like to write (inc x) and have the effect of (setq x (1+ x)). Here's a macro definition that does the job:

     (defmacro inc (var)
        (list 'setq var (list '1+ var)))

When this is called with (inc x), the argument var is the symbol x—not the value of x, as it would be in a function. The body of the macro uses this to construct the expansion, which is (setq x (1+ x)). Once the macro definition returns this expansion, Lisp proceeds to evaluate it, thus incrementing x.

13.2 Expansion of a Macro Call

A macro call looks just like a function call in that it is a list which starts with the name of the macro. The rest of the elements of the list are the arguments of the macro.

Evaluation of the macro call begins like evaluation of a function call except for one crucial difference: the macro arguments are the actual expressions appearing in the macro call. They are not evaluated before they are given to the macro definition. By contrast, the arguments of a function are results of evaluating the elements of the function call list.

Having obtained the arguments, Lisp invokes the macro definition just as a function is invoked. The argument variables of the macro are bound to the argument values from the macro call, or to a list of them in the case of a &rest argument. And the macro body executes and returns its value just as a function body does.

The second crucial difference between macros and functions is that the value returned by the macro body is an alternate Lisp expression, also known as the expansion of the macro. The Lisp interpreter proceeds to evaluate the expansion as soon as it comes back from the macro.

Since the expansion is evaluated in the normal manner, it may contain calls to other macros. It may even be a call to the same macro, though this is unusual.

Note that Emacs tries to expand macros when loading an uncompiled Lisp file. This is not always possible, but if it is, it speeds up subsequent execution. See How Programs Do Loading.

You can see the expansion of a given macro call by calling macroexpand.

13.3 Macros and Byte Compilation

You might ask why we take the trouble to compute an expansion for a macro and then evaluate the expansion. Why not have the macro body produce the desired results directly? The reason has to do with compilation.

When a macro call appears in a Lisp program being compiled, the Lisp compiler calls the macro definition just as the interpreter would, and receives an expansion. But instead of evaluating this expansion, it compiles the expansion as if it had appeared directly in the program. As a result, the compiled code produces the value and side effects intended for the macro, but executes at full compiled speed. This would not work if the macro body computed the value and side effects itself—they would be computed at compile time, which is not useful.

In order for compilation of macro calls to work, the macros must already be defined in Lisp when the calls to them are compiled. The compiler has a special feature to help you do this: if a file being compiled contains a defmacro form, the macro is defined temporarily for the rest of the compilation of that file.

Byte-compiling a file also executes any require calls at top-level in the file, so you can ensure that necessary macro definitions are available during compilation by requiring the files that define them (see Named Features). To avoid loading the macro definition files when someone runs the compiled program, write eval-when-compile around the require calls (see Eval During Compile).

13.4 Defining Macros

A Lisp macro object is a list whose car is macro, and whose cdr is a function. Expansion of the macro works by applying the function (with apply) to the list of unevaluated arguments from the macro call.

It is possible to use an anonymous Lisp macro just like an anonymous function, but this is never done, because it does not make sense to pass an anonymous macro to functionals such as mapcar. In practice, all Lisp macros have names, and they are almost always defined with the defmacro macro.

Macros often need to construct large list structures from a mixture of constants and nonconstant parts. To make this easier, use the ‘`’ syntax (see Backquote). For example:

     
          (defmacro t-becomes-nil (variable)
            `(if (eq ,variable t)
                 (setq ,variable nil)))
          
          (t-becomes-nil foo)
               == (if (eq foo t) (setq foo nil))

   

13.5 Common Problems Using Macros

Macro expansion can have counterintuitive consequences. This section describes some important consequences that can lead to trouble, and rules to follow to avoid trouble.

• Wrong Time: Do the work in the expansion, not in the macro.
• Argument Evaluation: The expansion should evaluate each macro arg once.
• Surprising Local Vars: Local variable bindings in the expansion require special care.
• Eval During Expansion: Don't evaluate them; put them in the expansion.
• Repeated Expansion: Avoid depending on how many times expansion is done.

13.5.1 Wrong Time

The most common problem in writing macros is doing some of the real work prematurely—while expanding the macro, rather than in the expansion itself. For instance, one real package had this macro definition:

     (defmacro my-set-buffer-multibyte (arg)
       (if (fboundp 'set-buffer-multibyte)
           (set-buffer-multibyte arg)))

With this erroneous macro definition, the program worked fine when interpreted but failed when compiled. This macro definition called set-buffer-multibyte during compilation, which was wrong, and then did nothing when the compiled package was run. The definition that the programmer really wanted was this:

     (defmacro my-set-buffer-multibyte (arg)
       (if (fboundp 'set-buffer-multibyte)
           `(set-buffer-multibyte ,arg)))

This macro expands, if appropriate, into a call to set-buffer-multibyte that will be executed when the compiled program is actually run.

13.5.2 Evaluating Macro Arguments Repeatedly

When defining a macro you must pay attention to the number of times the arguments will be evaluated when the expansion is executed. The following macro (used to facilitate iteration) illustrates the problem. This macro allows us to write a for-loop construct.

     (defmacro for (var from init to final do &rest body)
       "Execute a simple \"for\" loop.
     For example, (for i from 1 to 10 do (print i))."
       (list 'let (list (list var init))
             (cons 'while
                   (cons (list '<= var final)
                         (append body (list (list 'inc var)))))))
     
     (for i from 1 to 3 do
        (setq square (* i i))
        (princ (format "\n%d %d" i square)))
     ==>
     (let ((i 1))
       (while (<= i 3)
         (setq square (* i i))
         (princ (format "\n%d %d" i square))
         (inc i)))
     
          -|1       1
          -|2       4
          -|3       9
     ⇒ nil

The arguments from, to, and do in this macro are syntactic sugar; they are entirely ignored. The idea is that you will write noise words (such as from, to, and do) in those positions in the macro call.

Here's an equivalent definition simplified through use of backquote:

     (defmacro for (var from init to final do &rest body)
       "Execute a simple \"for\" loop.
     For example, (for i from 1 to 10 do (print i))."
       `(let ((,var ,init))
          (while (<= ,var ,final)
            ,@body
            (inc ,var))))

Both forms of this definition (with backquote and without) suffer from the defect that final is evaluated on every iteration. If final is a constant, this is not a problem. If it is a more complex form, say (long-complex-calculation x), this can slow down the execution significantly. If final has side effects, executing it more than once is probably incorrect.

A well-designed macro definition takes steps to avoid this problem by producing an expansion that evaluates the argument expressions exactly once unless repeated evaluation is part of the intended purpose of the macro. Here is a correct expansion for the for macro:

     (let ((i 1)
           (max 3))
       (while (<= i max)
         (setq square (* i i))
         (princ (format "%d      %d" i square))
         (inc i)))

Here is a macro definition that creates this expansion:

     (defmacro for (var from init to final do &rest body)
       "Execute a simple for loop: (for i from 1 to 10 do (print i))."
       `(let ((,var ,init)
              (max ,final))
          (while (<= ,var max)
            ,@body
            (inc ,var))))

Unfortunately, this fix introduces another problem, described in the following section.

13.5.3 Local Variables in Macro Expansions

In the previous section, the definition of for was fixed as follows to make the expansion evaluate the macro arguments the proper number of times:

     (defmacro for (var from init to final do &rest body)
       "Execute a simple for loop: (for i from 1 to 10 do (print i))."
       `(let ((,var ,init)
              (max ,final))
          (while (<= ,var max)
            ,@body
            (inc ,var))))

The new definition of for has a new problem: it introduces a local variable named max which the user does not expect. This causes trouble in examples such as the following:

     (let ((max 0))
       (for x from 0 to 10 do
         (let ((this (frob x)))
           (if (< max this)
               (setq max this)))))

The references to max inside the body of the for, which are supposed to refer to the user's binding of max, really access the binding made by for.

The way to correct this is to use an uninterned symbol instead of max (see Creating Symbols). The uninterned symbol can be bound and referred to just like any other symbol, but since it is created by for, we know that it cannot already appear in the user's program. Since it is not interned, there is no way the user can put it into the program later. It will never appear anywhere except where put by for. Here is a definition of for that works this way:

     (defmacro for (var from init to final do &rest body)
       "Execute a simple for loop: (for i from 1 to 10 do (print i))."
       (let ((tempvar (make-symbol "max")))
         `(let ((,var ,init)
                (,tempvar ,final))
            (while (<= ,var ,tempvar)
              ,@body
              (inc ,var)))))

This creates an uninterned symbol named max and puts it in the expansion instead of the usual interned symbol max that appears in expressions ordinarily.

13.5.4 Evaluating Macro Arguments in Expansion

Another problem can happen if the macro definition itself evaluates any of the macro argument expressions, such as by calling eval (see Eval). If the argument is supposed to refer to the user's variables, you may have trouble if the user happens to use a variable with the same name as one of the macro arguments. Inside the macro body, the macro argument binding is the most local binding of this variable, so any references inside the form being evaluated do refer to it. Here is an example:

     (defmacro foo (a)
       (list 'setq (eval a) t))
     (setq x 'b)
     (foo x) ==> (setq b t)
          ⇒ t                  ; and b has been set.
     ;; but
     (setq a 'c)
     (foo a) ==> (setq a t)
          ⇒ t                  ; but this set a, not c.

It makes a difference whether the user's variable is named a or x, because a conflicts with the macro argument variable a.

Another problem with calling eval in a macro definition is that it probably won't do what you intend in a compiled program. The byte compiler runs macro definitions while compiling the program, when the program's own computations (which you might have wished to access with eval) don't occur and its local variable bindings don't exist.

To avoid these problems, don't evaluate an argument expression while computing the macro expansion. Instead, substitute the expression into the macro expansion, so that its value will be computed as part of executing the expansion. This is how the other examples in this chapter work.

13.5.5 How Many Times is the Macro Expanded?

Occasionally problems result from the fact that a macro call is expanded each time it is evaluated in an interpreted function, but is expanded only once (during compilation) for a compiled function. If the macro definition has side effects, they will work differently depending on how many times the macro is expanded.

Therefore, you should avoid side effects in computation of the macro expansion, unless you really know what you are doing.

One special kind of side effect can't be avoided: constructing Lisp objects. Almost all macro expansions include constructed lists; that is the whole point of most macros. This is usually safe; there is just one case where you must be careful: when the object you construct is part of a quoted constant in the macro expansion.

If the macro is expanded just once, in compilation, then the object is constructed just once, during compilation. But in interpreted execution, the macro is expanded each time the macro call runs, and this means a new object is constructed each time.

In most clean Lisp code, this difference won't matter. It can matter only if you perform side-effects on the objects constructed by the macro definition. Thus, to avoid trouble, avoid side effects on objects constructed by macro definitions. Here is an example of how such side effects can get you into trouble:

     (defmacro empty-object ()
       (list 'quote (cons nil nil)))
     
     (defun initialize (condition)
       (let ((object (empty-object)))
         (if condition
             (setcar object condition))
         object))

If initialize is interpreted, a new list (nil) is constructed each time initialize is called. Thus, no side effect survives between calls. If initialize is compiled, then the macro empty-object is expanded during compilation, producing a single constant (nil) that is reused and altered each time initialize is called.

One way to avoid pathological cases like this is to think of empty-object as a funny kind of constant, not as a memory allocation construct. You wouldn't use setcar on a constant such as '(nil), so naturally you won't use it on (empty-object) either.

13.6 Indenting Macros

Within a macro definition, you can use the declare form (see Defining Macros) to specify how <TAB> should indent calls to the macro. An indentation specification is written like this:

     (declare (indent indent-spec))

This results in the lisp-indent-function property being set on the macro name.

Here are the possibilities for indent-spec:

nil

This is the same as no property—use the standard indentation pattern.

defun

Handle this function like a ‘def’ construct: treat the second line as the start of a body.

an integer, number

The first number arguments of the function are distinguished arguments; the rest are considered the body of the expression. A line in the expression is indented according to whether the first argument on it is distinguished or not. If the argument is part of the body, the line is indented lisp-body-indent more columns than the open-parenthesis starting the containing expression. If the argument is distinguished and is either the first or second argument, it is indented twice that many extra columns. If the argument is distinguished and not the first or second argument, the line uses the standard pattern.

a symbol, symbol

symbol should be a function name; that function is called to calculate the indentation of a line within this expression. The function receives two arguments:

pos

The position at which the line being indented begins.

state

The value returned by parse-partial-sexp (a Lisp primitive for indentation and nesting computation) when it parses up to the beginning of this line.

It should return either a number, which is the number of columns of indentation for that line, or a list whose car is such a number. The difference between returning a number and returning a list is that a number says that all following lines at the same nesting level should be indented just like this one; a list says that following lines might call for different indentations. This makes a difference when the indentation is being computed by C-M-q; if the value is a number, C-M-q need not recalculate indentation for the following lines until the end of the list.

14 Customization Settings

Users of Emacs can customize variables and faces without writing Lisp code, by using the Customize interface. See Easy Customization. This chapter describes how to define customization items that users can interact with through the Customize interface.

Customization items include customizable variables, which are defined with the defcustom macro; customizable faces, which are defined with defface (described separately in Defining Faces); and customization groups, defined with defgroup, which act as containers for groups of related customization items.

• Common Keywords: Common keyword arguments for all kinds of customization declarations.
• Group Definitions: Writing customization group definitions.
• Variable Definitions: Declaring user options.
• Customization Types: Specifying the type of a user option.
• Applying Customizations: Functions to apply customization settings.
• Custom Themes: Writing Custom themes.

14.1 Common Item Keywords

The customization declarations that we will describe in the next few sections—defcustom, defgroup, etc.—all accept keyword arguments (see Constant Variables) for specifying various information. This section describes keywords that apply to all types of customization declarations.

All of these keywords, except :tag, can be used more than once in a given item. Each use of the keyword has an independent effect. The keyword :tag is an exception because any given item can only display one name.

:tag label

Use label, a string, instead of the item's name, to label the item in customization menus and buffers. Don't use a tag which is substantially different from the item's real name; that would cause confusion.

:group group

Put this customization item in group group. If this keyword is missing from a customization item, it'll be placed in the same group that was last defined (in the current file).

When you use :group in a defgroup, it makes the new group a subgroup of group.

If you use this keyword more than once, you can put a single item into more than one group. Displaying any of those groups will show this item. Please don't overdo this, since the result would be annoying.

:link link-data

Include an external link after the documentation string for this item. This is a sentence containing a button that references some other documentation.

There are several alternatives you can use for link-data:

(custom-manual info-node)

Link to an Info node; info-node is a string which specifies the node name, as in "(emacs)Top". The link appears as ‘[Manual]’ in the customization buffer and enters the built-in Info reader on info-node.

(info-link info-node)

Like custom-manual except that the link appears in the customization buffer with the Info node name.

(url-link url)

Link to a web page; url is a string which specifies the URL. The link appears in the customization buffer as url and invokes the WWW browser specified by browse-url-browser-function.

(emacs-commentary-link library)

Link to the commentary section of a library; library is a string which specifies the library name. See Library Headers.

(emacs-library-link library)

Link to an Emacs Lisp library file; library is a string which specifies the library name.

(file-link file)

Link to a file; file is a string which specifies the name of the file to visit with find-file when the user invokes this link.

(function-link function)

Link to the documentation of a function; function is a string which specifies the name of the function to describe with describe-function when the user invokes this link.

(variable-link variable)

Link to the documentation of a variable; variable is a string which specifies the name of the variable to describe with describe-variable when the user invokes this link.

(custom-group-link group)

Link to another customization group. Invoking it creates a new customization buffer for group.

You can specify the text to use in the customization buffer by adding :tag name after the first element of the link-data; for example, (info-link :tag "foo" "(emacs)Top") makes a link to the Emacs manual which appears in the buffer as ‘foo’.

You can use this keyword more than once, to add multiple links.

:load file

Load file file (a string) before displaying this customization item (see Loading). Loading is done with load, and only if the file is not already loaded.

:require feature

Execute (require 'feature) when your saved customizations set the value of this item. feature should be a symbol.

The most common reason to use :require is when a variable enables a feature such as a minor mode, and just setting the variable won't have any effect unless the code which implements the mode is loaded.

:version version

This keyword specifies that the item was first introduced in Emacs version version, or that its default value was changed in that version. The value version must be a string.

:package-version '(package . version)

This keyword specifies that the item was first introduced in package version version, or that its meaning or default value was changed in that version. This keyword takes priority over :version.

package should be the official name of the package, as a symbol (e.g., MH-E). version should be a string. If the package package is released as part of Emacs, package and version should appear in the value of customize-package-emacs-version-alist.

Packages distributed as part of Emacs that use the :package-version keyword must also update the customize-package-emacs-version-alist variable.

14.2 Defining Customization Groups

Each Emacs Lisp package should have one main customization group which contains all the options, faces and other groups in the package. If the package has a small number of options and faces, use just one group and put everything in it. When there are more than twenty or so options and faces, then you should structure them into subgroups, and put the subgroups under the package's main customization group. It is OK to put some of the options and faces in the package's main group alongside the subgroups.

The package's main or only group should be a member of one or more of the standard customization groups. (To display the full list of them, use M-x customize.) Choose one or more of them (but not too many), and add your group to each of them using the :group keyword.

The way to declare new customization groups is with defgroup.

14.3 Defining Customization Variables

Customizable variables, also called user options, are global Lisp variables whose values can be set through the Customize interface. Unlike other global variables, which are defined with defvar (see Defining Variables), customizable variables are defined using the defcustom macro. In addition to calling defvar as a subroutine, defcustom states how the variable should be displayed in the Customize interface, the values it is allowed to take, etc.

In addition to the keywords listed in Common Keywords, this macro accepts the following keywords:

:type type

Use type as the data type for this option. It specifies which values are legitimate, and how to display the value (see Customization Types). Every defcustom should specify a value for this keyword.

:options value-list

Specify the list of reasonable values for use in this option. The user is not restricted to using only these values, but they are offered as convenient alternatives.

This is meaningful only for certain types, currently including hook, plist and alist. See the definition of the individual types for a description of how to use :options.

:set setfunction

Specify setfunction as the way to change the value of this option when using the Customize interface. The function setfunction should take two arguments, a symbol (the option name) and the new value, and should do whatever is necessary to update the value properly for this option (which may not mean simply setting the option as a Lisp variable); preferably, though, it should not modify its value argument destructively. The default for setfunction is set-default.

If you specify this keyword, the variable's documentation string should describe how to do the same job in hand-written Lisp code.

:get getfunction

Specify getfunction as the way to extract the value of this option. The function getfunction should take one argument, a symbol, and should return whatever customize should use as the current value for that symbol (which need not be the symbol's Lisp value). The default is default-value.

You have to really understand the workings of Custom to use :get correctly. It is meant for values that are treated in Custom as variables but are not actually stored in Lisp variables. It is almost surely a mistake to specify getfunction for a value that really is stored in a Lisp variable.

:initialize function

function should be a function used to initialize the variable when the defcustom is evaluated. It should take two arguments, the option name (a symbol) and the value. Here are some predefined functions meant for use in this way:

custom-initialize-set

Use the variable's :set function to initialize the variable, but do not reinitialize it if it is already non-void.

custom-initialize-default

Like custom-initialize-set, but use the function set-default to set the variable, instead of the variable's :set function. This is the usual choice for a variable whose :set function enables or disables a minor mode; with this choice, defining the variable will not call the minor mode function, but customizing the variable will do so.

custom-initialize-reset

Always use the :set function to initialize the variable. If the variable is already non-void, reset it by calling the :set function using the current value (returned by the :get method). This is the default :initialize function.

custom-initialize-changed

Use the :set function to initialize the variable, if it is already set or has been customized; otherwise, just use set-default.

custom-initialize-safe-set

custom-initialize-safe-default

These functions behave like custom-initialize-set (custom-initialize-default, respectively), but catch errors. If an error occurs during initialization, they set the variable to nil using set-default, and signal no error.

These functions are meant for options defined in pre-loaded files, where the standard expression may signal an error because some required variable or function is not yet defined. The value normally gets updated in startup.el, ignoring the value computed by defcustom. After startup, if one unsets the value and reevaluates the defcustom, the standard expression can be evaluated without error.

:risky value

Set the variable's risky-local-variable property to value (see File Local Variables).

:safe function

Set the variable's safe-local-variable property to function (see File Local Variables).

:set-after variables

When setting variables according to saved customizations, make sure to set the variables variables before this one; i.e., delay setting this variable until after those others have been handled. Use :set-after if setting this variable won't work properly unless those other variables already have their intended values.

It is useful to specify the :require keyword for an option that turns on a certain feature. This causes Emacs to load the feature, if it is not already loaded, whenever the option is set. See Common Keywords. Here is an example, from the library saveplace.el:

     (defcustom save-place nil
       "Non-nil means automatically save place in each file..."
       :type 'boolean
       :require 'saveplace
       :group 'save-place)

If a customization item has a type such as hook or alist, which supports :options, you can add additional values to the list from outside the defcustom declaration by calling custom-add-frequent-value. For example, if you define a function my-lisp-mode-initialization intended to be called from emacs-lisp-mode-hook, you might want to add that to the list of reasonable values for emacs-lisp-mode-hook, but not by editing its definition. You can do it thus:

     (custom-add-frequent-value 'emacs-lisp-mode-hook
        'my-lisp-mode-initialization)

Internally, defcustom uses the symbol property standard-value to record the expression for the standard value, saved-value to record the value saved by the user with the customization buffer, and customized-value to record the value set by the user with the customization buffer, but not saved. See Symbol Properties. These properties are lists, the car of which is an expression that evaluates to the value.

14.4 Customization Types

When you define a user option with defcustom, you must specify its customization type. That is a Lisp object which describes (1) which values are legitimate and (2) how to display the value in the customization buffer for editing.

You specify the customization type in defcustom with the :type keyword. The argument of :type is evaluated, but only once when the defcustom is executed, so it isn't useful for the value to vary. Normally we use a quoted constant. For example:

     (defcustom diff-command "diff"
       "The command to use to run diff."
       :type '(string)
       :group 'diff)

In general, a customization type is a list whose first element is a symbol, one of the customization type names defined in the following sections. After this symbol come a number of arguments, depending on the symbol. Between the type symbol and its arguments, you can optionally write keyword-value pairs (see Type Keywords).

Some type symbols do not use any arguments; those are called simple types. For a simple type, if you do not use any keyword-value pairs, you can omit the parentheses around the type symbol. For example just string as a customization type is equivalent to (string).

All customization types are implemented as widgets; see Introduction, for details.

• Simple Types: Simple customization types: sexp, integer, etc.
• Composite Types: Build new types from other types or data.
• Splicing into Lists: Splice elements into list with :inline.
• Type Keywords: Keyword-argument pairs in a customization type.
• Defining New Types: Give your type a name.

14.4.1 Simple Types

This section describes all the simple customization types. For several of these customization types, the customization widget provides inline completion with C-M-i or M-<TAB>.

sexp

The value may be any Lisp object that can be printed and read back. You can use sexp as a fall-back for any option, if you don't want to take the time to work out a more specific type to use.

integer

The value must be an integer.

number

The value must be a number (floating point or integer).

float

The value must be floating point.

string

The value must be a string. The customization buffer shows the string without delimiting ‘"’ characters or ‘\’ quotes.

regexp

Like string except that the string must be a valid regular expression.

character

The value must be a character code. A character code is actually an integer, but this type shows the value by inserting the character in the buffer, rather than by showing the number.

file

The value must be a file name. The widget provides completion.

(file :must-match t)

The value must be a file name for an existing file. The widget provides completion.

directory

The value must be a directory name. The widget provides completion.

hook

The value must be a list of functions. This customization type is used for hook variables. You can use the :options keyword in a hook variable's defcustom to specify a list of functions recommended for use in the hook; See Variable Definitions.

symbol

The value must be a symbol. It appears in the customization buffer as the symbol name. The widget provides completion.

function

The value must be either a lambda expression or a function name. The widget provides completion for function names.

variable

The value must be a variable name. The widget provides completion.

face

The value must be a symbol which is a face name. The widget provides completion.

boolean

The value is boolean—either nil or t. Note that by using choice and const together (see the next section), you can specify that the value must be nil or t, but also specify the text to describe each value in a way that fits the specific meaning of the alternative.

key-sequence

The value is a key sequence. The customization buffer shows the key sequence using the same syntax as the kbd function. See Key Sequences.

coding-system

The value must be a coding-system name, and you can do completion with M-<TAB>.

color

The value must be a valid color name. The widget provides completion for color names, as well as a sample and a button for selecting a color name from a list of color names shown in a *Colors* buffer.

14.4.2 Composite Types

When none of the simple types is appropriate, you can use composite types, which build new types from other types or from specified data. The specified types or data are called the arguments of the composite type. The composite type normally looks like this:

     (constructor arguments...)

but you can also add keyword-value pairs before the arguments, like this:

     (constructor {keyword value}... arguments...)

Here is a table of constructors and how to use them to write composite types:

(cons car-type cdr-type)

The value must be a cons cell, its car must fit car-type, and its cdr must fit cdr-type. For example, (cons string symbol) is a customization type which matches values such as ("foo" . foo).

In the customization buffer, the car and cdr are displayed and edited separately, each according to their specified type.

(list element-types...)

The value must be a list with exactly as many elements as the element-types given; and each element must fit the corresponding element-type.

For example, (list integer string function) describes a list of three elements; the first element must be an integer, the second a string, and the third a function.

In the customization buffer, each element is displayed and edited separately, according to the type specified for it.

(group element-types...)

This works like list except for the formatting of text in the Custom buffer. list labels each element value with its tag; group does not.

(vector element-types...)

Like list except that the value must be a vector instead of a list. The elements work the same as in list.

(alist :key-type key-type :value-type value-type)

The value must be a list of cons-cells, the car of each cell representing a key of customization type key-type, and the cdr of the same cell representing a value of customization type value-type. The user can add and delete key/value pairs, and edit both the key and the value of each pair.

If omitted, key-type and value-type default to sexp.

The user can add any key matching the specified key type, but you can give some keys a preferential treatment by specifying them with the :options (see Variable Definitions). The specified keys will always be shown in the customize buffer (together with a suitable value), with a checkbox to include or exclude or disable the key/value pair from the alist. The user will not be able to edit the keys specified by the :options keyword argument.

The argument to the :options keywords should be a list of specifications for reasonable keys in the alist. Ordinarily, they are simply atoms, which stand for themselves. For example:

          :options '("foo" "bar" "baz")

specifies that there are three known keys, namely "foo", "bar" and "baz", which will always be shown first.

You may want to restrict the value type for specific keys, for example, the value associated with the "bar" key can only be an integer. You can specify this by using a list instead of an atom in the list. The first element will specify the key, like before, while the second element will specify the value type. For example:

          :options '("foo" ("bar" integer) "baz")

Finally, you may want to change how the key is presented. By default, the key is simply shown as a const, since the user cannot change the special keys specified with the :options keyword. However, you may want to use a more specialized type for presenting the key, like function-item if you know it is a symbol with a function binding. This is done by using a customization type specification instead of a symbol for the key.

          :options '("foo"
                     ((function-item some-function) integer)
                     "baz")

Many alists use lists with two elements, instead of cons cells. For example,

          (defcustom list-alist
            '(("foo" 1) ("bar" 2) ("baz" 3))
            "Each element is a list of the form (KEY VALUE).")

instead of

          (defcustom cons-alist
            '(("foo" . 1) ("bar" . 2) ("baz" . 3))
            "Each element is a cons-cell (KEY . VALUE).")

Because of the way lists are implemented on top of cons cells, you can treat list-alist in the example above as a cons cell alist, where the value type is a list with a single element containing the real value.

          (defcustom list-alist '(("foo" 1) ("bar" 2) ("baz" 3))
            "Each element is a list of the form (KEY VALUE)."
            :type '(alist :value-type (group integer)))

The group widget is used here instead of list only because the formatting is better suited for the purpose.

Similarly, you can have alists with more values associated with each key, using variations of this trick:

          (defcustom person-data '(("brian"  50 t)
                                   ("dorith" 55 nil)
                                   ("ken"    52 t))
            "Alist of basic info about people.
          Each element has the form (NAME AGE MALE-FLAG)."
            :type '(alist :value-type (group integer boolean)))

(plist :key-type key-type :value-type value-type)

This customization type is similar to alist (see above), except that (i) the information is stored as a property list, (see Property Lists), and (ii) key-type, if omitted, defaults to symbol rather than sexp.

(choice alternative-types...)

The value must fit one of alternative-types. For example, (choice integer string) allows either an integer or a string.

In the customization buffer, the user selects an alternative using a menu, and can then edit the value in the usual way for that alternative.

Normally the strings in this menu are determined automatically from the choices; however, you can specify different strings for the menu by including the :tag keyword in the alternatives. For example, if an integer stands for a number of spaces, while a string is text to use verbatim, you might write the customization type this way,

          (choice (integer :tag "Number of spaces")
                  (string :tag "Literal text"))

so that the menu offers ‘Number of spaces’ and ‘Literal text’.

In any alternative for which nil is not a valid value, other than a const, you should specify a valid default for that alternative using the :value keyword. See Type Keywords.

If some values are covered by more than one of the alternatives, customize will choose the first alternative that the value fits. This means you should always list the most specific types first, and the most general last. Here's an example of proper usage:

          (choice (const :tag "Off" nil)
                  symbol (sexp :tag "Other"))

This way, the special value nil is not treated like other symbols, and symbols are not treated like other Lisp expressions.

(radio element-types...)

This is similar to choice, except that the choices are displayed using radio buttons rather than a menu. This has the advantage of displaying documentation for the choices when applicable and so is often a good choice for a choice between constant functions (function-item customization types).

(const value)

The value must be value—nothing else is allowed.

The main use of const is inside of choice. For example, (choice integer (const nil)) allows either an integer or nil.

:tag is often used with const, inside of choice. For example,

          (choice (const :tag "Yes" t)
                  (const :tag "No" nil)
                  (const :tag "Ask" foo))

describes a variable for which t means yes, nil means no, and foo means “ask”.

(other value)

This alternative can match any Lisp value, but if the user chooses this alternative, that selects the value value.

The main use of other is as the last element of choice. For example,

          (choice (const :tag "Yes" t)
                  (const :tag "No" nil)
                  (other :tag "Ask" foo))

describes a variable for which t means yes, nil means no, and anything else means “ask”. If the user chooses ‘Ask’ from the menu of alternatives, that specifies the value foo; but any other value (not t, nil or foo) displays as ‘Ask’, just like foo.

(function-item function)

Like const, but used for values which are functions. This displays the documentation string as well as the function name. The documentation string is either the one you specify with :doc, or function's own documentation string.

(variable-item variable)

Like const, but used for values which are variable names. This displays the documentation string as well as the variable name. The documentation string is either the one you specify with :doc, or variable's own documentation string.

(set types...)

The value must be a list, and each element of the list must match one of the types specified.

This appears in the customization buffer as a checklist, so that each of types may have either one corresponding element or none. It is not possible to specify two different elements that match the same one of types. For example, (set integer symbol) allows one integer and/or one symbol in the list; it does not allow multiple integers or multiple symbols. As a result, it is rare to use nonspecific types such as integer in a set.

Most often, the types in a set are const types, as shown here:

          (set (const :bold) (const :italic))

Sometimes they describe possible elements in an alist:

          (set (cons :tag "Height" (const height) integer)
               (cons :tag "Width" (const width) integer))

That lets the user specify a height value optionally and a width value optionally.

(repeat element-type)

The value must be a list and each element of the list must fit the type element-type. This appears in the customization buffer as a list of elements, with ‘[INS]’ and ‘[DEL]’ buttons for adding more elements or removing elements.

(restricted-sexp :match-alternatives criteria)

This is the most general composite type construct. The value may be any Lisp object that satisfies one of criteria. criteria should be a list, and each element should be one of these possibilities:

• A predicate—that is, a function of one argument that has no side effects, and returns either nil or non-nil according to the argument. Using a predicate in the list says that objects for which the predicate returns non-nil are acceptable.
• A quoted constant—that is, 'object. This sort of element in the list says that object itself is an acceptable value.

For example,

          (restricted-sexp :match-alternatives
                           (integerp 't 'nil))

allows integers, t and nil as legitimate values.

The customization buffer shows all legitimate values using their read syntax, and the user edits them textually.

Here is a table of the keywords you can use in keyword-value pairs in a composite type:

:tag tag

Use tag as the name of this alternative, for user communication purposes. This is useful for a type that appears inside of a choice.

:match-alternatives criteria

Use criteria to match possible values. This is used only in restricted-sexp.

:args argument-list

Use the elements of argument-list as the arguments of the type construct. For instance, (const :args (foo)) is equivalent to (const foo). You rarely need to write :args explicitly, because normally the arguments are recognized automatically as whatever follows the last keyword-value pair.

14.4.3 Splicing into Lists

The :inline feature lets you splice a variable number of elements into the middle of a list or vector customization type. You use it by adding :inline t to a type specification which is contained in a list or vector specification.

Normally, each entry in a list or vector type specification describes a single element type. But when an entry contains :inline t, the value it matches is merged directly into the containing sequence. For example, if the entry matches a list with three elements, those become three elements of the overall sequence. This is analogous to ‘,@’ in a backquote construct (see Backquote).

For example, to specify a list whose first element must be baz and whose remaining arguments should be zero or more of foo and bar, use this customization type:

     (list (const baz) (set :inline t (const foo) (const bar)))

This matches values such as (baz), (baz foo), (baz bar) and (baz foo bar).

When the element-type is a choice, you use :inline not in the choice itself, but in (some of) the alternatives of the choice. For example, to match a list which must start with a file name, followed either by the symbol t or two strings, use this customization type:

     (list file
           (choice (const t)
                   (list :inline t string string)))

If the user chooses the first alternative in the choice, then the overall list has two elements and the second element is t. If the user chooses the second alternative, then the overall list has three elements and the second and third must be strings.

14.4.4 Type Keywords

You can specify keyword-argument pairs in a customization type after the type name symbol. Here are the keywords you can use, and their meanings:

:value default

Provide a default value.

If nil is not a valid value for the alternative, then it is essential to specify a valid default with :value.

If you use this for a type that appears as an alternative inside of choice; it specifies the default value to use, at first, if and when the user selects this alternative with the menu in the customization buffer.

Of course, if the actual value of the option fits this alternative, it will appear showing the actual value, not default.

:format format-string

This string will be inserted in the buffer to represent the value corresponding to the type. The following ‘%’ escapes are available for use in format-string:

‘%[button%]’

Display the text button marked as a button. The :action attribute specifies what the button will do if the user invokes it; its value is a function which takes two arguments—the widget which the button appears in, and the event.

There is no way to specify two different buttons with different actions.

‘%{sample%}’

Show sample in a special face specified by :sample-face.

‘%v’

Substitute the item's value. How the value is represented depends on the kind of item, and (for variables) on the customization type.

‘%d’

Substitute the item's documentation string.

‘%h’

Like ‘%d’, but if the documentation string is more than one line, add a button to control whether to show all of it or just the first line.

‘%t’

Substitute the tag here. You specify the tag with the :tag keyword.

‘%%’

Display a literal ‘%’.

:action action

Perform action if the user clicks on a button.

:button-face face

Use the face face (a face name or a list of face names) for button text displayed with ‘%[...%]’.

:button-prefix prefix

:button-suffix suffix

These specify the text to display before and after a button. Each can be:

nil

No text is inserted.

a string

The string is inserted literally.

a symbol

The symbol's value is used.

:tag tag

Use tag (a string) as the tag for the value (or part of the value) that corresponds to this type.

:doc doc

Use doc as the documentation string for this value (or part of the value) that corresponds to this type. In order for this to work, you must specify a value for :format, and use ‘%d’ or ‘%h’ in that value.

The usual reason to specify a documentation string for a type is to provide more information about the meanings of alternatives inside a :choice type or the parts of some other composite type.

:help-echo motion-doc

When you move to this item with widget-forward or widget-backward, it will display the string motion-doc in the echo area. In addition, motion-doc is used as the mouse help-echo string and may actually be a function or form evaluated to yield a help string. If it is a function, it is called with one argument, the widget.

:match function

Specify how to decide whether a value matches the type. The corresponding value, function, should be a function that accepts two arguments, a widget and a value; it should return non-nil if the value is acceptable.

:validate function

Specify a validation function for input. function takes a widget as an argument, and should return nil if the widget's current value is valid for the widget. Otherwise, it should return the widget containing the invalid data, and set that widget's :error property to a string explaining the error.

14.4.5 Defining New Types

In the previous sections we have described how to construct elaborate type specifications for defcustom. In some cases you may want to give such a type specification a name. The obvious case is when you are using the same type for many user options: rather than repeat the specification for each option, you can give the type specification a name, and use that name each defcustom. The other case is when a user option's value is a recursive data structure. To make it possible for a datatype to refer to itself, it needs to have a name.

Since custom types are implemented as widgets, the way to define a new customize type is to define a new widget. We are not going to describe the widget interface here in details, see Introduction, for that. Instead we are going to demonstrate the minimal functionality needed for defining new customize types by a simple example.

     (define-widget 'binary-tree-of-string 'lazy
       "A binary tree made of cons-cells and strings."
       :offset 4
       :tag "Node"
       :type '(choice (string :tag "Leaf" :value "")
                      (cons :tag "Interior"
                            :value ("" . "")
                            binary-tree-of-string
                            binary-tree-of-string)))
     
     (defcustom foo-bar ""
       "Sample variable holding a binary tree of strings."
       :type 'binary-tree-of-string)

The function to define a new widget is called define-widget. The first argument is the symbol we want to make a new widget type. The second argument is a symbol representing an existing widget, the new widget is going to be defined in terms of difference from the existing widget. For the purpose of defining new customization types, the lazy widget is perfect, because it accepts a :type keyword argument with the same syntax as the keyword argument to defcustom with the same name. The third argument is a documentation string for the new widget. You will be able to see that string with the M-x widget-browse <RET> binary-tree-of-string <RET> command.

After these mandatory arguments follow the keyword arguments. The most important is :type, which describes the data type we want to match with this widget. Here a binary-tree-of-string is described as being either a string, or a cons-cell whose car and cdr are themselves both binary-tree-of-string. Note the reference to the widget type we are currently in the process of defining. The :tag attribute is a string to name the widget in the user interface, and the :offset argument is there to ensure that child nodes are indented four spaces relative to the parent node, making the tree structure apparent in the customization buffer.

The defcustom shows how the new widget can be used as an ordinary customization type.

The reason for the name lazy is that the other composite widgets convert their inferior widgets to internal form when the widget is instantiated in a buffer. This conversion is recursive, so the inferior widgets will convert their inferior widgets. If the data structure is itself recursive, this conversion is an infinite recursion. The lazy widget prevents the recursion: it convert its :type argument only when needed.

14.5 Applying Customizations

The following functions are responsible for installing the user's customization settings for variables and faces, respectively. When the user invokes ‘Save for future sessions’ in the Customize interface, that takes effect by writing a custom-set-variables and/or a custom-set-faces form into the custom file, to be evaluated the next time Emacs starts.

14.6 Custom Themes

Custom themes are collections of settings that can be enabled or disabled as a unit. See Custom Themes. Each Custom theme is defined by an Emacs Lisp source file, which should follow the conventions described in this section. (Instead of writing a Custom theme by hand, you can also create one using a Customize-like interface; see Creating Custom Themes.)

A Custom theme file should be named foo-theme.el, where foo is the theme name. The first Lisp form in the file should be a call to deftheme, and the last form should be a call to provide-theme.

In between deftheme and provide-theme are Lisp forms specifying the theme settings: usually a call to custom-theme-set-variables and/or a call to custom-theme-set-faces.

In theory, a theme file can also contain other Lisp forms, which would be evaluated when loading the theme, but that is bad form. To protect against loading themes containing malicious code, Emacs displays the source file and asks for confirmation from the user before loading any non-built-in theme for the first time.

The following functions are useful for programmatically enabling and disabling themes:

15 Loading

Loading a file of Lisp code means bringing its contents into the Lisp environment in the form of Lisp objects. Emacs finds and opens the file, reads the text, evaluates each form, and then closes the file. Such a file is also called a Lisp library.

The load functions evaluate all the expressions in a file just as the eval-buffer function evaluates all the expressions in a buffer. The difference is that the load functions read and evaluate the text in the file as found on disk, not the text in an Emacs buffer.

The loaded file must contain Lisp expressions, either as source code or as byte-compiled code. Each form in the file is called a top-level form. There is no special format for the forms in a loadable file; any form in a file may equally well be typed directly into a buffer and evaluated there. (Indeed, most code is tested this way.) Most often, the forms are function definitions and variable definitions.

Emacs can also load compiled dynamic modules: shared libraries that provide additional functionality for use in Emacs Lisp programs, just like a package written in Emacs Lisp would. When a dynamic module is loaded, Emacs calls a specially-named initialization function which the module needs to implement, and which exposes the additional functions and variables to Emacs Lisp programs.

For on-demand loading of external libraries which are known in advance to be required by certain Emacs primitives, see Dynamic Libraries.

• How Programs Do Loading: The load function and others.
• Load Suffixes: Details about the suffixes that load tries.
• Library Search: Finding a library to load.
• Loading Non-ASCII: Non-ASCII characters in Emacs Lisp files.
• Autoload: Setting up a function to autoload.
• Repeated Loading: Precautions about loading a file twice.
• Named Features: Loading a library if it isn't already loaded.
• Where Defined: Finding which file defined a certain symbol.
• Unloading: How to unload a library that was loaded.
• Hooks for Loading: Providing code to be run when particular libraries are loaded.
• Dynamic Modules: Modules provide additional Lisp primitives.

15.1 How Programs Do Loading

Emacs Lisp has several interfaces for loading. For example, autoload creates a placeholder object for a function defined in a file; trying to call the autoloading function loads the file to get the function's real definition (see Autoload). require loads a file if it isn't already loaded (see Named Features). Ultimately, all these facilities call the load function to do the work.

For information about how load is used in building Emacs, see Building Emacs.

15.2 Load Suffixes

We now describe some technical details about the exact suffixes that load tries.

To summarize, load normally first tries the suffixes in the value of (get-load-suffixes) and then those in load-file-rep-suffixes. If nosuffix is non-nil, it skips the former group, and if must-suffix is non-nil, it skips the latter group.

15.3 Library Search

When Emacs loads a Lisp library, it searches for the library in a list of directories specified by the variable load-path.

When Emacs starts up, it sets up the value of load-path in several steps. First, it initializes load-path using default locations set when Emacs was compiled. Normally, this is a directory something like

     "/usr/local/share/emacs/version/lisp"

(In this and the following examples, replace /usr/local with the installation prefix appropriate for your Emacs.) These directories contain the standard Lisp files that come with Emacs. If Emacs cannot find them, it will not start correctly.

If you run Emacs from the directory where it was built—that is, an executable that has not been formally installed—Emacs instead initializes load-path using the lisp directory in the directory containing the sources from which it was built. If you built Emacs in a separate directory from the sources, it also adds the lisp directories from the build directory. (In all cases, elements are represented as absolute file names.)

Unless you start Emacs with the --no-site-lisp option, it then adds two more site-lisp directories to the front of load-path. These are intended for locally installed Lisp files, and are normally of the form:

     "/usr/local/share/emacs/version/site-lisp"

and

     "/usr/local/share/emacs/site-lisp"

The first one is for locally installed files for a specific Emacs version; the second is for locally installed files meant for use with all installed Emacs versions. (If Emacs is running uninstalled, it also adds site-lisp directories from the source and build directories, if they exist. Normally these directories do not contain site-lisp directories.)

If the environment variable EMACSLOADPATH is set, it modifies the above initialization procedure. Emacs initializes load-path based on the value of the environment variable.

The syntax of EMACSLOADPATH is the same as used for PATH; directory names are separated by ‘:’ (or ‘;’, on some operating systems). Here is an example of how to set EMACSLOADPATH variable (from a sh-style shell):

     export EMACSLOADPATH=/home/foo/.emacs.d/lisp:

An empty element in the value of the environment variable, whether trailing (as in the above example), leading, or embedded, is replaced by the default value of load-path as determined by the standard initialization procedure. If there are no such empty elements, then EMACSLOADPATH specifies the entire load-path. You must include either an empty element, or the explicit path to the directory containing the standard Lisp files, else Emacs will not function. (Another way to modify load-path is to use the -L command-line option when starting Emacs; see below.)

For each directory in load-path, Emacs then checks to see if it contains a file subdirs.el, and if so, loads it. The subdirs.el file is created when Emacs is built/installed, and contains code that causes Emacs to add any subdirectories of those directories to load-path. Both immediate subdirectories and subdirectories multiple levels down are added. But it excludes subdirectories whose names do not start with a letter or digit, and subdirectories named RCS or CVS, and subdirectories containing a file named .nosearch.

Next, Emacs adds any extra load directories that you specify using the -L command-line option (see Action Arguments). It also adds the directories where optional packages are installed, if any (see Packaging Basics).

It is common to add code to one's init file (see Init File) to add one or more directories to load-path. For example:

     (push "~/.emacs.d/lisp" load-path)

Dumping Emacs uses a special value of load-path. If you use a site-load.el or site-init.el file to customize the dumped Emacs (see Building Emacs), any changes to load-path that these files make will be lost after dumping.

15.4 Loading Non-ASCII Characters

When Emacs Lisp programs contain string constants with non-ASCII characters, these can be represented within Emacs either as unibyte strings or as multibyte strings (see Text Representations). Which representation is used depends on how the file is read into Emacs. If it is read with decoding into multibyte representation, the text of the Lisp program will be multibyte text, and its string constants will be multibyte strings. If a file containing Latin-1 characters (for example) is read without decoding, the text of the program will be unibyte text, and its string constants will be unibyte strings. See Coding Systems.

In most Emacs Lisp programs, the fact that non-ASCII strings are multibyte strings should not be noticeable, since inserting them in unibyte buffers converts them to unibyte automatically. However, if this does make a difference, you can force a particular Lisp file to be interpreted as unibyte by writing ‘coding: raw-text’ in a local variables section. With that designator, the file will unconditionally be interpreted as unibyte. This can matter when making keybindings to non-ASCII characters written as ?vliteral.

15.5 Autoload

The autoload facility lets you register the existence of a function or macro, but put off loading the file that defines it. The first call to the function automatically loads the proper library, in order to install the real definition and other associated code, then runs the real definition as if it had been loaded all along. Autoloading can also be triggered by looking up the documentation of the function or macro (see Documentation Basics).

There are two ways to set up an autoloaded function: by calling autoload, and by writing a “magic” comment in the source before the real definition. autoload is the low-level primitive for autoloading; any Lisp program can call autoload at any time. Magic comments are the most convenient way to make a function autoload, for packages installed along with Emacs. These comments do nothing on their own, but they serve as a guide for the command update-file-autoloads, which constructs calls to autoload and arranges to execute them when Emacs is built.

The autoloaded file usually contains other definitions and may require or provide one or more features. If the file is not completely loaded (due to an error in the evaluation of its contents), any function definitions or provide calls that occurred during the load are undone. This is to ensure that the next attempt to call any function autoloading from this file will try again to load the file. If not for this, then some of the functions in the file might be defined by the aborted load, but fail to work properly for the lack of certain subroutines not loaded successfully because they come later in the file.

If the autoloaded file fails to define the desired Lisp function or macro, then an error is signaled with data "Autoloading failed to define function function-name".

A magic autoload comment (often called an autoload cookie) consists of ‘;;;###autoload’, on a line by itself, just before the real definition of the function in its autoloadable source file. The command M-x update-file-autoloads writes a corresponding autoload call into loaddefs.el. (The string that serves as the autoload cookie and the name of the file generated by update-file-autoloads can be changed from the above defaults, see below.) Building Emacs loads loaddefs.el and thus calls autoload. M-x update-directory-autoloads is even more powerful; it updates autoloads for all files in the current directory.

The same magic comment can copy any kind of form into loaddefs.el. The form following the magic comment is copied verbatim, except if it is one of the forms which the autoload facility handles specially (e.g., by conversion into an autoload call). The forms which are not copied verbatim are the following:

Definitions for function or function-like objects:

defun and defmacro; also cl-defun and cl-defmacro (see Argument Lists), and define-overloadable-function (see the commentary in mode-local.el).

Definitions for major or minor modes:

define-minor-mode, define-globalized-minor-mode, define-generic-mode, define-derived-mode, easy-mmode-define-minor-mode, easy-mmode-define-global-mode, define-compilation-mode, and define-global-minor-mode.

Other definition types:

defcustom, defgroup, defclass (see EIEIO), and define-skeleton (see Autotyping).

You can also use a magic comment to execute a form at build time without executing it when the file itself is loaded. To do this, write the form on the same line as the magic comment. Since it is in a comment, it does nothing when you load the source file; but M-x update-file-autoloads copies it to loaddefs.el, where it is executed while building Emacs.

The following example shows how doctor is prepared for autoloading with a magic comment:

     ;;;###autoload
     (defun doctor ()
       "Switch to *doctor* buffer and start giving psychotherapy."
       (interactive)
       (switch-to-buffer "*doctor*")
       (doctor-mode))

Here's what that produces in loaddefs.el:

     (autoload (quote doctor) "doctor" "\
     Switch to *doctor* buffer and start giving psychotherapy.
     
     \(fn)" t nil)

The backslash and newline immediately following the double-quote are a convention used only in the preloaded uncompiled Lisp files such as loaddefs.el; they tell make-docfile to put the documentation string in the etc/DOC file. See Building Emacs. See also the commentary in lib-src/make-docfile.c. ‘(fn)’ in the usage part of the documentation string is replaced with the function's name when the various help functions (see Help Functions) display it.

If you write a function definition with an unusual macro that is not one of the known and recognized function definition methods, use of an ordinary magic autoload comment would copy the whole definition into loaddefs.el. That is not desirable. You can put the desired autoload call into loaddefs.el instead by writing this:

     ;;;###autoload (autoload 'foo "myfile")
     (mydefunmacro foo
       ...)

You can use a non-default string as the autoload cookie and have the corresponding autoload calls written into a file whose name is different from the default loaddefs.el. Emacs provides two variables to control this:

The following function may be used to explicitly load the library specified by an autoload object:

15.6 Repeated Loading

You can load a given file more than once in an Emacs session. For example, after you have rewritten and reinstalled a function definition by editing it in a buffer, you may wish to return to the original version; you can do this by reloading the file it came from.

When you load or reload files, bear in mind that the load and load-library functions automatically load a byte-compiled file rather than a non-compiled file of similar name. If you rewrite a file that you intend to save and reinstall, you need to byte-compile the new version; otherwise Emacs will load the older, byte-compiled file instead of your newer, non-compiled file! If that happens, the message displayed when loading the file includes, ‘(compiled; note, source is newer)’, to remind you to recompile it.

When writing the forms in a Lisp library file, keep in mind that the file might be loaded more than once. For example, think about whether each variable should be reinitialized when you reload the library; defvar does not change the value if the variable is already initialized. (See Defining Variables.)

The simplest way to add an element to an alist is like this:

     (push '(leif-mode " Leif") minor-mode-alist)

But this would add multiple elements if the library is reloaded. To avoid the problem, use add-to-list (see List Variables):

     (add-to-list 'minor-mode-alist '(leif-mode " Leif"))

Occasionally you will want to test explicitly whether a library has already been loaded. If the library uses provide to provide a named feature, you can use featurep earlier in the file to test whether the provide call has been executed before (see Named Features). Alternatively, you could use something like this:

     (defvar foo-was-loaded nil)
     
     (unless foo-was-loaded
       execute-first-time-only
       (setq foo-was-loaded t))

15.7 Features

provide and require are an alternative to autoload for loading files automatically. They work in terms of named features. Autoloading is triggered by calling a specific function, but a feature is loaded the first time another program asks for it by name.

A feature name is a symbol that stands for a collection of functions, variables, etc. The file that defines them should provide the feature. Another program that uses them may ensure they are defined by requiring the feature. This loads the file of definitions if it hasn't been loaded already.

To require the presence of a feature, call require with the feature name as argument. require looks in the global variable features to see whether the desired feature has been provided already. If not, it loads the feature from the appropriate file. This file should call provide at the top level to add the feature to features; if it fails to do so, require signals an error.

For example, in idlwave.el, the definition for idlwave-complete-filename includes the following code:

     (defun idlwave-complete-filename ()
       "Use the comint stuff to complete a file name."
        (require 'comint)
        (let* ((comint-file-name-chars "~/A-Za-z0-9+@:_.$#%={}\\-")
               (comint-completion-addsuffix nil)
               ...)
            (comint-dynamic-complete-filename)))

The expression (require 'comint) loads the file comint.el if it has not yet been loaded, ensuring that comint-dynamic-complete-filename is defined. Features are normally named after the files that provide them, so that require need not be given the file name. (Note that it is important that the require statement be outside the body of the let. Loading a library while its variables are let-bound can have unintended consequences, namely the variables becoming unbound after the let exits.)

The comint.el file contains the following top-level expression:

     (provide 'comint)

This adds comint to the global features list, so that (require 'comint) will henceforth know that nothing needs to be done.

When require is used at top level in a file, it takes effect when you byte-compile that file (see Byte Compilation) as well as when you load it. This is in case the required package contains macros that the byte compiler must know about. It also avoids byte compiler warnings for functions and variables defined in the file loaded with require.

Although top-level calls to require are evaluated during byte compilation, provide calls are not. Therefore, you can ensure that a file of definitions is loaded before it is byte-compiled by including a provide followed by a require for the same feature, as in the following example.

     (provide 'my-feature)  ; Ignored by byte compiler,
                            ;   evaluated by load.
     (require 'my-feature)  ; Evaluated by byte compiler.

The compiler ignores the provide, then processes the require by loading the file in question. Loading the file does execute the provide call, so the subsequent require call does nothing when the file is loaded.

15.8 Which File Defined a Certain Symbol

The basis for symbol-file is the data in the variable load-history.

The command eval-region updates load-history, but does so by adding the symbols defined to the element for the file being visited, rather than replacing that element. See Eval.

15.9 Unloading

You can discard the functions and variables loaded by a library to reclaim memory for other Lisp objects. To do this, use the function unload-feature:

The unload-feature function is written in Lisp; its actions are based on the variable load-history.

15.10 Hooks for Loading

You can ask for code to be executed each time Emacs loads a library, by using the variable after-load-functions:

If you want code to be executed when a particular library is loaded, use the macro with-eval-after-load:

Normally, well-designed Lisp programs should not use with-eval-after-load. If you need to examine and set the variables defined in another library (those meant for outside use), you can do it immediately—there is no need to wait until the library is loaded. If you need to call functions defined by that library, you should load the library, preferably with require (see Named Features).

15.11 Emacs Dynamic Modules

A dynamic Emacs module is a shared library that provides additional functionality for use in Emacs Lisp programs, just like a package written in Emacs Lisp would.

Functions that load Emacs Lisp packages can also load dynamic modules. They recognize dynamic modules by looking at their file-name extension, a.k.a. “suffix”. This suffix is platform-dependent.

Every dynamic module should export a C-callable function named emacs_module_init, which Emacs will call as part of the call to load or require which loads the module. It should also export a symbol named plugin_is_GPL_compatible to indicate that its code is released under the GPL or compatible license; Emacs will refuse to load modules that don't export such a symbol.

If a module needs to call Emacs functions, it should do so through the API defined and documented in the header file emacs-module.h that is part of the Emacs distribution.

Modules can create user-ptr Lisp objects that embed pointers to C struct's defined by the module. This is useful for keeping around complex data structures created by a module, to be passed back to the module's functions. User-ptr objects can also have associated finalizers – functions to be run when the object is GC'ed; this is useful for freeing any resources allocated for the underlying data structure, such as memory, open file descriptors, etc.

Loadable modules in Emacs are enabled by using the --with-modules option at configure time.

16 Byte Compilation

Emacs Lisp has a compiler that translates functions written in Lisp into a special representation called byte-code that can be executed more efficiently. The compiler replaces Lisp function definitions with byte-code. When a byte-code function is called, its definition is evaluated by the byte-code interpreter.

Because the byte-compiled code is evaluated by the byte-code interpreter, instead of being executed directly by the machine's hardware (as true compiled code is), byte-code is completely transportable from machine to machine without recompilation. It is not, however, as fast as true compiled code.

In general, any version of Emacs can run byte-compiled code produced by recent earlier versions of Emacs, but the reverse is not true.

If you do not want a Lisp file to be compiled, ever, put a file-local variable binding for no-byte-compile into it, like this:

     ;; -*-no-byte-compile: t; -*-

• Speed of Byte-Code: An example of speedup from byte compilation.
• Compilation Functions: Byte compilation functions.
• Docs and Compilation: Dynamic loading of documentation strings.
• Dynamic Loading: Dynamic loading of individual functions.
• Eval During Compile: Code to be evaluated when you compile.
• Compiler Errors: Handling compiler error messages.
• Byte-Code Objects: The data type used for byte-compiled functions.
• Disassembly: Disassembling byte-code; how to read byte-code.

16.1 Performance of Byte-Compiled Code

A byte-compiled function is not as efficient as a primitive function written in C, but runs much faster than the version written in Lisp. Here is an example:

     (defun silly-loop (n)
       "Return the time, in seconds, to run N iterations of a loop."
       (let ((t1 (float-time)))
         (while (> (setq n (1- n)) 0))
         (- (float-time) t1)))
     ⇒ silly-loop
     
     (silly-loop 50000000)
     ⇒ 10.235304117202759
     
     (byte-compile 'silly-loop)
     ⇒ [Compiled code not shown]
     
     (silly-loop 50000000)
     ⇒ 3.705854892730713

In this example, the interpreted code required 10 seconds to run, whereas the byte-compiled code required less than 4 seconds. These results are representative, but actual results may vary.

16.2 Byte-Compilation Functions

You can byte-compile an individual function or macro definition with the byte-compile function. You can compile a whole file with byte-compile-file, or several files with byte-recompile-directory or batch-byte-compile.

Sometimes, the byte compiler produces warning and/or error messages (see Compiler Errors, for details). These messages are normally recorded in a buffer called *Compile-Log*, which uses Compilation mode. See Compilation Mode. However, if the variable byte-compile-debug is non-nil, error message will be signaled as Lisp errors instead (see Errors).

Be careful when writing macro calls in files that you intend to byte-compile. Since macro calls are expanded when they are compiled, the macros need to be loaded into Emacs or the byte compiler will not do the right thing. The usual way to handle this is with require forms which specify the files containing the needed macro definitions (see Named Features). Normally, the byte compiler does not evaluate the code that it is compiling, but it handles require forms specially, by loading the specified libraries. To avoid loading the macro definition files when someone runs the compiled program, write eval-when-compile around the require calls (see Eval During Compile). For more details, See Compiling Macros.

Inline (defsubst) functions are less troublesome; if you compile a call to such a function before its definition is known, the call will still work right, it will just run slower.

16.3 Documentation Strings and Compilation

When Emacs loads functions and variables from a byte-compiled file, it normally does not load their documentation strings into memory. Each documentation string is dynamically loaded from the byte-compiled file only when needed. This saves memory, and speeds up loading by skipping the processing of the documentation strings.

This feature has a drawback: if you delete, move, or alter the compiled file (such as by compiling a new version), Emacs may no longer be able to access the documentation string of previously-loaded functions or variables. Such a problem normally only occurs if you build Emacs yourself, and happen to edit and/or recompile the Lisp source files. To solve it, just reload each file after recompilation.

Dynamic loading of documentation strings from byte-compiled files is determined, at compile time, for each byte-compiled file. It can be disabled via the option byte-compile-dynamic-docstrings.

Internally, the dynamic loading of documentation strings is accomplished by writing compiled files with a special Lisp reader construct, ‘#@count’. This construct skips the next count characters. It also uses the ‘#$’ construct, which stands for the name of this file, as a string. Do not use these constructs in Lisp source files; they are not designed to be clear to humans reading the file.

16.4 Dynamic Loading of Individual Functions

When you compile a file, you can optionally enable the dynamic function loading feature (also known as lazy loading). With dynamic function loading, loading the file doesn't fully read the function definitions in the file. Instead, each function definition contains a place-holder which refers to the file. The first time each function is called, it reads the full definition from the file, to replace the place-holder.

The advantage of dynamic function loading is that loading the file becomes much faster. This is a good thing for a file which contains many separate user-callable functions, if using one of them does not imply you will probably also use the rest. A specialized mode which provides many keyboard commands often has that usage pattern: a user may invoke the mode, but use only a few of the commands it provides.

The dynamic loading feature has certain disadvantages:

• If you delete or move the compiled file after loading it, Emacs can no longer load the remaining function definitions not already loaded.
• If you alter the compiled file (such as by compiling a new version), then trying to load any function not already loaded will usually yield nonsense results.

These problems will never happen in normal circumstances with installed Emacs files. But they are quite likely to happen with Lisp files that you are changing. The easiest way to prevent these problems is to reload the new compiled file immediately after each recompilation.

The byte compiler uses the dynamic function loading feature if the variable byte-compile-dynamic is non-nil at compilation time. Do not set this variable globally, since dynamic loading is desirable only for certain files. Instead, enable the feature for specific source files with file-local variable bindings. For example, you could do it by writing this text in the source file's first line:

     -*-byte-compile-dynamic: t;-*-

16.5 Evaluation During Compilation

These features permit you to write code to be evaluated during compilation of a program.

16.6 Compiler Errors

Error and warning messages from byte compilation are printed in a buffer named *Compile-Log*. These messages include file names and line numbers identifying the location of the problem. The usual Emacs commands for operating on compiler output can be used on these messages.

When an error is due to invalid syntax in the program, the byte compiler might get confused about the error's exact location. One way to investigate is to switch to the buffer  *Compiler Input*. (This buffer name starts with a space, so it does not show up in the Buffer Menu.) This buffer contains the program being compiled, and point shows how far the byte compiler was able to read; the cause of the error might be nearby. See Syntax Errors, for some tips for locating syntax errors.

A common type of warning issued by the byte compiler is for functions and variables that were used but not defined. Such warnings report the line number for the end of the file, not the locations where the missing functions or variables were used; to find these, you must search the file manually.

If you are sure that a warning message about a missing function or variable is unjustified, there are several ways to suppress it:

• You can suppress the warning for a specific call to a function func by conditionalizing it on an fboundp test, like this:

            (if (fboundp 'func) ...(func ...)...)
  

  The call to func must be in the then-form of the if, and func must appear quoted in the call to fboundp. (This feature operates for cond as well.)

• Likewise, you can suppress the warning for a specific use of a variable variable by conditionalizing it on a boundp test:

            (if (boundp 'variable) ...variable...)
  

  The reference to variable must be in the then-form of the if, and variable must appear quoted in the call to boundp.

• You can tell the compiler that a function is defined using declare-function. See Declaring Functions.
• Likewise, you can tell the compiler that a variable is defined using defvar with no initial value. (Note that this marks the variable as special.) See Defining Variables.

You can also suppress any and all compiler warnings within a certain expression using the construct with-no-warnings:

Byte compiler warnings can be controlled more precisely by setting the variable byte-compile-warnings. See its documentation string for details.

16.7 Byte-Code Function Objects

Byte-compiled functions have a special data type: they are byte-code function objects. Whenever such an object appears as a function to be called, Emacs uses the byte-code interpreter to execute the byte-code.

Internally, a byte-code function object is much like a vector; its elements can be accessed using aref. Its printed representation is like that for a vector, with an additional ‘#’ before the opening ‘[’. It must have at least four elements; there is no maximum number, but only the first six elements have any normal use. They are:

argdesc

The descriptor of the arguments. This can either be a list of arguments, as described in Argument List, or an integer encoding the required number of arguments. In the latter case, the value of the descriptor specifies the minimum number of arguments in the bits zero to 6, and the maximum number of arguments in bits 8 to 14. If the argument list uses &rest, then bit 7 is set; otherwise it's cleared.

If argdesc is a list, the arguments will be dynamically bound before executing the byte code. If argdesc is an integer, the arguments will be instead pushed onto the stack of the byte-code interpreter, before executing the code.

byte-code

The string containing the byte-code instructions.

constants

The vector of Lisp objects referenced by the byte code. These include symbols used as function names and variable names.

stacksize

The maximum stack size this function needs.

docstring

The documentation string (if any); otherwise, nil. The value may be a number or a list, in case the documentation string is stored in a file. Use the function documentation to get the real documentation string (see Accessing Documentation).

interactive

The interactive spec (if any). This can be a string or a Lisp expression. It is nil for a function that isn't interactive.

Here's an example of a byte-code function object, in printed representation. It is the definition of the command backward-sexp.

     #[256
       "\211\204^G^@\300\262^A\301^A[!\207"
       [1 forward-sexp]
       3
       1793299
       "^p"]

The primitive way to create a byte-code object is with make-byte-code:

You should not try to come up with the elements for a byte-code function yourself, because if they are inconsistent, Emacs may crash when you call the function. Always leave it to the byte compiler to create these objects; it makes the elements consistent (we hope).

16.8 Disassembled Byte-Code

People do not write byte-code; that job is left to the byte compiler. But we provide a disassembler to satisfy a cat-like curiosity. The disassembler converts the byte-compiled code into human-readable form.

The byte-code interpreter is implemented as a simple stack machine. It pushes values onto a stack of its own, then pops them off to use them in calculations whose results are themselves pushed back on the stack. When a byte-code function returns, it pops a value off the stack and returns it as the value of the function.

In addition to the stack, byte-code functions can use, bind, and set ordinary Lisp variables, by transferring values between variables and the stack.

Here are two examples of using the disassemble function. We have added explanatory comments to help you relate the byte-code to the Lisp source; these do not appear in the output of disassemble.

     (defun factorial (integer)
       "Compute factorial of an integer."
       (if (= 1 integer) 1
         (* integer (factorial (1- integer)))))
          ⇒ factorial
     
     (factorial 4)
          ⇒ 24
     
     (disassemble 'factorial)
          -| byte-code for factorial:
      doc: Compute factorial of an integer.
      args: (integer)
     
     0   varref   integer      ; Get the value of integer and
                               ;   push it onto the stack.
     1   constant 1            ; Push 1 onto stack.
     2   eqlsign               ; Pop top two values off stack, compare
                               ;   them, and push result onto stack.
     3   goto-if-nil 1         ; Pop and test top of stack;
                               ;   if nil, go to 1, else continue.
     6   constant 1            ; Push 1 onto top of stack.
     7   return                ; Return the top element of the stack.
     8:1 varref   integer      ; Push value of integer onto stack.
     9   constant factorial    ; Push factorial onto stack.
     10  varref   integer      ; Push value of integer onto stack.
     11  sub1                  ; Pop integer, decrement value,
                               ;   push new value onto stack.
     12  call     1            ; Call function factorial using first
                               ;   (i.e., top) stack element as argument;
                               ;   push returned value onto stack.
     13 mult                   ; Pop top two values off stack, multiply
                               ;   them, and push result onto stack.
     14 return                 ; Return the top element of the stack.

The silly-loop function is somewhat more complex:

     (defun silly-loop (n)
       "Return time before and after N iterations of a loop."
       (let ((t1 (current-time-string)))
         (while (> (setq n (1- n))
                   0))
         (list t1 (current-time-string))))
          ⇒ silly-loop
     
     (disassemble 'silly-loop)
          -| byte-code for silly-loop:
      doc: Return time before and after N iterations of a loop.
      args: (n)
     
     0   constant current-time-string  ; Push current-time-string
                                       ;   onto top of stack.
     1   call     0            ; Call current-time-string with no
                               ;   argument, push result onto stack.
     2   varbind  t1           ; Pop stack and bind t1 to popped value.
     3:1 varref   n            ; Get value of n from the environment
                               ;   and push the value on the stack.
     4   sub1                  ; Subtract 1 from top of stack.
     5   dup                   ; Duplicate top of stack; i.e., copy the top
                               ;   of the stack and push copy onto stack.
     6   varset   n            ; Pop the top of the stack,
                               ;   and bind n to the value.
     
     ;; (In effect, the sequence dup varset copies the top of the stack
     ;; into the value of n without popping it.)
     
     7   constant 0            ; Push 0 onto stack.
     8   gtr                   ; Pop top two values off stack,
                               ;   test if n is greater than 0
                               ;   and push result onto stack.
     9   goto-if-not-nil 1     ; Goto 1 if n > 0
                               ;   (this continues the while loop)
                               ;   else continue.
     12  varref   t1           ; Push value of t1 onto stack.
     13  constant current-time-string  ; Push current-time-string
                                       ;   onto the top of the stack.
     14  call     0            ; Call current-time-string again.
     15  unbind   1            ; Unbind t1 in local environment.
     16  list2                 ; Pop top two elements off stack, create a
                               ;   list of them, and push it onto stack.
     17  return                ; Return value of the top of stack.

17 Debugging Lisp Programs

There are several ways to find and investigate problems in an Emacs Lisp program.

• If a problem occurs when you run the program, you can use the built-in Emacs Lisp debugger to suspend the Lisp evaluator, and examine and/or alter its internal state.
• You can use Edebug, a source-level debugger for Emacs Lisp.
• If a syntactic problem is preventing Lisp from even reading the program, you can locate it using Lisp editing commands.
• You can look at the error and warning messages produced by the byte compiler when it compiles the program. See Compiler Errors.
• You can use the Testcover package to perform coverage testing on the program.
• You can use the ERT package to write regression tests for the program. See the ERT manual.
• You can profile the program to get hints about how to make it more efficient.

Other useful tools for debugging input and output problems are the dribble file (see Terminal Input) and the open-termscript function (see Terminal Output).

• Debugger: A debugger for the Emacs Lisp evaluator.
• Edebug: A source-level Emacs Lisp debugger.
• Syntax Errors: How to find syntax errors.
• Test Coverage: Ensuring you have tested all branches in your code.
• Profiling: Measuring the resources that your code uses.

17.1 The Lisp Debugger

The ordinary Lisp debugger provides the ability to suspend evaluation of a form. While evaluation is suspended (a state that is commonly known as a break), you may examine the run time stack, examine the values of local or global variables, or change those values. Since a break is a recursive edit, all the usual editing facilities of Emacs are available; you can even run programs that will enter the debugger recursively. See Recursive Editing.

• Error Debugging: Entering the debugger when an error happens.
• Infinite Loops: Stopping and debugging a program that doesn't exit.
• Function Debugging: Entering it when a certain function is called.
• Explicit Debug: Entering it at a certain point in the program.
• Using Debugger: What the debugger does; what you see while in it.
• Debugger Commands: Commands used while in the debugger.
• Invoking the Debugger: How to call the function debug.
• Internals of Debugger: Subroutines of the debugger, and global variables.

17.1.1 Entering the Debugger on an Error

The most important time to enter the debugger is when a Lisp error happens. This allows you to investigate the immediate causes of the error.

However, entry to the debugger is not a normal consequence of an error. Many commands signal Lisp errors when invoked inappropriately, and during ordinary editing it would be very inconvenient to enter the debugger each time this happens. So if you want errors to enter the debugger, set the variable debug-on-error to non-nil. (The command toggle-debug-on-error provides an easy way to do this.)

To debug an error that happens during loading of the init file, use the option ‘--debug-init’. This binds debug-on-error to t while loading the init file, and bypasses the condition-case which normally catches errors in the init file.

17.1.2 Debugging Infinite Loops

When a program loops infinitely and fails to return, your first problem is to stop the loop. On most operating systems, you can do this with C-g, which causes a quit. See Quitting.

Ordinary quitting gives no information about why the program was looping. To get more information, you can set the variable debug-on-quit to non-nil. Once you have the debugger running in the middle of the infinite loop, you can proceed from the debugger using the stepping commands. If you step through the entire loop, you may get enough information to solve the problem.

Quitting with C-g is not considered an error, and debug-on-error has no effect on the handling of C-g. Likewise, debug-on-quit has no effect on errors.

17.1.3 Entering the Debugger on a Function Call

To investigate a problem that happens in the middle of a program, one useful technique is to enter the debugger whenever a certain function is called. You can do this to the function in which the problem occurs, and then step through the function, or you can do this to a function called shortly before the problem, step quickly over the call to that function, and then step through its caller.

17.1.4 Explicit Entry to the Debugger

You can cause the debugger to be called at a certain point in your program by writing the expression (debug) at that point. To do this, visit the source file, insert the text ‘(debug)’ at the proper place, and type C-M-x (eval-defun, a Lisp mode key binding). Warning: if you do this for temporary debugging purposes, be sure to undo this insertion before you save the file!

The place where you insert ‘(debug)’ must be a place where an additional form can be evaluated and its value ignored. (If the value of (debug) isn't ignored, it will alter the execution of the program!) The most common suitable places are inside a progn or an implicit progn (see Sequencing).

If you don't know exactly where in the source code you want to put the debug statement, but you want to display a backtrace when a certain message is displayed, you can set debug-on-message to a regular expression matching the desired message.

17.1.5 Using the Debugger

When the debugger is entered, it displays the previously selected buffer in one window and a buffer named *Backtrace* in another window. The backtrace buffer contains one line for each level of Lisp function execution currently going on. At the beginning of this buffer is a message describing the reason that the debugger was invoked (such as the error message and associated data, if it was invoked due to an error).

The backtrace buffer is read-only and uses a special major mode, Debugger mode, in which letters are defined as debugger commands. The usual Emacs editing commands are available; thus, you can switch windows to examine the buffer that was being edited at the time of the error, switch buffers, visit files, or do any other sort of editing. However, the debugger is a recursive editing level (see Recursive Editing) and it is wise to go back to the backtrace buffer and exit the debugger (with the q command) when you are finished with it. Exiting the debugger gets out of the recursive edit and buries the backtrace buffer. (You can customize what the q command does with the backtrace buffer by setting the variable debugger-bury-or-kill. For example, set it to kill if you prefer to kill the buffer rather than bury it. Consult the variable's documentation for more possibilities.)

When the debugger has been entered, the debug-on-error variable is temporarily set according to eval-expression-debug-on-error. If the latter variable is non-nil, debug-on-error will temporarily be set to t. This means that any further errors that occur while doing a debugging session will (by default) trigger another backtrace. If this is not what you want, you can either set eval-expression-debug-on-error to nil, or set debug-on-error to nil in debugger-mode-hook.

The backtrace buffer shows you the functions that are executing and their argument values. It also allows you to specify a stack frame by moving point to the line describing that frame. (A stack frame is the place where the Lisp interpreter records information about a particular invocation of a function.) The frame whose line point is on is considered the current frame. Some of the debugger commands operate on the current frame. If a line starts with a star, that means that exiting that frame will call the debugger again. This is useful for examining the return value of a function.

If a function name is underlined, that means the debugger knows where its source code is located. You can click with the mouse on that name, or move to it and type <RET>, to visit the source code.

The debugger itself must be run byte-compiled, since it makes assumptions about how many stack frames are used for the debugger itself. These assumptions are false if the debugger is running interpreted.

17.1.6 Debugger Commands

The debugger buffer (in Debugger mode) provides special commands in addition to the usual Emacs commands. The most important use of debugger commands is for stepping through code, so that you can see how control flows. The debugger can step through the control structures of an interpreted function, but cannot do so in a byte-compiled function. If you would like to step through a byte-compiled function, replace it with an interpreted definition of the same function. (To do this, visit the source for the function and type C-M-x on its definition.) You cannot use the Lisp debugger to step through a primitive function.

Here is a list of Debugger mode commands:

c

Exit the debugger and continue execution. This resumes execution of the program as if the debugger had never been entered (aside from any side-effects that you caused by changing variable values or data structures while inside the debugger).

d

Continue execution, but enter the debugger the next time any Lisp function is called. This allows you to step through the subexpressions of an expression, seeing what values the subexpressions compute, and what else they do.

The stack frame made for the function call which enters the debugger in this way will be flagged automatically so that the debugger will be called again when the frame is exited. You can use the u command to cancel this flag.

b

Flag the current frame so that the debugger will be entered when the frame is exited. Frames flagged in this way are marked with stars in the backtrace buffer.

u

Don't enter the debugger when the current frame is exited. This cancels a b command on that frame. The visible effect is to remove the star from the line in the backtrace buffer.

j

Flag the current frame like b. Then continue execution like c, but temporarily disable break-on-entry for all functions that are set up to do so by debug-on-entry.

e

Read a Lisp expression in the minibuffer, evaluate it (with the relevant lexical environment, if applicable), and print the value in the echo area. The debugger alters certain important variables, and the current buffer, as part of its operation; e temporarily restores their values from outside the debugger, so you can examine and change them. This makes the debugger more transparent. By contrast, M-: does nothing special in the debugger; it shows you the variable values within the debugger.

R

Like e, but also save the result of evaluation in the buffer *Debugger-record*.

q

Terminate the program being debugged; return to top-level Emacs command execution.

If the debugger was entered due to a C-g but you really want to quit, and not debug, use the q command.

r

Return a value from the debugger. The value is computed by reading an expression with the minibuffer and evaluating it.

The r command is useful when the debugger was invoked due to exit from a Lisp call frame (as requested with b or by entering the frame with d); then the value specified in the r command is used as the value of that frame. It is also useful if you call debug and use its return value. Otherwise, r has the same effect as c, and the specified return value does not matter.

You can't use r when the debugger was entered due to an error.

l

Display a list of functions that will invoke the debugger when called. This is a list of functions that are set to break on entry by means of debug-on-entry.

v

Toggle the display of local variables of the current stack frame.

17.1.7 Invoking the Debugger

Here we describe in full detail the function debug that is used to invoke the debugger.

17.1.8 Internals of the Debugger

This section describes functions and variables used internally by the debugger.

17.2 Edebug

Edebug is a source-level debugger for Emacs Lisp programs, with which you can:

• Step through evaluation, stopping before and after each expression.
• Set conditional or unconditional breakpoints.
• Stop when a specified condition is true (the global break event).
• Trace slow or fast, stopping briefly at each stop point, or at each breakpoint.
• Display expression results and evaluate expressions as if outside of Edebug.
• Automatically re-evaluate a list of expressions and display their results each time Edebug updates the display.
• Output trace information on function calls and returns.
• Stop when an error occurs.
• Display a backtrace, omitting Edebug's own frames.
• Specify argument evaluation for macros and defining forms.
• Obtain rudimentary coverage testing and frequency counts.

The first three sections below should tell you enough about Edebug to start using it.

• Using Edebug: Introduction to use of Edebug.
• Instrumenting: You must instrument your code in order to debug it with Edebug.
• Modes: Execution modes, stopping more or less often.
• Jumping: Commands to jump to a specified place.
• Misc: Miscellaneous commands.
• Breaks: Setting breakpoints to make the program stop.
• Trapping Errors: Trapping errors with Edebug.
• Views: Views inside and outside of Edebug.
• Eval: Evaluating expressions within Edebug.
• Eval List: Expressions whose values are displayed each time you enter Edebug.
• Printing in Edebug: Customization of printing.
• Trace Buffer: How to produce trace output in a buffer.
• Coverage Testing: How to test evaluation coverage.
• The Outside Context: Data that Edebug saves and restores.
• Edebug and Macros: Specifying how to handle macro calls.
• Options: Option variables for customizing Edebug.

17.2.1 Using Edebug

To debug a Lisp program with Edebug, you must first instrument the Lisp code that you want to debug. A simple way to do this is to first move point into the definition of a function or macro and then do C-u C-M-x (eval-defun with a prefix argument). See Instrumenting, for alternative ways to instrument code.

Once a function is instrumented, any call to the function activates Edebug. Depending on which Edebug execution mode you have selected, activating Edebug may stop execution and let you step through the function, or it may update the display and continue execution while checking for debugging commands. The default execution mode is step, which stops execution. See Edebug Execution Modes.

Within Edebug, you normally view an Emacs buffer showing the source of the Lisp code you are debugging. This is referred to as the source code buffer, and it is temporarily read-only.

An arrow in the left fringe indicates the line where the function is executing. Point initially shows where within the line the function is executing, but this ceases to be true if you move point yourself.

If you instrument the definition of fac (shown below) and then execute (fac 3), here is what you would normally see. Point is at the open-parenthesis before if.

     (defun fac (n)
     =>-!-(if (< 0 n)
           (* n (fac (1- n)))
         1))

The places within a function where Edebug can stop execution are called stop points. These occur both before and after each subexpression that is a list, and also after each variable reference. Here we use periods to show the stop points in the function fac:

     (defun fac (n)
       .(if .(< 0 n.).
           .(* n. .(fac .(1- n.).).).
         1).)

The special commands of Edebug are available in the source code buffer in addition to the commands of Emacs Lisp mode. For example, you can type the Edebug command <SPC> to execute until the next stop point. If you type <SPC> once after entry to fac, here is the display you will see:

     (defun fac (n)
     =>(if -!-(< 0 n)
           (* n (fac (1- n)))
         1))

When Edebug stops execution after an expression, it displays the expression's value in the echo area.

Other frequently used commands are b to set a breakpoint at a stop point, g to execute until a breakpoint is reached, and q to exit Edebug and return to the top-level command loop. Type ? to display a list of all Edebug commands.

17.2.2 Instrumenting for Edebug

In order to use Edebug to debug Lisp code, you must first instrument the code. Instrumenting code inserts additional code into it, to invoke Edebug at the proper places.

When you invoke command C-M-x (eval-defun) with a prefix argument on a function definition, it instruments the definition before evaluating it. (This does not modify the source code itself.) If the variable edebug-all-defs is non-nil, that inverts the meaning of the prefix argument: in this case, C-M-x instruments the definition unless it has a prefix argument. The default value of edebug-all-defs is nil. The command M-x edebug-all-defs toggles the value of the variable edebug-all-defs.

If edebug-all-defs is non-nil, then the commands eval-region, eval-current-buffer, and eval-buffer also instrument any definitions they evaluate. Similarly, edebug-all-forms controls whether eval-region should instrument any form, even non-defining forms. This doesn't apply to loading or evaluations in the minibuffer. The command M-x edebug-all-forms toggles this option.

Another command, M-x edebug-eval-top-level-form, is available to instrument any top-level form regardless of the values of edebug-all-defs and edebug-all-forms. edebug-defun is an alias for edebug-eval-top-level-form.

While Edebug is active, the command I (edebug-instrument-callee) instruments the definition of the function or macro called by the list form after point, if it is not already instrumented. This is possible only if Edebug knows where to find the source for that function; for this reason, after loading Edebug, eval-region records the position of every definition it evaluates, even if not instrumenting it. See also the i command (see Jumping), which steps into the call after instrumenting the function.

Edebug knows how to instrument all the standard special forms, interactive forms with an expression argument, anonymous lambda expressions, and other defining forms. However, Edebug cannot determine on its own what a user-defined macro will do with the arguments of a macro call, so you must provide that information using Edebug specifications; for details, see Edebug and Macros.

When Edebug is about to instrument code for the first time in a session, it runs the hook edebug-setup-hook, then sets it to nil. You can use this to load Edebug specifications associated with a package you are using, but only when you use Edebug.

To remove instrumentation from a definition, simply re-evaluate its definition in a way that does not instrument. There are two ways of evaluating forms that never instrument them: from a file with load, and from the minibuffer with eval-expression (M-:).

If Edebug detects a syntax error while instrumenting, it leaves point at the erroneous code and signals an invalid-read-syntax error.

See Edebug Eval, for other evaluation functions available inside of Edebug.

17.2.3 Edebug Execution Modes

Edebug supports several execution modes for running the program you are debugging. We call these alternatives Edebug execution modes; do not confuse them with major or minor modes. The current Edebug execution mode determines how far Edebug continues execution before stopping—whether it stops at each stop point, or continues to the next breakpoint, for example—and how much Edebug displays the progress of the evaluation before it stops.

Normally, you specify the Edebug execution mode by typing a command to continue the program in a certain mode. Here is a table of these commands; all except for S resume execution of the program, at least for a certain distance.

S

Stop: don't execute any more of the program, but wait for more Edebug commands (edebug-stop).

<SPC>

Step: stop at the next stop point encountered (edebug-step-mode).

n

Next: stop at the next stop point encountered after an expression (edebug-next-mode). Also see edebug-forward-sexp in Jumping.

t

Trace: pause (normally one second) at each Edebug stop point (edebug-trace-mode).

T

Rapid trace: update the display at each stop point, but don't actually pause (edebug-Trace-fast-mode).

g

Go: run until the next breakpoint (edebug-go-mode). See Breakpoints.

c

Continue: pause one second at each breakpoint, and then continue (edebug-continue-mode).

C

Rapid continue: move point to each breakpoint, but don't pause (edebug-Continue-fast-mode).

G

Go non-stop: ignore breakpoints (edebug-Go-nonstop-mode). You can still stop the program by typing S, or any editing command.

In general, the execution modes earlier in the above list run the program more slowly or stop sooner than the modes later in the list.

When you enter a new Edebug level, Edebug will normally stop at the first instrumented function it encounters. If you prefer to stop only at a break point, or not at all (for example, when gathering coverage data), change the value of edebug-initial-mode from its default step to go, or Go-nonstop, or one of its other values (see Edebug Options). You can do this readily with C-x C-a C-m (edebug-set-initial-mode):

Note that you may reenter the same Edebug level several times if, for example, an instrumented function is called several times from one command.

While executing or tracing, you can interrupt the execution by typing any Edebug command. Edebug stops the program at the next stop point and then executes the command you typed. For example, typing t during execution switches to trace mode at the next stop point. You can use S to stop execution without doing anything else.

If your function happens to read input, a character you type intending to interrupt execution may be read by the function instead. You can avoid such unintended results by paying attention to when your program wants input.

Keyboard macros containing the commands in this section do not completely work: exiting from Edebug, to resume the program, loses track of the keyboard macro. This is not easy to fix. Also, defining or executing a keyboard macro outside of Edebug does not affect commands inside Edebug. This is usually an advantage. See also the edebug-continue-kbd-macro option in Edebug Options.

17.2.4 Jumping

The commands described in this section execute until they reach a specified location. All except i make a temporary breakpoint to establish the place to stop, then switch to go mode. Any other breakpoint reached before the intended stop point will also stop execution. See Breakpoints, for the details on breakpoints.

These commands may fail to work as expected in case of nonlocal exit, as that can bypass the temporary breakpoint where you expected the program to stop.

h

Proceed to the stop point near where point is (edebug-goto-here).

f

Run the program for one expression (edebug-forward-sexp).

o

Run the program until the end of the containing sexp (edebug-step-out).

i

Step into the function or macro called by the form after point (edebug-step-in).

The h command proceeds to the stop point at or after the current location of point, using a temporary breakpoint.

The f command runs the program forward over one expression. More precisely, it sets a temporary breakpoint at the position that forward-sexp would reach, then executes in go mode so that the program will stop at breakpoints.

With a prefix argument n, the temporary breakpoint is placed n sexps beyond point. If the containing list ends before n more elements, then the place to stop is after the containing expression.

You must check that the position forward-sexp finds is a place that the program will really get to. In cond, for example, this may not be true.

For flexibility, the f command does forward-sexp starting at point, rather than at the stop point. If you want to execute one expression from the current stop point, first type w (edebug-where) to move point there, and then type f.

The o command continues out of an expression. It places a temporary breakpoint at the end of the sexp containing point. If the containing sexp is a function definition itself, o continues until just before the last sexp in the definition. If that is where you are now, it returns from the function and then stops. In other words, this command does not exit the currently executing function unless you are positioned after the last sexp.

The i command steps into the function or macro called by the list form after point, and stops at its first stop point. Note that the form need not be the one about to be evaluated. But if the form is a function call about to be evaluated, remember to use this command before any of the arguments are evaluated, since otherwise it will be too late.

The i command instruments the function or macro it's supposed to step into, if it isn't instrumented already. This is convenient, but keep in mind that the function or macro remains instrumented unless you explicitly arrange to deinstrument it.

17.2.5 Miscellaneous Edebug Commands

Some miscellaneous Edebug commands are described here.

?

Display the help message for Edebug (edebug-help).

C-]

Abort one level back to the previous command level (abort-recursive-edit).

q

Return to the top level editor command loop (top-level). This exits all recursive editing levels, including all levels of Edebug activity. However, instrumented code protected with unwind-protect or condition-case forms may resume debugging.

Q

Like q, but don't stop even for protected code (edebug-top-level-nonstop).

r

Redisplay the most recently known expression result in the echo area (edebug-previous-result).

d

Display a backtrace, excluding Edebug's own functions for clarity (edebug-backtrace).

You cannot use debugger commands in the backtrace buffer in Edebug as you would in the standard debugger.

The backtrace buffer is killed automatically when you continue execution.

You can invoke commands from Edebug that activate Edebug again recursively. Whenever Edebug is active, you can quit to the top level with q or abort one recursive edit level with C-]. You can display a backtrace of all the pending evaluations with d.

17.2.6 Breaks

Edebug's step mode stops execution when the next stop point is reached. There are three other ways to stop Edebug execution once it has started: breakpoints, the global break condition, and source breakpoints.

• Breakpoints: Breakpoints at stop points.
• Global Break Condition: Breaking on an event.
• Source Breakpoints: Embedding breakpoints in source code.

17.2.6.1 Edebug Breakpoints

While using Edebug, you can specify breakpoints in the program you are testing: these are places where execution should stop. You can set a breakpoint at any stop point, as defined in Using Edebug. For setting and unsetting breakpoints, the stop point that is affected is the first one at or after point in the source code buffer. Here are the Edebug commands for breakpoints:

b

Set a breakpoint at the stop point at or after point (edebug-set-breakpoint). If you use a prefix argument, the breakpoint is temporary—it turns off the first time it stops the program.

u

Unset the breakpoint (if any) at the stop point at or after point (edebug-unset-breakpoint).

x condition <RET>

Set a conditional breakpoint which stops the program only if evaluating condition produces a non-nil value (edebug-set-conditional-breakpoint). With a prefix argument, the breakpoint is temporary.

B

Move point to the next breakpoint in the current definition (edebug-next-breakpoint).

While in Edebug, you can set a breakpoint with b and unset one with u. First move point to the Edebug stop point of your choice, then type b or u to set or unset a breakpoint there. Unsetting a breakpoint where none has been set has no effect.

Re-evaluating or reinstrumenting a definition removes all of its previous breakpoints.

A conditional breakpoint tests a condition each time the program gets there. Any errors that occur as a result of evaluating the condition are ignored, as if the result were nil. To set a conditional breakpoint, use x, and specify the condition expression in the minibuffer. Setting a conditional breakpoint at a stop point that has a previously established conditional breakpoint puts the previous condition expression in the minibuffer so you can edit it.

You can make a conditional or unconditional breakpoint temporary by using a prefix argument with the command to set the breakpoint. When a temporary breakpoint stops the program, it is automatically unset.

Edebug always stops or pauses at a breakpoint, except when the Edebug mode is Go-nonstop. In that mode, it ignores breakpoints entirely.

To find out where your breakpoints are, use the B command, which moves point to the next breakpoint following point, within the same function, or to the first breakpoint if there are no following breakpoints. This command does not continue execution—it just moves point in the buffer.

17.2.6.2 Global Break Condition

A global break condition stops execution when a specified condition is satisfied, no matter where that may occur. Edebug evaluates the global break condition at every stop point; if it evaluates to a non-nil value, then execution stops or pauses depending on the execution mode, as if a breakpoint had been hit. If evaluating the condition gets an error, execution does not stop.

The condition expression is stored in edebug-global-break-condition. You can specify a new expression using the X command from the source code buffer while Edebug is active, or using C-x X X from any buffer at any time, as long as Edebug is loaded (edebug-set-global-break-condition).

The global break condition is the simplest way to find where in your code some event occurs, but it makes code run much more slowly. So you should reset the condition to nil when not using it.

17.2.6.3 Source Breakpoints

All breakpoints in a definition are forgotten each time you reinstrument it. If you wish to make a breakpoint that won't be forgotten, you can write a source breakpoint, which is simply a call to the function edebug in your source code. You can, of course, make such a call conditional. For example, in the fac function, you can insert the first line as shown below, to stop when the argument reaches zero:

     (defun fac (n)
       (if (= n 0) (edebug))
       (if (< 0 n)
           (* n (fac (1- n)))
         1))

When the fac definition is instrumented and the function is called, the call to edebug acts as a breakpoint. Depending on the execution mode, Edebug stops or pauses there.

If no instrumented code is being executed when edebug is called, that function calls debug.

17.2.7 Trapping Errors

Emacs normally displays an error message when an error is signaled and not handled with condition-case. While Edebug is active and executing instrumented code, it normally responds to all unhandled errors. You can customize this with the options edebug-on-error and edebug-on-quit; see Edebug Options.

When Edebug responds to an error, it shows the last stop point encountered before the error. This may be the location of a call to a function which was not instrumented, and within which the error actually occurred. For an unbound variable error, the last known stop point might be quite distant from the offending variable reference. In that case, you might want to display a full backtrace (see Edebug Misc).

If you change debug-on-error or debug-on-quit while Edebug is active, these changes will be forgotten when Edebug becomes inactive. Furthermore, during Edebug's recursive edit, these variables are bound to the values they had outside of Edebug.

17.2.8 Edebug Views

These Edebug commands let you view aspects of the buffer and window status as they were before entry to Edebug. The outside window configuration is the collection of windows and contents that were in effect outside of Edebug.

v

Switch to viewing the outside window configuration (edebug-view-outside). Type C-x X w to return to Edebug.

p

Temporarily display the outside current buffer with point at its outside position (edebug-bounce-point), pausing for one second before returning to Edebug. With a prefix argument n, pause for n seconds instead.

w

Move point back to the current stop point in the source code buffer (edebug-where).

If you use this command in a different window displaying the same buffer, that window will be used instead to display the current definition in the future.

W

Toggle whether Edebug saves and restores the outside window configuration (edebug-toggle-save-windows).

With a prefix argument, W only toggles saving and restoring of the selected window. To specify a window that is not displaying the source code buffer, you must use C-x X W from the global keymap.

You can view the outside window configuration with v or just bounce to the point in the current buffer with p, even if it is not normally displayed.

After moving point, you may wish to jump back to the stop point. You can do that with w from a source code buffer. You can jump back to the stop point in the source code buffer from any buffer using C-x X w.

Each time you use W to turn saving off, Edebug forgets the saved outside window configuration—so that even if you turn saving back on, the current window configuration remains unchanged when you next exit Edebug (by continuing the program). However, the automatic redisplay of *edebug* and *edebug-trace* may conflict with the buffers you wish to see unless you have enough windows open.

17.2.9 Evaluation

While within Edebug, you can evaluate expressions as if Edebug were not running. Edebug tries to be invisible to the expression's evaluation and printing. Evaluation of expressions that cause side effects will work as expected, except for changes to data that Edebug explicitly saves and restores. See The Outside Context, for details on this process.

e exp <RET>

Evaluate expression exp in the context outside of Edebug (edebug-eval-expression). That is, Edebug tries to minimize its interference with the evaluation.

M-: exp <RET>

Evaluate expression exp in the context of Edebug itself (eval-expression).

C-x C-e

Evaluate the expression before point, in the context outside of Edebug (edebug-eval-last-sexp).

Edebug supports evaluation of expressions containing references to lexically bound symbols created by the following constructs in cl.el: lexical-let, macrolet, and symbol-macrolet.

17.2.10 Evaluation List Buffer

You can use the evaluation list buffer, called *edebug*, to evaluate expressions interactively. You can also set up the evaluation list of expressions to be evaluated automatically each time Edebug updates the display.

E

Switch to the evaluation list buffer *edebug* (edebug-visit-eval-list).

In the *edebug* buffer you can use the commands of Lisp Interaction mode (see Lisp Interaction) as well as these special commands:

C-j

Evaluate the expression before point, in the outside context, and insert the value in the buffer (edebug-eval-print-last-sexp).

C-x C-e

Evaluate the expression before point, in the context outside of Edebug (edebug-eval-last-sexp).

C-c C-u

Build a new evaluation list from the contents of the buffer (edebug-update-eval-list).

C-c C-d

Delete the evaluation list group that point is in (edebug-delete-eval-item).

C-c C-w

Switch back to the source code buffer at the current stop point (edebug-where).

You can evaluate expressions in the evaluation list window with C-j or C-x C-e, just as you would in *scratch*; but they are evaluated in the context outside of Edebug.

The expressions you enter interactively (and their results) are lost when you continue execution; but you can set up an evaluation list consisting of expressions to be evaluated each time execution stops.

To do this, write one or more evaluation list groups in the evaluation list buffer. An evaluation list group consists of one or more Lisp expressions. Groups are separated by comment lines.

The command C-c C-u (edebug-update-eval-list) rebuilds the evaluation list, scanning the buffer and using the first expression of each group. (The idea is that the second expression of the group is the value previously computed and displayed.)

Each entry to Edebug redisplays the evaluation list by inserting each expression in the buffer, followed by its current value. It also inserts comment lines so that each expression becomes its own group. Thus, if you type C-c C-u again without changing the buffer text, the evaluation list is effectively unchanged.

If an error occurs during an evaluation from the evaluation list, the error message is displayed in a string as if it were the result. Therefore, expressions using variables that are not currently valid do not interrupt your debugging.

Here is an example of what the evaluation list window looks like after several expressions have been added to it:

     (current-buffer)
     #<buffer *scratch*>
     ;---------------------------------------------------------------
     (selected-window)
     #<window 16 on *scratch*>
     ;---------------------------------------------------------------
     (point)
     196
     ;---------------------------------------------------------------
     bad-var
     "Symbol's value as variable is void: bad-var"
     ;---------------------------------------------------------------
     (recursion-depth)
     0
     ;---------------------------------------------------------------
     this-command
     eval-last-sexp
     ;---------------------------------------------------------------

To delete a group, move point into it and type C-c C-d, or simply delete the text for the group and update the evaluation list with C-c C-u. To add a new expression to the evaluation list, insert the expression at a suitable place, insert a new comment line, then type C-c C-u. You need not insert dashes in the comment line—its contents don't matter.

After selecting *edebug*, you can return to the source code buffer with C-c C-w. The *edebug* buffer is killed when you continue execution, and recreated next time it is needed.

17.2.11 Printing in Edebug

If an expression in your program produces a value containing circular list structure, you may get an error when Edebug attempts to print it.

One way to cope with circular structure is to set print-length or print-level to truncate the printing. Edebug does this for you; it binds print-length and print-level to the values of the variables edebug-print-length and edebug-print-level (so long as they have non-nil values). See Output Variables.

You can also print circular structures and structures that share elements more informatively by binding print-circle to a non-nil value.

Here is an example of code that creates a circular structure:

     (setq a '(x y))
     (setcar a a)

Custom printing prints this as ‘Result: #1=(#1# y)’. The ‘#1=’ notation labels the structure that follows it with the label ‘1’, and the ‘#1#’ notation references the previously labeled structure. This notation is used for any shared elements of lists or vectors.

Other programs can also use custom printing; see cust-print.el for details.

17.2.12 Trace Buffer

Edebug can record an execution trace, storing it in a buffer named *edebug-trace*. This is a log of function calls and returns, showing the function names and their arguments and values. To enable trace recording, set edebug-trace to a non-nil value.

Making a trace buffer is not the same thing as using trace execution mode (see Edebug Execution Modes).

When trace recording is enabled, each function entry and exit adds lines to the trace buffer. A function entry record consists of ‘::::{’, followed by the function name and argument values. A function exit record consists of ‘::::}’, followed by the function name and result of the function.

The number of ‘:’s in an entry shows its recursion depth. You can use the braces in the trace buffer to find the matching beginning or end of function calls.

You can customize trace recording for function entry and exit by redefining the functions edebug-print-trace-before and edebug-print-trace-after.

edebug-tracing and edebug-trace insert lines in the trace buffer whenever they are called, even if Edebug is not active. Adding text to the trace buffer also scrolls its window to show the last lines inserted.

17.2.13 Coverage Testing

Edebug provides rudimentary coverage testing and display of execution frequency.

Coverage testing works by comparing the result of each expression with the previous result; each form in the program is considered covered if it has returned two different values since you began testing coverage in the current Emacs session. Thus, to do coverage testing on your program, execute it under various conditions and note whether it behaves correctly; Edebug will tell you when you have tried enough different conditions that each form has returned two different values.

Coverage testing makes execution slower, so it is only done if edebug-test-coverage is non-nil. Frequency counting is performed for all executions of an instrumented function, even if the execution mode is Go-nonstop, and regardless of whether coverage testing is enabled.

Use C-x X = (edebug-display-freq-count) to display both the coverage information and the frequency counts for a definition. Just = (edebug-temp-display-freq-count) displays the same information temporarily, only until you type another key.

For example, after evaluating (fac 5) with a source breakpoint, and setting edebug-test-coverage to t, when the breakpoint is reached, the frequency data looks like this:

     (defun fac (n)
       (if (= n 0) (edebug))
     ;#6           1      = =5
       (if (< 0 n)
     ;#5         =
           (* n (fac (1- n)))
     ;#    5               0
         1))
     ;#   0

The comment lines show that fac was called 6 times. The first if statement returned 5 times with the same result each time; the same is true of the condition on the second if. The recursive call of fac did not return at all.

17.2.14 The Outside Context

Edebug tries to be transparent to the program you are debugging, but it does not succeed completely. Edebug also tries to be transparent when you evaluate expressions with e or with the evaluation list buffer, by temporarily restoring the outside context. This section explains precisely what context Edebug restores, and how Edebug fails to be completely transparent.

• Checking Whether to Stop: When Edebug decides what to do.
• Edebug Display Update: When Edebug updates the display.
• Edebug Recursive Edit: When Edebug stops execution.

17.2.14.1 Checking Whether to Stop

Whenever Edebug is entered, it needs to save and restore certain data before even deciding whether to make trace information or stop the program.

• max-lisp-eval-depth and max-specpdl-size are both increased to reduce Edebug's impact on the stack. You could, however, still run out of stack space when using Edebug.
• The state of keyboard macro execution is saved and restored. While Edebug is active, executing-kbd-macro is bound to nil unless edebug-continue-kbd-macro is non-nil.

17.2.14.2 Edebug Display Update

When Edebug needs to display something (e.g., in trace mode), it saves the current window configuration from outside Edebug (see Window Configurations). When you exit Edebug, it restores the previous window configuration.

Emacs redisplays only when it pauses. Usually, when you continue execution, the program re-enters Edebug at a breakpoint or after stepping, without pausing or reading input in between. In such cases, Emacs never gets a chance to redisplay the outside configuration. Consequently, what you see is the same window configuration as the last time Edebug was active, with no interruption.

Entry to Edebug for displaying something also saves and restores the following data (though some of them are deliberately not restored if an error or quit signal occurs).

• Which buffer is current, and the positions of point and the mark in the current buffer, are saved and restored.
• The outside window configuration is saved and restored if edebug-save-windows is non-nil (see Edebug Options).

  The window configuration is not restored on error or quit, but the outside selected window is reselected even on error or quit in case a save-excursion is active. If the value of edebug-save-windows is a list, only the listed windows are saved and restored.

  The window start and horizontal scrolling of the source code buffer are not restored, however, so that the display remains coherent within Edebug.

• The value of point in each displayed buffer is saved and restored if edebug-save-displayed-buffer-points is non-nil.
• The variables overlay-arrow-position and overlay-arrow-string are saved and restored, so you can safely invoke Edebug from the recursive edit elsewhere in the same buffer.
• cursor-in-echo-area is locally bound to nil so that the cursor shows up in the window.

17.2.14.3 Edebug Recursive Edit

When Edebug is entered and actually reads commands from the user, it saves (and later restores) these additional data:

• The current match data. See Match Data.
• The variables last-command, this-command, last-command-event, last-input-event, last-event-frame, last-nonmenu-event, and track-mouse. Commands in Edebug do not affect these variables outside of Edebug.

  Executing commands within Edebug can change the key sequence that would be returned by this-command-keys, and there is no way to reset the key sequence from Lisp.

  Edebug cannot save and restore the value of unread-command-events. Entering Edebug while this variable has a nontrivial value can interfere with execution of the program you are debugging.

• Complex commands executed while in Edebug are added to the variable command-history. In rare cases this can alter execution.
• Within Edebug, the recursion depth appears one deeper than the recursion depth outside Edebug. This is not true of the automatically updated evaluation list window.
• standard-output and standard-input are bound to nil by the recursive-edit, but Edebug temporarily restores them during evaluations.
• The state of keyboard macro definition is saved and restored. While Edebug is active, defining-kbd-macro is bound to edebug-continue-kbd-macro.

17.2.15 Edebug and Macros

To make Edebug properly instrument expressions that call macros, some extra care is needed. This subsection explains the details.

• Instrumenting Macro Calls: The basic problem.
• Specification List: How to specify complex patterns of evaluation.
• Backtracking: What Edebug does when matching fails.
• Specification Examples: To help understand specifications.

17.2.15.1 Instrumenting Macro Calls

When Edebug instruments an expression that calls a Lisp macro, it needs additional information about the macro to do the job properly. This is because there is no a-priori way to tell which subexpressions of the macro call are forms to be evaluated. (Evaluation may occur explicitly in the macro body, or when the resulting expansion is evaluated, or any time later.)

Therefore, you must define an Edebug specification for each macro that Edebug will encounter, to explain the format of calls to that macro. To do this, add a debug declaration to the macro definition. Here is a simple example that shows the specification for the for example macro (see Argument Evaluation).

     (defmacro for (var from init to final do &rest body)
       "Execute a simple \"for\" loop.
     For example, (for i from 1 to 10 do (print i))."
       (declare (debug (symbolp "from" form "to" form "do" &rest form)))
       ...)

The Edebug specification says which parts of a call to the macro are forms to be evaluated. For simple macros, the specification often looks very similar to the formal argument list of the macro definition, but specifications are much more general than macro arguments. See Defining Macros, for more explanation of the declare form.

Take care to ensure that the specifications are known to Edebug when you instrument code. If you are instrumenting a function from a file that uses eval-when-compile to require another file containing macro definitions, you may need to explicitly load that file.

You can also define an edebug specification for a macro separately from the macro definition with def-edebug-spec. Adding debug declarations is preferred, and more convenient, for macro definitions in Lisp, but def-edebug-spec makes it possible to define Edebug specifications for special forms implemented in C.

Here is a table of the possibilities for specification and how each directs processing of arguments.

t

All arguments are instrumented for evaluation.

0

None of the arguments is instrumented.

a symbol

The symbol must have an Edebug specification, which is used instead. This indirection is repeated until another kind of specification is found. This allows you to inherit the specification from another macro.

a list

The elements of the list describe the types of the arguments of a calling form. The possible elements of a specification list are described in the following sections.

If a macro has no Edebug specification, neither through a debug declaration nor through a def-edebug-spec call, the variable edebug-eval-macro-args comes into play.

17.2.15.2 Specification List

A specification list is required for an Edebug specification if some arguments of a macro call are evaluated while others are not. Some elements in a specification list match one or more arguments, but others modify the processing of all following elements. The latter, called specification keywords, are symbols beginning with ‘&’ (such as &optional).

A specification list may contain sublists, which match arguments that are themselves lists, or it may contain vectors used for grouping. Sublists and groups thus subdivide the specification list into a hierarchy of levels. Specification keywords apply only to the remainder of the sublist or group they are contained in.

When a specification list involves alternatives or repetition, matching it against an actual macro call may require backtracking. For more details, see Backtracking.

Edebug specifications provide the power of regular expression matching, plus some context-free grammar constructs: the matching of sublists with balanced parentheses, recursive processing of forms, and recursion via indirect specifications.

Here's a table of the possible elements of a specification list, with their meanings (see Specification Examples, for the referenced examples):

sexp

A single unevaluated Lisp object, which is not instrumented.

form

A single evaluated expression, which is instrumented.

place

A generalized variable. See Generalized Variables.

body

Short for &rest form. See &rest below.

function-form

A function form: either a quoted function symbol, a quoted lambda expression, or a form (that should evaluate to a function symbol or lambda expression). This is useful when an argument that's a lambda expression might be quoted with quote rather than function, since it instruments the body of the lambda expression either way.

lambda-expr

A lambda expression with no quoting.

&optional

All following elements in the specification list are optional; as soon as one does not match, Edebug stops matching at this level.

To make just a few elements optional, followed by non-optional elements, use [&optional specs...]. To specify that several elements must all match or none, use &optional [specs...]. See the defun example.

&rest

All following elements in the specification list are repeated zero or more times. In the last repetition, however, it is not a problem if the expression runs out before matching all of the elements of the specification list.

To repeat only a few elements, use [&rest specs...]. To specify several elements that must all match on every repetition, use &rest [specs...].

&or

Each of the following elements in the specification list is an alternative. One of the alternatives must match, or the &or specification fails.

Each list element following &or is a single alternative. To group two or more list elements as a single alternative, enclose them in [...].

&not

Each of the following elements is matched as alternatives as if by using &or, but if any of them match, the specification fails. If none of them match, nothing is matched, but the &not specification succeeds.

&define

Indicates that the specification is for a defining form. The defining form itself is not instrumented (that is, Edebug does not stop before and after the defining form), but forms inside it typically will be instrumented. The &define keyword should be the first element in a list specification.

nil

This is successful when there are no more arguments to match at the current argument list level; otherwise it fails. See sublist specifications and the backquote example.

gate

No argument is matched but backtracking through the gate is disabled while matching the remainder of the specifications at this level. This is primarily used to generate more specific syntax error messages. See Backtracking, for more details. Also see the let example.

other-symbol

Any other symbol in a specification list may be a predicate or an indirect specification.

If the symbol has an Edebug specification, this indirect specification should be either a list specification that is used in place of the symbol, or a function that is called to process the arguments. The specification may be defined with def-edebug-spec just as for macros. See the defun example.

Otherwise, the symbol should be a predicate. The predicate is called with the argument, and if the predicate returns nil, the specification fails and the argument is not instrumented.

Some suitable predicates include symbolp, integerp, stringp, vectorp, and atom.

[elements...]

A vector of elements groups the elements into a single group specification. Its meaning has nothing to do with vectors.

"string"

The argument should be a symbol named string. This specification is equivalent to the quoted symbol, 'symbol, where the name of symbol is the string, but the string form is preferred.

(vector elements...)

The argument should be a vector whose elements must match the elements in the specification. See the backquote example.

(elements...)

Any other list is a sublist specification and the argument must be a list whose elements match the specification elements.

A sublist specification may be a dotted list and the corresponding list argument may then be a dotted list. Alternatively, the last cdr of a dotted list specification may be another sublist specification (via a grouping or an indirect specification, e.g., (spec . [(more specs...)])) whose elements match the non-dotted list arguments. This is useful in recursive specifications such as in the backquote example. Also see the description of a nil specification above for terminating such recursion.

Note that a sublist specification written as (specs . nil) is equivalent to (specs), and (specs . (sublist-elements...)) is equivalent to (specs sublist-elements...).

Here is a list of additional specifications that may appear only after &define. See the defun example.

name

The argument, a symbol, is the name of the defining form.

A defining form is not required to have a name field; and it may have multiple name fields.

:name

This construct does not actually match an argument. The element following :name should be a symbol; it is used as an additional name component for the definition. You can use this to add a unique, static component to the name of the definition. It may be used more than once.

arg

The argument, a symbol, is the name of an argument of the defining form. However, lambda-list keywords (symbols starting with ‘&’) are not allowed.

lambda-list

This matches a lambda list—the argument list of a lambda expression.

def-body

The argument is the body of code in a definition. This is like body, described above, but a definition body must be instrumented with a different Edebug call that looks up information associated with the definition. Use def-body for the highest level list of forms within the definition.

def-form

The argument is a single, highest-level form in a definition. This is like def-body, except it is used to match a single form rather than a list of forms. As a special case, def-form also means that tracing information is not output when the form is executed. See the interactive example.

17.2.15.3 Backtracking in Specifications

If a specification fails to match at some point, this does not necessarily mean a syntax error will be signaled; instead, backtracking will take place until all alternatives have been exhausted. Eventually every element of the argument list must be matched by some element in the specification, and every required element in the specification must match some argument.

When a syntax error is detected, it might not be reported until much later, after higher-level alternatives have been exhausted, and with the point positioned further from the real error. But if backtracking is disabled when an error occurs, it can be reported immediately. Note that backtracking is also reenabled automatically in several situations; when a new alternative is established by &optional, &rest, or &or, or at the start of processing a sublist, group, or indirect specification. The effect of enabling or disabling backtracking is limited to the remainder of the level currently being processed and lower levels.

Backtracking is disabled while matching any of the form specifications (that is, form, body, def-form, and def-body). These specifications will match any form so any error must be in the form itself rather than at a higher level.

Backtracking is also disabled after successfully matching a quoted symbol or string specification, since this usually indicates a recognized construct. But if you have a set of alternative constructs that all begin with the same symbol, you can usually work around this constraint by factoring the symbol out of the alternatives, e.g., ["foo" &or [first case] [second case] ...].

Most needs are satisfied by these two ways that backtracking is automatically disabled, but occasionally it is useful to explicitly disable backtracking by using the gate specification. This is useful when you know that no higher alternatives could apply. See the example of the let specification.

17.2.15.4 Specification Examples

It may be easier to understand Edebug specifications by studying the examples provided here.

A let special form has a sequence of bindings and a body. Each of the bindings is either a symbol or a sublist with a symbol and optional expression. In the specification below, notice the gate inside of the sublist to prevent backtracking once a sublist is found.

     (def-edebug-spec let
       ((&rest
         &or symbolp (gate symbolp &optional form))
        body))

Edebug uses the following specifications for defun and the associated argument list and interactive specifications. It is necessary to handle interactive forms specially since an expression argument is actually evaluated outside of the function body. (The specification for defmacro is very similar to that for defun, but allows for the declare statement.)

     (def-edebug-spec defun
       (&define name lambda-list
                [&optional stringp]   ; Match the doc string, if present.
                [&optional ("interactive" interactive)]
                def-body))
     
     (def-edebug-spec lambda-list
       (([&rest arg]
         [&optional ["&optional" arg &rest arg]]
         &optional ["&rest" arg]
         )))
     
     (def-edebug-spec interactive
       (&optional &or stringp def-form))    ; Notice: def-form

The specification for backquote below illustrates how to match dotted lists and use nil to terminate recursion. It also illustrates how components of a vector may be matched. (The actual specification defined by Edebug is a little different, and does not support dotted lists because doing so causes very deep recursion that could fail.)

     (def-edebug-spec \` (backquote-form))   ; Alias just for clarity.
     
     (def-edebug-spec backquote-form
       (&or ([&or "," ",@"] &or ("quote" backquote-form) form)
            (backquote-form . [&or nil backquote-form])
            (vector &rest backquote-form)
            sexp))

17.2.16 Edebug Options

These options affect the behavior of Edebug:

If you change the values of edebug-on-error or edebug-on-quit while Edebug is active, their values won't be used until the next time Edebug is invoked via a new command.

17.3 Debugging Invalid Lisp Syntax

The Lisp reader reports invalid syntax, but cannot say where the real problem is. For example, the error ‘End of file during parsing’ in evaluating an expression indicates an excess of open parentheses (or square brackets). The reader detects this imbalance at the end of the file, but it cannot figure out where the close parenthesis should have been. Likewise, ‘Invalid read syntax: ")"’ indicates an excess close parenthesis or missing open parenthesis, but does not say where the missing parenthesis belongs. How, then, to find what to change?

If the problem is not simply an imbalance of parentheses, a useful technique is to try C-M-e at the beginning of each defun, and see if it goes to the place where that defun appears to end. If it does not, there is a problem in that defun.

However, unmatched parentheses are the most common syntax errors in Lisp, and we can give further advice for those cases. (In addition, just moving point through the code with Show Paren mode enabled might find the mismatch.)

• Excess Open: How to find a spurious open paren or missing close.
• Excess Close: How to find a spurious close paren or missing open.

17.3.1 Excess Open Parentheses

The first step is to find the defun that is unbalanced. If there is an excess open parenthesis, the way to do this is to go to the end of the file and type C-u C-M-u. This will move you to the beginning of the first defun that is unbalanced.

The next step is to determine precisely what is wrong. There is no way to be sure of this except by studying the program, but often the existing indentation is a clue to where the parentheses should have been. The easiest way to use this clue is to reindent with C-M-q and see what moves. But don't do this yet! Keep reading, first.

Before you do this, make sure the defun has enough close parentheses. Otherwise, C-M-q will get an error, or will reindent all the rest of the file until the end. So move to the end of the defun and insert a close parenthesis there. Don't use C-M-e to move there, since that too will fail to work until the defun is balanced.

Now you can go to the beginning of the defun and type C-M-q. Usually all the lines from a certain point to the end of the function will shift to the right. There is probably a missing close parenthesis, or a superfluous open parenthesis, near that point. (However, don't assume this is true; study the code to make sure.) Once you have found the discrepancy, undo the C-M-q with C-_, since the old indentation is probably appropriate to the intended parentheses.

After you think you have fixed the problem, use C-M-q again. If the old indentation actually fit the intended nesting of parentheses, and you have put back those parentheses, C-M-q should not change anything.

17.3.2 Excess Close Parentheses

To deal with an excess close parenthesis, first go to the beginning of the file, then type C-u -1 C-M-u to find the end of the first unbalanced defun.

Then find the actual matching close parenthesis by typing C-M-f at the beginning of that defun. This will leave you somewhere short of the place where the defun ought to end. It is possible that you will find a spurious close parenthesis in that vicinity.

If you don't see a problem at that point, the next thing to do is to type C-M-q at the beginning of the defun. A range of lines will probably shift left; if so, the missing open parenthesis or spurious close parenthesis is probably near the first of those lines. (However, don't assume this is true; study the code to make sure.) Once you have found the discrepancy, undo the C-M-q with C-_, since the old indentation is probably appropriate to the intended parentheses.

After you think you have fixed the problem, use C-M-q again. If the old indentation actually fits the intended nesting of parentheses, and you have put back those parentheses, C-M-q should not change anything.

17.4 Test Coverage

You can do coverage testing for a file of Lisp code by loading the testcover library and using the command M-x testcover-start <RET> file <RET> to instrument the code. Then test your code by calling it one or more times. Then use the command M-x testcover-mark-all to display colored highlights on the code to show where coverage is insufficient. The command M-x testcover-next-mark will move point forward to the next highlighted spot.

Normally, a red highlight indicates the form was never completely evaluated; a brown highlight means it always evaluated to the same value (meaning there has been little testing of what is done with the result). However, the red highlight is skipped for forms that can't possibly complete their evaluation, such as error. The brown highlight is skipped for forms that are expected to always evaluate to the same value, such as (setq x 14).

For difficult cases, you can add do-nothing macros to your code to give advice to the test coverage tool.

Edebug also has a coverage testing feature (see Coverage Testing). These features partly duplicate each other, and it would be cleaner to combine them.

17.5 Profiling

If your program is working correctly, but you want to make it run more quickly or efficiently, the first thing to do is profile your code so that you know how it is using resources. If you find that one particular function is responsible for a significant portion of the runtime, you can start looking for ways to optimize that piece.

Emacs has built-in support for this. To begin profiling, type M-x profiler-start. You can choose to profile by processor usage, memory usage, or both. After doing some work, type M-x profiler-report to display a summary buffer for each resource that you chose to profile. The names of the report buffers include the times at which the reports were generated, so you can generate another report later on without erasing previous results. When you have finished profiling, type M-x profiler-stop (there is a small overhead associated with profiling).

The profiler report buffer shows, on each line, a function that was called, followed by how much resource (processor or memory) it used in absolute and percentage times since profiling started. If a given line has a ‘+’ symbol at the left-hand side, you can expand that line by typing <RET>, in order to see the function(s) called by the higher-level function. Use a prefix argument (<C-u RET>) to see the whole call tree below a function. Pressing <RET> again will collapse back to the original state.

Press j or mouse-2 to jump to the definition of a function. Press d to view a function's documentation. You can save a profile to a file using C-x C-w. You can compare two profiles using =.

The elp library offers an alternative approach. See the file elp.el for instructions.

You can check the speed of individual Emacs Lisp forms using the benchmark library. See the functions benchmark-run and benchmark-run-compiled in benchmark.el.

To profile Emacs at the level of its C code, you can build it using the --enable-profiling option of configure. When Emacs exits, it generates a file gmon.out that you can examine using the gprof utility. This feature is mainly useful for debugging Emacs. It actually stops the Lisp-level M-x profiler-... commands described above from working.

18 Reading and Printing Lisp Objects

Printing and reading are the operations of converting Lisp objects to textual form and vice versa. They use the printed representations and read syntax described in Lisp Data Types.

This chapter describes the Lisp functions for reading and printing. It also describes streams, which specify where to get the text (if reading) or where to put it (if printing).

• Streams Intro: Overview of streams, reading and printing.
• Input Streams: Various data types that can be used as input streams.
• Input Functions: Functions to read Lisp objects from text.
• Output Streams: Various data types that can be used as output streams.
• Output Functions: Functions to print Lisp objects as text.
• Output Variables: Variables that control what the printing functions do.

18.1 Introduction to Reading and Printing

Reading a Lisp object means parsing a Lisp expression in textual form and producing a corresponding Lisp object. This is how Lisp programs get into Lisp from files of Lisp code. We call the text the read syntax of the object. For example, the text ‘(a . 5)’ is the read syntax for a cons cell whose car is a and whose cdr is the number 5.

Printing a Lisp object means producing text that represents that object—converting the object to its printed representation (see Printed Representation). Printing the cons cell described above produces the text ‘(a . 5)’.

Reading and printing are more or less inverse operations: printing the object that results from reading a given piece of text often produces the same text, and reading the text that results from printing an object usually produces a similar-looking object. For example, printing the symbol foo produces the text ‘foo’, and reading that text returns the symbol foo. Printing a list whose elements are a and b produces the text ‘(a b)’, and reading that text produces a list (but not the same list) with elements a and b.

However, these two operations are not precisely inverse to each other. There are three kinds of exceptions:

• Printing can produce text that cannot be read. For example, buffers, windows, frames, subprocesses and markers print as text that starts with ‘#’; if you try to read this text, you get an error. There is no way to read those data types.
• One object can have multiple textual representations. For example, ‘1’ and ‘01’ represent the same integer, and ‘(a b)’ and ‘(a . (b))’ represent the same list. Reading will accept any of the alternatives, but printing must choose one of them.
• Comments can appear at certain points in the middle of an object's read sequence without affecting the result of reading it.

18.2 Input Streams

Most of the Lisp functions for reading text take an input stream as an argument. The input stream specifies where or how to get the characters of the text to be read. Here are the possible types of input stream:

buffer

The input characters are read from buffer, starting with the character directly after point. Point advances as characters are read.

marker

The input characters are read from the buffer that marker is in, starting with the character directly after the marker. The marker position advances as characters are read. The value of point in the buffer has no effect when the stream is a marker.

string

The input characters are taken from string, starting at the first character in the string and using as many characters as required.

function

The input characters are generated by function, which must support two kinds of calls:

• When it is called with no arguments, it should return the next character.
• When it is called with one argument (always a character), function should save the argument and arrange to return it on the next call. This is called unreading the character; it happens when the Lisp reader reads one character too many and wants to put it back where it came from. In this case, it makes no difference what value function returns.

t

t used as a stream means that the input is read from the minibuffer. In fact, the minibuffer is invoked once and the text given by the user is made into a string that is then used as the input stream. If Emacs is running in batch mode, standard input is used instead of the minibuffer. For example,

          (message "%s" (read t))

will read a Lisp expression from standard input and print the result to standard output.

nil

nil supplied as an input stream means to use the value of standard-input instead; that value is the default input stream, and must be a non-nil input stream.

symbol

A symbol as input stream is equivalent to the symbol's function definition (if any).

Here is an example of reading from a stream that is a buffer, showing where point is located before and after:

     ---------- Buffer: foo ----------
     This-!- is the contents of foo.
     ---------- Buffer: foo ----------
     
     (read (get-buffer "foo"))
          ⇒ is
     (read (get-buffer "foo"))
          ⇒ the
     
     ---------- Buffer: foo ----------
     This is the-!- contents of foo.
     ---------- Buffer: foo ----------

Note that the first read skips a space. Reading skips any amount of whitespace preceding the significant text.

Here is an example of reading from a stream that is a marker, initially positioned at the beginning of the buffer shown. The value read is the symbol This.

     
     ---------- Buffer: foo ----------
     This is the contents of foo.
     ---------- Buffer: foo ----------
     
     (setq m (set-marker (make-marker) 1 (get-buffer "foo")))
          ⇒ #<marker at 1 in foo>
     (read m)
          ⇒ This
     m
          ⇒ #<marker at 5 in foo>   ;; Before the first space.

Here we read from the contents of a string:

     (read "(When in) the course")
          ⇒ (When in)

The following example reads from the minibuffer. The prompt is: ‘Lisp expression: ’. (That is always the prompt used when you read from the stream t.) The user's input is shown following the prompt.

     (read t)
          ⇒ 23
     ---------- Buffer: Minibuffer ----------
     Lisp expression: 23 <RET>
     ---------- Buffer: Minibuffer ----------

Finally, here is an example of a stream that is a function, named useless-stream. Before we use the stream, we initialize the variable useless-list to a list of characters. Then each call to the function useless-stream obtains the next character in the list or unreads a character by adding it to the front of the list.

     (setq useless-list (append "XY()" nil))
          ⇒ (88 89 40 41)
     
     (defun useless-stream (&optional unread)
       (if unread
           (setq useless-list (cons unread useless-list))
         (prog1 (car useless-list)
                (setq useless-list (cdr useless-list)))))
          ⇒ useless-stream

Now we read using the stream thus constructed:

     (read 'useless-stream)
          ⇒ XY
     
     useless-list
          ⇒ (40 41)

Note that the open and close parentheses remain in the list. The Lisp reader encountered the open parenthesis, decided that it ended the input, and unread it. Another attempt to read from the stream at this point would read ‘()’ and return nil.

18.3 Input Functions

This section describes the Lisp functions and variables that pertain to reading.

In the functions below, stream stands for an input stream (see the previous section). If stream is nil or omitted, it defaults to the value of standard-input.

An end-of-file error is signaled if reading encounters an unterminated list, vector, or string.

When reading or writing from the standard input/output streams of the Emacs process in batch mode, it is sometimes required to make sure any arbitrary binary data will be read/written verbatim, and/or that no translation of newlines to or from CR-LF pairs is performed. This issue does not exist on Posix hosts, only on MS-Windows and MS-DOS. The following function allows you to control the I/O mode of any standard stream of the Emacs process.

18.4 Output Streams

An output stream specifies what to do with the characters produced by printing. Most print functions accept an output stream as an optional argument. Here are the possible types of output stream:

buffer

The output characters are inserted into buffer at point. Point advances as characters are inserted.

marker

The output characters are inserted into the buffer that marker points into, at the marker position. The marker position advances as characters are inserted. The value of point in the buffer has no effect on printing when the stream is a marker, and this kind of printing does not move point (except that if the marker points at or before the position of point, point advances with the surrounding text, as usual).

function

The output characters are passed to function, which is responsible for storing them away. It is called with a single character as argument, as many times as there are characters to be output, and is responsible for storing the characters wherever you want to put them.

t

The output characters are displayed in the echo area.

nil

nil specified as an output stream means to use the value of standard-output instead; that value is the default output stream, and must not be nil.

symbol

A symbol as output stream is equivalent to the symbol's function definition (if any).

Many of the valid output streams are also valid as input streams. The difference between input and output streams is therefore more a matter of how you use a Lisp object, than of different types of object.

Here is an example of a buffer used as an output stream. Point is initially located as shown immediately before the ‘h’ in ‘the’. At the end, point is located directly before that same ‘h’.

     ---------- Buffer: foo ----------
     This is t-!-he contents of foo.
     ---------- Buffer: foo ----------
     
     (print "This is the output" (get-buffer "foo"))
          ⇒ "This is the output"
     
     ---------- Buffer: foo ----------
     This is t
     "This is the output"
     -!-he contents of foo.
     ---------- Buffer: foo ----------

Now we show a use of a marker as an output stream. Initially, the marker is in buffer foo, between the ‘t’ and the ‘h’ in the word ‘the’. At the end, the marker has advanced over the inserted text so that it remains positioned before the same ‘h’. Note that the location of point, shown in the usual fashion, has no effect.

     ---------- Buffer: foo ----------
     This is the -!-output
     ---------- Buffer: foo ----------
     
     (setq m (copy-marker 10))
          ⇒ #<marker at 10 in foo>
     
     (print "More output for foo." m)
          ⇒ "More output for foo."
     
     ---------- Buffer: foo ----------
     This is t
     "More output for foo."
     he -!-output
     ---------- Buffer: foo ----------
     
     m
          ⇒ #<marker at 34 in foo>

The following example shows output to the echo area:

     (print "Echo Area output" t)
          ⇒ "Echo Area output"
     ---------- Echo Area ----------
     "Echo Area output"
     ---------- Echo Area ----------

Finally, we show the use of a function as an output stream. The function eat-output takes each character that it is given and conses it onto the front of the list last-output (see Building Lists). At the end, the list contains all the characters output, but in reverse order.

     (setq last-output nil)
          ⇒ nil
     
     (defun eat-output (c)
       (setq last-output (cons c last-output)))
          ⇒ eat-output
     
     (print "This is the output" 'eat-output)
          ⇒ "This is the output"
     
     last-output
          ⇒ (10 34 116 117 112 116 117 111 32 101 104
         116 32 115 105 32 115 105 104 84 34 10)

Now we can put the output in the proper order by reversing the list:

     (concat (nreverse last-output))
          ⇒ "
     \"This is the output\"
     "

Calling concat converts the list to a string so you can see its contents more clearly.

18.5 Output Functions

This section describes the Lisp functions for printing Lisp objects—converting objects into their printed representation.

Some of the Emacs printing functions add quoting characters to the output when necessary so that it can be read properly. The quoting characters used are ‘"’ and ‘\’; they distinguish strings from symbols, and prevent punctuation characters in strings and symbols from being taken as delimiters when reading. See Printed Representation, for full details. You specify quoting or no quoting by the choice of printing function.

If the text is to be read back into Lisp, then you should print with quoting characters to avoid ambiguity. Likewise, if the purpose is to describe a Lisp object clearly for a Lisp programmer. However, if the purpose of the output is to look nice for humans, then it is usually better to print without quoting.

Lisp objects can refer to themselves. Printing a self-referential object in the normal way would require an infinite amount of text, and the attempt could cause infinite recursion. Emacs detects such recursion and prints ‘#level’ instead of recursively printing an object already being printed. For example, here ‘#0’ indicates a recursive reference to the object at level 0 of the current print operation:

     (setq foo (list nil))
          ⇒ (nil)
     (setcar foo foo)
          ⇒ (#0)

In the functions below, stream stands for an output stream. (See the previous section for a description of output streams.) If stream is nil or omitted, it defaults to the value of standard-output.

If you need to use binary I/O in batch mode, e.g., use the functions described in this section to write out arbitrary binary data or avoid conversion of newlines on non-Posix hosts, see set-binary-mode.

18.6 Variables Affecting Output

These variables are used for detecting and reporting circular and shared structure:

19 Minibuffers

A minibuffer is a special buffer that Emacs commands use to read arguments more complicated than the single numeric prefix argument. These arguments include file names, buffer names, and command names (as in M-x). The minibuffer is displayed on the bottom line of the frame, in the same place as the echo area (see The Echo Area), but only while it is in use for reading an argument.

• Intro to Minibuffers: Basic information about minibuffers.
• Text from Minibuffer: How to read a straight text string.
• Object from Minibuffer: How to read a Lisp object or expression.
• Minibuffer History: Recording previous minibuffer inputs so the user can reuse them.
• Initial Input: Specifying initial contents for the minibuffer.
• Completion: How to invoke and customize completion.
• Yes-or-No Queries: Asking a question with a simple answer.
• Multiple Queries: Asking a series of similar questions.
• Reading a Password: Reading a password from the terminal.
• Minibuffer Commands: Commands used as key bindings in minibuffers.
• Minibuffer Windows: Operating on the special minibuffer windows.
• Minibuffer Contents: How such commands access the minibuffer text.
• Recursive Mini: Whether recursive entry to minibuffer is allowed.
• Minibuffer Misc: Various customization hooks and variables.

19.1 Introduction to Minibuffers

In most ways, a minibuffer is a normal Emacs buffer. Most operations within a buffer, such as editing commands, work normally in a minibuffer. However, many operations for managing buffers do not apply to minibuffers. The name of a minibuffer always has the form ‘ *Minibuf-number*’, and it cannot be changed. Minibuffers are displayed only in special windows used only for minibuffers; these windows always appear at the bottom of a frame. (Sometimes frames have no minibuffer window, and sometimes a special kind of frame contains nothing but a minibuffer window; see Minibuffers and Frames.)

The text in the minibuffer always starts with the prompt string, the text that was specified by the program that is using the minibuffer to tell the user what sort of input to type. This text is marked read-only so you won't accidentally delete or change it. It is also marked as a field (see Fields), so that certain motion functions, including beginning-of-line, forward-word, forward-sentence, and forward-paragraph, stop at the boundary between the prompt and the actual text.

The minibuffer's window is normally a single line; it grows automatically if the contents require more space. Whilst it is active, you can explicitly resize it temporarily with the window sizing commands; it reverts to its normal size when the minibuffer is exited. When the minibuffer is not active, you can resize it permanently by using the window sizing commands in the frame's other window, or dragging the mode line with the mouse. (Due to details of the current implementation, for this to work resize-mini-windows must be nil.) If the frame contains just a minibuffer, you can change the minibuffer's size by changing the frame's size.

Use of the minibuffer reads input events, and that alters the values of variables such as this-command and last-command (see Command Loop Info). Your program should bind them around the code that uses the minibuffer, if you do not want that to change them.

Under some circumstances, a command can use a minibuffer even if there is an active minibuffer; such a minibuffer is called a recursive minibuffer. The first minibuffer is named ‘ *Minibuf-1*’. Recursive minibuffers are named by incrementing the number at the end of the name. (The names begin with a space so that they won't show up in normal buffer lists.) Of several recursive minibuffers, the innermost (or most recently entered) is the active minibuffer. We usually call this the minibuffer. You can permit or forbid recursive minibuffers by setting the variable enable-recursive-minibuffers, or by putting properties of that name on command symbols (See Recursive Mini.)

Like other buffers, a minibuffer uses a local keymap (see Keymaps) to specify special key bindings. The function that invokes the minibuffer also sets up its local map according to the job to be done. See Text from Minibuffer, for the non-completion minibuffer local maps. See Completion Commands, for the minibuffer local maps for completion.

When a minibuffer is inactive, its major mode is minibuffer-inactive-mode, with keymap minibuffer-inactive-mode-map. This is only really useful if the minibuffer is in a separate frame. See Minibuffers and Frames.

When Emacs is running in batch mode, any request to read from the minibuffer actually reads a line from the standard input descriptor that was supplied when Emacs was started. This supports only basic input: none of the special minibuffer features (history, completion, etc.) are available in batch mode.

19.2 Reading Text Strings with the Minibuffer

The most basic primitive for minibuffer input is read-from-minibuffer, which can be used to read either a string or a Lisp object in textual form. The function read-regexp is used for reading regular expressions (see Regular Expressions), which are a special kind of string. There are also specialized functions for reading commands, variables, file names, etc. (see Completion).

In most cases, you should not call minibuffer input functions in the middle of a Lisp function. Instead, do all minibuffer input as part of reading the arguments for a command, in the interactive specification. See Defining Commands.

19.3 Reading Lisp Objects with the Minibuffer

This section describes functions for reading Lisp objects with the minibuffer.

19.4 Minibuffer History

A minibuffer history list records previous minibuffer inputs so the user can reuse them conveniently. It is a variable whose value is a list of strings (previous inputs), most recent first.

There are many separate minibuffer history lists, used for different kinds of inputs. It's the Lisp programmer's job to specify the right history list for each use of the minibuffer.

You specify a minibuffer history list with the optional history argument to read-from-minibuffer or completing-read. Here are the possible values for it:

variable

Use variable (a symbol) as the history list.

(variable . startpos)

Use variable (a symbol) as the history list, and assume that the initial history position is startpos (a nonnegative integer).

Specifying 0 for startpos is equivalent to just specifying the symbol variable. previous-history-element will display the most recent element of the history list in the minibuffer. If you specify a positive startpos, the minibuffer history functions behave as if (elt variable (1- startpos)) were the history element currently shown in the minibuffer.

For consistency, you should also specify that element of the history as the initial minibuffer contents, using the initial argument to the minibuffer input function (see Initial Input).

If you don't specify history, then the default history list minibuffer-history is used. For other standard history lists, see below. You can also create your own history list variable; just initialize it to nil before the first use.

Both read-from-minibuffer and completing-read add new elements to the history list automatically, and provide commands to allow the user to reuse items on the list. The only thing your program needs to do to use a history list is to initialize it and to pass its name to the input functions when you wish. But it is safe to modify the list by hand when the minibuffer input functions are not using it.

Emacs functions that add a new element to a history list can also delete old elements if the list gets too long. The variable history-length specifies the maximum length for most history lists. To specify a different maximum length for a particular history list, put the length in the history-length property of the history list symbol. The variable history-delete-duplicates specifies whether to delete duplicates in history.

Here are some of the standard minibuffer history list variables:

19.5 Initial Input

Several of the functions for minibuffer input have an argument called initial. This is a mostly-deprecated feature for specifying that the minibuffer should start out with certain text, instead of empty as usual.

If initial is a string, the minibuffer starts out containing the text of the string, with point at the end, when the user starts to edit the text. If the user simply types <RET> to exit the minibuffer, it will use the initial input string to determine the value to return.

We discourage use of a non-nil value for initial, because initial input is an intrusive interface. History lists and default values provide a much more convenient method to offer useful default inputs to the user.

There is just one situation where you should specify a string for an initial argument. This is when you specify a cons cell for the history argument. See Minibuffer History.

initial can also be a cons cell of the form (string . position). This means to insert string in the minibuffer but put point at position within the string's text.

As a historical accident, position was implemented inconsistently in different functions. In completing-read, position's value is interpreted as origin-zero; that is, a value of 0 means the beginning of the string, 1 means after the first character, etc. In read-minibuffer, and the other non-completion minibuffer input functions that support this argument, 1 means the beginning of the string, 2 means after the first character, etc.

Use of a cons cell as the value for initial arguments is deprecated.

19.6 Completion

Completion is a feature that fills in the rest of a name starting from an abbreviation for it. Completion works by comparing the user's input against a list of valid names and determining how much of the name is determined uniquely by what the user has typed. For example, when you type C-x b (switch-to-buffer) and then type the first few letters of the name of the buffer to which you wish to switch, and then type <TAB> (minibuffer-complete), Emacs extends the name as far as it can.

Standard Emacs commands offer completion for names of symbols, files, buffers, and processes; with the functions in this section, you can implement completion for other kinds of names.

The try-completion function is the basic primitive for completion: it returns the longest determined completion of a given initial string, with a given set of strings to match against.

The function completing-read provides a higher-level interface for completion. A call to completing-read specifies how to determine the list of valid names. The function then activates the minibuffer with a local keymap that binds a few keys to commands useful for completion. Other functions provide convenient simple interfaces for reading certain kinds of names with completion.

• Basic Completion: Low-level functions for completing strings.
• Minibuffer Completion: Invoking the minibuffer with completion.
• Completion Commands: Minibuffer commands that do completion.
• High-Level Completion: Convenient special cases of completion (reading buffer names, variable names, etc.).
• Reading File Names: Using completion to read file names and shell commands.
• Completion Variables: Variables controlling completion behavior.
• Programmed Completion: Writing your own completion function.
• Completion in Buffers: Completing text in ordinary buffers.

19.6.1 Basic Completion Functions

The following completion functions have nothing in themselves to do with minibuffers. We describe them here to keep them near the higher-level completion features that do use the minibuffer.

If you store a completion alist in a variable, you should mark the variable as risky by giving it a non-nil risky-local-variable property. See File Local Variables.

There are several functions that take an existing completion table and return a modified version. completion-table-case-fold returns a case-insensitive table. completion-table-in-turn and completion-table-merge combine multiple input tables in different ways. completion-table-subvert alters a table to use a different initial prefix. completion-table-with-quoting returns a table suitable for operating on quoted text. completion-table-with-predicate filters a table with a predicate function. completion-table-with-terminator adds a terminating string.

19.6.2 Completion and the Minibuffer

This section describes the basic interface for reading from the minibuffer with completion.

19.6.3 Minibuffer Commands that Do Completion

This section describes the keymaps, commands and user options used in the minibuffer to do completion.

19.6.4 High-Level Completion Functions

This section describes the higher-level convenience functions for reading certain sorts of names with completion.

In most cases, you should not call these functions in the middle of a Lisp function. When possible, do all minibuffer input as part of reading the arguments for a command, in the interactive specification. See Defining Commands.

See also the functions read-coding-system and read-non-nil-coding-system, in User-Chosen Coding Systems, and read-input-method-name, in Input Methods.

19.6.5 Reading File Names

The high-level completion functions read-file-name, read-directory-name, and read-shell-command are designed to read file names, directory names, and shell commands, respectively. They provide special features, including automatic insertion of the default directory.

19.6.6 Completion Variables

Here are some variables that can be used to alter the default completion behavior.

19.6.7 Programmed Completion

Sometimes it is not possible or convenient to create an alist or an obarray containing all the intended possible completions ahead of time. In such a case, you can supply your own function to compute the completion of a given string. This is called programmed completion. Emacs uses programmed completion when completing file names (see File Name Completion), among many other cases.

To use this feature, pass a function as the collection argument to completing-read. The function completing-read arranges to pass your completion function along to try-completion, all-completions, and other basic completion functions, which will then let your function do all the work.

The completion function should accept three arguments:

• The string to be completed.
• A predicate function with which to filter possible matches, or nil if none. The function should call the predicate for each possible match, and ignore the match if the predicate returns nil.
• A flag specifying the type of completion operation to perform. This flag may be one of the following values.

  nil

  This specifies a try-completion operation. The function should return t if the specified string is a unique and exact match; if there is more than one match, it should return the common substring of all matches (if the string is an exact match for one completion alternative but also matches other longer alternatives, the return value is the string); if there are no matches, it should return nil.
  

  t

  This specifies an all-completions operation. The function should return a list of all possible completions of the specified string.
  

  lambda

  This specifies a test-completion operation. The function should return t if the specified string is an exact match for some completion alternative; nil otherwise.
  

  (boundaries . suffix)

  This specifies a completion-boundaries operation. The function should return (boundaries start . end), where start is the position of the beginning boundary in the specified string, and end is the position of the end boundary in suffix.
  

  metadata

  This specifies a request for information about the state of the current completion. The return value should have the form (metadata . alist), where alist is an alist whose elements are described below.

  If the flag has any other value, the completion function should return nil.

The following is a list of metadata entries that a completion function may return in response to a metadata flag argument:

category

The value should be a symbol describing what kind of text the completion function is trying to complete. If the symbol matches one of the keys in completion-category-overrides, the usual completion behavior is overridden. See Completion Variables.

annotation-function

The value should be a function for annotating completions. The function should take one argument, string, which is a possible completion. It should return a string, which is displayed after the completion string in the *Completions* buffer.

display-sort-function

The value should be a function for sorting completions. The function should take one argument, a list of completion strings, and return a sorted list of completion strings. It is allowed to alter the input list destructively.

cycle-sort-function

The value should be a function for sorting completions, when completion-cycle-threshold is non-nil and the user is cycling through completion alternatives. See Completion Options. Its argument list and return value are the same as for display-sort-function.

19.6.8 Completion in Ordinary Buffers

Although completion is usually done in the minibuffer, the completion facility can also be used on the text in ordinary Emacs buffers. In many major modes, in-buffer completion is performed by the C-M-i or M-<TAB> command, bound to completion-at-point. See Symbol Completion. This command uses the abnormal hook variable completion-at-point-functions:

The following function provides a convenient way to perform completion on an arbitrary stretch of text in an Emacs buffer:

19.7 Yes-or-No Queries

This section describes functions used to ask the user a yes-or-no question. The function y-or-n-p can be answered with a single character; it is useful for questions where an inadvertent wrong answer will not have serious consequences. yes-or-no-p is suitable for more momentous questions, since it requires three or four characters to answer.

If either of these functions is called in a command that was invoked using the mouse—more precisely, if last-nonmenu-event (see Command Loop Info) is either nil or a list—then it uses a dialog box or pop-up menu to ask the question. Otherwise, it uses keyboard input. You can force use either of the mouse or of keyboard input by binding last-nonmenu-event to a suitable value around the call.

Strictly speaking, yes-or-no-p uses the minibuffer and y-or-n-p does not; but it seems best to describe them together.

19.8 Asking Multiple Y-or-N Questions

When you have a series of similar questions to ask, such as “Do you want to save this buffer?” for each buffer in turn, you should use map-y-or-n-p to ask the collection of questions, rather than asking each question individually. This gives the user certain convenient facilities such as the ability to answer the whole series at once.

19.9 Reading a Password

To read a password to pass to another program, you can use the function read-passwd.

19.10 Minibuffer Commands

This section describes some commands meant for use in the minibuffer.

19.11 Minibuffer Windows

These functions access and select minibuffer windows, test whether they are active and control how they get resized.

It is not correct to determine whether a given window is a minibuffer by comparing it with the result of (minibuffer-window), because there can be more than one minibuffer window if there is more than one frame.

The following two options control whether minibuffer windows are resized automatically and how large they can get in the process.

19.12 Minibuffer Contents

These functions access the minibuffer prompt and contents.

19.13 Recursive Minibuffers

These functions and variables deal with recursive minibuffers (see Recursive Editing):

If a command name has a property enable-recursive-minibuffers that is non-nil, then the command can use the minibuffer to read arguments even if it is invoked from the minibuffer. A command can also achieve this by binding enable-recursive-minibuffers to t in the interactive declaration (see Using Interactive). The minibuffer command next-matching-history-element (normally M-s in the minibuffer) does the latter.

19.14 Minibuffer Miscellany

20 Command Loop

When you run Emacs, it enters the editor command loop almost immediately. This loop reads key sequences, executes their definitions, and displays the results. In this chapter, we describe how these things are done, and the subroutines that allow Lisp programs to do them.

• Command Overview: How the command loop reads commands.
• Defining Commands: Specifying how a function should read arguments.
• Interactive Call: Calling a command, so that it will read arguments.
• Distinguish Interactive: Making a command distinguish interactive calls.
• Command Loop Info: Variables set by the command loop for you to examine.
• Adjusting Point: Adjustment of point after a command.
• Input Events: What input looks like when you read it.
• Reading Input: How to read input events from the keyboard or mouse.
• Special Events: Events processed immediately and individually.
• Waiting: Waiting for user input or elapsed time.
• Quitting: How C-g works. How to catch or defer quitting.
• Prefix Command Arguments: How the commands to set prefix args work.
• Recursive Editing: Entering a recursive edit, and why you usually shouldn't.
• Disabling Commands: How the command loop handles disabled commands.
• Command History: How the command history is set up, and how accessed.
• Keyboard Macros: How keyboard macros are implemented.

20.1 Command Loop Overview

The first thing the command loop must do is read a key sequence, which is a sequence of input events that translates into a command. It does this by calling the function read-key-sequence. Lisp programs can also call this function (see Key Sequence Input). They can also read input at a lower level with read-key or read-event (see Reading One Event), or discard pending input with discard-input (see Event Input Misc).

The key sequence is translated into a command through the currently active keymaps. See Key Lookup, for information on how this is done. The result should be a keyboard macro or an interactively callable function. If the key is M-x, then it reads the name of another command, which it then calls. This is done by the command execute-extended-command (see Interactive Call).

Prior to executing the command, Emacs runs undo-boundary to create an undo boundary. See Maintaining Undo.

To execute a command, Emacs first reads its arguments by calling command-execute (see Interactive Call). For commands written in Lisp, the interactive specification says how to read the arguments. This may use the prefix argument (see Prefix Command Arguments) or may read with prompting in the minibuffer (see Minibuffers). For example, the command find-file has an interactive specification which says to read a file name using the minibuffer. The function body of find-file does not use the minibuffer, so if you call find-file as a function from Lisp code, you must supply the file name string as an ordinary Lisp function argument.

If the command is a keyboard macro (i.e., a string or vector), Emacs executes it using execute-kbd-macro (see Keyboard Macros).

Quitting is suppressed while running pre-command-hook and post-command-hook. If an error happens while executing one of these hooks, it does not terminate execution of the hook; instead the error is silenced and the function in which the error occurred is removed from the hook.

A request coming into the Emacs server (see Emacs Server) runs these two hooks just as a keyboard command does.

20.2 Defining Commands

The special form interactive turns a Lisp function into a command. The interactive form must be located at top-level in the function body, usually as the first form in the body; this applies to both lambda expressions (see Lambda Expressions) and defun forms (see Defining Functions). This form does nothing during the actual execution of the function; its presence serves as a flag, telling the Emacs command loop that the function can be called interactively. The argument of the interactive form specifies how the arguments for an interactive call should be read.

Alternatively, an interactive form may be specified in a function symbol's interactive-form property. A non-nil value for this property takes precedence over any interactive form in the function body itself. This feature is seldom used.

Sometimes, a function is only intended to be called interactively, never directly from Lisp. In that case, give the function a non-nil interactive-only property, either directly or via declare (see Declare Form). This causes the byte compiler to warn if the command is called from Lisp. The output of describe-function will include similar information. The value of the property can be: a string, which the byte-compiler will use directly in its warning (it should end with a period, and not start with a capital, e.g., "use (system-name) instead."); t; any other symbol, which should be an alternative function to use in Lisp code.

• Using Interactive: General rules for interactive.
• Interactive Codes: The standard letter-codes for reading arguments in various ways.
• Interactive Examples: Examples of how to read interactive arguments.
• Generic Commands: Select among command alternatives.

20.2.1 Using interactive

This section describes how to write the interactive form that makes a Lisp function an interactively-callable command, and how to examine a command's interactive form.

There are three possibilities for the argument arg-descriptor:

• It may be omitted or nil; then the command is called with no arguments. This leads quickly to an error if the command requires one or more arguments.
• It may be a string; its contents are a sequence of elements separated by newlines, one for each argument¹⁰. Each element consists of a code character (see Interactive Codes) optionally followed by a prompt (which some code characters use and some ignore). Here is an example:

            (interactive "P\nbFrobnicate buffer: ")
  

  The code letter ‘P’ sets the command's first argument to the raw command prefix (see Prefix Command Arguments). ‘bFrobnicate buffer: ’ prompts the user with ‘Frobnicate buffer: ’ to enter the name of an existing buffer, which becomes the second and final argument.

  The prompt string can use ‘%’ to include previous argument values (starting with the first argument) in the prompt. This is done using format-message (see Formatting Strings). For example, here is how you could read the name of an existing buffer followed by a new name to give to that buffer:

            (interactive "bBuffer to rename: \nsRename buffer %s to: ")
  

  If ‘*’ appears at the beginning of the string, then an error is signaled if the buffer is read-only.

  If ‘@’ appears at the beginning of the string, and if the key sequence used to invoke the command includes any mouse events, then the window associated with the first of those events is selected before the command is run.

  If ‘^’ appears at the beginning of the string, and if the command was invoked through shift-translation, set the mark and activate the region temporarily, or extend an already active region, before the command is run. If the command was invoked without shift-translation, and the region is temporarily active, deactivate the region before the command is run. Shift-translation is controlled on the user level by shift-select-mode; see Shift Selection.

  You can use ‘*’, ‘@’, and ^ together; the order does not matter. Actual reading of arguments is controlled by the rest of the prompt string (starting with the first character that is not ‘*’, ‘@’, or ‘^’).

• It may be a Lisp expression that is not a string; then it should be a form that is evaluated to get a list of arguments to pass to the command. Usually this form will call various functions to read input from the user, most often through the minibuffer (see Minibuffers) or directly from the keyboard (see Reading Input).

  Providing point or the mark as an argument value is also common, but if you do this and read input (whether using the minibuffer or not), be sure to get the integer values of point or the mark after reading. The current buffer may be receiving subprocess output; if subprocess output arrives while the command is waiting for input, it could relocate point and the mark.

  Here's an example of what not to do:

            (interactive
             (list (region-beginning) (region-end)
                   (read-string "Foo: " nil 'my-history)))
  

  Here's how to avoid the problem, by examining point and the mark after reading the keyboard input:

            (interactive
             (let ((string (read-string "Foo: " nil 'my-history)))
               (list (region-beginning) (region-end) string)))
  

  Warning: the argument values should not include any data types that can't be printed and then read. Some facilities save command-history in a file to be read in the subsequent sessions; if a command's arguments contain a data type that prints using ‘#<...>’ syntax, those facilities won't work.

  There are, however, a few exceptions: it is ok to use a limited set of expressions such as (point), (mark), (region-beginning), and (region-end), because Emacs recognizes them specially and puts the expression (rather than its value) into the command history. To see whether the expression you wrote is one of these exceptions, run the command, then examine (car command-history).

20.2.2 Code Characters for interactive

The code character descriptions below contain a number of key words, defined here as follows:

Completion

Provide completion. <TAB>, <SPC>, and <RET> perform name completion because the argument is read using completing-read (see Completion). ? displays a list of possible completions.

Existing

Require the name of an existing object. An invalid name is not accepted; the commands to exit the minibuffer do not exit if the current input is not valid.

Default

A default value of some sort is used if the user enters no text in the minibuffer. The default depends on the code character.

No I/O

This code letter computes an argument without reading any input. Therefore, it does not use a prompt string, and any prompt string you supply is ignored.

Even though the code letter doesn't use a prompt string, you must follow it with a newline if it is not the last code character in the string.

Prompt

A prompt immediately follows the code character. The prompt ends either with the end of the string or with a newline.

Special

This code character is meaningful only at the beginning of the interactive string, and it does not look for a prompt or a newline. It is a single, isolated character.

Here are the code character descriptions for use with interactive:

‘*’

Signal an error if the current buffer is read-only. Special.

‘@’

Select the window mentioned in the first mouse event in the key sequence that invoked this command. Special.

‘^’

If the command was invoked through shift-translation, set the mark and activate the region temporarily, or extend an already active region, before the command is run. If the command was invoked without shift-translation, and the region is temporarily active, deactivate the region before the command is run. Special.

‘a’

A function name (i.e., a symbol satisfying fboundp). Existing, Completion, Prompt.

‘b’

The name of an existing buffer. By default, uses the name of the current buffer (see Buffers). Existing, Completion, Default, Prompt.

‘B’

A buffer name. The buffer need not exist. By default, uses the name of a recently used buffer other than the current buffer. Completion, Default, Prompt.

‘c’

A character. The cursor does not move into the echo area. Prompt.

‘C’

A command name (i.e., a symbol satisfying commandp). Existing, Completion, Prompt.

‘d’

The position of point, as an integer (see Point). No I/O.

‘D’

A directory name. The default is the current default directory of the current buffer, default-directory (see File Name Expansion). Existing, Completion, Default, Prompt.

‘e’

The first or next non-keyboard event in the key sequence that invoked the command. More precisely, ‘e’ gets events that are lists, so you can look at the data in the lists. See Input Events. No I/O.

You use ‘e’ for mouse events and for special system events (see Misc Events). The event list that the command receives depends on the event. See Input Events, which describes the forms of the list for each event in the corresponding subsections.

You can use ‘e’ more than once in a single command's interactive specification. If the key sequence that invoked the command has n events that are lists, the nth ‘e’ provides the nth such event. Events that are not lists, such as function keys and ASCII characters, do not count where ‘e’ is concerned.

‘f’

A file name of an existing file (see File Names). The default directory is default-directory. Existing, Completion, Default, Prompt.

‘F’

A file name. The file need not exist. Completion, Default, Prompt.

‘G’

A file name. The file need not exist. If the user enters just a directory name, then the value is just that directory name, with no file name within the directory added. Completion, Default, Prompt.

‘i’

An irrelevant argument. This code always supplies nil as the argument's value. No I/O.

‘k’

A key sequence (see Key Sequences). This keeps reading events until a command (or undefined command) is found in the current key maps. The key sequence argument is represented as a string or vector. The cursor does not move into the echo area. Prompt.

If ‘k’ reads a key sequence that ends with a down-event, it also reads and discards the following up-event. You can get access to that up-event with the ‘U’ code character.

This kind of input is used by commands such as describe-key and global-set-key.

‘K’

A key sequence, whose definition you intend to change. This works like ‘k’, except that it suppresses, for the last input event in the key sequence, the conversions that are normally used (when necessary) to convert an undefined key into a defined one.

‘m’

The position of the mark, as an integer. No I/O.

‘M’

Arbitrary text, read in the minibuffer using the current buffer's input method, and returned as a string (see Input Methods). Prompt.

‘n’

A number, read with the minibuffer. If the input is not a number, the user has to try again. ‘n’ never uses the prefix argument. Prompt.

‘N’

The numeric prefix argument; but if there is no prefix argument, read a number as with n. The value is always a number. See Prefix Command Arguments. Prompt.

‘p’

The numeric prefix argument. (Note that this ‘p’ is lower case.) No I/O.

‘P’

The raw prefix argument. (Note that this ‘P’ is upper case.) No I/O.

‘r’

Point and the mark, as two numeric arguments, smallest first. This is the only code letter that specifies two successive arguments rather than one. This will signal an error if the mark is not set in the buffer which is current when the command is invoked. No I/O.

‘s’

Arbitrary text, read in the minibuffer and returned as a string (see Text from Minibuffer). Terminate the input with either C-j or <RET>. (C-q may be used to include either of these characters in the input.) Prompt.

‘S’

An interned symbol whose name is read in the minibuffer. Terminate the input with either C-j or <RET>. Other characters that normally terminate a symbol (e.g., whitespace, parentheses and brackets) do not do so here. Prompt.

‘U’

A key sequence or nil. Can be used after a ‘k’ or ‘K’ argument to get the up-event that was discarded (if any) after ‘k’ or ‘K’ read a down-event. If no up-event has been discarded, ‘U’ provides nil as the argument. No I/O.

‘v’

A variable declared to be a user option (i.e., satisfying the predicate custom-variable-p). This reads the variable using read-variable. See Definition of read-variable. Existing, Completion, Prompt.

‘x’

A Lisp object, specified with its read syntax, terminated with a C-j or <RET>. The object is not evaluated. See Object from Minibuffer. Prompt.

‘X’

A Lisp form's value. ‘X’ reads as ‘x’ does, then evaluates the form so that its value becomes the argument for the command. Prompt.

‘z’

A coding system name (a symbol). If the user enters null input, the argument value is nil. See Coding Systems. Completion, Existing, Prompt.

‘Z’

A coding system name (a symbol)—but only if this command has a prefix argument. With no prefix argument, ‘Z’ provides nil as the argument value. Completion, Existing, Prompt.

20.2.3 Examples of Using interactive

Here are some examples of interactive:

     (defun foo1 ()              ; foo1 takes no arguments,
         (interactive)           ;   just moves forward two words.
         (forward-word 2))
          ⇒ foo1
     
     (defun foo2 (n)             ; foo2 takes one argument,
         (interactive "^p")      ;   which is the numeric prefix.
                                 ; under shift-select-mode,
                                 ;   will activate or extend region.
         (forward-word (* 2 n)))
          ⇒ foo2
     
     (defun foo3 (n)             ; foo3 takes one argument,
         (interactive "nCount:") ;   which is read with the Minibuffer.
         (forward-word (* 2 n)))
          ⇒ foo3
     
     (defun three-b (b1 b2 b3)
       "Select three existing buffers.
     Put them into three windows, selecting the last one."
         (interactive "bBuffer1:\nbBuffer2:\nbBuffer3:")
         (delete-other-windows)
         (split-window (selected-window) 8)
         (switch-to-buffer b1)
         (other-window 1)
         (split-window (selected-window) 8)
         (switch-to-buffer b2)
         (other-window 1)
         (switch-to-buffer b3))
          ⇒ three-b
     (three-b "*scratch*" "declarations.texi" "*mail*")
          ⇒ nil

20.2.4 Select among Command Alternatives

The macro define-alternatives can be used to define generic commands. These are interactive functions whose implementation can be selected from several alternatives, as a matter of user preference.

20.3 Interactive Call

After the command loop has translated a key sequence into a command, it invokes that command using the function command-execute. If the command is a function, command-execute calls call-interactively, which reads the arguments and calls the command. You can also call these functions yourself.

Note that the term “command”, in this context, refers to an interactively callable function (or function-like object), or a keyboard macro. It does not refer to the key sequence used to invoke a command (see Keymaps).

20.4 Distinguish Interactive Calls

Sometimes a command should display additional visual feedback (such as an informative message in the echo area) for interactive calls only. There are three ways to do this. The recommended way to test whether the function was called using call-interactively is to give it an optional argument print-message and use the interactive spec to make it non-nil in interactive calls. Here's an example:

     (defun foo (&optional print-message)
       (interactive "p")
       (when print-message
         (message "foo")))

We use "p" because the numeric prefix argument is never nil. Defined in this way, the function does display the message when called from a keyboard macro.

The above method with the additional argument is usually best, because it allows callers to say “treat this call as interactive”. But you can also do the job by testing called-interactively-p.

Here is an example of using called-interactively-p:

     (defun foo ()
       (interactive)
       (when (called-interactively-p 'any)
         (message "Interactive!")
         'foo-called-interactively))
     
     ;; Type M-x foo.
          -| Interactive!
     
     (foo)
          ⇒ nil

Here is another example that contrasts direct and indirect calls to called-interactively-p.

     (defun bar ()
       (interactive)
       (message "%s" (list (foo) (called-interactively-p 'any))))
     
     ;; Type M-x bar.
          -| (nil t)

20.5 Information from the Command Loop

The editor command loop sets several Lisp variables to keep status records for itself and for commands that are run. With the exception of this-command and last-command it's generally a bad idea to change any of these variables in a Lisp program.

If you do not want a particular command to be recognized as the previous command in the case where it got an error, you must code that command to prevent this. One way is to set this-command to t at the beginning of the command, and set this-command back to its proper value at the end, like this:

     (defun foo (args...)
       (interactive ...)
       (let ((old-this-command this-command))
         (setq this-command t)
         ...do the work...
         (setq this-command old-this-command)))

We do not bind this-command with let because that would restore the old value in case of error—a feature of let which in this case does precisely what we want to avoid.

20.6 Adjusting Point After Commands

It is not easy to display a value of point in the middle of a sequence of text that has the display, composition or is invisible. Therefore, after a command finishes and returns to the command loop, if point is within such a sequence, the command loop normally moves point to the edge of the sequence.

A command can inhibit this feature by setting the variable disable-point-adjustment:

20.7 Input Events

The Emacs command loop reads a sequence of input events that represent keyboard or mouse activity, or system events sent to Emacs. The events for keyboard activity are characters or symbols; other events are always lists. This section describes the representation and meaning of input events in detail.

• Keyboard Events: Ordinary characters -- keys with symbols on them.
• Function Keys: Function keys -- keys with names, not symbols.
• Mouse Events: Overview of mouse events.
• Click Events: Pushing and releasing a mouse button.
• Drag Events: Moving the mouse before releasing the button.
• Button-Down Events: A button was pushed and not yet released.
• Repeat Events: Double and triple click (or drag, or down).
• Motion Events: Just moving the mouse, not pushing a button.
• Focus Events: Moving the mouse between frames.
• Misc Events: Other events the system can generate.
• Event Examples: Examples of the lists for mouse events.
• Classifying Events: Finding the modifier keys in an event symbol. Event types.
• Accessing Mouse: Functions to extract info from mouse events.
• Accessing Scroll: Functions to get info from scroll bar events.
• Strings of Events: Special considerations for putting keyboard character events in a string.

20.7.1 Keyboard Events

There are two kinds of input you can get from the keyboard: ordinary keys, and function keys. Ordinary keys correspond to characters; the events they generate are represented in Lisp as characters. The event type of a character event is the character itself (an integer); see Classifying Events.

An input character event consists of a basic code between 0 and 524287, plus any or all of these modifier bits:

meta

The 2**27 bit in the character code indicates a character typed with the meta key held down.

control

The 2**26 bit in the character code indicates a non-ASCII control character.

ascii control characters such as C-a have special basic codes of their own, so Emacs needs no special bit to indicate them. Thus, the code for C-a is just 1.

But if you type a control combination not in ASCII, such as % with the control key, the numeric value you get is the code for % plus 2**26 (assuming the terminal supports non-ASCII control characters).

shift

The 2**25 bit in the character code indicates an ASCII control character typed with the shift key held down.

For letters, the basic code itself indicates upper versus lower case; for digits and punctuation, the shift key selects an entirely different character with a different basic code. In order to keep within the ASCII character set whenever possible, Emacs avoids using the 2**25 bit for those characters.

However, ASCII provides no way to distinguish C-A from C-a, so Emacs uses the 2**25 bit in C-A and not in C-a.

hyper

The 2**24 bit in the character code indicates a character typed with the hyper key held down.

super

The 2**23 bit in the character code indicates a character typed with the super key held down.

alt

The 2**22 bit in the character code indicates a character typed with the alt key held down. (The key labeled <Alt> on most keyboards is actually treated as the meta key, not this.)

It is best to avoid mentioning specific bit numbers in your program. To test the modifier bits of a character, use the function event-modifiers (see Classifying Events). When making key bindings, you can use the read syntax for characters with modifier bits (‘\C-’, ‘\M-’, and so on). For making key bindings with define-key, you can use lists such as (control hyper ?x) to specify the characters (see Changing Key Bindings). The function event-convert-list converts such a list into an event type (see Classifying Events).

20.7.2 Function Keys

Most keyboards also have function keys—keys that have names or symbols that are not characters. Function keys are represented in Emacs Lisp as symbols; the symbol's name is the function key's label, in lower case. For example, pressing a key labeled <F1> generates an input event represented by the symbol f1.

The event type of a function key event is the event symbol itself. See Classifying Events.

Here are a few special cases in the symbol-naming convention for function keys:

backspace, tab, newline, return, delete

These keys correspond to common ASCII control characters that have special keys on most keyboards.

In ASCII, C-i and <TAB> are the same character. If the terminal can distinguish between them, Emacs conveys the distinction to Lisp programs by representing the former as the integer 9, and the latter as the symbol tab.

Most of the time, it's not useful to distinguish the two. So normally local-function-key-map (see Translation Keymaps) is set up to map tab into 9. Thus, a key binding for character code 9 (the character C-i) also applies to tab. Likewise for the other symbols in this group. The function read-char likewise converts these events into characters.

In ASCII, <BS> is really C-h. But backspace converts into the character code 127 (<DEL>), not into code 8 (<BS>). This is what most users prefer.

left, up, right, down

Cursor arrow keys

kp-add, kp-decimal, kp-divide, ...

Keypad keys (to the right of the regular keyboard).

kp-0, kp-1, ...

Keypad keys with digits.

kp-f1, kp-f2, kp-f3, kp-f4

Keypad PF keys.

kp-home, kp-left, kp-up, kp-right, kp-down

Keypad arrow keys. Emacs normally translates these into the corresponding non-keypad keys home, left, ...

kp-prior, kp-next, kp-end, kp-begin, kp-insert, kp-delete

Additional keypad duplicates of keys ordinarily found elsewhere. Emacs normally translates these into the like-named non-keypad keys.

You can use the modifier keys <ALT>, <CTRL>, <HYPER>, <META>, <SHIFT>, and <SUPER> with function keys. The way to represent them is with prefixes in the symbol name:

‘A-’

The alt modifier.

‘C-’

The control modifier.

‘H-’

The hyper modifier.

‘M-’

The meta modifier.

‘S-’

The shift modifier.

‘s-’

The super modifier.

Thus, the symbol for the key <F3> with <META> held down is M-f3. When you use more than one prefix, we recommend you write them in alphabetical order; but the order does not matter in arguments to the key-binding lookup and modification functions.

20.7.3 Mouse Events

Emacs supports four kinds of mouse events: click events, drag events, button-down events, and motion events. All mouse events are represented as lists. The car of the list is the event type; this says which mouse button was involved, and which modifier keys were used with it. The event type can also distinguish double or triple button presses (see Repeat Events). The rest of the list elements give position and time information.

For key lookup, only the event type matters: two events of the same type necessarily run the same command. The command can access the full values of these events using the ‘e’ interactive code. See Interactive Codes.

A key sequence that starts with a mouse event is read using the keymaps of the buffer in the window that the mouse was in, not the current buffer. This does not imply that clicking in a window selects that window or its buffer—that is entirely under the control of the command binding of the key sequence.

20.7.4 Click Events

When the user presses a mouse button and releases it at the same location, that generates a click event. All mouse click event share the same format:

     (event-type position click-count)

event-type

This is a symbol that indicates which mouse button was used. It is one of the symbols mouse-1, mouse-2, ..., where the buttons are numbered left to right.

You can also use prefixes ‘A-’, ‘C-’, ‘H-’, ‘M-’, ‘S-’ and ‘s-’ for modifiers alt, control, hyper, meta, shift and super, just as you would with function keys.

This symbol also serves as the event type of the event. Key bindings describe events by their types; thus, if there is a key binding for mouse-1, that binding would apply to all events whose event-type is mouse-1.

position

This is a mouse position list specifying where the mouse click occurred; see below for details.

click-count

This is the number of rapid repeated presses so far of the same mouse button. See Repeat Events.

To access the contents of a mouse position list in the position slot of a click event, you should typically use the functions documented in Accessing Mouse. The explicit format of the list depends on where the click occurred. For clicks in the text area, mode line, header line, or in the fringe or marginal areas, the mouse position list has the form

     (window pos-or-area (x . y) timestamp
      object text-pos (col . row)
      image (dx . dy) (width . height))

The meanings of these list elements are as follows:

window

The window in which the click occurred.

pos-or-area

The buffer position of the character clicked on in the text area; or, if the click was outside the text area, the window area where it occurred. It is one of the symbols mode-line, header-line, vertical-line, left-margin, right-margin, left-fringe, or right-fringe.

In one special case, pos-or-area is a list containing a symbol (one of the symbols listed above) instead of just the symbol. This happens after the imaginary prefix keys for the event are registered by Emacs. See Key Sequence Input.

x, y

The relative pixel coordinates of the click. For clicks in the text area of a window, the coordinate origin (0 . 0) is taken to be the top left corner of the text area. See Window Sizes. For clicks in a mode line or header line, the coordinate origin is the top left corner of the window itself. For fringes, margins, and the vertical border, x does not have meaningful data. For fringes and margins, y is relative to the bottom edge of the header line. In all cases, the x and y coordinates increase rightward and downward respectively.

timestamp

The time at which the event occurred, as an integer number of milliseconds since a system-dependent initial time.

object

Either nil if there is no string-type text property at the click position, or a cons cell of the form (string . string-pos) if there is one:

string

The string which was clicked on, including any properties.

string-pos

The position in the string where the click occurred.

text-pos

For clicks on a marginal area or on a fringe, this is the buffer position of the first visible character in the corresponding line in the window. For clicks on the mode line or the header line, this is nil. For other events, it is the buffer position closest to the click.

col, row

These are the actual column and row coordinate numbers of the glyph under the x, y position. If x lies beyond the last column of actual text on its line, col is reported by adding fictional extra columns that have the default character width. Row 0 is taken to be the header line if the window has one, or the topmost row of the text area otherwise. Column 0 is taken to be the leftmost column of the text area for clicks on a window text area, or the leftmost mode line or header line column for clicks there. For clicks on fringes or vertical borders, these have no meaningful data. For clicks on margins, col is measured from the left edge of the margin area and row is measured from the top of the margin area.

image

This is the image object on which the click occurred. It is either nil if there is no image at the position clicked on, or it is an image object as returned by find-image if click was in an image.

dx, dy

These are the pixel coordinates of the click, relative to the top left corner of object, which is (0 . 0). If object is nil, the coordinates are relative to the top left corner of the character glyph clicked on.

width, height

These are the pixel width and height of object or, if this is nil, those of the character glyph clicked on.

For clicks on a scroll bar, position has this form:

     (window area (portion . whole) timestamp part)

window

The window whose scroll bar was clicked on.

area

This is the symbol vertical-scroll-bar.

portion

The number of pixels from the top of the scroll bar to the click position. On some toolkits, including GTK+, Emacs cannot extract this data, so the value is always 0.

whole

The total length, in pixels, of the scroll bar. On some toolkits, including GTK+, Emacs cannot extract this data, so the value is always 0.

timestamp

The time at which the event occurred, in milliseconds. On some toolkits, including GTK+, Emacs cannot extract this data, so the value is always 0.

part

The part of the scroll bar on which the click occurred. It is one of the symbols handle (the scroll bar handle), above-handle (the area above the handle), below-handle (the area below the handle), up (the up arrow at one end of the scroll bar), or down (the down arrow at one end of the scroll bar).

20.7.5 Drag Events

With Emacs, you can have a drag event without even changing your clothes. A drag event happens every time the user presses a mouse button and then moves the mouse to a different character position before releasing the button. Like all mouse events, drag events are represented in Lisp as lists. The lists record both the starting mouse position and the final position, like this:

     (event-type
      (window1 START-POSITION)
      (window2 END-POSITION))

For a drag event, the name of the symbol event-type contains the prefix ‘drag-’. For example, dragging the mouse with button 2 held down generates a drag-mouse-2 event. The second and third elements of the event give the starting and ending position of the drag, as mouse position lists (see Click Events). You can access the second element of any mouse event in the same way. However, the drag event may end outside the boundaries of the frame that was initially selected. In that case, the third element's position list contains that frame in place of a window.

The ‘drag-’ prefix follows the modifier key prefixes such as ‘C-’ and ‘M-’.

If read-key-sequence receives a drag event that has no key binding, and the corresponding click event does have a binding, it changes the drag event into a click event at the drag's starting position. This means that you don't have to distinguish between click and drag events unless you want to.

20.7.6 Button-Down Events

Click and drag events happen when the user releases a mouse button. They cannot happen earlier, because there is no way to distinguish a click from a drag until the button is released.

If you want to take action as soon as a button is pressed, you need to handle button-down events.¹¹ These occur as soon as a button is pressed. They are represented by lists that look exactly like click events (see Click Events), except that the event-type symbol name contains the prefix ‘down-’. The ‘down-’ prefix follows modifier key prefixes such as ‘C-’ and ‘M-’.

The function read-key-sequence ignores any button-down events that don't have command bindings; therefore, the Emacs command loop ignores them too. This means that you need not worry about defining button-down events unless you want them to do something. The usual reason to define a button-down event is so that you can track mouse motion (by reading motion events) until the button is released. See Motion Events.

20.7.7 Repeat Events

If you press the same mouse button more than once in quick succession without moving the mouse, Emacs generates special repeat mouse events for the second and subsequent presses.

The most common repeat events are double-click events. Emacs generates a double-click event when you click a button twice; the event happens when you release the button (as is normal for all click events).

The event type of a double-click event contains the prefix ‘double-’. Thus, a double click on the second mouse button with <meta> held down comes to the Lisp program as M-double-mouse-2. If a double-click event has no binding, the binding of the corresponding ordinary click event is used to execute it. Thus, you need not pay attention to the double click feature unless you really want to.

When the user performs a double click, Emacs generates first an ordinary click event, and then a double-click event. Therefore, you must design the command binding of the double click event to assume that the single-click command has already run. It must produce the desired results of a double click, starting from the results of a single click.

This is convenient, if the meaning of a double click somehow builds on the meaning of a single click—which is recommended user interface design practice for double clicks.

If you click a button, then press it down again and start moving the mouse with the button held down, then you get a double-drag event when you ultimately release the button. Its event type contains ‘double-drag’ instead of just ‘drag’. If a double-drag event has no binding, Emacs looks for an alternate binding as if the event were an ordinary drag.

Before the double-click or double-drag event, Emacs generates a double-down event when the user presses the button down for the second time. Its event type contains ‘double-down’ instead of just ‘down’. If a double-down event has no binding, Emacs looks for an alternate binding as if the event were an ordinary button-down event. If it finds no binding that way either, the double-down event is ignored.

To summarize, when you click a button and then press it again right away, Emacs generates a down event and a click event for the first click, a double-down event when you press the button again, and finally either a double-click or a double-drag event.

If you click a button twice and then press it again, all in quick succession, Emacs generates a triple-down event, followed by either a triple-click or a triple-drag. The event types of these events contain ‘triple’ instead of ‘double’. If any triple event has no binding, Emacs uses the binding that it would use for the corresponding double event.

If you click a button three or more times and then press it again, the events for the presses beyond the third are all triple events. Emacs does not have separate event types for quadruple, quintuple, etc. events. However, you can look at the event list to find out precisely how many times the button was pressed.

20.7.8 Motion Events

Emacs sometimes generates mouse motion events to describe motion of the mouse without any button activity. Mouse motion events are represented by lists that look like this:

     (mouse-movement POSITION)

position is a mouse position list (see Click Events), specifying the current position of the mouse cursor. As with the end-position of a drag event, this position list may represent a location outside the boundaries of the initially selected frame, in which case the list contains that frame in place of a window.

The special form track-mouse enables generation of motion events within its body. Outside of track-mouse forms, Emacs does not generate events for mere motion of the mouse, and these events do not appear. See Mouse Tracking.

20.7.9 Focus Events

Window systems provide general ways for the user to control which window gets keyboard input. This choice of window is called the focus. When the user does something to switch between Emacs frames, that generates a focus event. The normal definition of a focus event, in the global keymap, is to select a new frame within Emacs, as the user would expect. See Input Focus, which also describes hooks related to focus events.

Focus events are represented in Lisp as lists that look like this:

     (switch-frame new-frame)

where new-frame is the frame switched to.

Some X window managers are set up so that just moving the mouse into a window is enough to set the focus there. Usually, there is no need for a Lisp program to know about the focus change until some other kind of input arrives. Emacs generates a focus event only when the user actually types a keyboard key or presses a mouse button in the new frame; just moving the mouse between frames does not generate a focus event.

A focus event in the middle of a key sequence would garble the sequence. So Emacs never generates a focus event in the middle of a key sequence. If the user changes focus in the middle of a key sequence—that is, after a prefix key—then Emacs reorders the events so that the focus event comes either before or after the multi-event key sequence, and not within it.

20.7.10 Miscellaneous System Events

A few other event types represent occurrences within the system.

(delete-frame (frame))

This kind of event indicates that the user gave the window manager a command to delete a particular window, which happens to be an Emacs frame.

The standard definition of the delete-frame event is to delete frame.

(iconify-frame (frame))

This kind of event indicates that the user iconified frame using the window manager. Its standard definition is ignore; since the frame has already been iconified, Emacs has no work to do. The purpose of this event type is so that you can keep track of such events if you want to.

(make-frame-visible (frame))

This kind of event indicates that the user deiconified frame using the window manager. Its standard definition is ignore; since the frame has already been made visible, Emacs has no work to do.

(wheel-up position)

(wheel-down position)

These kinds of event are generated by moving a mouse wheel. The position element is a mouse position list (see Click Events), specifying the position of the mouse cursor when the event occurred.

This kind of event is generated only on some kinds of systems. On some systems, mouse-4 and mouse-5 are used instead. For portable code, use the variables mouse-wheel-up-event and mouse-wheel-down-event defined in mwheel.el to determine what event types to expect for the mouse wheel.

(drag-n-drop position files)

This kind of event is generated when a group of files is selected in an application outside of Emacs, and then dragged and dropped onto an Emacs frame.

The element position is a list describing the position of the event, in the same format as used in a mouse-click event (see Click Events), and files is the list of file names that were dragged and dropped. The usual way to handle this event is by visiting these files.

This kind of event is generated, at present, only on some kinds of systems.

help-echo

This kind of event is generated when a mouse pointer moves onto a portion of buffer text which has a help-echo text property. The generated event has this form:

          (help-echo frame help window object pos)

The precise meaning of the event parameters and the way these parameters are used to display the help-echo text are described in Text help-echo.

sigusr1

sigusr2

These events are generated when the Emacs process receives the signals SIGUSR1 and SIGUSR2. They contain no additional data because signals do not carry additional information. They can be useful for debugging (see Error Debugging).

To catch a user signal, bind the corresponding event to an interactive command in the special-event-map (see Active Keymaps). The command is called with no arguments, and the specific signal event is available in last-input-event. For example:

          (defun sigusr-handler ()
            (interactive)
            (message "Caught signal %S" last-input-event))
          
          (define-key special-event-map [sigusr1] 'sigusr-handler)

To test the signal handler, you can make Emacs send a signal to itself:

          (signal-process (emacs-pid) 'sigusr1)

language-change

This kind of event is generated on MS-Windows when the input language has changed. This typically means that the keyboard keys will send to Emacs characters from a different language. The generated event has this form:

          (language-change frame codepage language-id)

Here frame is the frame which was current when the input language changed; codepage is the new codepage number; and language-id is the numerical ID of the new input language. The coding-system (see Coding Systems) that corresponds to codepage is cpcodepage or windows-codepage. To convert language-id to a string (e.g., to use it for various language-dependent features, such as set-language-environment), use the w32-get-locale-info function, like this:

          ;; Get the abbreviated language name, such as "ENU" for English
          (w32-get-locale-info language-id)
          ;; Get the full English name of the language,
          ;; such as "English (United States)"
          (w32-get-locale-info language-id 4097)
          ;; Get the full localized name of the language
          (w32-get-locale-info language-id t)

If one of these events arrives in the middle of a key sequence—that is, after a prefix key—then Emacs reorders the events so that this event comes either before or after the multi-event key sequence, not within it.

20.7.11 Event Examples

If the user presses and releases the left mouse button over the same location, that generates a sequence of events like this:

     (down-mouse-1 (#<window 18 on NEWS> 2613 (0 . 38) -864320))
     (mouse-1      (#<window 18 on NEWS> 2613 (0 . 38) -864180))

While holding the control key down, the user might hold down the second mouse button, and drag the mouse from one line to the next. That produces two events, as shown here:

     (C-down-mouse-2 (#<window 18 on NEWS> 3440 (0 . 27) -731219))
     (C-drag-mouse-2 (#<window 18 on NEWS> 3440 (0 . 27) -731219)
                     (#<window 18 on NEWS> 3510 (0 . 28) -729648))

While holding down the meta and shift keys, the user might press the second mouse button on the window's mode line, and then drag the mouse into another window. That produces a pair of events like these:

     (M-S-down-mouse-2 (#<window 18 on NEWS> mode-line (33 . 31) -457844))
     (M-S-drag-mouse-2 (#<window 18 on NEWS> mode-line (33 . 31) -457844)
                       (#<window 20 on carlton-sanskrit.tex> 161 (33 . 3)
                        -453816))

The frame with input focus might not take up the entire screen, and the user might move the mouse outside the scope of the frame. Inside the track-mouse special form, that produces an event like this:

     (mouse-movement (#<frame *ielm* 0x102849a30> nil (563 . 205) 532301936))

To handle a SIGUSR1 signal, define an interactive function, and bind it to the signal usr1 event sequence:

     (defun usr1-handler ()
       (interactive)
       (message "Got USR1 signal"))
     (global-set-key [signal usr1] 'usr1-handler)

20.7.12 Classifying Events

Every event has an event type, which classifies the event for key binding purposes. For a keyboard event, the event type equals the event value; thus, the event type for a character is the character, and the event type for a function key symbol is the symbol itself. For events that are lists, the event type is the symbol in the car of the list. Thus, the event type is always a symbol or a character.

Two events of the same type are equivalent where key bindings are concerned; thus, they always run the same command. That does not necessarily mean they do the same things, however, as some commands look at the whole event to decide what to do. For example, some commands use the location of a mouse event to decide where in the buffer to act.

Sometimes broader classifications of events are useful. For example, you might want to ask whether an event involved the <META> key, regardless of which other key or mouse button was used.

The functions event-modifiers and event-basic-type are provided to get such information conveniently.

20.7.13 Accessing Mouse Events

This section describes convenient functions for accessing the data in a mouse button or motion event. Keyboard event data can be accessed using the same functions, but data elements that aren't applicable to keyboard events are zero or nil.

The following two functions return a mouse position list (see Click Events), specifying the position of a mouse event.

These functions take a mouse position list as argument, and return various parts of it:

These functions compute a position list given particular buffer position or screen position. You can access the data in this position list with the functions described above.

20.7.14 Accessing Scroll Bar Events

These functions are useful for decoding scroll bar events.

20.7.15 Putting Keyboard Events in Strings

In most of the places where strings are used, we conceptualize the string as containing text characters—the same kind of characters found in buffers or files. Occasionally Lisp programs use strings that conceptually contain keyboard characters; for example, they may be key sequences or keyboard macro definitions. However, storing keyboard characters in a string is a complex matter, for reasons of historical compatibility, and it is not always possible.

We recommend that new programs avoid dealing with these complexities by not storing keyboard events in strings. Here is how to do that:

• Use vectors instead of strings for key sequences, when you plan to use them for anything other than as arguments to lookup-key and define-key. For example, you can use read-key-sequence-vector instead of read-key-sequence, and this-command-keys-vector instead of this-command-keys.
• Use vectors to write key sequence constants containing meta characters, even when passing them directly to define-key.
• When you have to look at the contents of a key sequence that might be a string, use listify-key-sequence (see Event Input Misc) first, to convert it to a list.

The complexities stem from the modifier bits that keyboard input characters can include. Aside from the Meta modifier, none of these modifier bits can be included in a string, and the Meta modifier is allowed only in special cases.

The earliest GNU Emacs versions represented meta characters as codes in the range of 128 to 255. At that time, the basic character codes ranged from 0 to 127, so all keyboard character codes did fit in a string. Many Lisp programs used ‘\M-’ in string constants to stand for meta characters, especially in arguments to define-key and similar functions, and key sequences and sequences of events were always represented as strings.

When we added support for larger basic character codes beyond 127, and additional modifier bits, we had to change the representation of meta characters. Now the flag that represents the Meta modifier in a character is 2**27 and such numbers cannot be included in a string.

To support programs with ‘\M-’ in string constants, there are special rules for including certain meta characters in a string. Here are the rules for interpreting a string as a sequence of input characters:

• If the keyboard character value is in the range of 0 to 127, it can go in the string unchanged.
• The meta variants of those characters, with codes in the range of 2**27 to 2**27+127, can also go in the string, but you must change their numeric values. You must set the 2**7 bit instead of the 2**27 bit, resulting in a value between 128 and 255. Only a unibyte string can include these codes.
• Non-ASCII characters above 256 can be included in a multibyte string.
• Other keyboard character events cannot fit in a string. This includes keyboard events in the range of 128 to 255.

Functions such as read-key-sequence that construct strings of keyboard input characters follow these rules: they construct vectors instead of strings, when the events won't fit in a string.

When you use the read syntax ‘\M-’ in a string, it produces a code in the range of 128 to 255—the same code that you get if you modify the corresponding keyboard event to put it in the string. Thus, meta events in strings work consistently regardless of how they get into the strings.

However, most programs would do well to avoid these issues by following the recommendations at the beginning of this section.

20.8 Reading Input

The editor command loop reads key sequences using the function read-key-sequence, which uses read-event. These and other functions for event input are also available for use in Lisp programs. See also momentary-string-display in Temporary Displays, and sit-for in Waiting. See Terminal Input, for functions and variables for controlling terminal input modes and debugging terminal input.

For higher-level input facilities, see Minibuffers.

• Key Sequence Input: How to read one key sequence.
• Reading One Event: How to read just one event.
• Event Mod: How Emacs modifies events as they are read.
• Invoking the Input Method: How reading an event uses the input method.
• Quoted Character Input: Asking the user to specify a character.
• Event Input Misc: How to reread or throw away input events.

20.8.1 Key Sequence Input

The command loop reads input a key sequence at a time, by calling read-key-sequence. Lisp programs can also call this function; for example, describe-key uses it to read the key to describe.

If an input character is upper-case (or has the shift modifier) and has no key binding, but its lower-case equivalent has one, then read-key-sequence converts the character to lower case. Note that lookup-key does not perform case conversion in this way.

When reading input results in such a shift-translation, Emacs sets the variable this-command-keys-shift-translated to a non-nil value. Lisp programs can examine this variable if they need to modify their behavior when invoked by shift-translated keys. For example, the function handle-shift-selection examines the value of this variable to determine how to activate or deactivate the region (see handle-shift-selection).

The function read-key-sequence also transforms some mouse events. It converts unbound drag events into click events, and discards unbound button-down events entirely. It also reshuffles focus events and miscellaneous window events so that they never appear in a key sequence with any other events.

When mouse events occur in special parts of a window, such as a mode line or a scroll bar, the event type shows nothing special—it is the same symbol that would normally represent that combination of mouse button and modifier keys. The information about the window part is kept elsewhere in the event—in the coordinates. But read-key-sequence translates this information into imaginary prefix keys, all of which are symbols: header-line, horizontal-scroll-bar, menu-bar, mode-line, vertical-line, and vertical-scroll-bar. You can define meanings for mouse clicks in special window parts by defining key sequences using these imaginary prefix keys.

For example, if you call read-key-sequence and then click the mouse on the window's mode line, you get two events, like this:

     (read-key-sequence "Click on the mode line: ")
          ⇒ [mode-line
              (mouse-1
               (#<window 6 on NEWS> mode-line
                (40 . 63) 5959987))]

20.8.2 Reading One Event

The lowest level functions for command input are read-event, read-char, and read-char-exclusive.

None of the above functions suppress quitting.

We emphasize that, unlike read-key-sequence, the functions read-event, read-char, and read-char-exclusive do not perform the translations described in Translation Keymaps. If you wish to read a single key taking these translations into account, use the function read-key:

20.8.3 Modifying and Translating Input Events

Emacs modifies every event it reads according to extra-keyboard-modifiers, then translates it through keyboard-translate-table (if applicable), before returning it from read-event.

Here's an example of using the keyboard-translate-table to make C-x, C-c and C-v perform the cut, copy and paste operations:

     (keyboard-translate ?\C-x 'control-x)
     (keyboard-translate ?\C-c 'control-c)
     (keyboard-translate ?\C-v 'control-v)
     (global-set-key [control-x] 'kill-region)
     (global-set-key [control-c] 'kill-ring-save)
     (global-set-key [control-v] 'yank)

On a graphical terminal that supports extended ASCII input, you can still get the standard Emacs meanings of one of those characters by typing it with the shift key. That makes it a different character as far as keyboard translation is concerned, but it has the same usual meaning.

See Translation Keymaps, for mechanisms that translate event sequences at the level of read-key-sequence.

20.8.4 Invoking the Input Method

The event-reading functions invoke the current input method, if any (see Input Methods). If the value of input-method-function is non-nil, it should be a function; when read-event reads a printing character (including <SPC>) with no modifier bits, it calls that function, passing the character as an argument.

The input method function should return a list of events which should be used as input. (If the list is nil, that means there is no input, so read-event waits for another event.) These events are processed before the events in unread-command-events (see Event Input Misc). Events returned by the input method function are not passed to the input method function again, even if they are printing characters with no modifier bits.

If the input method function calls read-event or read-key-sequence, it should bind input-method-function to nil first, to prevent recursion.

The input method function is not called when reading the second and subsequent events of a key sequence. Thus, these characters are not subject to input method processing. The input method function should test the values of overriding-local-map and overriding-terminal-local-map; if either of these variables is non-nil, the input method should put its argument into a list and return that list with no further processing.

20.8.5 Quoted Character Input

You can use the function read-quoted-char to ask the user to specify a character, and allow the user to specify a control or meta character conveniently, either literally or as an octal character code. The command quoted-insert uses this function.

20.8.6 Miscellaneous Event Input Features

This section describes how to peek ahead at events without using them up, how to check for pending input, and how to discard pending input. See also the function read-passwd (see Reading a Password).

20.9 Special Events

Certain special events are handled at a very low level—as soon as they are read. The read-event function processes these events itself, and never returns them. Instead, it keeps waiting for the first event that is not special and returns that one.

Special events do not echo, they are never grouped into key sequences, and they never appear in the value of last-command-event or (this-command-keys). They do not discard a numeric argument, they cannot be unread with unread-command-events, they may not appear in a keyboard macro, and they are not recorded in a keyboard macro while you are defining one.

Special events do, however, appear in last-input-event immediately after they are read, and this is the way for the event's definition to find the actual event.

The events types iconify-frame, make-frame-visible, delete-frame, drag-n-drop, language-change, and user signals like sigusr1 are normally handled in this way. The keymap which defines how to handle special events—and which events are special—is in the variable special-event-map (see Active Keymaps).

20.10 Waiting for Elapsed Time or Input

The wait functions are designed to wait for a certain amount of time to pass or until there is input. For example, you may wish to pause in the middle of a computation to allow the user time to view the display. sit-for pauses and updates the screen, and returns immediately if input comes in, while sleep-for pauses without updating the screen.

See Time of Day, for functions to get the current time.

20.11 Quitting

Typing C-g while a Lisp function is running causes Emacs to quit whatever it is doing. This means that control returns to the innermost active command loop.

Typing C-g while the command loop is waiting for keyboard input does not cause a quit; it acts as an ordinary input character. In the simplest case, you cannot tell the difference, because C-g normally runs the command keyboard-quit, whose effect is to quit. However, when C-g follows a prefix key, they combine to form an undefined key. The effect is to cancel the prefix key as well as any prefix argument.

In the minibuffer, C-g has a different definition: it aborts out of the minibuffer. This means, in effect, that it exits the minibuffer and then quits. (Simply quitting would return to the command loop within the minibuffer.) The reason why C-g does not quit directly when the command reader is reading input is so that its meaning can be redefined in the minibuffer in this way. C-g following a prefix key is not redefined in the minibuffer, and it has its normal effect of canceling the prefix key and prefix argument. This too would not be possible if C-g always quit directly.

When C-g does directly quit, it does so by setting the variable quit-flag to t. Emacs checks this variable at appropriate times and quits if it is not nil. Setting quit-flag non-nil in any way thus causes a quit.

At the level of C code, quitting cannot happen just anywhere; only at the special places that check quit-flag. The reason for this is that quitting at other places might leave an inconsistency in Emacs's internal state. Because quitting is delayed until a safe place, quitting cannot make Emacs crash.

Certain functions such as read-key-sequence or read-quoted-char prevent quitting entirely even though they wait for input. Instead of quitting, C-g serves as the requested input. In the case of read-key-sequence, this serves to bring about the special behavior of C-g in the command loop. In the case of read-quoted-char, this is so that C-q can be used to quote a C-g.

You can prevent quitting for a portion of a Lisp function by binding the variable inhibit-quit to a non-nil value. Then, although C-g still sets quit-flag to t as usual, the usual result of this—a quit—is prevented. Eventually, inhibit-quit will become nil again, such as when its binding is unwound at the end of a let form. At that time, if quit-flag is still non-nil, the requested quit happens immediately. This behavior is ideal when you wish to make sure that quitting does not happen within a critical section of the program.

In some functions (such as read-quoted-char), C-g is handled in a special way that does not involve quitting. This is done by reading the input with inhibit-quit bound to t, and setting quit-flag to nil before inhibit-quit becomes nil again. This excerpt from the definition of read-quoted-char shows how this is done; it also shows that normal quitting is permitted after the first character of input.

     (defun read-quoted-char (&optional prompt)
       "...documentation..."
       (let ((message-log-max nil) done (first t) (code 0) char)
         (while (not done)
           (let ((inhibit-quit first)
                 ...)
             (and prompt (message "%s-" prompt))
             (setq char (read-event))
             (if inhibit-quit (setq quit-flag nil)))
           ...set the variable code...)
         code))

You can specify a character other than C-g to use for quitting. See the function set-input-mode in Input Modes.

20.12 Prefix Command Arguments

Most Emacs commands can use a prefix argument, a number specified before the command itself. (Don't confuse prefix arguments with prefix keys.) The prefix argument is at all times represented by a value, which may be nil, meaning there is currently no prefix argument. Each command may use the prefix argument or ignore it.

There are two representations of the prefix argument: raw and numeric. The editor command loop uses the raw representation internally, and so do the Lisp variables that store the information, but commands can request either representation.

Here are the possible values of a raw prefix argument:

• nil, meaning there is no prefix argument. Its numeric value is 1, but numerous commands make a distinction between nil and the integer 1.
• An integer, which stands for itself.
• A list of one element, which is an integer. This form of prefix argument results from one or a succession of C-us with no digits. The numeric value is the integer in the list, but some commands make a distinction between such a list and an integer alone.
• The symbol -. This indicates that M-- or C-u - was typed, without following digits. The equivalent numeric value is −1, but some commands make a distinction between the integer −1 and the symbol -.

We illustrate these possibilities by calling the following function with various prefixes:

     (defun display-prefix (arg)
       "Display the value of the raw prefix arg."
       (interactive "P")
       (message "%s" arg))

Here are the results of calling display-prefix with various raw prefix arguments:

             M-x display-prefix  -| nil
     
     C-u     M-x display-prefix  -| (4)
     
     C-u C-u M-x display-prefix  -| (16)
     
     C-u 3   M-x display-prefix  -| 3
     
     M-3     M-x display-prefix  -| 3      ; (Same as C-u 3.)
     
     C-u -   M-x display-prefix  -| -
     
     M--     M-x display-prefix  -| -      ; (Same as C-u -.)
     
     C-u - 7 M-x display-prefix  -| -7
     
     M-- 7   M-x display-prefix  -| -7     ; (Same as C-u -7.)

Emacs uses two variables to store the prefix argument: prefix-arg and current-prefix-arg. Commands such as universal-argument that set up prefix arguments for other commands store them in prefix-arg. In contrast, current-prefix-arg conveys the prefix argument to the current command, so setting it has no effect on the prefix arguments for future commands.

Normally, commands specify which representation to use for the prefix argument, either numeric or raw, in the interactive specification. (See Using Interactive.) Alternatively, functions may look at the value of the prefix argument directly in the variable current-prefix-arg, but this is less clean.

The following commands exist to set up prefix arguments for the following command. Do not call them for any other reason.

20.13 Recursive Editing

The Emacs command loop is entered automatically when Emacs starts up. This top-level invocation of the command loop never exits; it keeps running as long as Emacs does. Lisp programs can also invoke the command loop. Since this makes more than one activation of the command loop, we call it recursive editing. A recursive editing level has the effect of suspending whatever command invoked it and permitting the user to do arbitrary editing before resuming that command.

The commands available during recursive editing are the same ones available in the top-level editing loop and defined in the keymaps. Only a few special commands exit the recursive editing level; the others return to the recursive editing level when they finish. (The special commands for exiting are always available, but they do nothing when recursive editing is not in progress.)

All command loops, including recursive ones, set up all-purpose error handlers so that an error in a command run from the command loop will not exit the loop.

Minibuffer input is a special kind of recursive editing. It has a few special wrinkles, such as enabling display of the minibuffer and the minibuffer window, but fewer than you might suppose. Certain keys behave differently in the minibuffer, but that is only because of the minibuffer's local map; if you switch windows, you get the usual Emacs commands.

To invoke a recursive editing level, call the function recursive-edit. This function contains the command loop; it also contains a call to catch with tag exit, which makes it possible to exit the recursive editing level by throwing to exit (see Catch and Throw). If you throw a value other than t, then recursive-edit returns normally to the function that called it. The command C-M-c (exit-recursive-edit) does this. Throwing a t value causes recursive-edit to quit, so that control returns to the command loop one level up. This is called aborting, and is done by C-] (abort-recursive-edit).

Most applications should not use recursive editing, except as part of using the minibuffer. Usually it is more convenient for the user if you change the major mode of the current buffer temporarily to a special major mode, which should have a command to go back to the previous mode. (The e command in Rmail uses this technique.) Or, if you wish to give the user different text to edit recursively, create and select a new buffer in a special mode. In this mode, define a command to complete the processing and go back to the previous buffer. (The m command in Rmail does this.)

Recursive edits are useful in debugging. You can insert a call to debug into a function definition as a sort of breakpoint, so that you can look around when the function gets there. debug invokes a recursive edit but also provides the other features of the debugger.

Recursive editing levels are also used when you type C-r in query-replace or use C-x q (kbd-macro-query).

20.14 Disabling Commands

Disabling a command marks the command as requiring user confirmation before it can be executed. Disabling is used for commands which might be confusing to beginning users, to prevent them from using the commands by accident.

The low-level mechanism for disabling a command is to put a non-nil disabled property on the Lisp symbol for the command. These properties are normally set up by the user's init file (see Init File) with Lisp expressions such as this:

     (put 'upcase-region 'disabled t)

For a few commands, these properties are present by default (you can remove them in your init file if you wish).

If the value of the disabled property is a string, the message saying the command is disabled includes that string. For example:

     (put 'delete-region 'disabled
          "Text deleted this way cannot be yanked back!\n")

See Disabling, for the details on what happens when a disabled command is invoked interactively. Disabling a command has no effect on calling it as a function from Lisp programs.

20.15 Command History

The command loop keeps a history of the complex commands that have been executed, to make it convenient to repeat these commands. A complex command is one for which the interactive argument reading uses the minibuffer. This includes any M-x command, any M-: command, and any command whose interactive specification reads an argument from the minibuffer. Explicit use of the minibuffer during the execution of the command itself does not cause the command to be considered complex.

This history list is actually a special case of minibuffer history (see Minibuffer History), with one special twist: the elements are expressions rather than strings.

There are a number of commands devoted to the editing and recall of previous commands. The commands repeat-complex-command, and list-command-history are described in the user manual (see Repetition). Within the minibuffer, the usual minibuffer history commands are available.

20.16 Keyboard Macros

A keyboard macro is a canned sequence of input events that can be considered a command and made the definition of a key. The Lisp representation of a keyboard macro is a string or vector containing the events. Don't confuse keyboard macros with Lisp macros (see Macros).

21 Keymaps

The command bindings of input events are recorded in data structures called keymaps. Each entry in a keymap associates (or binds) an individual event type, either to another keymap or to a command. When an event type is bound to a keymap, that keymap is used to look up the next input event; this continues until a command is found. The whole process is called key lookup.

• Key Sequences: Key sequences as Lisp objects.
• Keymap Basics: Basic concepts of keymaps.
• Format of Keymaps: What a keymap looks like as a Lisp object.
• Creating Keymaps: Functions to create and copy keymaps.
• Inheritance and Keymaps: How one keymap can inherit the bindings of another keymap.
• Prefix Keys: Defining a key with a keymap as its definition.
• Active Keymaps: How Emacs searches the active keymaps for a key binding.
• Searching Keymaps: A pseudo-Lisp summary of searching active maps.
• Controlling Active Maps: Each buffer has a local keymap to override the standard (global) bindings. A minor mode can also override them.
• Key Lookup: Finding a key's binding in one keymap.
• Functions for Key Lookup: How to request key lookup.
• Changing Key Bindings: Redefining a key in a keymap.
• Remapping Commands: A keymap can translate one command to another.
• Translation Keymaps: Keymaps for translating sequences of events.
• Key Binding Commands: Interactive interfaces for redefining keys.
• Scanning Keymaps: Looking through all keymaps, for printing help.
• Menu Keymaps: Defining a menu as a keymap.

21.1 Key Sequences

A key sequence, or key for short, is a sequence of one or more input events that form a unit. Input events include characters, function keys, mouse actions, or system events external to Emacs, such as iconify-frame (see Input Events). The Emacs Lisp representation for a key sequence is a string or vector. Unless otherwise stated, any Emacs Lisp function that accepts a key sequence as an argument can handle both representations.

In the string representation, alphanumeric characters ordinarily stand for themselves; for example, "a" represents a and "2" represents 2. Control character events are prefixed by the substring "\C-", and meta characters by "\M-"; for example, "\C-x" represents the key C-x. In addition, the <TAB>, <RET>, <ESC>, and <DEL> events are represented by "\t", "\r", "\e", and "\d" respectively. The string representation of a complete key sequence is the concatenation of the string representations of the constituent events; thus, "\C-xl" represents the key sequence C-x l.

Key sequences containing function keys, mouse button events, system events, or non-ASCII characters such as C-= or H-a cannot be represented as strings; they have to be represented as vectors.

In the vector representation, each element of the vector represents an input event, in its Lisp form. See Input Events. For example, the vector [?\C-x ?l] represents the key sequence C-x l.

For examples of key sequences written in string and vector representations, Init Rebinding.

21.2 Keymap Basics

A keymap is a Lisp data structure that specifies key bindings for various key sequences.

A single keymap directly specifies definitions for individual events. When a key sequence consists of a single event, its binding in a keymap is the keymap's definition for that event. The binding of a longer key sequence is found by an iterative process: first find the definition of the first event (which must itself be a keymap); then find the second event's definition in that keymap, and so on until all the events in the key sequence have been processed.

If the binding of a key sequence is a keymap, we call the key sequence a prefix key. Otherwise, we call it a complete key (because no more events can be added to it). If the binding is nil, we call the key undefined. Examples of prefix keys are C-c, C-x, and C-x 4. Examples of defined complete keys are X, <RET>, and C-x 4 C-f. Examples of undefined complete keys are C-x C-g, and C-c 3. See Prefix Keys, for more details.

The rule for finding the binding of a key sequence assumes that the intermediate bindings (found for the events before the last) are all keymaps; if this is not so, the sequence of events does not form a unit—it is not really one key sequence. In other words, removing one or more events from the end of any valid key sequence must always yield a prefix key. For example, C-f C-n is not a key sequence; C-f is not a prefix key, so a longer sequence starting with C-f cannot be a key sequence.

The set of possible multi-event key sequences depends on the bindings for prefix keys; therefore, it can be different for different keymaps, and can change when bindings are changed. However, a one-event sequence is always a key sequence, because it does not depend on any prefix keys for its well-formedness.

At any time, several primary keymaps are active—that is, in use for finding key bindings. These are the global map, which is shared by all buffers; the local keymap, which is usually associated with a specific major mode; and zero or more minor mode keymaps, which belong to currently enabled minor modes. (Not all minor modes have keymaps.) The local keymap bindings shadow (i.e., take precedence over) the corresponding global bindings. The minor mode keymaps shadow both local and global keymaps. See Active Keymaps, for details.

21.3 Format of Keymaps

Each keymap is a list whose car is the symbol keymap. The remaining elements of the list define the key bindings of the keymap. A symbol whose function definition is a keymap is also a keymap. Use the function keymapp (see below) to test whether an object is a keymap.

Several kinds of elements may appear in a keymap, after the symbol keymap that begins it:

(type . binding)

This specifies one binding, for events of type type. Each ordinary binding applies to events of a particular event type, which is always a character or a symbol. See Classifying Events. In this kind of binding, binding is a command.

(type item-name . binding)

This specifies a binding which is also a simple menu item that displays as item-name in the menu. See Simple Menu Items.

(type item-name help-string . binding)

This is a simple menu item with help string help-string.

(type menu-item . details)

This specifies a binding which is also an extended menu item. This allows use of other features. See Extended Menu Items.

(t . binding)

This specifies a default key binding; any event not bound by other elements of the keymap is given binding as its binding. Default bindings allow a keymap to bind all possible event types without having to enumerate all of them. A keymap that has a default binding completely masks any lower-precedence keymap, except for events explicitly bound to nil (see below).

char-table

If an element of a keymap is a char-table, it counts as holding bindings for all character events with no modifier bits (see modifier bits): the element whose index is c is the binding for the character c. This is a compact way to record lots of bindings. A keymap with such a char-table is called a full keymap. Other keymaps are called sparse keymaps.

vector

This kind of element is similar to a char-table: the element whose index is c is the binding for the character c. Since the range of characters that can be bound this way is limited by the vector size, and vector creation allocates space for all character codes from 0 up, this format should not be used except for creating menu keymaps (see Menu Keymaps), where the bindings themselves don't matter.

string

Aside from elements that specify bindings for keys, a keymap can also have a string as an element. This is called the overall prompt string and makes it possible to use the keymap as a menu. See Defining Menus.

(keymap ...)

If an element of a keymap is itself a keymap, it counts as if this inner keymap were inlined in the outer keymap. This is used for multiple-inheritance, such as in make-composed-keymap.

When the binding is nil, it doesn't constitute a definition but it does take precedence over a default binding or a binding in the parent keymap. On the other hand, a binding of nil does not override lower-precedence keymaps; thus, if the local map gives a binding of nil, Emacs uses the binding from the global map.

Keymaps do not directly record bindings for the meta characters. Instead, meta characters are regarded for purposes of key lookup as sequences of two characters, the first of which is <ESC> (or whatever is currently the value of meta-prefix-char). Thus, the key M-a is internally represented as <ESC> a, and its global binding is found at the slot for a in esc-map (see Prefix Keys).

This conversion applies only to characters, not to function keys or other input events; thus, M-<end> has nothing to do with <ESC> <end>.

Here as an example is the local keymap for Lisp mode, a sparse keymap. It defines bindings for <DEL>, C-c C-z, C-M-q, and C-M-x (the actual value also contains a menu binding, which is omitted here for the sake of brevity).

     lisp-mode-map
     ⇒
     (keymap
      (3 keymap
         ;; C-c C-z
         (26 . run-lisp))
      (27 keymap
          ;; C-M-x, treated as <ESC> C-x
          (24 . lisp-send-defun))
      ;; This part is inherited from lisp-mode-shared-map.
      keymap
      ;; <DEL>
      (127 . backward-delete-char-untabify)
      (27 keymap
          ;; C-M-q, treated as <ESC> C-q
          (17 . indent-sexp)))

21.4 Creating Keymaps

Here we describe the functions for creating keymaps.

21.5 Inheritance and Keymaps

A keymap can inherit the bindings of another keymap, which we call the parent keymap. Such a keymap looks like this:

     (keymap elements... . parent-keymap)

The effect is that this keymap inherits all the bindings of parent-keymap, whatever they may be at the time a key is looked up, but can add to them or override them with elements.

If you change the bindings in parent-keymap using define-key or other key-binding functions, these changed bindings are visible in the inheriting keymap, unless shadowed by the bindings made by elements. The converse is not true: if you use define-key to change bindings in the inheriting keymap, these changes are recorded in elements, but have no effect on parent-keymap.

The proper way to construct a keymap with a parent is to use set-keymap-parent; if you have code that directly constructs a keymap with a parent, please convert the program to use set-keymap-parent instead.

Here is an example showing how to make a keymap that inherits from text-mode-map:

     (let ((map (make-sparse-keymap)))
       (set-keymap-parent map text-mode-map)
       map)

A non-sparse keymap can have a parent too, but this is not very useful. A non-sparse keymap always specifies something as the binding for every numeric character code without modifier bits, even if it is nil, so these character's bindings are never inherited from the parent keymap.

Sometimes you want to make a keymap that inherits from more than one map. You can use the function make-composed-keymap for this.

For example, here is how Emacs sets the parent of help-mode-map, such that it inherits from both button-buffer-map and special-mode-map:

     (defvar help-mode-map
       (let ((map (make-sparse-keymap)))
         (set-keymap-parent map
           (make-composed-keymap button-buffer-map special-mode-map))
         ... map) ... )

21.6 Prefix Keys

A prefix key is a key sequence whose binding is a keymap. The keymap defines what to do with key sequences that extend the prefix key. For example, C-x is a prefix key, and it uses a keymap that is also stored in the variable ctl-x-map. This keymap defines bindings for key sequences starting with C-x.

Some of the standard Emacs prefix keys use keymaps that are also found in Lisp variables:

• esc-map is the global keymap for the <ESC> prefix key. Thus, the global definitions of all meta characters are actually found here. This map is also the function definition of ESC-prefix.
• help-map is the global keymap for the C-h prefix key.
• mode-specific-map is the global keymap for the prefix key C-c. This map is actually global, not mode-specific, but its name provides useful information about C-c in the output of C-h b (display-bindings), since the main use of this prefix key is for mode-specific bindings.
• ctl-x-map is the global keymap used for the C-x prefix key. This map is found via the function cell of the symbol Control-X-prefix.
• mule-keymap is the global keymap used for the C-x <RET> prefix key.
• ctl-x-4-map is the global keymap used for the C-x 4 prefix key.
• ctl-x-5-map is the global keymap used for the C-x 5 prefix key.
• 2C-mode-map is the global keymap used for the C-x 6 prefix key.
• vc-prefix-map is the global keymap used for the C-x v prefix key.
• goto-map is the global keymap used for the M-g prefix key.
• search-map is the global keymap used for the M-s prefix key.
• facemenu-keymap is the global keymap used for the M-o prefix key.
• The other Emacs prefix keys are C-x @, C-x a i, C-x <ESC> and <ESC> <ESC>. They use keymaps that have no special names.

The keymap binding of a prefix key is used for looking up the event that follows the prefix key. (It may instead be a symbol whose function definition is a keymap. The effect is the same, but the symbol serves as a name for the prefix key.) Thus, the binding of C-x is the symbol Control-X-prefix, whose function cell holds the keymap for C-x commands. (The same keymap is also the value of ctl-x-map.)

Prefix key definitions can appear in any active keymap. The definitions of C-c, C-x, C-h and <ESC> as prefix keys appear in the global map, so these prefix keys are always available. Major and minor modes can redefine a key as a prefix by putting a prefix key definition for it in the local map or the minor mode's map. See Active Keymaps.

If a key is defined as a prefix in more than one active map, then its various definitions are in effect merged: the commands defined in the minor mode keymaps come first, followed by those in the local map's prefix definition, and then by those from the global map.

In the following example, we make C-p a prefix key in the local keymap, in such a way that C-p is identical to C-x. Then the binding for C-p C-f is the function find-file, just like C-x C-f. The key sequence C-p 6 is not found in any active keymap.

     (use-local-map (make-sparse-keymap))
         ⇒ nil
     (local-set-key "\C-p" ctl-x-map)
         ⇒ nil
     (key-binding "\C-p\C-f")
         ⇒ find-file
     
     (key-binding "\C-p6")
         ⇒ nil

21.7 Active Keymaps

Emacs contains many keymaps, but at any time only a few keymaps are active. When Emacs receives user input, it translates the input event (see Translation Keymaps), and looks for a key binding in the active keymaps.

Usually, the active keymaps are: (i) the keymap specified by the keymap property, (ii) the keymaps of enabled minor modes, (iii) the current buffer's local keymap, and (iv) the global keymap, in that order. Emacs searches for each input key sequence in all these keymaps.

Of these usual keymaps, the highest-precedence one is specified by the keymap text or overlay property at point, if any. (For a mouse input event, Emacs uses the event position instead of point; see Searching Keymaps.)

Next in precedence are keymaps specified by enabled minor modes. These keymaps, if any, are specified by the variables emulation-mode-map-alists, minor-mode-overriding-map-alist, and minor-mode-map-alist. See Controlling Active Maps.

Next in precedence is the buffer's local keymap, containing key bindings specific to the buffer. The minibuffer also has a local keymap (see Intro to Minibuffers). If there is a local-map text or overlay property at point, that specifies the local keymap to use, in place of the buffer's default local keymap.

The local keymap is normally set by the buffer's major mode, and every buffer with the same major mode shares the same local keymap. Hence, if you call local-set-key (see Key Binding Commands) to change the local keymap in one buffer, that also affects the local keymaps in other buffers with the same major mode.

Finally, the global keymap contains key bindings that are defined regardless of the current buffer, such as C-f. It is always active, and is bound to the variable global-map.

Apart from the above usual keymaps, Emacs provides special ways for programs to make other keymaps active. Firstly, the variable overriding-local-map specifies a keymap that replaces the usual active keymaps, except for the global keymap. Secondly, the terminal-local variable overriding-terminal-local-map specifies a keymap that takes precedence over all other keymaps (including overriding-local-map); this is normally used for modal/transient keybindings (the function set-transient-map provides a convenient interface for this). See Controlling Active Maps, for details.

Making keymaps active is not the only way to use them. Keymaps are also used in other ways, such as for translating events within read-key-sequence. See Translation Keymaps.

See Standard Keymaps, for a list of some standard keymaps.

21.8 Searching the Active Keymaps

Here is a pseudo-Lisp summary of how Emacs searches the active keymaps:

     (or (if overriding-terminal-local-map
             (find-in overriding-terminal-local-map))
         (if overriding-local-map
             (find-in overriding-local-map)
           (or (find-in (get-char-property (point) 'keymap))
               (find-in-any emulation-mode-map-alists)
               (find-in-any minor-mode-overriding-map-alist)
               (find-in-any minor-mode-map-alist)
               (if (get-text-property (point) 'local-map)
                   (find-in (get-char-property (point) 'local-map))
                 (find-in (current-local-map)))))
         (find-in (current-global-map)))

Here, find-in and find-in-any are pseudo functions that search in one keymap and in an alist of keymaps, respectively. Note that the set-transient-map function works by setting overriding-terminal-local-map (see Controlling Active Maps).

In the above pseudo-code, if a key sequence starts with a mouse event (see Mouse Events), that event's position is used instead of point, and the event's buffer is used instead of the current buffer. In particular, this affects how the keymap and local-map properties are looked up. If a mouse event occurs on a string embedded with a display, before-string, or after-string property (see Special Properties), and the string has a non-nil keymap or local-map property, that overrides the corresponding property in the underlying buffer text (i.e., the property specified by the underlying text is ignored).

When a key binding is found in one of the active keymaps, and that binding is a command, the search is over—the command is executed. However, if the binding is a symbol with a value or a string, Emacs replaces the input key sequences with the variable's value or the string, and restarts the search of the active keymaps. See Key Lookup.

The command which is finally found might also be remapped. See Remapping Commands.

21.9 Controlling the Active Keymaps

current-local-map returns a reference to the local keymap, not a copy of it; if you use define-key or other functions on it you will alter local bindings.

21.10 Key Lookup

Key lookup is the process of finding the binding of a key sequence from a given keymap. The execution or use of the binding is not part of key lookup.

Key lookup uses just the event type of each event in the key sequence; the rest of the event is ignored. In fact, a key sequence used for key lookup may designate a mouse event with just its types (a symbol) instead of the entire event (a list). See Input Events. Such a key sequence is insufficient for command-execute to run, but it is sufficient for looking up or rebinding a key.

When the key sequence consists of multiple events, key lookup processes the events sequentially: the binding of the first event is found, and must be a keymap; then the second event's binding is found in that keymap, and so on until all the events in the key sequence are used up. (The binding thus found for the last event may or may not be a keymap.) Thus, the process of key lookup is defined in terms of a simpler process for looking up a single event in a keymap. How that is done depends on the type of object associated with the event in that keymap.

Let's use the term keymap entry to describe the value found by looking up an event type in a keymap. (This doesn't include the item string and other extra elements in a keymap element for a menu item, because lookup-key and other key lookup functions don't include them in the returned value.) While any Lisp object may be stored in a keymap as a keymap entry, not all make sense for key lookup. Here is a table of the meaningful types of keymap entries:

nil

nil means that the events used so far in the lookup form an undefined key. When a keymap fails to mention an event type at all, and has no default binding, that is equivalent to a binding of nil for that event type.

command

The events used so far in the lookup form a complete key, and command is its binding. See What Is a Function.

array

The array (either a string or a vector) is a keyboard macro. The events used so far in the lookup form a complete key, and the array is its binding. See Keyboard Macros, for more information.

keymap

The events used so far in the lookup form a prefix key. The next event of the key sequence is looked up in keymap.

list

The meaning of a list depends on what it contains:

• If the car of list is the symbol keymap, then the list is a keymap, and is treated as a keymap (see above).
• If the car of list is lambda, then the list is a lambda expression. This is presumed to be a function, and is treated as such (see above). In order to execute properly as a key binding, this function must be a command—it must have an interactive specification. See Defining Commands.

symbol

The function definition of symbol is used in place of symbol. If that too is a symbol, then this process is repeated, any number of times. Ultimately this should lead to an object that is a keymap, a command, or a keyboard macro.

Note that keymaps and keyboard macros (strings and vectors) are not valid functions, so a symbol with a keymap, string, or vector as its function definition is invalid as a function. It is, however, valid as a key binding. If the definition is a keyboard macro, then the symbol is also valid as an argument to command-execute (see Interactive Call).

The symbol undefined is worth special mention: it means to treat the key as undefined. Strictly speaking, the key is defined, and its binding is the command undefined; but that command does the same thing that is done automatically for an undefined key: it rings the bell (by calling ding) but does not signal an error.

undefined is used in local keymaps to override a global key binding and make the key undefined locally. A local binding of nil would fail to do this because it would not override the global binding.

anything else

If any other type of object is found, the events used so far in the lookup form a complete key, and the object is its binding, but the binding is not executable as a command.

In short, a keymap entry may be a keymap, a command, a keyboard macro, a symbol that leads to one of them, or nil.

21.11 Functions for Key Lookup

Here are the functions and variables pertaining to key lookup.

21.12 Changing Key Bindings

The way to rebind a key is to change its entry in a keymap. If you change a binding in the global keymap, the change is effective in all buffers (though it has no direct effect in buffers that shadow the global binding with a local one). If you change the current buffer's local map, that usually affects all buffers using the same major mode. The global-set-key and local-set-key functions are convenient interfaces for these operations (see Key Binding Commands). You can also use define-key, a more general function; then you must explicitly specify the map to change.

When choosing the key sequences for Lisp programs to rebind, please follow the Emacs conventions for use of various keys (see Key Binding Conventions).

In writing the key sequence to rebind, it is good to use the special escape sequences for control and meta characters (see String Type). The syntax ‘\C-’ means that the following character is a control character and ‘\M-’ means that the following character is a meta character. Thus, the string "\M-x" is read as containing a single M-x, "\C-f" is read as containing a single C-f, and "\M-\C-x" and "\C-\M-x" are both read as containing a single C-M-x. You can also use this escape syntax in vectors, as well as others that aren't allowed in strings; one example is ‘[?\C-\H-x home]’. See Character Type.

The key definition and lookup functions accept an alternate syntax for event types in a key sequence that is a vector: you can use a list containing modifier names plus one base event (a character or function key name). For example, (control ?a) is equivalent to ?\C-a and (hyper control left) is equivalent to C-H-left. One advantage of such lists is that the precise numeric codes for the modifier bits don't appear in compiled files.

The functions below signal an error if keymap is not a keymap, or if key is not a string or vector representing a key sequence. You can use event types (symbols) as shorthand for events that are lists. The kbd function (see Key Sequences) is a convenient way to specify the key sequence.

This example creates a sparse keymap and makes a number of bindings in it:

     (setq map (make-sparse-keymap))
         ⇒ (keymap)
     (define-key map "\C-f" 'forward-char)
         ⇒ forward-char
     map
         ⇒ (keymap (6 . forward-char))
     
     ;; Build sparse submap for C-x and bind f in that.
     (define-key map (kbd "C-x f") 'forward-word)
         ⇒ forward-word
     map
     ⇒ (keymap
         (24 keymap                ; C-x
             (102 . forward-word)) ;      f
         (6 . forward-char))       ; C-f
     
     ;; Bind C-p to the ctl-x-map.
     (define-key map (kbd "C-p") ctl-x-map)
     ;; ctl-x-map
     ⇒ [nil ... find-file ... backward-kill-sentence]
     
     ;; Bind C-f to foo in the ctl-x-map.
     (define-key map (kbd "C-p C-f") 'foo)
     ⇒ 'foo
     map
     ⇒ (keymap     ; Note foo in ctl-x-map.
         (16 keymap [nil ... foo ... backward-kill-sentence])
         (24 keymap
             (102 . forward-word))
         (6 . forward-char))

Note that storing a new binding for C-p C-f actually works by changing an entry in ctl-x-map, and this has the effect of changing the bindings of both C-p C-f and C-x C-f in the default global map.

The function substitute-key-definition scans a keymap for keys that have a certain binding and rebinds them with a different binding. Another feature which is cleaner and can often produce the same results is to remap one command into another (see Remapping Commands).

21.13 Remapping Commands

A special kind of key binding can be used to remap one command to another, without having to refer to the key sequence(s) bound to the original command. To use this feature, make a key binding for a key sequence that starts with the dummy event remap, followed by the command name you want to remap; for the binding, specify the new definition (usually a command name, but possibly any other valid definition for a key binding).

For example, suppose My mode provides a special command my-kill-line, which should be invoked instead of kill-line. To establish this, its mode keymap should contain the following remapping:

     (define-key my-mode-map [remap kill-line] 'my-kill-line)

Then, whenever my-mode-map is active, if the user types C-k (the default global key sequence for kill-line) Emacs will instead run my-kill-line.

Note that remapping only takes place through active keymaps; for example, putting a remapping in a prefix keymap like ctl-x-map typically has no effect, as such keymaps are not themselves active. In addition, remapping only works through a single level; in the following example,

     (define-key my-mode-map [remap kill-line] 'my-kill-line)
     (define-key my-mode-map [remap my-kill-line] 'my-other-kill-line)

kill-line is not remapped to my-other-kill-line. Instead, if an ordinary key binding specifies kill-line, it is remapped to my-kill-line; if an ordinary binding specifies my-kill-line, it is remapped to my-other-kill-line.

To undo the remapping of a command, remap it to nil; e.g.,

     (define-key my-mode-map [remap kill-line] nil)

21.14 Keymaps for Translating Sequences of Events

When the read-key-sequence function reads a key sequence (see Key Sequence Input), it uses translation keymaps to translate certain event sequences into others. The translation keymaps are input-decode-map, local-function-key-map, and key-translation-map (in order of priority).

Translation keymaps have the same structure as other keymaps, but are used differently: they specify translations to make while reading key sequences, rather than bindings for complete key sequences. As each key sequence is read, it is checked against each translation keymap. If one of the translation keymaps binds k to a vector v, then whenever k appears as a sub-sequence anywhere in a key sequence, that sub-sequence is replaced with the events in v.

For example, VT100 terminals send <ESC> O P when the keypad key <PF1> is pressed. On such terminals, Emacs must translate that sequence of events into a single event pf1. This is done by binding <ESC> O P to [pf1] in input-decode-map. Thus, when you type C-c <PF1> on the terminal, the terminal emits the character sequence C-c <ESC> O P, and read-key-sequence translates this back into C-c <PF1> and returns it as the vector [?\C-c pf1].

Translation keymaps take effect only after Emacs has decoded the keyboard input (via the input coding system specified by keyboard-coding-system). See Terminal I/O Encoding.

You can use input-decode-map, local-function-key-map, and key-translation-map for more than simple aliases, by using a function, instead of a key sequence, as the translation of a key. Then this function is called to compute the translation of that key.

The key translation function receives one argument, which is the prompt that was specified in read-key-sequence—or nil if the key sequence is being read by the editor command loop. In most cases you can ignore the prompt value.

If the function reads input itself, it can have the effect of altering the event that follows. For example, here's how to define C-c h to turn the character that follows into a Hyper character:

     (defun hyperify (prompt)
       (let ((e (read-event)))
         (vector (if (numberp e)
                     (logior (lsh 1 24) e)
                   (if (memq 'hyper (event-modifiers e))
                       e
                     (add-event-modifier "H-" e))))))
     
     (defun add-event-modifier (string e)
       (let ((symbol (if (symbolp e) e (car e))))
         (setq symbol (intern (concat string
                                      (symbol-name symbol))))
         (if (symbolp e)
             symbol
           (cons symbol (cdr e)))))
     
     (define-key local-function-key-map "\C-ch" 'hyperify)

21.14.1 Interaction with normal keymaps

The end of a key sequence is detected when that key sequence either is bound to a command, or when Emacs determines that no additional event can lead to a sequence that is bound to a command.

This means that, while input-decode-map and key-translation-map apply regardless of whether the original key sequence would have a binding, the presence of such a binding can still prevent translation from taking place. For example, let us return to our VT100 example above and add a binding for C-c <ESC> to the global map; now when the user hits C-c <PF1> Emacs will fail to decode C-c <ESC> O P into C-c <PF1> because it will stop reading keys right after C-x <ESC>, leaving O P for later. This is in case the user really hit C-c <ESC>, in which case Emacs should not sit there waiting for the next key to decide whether the user really pressed <ESC> or <PF1>.

For that reason, it is better to avoid binding commands to key sequences where the end of the key sequence is a prefix of a key translation. The main such problematic suffixes/prefixes are <ESC>, M-O (which is really <ESC> O) and M-[ (which is really <ESC> [).

21.15 Commands for Binding Keys

This section describes some convenient interactive interfaces for changing key bindings. They work by calling define-key.

People often use global-set-key in their init files (see Init File) for simple customization. For example,

     (global-set-key (kbd "C-x C-\\") 'next-line)

or

     (global-set-key [?\C-x ?\C-\\] 'next-line)

or

     (global-set-key [(control ?x) (control ?\\)] 'next-line)

redefines C-x C-\ to move down a line.

     (global-set-key [M-mouse-1] 'mouse-set-point)

redefines the first (leftmost) mouse button, entered with the Meta key, to set point where you click.

Be careful when using non-ASCII text characters in Lisp specifications of keys to bind. If these are read as multibyte text, as they usually will be in a Lisp file (see Loading Non-ASCII), you must type the keys as multibyte too. For instance, if you use this:

     (global-set-key "ö" 'my-function) ; bind o-umlaut

or

     (global-set-key ?ö 'my-function) ; bind o-umlaut

and your language environment is multibyte Latin-1, these commands actually bind the multibyte character with code 246, not the byte code 246 (M-v) sent by a Latin-1 terminal. In order to use this binding, you need to teach Emacs how to decode the keyboard by using an appropriate input method (see Input Methods).

21.16 Scanning Keymaps

This section describes functions used to scan all the current keymaps for the sake of printing help information.

21.17 Menu Keymaps

A keymap can operate as a menu as well as defining bindings for keyboard keys and mouse buttons. Menus are usually actuated with the mouse, but they can function with the keyboard also. If a menu keymap is active for the next input event, that activates the keyboard menu feature.

• Defining Menus: How to make a keymap that defines a menu.
• Mouse Menus: How users actuate the menu with the mouse.
• Keyboard Menus: How users actuate the menu with the keyboard.
• Menu Example: Making a simple menu.
• Menu Bar: How to customize the menu bar.
• Tool Bar: A tool bar is a row of images.
• Modifying Menus: How to add new items to a menu.
• Easy Menu: A convenience macro for making menus.

21.17.1 Defining Menus

A keymap acts as a menu if it has an overall prompt string, which is a string that appears as an element of the keymap. (See Format of Keymaps.) The string should describe the purpose of the menu's commands. Emacs displays the overall prompt string as the menu title in some cases, depending on the toolkit (if any) used for displaying menus.¹² Keyboard menus also display the overall prompt string.

The easiest way to construct a keymap with a prompt string is to specify the string as an argument when you call make-keymap, make-sparse-keymap (see Creating Keymaps), or define-prefix-command (see Definition of define-prefix-command). If you do not want the keymap to operate as a menu, don't specify a prompt string for it.

The menu's items are the bindings in the keymap. Each binding associates an event type to a definition, but the event types have no significance for the menu appearance. (Usually we use pseudo-events, symbols that the keyboard cannot generate, as the event types for menu item bindings.) The menu is generated entirely from the bindings that correspond in the keymap to these events.

The order of items in the menu is the same as the order of bindings in the keymap. Since define-key puts new bindings at the front, you should define the menu items starting at the bottom of the menu and moving to the top, if you care about the order. When you add an item to an existing menu, you can specify its position in the menu using define-key-after (see Modifying Menus).

• Simple Menu Items: A simple kind of menu key binding.
• Extended Menu Items: More complex menu item definitions.
• Menu Separators: Drawing a horizontal line through a menu.
• Alias Menu Items: Using command aliases in menu items.

21.17.1.1 Simple Menu Items

The simpler (and original) way to define a menu item is to bind some event type (it doesn't matter what event type) to a binding like this:

     (item-string . real-binding)

The car, item-string, is the string to be displayed in the menu. It should be short—preferably one to three words. It should describe the action of the command it corresponds to. Note that not all graphical toolkits can display non-ASCII text in menus (it will work for keyboard menus and will work to a large extent with the GTK+ toolkit).

You can also supply a second string, called the help string, as follows:

     (item-string help . real-binding)

help specifies a help-echo string to display while the mouse is on that item in the same way as help-echo text properties (see Help display).

As far as define-key is concerned, item-string and help-string are part of the event's binding. However, lookup-key returns just real-binding, and only real-binding is used for executing the key.

If real-binding is nil, then item-string appears in the menu but cannot be selected.

If real-binding is a symbol and has a non-nil menu-enable property, that property is an expression that controls whether the menu item is enabled. Every time the keymap is used to display a menu, Emacs evaluates the expression, and it enables the menu item only if the expression's value is non-nil. When a menu item is disabled, it is displayed in a fuzzy fashion, and cannot be selected.

The menu bar does not recalculate which items are enabled every time you look at a menu. This is because the X toolkit requires the whole tree of menus in advance. To force recalculation of the menu bar, call force-mode-line-update (see Mode Line Format).

21.17.1.2 Extended Menu Items

An extended-format menu item is a more flexible and also cleaner alternative to the simple format. You define an event type with a binding that's a list starting with the symbol menu-item. For a non-selectable string, the binding looks like this:

     (menu-item item-name)

A string starting with two or more dashes specifies a separator line; see Menu Separators.

To define a real menu item which can be selected, the extended format binding looks like this:

     (menu-item item-name real-binding
         . item-property-list)

Here, item-name is an expression which evaluates to the menu item string. Thus, the string need not be a constant. The third element, real-binding, is the command to execute. The tail of the list, item-property-list, has the form of a property list which contains other information.

Here is a table of the properties that are supported:

:enable form

The result of evaluating form determines whether the item is enabled (non-nil means yes). If the item is not enabled, you can't really click on it.

:visible form

The result of evaluating form determines whether the item should actually appear in the menu (non-nil means yes). If the item does not appear, then the menu is displayed as if this item were not defined at all.

:help help

The value of this property, help, specifies a help-echo string to display while the mouse is on that item. This is displayed in the same way as help-echo text properties (see Help display). Note that this must be a constant string, unlike the help-echo property for text and overlays.

:button (type . selected)

This property provides a way to define radio buttons and toggle buttons. The car, type, says which: it should be :toggle or :radio. The cdr, selected, should be a form; the result of evaluating it says whether this button is currently selected.

A toggle is a menu item which is labeled as either on or off according to the value of selected. The command itself should toggle selected, setting it to t if it is nil, and to nil if it is t. Here is how the menu item to toggle the debug-on-error flag is defined:

          (menu-item "Debug on Error" toggle-debug-on-error
                     :button (:toggle
                              . (and (boundp 'debug-on-error)
                                     debug-on-error)))

This works because toggle-debug-on-error is defined as a command which toggles the variable debug-on-error.

Radio buttons are a group of menu items, in which at any time one and only one is selected. There should be a variable whose value says which one is selected at any time. The selected form for each radio button in the group should check whether the variable has the right value for selecting that button. Clicking on the button should set the variable so that the button you clicked on becomes selected.

:key-sequence key-sequence

This property specifies which key sequence is likely to be bound to the same command invoked by this menu item. If you specify the right key sequence, that makes preparing the menu for display run much faster.

If you specify the wrong key sequence, it has no effect; before Emacs displays key-sequence in the menu, it verifies that key-sequence is really equivalent to this menu item.

:key-sequence nil

This property indicates that there is normally no key binding which is equivalent to this menu item. Using this property saves time in preparing the menu for display, because Emacs does not need to search the keymaps for a keyboard equivalent for this menu item.

However, if the user has rebound this item's definition to a key sequence, Emacs ignores the :keys property and finds the keyboard equivalent anyway.

:keys string

This property specifies that string is the string to display as the keyboard equivalent for this menu item. You can use the ‘\\[...]’ documentation construct in string.

:filter filter-fn

This property provides a way to compute the menu item dynamically. The property value filter-fn should be a function of one argument; when it is called, its argument will be real-binding. The function should return the binding to use instead.

Emacs can call this function at any time that it does redisplay or operates on menu data structures, so you should write it so it can safely be called at any time.

21.17.1.3 Menu Separators

A menu separator is a kind of menu item that doesn't display any text—instead, it divides the menu into subparts with a horizontal line. A separator looks like this in the menu keymap:

     (menu-item separator-type)

where separator-type is a string starting with two or more dashes.

In the simplest case, separator-type consists of only dashes. That specifies the default kind of separator. (For compatibility, "" and - also count as separators.)

Certain other values of separator-type specify a different style of separator. Here is a table of them:

"--no-line"

"--space"

An extra vertical space, with no actual line.

"--single-line"

A single line in the menu's foreground color.

"--double-line"

A double line in the menu's foreground color.

"--single-dashed-line"

A single dashed line in the menu's foreground color.

"--double-dashed-line"

A double dashed line in the menu's foreground color.

"--shadow-etched-in"

A single line with a 3D sunken appearance. This is the default, used separators consisting of dashes only.

"--shadow-etched-out"

A single line with a 3D raised appearance.

"--shadow-etched-in-dash"

A single dashed line with a 3D sunken appearance.

"--shadow-etched-out-dash"

A single dashed line with a 3D raised appearance.

"--shadow-double-etched-in"

Two lines with a 3D sunken appearance.

"--shadow-double-etched-out"

Two lines with a 3D raised appearance.

"--shadow-double-etched-in-dash"

Two dashed lines with a 3D sunken appearance.

"--shadow-double-etched-out-dash"

Two dashed lines with a 3D raised appearance.

You can also give these names in another style, adding a colon after the double-dash and replacing each single dash with capitalization of the following word. Thus, "--:singleLine", is equivalent to "--single-line".

You can use a longer form to specify keywords such as :enable and :visible for a menu separator:

(menu-item separator-type nil . item-property-list)

For example:

     (menu-item "--" nil :visible (boundp 'foo))

Some systems and display toolkits don't really handle all of these separator types. If you use a type that isn't supported, the menu displays a similar kind of separator that is supported.

21.17.1.4 Alias Menu Items

Sometimes it is useful to make menu items that use the same command but with different enable conditions. The best way to do this in Emacs now is with extended menu items; before that feature existed, it could be done by defining alias commands and using them in menu items. Here's an example that makes two aliases for read-only-mode and gives them different enable conditions:

     (defalias 'make-read-only 'read-only-mode)
     (put 'make-read-only 'menu-enable '(not buffer-read-only))
     (defalias 'make-writable 'read-only-mode)
     (put 'make-writable 'menu-enable 'buffer-read-only)

When using aliases in menus, often it is useful to display the equivalent key bindings for the real command name, not the aliases (which typically don't have any key bindings except for the menu itself). To request this, give the alias symbol a non-nil menu-alias property. Thus,

     (put 'make-read-only 'menu-alias t)
     (put 'make-writable 'menu-alias t)

causes menu items for make-read-only and make-writable to show the keyboard bindings for read-only-mode.

21.17.2 Menus and the Mouse

The usual way to make a menu keymap produce a menu is to make it the definition of a prefix key. (A Lisp program can explicitly pop up a menu and receive the user's choice—see Pop-Up Menus.)

If the prefix key ends with a mouse event, Emacs handles the menu keymap by popping up a visible menu, so that the user can select a choice with the mouse. When the user clicks on a menu item, the event generated is whatever character or symbol has the binding that brought about that menu item. (A menu item may generate a series of events if the menu has multiple levels or comes from the menu bar.)

It's often best to use a button-down event to trigger the menu. Then the user can select a menu item by releasing the button.

If the menu keymap contains a binding to a nested keymap, the nested keymap specifies a submenu. There will be a menu item, labeled by the nested keymap's item string, and clicking on this item automatically pops up the specified submenu. As a special exception, if the menu keymap contains a single nested keymap and no other menu items, the menu shows the contents of the nested keymap directly, not as a submenu.

However, if Emacs is compiled without X toolkit support, or on text terminals, submenus are not supported. Each nested keymap is shown as a menu item, but clicking on it does not automatically pop up the submenu. If you wish to imitate the effect of submenus, you can do that by giving a nested keymap an item string which starts with ‘@’. This causes Emacs to display the nested keymap using a separate menu pane; the rest of the item string after the ‘@’ is the pane label. If Emacs is compiled without X toolkit support, or if a menu is displayed on a text terminal, menu panes are not used; in that case, a ‘@’ at the beginning of an item string is omitted when the menu label is displayed, and has no other effect.

21.17.3 Menus and the Keyboard

When a prefix key ending with a keyboard event (a character or function key) has a definition that is a menu keymap, the keymap operates as a keyboard menu; the user specifies the next event by choosing a menu item with the keyboard.

Emacs displays the keyboard menu with the map's overall prompt string, followed by the alternatives (the item strings of the map's bindings), in the echo area. If the bindings don't all fit at once, the user can type <SPC> to see the next line of alternatives. Successive uses of <SPC> eventually get to the end of the menu and then cycle around to the beginning. (The variable menu-prompt-more-char specifies which character is used for this; <SPC> is the default.)

When the user has found the desired alternative from the menu, he or she should type the corresponding character—the one whose binding is that alternative.

21.17.4 Menu Example

Here is a complete example of defining a menu keymap. It is the definition of the ‘Replace’ submenu in the ‘Edit’ menu in the menu bar, and it uses the extended menu item format (see Extended Menu Items). First we create the keymap, and give it a name:

     (defvar menu-bar-replace-menu (make-sparse-keymap "Replace"))

Next we define the menu items:

     (define-key menu-bar-replace-menu [tags-repl-continue]
       '(menu-item "Continue Replace" tags-loop-continue
                   :help "Continue last tags replace operation"))
     (define-key menu-bar-replace-menu [tags-repl]
       '(menu-item "Replace in tagged files" tags-query-replace
                   :help "Interactively replace a regexp in all tagged files"))
     (define-key menu-bar-replace-menu [separator-replace-tags]
       '(menu-item "--"))
     ;; ...

Note the symbols which the bindings are made for; these appear inside square brackets, in the key sequence being defined. In some cases, this symbol is the same as the command name; sometimes it is different. These symbols are treated as function keys, but they are not real function keys on the keyboard. They do not affect the functioning of the menu itself, but they are echoed in the echo area when the user selects from the menu, and they appear in the output of where-is and apropos.

The menu in this example is intended for use with the mouse. If a menu is intended for use with the keyboard, that is, if it is bound to a key sequence ending with a keyboard event, then the menu items should be bound to characters or real function keys, that can be typed with the keyboard.

The binding whose definition is ("--") is a separator line. Like a real menu item, the separator has a key symbol, in this case separator-replace-tags. If one menu has two separators, they must have two different key symbols.

Here is how we make this menu appear as an item in the parent menu:

     (define-key menu-bar-edit-menu [replace]
       (list 'menu-item "Replace" menu-bar-replace-menu))

Note that this incorporates the submenu keymap, which is the value of the variable menu-bar-replace-menu, rather than the symbol menu-bar-replace-menu itself. Using that symbol in the parent menu item would be meaningless because menu-bar-replace-menu is not a command.

If you wanted to attach the same replace menu to a mouse click, you can do it this way:

     (define-key global-map [C-S-down-mouse-1]
        menu-bar-replace-menu)

21.17.5 The Menu Bar

Emacs usually shows a menu bar at the top of each frame. See Menu Bars. Menu bar items are subcommands of the fake function key menu-bar, as defined in the active keymaps.

To add an item to the menu bar, invent a fake function key of your own (let's call it key), and make a binding for the key sequence [menu-bar key]. Most often, the binding is a menu keymap, so that pressing a button on the menu bar item leads to another menu.

When more than one active keymap defines the same function key for the menu bar, the item appears just once. If the user clicks on that menu bar item, it brings up a single, combined menu containing all the subcommands of that item—the global subcommands, the local subcommands, and the minor mode subcommands.

The variable overriding-local-map is normally ignored when determining the menu bar contents. That is, the menu bar is computed from the keymaps that would be active if overriding-local-map were nil. See Active Keymaps.

Here's an example of setting up a menu bar item:

     ;; Make a menu keymap (with a prompt string)
     ;; and make it the menu bar item's definition.
     (define-key global-map [menu-bar words]
       (cons "Words" (make-sparse-keymap "Words")))
     
     ;; Define specific subcommands in this menu.
     (define-key global-map
       [menu-bar words forward]
       '("Forward word" . forward-word))
     (define-key global-map
       [menu-bar words backward]
       '("Backward word" . backward-word))

A local keymap can cancel a menu bar item made by the global keymap by rebinding the same fake function key with undefined as the binding. For example, this is how Dired suppresses the ‘Edit’ menu bar item:

     (define-key dired-mode-map [menu-bar edit] 'undefined)

Here, edit is the fake function key used by the global map for the ‘Edit’ menu bar item. The main reason to suppress a global menu bar item is to regain space for mode-specific items.

Next to every menu bar item, Emacs displays a key binding that runs the same command (if such a key binding exists). This serves as a convenient hint for users who do not know the key binding. If a command has multiple bindings, Emacs normally displays the first one it finds. You can specify one particular key binding by assigning an :advertised-binding symbol property to the command. See Keys in Documentation.

21.17.6 Tool bars

A tool bar is a row of clickable icons at the top of a frame, just below the menu bar. See Tool Bars. Emacs normally shows a tool bar on graphical displays.

On each frame, the frame parameter tool-bar-lines controls how many lines' worth of height to reserve for the tool bar. A zero value suppresses the tool bar. If the value is nonzero, and auto-resize-tool-bars is non-nil, the tool bar expands and contracts automatically as needed to hold the specified contents. If the value is grow-only, the tool bar expands automatically, but does not contract automatically.

The tool bar contents are controlled by a menu keymap attached to a fake function key called tool-bar (much like the way the menu bar is controlled). So you define a tool bar item using define-key, like this:

     (define-key global-map [tool-bar key] item)

where key is a fake function key to distinguish this item from other items, and item is a menu item key binding (see Extended Menu Items), which says how to display this item and how it behaves.

The usual menu keymap item properties, :visible, :enable, :button, and :filter, are useful in tool bar bindings and have their normal meanings. The real-binding in the item must be a command, not a keymap; in other words, it does not work to define a tool bar icon as a prefix key.

The :help property specifies a help-echo string to display while the mouse is on that item. This is displayed in the same way as help-echo text properties (see Help display).

In addition, you should use the :image property; this is how you specify the image to display in the tool bar:

:image image

image is either a single image specification (see Images) or a vector of four image specifications. If you use a vector of four, one of them is used, depending on circumstances:

item 0

Used when the item is enabled and selected.

item 1

Used when the item is enabled and deselected.

item 2

Used when the item is disabled and selected.

item 3

Used when the item is disabled and deselected.

The GTK+ and NS versions of Emacs ignores items 1 to 3, because disabled and/or deselected images are autocomputed from item 0.

If image is a single image specification, Emacs draws the tool bar button in disabled state by applying an edge-detection algorithm to the image.

The :rtl property specifies an alternative image to use for right-to-left languages. Only the GTK+ version of Emacs supports this at present.

Like the menu bar, the tool bar can display separators (see Menu Separators). Tool bar separators are vertical rather than horizontal, though, and only a single style is supported. They are represented in the tool bar keymap by (menu-item "--") entries; properties like :visible are not supported for tool bar separators. Separators are rendered natively in GTK+ and Nextstep tool bars; in the other cases, they are rendered using an image of a vertical line.

The default tool bar is defined so that items specific to editing do not appear for major modes whose command symbol has a mode-class property of special (see Major Mode Conventions). Major modes may add items to the global bar by binding [tool-bar foo] in their local map. It makes sense for some major modes to replace the default tool bar items completely, since not many can be accommodated conveniently, and the default bindings make this easy by using an indirection through tool-bar-map.

There are two convenience functions for defining tool bar items, as follows.

You can define a special meaning for clicking on a tool bar item with the shift, control, meta, etc., modifiers. You do this by setting up additional items that relate to the original item through the fake function keys. Specifically, the additional items should use the modified versions of the same fake function key used to name the original item.

Thus, if the original item was defined this way,

     (define-key global-map [tool-bar shell]
       '(menu-item "Shell" shell
                   :image (image :type xpm :file "shell.xpm")))

then here is how you can define clicking on the same tool bar image with the shift modifier:

     (define-key global-map [tool-bar S-shell] 'some-command)

See Function Keys, for more information about how to add modifiers to function keys.

21.17.7 Modifying Menus

When you insert a new item in an existing menu, you probably want to put it in a particular place among the menu's existing items. If you use define-key to add the item, it normally goes at the front of the menu. To put it elsewhere in the menu, use define-key-after:

21.17.8 Easy Menu

The following macro provides a convenient way to define pop-up menus and/or menu bar menus.

Here is an example of using easy-menu-define to define a menu similar to the one defined in the example in Menu Bar:

     (easy-menu-define words-menu global-map
       "Menu for word navigation commands."
       '("Words"
          ["Forward word" forward-word]
          ["Backward word" backward-word]))

22 Major and Minor Modes

A mode is a set of definitions that customize Emacs behavior in useful ways. There are two varieties of modes: minor modes, which provide features that users can turn on and off while editing; and major modes, which are used for editing or interacting with a particular kind of text. Each buffer has exactly one major mode at a time.

This chapter describes how to write both major and minor modes, how to indicate them in the mode line, and how they run hooks supplied by the user. For related topics such as keymaps and syntax tables, see Keymaps, and Syntax Tables.

• Hooks: How to use hooks; how to write code that provides hooks.
• Major Modes: Defining major modes.
• Minor Modes: Defining minor modes.
• Mode Line Format: Customizing the text that appears in the mode line.
• Imenu: Providing a menu of definitions made in a buffer.
• Font Lock Mode: How modes can highlight text according to syntax.
• Auto-Indentation: How to teach Emacs to indent for a major mode.
• Desktop Save Mode: How modes can have buffer state saved between Emacs sessions.

22.1 Hooks

A hook is a variable where you can store a function or functions to be called on a particular occasion by an existing program. Emacs provides hooks for the sake of customization. Most often, hooks are set up in the init file (see Init File), but Lisp programs can set them also. See Standard Hooks, for a list of some standard hook variables.

Most of the hooks in Emacs are normal hooks. These variables contain lists of functions to be called with no arguments. By convention, whenever the hook name ends in ‘-hook’, that tells you it is normal. We try to make all hooks normal, as much as possible, so that you can use them in a uniform way.

Every major mode command is supposed to run a normal hook called the mode hook as one of the last steps of initialization. This makes it easy for a user to customize the behavior of the mode, by overriding the buffer-local variable assignments already made by the mode. Most minor mode functions also run a mode hook at the end. But hooks are used in other contexts too. For example, the hook suspend-hook runs just before Emacs suspends itself (see Suspending Emacs).

The recommended way to add a hook function to a hook is by calling add-hook (see Setting Hooks). The hook functions may be any of the valid kinds of functions that funcall accepts (see What Is a Function). Most normal hook variables are initially void; add-hook knows how to deal with this. You can add hooks either globally or buffer-locally with add-hook.

If the hook variable's name does not end with ‘-hook’, that indicates it is probably an abnormal hook. That means the hook functions are called with arguments, or their return values are used in some way. The hook's documentation says how the functions are called. You can use add-hook to add a function to an abnormal hook, but you must write the function to follow the hook's calling convention. By convention, abnormal hook names end in ‘-functions’.

If the variable's name ends in ‘-function’, then its value is just a single function, not a list of functions. add-hook cannot be used to modify such a single function hook, and you have to use add-function instead (see Advising Functions).

• Running Hooks: How to run a hook.
• Setting Hooks: How to put functions on a hook, or remove them.

22.1.1 Running Hooks

In this section, we document the run-hooks function, which is used to run a normal hook. We also document the functions for running various kinds of abnormal hooks.

22.1.2 Setting Hooks

Here's an example that uses a mode hook to turn on Auto Fill mode when in Lisp Interaction mode:

     (add-hook 'lisp-interaction-mode-hook 'auto-fill-mode)

22.2 Major Modes

Major modes specialize Emacs for editing or interacting with particular kinds of text. Each buffer has exactly one major mode at a time. Every major mode is associated with a major mode command, whose name should end in ‘-mode’. This command takes care of switching to that mode in the current buffer, by setting various buffer-local variables such as a local keymap. See Major Mode Conventions. Note that unlike minor modes there is no way to “turn off” a major mode, instead the buffer must be switched to a different one.

The least specialized major mode is called Fundamental mode, which has no mode-specific definitions or variable settings.

The easiest way to write a major mode is to use the macro define-derived-mode, which sets up the new mode as a variant of an existing major mode. See Derived Modes. We recommend using define-derived-mode even if the new mode is not an obvious derivative of another mode, as it automatically enforces many coding conventions for you. See Basic Major Modes, for common modes to derive from.

The standard GNU Emacs Lisp directory tree contains the code for several major modes, in files such as text-mode.el, texinfo.el, lisp-mode.el, and rmail.el. You can study these libraries to see how modes are written.

• Major Mode Conventions: Coding conventions for keymaps, etc.
• Auto Major Mode: How Emacs chooses the major mode automatically.
• Mode Help: Finding out how to use a mode.
• Derived Modes: Defining a new major mode based on another major mode.
• Basic Major Modes: Modes that other modes are often derived from.
• Mode Hooks: Hooks run at the end of major mode functions.
• Tabulated List Mode: Parent mode for buffers containing tabulated data.
• Generic Modes: Defining a simple major mode that supports comment syntax and Font Lock mode.
• Example Major Modes: Text mode and Lisp modes.

22.2.1 Major Mode Conventions

The code for every major mode should follow various coding conventions, including conventions for local keymap and syntax table initialization, function and variable names, and hooks.

If you use the define-derived-mode macro, it will take care of many of these conventions automatically. See Derived Modes. Note also that Fundamental mode is an exception to many of these conventions, because it represents the default state of Emacs.

The following list of conventions is only partial. Each major mode should aim for consistency in general with other Emacs major modes, as this makes Emacs as a whole more coherent. It is impossible to list here all the possible points where this issue might come up; if the Emacs developers point out an area where your major mode deviates from the usual conventions, please make it compatible.

• Define a major mode command whose name ends in ‘-mode’. When called with no arguments, this command should switch to the new mode in the current buffer by setting up the keymap, syntax table, and buffer-local variables in an existing buffer. It should not change the buffer's contents.
• Write a documentation string for this command that describes the special commands available in this mode. See Mode Help.

  The documentation string may include the special documentation substrings, ‘\[command]’, ‘\{keymap}’, and ‘\<keymap>’, which allow the help display to adapt automatically to the user's own key bindings. See Keys in Documentation.

• The major mode command should start by calling kill-all-local-variables. This runs the normal hook change-major-mode-hook, then gets rid of the buffer-local variables of the major mode previously in effect. See Creating Buffer-Local.
• The major mode command should set the variable major-mode to the major mode command symbol. This is how describe-mode discovers which documentation to print.
• The major mode command should set the variable mode-name to the “pretty” name of the mode, usually a string (but see Mode Line Data, for other possible forms). The name of the mode appears in the mode line.
• Since all global names are in the same name space, all the global variables, constants, and functions that are part of the mode should have names that start with the major mode name (or with an abbreviation of it if the name is long). See Coding Conventions.
• In a major mode for editing some kind of structured text, such as a programming language, indentation of text according to structure is probably useful. So the mode should set indent-line-function to a suitable function, and probably customize other variables for indentation. See Auto-Indentation.
• The major mode should usually have its own keymap, which is used as the local keymap in all buffers in that mode. The major mode command should call use-local-map to install this local map. See Active Keymaps, for more information.

  This keymap should be stored permanently in a global variable named modename-mode-map. Normally the library that defines the mode sets this variable.

  See Tips for Defining, for advice about how to write the code to set up the mode's keymap variable.

• The key sequences bound in a major mode keymap should usually start with C-c, followed by a control character, a digit, or {, }, <, >, : or ;. The other punctuation characters are reserved for minor modes, and ordinary letters are reserved for users.

  A major mode can also rebind the keys M-n, M-p and M-s. The bindings for M-n and M-p should normally be some kind of moving forward and backward, but this does not necessarily mean cursor motion.

  It is legitimate for a major mode to rebind a standard key sequence if it provides a command that does the same job in a way better suited to the text this mode is used for. For example, a major mode for editing a programming language might redefine C-M-a to move to the beginning of a function in a way that works better for that language.

  It is also legitimate for a major mode to rebind a standard key sequence whose standard meaning is rarely useful in that mode. For instance, minibuffer modes rebind M-r, whose standard meaning is rarely of any use in the minibuffer. Major modes such as Dired or Rmail that do not allow self-insertion of text can reasonably redefine letters and other printing characters as special commands.

• Major modes for editing text should not define <RET> to do anything other than insert a newline. However, it is ok for specialized modes for text that users don't directly edit, such as Dired and Info modes, to redefine <RET> to do something entirely different.
• Major modes should not alter options that are primarily a matter of user preference, such as whether Auto-Fill mode is enabled. Leave this to each user to decide. However, a major mode should customize other variables so that Auto-Fill mode will work usefully if the user decides to use it.
• The mode may have its own syntax table or may share one with other related modes. If it has its own syntax table, it should store this in a variable named modename-mode-syntax-table. See Syntax Tables.
• If the mode handles a language that has a syntax for comments, it should set the variables that define the comment syntax. See Options Controlling Comments.
• The mode may have its own abbrev table or may share one with other related modes. If it has its own abbrev table, it should store this in a variable named modename-mode-abbrev-table. If the major mode command defines any abbrevs itself, it should pass t for the system-flag argument to define-abbrev. See Defining Abbrevs.
• The mode should specify how to do highlighting for Font Lock mode, by setting up a buffer-local value for the variable font-lock-defaults (see Font Lock Mode).
• Each face that the mode defines should, if possible, inherit from an existing Emacs face. See Basic Faces, and Faces for Font Lock.
• The mode should specify how Imenu should find the definitions or sections of a buffer, by setting up a buffer-local value for the variable imenu-generic-expression, for the two variables imenu-prev-index-position-function and imenu-extract-index-name-function, or for the variable imenu-create-index-function (see Imenu).
• The mode can specify a local value for eldoc-documentation-function to tell ElDoc mode how to handle this mode.
• The mode can specify how to complete various keywords by adding one or more buffer-local entries to the special hook completion-at-point-functions. See Completion in Buffers.
• To make a buffer-local binding for an Emacs customization variable, use make-local-variable in the major mode command, not make-variable-buffer-local. The latter function would make the variable local to every buffer in which it is subsequently set, which would affect buffers that do not use this mode. It is undesirable for a mode to have such global effects. See Buffer-Local Variables.

  With rare exceptions, the only reasonable way to use make-variable-buffer-local in a Lisp package is for a variable which is used only within that package. Using it on a variable used by other packages would interfere with them.

• Each major mode should have a normal mode hook named modename-mode-hook. The very last thing the major mode command should do is to call run-mode-hooks. This runs the normal hook change-major-mode-after-body-hook, the mode hook, and then the normal hook after-change-major-mode-hook. See Mode Hooks.
• The major mode command may start by calling some other major mode command (called the parent mode) and then alter some of its settings. A mode that does this is called a derived mode. The recommended way to define one is to use the define-derived-mode macro, but this is not required. Such a mode should call the parent mode command inside a delay-mode-hooks form. (Using define-derived-mode does this automatically.) See Derived Modes, and Mode Hooks.
• If something special should be done if the user switches a buffer from this mode to any other major mode, this mode can set up a buffer-local value for change-major-mode-hook (see Creating Buffer-Local).
• If this mode is appropriate only for specially-prepared text produced by the mode itself (rather than by the user typing at the keyboard or by an external file), then the major mode command symbol should have a property named mode-class with value special, put on as follows:

            (put 'funny-mode 'mode-class 'special)
  

  This tells Emacs that new buffers created while the current buffer is in Funny mode should not be put in Funny mode, even though the default value of major-mode is nil. By default, the value of nil for major-mode means to use the current buffer's major mode when creating new buffers (see Auto Major Mode), but with such special modes, Fundamental mode is used instead. Modes such as Dired, Rmail, and Buffer List use this feature.

  The function view-buffer does not enable View mode in buffers whose mode-class is special, because such modes usually provide their own View-like bindings.

  The define-derived-mode macro automatically marks the derived mode as special if the parent mode is special. Special mode is a convenient parent for such modes to inherit from; See Basic Major Modes.

• If you want to make the new mode the default for files with certain recognizable names, add an element to auto-mode-alist to select the mode for those file names (see Auto Major Mode). If you define the mode command to autoload, you should add this element in the same file that calls autoload. If you use an autoload cookie for the mode command, you can also use an autoload cookie for the form that adds the element (see autoload cookie). If you do not autoload the mode command, it is sufficient to add the element in the file that contains the mode definition.
• The top-level forms in the file defining the mode should be written so that they may be evaluated more than once without adverse consequences. For instance, use defvar or defcustom to set mode-related variables, so that they are not reinitialized if they already have a value (see Defining Variables).

22.2.2 How Emacs Chooses a Major Mode

When Emacs visits a file, it automatically selects a major mode for the buffer based on information in the file name or in the file itself. It also processes local variables specified in the file text.

22.2.3 Getting Help about a Major Mode

The describe-mode function provides information about major modes. It is normally bound to C-h m. It uses the value of the variable major-mode (see Major Modes), which is why every major mode command needs to set that variable.

22.2.4 Defining Derived Modes

The recommended way to define a new major mode is to derive it from an existing one using define-derived-mode. If there is no closely related mode, you should inherit from either text-mode, special-mode, or prog-mode. See Basic Major Modes. If none of these are suitable, you can inherit from fundamental-mode (see Major Modes).

22.2.5 Basic Major Modes

Apart from Fundamental mode, there are three major modes that other major modes commonly derive from: Text mode, Prog mode, and Special mode. While Text mode is useful in its own right (e.g., for editing files ending in .txt), Prog mode and Special mode exist mainly to let other modes derive from them.

As far as possible, new major modes should be derived, either directly or indirectly, from one of these three modes. One reason is that this allows users to customize a single mode hook (e.g., prog-mode-hook) for an entire family of relevant modes (e.g., all programming language modes).

In addition, modes for buffers of tabulated data can inherit from Tabulated List mode, which is in turn derived from Special mode. See Tabulated List Mode.

22.2.6 Mode Hooks

Every major mode command should finish by running the mode-independent normal hook change-major-mode-after-body-hook, its mode hook, and the normal hook after-change-major-mode-hook. It does this by calling run-mode-hooks. If the major mode is a derived mode, that is if it calls another major mode (the parent mode) in its body, it should do this inside delay-mode-hooks so that the parent won't run these hooks itself. Instead, the derived mode's call to run-mode-hooks runs the parent's mode hook too. See Major Mode Conventions.

Emacs versions before Emacs 22 did not have delay-mode-hooks. Versions before 24 did not have change-major-mode-after-body-hook. When user-implemented major modes do not use run-mode-hooks and have not been updated to use these newer features, they won't entirely follow these conventions: they may run the parent's mode hook too early, or fail to run after-change-major-mode-hook. If you encounter such a major mode, please correct it to follow these conventions.

When you define a major mode using define-derived-mode, it automatically makes sure these conventions are followed. If you define a major mode “by hand”, not using define-derived-mode, use the following functions to handle these conventions automatically.

22.2.7 Tabulated List mode

Tabulated List mode is a major mode for displaying tabulated data, i.e., data consisting of entries, each entry occupying one row of text with its contents divided into columns. Tabulated List mode provides facilities for pretty-printing rows and columns, and sorting the rows according to the values in each column. It is derived from Special mode (see Basic Major Modes).

Tabulated List mode is intended to be used as a parent mode by a more specialized major mode. Examples include Process Menu mode (see Process Information) and Package Menu mode (see Package Menu).

Such a derived mode should use define-derived-mode in the usual way, specifying tabulated-list-mode as the second argument (see Derived Modes). The body of the define-derived-mode form should specify the format of the tabulated data, by assigning values to the variables documented below; optionally, it can then call the function tabulated-list-init-header, which will populate a header with the names of the columns.

The derived mode should also define a listing command. This, not the mode command, is what the user calls (e.g., M-x list-processes). The listing command should create or switch to a buffer, turn on the derived mode, specify the tabulated data, and finally call tabulated-list-print to populate the buffer.

22.2.8 Generic Modes

Generic modes are simple major modes with basic support for comment syntax and Font Lock mode. To define a generic mode, use the macro define-generic-mode. See the file generic-x.el for some examples of the use of define-generic-mode.

22.2.9 Major Mode Examples

Text mode is perhaps the simplest mode besides Fundamental mode. Here are excerpts from text-mode.el that illustrate many of the conventions listed above:

     ;; Create the syntax table for this mode.
     (defvar text-mode-syntax-table
       (let ((st (make-syntax-table)))
         (modify-syntax-entry ?\" ".   " st)
         (modify-syntax-entry ?\\ ".   " st)
         ;; Add 'p' so M-c on 'hello' leads to 'Hello', not 'hello'.
         (modify-syntax-entry ?' "w p" st)
         st)
       "Syntax table used while in `text-mode'.")
     
     ;; Create the keymap for this mode.
     (defvar text-mode-map
       (let ((map (make-sparse-keymap)))
         (define-key map "\e\t" 'ispell-complete-word)
         map)
       "Keymap for `text-mode'.
     Many other modes, such as `mail-mode', `outline-mode' and
     `indented-text-mode', inherit all the commands defined in this map.")

Here is how the actual mode command is defined now:

     (define-derived-mode text-mode nil "Text"
       "Major mode for editing text written for humans to read.
     In this mode, paragraphs are delimited only by blank or white lines.
     You can thus get the full benefit of adaptive filling
      (see the variable `adaptive-fill-mode').
     \\{text-mode-map}
     Turning on Text mode runs the normal hook `text-mode-hook'."
       (set (make-local-variable 'text-mode-variant) t)
       (set (make-local-variable 'require-final-newline)
            mode-require-final-newline)
       (set (make-local-variable 'indent-line-function) 'indent-relative))

(The last line is redundant nowadays, since indent-relative is the default value, and we'll delete it in a future version.)

The three Lisp modes (Lisp mode, Emacs Lisp mode, and Lisp Interaction mode) have more features than Text mode and the code is correspondingly more complicated. Here are excerpts from lisp-mode.el that illustrate how these modes are written.

Here is how the Lisp mode syntax and abbrev tables are defined:

     ;; Create mode-specific table variables.
     (defvar lisp-mode-abbrev-table nil)
     (define-abbrev-table 'lisp-mode-abbrev-table ())
     
     (defvar lisp-mode-syntax-table
       (let ((table (copy-syntax-table emacs-lisp-mode-syntax-table)))
         (modify-syntax-entry ?\[ "_   " table)
         (modify-syntax-entry ?\] "_   " table)
         (modify-syntax-entry ?# "' 14" table)
         (modify-syntax-entry ?| "\" 23bn" table)
         table)
       "Syntax table used in `lisp-mode'.")

The three modes for Lisp share much of their code. For instance, each calls the following function to set various variables:

     (defun lisp-mode-variables (&optional syntax keywords-case-insensitive)
       (when syntax
         (set-syntax-table lisp-mode-syntax-table))
       (setq local-abbrev-table lisp-mode-abbrev-table)
       ...

Amongst other things, this function sets up the comment-start variable to handle Lisp comments:

       (make-local-variable 'comment-start)
       (setq comment-start ";")
       ...

Each of the different Lisp modes has a slightly different keymap. For example, Lisp mode binds C-c C-z to run-lisp, but the other Lisp modes do not. However, all Lisp modes have some commands in common. The following code sets up the common commands:

     (defvar lisp-mode-shared-map
       (let ((map (make-sparse-keymap)))
         (define-key map "\e\C-q" 'indent-sexp)
         (define-key map "\177" 'backward-delete-char-untabify)
         map)
       "Keymap for commands shared by all sorts of Lisp modes.")

And here is the code to set up the keymap for Lisp mode:

     (defvar lisp-mode-map
       (let ((map (make-sparse-keymap))
     	(menu-map (make-sparse-keymap "Lisp")))
         (set-keymap-parent map lisp-mode-shared-map)
         (define-key map "\e\C-x" 'lisp-eval-defun)
         (define-key map "\C-c\C-z" 'run-lisp)
         ...
         map)
       "Keymap for ordinary Lisp mode.
     All commands in `lisp-mode-shared-map' are inherited by this map.")

Finally, here is the major mode command for Lisp mode:

     (define-derived-mode lisp-mode prog-mode "Lisp"
       "Major mode for editing Lisp code for Lisps other than GNU Emacs Lisp.
     Commands:
     Delete converts tabs to spaces as it moves back.
     Blank lines separate paragraphs.  Semicolons start comments.
     
     \\{lisp-mode-map}
     Note that `run-lisp' may be used either to start an inferior Lisp job
     or to switch back to an existing one.
     
     Entry to this mode calls the value of `lisp-mode-hook'
     if that value is non-nil."
       (lisp-mode-variables nil t)
       (set (make-local-variable 'find-tag-default-function)
            'lisp-find-tag-default)
       (set (make-local-variable 'comment-start-skip)
            "\\(\\(^\\|[^\\\\\n]\\)\\(\\\\\\\\\\)*\\)\\(;+\\|#|\\) *")
       (setq imenu-case-fold-search t))

22.3 Minor Modes

A minor mode provides optional features that users may enable or disable independently of the choice of major mode. Minor modes can be enabled individually or in combination.

Most minor modes implement features that are independent of the major mode, and can thus be used with most major modes. For example, Auto Fill mode works with any major mode that permits text insertion. A few minor modes, however, are specific to a particular major mode. For example, Diff Auto Refine mode is a minor mode that is intended to be used only with Diff mode.

Ideally, a minor mode should have its desired effect regardless of the other minor modes in effect. It should be possible to activate and deactivate minor modes in any order.

• Minor Mode Conventions: Tips for writing a minor mode.
• Keymaps and Minor Modes: How a minor mode can have its own keymap.
• Defining Minor Modes: A convenient facility for defining minor modes.

22.3.1 Conventions for Writing Minor Modes

There are conventions for writing minor modes just as there are for major modes. These conventions are described below. The easiest way to follow them is to use the macro define-minor-mode. See Defining Minor Modes.

• Define a variable whose name ends in ‘-mode’. We call this the mode variable. The minor mode command should set this variable. The value will be nil if the mode is disabled, and non-nil if the mode is enabled. The variable should be buffer-local if the minor mode is buffer-local.

  This variable is used in conjunction with the minor-mode-alist to display the minor mode name in the mode line. It also determines whether the minor mode keymap is active, via minor-mode-map-alist (see Controlling Active Maps). Individual commands or hooks can also check its value.

• Define a command, called the mode command, whose name is the same as the mode variable. Its job is to set the value of the mode variable, plus anything else that needs to be done to actually enable or disable the mode's features.

  The mode command should accept one optional argument. If called interactively with no prefix argument, it should toggle the mode (i.e., enable if it is disabled, and disable if it is enabled). If called interactively with a prefix argument, it should enable the mode if the argument is positive and disable it otherwise.

  If the mode command is called from Lisp (i.e., non-interactively), it should enable the mode if the argument is omitted or nil; it should toggle the mode if the argument is the symbol toggle; otherwise it should treat the argument in the same way as for an interactive call with a numeric prefix argument, as described above.

  The following example shows how to implement this behavior (it is similar to the code generated by the define-minor-mode macro):

            (interactive (list (or current-prefix-arg 'toggle)))
            (let ((enable (if (eq arg 'toggle)
                              (not foo-mode) ; this mode's mode variable
                            (> (prefix-numeric-value arg) 0))))
              (if enable
                  do-enable
                do-disable))
  

  The reason for this somewhat complex behavior is that it lets users easily toggle the minor mode interactively, and also lets the minor mode be easily enabled in a mode hook, like this:

            (add-hook 'text-mode-hook 'foo-mode)
  

  This behaves correctly whether or not foo-mode was already enabled, since the foo-mode mode command unconditionally enables the minor mode when it is called from Lisp with no argument. Disabling a minor mode in a mode hook is a little uglier:

            (add-hook 'text-mode-hook (lambda () (foo-mode -1)))
  

  However, this is not very commonly done.

• Add an element to minor-mode-alist for each minor mode (see Definition of minor-mode-alist), if you want to indicate the minor mode in the mode line. This element should be a list of the following form:

            (mode-variable string)
  

  Here mode-variable is the variable that controls enabling of the minor mode, and string is a short string, starting with a space, to represent the mode in the mode line. These strings must be short so that there is room for several of them at once.

  When you add an element to minor-mode-alist, use assq to check for an existing element, to avoid duplication. For example:

            (unless (assq 'leif-mode minor-mode-alist)
              (push '(leif-mode " Leif") minor-mode-alist))
  

  or like this, using add-to-list (see List Variables):

            (add-to-list 'minor-mode-alist '(leif-mode " Leif"))
  

In addition, several major mode conventions apply to minor modes as well: those regarding the names of global symbols, the use of a hook at the end of the initialization function, and the use of keymaps and other tables.

The minor mode should, if possible, support enabling and disabling via Custom (see Customization). To do this, the mode variable should be defined with defcustom, usually with :type 'boolean. If just setting the variable is not sufficient to enable the mode, you should also specify a :set method which enables the mode by invoking the mode command. Note in the variable's documentation string that setting the variable other than via Custom may not take effect. Also, mark the definition with an autoload cookie (see autoload cookie), and specify a :require so that customizing the variable will load the library that defines the mode. For example:

     ;;;###autoload
     (defcustom msb-mode nil
       "Toggle msb-mode.
     Setting this variable directly does not take effect;
     use either \\[customize] or the function `msb-mode'."
       :set 'custom-set-minor-mode
       :initialize 'custom-initialize-default
       :version "20.4"
       :type    'boolean
       :group   'msb
       :require 'msb)

22.3.2 Keymaps and Minor Modes

Each minor mode can have its own keymap, which is active when the mode is enabled. To set up a keymap for a minor mode, add an element to the alist minor-mode-map-alist. See Definition of minor-mode-map-alist.

One use of minor mode keymaps is to modify the behavior of certain self-inserting characters so that they do something else as well as self-insert. (Another way to customize self-insert-command is through post-self-insert-hook. Apart from this, the facilities for customizing self-insert-command are limited to special cases, designed for abbrevs and Auto Fill mode. Do not try substituting your own definition of self-insert-command for the standard one. The editor command loop handles this function specially.)

Minor modes may bind commands to key sequences consisting of C-c followed by a punctuation character. However, sequences consisting of C-c followed by one of {}<>:;, or a control character or digit, are reserved for major modes. Also, C-c letter is reserved for users. See Key Binding Conventions.

22.3.3 Defining Minor Modes

The macro define-minor-mode offers a convenient way of implementing a mode in one self-contained definition.

The initial value must be nil except in cases where (1) the mode is preloaded in Emacs, or (2) it is painless for loading to enable the mode even though the user did not request it. For instance, if the mode has no effect unless something else is enabled, and will always be loaded by that time, enabling it by default is harmless. But these are unusual circumstances. Normally, the initial value must be nil.

The name easy-mmode-define-minor-mode is an alias for this macro.

Here is an example of using define-minor-mode:

     (define-minor-mode hungry-mode
       "Toggle Hungry mode.
     Interactively with no argument, this command toggles the mode.
     A positive prefix argument enables the mode, any other prefix
     argument disables it.  From Lisp, argument omitted or nil enables
     the mode, `toggle' toggles the state.
     
     When Hungry mode is enabled, the control delete key
     gobbles all preceding whitespace except the last.
     See the command \\[hungry-electric-delete]."
      ;; The initial value.
      nil
      ;; The indicator for the mode line.
      " Hungry"
      ;; The minor mode bindings.
      '(([C-backspace] . hungry-electric-delete))
      :group 'hunger)

This defines a minor mode named “Hungry mode”, a command named hungry-mode to toggle it, a variable named hungry-mode which indicates whether the mode is enabled, and a variable named hungry-mode-map which holds the keymap that is active when the mode is enabled. It initializes the keymap with a key binding for C-<DEL>. It puts the variable hungry-mode into custom group hunger. There are no body forms—many minor modes don't need any.

Here's an equivalent way to write it:

     (define-minor-mode hungry-mode
       "Toggle Hungry mode.
     ...rest of documentation as before..."
      ;; The initial value.
      :init-value nil
      ;; The indicator for the mode line.
      :lighter " Hungry"
      ;; The minor mode bindings.
      :keymap
      '(([C-backspace] . hungry-electric-delete)
        ([C-M-backspace]
         . (lambda ()
             (interactive)
             (hungry-electric-delete t))))
      :group 'hunger)

22.4 Mode Line Format

Each Emacs window (aside from minibuffer windows) typically has a mode line at the bottom, which displays status information about the buffer displayed in the window. The mode line contains information about the buffer, such as its name, associated file, depth of recursive editing, and major and minor modes. A window can also have a header line, which is much like the mode line but appears at the top of the window.

This section describes how to control the contents of the mode line and header line. We include it in this chapter because much of the information displayed in the mode line relates to the enabled major and minor modes.

• Base: Basic ideas of mode line control.
• Data: The data structure that controls the mode line.
• Top: The top level variable, mode-line-format.
• Mode Line Variables: Variables used in that data structure.
• %-Constructs: Putting information into a mode line.
• Properties in Mode: Using text properties in the mode line.
• Header Lines: Like a mode line, but at the top.
• Emulating Mode Line: Formatting text as the mode line would.

22.4.1 Mode Line Basics

The contents of each mode line are specified by the buffer-local variable mode-line-format (see Mode Line Top). This variable holds a mode line construct: a template that controls what is displayed on the buffer's mode line. The value of header-line-format specifies the buffer's header line in the same way. All windows for the same buffer use the same mode-line-format and header-line-format.

For efficiency, Emacs does not continuously recompute each window's mode line and header line. It does so when circumstances appear to call for it—for instance, if you change the window configuration, switch buffers, narrow or widen the buffer, scroll, or modify the buffer. If you alter any of the variables referenced by mode-line-format or header-line-format (see Mode Line Variables), or any other data structures that affect how text is displayed (see Display), you should use the function force-mode-line-update to update the display.

The selected window's mode line is usually displayed in a different color using the face mode-line. Other windows' mode lines appear in the face mode-line-inactive instead. See Faces.

22.4.2 The Data Structure of the Mode Line

The mode line contents are controlled by a data structure called a mode line construct, made up of lists, strings, symbols, and numbers kept in buffer-local variables. Each data type has a specific meaning for the mode line appearance, as described below. The same data structure is used for constructing frame titles (see Frame Titles) and header lines (see Header Lines).

A mode line construct may be as simple as a fixed string of text, but it usually specifies how to combine fixed strings with variables' values to construct the text. Many of these variables are themselves defined to have mode line constructs as their values.

Here are the meanings of various data types as mode line constructs:

string

A string as a mode line construct appears verbatim except for %-constructs in it. These stand for substitution of other data; see %-Constructs.

If parts of the string have face properties, they control display of the text just as they would text in the buffer. Any characters which have no face properties are displayed, by default, in the face mode-line or mode-line-inactive (see Standard Faces). The help-echo and keymap properties in string have special meanings. See Properties in Mode.

symbol

A symbol as a mode line construct stands for its value. The value of symbol is used as a mode line construct, in place of symbol. However, the symbols t and nil are ignored, as is any symbol whose value is void.

There is one exception: if the value of symbol is a string, it is displayed verbatim: the %-constructs are not recognized.

Unless symbol is marked as risky (i.e., it has a non-nil risky-local-variable property), all text properties specified in symbol's value are ignored. This includes the text properties of strings in symbol's value, as well as all :eval and :propertize forms in it. (The reason for this is security: non-risky variables could be set automatically from file variables without prompting the user.)

(string rest...)

(list rest...)

A list whose first element is a string or list means to process all the elements recursively and concatenate the results. This is the most common form of mode line construct.

(:eval form)

A list whose first element is the symbol :eval says to evaluate form, and use the result as a string to display. Make sure this evaluation cannot load any files, as doing so could cause infinite recursion.

(:propertize elt props...)

A list whose first element is the symbol :propertize says to process the mode line construct elt recursively, then add the text properties specified by props to the result. The argument props should consist of zero or more pairs text-property value.

(symbol then else)

A list whose first element is a symbol that is not a keyword specifies a conditional. Its meaning depends on the value of symbol. If symbol has a non-nil value, the second element, then, is processed recursively as a mode line construct. Otherwise, the third element, else, is processed recursively. You may omit else; then the mode line construct displays nothing if the value of symbol is nil or void.

(width rest...)

A list whose first element is an integer specifies truncation or padding of the results of rest. The remaining elements rest are processed recursively as mode line constructs and concatenated together. When width is positive, the result is space filled on the right if its width is less than width. When width is negative, the result is truncated on the right to −width columns if its width exceeds −width.

For example, the usual way to show what percentage of a buffer is above the top of the window is to use a list like this: (-3 "%p").

22.4.3 The Top Level of Mode Line Control

The variable in overall control of the mode line is mode-line-format.

The default value of mode-line-format is designed to use the values of other variables such as mode-line-position and mode-line-modes (which in turn incorporates the values of the variables mode-name and minor-mode-alist). Very few modes need to alter mode-line-format itself. For most purposes, it is sufficient to alter some of the variables that mode-line-format either directly or indirectly refers to.

If you do alter mode-line-format itself, the new value should use the same variables that appear in the default value (see Mode Line Variables), rather than duplicating their contents or displaying the information in another fashion. This way, customizations made by the user or by Lisp programs (such as display-time and major modes) via changes to those variables remain effective.

Here is a hypothetical example of a mode-line-format that might be useful for Shell mode (in reality, Shell mode does not set mode-line-format):

     (setq mode-line-format
       (list "-"
        'mode-line-mule-info
        'mode-line-modified
        'mode-line-frame-identification
        "%b--"
        ;; Note that this is evaluated while making the list.
        ;; It makes a mode line construct which is just a string.
        (getenv "HOST")
        ":"
        'default-directory
        "   "
        'global-mode-string
        "   %[("
        '(:eval (mode-line-mode-name))
        'mode-line-process
        'minor-mode-alist
        "%n"
        ")%]--"
        '(which-func-mode ("" which-func-format "--"))
        '(line-number-mode "L%l--")
        '(column-number-mode "C%c--")
        '(-3 "%p")))

(The variables line-number-mode, column-number-mode and which-func-mode enable particular minor modes; as usual, these variable names are also the minor mode command names.)

22.4.4 Variables Used in the Mode Line

This section describes variables incorporated by the standard value of mode-line-format into the text of the mode line. There is nothing inherently special about these variables; any other variables could have the same effects on the mode line if the value of mode-line-format is changed to use them. However, various parts of Emacs set these variables on the understanding that they will control parts of the mode line; therefore, practically speaking, it is essential for the mode line to use them. Also see Optional Mode Line.

The following three variables are used in mode-line-modes:

Here is a simplified version of the default value of mode-line-format. The real default value also specifies addition of text properties.

     ("-"
      mode-line-mule-info
      mode-line-modified
      mode-line-frame-identification
      mode-line-buffer-identification
      "   "
      mode-line-position
      (vc-mode vc-mode)
      "   "
      mode-line-modes
      (which-func-mode ("" which-func-format "--"))
      (global-mode-string ("--" global-mode-string))
      "-%-")

22.4.5 %-Constructs in the Mode Line

Strings used as mode line constructs can use certain %-constructs to substitute various kinds of data. The following is a list of the defined %-constructs, and what they mean.

In any construct except ‘%%’, you can add a decimal integer after the ‘%’ to specify a minimum field width. If the width is less, the field is padded to that width. Purely numeric constructs (‘c’, ‘i’, ‘I’, and ‘l’) are padded by inserting spaces to the left, and others are padded by inserting spaces to the right.

%b

The current buffer name, obtained with the buffer-name function. See Buffer Names.

%c

The current column number of point.

%e

When Emacs is nearly out of memory for Lisp objects, a brief message saying so. Otherwise, this is empty.

%f

The visited file name, obtained with the buffer-file-name function. See Buffer File Name.

%F

The title (only on a window system) or the name of the selected frame. See Basic Parameters.

%i

The size of the accessible part of the current buffer; basically (- (point-max) (point-min)).

%I

Like ‘%i’, but the size is printed in a more readable way by using ‘k’ for 10^3, ‘M’ for 10^6, ‘G’ for 10^9, etc., to abbreviate.

%l

The current line number of point, counting within the accessible portion of the buffer.

%n

‘Narrow’ when narrowing is in effect; nothing otherwise (see narrow-to-region in Narrowing).

%p

The percentage of the buffer text above the top of window, or ‘Top’, ‘Bottom’ or ‘All’. Note that the default mode line construct truncates this to three characters.

%P

The percentage of the buffer text that is above the bottom of the window (which includes the text visible in the window, as well as the text above the top), plus ‘Top’ if the top of the buffer is visible on screen; or ‘Bottom’ or ‘All’.

%s

The status of the subprocess belonging to the current buffer, obtained with process-status. See Process Information.

%z

The mnemonics of keyboard, terminal, and buffer coding systems.

%Z

Like ‘%z’, but including the end-of-line format.

%*

‘%’ if the buffer is read only (see buffer-read-only);
‘*’ if the buffer is modified (see buffer-modified-p);
‘-’ otherwise. See Buffer Modification.

%+

‘*’ if the buffer is modified (see buffer-modified-p);
‘%’ if the buffer is read only (see buffer-read-only);
‘-’ otherwise. This differs from ‘%*’ only for a modified read-only buffer. See Buffer Modification.

%&

‘*’ if the buffer is modified, and ‘-’ otherwise.

%[

An indication of the depth of recursive editing levels (not counting minibuffer levels): one ‘[’ for each editing level. See Recursive Editing.

%]

One ‘]’ for each recursive editing level (not counting minibuffer levels).

%-

Dashes sufficient to fill the remainder of the mode line.

%%

The character ‘%’—this is how to include a literal ‘%’ in a string in which %-constructs are allowed.

The following two %-constructs are still supported, but they are obsolete, since you can get the same results with the variables mode-name and global-mode-string.

%m

The value of mode-name.

%M

The value of global-mode-string.

22.4.6 Properties in the Mode Line

Certain text properties are meaningful in the mode line. The face property affects the appearance of text; the help-echo property associates help strings with the text, and keymap can make the text mouse-sensitive.

There are four ways to specify text properties for text in the mode line:

1. Put a string with a text property directly into the mode line data structure.
2. Put a text property on a mode line %-construct such as ‘%12b’; then the expansion of the %-construct will have that same text property.
3. Use a (:propertize elt props...) construct to give elt a text property specified by props.
4. Use a list containing :eval form in the mode line data structure, and make form evaluate to a string that has a text property.

You can use the keymap property to specify a keymap. This keymap only takes real effect for mouse clicks; binding character keys and function keys to it has no effect, since it is impossible to move point into the mode line.

When the mode line refers to a variable which does not have a non-nil risky-local-variable property, any text properties given or specified within that variable's values are ignored. This is because such properties could otherwise specify functions to be called, and those functions could come from file local variables.

22.4.7 Window Header Lines

A window can have a header line at the top, just as it can have a mode line at the bottom. The header line feature works just like the mode line feature, except that it's controlled by header-line-format:

A window that is just one line tall never displays a header line. A window that is two lines tall cannot display both a mode line and a header line at once; if it has a mode line, then it does not display a header line.

22.4.8 Emulating Mode Line Formatting

You can use the function format-mode-line to compute the text that would appear in a mode line or header line based on a certain mode line construct.

22.5 Imenu

Imenu is a feature that lets users select a definition or section in the buffer, from a menu which lists all of them, to go directly to that location in the buffer. Imenu works by constructing a buffer index which lists the names and buffer positions of the definitions, or other named portions of the buffer; then the user can choose one of them and move point to it. Major modes can add a menu bar item to use Imenu using imenu-add-to-menubar.

The user-level commands for using Imenu are described in the Emacs Manual (see Imenu). This section explains how to customize Imenu's method of finding definitions or buffer portions for a particular major mode.

The usual and simplest way is to set the variable imenu-generic-expression:

Another way to customize Imenu for a major mode is to set the variables imenu-prev-index-position-function and imenu-extract-index-name-function:

The last way to customize Imenu for a major mode is to set the variable imenu-create-index-function:

22.6 Font Lock Mode

Font Lock mode is a buffer-local minor mode that automatically attaches face properties to certain parts of the buffer based on their syntactic role. How it parses the buffer depends on the major mode; most major modes define syntactic criteria for which faces to use in which contexts. This section explains how to customize Font Lock for a particular major mode.

Font Lock mode finds text to highlight in two ways: through syntactic parsing based on the syntax table, and through searching (usually for regular expressions). Syntactic fontification happens first; it finds comments and string constants and highlights them. Search-based fontification happens second.

• Font Lock Basics: Overview of customizing Font Lock.
• Search-based Fontification: Fontification based on regexps.
• Customizing Keywords: Customizing search-based fontification.
• Other Font Lock Variables: Additional customization facilities.
• Levels of Font Lock: Each mode can define alternative levels so that the user can select more or less.
• Precalculated Fontification: How Lisp programs that produce the buffer contents can also specify how to fontify it.
• Faces for Font Lock: Special faces specifically for Font Lock.
• Syntactic Font Lock: Fontification based on syntax tables.
• Multiline Font Lock: How to coerce Font Lock into properly highlighting multiline constructs.

22.6.1 Font Lock Basics

The Font Lock functionality is based on several basic functions. Each of these calls the function specified by the corresponding variable. This indirection allows major and minor modes to modify the way fontification works in the buffers of that mode, and even use the Font Lock mechanisms for features that have nothing to do with fontification. (This is why the description below says “should” when it describes what the functions do: the mode can customize the values of the corresponding variables to do something entirely different.) The variables mentioned below are described in Other Font Lock Variables.

font-lock-fontify-buffer

This function should fontify the current buffer's accessible portion, by calling the function specified by font-lock-fontify-buffer-function.

font-lock-unfontify-buffer

Used when turning Font Lock off to remove the fontification. Calls the function specified by font-lock-unfontify-buffer-function.

font-lock-fontify-region beg end &optional loudly

Should fontify the region between beg and end. If loudly is non-nil, should display status messages while fontifying. Calls the function specified by font-lock-fontify-region-function.

font-lock-unfontify-region beg end

Should remove fontification from the region between beg and end. Calls the function specified by font-lock-unfontify-region-function.

font-lock-flush &optional beg end

This function should mark the fontification of the region between beg and end as outdated. If not specified or nil, beg and end default to the beginning and end of the buffer's accessible portion. Calls the function specified by font-lock-flush-function.

font-lock-ensure &optional beg end

This function should make sure the region between beg and end has been fontified. The optional arguments beg and end default to the beginning and the end of the buffer's accessible portion. Calls the function specified by font-lock-ensure-function.

There are several variables that control how Font Lock mode highlights text. But major modes should not set any of these variables directly. Instead, they should set font-lock-defaults as a buffer-local variable. The value assigned to this variable is used, if and when Font Lock mode is enabled, to set all the other variables.

If your mode fontifies text explicitly by adding font-lock-face properties, it can specify (nil t) for font-lock-defaults to turn off all automatic fontification. However, this is not required; it is possible to fontify some things using font-lock-face properties and set up automatic fontification for other parts of the text.

22.6.2 Search-based Fontification

The variable which directly controls search-based fontification is font-lock-keywords, which is typically specified via the keywords element in font-lock-defaults.

Each element of font-lock-keywords specifies how to find certain cases of text, and how to highlight those cases. Font Lock mode processes the elements of font-lock-keywords one by one, and for each element, it finds and handles all matches. Ordinarily, once part of the text has been fontified already, this cannot be overridden by a subsequent match in the same text; but you can specify different behavior using the override element of a subexp-highlighter.

Each element of font-lock-keywords should have one of these forms:

regexp

Highlight all matches for regexp using font-lock-keyword-face. For example,

          ;; Highlight occurrences of the word ‘foo’
          ;; using font-lock-keyword-face.
          "\\<foo\\>"

Be careful when composing these regular expressions; a poorly written pattern can dramatically slow things down! The function regexp-opt (see Regexp Functions) is useful for calculating optimal regular expressions to match several keywords.

function

Find text by calling function, and highlight the matches it finds using font-lock-keyword-face.

When function is called, it receives one argument, the limit of the search; it should begin searching at point, and not search beyond the limit. It should return non-nil if it succeeds, and set the match data to describe the match that was found. Returning nil indicates failure of the search.

Fontification will call function repeatedly with the same limit, and with point where the previous invocation left it, until function fails. On failure, function need not reset point in any particular way.

(matcher . subexp)

In this kind of element, matcher is either a regular expression or a function, as described above. The cdr, subexp, specifies which subexpression of matcher should be highlighted (instead of the entire text that matcher matched).

          ;; Highlight the ‘bar’ in each occurrence of ‘fubar’,
          ;; using font-lock-keyword-face.
          ("fu\\(bar\\)" . 1)

If you use regexp-opt to produce the regular expression matcher, you can use regexp-opt-depth (see Regexp Functions) to calculate the value for subexp.

(matcher . facespec)

In this kind of element, facespec is an expression whose value specifies the face to use for highlighting. In the simplest case, facespec is a Lisp variable (a symbol) whose value is a face name.

          ;; Highlight occurrences of ‘fubar’,
          ;; using the face which is the value of fubar-face.
          ("fubar" . fubar-face)

However, facespec can also evaluate to a list of this form:

          (face face prop1 val1 prop2 val2...)

to specify the face face and various additional text properties to put on the text that matches. If you do this, be sure to add the other text property names that you set in this way to the value of font-lock-extra-managed-props so that the properties will also be cleared out when they are no longer appropriate. Alternatively, you can set the variable font-lock-unfontify-region-function to a function that clears these properties. See Other Font Lock Variables.

(matcher . subexp-highlighter)

In this kind of element, subexp-highlighter is a list which specifies how to highlight matches found by matcher. It has the form:

          (subexp facespec [override [laxmatch]])

The car, subexp, is an integer specifying which subexpression of the match to fontify (0 means the entire matching text). The second subelement, facespec, is an expression whose value specifies the face, as described above.

The last two values in subexp-highlighter, override and laxmatch, are optional flags. If override is t, this element can override existing fontification made by previous elements of font-lock-keywords. If it is keep, then each character is fontified if it has not been fontified already by some other element. If it is prepend, the face specified by facespec is added to the beginning of the font-lock-face property. If it is append, the face is added to the end of the font-lock-face property.

If laxmatch is non-nil, it means there should be no error if there is no subexpression numbered subexp in matcher. Obviously, fontification of the subexpression numbered subexp will not occur. However, fontification of other subexpressions (and other regexps) will continue. If laxmatch is nil, and the specified subexpression is missing, then an error is signaled which terminates search-based fontification.

Here are some examples of elements of this kind, and what they do:

          ;; Highlight occurrences of either ‘foo’ or ‘bar’, using
          ;; foo-bar-face, even if they have already been highlighted.
          ;; foo-bar-face should be a variable whose value is a face.
          ("foo\\|bar" 0 foo-bar-face t)
          
          ;; Highlight the first subexpression within each occurrence
          ;; that the function fubar-match finds,
          ;; using the face which is the value of fubar-face.
          (fubar-match 1 fubar-face)

(matcher . anchored-highlighter)

In this kind of element, anchored-highlighter specifies how to highlight text that follows a match found by matcher. So a match found by matcher acts as the anchor for further searches specified by anchored-highlighter. anchored-highlighter is a list of the following form:

          (anchored-matcher pre-form post-form
                                  subexp-highlighters...)

Here, anchored-matcher, like matcher, is either a regular expression or a function. After a match of matcher is found, point is at the end of the match. Now, Font Lock evaluates the form pre-form. Then it searches for matches of anchored-matcher and uses subexp-highlighters to highlight these. A subexp-highlighter is as described above. Finally, Font Lock evaluates post-form.

The forms pre-form and post-form can be used to initialize before, and cleanup after, anchored-matcher is used. Typically, pre-form is used to move point to some position relative to the match of matcher, before starting with anchored-matcher. post-form might be used to move back, before resuming with matcher.

After Font Lock evaluates pre-form, it does not search for anchored-matcher beyond the end of the line. However, if pre-form returns a buffer position that is greater than the position of point after pre-form is evaluated, then the position returned by pre-form is used as the limit of the search instead. It is generally a bad idea to return a position greater than the end of the line; in other words, the anchored-matcher search should not span lines.

For example,

          ;; Highlight occurrences of the word ‘item’ following
          ;; an occurrence of the word ‘anchor’ (on the same line)
          ;; in the value of item-face.
          ("\\<anchor\\>" "\\<item\\>" nil nil (0 item-face))

Here, pre-form and post-form are nil. Therefore searching for ‘item’ starts at the end of the match of ‘anchor’, and searching for subsequent instances of ‘anchor’ resumes from where searching for ‘item’ concluded.

(matcher highlighters...)

This sort of element specifies several highlighter lists for a single matcher. A highlighter list can be of the type subexp-highlighter or anchored-highlighter as described above.

For example,

          ;; Highlight occurrences of the word ‘anchor’ in the value
          ;; of anchor-face, and subsequent occurrences of the word
          ;; ‘item’ (on the same line) in the value of item-face.
          ("\\<anchor\\>" (0 anchor-face)
                          ("\\<item\\>" nil nil (0 item-face)))

(eval . form)

Here form is an expression to be evaluated the first time this value of font-lock-keywords is used in a buffer. Its value should have one of the forms described in this table.

Warning: Do not design an element of font-lock-keywords to match text which spans lines; this does not work reliably. For details, see See Multiline Font Lock.

You can use case-fold in font-lock-defaults to specify the value of font-lock-keywords-case-fold-search which says whether search-based fontification should be case-insensitive.

22.6.3 Customizing Search-Based Fontification

You can use font-lock-add-keywords to add additional search-based fontification rules to a major mode, and font-lock-remove-keywords to remove rules.

For example, the following code adds two fontification patterns for C mode: one to fontify the word ‘FIXME’, even in comments, and another to fontify the words ‘and’, ‘or’ and ‘not’ as keywords.

     (font-lock-add-keywords 'c-mode
      '(("\\<\\(FIXME\\):" 1 font-lock-warning-face prepend)
        ("\\<\\(and\\|or\\|not\\)\\>" . font-lock-keyword-face)))

This example affects only C mode proper. To add the same patterns to C mode and all modes derived from it, do this instead:

     (add-hook 'c-mode-hook
      (lambda ()
       (font-lock-add-keywords nil
        '(("\\<\\(FIXME\\):" 1 font-lock-warning-face prepend)
          ("\\<\\(and\\|or\\|not\\)\\>" .
           font-lock-keyword-face)))))

22.6.4 Other Font Lock Variables

This section describes additional variables that a major mode can set by means of other-vars in font-lock-defaults (see Font Lock Basics).

22.6.5 Levels of Font Lock

Some major modes offer three different levels of fontification. You can define multiple levels by using a list of symbols for keywords in font-lock-defaults. Each symbol specifies one level of fontification; it is up to the user to choose one of these levels, normally by setting font-lock-maximum-decoration (see Font Lock). The chosen level's symbol value is used to initialize font-lock-keywords.

Here are the conventions for how to define the levels of fontification:

• Level 1: highlight function declarations, file directives (such as include or import directives), strings and comments. The idea is speed, so only the most important and top-level components are fontified.
• Level 2: in addition to level 1, highlight all language keywords, including type names that act like keywords, as well as named constant values. The idea is that all keywords (either syntactic or semantic) should be fontified appropriately.
• Level 3: in addition to level 2, highlight the symbols being defined in function and variable declarations, and all builtin function names, wherever they appear.

22.6.6 Precalculated Fontification

Some major modes such as list-buffers and occur construct the buffer text programmatically. The easiest way for them to support Font Lock mode is to specify the faces of text when they insert the text in the buffer.

The way to do this is to specify the faces in the text with the special text property font-lock-face (see Special Properties). When Font Lock mode is enabled, this property controls the display, just like the face property. When Font Lock mode is disabled, font-lock-face has no effect on the display.

It is ok for a mode to use font-lock-face for some text and also use the normal Font Lock machinery. But if the mode does not use the normal Font Lock machinery, it should not set the variable font-lock-defaults.

22.6.7 Faces for Font Lock

Font Lock mode can highlight using any face, but Emacs defines several faces specifically for Font Lock to use to highlight text. These Font Lock faces are listed below. They can also be used by major modes for syntactic highlighting outside of Font Lock mode (see Major Mode Conventions).

Each of these symbols is both a face name, and a variable whose default value is the symbol itself. Thus, the default value of font-lock-comment-face is font-lock-comment-face.

The faces are listed with descriptions of their typical usage, and in order of greater to lesser prominence. If a mode's syntactic categories do not fit well with the usage descriptions, the faces can be assigned using the ordering as a guide.

font-lock-warning-face

for a construct that is peculiar, or that greatly changes the meaning of other text, like ‘;;;###autoload’ in Emacs Lisp and ‘#error’ in C.

font-lock-function-name-face

for the name of a function being defined or declared.

font-lock-variable-name-face

for the name of a variable being defined or declared.

font-lock-keyword-face

for a keyword with special syntactic significance, like ‘for’ and ‘if’ in C.

font-lock-comment-face

for comments.

font-lock-comment-delimiter-face

for comments delimiters, like ‘/*’ and ‘*/’ in C. On most terminals, this inherits from font-lock-comment-face.

font-lock-type-face

for the names of user-defined data types.

font-lock-constant-face

for the names of constants, like ‘NULL’ in C.

font-lock-builtin-face

for the names of built-in functions.

font-lock-preprocessor-face

for preprocessor commands. This inherits, by default, from font-lock-builtin-face.

font-lock-string-face

for string constants.

font-lock-doc-face

for documentation strings in the code. This inherits, by default, from font-lock-string-face.

font-lock-negation-char-face

for easily-overlooked negation characters.

22.6.8 Syntactic Font Lock

Syntactic fontification uses a syntax table (see Syntax Tables) to find and highlight syntactically relevant text. If enabled, it runs prior to search-based fontification. The variable font-lock-syntactic-face-function, documented below, determines which syntactic constructs to highlight. There are several variables that affect syntactic fontification; you should set them by means of font-lock-defaults (see Font Lock Basics).

Whenever Font Lock mode performs syntactic fontification on a stretch of text, it first calls the function specified by syntax-propertize-function. Major modes can use this to apply syntax-table text properties to override the buffer's syntax table in special cases. See Syntax Properties.

22.6.9 Multiline Font Lock Constructs

Normally, elements of font-lock-keywords should not match across multiple lines; that doesn't work reliably, because Font Lock usually scans just part of the buffer, and it can miss a multi-line construct that crosses the line boundary where the scan starts. (The scan normally starts at the beginning of a line.)

Making elements that match multiline constructs work properly has two aspects: correct identification and correct rehighlighting. The first means that Font Lock finds all multiline constructs. The second means that Font Lock will correctly rehighlight all the relevant text when a multiline construct is changed—for example, if some of the text that was previously part of a multiline construct ceases to be part of it. The two aspects are closely related, and often getting one of them to work will appear to make the other also work. However, for reliable results you must attend explicitly to both aspects.

There are three ways to ensure correct identification of multiline constructs:

• Add a function to font-lock-extend-region-functions that does the identification and extends the scan so that the scanned text never starts or ends in the middle of a multiline construct.
• Use the font-lock-fontify-region-function hook similarly to extend the scan so that the scanned text never starts or ends in the middle of a multiline construct.
• Somehow identify the multiline construct right when it gets inserted into the buffer (or at any point after that but before font-lock tries to highlight it), and mark it with a font-lock-multiline which will instruct font-lock not to start or end the scan in the middle of the construct.

There are three ways to do rehighlighting of multiline constructs:

• Place a font-lock-multiline property on the construct. This will rehighlight the whole construct if any part of it is changed. In some cases you can do this automatically by setting the font-lock-multiline variable, which see.
• Make sure jit-lock-contextually is set and rely on it doing its job. This will only rehighlight the part of the construct that follows the actual change, and will do it after a short delay. This only works if the highlighting of the various parts of your multiline construct never depends on text in subsequent lines. Since jit-lock-contextually is activated by default, this can be an attractive solution.
• Place a jit-lock-defer-multiline property on the construct. This works only if jit-lock-contextually is used, and with the same delay before rehighlighting, but like font-lock-multiline, it also handles the case where highlighting depends on subsequent lines.

• Font Lock Multiline: Marking multiline chunks with a text property.
• Region to Refontify: Controlling which region gets refontified after a buffer change.

22.6.9.1 Font Lock Multiline

One way to ensure reliable rehighlighting of multiline Font Lock constructs is to put on them the text property font-lock-multiline. It should be present and non-nil for text that is part of a multiline construct.

When Font Lock is about to highlight a range of text, it first extends the boundaries of the range as necessary so that they do not fall within text marked with the font-lock-multiline property. Then it removes any font-lock-multiline properties from the range, and highlights it. The highlighting specification (mostly font-lock-keywords) must reinstall this property each time, whenever it is appropriate.

Warning: don't use the font-lock-multiline property on large ranges of text, because that will make rehighlighting slow.

The font-lock-multiline property is meant to ensure proper refontification; it does not automatically identify new multiline constructs. Identifying the requires that Font Lock mode operate on large enough chunks at a time. This will happen by accident on many cases, which may give the impression that multiline constructs magically work. If you set the font-lock-multiline variable non-nil, this impression will be even stronger, since the highlighting of those constructs which are found will be properly updated from then on. But that does not work reliably.

To find multiline constructs reliably, you must either manually place the font-lock-multiline property on the text before Font Lock mode looks at it, or use font-lock-fontify-region-function.

22.6.9.2 Region to Fontify after a Buffer Change

When a buffer is changed, the region that Font Lock refontifies is by default the smallest sequence of whole lines that spans the change. While this works well most of the time, sometimes it doesn't—for example, when a change alters the syntactic meaning of text on an earlier line.

You can enlarge (or even reduce) the region to refontify by setting the following variable:

22.7 Automatic Indentation of code

For programming languages, an important feature of a major mode is to provide automatic indentation. There are two parts: one is to decide what is the right indentation of a line, and the other is to decide when to reindent a line. By default, Emacs reindents a line whenever you type a character in electric-indent-chars, which by default only includes Newline. Major modes can add chars to electric-indent-chars according to the syntax of the language.

Deciding what is the right indentation is controlled in Emacs by indent-line-function (see Mode-Specific Indent). For some modes, the right indentation cannot be known reliably, typically because indentation is significant so several indentations are valid but with different meanings. In that case, the mode should set electric-indent-inhibit to make sure the line is not constantly re-indented against the user's wishes.

Writing a good indentation function can be difficult and to a large extent it is still a black art. Many major mode authors will start by writing a simple indentation function that works for simple cases, for example by comparing with the indentation of the previous text line. For most programming languages that are not really line-based, this tends to scale very poorly: improving such a function to let it handle more diverse situations tends to become more and more difficult, resulting in the end with a large, complex, unmaintainable indentation function which nobody dares to touch.

A good indentation function will usually need to actually parse the text, according to the syntax of the language. Luckily, it is not necessary to parse the text in as much detail as would be needed for a compiler, but on the other hand, the parser embedded in the indentation code will want to be somewhat friendly to syntactically incorrect code.

Good maintainable indentation functions usually fall into two categories: either parsing forward from some safe starting point until the position of interest, or parsing backward from the position of interest. Neither of the two is a clearly better choice than the other: parsing backward is often more difficult than parsing forward because programming languages are designed to be parsed forward, but for the purpose of indentation it has the advantage of not needing to guess a safe starting point, and it generally enjoys the property that only a minimum of text will be analyzed to decide the indentation of a line, so indentation will tend to be less affected by syntax errors in some earlier unrelated piece of code. Parsing forward on the other hand is usually easier and has the advantage of making it possible to reindent efficiently a whole region at a time, with a single parse.

Rather than write your own indentation function from scratch, it is often preferable to try and reuse some existing ones or to rely on a generic indentation engine. There are sadly few such engines. The CC-mode indentation code (used with C, C++, Java, Awk and a few other such modes) has been made more generic over the years, so if your language seems somewhat similar to one of those languages, you might try to use that engine. Another one is SMIE which takes an approach in the spirit of Lisp sexps and adapts it to non-Lisp languages.

• SMIE: A simple minded indentation engine.

22.7.1 Simple Minded Indentation Engine

SMIE is a package that provides a generic navigation and indentation engine. Based on a very simple parser using an operator precedence grammar, it lets major modes extend the sexp-based navigation of Lisp to non-Lisp languages as well as provide a simple to use but reliable auto-indentation.

Operator precedence grammar is a very primitive technology for parsing compared to some of the more common techniques used in compilers. It has the following characteristics: its parsing power is very limited, and it is largely unable to detect syntax errors, but it has the advantage of being algorithmically efficient and able to parse forward just as well as backward. In practice that means that SMIE can use it for indentation based on backward parsing, that it can provide both forward-sexp and backward-sexp functionality, and that it will naturally work on syntactically incorrect code without any extra effort. The downside is that it also means that most programming languages cannot be parsed correctly using SMIE, at least not without resorting to some special tricks (see SMIE Tricks).

• SMIE setup: SMIE setup and features.
• Operator Precedence Grammars: A very simple parsing technique.
• SMIE Grammar: Defining the grammar of a language.
• SMIE Lexer: Defining tokens.
• SMIE Tricks: Working around the parser's limitations.
• SMIE Indentation: Specifying indentation rules.
• SMIE Indentation Helpers: Helper functions for indentation rules.
• SMIE Indentation Example: Sample indentation rules.
• SMIE Customization: Customizing indentation.

22.7.1.1 SMIE Setup and Features

SMIE is meant to be a one-stop shop for structural navigation and various other features which rely on the syntactic structure of code, in particular automatic indentation. The main entry point is smie-setup which is a function typically called while setting up a major mode.

Calling this function is sufficient to make commands such as forward-sexp, backward-sexp, and transpose-sexps be able to properly handle structural elements other than just the paired parentheses already handled by syntax tables. For example, if the provided grammar is precise enough, transpose-sexps can correctly transpose the two arguments of a + operator, taking into account the precedence rules of the language.

Calling smie-setup is also sufficient to make TAB indentation work in the expected way, extends blink-matching-paren to apply to elements like begin...end, and provides some commands that you can bind in the major mode keymap.

22.7.1.2 Operator Precedence Grammars

SMIE's precedence grammars simply give to each token a pair of precedences: the left-precedence and the right-precedence. We say T1 < T2 if the right-precedence of token T1 is less than the left-precedence of token T2. A good way to read this < is as a kind of parenthesis: if we find ... T1 something T2 ... then that should be parsed as ... T1 (something T2 ... rather than as ... T1 something) T2 .... The latter interpretation would be the case if we had T1 > T2. If we have T1 = T2, it means that token T2 follows token T1 in the same syntactic construction, so typically we have "begin" = "end". Such pairs of precedences are sufficient to express left-associativity or right-associativity of infix operators, nesting of tokens like parentheses and many other cases.

22.7.1.3 Defining the Grammar of a Language

The usual way to define the SMIE grammar of a language is by defining a new global variable that holds the precedence table by giving a set of BNF rules. For example, the grammar definition for a small Pascal-like language could look like:

     (require 'smie)
     (defvar sample-smie-grammar
       (smie-prec2->grammar
        (smie-bnf->prec2
         '((id)
           (inst ("begin" insts "end")
                 ("if" exp "then" inst "else" inst)
                 (id ":=" exp)
                 (exp))
           (insts (insts ";" insts) (inst))
           (exp (exp "+" exp)
                (exp "*" exp)
                ("(" exps ")"))
           (exps (exps "," exps) (exp)))
         '((assoc ";"))
         '((assoc ","))
         '((assoc "+") (assoc "*")))))

A few things to note:

• The above grammar does not explicitly mention the syntax of function calls: SMIE will automatically allow any sequence of sexps, such as identifiers, balanced parentheses, or begin ... end blocks to appear anywhere anyway.
• The grammar category id has no right hand side: this does not mean that it can match only the empty string, since as mentioned any sequence of sexps can appear anywhere anyway.
• Because non terminals cannot appear consecutively in the BNF grammar, it is difficult to correctly handle tokens that act as terminators, so the above grammar treats ";" as a statement separator instead, which SMIE can handle very well.
• Separators used in sequences (such as "," and ";" above) are best defined with BNF rules such as (foo (foo "separator" foo) ...) which generate precedence conflicts which are then resolved by giving them an explicit (assoc "separator").
• The ("(" exps ")") rule was not needed to pair up parens, since SMIE will pair up any characters that are marked as having paren syntax in the syntax table. What this rule does instead (together with the definition of exps) is to make it clear that "," should not appear outside of parentheses.
• Rather than have a single precs table to resolve conflicts, it is preferable to have several tables, so as to let the BNF part of the grammar specify relative precedences where possible.
• Unless there is a very good reason to prefer left or right, it is usually preferable to mark operators as associative, using assoc. For that reason "+" and "*" are defined above as assoc, although the language defines them formally as left associative.

22.7.1.4 Defining Tokens

SMIE comes with a predefined lexical analyzer which uses syntax tables in the following way: any sequence of characters that have word or symbol syntax is considered a token, and so is any sequence of characters that have punctuation syntax. This default lexer is often a good starting point but is rarely actually correct for any given language. For example, it will consider "2,+3" to be composed of 3 tokens: "2", ",+", and "3".

To describe the lexing rules of your language to SMIE, you need 2 functions, one to fetch the next token, and another to fetch the previous token. Those functions will usually first skip whitespace and comments and then look at the next chunk of text to see if it is a special token. If so it should skip the token and return a description of this token. Usually this is simply the string extracted from the buffer, but it can be anything you want. For example:

     (defvar sample-keywords-regexp
       (regexp-opt '("+" "*" "," ";" ">" ">=" "<" "<=" ":=" "=")))
     (defun sample-smie-forward-token ()
       (forward-comment (point-max))
       (cond
        ((looking-at sample-keywords-regexp)
         (goto-char (match-end 0))
         (match-string-no-properties 0))
        (t (buffer-substring-no-properties
            (point)
            (progn (skip-syntax-forward "w_")
                   (point))))))
     (defun sample-smie-backward-token ()
       (forward-comment (- (point)))
       (cond
        ((looking-back sample-keywords-regexp (- (point) 2) t)
         (goto-char (match-beginning 0))
         (match-string-no-properties 0))
        (t (buffer-substring-no-properties
            (point)
            (progn (skip-syntax-backward "w_")
                   (point))))))

Notice how those lexers return the empty string when in front of parentheses. This is because SMIE automatically takes care of the parentheses defined in the syntax table. More specifically if the lexer returns nil or an empty string, SMIE tries to handle the corresponding text as a sexp according to syntax tables.

22.7.1.5 Living With a Weak Parser

The parsing technique used by SMIE does not allow tokens to behave differently in different contexts. For most programming languages, this manifests itself by precedence conflicts when converting the BNF grammar.

Sometimes, those conflicts can be worked around by expressing the grammar slightly differently. For example, for Modula-2 it might seem natural to have a BNF grammar that looks like this:

       ...
       (inst ("IF" exp "THEN" insts "ELSE" insts "END")
             ("CASE" exp "OF" cases "END")
             ...)
       (cases (cases "|" cases)
              (caselabel ":" insts)
              ("ELSE" insts))
       ...

But this will create conflicts for "ELSE": on the one hand, the IF rule implies (among many other things) that "ELSE" = "END"; but on the other hand, since "ELSE" appears within cases, which appears left of "END", we also have "ELSE" > "END". We can solve the conflict either by using:

       ...
       (inst ("IF" exp "THEN" insts "ELSE" insts "END")
             ("CASE" exp "OF" cases "END")
             ("CASE" exp "OF" cases "ELSE" insts "END")
             ...)
       (cases (cases "|" cases) (caselabel ":" insts))
       ...

or

       ...
       (inst ("IF" exp "THEN" else "END")
             ("CASE" exp "OF" cases "END")
             ...)
       (else (insts "ELSE" insts))
       (cases (cases "|" cases) (caselabel ":" insts) (else))
       ...

Reworking the grammar to try and solve conflicts has its downsides, tho, because SMIE assumes that the grammar reflects the logical structure of the code, so it is preferable to keep the BNF closer to the intended abstract syntax tree.

Other times, after careful consideration you may conclude that those conflicts are not serious and simply resolve them via the resolvers argument of smie-bnf->prec2. Usually this is because the grammar is simply ambiguous: the conflict does not affect the set of programs described by the grammar, but only the way those programs are parsed. This is typically the case for separators and associative infix operators, where you want to add a resolver like '((assoc "|")). Another case where this can happen is for the classic dangling else problem, where you will use '((assoc "else" "then")). It can also happen for cases where the conflict is real and cannot really be resolved, but it is unlikely to pose a problem in practice.

Finally, in many cases some conflicts will remain despite all efforts to restructure the grammar. Do not despair: while the parser cannot be made more clever, you can make the lexer as smart as you want. So, the solution is then to look at the tokens involved in the conflict and to split one of those tokens into 2 (or more) different tokens. E.g., if the grammar needs to distinguish between two incompatible uses of the token "begin", make the lexer return different tokens (say "begin-fun" and "begin-plain") depending on which kind of "begin" it finds. This pushes the work of distinguishing the different cases to the lexer, which will thus have to look at the surrounding text to find ad-hoc clues.

22.7.1.6 Specifying Indentation Rules

Based on the provided grammar, SMIE will be able to provide automatic indentation without any extra effort. But in practice, this default indentation style will probably not be good enough. You will want to tweak it in many different cases.

SMIE indentation is based on the idea that indentation rules should be as local as possible. To this end, it relies on the idea of virtual indentation, which is the indentation that a particular program point would have if it were at the beginning of a line. Of course, if that program point is indeed at the beginning of a line, its virtual indentation is its current indentation. But if not, then SMIE uses the indentation algorithm to compute the virtual indentation of that point. Now in practice, the virtual indentation of a program point does not have to be identical to the indentation it would have if we inserted a newline before it. To see how this works, the SMIE rule for indentation after a { in C does not care whether the { is standing on a line of its own or is at the end of the preceding line. Instead, these different cases are handled in the indentation rule that decides how to indent before a {.

Another important concept is the notion of parent: The parent of a token, is the head token of the nearest enclosing syntactic construct. For example, the parent of an else is the if to which it belongs, and the parent of an if, in turn, is the lead token of the surrounding construct. The command backward-sexp jumps from a token to its parent, but there are some caveats: for openers (tokens which start a construct, like if), you need to start with point before the token, while for others you need to start with point after the token. backward-sexp stops with point before the parent token if that is the opener of the token of interest, and otherwise it stops with point after the parent token.

SMIE indentation rules are specified using a function that takes two arguments method and arg where the meaning of arg and the expected return value depend on method.

method can be:

• :after, in which case arg is a token and the function should return the offset to use for indentation after arg.
• :before, in which case arg is a token and the function should return the offset to use to indent arg itself.
• :elem, in which case the function should return either the offset to use to indent function arguments (if arg is the symbol arg) or the basic indentation step (if arg is the symbol basic).
• :list-intro, in which case arg is a token and the function should return non-nil if the token is followed by a list of expressions (not separated by any token) rather than an expression.

When arg is a token, the function is called with point just before that token. A return value of nil always means to fallback on the default behavior, so the function should return nil for arguments it does not expect.

offset can be:

• nil: use the default indentation rule.
• (column . column): indent to column column.
• number: offset by number, relative to a base token which is the current token for :after and its parent for :before.

22.7.1.7 Helper Functions for Indentation Rules

SMIE provides various functions designed specifically for use in the indentation rules function (several of those functions break if used in another context). These functions all start with the prefix smie-rule-.

22.7.1.8 Sample Indentation Rules

Here is an example of an indentation function:

     (defun sample-smie-rules (kind token)
       (pcase (cons kind token)
         (`(:elem . basic) sample-indent-basic)
         (`(,_ . ",") (smie-rule-separator kind))
         (`(:after . ":=") sample-indent-basic)
         (`(:before . ,(or `"begin" `"(" `"{")))
          (if (smie-rule-hanging-p) (smie-rule-parent)))
         (`(:before . "if")
          (and (not (smie-rule-bolp)) (smie-rule-prev-p "else")
               (smie-rule-parent)))))

A few things to note:

• The first case indicates the basic indentation increment to use. If sample-indent-basic is nil, then SMIE uses the global setting smie-indent-basic. The major mode could have set smie-indent-basic buffer-locally instead, but that is discouraged.
• The rule for the token "," make SMIE try to be more clever when the comma separator is placed at the beginning of lines. It tries to outdent the separator so as to align the code after the comma; for example:

            x = longfunctionname (
                    arg1
                  , arg2
                );
  

• The rule for indentation after ":=" exists because otherwise SMIE would treat ":=" as an infix operator and would align the right argument with the left one.
• The rule for indentation before "begin" is an example of the use of virtual indentation: This rule is used only when "begin" is hanging, which can happen only when "begin" is not at the beginning of a line. So this is not used when indenting "begin" itself but only when indenting something relative to this "begin". Concretely, this rule changes the indentation from:

                if x > 0 then begin
                        dosomething(x);
                    end
  

  to

                if x > 0 then begin
                    dosomething(x);
                end
  

• The rule for indentation before "if" is similar to the one for "begin", but where the purpose is to treat "else if" as a single unit, so as to align a sequence of tests rather than indent each test further to the right. This function does this only in the case where the "if" is not placed on a separate line, hence the smie-rule-bolp test.

  If we know that the "else" is always aligned with its "if" and is always at the beginning of a line, we can use a more efficient rule:

            ((equal token "if")
             (and (not (smie-rule-bolp))
                  (smie-rule-prev-p "else")
                  (save-excursion
                    (sample-smie-backward-token)
                    (cons 'column (current-column)))))
  

  The advantage of this formulation is that it reuses the indentation of the previous "else", rather than going all the way back to the first "if" of the sequence.

22.7.1.9 Customizing Indentation

If you are using a mode whose indentation is provided by SMIE, you can customize the indentation to suit your preferences. You can do this on a per-mode basis (using the option smie-config), or a per-file basis (using the function smie-config-local in a file-local variable specification).

22.8 Desktop Save Mode

Desktop Save Mode is a feature to save the state of Emacs from one session to another. The user-level commands for using Desktop Save Mode are described in the GNU Emacs Manual (see Saving Emacs Sessions). Modes whose buffers visit a file, don't have to do anything to use this feature.

For buffers not visiting a file to have their state saved, the major mode must bind the buffer local variable desktop-save-buffer to a non-nil value.

For buffers not visiting a file to be restored, the major mode must define a function to do the job, and that function must be listed in the alist desktop-buffer-mode-handlers.

23 Documentation

GNU Emacs has convenient built-in help facilities, most of which derive their information from documentation strings associated with functions and variables. This chapter describes how to access documentation strings in Lisp programs.

The contents of a documentation string should follow certain conventions. In particular, its first line should be a complete sentence (or two complete sentences) that briefly describes what the function or variable does. See Documentation Tips, for how to write good documentation strings.

Note that the documentation strings for Emacs are not the same thing as the Emacs manual. Manuals have their own source files, written in the Texinfo language; documentation strings are specified in the definitions of the functions and variables they apply to. A collection of documentation strings is not sufficient as a manual because a good manual is not organized in that fashion; it is organized in terms of topics of discussion.

For commands to display documentation strings, see Help.

• Documentation Basics: Where doc strings are defined and stored.
• Accessing Documentation: How Lisp programs can access doc strings.
• Keys in Documentation: Substituting current key bindings.
• Describing Characters: Making printable descriptions of non-printing characters and key sequences.
• Help Functions: Subroutines used by Emacs help facilities.

23.1 Documentation Basics

A documentation string is written using the Lisp syntax for strings, with double-quote characters surrounding the text. It is, in fact, an actual Lisp string. When the string appears in the proper place in a function or variable definition, it serves as the function's or variable's documentation.

In a function definition (a lambda or defun form), the documentation string is specified after the argument list, and is normally stored directly in the function object. See Function Documentation. You can also put function documentation in the function-documentation property of a function name (see Accessing Documentation).

In a variable definition (a defvar form), the documentation string is specified after the initial value. See Defining Variables. The string is stored in the variable's variable-documentation property.

Sometimes, Emacs does not keep documentation strings in memory. There are two such circumstances. Firstly, to save memory, the documentation for preloaded functions and variables (including primitives) is kept in a file named DOC, in the directory specified by doc-directory (see Accessing Documentation). Secondly, when a function or variable is loaded from a byte-compiled file, Emacs avoids loading its documentation string (see Docs and Compilation). In both cases, Emacs looks up the documentation string from the file only when needed, such as when the user calls C-h f (describe-function) for a function.

Documentation strings can contain special key substitution sequences, referring to key bindings which are looked up only when the user views the documentation. This allows the help commands to display the correct keys even if a user rearranges the default key bindings. See Keys in Documentation.

In the documentation string of an autoloaded command (see Autoload), these key-substitution sequences have an additional special effect: they cause C-h f on the command to trigger autoloading. (This is needed for correctly setting up the hyperlinks in the *Help* buffer.)

23.2 Access to Documentation Strings

Here is an example of using the two functions, documentation and documentation-property, to display the documentation strings for several symbols in a *Help* buffer.

     (defun describe-symbols (pattern)
       "Describe the Emacs Lisp symbols matching PATTERN.
     All symbols that have PATTERN in their name are described
     in the *Help* buffer."
       (interactive "sDescribe symbols matching: ")
       (let ((describe-func
              (function
               (lambda (s)
                 ;; Print description of symbol.
                 (if (fboundp s)             ; It is a function.
                     (princ
                      (format "%s\t%s\n%s\n\n" s
                        (if (commandp s)
                            (let ((keys (where-is-internal s)))
                              (if keys
                                  (concat
                                   "Keys: "
                                   (mapconcat 'key-description
                                              keys " "))
                                "Keys: none"))
                          "Function")
                        (or (documentation s)
                            "not documented"))))
     
                 (if (boundp s)              ; It is a variable.
                     (princ
                      (format "%s\t%s\n%s\n\n" s
                        (if (custom-variable-p s)
                            "Option " "Variable")
                        (or (documentation-property
                              s 'variable-documentation)
                            "not documented")))))))
             sym-list)
     
         ;; Build a list of symbols that match pattern.
         (mapatoms (function
                    (lambda (sym)
                      (if (string-match pattern (symbol-name sym))
                          (setq sym-list (cons sym sym-list))))))
     
         ;; Display the data.
         (help-setup-xref (list 'describe-symbols pattern) (interactive-p))
         (with-help-window (help-buffer)
           (mapcar describe-func (sort sym-list 'string<)))))

The describe-symbols function works like apropos, but provides more information.

     (describe-symbols "goal")
     
     ---------- Buffer: *Help* ----------
     goal-column     Option
     Semipermanent goal column for vertical motion, as set by ...
     
     
     
     minibuffer-temporary-goal-position      Variable
     not documented
     
     set-goal-column Keys: C-x C-n
     Set the current horizontal position as a goal for C-n and C-p.
     
     Those commands will move to this position in the line moved to
     rather than trying to keep the same horizontal position.
     With a non-nil argument ARG, clears out the goal column
     so that C-n and C-p resume vertical motion.
     The goal column is stored in the variable ‘goal-column’.
     
     (fn ARG)
     
     temporary-goal-column   Variable
     Current goal column for vertical motion.
     It is the column where point was at the start of the current run
     of vertical motion commands.
     
     When moving by visual lines via the function ‘line-move-visual’, it is a cons
     cell (COL . HSCROLL), where COL is the x-position, in pixels,
     divided by the default column width, and HSCROLL is the number of
     columns by which window is scrolled from left margin.
     
     When the ‘track-eol’ feature is doing its job, the value is
     ‘most-positive-fixnum’.
     ---------- Buffer: *Help* ----------

23.3 Substituting Key Bindings in Documentation

When documentation strings refer to key sequences, they should use the current, actual key bindings. They can do so using certain special text sequences described below. Accessing documentation strings in the usual way substitutes current key binding information for these special sequences. This works by calling substitute-command-keys. You can also call that function yourself.

Here is a list of the special sequences and what they mean:

\[command]

stands for a key sequence that will invoke command, or ‘M-x command’ if command has no key bindings.

\{mapvar}

stands for a summary of the keymap which is the value of the variable mapvar. The summary is made using describe-bindings.

\<mapvar>

stands for no text itself. It is used only for a side effect: it specifies mapvar's value as the keymap for any following ‘\[command]’ sequences in this documentation string.

‘

`

(left single quotation mark and grave accent) both stand for a left quote. This generates a left single quotation mark, an apostrophe, or a grave accent depending on the value of text-quoting-style.

’

'

(right single quotation mark and apostrophe) both stand for a right quote. This generates a right single quotation mark or an apostrophe depending on the value of text-quoting-style.

\=

quotes the following character and is discarded; thus, ‘\=`’ puts ‘`’ into the output, ‘\=\[’ puts ‘\[’ into the output, and ‘\=\=’ puts ‘\=’ into the output.

Please note: Each ‘\’ must be doubled when written in a string in Emacs Lisp.

Here are examples of the special sequences:

     (substitute-command-keys
        "To abort recursive edit, type `\\[abort-recursive-edit]'.")
     ⇒ "To abort recursive edit, type ‘C-]’."
     
     (substitute-command-keys
        "The keys that are defined for the minibuffer here are:
       \\{minibuffer-local-must-match-map}")
     ⇒ "The keys that are defined for the minibuffer here are:
     
     ?               minibuffer-completion-help
     SPC             minibuffer-complete-word
     TAB             minibuffer-complete
     C-j             minibuffer-complete-and-exit
     RET             minibuffer-complete-and-exit
     C-g             abort-recursive-edit
     "
     
     (substitute-command-keys
        "To abort a recursive edit from the minibuffer, type \
     `\\<minibuffer-local-must-match-map>\\[abort-recursive-edit]'.")
     ⇒ "To abort a recursive edit from the minibuffer, type ‘C-g’."

There are other special conventions for the text in documentation strings—for instance, you can refer to functions, variables, and sections of this manual. See Documentation Tips, for details.

23.4 Describing Characters for Help Messages

These functions convert events, key sequences, or characters to textual descriptions. These descriptions are useful for including arbitrary text characters or key sequences in messages, because they convert non-printing and whitespace characters to sequences of printing characters. The description of a non-whitespace printing character is the character itself.

23.5 Help Functions

Emacs provides a variety of built-in help functions, all accessible to the user as subcommands of the prefix C-h. For more information about them, see Help. Here we describe some program-level interfaces to the same information.

The following two functions are meant for modes that want to provide help without relinquishing control, such as the electric modes. Their names begin with ‘Helper’ to distinguish them from the ordinary help functions.

See describe-symbols example, for an example of using help-buffer, with-help-window, and help-setup-xref.

24 Files

This chapter describes the Emacs Lisp functions and variables to find, create, view, save, and otherwise work with files and directories. A few other file-related functions are described in Buffers, and those related to backups and auto-saving are described in Backups and Auto-Saving.

Many of the file functions take one or more arguments that are file names. A file name is a string. Most of these functions expand file name arguments using the function expand-file-name, so that ~ is handled correctly, as are relative file names (including ../). See File Name Expansion.

In addition, certain magic file names are handled specially. For example, when a remote file name is specified, Emacs accesses the file over the network via an appropriate protocol. See Remote Files. This handling is done at a very low level, so you may assume that all the functions described in this chapter accept magic file names as file name arguments, except where noted. See Magic File Names, for details.

When file I/O functions signal Lisp errors, they usually use the condition file-error (see Handling Errors). The error message is in most cases obtained from the operating system, according to locale system-messages-locale, and decoded using coding system locale-coding-system (see Locales).

• Visiting Files: Reading files into Emacs buffers for editing.
• Saving Buffers: Writing changed buffers back into files.
• Reading from Files: Reading files into buffers without visiting.
• Writing to Files: Writing new files from parts of buffers.
• File Locks: Locking and unlocking files, to prevent simultaneous editing by two people.
• Information about Files: Testing existence, accessibility, size of files.
• Changing Files: Renaming files, changing permissions, etc.
• File Names: Decomposing and expanding file names.
• Contents of Directories: Getting a list of the files in a directory.
• Create/Delete Dirs: Creating and Deleting Directories.
• Magic File Names: Special handling for certain file names.
• Format Conversion: Conversion to and from various file formats.

24.1 Visiting Files

Visiting a file means reading a file into a buffer. Once this is done, we say that the buffer is visiting that file, and call the file the visited file of the buffer.

A file and a buffer are two different things. A file is information recorded permanently in the computer (unless you delete it). A buffer, on the other hand, is information inside of Emacs that will vanish at the end of the editing session (or when you kill the buffer). When a buffer is visiting a file, it contains information copied from the file. The copy in the buffer is what you modify with editing commands. Changes to the buffer do not change the file; to make the changes permanent, you must save the buffer, which means copying the altered buffer contents back into the file.

Despite the distinction between files and buffers, people often refer to a file when they mean a buffer and vice-versa. Indeed, we say, “I am editing a file”, rather than, “I am editing a buffer that I will soon save as a file of the same name”. Humans do not usually need to make the distinction explicit. When dealing with a computer program, however, it is good to keep the distinction in mind.

• Visiting Functions: The usual interface functions for visiting.
• Subroutines of Visiting: Lower-level subroutines that they use.

24.1.1 Functions for Visiting Files

This section describes the functions normally used to visit files. For historical reasons, these functions have names starting with ‘find-’ rather than ‘visit-’. See Buffer File Name, for functions and variables that access the visited file name of a buffer or that find an existing buffer by its visited file name.

In a Lisp program, if you want to look at the contents of a file but not alter it, the fastest way is to use insert-file-contents in a temporary buffer. Visiting the file is not necessary and takes longer. See Reading from Files.

24.1.2 Subroutines of Visiting

The find-file-noselect function uses two important subroutines which are sometimes useful in user Lisp code: create-file-buffer and after-find-file. This section explains how to use them.

24.2 Saving Buffers

When you edit a file in Emacs, you are actually working on a buffer that is visiting that file—that is, the contents of the file are copied into the buffer and the copy is what you edit. Changes to the buffer do not change the file until you save the buffer, which means copying the contents of the buffer into the file.

Saving a buffer runs several hooks. It also performs format conversion (see Format Conversion). Note that these hooks, described below, are only run by save-buffer, they are not run by other primitives and functions that write buffer text to files, and in particular auto-saving (see Auto-Saving) doesn't run these hooks.

See also the function set-visited-file-name (see Buffer File Name).

24.3 Reading from Files

To copy the contents of a file into a buffer, use the function insert-file-contents. (Don't use the command insert-file in a Lisp program, as that sets the mark.)

If you want to pass a file name to another process so that another program can read the file, use the function file-local-copy; see Magic File Names.

24.4 Writing to Files

You can write the contents of a buffer, or part of a buffer, directly to a file on disk using the append-to-file and write-region functions. Don't use these functions to write to files that are being visited; that could cause confusion in the mechanisms for visiting.

24.5 File Locks

When two users edit the same file at the same time, they are likely to interfere with each other. Emacs tries to prevent this situation from arising by recording a file lock when a file is being modified. Emacs can then detect the first attempt to modify a buffer visiting a file that is locked by another Emacs job, and ask the user what to do. The file lock is really a file, a symbolic link with a special name, stored in the same directory as the file you are editing. (On file systems that do not support symbolic links, a regular file is used.)

When you access files using NFS, there may be a small probability that you and another user will both lock the same file simultaneously. If this happens, it is possible for the two users to make changes simultaneously, but Emacs will still warn the user who saves second. Also, the detection of modification of a buffer visiting a file changed on disk catches some cases of simultaneous editing; see Modification Time.

24.6 Information about Files

This section describes the functions for retrieving various types of information about files (or directories or symbolic links), such as whether a file is readable or writable, and its size. These functions all take arguments which are file names. Except where noted, these arguments need to specify existing files, or an error is signaled.

Be careful with file names that end in spaces. On some filesystems (notably, MS-Windows), trailing whitespace characters in file names are silently and automatically ignored.

• Testing Accessibility: Is a given file readable? Writable?
• Kinds of Files: Is it a directory? A symbolic link?
• Truenames: Eliminating symbolic links from a file name.
• File Attributes: File sizes, modification times, etc.
• Extended Attributes: Extended file attributes for access control.
• Locating Files: How to find a file in standard places.

24.6.1 Testing Accessibility

These functions test for permission to access a file for reading, writing, or execution. Unless explicitly stated otherwise, they recursively follow symbolic links for their file name arguments, at all levels (at the level of the file itself and at all levels of parent directories).

On some operating systems, more complex sets of access permissions can be specified, via mechanisms such as Access Control Lists (ACLs). See Extended Attributes, for how to query and set those permissions.

24.6.2 Distinguishing Kinds of Files

This section describes how to distinguish various kinds of files, such as directories, symbolic links, and ordinary files.

The next two functions recursively follow symbolic links at all levels for filename.

24.6.3 Truenames

The truename of a file is the name that you get by following symbolic links at all levels until none remain, then simplifying away ‘.’ and ‘..’ appearing as name components. This results in a sort of canonical name for the file. A file does not always have a unique truename; the number of distinct truenames a file has is equal to the number of hard links to the file. However, truenames are useful because they eliminate symbolic links as a cause of name variation.

To illustrate the difference between file-chase-links and file-truename, suppose that /usr/foo is a symbolic link to the directory /home/foo, and /home/foo/hello is an ordinary file (or at least, not a symbolic link) or nonexistent. Then we would have:

     (file-chase-links "/usr/foo/hello")
          ;; This does not follow the links in the parent directories.
          ⇒ "/usr/foo/hello"
     (file-truename "/usr/foo/hello")
          ;; Assuming that /home is not a symbolic link.
          ⇒ "/home/foo/hello"

24.6.4 File Attributes

This section describes the functions for getting detailed information about a file, including the owner and group numbers, the number of names, the inode number, the size, and the times of access and modification.

If the filename argument to the next two functions is a symbolic link, then these function do not replace it with its target. However, they both recursively follow symbolic links at all levels of parent directories.

24.6.5 Extended File Attributes

On some operating systems, each file can be associated with arbitrary extended file attributes. At present, Emacs supports querying and setting two specific sets of extended file attributes: Access Control Lists (ACLs) and SELinux contexts. These extended file attributes are used, on some systems, to impose more sophisticated file access controls than the basic Unix-style permissions discussed in the previous sections.

A detailed explanation of ACLs and SELinux is beyond the scope of this manual. For our purposes, each file can be associated with an ACL, which specifies its properties under an ACL-based file control system, and/or an SELinux context, which specifies its properties under the SELinux system.

24.6.6 Locating Files in Standard Places

This section explains how to search for a file in a list of directories (a path), or for an executable file in the standard list of executable file directories.

To search for a user-specific configuration file, See Standard File Names, for the locate-user-emacs-file function.

24.7 Changing File Names and Attributes

The functions in this section rename, copy, delete, link, and set the modes (permissions) of files. They all signal a file-error error if they fail to perform their function, reporting the system-dependent error message that describes the reason for the failure.

In the functions that have an argument newname, if a file by the name of newname already exists, the actions taken depend on the value of the argument ok-if-already-exists:

• Signal a file-already-exists error if ok-if-already-exists is nil.
• Request confirmation if ok-if-already-exists is a number.
• Replace the old file without confirmation if ok-if-already-exists is any other value.

The next four commands all recursively follow symbolic links at all levels of parent directories for their first argument, but, if that argument is itself a symbolic link, then only copy-file replaces it with its (recursive) target.

24.8 File Names

Files are generally referred to by their names, in Emacs as elsewhere. File names in Emacs are represented as strings. The functions that operate on a file all expect a file name argument.

In addition to operating on files themselves, Emacs Lisp programs often need to operate on file names; i.e., to take them apart and to use part of a name to construct related file names. This section describes how to manipulate file names.

The functions in this section do not actually access files, so they can operate on file names that do not refer to an existing file or directory.

On MS-DOS and MS-Windows, these functions (like the function that actually operate on files) accept MS-DOS or MS-Windows file-name syntax, where backslashes separate the components, as well as Unix syntax; but they always return Unix syntax. This enables Lisp programs to specify file names in Unix syntax and work properly on all systems without change.¹³

• File Name Components: The directory part of a file name, and the rest.
• Relative File Names: Some file names are relative to a current directory.
• Directory Names: A directory's name as a directory is different from its name as a file.
• File Name Expansion: Converting relative file names to absolute ones.
• Unique File Names: Generating names for temporary files.
• File Name Completion: Finding the completions for a given file name.
• Standard File Names: If your package uses a fixed file name, how to handle various operating systems simply.

24.8.1 File Name Components

The operating system groups files into directories. To specify a file, you must specify the directory and the file's name within that directory. Therefore, Emacs considers a file name as having two main parts: the directory name part, and the nondirectory part (or file name within the directory). Either part may be empty. Concatenating these two parts reproduces the original file name.

On most systems, the directory part is everything up to and including the last slash (backslash is also allowed in input on MS-DOS or MS-Windows); the nondirectory part is the rest.

For some purposes, the nondirectory part is further subdivided into the name proper and the version number. On most systems, only backup files have version numbers in their names.

24.8.2 Absolute and Relative File Names

All the directories in the file system form a tree starting at the root directory. A file name can specify all the directory names starting from the root of the tree; then it is called an absolute file name. Or it can specify the position of the file in the tree relative to a default directory; then it is called a relative file name. On Unix and GNU/Linux, an absolute file name starts with a ‘/’ or a ‘~’ (see abbreviate-file-name), and a relative one does not. On MS-DOS and MS-Windows, an absolute file name starts with a slash or a backslash, or with a drive specification ‘x:/’, where x is the drive letter.

Given a possibly relative file name, you can convert it to an absolute name using expand-file-name (see File Name Expansion). This function converts absolute file names to relative names:

24.8.3 Directory Names

A directory name is the name of a directory. A directory is actually a kind of file, so it has a file name (called the directory file name, which is related to the directory name but not identical to it. (This is not quite the same as the usual Unix terminology.) These two different names for the same entity are related by a syntactic transformation. On GNU and Unix systems, this is simple: a directory name ends in a slash, whereas the directory file name lacks that slash. On MS-DOS the relationship is more complicated.

The difference between directory name and directory file name is subtle but crucial. When an Emacs variable or function argument is described as being a directory name, a directory file name is not acceptable. When file-name-directory returns a string, that is always a directory name.

The following two functions convert between directory names and directory file names. They do nothing special with environment variable substitutions such as ‘$HOME’, and the constructs ‘~’, ‘.’ and ‘..’.

Given a directory name, you can combine it with a relative file name using concat:

     (concat dirname relfile)

Be sure to verify that the file name is relative before doing that. If you use an absolute file name, the results could be syntactically invalid or refer to the wrong file.

If you want to use a directory file name in making such a combination, you must first convert it to a directory name using file-name-as-directory:

     (concat (file-name-as-directory dirfile) relfile)

Don't try concatenating a slash by hand, as in

     ;;; Wrong!
     (concat dirfile "/" relfile)

because this is not portable. Always use file-name-as-directory.

To avoid the issues mentioned above, or if the dirname value might be nil (for example, from an element of load-path), use:

     (expand-file-name relfile dirname)

To convert a directory name to its abbreviation, use this function:

24.8.4 Functions that Expand Filenames

Expanding a file name means converting a relative file name to an absolute one. Since this is done relative to a default directory, you must specify the default directory name as well as the file name to be expanded. It also involves expanding abbreviations like ~/ (see abbreviate-file-name), and eliminating redundancies like ./ and name/../.

24.8.5 Generating Unique File Names

Some programs need to write temporary files. Here is the usual way to construct a name for such a file:

     (make-temp-file name-of-application)

The job of make-temp-file is to prevent two different users or two different jobs from trying to use the exact same file name.

The default directory for temporary files is controlled by the variable temporary-file-directory. This variable gives the user a uniform way to specify the directory for all temporary files. Some programs use small-temporary-file-directory instead, if that is non-nil. To use it, you should expand the prefix against the proper directory before calling make-temp-file.

24.8.6 File Name Completion

This section describes low-level subroutines for completing a file name. For higher level functions, see Reading File Names.

24.8.7 Standard File Names

Sometimes, an Emacs Lisp program needs to specify a standard file name for a particular use—typically, to hold configuration data specified by the current user. Usually, such files should be located in the directory specified by user-emacs-directory, which is ~/.emacs.d by default (see Init File). For example, abbrev definitions are stored by default in ~/.emacs.d/abbrev_defs. The easiest way to specify such a file name is to use the function locate-user-emacs-file.

A lower-level function for standardizing file names, which locate-user-emacs-file uses as a subroutine, is convert-standard-filename.

24.9 Contents of Directories

A directory is a kind of file that contains other files entered under various names. Directories are a feature of the file system.

Emacs can list the names of the files in a directory as a Lisp list, or display the names in a buffer using the ls shell command. In the latter case, it can optionally display information about each file, depending on the options passed to the ls command.

24.10 Creating, Copying and Deleting Directories

Most Emacs Lisp file-manipulation functions get errors when used on files that are directories. For example, you cannot delete a directory with delete-file. These special functions exist to create and delete directories.

24.11 Making Certain File Names “Magic”

You can implement special handling for certain file names. This is called making those names magic. The principal use for this feature is in implementing access to remote files (see Remote Files).

To define a kind of magic file name, you must supply a regular expression to define the class of names (all those that match the regular expression), plus a handler that implements all the primitive Emacs file operations for file names that match.

The variable file-name-handler-alist holds a list of handlers, together with regular expressions that determine when to apply each handler. Each element has this form:

     (regexp . handler)

All the Emacs primitives for file access and file name transformation check the given file name against file-name-handler-alist. If the file name matches regexp, the primitives handle that file by calling handler.

The first argument given to handler is the name of the primitive, as a symbol; the remaining arguments are the arguments that were passed to that primitive. (The first of these arguments is most often the file name itself.) For example, if you do this:

     (file-exists-p filename)

and filename has handler handler, then handler is called like this:

     (funcall handler 'file-exists-p filename)

When a function takes two or more arguments that must be file names, it checks each of those names for a handler. For example, if you do this:

     (expand-file-name filename dirname)

then it checks for a handler for filename and then for a handler for dirname. In either case, the handler is called like this:

     (funcall handler 'expand-file-name filename dirname)

The handler then needs to figure out whether to handle filename or dirname.

If the specified file name matches more than one handler, the one whose match starts last in the file name gets precedence. This rule is chosen so that handlers for jobs such as uncompression are handled first, before handlers for jobs such as remote file access.

Here are the operations that a magic file name handler gets to handle:

access-file, add-name-to-file, byte-compiler-base-file-name,
copy-directory, copy-file, delete-directory, delete-file, diff-latest-backup-file, directory-file-name, directory-files, directory-files-and-attributes, dired-compress-file, dired-uncache,
expand-file-name, file-accessible-directory-p, file-acl, file-attributes, file-directory-p, file-equal-p, file-executable-p, file-exists-p, file-in-directory-p, file-local-copy, file-modes, file-name-all-completions, file-name-as-directory, file-name-completion, file-name-directory, file-name-nondirectory, file-name-sans-versions, file-newer-than-file-p, file-notify-add-watch, file-notify-rm-watch, file-notify-valid-p, file-ownership-preserved-p, file-readable-p, file-regular-p, file-remote-p, file-selinux-context, file-symlink-p, file-truename, file-writable-p, find-backup-file-name, get-file-buffer, insert-directory, insert-file-contents,
load, make-auto-save-file-name, make-directory, make-directory-internal, make-symbolic-link,
process-file, rename-file, set-file-acl, set-file-modes, set-file-selinux-context, set-file-times, set-visited-file-modtime, shell-command, start-file-process, substitute-in-file-name,
unhandled-file-name-directory, vc-registered, verify-visited-file-modtime,
write-region.

Handlers for insert-file-contents typically need to clear the buffer's modified flag, with (set-buffer-modified-p nil), if the visit argument is non-nil. This also has the effect of unlocking the buffer if it is locked.

The handler function must handle all of the above operations, and possibly others to be added in the future. It need not implement all these operations itself—when it has nothing special to do for a certain operation, it can reinvoke the primitive, to handle the operation in the usual way. It should always reinvoke the primitive for an operation it does not recognize. Here's one way to do this:

     (defun my-file-handler (operation &rest args)
       ;; First check for the specific operations
       ;; that we have special handling for.
       (cond ((eq operation 'insert-file-contents) ...)
             ((eq operation 'write-region) ...)
             ...
             ;; Handle any operation we don't know about.
             (t (let ((inhibit-file-name-handlers
                       (cons 'my-file-handler
                             (and (eq inhibit-file-name-operation operation)
                                  inhibit-file-name-handlers)))
                      (inhibit-file-name-operation operation))
                  (apply operation args)))))

When a handler function decides to call the ordinary Emacs primitive for the operation at hand, it needs to prevent the primitive from calling the same handler once again, thus leading to an infinite recursion. The example above shows how to do this, with the variables inhibit-file-name-handlers and inhibit-file-name-operation. Be careful to use them exactly as shown above; the details are crucial for proper behavior in the case of multiple handlers, and for operations that have two file names that may each have handlers.

Handlers that don't really do anything special for actual access to the file—such as the ones that implement completion of host names for remote file names—should have a non-nil safe-magic property. For instance, Emacs normally protects directory names it finds in PATH from becoming magic, if they look like magic file names, by prefixing them with ‘/:’. But if the handler that would be used for them has a non-nil safe-magic property, the ‘/:’ is not added.

A file name handler can have an operations property to declare which operations it handles in a nontrivial way. If this property has a non-nil value, it should be a list of operations; then only those operations will call the handler. This avoids inefficiency, but its main purpose is for autoloaded handler functions, so that they won't be loaded except when they have real work to do.

Simply deferring all operations to the usual primitives does not work. For instance, if the file name handler applies to file-exists-p, then it must handle load itself, because the usual load code won't work properly in that case. However, if the handler uses the operations property to say it doesn't handle file-exists-p, then it need not handle load nontrivially.

24.12 File Format Conversion

Emacs performs several steps to convert the data in a buffer (text, text properties, and possibly other information) to and from a representation suitable for storing into a file. This section describes the fundamental functions that perform this format conversion, namely insert-file-contents for reading a file into a buffer, and write-region for writing a buffer into a file.

• Overview: insert-file-contents and write-region.
• Round-Trip: Using format-alist.
• Piecemeal: Specifying non-paired conversion.

24.12.1 Overview

The function insert-file-contents:

• initially, inserts bytes from the file into the buffer;
• decodes bytes to characters as appropriate;
• processes formats as defined by entries in format-alist; and
• calls functions in after-insert-file-functions.

The function write-region:

• initially, calls functions in write-region-annotate-functions;
• processes formats as defined by entries in format-alist;
• encodes characters to bytes as appropriate; and
• modifies the file with the bytes.

This shows the symmetry of the lowest-level operations; reading and writing handle things in opposite order. The rest of this section describes the two facilities surrounding the three variables named above, as well as some related functions. Coding Systems, for details on character encoding and decoding.

24.12.2 Round-Trip Specification

The most general of the two facilities is controlled by the variable format-alist, a list of file format specifications, which describe textual representations used in files for the data in an Emacs buffer. The descriptions for reading and writing are paired, which is why we call this “round-trip” specification (see Format Conversion Piecemeal, for non-paired specification).

Here is what the elements in a format definition mean:

name

The name of this format.

doc-string

A documentation string for the format.

regexp

A regular expression which is used to recognize files represented in this format. If nil, the format is never applied automatically.

from-fn

A shell command or function to decode data in this format (to convert file data into the usual Emacs data representation).

A shell command is represented as a string; Emacs runs the command as a filter to perform the conversion.

If from-fn is a function, it is called with two arguments, begin and end, which specify the part of the buffer it should convert. It should convert the text by editing it in place. Since this can change the length of the text, from-fn should return the modified end position.

One responsibility of from-fn is to make sure that the beginning of the file no longer matches regexp. Otherwise it is likely to get called again.

to-fn

A shell command or function to encode data in this format—that is, to convert the usual Emacs data representation into this format.

If to-fn is a string, it is a shell command; Emacs runs the command as a filter to perform the conversion.

If to-fn is a function, it is called with three arguments: begin and end, which specify the part of the buffer it should convert, and buffer, which specifies which buffer. There are two ways it can do the conversion:

• By editing the buffer in place. In this case, to-fn should return the end-position of the range of text, as modified.
• By returning a list of annotations. This is a list of elements of the form (position . string), where position is an integer specifying the relative position in the text to be written, and string is the annotation to add there. The list must be sorted in order of position when to-fn returns it.

  When write-region actually writes the text from the buffer to the file, it intermixes the specified annotations at the corresponding positions. All this takes place without modifying the buffer.

modify

A flag, t if the encoding function modifies the buffer, and nil if it works by returning a list of annotations.

mode-fn

A minor-mode function to call after visiting a file converted from this format. The function is called with one argument, the integer 1; that tells a minor-mode function to enable the mode.

preserve

A flag, t if format-write-file should not remove this format from buffer-file-format.

The function insert-file-contents automatically recognizes file formats when it reads the specified file. It checks the text of the beginning of the file against the regular expressions of the format definitions, and if it finds a match, it calls the decoding function for that format. Then it checks all the known formats over again. It keeps checking them until none of them is applicable.

Visiting a file, with find-file-noselect or the commands that use it, performs conversion likewise (because it calls insert-file-contents); it also calls the mode function for each format that it decodes. It stores a list of the format names in the buffer-local variable buffer-file-format.

When write-region writes data into a file, it first calls the encoding functions for the formats listed in buffer-file-format, in the order of appearance in the list.

24.12.3 Piecemeal Specification

In contrast to the round-trip specification described in the previous subsection (see Format Conversion Round-Trip), you can use the variables after-insert-file-functions and write-region-annotate-functions to separately control the respective reading and writing conversions.

Conversion starts with one representation and produces another representation. When there is only one conversion to do, there is no conflict about what to start with. However, when there are multiple conversions involved, conflict may arise when two conversions need to start with the same data.

This situation is best understood in the context of converting text properties during write-region. For example, the character at position 42 in a buffer is ‘X’ with a text property foo. If the conversion for foo is done by inserting into the buffer, say, ‘FOO:’, then that changes the character at position 42 from ‘X’ to ‘F’. The next conversion will start with the wrong data straight away.

To avoid conflict, cooperative conversions do not modify the buffer, but instead specify annotations, a list of elements of the form (position . string), sorted in order of increasing position.

If there is more than one conversion, write-region merges their annotations destructively into one sorted list. Later, when the text from the buffer is actually written to the file, it intermixes the specified annotations at the corresponding positions. All this takes place without modifying the buffer.

In contrast, when reading, the annotations intermixed with the text are handled immediately. insert-file-contents sets point to the beginning of some text to be converted, then calls the conversion functions with the length of that text. These functions should always return with point at the beginning of the inserted text. This approach makes sense for reading because annotations removed by the first converter can't be mistakenly processed by a later converter. Each conversion function should scan for the annotations it recognizes, remove the annotation, modify the buffer text (to set a text property, for example), and return the updated length of the text, as it stands after those changes. The value returned by one function becomes the argument to the next function.

We invite users to write Lisp programs to store and retrieve text properties in files, using these hooks, and thus to experiment with various data formats and find good ones. Eventually we hope users will produce good, general extensions we can install in Emacs.

We suggest not trying to handle arbitrary Lisp objects as text property names or values—because a program that general is probably difficult to write, and slow. Instead, choose a set of possible data types that are reasonably flexible, and not too hard to encode.

25 Backups and Auto-Saving

Backup files and auto-save files are two methods by which Emacs tries to protect the user from the consequences of crashes or of the user's own errors. Auto-saving preserves the text from earlier in the current editing session; backup files preserve file contents prior to the current session.

• Backup Files: How backup files are made; how their names are chosen.
• Auto-Saving: How auto-save files are made; how their names are chosen.
• Reverting: revert-buffer, and how to customize what it does.

25.1 Backup Files

A backup file is a copy of the old contents of a file you are editing. Emacs makes a backup file the first time you save a buffer into its visited file. Thus, normally, the backup file contains the contents of the file as it was before the current editing session. The contents of the backup file normally remain unchanged once it exists.

Backups are usually made by renaming the visited file to a new name. Optionally, you can specify that backup files should be made by copying the visited file. This choice makes a difference for files with multiple names; it also can affect whether the edited file remains owned by the original owner or becomes owned by the user editing it.

By default, Emacs makes a single backup file for each file edited. You can alternatively request numbered backups; then each new backup file gets a new name. You can delete old numbered backups when you don't want them any more, or Emacs can delete them automatically.

• Making Backups: How Emacs makes backup files, and when.
• Rename or Copy: Two alternatives: renaming the old file or copying it.
• Numbered Backups: Keeping multiple backups for each source file.
• Backup Names: How backup file names are computed; customization.

25.1.1 Making Backup Files

25.1.2 Backup by Renaming or by Copying?

There are two ways that Emacs can make a backup file:

• Emacs can rename the original file so that it becomes a backup file, and then write the buffer being saved into a new file. After this procedure, any other names (i.e., hard links) of the original file now refer to the backup file. The new file is owned by the user doing the editing, and its group is the default for new files written by the user in that directory.
• Emacs can copy the original file into a backup file, and then overwrite the original file with new contents. After this procedure, any other names (i.e., hard links) of the original file continue to refer to the current (updated) version of the file. The file's owner and group will be unchanged.

The first method, renaming, is the default.

The variable backup-by-copying, if non-nil, says to use the second method, which is to copy the original file and overwrite it with the new buffer contents. The variable file-precious-flag, if non-nil, also has this effect (as a sideline of its main significance). See Saving Buffers.

The following three variables, when non-nil, cause the second method to be used in certain special cases. They have no effect on the treatment of files that don't fall into the special cases.

25.1.3 Making and Deleting Numbered Backup Files

If a file's name is foo, the names of its numbered backup versions are foo.~v~, for various integers v, like this: foo.~1~, foo.~2~, foo.~3~, ..., foo.~259~, and so on.

The use of numbered backups ultimately leads to a large number of backup versions, which must then be deleted. Emacs can do this automatically or it can ask the user whether to delete them.

If there are backups numbered 1, 2, 3, 5, and 7, and both of these variables have the value 2, then the backups numbered 1 and 2 are kept as old versions and those numbered 5 and 7 are kept as new versions; backup version 3 is excess. The function find-backup-file-name (see Backup Names) is responsible for determining which backup versions to delete, but does not delete them itself.

25.1.4 Naming Backup Files

The functions in this section are documented mainly because you can customize the naming conventions for backup files by redefining them. If you change one, you probably need to change the rest.

25.2 Auto-Saving

Emacs periodically saves all files that you are visiting; this is called auto-saving. Auto-saving prevents you from losing more than a limited amount of work if the system crashes. By default, auto-saves happen every 300 keystrokes, or after around 30 seconds of idle time. See Auto Save, for information on auto-save for users. Here we describe the functions used to implement auto-saving and the variables that control them.

25.3 Reverting

If you have made extensive changes to a file and then change your mind about them, you can get rid of them by reading in the previous version of the file with the revert-buffer command. See Reverting a Buffer.

You can customize how revert-buffer does its work by setting the variables described in the rest of this section.

Some major modes customize revert-buffer by making buffer-local bindings for these variables:

26 Buffers

A buffer is a Lisp object containing text to be edited. Buffers are used to hold the contents of files that are being visited; there may also be buffers that are not visiting files. While several buffers may exist at one time, only one buffer is designated the current buffer at any time. Most editing commands act on the contents of the current buffer. Each buffer, including the current buffer, may or may not be displayed in any windows.

• Buffer Basics: What is a buffer?
• Current Buffer: Designating a buffer as current so that primitives will access its contents.
• Buffer Names: Accessing and changing buffer names.
• Buffer File Name: The buffer file name indicates which file is visited.
• Buffer Modification: A buffer is modified if it needs to be saved.
• Modification Time: Determining whether the visited file was changed behind Emacs's back.
• Read Only Buffers: Modifying text is not allowed in a read-only buffer.
• Buffer List: How to look at all the existing buffers.
• Creating Buffers: Functions that create buffers.
• Killing Buffers: Buffers exist until explicitly killed.
• Indirect Buffers: An indirect buffer shares text with some other buffer.
• Swapping Text: Swapping text between two buffers.
• Buffer Gap: The gap in the buffer.

26.1 Buffer Basics

A buffer is a Lisp object containing text to be edited. Buffers are used to hold the contents of files that are being visited; there may also be buffers that are not visiting files. Although several buffers normally exist, only one buffer is designated the current buffer at any time. Most editing commands act on the contents of the current buffer. Each buffer, including the current buffer, may or may not be displayed in any windows.

Buffers in Emacs editing are objects that have distinct names and hold text that can be edited. Buffers appear to Lisp programs as a special data type. You can think of the contents of a buffer as a string that you can extend; insertions and deletions may occur in any part of the buffer. See Text.

A Lisp buffer object contains numerous pieces of information. Some of this information is directly accessible to the programmer through variables, while other information is accessible only through special-purpose functions. For example, the visited file name is directly accessible through a variable, while the value of point is accessible only through a primitive function.

Buffer-specific information that is directly accessible is stored in buffer-local variable bindings, which are variable values that are effective only in a particular buffer. This feature allows each buffer to override the values of certain variables. Most major modes override variables such as fill-column or comment-column in this way. For more information about buffer-local variables and functions related to them, see Buffer-Local Variables.

For functions and variables related to visiting files in buffers, see Visiting Files and Saving Buffers. For functions and variables related to the display of buffers in windows, see Buffers and Windows.

26.2 The Current Buffer

There are, in general, many buffers in an Emacs session. At any time, one of them is designated the current buffer—the buffer in which most editing takes place. Most of the primitives for examining or changing text operate implicitly on the current buffer (see Text).

Normally, the buffer displayed in the selected window is the current buffer, but this is not always so: a Lisp program can temporarily designate any buffer as current in order to operate on its contents, without changing what is displayed on the screen. The most basic function for designating a current buffer is set-buffer.

When an editing command returns to the editor command loop, Emacs automatically calls set-buffer on the buffer shown in the selected window. This is to prevent confusion: it ensures that the buffer that the cursor is in, when Emacs reads a command, is the buffer to which that command applies (see Command Loop). Thus, you should not use set-buffer to switch visibly to a different buffer; for that, use the functions described in Switching Buffers.

When writing a Lisp function, do not rely on this behavior of the command loop to restore the current buffer after an operation. Editing commands can also be called as Lisp functions by other programs, not just from the command loop; it is convenient for the caller if the subroutine does not change which buffer is current (unless, of course, that is the subroutine's purpose).

To operate temporarily on another buffer, put the set-buffer within a save-current-buffer form. Here, as an example, is a simplified version of the command append-to-buffer:

     (defun append-to-buffer (buffer start end)
       "Append the text of the region to BUFFER."
       (interactive "BAppend to buffer: \nr")
       (let ((oldbuf (current-buffer)))
         (save-current-buffer
           (set-buffer (get-buffer-create buffer))
           (insert-buffer-substring oldbuf start end))))

Here, we bind a local variable to record the current buffer, and then save-current-buffer arranges to make it current again later. Next, set-buffer makes the specified buffer current, and insert-buffer-substring copies the string from the original buffer to the specified (and now current) buffer.

Alternatively, we can use the with-current-buffer macro:

     (defun append-to-buffer (buffer start end)
       "Append the text of the region to BUFFER."
       (interactive "BAppend to buffer: \nr")
       (let ((oldbuf (current-buffer)))
         (with-current-buffer (get-buffer-create buffer)
           (insert-buffer-substring oldbuf start end))))

In either case, if the buffer appended to happens to be displayed in some window, the next redisplay will show how its text has changed. If it is not displayed in any window, you will not see the change immediately on the screen. The command causes the buffer to become current temporarily, but does not cause it to be displayed.

If you make local bindings (with let or function arguments) for a variable that may also have buffer-local bindings, make sure that the same buffer is current at the beginning and at the end of the local binding's scope. Otherwise you might bind it in one buffer and unbind it in another!

Do not rely on using set-buffer to change the current buffer back, because that won't do the job if a quit happens while the wrong buffer is current. For instance, in the previous example, it would have been wrong to do this:

       (let ((oldbuf (current-buffer)))
         (set-buffer (get-buffer-create buffer))
         (insert-buffer-substring oldbuf start end)
         (set-buffer oldbuf))

Using save-current-buffer or with-current-buffer, as we did, correctly handles quitting, errors, and throw, as well as ordinary evaluation.

26.3 Buffer Names

Each buffer has a unique name, which is a string. Many of the functions that work on buffers accept either a buffer or a buffer name as an argument. Any argument called buffer-or-name is of this sort, and an error is signaled if it is neither a string nor a buffer. Any argument called buffer must be an actual buffer object, not a name.

Buffers that are ephemeral and generally uninteresting to the user have names starting with a space, so that the list-buffers and buffer-menu commands don't mention them (but if such a buffer visits a file, it is mentioned). A name starting with space also initially disables recording undo information; see Undo.

26.4 Buffer File Name

The buffer file name is the name of the file that is visited in that buffer. When a buffer is not visiting a file, its buffer file name is nil. Most of the time, the buffer name is the same as the nondirectory part of the buffer file name, but the buffer file name and the buffer name are distinct and can be set independently. See Visiting Files.

26.5 Buffer Modification

Emacs keeps a flag called the modified flag for each buffer, to record whether you have changed the text of the buffer. This flag is set to t whenever you alter the contents of the buffer, and cleared to nil when you save it. Thus, the flag shows whether there are unsaved changes. The flag value is normally shown in the mode line (see Mode Line Variables), and controls saving (see Saving Buffers) and auto-saving (see Auto-Saving).

Some Lisp programs set the flag explicitly. For example, the function set-visited-file-name sets the flag to t, because the text does not match the newly-visited file, even if it is unchanged from the file formerly visited.

The functions that modify the contents of buffers are described in Text.

26.6 Buffer Modification Time

Suppose that you visit a file and make changes in its buffer, and meanwhile the file itself is changed on disk. At this point, saving the buffer would overwrite the changes in the file. Occasionally this may be what you want, but usually it would lose valuable information. Emacs therefore checks the file's modification time using the functions described below before saving the file. (See File Attributes, for how to examine a file's modification time.)

26.7 Read-Only Buffers

If a buffer is read-only, then you cannot change its contents, although you may change your view of the contents by scrolling and narrowing.

Read-only buffers are used in two kinds of situations:

• A buffer visiting a write-protected file is normally read-only.

  Here, the purpose is to inform the user that editing the buffer with the aim of saving it in the file may be futile or undesirable. The user who wants to change the buffer text despite this can do so after clearing the read-only flag with C-x C-q.

• Modes such as Dired and Rmail make buffers read-only when altering the contents with the usual editing commands would probably be a mistake.

  The special commands of these modes bind buffer-read-only to nil (with let) or bind inhibit-read-only to t around the places where they themselves change the text.

26.8 The Buffer List

The buffer list is a list of all live buffers. The order of the buffers in this list is based primarily on how recently each buffer has been displayed in a window. Several functions, notably other-buffer, use this ordering. A buffer list displayed for the user also follows this order.

Creating a buffer adds it to the end of the buffer list, and killing a buffer removes it from that list. A buffer moves to the front of this list whenever it is chosen for display in a window (see Switching Buffers) or a window displaying it is selected (see Selecting Windows). A buffer moves to the end of the list when it is buried (see bury-buffer, below). There are no functions available to the Lisp programmer which directly manipulate the buffer list.

In addition to the fundamental buffer list just described, Emacs maintains a local buffer list for each frame, in which the buffers that have been displayed (or had their windows selected) in that frame come first. (This order is recorded in the frame's buffer-list frame parameter; see Buffer Parameters.) Buffers never displayed in that frame come afterward, ordered according to the fundamental buffer list.

The list returned by buffer-list is constructed specifically; it is not an internal Emacs data structure, and modifying it has no effect on the order of buffers. If you want to change the order of buffers in the fundamental buffer list, here is an easy way:

     (defun reorder-buffer-list (new-list)
       (while new-list
         (bury-buffer (car new-list))
         (setq new-list (cdr new-list))))

With this method, you can specify any order for the list, but there is no danger of losing a buffer or adding something that is not a valid live buffer.

To change the order or value of a specific frame's buffer list, set that frame's buffer-list parameter with modify-frame-parameters (see Parameter Access).

26.9 Creating Buffers

This section describes the two primitives for creating buffers. get-buffer-create creates a buffer if it finds no existing buffer with the specified name; generate-new-buffer always creates a new buffer and gives it a unique name.

Other functions you can use to create buffers include with-output-to-temp-buffer (see Temporary Displays) and create-file-buffer (see Visiting Files). Starting a subprocess can also create a buffer (see Processes).

26.10 Killing Buffers

Killing a buffer makes its name unknown to Emacs and makes the memory space it occupied available for other use.

The buffer object for the buffer that has been killed remains in existence as long as anything refers to it, but it is specially marked so that you cannot make it current or display it. Killed buffers retain their identity, however; if you kill two distinct buffers, they remain distinct according to eq although both are dead.

If you kill a buffer that is current or displayed in a window, Emacs automatically selects or displays some other buffer instead. This means that killing a buffer can change the current buffer. Therefore, when you kill a buffer, you should also take the precautions associated with changing the current buffer (unless you happen to know that the buffer being killed isn't current). See Current Buffer.

If you kill a buffer that is the base buffer of one or more indirect buffers (see Indirect Buffers), the indirect buffers are automatically killed as well.

The buffer-name of a buffer is nil if, and only if, the buffer is killed. A buffer that has not been killed is called a live buffer. To test whether a buffer is live or killed, use the function buffer-live-p (see below).

26.11 Indirect Buffers

An indirect buffer shares the text of some other buffer, which is called the base buffer of the indirect buffer. In some ways it is the analogue, for buffers, of a symbolic link among files. The base buffer may not itself be an indirect buffer.

The text of the indirect buffer is always identical to the text of its base buffer; changes made by editing either one are visible immediately in the other. This includes the text properties as well as the characters themselves.

In all other respects, the indirect buffer and its base buffer are completely separate. They have different names, independent values of point, independent narrowing, independent markers and overlays (though inserting or deleting text in either buffer relocates the markers and overlays for both), independent major modes, and independent buffer-local variable bindings.

An indirect buffer cannot visit a file, but its base buffer can. If you try to save the indirect buffer, that actually saves the base buffer.

Killing an indirect buffer has no effect on its base buffer. Killing the base buffer effectively kills the indirect buffer in that it cannot ever again be the current buffer.

26.12 Swapping Text Between Two Buffers

Specialized modes sometimes need to let the user access from the same buffer several vastly different types of text. For example, you may need to display a summary of the buffer text, in addition to letting the user access the text itself.

This could be implemented with multiple buffers (kept in sync when the user edits the text), or with narrowing (see Narrowing). But these alternatives might sometimes become tedious or prohibitively expensive, especially if each type of text requires expensive buffer-global operations in order to provide correct display and editing commands.

Emacs provides another facility for such modes: you can quickly swap buffer text between two buffers with buffer-swap-text. This function is very fast because it doesn't move any text, it only changes the internal data structures of the buffer object to point to a different chunk of text. Using it, you can pretend that a group of two or more buffers are actually a single virtual buffer that holds the contents of all the individual buffers together.

If you use buffer-swap-text on a file-visiting buffer, you should set up a hook to save the buffer's original text rather than what it was swapped with. write-region-annotate-functions works for this purpose. You should probably set buffer-saved-size to −2 in the buffer, so that changes in the text it is swapped with will not interfere with auto-saving.

26.13 The Buffer Gap

Emacs buffers are implemented using an invisible gap to make insertion and deletion faster. Insertion works by filling in part of the gap, and deletion adds to the gap. Of course, this means that the gap must first be moved to the locus of the insertion or deletion. Emacs moves the gap only when you try to insert or delete. This is why your first editing command in one part of a large buffer, after previously editing in another far-away part, sometimes involves a noticeable delay.

This mechanism works invisibly, and Lisp code should never be affected by the gap's current location, but these functions are available for getting information about the gap status.

27 Windows

This chapter describes the functions and variables related to Emacs windows. See Frames, for how windows are assigned an area of screen available for Emacs to use. See Display, for information on how text is displayed in windows.

• Basic Windows: Basic information on using windows.
• Windows and Frames: Relating windows to the frame they appear on.
• Window Sizes: Accessing a window's size.
• Resizing Windows: Changing the sizes of windows.
• Preserving Window Sizes: Preserving the size of windows.
• Splitting Windows: Creating a new window.
• Deleting Windows: Removing a window from its frame.
• Recombining Windows: Preserving the frame layout when splitting and deleting windows.
• Selecting Windows: The selected window is the one that you edit in.
• Cyclic Window Ordering: Moving around the existing windows.
• Buffers and Windows: Each window displays the contents of a buffer.
• Switching Buffers: Higher-level functions for switching to a buffer.
• Choosing Window: How to choose a window for displaying a buffer.
• Display Action Functions: Subroutines for display-buffer.
• Choosing Window Options: Extra options affecting how buffers are displayed.
• Window History: Each window remembers the buffers displayed in it.
• Dedicated Windows: How to avoid displaying another buffer in a specific window.
• Quitting Windows: How to restore the state prior to displaying a buffer.
• Window Point: Each window has its own location of point.
• Window Start and End: Buffer positions indicating which text is on-screen in a window.
• Textual Scrolling: Moving text up and down through the window.
• Vertical Scrolling: Moving the contents up and down on the window.
• Horizontal Scrolling: Moving the contents sideways on the window.
• Coordinates and Windows: Converting coordinates to windows.
• Window Configurations: Saving and restoring the state of the screen.
• Window Parameters: Associating additional information with windows.
• Window Hooks: Hooks for scrolling, window size changes, redisplay going past a certain point, or window configuration changes.

27.1 Basic Concepts of Emacs Windows

A window is an area of the screen that is used to display a buffer (see Buffers). In Emacs Lisp, windows are represented by a special Lisp object type.

Windows are grouped into frames (see Frames). Each frame contains at least one window; the user can subdivide it into multiple, non-overlapping windows to view several buffers at once. Lisp programs can use multiple windows for a variety of purposes. In Rmail, for example, you can view a summary of message titles in one window, and the contents of the selected message in another window.

Emacs uses the word “window” with a different meaning than in graphical desktop environments and window systems, such as the X Window System. When Emacs is run on X, each of its graphical X windows is an Emacs frame (containing one or more Emacs windows). When Emacs is run on a text terminal, the frame fills the entire terminal screen.

Unlike X windows, Emacs windows are tiled; they never overlap within the area of the frame. When a window is created, resized, or deleted, the change in window space is taken from or given to the adjacent windows, so that the total area of the frame is unchanged.

A live window is one that is actually displaying a buffer in a frame.

The windows in each frame are organized into a window tree. See Windows and Frames. The leaf nodes of each window tree are live windows—the ones actually displaying buffers. The internal nodes of the window tree are internal windows, which are not live.

A valid window is one that is either live or internal. A valid window can be deleted, i.e., removed from its frame (see Deleting Windows); then it is no longer valid, but the Lisp object representing it might be still referenced from other Lisp objects. A deleted window may be made valid again by restoring a saved window configuration (see Window Configurations).

You can distinguish valid windows from deleted windows with window-valid-p.

In each frame, at any time, exactly one Emacs window is designated as selected within the frame. For the selected frame, that window is called the selected window—the one in which most editing takes place, and in which the cursor for selected windows appears (see Cursor Parameters). The selected window's buffer is usually also the current buffer, except when set-buffer has been used (see Current Buffer). As for non-selected frames, the window selected within the frame becomes the selected window if the frame is ever selected. See Selecting Windows.

Sometimes several windows collectively and cooperatively display a buffer, for example, under the management of Follow Mode (see Follow Mode), where the windows together display a bigger portion of the buffer than one window could alone. It is often useful to consider such a window group as a single entity. Several functions such as window-group-start (see Window Start and End) allow you to do this by supplying, as an argument, one of the windows as a stand in for the whole group.

27.2 Windows and Frames

Each window belongs to exactly one frame (see Frames).

Windows in the same frame are organized into a window tree, whose leaf nodes are the live windows. The internal nodes of a window tree are not live; they exist for the purpose of organizing the relationships between live windows. The root node of a window tree is called the root window. It can be either a live window (if the frame has just one window), or an internal window.

A minibuffer window (see Minibuffer Windows) is not part of its frame's window tree unless the frame is a minibuffer-only frame. Nonetheless, most of the functions in this section accept the minibuffer window as an argument. Also, the function window-tree described at the end of this section lists the minibuffer window alongside the actual window tree.

When a window is split, there are two live windows where previously there was one. One of these is represented by the same Lisp window object as the original window, and the other is represented by a newly-created Lisp window object. Both of these live windows become leaf nodes of the window tree, as child windows of a single internal window. If necessary, Emacs automatically creates this internal window, which is also called the parent window, and assigns it to the appropriate position in the window tree. A set of windows that share the same parent are called siblings.

Each internal window always has at least two child windows. If this number falls to one as a result of window deletion, Emacs automatically deletes the internal window, and its sole remaining child window takes its place in the window tree.

Each child window can be either a live window, or an internal window (which in turn would have its own child windows). Therefore, each internal window can be thought of as occupying a certain rectangular screen area—the union of the areas occupied by the live windows that are ultimately descended from it.

For each internal window, the screen areas of the immediate children are arranged either vertically or horizontally (never both). If the child windows are arranged one above the other, they are said to form a vertical combination; if they are arranged side by side, they are said to form a horizontal combination. Consider the following example:

          ______________________________________
         | ______  ____________________________ |
         ||      || __________________________ ||
         ||      |||                          |||
         ||      |||                          |||
         ||      |||                          |||
         ||      |||____________W4____________|||
         ||      || __________________________ ||
         ||      |||                          |||
         ||      |||                          |||
         ||      |||____________W5____________|||
         ||__W2__||_____________W3_____________ |
         |__________________W1__________________|

The root window of this frame is an internal window, W1. Its child windows form a horizontal combination, consisting of the live window W2 and the internal window W3. The child windows of W3 form a vertical combination, consisting of the live windows W4 and W5. Hence, the live windows in this window tree are W2, W4, and W5.

The following functions can be used to retrieve a child window of an internal window, and the siblings of a child window.

The functions window-next-sibling and window-prev-sibling should not be confused with the functions next-window and previous-window, which return the next and previous window, respectively, in the cyclic ordering of windows (see Cyclic Window Ordering).

You can use the following functions to find the first live window on a frame and the window nearest to a given window.

The following function allows the entire window tree of a frame to be retrieved:

27.3 Window Sizes

The following schematic shows the structure of a live window:

             ____________________________________________
            |______________ Header Line ______________|RD| ^
          ^ |LS|LM|LF|                       |RF|RM|RS|  | |
          | |  |  |  |                       |  |  |  |  | |
     Window |  |  |  |       Text Area       |  |  |  |  | Window
     Body | |  |  |  |     (Window Body)     |  |  |  |  | Total
     Height |  |  |  |                       |  |  |  |  | Height
          | |  |  |  |<- Window Body Width ->|  |  |  |  | |
          v |__|__|__|_______________________|__|__|__|  | |
            |_________ Horizontal Scroll Bar _________|  | |
            |_______________ Mode Line _______________|__| |
            |_____________ Bottom Divider _______________| v
             <---------- Window Total Width ------------>

At the center of the window is the text area, or body, where the buffer text is displayed. The text area can be surrounded by a series of optional areas. On the left and right, from innermost to outermost, these are the left and right fringes, denoted by LF and RF (see Fringes); the left and right margins, denoted by LM and RM in the schematic (see Display Margins); the left or right vertical scroll bar, only one of which is present at any time, denoted by LS and RS (see Scroll Bars); and the right divider, denoted by RD (see Window Dividers). At the top of the window is the header line (see Header Lines). At the bottom of the window are the horizontal scroll bar (see Scroll Bars); the mode line (see Mode Line Format); and the bottom divider (see Window Dividers).

Emacs provides miscellaneous functions for finding the height and width of a window. The return value of many of these functions can be specified either in units of pixels or in units of lines and columns. On a graphical display, the latter actually correspond to the height and width of a default character specified by the frame's default font as returned by frame-char-height and frame-char-width (see Frame Font). Thus, if a window is displaying text with a different font or size, the reported line height and column width for that window may differ from the actual number of text lines or columns displayed within it.

The total height of a window is the number of lines comprising the window's body, the header line, the horizontal scroll bar, the mode line and the bottom divider (if any).

The total width of a window is the number of lines comprising the window's body, its margins, fringes, scroll bars and a right divider (if any).

The following two functions can be used to return the total size of a window in units of pixels.

The following functions can be used to determine whether a given window has any adjacent windows.

The body height of a window is the height of its text area, which does not include a mode or header line, a horizontal scroll bar, or a bottom divider.

The body width of a window is the width of its text area, which does not include the scroll bar, fringes, margins or a right divider. Note that when one or both fringes are removed (by setting their width to zero), the display engine reserves two character cells, one on each side of the window, for displaying the continuation and truncation glyphs, which leaves 2 columns less for text display. (The function window-max-chars-per-line, described below, takes this peculiarity into account.)

For compatibility with previous versions of Emacs, window-height is an alias for window-total-height, and window-width is an alias for window-body-width. These aliases are considered obsolete and will be removed in the future.

The pixel heights of a window's mode and header line can be retrieved with the functions given below. Their return value is usually accurate unless the window has not been displayed before: In that case, the return value is based on an estimate of the font used for the window's frame.

Functions for retrieving the height and/or width of window dividers (see Window Dividers), fringes (see Fringes), scroll bars (see Scroll Bars), and display margins (see Display Margins) are described in the corresponding sections.

If your Lisp program needs to make layout decisions, you will find the following function useful:

Commands that change the size of windows (see Resizing Windows), or split them (see Splitting Windows), obey the variables window-min-height and window-min-width, which specify the smallest allowable window height and width. They also obey the variable window-size-fixed, with which a window can be fixed in size (see Preserving Window Sizes).

The following function tells how small a specific window can get taking into account the sizes of its areas and the values of window-min-height, window-min-width and window-size-fixed (see Preserving Window Sizes).

27.4 Resizing Windows

This section describes functions for resizing a window without changing the size of its frame. Because live windows do not overlap, these functions are meaningful only on frames that contain two or more windows: resizing a window also changes the size of a neighboring window. If there is just one window on a frame, its size cannot be changed except by resizing the frame (see Size and Position).

Except where noted, these functions also accept internal windows as arguments. Resizing an internal window causes its child windows to be resized to fit the same space.

The following commands resize windows in more specific ways. When called interactively, they act on the selected window.

If you have a frame that displays only one window, you can fit that frame to its buffer using the command fit-frame-to-buffer.

The behavior of fit-frame-to-buffer can be controlled with the help of the two options listed next.

27.5 Preserving Window Sizes

A window can get resized explicitly by using one of the functions from the preceding section or implicitly, for example, when resizing an adjacent window, when splitting or deleting a window (see Splitting Windows, see Deleting Windows) or when resizing the window's frame (see Size and Position).

It is possible to avoid implicit resizing of a specific window when there are one or more other resizable windows on the same frame. For this purpose, Emacs must be advised to preserve the size of that window. There are two basic ways to do that.

Often window-size-fixed is overly aggressive because it inhibits any attempt to explicitly resize or split an affected window as well. This may even happen after the window has been resized implicitly, for example, when deleting an adjacent window or resizing the window's frame. The following function tries hard to never disallow resizing such a window explicitly:

window-preserve-size is currently invoked by the following functions:

fit-window-to-buffer

If the optional argument preserve-size of that function (see Resizing Windows) is non-nil, the size established by that function is preserved.

display-buffer

If the alist argument of that function (see Choosing Window) contains a preserve-size entry, the size of the window produced by that function is preserved.

window-preserve-size installs a window parameter (see Window Parameters) called preserved-size which is consulted by the window resizing functions. This parameter will not prevent resizing the window when the window shows another buffer than the one when window-preserve-size was invoked or if its size has changed since then.

The following function can be used to check whether the height of a particular window is preserved:

27.6 Splitting Windows

This section describes functions for creating a new window by splitting an existing one.

As an example, here is a sequence of split-window calls that yields the window configuration discussed in Windows and Frames. This example demonstrates splitting a live window as well as splitting an internal window. We begin with a frame containing a single window (a live root window), which we denote by W4. Calling (split-window W4) yields this window configuration:

          ______________________________________
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||_________________W4_________________||
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||_________________W5_________________||
         |__________________W3__________________|

The split-window call has created a new live window, denoted by W5. It has also created a new internal window, denoted by W3, which becomes the root window and the parent of both W4 and W5.

Next, we call (split-window W3 nil 'left), passing the internal window W3 as the argument. The result:

          ______________________________________
         | ______  ____________________________ |
         ||      || __________________________ ||
         ||      |||                          |||
         ||      |||                          |||
         ||      |||                          |||
         ||      |||____________W4____________|||
         ||      || __________________________ ||
         ||      |||                          |||
         ||      |||                          |||
         ||      |||____________W5____________|||
         ||__W2__||_____________W3_____________ |
         |__________________W1__________________|

A new live window W2 is created, to the left of the internal window W3. A new internal window W1 is created, becoming the new root window.

For interactive use, Emacs provides two commands which always split the selected window. These call split-window internally.

27.7 Deleting Windows

Deleting a window removes it from the frame's window tree. If the window is a live window, it disappears from the screen. If the window is an internal window, its child windows are deleted too.

Even after a window is deleted, it continues to exist as a Lisp object, until there are no more references to it. Window deletion can be reversed, by restoring a saved window configuration (see Window Configurations).

27.8 Recombining Windows

When deleting the last sibling of a window W, its parent window is deleted too, with W replacing it in the window tree. This means that W must be recombined with its parent's siblings to form a new window combination (see Windows and Frames). In some occasions, deleting a live window may even entail the deletion of two internal windows.

          ______________________________________
         | ______  ____________________________ |
         ||      || __________________________ ||
         ||      ||| ___________  ___________ |||
         ||      ||||           ||           ||||
         ||      ||||____W6_____||_____W7____||||
         ||      |||____________W4____________|||
         ||      || __________________________ ||
         ||      |||                          |||
         ||      |||                          |||
         ||      |||____________W5____________|||
         ||__W2__||_____________W3_____________ |
         |__________________W1__________________|

Deleting W5 in this configuration normally causes the deletion of W3 and W4. The remaining live windows W2, W6 and W7 are recombined to form a new horizontal combination with parent W1.

Sometimes, however, it makes sense to not delete a parent window like W4. In particular, a parent window should not be removed when it was used to preserve a combination embedded in a combination of the same type. Such embeddings make sense to assure that when you split a window and subsequently delete the new window, Emacs reestablishes the layout of the associated frame as it existed before the splitting.

Consider a scenario starting with two live windows W2 and W3 and their parent W1.

          ______________________________________
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||_________________W2_________________||
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||_________________W3_________________||
         |__________________W1__________________|

Split W2 to make a new window W4 as follows.

          ______________________________________
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||_________________W2_________________||
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||_________________W4_________________||
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||_________________W3_________________||
         |__________________W1__________________|

Now, when enlarging a window vertically, Emacs tries to obtain the corresponding space from its lower sibling, provided such a window exists. In our scenario, enlarging W4 will steal space from W3.

          ______________________________________
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||_________________W2_________________||
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||_________________W4_________________||
         | ____________________________________ |
         ||_________________W3_________________||
         |__________________W1__________________|

Deleting W4 will now give its entire space to W2, including the space earlier stolen from W3.

          ______________________________________
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||_________________W2_________________||
         | ____________________________________ |
         ||_________________W3_________________||
         |__________________W1__________________|

This can be counterintuitive, in particular if W4 were used for displaying a buffer only temporarily (see Temporary Displays), and you want to continue working with the initial layout.

The behavior can be fixed by making a new parent window when splitting W2. The variable described next allows that to be done.

If window-combination-limit is t, splitting W2 in the initial configuration of our scenario would have produced this:

          ______________________________________
         | ____________________________________ |
         || __________________________________ ||
         |||                                  |||
         |||________________W2________________|||
         || __________________________________ ||
         |||                                  |||
         |||________________W4________________|||
         ||_________________W5_________________||
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||_________________W3_________________||
         |__________________W1__________________|

A new internal window W5 has been created; its children are W2 and the new live window W4. Now, W2 is the only sibling of W4, so enlarging W4 will try to shrink W2, leaving W3 unaffected. Observe that W5 represents a vertical combination of two windows embedded in the vertical combination W1.

Alternatively, the problems sketched above can be avoided by always resizing all windows in the same combination whenever one of its windows is split or deleted. This also permits splitting windows that would be otherwise too small for such an operation.

To illustrate the effect of window-combination-resize, consider the following frame layout.

          ______________________________________
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||_________________W2_________________||
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||_________________W3_________________||
         |__________________W1__________________|

If window-combination-resize is nil, splitting window W3 leaves the size of W2 unchanged:

          ______________________________________
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||_________________W2_________________||
         | ____________________________________ |
         ||                                    ||
         ||_________________W3_________________||
         | ____________________________________ |
         ||                                    ||
         ||_________________W4_________________||
         |__________________W1__________________|

If window-combination-resize is t, splitting W3 instead leaves all three live windows with approximately the same height:

          ______________________________________
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||_________________W2_________________||
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||_________________W3_________________||
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||_________________W4_________________||
         |__________________W1__________________|

Deleting any of the live windows W2, W3 or W4 will distribute its space proportionally among the two remaining live windows.

27.9 Selecting Windows

The sequence of calls to select-window with a non-nil norecord argument determines an ordering of windows by their selection time. The function get-lru-window can be used to retrieve the least recently selected live window (see Cyclic Window Ordering).

27.10 Cyclic Ordering of Windows

When you use the command C-x o (other-window) to select some other window, it moves through live windows in a specific order. For any given configuration of windows, this order never varies. It is called the cyclic ordering of windows.

The ordering is determined by a depth-first traversal of each frame's window tree, retrieving the live windows which are the leaf nodes of the tree (see Windows and Frames). If the minibuffer is active, the minibuffer window is included too. The ordering is cyclic, so the last window in the sequence is followed by the first one.

The following functions return a window which satisfies some criterion, without selecting it:

27.11 Buffers and Windows

This section describes low-level functions for examining and setting the contents of windows. See Switching Buffers, for higher-level functions for displaying a specific buffer in a window.

27.12 Switching to a Buffer in a Window

This section describes high-level functions for switching to a specified buffer in some window. In general, “switching to a buffer” means to (1) show the buffer in some window, (2) make that window the selected window (and its frame the selected frame), and (3) make the buffer the current buffer.

Do not use these functions to make a buffer temporarily current just so a Lisp program can access or modify it. They have side-effects, such as changing window histories (see Window History), which will surprise the user if used that way. If you want to make a buffer current to modify it in Lisp, use with-current-buffer, save-current-buffer, or set-buffer. See Current Buffer.

By default, switch-to-buffer shows the buffer at its position of point. This behavior can be tuned using the following option.

The next two commands are similar to switch-to-buffer, except for the described features.

The above commands use the function pop-to-buffer, which flexibly displays a buffer in some window and selects that window for editing. In turn, pop-to-buffer uses display-buffer for displaying the buffer. Hence, all the variables affecting display-buffer will affect it as well. See Choosing Window, for the documentation of display-buffer.

27.13 Choosing a Window for Display

The command display-buffer flexibly chooses a window for display, and displays a specified buffer in that window. It can be called interactively, via the key binding C-x 4 C-o. It is also used as a subroutine by many functions and commands, including switch-to-buffer and pop-to-buffer (see Switching Buffers).

This command performs several complex steps to find a window to display in. These steps are described by means of display actions, which have the form (function . alist). Here, function is either a function or a list of functions, which we refer to as action functions; alist is an association list, which we refer to as an action alist.

An action function accepts two arguments: the buffer to display and an action alist. It attempts to display the buffer in some window, picking or creating a window according to its own criteria. If successful, it returns the window; otherwise, it returns nil. See Display Action Functions, for a list of predefined action functions.

display-buffer works by combining display actions from several sources, and calling the action functions in turn, until one of them manages to display the buffer and returns a non-nil value.

27.14 Action Functions for display-buffer

The following basic action functions are defined in Emacs. Each of these functions takes two arguments: buffer, the buffer to display, and alist, an action alist. Each action function returns the window if it succeeds, and nil if it fails.

To illustrate the use of action functions, consider the following example.

     (display-buffer
      (get-buffer-create "*foo*")
      '((display-buffer-reuse-window
         display-buffer-pop-up-window
         display-buffer-pop-up-frame)
        (reusable-frames . 0)
        (window-height . 10) (window-width . 40)))

Evaluating the form above will cause display-buffer to proceed as follows: If a buffer called *foo* already appears on a visible or iconified frame, it will reuse its window. Otherwise, it will try to pop up a new window or, if that is impossible, a new frame and show the buffer there. If all these steps fail, it will proceed using whatever display-buffer-base-action and display-buffer-fallback-action prescribe.

Furthermore, display-buffer will try to adjust a reused window (provided *foo* was put by display-buffer there before) or a popped-up window as follows: If the window is part of a vertical combination, it will set its height to ten lines. Note that if, instead of the number 10, we specified the function fit-window-to-buffer, display-buffer would come up with a one-line window to fit the empty buffer. If the window is part of a horizontal combination, it sets its width to 40 columns. Whether a new window is vertically or horizontally combined depends on the shape of the window split and the values of split-window-preferred-function, split-height-threshold and split-width-threshold (see Choosing Window Options).

Now suppose we combine this call with a preexisting setup for display-buffer-alist as follows.

     (let ((display-buffer-alist
            (cons
             '("\\*foo\\*"
               (display-buffer-reuse-window display-buffer-below-selected)
               (reusable-frames)
               (window-height . 5))
             display-buffer-alist)))
       (display-buffer
        (get-buffer-create "*foo*")
        '((display-buffer-reuse-window
           display-buffer-pop-up-window
           display-buffer-pop-up-frame)
          (reusable-frames . 0)
          (window-height . 10) (window-width . 40))))

This form will have display-buffer first try reusing a window that shows *foo* on the selected frame. If there's no such window, it will try to split the selected window or, if that is impossible, use the window below the selected window.

If there's no window below the selected one, or the window below the selected one is dedicated to its buffer, display-buffer will proceed as described in the previous example. Note, however, that when it tries to adjust the height of any reused or popped-up window, it will in any case try to set its number of lines to 5 since that value overrides the corresponding specification in the action argument of display-buffer.

27.15 Additional Options for Displaying Buffers

The behavior of the standard display actions of display-buffer (see Choosing Window) can be modified by a variety of user options.

27.16 Window History

Each window remembers in a list the buffers it has previously displayed, and the order in which these buffers were removed from it. This history is used, for example, by replace-buffer-in-windows (see Buffers and Windows), and when quitting windows (see Quitting Windows). The list is automatically maintained by Emacs, but you can use the following functions to explicitly inspect or alter it:

In addition, each buffer maintains a list of next buffers, which is a list of buffers re-shown by switch-to-prev-buffer (see below). This list is mainly used by switch-to-prev-buffer and switch-to-next-buffer for choosing buffers to switch to.

The following commands can be used to cycle through the global buffer list, much like bury-buffer and unbury-buffer. However, they cycle according to the specified window's history list, rather than the global buffer list. In addition, they restore window-specific window start and point positions, and may show a buffer even if it is already shown in another window. The switch-to-prev-buffer command, in particular, is used by replace-buffer-in-windows, bury-buffer and quit-window to find a replacement buffer for a window.

By default switch-to-prev-buffer and switch-to-next-buffer can switch to a buffer that is already shown in another window on the same frame. The following option can be used to override this behavior.

27.17 Dedicated Windows

Functions for displaying a buffer can be told to not use specific windows by marking these windows as dedicated to their buffers. display-buffer (see Choosing Window) never uses a dedicated window for displaying another buffer in it. get-lru-window and get-largest-window (see Cyclic Window Ordering) do not consider dedicated windows as candidates when their dedicated argument is non-nil. The behavior of set-window-buffer (see Buffers and Windows) with respect to dedicated windows is slightly different, see below.

Functions supposed to remove a buffer from a window or a window from a frame can behave specially when a window they operate on is dedicated. We will distinguish three basic cases, namely where (1) the window is not the only window on its frame, (2) the window is the only window on its frame but there are other frames on the same terminal left, and (3) the window is the only window on the only frame on the same terminal.

In particular, delete-windows-on (see Deleting Windows) handles case (2) by deleting the associated frame and case (3) by showing another buffer in that frame's only window. The function replace-buffer-in-windows (see Buffers and Windows) which is called when a buffer gets killed, deletes the window in case (1) and behaves like delete-windows-on otherwise.

When bury-buffer (see Buffer List) operates on the selected window (which shows the buffer that shall be buried), it handles case (2) by calling frame-auto-hide-function (see Quitting Windows) to deal with the selected frame. The other two cases are handled as with replace-buffer-in-windows.

27.18 Quitting Windows

When you want to get rid of a window used for displaying a buffer, you can call delete-window or delete-windows-on (see Deleting Windows) to remove that window from its frame. If the buffer is shown on a separate frame, you might want to call delete-frame (see Deleting Frames) instead. If, on the other hand, a window has been reused for displaying the buffer, you might prefer showing the buffer previously shown in that window, by calling the function switch-to-prev-buffer (see Window History). Finally, you might want to either bury (see Buffer List) or kill (see Killing Buffers) the window's buffer.

The following command uses information on how the window for displaying the buffer was obtained in the first place, thus attempting to automate the above decisions for you.

The following option specifies how to deal with a frame containing just one window that should be either quit, or whose buffer should be buried.

27.19 Windows and Point

Each window has its own value of point (see Point), independent of the value of point in other windows displaying the same buffer. This makes it useful to have multiple windows showing one buffer.

• The window point is established when a window is first created; it is initialized from the buffer's point, or from the window point of another window opened on the buffer if such a window exists.
• Selecting a window sets the value of point in its buffer from the window's value of point. Conversely, deselecting a window sets the window's value of point from that of the buffer. Thus, when you switch between windows that display a given buffer, the point value for the selected window is in effect in the buffer, while the point values for the other windows are stored in those windows.
• As long as the selected window displays the current buffer, the window's point and the buffer's point always move together; they remain equal.

Emacs displays the cursor, by default as a rectangular block, in each window at the position of that window's point. When the user switches to another buffer in a window, Emacs moves that window's cursor to where point is in that buffer. If the exact position of point is hidden behind some display element, such as a display string or an image, Emacs displays the cursor immediately before or after that display element.

27.20 The Window Start and End Positions

Each window maintains a marker used to keep track of a buffer position that specifies where in the buffer display should start. This position is called the display-start position of the window (or just the start). The character after this position is the one that appears at the upper left corner of the window. It is usually, but not inevitably, at the beginning of a text line.

After switching windows or buffers, and in some other cases, if the window start is in the middle of a line, Emacs adjusts the window start to the start of a line. This prevents certain operations from leaving the window start at a meaningless point within a line. This feature may interfere with testing some Lisp code by executing it using the commands of Lisp mode, because they trigger this readjustment. To test such code, put it into a command and bind the command to a key.

27.21 Textual Scrolling

Textual scrolling means moving the text up or down through a window. It works by changing the window's display-start location. It may also change the value of window-point to keep point on the screen (see Window Point).

The basic textual scrolling functions are scroll-up (which scrolls forward) and scroll-down (which scrolls backward). In these function names, “up” and “down” refer to the direction of motion of the buffer text relative to the window. Imagine that the text is written on a long roll of paper and that the scrolling commands move the paper up and down. Thus, if you are looking at the middle of a buffer and repeatedly call scroll-down, you will eventually see the beginning of the buffer.

Unfortunately, this sometimes causes confusion, because some people tend to think in terms of the opposite convention: they imagine the window moving over text that remains in place, so that “down” commands take you to the end of the buffer. This convention is consistent with fact that such a command is bound to a key named <PageDown> on modern keyboards.

Textual scrolling functions (aside from scroll-other-window) have unpredictable results if the current buffer is not the one displayed in the selected window. See Current Buffer.

If the window contains a row taller than the height of the window (for example in the presence of a large image), the scroll functions will adjust the window's vertical scroll position to scroll the partially visible row. Lisp callers can disable this feature by binding the variable auto-window-vscroll to nil (see Vertical Scrolling).

27.22 Vertical Fractional Scrolling

Vertical fractional scrolling means shifting text in a window up or down by a specified multiple or fraction of a line. Each window has a vertical scroll position, which is a number, never less than zero. It specifies how far to raise the contents of the window. Raising the window contents generally makes all or part of some lines disappear off the top, and all or part of some other lines appear at the bottom. The usual value is zero.

The vertical scroll position is measured in units of the normal line height, which is the height of the default font. Thus, if the value is .5, that means the window contents are scrolled up half the normal line height. If it is 3.3, that means the window contents are scrolled up somewhat over three times the normal line height.

What fraction of a line the vertical scrolling covers, or how many lines, depends on what the lines contain. A value of .5 could scroll a line whose height is very short off the screen, while a value of 3.3 could scroll just part of the way through a tall line or an image.

27.23 Horizontal Scrolling

Horizontal scrolling means shifting the image in the window left or right by a specified multiple of the normal character width. Each window has a horizontal scroll position, which is a number, never less than zero. It specifies how far to shift the contents left. Shifting the window contents left generally makes all or part of some characters disappear off the left, and all or part of some other characters appear at the right. The usual value is zero.

The horizontal scroll position is measured in units of the normal character width, which is the width of space in the default font. Thus, if the value is 5, that means the window contents are scrolled left by 5 times the normal character width. How many characters actually disappear off to the left depends on their width, and could vary from line to line.

Because we read from side to side in the inner loop, and from top to bottom in the outer loop, the effect of horizontal scrolling is not like that of textual or vertical scrolling. Textual scrolling involves selection of a portion of text to display, and vertical scrolling moves the window contents contiguously; but horizontal scrolling causes part of each line to go off screen.

Usually, no horizontal scrolling is in effect; then the leftmost column is at the left edge of the window. In this state, scrolling to the right is meaningless, since there is no data to the left of the edge to be revealed by it; so this is not allowed. Scrolling to the left is allowed; it scrolls the first columns of text off the edge of the window and can reveal additional columns on the right that were truncated before. Once a window has a nonzero amount of leftward horizontal scrolling, you can scroll it back to the right, but only so far as to reduce the net horizontal scroll to zero. There is no limit to how far left you can scroll, but eventually all the text will disappear off the left edge.

If auto-hscroll-mode is set, redisplay automatically alters the horizontal scrolling of a window as necessary to ensure that point is always visible. However, you can still set the horizontal scrolling value explicitly. The value you specify serves as a lower bound for automatic scrolling, i.e., automatic scrolling will not scroll a window to a column less than the specified one.

Here is how you can determine whether a given position position is off the screen due to horizontal scrolling:

     (defun hscroll-on-screen (window position)
       (save-excursion
         (goto-char position)
         (and
          (>= (- (current-column) (window-hscroll window)) 0)
          (< (- (current-column) (window-hscroll window))
             (window-width window)))))

27.24 Coordinates and Windows

This section describes functions that report the position of a window. Most of these functions report positions relative to an origin at the native position of the window's frame (see Frame Geometry). Some functions report positions relative to the origin of the display of the window's frame. In any case, the origin has the coordinates (0, 0) and X and Y coordinates increase rightward and downward respectively.

For the following functions, X and Y coordinates are reported in integer character units, i.e., numbers of lines and columns respectively. On a graphical display, each “line” and “column” corresponds to the height and width of the default character specified by the frame's default font (see Frame Font).

The following functions can be used to relate a set of frame-relative coordinates to a window:

The following functions return window positions in pixels, rather than character units. Though mostly useful on graphical displays, they can also be called on text terminals, where the screen area of each text character is taken to be one pixel.

The following functions return window positions in pixels, relative to the origin of the display screen rather than that of the frame:

The following function returns the screen coordinates of a buffer position visible in a window:

27.25 Window Configurations

A window configuration records the entire layout of one frame—all windows, their sizes, which buffers they contain, how those buffers are scrolled, and their value of point; also their fringes, margins, and scroll bar settings. It also includes the value of minibuffer-scroll-window. As a special exception, the window configuration does not record the value of point in the selected window for the current buffer.

You can bring back an entire frame layout by restoring a previously saved window configuration. If you want to record the layout of all frames instead of just one, use a frame configuration instead of a window configuration. See Frame Configurations.

Other primitives to look inside of window configurations would make sense, but are not implemented because we did not need them. See the file winner.el for some more operations on windows configurations.

The objects returned by current-window-configuration die together with the Emacs process. In order to store a window configuration on disk and read it back in another Emacs session, you can use the functions described next. These functions are also useful to clone the state of a frame into an arbitrary live window (set-window-configuration effectively clones the windows of a frame into the root window of that very frame only).

The value returned by window-state-get can be used in the same session to make a clone of a window in another window. It can be also written to disk and read back in another session. In either case, use the following function to restore the state of the window.

27.26 Window Parameters

This section describes how window parameters can be used to associate additional information with windows.

By default, the functions that save and restore window configurations or the states of windows (see Window Configurations) do not care about window parameters. This means that when you change the value of a parameter within the body of a save-window-excursion, the previous value is not restored when that macro exits. It also means that when you restore via window-state-put a window state saved earlier by window-state-get, all cloned windows have their parameters reset to nil. The following variable allows you to override the standard behavior:

Some functions (notably delete-window, delete-other-windows and split-window), may behave specially when their window argument has a parameter set. You can override such special behavior by binding the following variable to a non-nil value:

The following parameters are currently used by the window management code:

delete-window

This parameter affects the execution of delete-window (see Deleting Windows).

delete-other-windows

This parameter affects the execution of delete-other-windows (see Deleting Windows).

split-window

This parameter affects the execution of split-window (see Splitting Windows).

other-window

This parameter affects the execution of other-window (see Cyclic Window Ordering).

no-other-window

This parameter marks the window as not selectable by other-window (see Cyclic Window Ordering).

clone-of

This parameter specifies the window that this one has been cloned from. It is installed by window-state-get (see Window Configurations).

preserved-size

This parameter specifies a buffer, a direction where nil means vertical and t horizontal, and a size in pixels. If this window displays the specified buffer and its size in the indicated direction equals the size specified by this parameter, then Emacs will try to preserve the size of this window in the indicated direction. This parameter is installed and updated by the function window-preserve-size (see Preserving Window Sizes).

quit-restore

This parameter is installed by the buffer display functions (see Choosing Window) and consulted by quit-restore-window (see Quitting Windows). It contains four elements:

The first element is one of the symbols window, meaning that the window has been specially created by display-buffer; frame, a separate frame has been created; same, the window has only ever displayed this buffer; or other, the window showed another buffer before. frame and window affect how the window is quit, while same and other affect the redisplay of buffers previously shown in this window.

The second element is either one of the symbols window or frame, or a list whose elements are the buffer shown in the window before, that buffer's window start and window point positions, and the window's height at that time. If that buffer is still live when the window is quit, then the function quit-restore-window reuses the window to display the buffer.

The third element is the window selected at the time the parameter was created. If quit-restore-window deletes the window passed to it as argument, it then tries to reselect this window.

The fourth element is the buffer whose display caused the creation of this parameter. quit-restore-window deletes the specified window only if it still shows that buffer.

min-margins

The value of this parameter is a cons cell whose car and cdr, if non-nil, specify the minimum values (in columns) for the left and right margin of this window. When present, Emacs will use these values instead of the actual margin widths for determining whether a window can be split or shrunk horizontally.

Emacs never auto-adjusts the margins of any window after splitting or resizing it. It is the sole responsibility of any application setting this parameter to adjust the margins of this window as well as those of any new window that inherits this window's margins due to a split. Both window-configuration-change-hook and window-size-change-functions (see Window Hooks) should be employed for this purpose.

This parameter was introduced in Emacs version 25.1 to support applications that use large margins to center buffer text within a window and should be used, with due care, exclusively by those applications. It might be replaced by an improved solution in future versions of Emacs.

There are additional parameters window-atom and window-side; these are reserved and should not be used by applications.

27.27 Hooks for Window Scrolling and Changes

This section describes how a Lisp program can take action whenever a window displays a different part of its buffer or a different buffer. There are three actions that can change this: scrolling the window, switching buffers in the window, and changing the size of the window. The first two actions run window-scroll-functions; the last runs window-size-change-functions.

In addition, you can use jit-lock-register to register a Font Lock fontification function, which will be called whenever parts of a buffer are (re)fontified because a window was scrolled or its size changed. See Other Font Lock Variables.

28 Frames

A frame is a screen object that contains one or more Emacs windows (see Windows). It is the kind of object called a “window” in the terminology of graphical environments; but we can't call it a “window” here, because Emacs uses that word in a different way. In Emacs Lisp, a frame object is a Lisp object that represents a frame on the screen. See Frame Type.

A frame initially contains a single main window and/or a minibuffer window; you can subdivide the main window vertically or horizontally into smaller windows. See Splitting Windows.

A terminal is a display device capable of displaying one or more Emacs frames. In Emacs Lisp, a terminal object is a Lisp object that represents a terminal. See Terminal Type.

There are two classes of terminals: text terminals and graphical terminals. Text terminals are non-graphics-capable displays, including xterm and other terminal emulators. On a text terminal, each Emacs frame occupies the terminal's entire screen; although you can create additional frames and switch between them, the terminal only shows one frame at a time. Graphical terminals, on the other hand, are managed by graphical display systems such as the X Window System, which allow Emacs to show multiple frames simultaneously on the same display.

On GNU and Unix systems, you can create additional frames on any available terminal, within a single Emacs session, regardless of whether Emacs was started on a text or graphical terminal. Emacs can display on both graphical and text terminals simultaneously. This comes in handy, for instance, when you connect to the same session from several remote locations. See Multiple Terminals.

• Creating Frames: Creating additional frames.
• Multiple Terminals: Displaying on several different devices.
• Frame Geometry: Geometric properties of frames.
• Frame Parameters: Controlling frame size, position, font, etc.
• Terminal Parameters: Parameters common for all frames on terminal.
• Frame Titles: Automatic updating of frame titles.
• Deleting Frames: Frames last until explicitly deleted.
• Finding All Frames: How to examine all existing frames.
• Minibuffers and Frames: How a frame finds the minibuffer to use.
• Input Focus: Specifying the selected frame.
• Visibility of Frames: Frames may be visible or invisible, or icons.
• Raising and Lowering: Raising a frame makes it hide other windows; lowering it makes the others hide it.
• Frame Configurations: Saving the state of all frames.
• Mouse Tracking: Getting events that say when the mouse moves.
• Mouse Position: Asking where the mouse is, or moving it.
• Pop-Up Menus: Displaying a menu for the user to select from.
• Dialog Boxes: Displaying a box to ask yes or no.
• Pointer Shape: Specifying the shape of the mouse pointer.
• Window System Selections: Transferring text to and from other X clients.
• Drag and Drop: Internals of Drag-and-Drop implementation.
• Color Names: Getting the definitions of color names.
• Text Terminal Colors: Defining colors for text terminals.
• Resources: Getting resource values from the server.
• Display Feature Testing: Determining the features of a terminal.

28.1 Creating Frames

To create a new frame, call the function make-frame.

28.2 Multiple Terminals

Emacs represents each terminal as a terminal object data type (see Terminal Type). On GNU and Unix systems, Emacs can use multiple terminals simultaneously in each session. On other systems, it can only use a single terminal. Each terminal object has the following attributes:

• The name of the device used by the terminal (e.g., ‘:0.0’ or /dev/tty).
• The terminal and keyboard coding systems used on the terminal. See Terminal I/O Encoding.
• The kind of display associated with the terminal. This is the symbol returned by the function terminal-live-p (i.e., x, t, w32, ns, or pc). See Frames.
• A list of terminal parameters. See Terminal Parameters.

There is no primitive for creating terminal objects. Emacs creates them as needed, such as when you call make-frame-on-display (described below).

A few Lisp variables are terminal-local; that is, they have a separate binding for each terminal. The binding in effect at any time is the one for the terminal that the currently selected frame belongs to. These variables include default-minibuffer-frame, defining-kbd-macro, last-kbd-macro, and system-key-alist. They are always terminal-local, and can never be buffer-local (see Buffer-Local Variables).

On GNU and Unix systems, each X display is a separate graphical terminal. When Emacs is started from within the X window system, it uses the X display specified by the DISPLAY environment variable, or by the ‘--display’ option (see Initial Options). Emacs can connect to other X displays via the command make-frame-on-display. Each X display has its own selected frame and its own minibuffer windows; however, only one of those frames is the selected frame at any given moment (see Input Focus). Emacs can even connect to other text terminals, by interacting with the emacsclient program. See Emacs Server.

A single X server can handle more than one display. Each X display has a three-part name, ‘hostname:displaynumber.screennumber’. The first part, hostname, specifies the name of the machine to which the display is physically connected. The second part, displaynumber, is a zero-based number that identifies one or more monitors connected to that machine that share a common keyboard and pointing device (mouse, tablet, etc.). The third part, screennumber, identifies a zero-based screen number (a separate monitor) that is part of a single monitor collection on that X server. When you use two or more screens belonging to one server, Emacs knows by the similarity in their names that they share a single keyboard.

Systems that don't use the X window system, such as MS-Windows, don't support the notion of X displays, and have only one display on each host. The display name on these systems doesn't follow the above 3-part format; for example, the display name on MS-Windows systems is a constant string ‘w32’, and exists for compatibility, so that you could pass it to functions that expect a display name.

On some multi-monitor setups, a single X display outputs to more than one physical monitor. You can use the functions display-monitor-attributes-list and frame-monitor-attributes to obtain information about such setups.

28.3 Frame Geometry

The geometry of a frame depends on the toolkit that was used to build this instance of Emacs and the terminal that displays the frame. This chapter describes these dependencies and some of the functions to deal with them. Note that the frame argument of all of these functions has to specify a live frame (see Deleting Frames). If omitted or nil, it specifies the selected frame (see Input Focus).

• Frame Layout: Basic layout of frames.
• Frame Font: The default font of a frame and how to set it.
• Size and Position: Changing the size and position of a frame.
• Implied Frame Resizing: Implied resizing of frames and how to prevent it.

28.3.1 Frame Layout

The drawing below sketches the layout of a frame on a graphical terminal:

     
             <------------ Outer Frame Width ----------->
             ___________________________________________
          ^(0)  ___________ External Border __________   |
          | |  |_____________ Title Bar ______________|  |
          | | (1)_____________ Menu Bar ______________|  | ^
          | | (2)_____________ Tool Bar ______________|  | ^
          | | (3) _________ Internal Border ________  |  | ^
          | |  | |   ^                              | |  | |
          | |  | |   |                              | |  | |
     Outer  |  | | Inner                            | |  | Native
     Frame  |  | | Frame                            | |  | Frame
     Height |  | | Height                           | |  | Height
          | |  | |   |                              | |  | |
          | |  | |<--+--- Inner Frame Width ------->| |  | |
          | |  | |   |                              | |  | |
          | |  | |___v______________________________| |  | |
          | |  |___________ Internal Border __________|  | v
          v |______________ External Border _____________|
                <-------- Native Frame Width -------->

In practice not all of the areas shown in the drawing will or may be present. The meaning of these areas is:

‘Outer Frame’

The outer frame is a rectangle comprising all areas shown in the drawing. The edges of that rectangle are called the outer edges of the frame. The outer width and outer height of the frame specify the size of that rectangle.

The upper left corner of the outer frame (indicated by ‘(0)’ in the drawing above) is the outer position or the frame. It is specified by and settable via the left and top frame parameters (see Position Parameters) as well as the functions frame-position and set-frame-position (see Size and Position).

‘External Border’

The external border is part of the decorations supplied by the window manager. It's typically used for resizing the frame with the mouse. The external border is normally not shown on “fullboth” and maximized frames (see Size Parameters) and doesn't exist for text terminal frames.

The external border should not be confused with the outer border specified by the border-width frame parameter (see Layout Parameters). Since the outer border is usually ignored on most platforms it is not covered here.

‘Title Bar’

The title bar is also part of the window manager's decorations and typically displays the title of the frame (see Frame Titles) as well as buttons for minimizing, maximizing and deleting the frame. The title bar is usually not displayed on fullboth (see Size Parameters) or tooltip frames. Title bars don't exist for text terminal frames.

‘Menu Bar’

The menu bar (see Menu Bar) can be either internal (drawn by Emacs itself) or external (drawn by a toolkit). Most builds (GTK+, Lucid, Motif and Windows) rely on an external menu bar. NS also uses an external menu bar which, however, is not part of the outer frame. Non-toolkit builds can provide an internal menu bar. On text terminal frames, the menu bar is part of the frame's root window (see Windows and Frames).

‘Tool Bar’

Like the menu bar, the tool bar (see Tool Bar) can be either internal (drawn by Emacs itself) or external (drawn by a toolkit). The GTK+ and NS builds have the tool bar drawn by the toolkit. The remaining builds use internal tool bars. With GTK+ the tool bar can be located on either side of the frame, immediately outside the internal border, see below.

‘Native Frame’

The native frame is a rectangle located entirely within the outer frame. It excludes the areas occupied by the external border, the title bar and any external menu or external tool bar. The area enclosed by the native frame is sometimes also referred to as the display area of the frame. The edges of the native frame are called the native edges of the frame. The native width and native height of the frame specify the size of the rectangle.

The top left corner of the native frame specifies the native position of the frame. (1)–(3) in the drawing above indicate that position for the various builds:

• (1) non-toolkit and terminal frames
• (2) Lucid, Motif and Windows frames
• (3) GTK+ and NS frames

Accordingly, the native height of a frame includes the height of the tool bar but not that of the menu bar (Lucid, Motif, Windows) or those of the menu bar and the tool bar (non-toolkit and text terminal frames).

The native position of a frame is the reference position of functions that set or return the current position of the mouse (see Mouse Position) and for functions dealing with the position of windows like window-edges, window-at or coordinates-in-window-p (see Coordinates and Windows).

‘Internal Border’

The internal border (see Layout Parameters) is a border drawn by Emacs around the inner frame (see below).

‘Inner Frame’

The inner frame is the rectangle reserved for the frame's windows. It's enclosed by the internal border which, however, is not part of the inner frame. Its edges are called the inner edges of the frame. The inner width and inner height specify the size of the rectangle.

As a rule, the inner frame is subdivided into the frame's root window (see Windows and Frames) and the frame's minibuffer window (see Minibuffer Windows). There are two notable exceptions to this rule: A minibuffer-less frame contains a root window only and does not contain a minibuffer window. A minibuffer-only frame contains only a minibuffer window which also serves as that frame's root window. See Initial Parameters for how to create such frame configurations.

‘Text Area’

The text area of a frame is a somewhat fictitious area located entirely within the native frame. It can be obtained by removing from the native frame any internal borders, one vertical and one horizontal scroll bar, and one left and one right fringe as specified for this frame, see Layout Parameters.

The absolute position of a frame or its edges is usually given in terms of pixels counted from an origin at position (0, 0) of the frame's display. Note that with multiple monitors the origin does not necessarily coincide with the top left corner of the entire usable display area. Hence the absolute outer position of a frame or the absolute positions of the edges of the outer, native or inner frame can be negative in such an environment even when that frame is completely visible.

For a frame on a graphical terminal the following function returns the sizes of the areas described above:

The following function can be used to retrieve the edges of the outer, native and inner frame.

28.3.2 Frame Font

Each frame has a default font which specifies the default character size for that frame. This size is meant when retrieving or changing the size of a frame in terms of columns or lines (see Size Parameters). It is also used when resizing (see Window Sizes) or splitting (see Splitting Windows) windows.

The terms line height and canonical character height are sometimes used instead of “default character height”. Similarly, the terms column width and canonical character width are used instead of “default character width”.

The default font can be also set directly with the following function:

28.3.3 Size and Position

You can read or change the position of a frame using the frame parameters left and top (see Position Parameters) and its size using the height and width parameters (see Size Parameters). Here are some special features for working with sizes and positions. For all of these functions the argument frame must denote a live frame and defaults to the selected frame.

None of these three functions will make a frame smaller than needed to display all of its windows together with their scroll bars, fringes, margins, dividers, mode and header lines. This contrasts with requests by the window manager triggered, for example, by dragging the external border of a frame with the mouse. Such requests are always honored by clipping, if necessary, portions that cannot be displayed at the right, bottom corner of the frame.

28.3.4 Implied Frame Resizing

By default, Emacs tries to keep the number of lines and columns of a frame's text area unaltered when, for example, adding or removing the menu bar, changing the default font or setting the width of the frame's scroll bars. This means, however, that in such case Emacs must ask the window manager to resize the outer frame in order to accommodate the size change. Note that wrapping a menu or tool bar usually does not resize the frame's outer size, hence this will alter the number of displayed lines.

Occasionally, such implied frame resizing may be unwanted, for example, when the frame is maximized or made full-screen (where it's turned off by default). In other cases you can disable implied resizing with the following option:

28.4 Frame Parameters

A frame has many parameters that control its appearance and behavior. Just what parameters a frame has depends on what display mechanism it uses.

Frame parameters exist mostly for the sake of graphical displays. Most frame parameters have no effect when applied to a frame on a text terminal; only the height, width, name, title, menu-bar-lines, buffer-list and buffer-predicate parameters do something special. If the terminal supports colors, the parameters foreground-color, background-color, background-mode and display-type are also meaningful. If the terminal supports frame transparency, the parameter alpha is also meaningful.

• Parameter Access: How to change a frame's parameters.
• Initial Parameters: Specifying frame parameters when you make a frame.
• Window Frame Parameters: List of frame parameters for window systems.
• Geometry: Parsing geometry specifications.

28.4.1 Access to Frame Parameters

These functions let you read and change the parameter values of a frame.

28.4.2 Initial Frame Parameters

You can specify the parameters for the initial startup frame by setting initial-frame-alist in your init file (see Init File).

If these parameters include (minibuffer . nil), that indicates that the initial frame should have no minibuffer. In this case, Emacs creates a separate minibuffer-only frame as well.

If you invoke Emacs with command-line options that specify frame appearance, those options take effect by adding elements to either initial-frame-alist or default-frame-alist. Options which affect just the initial frame, such as ‘--geometry’ and ‘--maximized’, add to initial-frame-alist; the others add to default-frame-alist. see Command Line Arguments for Emacs Invocation.

28.4.3 Window Frame Parameters

Just what parameters a frame has depends on what display mechanism it uses. This section describes the parameters that have special meanings on some or all kinds of terminals. Of these, name, title, height, width, buffer-list and buffer-predicate provide meaningful information in terminal frames, and tty-color-mode is meaningful only for frames on text terminals.

• Basic Parameters: Parameters that are fundamental.
• Position Parameters: The position of the frame on the screen.
• Size Parameters: Frame's size.
• Layout Parameters: Size of parts of the frame, and enabling or disabling some parts.
• Buffer Parameters: Which buffers have been or should be shown.
• Management Parameters: Communicating with the window manager.
• Cursor Parameters: Controlling the cursor appearance.
• Font and Color Parameters: Fonts and colors for the frame text.

28.4.3.1 Basic Parameters

These frame parameters give the most basic information about the frame. title and name are meaningful on all terminals.

display

The display on which to open this frame. It should be a string of the form ‘host:dpy.screen’, just like the DISPLAY environment variable. See Multiple Terminals, for more details about display names.

display-type

This parameter describes the range of possible colors that can be used in this frame. Its value is color, grayscale or mono.

title

If a frame has a non-nil title, it appears in the window system's title bar at the top of the frame, and also in the mode line of windows in that frame if mode-line-frame-identification uses ‘%F’ (see %-Constructs). This is normally the case when Emacs is not using a window system, and can only display one frame at a time. See Frame Titles.

name

The name of the frame. The frame name serves as a default for the frame title, if the title parameter is unspecified or nil. If you don't specify a name, Emacs sets the frame name automatically (see Frame Titles).

If you specify the frame name explicitly when you create the frame, the name is also used (instead of the name of the Emacs executable) when looking up X resources for the frame.

explicit-name

If the frame name was specified explicitly when the frame was created, this parameter will be that name. If the frame wasn't explicitly named, this parameter will be nil.

28.4.3.2 Position Parameters

Position parameters' values are measured in pixels. (Note that none of these parameters exist on TTY frames.)

left

The position, in pixels, of the left (or right) edge of the frame with respect to the left (or right) edge of the screen. The value may be:

an integer

A positive integer relates the left edge of the frame to the left edge of the screen. A negative integer relates the right frame edge to the right screen edge.

(+ pos)

This specifies the position of the left frame edge relative to the left screen edge. The integer pos may be positive or negative; a negative value specifies a position outside the screen or on a monitor other than the primary one (for multi-monitor displays).

(- pos)

This specifies the position of the right frame edge relative to the right screen edge. The integer pos may be positive or negative; a negative value specifies a position outside the screen or on a monitor other than the primary one (for multi-monitor displays).

Some window managers ignore program-specified positions. If you want to be sure the position you specify is not ignored, specify a non-nil value for the user-position parameter as well.

If the window manager refuses to align a frame at the left or top screen edge, combining position notation and user-position as in

          (modify-frame-parameters
            nil '((user-position . t) (left . (+ -4))))

may help to override that.

top

The screen position of the top (or bottom) edge, in pixels, with respect to the top (or bottom) edge of the screen. It works just like left, except vertically instead of horizontally.

icon-left

The screen position of the left edge of the frame's icon, in pixels, counting from the left edge of the screen. This takes effect when the frame is iconified, if the window manager supports this feature. If you specify a value for this parameter, then you must also specify a value for icon-top and vice versa.

icon-top

The screen position of the top edge of the frame's icon, in pixels, counting from the top edge of the screen. This takes effect when the frame is iconified, if the window manager supports this feature.

user-position

When you create a frame and specify its screen position with the left and top parameters, use this parameter to say whether the specified position was user-specified (explicitly requested in some way by a human user) or merely program-specified (chosen by a program). A non-nil value says the position was user-specified.

Window managers generally heed user-specified positions, and some heed program-specified positions too. But many ignore program-specified positions, placing the window in a default fashion or letting the user place it with the mouse. Some window managers, including twm, let the user specify whether to obey program-specified positions or ignore them.

When you call make-frame, you should specify a non-nil value for this parameter if the values of the left and top parameters represent the user's stated preference; otherwise, use nil.

28.4.3.3 Size Parameters

Frame parameters specify frame sizes in character units. On graphical displays, the default face determines the actual pixel sizes of these character units (see Face Attributes).

height

The height of the frame's text area (see Frame Geometry), in characters.

width

The width of the frame's text area (see Frame Geometry), in characters.

user-size

This does for the size parameters height and width what the user-position parameter (see user-position) does for the position parameters top and left.

fullscreen

This parameter specifies whether to maximize the frame's width, height or both. Its value can be fullwidth, fullheight, fullboth, or maximized. A fullwidth frame is as wide as possible, a fullheight frame is as tall as possible, and a fullboth frame is both as wide and as tall as possible. A maximized frame is like a “fullboth” frame, except that it usually keeps its title bar and the buttons for resizing and closing the frame. Also, maximized frames typically avoid hiding any task bar or panels displayed on the desktop. A “fullboth” frame, on the other hand, usually omits the title bar and occupies the entire available screen space.

Full-height and full-width frames are more similar to maximized frames in this regard. However, these typically display an external border which might be absent with maximized frames. Hence the heights of maximized and full-height frames and the widths of maximized and full-width frames often differ by a few pixels.

With some window managers you may have to customize the variable frame-resize-pixelwise (see Size and Position) in order to make a frame truly appear maximized or full-screen. Moreover, some window managers might not support smooth transition between the various full-screen or maximization states. Customizing the variable x-frame-normalize-before-maximize can help to overcome that.

fullscreen-restore

This parameter specifies the desired fullscreen state of the frame after invoking the toggle-frame-fullscreen command (see Frame Commands) in the “fullboth” state. Normally this parameter is installed automatically by that command when toggling the state to fullboth. If, however, you start Emacs in the “fullboth” state, you have to specify the desired behavior in your initial file as, for example

          (setq default-frame-alist
              '((fullscreen . fullboth) (fullscreen-restore . fullheight)))

This will give a new frame full height after typing in it <F11> for the first time.

28.4.3.4 Layout Parameters

These frame parameters enable or disable various parts of the frame, or control their sizes.

border-width

The width in pixels of the frame's border.

internal-border-width

The distance in pixels between text (or fringe) and the frame's border.

vertical-scroll-bars

Whether the frame has scroll bars for vertical scrolling, and which side of the frame they should be on. The possible values are left, right, and nil for no scroll bars.

horizontal-scroll-bars

Whether the frame has scroll bars for horizontal scrolling (t and bottom mean yes, nil means no).

scroll-bar-width

The width of vertical scroll bars, in pixels, or nil meaning to use the default width.

scroll-bar-height

The height of horizontal scroll bars, in pixels, or nil meaning to use the default height.

left-fringe

right-fringe

The default width of the left and right fringes of windows in this frame (see Fringes). If either of these is zero, that effectively removes the corresponding fringe.

When you use frame-parameter to query the value of either of these two frame parameters, the return value is always an integer. When using set-frame-parameter, passing a nil value imposes an actual default value of 8 pixels.

right-divider-width

The width (thickness) reserved for the right divider (see Window Dividers) of any window on the frame, in pixels. A value of zero means to not draw right dividers.

bottom-divider-width

The width (thickness) reserved for the bottom divider (see Window Dividers) of any window on the frame, in pixels. A value of zero means to not draw bottom dividers.

menu-bar-lines

The number of lines to allocate at the top of the frame for a menu bar. The default is 1 if Menu Bar mode is enabled, and 0 otherwise. See Menu Bars.

tool-bar-lines

The number of lines to use for the tool bar. The default is 1 if Tool Bar mode is enabled, and 0 otherwise. See Tool Bars.

tool-bar-position

The position of the tool bar. Currently only for the GTK tool bar. Value can be one of top, bottom left, right. The default is top.

line-spacing

Additional space to leave below each text line, in pixels (a positive integer). See Line Height, for more information.

28.4.3.5 Buffer Parameters

These frame parameters, meaningful on all kinds of terminals, deal with which buffers have been, or should, be displayed in the frame.

minibuffer

Whether this frame has its own minibuffer. The value t means yes, nil means no, only means this frame is just a minibuffer. If the value is a minibuffer window (in some other frame), the frame uses that minibuffer.

This frame parameter takes effect when the frame is created, and can not be changed afterwards.

buffer-predicate

The buffer-predicate function for this frame. The function other-buffer uses this predicate (from the selected frame) to decide which buffers it should consider, if the predicate is not nil. It calls the predicate with one argument, a buffer, once for each buffer; if the predicate returns a non-nil value, it considers that buffer.

buffer-list

A list of buffers that have been selected in this frame, ordered most-recently-selected first.

unsplittable

If non-nil, this frame's window is never split automatically.

28.4.3.6 Window Management Parameters

The following frame parameters control various aspects of the frame's interaction with the window manager. They have no effect on text terminals.

visibility

The state of visibility of the frame. There are three possibilities: nil for invisible, t for visible, and icon for iconified. See Visibility of Frames.

auto-raise

If non-nil, Emacs automatically raises the frame when it is selected. Some window managers do not allow this.

auto-lower

If non-nil, Emacs automatically lowers the frame when it is deselected. Some window managers do not allow this.

icon-type

The type of icon to use for this frame. If the value is a string, that specifies a file containing a bitmap to use; nil specifies no icon (in which case the window manager decides what to show); any other non-nil value specifies the default Emacs icon.

icon-name

The name to use in the icon for this frame, when and if the icon appears. If this is nil, the frame's title is used.

window-id

The ID number which the graphical display uses for this frame. Emacs assigns this parameter when the frame is created; changing the parameter has no effect on the actual ID number.

outer-window-id

The ID number of the outermost window-system window in which the frame exists. As with window-id, changing this parameter has no actual effect.

wait-for-wm

If non-nil, tell Xt to wait for the window manager to confirm geometry changes. Some window managers, including versions of Fvwm2 and KDE, fail to confirm, so Xt hangs. Set this to nil to prevent hanging with those window managers.

sticky

If non-nil, the frame is visible on all virtual desktops on systems with virtual desktops.

28.4.3.7 Cursor Parameters

This frame parameter controls the way the cursor looks.

cursor-type

How to display the cursor. Legitimate values are:

box

Display a filled box. (This is the default.)

hollow

Display a hollow box.

nil

Don't display a cursor.

bar

Display a vertical bar between characters.

(bar . width)

Display a vertical bar width pixels wide between characters.

hbar

Display a horizontal bar.

(hbar . height)

Display a horizontal bar height pixels high.

The cursor-type frame parameter may be overridden by the variables cursor-type and cursor-in-non-selected-windows:

28.4.3.8 Font and Color Parameters

These frame parameters control the use of fonts and colors.

font-backend

A list of symbols, specifying the font backends to use for drawing fonts in the frame, in order of priority. On X, there are currently two available font backends: x (the X core font driver) and xft (the Xft font driver). On MS-Windows, there are currently two available font backends: gdi and uniscribe (see Windows Fonts). On other systems, there is only one available font backend, so it does not make sense to modify this frame parameter.

background-mode

This parameter is either dark or light, according to whether the background color is a light one or a dark one.

tty-color-mode

This parameter overrides the terminal's color support as given by the system's terminal capabilities database in that this parameter's value specifies the color mode to use on a text terminal. The value can be either a symbol or a number. A number specifies the number of colors to use (and, indirectly, what commands to issue to produce each color). For example, (tty-color-mode . 8) specifies use of the ANSI escape sequences for 8 standard text colors. A value of -1 turns off color support.

If the parameter's value is a symbol, it specifies a number through the value of tty-color-mode-alist, and the associated number is used instead.

screen-gamma

If this is a number, Emacs performs gamma correction which adjusts the brightness of all colors. The value should be the screen gamma of your display.

Usual PC monitors have a screen gamma of 2.2, so color values in Emacs, and in X windows generally, are calibrated to display properly on a monitor with that gamma value. If you specify 2.2 for screen-gamma, that means no correction is needed. Other values request correction, designed to make the corrected colors appear on your screen the way they would have appeared without correction on an ordinary monitor with a gamma value of 2.2.

If your monitor displays colors too light, you should specify a screen-gamma value smaller than 2.2. This requests correction that makes colors darker. A screen gamma value of 1.5 may give good results for LCD color displays.

alpha

This parameter specifies the opacity of the frame, on graphical displays that support variable opacity. It should be an integer between 0 and 100, where 0 means completely transparent and 100 means completely opaque. It can also have a nil value, which tells Emacs not to set the frame opacity (leaving it to the window manager).

To prevent the frame from disappearing completely from view, the variable frame-alpha-lower-limit defines a lower opacity limit. If the value of the frame parameter is less than the value of this variable, Emacs uses the latter. By default, frame-alpha-lower-limit is 20.

The alpha frame parameter can also be a cons cell (active . inactive), where active is the opacity of the frame when it is selected, and inactive is the opacity when it is not selected.

The following frame parameters are semi-obsolete in that they are automatically equivalent to particular face attributes of particular faces (see Standard Faces):

font

The name of the font for displaying text in the frame. This is a string, either a valid font name for your system or the name of an Emacs fontset (see Fontsets). It is equivalent to the font attribute of the default face.

foreground-color

The color to use for the image of a character. It is equivalent to the :foreground attribute of the default face.

background-color

The color to use for the background of characters. It is equivalent to the :background attribute of the default face.

mouse-color

The color for the mouse pointer. It is equivalent to the :background attribute of the mouse face.

cursor-color

The color for the cursor that shows point. It is equivalent to the :background attribute of the cursor face.

border-color

The color for the border of the frame. It is equivalent to the :background attribute of the border face.

scroll-bar-foreground

If non-nil, the color for the foreground of scroll bars. It is equivalent to the :foreground attribute of the scroll-bar face.

scroll-bar-background

If non-nil, the color for the background of scroll bars. It is equivalent to the :background attribute of the scroll-bar face.

28.4.4 Geometry

Here's how to examine the data in an X-style window geometry specification:

28.5 Terminal Parameters

Each terminal has a list of associated parameters. These terminal parameters are mostly a convenient way of storage for terminal-local variables, but some terminal parameters have a special meaning.

This section describes functions to read and change the parameter values of a terminal. They all accept as their argument either a terminal or a frame; the latter means use that frame's terminal. An argument of nil means the selected frame's terminal.

Here's a list of a few terminal parameters that have a special meaning:

background-mode

The classification of the terminal's background color, either light or dark.

normal-erase-is-backspace

Value is either 1 or 0, depending on whether normal-erase-is-backspace-mode is turned on or off on this terminal. See DEL Does Not Delete.

terminal-initted

After the terminal is initialized, this is set to the terminal-specific initialization function.

tty-mode-set-strings

When present, a list of strings containing escape sequences that Emacs will output while configuring a tty for rendering. Emacs emits these strings only when configuring a terminal: if you want to enable a mode on a terminal that is already active (for example, while in tty-setup-hook), explicitly output the necessary escape sequence using send-string-to-terminal in addition to adding the sequence to tty-mode-set-strings.

tty-mode-reset-strings

When present, a list of strings that undo the effects of the strings in tty-mode-set-strings. Emacs emits these strings when exiting, deleting a terminal, or suspending itself.

28.6 Frame Titles

Every frame has a name parameter; this serves as the default for the frame title which window systems typically display at the top of the frame. You can specify a name explicitly by setting the name frame property.

Normally you don't specify the name explicitly, and Emacs computes the frame name automatically based on a template stored in the variable frame-title-format. Emacs recomputes the name each time the frame is redisplayed.

28.7 Deleting Frames

A live frame is one that has not been deleted. When a frame is deleted, it is removed from its terminal display, although it may continue to exist as a Lisp object until there are no more references to it.

Some window managers provide a command to delete a window. These work by sending a special message to the program that operates the window. When Emacs gets one of these commands, it generates a delete-frame event, whose normal definition is a command that calls the function delete-frame. See Misc Events.

28.8 Finding All Frames

See also next-window and previous-window, in Cyclic Window Ordering.

28.9 Minibuffers and Frames

Normally, each frame has its own minibuffer window at the bottom, which is used whenever that frame is selected. If the frame has a minibuffer, you can get it with minibuffer-window (see Minibuffer Windows).

However, you can also create a frame without a minibuffer. Such a frame must use the minibuffer window of some other frame. That other frame will serve as surrogate minibuffer frame for this frame and cannot be deleted via delete-frame (see Deleting Frames) as long as this frame is live.

When you create the frame, you can explicitly specify the minibuffer window to use (in some other frame). If you don't, then the minibuffer is found in the frame which is the value of the variable default-minibuffer-frame. Its value should be a frame that does have a minibuffer.

If you use a minibuffer-only frame, you might want that frame to raise when you enter the minibuffer. If so, set the variable minibuffer-auto-raise to t. See Raising and Lowering.

28.10 Input Focus

At any time, one frame in Emacs is the selected frame. The selected window always resides on the selected frame.

When Emacs displays its frames on several terminals (see Multiple Terminals), each terminal has its own selected frame. But only one of these is the selected frame: it's the frame that belongs to the terminal from which the most recent input came. That is, when Emacs runs a command that came from a certain terminal, the selected frame is the one of that terminal. Since Emacs runs only a single command at any given time, it needs to consider only one selected frame at a time; this frame is what we call the selected frame in this manual. The display on which the selected frame is shown is the selected frame's display.

Some window systems and window managers direct keyboard input to the window object that the mouse is in; others require explicit clicks or commands to shift the focus to various window objects. Either way, Emacs automatically keeps track of which frame has the focus. To explicitly switch to a different frame from a Lisp function, call select-frame-set-input-focus.

Lisp programs can also switch frames temporarily by calling the function select-frame. This does not alter the window system's concept of focus; rather, it escapes from the window manager's control until that control is somehow reasserted.

When using a text terminal, only one frame can be displayed at a time on the terminal, so after a call to select-frame, the next redisplay actually displays the newly selected frame. This frame remains selected until a subsequent call to select-frame. Each frame on a text terminal has a number which appears in the mode line before the buffer name (see Mode Line Variables).

Emacs cooperates with the window system by arranging to select frames as the server and window manager request. It does so by generating a special kind of input event, called a focus event, when appropriate. The command loop handles a focus event by calling handle-switch-frame. See Focus Events.

28.11 Visibility of Frames

A frame on a graphical display may be visible, invisible, or iconified. If it is visible, its contents are displayed in the usual manner. If it is iconified, its contents are not displayed, but there is a little icon somewhere to bring the frame back into view (some window managers refer to this state as minimized rather than iconified, but from Emacs' point of view they are the same thing). If a frame is invisible, it is not displayed at all.

Visibility is meaningless on text terminals, since only the selected one is actually displayed in any case.

The visibility status of a frame is also available as a frame parameter. You can read or change it as such. See Management Parameters. The user can also iconify and deiconify frames with the window manager. This happens below the level at which Emacs can exert any control, but Emacs does provide events that you can use to keep track of such changes. See Misc Events.

28.12 Raising and Lowering Frames

Most window systems use a desktop metaphor. Part of this metaphor is the idea that system-level windows (e.g., Emacs frames) are stacked in a notional third dimension perpendicular to the screen surface. Where two overlap, the one higher up covers the one underneath. You can raise or lower a frame using the functions raise-frame and lower-frame.

On window systems, you can also enable auto-raising (on frame selection) or auto-lowering (on frame deselection) using frame parameters. See Management Parameters.

The concept of raising and lowering frames also applies to text terminal frames. On each text terminal, only the top frame is displayed at any one time.

28.13 Frame Configurations

A frame configuration records the current arrangement of frames, all their properties, and the window configuration of each one. (See Window Configurations.)

28.14 Mouse Tracking

Sometimes it is useful to track the mouse, which means to display something to indicate where the mouse is and move the indicator as the mouse moves. For efficient mouse tracking, you need a way to wait until the mouse actually moves.

The convenient way to track the mouse is to ask for events to represent mouse motion. Then you can wait for motion by waiting for an event. In addition, you can easily handle any other sorts of events that may occur. That is useful, because normally you don't want to track the mouse forever—only until some other event, such as the release of a button.

The usual purpose of tracking mouse motion is to indicate on the screen the consequences of pushing or releasing a button at the current position.

In many cases, you can avoid the need to track the mouse by using the mouse-face text property (see Special Properties). That works at a much lower level and runs more smoothly than Lisp-level mouse tracking.

28.15 Mouse Position

The functions mouse-position and set-mouse-position give access to the current position of the mouse.

On a graphical terminal the following two functions allow the absolute position of the mouse cursor to be retrieved and set.

The following function can tell whether the mouse cursor is currently visible on a frame:

28.16 Pop-Up Menus

A Lisp program can pop up a menu so that the user can choose an alternative with the mouse. On a text terminal, if the mouse is not available, the user can choose an alternative using the keyboard motion keys—C-n, C-p, or up- and down-arrow keys.

Usage note: Don't use x-popup-menu to display a menu if you could do the job with a prefix key defined with a menu keymap. If you use a menu keymap to implement a menu, C-h c and C-h a can see the individual items in that menu and provide help for them. If instead you implement the menu by defining a command that calls x-popup-menu, the help facilities cannot know what happens inside that command, so they cannot give any help for the menu's items.

The menu bar mechanism, which lets you switch between submenus by moving the mouse, cannot look within the definition of a command to see that it calls x-popup-menu. Therefore, if you try to implement a submenu using x-popup-menu, it cannot work with the menu bar in an integrated fashion. This is why all menu bar submenus are implemented with menu keymaps within the parent menu, and never with x-popup-menu. See Menu Bar.

If you want a menu bar submenu to have contents that vary, you should still use a menu keymap to implement it. To make the contents vary, add a hook function to menu-bar-update-hook to update the contents of the menu keymap as necessary.

28.17 Dialog Boxes

A dialog box is a variant of a pop-up menu—it looks a little different, it always appears in the center of a frame, and it has just one level and one or more buttons. The main use of dialog boxes is for asking questions that the user can answer with “yes”, “no”, and a few other alternatives. With a single button, they can also force the user to acknowledge important information. The functions y-or-n-p and yes-or-no-p use dialog boxes instead of the keyboard, when called from commands invoked by mouse clicks.

28.18 Pointer Shape

You can specify the mouse pointer style for particular text or images using the pointer text property, and for images with the :pointer and :map image properties. The values you can use in these properties are text (or nil), arrow, hand, vdrag, hdrag, modeline, and hourglass. text stands for the usual mouse pointer style used over text.

Over void parts of the window (parts that do not correspond to any of the buffer contents), the mouse pointer usually uses the arrow style, but you can specify a different style (one of those above) by setting void-text-area-pointer.

When using X, you can specify what the text pointer style really looks like by setting the variable x-pointer-shape.

These variables affect newly created frames. They do not normally affect existing frames; however, if you set the mouse color of a frame, that also installs the current value of those two variables. See Font and Color Parameters.

The values you can use, to specify either of these pointer shapes, are defined in the file lisp/term/x-win.el. Use M-x apropos <RET> x-pointer <RET> to see a list of them.

28.19 Window System Selections

In window systems, such as X, data can be transferred between different applications by means of selections. X defines an arbitrary number of selection types, each of which can store its own data; however, only three are commonly used: the clipboard, primary selection, and secondary selection. Other window systems support only the clipboard. See Cut and Paste, for Emacs commands that make use of these selections. This section documents the low-level functions for reading and setting window-system selections.

When Emacs runs on MS-Windows, it does not implement X selections in general, but it does support the clipboard. gui-get-selection and gui-set-selection on MS-Windows support the text data type only; if the clipboard holds other types of data, Emacs treats the clipboard as empty. The supported data type is STRING.

For backward compatibility, there are obsolete aliases x-get-selection and x-set-selection, which were the names of gui-get-selection and gui-set-selection before Emacs 25.1.

28.20 Drag and Drop

When a user drags something from another application over Emacs, that other application expects Emacs to tell it if Emacs can handle the data that is dragged. The variable x-dnd-test-function is used by Emacs to determine what to reply. The default value is x-dnd-default-test-function which accepts drops if the type of the data to be dropped is present in x-dnd-known-types. You can customize x-dnd-test-function and/or x-dnd-known-types if you want Emacs to accept or reject drops based on some other criteria.

If you want to change the way Emacs handles drop of different types or add a new type, customize x-dnd-types-alist. This requires detailed knowledge of what types other applications use for drag and drop.

When an URL is dropped on Emacs it may be a file, but it may also be another URL type (ftp, http, etc.). Emacs first checks dnd-protocol-alist to determine what to do with the URL. If there is no match there and if browse-url-browser-function is an alist, Emacs looks for a match there. If no match is found the text for the URL is inserted. If you want to alter Emacs behavior, you can customize these variables.

28.21 Color Names

A color name is text (usually in a string) that specifies a color. Symbolic names such as ‘black’, ‘white’, ‘red’, etc., are allowed; use M-x list-colors-display to see a list of defined names. You can also specify colors numerically in forms such as ‘#rgb’ and ‘RGB:r/g/b’, where r specifies the red level, g specifies the green level, and b specifies the blue level. You can use either one, two, three, or four hex digits for r; then you must use the same number of hex digits for all g and b as well, making either 3, 6, 9 or 12 hex digits in all. (See the documentation of the X Window System for more details about numerical RGB specification of colors.)

These functions provide a way to determine which color names are valid, and what they look like. In some cases, the value depends on the selected frame, as described below; see Input Focus, for the meaning of the term “selected frame”.

To read user input of color names with completion, use read-color (see read-color).

28.22 Text Terminal Colors

Text terminals usually support only a small number of colors, and the computer uses small integers to select colors on the terminal. This means that the computer cannot reliably tell what the selected color looks like; instead, you have to inform your application which small integers correspond to which colors. However, Emacs does know the standard set of colors and will try to use them automatically.

The functions described in this section control how terminal colors are used by Emacs.

Several of these functions use or return rgb values, described in Color Names.

These functions accept a display (either a frame or the name of a terminal) as an optional argument. We hope in the future to make Emacs support different colors on different text terminals; then this argument will specify which terminal to operate on (the default being the selected frame's terminal; see Input Focus). At present, though, the frame argument has no effect.

28.23 X Resources

This section describes some of the functions and variables for querying and using X resources, or their equivalent on your operating system. See X Resources, for more information about X resources.

To illustrate some of the above, suppose that you have the line:

     xterm.vt100.background: yellow

in your X resources file (whose name is usually ~/.Xdefaults or ~/.Xresources). Then:

     (let ((x-resource-class "XTerm") (x-resource-name "xterm"))
       (x-get-resource "vt100.background" "VT100.Background"))
          ⇒ "yellow"
     (let ((x-resource-class "XTerm") (x-resource-name "xterm"))
       (x-get-resource "background" "VT100" "vt100" "Background"))
          ⇒ "yellow"

28.24 Display Feature Testing

The functions in this section describe the basic capabilities of a particular display. Lisp programs can use them to adapt their behavior to what the display can do. For example, a program that ordinarily uses a popup menu could use the minibuffer if popup menus are not supported.

The optional argument display in these functions specifies which display to ask the question about. It can be a display name, a frame (which designates the display that frame is on), or nil (which refers to the selected frame's display, see Input Focus).

See Color Names, Text Terminal Colors, for other functions to obtain information about displays.

These functions obtain additional information about the window system in use where Emacs shows the specified display. (Their names begin with x- for historical reasons.)

29 Positions

A position is the index of a character in the text of a buffer. More precisely, a position identifies the place between two characters (or before the first character, or after the last character), so we can speak of the character before or after a given position. However, we often speak of the character “at” a position, meaning the character after that position.

Positions are usually represented as integers starting from 1, but can also be represented as markers—special objects that relocate automatically when text is inserted or deleted so they stay with the surrounding characters. Functions that expect an argument to be a position (an integer), but accept a marker as a substitute, normally ignore which buffer the marker points into; they convert the marker to an integer, and use that integer, exactly as if you had passed the integer as the argument, even if the marker points to the wrong buffer. A marker that points nowhere cannot convert to an integer; using it instead of an integer causes an error. See Markers.

See also the field feature (see Fields), which provides functions that are used by many cursor-motion commands.

• Point: The special position where editing takes place.
• Motion: Changing point.
• Excursions: Temporary motion and buffer changes.
• Narrowing: Restricting editing to a portion of the buffer.

29.1 Point

Point is a special buffer position used by many editing commands, including the self-inserting typed characters and text insertion functions. Other commands move point through the text to allow editing and insertion at different places.

Like other positions, point designates a place between two characters (or before the first character, or after the last character), rather than a particular character. Usually terminals display the cursor over the character that immediately follows point; point is actually before the character on which the cursor sits.

The value of point is a number no less than 1, and no greater than the buffer size plus 1. If narrowing is in effect (see Narrowing), then point is constrained to fall within the accessible portion of the buffer (possibly at one end of it).

Each buffer has its own value of point, which is independent of the value of point in other buffers. Each window also has a value of point, which is independent of the value of point in other windows on the same buffer. This is why point can have different values in various windows that display the same buffer. When a buffer appears in only one window, the buffer's point and the window's point normally have the same value, so the distinction is rarely important. See Window Point, for more details.

29.2 Motion

Motion functions change the value of point, either relative to the current value of point, relative to the beginning or end of the buffer, or relative to the edges of the selected window. See Point.

• Character Motion: Moving in terms of characters.
• Word Motion: Moving in terms of words.
• Buffer End Motion: Moving to the beginning or end of the buffer.
• Text Lines: Moving in terms of lines of text.
• Screen Lines: Moving in terms of lines as displayed.
• List Motion: Moving by parsing lists and sexps.
• Skipping Characters: Skipping characters belonging to a certain set.

29.2.1 Motion by Characters

These functions move point based on a count of characters. goto-char is the fundamental primitive; the other functions use that.

29.2.2 Motion by Words

The functions for parsing words described below use the syntax table and char-script-table to decide whether a given character is part of a word. See Syntax Tables, and see Character Properties.

29.2.3 Motion to an End of the Buffer

To move point to the beginning of the buffer, write:

     (goto-char (point-min))

Likewise, to move to the end of the buffer, use:

     (goto-char (point-max))

Here are two commands that users use to do these things. They are documented here to warn you not to use them in Lisp programs, because they set the mark and display messages in the echo area.

29.2.4 Motion by Text Lines

Text lines are portions of the buffer delimited by newline characters, which are regarded as part of the previous line. The first text line begins at the beginning of the buffer, and the last text line ends at the end of the buffer whether or not the last character is a newline. The division of the buffer into text lines is not affected by the width of the window, by line continuation in display, or by how tabs and control characters are displayed.

Also see the functions bolp and eolp in Near Point. These functions do not move point, but test whether it is already at the beginning or end of a line.

29.2.5 Motion by Screen Lines

The line functions in the previous section count text lines, delimited only by newline characters. By contrast, these functions count screen lines, which are defined by the way the text appears on the screen. A text line is a single screen line if it is short enough to fit the width of the selected window, but otherwise it may occupy several screen lines.

In some cases, text lines are truncated on the screen rather than continued onto additional screen lines. In these cases, vertical-motion moves point much like forward-line. See Truncation.

Because the width of a given string depends on the flags that control the appearance of certain characters, vertical-motion behaves differently, for a given piece of text, depending on the buffer it is in, and even on the selected window (because the width, the truncation flag, and display table may vary between windows). See Usual Display.

These functions scan text to determine where screen lines break, and thus take time proportional to the distance scanned.

29.2.6 Moving over Balanced Expressions

Here are several functions concerned with balanced-parenthesis expressions (also called sexps in connection with moving across them in Emacs). The syntax table controls how these functions interpret various characters; see Syntax Tables. See Parsing Expressions, for lower-level primitives for scanning sexps or parts of sexps. For user-level commands, see Commands for Editing with Parentheses.

29.2.7 Skipping Characters

The following two functions move point over a specified set of characters. For example, they are often used to skip whitespace. For related functions, see Motion and Syntax.

These functions convert the set string to multibyte if the buffer is multibyte, and they convert it to unibyte if the buffer is unibyte, as the search functions do (see Searching and Matching).

29.3 Excursions

It is often useful to move point temporarily within a localized portion of the program. This is called an excursion, and it is done with the save-excursion special form. This construct remembers the initial identity of the current buffer, and its value of point, and restores them after the excursion completes. It is the standard way to move point within one part of a program and avoid affecting the rest of the program, and is used thousands of times in the Lisp sources of Emacs.

If you only need to save and restore the identity of the current buffer, use save-current-buffer or with-current-buffer instead (see Current Buffer). If you need to save or restore window configurations, see the forms described in Window Configurations and in Frame Configurations.

Because save-excursion only saves point for the buffer that was current at the start of the excursion, any changes made to point in other buffers, during the excursion, will remain in effect afterward. This frequently leads to unintended consequences, so the byte compiler warns if you call set-buffer during an excursion:

     Warning: Use ‘with-current-buffer’ rather than
              save-excursion+set-buffer

To avoid such problems, you should call save-excursion only after setting the desired current buffer, as in the following example:

     (defun append-string-to-buffer (string buffer)
       "Append STRING to the end of BUFFER."
       (with-current-buffer buffer
         (save-excursion
           (goto-char (point-max))
           (insert string))))

Likewise, save-excursion does not restore window-buffer correspondences altered by functions such as switch-to-buffer.

Warning: Ordinary insertion of text adjacent to the saved point value relocates the saved value, just as it relocates all markers. More precisely, the saved value is a marker with insertion type nil. See Marker Insertion Types. Therefore, when the saved point value is restored, it normally comes before the inserted text.

29.4 Narrowing

Narrowing means limiting the text addressable by Emacs editing commands to a limited range of characters in a buffer. The text that remains addressable is called the accessible portion of the buffer.

Narrowing is specified with two buffer positions, which become the beginning and end of the accessible portion. For most editing commands and primitives, these positions replace the values of the beginning and end of the buffer. While narrowing is in effect, no text outside the accessible portion is displayed, and point cannot move outside the accessible portion. Note that narrowing does not alter actual buffer positions (see Point); it only determines which positions are considered the accessible portion of the buffer. Most functions refuse to operate on text that is outside the accessible portion.

Commands for saving buffers are unaffected by narrowing; they save the entire buffer regardless of any narrowing.

If you need to display in a single buffer several very different types of text, consider using an alternative facility described in Swapping Text.

30 Markers

A marker is a Lisp object used to specify a position in a buffer relative to the surrounding text. A marker changes its offset from the beginning of the buffer automatically whenever text is inserted or deleted, so that it stays with the two characters on either side of it.

• Overview of Markers: The components of a marker, and how it relocates.
• Predicates on Markers: Testing whether an object is a marker.
• Creating Markers: Making empty markers or markers at certain places.
• Information from Markers: Finding the marker's buffer or character position.
• Marker Insertion Types: Two ways a marker can relocate when you insert where it points.
• Moving Markers: Moving the marker to a new buffer or position.
• The Mark: How the mark is implemented with a marker.
• The Region: How to access the region.

30.1 Overview of Markers

A marker specifies a buffer and a position in that buffer. A marker can be used to represent a position in functions that require one, just as an integer could be used. In that case, the marker's buffer is normally ignored. Of course, a marker used in this way usually points to a position in the buffer that the function operates on, but that is entirely the programmer's responsibility. See Positions, for a complete description of positions.

A marker has three attributes: the marker position, the marker buffer, and the insertion type. The marker position is an integer that is equivalent (at a given time) to the marker as a position in that buffer. But the marker's position value can change during the life of the marker, and often does. Insertion and deletion of text in the buffer relocate the marker. The idea is that a marker positioned between two characters remains between those two characters despite insertion and deletion elsewhere in the buffer. Relocation changes the integer equivalent of the marker.

Deleting text around a marker's position leaves the marker between the characters immediately before and after the deleted text. Inserting text at the position of a marker normally leaves the marker either in front of or after the new text, depending on the marker's insertion type (see Marker Insertion Types)—unless the insertion is done with insert-before-markers (see Insertion).

Insertion and deletion in a buffer must check all the markers and relocate them if necessary. This slows processing in a buffer with a large number of markers. For this reason, it is a good idea to make a marker point nowhere if you are sure you don't need it any more. Markers that can no longer be accessed are eventually removed (see Garbage Collection).

Because it is common to perform arithmetic operations on a marker position, most of these operations (including + and -) accept markers as arguments. In such cases, the marker stands for its current position.

Here are examples of creating markers, setting markers, and moving point to markers:

     ;; Make a new marker that initially does not point anywhere:
     (setq m1 (make-marker))
          ⇒ #<marker in no buffer>
     
     ;; Set m1 to point between the 99th and 100th characters
     ;;   in the current buffer:
     (set-marker m1 100)
          ⇒ #<marker at 100 in markers.texi>
     
     ;; Now insert one character at the beginning of the buffer:
     (goto-char (point-min))
          ⇒ 1
     (insert "Q")
          ⇒ nil
     
     ;; m1 is updated appropriately.
     m1
          ⇒ #<marker at 101 in markers.texi>
     
     ;; Two markers that point to the same position
     ;;   are not eq, but they are equal.
     (setq m2 (copy-marker m1))
          ⇒ #<marker at 101 in markers.texi>
     (eq m1 m2)
          ⇒ nil
     (equal m1 m2)
          ⇒ t
     
     ;; When you are finished using a marker, make it point nowhere.
     (set-marker m1 nil)
          ⇒ #<marker in no buffer>

30.2 Predicates on Markers

You can test an object to see whether it is a marker, or whether it is either an integer or a marker. The latter test is useful in connection with the arithmetic functions that work with both markers and integers.

30.3 Functions that Create Markers

When you create a new marker, you can make it point nowhere, or point to the present position of point, or to the beginning or end of the accessible portion of the buffer, or to the same place as another given marker.

The next four functions all return markers with insertion type nil. See Marker Insertion Types.

Two distinct markers are considered equal (even though not eq) to each other if they have the same position and buffer, or if they both point nowhere.

     (setq p (point-marker))
          ⇒ #<marker at 2139 in markers.texi>
     
     (setq q (copy-marker p))
          ⇒ #<marker at 2139 in markers.texi>
     
     (eq p q)
          ⇒ nil
     
     (equal p q)
          ⇒ t

30.4 Information from Markers

This section describes the functions for accessing the components of a marker object.

30.5 Marker Insertion Types

When you insert text directly at the place where a marker points, there are two possible ways to relocate that marker: it can point before the inserted text, or point after it. You can specify which one a given marker should do by setting its insertion type. Note that use of insert-before-markers ignores markers' insertion types, always relocating a marker to point after the inserted text.

All functions that create markers without accepting an argument that specifies the insertion type, create them with insertion type nil (see Creating Markers). Also, the mark has, by default, insertion type nil.

30.6 Moving Marker Positions

This section describes how to change the position of an existing marker. When you do this, be sure you know whether the marker is used outside of your program, and, if so, what effects will result from moving it—otherwise, confusing things may happen in other parts of Emacs.

30.7 The Mark

Each buffer has a special marker, which is designated the mark. When a buffer is newly created, this marker exists but does not point anywhere; this means that the mark doesn't exist in that buffer yet. Subsequent commands can set the mark.

The mark specifies a position to bound a range of text for many commands, such as kill-region and indent-rigidly. These commands typically act on the text between point and the mark, which is called the region. If you are writing a command that operates on the region, don't examine the mark directly; instead, use interactive with the ‘r’ specification. This provides the values of point and the mark as arguments to the command in an interactive call, but permits other Lisp programs to specify arguments explicitly. See Interactive Codes.

Some commands set the mark as a side-effect. Commands should do this only if it has a potential use to the user, and never for their own internal purposes. For example, the replace-regexp command sets the mark to the value of point before doing any replacements, because this enables the user to move back there conveniently after the replace is finished.

Once the mark exists in a buffer, it normally never ceases to exist. However, it may become inactive, if Transient Mark mode is enabled. The buffer-local variable mark-active, if non-nil, means that the mark is active. A command can call the function deactivate-mark to deactivate the mark directly, or it can request deactivation of the mark upon return to the editor command loop by setting the variable deactivate-mark to a non-nil value.

If Transient Mark mode is enabled, certain editing commands that normally apply to text near point, apply instead to the region when the mark is active. This is the main motivation for using Transient Mark mode. (Another is that this enables highlighting of the region when the mark is active. See Display.)

In addition to the mark, each buffer has a mark ring which is a list of markers containing previous values of the mark. When editing commands change the mark, they should normally save the old value of the mark on the mark ring. The variable mark-ring-max specifies the maximum number of entries in the mark ring; once the list becomes this long, adding a new element deletes the last element.

There is also a separate global mark ring, but that is used only in a few particular user-level commands, and is not relevant to Lisp programming. So we do not describe it here.

When Delete Selection mode (see Delete Selection) is enabled, commands that operate on the active region (a.k.a. “selection”) behave slightly differently. This works by adding the function delete-selection-pre-hook to the pre-command-hook (see Command Overview). That function calls delete-selection-helper to delete the selection as appropriate for the command. If you want to adapt a command to Delete Selection mode, put the delete-selection property on the function's symbol (see Symbol Plists); commands that don't have this property on their symbol won't delete the selection. This property can have one of several values to tailor the behavior to what the command is supposed to do; see the doc strings of delete-selection-pre-hook and delete-selection-helper for the details.

30.8 The Region

The text between point and the mark is known as the region. Various functions operate on text delimited by point and the mark, but only those functions specifically related to the region itself are described here.

The next two functions signal an error if the mark does not point anywhere. If Transient Mark mode is enabled and mark-even-if-inactive is nil, they also signal an error if the mark is inactive.

Instead of using region-beginning and region-end, a command designed to operate on a region should normally use interactive with the ‘r’ specification to find the beginning and end of the region. This lets other Lisp programs specify the bounds explicitly as arguments. See Interactive Codes.

31 Text

This chapter describes the functions that deal with the text in a buffer. Most examine, insert, or delete text in the current buffer, often operating at point or on text adjacent to point. Many are interactive. All the functions that change the text provide for undoing the changes (see Undo).

Many text-related functions operate on a region of text defined by two buffer positions passed in arguments named start and end. These arguments should be either markers (see Markers) or numeric character positions (see Positions). The order of these arguments does not matter; it is all right for start to be the end of the region and end the beginning. For example, (delete-region 1 10) and (delete-region 10 1) are equivalent. An args-out-of-range error is signaled if either start or end is outside the accessible portion of the buffer. In an interactive call, point and the mark are used for these arguments.

Throughout this chapter, “text” refers to the characters in the buffer, together with their properties (when relevant). Keep in mind that point is always between two characters, and the cursor appears on the character after point.

• Near Point: Examining text in the vicinity of point.
• Buffer Contents: Examining text in a general fashion.
• Comparing Text: Comparing substrings of buffers.
• Insertion: Adding new text to a buffer.
• Commands for Insertion: User-level commands to insert text.
• Deletion: Removing text from a buffer.
• User-Level Deletion: User-level commands to delete text.
• The Kill Ring: Where removed text sometimes is saved for later use.
• Undo: Undoing changes to the text of a buffer.
• Maintaining Undo: How to enable and disable undo information. How to control how much information is kept.
• Filling: Functions for explicit filling.
• Margins: How to specify margins for filling commands.
• Adaptive Fill: Adaptive Fill mode chooses a fill prefix from context.
• Auto Filling: How auto-fill mode is implemented to break lines.
• Sorting: Functions for sorting parts of the buffer.
• Columns: Computing horizontal positions, and using them.
• Indentation: Functions to insert or adjust indentation.
• Case Changes: Case conversion of parts of the buffer.
• Text Properties: Assigning Lisp property lists to text characters.
• Substitution: Replacing a given character wherever it appears.
• Registers: How registers are implemented. Accessing the text or position stored in a register.
• Transposition: Swapping two portions of a buffer.
• Decompression: Dealing with compressed data.
• Base 64: Conversion to or from base 64 encoding.
• Checksum/Hash: Computing cryptographic hashes.
• Parsing HTML/XML: Parsing HTML and XML.
• Atomic Changes: Installing several buffer changes atomically.
• Change Hooks: Supplying functions to be run when text is changed.

31.1 Examining Text Near Point

Many functions are provided to look at the characters around point. Several simple functions are described here. See also looking-at in Regexp Search.

In the following four functions, “beginning” or “end” of buffer refers to the beginning or end of the accessible portion.

31.2 Examining Buffer Contents

This section describes functions that allow a Lisp program to convert any portion of the text in the buffer into a string.

If you need to make sure the resulting string, when copied to a different location, will not change its visual appearance due to reordering of bidirectional text, use the buffer-substring-with-bidi-context function (see buffer-substring-with-bidi-context).

The following two variables are obsoleted by filter-buffer-substring-function, but are still supported for backward compatibility.

31.3 Comparing Text

This function lets you compare portions of the text in a buffer, without copying them into strings first.

31.4 Inserting Text

Insertion means adding new text to a buffer. The inserted text goes at point—between the character before point and the character after point. Some insertion functions leave point before the inserted text, while other functions leave it after. We call the former insertion after point and the latter insertion before point.

Insertion moves markers located at positions after the insertion point, so that they stay with the surrounding text (see Markers). When a marker points at the place of insertion, insertion may or may not relocate the marker, depending on the marker's insertion type (see Marker Insertion Types). Certain special functions such as insert-before-markers relocate all such markers to point after the inserted text, regardless of the markers' insertion type.

Insertion functions signal an error if the current buffer is read-only (see Read Only Buffers) or if they insert within read-only text (see Special Properties).

These functions copy text characters from strings and buffers along with their properties. The inserted characters have exactly the same properties as the characters they were copied from. By contrast, characters specified as separate arguments, not part of a string or buffer, inherit their text properties from the neighboring text.

The insertion functions convert text from unibyte to multibyte in order to insert in a multibyte buffer, and vice versa—if the text comes from a string or from a buffer. However, they do not convert unibyte character codes 128 through 255 to multibyte characters, not even if the current buffer is a multibyte buffer. See Converting Representations.

See Sticky Properties, for other insertion functions that inherit text properties from the nearby text in addition to inserting it. Whitespace inserted by indentation functions also inherits text properties.

31.5 User-Level Insertion Commands

This section describes higher-level commands for inserting text, commands intended primarily for the user but useful also in Lisp programs.

31.6 Deleting Text

Deletion means removing part of the text in a buffer, without saving it in the kill ring (see The Kill Ring). Deleted text can't be yanked, but can be reinserted using the undo mechanism (see Undo). Some deletion functions do save text in the kill ring in some special cases.

All of the deletion functions operate on the current buffer.

31.7 User-Level Deletion Commands

This section describes higher-level commands for deleting text, commands intended primarily for the user but useful also in Lisp programs.

31.8 The Kill Ring

Kill functions delete text like the deletion functions, but save it so that the user can reinsert it by yanking. Most of these functions have ‘kill-’ in their name. By contrast, the functions whose names start with ‘delete-’ normally do not save text for yanking (though they can still be undone); these are deletion functions.

Most of the kill commands are primarily for interactive use, and are not described here. What we do describe are the functions provided for use in writing such commands. You can use these functions to write commands for killing text. When you need to delete text for internal purposes within a Lisp function, you should normally use deletion functions, so as not to disturb the kill ring contents. See Deletion.

Killed text is saved for later yanking in the kill ring. This is a list that holds a number of recent kills, not just the last text kill. We call this a “ring” because yanking treats it as having elements in a cyclic order. The list is kept in the variable kill-ring, and can be operated on with the usual functions for lists; there are also specialized functions, described in this section, that treat it as a ring.

Some people think this use of the word “kill” is unfortunate, since it refers to operations that specifically do not destroy the entities killed. This is in sharp contrast to ordinary life, in which death is permanent and killed entities do not come back to life. Therefore, other metaphors have been proposed. For example, the term “cut ring” makes sense to people who, in pre-computer days, used scissors and paste to cut up and rearrange manuscripts. However, it would be difficult to change the terminology now.

• Kill Ring Concepts: What text looks like in the kill ring.
• Kill Functions: Functions that kill text.
• Yanking: How yanking is done.
• Yank Commands: Commands that access the kill ring.
• Low-Level Kill Ring: Functions and variables for kill ring access.
• Internals of Kill Ring: Variables that hold kill ring data.

31.8.1 Kill Ring Concepts

The kill ring records killed text as strings in a list, most recent first. A short kill ring, for example, might look like this:

     ("some text" "a different piece of text" "even older text")

When the list reaches kill-ring-max entries in length, adding a new entry automatically deletes the last entry.

When kill commands are interwoven with other commands, each kill command makes a new entry in the kill ring. Multiple kill commands in succession build up a single kill ring entry, which would be yanked as a unit; the second and subsequent consecutive kill commands add text to the entry made by the first one.

For yanking, one entry in the kill ring is designated the front of the ring. Some yank commands rotate the ring by designating a different element as the front. But this virtual rotation doesn't change the list itself—the most recent entry always comes first in the list.

31.8.2 Functions for Killing

kill-region is the usual subroutine for killing text. Any command that calls this function is a kill command (and should probably have ‘kill’ in its name). kill-region puts the newly killed text in a new element at the beginning of the kill ring or adds it to the most recent element. It determines automatically (using last-command) whether the previous command was a kill command, and if so appends the killed text to the most recent entry.

The commands described below can filter the killed text before they save it in the kill ring. They call filter-buffer-substring (see Buffer Contents) to perform the filtering. By default, there's no filtering, but major and minor modes and hook functions can set up filtering, so that text saved in the kill ring is different from what was in the buffer.

31.8.3 Yanking

Yanking means inserting text from the kill ring, but it does not insert the text blindly. The yank command, and related commands, use insert-for-yank to perform special processing on the text before it is inserted.

If you put a yank-handler text property on all or part of a string, that alters how insert-for-yank inserts the string. If different parts of the string have different yank-handler values (comparison being done with eq), each substring is handled separately. The property value must be a list of one to four elements, with the following format (where elements after the first may be omitted):

     (function param noexclude undo)

Here is what the elements do:

function

When function is non-nil, it is called instead of insert to insert the string, with one argument—the string to insert.

param

If param is present and non-nil, it replaces string (or the substring of string being processed) as the object passed to function (or insert). For example, if function is yank-rectangle, param should be a list of strings to insert as a rectangle.

noexclude

If noexclude is present and non-nil, that disables the normal action of yank-handled-properties and yank-excluded-properties on the inserted string.

undo

If undo is present and non-nil, it is a function that will be called by yank-pop to undo the insertion of the current object. It is called with two arguments, the start and end of the current region. function can set yank-undo-function to override the undo value.

31.8.4 Functions for Yanking

This section describes higher-level commands for yanking, which are intended primarily for the user but useful also in Lisp programs. Both yank and yank-pop honor the yank-excluded-properties variable and yank-handler text property (see Yanking).

31.8.5 Low-Level Kill Ring

These functions and variables provide access to the kill ring at a lower level, but are still convenient for use in Lisp programs, because they take care of interaction with window system selections (see Window System Selections).

31.8.6 Internals of the Kill Ring

The variable kill-ring holds the kill ring contents, in the form of a list of strings. The most recent kill is always at the front of the list.

The kill-ring-yank-pointer variable points to a link in the kill ring list, whose car is the text to yank next. We say it identifies the front of the ring. Moving kill-ring-yank-pointer to a different link is called rotating the kill ring. We call the kill ring a “ring” because the functions that move the yank pointer wrap around from the end of the list to the beginning, or vice-versa. Rotation of the kill ring is virtual; it does not change the value of kill-ring.

Both kill-ring and kill-ring-yank-pointer are Lisp variables whose values are normally lists. The word “pointer” in the name of the kill-ring-yank-pointer indicates that the variable's purpose is to identify one element of the list for use by the next yank command.

The value of kill-ring-yank-pointer is always eq to one of the links in the kill ring list. The element it identifies is the car of that link. Kill commands, which change the kill ring, also set this variable to the value of kill-ring. The effect is to rotate the ring so that the newly killed text is at the front.

Here is a diagram that shows the variable kill-ring-yank-pointer pointing to the second entry in the kill ring ("some text" "a different piece of text" "yet older text").

     kill-ring                  ---- kill-ring-yank-pointer
       |                       |
       |                       v
       |     --- ---          --- ---      --- ---
        --> |   |   |------> |   |   |--> |   |   |--> nil
             --- ---          --- ---      --- ---
              |                |            |
              |                |            |
              |                |             -->"yet older text"
              |                |
              |                 --> "a different piece of text"
              |
               --> "some text"

This state of affairs might occur after C-y (yank) immediately followed by M-y (yank-pop).

31.9 Undo

Most buffers have an undo list, which records all changes made to the buffer's text so that they can be undone. (The buffers that don't have one are usually special-purpose buffers for which Emacs assumes that undoing is not useful. In particular, any buffer whose name begins with a space has its undo recording off by default; see Buffer Names.) All the primitives that modify the text in the buffer automatically add elements to the front of the undo list, which is in the variable buffer-undo-list.

Here are the kinds of elements an undo list can have:

position

This kind of element records a previous value of point; undoing this element moves point to position. Ordinary cursor motion does not make any sort of undo record, but deletion operations use these entries to record where point was before the command.

(beg . end)

This kind of element indicates how to delete text that was inserted. Upon insertion, the text occupied the range beg–end in the buffer.

(text . position)

This kind of element indicates how to reinsert text that was deleted. The deleted text itself is the string text. The place to reinsert it is (abs position). If position is positive, point was at the beginning of the deleted text, otherwise it was at the end. Zero or more (marker . adjustment) elements follow immediately after this element.

(t . time-flag)

This kind of element indicates that an unmodified buffer became modified. A time-flag of the form (sec-high sec-low microsec picosec) represents the visited file's modification time as of when it was previously visited or saved, using the same format as current-time; see Time of Day. A time-flag of 0 means the buffer does not correspond to any file; −1 means the visited file previously did not exist. primitive-undo uses these values to determine whether to mark the buffer as unmodified once again; it does so only if the file's status matches that of time-flag.

(nil property value beg . end)

This kind of element records a change in a text property. Here's how you might undo the change:

          (put-text-property beg end property value)

(marker . adjustment)

This kind of element records the fact that the marker marker was relocated due to deletion of surrounding text, and that it moved adjustment character positions. If the marker's location is consistent with the (text . position) element preceding it in the undo list, then undoing this element moves marker − adjustment characters.

(apply funname . args)

This is an extensible undo item, which is undone by calling funname with arguments args.

(apply delta beg end funname . args)

This is an extensible undo item, which records a change limited to the range beg to end, which increased the size of the buffer by delta characters. It is undone by calling funname with arguments args.

This kind of element enables undo limited to a region to determine whether the element pertains to that region.

nil

This element is a boundary. The elements between two boundaries are called a change group; normally, each change group corresponds to one keyboard command, and undo commands normally undo an entire group as a unit.

31.10 Maintaining Undo Lists

This section describes how to enable and disable undo information for a given buffer. It also explains how the undo list is truncated automatically so it doesn't get too big.

Recording of undo information in a newly created buffer is normally enabled to start with; but if the buffer name starts with a space, the undo recording is initially disabled. You can explicitly enable or disable undo recording with the following two functions, or by setting buffer-undo-list yourself.

As editing continues, undo lists get longer and longer. To prevent them from using up all available memory space, garbage collection trims them back to size limits you can set. (For this purpose, the size of an undo list measures the cons cells that make up the list, plus the strings of deleted text.) Three variables control the range of acceptable sizes: undo-limit, undo-strong-limit and undo-outer-limit. In these variables, size is counted as the number of bytes occupied, which includes both saved text and other data.

31.11 Filling

Filling means adjusting the lengths of lines (by moving the line breaks) so that they are nearly (but no greater than) a specified maximum width. Additionally, lines can be justified, which means inserting spaces to make the left and/or right margins line up precisely. The width is controlled by the variable fill-column. For ease of reading, lines should be no longer than 70 or so columns.

You can use Auto Fill mode (see Auto Filling) to fill text automatically as you insert it, but changes to existing text may leave it improperly filled. Then you must fill the text explicitly.

Most of the commands in this section return values that are not meaningful. All the functions that do filling take note of the current left margin, current right margin, and current justification style (see Margins). If the current justification style is none, the filling functions don't actually do anything.

Several of the filling functions have an argument justify. If it is non-nil, that requests some kind of justification. It can be left, right, full, or center, to request a specific style of justification. If it is t, that means to use the current justification style for this part of the text (see current-justification, below). Any other value is treated as full.

When you call the filling functions interactively, using a prefix argument implies the value full for justify.

31.12 Margins for Filling

31.13 Adaptive Fill Mode

When Adaptive Fill Mode is enabled, Emacs determines the fill prefix automatically from the text in each paragraph being filled rather than using a predetermined value. During filling, this fill prefix gets inserted at the start of the second and subsequent lines of the paragraph as described in Filling, and in Auto Filling.

31.14 Auto Filling

Auto Fill mode is a minor mode that fills lines automatically as text is inserted. This section describes the hook used by Auto Fill mode. For a description of functions that you can call explicitly to fill and justify existing text, see Filling.

Auto Fill mode also enables the functions that change the margins and justification style to refill portions of the text. See Margins.

31.15 Sorting Text

The sorting functions described in this section all rearrange text in a buffer. This is in contrast to the function sort, which rearranges the order of the elements of a list (see Rearrangement). The values returned by these functions are not meaningful.

31.16 Counting Columns

The column functions convert between a character position (counting characters from the beginning of the buffer) and a column position (counting screen characters from the beginning of a line).

These functions count each character according to the number of columns it occupies on the screen. This means control characters count as occupying 2 or 4 columns, depending upon the value of ctl-arrow, and tabs count as occupying a number of columns that depends on the value of tab-width and on the column where the tab begins. See Usual Display.

Column number computations ignore the width of the window and the amount of horizontal scrolling. Consequently, a column value can be arbitrarily high. The first (or leftmost) column is numbered 0. They also ignore overlays and text properties, aside from invisibility.

31.17 Indentation

The indentation functions are used to examine, move to, and change whitespace that is at the beginning of a line. Some of the functions can also change whitespace elsewhere on a line. Columns and indentation count from zero at the left margin.

• Primitive Indent: Functions used to count and insert indentation.
• Mode-Specific Indent: Customize indentation for different modes.
• Region Indent: Indent all the lines in a region.
• Relative Indent: Indent the current line based on previous lines.
• Indent Tabs: Adjustable, typewriter-like tab stops.
• Motion by Indent: Move to first non-blank character.

31.17.1 Indentation Primitives

This section describes the primitive functions used to count and insert indentation. The functions in the following sections use these primitives. See Size of Displayed Text, for related functions.

31.17.2 Indentation Controlled by Major Mode

An important function of each major mode is to customize the <TAB> key to indent properly for the language being edited. This section describes the mechanism of the <TAB> key and how to control it. The functions in this section return unpredictable values.

31.17.3 Indenting an Entire Region

This section describes commands that indent all the lines in the region. They return unpredictable values.

31.17.4 Indentation Relative to Previous Lines

This section describes two commands that indent the current line based on the contents of previous lines.

31.17.5 Adjustable Tab Stops

This section explains the mechanism for user-specified tab stops and the mechanisms that use and set them. The name “tab stops” is used because the feature is similar to that of the tab stops on a typewriter. The feature works by inserting an appropriate number of spaces and tab characters to reach the next tab stop column; it does not affect the display of tab characters in the buffer (see Usual Display). Note that the <TAB> character as input uses this tab stop feature only in a few major modes, such as Text mode. See Tab Stops.

31.17.6 Indentation-Based Motion Commands

These commands, primarily for interactive use, act based on the indentation in the text.

31.18 Case Changes

The case change commands described here work on text in the current buffer. See Case Conversion, for case conversion functions that work on strings and characters. See Case Tables, for how to customize which characters are upper or lower case and how to convert them.

31.19 Text Properties

Each character position in a buffer or a string can have a text property list, much like the property list of a symbol (see Property Lists). The properties belong to a particular character at a particular place, such as, the letter ‘T’ at the beginning of this sentence or the first ‘o’ in ‘foo’—if the same character occurs in two different places, the two occurrences in general have different properties.

Each property has a name and a value. Both of these can be any Lisp object, but the name is normally a symbol. Typically each property name symbol is used for a particular purpose; for instance, the text property face specifies the faces for displaying the character (see Special Properties). The usual way to access the property list is to specify a name and ask what value corresponds to it.

If a character has a category property, we call it the property category of the character. It should be a symbol. The properties of the symbol serve as defaults for the properties of the character.

Copying text between strings and buffers preserves the properties along with the characters; this includes such diverse functions as substring, insert, and buffer-substring.

• Examining Properties: Looking at the properties of one character.
• Changing Properties: Setting the properties of a range of text.
• Property Search: Searching for where a property changes value.
• Special Properties: Particular properties with special meanings.
• Format Properties: Properties for representing formatting of text.
• Sticky Properties: How inserted text gets properties from neighboring text.
• Lazy Properties: Computing text properties in a lazy fashion only when text is examined.
• Clickable Text: Using text properties to make regions of text do something when you click on them.
• Fields: The field property defines fields within the buffer.
• Not Intervals: Why text properties do not use Lisp-visible text intervals.

31.19.1 Examining Text Properties

The simplest way to examine text properties is to ask for the value of a particular property of a particular character. For that, use get-text-property. Use text-properties-at to get the entire property list of a character. See Property Search, for functions to examine the properties of a number of characters at once.

These functions handle both strings and buffers. Keep in mind that positions in a string start from 0, whereas positions in a buffer start from 1.

31.19.2 Changing Text Properties

The primitives for changing properties apply to a specified range of text in a buffer or string. The function set-text-properties (see end of section) sets the entire property list of the text in that range; more often, it is useful to add, change, or delete just certain properties specified by name.

Since text properties are considered part of the contents of the buffer (or string), and can affect how a buffer looks on the screen, any change in buffer text properties marks the buffer as modified. Buffer text property changes are undoable also (see Undo). Positions in a string start from 0, whereas positions in a buffer start from 1.

The easiest way to make a string with text properties is with propertize:

See Buffer Contents, for the function buffer-substring-no-properties, which copies text from the buffer but does not copy its properties.

If you wish to add or remove text properties to a buffer without marking the buffer as modified, you can wrap the calls above in the with-silent-modifications macro.

31.19.3 Text Property Search Functions

In typical use of text properties, most of the time several or many consecutive characters have the same value for a property. Rather than writing your programs to examine characters one by one, it is much faster to process chunks of text that have the same property value.

Here are functions you can use to do this. They use eq for comparing property values. In all cases, object defaults to the current buffer.

For good performance, it's very important to use the limit argument to these functions, especially the ones that search for a single property—otherwise, they may spend a long time scanning to the end of the buffer, if the property you are interested in does not change.

These functions do not move point; instead, they return a position (or nil). Remember that a position is always between two characters; the position returned by these functions is between two characters with different properties.

31.19.4 Properties with Special Meanings

Here is a table of text property names that have special built-in meanings. The following sections list a few additional special property names that control filling and property inheritance. All other names have no standard meaning, and you can use them as you like.

Note: the properties composition, display, invisible and intangible can also cause point to move to an acceptable place, after each Emacs command. See Adjusting Point.

category

If a character has a category property, we call it the property category of the character. It should be a symbol. The properties of this symbol serve as defaults for the properties of the character.

face

The face property controls the appearance of the character (see Faces). The value of the property can be the following:

• A face name (a symbol or string).
• An anonymous face: a property list of the form (keyword value ...), where each keyword is a face attribute name and value is a value for that attribute.
• A list of faces. Each list element should be either a face name or an anonymous face. This specifies a face which is an aggregate of the attributes of each of the listed faces. Faces occurring earlier in the list have higher priority.
• A cons cell of the form (foreground-color . color-name) or (background-color . color-name). This specifies the foreground or background color, similar to (:foreground color-name) or (:background color-name). This form is supported for backward compatibility only, and should be avoided.

Font Lock mode (see Font Lock Mode) works in most buffers by dynamically updating the face property of characters based on the context.

The add-face-text-property function provides a convenient way to set this text property. See Changing Properties.

font-lock-face

This property specifies a value for the face property that Font Lock mode should apply to the underlying text. It is one of the fontification methods used by Font Lock mode, and is useful for special modes that implement their own highlighting. See Precalculated Fontification. When Font Lock mode is disabled, font-lock-face has no effect.

mouse-face

This property is used instead of face when the mouse is on or near the character. For this purpose, “near” means that all text between the character and where the mouse is have the same mouse-face property value.

Emacs ignores all face attributes from the mouse-face property that alter the text size (e.g., :height, :weight, and :slant). Those attributes are always the same as for the unhighlighted text.

fontified

This property says whether the text is ready for display. If nil, Emacs's redisplay routine calls the functions in fontification-functions (see Auto Faces) to prepare this part of the buffer before it is displayed. It is used internally by the just-in-time font locking code.

display

This property activates various features that change the way text is displayed. For example, it can make text appear taller or shorter, higher or lower, wider or narrow, or replaced with an image. See Display Property.

help-echo

If text has a string as its help-echo property, then when you move the mouse onto that text, Emacs displays that string in the echo area, or in the tooltip window (see Tooltips).

If the value of the help-echo property is a function, that function is called with three arguments, window, object and pos and should return a help string or nil for none. The first argument, window is the window in which the help was found. The second, object, is the buffer, overlay or string which had the help-echo property. The pos argument is as follows:

• If object is a buffer, pos is the position in the buffer.
• If object is an overlay, that overlay has a help-echo property, and pos is the position in the overlay's buffer.
• If object is a string (an overlay string or a string displayed with the display property), pos is the position in that string.

If the value of the help-echo property is neither a function nor a string, it is evaluated to obtain a help string.

You can alter the way help text is displayed by setting the variable show-help-function (see Help display).

This feature is used in the mode line and for other active text.

keymap

The keymap property specifies an additional keymap for commands. When this keymap applies, it is used for key lookup before the minor mode keymaps and before the buffer's local map. See Active Keymaps. If the property value is a symbol, the symbol's function definition is used as the keymap.

The property's value for the character before point applies if it is non-nil and rear-sticky, and the property's value for the character after point applies if it is non-nil and front-sticky. (For mouse clicks, the position of the click is used instead of the position of point.)

local-map

This property works like keymap except that it specifies a keymap to use instead of the buffer's local map. For most purposes (perhaps all purposes), it is better to use the keymap property.

syntax-table

The syntax-table property overrides what the syntax table says about this particular character. See Syntax Properties.

read-only

If a character has the property read-only, then modifying that character is not allowed. Any command that would do so gets an error, text-read-only. If the property value is a string, that string is used as the error message.

Insertion next to a read-only character is an error if inserting ordinary text there would inherit the read-only property due to stickiness. Thus, you can control permission to insert next to read-only text by controlling the stickiness. See Sticky Properties.

Since changing properties counts as modifying the buffer, it is not possible to remove a read-only property unless you know the special trick: bind inhibit-read-only to a non-nil value and then remove the property. See Read Only Buffers.

inhibit-read-only

Characters that have the property inhibit-read-only can be edited even in read-only buffers. See Read Only Buffers.

invisible

A non-nil invisible property can make a character invisible on the screen. See Invisible Text, for details.

intangible

If a group of consecutive characters have equal and non-nil intangible properties, then you cannot place point between them. If you try to move point forward into the group, point actually moves to the end of the group. If you try to move point backward into the group, point actually moves to the start of the group.

If consecutive characters have unequal non-nil intangible properties, they belong to separate groups; each group is separately treated as described above.

When the variable inhibit-point-motion-hooks is non-nil (as it is by default), the intangible property is ignored.

Beware: this property operates at a very low level, and affects a lot of code in unexpected ways. So use it with extreme caution. A common misuse is to put an intangible property on invisible text, which is actually unnecessary since the command loop will move point outside of the invisible text at the end of each command anyway. See Adjusting Point. For these reasons, this property is obsolete; use the cursor-intangible property instead.

cursor-intangible

When the minor mode cursor-intangible-mode is turned on, point is moved away of any position that has a non-nil cursor-intangible property, just before redisplay happens.

field

Consecutive characters with the same field property constitute a field. Some motion functions including forward-word and beginning-of-line stop moving at a field boundary. See Fields.

cursor

Normally, the cursor is displayed at the beginning or the end of any overlay and text property strings present at the current buffer position. You can place the cursor on any desired character of these strings by giving that character a non-nil cursor text property. In addition, if the value of the cursor property is an integer, it specifies the number of buffer's character positions, starting with the position where the overlay or the display property begins, for which the cursor should be displayed on that character. Specifically, if the value of the cursor property of a character is the number n, the cursor will be displayed on this character for any buffer position in the range [ovpos..ovpos+n), where ovpos is the overlay's starting position given by overlay-start (see Managing Overlays), or the position where the display text property begins in the buffer.

In other words, the string character with the cursor property of any non-nil value is the character where to display the cursor. The value of the property says for which buffer positions to display the cursor there. If the value is an integer n, the cursor is displayed there when point is anywhere between the beginning of the overlay or display property and n positions after that. If the value is anything else and non-nil, the cursor is displayed there only when point is at the beginning of the display property or at overlay-start.

When the buffer has many overlay strings (e.g., see before-string) that conceal some of the buffer text or display properties that are strings, it is a good idea to use the cursor property on these strings to cue the Emacs display about the places where to put the cursor while traversing these strings. This directly communicates to the display engine where the Lisp program wants to put the cursor, or where the user would expect the cursor, when point is located on some buffer position that is “covered” by the display or overlay string.

pointer

This specifies a specific pointer shape when the mouse pointer is over this text or image. See Pointer Shape, for possible pointer shapes.

line-spacing

A newline can have a line-spacing text or overlay property that controls the height of the display line ending with that newline. The property value overrides the default frame line spacing and the buffer local line-spacing variable. See Line Height.

line-height

A newline can have a line-height text or overlay property that controls the total height of the display line ending in that newline. See Line Height.

wrap-prefix

If text has a wrap-prefix property, the prefix it defines will be added at display time to the beginning of every continuation line due to text wrapping (so if lines are truncated, the wrap-prefix is never used). It may be a string or an image (see Other Display Specs), or a stretch of whitespace such as specified by the :width or :align-to display properties (see Specified Space).

A wrap-prefix may also be specified for an entire buffer using the wrap-prefix buffer-local variable (however, a wrap-prefix text-property takes precedence over the value of the wrap-prefix variable). See Truncation.

line-prefix

If text has a line-prefix property, the prefix it defines will be added at display time to the beginning of every non-continuation line. It may be a string or an image (see Other Display Specs), or a stretch of whitespace such as specified by the :width or :align-to display properties (see Specified Space).

A line-prefix may also be specified for an entire buffer using the line-prefix buffer-local variable (however, a line-prefix text-property takes precedence over the value of the line-prefix variable). See Truncation.

modification-hooks

If a character has the property modification-hooks, then its value should be a list of functions; modifying that character calls all of those functions before the actual modification. Each function receives two arguments: the beginning and end of the part of the buffer being modified. Note that if a particular modification hook function appears on several characters being modified by a single primitive, you can't predict how many times the function will be called. Furthermore, insertion will not modify any existing character, so this hook will only be run when removing some characters, replacing them with others, or changing their text-properties.

If these functions modify the buffer, they should bind inhibit-modification-hooks to t around doing so, to avoid confusing the internal mechanism that calls these hooks.

Overlays also support the modification-hooks property, but the details are somewhat different (see Overlay Properties).

insert-in-front-hooks

insert-behind-hooks

The operation of inserting text in a buffer also calls the functions listed in the insert-in-front-hooks property of the following character and in the insert-behind-hooks property of the preceding character. These functions receive two arguments, the beginning and end of the inserted text. The functions are called after the actual insertion takes place.

See also Change Hooks, for other hooks that are called when you change text in a buffer.

point-entered

point-left

The special properties point-entered and point-left record hook functions that report motion of point. Each time point moves, Emacs compares these two property values:

• the point-left property of the character after the old location, and
• the point-entered property of the character after the new location.

If these two values differ, each of them is called (if not nil) with two arguments: the old value of point, and the new one.

The same comparison is made for the characters before the old and new locations. The result may be to execute two point-left functions (which may be the same function) and/or two point-entered functions (which may be the same function). In any case, all the point-left functions are called first, followed by all the point-entered functions.

It is possible to use char-after to examine characters at various buffer positions without moving point to those positions. Only an actual change in the value of point runs these hook functions.

The variable inhibit-point-motion-hooks by default inhibits running the point-left and point-entered hooks, see Inhibit point motion hooks.

These properties are obsolete; please use cursor-sensor-functions instead.

cursor-sensor-functions

This special property records a list of functions that react to cursor motion. Each function in the list is called, just before redisplay, with 3 arguments: the affected window, the previous known position of the cursor, and one of the symbols entered or left, depending on whether the cursor is entering the text that has this property or leaving it. The functions are called only when the minor mode cursor-sensor-mode is turned on.

composition

This text property is used to display a sequence of characters as a single glyph composed from components. But the value of the property itself is completely internal to Emacs and should not be manipulated directly by, for instance, put-text-property.

31.19.5 Formatted Text Properties

These text properties affect the behavior of the fill commands. They are used for representing formatted text. See Filling, and Margins.

hard

If a newline character has this property, it is a “hard” newline. The fill commands do not alter hard newlines and do not move words across them. However, this property takes effect only if the use-hard-newlines minor mode is enabled. See Hard and Soft Newlines.

right-margin

This property specifies an extra right margin for filling this part of the text.

left-margin

This property specifies an extra left margin for filling this part of the text.

justification

This property specifies the style of justification for filling this part of the text.

31.19.6 Stickiness of Text Properties

Self-inserting characters, the ones that get inserted into a buffer when the user types them (see Commands for Insertion), normally take on the same properties as the preceding character. This is called inheritance of properties.

By contrast, a Lisp program can do insertion with inheritance or without, depending on the choice of insertion primitive. The ordinary text insertion functions, such as insert, do not inherit any properties. They insert text with precisely the properties of the string being inserted, and no others. This is correct for programs that copy text from one context to another—for example, into or out of the kill ring. To insert with inheritance, use the special primitives described in this section. Self-inserting characters inherit properties because they work using these primitives.

When you do insertion with inheritance, which properties are inherited, and from where, depends on which properties are sticky. Insertion after a character inherits those of its properties that are rear-sticky. Insertion before a character inherits those of its properties that are front-sticky. When both sides offer different sticky values for the same property, the previous character's value takes precedence.

By default, a text property is rear-sticky but not front-sticky; thus, the default is to inherit all the properties of the preceding character, and nothing from the following character.

You can control the stickiness of various text properties with two specific text properties, front-sticky and rear-nonsticky, and with the variable text-property-default-nonsticky. You can use the variable to specify a different default for a given property. You can use those two text properties to make any specific properties sticky or nonsticky in any particular part of the text.

If a character's front-sticky property is t, then all its properties are front-sticky. If the front-sticky property is a list, then the sticky properties of the character are those whose names are in the list. For example, if a character has a front-sticky property whose value is (face read-only), then insertion before the character can inherit its face property and its read-only property, but no others.

The rear-nonsticky property works the opposite way. Most properties are rear-sticky by default, so the rear-nonsticky property says which properties are not rear-sticky. If a character's rear-nonsticky property is t, then none of its properties are rear-sticky. If the rear-nonsticky property is a list, properties are rear-sticky unless their names are in the list.

Here are the functions that insert text with inheritance of properties:

See Insertion, for the ordinary insertion functions which do not inherit.

31.19.7 Lazy Computation of Text Properties

Instead of computing text properties for all the text in the buffer, you can arrange to compute the text properties for parts of the text when and if something depends on them.

The primitive that extracts text from the buffer along with its properties is buffer-substring. Before examining the properties, this function runs the abnormal hook buffer-access-fontify-functions.

The function buffer-substring-no-properties does not call these functions, since it ignores text properties anyway.

In order to prevent the hook functions from being called more than once for the same part of the buffer, you can use the variable buffer-access-fontified-property.

31.19.8 Defining Clickable Text

Clickable text is text that can be clicked, with either the mouse or via a keyboard command, to produce some result. Many major modes use clickable text to implement textual hyper-links, or links for short.

The easiest way to insert and manipulate links is to use the button package. See Buttons. In this section, we will explain how to manually set up clickable text in a buffer, using text properties. For simplicity, we will refer to the clickable text as a link.

Implementing a link involves three separate steps: (1) indicating clickability when the mouse moves over the link; (2) making <RET> or mouse-2 on that link do something; and (3) setting up a follow-link condition so that the link obeys mouse-1-click-follows-link.

To indicate clickability, add the mouse-face text property to the text of the link; then Emacs will highlight the link when the mouse moves over it. In addition, you should define a tooltip or echo area message, using the help-echo text property. See Special Properties. For instance, here is how Dired indicates that file names are clickable:

      (if (dired-move-to-filename)
          (add-text-properties
            (point)
            (save-excursion
              (dired-move-to-end-of-filename)
              (point))
            '(mouse-face highlight
              help-echo "mouse-2: visit this file in other window")))

To make the link clickable, bind <RET> and mouse-2 to commands that perform the desired action. Each command should check to see whether it was called on a link, and act accordingly. For instance, Dired's major mode keymap binds mouse-2 to the following command:

     (defun dired-mouse-find-file-other-window (event)
       "In Dired, visit the file or directory name you click on."
       (interactive "e")
       (let ((window (posn-window (event-end event)))
             (pos (posn-point (event-end event)))
             file)
         (if (not (windowp window))
             (error "No file chosen"))
         (with-current-buffer (window-buffer window)
           (goto-char pos)
           (setq file (dired-get-file-for-visit)))
         (if (file-directory-p file)
             (or (and (cdr dired-subdir-alist)
                      (dired-goto-subdir file))
                 (progn
                   (select-window window)
                   (dired-other-window file)))
           (select-window window)
           (find-file-other-window (file-name-sans-versions file t)))))

This command uses the functions posn-window and posn-point to determine where the click occurred, and dired-get-file-for-visit to determine which file to visit.

Instead of binding the mouse command in a major mode keymap, you can bind it within the link text, using the keymap text property (see Special Properties). For instance:

     (let ((map (make-sparse-keymap)))
       (define-key map [mouse-2] 'operate-this-button)
       (put-text-property link-start link-end 'keymap map))

With this method, you can easily define different commands for different links. Furthermore, the global definition of <RET> and mouse-2 remain available for the rest of the text in the buffer.

The basic Emacs command for clicking on links is mouse-2. However, for compatibility with other graphical applications, Emacs also recognizes mouse-1 clicks on links, provided the user clicks on the link quickly without moving the mouse. This behavior is controlled by the user option mouse-1-click-follows-link. See Mouse References.

To set up the link so that it obeys mouse-1-click-follows-link, you must either (1) apply a follow-link text or overlay property to the link text, or (2) bind the follow-link event to a keymap (which can be a major mode keymap or a local keymap specified via the keymap text property). The value of the follow-link property, or the binding for the follow-link event, acts as a condition for the link action. This condition tells Emacs two things: the circumstances under which a mouse-1 click should be regarded as occurring inside the link, and how to compute an action code that says what to translate the mouse-1 click into. The link action condition can be one of the following:

mouse-face

If the condition is the symbol mouse-face, a position is inside a link if there is a non-nil mouse-face property at that position. The action code is always t.

For example, here is how Info mode handles <mouse-1>:

          (define-key Info-mode-map [follow-link] 'mouse-face)

a function

If the condition is a function, func, then a position pos is inside a link if (func pos) evaluates to non-nil. The value returned by func serves as the action code.

For example, here is how pcvs enables mouse-1 to follow links on file names only:

          (define-key map [follow-link]
            (lambda (pos)
              (eq (get-char-property pos 'face) 'cvs-filename-face)))

anything else

If the condition value is anything else, then the position is inside a link and the condition itself is the action code. Clearly, you should specify this kind of condition only when applying the condition via a text or property overlay on the link text (so that it does not apply to the entire buffer).

The action code tells mouse-1 how to follow the link:

a string or vector

If the action code is a string or vector, the mouse-1 event is translated into the first element of the string or vector; i.e., the action of the mouse-1 click is the local or global binding of that character or symbol. Thus, if the action code is "foo", mouse-1 translates into f. If it is [foo], mouse-1 translates into <foo>.

anything else

For any other non-nil action code, the mouse-1 event is translated into a mouse-2 event at the same position.

To define mouse-1 to activate a button defined with define-button-type, give the button a follow-link property. The property value should be a link action condition, as described above. See Buttons. For example, here is how Help mode handles mouse-1:

     (define-button-type 'help-xref
       'follow-link t
       'action #'help-button-action)

To define mouse-1 on a widget defined with define-widget, give the widget a :follow-link property. The property value should be a link action condition, as described above. For example, here is how the link widget specifies that a <mouse-1> click shall be translated to <RET>:

     (define-widget 'link 'item
       "An embedded link."
       :button-prefix 'widget-link-prefix
       :button-suffix 'widget-link-suffix
       :follow-link "\C-m"
       :help-echo "Follow the link."
       :format "%[%t%]")

31.19.9 Defining and Using Fields

A field is a range of consecutive characters in the buffer that are identified by having the same value (comparing with eq) of the field property (either a text-property or an overlay property). This section describes special functions that are available for operating on fields.

You specify a field with a buffer position, pos. We think of each field as containing a range of buffer positions, so the position you specify stands for the field containing that position.

When the characters before and after pos are part of the same field, there is no doubt which field contains pos: the one those characters both belong to. When pos is at a boundary between fields, which field it belongs to depends on the stickiness of the field properties of the two surrounding characters (see Sticky Properties). The field whose property would be inherited by text inserted at pos is the field that contains pos.

There is an anomalous case where newly inserted text at pos would not inherit the field property from either side. This happens if the previous character's field property is not rear-sticky, and the following character's field property is not front-sticky. In this case, pos belongs to neither the preceding field nor the following field; the field functions treat it as belonging to an empty field whose beginning and end are both at pos.

In all of these functions, if pos is omitted or nil, the value of point is used by default. If narrowing is in effect, then pos should fall within the accessible portion. See Narrowing.

31.19.10 Why Text Properties are not Intervals

Some editors that support adding attributes to text in the buffer do so by letting the user specify intervals within the text, and adding the properties to the intervals. Those editors permit the user or the programmer to determine where individual intervals start and end. We deliberately provided a different sort of interface in Emacs Lisp to avoid certain paradoxical behavior associated with text modification.

If the actual subdivision into intervals is meaningful, that means you can distinguish between a buffer that is just one interval with a certain property, and a buffer containing the same text subdivided into two intervals, both of which have that property.

Suppose you take the buffer with just one interval and kill part of the text. The text remaining in the buffer is one interval, and the copy in the kill ring (and the undo list) becomes a separate interval. Then if you yank back the killed text, you get two intervals with the same properties. Thus, editing does not preserve the distinction between one interval and two.

Suppose we attempt to fix this problem by coalescing the two intervals when the text is inserted. That works fine if the buffer originally was a single interval. But suppose instead that we have two adjacent intervals with the same properties, and we kill the text of one interval and yank it back. The same interval-coalescence feature that rescues the other case causes trouble in this one: after yanking, we have just one interval. Once again, editing does not preserve the distinction between one interval and two.

Insertion of text at the border between intervals also raises questions that have no satisfactory answer.

However, it is easy to arrange for editing to behave consistently for questions of the form, “What are the properties of text at this buffer or string position?” So we have decided these are the only questions that make sense; we have not implemented asking questions about where intervals start or end.

In practice, you can usually use the text property search functions in place of explicit interval boundaries. You can think of them as finding the boundaries of intervals, assuming that intervals are always coalesced whenever possible. See Property Search.

Emacs also provides explicit intervals as a presentation feature; see Overlays.

31.20 Substituting for a Character Code

The following functions replace characters within a specified region based on their character codes.

31.21 Registers

A register is a sort of variable used in Emacs editing that can hold a variety of different kinds of values. Each register is named by a single character. All ASCII characters and their meta variants (but with the exception of C-g) can be used to name registers. Thus, there are 255 possible registers. A register is designated in Emacs Lisp by the character that is its name.

The contents of a register can have several possible types:

a number

A number stands for itself. If insert-register finds a number in the register, it converts the number to decimal.

a marker

A marker represents a buffer position to jump to.

a string

A string is text saved in the register.

a rectangle

A rectangle is represented by a list of strings.

(window-configuration position)

This represents a window configuration to restore in one frame, and a position to jump to in the current buffer.

(frame-configuration position)

This represents a frame configuration to restore, and a position to jump to in the current buffer.

(file filename)

This represents a file to visit; jumping to this value visits file filename.

(file-query filename position)

This represents a file to visit and a position in it; jumping to this value visits file filename and goes to buffer position position. Restoring this type of position asks the user for confirmation first.

The functions in this section return unpredictable values unless otherwise stated.

31.22 Transposition of Text

This function can be used to transpose stretches of text:

31.23 Dealing With Compressed Data

When auto-compression-mode is enabled, Emacs automatically uncompresses compressed files when you visit them, and automatically recompresses them if you alter and save them. See Compressed Files.

The above feature works by calling an external executable (e.g., gzip). Emacs can also be compiled with support for built-in decompression using the zlib library, which is faster than calling an external program.

31.24 Base 64 Encoding

Base 64 code is used in email to encode a sequence of 8-bit bytes as a longer sequence of ASCII graphic characters. It is defined in Internet RFC¹⁴2045. This section describes the functions for converting to and from this code.

31.25 Checksum/Hash

Emacs has built-in support for computing cryptographic hashes. A cryptographic hash, or checksum, is a digital fingerprint of a piece of data (e.g., a block of text) which can be used to check that you have an unaltered copy of that data.

Emacs supports several common cryptographic hash algorithms: MD5, SHA-1, SHA-2, SHA-224, SHA-256, SHA-384 and SHA-512. MD5 is the oldest of these algorithms, and is commonly used in message digests to check the integrity of messages transmitted over a network. MD5 is not collision resistant (i.e., it is possible to deliberately design different pieces of data which have the same MD5 hash), so you should not used it for anything security-related. A similar theoretical weakness also exists in SHA-1. Therefore, for security-related applications you should use the other hash types, such as SHA-2.

31.26 Parsing HTML and XML

When Emacs is compiled with libxml2 support, the following functions are available to parse HTML or XML text into Lisp object trees.

• Document Object Model: Access, manipulate and search the DOM.

31.26.1 Document Object Model

The DOM returned by libxml-parse-html-region (and the other XML parsing functions) is a tree structure where each node has a node name (called a tag), and optional key/value attribute list, and then a list of child nodes. The child nodes are either strings or DOM objects.

     (body ((width . "101"))
      (div ((class . "thing"))
       "Foo"
       (div nil
        "Yes")))

The following functions can be used to work with this structure. Each function takes a DOM node, or a list of nodes. In the latter case, only the first node in the list is used.

Simple accessors:

dom-tag node

Return the tag (also called “node name”) of the node.

dom-attr node attribute

Return the value of attribute in the node. A common usage would be:

          (dom-attr img 'href)
          => "http://fsf.org/logo.png"

dom-children node

Return all the children of the node.

dom-non-text-children node

Return all the non-string children of the node.

dom-attributes node

Return the key/value pair list of attributes of the node.

dom-text node

Return all the textual elements of the node as a concatenated string.

dom-texts node

Return all the textual elements of the node, as well as the textual elements of all the children of the node, recursively, as a concatenated string. This function also takes an optional separator to be inserted between the textual elements.

dom-parent dom node

Return the parent of node in dom.

The following are functions for altering the DOM.

dom-set-attribute node attribute value

Set the attribute of the node to value.

dom-append-child node child

Append child as the last child of node.

dom-add-child-before node child before

Add child to node's child list before the before node. If before is nil, make child the first child.

dom-set-attributes node attributes

Replace all the attributes of the node with a new key/value list.

The following are functions for searching for elements in the DOM. They all return lists of matching nodes.

dom-by-tag dom tag

Return all nodes in dom that are of type tag. A typical use would be:

          (dom-by-tag dom 'td)
          => '((td ...) (td ...) (td ...))

dom-by-class dom match

Return all nodes in dom that have class names that match match, which is a regular expression.

dom-by-style dom style

Return all nodes in dom that have styles that match match, which is a regular expression.

dom-by-id dom style

Return all nodes in dom that have IDs that match match, which is a regular expression.

dom-strings dom

Return all strings in dom.

Utility functions:

dom-pp dom &optional remove-empty

Pretty-print dom at point. If remove-empty, don't print textual nodes that just contain white-space.

31.27 Atomic Change Groups

In database terminology, an atomic change is an indivisible change—it can succeed entirely or it can fail entirely, but it cannot partly succeed. A Lisp program can make a series of changes to one or several buffers as an atomic change group, meaning that either the entire series of changes will be installed in their buffers or, in case of an error, none of them will be.

To do this for one buffer, the one already current, simply write a call to atomic-change-group around the code that makes the changes, like this:

     (atomic-change-group
       (insert foo)
       (delete-region x y))

If an error (or other nonlocal exit) occurs inside the body of atomic-change-group, it unmakes all the changes in that buffer that were during the execution of the body. This kind of change group has no effect on any other buffers—any such changes remain.

If you need something more sophisticated, such as to make changes in various buffers constitute one atomic group, you must directly call lower-level functions that atomic-change-group uses.

To use the change group, you must activate it. You must do this before making any changes in the text of buffer.

After you activate the change group, any changes you make in that buffer become part of it. Once you have made all the desired changes in the buffer, you must finish the change group. There are two ways to do this: you can either accept (and finalize) all the changes, or cancel them all.

Your code should use unwind-protect to make sure the group is always finished. The call to activate-change-group should be inside the unwind-protect, in case the user types C-g just after it runs. (This is one reason why prepare-change-group and activate-change-group are separate functions, because normally you would call prepare-change-group before the start of that unwind-protect.) Once you finish the group, don't use the handle again—in particular, don't try to finish the same group twice.

To make a multibuffer change group, call prepare-change-group once for each buffer you want to cover, then use nconc to combine the returned values, like this:

     (nconc (prepare-change-group buffer-1)
            (prepare-change-group buffer-2))

You can then activate the multibuffer change group with a single call to activate-change-group, and finish it with a single call to accept-change-group or cancel-change-group.

Nested use of several change groups for the same buffer works as you would expect. Non-nested use of change groups for the same buffer will get Emacs confused, so don't let it happen; the first change group you start for any given buffer should be the last one finished.

31.28 Change Hooks

These hook variables let you arrange to take notice of changes in buffers (or in a particular buffer, if you make them buffer-local). See also Special Properties, for how to detect changes to specific parts of the text.

The functions you use in these hooks should save and restore the match data if they do anything that uses regular expressions; otherwise, they will interfere in bizarre ways with the editing operations that call them.

Output of messages into the *Messages* buffer does not call these functions, and neither do certain internal buffer changes, such as changes in buffers created by Emacs internally for certain jobs, that should not be visible to Lisp programs.

Do not expect the before-change hooks and the after-change hooks be called in balanced pairs around each buffer change. Also don't expect the before-change hooks to be called for every chunk of text Emacs is about to delete. These hooks are provided on the assumption that Lisp programs will use either before- or the after-change hooks, but not both, and the boundaries of the region where the changes happen might include more than just the actual changed text, or even lump together several changes done piecemeal.

32 Non-ASCII Characters

This chapter covers the special issues relating to characters and how they are stored in strings and buffers.

• Text Representations: How Emacs represents text.
• Disabling Multibyte: Controlling whether to use multibyte characters.
• Converting Representations: Converting unibyte to multibyte and vice versa.
• Selecting a Representation: Treating a byte sequence as unibyte or multi.
• Character Codes: How unibyte and multibyte relate to codes of individual characters.
• Character Properties: Character attributes that define their behavior and handling.
• Character Sets: The space of possible character codes is divided into various character sets.
• Scanning Charsets: Which character sets are used in a buffer?
• Translation of Characters: Translation tables are used for conversion.
• Coding Systems: Coding systems are conversions for saving files.
• Input Methods: Input methods allow users to enter various non-ASCII characters without special keyboards.
• Locales: Interacting with the POSIX locale.

32.1 Text Representations

Emacs buffers and strings support a large repertoire of characters from many different scripts, allowing users to type and display text in almost any known written language.

To support this multitude of characters and scripts, Emacs closely follows the Unicode Standard. The Unicode Standard assigns a unique number, called a codepoint, to each and every character. The range of codepoints defined by Unicode, or the Unicode codespace, is 0..#x10FFFF (in hexadecimal notation), inclusive. Emacs extends this range with codepoints in the range #x110000..#x3FFFFF, which it uses for representing characters that are not unified with Unicode and raw 8-bit bytes that cannot be interpreted as characters. Thus, a character codepoint in Emacs is a 22-bit integer.

To conserve memory, Emacs does not hold fixed-length 22-bit numbers that are codepoints of text characters within buffers and strings. Rather, Emacs uses a variable-length internal representation of characters, that stores each character as a sequence of 1 to 5 8-bit bytes, depending on the magnitude of its codepoint¹⁵. For example, any ASCII character takes up only 1 byte, a Latin-1 character takes up 2 bytes, etc. We call this representation of text multibyte.

Outside Emacs, characters can be represented in many different encodings, such as ISO-8859-1, GB-2312, Big-5, etc. Emacs converts between these external encodings and its internal representation, as appropriate, when it reads text into a buffer or a string, or when it writes text to a disk file or passes it to some other process.

Occasionally, Emacs needs to hold and manipulate encoded text or binary non-text data in its buffers or strings. For example, when Emacs visits a file, it first reads the file's text verbatim into a buffer, and only then converts it to the internal representation. Before the conversion, the buffer holds encoded text.

Encoded text is not really text, as far as Emacs is concerned, but rather a sequence of raw 8-bit bytes. We call buffers and strings that hold encoded text unibyte buffers and strings, because Emacs treats them as a sequence of individual bytes. Usually, Emacs displays unibyte buffers and strings as octal codes such as \237. We recommend that you never use unibyte buffers and strings except for manipulating encoded text or binary non-text data.

In a buffer, the buffer-local value of the variable enable-multibyte-characters specifies the representation used. The representation for a string is determined and recorded in the string when the string is constructed.

The following two functions are useful when a Lisp program needs to map buffer positions to byte offsets in a file visited by the buffer.

32.2 Disabling Multibyte Characters

By default, Emacs starts in multibyte mode: it stores the contents of buffers and strings using an internal encoding that represents non-ASCII characters using multi-byte sequences. Multibyte mode allows you to use all the supported languages and scripts without limitations.

Under very special circumstances, you may want to disable multibyte character support, for a specific buffer. When multibyte characters are disabled in a buffer, we call that unibyte mode. In unibyte mode, each character in the buffer has a character code ranging from 0 through 255 (0377 octal); 0 through 127 (0177 octal) represent ASCII characters, and 128 (0200 octal) through 255 (0377 octal) represent non-ASCII characters.

To edit a particular file in unibyte representation, visit it using find-file-literally. See Visiting Functions. You can convert a multibyte buffer to unibyte by saving it to a file, killing the buffer, and visiting the file again with find-file-literally. Alternatively, you can use C-x <RET> c (universal-coding-system-argument) and specify ‘raw-text’ as the coding system with which to visit or save a file. See Specifying a Coding System for File Text. Unlike find-file-literally, finding a file as ‘raw-text’ doesn't disable format conversion, uncompression, or auto mode selection.

The buffer-local variable enable-multibyte-characters is non-nil in multibyte buffers, and nil in unibyte ones. The mode line also indicates whether a buffer is multibyte or not. With a graphical display, in a multibyte buffer, the portion of the mode line that indicates the character set has a tooltip that (amongst other things) says that the buffer is multibyte. In a unibyte buffer, the character set indicator is absent. Thus, in a unibyte buffer (when using a graphical display) there is normally nothing before the indication of the visited file's end-of-line convention (colon, backslash, etc.), unless you are using an input method.

You can turn off multibyte support in a specific buffer by invoking the command toggle-enable-multibyte-characters in that buffer.

32.3 Converting Text Representations

Emacs can convert unibyte text to multibyte; it can also convert multibyte text to unibyte, provided that the multibyte text contains only ASCII and 8-bit raw bytes. In general, these conversions happen when inserting text into a buffer, or when putting text from several strings together in one string. You can also explicitly convert a string's contents to either representation.

Emacs chooses the representation for a string based on the text from which it is constructed. The general rule is to convert unibyte text to multibyte text when combining it with other multibyte text, because the multibyte representation is more general and can hold whatever characters the unibyte text has.

When inserting text into a buffer, Emacs converts the text to the buffer's representation, as specified by enable-multibyte-characters in that buffer. In particular, when you insert multibyte text into a unibyte buffer, Emacs converts the text to unibyte, even though this conversion cannot in general preserve all the characters that might be in the multibyte text. The other natural alternative, to convert the buffer contents to multibyte, is not acceptable because the buffer's representation is a choice made by the user that cannot be overridden automatically.

Converting unibyte text to multibyte text leaves ASCII characters unchanged, and converts bytes with codes 128 through 255 to the multibyte representation of raw eight-bit bytes.

Converting multibyte text to unibyte converts all ASCII and eight-bit characters to their single-byte form, but loses information for non-ASCII characters by discarding all but the low 8 bits of each character's codepoint. Converting unibyte text to multibyte and back to unibyte reproduces the original unibyte text.

The next two functions either return the argument string, or a newly created string with no text properties.

32.4 Selecting a Representation

Sometimes it is useful to examine an existing buffer or string as multibyte when it was unibyte, or vice versa.

32.5 Character Codes

The unibyte and multibyte text representations use different character codes. The valid character codes for unibyte representation range from 0 to #xFF (255)—the values that can fit in one byte. The valid character codes for multibyte representation range from 0 to #x3FFFFF. In this code space, values 0 through #x7F (127) are for ASCII characters, and values #x80 (128) through #x3FFF7F (4194175) are for non-ASCII characters.

Emacs character codes are a superset of the Unicode standard. Values 0 through #x10FFFF (1114111) correspond to Unicode characters of the same codepoint; values #x110000 (1114112) through #x3FFF7F (4194175) represent characters that are not unified with Unicode; and values #x3FFF80 (4194176) through #x3FFFFF (4194303) represent eight-bit raw bytes.

32.6 Character Properties

A character property is a named attribute of a character that specifies how the character behaves and how it should be handled during text processing and display. Thus, character properties are an important part of specifying the character's semantics.

On the whole, Emacs follows the Unicode Standard in its implementation of character properties. In particular, Emacs supports the Unicode Character Property Model, and the Emacs character property database is derived from the Unicode Character Database (UCD). See the Character Properties chapter of the Unicode Standard, for a detailed description of Unicode character properties and their meaning. This section assumes you are already familiar with that chapter of the Unicode Standard, and want to apply that knowledge to Emacs Lisp programs.

In Emacs, each property has a name, which is a symbol, and a set of possible values, whose types depend on the property; if a character does not have a certain property, the value is nil. As a general rule, the names of character properties in Emacs are produced from the corresponding Unicode properties by downcasing them and replacing each ‘_’ character with a dash ‘-’. For example, Canonical_Combining_Class becomes canonical-combining-class. However, sometimes we shorten the names to make their use easier.

Some codepoints are left unassigned by the UCD—they don't correspond to any character. The Unicode Standard defines default values of properties for such codepoints; they are mentioned below for each property.

Here is the full list of value types for all the character properties that Emacs knows about:

name

Corresponds to the Name Unicode property. The value is a string consisting of upper-case Latin letters A to Z, digits, spaces, and hyphen ‘-’ characters. For unassigned codepoints, the value is nil.

general-category

Corresponds to the General_Category Unicode property. The value is a symbol whose name is a 2-letter abbreviation of the character's classification. For unassigned codepoints, the value is Cn.

canonical-combining-class

Corresponds to the Canonical_Combining_Class Unicode property. The value is an integer. For unassigned codepoints, the value is zero.

bidi-class

Corresponds to the Unicode Bidi_Class property. The value is a symbol whose name is the Unicode directional type of the character. Emacs uses this property when it reorders bidirectional text for display (see Bidirectional Display). For unassigned codepoints, the value depends on the code blocks to which the codepoint belongs: most unassigned codepoints get the value of L (strong L), but some get values of AL (Arabic letter) or R (strong R).

decomposition

Corresponds to the Unicode properties Decomposition_Type and Decomposition_Value. The value is a list, whose first element may be a symbol representing a compatibility formatting tag, such as small¹⁶; the other elements are characters that give the compatibility decomposition sequence of this character. For characters that don't have decomposition sequences, and for unassigned codepoints, the value is a list with a single member, the character itself.

decimal-digit-value

Corresponds to the Unicode Numeric_Value property for characters whose Numeric_Type is ‘Decimal’. The value is an integer, or nil if the character has no decimal digit value. For unassigned codepoints, the value is nil, which means NaN, or “not a number”.

digit-value

Corresponds to the Unicode Numeric_Value property for characters whose Numeric_Type is ‘Digit’. The value is an integer. Examples of such characters include compatibility subscript and superscript digits, for which the value is the corresponding number. For characters that don't have any numeric value, and for unassigned codepoints, the value is nil, which means NaN.

numeric-value

Corresponds to the Unicode Numeric_Value property for characters whose Numeric_Type is ‘Numeric’. The value of this property is a number. Examples of characters that have this property include fractions, subscripts, superscripts, Roman numerals, currency numerators, and encircled numbers. For example, the value of this property for the character U+2155 (vulgar fraction one fifth) is 0.2. For characters that don't have any numeric value, and for unassigned codepoints, the value is nil, which means NaN.

mirrored

Corresponds to the Unicode Bidi_Mirrored property. The value of this property is a symbol, either Y or N. For unassigned codepoints, the value is N.

mirroring

Corresponds to the Unicode Bidi_Mirroring_Glyph property. The value of this property is a character whose glyph represents the mirror image of the character's glyph, or nil if there's no defined mirroring glyph. All the characters whose mirrored property is N have nil as their mirroring property; however, some characters whose mirrored property is Y also have nil for mirroring, because no appropriate characters exist with mirrored glyphs. Emacs uses this property to display mirror images of characters when appropriate (see Bidirectional Display). For unassigned codepoints, the value is nil.

paired-bracket

Corresponds to the Unicode Bidi_Paired_Bracket property. The value of this property is the codepoint of a character's paired bracket, or nil if the character is not a bracket character. This establishes a mapping between characters that are treated as bracket pairs by the Unicode Bidirectional Algorithm; Emacs uses this property when it decides how to reorder for display parentheses, braces, and other similar characters (see Bidirectional Display).

bracket-type

Corresponds to the Unicode Bidi_Paired_Bracket_Type property. For characters whose paired-bracket property is non-nil, the value of this property is a symbol, either o (for opening bracket characters) or c (for closing bracket characters). For characters whose paired-bracket property is nil, the value is the symbol n (None). Like paired-bracket, this property is used for bidirectional display.

old-name

Corresponds to the Unicode Unicode_1_Name property. The value is a string. For unassigned codepoints, and characters that have no value for this property, the value is nil.

iso-10646-comment

Corresponds to the Unicode ISO_Comment property. The value is either a string or nil. For unassigned codepoints, the value is nil.

uppercase

Corresponds to the Unicode Simple_Uppercase_Mapping property. The value of this property is a single character. For unassigned codepoints, the value is nil, which means the character itself.

lowercase

Corresponds to the Unicode Simple_Lowercase_Mapping property. The value of this property is a single character. For unassigned codepoints, the value is nil, which means the character itself.

titlecase

Corresponds to the Unicode Simple_Titlecase_Mapping property. Title case is a special form of a character used when the first character of a word needs to be capitalized. The value of this property is a single character. For unassigned codepoints, the value is nil, which means the character itself.

32.7 Character Sets

An Emacs character set, or charset, is a set of characters in which each character is assigned a numeric code point. (The Unicode Standard calls this a coded character set.) Each Emacs charset has a name which is a symbol. A single character can belong to any number of different character sets, but it will generally have a different code point in each charset. Examples of character sets include ascii, iso-8859-1, greek-iso8859-7, and windows-1255. The code point assigned to a character in a charset is usually different from its code point used in Emacs buffers and strings.

Emacs defines several special character sets. The character set unicode includes all the characters whose Emacs code points are in the range 0..#x10FFFF. The character set emacs includes all ASCII and non-ASCII characters. Finally, the eight-bit charset includes the 8-bit raw bytes; Emacs uses it to represent raw bytes encountered in text.

Emacs can convert between its internal representation of a character and the character's codepoint in a specific charset. The following two functions support these conversions.

The following function comes in handy for applying a certain function to all or part of the characters in a charset:

32.8 Scanning for Character Sets

Sometimes it is useful to find out which character set a particular character belongs to. One use for this is in determining which coding systems (see Coding Systems) are capable of representing all of the text in question; another is to determine the font(s) for displaying that text.

32.9 Translation of Characters

A translation table is a char-table (see Char-Tables) that specifies a mapping of characters into characters. These tables are used in encoding and decoding, and for other purposes. Some coding systems specify their own particular translation tables; there are also default translation tables which apply to all other coding systems.

A translation table has two extra slots. The first is either nil or a translation table that performs the reverse translation; the second is the maximum number of characters to look up for translating sequences of characters (see the description of make-translation-table-from-alist below).

During decoding, the translation table's translations are applied to the characters that result from ordinary decoding. If a coding system has the property :decode-translation-table, that specifies the translation table to use, or a list of translation tables to apply in sequence. (This is a property of the coding system, as returned by coding-system-get, not a property of the symbol that is the coding system's name. See Basic Concepts of Coding Systems.) Finally, if standard-translation-table-for-decode is non-nil, the resulting characters are translated by that table.

During encoding, the translation table's translations are applied to the characters in the buffer, and the result of translation is actually encoded. If a coding system has property :encode-translation-table, that specifies the translation table to use, or a list of translation tables to apply in sequence. In addition, if the variable standard-translation-table-for-encode is non-nil, it specifies the translation table to use for translating the result.

32.10 Coding Systems

When Emacs reads or writes a file, and when Emacs sends text to a subprocess or receives text from a subprocess, it normally performs character code conversion and end-of-line conversion as specified by a particular coding system.

How to define a coding system is an arcane matter, and is not documented here.

• Coding System Basics: Basic concepts.
• Encoding and I/O: How file I/O functions handle coding systems.
• Lisp and Coding Systems: Functions to operate on coding system names.
• User-Chosen Coding Systems: Asking the user to choose a coding system.
• Default Coding Systems: Controlling the default choices.
• Specifying Coding Systems: Requesting a particular coding system for a single file operation.
• Explicit Encoding: Encoding or decoding text without doing I/O.
• Terminal I/O Encoding: Use of encoding for terminal I/O.

32.10.1 Basic Concepts of Coding Systems

Character code conversion involves conversion between the internal representation of characters used inside Emacs and some other encoding. Emacs supports many different encodings, in that it can convert to and from them. For example, it can convert text to or from encodings such as Latin 1, Latin 2, Latin 3, Latin 4, Latin 5, and several variants of ISO 2022. In some cases, Emacs supports several alternative encodings for the same characters; for example, there are three coding systems for the Cyrillic (Russian) alphabet: ISO, Alternativnyj, and KOI8.

Every coding system specifies a particular set of character code conversions, but the coding system undecided is special: it leaves the choice unspecified, to be chosen heuristically for each file, based on the file's data. The coding system prefer-utf-8 is like undecided, but it prefers to choose utf-8 when possible.

In general, a coding system doesn't guarantee roundtrip identity: decoding a byte sequence using coding system, then encoding the resulting text in the same coding system, can produce a different byte sequence. But some coding systems do guarantee that the byte sequence will be the same as what you originally decoded. Here are a few examples:

iso-8859-1, utf-8, big5, shift_jis, euc-jp

Encoding buffer text and then decoding the result can also fail to reproduce the original text. For instance, if you encode a character with a coding system which does not support that character, the result is unpredictable, and thus decoding it using the same coding system may produce a different text. Currently, Emacs can't report errors that result from encoding unsupported characters.

End of line conversion handles three different conventions used on various systems for representing end of line in files. The Unix convention, used on GNU and Unix systems, is to use the linefeed character (also called newline). The DOS convention, used on MS-Windows and MS-DOS systems, is to use a carriage-return and a linefeed at the end of a line. The Mac convention is to use just carriage-return. (This was the convention used in Classic Mac OS.)

Base coding systems such as latin-1 leave the end-of-line conversion unspecified, to be chosen based on the data. Variant coding systems such as latin-1-unix, latin-1-dos and latin-1-mac specify the end-of-line conversion explicitly as well. Most base coding systems have three corresponding variants whose names are formed by adding ‘-unix’, ‘-dos’ and ‘-mac’.

The coding system raw-text is special in that it prevents character code conversion, and causes the buffer visited with this coding system to be a unibyte buffer. For historical reasons, you can save both unibyte and multibyte text with this coding system. When you use raw-text to encode multibyte text, it does perform one character code conversion: it converts eight-bit characters to their single-byte external representation. raw-text does not specify the end-of-line conversion, allowing that to be determined as usual by the data, and has the usual three variants which specify the end-of-line conversion.

no-conversion (and its alias binary) is equivalent to raw-text-unix: it specifies no conversion of either character codes or end-of-line.

The coding system utf-8-emacs specifies that the data is represented in the internal Emacs encoding (see Text Representations). This is like raw-text in that no code conversion happens, but different in that the result is multibyte data. The name emacs-internal is an alias for utf-8-emacs.

32.10.2 Encoding and I/O

The principal purpose of coding systems is for use in reading and writing files. The function insert-file-contents uses a coding system to decode the file data, and write-region uses one to encode the buffer contents.

You can specify the coding system to use either explicitly (see Specifying Coding Systems), or implicitly using a default mechanism (see Default Coding Systems). But these methods may not completely specify what to do. For example, they may choose a coding system such as undecided which leaves the character code conversion to be determined from the data. In these cases, the I/O operation finishes the job of choosing a coding system. Very often you will want to find out afterwards which coding system was chosen.

The variable selection-coding-system specifies how to encode selections for the window system. See Window System Selections.

Warning: if you change file-name-coding-system (or the language environment) in the middle of an Emacs session, problems can result if you have already visited files whose names were encoded using the earlier coding system and are handled differently under the new coding system. If you try to save one of these buffers under the visited file name, saving may use the wrong file name, or it may get an error. If such a problem happens, use C-x C-w to specify a new file name for that buffer.

On Windows 2000 and later, Emacs by default uses Unicode APIs to pass file names to the OS, so the value of file-name-coding-system is largely ignored. Lisp applications that need to encode or decode file names on the Lisp level should use utf-8 coding-system when system-type is windows-nt; the conversion of UTF-8 encoded file names to the encoding appropriate for communicating with the OS is performed internally by Emacs.

32.10.3 Coding Systems in Lisp

Here are the Lisp facilities for working with coding systems:

See Process Information, in particular the description of the functions process-coding-system and set-process-coding-system, for how to examine or set the coding systems used for I/O to a subprocess.

32.10.4 User-Chosen Coding Systems

Here are two functions you can use to let the user specify a coding system, with completion. See Completion.

32.10.5 Default Coding Systems

This section describes variables that specify the default coding system for certain files or when running certain subprograms, and the function that I/O operations use to access them.

The idea of these variables is that you set them once and for all to the defaults you want, and then do not change them again. To specify a particular coding system for a particular operation in a Lisp program, don't change these variables; instead, override them using coding-system-for-read and coding-system-for-write (see Specifying Coding Systems).

Warning: Coding systems such as undecided, which determine the coding system from the data, do not work entirely reliably with asynchronous subprocess output. This is because Emacs handles asynchronous subprocess output in batches, as it arrives. If the coding system leaves the character code conversion unspecified, or leaves the end-of-line conversion unspecified, Emacs must try to detect the proper conversion from one batch at a time, and this does not always work.

Therefore, with an asynchronous subprocess, if at all possible, use a coding system which determines both the character code conversion and the end of line conversion—that is, one like latin-1-unix, rather than undecided or latin-1.

32.10.6 Specifying a Coding System for One Operation

You can specify the coding system for a specific operation by binding the variables coding-system-for-read and/or coding-system-for-write.

Sometimes, you need to prefer several coding systems for some operation, rather than fix a single one. Emacs lets you specify a priority order for using coding systems. This ordering affects the sorting of lists of coding systems returned by functions such as find-coding-systems-region (see Lisp and Coding Systems).

32.10.7 Explicit Encoding and Decoding

All the operations that transfer text in and out of Emacs have the ability to use a coding system to encode or decode the text. You can also explicitly encode and decode text using the functions in this section.

The result of encoding, and the input to decoding, are not ordinary text. They logically consist of a series of byte values; that is, a series of ASCII and eight-bit characters. In unibyte buffers and strings, these characters have codes in the range 0 through #xFF (255). In a multibyte buffer or string, eight-bit characters have character codes higher than #xFF (see Text Representations), but Emacs transparently converts them to their single-byte values when you encode or decode such text.

The usual way to read a file into a buffer as a sequence of bytes, so you can decode the contents explicitly, is with insert-file-contents-literally (see Reading from Files); alternatively, specify a non-nil rawfile argument when visiting a file with find-file-noselect. These methods result in a unibyte buffer.

The usual way to use the byte sequence that results from explicitly encoding text is to copy it to a file or process—for example, to write it with write-region (see Writing to Files), and suppress encoding by binding coding-system-for-write to no-conversion.

Here are the functions to perform explicit encoding or decoding. The encoding functions produce sequences of bytes; the decoding functions are meant to operate on sequences of bytes. All of these functions discard text properties. They also set last-coding-system-used to the precise coding system they used.

32.10.8 Terminal I/O Encoding

Emacs can use coding systems to decode keyboard input and encode terminal output. This is useful for terminals that transmit or display text using a particular encoding, such as Latin-1. Emacs does not set last-coding-system-used when encoding or decoding terminal I/O.

32.11 Input Methods

Input methods provide convenient ways of entering non-ASCII characters from the keyboard. Unlike coding systems, which translate non-ASCII characters to and from encodings meant to be read by programs, input methods provide human-friendly commands. (See Input Methods, for information on how users use input methods to enter text.) How to define input methods is not yet documented in this manual, but here we describe how to use them.

Each input method has a name, which is currently a string; in the future, symbols may also be usable as input method names.

The fundamental interface to input methods is through the variable input-method-function. See Reading One Event, and Invoking the Input Method.

32.12 Locales

In POSIX, locales control which language to use in language-related features. These Emacs variables control how Emacs interacts with these features.

33 Searching and Matching

GNU Emacs provides two ways to search through a buffer for specified text: exact string searches and regular expression searches. After a regular expression search, you can examine the match data to determine which text matched the whole regular expression or various portions of it.

• String Search: Search for an exact match.
• Searching and Case: Case-independent or case-significant searching.
• Regular Expressions: Describing classes of strings.
• Regexp Search: Searching for a match for a regexp.
• POSIX Regexps: Searching POSIX-style for the longest match.
• Match Data: Finding out which part of the text matched, after a string or regexp search.
• Search and Replace: Commands that loop, searching and replacing.
• Standard Regexps: Useful regexps for finding sentences, pages,...

The ‘skip-chars...’ functions also perform a kind of searching. See Skipping Characters. To search for changes in character properties, see Property Search.

33.1 Searching for Strings

These are the primitive functions for searching through the text in a buffer. They are meant for use in programs, but you may call them interactively. If you do so, they prompt for the search string; the arguments limit and noerror are nil, and repeat is 1. For more details on interactive searching, see Searching and Replacement.

These search functions convert the search string to multibyte if the buffer is multibyte; they convert the search string to unibyte if the buffer is unibyte. See Text Representations.

33.2 Searching and Case

By default, searches in Emacs ignore the case of the text they are searching through; if you specify searching for ‘FOO’, then ‘Foo’ or ‘foo’ is also considered a match. This applies to regular expressions, too; thus, ‘[aB]’ would match ‘a’ or ‘A’ or ‘b’ or ‘B’.

If you do not want this feature, set the variable case-fold-search to nil. Then all letters must match exactly, including case. This is a buffer-local variable; altering the variable affects only the current buffer. (See Intro to Buffer-Local.) Alternatively, you may change the default value. In Lisp code, you will more typically use let to bind case-fold-search to the desired value.

Note that the user-level incremental search feature handles case distinctions differently. When the search string contains only lower case letters, the search ignores case, but when the search string contains one or more upper case letters, the search becomes case-sensitive. But this has nothing to do with the searching functions used in Lisp code. See Incremental Search.

33.3 Regular Expressions

A regular expression, or regexp for short, is a pattern that denotes a (possibly infinite) set of strings. Searching for matches for a regexp is a very powerful operation. This section explains how to write regexps; the following section says how to search for them.

For interactive development of regular expressions, you can use the M-x re-builder command. It provides a convenient interface for creating regular expressions, by giving immediate visual feedback in a separate buffer. As you edit the regexp, all its matches in the target buffer are highlighted. Each parenthesized sub-expression of the regexp is shown in a distinct face, which makes it easier to verify even very complex regexps.

• Syntax of Regexps: Rules for writing regular expressions.
• Regexp Example: Illustrates regular expression syntax.
• Regexp Functions: Functions for operating on regular expressions.

33.3.1 Syntax of Regular Expressions

Regular expressions have a syntax in which a few characters are special constructs and the rest are ordinary. An ordinary character is a simple regular expression that matches that character and nothing else. The special characters are ‘.’, ‘*’, ‘+’, ‘?’, ‘[’, ‘^’, ‘$’, and ‘\’; no new special characters will be defined in the future. The character ‘]’ is special if it ends a character alternative (see later). The character ‘-’ is special inside a character alternative. A ‘[:’ and balancing ‘:]’ enclose a character class inside a character alternative. Any other character appearing in a regular expression is ordinary, unless a ‘\’ precedes it.

For example, ‘f’ is not a special character, so it is ordinary, and therefore ‘f’ is a regular expression that matches the string ‘f’ and no other string. (It does not match the string ‘fg’, but it does match a part of that string.) Likewise, ‘o’ is a regular expression that matches only ‘o’.

Any two regular expressions a and b can be concatenated. The result is a regular expression that matches a string if a matches some amount of the beginning of that string and b matches the rest of the string.

As a simple example, we can concatenate the regular expressions ‘f’ and ‘o’ to get the regular expression ‘fo’, which matches only the string ‘fo’. Still trivial. To do something more powerful, you need to use one of the special regular expression constructs.

• Regexp Special: Special characters in regular expressions.
• Char Classes: Character classes used in regular expressions.
• Regexp Backslash: Backslash-sequences in regular expressions.

33.3.1.1 Special Characters in Regular Expressions

Here is a list of the characters that are special in a regular expression.

‘.’ (Period)

is a special character that matches any single character except a newline. Using concatenation, we can make regular expressions like ‘a.b’, which matches any three-character string that begins with ‘a’ and ends with ‘b’.

‘*’

is not a construct by itself; it is a postfix operator that means to match the preceding regular expression repetitively as many times as possible. Thus, ‘o*’ matches any number of ‘o’s (including no ‘o’s).

‘*’ always applies to the smallest possible preceding expression. Thus, ‘fo*’ has a repeating ‘o’, not a repeating ‘fo’. It matches ‘f’, ‘fo’, ‘foo’, and so on.

The matcher processes a ‘*’ construct by matching, immediately, as many repetitions as can be found. Then it continues with the rest of the pattern. If that fails, backtracking occurs, discarding some of the matches of the ‘*’-modified construct in the hope that that will make it possible to match the rest of the pattern. For example, in matching ‘ca*ar’ against the string ‘caaar’, the ‘a*’ first tries to match all three ‘a’s; but the rest of the pattern is ‘ar’ and there is only ‘r’ left to match, so this try fails. The next alternative is for ‘a*’ to match only two ‘a’s. With this choice, the rest of the regexp matches successfully.

Warning: Nested repetition operators can run for an indefinitely long time, if they lead to ambiguous matching. For example, trying to match the regular expression ‘\(x+y*\)*a’ against the string ‘xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxz’ could take hours before it ultimately fails. Emacs must try each way of grouping the ‘x’s before concluding that none of them can work. Even worse, ‘\(x*\)*’ can match the null string in infinitely many ways, so it causes an infinite loop. To avoid these problems, check nested repetitions carefully, to make sure that they do not cause combinatorial explosions in backtracking.

‘+’

is a postfix operator, similar to ‘*’ except that it must match the preceding expression at least once. So, for example, ‘ca+r’ matches the strings ‘car’ and ‘caaaar’ but not the string ‘cr’, whereas ‘ca*r’ matches all three strings.

‘?’

is a postfix operator, similar to ‘*’ except that it must match the preceding expression either once or not at all. For example, ‘ca?r’ matches ‘car’ or ‘cr’; nothing else.

‘*?’, ‘+?’, ‘??’

These are non-greedy variants of the operators ‘*’, ‘+’ and ‘?’. Where those operators match the largest possible substring (consistent with matching the entire containing expression), the non-greedy variants match the smallest possible substring (consistent with matching the entire containing expression).

For example, the regular expression ‘c[ad]*a’ when applied to the string ‘cdaaada’ matches the whole string; but the regular expression ‘c[ad]*?a’, applied to that same string, matches just ‘cda’. (The smallest possible match here for ‘[ad]*?’ that permits the whole expression to match is ‘d’.)

‘[ ... ]’

is a character alternative, which begins with ‘[’ and is terminated by ‘]’. In the simplest case, the characters between the two brackets are what this character alternative can match.

Thus, ‘[ad]’ matches either one ‘a’ or one ‘d’, and ‘[ad]*’ matches any string composed of just ‘a’s and ‘d’s (including the empty string). It follows that ‘c[ad]*r’ matches ‘cr’, ‘car’, ‘cdr’, ‘caddaar’, etc.

You can also include character ranges in a character alternative, by writing the starting and ending characters with a ‘-’ between them. Thus, ‘[a-z]’ matches any lower-case ASCII letter. Ranges may be intermixed freely with individual characters, as in ‘[a-z$%.]’, which matches any lower case ASCII letter or ‘$’, ‘%’ or period.

If case-fold-search is non-nil, ‘[a-z]’ also matches upper-case letters. Note that a range like ‘[a-z]’ is not affected by the locale's collation sequence, it always represents a sequence in ASCII order.

Note also that the usual regexp special characters are not special inside a character alternative. A completely different set of characters is special inside character alternatives: ‘]’, ‘-’ and ‘^’.

To include a ‘]’ in a character alternative, you must make it the first character. For example, ‘[]a]’ matches ‘]’ or ‘a’. To include a ‘-’, write ‘-’ as the first or last character of the character alternative, or put it after a range. Thus, ‘[]-]’ matches both ‘]’ and ‘-’. (As explained below, you cannot use ‘\]’ to include a ‘]’ inside a character alternative, since ‘\’ is not special there.)

To include ‘^’ in a character alternative, put it anywhere but at the beginning.

If a range starts with a unibyte character c and ends with a multibyte character c2, the range is divided into two parts: one spans the unibyte characters ‘c..?\377’, the other the multibyte characters ‘c1..c2’, where c1 is the first character of the charset to which c2 belongs.

A character alternative can also specify named character classes (see Char Classes). This is a POSIX feature. For example, ‘[[:ascii:]]’ matches any ASCII character. Using a character class is equivalent to mentioning each of the characters in that class; but the latter is not feasible in practice, since some classes include thousands of different characters.

‘[^ ... ]’

‘[^’ begins a complemented character alternative. This matches any character except the ones specified. Thus, ‘[^a-z0-9A-Z]’ matches all characters except letters and digits.

‘^’ is not special in a character alternative unless it is the first character. The character following the ‘^’ is treated as if it were first (in other words, ‘-’ and ‘]’ are not special there).

A complemented character alternative can match a newline, unless newline is mentioned as one of the characters not to match. This is in contrast to the handling of regexps in programs such as grep.

You can specify named character classes, just like in character alternatives. For instance, ‘[^[:ascii:]]’ matches any non-ASCII character. See Char Classes.

‘^’

When matching a buffer, ‘^’ matches the empty string, but only at the beginning of a line in the text being matched (or the beginning of the accessible portion of the buffer). Otherwise it fails to match anything. Thus, ‘^foo’ matches a ‘foo’ that occurs at the beginning of a line.

When matching a string instead of a buffer, ‘^’ matches at the beginning of the string or after a newline character.

For historical compatibility reasons, ‘^’ can be used only at the beginning of the regular expression, or after ‘\(’, ‘\(?:’ or ‘\|’.

‘$’

is similar to ‘^’ but matches only at the end of a line (or the end of the accessible portion of the buffer). Thus, ‘x+$’ matches a string of one ‘x’ or more at the end of a line.

When matching a string instead of a buffer, ‘$’ matches at the end of the string or before a newline character.

For historical compatibility reasons, ‘$’ can be used only at the end of the regular expression, or before ‘\)’ or ‘\|’.

‘\’

has two functions: it quotes the special characters (including ‘\’), and it introduces additional special constructs.

Because ‘\’ quotes special characters, ‘\$’ is a regular expression that matches only ‘$’, and ‘\[’ is a regular expression that matches only ‘[’, and so on.

Note that ‘\’ also has special meaning in the read syntax of Lisp strings (see String Type), and must be quoted with ‘\’. For example, the regular expression that matches the ‘\’ character is ‘\\’. To write a Lisp string that contains the characters ‘\\’, Lisp syntax requires you to quote each ‘\’ with another ‘\’. Therefore, the read syntax for a regular expression matching ‘\’ is "\\\\".

Please note: For historical compatibility, special characters are treated as ordinary ones if they are in contexts where their special meanings make no sense. For example, ‘*foo’ treats ‘*’ as ordinary since there is no preceding expression on which the ‘*’ can act. It is poor practice to depend on this behavior; quote the special character anyway, regardless of where it appears.

As a ‘\’ is not special inside a character alternative, it can never remove the special meaning of ‘-’ or ‘]’. So you should not quote these characters when they have no special meaning either. This would not clarify anything, since backslashes can legitimately precede these characters where they have special meaning, as in ‘[^\]’ ("[^\\]" for Lisp string syntax), which matches any single character except a backslash.

In practice, most ‘]’ that occur in regular expressions close a character alternative and hence are special. However, occasionally a regular expression may try to match a complex pattern of literal ‘[’ and ‘]’. In such situations, it sometimes may be necessary to carefully parse the regexp from the start to determine which square brackets enclose a character alternative. For example, ‘[^][]]’ consists of the complemented character alternative ‘[^][]’ (which matches any single character that is not a square bracket), followed by a literal ‘]’.

The exact rules are that at the beginning of a regexp, ‘[’ is special and ‘]’ not. This lasts until the first unquoted ‘[’, after which we are in a character alternative; ‘[’ is no longer special (except when it starts a character class) but ‘]’ is special, unless it immediately follows the special ‘[’ or that ‘[’ followed by a ‘^’. This lasts until the next special ‘]’ that does not end a character class. This ends the character alternative and restores the ordinary syntax of regular expressions; an unquoted ‘[’ is special again and a ‘]’ not.

33.3.1.2 Character Classes

Here is a table of the classes you can use in a character alternative, and what they mean:

‘[:ascii:]’

This matches any ASCII character (codes 0–127).

‘[:alnum:]’

This matches any letter or digit. For multibyte characters, it matches characters whose Unicode ‘general-category’ property (see Character Properties) indicates they are alphabetic or decimal number characters.

‘[:alpha:]’

This matches any letter. For multibyte characters, it matches characters whose Unicode ‘general-category’ property (see Character Properties) indicates they are alphabetic characters.

‘[:blank:]’

This matches space and tab only.

‘[:cntrl:]’

This matches any ASCII control character.

‘[:digit:]’

This matches ‘0’ through ‘9’. Thus, ‘[-+[:digit:]]’ matches any digit, as well as ‘+’ and ‘-’.

‘[:graph:]’

This matches graphic characters—everything except whitespace, ASCII and non-ASCII control characters, surrogates, and codepoints unassigned by Unicode, as indicated by the Unicode ‘general-category’ property (see Character Properties).

‘[:lower:]’

This matches any lower-case letter, as determined by the current case table (see Case Tables). If case-fold-search is non-nil, this also matches any upper-case letter.

‘[:multibyte:]’

This matches any multibyte character (see Text Representations).

‘[:nonascii:]’

This matches any non-ASCII character.

‘[:print:]’

This matches any printing character—either whitespace, or a graphic character matched by ‘[:graph:]’.

‘[:punct:]’

This matches any punctuation character. (At present, for multibyte characters, it matches anything that has non-word syntax.)

‘[:space:]’

This matches any character that has whitespace syntax (see Syntax Class Table).

‘[:unibyte:]’

This matches any unibyte character (see Text Representations).

‘[:upper:]’

This matches any upper-case letter, as determined by the current case table (see Case Tables). If case-fold-search is non-nil, this also matches any lower-case letter.

‘[:word:]’

This matches any character that has word syntax (see Syntax Class Table).

‘[:xdigit:]’

This matches the hexadecimal digits: ‘0’ through ‘9’, ‘a’ through ‘f’ and ‘A’ through ‘F’.

33.3.1.3 Backslash Constructs in Regular Expressions

For the most part, ‘\’ followed by any character matches only that character. However, there are several exceptions: certain sequences starting with ‘\’ that have special meanings. Here is a table of the special ‘\’ constructs.

‘\|’

specifies an alternative. Two regular expressions a and b with ‘\|’ in between form an expression that matches anything that either a or b matches.

Thus, ‘foo\|bar’ matches either ‘foo’ or ‘bar’ but no other string.

‘\|’ applies to the largest possible surrounding expressions. Only a surrounding ‘\( ... \)’ grouping can limit the grouping power of ‘\|’.

If you need full backtracking capability to handle multiple uses of ‘\|’, use the POSIX regular expression functions (see POSIX Regexps).

‘\{m\}’

is a postfix operator that repeats the previous pattern exactly m times. Thus, ‘x\{5\}’ matches the string ‘xxxxx’ and nothing else. ‘c[ad]\{3\}r’ matches string such as ‘caaar’, ‘cdddr’, ‘cadar’, and so on.

‘\{m,n\}’

is a more general postfix operator that specifies repetition with a minimum of m repeats and a maximum of n repeats. If m is omitted, the minimum is 0; if n is omitted, there is no maximum.

For example, ‘c[ad]\{1,2\}r’ matches the strings ‘car’, ‘cdr’, ‘caar’, ‘cadr’, ‘cdar’, and ‘cddr’, and nothing else.
‘\{0,1\}’ or ‘\{,1\}’ is equivalent to ‘?’.
‘\{0,\}’ or ‘\{,\}’ is equivalent to ‘*’.
‘\{1,\}’ is equivalent to ‘+’.

‘\( ... \)’

is a grouping construct that serves three purposes:

1. To enclose a set of ‘\|’ alternatives for other operations. Thus, the regular expression ‘\(foo\|bar\)x’ matches either ‘foox’ or ‘barx’.
2. To enclose a complicated expression for the postfix operators ‘*’, ‘+’ and ‘?’ to operate on. Thus, ‘ba\(na\)*’ matches ‘ba’, ‘bana’, ‘banana’, ‘bananana’, etc., with any number (zero or more) of ‘na’ strings.
3. To record a matched substring for future reference with ‘\digit’ (see below).

This last application is not a consequence of the idea of a parenthetical grouping; it is a separate feature that was assigned as a second meaning to the same ‘\( ... \)’ construct because, in practice, there was usually no conflict between the two meanings. But occasionally there is a conflict, and that led to the introduction of shy groups.

‘\(?: ... \)’

is the shy group construct. A shy group serves the first two purposes of an ordinary group (controlling the nesting of other operators), but it does not get a number, so you cannot refer back to its value with ‘\digit’. Shy groups are particularly useful for mechanically-constructed regular expressions, because they can be added automatically without altering the numbering of ordinary, non-shy groups.

Shy groups are also called non-capturing or unnumbered groups.

‘\(?num: ... \)’

is the explicitly numbered group construct. Normal groups get their number implicitly, based on their position, which can be inconvenient. This construct allows you to force a particular group number. There is no particular restriction on the numbering, e.g., you can have several groups with the same number in which case the last one to match (i.e., the rightmost match) will win. Implicitly numbered groups always get the smallest integer larger than the one of any previous group.

‘\digit’

matches the same text that matched the digitth occurrence of a grouping (‘\( ... \)’) construct.

In other words, after the end of a group, the matcher remembers the beginning and end of the text matched by that group. Later on in the regular expression you can use ‘\’ followed by digit to match that same text, whatever it may have been.

The strings matching the first nine grouping constructs appearing in the entire regular expression passed to a search or matching function are assigned numbers 1 through 9 in the order that the open parentheses appear in the regular expression. So you can use ‘\1’ through ‘\9’ to refer to the text matched by the corresponding grouping constructs.

For example, ‘\(.*\)\1’ matches any newline-free string that is composed of two identical halves. The ‘\(.*\)’ matches the first half, which may be anything, but the ‘\1’ that follows must match the same exact text.

If a ‘\( ... \)’ construct matches more than once (which can happen, for instance, if it is followed by ‘*’), only the last match is recorded.

If a particular grouping construct in the regular expression was never matched—for instance, if it appears inside of an alternative that wasn't used, or inside of a repetition that repeated zero times—then the corresponding ‘\digit’ construct never matches anything. To use an artificial example, ‘\(foo\(b*\)\|lose\)\2’ cannot match ‘lose’: the second alternative inside the larger group matches it, but then ‘\2’ is undefined and can't match anything. But it can match ‘foobb’, because the first alternative matches ‘foob’ and ‘\2’ matches ‘b’.

‘\w’

matches any word-constituent character. The editor syntax table determines which characters these are. See Syntax Tables.

‘\W’

matches any character that is not a word constituent.

‘\scode’

matches any character whose syntax is code. Here code is a character that represents a syntax code: thus, ‘w’ for word constituent, ‘-’ for whitespace, ‘(’ for open parenthesis, etc. To represent whitespace syntax, use either ‘-’ or a space character. See Syntax Class Table, for a list of syntax codes and the characters that stand for them.

‘\Scode’

matches any character whose syntax is not code.

‘\cc’

matches any character whose category is c. Here c is a character that represents a category: thus, ‘c’ for Chinese characters or ‘g’ for Greek characters in the standard category table. You can see the list of all the currently defined categories with M-x describe-categories <RET>. You can also define your own categories in addition to the standard ones using the define-category function (see Categories).

‘\Cc’

matches any character whose category is not c.

The following regular expression constructs match the empty string—that is, they don't use up any characters—but whether they match depends on the context. For all, the beginning and end of the accessible portion of the buffer are treated as if they were the actual beginning and end of the buffer.

‘\`’

matches the empty string, but only at the beginning of the buffer or string being matched against.

‘\'’

matches the empty string, but only at the end of the buffer or string being matched against.

‘\=’

matches the empty string, but only at point. (This construct is not defined when matching against a string.)

‘\b’

matches the empty string, but only at the beginning or end of a word. Thus, ‘\bfoo\b’ matches any occurrence of ‘foo’ as a separate word. ‘\bballs?\b’ matches ‘ball’ or ‘balls’ as a separate word.

‘\b’ matches at the beginning or end of the buffer (or string) regardless of what text appears next to it.

‘\B’

matches the empty string, but not at the beginning or end of a word, nor at the beginning or end of the buffer (or string).

‘\<’

matches the empty string, but only at the beginning of a word. ‘\<’ matches at the beginning of the buffer (or string) only if a word-constituent character follows.

‘\>’

matches the empty string, but only at the end of a word. ‘\>’ matches at the end of the buffer (or string) only if the contents end with a word-constituent character.

‘\_<’

matches the empty string, but only at the beginning of a symbol. A symbol is a sequence of one or more word or symbol constituent characters. ‘\_<’ matches at the beginning of the buffer (or string) only if a symbol-constituent character follows.

‘\_>’

matches the empty string, but only at the end of a symbol. ‘\_>’ matches at the end of the buffer (or string) only if the contents end with a symbol-constituent character.

Not every string is a valid regular expression. For example, a string that ends inside a character alternative without a terminating ‘]’ is invalid, and so is a string that ends with a single ‘\’. If an invalid regular expression is passed to any of the search functions, an invalid-regexp error is signaled.

33.3.2 Complex Regexp Example

Here is a complicated regexp which was formerly used by Emacs to recognize the end of a sentence together with any whitespace that follows. (Nowadays Emacs uses a similar but more complex default regexp constructed by the function sentence-end. See Standard Regexps.)

Below, we show first the regexp as a string in Lisp syntax (to distinguish spaces from tab characters), and then the result of evaluating it. The string constant begins and ends with a double-quote. ‘\"’ stands for a double-quote as part of the string, ‘\\’ for a backslash as part of the string, ‘\t’ for a tab and ‘\n’ for a newline.

     "[.?!][]\"')}]*\\($\\| $\\|\t\\|  \\)[ \t\n]*"
          ⇒ "[.?!][]\"')}]*\\($\\| $\\|  \\|  \\)[
     ]*"

In the output, tab and newline appear as themselves.

This regular expression contains four parts in succession and can be deciphered as follows:

[.?!]

The first part of the pattern is a character alternative that matches any one of three characters: period, question mark, and exclamation mark. The match must begin with one of these three characters. (This is one point where the new default regexp used by Emacs differs from the old. The new value also allows some non-ASCII characters that end a sentence without any following whitespace.)

[]\"')}]*

The second part of the pattern matches any closing braces and quotation marks, zero or more of them, that may follow the period, question mark or exclamation mark. The \" is Lisp syntax for a double-quote in a string. The ‘*’ at the end indicates that the immediately preceding regular expression (a character alternative, in this case) may be repeated zero or more times.

\\($\\| $\\|\t\\| \\)

The third part of the pattern matches the whitespace that follows the end of a sentence: the end of a line (optionally with a space), or a tab, or two spaces. The double backslashes mark the parentheses and vertical bars as regular expression syntax; the parentheses delimit a group and the vertical bars separate alternatives. The dollar sign is used to match the end of a line.

[ \t\n]*

Finally, the last part of the pattern matches any additional whitespace beyond the minimum needed to end a sentence.

33.3.3 Regular Expression Functions

These functions operate on regular expressions.

33.4 Regular Expression Searching

In GNU Emacs, you can search for the next match for a regular expression (see Syntax of Regexps) either incrementally or not. For incremental search commands, see Regular Expression Search. Here we describe only the search functions useful in programs. The principal one is re-search-forward.

These search functions convert the regular expression to multibyte if the buffer is multibyte; they convert the regular expression to unibyte if the buffer is unibyte. See Text Representations.

33.5 POSIX Regular Expression Searching

The usual regular expression functions do backtracking when necessary to handle the ‘\|’ and repetition constructs, but they continue this only until they find some match. Then they succeed and report the first match found.

This section describes alternative search functions which perform the full backtracking specified by the POSIX standard for regular expression matching. They continue backtracking until they have tried all possibilities and found all matches, so they can report the longest match, as required by POSIX. This is much slower, so use these functions only when you really need the longest match.

The POSIX search and match functions do not properly support the non-greedy repetition operators (see non-greedy). This is because POSIX backtracking conflicts with the semantics of non-greedy repetition.

33.6 The Match Data

Emacs keeps track of the start and end positions of the segments of text found during a search; this is called the match data. Thanks to the match data, you can search for a complex pattern, such as a date in a mail message, and then extract parts of the match under control of the pattern.

Because the match data normally describe the most recent search only, you must be careful not to do another search inadvertently between the search you wish to refer back to and the use of the match data. If you can't avoid another intervening search, you must save and restore the match data around it, to prevent it from being overwritten.

Notice that all functions are allowed to overwrite the match data unless they're explicitly documented not to do so. A consequence is that functions that are run implicitly in the background (see Timers, and Idle Timers) should likely save and restore the match data explicitly.

• Replacing Match: Replacing a substring that was matched.
• Simple Match Data: Accessing single items of match data, such as where a particular subexpression started.
• Entire Match Data: Accessing the entire match data at once, as a list.
• Saving Match Data: Saving and restoring the match data.

33.6.1 Replacing the Text that Matched

This function replaces all or part of the text matched by the last search. It works by means of the match data.

33.6.2 Simple Match Data Access

This section explains how to use the match data to find out what was matched by the last search or match operation, if it succeeded.

You can ask about the entire matching text, or about a particular parenthetical subexpression of a regular expression. The count argument in the functions below specifies which. If count is zero, you are asking about the entire match. If count is positive, it specifies which subexpression you want.

Recall that the subexpressions of a regular expression are those expressions grouped with escaped parentheses, ‘\(...\)’. The countth subexpression is found by counting occurrences of ‘\(’ from the beginning of the whole regular expression. The first subexpression is numbered 1, the second 2, and so on. Only regular expressions can have subexpressions—after a simple string search, the only information available is about the entire match.

Every successful search sets the match data. Therefore, you should query the match data immediately after searching, before calling any other function that might perform another search. Alternatively, you may save and restore the match data (see Saving Match Data) around the call to functions that could perform another search. Or use the functions that explicitly do not modify the match data; e.g., string-match-p.

A search which fails may or may not alter the match data. In the current implementation, it does not, but we may change it in the future. Don't try to rely on the value of the match data after a failing search.

Here is an example of using the match data, with a comment showing the positions within the text:

     (string-match "\\(qu\\)\\(ick\\)"
                   "The quick fox jumped quickly.")
                   ;0123456789
          ⇒ 4
     
     (match-string 0 "The quick fox jumped quickly.")
          ⇒ "quick"
     (match-string 1 "The quick fox jumped quickly.")
          ⇒ "qu"
     (match-string 2 "The quick fox jumped quickly.")
          ⇒ "ick"
     
     (match-beginning 1)       ; The beginning of the match
          ⇒ 4                 ;   with ‘qu’ is at index 4.
     
     (match-beginning 2)       ; The beginning of the match
          ⇒ 6                 ;   with ‘ick’ is at index 6.
     
     (match-end 1)             ; The end of the match
          ⇒ 6                 ;   with ‘qu’ is at index 6.
     
     (match-end 2)             ; The end of the match
          ⇒ 9                 ;   with ‘ick’ is at index 9.

Here is another example. Point is initially located at the beginning of the line. Searching moves point to between the space and the word ‘in’. The beginning of the entire match is at the 9th character of the buffer (‘T’), and the beginning of the match for the first subexpression is at the 13th character (‘c’).

     (list
       (re-search-forward "The \\(cat \\)")
       (match-beginning 0)
       (match-beginning 1))
         ⇒ (17 9 13)
     
     ---------- Buffer: foo ----------
     I read "The cat -!-in the hat comes back" twice.
             ^   ^
             9  13
     ---------- Buffer: foo ----------

(In this case, the index returned is a buffer position; the first character of the buffer counts as 1.)

33.6.3 Accessing the Entire Match Data

The functions match-data and set-match-data read or write the entire match data, all at once.

33.6.4 Saving and Restoring the Match Data

When you call a function that may search, you may need to save and restore the match data around that call, if you want to preserve the match data from an earlier search for later use. Here is an example that shows the problem that arises if you fail to save the match data:

     (re-search-forward "The \\(cat \\)")
          ⇒ 48
     (foo)                   ; foo does more searching.
     (match-end 0)
          ⇒ 61              ; Unexpected result---not 48!

You can save and restore the match data with save-match-data:

You could use set-match-data together with match-data to imitate the effect of the special form save-match-data. Here is how:

     (let ((data (match-data)))
       (unwind-protect
           ...   ; Ok to change the original match data.
         (set-match-data data)))

Emacs automatically saves and restores the match data when it runs process filter functions (see Filter Functions) and process sentinels (see Sentinels).

33.7 Search and Replace

If you want to find all matches for a regexp in part of the buffer, and replace them, the best way is to write an explicit loop using re-search-forward and replace-match, like this:

     (while (re-search-forward "foo[ \t]+bar" nil t)
       (replace-match "foobar"))

See Replacing the Text that Matched, for a description of replace-match.

However, replacing matches in a string is more complex, especially if you want to do it efficiently. So Emacs provides a function to do this.

If you want to write a command along the lines of query-replace, you can use perform-replace to do the work.

Here are the meaningful bindings for query-replace-map. Several of them are meaningful only for query-replace and friends.

act

Do take the action being considered—in other words, “yes”.

skip

Do not take action for this question—in other words, “no”.

exit

Answer this question “no”, and give up on the entire series of questions, assuming that the answers will be “no”.

exit-prefix

Like exit, but add the key that was pressed to unread-command-events (see Event Input Misc).

act-and-exit

Answer this question “yes”, and give up on the entire series of questions, assuming that subsequent answers will be “no”.

act-and-show

Answer this question “yes”, but show the results—don't advance yet to the next question.

automatic

Answer this question and all subsequent questions in the series with “yes”, without further user interaction.

backup

Move back to the previous place that a question was asked about.

edit

Enter a recursive edit to deal with this question—instead of any other action that would normally be taken.

edit-replacement

Edit the replacement for this question in the minibuffer.

delete-and-edit

Delete the text being considered, then enter a recursive edit to replace it.

recenter

scroll-up

scroll-down

scroll-other-window

scroll-other-window-down

Perform the specified window scroll operation, then ask the same question again. Only y-or-n-p and related functions use this answer.

quit

Perform a quit right away. Only y-or-n-p and related functions use this answer.

help

Display some help, then ask again.

33.8 Standard Regular Expressions Used in Editing

This section describes some variables that hold regular expressions used for certain purposes in editing:

The following two regular expressions should not assume the match always starts at the beginning of a line; they should not use ‘^’ to anchor the match. Most often, the paragraph commands do check for a match only at the beginning of a line, which means that ‘^’ would be superfluous. When there is a nonzero left margin, they accept matches that start after the left margin. In that case, a ‘^’ would be incorrect. However, a ‘^’ is harmless in modes where a left margin is never used.

34 Syntax Tables

A syntax table specifies the syntactic role of each character in a buffer. It can be used to determine where words, symbols, and other syntactic constructs begin and end. This information is used by many Emacs facilities, including Font Lock mode (see Font Lock Mode) and the various complex movement commands (see Motion).

• Basics: Basic concepts of syntax tables.
• Syntax Descriptors: How characters are classified.
• Syntax Table Functions: How to create, examine and alter syntax tables.
• Syntax Properties: Overriding syntax with text properties.
• Motion and Syntax: Moving over characters with certain syntaxes.
• Parsing Expressions: Parsing balanced expressions using the syntax table.
• Syntax Table Internals: How syntax table information is stored.
• Categories: Another way of classifying character syntax.

34.1 Syntax Table Concepts

A syntax table is a data structure which can be used to look up the syntax class and other syntactic properties of each character. Syntax tables are used by Lisp programs for scanning and moving across text.

Internally, a syntax table is a char-table (see Char-Tables). The element at index c describes the character with code c; its value is a cons cell which specifies the syntax of the character in question. See Syntax Table Internals, for details. However, instead of using aset and aref to modify and inspect syntax table contents, you should usually use the higher-level functions char-syntax and modify-syntax-entry, which are described in Syntax Table Functions.

Each buffer has its own major mode, and each major mode has its own idea of the syntax class of various characters. For example, in Lisp mode, the character ‘;’ begins a comment, but in C mode, it terminates a statement. To support these variations, the syntax table is local to each buffer. Typically, each major mode has its own syntax table, which it installs in all buffers that use that mode. For example, the variable emacs-lisp-mode-syntax-table holds the syntax table used by Emacs Lisp mode, and c-mode-syntax-table holds the syntax table used by C mode. Changing a major mode's syntax table alters the syntax in all of that mode's buffers, as well as in any buffers subsequently put in that mode. Occasionally, several similar modes share one syntax table. See Example Major Modes, for an example of how to set up a syntax table.

A syntax table can inherit from another syntax table, which is called its parent syntax table. A syntax table can leave the syntax class of some characters unspecified, by giving them the “inherit” syntax class; such a character then acquires the syntax class specified by the parent syntax table (see Syntax Class Table). Emacs defines a standard syntax table, which is the default parent syntax table, and is also the syntax table used by Fundamental mode.

Syntax tables are not used by the Emacs Lisp reader, which has its own built-in syntactic rules which cannot be changed. (Some Lisp systems provide ways to redefine the read syntax, but we decided to leave this feature out of Emacs Lisp for simplicity.)

34.2 Syntax Descriptors

The syntax class of a character describes its syntactic role. Each syntax table specifies the syntax class of each character. There is no necessary relationship between the class of a character in one syntax table and its class in any other table.

Each syntax class is designated by a mnemonic character, which serves as the name of the class when you need to specify a class. Usually, this designator character is one that is often assigned that class; however, its meaning as a designator is unvarying and independent of what syntax that character currently has. Thus, ‘\’ as a designator character always stands for escape character syntax, regardless of whether the ‘\’ character actually has that syntax in the current syntax table. See Syntax Class Table, for a list of syntax classes and their designator characters.

A syntax descriptor is a Lisp string that describes the syntax class and other syntactic properties of a character. When you want to modify the syntax of a character, that is done by calling the function modify-syntax-entry and passing a syntax descriptor as one of its arguments (see Syntax Table Functions).

The first character in a syntax descriptor must be a syntax class designator character. The second character, if present, specifies a matching character (e.g., in Lisp, the matching character for ‘(’ is ‘)’); a space specifies that there is no matching character. Then come characters specifying additional syntax properties (see Syntax Flags).

If no matching character or flags are needed, only one character (specifying the syntax class) is sufficient.

For example, the syntax descriptor for the character ‘*’ in C mode is ". 23" (i.e., punctuation, matching character slot unused, second character of a comment-starter, first character of a comment-ender), and the entry for ‘/’ is ‘. 14’ (i.e., punctuation, matching character slot unused, first character of a comment-starter, second character of a comment-ender).

Emacs also defines raw syntax descriptors, which are used to describe syntax classes at a lower level. See Syntax Table Internals.

• Syntax Class Table: Table of syntax classes.
• Syntax Flags: Additional flags each character can have.

34.2.1 Table of Syntax Classes

Here is a table of syntax classes, the characters that designate them, their meanings, and examples of their use.

Whitespace characters: ‘ ’ or ‘-’

Characters that separate symbols and words from each other. Typically, whitespace characters have no other syntactic significance, and multiple whitespace characters are syntactically equivalent to a single one. Space, tab, and formfeed are classified as whitespace in almost all major modes.

This syntax class can be designated by either ‘ ’ or ‘-’. Both designators are equivalent.

Word constituents: ‘w’

Parts of words in human languages. These are typically used in variable and command names in programs. All upper- and lower-case letters, and the digits, are typically word constituents.

Symbol constituents: ‘_’

Extra characters used in variable and command names along with word constituents. Examples include the characters ‘$&*+-_<>’ in Lisp mode, which may be part of a symbol name even though they are not part of English words. In standard C, the only non-word-constituent character that is valid in symbols is underscore (‘_’).

Punctuation characters: ‘.’

Characters used as punctuation in a human language, or used in a programming language to separate symbols from one another. Some programming language modes, such as Emacs Lisp mode, have no characters in this class since the few characters that are not symbol or word constituents all have other uses. Other programming language modes, such as C mode, use punctuation syntax for operators.

Open parenthesis characters: ‘(’

Close parenthesis characters: ‘)’

Characters used in dissimilar pairs to surround sentences or expressions. Such a grouping is begun with an open parenthesis character and terminated with a close. Each open parenthesis character matches a particular close parenthesis character, and vice versa. Normally, Emacs indicates momentarily the matching open parenthesis when you insert a close parenthesis. See Blinking.

In human languages, and in C code, the parenthesis pairs are ‘()’, ‘[]’, and ‘{}’. In Emacs Lisp, the delimiters for lists and vectors (‘()’ and ‘[]’) are classified as parenthesis characters.

String quotes: ‘"’

Characters used to delimit string constants. The same string quote character appears at the beginning and the end of a string. Such quoted strings do not nest.

The parsing facilities of Emacs consider a string as a single token. The usual syntactic meanings of the characters in the string are suppressed.

The Lisp modes have two string quote characters: double-quote (‘"’) and vertical bar (‘|’). ‘|’ is not used in Emacs Lisp, but it is used in Common Lisp. C also has two string quote characters: double-quote for strings, and apostrophe (‘'’) for character constants.

Human text has no string quote characters. We do not want quotation marks to turn off the usual syntactic properties of other characters in the quotation.

Escape-syntax characters: ‘\’

Characters that start an escape sequence, such as is used in string and character constants. The character ‘\’ belongs to this class in both C and Lisp. (In C, it is used thus only inside strings, but it turns out to cause no trouble to treat it this way throughout C code.)

Characters in this class count as part of words if words-include-escapes is non-nil. See Word Motion.

Character quotes: ‘/’

Characters used to quote the following character so that it loses its normal syntactic meaning. This differs from an escape character in that only the character immediately following is ever affected.

Characters in this class count as part of words if words-include-escapes is non-nil. See Word Motion.

This class is used for backslash in TeX mode.

Paired delimiters: ‘$’

Similar to string quote characters, except that the syntactic properties of the characters between the delimiters are not suppressed. Only TeX mode uses a paired delimiter presently—the ‘$’ that both enters and leaves math mode.

Expression prefixes: ‘'’

Characters used for syntactic operators that are considered as part of an expression if they appear next to one. In Lisp modes, these characters include the apostrophe, ‘'’ (used for quoting), the comma, ‘,’ (used in macros), and ‘#’ (used in the read syntax for certain data types).

Comment starters: ‘<’

Comment enders: ‘>’

Characters used in various languages to delimit comments. Human text has no comment characters. In Lisp, the semicolon (‘;’) starts a comment and a newline or formfeed ends one.

Inherit standard syntax: ‘@’

This syntax class does not specify a particular syntax. It says to look in the standard syntax table to find the syntax of this character.

Generic comment delimiters: ‘!’

Characters that start or end a special kind of comment. Any generic comment delimiter matches any generic comment delimiter, but they cannot match a comment starter or comment ender; generic comment delimiters can only match each other.

This syntax class is primarily meant for use with the syntax-table text property (see Syntax Properties). You can mark any range of characters as forming a comment, by giving the first and last characters of the range syntax-table properties identifying them as generic comment delimiters.

Generic string delimiters: ‘|’

Characters that start or end a string. This class differs from the string quote class in that any generic string delimiter can match any other generic string delimiter; but they do not match ordinary string quote characters.

This syntax class is primarily meant for use with the syntax-table text property (see Syntax Properties). You can mark any range of characters as forming a string constant, by giving the first and last characters of the range syntax-table properties identifying them as generic string delimiters.

34.2.2 Syntax Flags

In addition to the classes, entries for characters in a syntax table can specify flags. There are eight possible flags, represented by the characters ‘1’, ‘2’, ‘3’, ‘4’, ‘b’, ‘c’, ‘n’, and ‘p’.

All the flags except ‘p’ are used to describe comment delimiters. The digit flags are used for comment delimiters made up of 2 characters. They indicate that a character can also be part of a comment sequence, in addition to the syntactic properties associated with its character class. The flags are independent of the class and each other for the sake of characters such as ‘*’ in C mode, which is a punctuation character, and the second character of a start-of-comment sequence (‘/*’), and the first character of an end-of-comment sequence (‘*/’). The flags ‘b’, ‘c’, and ‘n’ are used to qualify the corresponding comment delimiter.

Here is a table of the possible flags for a character c, and what they mean:

• ‘1’ means c is the start of a two-character comment-start sequence.
• ‘2’ means c is the second character of such a sequence.
• ‘3’ means c is the start of a two-character comment-end sequence.
• ‘4’ means c is the second character of such a sequence.
• ‘b’ means that c as a comment delimiter belongs to the alternative “b” comment style. For a two-character comment starter, this flag is only significant on the second char, and for a 2-character comment ender it is only significant on the first char.
• ‘c’ means that c as a comment delimiter belongs to the alternative “c” comment style. For a two-character comment delimiter, ‘c’ on either character makes it of style “c”.
• ‘n’ on a comment delimiter character specifies that this kind of comment can be nested. For a two-character comment delimiter, ‘n’ on either character makes it nestable.

  Emacs supports several comment styles simultaneously in any one syntax table. A comment style is a set of flags ‘b’, ‘c’, and ‘n’, so there can be up to 8 different comment styles. Each comment delimiter has a style and only matches comment delimiters of the same style. Thus if a comment starts with the comment-start sequence of style “bn”, it will extend until the next matching comment-end sequence of style “bn”.

  The appropriate comment syntax settings for C++ can be as follows:

  ‘/’

  ‘124’
  

  ‘*’

  ‘23b’
  

  newline

  ‘>’

  This defines four comment-delimiting sequences:

  ‘/*’

  This is a comment-start sequence for “b” style because the second character, ‘*’, has the ‘b’ flag.
  

  ‘//’

  This is a comment-start sequence for “a” style because the second character, ‘/’, does not have the ‘b’ flag.
  

  ‘*/’

  This is a comment-end sequence for “b” style because the first character, ‘*’, has the ‘b’ flag.
  

  newline

  This is a comment-end sequence for “a” style, because the newline character does not have the ‘b’ flag.

• ‘p’ identifies an additional prefix character for Lisp syntax. These characters are treated as whitespace when they appear between expressions. When they appear within an expression, they are handled according to their usual syntax classes.

  The function backward-prefix-chars moves back over these characters, as well as over characters whose primary syntax class is prefix (‘'’). See Motion and Syntax.

34.3 Syntax Table Functions

In this section we describe functions for creating, accessing and altering syntax tables.

34.4 Syntax Properties

When the syntax table is not flexible enough to specify the syntax of a language, you can override the syntax table for specific character occurrences in the buffer, by applying a syntax-table text property. See Text Properties, for how to apply text properties.

The valid values of syntax-table text property are:

syntax-table

If the property value is a syntax table, that table is used instead of the current buffer's syntax table to determine the syntax for the underlying text character.

(syntax-code . matching-char)

A cons cell of this format is a raw syntax descriptor (see Syntax Table Internals), which directly specifies a syntax class for the underlying text character.

nil

If the property is nil, the character's syntax is determined from the current syntax table in the usual way.

34.5 Motion and Syntax

This section describes functions for moving across characters that have certain syntax classes.

34.6 Parsing Expressions

This section describes functions for parsing and scanning balanced expressions. We will refer to such expressions as sexps, following the terminology of Lisp, even though these functions can act on languages other than Lisp. Basically, a sexp is either a balanced parenthetical grouping, a string, or a symbol (i.e., a sequence of characters whose syntax is either word constituent or symbol constituent). However, characters in the expression prefix syntax class (see Syntax Class Table) are treated as part of the sexp if they appear next to it.

The syntax table controls the interpretation of characters, so these functions can be used for Lisp expressions when in Lisp mode and for C expressions when in C mode. See List Motion, for convenient higher-level functions for moving over balanced expressions.

A character's syntax controls how it changes the state of the parser, rather than describing the state itself. For example, a string delimiter character toggles the parser state between in-string and in-code, but the syntax of characters does not directly say whether they are inside a string. For example (note that 15 is the syntax code for generic string delimiters),

     (put-text-property 1 9 'syntax-table '(15 . nil))

does not tell Emacs that the first eight chars of the current buffer are a string, but rather that they are all string delimiters. As a result, Emacs treats them as four consecutive empty string constants.

• Motion via Parsing: Motion functions that work by parsing.
• Position Parse: Determining the syntactic state of a position.
• Parser State: How Emacs represents a syntactic state.
• Low-Level Parsing: Parsing across a specified region.
• Control Parsing: Parameters that affect parsing.

34.6.1 Motion Commands Based on Parsing

This section describes simple point-motion functions that operate based on parsing expressions.

34.6.2 Finding the Parse State for a Position

For syntactic analysis, such as in indentation, often the useful thing is to compute the syntactic state corresponding to a given buffer position. This function does that conveniently.

34.6.3 Parser State

A parser state is a list of ten elements describing the state of the syntactic parser, after it parses the text between a specified starting point and a specified end point in the buffer. Parsing functions such as syntax-ppss (see Position Parse) return a parser state as the value. Some parsing functions accept a parser state as an argument, for resuming parsing.

Here are the meanings of the elements of the parser state:

0. The depth in parentheses, counting from 0. Warning: this can be negative if there are more close parens than open parens between the parser's starting point and end point.
1. The character position of the start of the innermost parenthetical grouping containing the stopping point; nil if none.
2. The character position of the start of the last complete subexpression terminated; nil if none.
3. Non-nil if inside a string. More precisely, this is the character that will terminate the string, or t if a generic string delimiter character should terminate it.
4. t if inside a non-nestable comment (of any comment style; see Syntax Flags); or the comment nesting level if inside a comment that can be nested.
5. t if the end point is just after a quote character.
6. The minimum parenthesis depth encountered during this scan.
7. What kind of comment is active: nil if not in a comment or in a comment of style ‘a’; 1 for a comment of style ‘b’; 2 for a comment of style ‘c’; and syntax-table for a comment that should be ended by a generic comment delimiter character.
8. The string or comment start position. While inside a comment, this is the position where the comment began; while inside a string, this is the position where the string began. When outside of strings and comments, this element is nil.
9. Internal data for continuing the parsing. The meaning of this data is subject to change; it is used if you pass this list as the state argument to another call.

Elements 1, 2, and 6 are ignored in a state which you pass as an argument to continue parsing, and elements 8 and 9 are used only in trivial cases. Those elements are mainly used internally by the parser code.

One additional piece of useful information is available from a parser state using this function:

34.6.4 Low-Level Parsing

The most basic way to use the expression parser is to tell it to start at a given position with a certain state, and parse up to a specified end position.

34.6.5 Parameters to Control Parsing

The behavior of parse-partial-sexp is also affected by parse-sexp-lookup-properties (see Syntax Properties).

You can use forward-comment to move forward or backward over one comment or several comments.

34.7 Syntax Table Internals

Syntax tables are implemented as char-tables (see Char-Tables), but most Lisp programs don't work directly with their elements. Syntax tables do not store syntax data as syntax descriptors (see Syntax Descriptors); they use an internal format, which is documented in this section. This internal format can also be assigned as syntax properties (see Syntax Properties).

Each entry in a syntax table is a raw syntax descriptor: a cons cell of the form (syntax-code . matching-char). syntax-code is an integer which encodes the syntax class and syntax flags, according to the table below. matching-char, if non-nil, specifies a matching character (similar to the second character in a syntax descriptor).

Here are the syntax codes corresponding to the various syntax classes:

Code	Class	Code	Class
0	whitespace	8	paired delimiter
1	punctuation	9	escape
2	word	10	character quote
3	symbol	11	comment-start
4	open parenthesis	12	comment-end
5	close parenthesis	13	inherit
6	expression prefix	14	generic comment
7	string quote	15	generic string

For example, in the standard syntax table, the entry for ‘(’ is (4 . 41). 41 is the character code for ‘)’.

Syntax flags are encoded in higher order bits, starting 16 bits from the least significant bit. This table gives the power of two which corresponds to each syntax flag.

Prefix	Flag	Prefix	Flag
‘1’	(lsh 1 16)	‘p’	(lsh 1 20)
‘2’	(lsh 1 17)	‘b’	(lsh 1 21)
‘3’	(lsh 1 18)	‘n’	(lsh 1 22)
‘4’	(lsh 1 19)

34.8 Categories

Categories provide an alternate way of classifying characters syntactically. You can define several categories as needed, then independently assign each character to one or more categories. Unlike syntax classes, categories are not mutually exclusive; it is normal for one character to belong to several categories.

Each buffer has a category table which records which categories are defined and also which characters belong to each category. Each category table defines its own categories, but normally these are initialized by copying from the standard categories table, so that the standard categories are available in all modes.

Each category has a name, which is an ASCII printing character in the range ‘ ’ to ‘~’. You specify the name of a category when you define it with define-category.

The category table is actually a char-table (see Char-Tables). The element of the category table at index c is a category set—a bool-vector—that indicates which categories character c belongs to. In this category set, if the element at index cat is t, that means category cat is a member of the set, and that character c belongs to category cat.

For the next three functions, the optional argument table defaults to the current buffer's category table.

35 Abbrevs and Abbrev Expansion

An abbreviation or abbrev is a string of characters that may be expanded to a longer string. The user can insert the abbrev string and find it replaced automatically with the expansion of the abbrev. This saves typing.

The set of abbrevs currently in effect is recorded in an abbrev table. Each buffer has a local abbrev table, but normally all buffers in the same major mode share one abbrev table. There is also a global abbrev table. Normally both are used.

An abbrev table is represented as an obarray. See Creating Symbols, for information about obarrays. Each abbreviation is represented by a symbol in the obarray. The symbol's name is the abbreviation; its value is the expansion; its function definition is the hook function for performing the expansion (see Defining Abbrevs); and its property list cell contains various additional properties, including the use count and the number of times the abbreviation has been expanded (see Abbrev Properties).

Certain abbrevs, called system abbrevs, are defined by a major mode instead of the user. A system abbrev is identified by its non-nil :system property (see Abbrev Properties). When abbrevs are saved to an abbrev file, system abbrevs are omitted. See Abbrev Files.

Because the symbols used for abbrevs are not interned in the usual obarray, they will never appear as the result of reading a Lisp expression; in fact, normally they are never used except by the code that handles abbrevs. Therefore, it is safe to use them in a nonstandard way.

If the minor mode Abbrev mode is enabled, the buffer-local variable abbrev-mode is non-nil, and abbrevs are automatically expanded in the buffer. For the user-level commands for abbrevs, see Abbrev Mode.

• Tables: Creating and working with abbrev tables.
• Defining Abbrevs: Specifying abbreviations and their expansions.
• Files: Saving abbrevs in files.
• Expansion: Controlling expansion; expansion subroutines.
• Standard Abbrev Tables: Abbrev tables used by various major modes.
• Abbrev Properties: How to read and set abbrev properties. Which properties have which effect.
• Abbrev Table Properties: How to read and set abbrev table properties. Which properties have which effect.

35.1 Abbrev Tables

This section describes how to create and manipulate abbrev tables.

35.2 Defining Abbrevs

define-abbrev is the low-level basic function for defining an abbrev in an abbrev table.

When a major mode defines a system abbrev, it should call define-abbrev and specify t for the :system property. Be aware that any saved non-system abbrevs are restored at startup, i.e., before some major modes are loaded. Therefore, major modes should not assume that their abbrev tables are empty when they are first loaded.

35.3 Saving Abbrevs in Files

A file of saved abbrev definitions is actually a file of Lisp code. The abbrevs are saved in the form of a Lisp program to define the same abbrev tables with the same contents. Therefore, you can load the file with load (see How Programs Do Loading). However, the function quietly-read-abbrev-file is provided as a more convenient interface. Emacs automatically calls this function at startup.

User-level facilities such as save-some-buffers can save abbrevs in a file automatically, under the control of variables described here.

35.4 Looking Up and Expanding Abbreviations

Abbrevs are usually expanded by certain interactive commands, including self-insert-command. This section describes the subroutines used in writing such commands, as well as the variables they use for communication.

The following sample code shows a simple use of abbrev-expand-function. It assumes that foo-mode is a mode for editing certain files in which lines that start with ‘#’ are comments. You want to use Text mode abbrevs for those lines. The regular local abbrev table, foo-mode-abbrev-table is appropriate for all other lines. See Standard Abbrev Tables, for the definitions of local-abbrev-table and text-mode-abbrev-table. See Advising Functions, for details of add-function.

     (defun foo-mode-abbrev-expand-function (expand)
       (if (not (save-excursion (forward-line 0) (eq (char-after) ?#)))
           ;; Performs normal expansion.
           (funcall expand)
         ;; We're inside a comment: use the text-mode abbrevs.
         (let ((local-abbrev-table text-mode-abbrev-table))
           (funcall expand))))
     
     (add-hook 'foo-mode-hook
               #'(lambda ()
                   (add-function :around (local 'abbrev-expand-function)
                                 #'foo-mode-abbrev-expand-function)))

35.5 Standard Abbrev Tables

Here we list the variables that hold the abbrev tables for the preloaded major modes of Emacs.

35.6 Abbrev Properties

Abbrevs have properties, some of which influence the way they work. You can provide them as arguments to define-abbrev, and manipulate them with the following functions:

The following properties have special meanings:

:count

This property counts the number of times the abbrev has been expanded. If not explicitly set, it is initialized to 0 by define-abbrev.

:system

If non-nil, this property marks the abbrev as a system abbrev. Such abbrevs are not saved (see Abbrev Files).

:enable-function

If non-nil, this property should be a function of no arguments which returns nil if the abbrev should not be used and t otherwise.

:case-fixed

If non-nil, this property indicates that the case of the abbrev's name is significant and should only match a text with the same pattern of capitalization. It also disables the code that modifies the capitalization of the expansion.

35.7 Abbrev Table Properties

Like abbrevs, abbrev tables have properties, some of which influence the way they work. You can provide them as arguments to define-abbrev-table, and manipulate them with the functions:

The following properties have special meaning:

:enable-function

This is like the :enable-function abbrev property except that it applies to all abbrevs in the table. It is used before even trying to find the abbrev before point, so it can dynamically modify the abbrev table.

:case-fixed

This is like the :case-fixed abbrev property except that it applies to all abbrevs in the table.

:regexp

If non-nil, this property is a regular expression that indicates how to extract the name of the abbrev before point, before looking it up in the table. When the regular expression matches before point, the abbrev name is expected to be in submatch 1. If this property is nil, the default is to use backward-word and forward-word to find the name. This property allows the use of abbrevs whose name contains characters of non-word syntax.

:parents

This property holds a list of tables from which to inherit other abbrevs.

:abbrev-table-modiff

This property holds a counter incremented each time a new abbrev is added to the table.

36 Processes

In the terminology of operating systems, a process is a space in which a program can execute. Emacs runs in a process. Emacs Lisp programs can invoke other programs in processes of their own. These are called subprocesses or child processes of the Emacs process, which is their parent process.

A subprocess of Emacs may be synchronous or asynchronous, depending on how it is created. When you create a synchronous subprocess, the Lisp program waits for the subprocess to terminate before continuing execution. When you create an asynchronous subprocess, it can run in parallel with the Lisp program. This kind of subprocess is represented within Emacs by a Lisp object which is also called a “process”. Lisp programs can use this object to communicate with the subprocess or to control it. For example, you can send signals, obtain status information, receive output from the process, or send input to it.

In addition to processes that run programs, Lisp programs can open connections of several types to devices or processes running on the same machine or on other machines. The supported connection types are: TCP and UDP network connections, serial port connections, and pipe connections. Each such connection is also represented by a process object.

In addition to subprocesses of the current Emacs session, you can also access other processes running on your machine. See System Processes.

• Subprocess Creation: Functions that start subprocesses.
• Shell Arguments: Quoting an argument to pass it to a shell.
• Synchronous Processes: Details of using synchronous subprocesses.
• Asynchronous Processes: Starting up an asynchronous subprocess.
• Deleting Processes: Eliminating an asynchronous subprocess.
• Process Information: Accessing run-status and other attributes.
• Input to Processes: Sending input to an asynchronous subprocess.
• Signals to Processes: Stopping, continuing or interrupting an asynchronous subprocess.
• Output from Processes: Collecting output from an asynchronous subprocess.
• Sentinels: Sentinels run when process run-status changes.
• Query Before Exit: Whether to query if exiting will kill a process.
• System Processes: Accessing other processes running on your system.
• Transaction Queues: Transaction-based communication with subprocesses.
• Network: Opening network connections.
• Network Servers: Network servers let Emacs accept net connections.
• Datagrams: UDP network connections.
• Low-Level Network: Lower-level but more general function to create connections and servers.
• Misc Network: Additional relevant functions for net connections.
• Serial Ports: Communicating with serial ports.
• Byte Packing: Using bindat to pack and unpack binary data.

36.1 Functions that Create Subprocesses

There are three primitives that create a new subprocess in which to run a program. One of them, make-process, creates an asynchronous process and returns a process object (see Asynchronous Processes). The other two, call-process and call-process-region, create a synchronous process and do not return a process object (see Synchronous Processes). There are various higher-level functions that make use of these primitives to run particular types of process.

Synchronous and asynchronous processes are explained in the following sections. Since the three functions are all called in a similar fashion, their common arguments are described here.

In all cases, the functions specify the program to be run. An error is signaled if the file is not found or cannot be executed. If the file name is relative, the variable exec-path contains a list of directories to search. Emacs initializes exec-path when it starts up, based on the value of the environment variable PATH. The standard file name constructs, ‘~’, ‘.’, and ‘..’, are interpreted as usual in exec-path, but environment variable substitutions (‘$HOME’, etc.) are not recognized; use substitute-in-file-name to perform them (see File Name Expansion). nil in this list refers to default-directory.

Executing a program can also try adding suffixes to the specified name:

Please note: The argument program contains only the name of the program file; it may not contain any command-line arguments. You must use a separate argument, args, to provide those, as described below.

Each of the subprocess-creating functions has a buffer-or-name argument that specifies where the output from the program will go. It should be a buffer or a buffer name; if it is a buffer name, that will create the buffer if it does not already exist. It can also be nil, which says to discard the output, unless a custom filter function handles it. (See Filter Functions, and Read and Print.) Normally, you should avoid having multiple processes send output to the same buffer because their output would be intermixed randomly. For synchronous processes, you can send the output to a file instead of a buffer (and the corresponding argument is therefore more appropriately called destination). By default, both standard output and standard error streams go to the same destination, but all the 3 primitives allow optionally to direct the standard error stream to a different destination.

All three of the subprocess-creating functions allow to specify command-line arguments for the process to run. For call-process and call-process-region, these come in the form of a &rest argument, args. For make-process, both the program to run and its command-line arguments are specified as a list of strings. The command-line arguments must all be strings, and they are supplied to the program as separate argument strings. Wildcard characters and other shell constructs have no special meanings in these strings, since the strings are passed directly to the specified program.

The subprocess inherits its environment from Emacs, but you can specify overrides for it with process-environment. See System Environment. The subprocess gets its current directory from the value of default-directory.

36.2 Shell Arguments

Lisp programs sometimes need to run a shell and give it a command that contains file names that were specified by the user. These programs ought to be able to support any valid file name. But the shell gives special treatment to certain characters, and if these characters occur in the file name, they will confuse the shell. To handle these characters, use the function shell-quote-argument:

The following two functions are useful for combining a list of individual command-line argument strings into a single string, and taking a string apart into a list of individual command-line arguments. These functions are mainly intended for converting user input in the minibuffer, a Lisp string, into a list of string arguments to be passed to make-process, call-process or start-process, or for converting such lists of arguments into a single Lisp string to be presented in the minibuffer or echo area. Note that if a shell is involved (e.g., if using call-process-shell-command), arguments should still be protected by shell-quote-argument; combine-and-quote-strings is not intended to protect special characters from shell evaluation.

36.3 Creating a Synchronous Process

After a synchronous process is created, Emacs waits for the process to terminate before continuing. Starting Dired on GNU or Unix¹⁸ is an example of this: it runs ls in a synchronous process, then modifies the output slightly. Because the process is synchronous, the entire directory listing arrives in the buffer before Emacs tries to do anything with it.

While Emacs waits for the synchronous subprocess to terminate, the user can quit by typing C-g. The first C-g tries to kill the subprocess with a SIGINT signal; but it waits until the subprocess actually terminates before quitting. If during that time the user types another C-g, that kills the subprocess instantly with SIGKILL and quits immediately (except on MS-DOS, where killing other processes doesn't work). See Quitting.

The synchronous subprocess functions return an indication of how the process terminated.

The output from a synchronous subprocess is generally decoded using a coding system, much like text read from a file. The input sent to a subprocess by call-process-region is encoded using a coding system, much like text written into a file. See Coding Systems.

36.4 Creating an Asynchronous Process

In this section, we describe how to create an asynchronous process. After an asynchronous process is created, it runs in parallel with Emacs, and Emacs can communicate with it using the functions described in the following sections (see Input to Processes, and see Output from Processes). Note that process communication is only partially asynchronous: Emacs sends data to the process only when certain functions are called, and Emacs accepts data from the process only while waiting for input or for a time delay.

An asynchronous process is controlled either via a pty (pseudo-terminal) or a pipe. The choice of pty or pipe is made when creating the process, by default based on the value of the variable process-connection-type (see below). If available, ptys are usually preferable for processes visible to the user, as in Shell mode, because they allow for job control (C-c, C-z, etc.) between the process and its children, and because interactive programs treat ptys as terminal devices, whereas pipes don't support these features. However, for subprocesses used by Lisp programs for internal purposes, it is often better to use a pipe, because pipes are more efficient, and because they are immune to stray character injections that ptys introduce for large (around 500 byte) messages. Also, the total number of ptys is limited on many systems and it is good not to waste them.

36.5 Deleting Processes

Deleting a process disconnects Emacs immediately from the subprocess. Processes are deleted automatically after they terminate, but not necessarily right away. You can delete a process explicitly at any time. If you explicitly delete a terminated process before it is deleted automatically, no harm results. Deleting a running process sends a signal to terminate it (and its child processes, if any), and calls the process sentinel. See Sentinels.

When a process is deleted, the process object itself continues to exist as long as other Lisp objects point to it. All the Lisp primitives that work on process objects accept deleted processes, but those that do I/O or send signals will report an error. The process mark continues to point to the same place as before, usually into a buffer where output from the process was being inserted.

36.6 Process Information

Several functions return information about processes.

Every process also has a property list that you can use to store miscellaneous values associated with the process.

36.7 Sending Input to Processes

Asynchronous subprocesses receive input when it is sent to them by Emacs, which is done with the functions in this section. You must specify the process to send input to, and the input data to send. If the subprocess runs a program, the data appears on the standard input of that program; for connections, the data is sent to the connected device or program.

Some operating systems have limited space for buffered input in a pty. On these systems, Emacs sends an EOF periodically amidst the other characters, to force them through. For most programs, these EOFs do no harm.

Subprocess input is normally encoded using a coding system before the subprocess receives it, much like text written into a file. You can use set-process-coding-system to specify which coding system to use (see Process Information). Otherwise, the coding system comes from coding-system-for-write, if that is non-nil; or else from the defaulting mechanism (see Default Coding Systems).

Sometimes the system is unable to accept input for that process, because the input buffer is full. When this happens, the send functions wait a short while, accepting output from subprocesses, and then try again. This gives the subprocess a chance to read more of its pending input and make space in the buffer. It also allows filters, sentinels and timers to run—so take account of that in writing your code.

In these functions, the process argument can be a process or the name of a process, or a buffer or buffer name (which stands for a process via get-buffer-process). nil means the current buffer's process.

36.8 Sending Signals to Processes

Sending a signal to a subprocess is a way of interrupting its activities. There are several different signals, each with its own meaning. The set of signals and their names is defined by the operating system. For example, the signal SIGINT means that the user has typed C-c, or that some analogous thing has happened.

Each signal has a standard effect on the subprocess. Most signals kill the subprocess, but some stop (or resume) execution instead. Most signals can optionally be handled by programs; if the program handles the signal, then we can say nothing in general about its effects.

You can send signals explicitly by calling the functions in this section. Emacs also sends signals automatically at certain times: killing a buffer sends a SIGHUP signal to all its associated processes; killing Emacs sends a SIGHUP signal to all remaining processes. (SIGHUP is a signal that usually indicates that the user “hung up the phone”, i.e., disconnected.)

Each of the signal-sending functions takes two optional arguments: process and current-group.

The argument process must be either a process, a process name, a buffer, a buffer name, or nil. A buffer or buffer name stands for a process through get-buffer-process. nil stands for the process associated with the current buffer. Except with stop-process and continue-process, an error is signaled if process does not identify an active process, or if it represents a network, serial, or pipe connection.

The argument current-group is a flag that makes a difference when you are running a job-control shell as an Emacs subprocess. If it is non-nil, then the signal is sent to the current process-group of the terminal that Emacs uses to communicate with the subprocess. If the process is a job-control shell, this means the shell's current subjob. If current-group is nil, the signal is sent to the process group of the immediate subprocess of Emacs. If the subprocess is a job-control shell, this is the shell itself. If current-group is lambda, the signal is sent to the process-group that owns the terminal, but only if it is not the shell itself.

The flag current-group has no effect when a pipe is used to communicate with the subprocess, because the operating system does not support the distinction in the case of pipes. For the same reason, job-control shells won't work when a pipe is used. See process-connection-type in Asynchronous Processes.

36.9 Receiving Output from Processes

The output that an asynchronous subprocess writes to its standard output stream is passed to a function called the filter function. The default filter function simply inserts the output into a buffer, which is called the associated buffer of the process (see Process Buffers). If the process has no buffer then the default filter discards the output.

If the subprocess writes to its standard error stream, by default the error output is also passed to the process filter function. If Emacs uses a pseudo-TTY (pty) for communication with the subprocess, then it is impossible to separate the standard output and standard error streams of the subprocess, because a pseudo-TTY has only one output channel. In that case, if you want to keep the output to those streams separate, you should redirect one of them to a file—for example, by using an appropriate shell command via start-process-shell-command or a similar function.

Alternatively, you could use the :stderr parameter with a non-nil value in a call to make-process (see make-process) to make the destination of the error output separate from the standard output; in that case, Emacs will use pipes for communicating with the subprocess.

When a subprocess terminates, Emacs reads any pending output, then stops reading output from that subprocess. Therefore, if the subprocess has children that are still live and still producing output, Emacs won't receive that output.

Output from a subprocess can arrive only while Emacs is waiting: when reading terminal input (see the function waiting-for-user-input-p), in sit-for and sleep-for (see Waiting), and in accept-process-output (see Accepting Output). This minimizes the problem of timing errors that usually plague parallel programming. For example, you can safely create a process and only then specify its buffer or filter function; no output can arrive before you finish, if the code in between does not call any primitive that waits.

• Process Buffers: By default, output is put in a buffer.
• Filter Functions: Filter functions accept output from the process.
• Decoding Output: Filters can get unibyte or multibyte strings.
• Accepting Output: How to wait until process output arrives.

36.9.1 Process Buffers

A process can (and usually does) have an associated buffer, which is an ordinary Emacs buffer that is used for two purposes: storing the output from the process, and deciding when to kill the process. You can also use the buffer to identify a process to operate on, since in normal practice only one process is associated with any given buffer. Many applications of processes also use the buffer for editing input to be sent to the process, but this is not built into Emacs Lisp.

By default, process output is inserted in the associated buffer. (You can change this by defining a custom filter function, see Filter Functions.) The position to insert the output is determined by the process-mark, which is then updated to point to the end of the text just inserted. Usually, but not always, the process-mark is at the end of the buffer.

Killing the associated buffer of a process also kills the process. Emacs asks for confirmation first, if the process's process-query-on-exit-flag is non-nil (see Query Before Exit). This confirmation is done by the function process-kill-buffer-query-function, which is run from kill-buffer-query-functions (see Killing Buffers).

If the process's buffer is displayed in a window, your Lisp program may wish to tell the process the dimensions of that window, so that the process could adapt its output to those dimensions, much as it adapts to the screen dimensions. The following functions allow communicating this kind of information to processes; however, not all systems support the underlying functionality, so it is best to provide fallbacks, e.g., via command-line arguments or environment variables.

When windows that display buffers associated with process change their dimensions, the affected processes should be told about these changes. By default, when the window configuration changes, Emacs will automatically call set-process-window-size on behalf of every process whose buffer is displayed in a window, passing it the smallest dimensions of all the windows displaying the process's buffer. This works via window-configuration-change-hook (see Window Hooks), which is told to invoke the function that is the value of the variable window-adjust-process-window-size-function for each process whose buffer is displayed in at least one window. You can customize this behavior by setting the value of that variable.

If the process has the adjust-window-size-function property (see Process Information), its value overrides the global and buffer-local values of window-adjust-process-window-size-function.

36.9.2 Process Filter Functions

A process filter function is a function that receives the standard output from the associated process. All output from that process is passed to the filter. The default filter simply outputs directly to the process buffer.

By default, the error output from the process, if any, is also passed to the filter function, unless the destination for the standard error stream of the process was separated from the standard output when the process was created (see Output from Processes).

The filter function can only be called when Emacs is waiting for something, because process output arrives only at such times. Emacs waits when reading terminal input (see the function waiting-for-user-input-p), in sit-for and sleep-for (see Waiting), and in accept-process-output (see Accepting Output).

A filter function must accept two arguments: the associated process and a string, which is output just received from it. The function is then free to do whatever it chooses with the output.

Quitting is normally inhibited within a filter function—otherwise, the effect of typing C-g at command level or to quit a user command would be unpredictable. If you want to permit quitting inside a filter function, bind inhibit-quit to nil. In most cases, the right way to do this is with the macro with-local-quit. See Quitting.

If an error happens during execution of a filter function, it is caught automatically, so that it doesn't stop the execution of whatever program was running when the filter function was started. However, if debug-on-error is non-nil, errors are not caught. This makes it possible to use the Lisp debugger to debug filter functions. See Debugger.

Many filter functions sometimes (or always) insert the output in the process's buffer, mimicking the actions of the default filter. Such filter functions need to make sure that they save the current buffer, select the correct buffer (if different) before inserting output, and then restore the original buffer. They should also check whether the buffer is still alive, update the process marker, and in some cases update the value of point. Here is how to do these things:

     (defun ordinary-insertion-filter (proc string)
       (when (buffer-live-p (process-buffer proc))
         (with-current-buffer (process-buffer proc)
           (let ((moving (= (point) (process-mark proc))))
             (save-excursion
               ;; Insert the text, advancing the process marker.
               (goto-char (process-mark proc))
               (insert string)
               (set-marker (process-mark proc) (point)))
             (if moving (goto-char (process-mark proc)))))))

To make the filter force the process buffer to be visible whenever new text arrives, you could insert a line like the following just before the with-current-buffer construct:

     (display-buffer (process-buffer proc))

To force point to the end of the new output, no matter where it was previously, eliminate the variable moving from the example and call goto-char unconditionally.

Note that Emacs automatically saves and restores the match data while executing filter functions. See Match Data.

The output to the filter may come in chunks of any size. A program that produces the same output twice in a row may send it as one batch of 200 characters one time, and five batches of 40 characters the next. If the filter looks for certain text strings in the subprocess output, make sure to handle the case where one of these strings is split across two or more batches of output; one way to do this is to insert the received text into a temporary buffer, which can then be searched.

In case the process's output needs to be passed to several filters, you can use add-function to combine an existing filter with a new one. See Advising Functions.

Here is an example of the use of a filter function:

     (defun keep-output (process output)
        (setq kept (cons output kept)))
          ⇒ keep-output
     (setq kept nil)
          ⇒ nil
     (set-process-filter (get-process "shell") 'keep-output)
          ⇒ keep-output
     (process-send-string "shell" "ls ~/other\n")
          ⇒ nil
     kept
          ⇒ ("lewis@slug:$ "
     "FINAL-W87-SHORT.MSS    backup.otl              kolstad.mss~
     address.txt             backup.psf              kolstad.psf
     backup.bib~             david.mss               resume-Dec-86.mss~
     backup.err              david.psf               resume-Dec.psf
     backup.mss              dland                   syllabus.mss
     "
     "#backups.mss#          backup.mss~             kolstad.mss
     ")

36.9.3 Decoding Process Output

When Emacs writes process output directly into a multibyte buffer, it decodes the output according to the process output coding system. If the coding system is raw-text or no-conversion, Emacs converts the unibyte output to multibyte using string-to-multibyte, and inserts the resulting multibyte text.

You can use set-process-coding-system to specify which coding system to use (see Process Information). Otherwise, the coding system comes from coding-system-for-read, if that is non-nil; or else from the defaulting mechanism (see Default Coding Systems). If the text output by a process contains null bytes, Emacs by default uses no-conversion for it; see inhibit-null-byte-detection, for how to control this behavior.

Warning: Coding systems such as undecided, which determine the coding system from the data, do not work entirely reliably with asynchronous subprocess output. This is because Emacs has to process asynchronous subprocess output in batches, as it arrives. Emacs must try to detect the proper coding system from one batch at a time, and this does not always work. Therefore, if at all possible, specify a coding system that determines both the character code conversion and the end of line conversion—that is, one like latin-1-unix, rather than undecided or latin-1.

When Emacs calls a process filter function, it provides the process output as a multibyte string or as a unibyte string according to the process's filter coding system. Emacs decodes the output according to the process output coding system, which usually produces a multibyte string, except for coding systems such as binary and raw-text.

36.9.4 Accepting Output from Processes

Output from asynchronous subprocesses normally arrives only while Emacs is waiting for some sort of external event, such as elapsed time or terminal input. Occasionally it is useful in a Lisp program to explicitly permit output to arrive at a specific point, or even to wait until output arrives from a process.

36.10 Sentinels: Detecting Process Status Changes

A process sentinel is a function that is called whenever the associated process changes status for any reason, including signals (whether sent by Emacs or caused by the process's own actions) that terminate, stop, or continue the process. The process sentinel is also called if the process exits. The sentinel receives two arguments: the process for which the event occurred, and a string describing the type of event.

If no sentinel function was specified for a process, it will use the default sentinel function, which inserts a message in the process's buffer with the process name and the string describing the event.

The string describing the event looks like one of the following:

• "finished\n".
• "deleted\n".
• "exited abnormally with code exitcode (core dumped)\n". The “core dumped” part is optional, and only appears if the process dumped core.
• "failed with code fail-code\n".
• "signal-description (core dumped)\n". The signal-description is a system-dependent textual description of a signal, e.g., "killed" for SIGKILL. The “core dumped” part is optional, and only appears if the process dumped core.
• "open from host-name\n".
• "open\n".
• "connection broken by remote peer\n".

A sentinel runs only while Emacs is waiting (e.g., for terminal input, or for time to elapse, or for process output). This avoids the timing errors that could result from running sentinels at random places in the middle of other Lisp programs. A program can wait, so that sentinels will run, by calling sit-for or sleep-for (see Waiting), or accept-process-output (see Accepting Output). Emacs also allows sentinels to run when the command loop is reading input. delete-process calls the sentinel when it terminates a running process.

Emacs does not keep a queue of multiple reasons to call the sentinel of one process; it records just the current status and the fact that there has been a change. Therefore two changes in status, coming in quick succession, can call the sentinel just once. However, process termination will always run the sentinel exactly once. This is because the process status can't change again after termination.

Emacs explicitly checks for output from the process before running the process sentinel. Once the sentinel runs due to process termination, no further output can arrive from the process.

A sentinel that writes the output into the buffer of the process should check whether the buffer is still alive. If it tries to insert into a dead buffer, it will get an error. If the buffer is dead, (buffer-name (process-buffer process)) returns nil.

Quitting is normally inhibited within a sentinel—otherwise, the effect of typing C-g at command level or to quit a user command would be unpredictable. If you want to permit quitting inside a sentinel, bind inhibit-quit to nil. In most cases, the right way to do this is with the macro with-local-quit. See Quitting.

If an error happens during execution of a sentinel, it is caught automatically, so that it doesn't stop the execution of whatever programs was running when the sentinel was started. However, if debug-on-error is non-nil, errors are not caught. This makes it possible to use the Lisp debugger to debug the sentinel. See Debugger.

While a sentinel is running, the process sentinel is temporarily set to nil so that the sentinel won't run recursively. For this reason it is not possible for a sentinel to specify a new sentinel.

Note that Emacs automatically saves and restores the match data while executing sentinels. See Match Data.

In case a process status changes need to be passed to several sentinels, you can use add-function to combine an existing sentinel with a new one. See Advising Functions.

36.11 Querying Before Exit

When Emacs exits, it terminates all its subprocesses. For subprocesses that run a program, it sends them the SIGHUP signal; connections are simply closed. Because subprocesses may be doing valuable work, Emacs normally asks the user to confirm that it is ok to terminate them. Each process has a query flag, which, if non-nil, says that Emacs should ask for confirmation before exiting and thus killing that process. The default for the query flag is t, meaning do query.

36.12 Accessing Other Processes

In addition to accessing and manipulating processes that are subprocesses of the current Emacs session, Emacs Lisp programs can also access other processes running on the same machine. We call these system processes, to distinguish them from Emacs subprocesses.

Emacs provides several primitives for accessing system processes. Not all platforms support these primitives; on those which don't, these primitives return nil.

36.13 Transaction Queues

You can use a transaction queue to communicate with a subprocess using transactions. First use tq-create to create a transaction queue communicating with a specified process. Then you can call tq-enqueue to send a transaction.

Transaction queues are implemented by means of a filter function. See Filter Functions.

36.14 Network Connections

Emacs Lisp programs can open stream (TCP) and datagram (UDP) network connections (see Datagrams) to other processes on the same machine or other machines. A network connection is handled by Lisp much like a subprocess, and is represented by a process object. However, the process you are communicating with is not a child of the Emacs process, has no process ID, and you can't kill it or send it signals. All you can do is send and receive data. delete-process closes the connection, but does not kill the program at the other end; that program must decide what to do about closure of the connection.

Lisp programs can listen for connections by creating network servers. A network server is also represented by a kind of process object, but unlike a network connection, the network server never transfers data itself. When it receives a connection request, it creates a new network connection to represent the connection just made. (The network connection inherits certain information, including the process plist, from the server.) The network server then goes back to listening for more connection requests.

Network connections and servers are created by calling make-network-process with an argument list consisting of keyword/argument pairs, for example :server t to create a server process, or :type 'datagram to create a datagram connection. See Low-Level Network, for details. You can also use the open-network-stream function described below.

To distinguish the different types of processes, the process-type function returns the symbol network for a network connection or server, serial for a serial port connection, pipe for a pipe connection, or real for a real subprocess.

The process-status function returns open, closed, connect, stop, or failed for network connections. For a network server, the status is always listen. Except for stop, none of those values is possible for a real subprocess. See Process Information.

You can stop and resume operation of a network process by calling stop-process and continue-process. For a server process, being stopped means not accepting new connections. (Up to 5 connection requests will be queued for when you resume the server; you can increase this limit, unless it is imposed by the operating system—see the :server keyword of make-network-process, Network Processes.) For a network stream connection, being stopped means not processing input (any arriving input waits until you resume the connection). For a datagram connection, some number of packets may be queued but input may be lost. You can use the function process-command to determine whether a network connection or server is stopped; a non-nil value means yes.

Emacs can create encrypted network connections, using either built-in or external support. The built-in support uses the GnuTLS Transport Layer Security Library; see the GnuTLS project page. If your Emacs was compiled with GnuTLS support, the function gnutls-available-p is defined and returns non-nil. For more details, see Overview. The external support uses the starttls.el library, which requires a helper utility such as gnutls-cli to be installed on the system. The open-network-stream function can transparently handle the details of creating encrypted connections for you, using whatever support is available.

36.15 Network Servers

You create a server by calling make-network-process (see Network Processes) with :server t. The server will listen for connection requests from clients. When it accepts a client connection request, that creates a new network connection, itself a process object, with the following parameters:

• The connection's process name is constructed by concatenating the server process's name with a client identification string. The client identification string for an IPv4 connection looks like ‘<a.b.c.d:p>’, which represents an address and port number. Otherwise, it is a unique number in brackets, as in ‘<nnn>’. The number is unique for each connection in the Emacs session.
• If the server has a non-default filter, the connection process does not get a separate process buffer; otherwise, Emacs creates a new buffer for the purpose. The buffer name is the server's buffer name or process name, concatenated with the client identification string.

  The server's process buffer value is never used directly, but the log function can retrieve it and use it to log connections by inserting text there.

• The communication type and the process filter and sentinel are inherited from those of the server. The server never directly uses its filter and sentinel; their sole purpose is to initialize connections made to the server.
• The connection's process contact information is set according to the client's addressing information (typically an IP address and a port number). This information is associated with the process-contact keywords :host, :service, :remote.
• The connection's local address is set up according to the port number used for the connection.
• The client process's plist is initialized from the server's plist.

36.16 Datagrams

A datagram connection communicates with individual packets rather than streams of data. Each call to process-send sends one datagram packet (see Input to Processes), and each datagram received results in one call to the filter function.

The datagram connection doesn't have to talk with the same remote peer all the time. It has a remote peer address which specifies where to send datagrams to. Each time an incoming datagram is passed to the filter function, the peer address is set to the address that datagram came from; that way, if the filter function sends a datagram, it will go back to that place. You can specify the remote peer address when you create the datagram connection using the :remote keyword. You can change it later on by calling set-process-datagram-address.

36.17 Low-Level Network Access

You can also create network connections by operating at a lower level than that of open-network-stream, using make-network-process.

• Proc: Using make-network-process.
• Options: Further control over network connections.
• Features Determining which network features work on the machine you are using.

36.17.1 make-network-process

The basic function for creating network connections and network servers is make-network-process. It can do either of those jobs, depending on the arguments you give it.

36.17.2 Network Options

The following network options can be specified when you create a network process. Except for :reuseaddr, you can also set or modify these options later, using set-network-process-option.

For a server process, the options specified with make-network-process are not inherited by the client connections, so you will need to set the necessary options for each child connection as it is created.

:bindtodevice device-name

If device-name is a non-empty string identifying a network interface name (see network-interface-list), only handle packets received on that interface. If device-name is nil (the default), handle packets received on any interface.

Using this option may require special privileges on some systems.

:broadcast broadcast-flag

If broadcast-flag is non-nil for a datagram process, the process will receive datagram packet sent to a broadcast address, and be able to send packets to a broadcast address. This is ignored for a stream connection.

:dontroute dontroute-flag

If dontroute-flag is non-nil, the process can only send to hosts on the same network as the local host.

:keepalive keepalive-flag

If keepalive-flag is non-nil for a stream connection, enable exchange of low-level keep-alive messages.

:linger linger-arg

If linger-arg is non-nil, wait for successful transmission of all queued packets on the connection before it is deleted (see delete-process). If linger-arg is an integer, it specifies the maximum time in seconds to wait for queued packets to be sent before closing the connection. The default is nil, which means to discard unsent queued packets when the process is deleted.

:oobinline oobinline-flag

If oobinline-flag is non-nil for a stream connection, receive out-of-band data in the normal data stream. Otherwise, ignore out-of-band data.

:priority priority

Set the priority for packets sent on this connection to the integer priority. The interpretation of this number is protocol specific; such as setting the TOS (type of service) field on IP packets sent on this connection. It may also have system dependent effects, such as selecting a specific output queue on the network interface.

:reuseaddr reuseaddr-flag

If reuseaddr-flag is non-nil (the default) for a stream server process, allow this server to reuse a specific port number (see :service), unless another process on this host is already listening on that port. If reuseaddr-flag is nil, there may be a period of time after the last use of that port (by any process on the host) where it is not possible to make a new server on that port.

36.17.3 Testing Availability of Network Features

To test for the availability of a given network feature, use featurep like this:

     (featurep 'make-network-process '(keyword value))

The result of this form is t if it works to specify keyword with value value in make-network-process. Here are some of the keyword—value pairs you can test in this way.

(:nowait t)

Non-nil if non-blocking connect is supported.

(:type datagram)

Non-nil if datagrams are supported.

(:family local)

Non-nil if local (a.k.a. “UNIX domain”) sockets are supported.

(:family ipv6)

Non-nil if IPv6 is supported.

(:service t)

Non-nil if the system can select the port for a server.

To test for the availability of a given network option, use featurep like this:

     (featurep 'make-network-process 'keyword)

The accepted keyword values are :bindtodevice, etc. For the complete list, see Network Options. This form returns non-nil if that particular network option is supported by make-network-process (or set-network-process-option).

36.18 Misc Network Facilities

These additional functions are useful for creating and operating on network connections. Note that they are supported only on some systems.

36.19 Communicating with Serial Ports

Emacs can communicate with serial ports. For interactive use, M-x serial-term opens a terminal window. In a Lisp program, make-serial-process creates a process object.

The serial port can be configured at run-time, without having to close and re-open it. The function serial-process-configure lets you change the speed, bytesize, and other parameters. In a terminal window created by serial-term, you can click on the mode line for configuration.

A serial connection is represented by a process object, which can be used in a similar way to a subprocess or network process. You can send and receive data, and configure the serial port. A serial process object has no process ID, however, and you can't send signals to it, and the status codes are different from other types of processes. delete-process on the process object or kill-buffer on the process buffer close the connection, but this does not affect the device connected to the serial port.

The function process-type returns the symbol serial for a process object representing a serial port connection.

Serial ports are available on GNU/Linux, Unix, and MS Windows systems.

36.20 Packing and Unpacking Byte Arrays

This section describes how to pack and unpack arrays of bytes, usually for binary network protocols. These functions convert byte arrays to alists, and vice versa. The byte array can be represented as a unibyte string or as a vector of integers, while the alist associates symbols either with fixed-size objects or with recursive sub-alists. To use the functions referred to in this section, load the bindat library.

Conversion from byte arrays to nested alists is also known as deserializing or unpacking, while going in the opposite direction is also known as serializing or packing.

• Bindat Spec: Describing data layout.
• Bindat Functions: Doing the unpacking and packing.
• Bindat Examples: Samples of what bindat.el can do for you!

36.20.1 Describing Data Layout

To control unpacking and packing, you write a data layout specification, a special nested list describing named and typed fields. This specification controls the length of each field to be processed, and how to pack or unpack it. We normally keep bindat specs in variables whose names end in ‘-bindat-spec’; that kind of name is automatically recognized as risky.

A field's type describes the size (in bytes) of the object that the field represents and, in the case of multibyte fields, how the bytes are ordered within the field. The two possible orderings are big endian (also known as “network byte ordering”) and little endian. For instance, the number #x23cd (decimal 9165) in big endian would be the two bytes #x23 #xcd; and in little endian, #xcd #x23. Here are the possible type values:

u8

byte

Unsigned byte, with length 1.

u16

word

short

Unsigned integer in network byte order, with length 2.

u24

Unsigned integer in network byte order, with length 3.

u32

dword

long

Unsigned integer in network byte order, with length 4. Note: These values may be limited by Emacs's integer implementation limits.

u16r

u24r

u32r

Unsigned integer in little endian order, with length 2, 3 and 4, respectively.

str len

String of length len.

strz len

Zero-terminated string, in a fixed-size field with length len.

vec len [type]

Vector of len elements of type type, defaulting to bytes. The type is any of the simple types above, or another vector specified as a list of the form (vec len [type]).

ip

Four-byte vector representing an Internet address. For example: [127 0 0 1] for localhost.

bits len

List of set bits in len bytes. The bytes are taken in big endian order and the bits are numbered starting with 8 * len − 1 and ending with zero. For example: bits 2 unpacks #x28 #x1c to (2 3 4 11 13) and #x1c #x28 to (3 5 10 11 12).

(eval form)

form is a Lisp expression evaluated at the moment the field is unpacked or packed. The result of the evaluation should be one of the above-listed type specifications.

For a fixed-size field, the length len is given as an integer specifying the number of bytes in the field.

When the length of a field is not fixed, it typically depends on the value of a preceding field. In this case, the length len can be given either as a list (name ...) identifying a field name in the format specified for bindat-get-field below, or by an expression (eval form) where form should evaluate to an integer, specifying the field length.

A field specification generally has the form ([name] handler), where name is optional. Don't use names that are symbols meaningful as type specifications (above) or handler specifications (below), since that would be ambiguous. name can be a symbol or an expression (eval form), in which case form should evaluate to a symbol.

handler describes how to unpack or pack the field and can be one of the following:

type

Unpack/pack this field according to the type specification type.

eval form

Evaluate form, a Lisp expression, for side-effect only. If the field name is specified, the value is bound to that field name.

fill len

Skip len bytes. In packing, this leaves them unchanged, which normally means they remain zero. In unpacking, this means they are ignored.

align len

Skip to the next multiple of len bytes.

struct spec-name

Process spec-name as a sub-specification. This describes a structure nested within another structure.

union form (tag spec)...

Evaluate form, a Lisp expression, find the first tag that matches it, and process its associated data layout specification spec. Matching can occur in one of three ways:

• If a tag has the form (eval expr), evaluate expr with the variable tag dynamically bound to the value of form. A non-nil result indicates a match.
• tag matches if it is equal to the value of form.
• tag matches unconditionally if it is t.

repeat count field-specs...

Process the field-specs recursively, in order, then repeat starting from the first one, processing all the specifications count times overall. The count is given using the same formats as a field length—if an eval form is used, it is evaluated just once. For correct operation, each specification in field-specs must include a name.

For the (eval form) forms used in a bindat specification, the form can access and update these dynamically bound variables during evaluation:

last

Value of the last field processed.

bindat-raw

The data as a byte array.

bindat-idx

Current index (within bindat-raw) for unpacking or packing.

struct

The alist containing the structured data that have been unpacked so far, or the entire structure being packed. You can use bindat-get-field to access specific fields of this structure.

count

index

Inside a repeat block, these contain the maximum number of repetitions (as specified by the count parameter), and the current repetition number (counting from 0). Setting count to zero will terminate the inner-most repeat block after the current repetition has completed.

36.20.2 Functions to Unpack and Pack Bytes

In the following documentation, spec refers to a data layout specification, bindat-raw to a byte array, and struct to an alist representing unpacked field data.

Although packing and unpacking operations change the organization of data (in memory), they preserve the data's total length, which is the sum of all the fields' lengths, in bytes. This value is not generally inherent in either the specification or alist alone; instead, both pieces of information contribute to its calculation. Likewise, the length of a string or array being unpacked may be longer than the data's total length as described by the specification.

36.20.3 Examples of Byte Unpacking and Packing

Here is a complete example of byte unpacking and packing:

     (require 'bindat)
     
     (defvar fcookie-index-spec
       '((:version  u32)
         (:count    u32)
         (:longest  u32)
         (:shortest u32)
         (:flags    u32)
         (:delim    u8)
         (:ignored  fill 3)
         (:offset   repeat (:count) (:foo u32)))
       "Description of a fortune cookie index file's contents.")
     
     (defun fcookie (cookies &optional index)
       "Display a random fortune cookie from file COOKIES.
     Optional second arg INDEX specifies the associated index
     filename, by default \"COOKIES.dat\".  Display cookie text
     in buffer \"*Fortune Cookie: BASENAME*\", where BASENAME
     is COOKIES without the directory part."
       (interactive "fCookies file: ")
       (let* ((info (with-temp-buffer
                      (insert-file-contents-literally
                       (or index (concat cookies ".dat")))
                      (bindat-unpack fcookie-index-spec
                                     (buffer-string))))
              (sel (random (bindat-get-field info :count)))
              (beg (cdar (bindat-get-field info :offset sel)))
              (end (or (cdar (bindat-get-field info
                                               :offset (1+ sel)))
                       (nth 7 (file-attributes cookies)))))
         (switch-to-buffer
          (get-buffer-create
           (format "*Fortune Cookie: %s*"
                   (file-name-nondirectory cookies))))
         (erase-buffer)
         (insert-file-contents-literally
          cookies nil beg (- end 3))))
     
     (defun fcookie-create-index (cookies &optional index delim)
       "Scan file COOKIES, and write out its index file.
     Optional arg INDEX specifies the index filename, which by
     default is \"COOKIES.dat\".  Optional arg DELIM specifies the
     unibyte character that, when found on a line of its own in
     COOKIES, indicates the border between entries."
       (interactive "fCookies file: ")
       (setq delim (or delim ?%))
       (let ((delim-line (format "\n%c\n" delim))
             (count 0)
             (max 0)
             min p q len offsets)
         (unless (= 3 (string-bytes delim-line))
           (error "Delimiter cannot be represented in one byte"))
         (with-temp-buffer
           (insert-file-contents-literally cookies)
           (while (and (setq p (point))
                       (search-forward delim-line (point-max) t)
                       (setq len (- (point) 3 p)))
             (setq count (1+ count)
                   max (max max len)
                   min (min (or min max) len)
                   offsets (cons (1- p) offsets))))
         (with-temp-buffer
           (set-buffer-multibyte nil)
           (insert
            (bindat-pack
             fcookie-index-spec
             `((:version . 2)
               (:count . ,count)
               (:longest . ,max)
               (:shortest . ,min)
               (:flags . 0)
               (:delim . ,delim)
               (:offset . ,(mapcar (lambda (o)
                                     (list (cons :foo o)))
                                   (nreverse offsets))))))
           (let ((coding-system-for-write 'raw-text-unix))
             (write-file (or index (concat cookies ".dat")))))))

The following is an example of defining and unpacking a complex structure. Consider the following C structures:

     struct header {
         unsigned long    dest_ip;
         unsigned long    src_ip;
         unsigned short   dest_port;
         unsigned short   src_port;
     };
     
     struct data {
         unsigned char    type;
         unsigned char    opcode;
         unsigned short   length;  /* in network byte order  */
         unsigned char    id[8];   /* null-terminated string  */
         unsigned char    data[/* (length + 3) & ~3 */];
     };
     
     struct packet {
         struct header    header;
         unsigned long    counters[2];  /* in little endian order  */
         unsigned char    items;
         unsigned char    filler[3];
         struct data      item[/* items */];
     
     };

The corresponding data layout specification is:

     (setq header-spec
           '((dest-ip   ip)
             (src-ip    ip)
             (dest-port u16)
             (src-port  u16)))
     
     (setq data-spec
           '((type      u8)
             (opcode    u8)
             (length    u16)  ; network byte order
             (id        strz 8)
             (data      vec (length))
             (align     4)))
     
     (setq packet-spec
           '((header    struct header-spec)
             (counters  vec 2 u32r)   ; little endian order
             (items     u8)
             (fill      3)
             (item      repeat (items)
                        (struct data-spec))))

A binary data representation is:

     (setq binary-data
           [ 192 168 1 100 192 168 1 101 01 28 21 32
             160 134 1 0 5 1 0 0 2 0 0 0
             2 3 0 5 ?A ?B ?C ?D ?E ?F 0 0 1 2 3 4 5 0 0 0
             1 4 0 7 ?B ?C ?D ?E ?F ?G 0 0 6 7 8 9 10 11 12 0 ])

The corresponding decoded structure is:

     (setq decoded (bindat-unpack packet-spec binary-data))
          ⇒
     ((header
       (dest-ip   . [192 168 1 100])
       (src-ip    . [192 168 1 101])
       (dest-port . 284)
       (src-port  . 5408))
      (counters . [100000 261])
      (items . 2)
      (item ((data . [1 2 3 4 5])
             (id . "ABCDEF")
             (length . 5)
             (opcode . 3)
             (type . 2))
            ((data . [6 7 8 9 10 11 12])
             (id . "BCDEFG")
             (length . 7)
             (opcode . 4)
             (type . 1))))

An example of fetching data from this structure:

     (bindat-get-field decoded 'item 1 'id)
          ⇒ "BCDEFG"

37 Emacs Display

This chapter describes a number of features related to the display that Emacs presents to the user.

• Refresh Screen: Clearing the screen and redrawing everything on it.
• Forcing Redisplay: Forcing redisplay.
• Truncation: Folding or wrapping long text lines.
• The Echo Area: Displaying messages at the bottom of the screen.
• Warnings: Displaying warning messages for the user.
• Invisible Text: Hiding part of the buffer text.
• Selective Display: Hiding part of the buffer text (the old way).
• Temporary Displays: Displays that go away automatically.
• Overlays: Use overlays to highlight parts of the buffer.
• Size of Displayed Text: How large displayed text is.
• Line Height: Controlling the height of lines.
• Faces: A face defines a graphics style for text characters: font, colors, etc.
• Fringes: Controlling window fringes.
• Scroll Bars: Controlling scroll bars.
• Window Dividers: Separating windows visually.
• Display Property: Enabling special display features.
• Images: Displaying images in Emacs buffers.
• Xwidgets: Displaying native widgets in Emacs buffers.
• Buttons: Adding clickable buttons to Emacs buffers.
• Abstract Display: Emacs's Widget for Object Collections.
• Blinking: How Emacs shows the matching open parenthesis.
• Character Display: How Emacs displays individual characters.
• Beeping: Audible signal to the user.
• Window Systems: Which window system is being used.
• Tooltips: Tooltip display in Emacs.
• Bidirectional Display: Display of bidirectional scripts, such as Arabic and Farsi.

37.1 Refreshing the Screen

The function redraw-frame clears and redisplays the entire contents of a given frame (see Frames). This is useful if the screen is corrupted.

Even more powerful is redraw-display:

In Emacs, processing user input takes priority over redisplay. If you call these functions when input is available, they don't redisplay immediately, but the requested redisplay does happen eventually—after all the input has been processed.

On text terminals, suspending and resuming Emacs normally also refreshes the screen. Some terminal emulators record separate contents for display-oriented programs such as Emacs and for ordinary sequential display. If you are using such a terminal, you might want to inhibit the redisplay on resumption.

37.2 Forcing Redisplay

Emacs normally tries to redisplay the screen whenever it waits for input. With the following function, you can request an immediate attempt to redisplay, in the middle of Lisp code, without actually waiting for input.

Although redisplay tries immediately to redisplay, it does not change how Emacs decides which parts of its frame(s) to redisplay. By contrast, the following function adds certain windows to the pending redisplay work (as if their contents had completely changed), but does not immediately try to perform redisplay.

37.3 Truncation

When a line of text extends beyond the right edge of a window, Emacs can continue the line (make it wrap to the next screen line), or truncate the line (limit it to one screen line). The additional screen lines used to display a long text line are called continuation lines. Continuation is not the same as filling; continuation happens on the screen only, not in the buffer contents, and it breaks a line precisely at the right margin, not at a word boundary. See Filling.

On a graphical display, tiny arrow images in the window fringes indicate truncated and continued lines (see Fringes). On a text terminal, a ‘$’ in the rightmost column of the window indicates truncation; a ‘\’ on the rightmost column indicates a line that wraps. (The display table can specify alternate characters to use for this; see Display Tables).

When horizontal scrolling (see Horizontal Scrolling) is in use in a window, that forces truncation.

37.4 The Echo Area

The echo area is used for displaying error messages (see Errors), for messages made with the message primitive, and for echoing keystrokes. It is not the same as the minibuffer, despite the fact that the minibuffer appears (when active) in the same place on the screen as the echo area. See The Minibuffer.

Apart from the functions documented in this section, you can print Lisp objects to the echo area by specifying t as the output stream. See Output Streams.

• Displaying Messages: Explicitly displaying text in the echo area.
• Progress: Informing user about progress of a long operation.
• Logging Messages: Echo area messages are logged for the user.
• Echo Area Customization: Controlling the echo area.

37.4.1 Displaying Messages in the Echo Area

This section describes the standard functions for displaying messages in the echo area.

37.4.2 Reporting Operation Progress

When an operation can take a while to finish, you should inform the user about the progress it makes. This way the user can estimate remaining time and clearly see that Emacs is busy working, not hung. A convenient way to do this is to use a progress reporter.

Here is a working example that does nothing useful:

     (let ((progress-reporter
            (make-progress-reporter "Collecting mana for Emacs..."
                                    0  500)))
       (dotimes (k 500)
         (sit-for 0.01)
         (progress-reporter-update progress-reporter k))
       (progress-reporter-done progress-reporter))

37.4.3 Logging Messages in *Messages*

Almost all the messages displayed in the echo area are also recorded in the *Messages* buffer so that the user can refer back to them. This includes all the messages that are output with message. By default, this buffer is read-only and uses the major mode messages-buffer-mode. Nothing prevents the user from killing the *Messages* buffer, but the next display of a message recreates it. Any Lisp code that needs to access the *Messages* buffer directly and wants to ensure that it exists should use the function messages-buffer.

To make *Messages* more convenient for the user, the logging facility combines successive identical messages. It also combines successive related messages for the sake of two cases: question followed by answer, and a series of progress messages.

A question followed by an answer has two messages like the ones produced by y-or-n-p: the first is ‘question’, and the second is ‘question...answer’. The first message conveys no additional information beyond what's in the second, so logging the second message discards the first from the log.

A series of progress messages has successive messages like those produced by make-progress-reporter. They have the form ‘base...how-far’, where base is the same each time, while how-far varies. Logging each message in the series discards the previous one, provided they are consecutive.

The functions make-progress-reporter and y-or-n-p don't have to do anything special to activate the message log combination feature. It operates whenever two consecutive messages are logged that share a common prefix ending in ‘...’.

37.4.4 Echo Area Customization

These variables control details of how the echo area works.

The variable max-mini-window-height, which specifies the maximum height for resizing minibuffer windows, also applies to the echo area (which is really a special use of the minibuffer window; see Minibuffer Misc).

37.5 Reporting Warnings

Warnings are a facility for a program to inform the user of a possible problem, but continue running.

• Warning Basics: Warnings concepts and functions to report them.
• Warning Variables: Variables programs bind to customize their warnings.
• Warning Options: Variables users set to control display of warnings.
• Delayed Warnings: Deferring a warning until the end of a command.

37.5.1 Warning Basics

Every warning has a textual message, which explains the problem for the user, and a severity level which is a symbol. Here are the possible severity levels, in order of decreasing severity, and their meanings:

:emergency

A problem that will seriously impair Emacs operation soon if you do not attend to it promptly.

:error

A report of data or circumstances that are inherently wrong.

:warning

A report of data or circumstances that are not inherently wrong, but raise suspicion of a possible problem.

:debug

A report of information that may be useful if you are debugging.

When your program encounters invalid input data, it can either signal a Lisp error by calling error or signal or report a warning with severity :error. Signaling a Lisp error is the easiest thing to do, but it means the program cannot continue processing. If you want to take the trouble to implement a way to continue processing despite the bad data, then reporting a warning of severity :error is the right way to inform the user of the problem. For instance, the Emacs Lisp byte compiler can report an error that way and continue compiling other functions. (If the program signals a Lisp error and then handles it with condition-case, the user won't see the error message; it could show the message to the user by reporting it as a warning.)

Each warning has a warning type to classify it. The type is a list of symbols. The first symbol should be the custom group that you use for the program's user options. For example, byte compiler warnings use the warning type (bytecomp). You can also subcategorize the warnings, if you wish, by using more symbols in the list.

37.5.2 Warning Variables

Programs can customize how their warnings appear by binding the variables described in this section.

37.5.3 Warning Options

These variables are used by users to control what happens when a Lisp program reports a warning.

37.5.4 Delayed Warnings

Sometimes, you may wish to avoid showing a warning while a command is running, and only show it only after the end of the command. You can use the variable delayed-warnings-list for this.

Programs which need to further customize the delayed warnings mechanism can change the variable delayed-warnings-hook:

37.6 Invisible Text

You can make characters invisible, so that they do not appear on the screen, with the invisible property. This can be either a text property (see Text Properties) or an overlay property (see Overlays). Cursor motion also partly ignores these characters; if the command loop finds that point is inside a range of invisible text after a command, it relocates point to the other side of the text.

In the simplest case, any non-nil invisible property makes a character invisible. This is the default case—if you don't alter the default value of buffer-invisibility-spec, this is how the invisible property works. You should normally use t as the value of the invisible property if you don't plan to set buffer-invisibility-spec yourself.

More generally, you can use the variable buffer-invisibility-spec to control which values of the invisible property make text invisible. This permits you to classify the text into different subsets in advance, by giving them different invisible values, and subsequently make various subsets visible or invisible by changing the value of buffer-invisibility-spec.

Controlling visibility with buffer-invisibility-spec is especially useful in a program to display the list of entries in a database. It permits the implementation of convenient filtering commands to view just a part of the entries in the database. Setting this variable is very fast, much faster than scanning all the text in the buffer looking for properties to change.

Two functions are specifically provided for adding elements to buffer-invisibility-spec and removing elements from it.

A convention for use of buffer-invisibility-spec is that a major mode should use the mode's own name as an element of buffer-invisibility-spec and as the value of the invisible property:

     ;; If you want to display an ellipsis:
     (add-to-invisibility-spec '(my-symbol . t))
     ;; If you don't want ellipsis:
     (add-to-invisibility-spec 'my-symbol)
     
     (overlay-put (make-overlay beginning end)
                  'invisible 'my-symbol)
     
     ;; When done with the invisibility:
     (remove-from-invisibility-spec '(my-symbol . t))
     ;; Or respectively:
     (remove-from-invisibility-spec 'my-symbol)

You can check for invisibility using the following function:

Ordinarily, functions that operate on text or move point do not care whether the text is invisible, they process invisible characters and visible characters alike. The user-level line motion commands, such as next-line, previous-line, ignore invisible newlines if line-move-ignore-invisible is non-nil (the default), i.e., behave like these invisible newlines didn't exist in the buffer, but only because they are explicitly programmed to do so.

If a command ends with point inside or at the boundary of invisible text, the main editing loop relocates point to one of the two ends of the invisible text. Emacs chooses the direction of relocation so that it is the same as the overall movement direction of the command; if in doubt, it prefers a position where an inserted char would not inherit the invisible property. Additionally, if the text is not replaced by an ellipsis and the command only moved within the invisible text, then point is moved one extra character so as to try and reflect the command's movement by a visible movement of the cursor.

Thus, if the command moved point back to an invisible range (with the usual stickiness), Emacs moves point back to the beginning of that range. If the command moved point forward into an invisible range, Emacs moves point forward to the first visible character that follows the invisible text and then forward one more character.

These adjustments of point that ended up in the middle of invisible text can be disabled by setting disable-point-adjustment to a non-nil value. See Adjusting Point.

Incremental search can make invisible overlays visible temporarily and/or permanently when a match includes invisible text. To enable this, the overlay should have a non-nil isearch-open-invisible property. The property value should be a function to be called with the overlay as an argument. This function should make the overlay visible permanently; it is used when the match overlaps the overlay on exit from the search.

During the search, such overlays are made temporarily visible by temporarily modifying their invisible and intangible properties. If you want this to be done differently for a certain overlay, give it an isearch-open-invisible-temporary property which is a function. The function is called with two arguments: the first is the overlay, and the second is nil to make the overlay visible, or t to make it invisible again.

37.7 Selective Display

Selective display refers to a pair of related features for hiding certain lines on the screen.

The first variant, explicit selective display, was designed for use in a Lisp program: it controls which lines are hidden by altering the text. This kind of hiding is now obsolete; instead you can get the same effect with the invisible property (see Invisible Text).

In the second variant, the choice of lines to hide is made automatically based on indentation. This variant is designed to be a user-level feature.

The way you control explicit selective display is by replacing a newline (control-j) with a carriage return (control-m). The text that was formerly a line following that newline is now hidden. Strictly speaking, it is temporarily no longer a line at all, since only newlines can separate lines; it is now part of the previous line.

Selective display does not directly affect editing commands. For example, C-f (forward-char) moves point unhesitatingly into hidden text. However, the replacement of newline characters with carriage return characters affects some editing commands. For example, next-line skips hidden lines, since it searches only for newlines. Modes that use selective display can also define commands that take account of the newlines, or that control which parts of the text are hidden.

When you write a selectively displayed buffer into a file, all the control-m's are output as newlines. This means that when you next read in the file, it looks OK, with nothing hidden. The selective display effect is seen only within Emacs.

37.8 Temporary Displays

Temporary displays are used by Lisp programs to put output into a buffer and then present it to the user for perusal rather than for editing. Many help commands use this feature.

The two constructs described next are mostly identical to with-temp-buffer-window but differ from it as specified:

A window showing a temporary buffer can be fit to the size of that buffer using the following mode:

The following function uses the current buffer for temporal display:

37.9 Overlays

You can use overlays to alter the appearance of a buffer's text on the screen, for the sake of presentation features. An overlay is an object that belongs to a particular buffer, and has a specified beginning and end. It also has properties that you can examine and set; these affect the display of the text within the overlay.

The visual effect of an overlay is the same as of the corresponding text property (see Text Properties). However, due to a different implementation, overlays generally don't scale well (many operations take a time that is proportional to the number of overlays in the buffer). If you need to affect the visual appearance of many portions in the buffer, we recommend using text properties.

An overlay uses markers to record its beginning and end; thus, editing the text of the buffer adjusts the beginning and end of each overlay so that it stays with the text. When you create the overlay, you can specify whether text inserted at the beginning should be inside the overlay or outside, and likewise for the end of the overlay.

• Managing Overlays: Creating and moving overlays.
• Overlay Properties: How to read and set properties. What properties do to the screen display.
• Finding Overlays: Searching for overlays.

37.9.1 Managing Overlays

This section describes the functions to create, delete and move overlays, and to examine their contents. Overlay changes are not recorded in the buffer's undo list, since the overlays are not part of the buffer's contents.

Here are some examples:

     ;; Create an overlay.
     (setq foo (make-overlay 1 10))
          ⇒ #<overlay from 1 to 10 in display.texi>
     (overlay-start foo)
          ⇒ 1
     (overlay-end foo)
          ⇒ 10
     (overlay-buffer foo)
          ⇒ #<buffer display.texi>
     ;; Give it a property we can check later.
     (overlay-put foo 'happy t)
          ⇒ t
     ;; Verify the property is present.
     (overlay-get foo 'happy)
          ⇒ t
     ;; Move the overlay.
     (move-overlay foo 5 20)
          ⇒ #<overlay from 5 to 20 in display.texi>
     (overlay-start foo)
          ⇒ 5
     (overlay-end foo)
          ⇒ 20
     ;; Delete the overlay.
     (delete-overlay foo)
          ⇒ nil
     ;; Verify it is deleted.
     foo
          ⇒ #<overlay in no buffer>
     ;; A deleted overlay has no position.
     (overlay-start foo)
          ⇒ nil
     (overlay-end foo)
          ⇒ nil
     (overlay-buffer foo)
          ⇒ nil
     ;; Undelete the overlay.
     (move-overlay foo 1 20)
          ⇒ #<overlay from 1 to 20 in display.texi>
     ;; Verify the results.
     (overlay-start foo)
          ⇒ 1
     (overlay-end foo)
          ⇒ 20
     (overlay-buffer foo)
          ⇒ #<buffer display.texi>
     ;; Moving and deleting the overlay does not change its properties.
     (overlay-get foo 'happy)
          ⇒ t

Emacs stores the overlays of each buffer in two lists, divided around an arbitrary center position. One list extends backwards through the buffer from that center position, and the other extends forwards from that center position. The center position can be anywhere in the buffer.

A loop that scans the buffer forwards, creating overlays, can run faster if you do (overlay-recenter (point-max)) first.

37.9.2 Overlay Properties

Overlay properties are like text properties in that the properties that alter how a character is displayed can come from either source. But in most respects they are different. See Text Properties, for comparison.

Text properties are considered a part of the text; overlays and their properties are specifically considered not to be part of the text. Thus, copying text between various buffers and strings preserves text properties, but does not try to preserve overlays. Changing a buffer's text properties marks the buffer as modified, while moving an overlay or changing its properties does not. Unlike text property changes, overlay property changes are not recorded in the buffer's undo list.

Since more than one overlay can specify a property value for the same character, Emacs lets you specify a priority value of each overlay. The priority value is used to decide which of the overlapping overlays will “win”.

These functions read and set the properties of an overlay:

See also the function get-char-property which checks both overlay properties and text properties for a given character. See Examining Properties.

Many overlay properties have special meanings; here is a table of them:

priority

This property's value determines the priority of the overlay. If you want to specify a priority value, use either nil (or zero), or a positive integer. Any other value has undefined behavior.

The priority matters when two or more overlays cover the same character and both specify the same property; the one whose priority value is larger overrides the other. (For the face property, the higher priority overlay's value does not completely override the other value; instead, its face attributes override the face attributes of the lower priority face property.) If two overlays have the same priority value, and one is nested in the other, then the inner one will prevail over the outer one. If neither is nested in the other then you should not make assumptions about which overlay will prevail.

Currently, all overlays take priority over text properties.

Note that Emacs sometimes uses non-numeric priority values for some of its internal overlays, so do not try to do arithmetic on the priority of an overlay (unless it is one that you created). In particular, the overlay used for showing the region uses a priority value of the form (primary . secondary), where the primary value is used as described above, and secondary is the fallback value used when primary and the nesting considerations fail to resolve the precedence between overlays. However, you are advised not to design Lisp programs based on this implementation detail; if you need to put overlays in priority order, use the sorted argument of overlays-at. See Finding Overlays.

window

If the window property is non-nil, then the overlay applies only on that window.

category

If an overlay has a category property, we call it the category of the overlay. It should be a symbol. The properties of the symbol serve as defaults for the properties of the overlay.

face

This property controls the appearance of the text (see Faces). The value of the property can be the following:

• A face name (a symbol or string).
• An anonymous face: a property list of the form (keyword value ...), where each keyword is a face attribute name and value is a value for that attribute.
• A list of faces. Each list element should be either a face name or an anonymous face. This specifies a face which is an aggregate of the attributes of each of the listed faces. Faces occurring earlier in the list have higher priority.
• A cons cell of the form (foreground-color . color-name) or (background-color . color-name). This specifies the foreground or background color, similar to (:foreground color-name) or (:background color-name). This form is supported for backward compatibility only, and should be avoided.

mouse-face

This property is used instead of face when the mouse is within the range of the overlay. However, Emacs ignores all face attributes from this property that alter the text size (e.g., :height, :weight, and :slant). Those attributes are always the same as in the unhighlighted text.

display

This property activates various features that change the way text is displayed. For example, it can make text appear taller or shorter, higher or lower, wider or narrower, or replaced with an image. See Display Property.

help-echo

If an overlay has a help-echo property, then when you move the mouse onto the text in the overlay, Emacs displays a help string in the echo area, or in the tooltip window. For details see Text help-echo.

field

Consecutive characters with the same field property constitute a field. Some motion functions including forward-word and beginning-of-line stop moving at a field boundary. See Fields.

modification-hooks

This property's value is a list of functions to be called if any character within the overlay is changed or if text is inserted strictly within the overlay.

The hook functions are called both before and after each change. If the functions save the information they receive, and compare notes between calls, they can determine exactly what change has been made in the buffer text.

When called before a change, each function receives four arguments: the overlay, nil, and the beginning and end of the text range to be modified.

When called after a change, each function receives five arguments: the overlay, t, the beginning and end of the text range just modified, and the length of the pre-change text replaced by that range. (For an insertion, the pre-change length is zero; for a deletion, that length is the number of characters deleted, and the post-change beginning and end are equal.)

If these functions modify the buffer, they should bind inhibit-modification-hooks to t around doing so, to avoid confusing the internal mechanism that calls these hooks.

Text properties also support the modification-hooks property, but the details are somewhat different (see Special Properties).

insert-in-front-hooks

This property's value is a list of functions to be called before and after inserting text right at the beginning of the overlay. The calling conventions are the same as for the modification-hooks functions.

insert-behind-hooks

This property's value is a list of functions to be called before and after inserting text right at the end of the overlay. The calling conventions are the same as for the modification-hooks functions.

invisible

The invisible property can make the text in the overlay invisible, which means that it does not appear on the screen. See Invisible Text, for details.

intangible

The intangible property on an overlay works just like the intangible text property. It is obsolete. See Special Properties, for details.

isearch-open-invisible

This property tells incremental search how to make an invisible overlay visible, permanently, if the final match overlaps it. See Invisible Text.

isearch-open-invisible-temporary

This property tells incremental search how to make an invisible overlay visible, temporarily, during the search. See Invisible Text.

before-string

This property's value is a string to add to the display at the beginning of the overlay. The string does not appear in the buffer in any sense—only on the screen.

after-string

This property's value is a string to add to the display at the end of the overlay. The string does not appear in the buffer in any sense—only on the screen.

line-prefix

This property specifies a display spec to prepend to each non-continuation line at display-time. See Truncation.

wrap-prefix

This property specifies a display spec to prepend to each continuation line at display-time. See Truncation.

evaporate

If this property is non-nil, the overlay is deleted automatically if it becomes empty (i.e., if its length becomes zero). If you give an empty overlay (see empty overlay) a non-nil evaporate property, that deletes it immediately. Note that, unless an overlay has this property, it will not be deleted when the text between its starting and ending positions is deleted from the buffer.

keymap

If this property is non-nil, it specifies a keymap for a portion of the text. This keymap is used when the character after point is within the overlay, and takes precedence over most other keymaps. See Active Keymaps.

local-map

The local-map property is similar to keymap but replaces the buffer's local map rather than augmenting existing keymaps. This also means it has lower precedence than minor mode keymaps.

The keymap and local-map properties do not affect a string displayed by the before-string, after-string, or display properties. This is only relevant for mouse clicks and other mouse events that fall on the string, since point is never on the string. To bind special mouse events for the string, assign it a keymap or local-map text property. See Special Properties.

37.9.3 Searching for Overlays

As an example, here's a simplified (and inefficient) version of the primitive function next-single-char-property-change (see Property Search). It searches forward from position pos for the next position where the value of a given property prop, as obtained from either overlays or text properties, changes.

     (defun next-single-char-property-change (position prop)
       (save-excursion
         (goto-char position)
         (let ((propval (get-char-property (point) prop)))
           (while (and (not (eobp))
                       (eq (get-char-property (point) prop) propval))
             (goto-char (min (next-overlay-change (point))
                             (next-single-property-change (point) prop)))))
         (point)))

37.10 Size of Displayed Text

Since not all characters have the same width, these functions let you check the width of a character. See Primitive Indent, and Screen Lines, for related functions.

The following function returns the size in pixels of text as if it were displayed in a given window. This function is used by fit-window-to-buffer and fit-frame-to-buffer (see Resizing Windows) to make a window exactly as large as the text it contains.

37.11 Line Height

The total height of each display line consists of the height of the contents of the line, plus optional additional vertical line spacing above or below the display line.

The height of the line contents is the maximum height of any character or image on that display line, including the final newline if there is one. (A display line that is continued doesn't include a final newline.) That is the default line height, if you do nothing to specify a greater height. (In the most common case, this equals the height of the corresponding frame's default font, see Frame Font.)

There are several ways to explicitly specify a larger line height, either by specifying an absolute height for the display line, or by specifying vertical space. However, no matter what you specify, the actual line height can never be less than the default.

A newline can have a line-height text or overlay property that controls the total height of the display line ending in that newline.

If the property value is t, the newline character has no effect on the displayed height of the line—the visible contents alone determine the height. The line-spacing property, described below, is also ignored in this case. This is useful for tiling small images (or image slices) without adding blank areas between the images.

If the property value is a list of the form (height total), that adds extra space below the display line. First Emacs uses height as a height spec to control extra space above the line; then it adds enough space below the line to bring the total line height up to total. In this case, any value of line-spacing property for the newline is ignored.

Any other kind of property value is a height spec, which translates into a number—the specified line height. There are several ways to write a height spec; here's how each of them translates into a number:

integer

If the height spec is a positive integer, the height value is that integer.

float

If the height spec is a float, float, the numeric height value is float times the frame's default line height.

(face . ratio)

If the height spec is a cons of the format shown, the numeric height is ratio times the height of face face. ratio can be any type of number, or nil which means a ratio of 1. If face is t, it refers to the current face.

(nil . ratio)

If the height spec is a cons of the format shown, the numeric height is ratio times the height of the contents of the line.

Thus, any valid height spec determines the height in pixels, one way or another. If the line contents' height is less than that, Emacs adds extra vertical space above the line to achieve the specified total height.

If you don't specify the line-height property, the line's height consists of the contents' height plus the line spacing. There are several ways to specify the line spacing for different parts of Emacs text.

On graphical terminals, you can specify the line spacing for all lines in a frame, using the line-spacing frame parameter (see Layout Parameters). However, if the default value of line-spacing is non-nil, it overrides the frame's line-spacing parameter. An integer specifies the number of pixels put below lines. A floating-point number specifies the spacing relative to the frame's default line height.

You can specify the line spacing for all lines in a buffer via the buffer-local line-spacing variable. An integer specifies the number of pixels put below lines. A floating-point number specifies the spacing relative to the default frame line height. This overrides line spacings specified for the frame.

Finally, a newline can have a line-spacing text or overlay property that can enlarge the default frame line spacing and the buffer local line-spacing variable: if its value is larger than the buffer or frame defaults, that larger value is used instead, for the display line ending in that newline.

One way or another, these mechanisms specify a Lisp value for the spacing of each line. The value is a height spec, and it translates into a Lisp value as described above. However, in this case the numeric height value specifies the line spacing, rather than the line height.

On text terminals, the line spacing cannot be altered.

37.12 Faces

A face is a collection of graphical attributes for displaying text: font, foreground color, background color, optional underlining, etc. Faces control how Emacs displays text in buffers, as well as other parts of the frame such as the mode line.

One way to represent a face is as a property list of attributes, like (:foreground "red" :weight bold). Such a list is called an anonymous face. For example, you can assign an anonymous face as the value of the face text property, and Emacs will display the underlying text with the specified attributes. See Special Properties.

More commonly, a face is referred to via a face name: a Lisp symbol associated with a set of face attributes¹⁹. Named faces are defined using the defface macro (see Defining Faces). Emacs comes with several standard named faces (see Basic Faces).

Many parts of Emacs required named faces, and do not accept anonymous faces. These include the functions documented in Attribute Functions, and the variable font-lock-keywords (see Search-based Fontification). Unless otherwise stated, we will use the term face to refer only to named faces.

• Face Attributes: What is in a face?
• Defining Faces: How to define a face.
• Attribute Functions: Functions to examine and set face attributes.
• Displaying Faces: How Emacs combines the faces specified for a character.
• Face Remapping: Remapping faces to alternative definitions.
• Face Functions: How to define and examine faces.
• Auto Faces: Hook for automatic face assignment.
• Basic Faces: Faces that are defined by default.
• Font Selection: Finding the best available font for a face.
• Font Lookup: Looking up the names of available fonts and information about them.
• Fontsets: A fontset is a collection of fonts that handle a range of character sets.
• Low-Level Font: Lisp representation for character display fonts.

37.12.1 Face Attributes

Face attributes determine the visual appearance of a face. The following table lists all the face attributes, their possible values, and their effects.

Apart from the values given below, each face attribute can have the value unspecified. This special value means that the face doesn't specify that attribute directly. An unspecified attribute tells Emacs to refer instead to a parent face (see the description :inherit attribute below); or, failing that, to an underlying face (see Displaying Faces). The default face must specify all attributes.

Some of these attributes are meaningful only on certain kinds of displays. If your display cannot handle a certain attribute, the attribute is ignored.

:family

Font family or fontset (a string). See Fonts, for more information about font families. The function font-family-list (see below) returns a list of available family names. See Fontsets, for information about fontsets.

:foundry

The name of the font foundry for the font family specified by the :family attribute (a string). See Fonts.

:width

Relative character width. This should be one of the symbols ultra-condensed, extra-condensed, condensed, semi-condensed, normal, semi-expanded, expanded, extra-expanded, or ultra-expanded.

:height

The height of the font. In the simplest case, this is an integer in units of 1/10 point.

The value can also be floating point or a function, which specifies the height relative to an underlying face (see Displaying Faces). A floating-point value specifies the amount by which to scale the height of the underlying face. A function value is called with one argument, the height of the underlying face, and returns the height of the new face. If the function is passed an integer argument, it must return an integer.

The height of the default face must be specified using an integer; floating point and function values are not allowed.

:weight

Font weight—one of the symbols (from densest to faintest) ultra-bold, extra-bold, bold, semi-bold, normal, semi-light, light, extra-light, or ultra-light. On text terminals which support variable-brightness text, any weight greater than normal is displayed as extra bright, and any weight less than normal is displayed as half-bright.

:slant

Font slant—one of the symbols italic, oblique, normal, reverse-italic, or reverse-oblique. On text terminals that support variable-brightness text, slanted text is displayed as half-bright.

:foreground

Foreground color, a string. The value can be a system-defined color name, or a hexadecimal color specification. See Color Names. On black-and-white displays, certain shades of gray are implemented by stipple patterns.

:distant-foreground

Alternative foreground color, a string. This is like :foreground but the color is only used as a foreground when the background color is near to the foreground that would have been used. This is useful for example when marking text (i.e., the region face). If the text has a foreground that is visible with the region face, that foreground is used. If the foreground is near the region face background, :distant-foreground is used instead so the text is readable.

:background

Background color, a string. The value can be a system-defined color name, or a hexadecimal color specification. See Color Names.

:underline

Whether or not characters should be underlined, and in what way. The possible values of the :underline attribute are:

nil

Don't underline.

t

Underline with the foreground color of the face.

color

Underline in color color, a string specifying a color.

(:color color :style style)

color is either a string, or the symbol foreground-color, meaning the foreground color of the face. Omitting the attribute :color means to use the foreground color of the face. style should be a symbol line or wave, meaning to use a straight or wavy line. Omitting the attribute :style means to use a straight line.

:overline

Whether or not characters should be overlined, and in what color. If the value is t, overlining uses the foreground color of the face. If the value is a string, overlining uses that color. The value nil means do not overline.

:strike-through

Whether or not characters should be strike-through, and in what color. The value is used like that of :overline.

:box

Whether or not a box should be drawn around characters, its color, the width of the box lines, and 3D appearance. Here are the possible values of the :box attribute, and what they mean:

nil

Don't draw a box.

t

Draw a box with lines of width 1, in the foreground color.

color

Draw a box with lines of width 1, in color color.

(:line-width width :color color :style style)

This way you can explicitly specify all aspects of the box. The value width specifies the width of the lines to draw; it defaults to 1. A negative width -n means to draw a line of width n that occupies the space of the underlying text, thus avoiding any increase in the character height or width.

The value color specifies the color to draw with. The default is the foreground color of the face for simple boxes, and the background color of the face for 3D boxes.

The value style specifies whether to draw a 3D box. If it is released-button, the box looks like a 3D button that is not being pressed. If it is pressed-button, the box looks like a 3D button that is being pressed. If it is nil or omitted, a plain 2D box is used.

:inverse-video

Whether or not characters should be displayed in inverse video. The value should be t (yes) or nil (no).

:stipple

The background stipple, a bitmap.

The value can be a string; that should be the name of a file containing external-format X bitmap data. The file is found in the directories listed in the variable x-bitmap-file-path.

Alternatively, the value can specify the bitmap directly, with a list of the form (width height data). Here, width and height specify the size in pixels, and data is a string containing the raw bits of the bitmap, row by row. Each row occupies (width + 7) / 8 consecutive bytes in the string (which should be a unibyte string for best results). This means that each row always occupies at least one whole byte.

If the value is nil, that means use no stipple pattern.

Normally you do not need to set the stipple attribute, because it is used automatically to handle certain shades of gray.

:font

The font used to display the face. Its value should be a font object. See Low-Level Font, for information about font objects, font specs, and font entities.

When specifying this attribute using set-face-attribute (see Attribute Functions), you may also supply a font spec, a font entity, or a string. Emacs converts such values to an appropriate font object, and stores that font object as the actual attribute value. If you specify a string, the contents of the string should be a font name (see Fonts); if the font name is an XLFD containing wildcards, Emacs chooses the first font matching those wildcards. Specifying this attribute also changes the values of the :family, :foundry, :width, :height, :weight, and :slant attributes.

:inherit

The name of a face from which to inherit attributes, or a list of face names. Attributes from inherited faces are merged into the face like an underlying face would be, with higher priority than underlying faces (see Displaying Faces). If a list of faces is used, attributes from faces earlier in the list override those from later faces.

37.12.2 Defining Faces

The usual way to define a face is through the defface macro. This macro associates a face name (a symbol) with a default face spec. A face spec is a construct which specifies what attributes a face should have on any given terminal; for example, a face spec might specify one foreground color on high-color terminals, and a different foreground color on low-color terminals.

People are sometimes tempted to create a variable whose value is a face name. In the vast majority of cases, this is not necessary; the usual procedure is to define a face with defface, and then use its name directly.

For example, here's the definition of the standard face highlight:

     (defface highlight
       '((((class color) (min-colors 88) (background light))
          :background "darkseagreen2")
         (((class color) (min-colors 88) (background dark))
          :background "darkolivegreen")
         (((class color) (min-colors 16) (background light))
          :background "darkseagreen2")
         (((class color) (min-colors 16) (background dark))
          :background "darkolivegreen")
         (((class color) (min-colors 8))
          :background "green" :foreground "black")
         (t :inverse-video t))
       "Basic face for highlighting."
       :group 'basic-faces)

Internally, Emacs stores each face's default spec in its face-defface-spec symbol property (see Symbol Properties). The saved-face property stores any face spec saved by the user using the customization buffer; the customized-face property stores the face spec customized for the current session, but not saved; and the theme-face property stores an alist associating the active customization settings and Custom themes with the face specs for that face. The face's documentation string is stored in the face-documentation property.

Normally, a face is declared just once, using defface, and any further changes to its appearance are applied using the Customize framework (e.g., via the Customize user interface or via the custom-set-faces function; see Applying Customizations), or by face remapping (see Face Remapping). In the rare event that you need to change a face spec directly from Lisp, you can use the face-spec-set function.

37.12.3 Face Attribute Functions

This section describes functions for directly accessing and modifying the attributes of a named face.

Normally, Emacs uses the face specs of each face to automatically calculate its attributes on each frame (see Defining Faces). The function set-face-attribute can override this calculation by directly assigning attributes to a face, either on a specific frame or for all frames. This function is mostly intended for internal usage.

The following commands and functions mostly provide compatibility with old versions of Emacs. They work by calling set-face-attribute. Values of t and nil (or omitted) for their frame argument are handled just like set-face-attribute and face-attribute. The commands read their arguments using the minibuffer, if called interactively.

The following functions examine the attributes of a face. They mostly provide compatibility with old versions of Emacs. If you don't specify frame, they refer to the selected frame; t refers to the default data for new frames. They return unspecified if the face doesn't define any value for that attribute. If inherit is nil, only an attribute directly defined by the face is returned. If inherit is non-nil, any faces specified by its :inherit attribute are considered as well, and if inherit is a face or a list of faces, then they are also considered, until a specified attribute is found. To ensure that the return value is always specified, use a value of default for inherit.

37.12.4 Displaying Faces

When Emacs displays a given piece of text, the visual appearance of the text may be determined by faces drawn from different sources. If these various sources together specify more than one face for a particular character, Emacs merges the attributes of the various faces. Here is the order in which Emacs merges the faces, from highest to lowest priority:

• If the text consists of a special glyph, the glyph can specify a particular face. See Glyphs.
• If the text lies within an active region, Emacs highlights it using the region face. See Standard Faces.
• If the text lies within an overlay with a non-nil face property, Emacs applies the face(s) specified by that property. If the overlay has a mouse-face property and the mouse is near enough to the overlay, Emacs applies the face or face attributes specified by the mouse-face property instead. See Overlay Properties.

  When multiple overlays cover one character, an overlay with higher priority overrides those with lower priority. See Overlays.

• If the text contains a face or mouse-face property, Emacs applies the specified faces and face attributes. See Special Properties. (This is how Font Lock mode faces are applied. See Font Lock Mode.)
• If the text lies within the mode line of the selected window, Emacs applies the mode-line face. For the mode line of a non-selected window, Emacs applies the mode-line-inactive face. For a header line, Emacs applies the header-line face.
• If any given attribute has not been specified during the preceding steps, Emacs applies the attribute of the default face.

At each stage, if a face has a valid :inherit attribute, Emacs treats any attribute with an unspecified value as having the corresponding value drawn from the parent face(s). see Face Attributes. Note that the parent face(s) may also leave the attribute unspecified; in that case, the attribute remains unspecified at the next level of face merging.

37.12.5 Face Remapping

The variable face-remapping-alist is used for buffer-local or global changes in the appearance of a face. For instance, it is used to implement the text-scale-adjust command (see Text Scale).

The following functions implement a higher-level interface to face-remapping-alist. Most Lisp code should use these functions instead of setting face-remapping-alist directly, to avoid trampling on remappings applied elsewhere. These functions are intended for buffer-local remappings, so they all make face-remapping-alist buffer-local as a side-effect. They manage face-remapping-alist entries of the form

       (face relative-spec-1 relative-spec-2 ... base-spec)

where, as explained above, each of the relative-spec-N and base-spec is either a face name, or a property list of attribute/value pairs. Each of the relative remapping entries, relative-spec-N, is managed by the face-remap-add-relative and face-remap-remove-relative functions; these are intended for simple modifications like changing the text size. The base remapping entry, base-spec, has the lowest priority and is managed by the face-remap-set-base and face-remap-reset-base functions; it is intended for major modes to remap faces in the buffers they control.

37.12.6 Functions for Working with Faces

Here are additional functions for creating and working with faces.

A face alias provides an equivalent name for a face. You can define a face alias by giving the alias symbol the face-alias property, with a value of the target face name. The following example makes modeline an alias for the mode-line face.

     (put 'modeline 'face-alias 'mode-line)

37.12.7 Automatic Face Assignment

This hook is used for automatically assigning faces to text in the buffer. It is part of the implementation of Jit-Lock mode, used by Font-Lock.

37.12.8 Basic Faces

If your Emacs Lisp program needs to assign some faces to text, it is often a good idea to use certain existing faces or inherit from them, rather than defining entirely new faces. This way, if other users have customized the basic faces to give Emacs a certain look, your program will fit in without additional customization.

Some of the basic faces defined in Emacs are listed below. In addition to these, you might want to make use of the Font Lock faces for syntactic highlighting, if highlighting is not already handled by Font Lock mode, or if some Font Lock faces are not in use. See Faces for Font Lock.

default

The default face, whose attributes are all specified. All other faces implicitly inherit from it: any unspecified attribute defaults to the attribute on this face (see Face Attributes).

bold

italic

bold-italic

underline

fixed-pitch

fixed-pitch-serif

variable-pitch

These have the attributes indicated by their names (e.g., bold has a bold :weight attribute), with all other attributes unspecified (and so given by default).

shadow

For dimmed-out text. For example, it is used for the ignored part of a filename in the minibuffer (see Minibuffers for File Names).

link

link-visited

For clickable text buttons that send the user to a different buffer or location.

highlight

For stretches of text that should temporarily stand out. For example, it is commonly assigned to the mouse-face property for cursor highlighting (see Special Properties).

match

isearch

lazy-highlight

For text matching (respectively) permanent search matches, interactive search matches, and lazy highlighting other matches than the current interactive one.

error

warning

success

For text concerning errors, warnings, or successes. For example, these are used for messages in *Compilation* buffers.

37.12.9 Font Selection

Before Emacs can draw a character on a graphical display, it must select a font for that character²⁰. See Fonts. Normally, Emacs automatically chooses a font based on the faces assigned to that character—specifically, the face attributes :family, :weight, :slant, and :width (see Face Attributes). The choice of font also depends on the character to be displayed; some fonts can only display a limited set of characters. If no available font exactly fits the requirements, Emacs looks for the closest matching font. The variables in this section control how Emacs makes this selection.

Emacs can make use of scalable fonts, but by default it does not use them.

37.12.10 Looking Up Fonts

37.12.11 Fontsets

A fontset is a list of fonts, each assigned to a range of character codes. An individual font cannot display the whole range of characters that Emacs supports, but a fontset can. Fontsets have names, just as fonts do, and you can use a fontset name in place of a font name when you specify the font for a frame or a face. Here is information about defining a fontset under Lisp program control.

The construct ‘charset:font’ specifies which font to use (in this fontset) for one particular character set. Here, charset is the name of a character set, and font is the font to use for that character set. You can use this construct any number of times in the specification string.

For the remaining character sets, those that you don't specify explicitly, Emacs chooses a font based on fontpattern: it replaces ‘fontset-alias’ with a value that names one character set. For the ASCII character set, ‘fontset-alias’ is replaced with ‘ISO8859-1’.

In addition, when several consecutive fields are wildcards, Emacs collapses them into a single wildcard. This is to prevent use of auto-scaled fonts. Fonts made by scaling larger fonts are not usable for editing, and scaling a smaller font is not useful because it is better to use the smaller font in its own size, which Emacs does.

Thus if fontpattern is this,

     -*-fixed-medium-r-normal-*-24-*-*-*-*-*-fontset-24

the font specification for ASCII characters would be this:

     -*-fixed-medium-r-normal-*-24-*-ISO8859-1

and the font specification for Chinese GB2312 characters would be this:

     -*-fixed-medium-r-normal-*-24-*-gb2312*-*

You may not have any Chinese font matching the above font specification. Most X distributions include only Chinese fonts that have ‘song ti’ or ‘fangsong ti’ in the family field. In such a case, ‘Fontset-n’ can be specified as below:

     Emacs.Fontset-0: -*-fixed-medium-r-normal-*-24-*-*-*-*-*-fontset-24,\
             chinese-gb2312:-*-*-medium-r-normal-*-24-*-gb2312*-*

Then, the font specifications for all but Chinese GB2312 characters have ‘fixed’ in the family field, and the font specification for Chinese GB2312 characters has a wild card ‘*’ in the family field.

37.12.12 Low-Level Font Representation

Normally, it is not necessary to manipulate fonts directly. In case you need to do so, this section explains how.

In Emacs Lisp, fonts are represented using three different Lisp object types: font objects, font specs, and font entities.

A font object is a Lisp object that represents a font that Emacs has opened. Font objects cannot be modified in Lisp, but they can be inspected.

A font spec is a Lisp object that contains a set of specifications that can be used to find a font. More than one font may match the specifications in a font spec.

A font entity is a reference to a font that need not be open. Its properties are intermediate between a font object and a font spec: like a font object, and unlike a font spec, it refers to a single, specific font. Unlike a font object, creating a font entity does not load the contents of that font into computer memory. Emacs may open multiple font objects of different sizes from a single font entity referring to a scalable font.

If you call set-face-attribute and pass a font spec, font entity, or font name string as the value of the :font attribute, Emacs opens the best matching font that is available for display. It then stores the corresponding font object as the actual value of the :font attribute for that face.

The following functions can be used to obtain information about a font. For these functions, the font argument can be a font object, a font entity, or a font spec.

The following two functions return important information about a font.

The following four functions return size information about fonts used by various faces, allowing various layout considerations in Lisp programs. These functions take face remapping into consideration, returning information about the remapped face, if the face in question was remapped. See Face Remapping.

37.13 Fringes

On graphical displays, Emacs draws fringes next to each window: thin vertical strips down the sides which can display bitmaps indicating truncation, continuation, horizontal scrolling, and so on.

• Fringe Size/Pos: Specifying where to put the window fringes.
• Fringe Indicators: Displaying indicator icons in the window fringes.
• Fringe Cursors: Displaying cursors in the right fringe.
• Fringe Bitmaps: Specifying bitmaps for fringe indicators.
• Customizing Bitmaps: Specifying your own bitmaps to use in the fringes.
• Overlay Arrow: Display of an arrow to indicate position.

37.13.1 Fringe Size and Position

The following buffer-local variables control the position and width of fringes in windows showing that buffer.

Any buffer which does not specify values for these variables uses the values specified by the left-fringe and right-fringe frame parameters (see Layout Parameters).

The above variables actually take effect via the function set-window-buffer (see Buffers and Windows), which calls set-window-fringes as a subroutine. If you change one of these variables, the fringe display is not updated in existing windows showing the buffer, unless you call set-window-buffer again in each affected window. You can also use set-window-fringes to control the fringe display in individual windows.

37.13.2 Fringe Indicators

Fringe indicators are tiny icons displayed in the window fringe to indicate truncated or continued lines, buffer boundaries, etc.

37.13.3 Fringe Cursors

When a line is exactly as wide as the window, Emacs displays the cursor in the right fringe instead of using two lines. Different bitmaps are used to represent the cursor in the fringe depending on the current buffer's cursor type.

37.13.4 Fringe Bitmaps

The fringe bitmaps are the actual bitmaps which represent the logical fringe indicators for truncated or continued lines, buffer boundaries, overlay arrows, etc. Each bitmap is represented by a symbol. These symbols are referred to by the variable fringe-indicator-alist, which maps fringe indicators to bitmaps (see Fringe Indicators), and the variable fringe-cursor-alist, which maps fringe cursors to bitmaps (see Fringe Cursors).

Lisp programs can also directly display a bitmap in the left or right fringe, by using a display property for one of the characters appearing in the line (see Other Display Specs). Such a display specification has the form

     (fringe bitmap [face])

fringe is either the symbol left-fringe or right-fringe. bitmap is a symbol identifying the bitmap to display. The optional face names a face whose foreground color is used to display the bitmap; this face is automatically merged with the fringe face.

Here is a list of the standard fringe bitmaps defined in Emacs, and how they are currently used in Emacs (via fringe-indicator-alist and fringe-cursor-alist):

left-arrow, right-arrow

Used to indicate truncated lines.

left-curly-arrow, right-curly-arrow

Used to indicate continued lines.

right-triangle, left-triangle

The former is used by overlay arrows. The latter is unused.

up-arrow, down-arrow, top-left-angle top-right-angle

bottom-left-angle, bottom-right-angle

top-right-angle, top-left-angle

left-bracket, right-bracket, top-right-angle, top-left-angle

Used to indicate buffer boundaries.

filled-rectangle, hollow-rectangle

filled-square, hollow-square

vertical-bar, horizontal-bar

Used for different types of fringe cursors.

empty-line, exclamation-mark, question-mark, exclamation-mark

Not used by core Emacs features.

The next subsection describes how to define your own fringe bitmaps.

37.13.5 Customizing Fringe Bitmaps

37.13.6 The Overlay Arrow

The overlay arrow is useful for directing the user's attention to a particular line in a buffer. For example, in the modes used for interface to debuggers, the overlay arrow indicates the line of code about to be executed. This feature has nothing to do with overlays (see Overlays).

You can do a similar job by creating an overlay with a before-string property. See Overlay Properties.

You can define multiple overlay arrows via the variable overlay-arrow-variable-list.

Each variable on this list can have properties overlay-arrow-string and overlay-arrow-bitmap that specify an overlay arrow string (for text terminals) or fringe bitmap (for graphical terminals) to display at the corresponding overlay arrow position. If either property is not set, the default overlay-arrow-string or overlay-arrow fringe indicator is used.

37.14 Scroll Bars

Normally the frame parameter vertical-scroll-bars controls whether the windows in the frame have vertical scroll bars, and whether they are on the left or right. The frame parameter scroll-bar-width specifies how wide they are (nil meaning the default).

The frame parameter horizontal-scroll-bars controls whether the windows in the frame have horizontal scroll bars. The frame parameter scroll-bar-height specifies how high they are (nil meaning the default). See Layout Parameters.

Horizontal scroll bars are not available on all platforms. The function horizontal-scroll-bars-available-p which takes no argument returns non-nil if they are available on your system.

The following three functions take as argument a live frame which defaults to the selected one.

You can override the frame specific settings for individual windows by using the following function:

The following four functions take as argument a live window which defaults to the selected one.

If you don't specify these values for a window with set-window-scroll-bars, the buffer-local variables vertical-scroll-bar, horizontal-scroll-bar, scroll-bar-width and scroll-bar-height in the buffer being displayed control the window's scroll bars. The function set-window-buffer examines these variables. If you change them in a buffer that is already visible in a window, you can make the window take note of the new values by calling set-window-buffer specifying the same buffer that is already displayed.

You can control the appearance of scroll bars for a particular buffer by setting the following variables which automatically become buffer-local when set.

Finally you can toggle the display of scroll bars on all frames by customizing the variables scroll-bar-mode and horizontal-scroll-bar-mode.

37.15 Window Dividers

Window dividers are bars drawn between a frame's windows. A right divider is drawn between a window and any adjacent windows on the right. Its width (thickness) is specified by the frame parameter right-divider-width. A bottom divider is drawn between a window and adjacent windows on the bottom or the echo area. Its width is specified by the frame parameter bottom-divider-width. In either case, specifying a width of zero means to not draw such dividers. See Layout Parameters.

Technically, a right divider belongs to the window on its left, which means that its width contributes to the total width of that window. A bottom divider belongs to the window above it, which means that its width contributes to the total height of that window. See Window Sizes. When a window has both, a right and a bottom divider, the bottom divider prevails. This means that a bottom divider is drawn over the full total width of its window while the right divider ends above the bottom divider.

Dividers can be dragged with the mouse and are therefore useful for adjusting the sizes of adjacent windows with the mouse. They also serve to visually set apart adjacent windows when no scroll bars or mode lines are present. The following three faces allow the customization of the appearance of dividers:

window-divider

When a divider is less than three pixels wide, it is drawn solidly with the foreground of this face. For larger dividers this face is used for the inner part only, excluding the first and last pixel.

window-divider-first-pixel

This is the face used for drawing the first pixel of a divider that is at least three pixels wide. To obtain a solid appearance, set this to the same value used for the window-divider face.

window-divider-last-pixel

This is the face used for drawing the last pixel of a divider that is at least three pixels wide. To obtain a solid appearance, set this to the same value used for the window-divider face.

You can get the sizes of the dividers of a specific window with the following two functions.

37.16 The display Property

The display text property (or overlay property) is used to insert images into text, and to control other aspects of how text displays. The value of the display property should be a display specification, or a list or vector containing several display specifications. Display specifications in the same display property value generally apply in parallel to the text they cover.

If several sources (overlays and/or a text property) specify values for the display property, only one of the values takes effect, following the rules of get-char-property. See Examining Properties.

The rest of this section describes several kinds of display specifications and what they mean.

• Replacing Specs: Display specs that replace the text.
• Specified Space: Displaying one space with a specified width.
• Pixel Specification: Specifying space width or height in pixels.
• Other Display Specs: Displaying an image; adjusting the height, spacing, and other properties of text.
• Display Margins: Displaying text or images to the side of the main text.

37.16.1 Display Specs That Replace The Text

Some kinds of display specifications specify something to display instead of the text that has the property. These are called replacing display specifications. Emacs does not allow the user to interactively move point into the middle of buffer text that is replaced in this way.

If a list of display specifications includes more than one replacing display specification, the first overrides the rest. Replacing display specifications make most other display specifications irrelevant, since those don't apply to the replacement.

For replacing display specifications, the text that has the property means all the consecutive characters that have the same Lisp object as their display property; these characters are replaced as a single unit. If two characters have different Lisp objects as their display properties (i.e., objects which are not eq), they are handled separately.

Here is an example which illustrates this point. A string serves as a replacing display specification, which replaces the text that has the property with the specified string (see Other Display Specs). Consider the following function:

     (defun foo ()
       (dotimes (i 5)
         (let ((string (concat "A"))
               (start (+ i i (point-min))))
           (put-text-property start (1+ start) 'display string)
           (put-text-property start (+ 2 start) 'display string))))

This function gives each of the first ten characters in the buffer a display property which is a string "A", but they don't all get the same string object. The first two characters get the same string object, so they are replaced with one ‘A’; the fact that the display property was assigned in two separate calls to put-text-property is irrelevant. Similarly, the next two characters get a second string (concat creates a new string object), so they are replaced with one ‘A’; and so on. Thus, the ten characters appear as five A's.

37.16.2 Specified Spaces

To display a space of specified width and/or height, use a display specification of the form (space . props), where props is a property list (a list of alternating properties and values). You can put this property on one or more consecutive characters; a space of the specified height and width is displayed in place of all of those characters. These are the properties you can use in props to specify the weight of the space:

:width width

If width is a number, it specifies that the space width should be width times the normal character width. width can also be a pixel width specification (see Pixel Specification).

:relative-width factor

Specifies that the width of the stretch should be computed from the first character in the group of consecutive characters that have the same display property. The space width is the pixel width of that character, multiplied by factor. (On text-mode terminals, the “pixel width” of a character is usually 1, but it could be more for TABs and double-width CJK characters.)

:align-to hpos

Specifies that the space should be wide enough to reach hpos. If hpos is a number, it is measured in units of the normal character width. hpos can also be a pixel width specification (see Pixel Specification).

You should use one and only one of the above properties. You can also specify the height of the space, with these properties:

:height height

Specifies the height of the space. If height is a number, it specifies that the space height should be height times the normal character height. The height may also be a pixel height specification (see Pixel Specification).

:relative-height factor

Specifies the height of the space, multiplying the ordinary height of the text having this display specification by factor.

:ascent ascent

If the value of ascent is a non-negative number no greater than 100, it specifies that ascent percent of the height of the space should be considered as the ascent of the space—that is, the part above the baseline. The ascent may also be specified in pixel units with a pixel ascent specification (see Pixel Specification).

Don't use both :height and :relative-height together.

The :width and :align-to properties are supported on non-graphic terminals, but the other space properties in this section are not.

Note that space properties are treated as paragraph separators for the purposes of reordering bidirectional text for display. See Bidirectional Display, for the details.

37.16.3 Pixel Specification for Spaces

The value of the :width, :align-to, :height, and :ascent properties can be a special kind of expression that is evaluated during redisplay. The result of the evaluation is used as an absolute number of pixels.

The following expressions are supported:

       expr ::= num | (num) | unit | elem | pos | image | form
       num  ::= integer | float | symbol
       unit ::= in | mm | cm | width | height
       elem ::= left-fringe | right-fringe | left-margin | right-margin
             |  scroll-bar | text
       pos  ::= left | center | right
       form ::= (num . expr) | (op expr ...)
       op   ::= + | -

The form num specifies a fraction of the default frame font height or width. The form (num) specifies an absolute number of pixels. If num is a symbol, symbol, its buffer-local variable binding is used.

The in, mm, and cm units specify the number of pixels per inch, millimeter, and centimeter, respectively. The width and height units correspond to the default width and height of the current face. An image specification image corresponds to the width or height of the image.

The elements left-fringe, right-fringe, left-margin, right-margin, scroll-bar, and text specify to the width of the corresponding area of the window.

The left, center, and right positions can be used with :align-to to specify a position relative to the left edge, center, or right edge of the text area.

Any of the above window elements (except text) can also be used with :align-to to specify that the position is relative to the left edge of the given area. Once the base offset for a relative position has been set (by the first occurrence of one of these symbols), further occurrences of these symbols are interpreted as the width of the specified area. For example, to align to the center of the left-margin, use

     :align-to (+ left-margin (0.5 . left-margin))

If no specific base offset is set for alignment, it is always relative to the left edge of the text area. For example, ‘:align-to 0’ in a header-line aligns with the first text column in the text area.

A value of the form (num . expr) stands for the product of the values of num and expr. For example, (2 . in) specifies a width of 2 inches, while (0.5 . image) specifies half the width (or height) of the specified image.

The form (+ expr ...) adds up the value of the expressions. The form (- expr ...) negates or subtracts the value of the expressions.

37.16.4 Other Display Specifications

Here are the other sorts of display specifications that you can use in the display text property.

string

Display string instead of the text that has this property.

Recursive display specifications are not supported—string's display properties, if any, are not used.

(image . image-props)

This kind of display specification is an image descriptor (see Images). When used as a display specification, it means to display the image instead of the text that has the display specification.

(slice x y width height)

This specification together with image specifies a slice (a partial area) of the image to display. The elements y and x specify the top left corner of the slice, within the image; width and height specify the width and height of the slice. Integers are numbers of pixels. A floating-point number in the range 0.0–1.0 stands for that fraction of the width or height of the entire image.

((margin nil) string)

A display specification of this form means to display string instead of the text that has the display specification, at the same position as that text. It is equivalent to using just string, but it is done as a special case of marginal display (see Display Margins).

(left-fringe bitmap [face])

(right-fringe bitmap [face])

This display specification on any character of a line of text causes the specified bitmap be displayed in the left or right fringes for that line, instead of the characters that have the display specification. The optional face specifies the colors to be used for the bitmap. See Fringe Bitmaps, for the details.

(space-width factor)

This display specification affects all the space characters within the text that has the specification. It displays all of these spaces factor times as wide as normal. The element factor should be an integer or float. Characters other than spaces are not affected at all; in particular, this has no effect on tab characters.

(height height)

This display specification makes the text taller or shorter. Here are the possibilities for height:

(+ n)

This means to use a font that is n steps larger. A step is defined by the set of available fonts—specifically, those that match what was otherwise specified for this text, in all attributes except height. Each size for which a suitable font is available counts as another step. n should be an integer.

(- n)

This means to use a font that is n steps smaller.

a number, factor

A number, factor, means to use a font that is factor times as tall as the default font.

a symbol, function

A symbol is a function to compute the height. It is called with the current height as argument, and should return the new height to use.

anything else, form

If the height value doesn't fit the previous possibilities, it is a form. Emacs evaluates it to get the new height, with the symbol height bound to the current specified font height.

(raise factor)

This kind of display specification raises or lowers the text it applies to, relative to the baseline of the line. It is mainly meant to support display of subscripts and superscripts.

The factor must be a number, which is interpreted as a multiple of the height of the affected text. If it is positive, that means to display the characters raised. If it is negative, that means to display them lower down.

Note that if the text also has a height display specification, which was specified before (i.e. to the left of) raise, the latter will affect the amount of raising or lowering in pixels, because that is based on the height of the text being raised. Therefore, if you want to display a sub- or superscript that is smaller than the normal text height, consider specifying raise before height.

You can make any display specification conditional. To do that, package it in another list of the form (when condition . spec). Then the specification spec applies only when condition evaluates to a non-nil value. During the evaluation, object is bound to the string or buffer having the conditional display property. position and buffer-position are bound to the position within object and the buffer position where the display property was found, respectively. Both positions can be different when object is a string.

37.16.5 Displaying in the Margins

A buffer can have blank areas called display margins on the left and on the right. Ordinary text never appears in these areas, but you can put things into the display margins using the display property. There is currently no way to make text or images in the margin mouse-sensitive.

The way to display something in the margins is to specify it in a margin display specification in the display property of some text. This is a replacing display specification, meaning that the text you put it on does not get displayed; the margin display appears, but that text does not.

A margin display specification looks like ((margin right-margin) spec) or ((margin left-margin) spec). Here, spec is another display specification that says what to display in the margin. Typically it is a string of text to display, or an image descriptor.

To display something in the margin in association with certain buffer text, without altering or preventing the display of that text, put a before-string property on the text and put the margin display specification on the contents of the before-string.

Before the display margins can display anything, you must give them a nonzero width. The usual way to do that is to set these variables:

Setting these variables does not immediately affect the window. These variables are checked when a new buffer is displayed in the window. Thus, you can make changes take effect by calling set-window-buffer.

You can also set the margin widths immediately.

37.17 Images

To display an image in an Emacs buffer, you must first create an image descriptor, then use it as a display specifier in the display property of text that is displayed (see Display Property).

Emacs is usually able to display images when it is run on a graphical terminal. Images cannot be displayed in a text terminal, on certain graphical terminals that lack the support for this, or if Emacs is compiled without image support. You can use the function display-images-p to determine if images can in principle be displayed (see Display Feature Testing).

• Image Formats: Supported image formats.
• Image Descriptors: How to specify an image for use in :display.
• XBM Images: Special features for XBM format.
• XPM Images: Special features for XPM format.
• PostScript Images: Special features for PostScript format.
• ImageMagick Images: Special features available through ImageMagick.
• Other Image Types: Various other formats are supported.
• Defining Images: Convenient ways to define an image for later use.
• Showing Images: Convenient ways to display an image once it is defined.
• Multi-Frame Images: Some images contain more than one frame.
• Image Cache: Internal mechanisms of image display.

37.17.1 Image Formats

Emacs can display a number of different image formats. Some of these image formats are supported only if particular support libraries are installed. On some platforms, Emacs can load support libraries on demand; if so, the variable dynamic-library-alist can be used to modify the set of known names for these dynamic libraries. See Dynamic Libraries.

Supported image formats (and the required support libraries) include PBM and XBM (which do not depend on support libraries and are always available), XPM (libXpm), GIF (libgif or libungif), PostScript (gs), JPEG (libjpeg), TIFF (libtiff), PNG (libpng), and SVG (librsvg).

Each of these image formats is associated with an image type symbol. The symbols for the above formats are, respectively, pbm, xbm, xpm, gif, postscript, jpeg, tiff, png, and svg.

Furthermore, if you build Emacs with ImageMagick (libMagickWand) support, Emacs can display any image format that ImageMagick can. See ImageMagick Images. All images displayed via ImageMagick have type symbol imagemagick.

37.17.2 Image Descriptors

An image descriptor is a list which specifies the underlying data for an image, and how to display it. It is typically used as the value of a display overlay or text property (see Other Display Specs); but See Showing Images, for convenient helper functions to insert images into buffers.

Each image descriptor has the form (image . props), where props is a property list of alternating keyword symbols and values, including at least the pair :type type that specifies the image type.

The following is a list of properties that are meaningful for all image types (there are also properties which are meaningful only for certain image types, as documented in the following subsections):

:type type

The image type. See Image Formats. Every image descriptor must include this property.

:file file

This says to load the image from file file. If file is not an absolute file name, it is expanded in data-directory.

:data data

This specifies the raw image data. Each image descriptor must have either :data or :file, but not both.

For most image types, the value of a :data property should be a string containing the image data. Some image types do not support :data; for some others, :data alone is not enough, so you need to use other image properties along with :data. See the following subsections for details.

:margin margin

This specifies how many pixels to add as an extra margin around the image. The value, margin, must be a non-negative number, or a pair (x . y) of such numbers. If it is a pair, x specifies how many pixels to add horizontally, and y specifies how many pixels to add vertically. If :margin is not specified, the default is zero.

:ascent ascent

This specifies the amount of the image's height to use for its ascent—that is, the part above the baseline. The value, ascent, must be a number in the range 0 to 100, or the symbol center.

If ascent is a number, that percentage of the image's height is used for its ascent.

If ascent is center, the image is vertically centered around a centerline which would be the vertical centerline of text drawn at the position of the image, in the manner specified by the text properties and overlays that apply to the image.

If this property is omitted, it defaults to 50.

:relief relief

This adds a shadow rectangle around the image. The value, relief, specifies the width of the shadow lines, in pixels. If relief is negative, shadows are drawn so that the image appears as a pressed button; otherwise, it appears as an unpressed button.

:conversion algorithm

This specifies a conversion algorithm that should be applied to the image before it is displayed; the value, algorithm, specifies which algorithm.

laplace

emboss

Specifies the Laplace edge detection algorithm, which blurs out small differences in color while highlighting larger differences. People sometimes consider this useful for displaying the image for a disabled button.

(edge-detection :matrix matrix :color-adjust adjust)

Specifies a general edge-detection algorithm. matrix must be either a nine-element list or a nine-element vector of numbers. A pixel at position x/y in the transformed image is computed from original pixels around that position. matrix specifies, for each pixel in the neighborhood of x/y, a factor with which that pixel will influence the transformed pixel; element 0 specifies the factor for the pixel at x-1/y-1, element 1 the factor for the pixel at x/y-1 etc., as shown below:

                 (x-1/y-1  x/y-1  x+1/y-1
                  x-1/y    x/y    x+1/y
                  x-1/y+1  x/y+1  x+1/y+1)

The resulting pixel is computed from the color intensity of the color resulting from summing up the RGB values of surrounding pixels, multiplied by the specified factors, and dividing that sum by the sum of the factors' absolute values.

Laplace edge-detection currently uses a matrix of

                 (1  0  0
                  0  0  0
                  0  0 -1)

Emboss edge-detection uses a matrix of

                 ( 2 -1  0
                  -1  0  1
                   0  1 -2)

disabled

Specifies transforming the image so that it looks disabled.

:mask mask

If mask is heuristic or (heuristic bg), build a clipping mask for the image, so that the background of a frame is visible behind the image. If bg is not specified, or if bg is t, determine the background color of the image by looking at the four corners of the image, assuming the most frequently occurring color from the corners is the background color of the image. Otherwise, bg must be a list (red green blue) specifying the color to assume for the background of the image.

If mask is nil, remove a mask from the image, if it has one. Images in some formats include a mask which can be removed by specifying :mask nil.

:pointer shape

This specifies the pointer shape when the mouse pointer is over this image. See Pointer Shape, for available pointer shapes.

:map map

This associates an image map of hot spots with this image.

An image map is an alist where each element has the format (area id plist). An area is specified as either a rectangle, a circle, or a polygon.

A rectangle is a cons (rect . ((x0 . y0) . (x1 . y1))) which specifies the pixel coordinates of the upper left and bottom right corners of the rectangle area.

A circle is a cons (circle . ((x0 . y0) . r)) which specifies the center and the radius of the circle; r may be a float or integer.

A polygon is a cons (poly . [x0 y0 x1 y1 ...]) where each pair in the vector describes one corner in the polygon.

When the mouse pointer lies on a hot-spot area of an image, the plist of that hot-spot is consulted; if it contains a help-echo property, that defines a tool-tip for the hot-spot, and if it contains a pointer property, that defines the shape of the mouse cursor when it is on the hot-spot. See Pointer Shape, for available pointer shapes.

When you click the mouse when the mouse pointer is over a hot-spot, an event is composed by combining the id of the hot-spot with the mouse event; for instance, [area4 mouse-1] if the hot-spot's id is area4.

37.17.3 XBM Images

To use XBM format, specify xbm as the image type. This image format doesn't require an external library, so images of this type are always supported.

Additional image properties supported for the xbm image type are:

:foreground foreground

The value, foreground, should be a string specifying the image foreground color, or nil for the default color. This color is used for each pixel in the XBM that is 1. The default is the frame's foreground color.

:background background

The value, background, should be a string specifying the image background color, or nil for the default color. This color is used for each pixel in the XBM that is 0. The default is the frame's background color.

If you specify an XBM image using data within Emacs instead of an external file, use the following three properties:

:data data

The value, data, specifies the contents of the image. There are three formats you can use for data:

• A vector of strings or bool-vectors, each specifying one line of the image. Do specify :height and :width.
• A string containing the same byte sequence as an XBM file would contain. You must not specify :height and :width in this case, because omitting them is what indicates the data has the format of an XBM file. The file contents specify the height and width of the image.
• A string or a bool-vector containing the bits of the image (plus perhaps some extra bits at the end that will not be used). It should contain at least width * height bits. In this case, you must specify :height and :width, both to indicate that the string contains just the bits rather than a whole XBM file, and to specify the size of the image.

:width width

The value, width, specifies the width of the image, in pixels.

:height height

The value, height, specifies the height of the image, in pixels.

37.17.4 XPM Images

To use XPM format, specify xpm as the image type. The additional image property :color-symbols is also meaningful with the xpm image type:

:color-symbols symbols

The value, symbols, should be an alist whose elements have the form (name . color). In each element, name is the name of a color as it appears in the image file, and color specifies the actual color to use for displaying that name.

37.17.5 PostScript Images

To use PostScript for an image, specify image type postscript. This works only if you have Ghostscript installed. You must always use these three properties:

:pt-width width

The value, width, specifies the width of the image measured in points (1/72 inch). width must be an integer.

:pt-height height

The value, height, specifies the height of the image in points (1/72 inch). height must be an integer.

:bounding-box box

The value, box, must be a list or vector of four integers, which specifying the bounding box of the PostScript image, analogous to the ‘BoundingBox’ comment found in PostScript files.

          %%BoundingBox: 22 171 567 738

37.17.6 ImageMagick Images

If you build Emacs with ImageMagick support, you can use the ImageMagick library to load many image formats (see File Conveniences). The image type symbol for images loaded via ImageMagick is imagemagick, regardless of the actual underlying image format.

To check for ImageMagick support, use the following:

     (image-type-available-p 'imagemagick)

Images loaded with ImageMagick support the following additional image descriptor properties:

:background background

background, if non-nil, should be a string specifying a color, which is used as the image's background color if the image supports transparency. If the value is nil, it defaults to the frame's background color.

:width width, :height height

The :width and :height keywords are used for scaling the image. If only one of them is specified, the other one will be calculated so as to preserve the aspect ratio. If both are specified, aspect ratio may not be preserved.

:max-width max-width, :max-height max-height

The :max-width and :max-height keywords are used for scaling if the size of the image of the image exceeds these values. If :width is set it will have precedence over max-width, and if :height is set it will have precedence over max-height, but you can otherwise mix these keywords as you wish. :max-width and :max-height will always preserve the aspect ratio.

:format type

The value, type, should be a symbol specifying the type of the image data, as found in image-format-suffixes. This is used when the image does not have an associated file name, to provide a hint to ImageMagick to help it detect the image type.

:rotation angle

Specifies a rotation angle in degrees.

:index frame

See Multi-Frame Images.

37.17.7 Other Image Types

For PBM images, specify image type pbm. Color, gray-scale and monochromatic images are supported. For mono PBM images, two additional image properties are supported.

:foreground foreground

The value, foreground, should be a string specifying the image foreground color, or nil for the default color. This color is used for each pixel in the PBM that is 1. The default is the frame's foreground color.

:background background

The value, background, should be a string specifying the image background color, or nil for the default color. This color is used for each pixel in the PBM that is 0. The default is the frame's background color.

The remaining image types that Emacs can support are:

GIF

Image type gif. Supports the :index property. See Multi-Frame Images.

JPEG

Image type jpeg.

PNG

Image type png.

SVG

Image type svg.

TIFF

Image type tiff. Supports the :index property. See Multi-Frame Images.

37.17.8 Defining Images

The functions create-image, defimage and find-image provide convenient ways to create image descriptors.

37.17.9 Showing Images

You can use an image descriptor by setting up the display property yourself, but it is easier to use the functions in this section.

37.17.10 Multi-Frame Images

Some image files can contain more than one image. We say that there are multiple “frames” in the image. At present, Emacs supports multiple frames for GIF, TIFF, and certain ImageMagick formats such as DJVM.

The frames can be used either to represent multiple pages (this is usually the case with multi-frame TIFF files, for example), or to create animation (usually the case with multi-frame GIF files).

A multi-frame image has a property :index, whose value is an integer (counting from 0) that specifies which frame is being displayed.

Animation operates by means of a timer. Note that Emacs imposes a minimum frame delay of 0.01 (image-minimum-frame-delay) seconds. If the image itself does not specify a delay, Emacs uses image-default-frame-delay.

37.17.11 Image Cache

Emacs caches images so that it can display them again more efficiently. When Emacs displays an image, it searches the image cache for an existing image specification equal to the desired specification. If a match is found, the image is displayed from the cache. Otherwise, Emacs loads the image normally.

One use for image-flush is to tell Emacs about a change in an image file. If an image specification contains a :file property, the image is cached based on the file's contents when the image is first displayed. Even if the file subsequently changes, Emacs continues displaying the old version of the image. Calling image-flush flushes the image from the cache, forcing Emacs to re-read the file the next time it needs to display that image.

Another use for image-flush is for memory conservation. If your Lisp program creates a large number of temporary images over a period much shorter than image-cache-eviction-delay (see below), you can opt to flush unused images yourself, instead of waiting for Emacs to do it automatically.

If an image in the image cache has not been displayed for a specified period of time, Emacs removes it from the cache and frees the associated memory.

37.18 Embedded Native Widgets

Emacs is able to display native widgets, such as GTK WebKit widgets, in Emacs buffers when it was built with the necessary support libraries and is running on a graphical terminal. To test whether Emacs supports display of embedded widgets, check that the xwidget-internal feature is available (see Named Features).

To display an embedded widget in a buffer, you must first create an xwidget object, and then use that object as the display specifier in a display text or overlay property (see Display Property).

37.19 Buttons

The Button package defines functions for inserting and manipulating buttons that can be activated with the mouse or via keyboard commands. These buttons are typically used for various kinds of hyperlinks.

A button is essentially a set of text or overlay properties, attached to a stretch of text in a buffer. These properties are called button properties. One of these properties, the action property, specifies a function which is called when the user invokes the button using the keyboard or the mouse. The action function may examine the button and use its other properties as desired.

In some ways, the Button package duplicates the functionality in the Widget package. See Introduction. The advantage of the Button package is that it is faster, smaller, and simpler to program. From the point of view of the user, the interfaces produced by the two packages are very similar.

• Button Properties: Button properties with special meanings.
• Button Types: Defining common properties for classes of buttons.
• Making Buttons: Adding buttons to Emacs buffers.
• Manipulating Buttons: Getting and setting properties of buttons.
• Button Buffer Commands: Buffer-wide commands and bindings for buttons.

37.19.1 Button Properties

Each button has an associated list of properties defining its appearance and behavior, and other arbitrary properties may be used for application specific purposes. The following properties have special meaning to the Button package:

action

The function to call when the user invokes the button, which is passed the single argument button. By default this is ignore, which does nothing.

mouse-action

This is similar to action, and when present, will be used instead of action for button invocations resulting from mouse-clicks (instead of the user hitting <RET>). If not present, mouse-clicks use action instead.

face

This is an Emacs face controlling how buttons of this type are displayed; by default this is the button face.

mouse-face

This is an additional face which controls appearance during mouse-overs (merged with the usual button face); by default this is the usual Emacs highlight face.

keymap

The button's keymap, defining bindings active within the button region. By default this is the usual button region keymap, stored in the variable button-map, which defines <RET> and <mouse-2> to invoke the button.

type

The button type. See Button Types.

help-echo

A string displayed by the Emacs tool-tip help system; by default, "mouse-2, RET: Push this button".

follow-link

The follow-link property, defining how a <mouse-1> click behaves on this button, See Clickable Text.

button

All buttons have a non-nil button property, which may be useful in finding regions of text that comprise buttons (which is what the standard button functions do).

There are other properties defined for the regions of text in a button, but these are not generally interesting for typical uses.

37.19.2 Button Types

Every button has a button type, which defines default values for the button's properties. Button types are arranged in a hierarchy, with specialized types inheriting from more general types, so that it's easy to define special-purpose types of buttons for specific tasks.

Using define-button-type to define default properties for buttons is not necessary—buttons without any specified type use the built-in button-type button—but it is encouraged, since doing so usually makes the resulting code clearer and more efficient.

37.19.3 Making Buttons

Buttons are associated with a region of text, using an overlay or text properties to hold button-specific information, all of which are initialized from the button's type (which defaults to the built-in button type button). Like all Emacs text, the appearance of the button is governed by the face property; by default (via the face property inherited from the button button-type) this is a simple underline, like a typical web-page link.

For convenience, there are two sorts of button-creation functions, those that add button properties to an existing region of a buffer, called make-...button, and those that also insert the button text, called insert-...button.

The button-creation functions all take the &rest argument properties, which should be a sequence of property value pairs, specifying properties to add to the button; see Button Properties. In addition, the keyword argument :type may be used to specify a button-type from which to inherit other properties; see Button Types. Any properties not explicitly specified during creation will be inherited from the button's type (if the type defines such a property).

The following functions add a button using an overlay (see Overlays) to hold the button properties:

The following functions are similar, but using text properties (see Text Properties) to hold the button properties. Such buttons do not add markers to the buffer, so editing in the buffer does not slow down if there is an extremely large numbers of buttons. However, if there is an existing face text property on the text (e.g., a face assigned by Font Lock mode), the button face may not be visible. Both of these functions return the starting position of the new button.

37.19.4 Manipulating Buttons

These are functions for getting and setting properties of buttons. Often these are used by a button's invocation function to determine what to do.

Where a button parameter is specified, it means an object referring to a specific button, either an overlay (for overlay buttons), or a buffer-position or marker (for text property buttons). Such an object is passed as the first argument to a button's invocation function when it is invoked.

37.19.5 Button Buffer Commands

These are commands and functions for locating and operating on buttons in an Emacs buffer.

push-button is the command that a user uses to actually push a button, and is bound by default in the button itself to <RET> and to <mouse-2> using a local keymap in the button's overlay or text properties. Commands that are useful outside the buttons itself, such as forward-button and backward-button are additionally available in the keymap stored in button-buffer-map; a mode which uses buttons may want to use button-buffer-map as a parent keymap for its keymap.

If the button has a non-nil follow-link property, and mouse-1-click-follows-link is set, a quick <mouse-1> click will also activate the push-button command. See Clickable Text.

37.20 Abstract Display

The Ewoc package constructs buffer text that represents a structure of Lisp objects, and updates the text to follow changes in that structure. This is like the “view” component in the “model–view–controller” design paradigm. Ewoc means “Emacs's Widget for Object Collections”.

An ewoc is a structure that organizes information required to construct buffer text that represents certain Lisp data. The buffer text of the ewoc has three parts, in order: first, fixed header text; next, textual descriptions of a series of data elements (Lisp objects that you specify); and last, fixed footer text. Specifically, an ewoc contains information on:

• The buffer which its text is generated in.
• The text's start position in the buffer.
• The header and footer strings.
• A doubly-linked chain of nodes, each of which contains:
  • A data element, a single Lisp object.
  • Links to the preceding and following nodes in the chain.
• A pretty-printer function which is responsible for inserting the textual representation of a data element value into the current buffer.

Typically, you define an ewoc with ewoc-create, and then pass the resulting ewoc structure to other functions in the Ewoc package to build nodes within it, and display it in the buffer. Once it is displayed in the buffer, other functions determine the correspondence between buffer positions and nodes, move point from one node's textual representation to another, and so forth. See Abstract Display Functions.

A node encapsulates a data element much the way a variable holds a value. Normally, encapsulation occurs as a part of adding a node to the ewoc. You can retrieve the data element value and place a new value in its place, like so:

     (ewoc-data node)
     ⇒ value
     
     (ewoc-set-data node new-value)
     ⇒ new-value

You can also use, as the data element value, a Lisp object (list or vector) that is a container for the real value, or an index into some other structure. The example (see Abstract Display Example) uses the latter approach.

When the data changes, you will want to update the text in the buffer. You can update all nodes by calling ewoc-refresh, or just specific nodes using ewoc-invalidate, or all nodes satisfying a predicate using ewoc-map. Alternatively, you can delete invalid nodes using ewoc-delete or ewoc-filter, and add new nodes in their place. Deleting a node from an ewoc deletes its associated textual description from buffer, as well.

• Abstract Display Functions: Functions in the Ewoc package.
• Abstract Display Example: Example of using Ewoc.

37.20.1 Abstract Display Functions

In this subsection, ewoc and node stand for the structures described above (see Abstract Display), while data stands for an arbitrary Lisp object used as a data element.

37.20.2 Abstract Display Example

Here is a simple example using functions of the ewoc package to implement a color components display, an area in a buffer that represents a vector of three integers (itself representing a 24-bit RGB value) in various ways.

     (setq colorcomp-ewoc nil
           colorcomp-data nil
           colorcomp-mode-map nil
           colorcomp-labels ["Red" "Green" "Blue"])
     
     (defun colorcomp-pp (data)
       (if data
           (let ((comp (aref colorcomp-data data)))
             (insert (aref colorcomp-labels data) "\t: #x"
                     (format "%02X" comp) " "
                     (make-string (ash comp -2) ?#) "\n"))
         (let ((cstr (format "#%02X%02X%02X"
                             (aref colorcomp-data 0)
                             (aref colorcomp-data 1)
                             (aref colorcomp-data 2)))
               (samp " (sample text) "))
           (insert "Color\t: "
                   (propertize samp 'face
                               `(foreground-color . ,cstr))
                   (propertize samp 'face
                               `(background-color . ,cstr))
                   "\n"))))
     
     (defun colorcomp (color)
       "Allow fiddling with COLOR in a new buffer.
     The buffer is in Color Components mode."
       (interactive "sColor (name or #RGB or #RRGGBB): ")
       (when (string= "" color)
         (setq color "green"))
       (unless (color-values color)
         (error "No such color: %S" color))
       (switch-to-buffer
        (generate-new-buffer (format "originally: %s" color)))
       (kill-all-local-variables)
       (setq major-mode 'colorcomp-mode
             mode-name "Color Components")
       (use-local-map colorcomp-mode-map)
       (erase-buffer)
       (buffer-disable-undo)
       (let ((data (apply 'vector (mapcar (lambda (n) (ash n -8))
                                          (color-values color))))
             (ewoc (ewoc-create 'colorcomp-pp
                                "\nColor Components\n\n"
                                (substitute-command-keys
                                 "\n\\{colorcomp-mode-map}"))))
         (set (make-local-variable 'colorcomp-data) data)
         (set (make-local-variable 'colorcomp-ewoc) ewoc)
         (ewoc-enter-last ewoc 0)
         (ewoc-enter-last ewoc 1)
         (ewoc-enter-last ewoc 2)
         (ewoc-enter-last ewoc nil)))

This example can be extended to be a color selection widget (in other words, the “controller” part of the “model–view–controller” design paradigm) by defining commands to modify colorcomp-data and to finish the selection process, and a keymap to tie it all together conveniently.

     (defun colorcomp-mod (index limit delta)
       (let ((cur (aref colorcomp-data index)))
         (unless (= limit cur)
           (aset colorcomp-data index (+ cur delta)))
         (ewoc-invalidate
          colorcomp-ewoc
          (ewoc-nth colorcomp-ewoc index)
          (ewoc-nth colorcomp-ewoc -1))))
     
     (defun colorcomp-R-more () (interactive) (colorcomp-mod 0 255 1))
     (defun colorcomp-G-more () (interactive) (colorcomp-mod 1 255 1))
     (defun colorcomp-B-more () (interactive) (colorcomp-mod 2 255 1))
     (defun colorcomp-R-less () (interactive) (colorcomp-mod 0 0 -1))
     (defun colorcomp-G-less () (interactive) (colorcomp-mod 1 0 -1))
     (defun colorcomp-B-less () (interactive) (colorcomp-mod 2 0 -1))
     
     (defun colorcomp-copy-as-kill-and-exit ()
       "Copy the color components into the kill ring and kill the buffer.
     The string is formatted #RRGGBB (hash followed by six hex digits)."
       (interactive)
       (kill-new (format "#%02X%02X%02X"
                         (aref colorcomp-data 0)
                         (aref colorcomp-data 1)
                         (aref colorcomp-data 2)))
       (kill-buffer nil))
     
     (setq colorcomp-mode-map
           (let ((m (make-sparse-keymap)))
             (suppress-keymap m)
             (define-key m "i" 'colorcomp-R-less)
             (define-key m "o" 'colorcomp-R-more)
             (define-key m "k" 'colorcomp-G-less)
             (define-key m "l" 'colorcomp-G-more)
             (define-key m "," 'colorcomp-B-less)
             (define-key m "." 'colorcomp-B-more)
             (define-key m " " 'colorcomp-copy-as-kill-and-exit)
             m))

Note that we never modify the data in each node, which is fixed when the ewoc is created to be either nil or an index into the vector colorcomp-data, the actual color components.

37.21 Blinking Parentheses

This section describes the mechanism by which Emacs shows a matching open parenthesis when the user inserts a close parenthesis.

37.22 Character Display

This section describes how characters are actually displayed by Emacs. Typically, a character is displayed as a glyph (a graphical symbol which occupies one character position on the screen), whose appearance corresponds to the character itself. For example, the character ‘a’ (character code 97) is displayed as ‘a’. Some characters, however, are displayed specially. For example, the formfeed character (character code 12) is usually displayed as a sequence of two glyphs, ‘^L’, while the newline character (character code 10) starts a new screen line.

You can modify how each character is displayed by defining a display table, which maps each character code into a sequence of glyphs. See Display Tables.

• Usual Display: The usual conventions for displaying characters.
• Display Tables: What a display table consists of.
• Active Display Table: How Emacs selects a display table to use.
• Glyphs: How to define a glyph, and what glyphs mean.
• Glyphless Chars: How glyphless characters are drawn.

37.22.1 Usual Display Conventions

Here are the conventions for displaying each character code (in the absence of a display table, which can override these conventions; see Display Tables).

• The printable ASCII characters, character codes 32 through 126 (consisting of numerals, English letters, and symbols like ‘#’) are displayed literally.
• The tab character (character code 9) displays as whitespace stretching up to the next tab stop column. See Text Display. The variable tab-width controls the number of spaces per tab stop (see below).
• The newline character (character code 10) has a special effect: it ends the preceding line and starts a new line.

• The non-printable ASCII control characters—character codes 0 through 31, as well as the <DEL> character (character code 127)—display in one of two ways according to the variable ctl-arrow. If this variable is non-nil (the default), these characters are displayed as sequences of two glyphs, where the first glyph is ‘^’ (a display table can specify a glyph to use instead of ‘^’); e.g., the <DEL> character is displayed as ‘^?’.

  If ctl-arrow is nil, these characters are displayed as octal escapes (see below).

  This rule also applies to carriage return (character code 13), if that character appears in the buffer. But carriage returns usually do not appear in buffer text; they are eliminated as part of end-of-line conversion (see Coding System Basics).

• Raw bytes are non-ASCII characters with codes 128 through 255 (see Text Representations). These characters display as octal escapes: sequences of four glyphs, where the first glyph is the ASCII code for ‘\’, and the others are digit characters representing the character code in octal. (A display table can specify a glyph to use instead of ‘\’.)
• Each non-ASCII character with code above 255 is displayed literally, if the terminal supports it. If the terminal does not support it, the character is said to be glyphless, and it is usually displayed using a placeholder glyph. For example, if a graphical terminal has no font for a character, Emacs usually displays a box containing the character code in hexadecimal. See Glyphless Chars.

The above display conventions apply even when there is a display table, for any character whose entry in the active display table is nil. Thus, when you set up a display table, you need only specify the characters for which you want special behavior.

The following variables affect how certain characters are displayed on the screen. Since they change the number of columns the characters occupy, they also affect the indentation functions. They also affect how the mode line is displayed; if you want to force redisplay of the mode line using the new values, call the function force-mode-line-update (see Mode Line Format).

37.22.2 Display Tables

A display table is a special-purpose char-table (see Char-Tables), with display-table as its subtype, which is used to override the usual character display conventions. This section describes how to make, inspect, and assign elements to a display table object.

The ordinary elements of the display table are indexed by character codes; the element at index c says how to display the character code c. The value should be nil (which means to display the character c according to the usual display conventions; see Usual Display), or a vector of glyph codes (which means to display the character c as those glyphs; see Glyphs).

Warning: if you use the display table to change the display of newline characters, the whole buffer will be displayed as one long line.

The display table also has six extra slots which serve special purposes. Here is a table of their meanings; nil in any slot means to use the default for that slot, as stated below.

0

The glyph for the end of a truncated screen line (the default for this is ‘$’). See Glyphs. On graphical terminals, Emacs uses arrows in the fringes to indicate truncation, so the display table has no effect.

1

The glyph for the end of a continued line (the default is ‘\’). On graphical terminals, Emacs uses curved arrows in the fringes to indicate continuation, so the display table has no effect.

2

The glyph for indicating a character displayed as an octal character code (the default is ‘\’).

3

The glyph for indicating a control character (the default is ‘^’).

4

A vector of glyphs for indicating the presence of invisible lines (the default is ‘...’). See Selective Display.

5

The glyph used to draw the border between side-by-side windows (the default is ‘|’). See Splitting Windows. This takes effect only when there are no scroll bars; if scroll bars are supported and in use, a scroll bar separates the two windows.

For example, here is how to construct a display table that mimics the effect of setting ctl-arrow to a non-nil value (see Glyphs, for the function make-glyph-code):

     (setq disptab (make-display-table))
     (dotimes (i 32)
       (or (= i ?\t)
           (= i ?\n)
           (aset disptab i
                 (vector (make-glyph-code ?^ 'escape-glyph)
                         (make-glyph-code (+ i 64) 'escape-glyph)))))
     (aset disptab 127
           (vector (make-glyph-code ?^ 'escape-glyph)
                   (make-glyph-code ?? 'escape-glyph)))))

37.22.3 Active Display Table

Each window can specify a display table, and so can each buffer. The window's display table, if there is one, takes precedence over the buffer's display table. If neither exists, Emacs tries to use the standard display table; if that is nil, Emacs uses the usual character display conventions (see Usual Display).

Note that display tables affect how the mode line is displayed, so if you want to force redisplay of the mode line using a new display table, call force-mode-line-update (see Mode Line Format).

The disp-table library defines several functions for changing the standard display table.

37.22.4 Glyphs

A glyph is a graphical symbol which occupies a single character position on the screen. Each glyph is represented in Lisp as a glyph code, which specifies a character and optionally a face to display it in (see Faces). The main use of glyph codes is as the entries of display tables (see Display Tables). The following functions are used to manipulate glyph codes:

You can set up a glyph table to change how glyph codes are actually displayed on text terminals. This feature is semi-obsolete; use glyphless-char-display instead (see Glyphless Chars).

37.22.5 Glyphless Character Display

Glyphless characters are characters which are displayed in a special way, e.g., as a box containing a hexadecimal code, instead of being displayed literally. These include characters which are explicitly defined to be glyphless, as well as characters for which there is no available font (on a graphical display), and characters which cannot be encoded by the terminal's coding system (on a text terminal).

37.23 Beeping

This section describes how to make Emacs ring the bell (or blink the screen) to attract the user's attention. Be conservative about how often you do this; frequent bells can become irritating. Also be careful not to use just beeping when signaling an error is more appropriate (see Errors).

37.24 Window Systems

Emacs works with several window systems, most notably the X Window System. Both Emacs and X use the term “window”, but use it differently. An Emacs frame is a single window as far as X is concerned; the individual Emacs windows are not known to X at all.

Do not use window-system and initial-window-system as predicates or boolean flag variables, if you want to write code that works differently on text terminals and graphic displays. That is because window-system is not a good indicator of Emacs capabilities on a given display type. Instead, use display-graphic-p or any of the other display-*-p predicates described in Display Feature Testing.

37.25 Tooltips

Tooltips are special frames (see Frames) that are used to display helpful hints (a.k.a. “tips”) related to the current position of the mouse pointer. Emacs uses tooltips to display help strings about active portions of text (see Special Properties) and about various UI elements, such as menu items (see Extended Menu Items) and tool-bar buttons (see Tool Bar).

When Emacs is built with GTK+ support, it by default displays tooltips using GTK+ functions, and the appearance of the tooltips is then controlled by GTK+ settings. GTK+ tooltips can be disabled by changing the value of the variable x-gtk-use-system-tooltips to nil. The rest of this subsection describes how to control non-GTK+ tooltips, which are presented by Emacs itself.

Since tooltips are special frames, they have their frame parameters (see Frame Parameters). Unlike other frames, the frame parameters for tooltips are stored in a special variable.

The tooltip face determines the appearance of text shown in tooltips. It should generally use a variable-pitch font of size that is preferably smaller than the default frame font.

If you write your own function to be put on the tooltip-functions list, you may need to know the buffer of the mouse event that triggered the tooltip display. The following function provides that information.

Other aspects of tooltip display are controlled by several customizable settings; see Tooltips.

37.26 Bidirectional Display

Emacs can display text written in scripts, such as Arabic, Farsi, and Hebrew, whose natural ordering for horizontal text display runs from right to left. Furthermore, segments of Latin script and digits embedded in right-to-left text are displayed left-to-right, while segments of right-to-left script embedded in left-to-right text (e.g., Arabic or Hebrew text in comments or strings in a program source file) are appropriately displayed right-to-left. We call such mixtures of left-to-right and right-to-left text bidirectional text. This section describes the facilities and options for editing and displaying bidirectional text.

Text is stored in Emacs buffers and strings in logical (or reading) order, i.e., the order in which a human would read each character. In right-to-left and bidirectional text, the order in which characters are displayed on the screen (called visual order) is not the same as logical order; the characters' screen positions do not increase monotonically with string or buffer position. In performing this bidirectional reordering, Emacs follows the Unicode Bidirectional Algorithm (a.k.a. UBA), which is described in Annex #9 of the Unicode standard (http://www.unicode.org/reports/tr9/). Emacs provides a “Full Bidirectionality” class implementation of the UBA, consistent with the requirements of the Unicode Standard v8.0.

Emacs never reorders the text of a unibyte buffer, even if bidi-display-reordering is non-nil in the buffer. This is because unibyte buffers contain raw bytes, not characters, and thus lack the directionality properties required for reordering. Therefore, to test whether text in a buffer will be reordered for display, it is not enough to test the value of bidi-display-reordering alone. The correct test is this:

      (if (and enable-multibyte-characters
               bidi-display-reordering)
          ;; Buffer is being reordered for display
        )

However, unibyte display and overlay strings are reordered if their parent buffer is reordered. This is because plain-ascii strings are stored by Emacs as unibyte strings. If a unibyte display or overlay string includes non-ascii characters, these characters are assumed to have left-to-right direction.

Text covered by display text properties, by overlays with display properties whose value is a string, and by any other properties that replace buffer text, is treated as a single unit when it is reordered for display. That is, the entire chunk of text covered by these properties is reordered together. Moreover, the bidirectional properties of the characters in such a chunk of text are ignored, and Emacs reorders them as if they were replaced with a single character U+FFFC, known as the Object Replacement Character. This means that placing a display property over a portion of text may change the way that the surrounding text is reordered for display. To prevent this unexpected effect, always place such properties on text whose directionality is identical with text that surrounds it.

Each paragraph of bidirectional text has a base direction, either right-to-left or left-to-right. Left-to-right paragraphs are displayed beginning at the left margin of the window, and are truncated or continued when the text reaches the right margin. Right-to-left paragraphs are displayed beginning at the right margin, and are continued or truncated at the left margin.

By default, Emacs determines the base direction of each paragraph by looking at the text at its beginning. The precise method of determining the base direction is specified by the UBA; in a nutshell, the first character in a paragraph that has an explicit directionality determines the base direction of the paragraph. However, sometimes a buffer may need to force a certain base direction for its paragraphs. For example, buffers containing program source code should force all paragraphs to be displayed left-to-right. You can use following variable to do this:

Sometimes there's a need to move point in strict visual order, either to the left or to the right of its current screen position. Emacs provides a primitive to do that.

Bidirectional reordering can have surprising and unpleasant effects when two strings with bidirectional content are juxtaposed in a buffer, or otherwise programmatically concatenated into a string of text. A typical problematic case is when a buffer consists of sequences of text fields separated by whitespace or punctuation characters, like Buffer Menu mode or Rmail Summary Mode. Because the punctuation characters used as separators have weak directionality, they take on the directionality of surrounding text. As result, a numeric field that follows a field with bidirectional content can be displayed to the left of the preceding field, messing up the expected layout. There are several ways to avoid this problem:

• Append the special character U+200E, LEFT-TO-RIGHT MARK, or LRM, to the end of each field that may have bidirectional content, or prepend it to the beginning of the following field. The function bidi-string-mark-left-to-right, described below, comes in handy for this purpose. (In a right-to-left paragraph, use U+200F, RIGHT-TO-LEFT MARK, or RLM, instead.) This is one of the solutions recommended by the UBA.
• Include the tab character in the field separator. The tab character plays the role of segment separator in bidirectional reordering, causing the text on either side to be reordered separately.

• Separate fields with a display property or overlay with a property value of the form (space . PROPS) (see Specified Space). Emacs treats this display specification as a paragraph separator, and reorders the text on either side separately.

The reordering algorithm uses the bidirectional properties of the characters stored as their bidi-class property (see Character Properties). Lisp programs can change these properties by calling the put-char-code-property function. However, doing this requires a thorough understanding of the UBA, and is therefore not recommended. Any changes to the bidirectional properties of a character have global effect: they affect all Emacs frames and windows.

Similarly, the mirroring property is used to display the appropriate mirrored character in the reordered text. Lisp programs can affect the mirrored display by changing this property. Again, any such changes affect all of Emacs display.

The bidirectional properties of characters can be overridden by inserting into the text special directional control characters, LEFT-TO-RIGHT OVERRIDE (LRO) and RIGHT-TO-LEFT OVERRIDE (RLO). Any characters between a RLO and the following newline or POP DIRECTIONAL FORMATTING (PDF) control character, whichever comes first, will be displayed as if they were strong right-to-left characters, i.e. they will be reversed on display. Similarly, any characters between LRO and PDF or newline will display as if they were strong left-to-right, and will not be reversed even if they are strong right-to-left characters.

These overrides are useful when you want to make some text unaffected by the reordering algorithm, and instead directly control the display order. But they can also be used for malicious purposes, known as phishing. Specifically, a URL on a Web page or a link in an email message can be manipulated to make its visual appearance unrecognizable, or similar to some popular benign location, while the real location, interpreted by a browser in the logical order, is very different.

Emacs provides a primitive that applications can use to detect instances of text whose bidirectional properties were overridden so as to make a left-to-right character display as if it were a right-to-left character, or vise versa.

When text that includes mixed right-to-left and left-to-right characters and bidirectional controls is copied into a different location, it can change its visual appearance, and also can affect the visual appearance of the surrounding text at destination. This is because reordering of bidirectional text specified by the UBA has non-trivial context-dependent effects both on the copied text and on the text at copy destination that will surround it.

Sometimes, a Lisp program may need to preserve the exact visual appearance of the copied text at destination, and of the text that surrounds the copy. Lisp programs can use the following function to achieve that effect.

38 Operating System Interface

This chapter is about starting and getting out of Emacs, access to values in the operating system environment, and terminal input, output.

See Building Emacs, for related information. See Display, for additional operating system status information pertaining to the terminal and the screen.

• Starting Up: Customizing Emacs startup processing.
• Getting Out: How exiting works (permanent or temporary).
• System Environment: Distinguish the name and kind of system.
• User Identification: Finding the name and user id of the user.
• Time of Day: Getting the current time.
• Time Zone Rules: Rules for time zones and daylight saving time.
• Time Conversion: Converting a time from numeric form to calendrical data and vice versa.
• Time Parsing: Converting a time from numeric form to text and vice versa.
• Processor Run Time: Getting the run time used by Emacs.
• Time Calculations: Adding, subtracting, comparing times, etc.
• Timers: Setting a timer to call a function at a certain time.
• Idle Timers: Setting a timer to call a function when Emacs has been idle for a certain length of time.
• Terminal Input: Accessing and recording terminal input.
• Terminal Output: Controlling and recording terminal output.
• Sound Output: Playing sounds on the computer's speaker.
• X11 Keysyms: Operating on key symbols for X Windows.
• Batch Mode: Running Emacs without terminal interaction.
• Session Management: Saving and restoring state with X Session Management.
• Desktop Notifications: Desktop notifications.
• File Notifications: File notifications.
• Dynamic Libraries: On-demand loading of support libraries.
• Security Considerations: Running Emacs in an unfriendly environment.

38.1 Starting Up Emacs

This section describes what Emacs does when it is started, and how you can customize these actions.

• Startup Summary: Sequence of actions Emacs performs at startup.
• Init File: Details on reading the init file.
• Terminal-Specific: How the terminal-specific Lisp file is read.
• Command-Line Arguments: How command-line arguments are processed, and how you can customize them.

38.1.1 Summary: Sequence of Actions at Startup

When Emacs is started up, it performs the following operations (see normal-top-level in startup.el):

1. It adds subdirectories to load-path, by running the file named subdirs.el in each directory in the list. Normally, this file adds the directory's subdirectories to the list, and those are scanned in their turn. The files subdirs.el are normally generated automatically when Emacs is installed.
2. It loads any leim-list.el that it finds in the load-path directories. This file is intended for registering input methods. The search is only for any personal leim-list.el files that you may have created; it skips the directories containing the standard Emacs libraries (these should contain only a single leim-list.el file, which is compiled into the Emacs executable).

3. It sets the variable before-init-time to the value of current-time (see Time of Day). It also sets after-init-time to nil, which signals to Lisp programs that Emacs is being initialized.
4. It sets the language environment and the terminal coding system, if requested by environment variables such as LANG.
5. It does some basic parsing of the command-line arguments.

6. If not running in batch mode, it initializes the window system that the variable initial-window-system specifies (see initial-window-system). The initialization function for each supported window system is specified by window-system-initialization-alist. If the value of initial-window-system is windowsystem, then the appropriate initialization function is defined in the file term/windowsystem-win.el. This file should have been compiled into the Emacs executable when it was built.
7. It runs the normal hook before-init-hook.
8. If appropriate, it creates a graphical frame. This is not done if the options ‘--batch’ or ‘--daemon’ were specified.
9. It initializes the initial frame's faces, and sets up the menu bar and tool bar if needed. If graphical frames are supported, it sets up the tool bar even if the current frame is not a graphical one, since a graphical frame may be created later on.
10. It use custom-reevaluate-setting to re-initialize the members of the list custom-delayed-init-variables. These are any pre-loaded user options whose default value depends on the run-time, rather than build-time, context. See custom-initialize-delay.
11. It loads the library site-start, if it exists. This is not done if the options ‘-Q’ or ‘--no-site-file’ were specified.
12. It loads your init file (see Init File). This is not done if the options ‘-q’, ‘-Q’, or ‘--batch’ were specified. If the ‘-u’ option was specified, Emacs looks for the init file in that user's home directory instead.
13. It loads the library default, if it exists. This is not done if inhibit-default-init is non-nil, nor if the options ‘-q’, ‘-Q’, or ‘--batch’ were specified.
14. It loads your abbrevs from the file specified by abbrev-file-name, if that file exists and can be read (see abbrev-file-name). This is not done if the option ‘--batch’ was specified.
15. It calls the function package-initialize to activate any optional Emacs Lisp package that has been installed. See Packaging Basics. However, Emacs doesn't initialize packages when package-enable-at-startup is nil or when it's started with one of the options ‘-q’, ‘-Q’, or ‘--batch’. To initialize packages in the latter case, package-initialize should be called explicitly (e.g., via the ‘--funcall’ option).

16. It sets the variable after-init-time to the value of current-time. This variable was set to nil earlier; setting it to the current time signals that the initialization phase is over, and, together with before-init-time, provides the measurement of how long it took.
17. It runs the normal hook after-init-hook.
18. If the buffer *scratch* exists and is still in Fundamental mode (as it should be by default), it sets its major mode according to initial-major-mode.
19. If started on a text terminal, it loads the terminal-specific Lisp library (see Terminal-Specific), and runs the hook tty-setup-hook. This is not done in --batch mode, nor if term-file-prefix is nil.
20. It displays the initial echo area message, unless you have suppressed that with inhibit-startup-echo-area-message.
21. It processes any command-line options that were not handled earlier.
22. It now exits if the option --batch was specified.
23. If the *scratch* buffer exists and is empty, it inserts (substitute-command-keys initial-scratch-message) into that buffer.
24. If initial-buffer-choice is a string, it visits the file (or directory) with that name. If it is a function, it calls the function with no arguments and selects the buffer that it returns. If one file is given as a command line argument, that file is visited and its buffer displayed alongside initial-buffer-choice. If more than one file is given, all of the files are visited and the *Buffer List* buffer is displayed alongside initial-buffer-choice.
25. It runs emacs-startup-hook.
26. It calls frame-notice-user-settings, which modifies the parameters of the selected frame according to whatever the init files specify.
27. It runs window-setup-hook. The only difference between this hook and emacs-startup-hook is that this one runs after the previously mentioned modifications to the frame parameters.
28. It displays the startup screen, which is a special buffer that contains information about copyleft and basic Emacs usage. This is not done if inhibit-startup-screen or initial-buffer-choice are non-nil, or if the ‘--no-splash’ or ‘-Q’ command-line options were specified.
29. If the option --daemon was specified, it calls server-start, and on Posix systems also detaches from the controlling terminal. See Emacs Server.
30. If started by the X session manager, it calls emacs-session-restore passing it as argument the ID of the previous session. See Session Management.

The following options affect some aspects of the startup sequence.

The following command-line options affect some aspects of the startup sequence. See Initial Options.

--no-splash

Do not display a splash screen.

--batch

Run without an interactive terminal. See Batch Mode.

--daemon

Do not initialize any display; just start a server in the background.

--no-init-file

-q

Do not load either the init file, or the default library.

--no-site-file

Do not load the site-start library.

--quick

-Q

Equivalent to ‘-q --no-site-file --no-splash’.

38.1.2 The Init File

When you start Emacs, it normally attempts to load your init file. This is either a file named .emacs or .emacs.el in your home directory, or a file named init.el in a subdirectory named .emacs.d in your home directory.

The command-line switches ‘-q’, ‘-Q’, and ‘-u’ control whether and where to find the init file; ‘-q’ (and the stronger ‘-Q’) says not to load an init file, while ‘-u user’ says to load user's init file instead of yours. See Entering Emacs. If neither option is specified, Emacs uses the LOGNAME environment variable, or the USER (most systems) or USERNAME (MS systems) variable, to find your home directory and thus your init file; this way, even if you have su'd, Emacs still loads your own init file. If those environment variables are absent, though, Emacs uses your user-id to find your home directory.

An Emacs installation may have a default init file, which is a Lisp library named default.el. Emacs finds this file through the standard search path for libraries (see How Programs Do Loading). The Emacs distribution does not come with this file; it is intended for local customizations. If the default init file exists, it is loaded whenever you start Emacs. But your own personal init file, if any, is loaded first; if it sets inhibit-default-init to a non-nil value, then Emacs does not subsequently load the default.el file. In batch mode, or if you specify ‘-q’ (or ‘-Q’), Emacs loads neither your personal init file nor the default init file.

Another file for site-customization is site-start.el. Emacs loads this before the user's init file. You can inhibit the loading of this file with the option ‘--no-site-file’.

See Init File Examples, for examples of how to make various commonly desired customizations in your .emacs file.

38.1.3 Terminal-Specific Initialization

Each terminal type can have its own Lisp library that Emacs loads when run on that type of terminal. The library's name is constructed by concatenating the value of the variable term-file-prefix and the terminal type (specified by the environment variable TERM). Normally, term-file-prefix has the value "term/"; changing this is not recommended. If there is an entry matching TERM in the term-file-aliases association list, Emacs uses the associated value in place of TERM. Emacs finds the file in the normal manner, by searching the load-path directories, and trying the ‘.elc’ and ‘.el’ suffixes.

The usual role of a terminal-specific library is to enable special keys to send sequences that Emacs can recognize. It may also need to set or add to input-decode-map if the Termcap or Terminfo entry does not specify all the terminal's function keys. See Terminal Input.

When the name of the terminal type contains a hyphen or underscore, and no library is found whose name is identical to the terminal's name, Emacs strips from the terminal's name the last hyphen or underscore and everything that follows it, and tries again. This process is repeated until Emacs finds a matching library, or until there are no more hyphens or underscores in the name (i.e., there is no terminal-specific library). For example, if the terminal name is ‘xterm-256color’ and there is no term/xterm-256color.el library, Emacs tries to load term/xterm.el. If necessary, the terminal library can evaluate (getenv "TERM") to find the full name of the terminal type.

Your init file can prevent the loading of the terminal-specific library by setting the variable term-file-prefix to nil.

You can also arrange to override some of the actions of the terminal-specific library by using tty-setup-hook. This is a normal hook that Emacs runs after initializing a new text terminal. You could use this hook to define initializations for terminals that do not have their own libraries. See Hooks.

38.1.4 Command-Line Arguments

You can use command-line arguments to request various actions when you start Emacs. Note that the recommended way of using Emacs is to start it just once, after logging in, and then do all editing in the same Emacs session (see Entering Emacs). For this reason, you might not use command-line arguments very often; nonetheless, they can be useful when invoking Emacs from session scripts or debugging Emacs. This section describes how Emacs processes command-line arguments.

38.2 Getting Out of Emacs

There are two ways to get out of Emacs: you can kill the Emacs job, which exits permanently, or you can suspend it, which permits you to reenter the Emacs process later. (In a graphical environment, you can of course simply switch to another application without doing anything special to Emacs, then switch back to Emacs when you want.)

• Killing Emacs: Exiting Emacs irreversibly.
• Suspending Emacs: Exiting Emacs reversibly.

38.2.1 Killing Emacs

Killing Emacs means ending the execution of the Emacs process. If you started Emacs from a terminal, the parent process normally resumes control. The low-level primitive for killing Emacs is kill-emacs.

The kill-emacs function is normally called via the higher-level command C-x C-c (save-buffers-kill-terminal). See Exiting. It is also called automatically if Emacs receives a SIGTERM or SIGHUP operating system signal (e.g., when the controlling terminal is disconnected), or if it receives a SIGINT signal while running in batch mode (see Batch Mode).

When Emacs is killed, all the information in the Emacs process, aside from files that have been saved, is lost. Because killing Emacs inadvertently can lose a lot of work, the save-buffers-kill-terminal command queries for confirmation if you have buffers that need saving or subprocesses that are running. It also runs the abnormal hook kill-emacs-query-functions:

38.2.2 Suspending Emacs

On text terminals, it is possible to suspend Emacs, which means stopping Emacs temporarily and returning control to its superior process, which is usually the shell. This allows you to resume editing later in the same Emacs process, with the same buffers, the same kill ring, the same undo history, and so on. To resume Emacs, use the appropriate command in the parent shell—most likely fg.

Suspending works only on a terminal device from which the Emacs session was started. We call that device the controlling terminal of the session. Suspending is not allowed if the controlling terminal is a graphical terminal. Suspending is usually not relevant in graphical environments, since you can simply switch to another application without doing anything special to Emacs.

Some operating systems (those without SIGTSTP, or MS-DOS) do not support suspension of jobs; on these systems, suspension actually creates a new shell temporarily as a subprocess of Emacs. Then you would exit the shell to return to Emacs.

38.3 Operating System Environment

Emacs provides access to variables in the operating system environment through various functions. These variables include the name of the system, the user's UID, and so on.

38.4 User Identification

The symbols user-login-name, user-real-login-name and user-full-name are variables as well as functions. The functions return the same values that the variables hold. These variables allow you to fake out Emacs by telling the functions what to return. The variables are also useful for constructing frame titles (see Frame Titles).

38.5 Time of Day

This section explains how to determine the current time and time zone.

Most of these functions represent time as a list of four integers (sec-high sec-low microsec picosec). This represents the number of seconds from the epoch (January 1, 1970 at 00:00 UTC), using the formula: high * 2**16 + low + micro * 10**−6 + pico * 10**−12. The return value of current-time represents time using this form, as do the timestamps in the return values of other functions such as file-attributes (see Definition of file-attributes). In some cases, functions may return two- or three-element lists, with omitted microsec and picosec components defaulting to zero.

Function arguments, e.g., the time argument to current-time-string, accept a more-general time value format, which can be a list of integers as above, or a single number for seconds since the epoch, or nil for the current time. You can convert a time value into a human-readable string using current-time-string and format-time-string, into a list of integers using seconds-to-time, and into other forms using decode-time and float-time. These functions are described in the following sections.

38.6 Time Zone Rules

The default time zone is determined by the TZ environment variable. See System Environment. For example, you can tell Emacs to default to Universal Time with (setenv "TZ" "UTC0"). If TZ is not in the environment, Emacs uses system wall clock time, which is a platform-dependent default time zone.

The set of supported TZ strings is system-dependent. GNU and many other systems support the tzdata database, e.g., ‘"America/New_York"’ specifies the time zone and daylight saving time history for locations near New York City. GNU and most other systems support POSIX-style TZ strings, e.g., ‘"EST+5EDT,M4.1.0/2,M10.5.0/2"’ specifies the rules used in New York from 1987 through 2006. All systems support the string ‘"UTC0"’ meaning Universal Time.

Functions that convert to and from local time accept an optional time zone rule argument, which specifies the conversion's time zone and daylight saving time history. If the time zone rule is omitted or nil, the conversion uses Emacs's default time zone. If it is t, the conversion uses Universal Time. If it is wall, the conversion uses the system wall clock time. If it is a string, the conversion uses the time zone rule equivalent to setting TZ to that string.

38.7 Time Conversion

These functions convert time values (see Time of Day) into calendrical information and vice versa.

Many 32-bit operating systems are limited to system times containing 32 bits of information in their seconds component; these systems typically handle only the times from 1901-12-13 20:45:52 through 2038-01-19 03:14:07 Universal Time. However, 64-bit and some 32-bit operating systems have larger seconds components, and can represent times far in the past or future.

Time conversion functions always use the Gregorian calendar, even for dates before the Gregorian calendar was introduced. Year numbers count the number of years since the year 1 B.C., and do not skip zero as traditional Gregorian years do; for example, the year number −37 represents the Gregorian year 38 B.C.

38.8 Parsing and Formatting Times

These functions convert time values to text in a string, and vice versa. Time values are lists of two to four integers (see Time of Day).

38.9 Processor Run time

Emacs provides several functions and primitives that return time, both elapsed and processor time, used by the Emacs process.

38.10 Time Calculations

These functions perform calendrical computations using time values (see Time of Day). A value of nil for any of their time-value arguments stands for the current system time, and a single integer number stands for the number of seconds since the epoch.

38.11 Timers for Delayed Execution

You can set up a timer to call a function at a specified future time or after a certain length of idleness. A timer is a special object that stores the information about the next invocation times and the function to invoke.

Emacs cannot run timers at any arbitrary point in a Lisp program; it can run them only when Emacs could accept output from a subprocess: namely, while waiting or inside certain primitive functions such as sit-for or read-event which can wait. Therefore, a timer's execution may be delayed if Emacs is busy. However, the time of execution is very precise if Emacs is idle.

Emacs binds inhibit-quit to t before calling the timer function, because quitting out of many timer functions can leave things in an inconsistent state. This is normally unproblematical because most timer functions don't do a lot of work. Indeed, for a timer to call a function that takes substantial time to run is likely to be annoying. If a timer function needs to allow quitting, it should use with-local-quit (see Quitting). For example, if a timer function calls accept-process-output to receive output from an external process, that call should be wrapped inside with-local-quit, to ensure that C-g works if the external process hangs.

It is usually a bad idea for timer functions to alter buffer contents. When they do, they usually should call undo-boundary both before and after changing the buffer, to separate the timer's changes from user commands' changes and prevent a single undo entry from growing to be quite large.

Timer functions should also avoid calling functions that cause Emacs to wait, such as sit-for (see Waiting). This can lead to unpredictable effects, since other timers (or even the same timer) can run while waiting. If a timer function needs to perform an action after a certain time has elapsed, it can do this by scheduling a new timer.

If a timer function calls functions that can change the match data, it should save and restore the match data. See Saving Match Data.

A repeating timer nominally ought to run every repeat seconds, but remember that any invocation of a timer can be late. Lateness of one repetition has no effect on the scheduled time of the next repetition. For instance, if Emacs is busy computing for long enough to cover three scheduled repetitions of the timer, and then starts to wait, it will immediately call the timer function three times in immediate succession (presuming no other timers trigger before or between them). If you want a timer to run again no less than n seconds after the last invocation, don't use the repeat argument. Instead, the timer function should explicitly reschedule the timer.

The function y-or-n-p-with-timeout provides a simple way to use a timer to avoid waiting too long for an answer. See Yes-or-No Queries.

38.12 Idle Timers

Here is how to set up a timer that runs when Emacs is idle for a certain length of time. Aside from how to set them up, idle timers work just like ordinary timers.

Emacs becomes idle when it starts waiting for user input, and it remains idle until the user provides some input. If a timer is set for five seconds of idleness, it runs approximately five seconds after Emacs first becomes idle. Even if repeat is non-nil, this timer will not run again as long as Emacs remains idle, because the duration of idleness will continue to increase and will not go down to five seconds again.

Emacs can do various things while idle: garbage collect, autosave or handle data from a subprocess. But these interludes during idleness do not interfere with idle timers, because they do not reset the clock of idleness to zero. An idle timer set for 600 seconds will run when ten minutes have elapsed since the last user command was finished, even if subprocess output has been accepted thousands of times within those ten minutes, and even if there have been garbage collections and autosaves.

When the user supplies input, Emacs becomes non-idle while executing the input. Then it becomes idle again, and all the idle timers that are set up to repeat will subsequently run another time, one by one.

Do not write an idle timer function containing a loop which does a certain amount of processing each time around, and exits when (input-pending-p) is non-nil. This approach seems very natural but has two problems:

• It blocks out all process output (since Emacs accepts process output only while waiting).
• It blocks out any idle timers that ought to run during that time.

Similarly, do not write an idle timer function that sets up another idle timer (including the same idle timer) with secs argument less than or equal to the current idleness time. Such a timer will run almost immediately, and continue running again and again, instead of waiting for the next time Emacs becomes idle. The correct approach is to reschedule with an appropriate increment of the current value of the idleness time, as described below.

The main use of current-idle-time is when an idle timer function wants to “take a break” for a while. It can set up another idle timer to call the same function again, after a few seconds more idleness. Here's an example:

     (defvar my-resume-timer nil
       "Timer for `my-timer-function' to reschedule itself, or nil.")
     
     (defun my-timer-function ()
       ;; If the user types a command while my-resume-timer
       ;; is active, the next time this function is called from
       ;; its main idle timer, deactivate my-resume-timer.
       (when my-resume-timer
         (cancel-timer my-resume-timer))
       ...do the work for a while...
       (when taking-a-break
         (setq my-resume-timer
               (run-with-idle-timer
                 ;; Compute an idle time break-length
                 ;; more than the current value.
                 (time-add (current-idle-time) break-length)
                 nil
                 'my-timer-function))))

38.13 Terminal Input

This section describes functions and variables for recording or manipulating terminal input. See Display, for related functions.

• Input Modes: Options for how input is processed.
• Recording Input: Saving histories of recent or all input events.

38.13.1 Input Modes

The current-input-mode function returns the input mode settings Emacs is currently using.

38.13.2 Recording Input

See also the open-termscript function (see Terminal Output).

38.14 Terminal Output

The terminal output functions send output to a text terminal, or keep track of output sent to the terminal. The variable baud-rate tells you what Emacs thinks is the output speed of the terminal.

If you are running across a network, and different parts of the network work at different baud rates, the value returned by Emacs may be different from the value used by your local terminal. Some network protocols communicate the local terminal speed to the remote machine, so that Emacs and other programs can get the proper value, but others do not. If Emacs has the wrong value, it makes decisions that are less than optimal. To fix the problem, set baud-rate.

38.15 Sound Output

To play sound using Emacs, use the function play-sound. Only certain systems are supported; if you call play-sound on a system which cannot really do the job, it gives an error.

The sound must be stored as a file in RIFF-WAVE format (‘.wav’) or Sun Audio format (‘.au’).

38.16 Operating on X11 Keysyms

To define system-specific X11 keysyms, set the variable system-key-alist.

You can specify which keysyms Emacs should use for the Meta, Alt, Hyper, and Super modifiers by setting these variables:

38.17 Batch Mode

The command-line option ‘-batch’ causes Emacs to run noninteractively. In this mode, Emacs does not read commands from the terminal, it does not alter the terminal modes, and it does not expect to be outputting to an erasable screen. The idea is that you specify Lisp programs to run; when they are finished, Emacs should exit. The way to specify the programs to run is with ‘-l file’, which loads the library named file, or ‘-f function’, which calls function with no arguments, or ‘--eval form’.

Any Lisp program output that would normally go to the echo area, either using message, or using prin1, etc., with t as the stream, goes instead to Emacs's standard descriptors when in batch mode: message writes to the standard error descriptor, while prin1 and other print functions write to the standard output. Similarly, input that would normally come from the minibuffer is read from the standard input descriptor. Thus, Emacs behaves much like a noninteractive application program. (The echo area output that Emacs itself normally generates, such as command echoing, is suppressed entirely.)

Non-ASCII text written to the standard output or error descriptors is by default encoded using locale-coding-system (see Locales) if it is non-nil; this can be overridden by binding coding-system-for-write to a coding system of you choice (see Explicit Encoding).

38.18 Session Management

Emacs supports the X Session Management Protocol, which is used to suspend and restart applications. In the X Window System, a program called the session manager is responsible for keeping track of the applications that are running. When the X server shuts down, the session manager asks applications to save their state, and delays the actual shutdown until they respond. An application can also cancel the shutdown.

When the session manager restarts a suspended session, it directs these applications to individually reload their saved state. It does this by specifying a special command-line argument that says what saved session to restore. For Emacs, this argument is ‘--smid session’.

Here is an example that just inserts some text into *scratch* when Emacs is restarted by the session manager.

     (add-hook 'emacs-save-session-functions 'save-yourself-test)
     
     (defun save-yourself-test ()
       (insert "(save-current-buffer
       (switch-to-buffer \"*scratch*\")
       (insert \"I am restored\"))")
       nil)

38.19 Desktop Notifications

Emacs is able to send notifications on systems that support the freedesktop.org Desktop Notifications Specification and on MS-Windows. In order to use this functionality on Posix hosts, Emacs must have been compiled with D-Bus support, and the notifications library must be loaded. See D-Bus. The following function is supported when D-Bus support is available:

When Emacs runs on MS-Windows as a GUI session, it supports a small subset of the D-Bus notifications functionality via a native primitive:

To remove the notification and its icon from the taskbar, use the following function:

38.20 Notifications on File Changes

Several operating systems support watching of filesystems for changes of files. If configured properly, Emacs links a respective library like inotify, kqueue, gfilenotify, or w32notify statically. These libraries enable watching of filesystems on the local machine.

It is also possible to watch filesystems on remote machines, see Remote Files This does not depend on one of the libraries linked to Emacs.

Since all these libraries emit different events on notified file changes, there is the Emacs library filenotify which provides a unique interface.

38.21 Dynamically Loaded Libraries

A dynamically loaded library is a library that is loaded on demand, when its facilities are first needed. Emacs supports such on-demand loading of support libraries for some of its features.

38.22 Security Considerations

Like any application, Emacs can be run in a secure environment, where the operating system enforces rules about access and the like. With some care, Emacs-based applications can also be part of a security perimeter that checks such rules. Although the default settings for Emacs work well for a typical software development environment, they may require adjustment in environments containing untrusted users that may include attackers. Here is a compendium of security issues that may be helpful if you are developing such applications. It is by no means complete; it is intended to give you an idea of the security issues involved, rather than to be a security checklist.

File local variables

A file that Emacs visits can contain variable settings that affects the buffer visiting that file; See File Local Variables. Similarly, a directory can specify local variable values common to all files in that directory; See Directory Local Variables. Although Emacs takes some effort to protect against misuse of these variables, a security hole can be created merely by a package setting safe-local-variable too optimistically, a problem that is all too common. To disable this feature for both files and directories, set enable-local-variables to nil.

Access control

Although Emacs normally respects access permissions of the underlying operating system, in some cases it handles accesses specially. For example, file names can have handlers that treat the files specially, with their own access checking. See Magic File Names. Also, a buffer can be read-only even if the corresponding file is writeable, and vice versa, which can result in messages such as ‘File passwd is write-protected; try to save anyway? (yes or no)’. See Read Only Buffers.

Authentication

Emacs has several functions that deal with passwords, e.g., read-passwd. See Reading a Password. Although these functions do not attempt to broadcast passwords to the world, their implementations are not proof against determined attackers with access to Emacs internals. For example, even if Elisp code uses clear-string to scrub a password from its memory after using it, remnants of the password may still reside in the garbage-collected free list. See Modifying Strings.

Code injection

Emacs can send commands to many other applications, and applications should take care that strings sent as operands of these commands are not misinterpreted as directives. For example, when using a shell command to rename a file a to b, do not simply use the string mv a b, because either file name might start with ‘-’, or might contain shell metacharacters like ‘;’. Although functions like shell-quote-argument can help avoid this sort of problem, they are not panaceas; for example, on a POSIX platform shell-quote-argument quotes shell metacharacters but not leading ‘-’. See Shell Arguments. Typically it is safer to use call-process than a subshell. See Synchronous Processes. And it is safer yet to use builtin Emacs functions; for example, use (rename-file "a" "b" t) instead of invoking mv. See Changing Files.

Coding systems

Emacs attempts to infer the coding systems of the files and network connections it accesses. See Coding Systems. If Emacs infers incorrectly, or if the other parties to the network connection disagree with Emacs's inferences, the resulting system could be unreliable. Also, even when it infers correctly, Emacs often can use bytes that other programs cannot. For example, although to Emacs the null byte is just a character like any other, many other applications treat it as a string terminator and mishandle strings or files containing null bytes.

Environment and configuration variables

POSIX specifies several environment variables that can affect how Emacs behaves. Any environment variable whose name consists entirely of uppercase ASCII letters, digits, and the underscore may affect the internal behavior of Emacs. Emacs uses several such variables, e.g., EMACSLOADPATH. See Library Search. On some platforms some environment variables (e.g., PATH, POSIXLY_CORRECT, SHELL, TMPDIR) need to have properly-configured values in order to get standard behavior for any utility Emacs might invoke. Even seemingly-benign variables like TZ may have security implications. See System Environment.

Emacs has customization and other variables with similar considerations. For example, if the variable shell-file-name specifies a shell with nonstandard behavior, an Emacs-based application may misbehave.

Installation

When Emacs is installed, if the installation directory hierarchy can be modified by untrusted users, the application cannot be trusted. This applies also to the directory hierarchies of the programs that Emacs uses, and of the files that Emacs reads and writes.

Network access

Emacs often accesses the network, and you may want to configure it to avoid network accesses that it would normally do. For example, unless you set tramp-mode to nil, file names using a certain syntax are interpreted as being network files, and are retrieved across the network. See The Tramp Manual.

Race conditions

Emacs applications have the same sort of race-condition issues that other applications do. For example, even when (file-readable-p "foo.txt") returns t, it could be that foo.txt is unreadable because some other program changed the file's permissions between the call to file-readable-p and now. See Testing Accessibility.

Resource limits

When Emacs exhausts memory or other operating system resources, its behavior can be less reliable, in that computations that ordinarily run to completion may abort back to the top level. This may cause Emacs to neglect operations that it normally would have done.

39 Preparing Lisp code for distribution

Emacs provides a standard way to distribute Emacs Lisp code to users. A package is a collection of one or more files, formatted and bundled in such a way that users can easily download, install, uninstall, and upgrade it.

The following sections describe how to create a package, and how to put it in a package archive for others to download. See Packages, for a description of user-level features of the packaging system.

• Packaging Basics: The basic concepts of Emacs Lisp packages.
• Simple Packages: How to package a single .el file.
• Multi-file Packages: How to package multiple files.
• Package Archives: Maintaining package archives.

39.1 Packaging Basics

A package is either a simple package or a multi-file package. A simple package is stored in a package archive as a single Emacs Lisp file, while a multi-file package is stored as a tar file (containing multiple Lisp files, and possibly non-Lisp files such as a manual).

In ordinary usage, the difference between simple packages and multi-file packages is relatively unimportant; the Package Menu interface makes no distinction between them. However, the procedure for creating them differs, as explained in the following sections.

Each package (whether simple or multi-file) has certain attributes:

Name

A short word (e.g., ‘auctex’). This is usually also the symbol prefix used in the program (see Coding Conventions).

Version

A version number, in a form that the function version-to-list understands (e.g., ‘11.86’). Each release of a package should be accompanied by an increase in the version number so that it will be recognized as an upgrade by users querying the package archive.

Brief description

This is shown when the package is listed in the Package Menu. It should occupy a single line, ideally in 36 characters or less.

Long description

This is shown in the buffer created by C-h P (describe-package), following the package's brief description and installation status. It normally spans multiple lines, and should fully describe the package's capabilities and how to begin using it once it is installed.

Dependencies

A list of other packages (possibly including minimal acceptable version numbers) on which this package depends. The list may be empty, meaning this package has no dependencies. Otherwise, installing this package also automatically installs its dependencies, recursively; if any dependency cannot be found, the package cannot be installed.

Installing a package, either via the command package-install-file, or via the Package Menu, creates a subdirectory of package-user-dir named name-version, where name is the package's name and version its version (e.g., ~/.emacs.d/elpa/auctex-11.86/). We call this the package's content directory. It is where Emacs puts the package's contents (the single Lisp file for a simple package, or the files extracted from a multi-file package).

Emacs then searches every Lisp file in the content directory for autoload magic comments (see Autoload). These autoload definitions are saved to a file named name-autoloads.el in the content directory. They are typically used to autoload the principal user commands defined in the package, but they can also perform other tasks, such as adding an element to auto-mode-alist (see Auto Major Mode). Note that a package typically does not autoload every function and variable defined within it—only the handful of commands typically called to begin using the package. Emacs then byte-compiles every Lisp file in the package.

After installation, the installed package is loaded: Emacs adds the package's content directory to load-path, and evaluates the autoload definitions in name-autoloads.el.

Whenever Emacs starts up, it automatically calls the function package-initialize to load installed packages. This is done after loading the init file and abbrev file (if any) and before running after-init-hook (see Startup Summary). Automatic package loading is disabled if the user option package-enable-at-startup is nil.

39.2 Simple Packages

A simple package consists of a single Emacs Lisp source file. The file must conform to the Emacs Lisp library header conventions (see Library Headers). The package's attributes are taken from the various headers, as illustrated by the following example:

     ;;; superfrobnicator.el --- Frobnicate and bifurcate flanges
     
     ;; Copyright (C) 2011 Free Software Foundation, Inc.
     
     ;; Author: J. R. Hacker <jrh@example.com>
     ;; Version: 1.3
     ;; Package-Requires: ((flange "1.0"))
     ;; Keywords: multimedia, frobnicate
     ;; URL: http://example.com/jrhacker/superfrobnicate
     
     ...
     
     ;;; Commentary:
     
     ;; This package provides a minor mode to frobnicate and/or
     ;; bifurcate any flanges you desire.  To activate it, just type
     ...
     
     ;;;###autoload
     (define-minor-mode superfrobnicator-mode
     ...

The name of the package is the same as the base name of the file, as written on the first line. Here, it is ‘superfrobnicator’.

The brief description is also taken from the first line. Here, it is ‘Frobnicate and bifurcate flanges’.

The version number comes from the ‘Package-Version’ header, if it exists, or from the ‘Version’ header otherwise. One or the other must be present. Here, the version number is 1.3.

If the file has a ‘;;; Commentary:’ section, this section is used as the long description. (When displaying the description, Emacs omits the ‘;;; Commentary:’ line, as well as the leading comment characters in the commentary itself.)

If the file has a ‘Package-Requires’ header, that is used as the package dependencies. In the above example, the package depends on the ‘flange’ package, version 1.0 or higher. See Library Headers, for a description of the ‘Package-Requires’ header. If the header is omitted, the package has no dependencies.

The ‘Keywords’ and ‘URL’ headers are optional, but recommended. The command describe-package uses these to add links to its output. The ‘Keywords’ header should contain at least one standard keyword from the finder-known-keywords list.

The file ought to also contain one or more autoload magic comments, as explained in Packaging Basics. In the above example, a magic comment autoloads superfrobnicator-mode.

See Package Archives, for a explanation of how to add a single-file package to a package archive.

39.3 Multi-file Packages

A multi-file package is less convenient to create than a single-file package, but it offers more features: it can include multiple Emacs Lisp files, an Info manual, and other file types (such as images).

Prior to installation, a multi-file package is stored in a package archive as a tar file. The tar file must be named name-version.tar, where name is the package name and version is the version number. Its contents, once extracted, must all appear in a directory named name-version, the content directory (see Packaging Basics). Files may also extract into subdirectories of the content directory.

One of the files in the content directory must be named name-pkg.el. It must contain a single Lisp form, consisting of a call to the function define-package, described below. This defines the package's attributes: version, brief description, and requirements.

For example, if we distribute version 1.3 of the superfrobnicator as a multi-file package, the tar file would be superfrobnicator-1.3.tar. Its contents would extract into the directory superfrobnicator-1.3, and one of these would be the file superfrobnicator-pkg.el.

If the content directory contains a file named README, this file is used as the long description.

If the content directory contains a file named dir, this is assumed to be an Info directory file made with install-info. See Invoking install-info. The relevant Info files should also be present in the content directory. In this case, Emacs will automatically add the content directory to Info-directory-list when the package is activated.

Do not include any .elc files in the package. Those are created when the package is installed. Note that there is no way to control the order in which files are byte-compiled.

Do not include any file named name-autoloads.el. This file is reserved for the package's autoload definitions (see Packaging Basics). It is created automatically when the package is installed, by searching all the Lisp files in the package for autoload magic comments.

If the multi-file package contains auxiliary data files (such as images), the package's Lisp code can refer to these files via the variable load-file-name (see Loading). Here is an example:

     (defconst superfrobnicator-base (file-name-directory load-file-name))
     
     (defun superfrobnicator-fetch-image (file)
       (expand-file-name file superfrobnicator-base))

39.4 Creating and Maintaining Package Archives

Via the Package Menu, users may download packages from package archives. Such archives are specified by the variable package-archives, whose default value contains a single entry: the archive hosted by the GNU project at http://elpa.gnu.org. This section describes how to set up and maintain a package archive.

A package archive is simply a directory in which the package files, and associated files, are stored. If you want the archive to be reachable via HTTP, this directory must be accessible to a web server. How to accomplish this is beyond the scope of this manual.

A convenient way to set up and update a package archive is via the package-x library. This is included with Emacs, but not loaded by default; type M-x load-library <RET> package-x <RET> to load it, or add (require 'package-x) to your init file. See Lisp Libraries. Once loaded, you can make use of the following:

After you create an archive, remember that it is not accessible in the Package Menu interface unless it is in package-archives.

Maintaining a public package archive entails a degree of responsibility. When Emacs users install packages from your archive, those packages can cause Emacs to run arbitrary code with the permissions of the installing user. (This is true for Emacs code in general, not just for packages.) So you should ensure that your archive is well-maintained and keep the hosting system secure.

One way to increase the security of your packages is to sign them using a cryptographic key. If you have generated a private/public gpg key pair, you can use gpg to sign the package like this:

     gpg -ba -o file.sig file

For a single-file package, file is the package Lisp file; for a multi-file package, it is the package tar file. You can also sign the archive's contents file in the same way. Make the .sig files available in the same location as the packages. You should also make your public key available for people to download; e.g., by uploading it to a key server such as http://pgp.mit.edu/. When people install packages from your archive, they can use your public key to verify the signatures.

A full explanation of these matters is outside the scope of this manual. For more information on cryptographic keys and signing, see GnuPG. Emacs comes with an interface to GNU Privacy Guard, see EasyPG.

Appendix A Emacs 24 Antinews

For those users who live backwards in time, here is information about downgrading to Emacs version 24.5. We hope you will enjoy the greater simplicity that results from the absence of many Emacs 25.2 features.

A.1 Old Lisp Features in Emacs 24

• The requirement that setq and setf must be called with an even number of arguments has been removed. You can now call them with an odd number of arguments, and Emacs will helpfully supply a nil for the missing one. Simplicity rules!
• M-x shell and M-x compile set the EMACS environment variable, as they should, to indicate that the subprocess is run by Emacs. This is so packages that took years to learn how to work around that setting could continue using their code to that effect.
• The save-excursion form saves and restores the mark, as expected. No more need for the new save-mark-and-excursion, which has been deleted.
• We have removed the text-quoting-style variable and the associated functionality that translates quote characters in messages displayed to the user and in help buffers. Emacs now shows exactly the same quote characters as you wrote in your code! Likewise, substitute-command-keys leaves the quote characters alone. As you move back in time, Unicode support becomes less and less important, so no need to display those fancy new quotes the Unicode Standard invented.
• Regular expressions have been simplified by removing support for Unicode character properties in regexp classes. As result, [:alpha:] and [:alnum:] will match any character with a word syntax, and [:graph:] and [:print:] will match any multibyte character, including surrogates and unassigned codepoints. Once again, this is in line with diminishing importance of Unicode as you move back in time.
• Evaluating ‘(/ n)’ will now yield n. We have realized that interpreting that as in Common Lisp was a bad mistake that needed to be corrected.
• The pcase form was significantly simplified by removing the UPatterns quote and app. To further simplify this facility, we've removed pcase-defmacro, since we found no need for letting Lisp programs define new UPatterns.
• We've removed the text properties cursor-intangible and cursor-sensor-functions, replacing them by the much simpler intangible, point-entered, and point-left properties. The latter are implemented on a much lower level, and therefore are better integrated with user expectations. For similar reasons, cursor-intangible-mode and cursor-sensor-mode were removed; use the hook variable inhibit-point-motion-hooks which is no longer obsolete.
• Process creation and management functions were significantly improved and simplified by removing make-process and the pipe connection type. Redirecting stderr of a subprocess should be done with shell facilities, not by Emacs.
• We decided that shutting up informative messages is bad for user interaction, so we've removed the inhibit-message variable which could be used to that effect.
• Support for generators and for finalizers has been removed, as we found no real need for these facilities.
• Due to excessive complexity and the diminishing need for Unicode support, the functions string-collate-lessp and string-collate-equalp were removed. Their locale-independent counterparts string-lessp and string-equal are so much more simple and yield predictable results that we don't see any situation where the locale-dependent collation could be useful in Emacs. As result, the ls-lisp.el package sorts files in a locale-independent manner.
• In preparation for removal in some past version of Emacs of the bidirectional editing support, we started by deleting two functions bidi-find-overridden-directionality and buffer-substring-with-bidi-context.
• Time conversion functions, such as current-time-string, no longer accept an optional zone argument. If you need to change the current time zone (why?), do that explicitly with set-time-zone-rule.
• As part of the ongoing quest for simplicity, many other functions and variables have been eliminated.

Appendix B GNU Free Documentation License

     Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
     http://fsf.org/
     
     Everyone is permitted to copy and distribute verbatim copies
     of this license document, but changing it is not allowed.

0. PREAMBLE

   The purpose of this License is to make a manual, textbook, or other functional and useful document free in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.

   This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

   We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.

1. APPLICABILITY AND DEFINITIONS

   This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein. The “Document”, below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as “you”. You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law.

   A “Modified Version” of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language.

   A “Secondary Section” is a named appendix or a front-matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document's overall subject (or to related matters) and contains nothing that could fall directly within that overall subject. (Thus, if the Document is in part a textbook of mathematics, a Secondary Section may not explain any mathematics.) The relationship could be a matter of historical connection with the subject or with related matters, or of legal, commercial, philosophical, ethical or political position regarding them.

   The “Invariant Sections” are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License. If a section does not fit the above definition of Secondary then it is not allowed to be designated as Invariant. The Document may contain zero Invariant Sections. If the Document does not identify any Invariant Sections then there are none.

   The “Cover Texts” are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in the notice that says that the Document is released under this License. A Front-Cover Text may be at most 5 words, and a Back-Cover Text may be at most 25 words.

   A “Transparent” copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, that is suitable for revising the document straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file format whose markup, or absence of markup, has been arranged to thwart or discourage subsequent modification by readers is not Transparent. An image format is not Transparent if used for any substantial amount of text. A copy that is not “Transparent” is called “Opaque”.

   Examples of suitable formats for Transparent copies include plain ASCII without markup, Texinfo input format, LaTeX input format, SGML or XML using a publicly available DTD, and standard-conforming simple HTML, PostScript or PDF designed for human modification. Examples of transparent image formats include PNG, XCF and JPG. Opaque formats include proprietary formats that can be read and edited only by proprietary word processors, SGML or XML for which the DTD and/or processing tools are not generally available, and the machine-generated HTML, PostScript or PDF produced by some word processors for output purposes only.

   The “Title Page” means, for a printed book, the title page itself, plus such following pages as are needed to hold, legibly, the material this License requires to appear in the title page. For works in formats which do not have any title page as such, “Title Page” means the text near the most prominent appearance of the work's title, preceding the beginning of the body of the text.

   The “publisher” means any person or entity that distributes copies of the Document to the public.

   A section “Entitled XYZ” means a named subunit of the Document whose title either is precisely XYZ or contains XYZ in parentheses following text that translates XYZ in another language. (Here XYZ stands for a specific section name mentioned below, such as “Acknowledgements”, “Dedications”, “Endorsements”, or “History”.) To “Preserve the Title” of such a section when you modify the Document means that it remains a section “Entitled XYZ” according to this definition.

   The Document may include Warranty Disclaimers next to the notice which states that this License applies to the Document. These Warranty Disclaimers are considered to be included by reference in this License, but only as regards disclaiming warranties: any other implication that these Warranty Disclaimers may have is void and has no effect on the meaning of this License.

2. VERBATIM COPYING

   You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow the conditions in section 3.

   You may also lend copies, under the same conditions stated above, and you may publicly display copies.

3. COPYING IN QUANTITY

   If you publish printed copies (or copies in media that commonly have printed covers) of the Document, numbering more than 100, and the Document's license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects.

   If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages.

   If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a computer-network location from which the general network-using public has access to download using public-standard network protocols a complete Transparent copy of the Document, free of added material. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public.

   It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document.

4. MODIFICATIONS

   You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:

   1. Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of previous versions (which should, if there were any, be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission.
   2. List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of its principal authors, if it has fewer than five), unless they release you from this requirement.
   3. State on the Title page the name of the publisher of the Modified Version, as the publisher.
   4. Preserve all the copyright notices of the Document.
   5. Add an appropriate copyright notice for your modifications adjacent to the other copyright notices.
   6. Include, immediately after the copyright notices, a license notice giving the public permission to use the Modified Version under the terms of this License, in the form shown in the Addendum below.
   7. Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document's license notice.
   8. Include an unaltered copy of this License.
   9. Preserve the section Entitled “History”, Preserve its Title, and add to it an item stating at least the title, year, new authors, and publisher of the Modified Version as given on the Title Page. If there is no section Entitled “History” in the Document, create one stating the title, year, authors, and publisher of the Document as given on its Title Page, then add an item describing the Modified Version as stated in the previous sentence.
   10. Preserve the network location, if any, given in the Document for public access to a Transparent copy of the Document, and likewise the network locations given in the Document for previous versions it was based on. These may be placed in the “History” section. You may omit a network location for a work that was published at least four years before the Document itself, or if the original publisher of the version it refers to gives permission.
   11. For any section Entitled “Acknowledgements” or “Dedications”, Preserve the Title of the section, and preserve in the section all the substance and tone of each of the contributor acknowledgements and/or dedications given therein.
   12. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles.
   13. Delete any section Entitled “Endorsements”. Such a section may not be included in the Modified Version.
   14. Do not retitle any existing section to be Entitled “Endorsements” or to conflict in title with any Invariant Section.
   15. Preserve any Warranty Disclaimers.

   If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version's license notice. These titles must be distinct from any other section titles.

   You may add a section Entitled “Endorsements”, provided it contains nothing but endorsements of your Modified Version by various parties—for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.

   You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.

   The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version.

5. COMBINING DOCUMENTS

   You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers.

   The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.

   In the combination, you must combine any sections Entitled “History” in the various original documents, forming one section Entitled “History”; likewise combine any sections Entitled “Acknowledgements”, and any sections Entitled “Dedications”. You must delete all sections Entitled “Endorsements.”

6. COLLECTIONS OF DOCUMENTS

   You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.

   You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.

7. AGGREGATION WITH INDEPENDENT WORKS

   A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an “aggregate” if the copyright resulting from the compilation is not used to limit the legal rights of the compilation's users beyond what the individual works permit. When the Document is included in an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document.

   If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document's Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.

8. TRANSLATION

   Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.

   If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.

9. TERMINATION

   You may not copy, modify, sublicense, or distribute the Document except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense, or distribute it is void, and will automatically terminate your rights under this License.

   However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b) permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation.

   Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice.

   Termination of your rights under this section does not terminate the licenses of parties who have received copies or rights from you under this License. If your rights have been terminated and not permanently reinstated, receipt of a copy of some or all of the same material does not give you any rights to use it.

10. FUTURE REVISIONS OF THIS LICENSE

    The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.

    Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation. If the Document specifies that a proxy can decide which future versions of this License can be used, that proxy's public statement of acceptance of a version permanently authorizes you to choose that version for the Document.

11. RELICENSING

    “Massive Multiauthor Collaboration Site” (or “MMC Site”) means any World Wide Web server that publishes copyrightable works and also provides prominent facilities for anybody to edit those works. A public wiki that anybody can edit is an example of such a server. A “Massive Multiauthor Collaboration” (or “MMC”) contained in the site means any set of copyrightable works thus published on the MMC site.

    “CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0 license published by Creative Commons Corporation, a not-for-profit corporation with a principal place of business in San Francisco, California, as well as future copyleft versions of that license published by that same organization.

    “Incorporate” means to publish or republish a Document, in whole or in part, as part of another Document.

    An MMC is “eligible for relicensing” if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008.

    The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing.

ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

       Copyright (C)  year  your name.
       Permission is granted to copy, distribute and/or modify this document
       under the terms of the GNU Free Documentation License, Version 1.3
       or any later version published by the Free Software Foundation;
       with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
       Texts.  A copy of the license is included in the section entitled ``GNU
       Free Documentation License''.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with...Texts.” line with this:

         with the Invariant Sections being list their titles, with
         the Front-Cover Texts being list, and with the Back-Cover Texts
         being list.

If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.

Appendix C GNU General Public License

     Copyright © 2007 Free Software Foundation, Inc. http://fsf.org/
     
     Everyone is permitted to copy and distribute verbatim copies of this
     license document, but changing it is not allowed.

Preamble

The GNU General Public License is a free, copyleft license for software and other kinds of works.

The licenses for most software and other practical works are designed to take away your freedom to share and change the works. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change all versions of a program—to make sure it remains free software for all its users. We, the Free Software Foundation, use the GNU General Public License for most of our software; it applies also to any other work released this way by its authors. You can apply it to your programs, too.

When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for them if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs, and that you know you can do these things.

To protect your rights, we need to prevent others from denying you these rights or asking you to surrender the rights. Therefore, you have certain responsibilities if you distribute copies of the software, or if you modify it: responsibilities to respect the freedom of others.

For example, if you distribute copies of such a program, whether gratis or for a fee, you must pass on to the recipients the same freedoms that you received. You must make sure that they, too, receive or can get the source code. And you must show them these terms so they know their rights.

Developers that use the GNU GPL protect your rights with two steps: (1) assert copyright on the software, and (2) offer you this License giving you legal permission to copy, distribute and/or modify it.

For the developers' and authors' protection, the GPL clearly explains that there is no warranty for this free software. For both users' and authors' sake, the GPL requires that modified versions be marked as changed, so that their problems will not be attributed erroneously to authors of previous versions.

Some devices are designed to deny users access to install or run modified versions of the software inside them, although the manufacturer can do so. This is fundamentally incompatible with the aim of protecting users' freedom to change the software. The systematic pattern of such abuse occurs in the area of products for individuals to use, which is precisely where it is most unacceptable. Therefore, we have designed this version of the GPL to prohibit the practice for those products. If such problems arise substantially in other domains, we stand ready to extend this provision to those domains in future versions of the GPL, as needed to protect the freedom of users.

Finally, every program is threatened constantly by software patents. States should not allow patents to restrict development and use of software on general-purpose computers, but in those that do, we wish to avoid the special danger that patents applied to a free program could make it effectively proprietary. To prevent this, the GPL assures that patents cannot be used to render the program non-free.

The precise terms and conditions for copying, distribution and modification follow.

TERMS AND CONDITIONS

0. Definitions.

   “This License” refers to version 3 of the GNU General Public License.

   “Copyright” also means copyright-like laws that apply to other kinds of works, such as semiconductor masks.

   “The Program” refers to any copyrightable work licensed under this License. Each licensee is addressed as “you”. “Licensees” and “recipients” may be individuals or organizations.

   To “modify” a work means to copy from or adapt all or part of the work in a fashion requiring copyright permission, other than the making of an exact copy. The resulting work is called a “modified version” of the earlier work or a work “based on” the earlier work.

   A “covered work” means either the unmodified Program or a work based on the Program.

   To “propagate” a work means to do anything with it that, without permission, would make you directly or secondarily liable for infringement under applicable copyright law, except executing it on a computer or modifying a private copy. Propagation includes copying, distribution (with or without modification), making available to the public, and in some countries other activities as well.

   To “convey” a work means any kind of propagation that enables other parties to make or receive copies. Mere interaction with a user through a computer network, with no transfer of a copy, is not conveying.

   An interactive user interface displays “Appropriate Legal Notices” to the extent that it includes a convenient and prominently visible feature that (1) displays an appropriate copyright notice, and (2) tells the user that there is no warranty for the work (except to the extent that warranties are provided), that licensees may convey the work under this License, and how to view a copy of this License. If the interface presents a list of user commands or options, such as a menu, a prominent item in the list meets this criterion.

1. Source Code.

   The “source code” for a work means the preferred form of the work for making modifications to it. “Object code” means any non-source form of a work.

   A “Standard Interface” means an interface that either is an official standard defined by a recognized standards body, or, in the case of interfaces specified for a particular programming language, one that is widely used among developers working in that language.

   The “System Libraries” of an executable work include anything, other than the work as a whole, that (a) is included in the normal form of packaging a Major Component, but which is not part of that Major Component, and (b) serves only to enable use of the work with that Major Component, or to implement a Standard Interface for which an implementation is available to the public in source code form. A “Major Component”, in this context, means a major essential component (kernel, window system, and so on) of the specific operating system (if any) on which the executable work runs, or a compiler used to produce the work, or an object code interpreter used to run it.

   The “Corresponding Source” for a work in object code form means all the source code needed to generate, install, and (for an executable work) run the object code and to modify the work, including scripts to control those activities. However, it does not include the work's System Libraries, or general-purpose tools or generally available free programs which are used unmodified in performing those activities but which are not part of the work. For example, Corresponding Source includes interface definition files associated with source files for the work, and the source code for shared libraries and dynamically linked subprograms that the work is specifically designed to require, such as by intimate data communication or control flow between those subprograms and other parts of the work.

   The Corresponding Source need not include anything that users can regenerate automatically from other parts of the Corresponding Source.

   The Corresponding Source for a work in source code form is that same work.

2. Basic Permissions.

   All rights granted under this License are granted for the term of copyright on the Program, and are irrevocable provided the stated conditions are met. This License explicitly affirms your unlimited permission to run the unmodified Program. The output from running a covered work is covered by this License only if the output, given its content, constitutes a covered work. This License acknowledges your rights of fair use or other equivalent, as provided by copyright law.

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7. Additional Terms.

   “Additional permissions” are terms that supplement the terms of this License by making exceptions from one or more of its conditions. Additional permissions that are applicable to the entire Program shall be treated as though they were included in this License, to the extent that they are valid under applicable law. If additional permissions apply only to part of the Program, that part may be used separately under those permissions, but the entire Program remains governed by this License without regard to the additional permissions.

   When you convey a copy of a covered work, you may at your option remove any additional permissions from that copy, or from any part of it. (Additional permissions may be written to require their own removal in certain cases when you modify the work.) You may place additional permissions on material, added by you to a covered work, for which you have or can give appropriate copyright permission.

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   Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice.

   Termination of your rights under this section does not terminate the licenses of parties who have received copies or rights from you under this License. If your rights have been terminated and not permanently reinstated, you do not qualify to receive new licenses for the same material under section 10.

9. Acceptance Not Required for Having Copies.

   You are not required to accept this License in order to receive or run a copy of the Program. Ancillary propagation of a covered work occurring solely as a consequence of using peer-to-peer transmission to receive a copy likewise does not require acceptance. However, nothing other than this License grants you permission to propagate or modify any covered work. These actions infringe copyright if you do not accept this License. Therefore, by modifying or propagating a covered work, you indicate your acceptance of this License to do so.

10. Automatic Licensing of Downstream Recipients.

    Each time you convey a covered work, the recipient automatically receives a license from the original licensors, to run, modify and propagate that work, subject to this License. You are not responsible for enforcing compliance by third parties with this License.

    An “entity transaction” is a transaction transferring control of an organization, or substantially all assets of one, or subdividing an organization, or merging organizations. If propagation of a covered work results from an entity transaction, each party to that transaction who receives a copy of the work also receives whatever licenses to the work the party's predecessor in interest had or could give under the previous paragraph, plus a right to possession of the Corresponding Source of the work from the predecessor in interest, if the predecessor has it or can get it with reasonable efforts.

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    A “contributor” is a copyright holder who authorizes use under this License of the Program or a work on which the Program is based. The work thus licensed is called the contributor's “contributor version”.

    A contributor's “essential patent claims” are all patent claims owned or controlled by the contributor, whether already acquired or hereafter acquired, that would be infringed by some manner, permitted by this License, of making, using, or selling its contributor version, but do not include claims that would be infringed only as a consequence of further modification of the contributor version. For purposes of this definition, “control” includes the right to grant patent sublicenses in a manner consistent with the requirements of this License.

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    Nothing in this License shall be construed as excluding or limiting any implied license or other defenses to infringement that may otherwise be available to you under applicable patent law.

12. No Surrender of Others' Freedom.

    If conditions are imposed on you (whether by court order, agreement or otherwise) that contradict the conditions of this License, they do not excuse you from the conditions of this License. If you cannot convey a covered work so as to satisfy simultaneously your obligations under this License and any other pertinent obligations, then as a consequence you may not convey it at all. For example, if you agree to terms that obligate you to collect a royalty for further conveying from those to whom you convey the Program, the only way you could satisfy both those terms and this License would be to refrain entirely from conveying the Program.

13. Use with the GNU Affero General Public License.

    Notwithstanding any other provision of this License, you have permission to link or combine any covered work with a work licensed under version 3 of the GNU Affero General Public License into a single combined work, and to convey the resulting work. The terms of this License will continue to apply to the part which is the covered work, but the special requirements of the GNU Affero General Public License, section 13, concerning interaction through a network will apply to the combination as such.

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    The Free Software Foundation may publish revised and/or new versions of the GNU General Public License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns.

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17. Interpretation of Sections 15 and 16.

    If the disclaimer of warranty and limitation of liability provided above cannot be given local legal effect according to their terms, reviewing courts shall apply local law that most closely approximates an absolute waiver of all civil liability in connection with the Program, unless a warranty or assumption of liability accompanies a copy of the Program in return for a fee.

END OF TERMS AND CONDITIONS

How to Apply These Terms to Your New Programs

If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms.

To do so, attach the following notices to the program. It is safest to attach them to the start of each source file to most effectively state the exclusion of warranty; and each file should have at least the “copyright” line and a pointer to where the full notice is found.

     one line to give the program's name and a brief idea of what it does.
     Copyright (C) year name of author
     
     This program is free software: you can redistribute it and/or modify
     it under the terms of the GNU General Public License as published by
     the Free Software Foundation, either version 3 of the License, or (at
     your option) any later version.
     
     This program is distributed in the hope that it will be useful, but
     WITHOUT ANY WARRANTY; without even the implied warranty of
     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
     General Public License for more details.
     
     You should have received a copy of the GNU General Public License
     along with this program.  If not, see http://www.gnu.org/licenses/.

Also add information on how to contact you by electronic and paper mail.

If the program does terminal interaction, make it output a short notice like this when it starts in an interactive mode:

     program Copyright (C) year name of author
     This program comes with ABSOLUTELY NO WARRANTY; for details type ‘show w’.
     This is free software, and you are welcome to redistribute it
     under certain conditions; type ‘show c’ for details.

The hypothetical commands ‘show w’ and ‘show c’ should show the appropriate parts of the General Public License. Of course, your program's commands might be different; for a GUI interface, you would use an “about box”.

You should also get your employer (if you work as a programmer) or school, if any, to sign a “copyright disclaimer” for the program, if necessary. For more information on this, and how to apply and follow the GNU GPL, see http://www.gnu.org/licenses/.

The GNU General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Lesser General Public License instead of this License. But first, please read http://www.gnu.org/philosophy/why-not-lgpl.html.

Appendix D Tips and Conventions

This chapter describes no additional features of Emacs Lisp. Instead it gives advice on making effective use of the features described in the previous chapters, and describes conventions Emacs Lisp programmers should follow.

You can automatically check some of the conventions described below by running the command M-x checkdoc RET when visiting a Lisp file. It cannot check all of the conventions, and not all the warnings it gives necessarily correspond to problems, but it is worth examining them all. Alternatively, use the command M-x checkdoc-current-buffer RET to check the conventions in the current buffer, or checkdoc-file when you want to check a file in batch mode, e.g., with a command run by M-x compile RET.

• Coding Conventions: Conventions for clean and robust programs.
• Key Binding Conventions: Which keys should be bound by which programs.
• Programming Tips: Making Emacs code fit smoothly in Emacs.
• Compilation Tips: Making compiled code run fast.
• Warning Tips: Turning off compiler warnings.
• Documentation Tips: Writing readable documentation strings.
• Comment Tips: Conventions for writing comments.
• Library Headers: Standard headers for library packages.

D.1 Emacs Lisp Coding Conventions

Here are conventions that you should follow when writing Emacs Lisp code intended for widespread use:

• Simply loading a package should not change Emacs's editing behavior. Include a command or commands to enable and disable the feature, or to invoke it.

  This convention is mandatory for any file that includes custom definitions. If fixing such a file to follow this convention requires an incompatible change, go ahead and make the incompatible change; don't postpone it.

• You should choose a short word to distinguish your program from other Lisp programs. The names of all global symbols in your program, that is the names of variables, constants, and functions, should begin with that chosen prefix. Separate the prefix from the rest of the name with a hyphen, ‘-’. This practice helps avoid name conflicts, since all global variables in Emacs Lisp share the same name space, and all functions share another name space²¹. Use two hyphens to separate prefix and name if the symbol is not meant to be used by other packages.

  Occasionally, for a command name intended for users to use, it is more convenient if some words come before the package's name prefix. And constructs that define functions, variables, etc., work better if they start with ‘defun’ or ‘defvar’, so put the name prefix later on in the name.

  This recommendation applies even to names for traditional Lisp primitives that are not primitives in Emacs Lisp—such as copy-list. Believe it or not, there is more than one plausible way to define copy-list. Play it safe; append your name prefix to produce a name like foo-copy-list or mylib-copy-list instead.

  If you write a function that you think ought to be added to Emacs under a certain name, such as twiddle-files, don't call it by that name in your program. Call it mylib-twiddle-files in your program, and send mail to ‘bug-gnu-emacs@gnu.org’ suggesting we add it to Emacs. If and when we do, we can change the name easily enough.

  If one prefix is insufficient, your package can use two or three alternative common prefixes, so long as they make sense.

• Put a call to provide at the end of each separate Lisp file. See Named Features.
• If a file requires certain other Lisp programs to be loaded beforehand, then the comments at the beginning of the file should say so. Also, use require to make sure they are loaded. See Named Features.
• If a file foo uses a macro defined in another file bar, but does not use any functions or variables defined in bar, then foo should contain the following expression:

            (eval-when-compile (require 'bar))
  

  This tells Emacs to load bar just before byte-compiling foo, so that the macro definition is available during compilation. Using eval-when-compile avoids loading bar when the compiled version of foo is used. It should be called before the first use of the macro in the file. See Compiling Macros.

• Avoid loading additional libraries at run time unless they are really needed. If your file simply cannot work without some other library, then just require that library at the top-level and be done with it. But if your file contains several independent features, and only one or two require the extra library, then consider putting require statements inside the relevant functions rather than at the top-level. Or use autoload statements to load the extra library when needed. This way people who don't use those aspects of your file do not need to load the extra library.
• If you need Common Lisp extensions, use the cl-lib library rather than the old cl library. The latter does not use a clean namespace (i.e., its definitions do not start with a ‘cl-’ prefix). If your package loads cl at run time, that could cause name clashes for users who don't use that package.

  There is no problem with using the cl package at compile time, with (eval-when-compile (require 'cl)). That's sufficient for using the macros in the cl package, because the compiler expands them before generating the byte-code. It is still better to use the more modern cl-lib in this case, though.

• When defining a major mode, please follow the major mode conventions. See Major Mode Conventions.
• When defining a minor mode, please follow the minor mode conventions. See Minor Mode Conventions.
• If the purpose of a function is to tell you whether a certain condition is true or false, give the function a name that ends in ‘p’ (which stands for “predicate”). If the name is one word, add just ‘p’; if the name is multiple words, add ‘-p’. Examples are framep and frame-live-p.
• If the purpose of a variable is to store a single function, give it a name that ends in ‘-function’. If the purpose of a variable is to store a list of functions (i.e., the variable is a hook), please follow the naming conventions for hooks. See Hooks.
• If loading the file adds functions to hooks, define a function feature-unload-function, where feature is the name of the feature the package provides, and make it undo any such changes. Using unload-feature to unload the file will run this function. See Unloading.
• It is a bad idea to define aliases for the Emacs primitives. Normally you should use the standard names instead. The case where an alias may be useful is where it facilitates backwards compatibility or portability.
• If a package needs to define an alias or a new function for compatibility with some other version of Emacs, name it with the package prefix, not with the raw name with which it occurs in the other version. Here is an example from Gnus, which provides many examples of such compatibility issues.

            (defalias 'gnus-point-at-bol
              (if (fboundp 'point-at-bol)
                  'point-at-bol
                'line-beginning-position))
  

• Redefining or advising an Emacs primitive is a bad idea. It may do the right thing for a particular program, but there is no telling what other programs might break as a result.
• It is likewise a bad idea for one Lisp package to advise a function in another Lisp package (see Advising Functions).
• Avoid using eval-after-load and with-eval-after-load in libraries and packages (see Hooks for Loading). This feature is meant for personal customizations; using it in a Lisp program is unclean, because it modifies the behavior of another Lisp file in a way that's not visible in that file. This is an obstacle for debugging, much like advising a function in the other package.
• If a file does replace any of the standard functions or library programs of Emacs, prominent comments at the beginning of the file should say which functions are replaced, and how the behavior of the replacements differs from that of the originals.
• Constructs that define a function or variable should be macros, not functions, and their names should start with ‘define-’. The macro should receive the name to be defined as the first argument. That will help various tools find the definition automatically. Avoid constructing the names in the macro itself, since that would confuse these tools.
• In some other systems there is a convention of choosing variable names that begin and end with ‘*’. We don't use that convention in Emacs Lisp, so please don't use it in your programs. (Emacs uses such names only for special-purpose buffers.) People will find Emacs more coherent if all libraries use the same conventions.
• The default file coding system for Emacs Lisp source files is UTF-8 (see Text Representations). In the rare event that your program contains characters which are not in UTF-8, you should specify an appropriate coding system in the source file's ‘-*-’ line or local variables list. See Local Variables in Files.
• Indent the file using the default indentation parameters.
• Don't make a habit of putting close-parentheses on lines by themselves; Lisp programmers find this disconcerting.
• Please put a copyright notice and copying permission notice on the file if you distribute copies. See Library Headers.

D.2 Key Binding Conventions

• Many special major modes, like Dired, Info, Compilation, and Occur, are designed to handle read-only text that contains hyper-links. Such a major mode should redefine mouse-2 and <RET> to follow the links. It should also set up a follow-link condition, so that the link obeys mouse-1-click-follows-link. See Clickable Text. See Buttons, for an easy method of implementing such clickable links.
• Don't define C-c letter as a key in Lisp programs. Sequences consisting of C-c and a letter (either upper or lower case) are reserved for users; they are the only sequences reserved for users, so do not block them.

  Changing all the Emacs major modes to respect this convention was a lot of work; abandoning this convention would make that work go to waste, and inconvenience users. Please comply with it.

• Function keys <F5> through <F9> without modifier keys are also reserved for users to define.
• Sequences consisting of C-c followed by a control character or a digit are reserved for major modes.
• Sequences consisting of C-c followed by {, }, <, >, : or ; are also reserved for major modes.
• Sequences consisting of C-c followed by any other ASCII punctuation or symbol character are allocated for minor modes. Using them in a major mode is not absolutely prohibited, but if you do that, the major mode binding may be shadowed from time to time by minor modes.
• Don't bind C-h following any prefix character (including C-c). If you don't bind C-h, it is automatically available as a help character for listing the subcommands of the prefix character.
• Don't bind a key sequence ending in <ESC> except following another <ESC>. (That is, it is OK to bind a sequence ending in <ESC> <ESC>.)

  The reason for this rule is that a non-prefix binding for <ESC> in any context prevents recognition of escape sequences as function keys in that context.

• Similarly, don't bind a key sequence ending in <C-g>, since that is commonly used to cancel a key sequence.
• Anything that acts like a temporary mode or state that the user can enter and leave should define <ESC> <ESC> or <ESC> <ESC> <ESC> as a way to escape.

  For a state that accepts ordinary Emacs commands, or more generally any kind of state in which <ESC> followed by a function key or arrow key is potentially meaningful, then you must not define <ESC> <ESC>, since that would preclude recognizing an escape sequence after <ESC>. In these states, you should define <ESC> <ESC> <ESC> as the way to escape. Otherwise, define <ESC> <ESC> instead.

D.3 Emacs Programming Tips

Following these conventions will make your program fit better into Emacs when it runs.

• Don't use next-line or previous-line in programs; nearly always, forward-line is more convenient as well as more predictable and robust. See Text Lines.
• Don't call functions that set the mark, unless setting the mark is one of the intended features of your program. The mark is a user-level feature, so it is incorrect to change the mark except to supply a value for the user's benefit. See The Mark.

  In particular, don't use any of these functions:

  • beginning-of-buffer, end-of-buffer
  • replace-string, replace-regexp
  • insert-file, insert-buffer

  If you just want to move point, or replace a certain string, or insert a file or buffer's contents, without any of the other features intended for interactive users, you can replace these functions with one or two lines of simple Lisp code.

• Use lists rather than vectors, except when there is a particular reason to use a vector. Lisp has more facilities for manipulating lists than for vectors, and working with lists is usually more convenient.

  Vectors are advantageous for tables that are substantial in size and are accessed in random order (not searched front to back), provided there is no need to insert or delete elements (only lists allow that).

• The recommended way to show a message in the echo area is with the message function, not princ. See The Echo Area.
• When you encounter an error condition, call the function error (or signal). The function error does not return. See Signaling Errors.

  Don't use message, throw, sleep-for, or beep to report errors.

• An error message should start with a capital letter but should not end with a period.
• A question asked in the minibuffer with yes-or-no-p or y-or-n-p should start with a capital letter and end with ‘? ’.
• When you mention a default value in a minibuffer prompt, put it and the word ‘default’ inside parentheses. It should look like this:

            Enter the answer (default 42):
  

• In interactive, if you use a Lisp expression to produce a list of arguments, don't try to provide the correct default values for region or position arguments. Instead, provide nil for those arguments if they were not specified, and have the function body compute the default value when the argument is nil. For instance, write this:

            (defun foo (pos)
              (interactive
               (list (if specified specified-pos)))
              (unless pos (setq pos default-pos))
              ...)
  

  rather than this:

            (defun foo (pos)
              (interactive
               (list (if specified specified-pos
                         default-pos)))
              ...)
  

  This is so that repetition of the command will recompute these defaults based on the current circumstances.

  You do not need to take such precautions when you use interactive specs ‘d’, ‘m’ and ‘r’, because they make special arrangements to recompute the argument values on repetition of the command.

• Many commands that take a long time to execute display a message that says something like ‘Operating...’ when they start, and change it to ‘Operating...done’ when they finish. Please keep the style of these messages uniform: no space around the ellipsis, and no period after ‘done’. See Progress, for an easy way to generate such messages.
• Try to avoid using recursive edits. Instead, do what the Rmail e command does: use a new local keymap that contains a command defined to switch back to the old local keymap. Or simply switch to another buffer and let the user switch back at will. See Recursive Editing.

D.4 Tips for Making Compiled Code Fast

Here are ways of improving the execution speed of byte-compiled Lisp programs.

• Profile your program, to find out where the time is being spent. See Profiling.
• Use iteration rather than recursion whenever possible. Function calls are slow in Emacs Lisp even when a compiled function is calling another compiled function.
• Using the primitive list-searching functions memq, member, assq, or assoc is even faster than explicit iteration. It can be worth rearranging a data structure so that one of these primitive search functions can be used.
• Certain built-in functions are handled specially in byte-compiled code, avoiding the need for an ordinary function call. It is a good idea to use these functions rather than alternatives. To see whether a function is handled specially by the compiler, examine its byte-compile property. If the property is non-nil, then the function is handled specially.

  For example, the following input will show you that aref is compiled specially (see Array Functions):

            (get 'aref 'byte-compile)
                 ⇒ byte-compile-two-args
  

  Note that in this case (and many others), you must first load the bytecomp library, which defines the byte-compile property.

• If calling a small function accounts for a substantial part of your program's running time, make the function inline. This eliminates the function call overhead. Since making a function inline reduces the flexibility of changing the program, don't do it unless it gives a noticeable speedup in something slow enough that users care about the speed. See Inline Functions.

D.5 Tips for Avoiding Compiler Warnings

• Try to avoid compiler warnings about undefined free variables, by adding dummy defvar definitions for these variables, like this:

            (defvar foo)
  

  Such a definition has no effect except to tell the compiler not to warn about uses of the variable foo in this file.

• Similarly, to avoid a compiler warning about an undefined function that you know will be defined, use a declare-function statement (see Declaring Functions).
• If you use many functions and variables from a certain file, you can add a require for that package to avoid compilation warnings for them. For instance,

            (eval-when-compile
              (require 'foo))
  

• If you bind a variable in one function, and use it or set it in another function, the compiler warns about the latter function unless the variable has a definition. But adding a definition would be unclean if the variable has a short name, since Lisp packages should not define short variable names. The right thing to do is to rename this variable to start with the name prefix used for the other functions and variables in your package.
• The last resort for avoiding a warning, when you want to do something that is usually a mistake but you know is not a mistake in your usage, is to put it inside with-no-warnings. See Compiler Errors.

D.6 Tips for Documentation Strings

Here are some tips and conventions for the writing of documentation strings. You can check many of these conventions by running the command M-x checkdoc-minor-mode.

• Every command, function, or variable intended for users to know about should have a documentation string.
• An internal variable or subroutine of a Lisp program might as well have a documentation string. Documentation strings take up very little space in a running Emacs.
• Format the documentation string so that it fits in an Emacs window on an 80-column screen. It is a good idea for most lines to be no wider than 60 characters. The first line should not be wider than 67 characters or it will look bad in the output of apropos.

  You can fill the text if that looks good. Emacs Lisp mode fills documentation strings to the width specified by emacs-lisp-docstring-fill-column. However, you can sometimes make a documentation string much more readable by adjusting its line breaks with care. Use blank lines between sections if the documentation string is long.

• The first line of the documentation string should consist of one or two complete sentences that stand on their own as a summary. M-x apropos displays just the first line, and if that line's contents don't stand on their own, the result looks bad. In particular, start the first line with a capital letter and end it with a period.

  For a function, the first line should briefly answer the question, “What does this function do?” For a variable, the first line should briefly answer the question, “What does this value mean?”

  Don't limit the documentation string to one line; use as many lines as you need to explain the details of how to use the function or variable. Please use complete sentences for the rest of the text too.

• When the user tries to use a disabled command, Emacs displays just the first paragraph of its documentation string—everything through the first blank line. If you wish, you can choose which information to include before the first blank line so as to make this display useful.
• The first line should mention all the important arguments of the function, and should mention them in the order that they are written in a function call. If the function has many arguments, then it is not feasible to mention them all in the first line; in that case, the first line should mention the first few arguments, including the most important arguments.
• When a function's documentation string mentions the value of an argument of the function, use the argument name in capital letters as if it were a name for that value. Thus, the documentation string of the function eval refers to its first argument as ‘FORM’, because the actual argument name is form:

            Evaluate FORM and return its value.
  

  Also write metasyntactic variables in capital letters, such as when you show the decomposition of a list or vector into subunits, some of which may vary. ‘KEY’ and ‘VALUE’ in the following example illustrate this practice:

            The argument TABLE should be an alist whose elements
            have the form (KEY . VALUE).  Here, KEY is ...
  

• Never change the case of a Lisp symbol when you mention it in a doc string. If the symbol's name is foo, write “foo”, not “Foo” (which is a different symbol).

  This might appear to contradict the policy of writing function argument values, but there is no real contradiction; the argument value is not the same thing as the symbol that the function uses to hold the value.

  If this puts a lower-case letter at the beginning of a sentence and that annoys you, rewrite the sentence so that the symbol is not at the start of it.

• Do not start or end a documentation string with whitespace.
• Do not indent subsequent lines of a documentation string so that the text is lined up in the source code with the text of the first line. This looks nice in the source code, but looks bizarre when users view the documentation. Remember that the indentation before the starting double-quote is not part of the string!

• When a documentation string refers to a Lisp symbol, write it as it would be printed (which usually means in lower case), surrounding it with curved single quotes (‘ and ’). There are two exceptions: write t and nil without surrounding punctuation. For example: ‘CODE can be ‘lambda’, nil, or t’. See Quotation Marks, for how to enter curved single quotes.

  Documentation strings can also use an older single-quoting convention, which quotes symbols with grave accent ` and apostrophe ': `like-this' rather than ‘like-this’. This older convention was designed for now-obsolete displays in which grave accent and apostrophe were mirror images.

  Documentation using either convention is converted to the user's preferred format when it is copied into a help buffer. See Keys in Documentation.

  Help mode automatically creates a hyperlink when a documentation string uses a single-quoted symbol name, if the symbol has either a function or a variable definition. You do not need to do anything special to make use of this feature. However, when a symbol has both a function definition and a variable definition, and you want to refer to just one of them, you can specify which one by writing one of the words ‘variable’, ‘option’, ‘function’, or ‘command’, immediately before the symbol name. (Case makes no difference in recognizing these indicator words.) For example, if you write

            This function sets the variable `buffer-file-name'.
  

  then the hyperlink will refer only to the variable documentation of buffer-file-name, and not to its function documentation.

  If a symbol has a function definition and/or a variable definition, but those are irrelevant to the use of the symbol that you are documenting, you can write the words ‘symbol’ or ‘program’ before the symbol name to prevent making any hyperlink. For example,

            If the argument KIND-OF-RESULT is the symbol `list',
            this function returns a list of all the objects
            that satisfy the criterion.
  

  does not make a hyperlink to the documentation, irrelevant here, of the function list.

  Normally, no hyperlink is made for a variable without variable documentation. You can force a hyperlink for such variables by preceding them with one of the words ‘variable’ or ‘option’.

  Hyperlinks for faces are only made if the face name is preceded or followed by the word ‘face’. In that case, only the face documentation will be shown, even if the symbol is also defined as a variable or as a function.

  To make a hyperlink to Info documentation, write the single-quoted name of the Info node (or anchor), preceded by ‘info node’, ‘Info node’, ‘info anchor’ or ‘Info anchor’. The Info file name defaults to ‘emacs’. For example,

            See Info node `Font Lock' and Info node `(elisp)Font Lock Basics'.
  

  Finally, to create a hyperlink to URLs, write the single-quoted URL, preceded by ‘URL’. For example,

            The home page for the GNU project has more information (see URL
            `http://www.gnu.org/').
  

• Don't write key sequences directly in documentation strings. Instead, use the ‘\\[...]’ construct to stand for them. For example, instead of writing ‘C-f’, write the construct ‘\\[forward-char]’. When Emacs displays the documentation string, it substitutes whatever key is currently bound to forward-char. (This is normally ‘C-f’, but it may be some other character if the user has moved key bindings.) See Keys in Documentation.
• In documentation strings for a major mode, you will want to refer to the key bindings of that mode's local map, rather than global ones. Therefore, use the construct ‘\\<...>’ once in the documentation string to specify which key map to use. Do this before the first use of ‘\\[...]’. The text inside the ‘\\<...>’ should be the name of the variable containing the local keymap for the major mode.

  It is not practical to use ‘\\[...]’ very many times, because display of the documentation string will become slow. So use this to describe the most important commands in your major mode, and then use ‘\\{...}’ to display the rest of the mode's keymap.

• For consistency, phrase the verb in the first sentence of a function's documentation string as an imperative—for instance, use “Return the cons of A and B.” in preference to “Returns the cons of A and B.” Usually it looks good to do likewise for the rest of the first paragraph. Subsequent paragraphs usually look better if each sentence is indicative and has a proper subject.
• The documentation string for a function that is a yes-or-no predicate should start with words such as “Return t if”, to indicate explicitly what constitutes truth. The word “return” avoids starting the sentence with lower-case “t”, which could be somewhat distracting.
• If a line in a documentation string begins with an open-parenthesis, write a backslash before the open-parenthesis, like this:

            The argument FOO can be either a number
            \(a buffer position) or a string (a file name).
  

  This prevents the open-parenthesis from being treated as the start of a defun (see Defuns).

• Write documentation strings in the active voice, not the passive, and in the present tense, not the future. For instance, use “Return a list containing A and B.” instead of “A list containing A and B will be returned.”
• Avoid using the word “cause” (or its equivalents) unnecessarily. Instead of, “Cause Emacs to display text in boldface”, write just “Display text in boldface”.
• Avoid using “iff” (a mathematics term meaning “if and only if”), since many people are unfamiliar with it and mistake it for a typo. In most cases, the meaning is clear with just “if”. Otherwise, try to find an alternate phrasing that conveys the meaning.
• When a command is meaningful only in a certain mode or situation, do mention that in the documentation string. For example, the documentation of dired-find-file is:

            In Dired, visit the file or directory named on this line.
  

• When you define a variable that represents an option users might want to set, use defcustom. See Defining Variables.
• The documentation string for a variable that is a yes-or-no flag should start with words such as “Non-nil means”, to make it clear that all non-nil values are equivalent and indicate explicitly what nil and non-nil mean.

D.7 Tips on Writing Comments

We recommend these conventions for comments:

‘;’

Comments that start with a single semicolon, ‘;’, should all be aligned to the same column on the right of the source code. Such comments usually explain how the code on that line does its job. For example:

          (setq base-version-list                 ; There was a base
                (assoc (substring fn 0 start-vn)  ; version to which
                       file-version-assoc-list))  ; this looks like
                                                  ; a subversion.

‘;;’

Comments that start with two semicolons, ‘;;’, should be aligned to the same level of indentation as the code. Such comments usually describe the purpose of the following lines or the state of the program at that point. For example:

          (prog1 (setq auto-fill-function
                       ...
                       ...
            ;; Update mode line.
            (force-mode-line-update)))

We also normally use two semicolons for comments outside functions.

          ;; This Lisp code is run in Emacs when it is to operate as
          ;; a server for other processes.

If a function has no documentation string, it should instead have a two-semicolon comment right before the function, explaining what the function does and how to call it properly. Explain precisely what each argument means and how the function interprets its possible values. It is much better to convert such comments to documentation strings, though.

‘;;;’

Comments that start with three semicolons, ‘;;;’, should start at the left margin. We use them for comments which should be considered a heading by Outline minor mode. By default, comments starting with at least three semicolons (followed by a single space and a non-whitespace character) are considered headings, comments starting with two or fewer are not. Historically, triple-semicolon comments have also been used for commenting out lines within a function, but this use is discouraged.

When commenting out entire functions, use two semicolons.

‘;;;;’

Comments that start with four semicolons, ‘;;;;’, should be aligned to the left margin and are used for headings of major sections of a program. For example:

          ;;;; The kill ring

Generally speaking, the M-; (comment-dwim) command automatically starts a comment of the appropriate type; or indents an existing comment to the right place, depending on the number of semicolons. See Manipulating Comments.

D.8 Conventional Headers for Emacs Libraries

Emacs has conventions for using special comments in Lisp libraries to divide them into sections and give information such as who wrote them. Using a standard format for these items makes it easier for tools (and people) to extract the relevant information. This section explains these conventions, starting with an example:

     ;;; foo.el --- Support for the Foo programming language
     
     ;; Copyright (C) 2010-2017 Your Name
     
     ;; Author: Your Name <yourname@example.com>
     ;; Maintainer: Someone Else <someone@example.com>
     ;; Created: 14 Jul 2010
     ;; Keywords: languages
     ;; Homepage: http://example.com/foo
     
     ;; This file is not part of GNU Emacs.
     
     ;; This file is free software...
     ...
     ;; along with this file.  If not, see <http://www.gnu.org/licenses/>.

The very first line should have this format:

     ;;; filename --- description

The description should be contained in one line. If the file needs a ‘-*-’ specification, put it after description. If this would make the first line too long, use a Local Variables section at the end of the file.

The copyright notice usually lists your name (if you wrote the file). If you have an employer who claims copyright on your work, you might need to list them instead. Do not say that the copyright holder is the Free Software Foundation (or that the file is part of GNU Emacs) unless your file has been accepted into the Emacs distribution. For more information on the form of copyright and license notices, see the guide on the GNU website.

After the copyright notice come several header comment lines, each beginning with ‘;; header-name:’. Here is a table of the conventional possibilities for header-name:

‘Author’

This line states the name and email address of at least the principal author of the library. If there are multiple authors, list them on continuation lines led by ;; and a tab or at least two spaces. We recommend including a contact email address, of the form ‘<...>’. For example:

          ;; Author: Your Name <yourname@example.com>
          ;;      Someone Else <someone@example.com>
          ;;      Another Person <another@example.com>

‘Maintainer’

This header has the same format as the Author header. It lists the person(s) who currently maintain(s) the file (respond to bug reports, etc.).

If there is no maintainer line, the person(s) in the Author field is/are presumed to be the maintainers. Some files in Emacs use ‘FSF’ for the maintainer. This means that the original author is no longer responsible for the file, and that it is maintained as part of Emacs.

‘Created’

This optional line gives the original creation date of the file, and is for historical interest only.

‘Version’

If you wish to record version numbers for the individual Lisp program, put them in this line. Lisp files distributed with Emacs generally do not have a ‘Version’ header, since the version number of Emacs itself serves the same purpose. If you are distributing a collection of multiple files, we recommend not writing the version in every file, but only the main one.

‘Keywords’

This line lists keywords for the finder-by-keyword help command. Please use that command to see a list of the meaningful keywords. The command M-x checkdoc-package-keywords RET will find and display any keywords that are not in finder-known-keywords. If you set the variable checkdoc-package-keywords-flag non-nil, checkdoc commands will include the keyword verification in its checks.

This field is how people will find your package when they're looking for things by topic. To separate the keywords, you can use spaces, commas, or both.

The name of this field is unfortunate, since people often assume it is the place to write arbitrary keywords that describe their package, rather than just the relevant Finder keywords.

‘Homepage’

This line states the homepage of the library.

‘Package-Version’

If ‘Version’ is not suitable for use by the package manager, then a package can define ‘Package-Version’; it will be used instead. This is handy if ‘Version’ is an RCS id or something else that cannot be parsed by version-to-list. See Packaging Basics.

‘Package-Requires’

If this exists, it names packages on which the current package depends for proper operation. See Packaging Basics. This is used by the package manager both at download time (to ensure that a complete set of packages is downloaded) and at activation time (to ensure that a package is only activated if all its dependencies have been).

Its format is a list of lists on a single line. The car of each sub-list is the name of a package, as a symbol. The cadr of each sub-list is the minimum acceptable version number, as a string that can be parse by version-to-list. An entry that lacks a version (i.e., an entry which is just a symbol, or a sub-list of one element) is equivalent to entry with version "0". For instance:

          ;; Package-Requires: ((gnus "1.0") (bubbles "2.7.2") cl-lib (seq))

The package code automatically defines a package named ‘emacs’ with the version number of the currently running Emacs. This can be used to require a minimal version of Emacs for a package.

Just about every Lisp library ought to have the ‘Author’ and ‘Keywords’ header comment lines. Use the others if they are appropriate. You can also put in header lines with other header names—they have no standard meanings, so they can't do any harm.

We use additional stylized comments to subdivide the contents of the library file. These should be separated from anything else by blank lines. Here is a table of them:

‘;;; Commentary:’

This begins introductory comments that explain how the library works. It should come right after the copying permissions, terminated by a ‘Change Log’, ‘History’ or ‘Code’ comment line. This text is used by the Finder package, so it should make sense in that context.

‘;;; Change Log:’

This begins an optional log of changes to the file over time. Don't put too much information in this section—it is better to keep the detailed logs in a version control system (as Emacs does) or in a separate ChangeLog file. ‘History’ is an alternative to ‘Change Log’.

‘;;; Code:’

This begins the actual code of the program.

‘;;; filename ends here’

This is the footer line; it appears at the very end of the file. Its purpose is to enable people to detect truncated versions of the file from the lack of a footer line.

Appendix E GNU Emacs Internals

This chapter describes how the runnable Emacs executable is dumped with the preloaded Lisp libraries in it, how storage is allocated, and some internal aspects of GNU Emacs that may be of interest to C programmers.

• Building Emacs: How the dumped Emacs is made.
• Pure Storage: Kludge to make preloaded Lisp functions shareable.
• Garbage Collection: Reclaiming space for Lisp objects no longer used.
• Stack-allocated Objects: Temporary conses and strings on C stack.
• Memory Usage: Info about total size of Lisp objects made so far.
• C Dialect: What C variant Emacs is written in.
• Writing Emacs Primitives: Writing C code for Emacs.
• Object Internals: Data formats of buffers, windows, processes.
• C Integer Types: How C integer types are used inside Emacs.

E.1 Building Emacs

This section explains the steps involved in building the Emacs executable. You don't have to know this material to build and install Emacs, since the makefiles do all these things automatically. This information is pertinent to Emacs developers.

Building Emacs requires GNU Make version 3.81 or later.

Compilation of the C source files in the src directory produces an executable file called temacs, also called a bare impure Emacs. It contains the Emacs Lisp interpreter and I/O routines, but not the editing commands.

The command temacs -l loadup would run temacs and direct it to load loadup.el. The loadup library loads additional Lisp libraries, which set up the normal Emacs editing environment. After this step, the Emacs executable is no longer bare.

Because it takes some time to load the standard Lisp files, the temacs executable usually isn't run directly by users. Instead, as one of the last steps of building Emacs, the command ‘temacs -batch -l loadup dump’ is run. The special ‘dump’ argument causes temacs to dump out an executable program, called emacs, which has all the standard Lisp files preloaded. (The ‘-batch’ argument prevents temacs from trying to initialize any of its data on the terminal, so that the tables of terminal information are empty in the dumped Emacs.)

The dumped emacs executable (also called a pure Emacs) is the one which is installed. The variable preloaded-file-list stores a list of the Lisp files preloaded into the dumped Emacs. If you port Emacs to a new operating system, and are not able to implement dumping, then Emacs must load loadup.el each time it starts.

You can specify additional files to preload by writing a library named site-load.el that loads them. You may need to rebuild Emacs with an added definition

     #define SITELOAD_PURESIZE_EXTRA n

to make n added bytes of pure space to hold the additional files; see src/puresize.h. (Try adding increments of 20000 until it is big enough.) However, the advantage of preloading additional files decreases as machines get faster. On modern machines, it is usually not advisable.

After loadup.el reads site-load.el, it finds the documentation strings for primitive and preloaded functions (and variables) in the file etc/DOC where they are stored, by calling Snarf-documentation (see Accessing Documentation).

You can specify other Lisp expressions to execute just before dumping by putting them in a library named site-init.el. This file is executed after the documentation strings are found.

If you want to preload function or variable definitions, there are three ways you can do this and make their documentation strings accessible when you subsequently run Emacs:

• Arrange to scan these files when producing the etc/DOC file, and load them with site-load.el.
• Load the files with site-init.el, then copy the files into the installation directory for Lisp files when you install Emacs.
• Specify a nil value for byte-compile-dynamic-docstrings as a local variable in each of these files, and load them with either site-load.el or site-init.el. (This method has the drawback that the documentation strings take up space in Emacs all the time.)

It is not advisable to put anything in site-load.el or site-init.el that would alter any of the features that users expect in an ordinary unmodified Emacs. If you feel you must override normal features for your site, do it with default.el, so that users can override your changes if they wish. See Startup Summary. Note that if either site-load.el or site-init.el changes load-path, the changes will be lost after dumping. See Library Search. To make a permanent change to load-path, use the --enable-locallisppath option of configure.

In a package that can be preloaded, it is sometimes necessary (or useful) to delay certain evaluations until Emacs subsequently starts up. The vast majority of such cases relate to the values of customizable variables. For example, tutorial-directory is a variable defined in startup.el, which is preloaded. The default value is set based on data-directory. The variable needs to access the value of data-directory when Emacs starts, not when it is dumped, because the Emacs executable has probably been installed in a different location since it was dumped.

In the unlikely event that you need a more general functionality than custom-initialize-delay provides, you can use before-init-hook (see Startup Summary).

E.2 Pure Storage

Emacs Lisp uses two kinds of storage for user-created Lisp objects: normal storage and pure storage. Normal storage is where all the new data created during an Emacs session are kept (see Garbage Collection). Pure storage is used for certain data in the preloaded standard Lisp files—data that should never change during actual use of Emacs.

Pure storage is allocated only while temacs is loading the standard preloaded Lisp libraries. In the file emacs, it is marked as read-only (on operating systems that permit this), so that the memory space can be shared by all the Emacs jobs running on the machine at once. Pure storage is not expandable; a fixed amount is allocated when Emacs is compiled, and if that is not sufficient for the preloaded libraries, temacs allocates dynamic memory for the part that didn't fit. The resulting image will work, but garbage collection (see Garbage Collection) is disabled in this situation, causing a memory leak. Such an overflow normally won't happen unless you try to preload additional libraries or add features to the standard ones. Emacs will display a warning about the overflow when it starts. If this happens, you should increase the compilation parameter SYSTEM_PURESIZE_EXTRA in the file src/puresize.h and rebuild Emacs.

E.3 Garbage Collection

When a program creates a list or the user defines a new function (such as by loading a library), that data is placed in normal storage. If normal storage runs low, then Emacs asks the operating system to allocate more memory. Different types of Lisp objects, such as symbols, cons cells, small vectors, markers, etc., are segregated in distinct blocks in memory. (Large vectors, long strings, buffers and certain other editing types, which are fairly large, are allocated in individual blocks, one per object; small strings are packed into blocks of 8k bytes, and small vectors are packed into blocks of 4k bytes).

Beyond the basic vector, a lot of objects like window, buffer, and frame are managed as if they were vectors. The corresponding C data structures include the struct vectorlike_header field whose size member contains the subtype enumerated by enum pvec_type and an information about how many Lisp_Object fields this structure contains and what the size of the rest data is. This information is needed to calculate the memory footprint of an object, and used by the vector allocation code while iterating over the vector blocks.

It is quite common to use some storage for a while, then release it by (for example) killing a buffer or deleting the last pointer to an object. Emacs provides a garbage collector to reclaim this abandoned storage. The garbage collector operates by finding and marking all Lisp objects that are still accessible to Lisp programs. To begin with, it assumes all the symbols, their values and associated function definitions, and any data presently on the stack, are accessible. Any objects that can be reached indirectly through other accessible objects are also accessible.

When marking is finished, all objects still unmarked are garbage. No matter what the Lisp program or the user does, it is impossible to refer to them, since there is no longer a way to reach them. Their space might as well be reused, since no one will miss them. The second (sweep) phase of the garbage collector arranges to reuse them.

The sweep phase puts unused cons cells onto a free list for future allocation; likewise for symbols and markers. It compacts the accessible strings so they occupy fewer 8k blocks; then it frees the other 8k blocks. Unreachable vectors from vector blocks are coalesced to create largest possible free areas; if a free area spans a complete 4k block, that block is freed. Otherwise, the free area is recorded in a free list array, where each entry corresponds to a free list of areas of the same size. Large vectors, buffers, and other large objects are allocated and freed individually.

Common Lisp note: Unlike other Lisps, GNU Emacs Lisp does not call the garbage collector when the free list is empty. Instead, it simply requests the operating system to allocate more storage, and processing continues until gc-cons-threshold bytes have been used.

This means that you can make sure that the garbage collector will not run during a certain portion of a Lisp program by calling the garbage collector explicitly just before it (provided that portion of the program does not use so much space as to force a second garbage collection).

The value returned by garbage-collect describes the amount of memory used by Lisp data, broken down by data type. By contrast, the function memory-limit provides information on the total amount of memory Emacs is currently using.

E.4 Stack-allocated Objects

The garbage collector described above is used to manage data visible from Lisp programs, as well as most of the data internally used by the Lisp interpreter. Sometimes it may be useful to allocate temporary internal objects using the C stack of the interpreter. This can help performance, as stack allocation is typically faster than using heap memory to allocate and the garbage collector to free. The downside is that using such objects after they are freed results in undefined behavior, so uses should be well thought out and carefully debugged by using the GC_CHECK_MARKED_OBJECTS feature (see src/alloc.c). In particular, stack-allocated objects should never be made visible to user Lisp code.

Currently, cons cells and strings can be allocated this way. This is implemented by C macros like AUTO_CONS and AUTO_STRING that define a named Lisp_Object with block lifetime. These objects are not freed by the garbage collector; instead, they have automatic storage duration, i.e., they are allocated like local variables and are automatically freed at the end of execution of the C block that defined the object.

For performance reasons, stack-allocated strings are limited to ASCII characters, and many of these strings are immutable, i.e., calling ASET on them produces undefined behavior.

E.5 Memory Usage

These functions and variables give information about the total amount of memory allocation that Emacs has done, broken down by data type. Note the difference between these and the values returned by garbage-collect; those count objects that currently exist, but these count the number or size of all allocations, including those for objects that have since been freed.

E.6 C Dialect

The C part of Emacs is portable to C99 or later: C11-specific features such as ‘<stdalign.h>’ and ‘_Noreturn’ are not used without a check, typically at configuration time, and the Emacs build procedure provides a substitute implementation if necessary. Some C11 features, such as anonymous structures and unions, are too difficult to emulate, so they are avoided entirely.

At some point in the future the base C dialect will no doubt change to C11.

E.7 Writing Emacs Primitives

Lisp primitives are Lisp functions implemented in C. The details of interfacing the C function so that Lisp can call it are handled by a few C macros. The only way to really understand how to write new C code is to read the source, but we can explain some things here.

An example of a special form is the definition of or, from eval.c. (An ordinary function would have the same general appearance.)

     DEFUN ("or", For, Sor, 0, UNEVALLED, 0,
       doc: /* Eval args until one of them yields non-nil, then return
     that value.
     The remaining args are not evalled at all.
     If all args return nil, return nil.
     usage: (or CONDITIONS...)  */)
       (Lisp_Object args)
     {
       Lisp_Object val = Qnil;
     
       while (CONSP (args))
         {
           val = eval_sub (XCAR (args));
           if (!NILP (val))
             break;
           args = XCDR (args);
           QUIT;
         }
     
       return val;
     }

Let's start with a precise explanation of the arguments to the DEFUN macro. Here is a template for them:

     DEFUN (lname, fname, sname, min, max, interactive, doc)

lname

This is the name of the Lisp symbol to define as the function name; in the example above, it is or.

fname

This is the C function name for this function. This is the name that is used in C code for calling the function. The name is, by convention, ‘F’ prepended to the Lisp name, with all dashes (‘-’) in the Lisp name changed to underscores. Thus, to call this function from C code, call For.

sname

This is a C variable name to use for a structure that holds the data for the subr object that represents the function in Lisp. This structure conveys the Lisp symbol name to the initialization routine that will create the symbol and store the subr object as its definition. By convention, this name is always fname with ‘F’ replaced with ‘S’.

min

This is the minimum number of arguments that the function requires. The function or allows a minimum of zero arguments.

max

This is the maximum number of arguments that the function accepts, if there is a fixed maximum. Alternatively, it can be UNEVALLED, indicating a special form that receives unevaluated arguments, or MANY, indicating an unlimited number of evaluated arguments (the equivalent of &rest). Both UNEVALLED and MANY are macros. If max is a number, it must be more than min but less than 8.

interactive

This is an interactive specification, a string such as might be used as the argument of interactive in a Lisp function. In the case of or, it is 0 (a null pointer), indicating that or cannot be called interactively. A value of "" indicates a function that should receive no arguments when called interactively. If the value begins with a ‘"(’, the string is evaluated as a Lisp form. For example:

          DEFUN ("foo", Ffoo, Sfoo, 0, UNEVALLED,
                 "(list (read-char-by-name \"Insert character: \")\
                        (prefix-numeric-value current-prefix-arg)\
                        t))",
            doc: /* ... /*)

doc

This is the documentation string. It uses C comment syntax rather than C string syntax because comment syntax requires nothing special to include multiple lines. The ‘doc:’ identifies the comment that follows as the documentation string. The ‘/*’ and ‘*/’ delimiters that begin and end the comment are not part of the documentation string.

If the last line of the documentation string begins with the keyword ‘usage:’, the rest of the line is treated as the argument list for documentation purposes. This way, you can use different argument names in the documentation string from the ones used in the C code. ‘usage:’ is required if the function has an unlimited number of arguments.

All the usual rules for documentation strings in Lisp code (see Documentation Tips) apply to C code documentation strings too.

After the call to the DEFUN macro, you must write the argument list for the C function, including the types for the arguments. If the primitive accepts a fixed maximum number of Lisp arguments, there must be one C argument for each Lisp argument, and each argument must be of type Lisp_Object. (Various macros and functions for creating values of type Lisp_Object are declared in the file lisp.h.) If the primitive has no upper limit on the number of Lisp arguments, it must have exactly two C arguments: the first is the number of Lisp arguments, and the second is the address of a block containing their values. These have types int and Lisp_Object * respectively. Since Lisp_Object can hold any Lisp object of any data type, you can determine the actual data type only at run time; so if you want a primitive to accept only a certain type of argument, you must check the type explicitly using a suitable predicate (see Type Predicates). Within the function For itself, the local variable args refers to objects controlled by Emacs's stack-marking garbage collector. Although the garbage collector does not reclaim objects reachable from C Lisp_Object stack variables, it may move non-object components of an object, such as string contents; so functions that access non-object components must take care to refetch their addresses after performing Lisp evaluation. Lisp evaluation can occur via calls to eval_sub or Feval, either directly or indirectly.

Note the call to the QUIT macro inside the loop: this macro checks whether the user pressed C-g, and if so, aborts the processing. You should do that in any loop that can potentially require a large number of iterations; in this case, the list of arguments could be very long. This increases Emacs responsiveness and improves user experience.

You must not use C initializers for static or global variables unless the variables are never written once Emacs is dumped. These variables with initializers are allocated in an area of memory that becomes read-only (on certain operating systems) as a result of dumping Emacs. See Pure Storage.

Defining the C function is not enough to make a Lisp primitive available; you must also create the Lisp symbol for the primitive and store a suitable subr object in its function cell. The code looks like this:

     defsubr (&sname);

Here sname is the name you used as the third argument to DEFUN.

If you add a new primitive to a file that already has Lisp primitives defined in it, find the function (near the end of the file) named syms_of_something, and add the call to defsubr there. If the file doesn't have this function, or if you create a new file, add to it a syms_of_filename (e.g., syms_of_myfile). Then find the spot in emacs.c where all of these functions are called, and add a call to syms_of_filename there.

The function syms_of_filename is also the place to define any C variables that are to be visible as Lisp variables. DEFVAR_LISP makes a C variable of type Lisp_Object visible in Lisp. DEFVAR_INT makes a C variable of type int visible in Lisp with a value that is always an integer. DEFVAR_BOOL makes a C variable of type int visible in Lisp with a value that is either t or nil. Note that variables defined with DEFVAR_BOOL are automatically added to the list byte-boolean-vars used by the byte compiler.

If you want to make a Lisp variables that is defined in C behave like one declared with defcustom, add an appropriate entry to cus-start.el.

If you define a file-scope C variable of type Lisp_Object, you must protect it from garbage-collection by calling staticpro in syms_of_filename, like this:

     staticpro (&variable);

Here is another example function, with more complicated arguments. This comes from the code in window.c, and it demonstrates the use of macros and functions to manipulate Lisp objects.

     DEFUN ("coordinates-in-window-p", Fcoordinates_in_window_p,
       Scoordinates_in_window_p, 2, 2, 0,
       doc: /* Return non-nil if COORDINATES are in WINDOW.
       ...
       or `right-margin' is returned.  */)
       (register Lisp_Object coordinates, Lisp_Object window)
     {
       struct window *w;
       struct frame *f;
       int x, y;
       Lisp_Object lx, ly;
     
       CHECK_LIVE_WINDOW (window);
       w = XWINDOW (window);
       f = XFRAME (w->frame);
       CHECK_CONS (coordinates);
       lx = Fcar (coordinates);
       ly = Fcdr (coordinates);
       CHECK_NUMBER_OR_FLOAT (lx);
       CHECK_NUMBER_OR_FLOAT (ly);
       x = FRAME_PIXEL_X_FROM_CANON_X (f, lx) + FRAME_INTERNAL_BORDER_WIDTH(f);
       y = FRAME_PIXEL_Y_FROM_CANON_Y (f, ly) + FRAME_INTERNAL_BORDER_WIDTH(f);
     
       switch (coordinates_in_window (w, x, y))
         {
         case ON_NOTHING:            /* NOT in window at all.  */
           return Qnil;
     
         ...
     
         case ON_MODE_LINE:          /* In mode line of window.  */
           return Qmode_line;
     
         ...
     
         case ON_SCROLL_BAR:         /* On scroll-bar of window.  */
           /* Historically we are supposed to return nil in this case.  */
           return Qnil;
     
         default:
           abort ();
         }
     }

Note that C code cannot call functions by name unless they are defined in C. The way to call a function written in Lisp is to use Ffuncall, which embodies the Lisp function funcall. Since the Lisp function funcall accepts an unlimited number of arguments, in C it takes two: the number of Lisp-level arguments, and a one-dimensional array containing their values. The first Lisp-level argument is the Lisp function to call, and the rest are the arguments to pass to it.

The C functions call0, call1, call2, and so on, provide handy ways to call a Lisp function conveniently with a fixed number of arguments. They work by calling Ffuncall.

eval.c is a very good file to look through for examples; lisp.h contains the definitions for some important macros and functions.

If you define a function which is side-effect free, update the code in byte-opt.el that binds side-effect-free-fns and side-effect-and-error-free-fns so that the compiler optimizer knows about it.

E.8 Object Internals

Emacs Lisp provides a rich set of the data types. Some of them, like cons cells, integers and strings, are common to nearly all Lisp dialects. Some others, like markers and buffers, are quite special and needed to provide the basic support to write editor commands in Lisp. To implement such a variety of object types and provide an efficient way to pass objects between the subsystems of an interpreter, there is a set of C data structures and a special type to represent the pointers to all of them, which is known as tagged pointer.

In C, the tagged pointer is an object of type Lisp_Object. Any initialized variable of such a type always holds the value of one of the following basic data types: integer, symbol, string, cons cell, float, vectorlike or miscellaneous object. Each of these data types has the corresponding tag value. All tags are enumerated by enum Lisp_Type and placed into a 3-bit bitfield of the Lisp_Object. The rest of the bits is the value itself. Integers are immediate, i.e., directly represented by those value bits, and all other objects are represented by the C pointers to a corresponding object allocated from the heap. Width of the Lisp_Object is platform- and configuration-dependent: usually it's equal to the width of an underlying platform pointer (i.e., 32-bit on a 32-bit machine and 64-bit on a 64-bit one), but also there is a special configuration where Lisp_Object is 64-bit but all pointers are 32-bit. The latter trick was designed to overcome the limited range of values for Lisp integers on a 32-bit system by using 64-bit long long type for Lisp_Object.

The following C data structures are defined in lisp.h to represent the basic data types beyond integers:

struct Lisp_Cons

Cons cell, an object used to construct lists.

struct Lisp_String

String, the basic object to represent a sequence of characters.

struct Lisp_Vector

Array, a fixed-size set of Lisp objects which may be accessed by an index.

struct Lisp_Symbol

Symbol, the unique-named entity commonly used as an identifier.

struct Lisp_Float

Floating-point value.

union Lisp_Misc

Miscellaneous kinds of objects which don't fit into any of the above.

These types are the first-class citizens of an internal type system. Since the tag space is limited, all other types are the subtypes of either Lisp_Vectorlike or Lisp_Misc. Vector subtypes are enumerated by enum pvec_type, and nearly all complex objects like windows, buffers, frames, and processes fall into this category. The rest of special types, including markers and overlays, are enumerated by enum Lisp_Misc_Type and form the set of subtypes of Lisp_Misc.

Below there is a description of a few subtypes of Lisp_Vectorlike. Buffer object represents the text to display and edit. Window is the part of display structure which shows the buffer or used as a container to recursively place other windows on the same frame. (Do not confuse Emacs Lisp window object with the window as an entity managed by the user interface system like X; in Emacs terminology, the latter is called frame.) Finally, process object is used to manage the subprocesses.

• Buffer Internals: Components of a buffer structure.
• Window Internals: Components of a window structure.
• Process Internals: Components of a process structure.

E.8.1 Buffer Internals

Two structures (see buffer.h) are used to represent buffers in C. The buffer_text structure contains fields describing the text of a buffer; the buffer structure holds other fields. In the case of indirect buffers, two or more buffer structures reference the same buffer_text structure.

Here are some of the fields in struct buffer_text:

beg

The address of the buffer contents.

gpt

gpt_byte

The character and byte positions of the buffer gap. See Buffer Gap.

z

z_byte

The character and byte positions of the end of the buffer text.

gap_size

The size of buffer's gap. See Buffer Gap.

modiff

save_modiff

chars_modiff

overlay_modiff

These fields count the number of buffer-modification events performed in this buffer. modiff is incremented after each buffer-modification event, and is never otherwise changed; save_modiff contains the value of modiff the last time the buffer was visited or saved; chars_modiff counts only modifications to the characters in the buffer, ignoring all other kinds of changes; and overlay_modiff counts only modifications to the overlays.

beg_unchanged

end_unchanged

The number of characters at the start and end of the text that are known to be unchanged since the last complete redisplay.

unchanged_modified

overlay_unchanged_modified

The values of modiff and overlay_modiff, respectively, after the last complete redisplay. If their current values match modiff or overlay_modiff, that means beg_unchanged and end_unchanged contain no useful information.

markers

The markers that refer to this buffer. This is actually a single marker, and successive elements in its marker chain are the other markers referring to this buffer text.

intervals

The interval tree which records the text properties of this buffer.

Some of the fields of struct buffer are:

header

A header of type struct vectorlike_header is common to all vectorlike objects.

own_text

A struct buffer_text structure that ordinarily holds the buffer contents. In indirect buffers, this field is not used.

text

A pointer to the buffer_text structure for this buffer. In an ordinary buffer, this is the own_text field above. In an indirect buffer, this is the own_text field of the base buffer.

next

A pointer to the next buffer, in the chain of all buffers, including killed buffers. This chain is used only for allocation and garbage collection, in order to collect killed buffers properly.

pt

pt_byte

The character and byte positions of point in a buffer.

begv

begv_byte

The character and byte positions of the beginning of the accessible range of text in the buffer.

zv

zv_byte

The character and byte positions of the end of the accessible range of text in the buffer.

base_buffer

In an indirect buffer, this points to the base buffer. In an ordinary buffer, it is null.

local_flags

This field contains flags indicating that certain variables are local in this buffer. Such variables are declared in the C code using DEFVAR_PER_BUFFER, and their buffer-local bindings are stored in fields in the buffer structure itself. (Some of these fields are described in this table.)

modtime

The modification time of the visited file. It is set when the file is written or read. Before writing the buffer into a file, this field is compared to the modification time of the file to see if the file has changed on disk. See Buffer Modification.

auto_save_modified

The time when the buffer was last auto-saved.

last_window_start

The window-start position in the buffer as of the last time the buffer was displayed in a window.

clip_changed

This flag indicates that narrowing has changed in the buffer. See Narrowing.

prevent_redisplay_optimizations_p

This flag indicates that redisplay optimizations should not be used to display this buffer.

overlay_center

This field holds the current overlay center position. See Managing Overlays.

overlays_before

overlays_after

These fields hold, respectively, a list of overlays that end at or before the current overlay center, and a list of overlays that end after the current overlay center. See Managing Overlays. overlays_before is sorted in order of decreasing end position, and overlays_after is sorted in order of increasing beginning position.

name

A Lisp string that names the buffer. It is guaranteed to be unique. See Buffer Names.

save_length

The length of the file this buffer is visiting, when last read or saved. This and other fields concerned with saving are not kept in the buffer_text structure because indirect buffers are never saved.

directory

The directory for expanding relative file names. This is the value of the buffer-local variable default-directory (see File Name Expansion).

filename

The name of the file visited in this buffer, or nil. This is the value of the buffer-local variable buffer-file-name (see Buffer File Name).

undo_list

backed_up

auto_save_file_name

auto_save_file_format

read_only

file_format

file_truename

invisibility_spec

display_count

display_time

These fields store the values of Lisp variables that are automatically buffer-local (see Buffer-Local Variables), whose corresponding variable names have the additional prefix buffer- and have underscores replaced with dashes. For instance, undo_list stores the value of buffer-undo-list.

mark

The mark for the buffer. The mark is a marker, hence it is also included on the list markers. See The Mark.

local_var_alist

The association list describing the buffer-local variable bindings of this buffer, not including the built-in buffer-local bindings that have special slots in the buffer object. (Those slots are omitted from this table.) See Buffer-Local Variables.

major_mode

Symbol naming the major mode of this buffer, e.g., lisp-mode.

mode_name

Pretty name of the major mode, e.g., "Lisp".

keymap

abbrev_table

syntax_table

category_table

display_table

These fields store the buffer's local keymap (see Keymaps), abbrev table (see Abbrev Tables), syntax table (see Syntax Tables), category table (see Categories), and display table (see Display Tables).

downcase_table

upcase_table

case_canon_table

These fields store the conversion tables for converting text to lower case, upper case, and for canonicalizing text for case-fold search. See Case Tables.

minor_modes

An alist of the minor modes of this buffer.

pt_marker

begv_marker

zv_marker

These fields are only used in an indirect buffer, or in a buffer that is the base of an indirect buffer. Each holds a marker that records pt, begv, and zv respectively, for this buffer when the buffer is not current.

mode_line_format

header_line_format

case_fold_search

tab_width

fill_column

left_margin

auto_fill_function

truncate_lines

word_wrap

ctl_arrow

bidi_display_reordering

bidi_paragraph_direction

selective_display

selective_display_ellipses

overwrite_mode

abbrev_mode

mark_active

enable_multibyte_characters

buffer_file_coding_system

cache_long_line_scans

point_before_scroll

left_fringe_width

right_fringe_width

fringes_outside_margins

scroll_bar_width

indicate_empty_lines

indicate_buffer_boundaries

fringe_indicator_alist

fringe_cursor_alist

scroll_up_aggressively

scroll_down_aggressively

cursor_type

cursor_in_non_selected_windows

These fields store the values of Lisp variables that are automatically buffer-local (see Buffer-Local Variables), whose corresponding variable names have underscores replaced with dashes. For instance, mode_line_format stores the value of mode-line-format.

last_selected_window

This is the last window that was selected with this buffer in it, or nil if that window no longer displays this buffer.

E.8.2 Window Internals

The fields of a window (for a complete list, see the definition of struct window in window.h) include:

frame

The frame that this window is on.

mini_p

Non-nil if this window is a minibuffer window.

parent

Internally, Emacs arranges windows in a tree; each group of siblings has a parent window whose area includes all the siblings. This field points to a window's parent.

Parent windows do not display buffers, and play little role in display except to shape their child windows. Emacs Lisp programs usually have no access to the parent windows; they operate on the windows at the leaves of the tree, which actually display buffers.

hchild

vchild

These fields contain the window's leftmost child and its topmost child respectively. hchild is used if the window is subdivided horizontally by child windows, and vchild if it is subdivided vertically. In a live window, only one of hchild, vchild, and buffer (q.v.) is non-nil.

next

prev

The next sibling and previous sibling of this window. next is nil if the window is the right-most or bottom-most in its group; prev is nil if it is the left-most or top-most in its group.

left_col

The left-hand edge of the window, measured in columns, relative to the leftmost column in the frame (column 0).

top_line

The top edge of the window, measured in lines, relative to the topmost line in the frame (line 0).

total_cols

total_lines

The width and height of the window, measured in columns and lines respectively. The width includes the scroll bar and fringes, and/or the separator line on the right of the window (if any).

buffer

The buffer that the window is displaying.

start

A marker pointing to the position in the buffer that is the first character displayed in the window.

pointm

This is the value of point in the current buffer when this window is selected; when it is not selected, it retains its previous value.

force_start

If this flag is non-nil, it says that the window has been scrolled explicitly by the Lisp program. This affects what the next redisplay does if point is off the screen: instead of scrolling the window to show the text around point, it moves point to a location that is on the screen.

frozen_window_start_p

This field is set temporarily to 1 to indicate to redisplay that start of this window should not be changed, even if point gets invisible.

start_at_line_beg

Non-nil means current value of start was the beginning of a line when it was chosen.

use_time

This is the last time that the window was selected. The function get-lru-window uses this field.

sequence_number

A unique number assigned to this window when it was created.

last_modified

The modiff field of the window's buffer, as of the last time a redisplay completed in this window.

last_overlay_modified

The overlay_modiff field of the window's buffer, as of the last time a redisplay completed in this window.

last_point

The buffer's value of point, as of the last time a redisplay completed in this window.

last_had_star

A non-nil value means the window's buffer was modified when the window was last updated.

vertical_scroll_bar

This window's vertical scroll bar.

left_margin_cols

right_margin_cols

The widths of the left and right margins in this window. A value of nil means no margin.

left_fringe_width

right_fringe_width

The widths of the left and right fringes in this window. A value of nil or t means use the values of the frame.

fringes_outside_margins

A non-nil value means the fringes outside the display margins; othersize they are between the margin and the text.

window_end_pos

This is computed as z minus the buffer position of the last glyph in the current matrix of the window. The value is only valid if window_end_valid is not nil.

window_end_bytepos

The byte position corresponding to window_end_pos.

window_end_vpos

The window-relative vertical position of the line containing window_end_pos.

window_end_valid

This field is set to a non-nil value if window_end_pos is truly valid. This is nil if nontrivial redisplay is pre-empted, since in that case the display that window_end_pos was computed for did not get onto the screen.

cursor

A structure describing where the cursor is in this window.

last_cursor

The value of cursor as of the last redisplay that finished.

phys_cursor

A structure describing where the cursor of this window physically is.

phys_cursor_type

phys_cursor_height

phys_cursor_width

The type, height, and width of the cursor that was last displayed on this window.

phys_cursor_on_p

This field is non-zero if the cursor is physically on.

cursor_off_p

Non-zero means the cursor in this window is logically off. This is used for blinking the cursor.

last_cursor_off_p

This field contains the value of cursor_off_p as of the time of the last redisplay.

must_be_updated_p

This is set to 1 during redisplay when this window must be updated.

hscroll

This is the number of columns that the display in the window is scrolled horizontally to the left. Normally, this is 0.

vscroll

Vertical scroll amount, in pixels. Normally, this is 0.

dedicated

Non-nil if this window is dedicated to its buffer.

display_table

The window's display table, or nil if none is specified for it.

update_mode_line

Non-nil means this window's mode line needs to be updated.

base_line_number

The line number of a certain position in the buffer, or nil. This is used for displaying the line number of point in the mode line.

base_line_pos

The position in the buffer for which the line number is known, or nil meaning none is known. If it is a buffer, don't display the line number as long as the window shows that buffer.

column_number_displayed

The column number currently displayed in this window's mode line, or nil if column numbers are not being displayed.

current_matrix

desired_matrix

Glyph matrices describing the current and desired display of this window.

E.8.3 Process Internals

The fields of a process (for a complete list, see the definition of struct Lisp_Process in process.h) include:

name

A string, the name of the process.

command

A list containing the command arguments that were used to start this process. For a network or serial process, it is nil if the process is running or t if the process is stopped.

filter

A function used to accept output from the process.

sentinel

A function called whenever the state of the process changes.

buffer

The associated buffer of the process.

pid

An integer, the operating system's process ID. Pseudo-processes such as network or serial connections use a value of 0.

childp

A flag, t if this is really a child process. For a network or serial connection, it is a plist based on the arguments to make-network-process or make-serial-process.

mark

A marker indicating the position of the end of the last output from this process inserted into the buffer. This is often but not always the end of the buffer.

kill_without_query

If this is non-zero, killing Emacs while this process is still running does not ask for confirmation about killing the process.

raw_status

The raw process status, as returned by the wait system call.

status

The process status, as process-status should return it.

tick

update_tick

If these two fields are not equal, a change in the status of the process needs to be reported, either by running the sentinel or by inserting a message in the process buffer.

pty_flag

Non-nil if communication with the subprocess uses a pty; nil if it uses a pipe.

infd

The file descriptor for input from the process.

outfd

The file descriptor for output to the process.

tty_name

The name of the terminal that the subprocess is using, or nil if it is using pipes.

decode_coding_system

Coding-system for decoding the input from this process.

decoding_buf

A working buffer for decoding.

decoding_carryover

Size of carryover in decoding.

encode_coding_system

Coding-system for encoding the output to this process.

encoding_buf

A working buffer for encoding.

inherit_coding_system_flag

Flag to set coding-system of the process buffer from the coding system used to decode process output.

type

Symbol indicating the type of process: real, network, serial.

E.9 C Integer Types

Here are some guidelines for use of integer types in the Emacs C source code. These guidelines sometimes give competing advice; common sense is advised.

• Avoid arbitrary limits. For example, avoid int len = strlen (s); unless the length of s is required for other reasons to fit in int range.
• Do not assume that signed integer arithmetic wraps around on overflow. This is no longer true of Emacs porting targets: signed integer overflow has undefined behavior in practice, and can dump core or even cause earlier or later code to behave illogically. Unsigned overflow does wrap around reliably, modulo a power of two.
• Prefer signed types to unsigned, as code gets confusing when signed and unsigned types are combined. Many other guidelines assume that types are signed; in the rarer cases where unsigned types are needed, similar advice may apply to the unsigned counterparts (e.g., size_t instead of ptrdiff_t, or uintptr_t instead of intptr_t).
• Prefer int for Emacs character codes, in the range 0 .. 0x3FFFFF. More generally, prefer int for integers known to be in int range, e.g., screen column counts.
• Prefer ptrdiff_t for sizes, i.e., for integers bounded by the maximum size of any individual C object or by the maximum number of elements in any C array. This is part of Emacs's general preference for signed types. Using ptrdiff_t limits objects to PTRDIFF_MAX bytes, but larger objects would cause trouble anyway since they would break pointer subtraction, so this does not impose an arbitrary limit.
• Avoid ssize_t except when communicating to low-level APIs that have ssize_t-related limitations. Although it's equivalent to ptrdiff_t on typical platforms, ssize_t is occasionally narrower, so using it for size-related calculations could overflow. Also, ptrdiff_t is more ubiquitous and better-standardized, has standard printf formats, and is the basis for Emacs's internal size-overflow checking. When using ssize_t, please note that POSIX requires support only for values in the range −1 .. SSIZE_MAX.
• Prefer intptr_t for internal representations of pointers, or for integers bounded only by the number of objects that can exist at any given time or by the total number of bytes that can be allocated. Currently Emacs sometimes uses other types when intptr_t would be better; fixing this is lower priority, as the code works as-is on Emacs's current porting targets.
• Prefer the Emacs-defined type EMACS_INT for representing values converted to or from Emacs Lisp fixnums, as fixnum arithmetic is based on EMACS_INT.
• When representing a system value (such as a file size or a count of seconds since the Epoch), prefer the corresponding system type (e.g., off_t, time_t). Do not assume that a system type is signed, unless this assumption is known to be safe. For example, although off_t is always signed, time_t need not be.
• Prefer the Emacs-defined type printmax_t for representing values that might be any signed integer that can be printed, using a printf-family function.
• Prefer intmax_t for representing values that might be any signed integer value.
• Prefer bool, false and true for booleans. Using bool can make programs easier to read and a bit faster than using int. Although it is also OK to use int, 0 and 1, this older style is gradually being phased out. When using bool, respect the limitations of the replacement implementation of bool, as documented in the source file lib/stdbool.in.h. In particular, boolean bitfields should be of type bool_bf, not bool, so that they work correctly even when compiling Objective C with standard GCC.
• In bitfields, prefer unsigned int or signed int to int, as int is less portable: it might be signed, and might not be. Single-bit bit fields should be unsigned int or bool_bf so that their values are 0 or 1.

Appendix F Standard Errors

Here is a list of the more important error symbols in standard Emacs, grouped by concept. The list includes each symbol's message and a cross reference to a description of how the error can occur.

Each error symbol has an set of parent error conditions that is a list of symbols. Normally this list includes the error symbol itself and the symbol error. Occasionally it includes additional symbols, which are intermediate classifications, narrower than error but broader than a single error symbol. For example, all the errors in accessing files have the condition file-error. If we do not say here that a certain error symbol has additional error conditions, that means it has none.

As a special exception, the error symbol quit does not have the condition error, because quitting is not considered an error.

Most of these error symbols are defined in C (mainly data.c), but some are defined in Lisp. For example, the file userlock.el defines the file-locked and file-supersession errors. Several of the specialized Lisp libraries distributed with Emacs define their own error symbols. We do not attempt to list of all those here.

See Errors, for an explanation of how errors are generated and handled.

error

The message is ‘error’. See Errors.

quit

The message is ‘Quit’. See Quitting.

args-out-of-range

The message is ‘Args out of range’. This happens when trying to access an element beyond the range of a sequence, buffer, or other container-like object. See Sequences Arrays Vectors, and See Text.

arith-error

The message is ‘Arithmetic error’. This occurs when trying to perform integer division by zero. See Numeric Conversions, and See Arithmetic Operations.

beginning-of-buffer

The message is ‘Beginning of buffer’. See Character Motion.

buffer-read-only

The message is ‘Buffer is read-only’. See Read Only Buffers.

circular-list

The message is ‘List contains a loop’. This happens when a circular structure is encountered. See Circular Objects.

cl-assertion-failed

The message is ‘Assertion failed’. This happens when the cl-assert macro fails a test. See Assertions.

coding-system-error

The message is ‘Invalid coding system’. See Lisp and Coding Systems.

cyclic-function-indirection

The message is ‘Symbol's chain of function indirections contains a loop’. See Function Indirection.

cyclic-variable-indirection

The message is ‘Symbol's chain of variable indirections contains a loop’. See Variable Aliases.

dbus-error

The message is ‘D-Bus error’. This is only defined if Emacs was compiled with D-Bus support. See Errors and Events.

end-of-buffer

The message is ‘End of buffer’. See Character Motion.

end-of-file

The message is ‘End of file during parsing’. Note that this is not a subcategory of file-error, because it pertains to the Lisp reader, not to file I/O. See Input Functions.

file-already-exists

This is a subcategory of file-error. See Writing to Files.

file-date-error

This is a subcategory of file-error. It occurs when copy-file tries and fails to set the last-modification time of the output file. See Changing Files.

file-error

We do not list the error-strings of this error and its subcategories, because the error message is normally constructed from the data items alone when the error condition file-error is present. Thus, the error-strings are not very relevant. However, these error symbols do have error-message properties, and if no data is provided, the error-message property is used. See Files.

compression-error

This is a subcategory of file-error, which results from problems handling a compressed file. See How Programs Do Loading.

file-locked

This is a subcategory of file-error. See File Locks.

file-supersession

This is a subcategory of file-error. See Modification Time.

file-notify-error

This is a subcategory of file-error. It happens, when a file could not be watched for changes. See File Notifications.

ftp-error

This is a subcategory of file-error, which results from problems in accessing a remote file using ftp. See Remote Files.

invalid-function

The message is ‘Invalid function’. See Function Indirection.

invalid-read-syntax

The message is ‘Invalid read syntax’. See Printed Representation.

invalid-regexp

The message is ‘Invalid regexp’. See Regular Expressions.

mark-inactive

The message is ‘The mark is not active now’. See The Mark.

no-catch

The message is ‘No catch for tag’. See Catch and Throw.

scan-error

The message is ‘Scan error’. This happens when certain syntax-parsing functions find invalid syntax or mismatched parentheses. Conventionally raised with three argument: a human-readable error message, the start of the obstacle that cannot be moved over, and the end of the obstacle. See List Motion, and See Parsing Expressions.

search-failed

The message is ‘Search failed’. See Searching and Matching.

setting-constant

The message is ‘Attempt to set a constant symbol’. This happens when attempting to assign values to nil, t, and keyword symbols. See Constant Variables.

text-read-only

The message is ‘Text is read-only’. This is a subcategory of buffer-read-only. See Special Properties.

undefined-color

The message is ‘Undefined color’. See Color Names.

user-error

The message is the empty string. See Signaling Errors.

void-function

The message is ‘Symbol's function definition is void’. See Function Cells.

void-variable

The message is ‘Symbol's value as variable is void’. See Accessing Variables.

wrong-number-of-arguments

The message is ‘Wrong number of arguments’. See Argument List.

wrong-type-argument

The message is ‘Wrong type argument’. See Type Predicates.

Appendix G Standard Keymaps

In this section we list some of the more general keymaps. Many of these exist when Emacs is first started, but some are loaded only when the respective feature is accessed.

There are many other, more specialized, maps than these; in particular those associated with major and minor modes. The minibuffer uses several keymaps (see Completion Commands). For more details on keymaps, see Keymaps.

2C-mode-map

A sparse keymap for subcommands of the prefix C-x 6.
See Two-Column Editing.

abbrev-map

A sparse keymap for subcommands of the prefix C-x a.
See Defining Abbrevs.

button-buffer-map

A sparse keymap useful for buffers containing buffers.
You may want to use this as a parent keymap. See Buttons.

button-map

A sparse keymap used by buttons.

ctl-x-4-map

A sparse keymap for subcommands of the prefix C-x 4.

ctl-x-5-map

A sparse keymap for subcommands of the prefix C-x 5.

ctl-x-map

A full keymap for C-x commands.

ctl-x-r-map

A sparse keymap for subcommands of the prefix C-x r.
See Registers.

esc-map

A full keymap for ESC (or Meta) commands.

facemenu-keymap

A sparse keymap used for the M-o prefix key.

function-key-map

The parent keymap of all local-function-key-map (q.v.) instances.

global-map

The full keymap containing default global key bindings.
Modes should not modify the Global map.

goto-map

A sparse keymap used for the M-g prefix key.

help-map

A sparse keymap for the keys following the help character C-h.
See Help Functions.

Helper-help-map

A full keymap used by the help utility package.
It has the same keymap in its value cell and in its function cell.

input-decode-map

The keymap for translating keypad and function keys.
If there are none, then it contains an empty sparse keymap. See Translation Keymaps.

key-translation-map

A keymap for translating keys. This one overrides ordinary key bindings, unlike local-function-key-map. See Translation Keymaps.

kmacro-keymap

A sparse keymap for keys that follows the C-x C-k prefix search.
See Keyboard Macros.

local-function-key-map

The keymap for translating key sequences to preferred alternatives.
If there are none, then it contains an empty sparse keymap. See Translation Keymaps.

menu-bar-file-menu

menu-bar-edit-menu

menu-bar-options-menu

global-buffers-menu-map

menu-bar-tools-menu

menu-bar-help-menu

These keymaps display the main, top-level menus in the menu bar.
Some of them contain sub-menus. For example, the Edit menu contains menu-bar-search-menu, etc. See Menu Bar.

minibuffer-inactive-mode-map

A full keymap used in the minibuffer when it is not active.
See Editing in the Minibuffer.

mode-line-coding-system-map

mode-line-input-method-map

mode-line-column-line-number-mode-map

These keymaps control various areas of the mode line.
See Mode Line Format.

mode-specific-map

The keymap for characters following C-c. Note, this is in the global map. This map is not actually mode-specific: its name was chosen to be informative in C-h b (display-bindings), where it describes the main use of the C-c prefix key.

mouse-appearance-menu-map

A sparse keymap used for the S-mouse-1 key.

mule-keymap

The global keymap used for the C-x <RET> prefix key.

narrow-map

A sparse keymap for subcommands of the prefix C-x n.

prog-mode-map

The keymap used by Prog mode.
See Basic Major Modes.

query-replace-map

multi-query-replace-map

A sparse keymap used for responses in query-replace and related commands; also for y-or-n-p and map-y-or-n-p. The functions that use this map do not support prefix keys; they look up one event at a time. multi-query-replace-map extends query-replace-map for multi-buffer replacements. See query-replace-map.

search-map

A sparse keymap that provides global bindings for search-related commands.

special-mode-map

The keymap used by Special mode.
See Basic Major Modes.

tool-bar-map

The keymap defining the contents of the tool bar.
See Tool Bar.

universal-argument-map

A sparse keymap used while processing C-u.
See Prefix Command Arguments.

vc-prefix-map

The global keymap used for the C-x v prefix key.

x-alternatives-map

A sparse keymap used to map certain keys under graphical frames.
The function x-setup-function-keys uses this.

Appendix H Standard Hooks

The following is a list of some hook variables that let you provide functions to be called from within Emacs on suitable occasions.

Most of these variables have names ending with ‘-hook’. They are normal hooks, run by means of run-hooks. The value of such a hook is a list of functions; the functions are called with no arguments and their values are completely ignored. The recommended way to put a new function on such a hook is to call add-hook. See Hooks, for more information about using hooks.

The variables whose names end in ‘-functions’ are usually abnormal hooks (some old code may also use the deprecated ‘-hooks’ suffix); their values are lists of functions, but these functions are called in a special way (they are passed arguments, or their return values are used). The variables whose names end in ‘-function’ have single functions as their values.

This is not an exhaustive list, it only covers the more general hooks. For example, every major mode defines a hook named ‘modename-mode-hook’. The major mode command runs this normal hook with run-mode-hooks as the very last thing it does. See Mode Hooks. Most minor modes have mode hooks too.

A special feature allows you to specify expressions to evaluate if and when a file is loaded (see Hooks for Loading). That feature is not exactly a hook, but does a similar job.

activate-mark-hook

deactivate-mark-hook

See The Mark.

after-change-functions

before-change-functions

first-change-hook

See Change Hooks.

after-change-major-mode-hook

change-major-mode-after-body-hook

See Mode Hooks.

after-init-hook

before-init-hook

emacs-startup-hook

window-setup-hook

See Init File.

after-insert-file-functions

write-region-annotate-functions

write-region-post-annotation-function

See Format Conversion.

after-make-frame-functions

before-make-frame-hook

See Creating Frames.

after-save-hook

before-save-hook

write-contents-functions

write-file-functions

See Saving Buffers.

after-setting-font-hook

Hook run after a frame's font changes.

auto-save-hook

See Auto-Saving.

before-hack-local-variables-hook

hack-local-variables-hook

See File Local Variables.

buffer-access-fontify-functions

See Lazy Properties.

buffer-list-update-hook

Hook run when the buffer list changes (see Buffer List).

buffer-quit-function

Function to call to quit the current buffer.

change-major-mode-hook

See Creating Buffer-Local.

command-line-functions

See Command-Line Arguments.

delayed-warnings-hook

The command loop runs this soon after post-command-hook (q.v.).

focus-in-hook

focus-out-hook

See Input Focus.

delete-frame-functions

See Deleting Frames.

delete-terminal-functions

See Multiple Terminals.

pop-up-frame-function

split-window-preferred-function

See Choosing Window Options.

echo-area-clear-hook

See Echo Area Customization.

find-file-hook

find-file-not-found-functions

See Visiting Functions.

font-lock-extend-after-change-region-function

See Region to Refontify.

font-lock-extend-region-functions

See Multiline Font Lock.

font-lock-fontify-buffer-function

font-lock-fontify-region-function

font-lock-mark-block-function

font-lock-unfontify-buffer-function

font-lock-unfontify-region-function

See Other Font Lock Variables.

fontification-functions

See Automatic Face Assignment.

frame-auto-hide-function

See Quitting Windows.

kill-buffer-hook

kill-buffer-query-functions

See Killing Buffers.

kill-emacs-hook

kill-emacs-query-functions

See Killing Emacs.

menu-bar-update-hook

See Menu Bar.

minibuffer-setup-hook

minibuffer-exit-hook

See Minibuffer Misc.

mouse-leave-buffer-hook

Hook run when about to switch windows with a mouse command.

mouse-position-function

See Mouse Position.

prefix-command-echo-keystrokes-functions

An abnormal hook run by prefix commands (such as C-u) which should return a string describing the current prefix state. For example, C-u produces ‘C-u-’ and ‘C-u 1 2 3-’. Each hook function is called with no arguments and should return a string describing the current prefix state, or nil if there's no prefix state. See Prefix Command Arguments.

prefix-command-preserve-state-hook

Hook run when a prefix command needs to preserve the prefix by passing the current prefix command state to the next command. For example, C-u needs to pass the state to the next command when the user types C-u - or follows C-u with a digit.

pre-redisplay-functions

Hook run in each window just before redisplaying it. See Forcing Redisplay.

post-command-hook

pre-command-hook

See Command Overview.

post-gc-hook

See Garbage Collection.

post-self-insert-hook

See Keymaps and Minor Modes.

suspend-hook

suspend-resume-hook

suspend-tty-functions

resume-tty-functions

See Suspending Emacs.

syntax-begin-function

syntax-propertize-extend-region-functions

syntax-propertize-function

font-lock-syntactic-face-function

See Syntactic Font Lock. See Syntax Properties.

temp-buffer-setup-hook

temp-buffer-show-function

temp-buffer-show-hook

See Temporary Displays.

tty-setup-hook

See Terminal-Specific.

window-configuration-change-hook

window-scroll-functions

window-size-change-functions

See Window Hooks.

window-text-change-functions

Functions to call in redisplay when text in the window might change.

Index

• ‘"’ in printing: Output Functions
• ‘"’ in strings: Syntax for Strings
• ‘##’ read syntax: Symbol Type
• ‘#$’: Docs and Compilation
• ‘#'’ syntax: Anonymous Functions
• ‘#(’ read syntax: Text Props and Strings
• ‘#:’ read syntax: Symbol Type
• ‘#@count’: Docs and Compilation
• ‘#^’ read syntax: Char-Table Type
• ‘#n#’ read syntax: Circular Objects
• ‘#n=’ read syntax: Circular Objects
• ‘$’ in display: Truncation
• ‘$’ in regexp: Regexp Special
• %: Arithmetic Operations
• ‘%’ in format: Formatting Strings
• ‘&’ in replacement: Replacing Match
• &optional: Argument List
• &rest: Argument List
• ‘'’ for quoting: Quoting
• ‘(’ in regexp: Regexp Backslash
• ‘(...)’ in lists: Cons Cell Type
• ‘(?:’ in regexp: Regexp Backslash
• ‘)’ in regexp: Regexp Backslash
• *: Arithmetic Operations
• ‘*’ in interactive: Using Interactive
• ‘*’ in regexp: Regexp Special
• *scratch*: Auto Major Mode
• +: Arithmetic Operations
• ‘+’ in regexp: Regexp Special
• , (with backquote): Backquote
• ,@ (with backquote): Backquote
• -: Arithmetic Operations
• --enable-locallisppath option to configure: Building Emacs
• –enable-profiling option of configure: Profiling
• ‘.’ in lists: Dotted Pair Notation
• ‘.’ in regexp: Regexp Special
• .emacs: Init File
• /: Arithmetic Operations
• /=: Comparison of Numbers
• /dev/tty: Serial Ports
• 1+: Arithmetic Operations
• 1-: Arithmetic Operations
• 1value: Test Coverage
• 2C-mode-map: Prefix Keys
• 2D box: Face Attributes
• 3D box: Face Attributes
• ‘;’ in comment: Comments
• <: Comparison of Numbers
• <=: Comparison of Numbers
• <ESC>: Functions for Key Lookup
• <SPC> in minibuffer: Text from Minibuffer
• <TAB> in minibuffer: Text from Minibuffer
• =: Comparison of Numbers
• >: Comparison of Numbers
• >=: Comparison of Numbers
• ‘?’ in character constant: Basic Char Syntax
• ? in minibuffer: Text from Minibuffer
• ‘?’ in regexp: Regexp Special
• ‘@’ in interactive: Using Interactive
• ‘[’ in regexp: Regexp Special
• [...] (Edebug): Specification List
• ‘\’ in character constant: General Escape Syntax
• ‘\’ in display: Truncation
• ‘\’ in printing: Output Functions
• ‘\’ in regexp: Regexp Special
• ‘\’ in replacement: Replacing Match
• ‘\’ in strings: Syntax for Strings
• ‘\’ in symbols: Symbol Type
• ‘\'’ in regexp: Regexp Backslash
• ‘\<’ in regexp: Regexp Backslash
• ‘\=’ in regexp: Regexp Backslash
• ‘\>’ in regexp: Regexp Backslash
• ‘\_<’ in regexp: Regexp Backslash
• ‘\_>’ in regexp: Regexp Backslash
• ‘\`’ in regexp: Regexp Backslash
• ‘\a’: Basic Char Syntax
• ‘\b’: Basic Char Syntax
• ‘\B’ in regexp: Regexp Backslash
• ‘\b’ in regexp: Regexp Backslash
• ‘\e’: Basic Char Syntax
• ‘\f’: Basic Char Syntax
• ‘\n’: Basic Char Syntax
• ‘\n’ in print: Output Variables
• ‘\n’ in replacement: Replacing Match
• ‘\r’: Basic Char Syntax
• ‘\s’: Basic Char Syntax
• ‘\S’ in regexp: Regexp Backslash
• ‘\s’ in regexp: Regexp Backslash
• ‘\t’: Basic Char Syntax
• ‘\v’: Basic Char Syntax
• ‘\W’ in regexp: Regexp Backslash
• ‘\w’ in regexp: Regexp Backslash
• ‘]’ in regexp: Regexp Special
• ‘^’ in interactive: Using Interactive
• ‘^’ in regexp: Regexp Special
• `: Backquote
• ` (list substitution): Backquote
• abbrev: Abbrevs
• abbrev properties: Abbrev Properties
• abbrev table properties: Abbrev Table Properties
• abbrev tables: Abbrev Tables
• abbrev tables in modes: Major Mode Conventions
• abbrev-all-caps: Abbrev Expansion
• abbrev-expand-function: Abbrev Expansion
• abbrev-expansion: Abbrev Expansion
• abbrev-file-name: Abbrev Files
• abbrev-get: Abbrev Properties
• abbrev-insert: Abbrev Expansion
• abbrev-map: Standard Keymaps
• abbrev-minor-mode-table-alist: Standard Abbrev Tables
• abbrev-prefix-mark: Abbrev Expansion
• abbrev-put: Abbrev Properties
• abbrev-start-location: Abbrev Expansion
• abbrev-start-location-buffer: Abbrev Expansion
• abbrev-symbol: Abbrev Expansion
• abbrev-table-get: Abbrev Table Properties
• abbrev-table-name-list: Abbrev Tables
• abbrev-table-p: Abbrev Tables
• abbrev-table-put: Abbrev Table Properties
• abbreviate-file-name: Directory Names
• abbreviated file names: Directory Names
• abbrevs, looking up and expanding: Abbrev Expansion
• abbrevs-changed: Abbrev Files
• abnormal hook: Hooks
• abort-recursive-edit: Recursive Editing
• aborting: Recursive Editing
• abs: Comparison of Numbers
• absolute file name: Relative File Names
• absolute position: Frame Layout
• accept input from processes: Accepting Output
• accept-change-group: Atomic Changes
• accept-process-output: Accepting Output
• access control list: Extended Attributes
• access minibuffer contents: Minibuffer Contents
• access-file: Testing Accessibility
• accessibility of a file: Testing Accessibility
• accessible portion (of a buffer): Narrowing
• accessible-keymaps: Scanning Keymaps
• accessing documentation strings: Accessing Documentation
• accessing hash tables: Hash Access
• accessing plist properties: Plist Access
• ACL entries: Extended Attributes
• acos: Math Functions
• action (button property): Button Properties
• action alist, for display-buffer: Choosing Window
• action function, for display-buffer: Choosing Window
• action, customization keyword: Type Keywords
• activate-change-group: Atomic Changes
• activate-mark-hook: The Mark
• active display table: Active Display Table
• active keymap: Active Keymaps
• active keymap, controlling: Controlling Active Maps
• active-minibuffer-window: Minibuffer Windows
• adaptive-fill-first-line-regexp: Adaptive Fill
• adaptive-fill-function: Adaptive Fill
• adaptive-fill-mode: Adaptive Fill
• adaptive-fill-regexp: Adaptive Fill
• add-face-text-property: Changing Properties
• add-function: Core Advising Primitives
• add-hook: Setting Hooks
• add-name-to-file: Changing Files
• add-text-properties: Changing Properties
• add-to-history: Minibuffer History
• add-to-invisibility-spec: Invisible Text
• add-to-list: List Variables
• add-to-ordered-list: List Variables
• address field of register: Cons Cell Type
• adjust-window-trailing-edge: Resizing Windows
• adjusting point: Adjusting Point
• advertised binding: Keys in Documentation
• advice, add and remove: Core Advising Primitives
• advice-add: Advising Named Functions
• advice-eval-interactive-spec: Core Advising Primitives
• advice-function-mapc: Core Advising Primitives
• advice-function-member-p: Core Advising Primitives
• advice-mapc: Advising Named Functions
• advice-member-p: Advising Named Functions
• advice-remove: Advising Named Functions
• advising functions: Advising Functions
• advising named functions: Advising Named Functions
• after-change-functions: Change Hooks
• after-change-major-mode-hook: Mode Hooks
• after-find-file: Subroutines of Visiting
• after-init-hook: Init File
• after-init-time: Startup Summary
• after-insert-file-functions: Format Conversion Piecemeal
• after-load-functions: Hooks for Loading
• after-make-frame-functions: Creating Frames
• after-revert-hook: Reverting
• after-save-hook: Saving Buffers
• after-setting-font-hook: Standard Hooks
• after-string (overlay property): Overlay Properties
• alias, for coding systems: Coding System Basics
• alias, for faces: Face Functions
• alias, for functions: Defining Functions
• alias, for variables: Variable Aliases
• alist: Association Lists
• alist vs. plist: Plists and Alists
• alist-get: Association Lists
• all-completions: Basic Completion
• alpha, a frame parameter: Font and Color Parameters
• alt characters: Other Char Bits
• alternatives, defining: Generic Commands
• amalgamating commands, and undo: Undo
• and: Combining Conditions
• animation: Multi-Frame Images
• anonymous face: Faces
• anonymous function: Anonymous Functions
• apostrophe for quoting: Quoting
• append: Building Lists
• append-to-file: Writing to Files
• apply: Calling Functions
• apply, and debugging: Internals of Debugger
• apply-partially: Calling Functions
• applying customizations: Applying Customizations
• apropos: Help Functions
• aref: Array Functions
• args, customization keyword: Composite Types
• argument: What Is a Function
• argument binding: Argument List
• argument lists, features: Argument List
• arguments for shell commands: Shell Arguments
• arguments, interactive entry: Using Interactive
• arguments, reading: Minibuffers
• argv: Command-Line Arguments
• arith-error example: Handling Errors
• arith-error in division: Arithmetic Operations
• arithmetic operations: Arithmetic Operations
• arithmetic shift: Bitwise Operations
• array: Arrays
• array elements: Array Functions
• arrayp: Array Functions
• ASCII character codes: Character Type
• ASCII control characters: Usual Display
• ascii-case-table: Case Tables
• aset: Array Functions
• ash: Bitwise Operations
• asin: Math Functions
• ask-user-about-lock: File Locks
• ask-user-about-supersession-threat: Modification Time
• asking the user questions: Yes-or-No Queries
• assoc: Association Lists
• assoc-default: Association Lists
• assoc-string: Text Comparison
• association list: Association Lists
• assq: Association Lists
• assq-delete-all: Association Lists
• asynchronous subprocess: Asynchronous Processes
• atan: Math Functions
• atom: List-related Predicates
• atomic changes: Atomic Changes
• atoms: Cons Cell Type
• attributes of text: Text Properties
• Auto Fill mode: Auto Filling
• auto-coding-alist: Default Coding Systems
• auto-coding-functions: Default Coding Systems
• auto-coding-regexp-alist: Default Coding Systems
• auto-fill-chars: Auto Filling
• auto-fill-function: Auto Filling
• auto-hscroll-mode: Horizontal Scrolling
• auto-lower, a frame parameter: Management Parameters
• auto-mode-alist: Auto Major Mode
• auto-raise, a frame parameter: Management Parameters
• auto-raise-tool-bar-buttons: Tool Bar
• auto-resize-tool-bars: Tool Bar
• auto-save-default: Auto-Saving
• auto-save-file-name-p: Auto-Saving
• auto-save-hook: Auto-Saving
• auto-save-interval: Auto-Saving
• auto-save-list-file-name: Auto-Saving
• auto-save-list-file-prefix: Auto-Saving
• auto-save-mode: Auto-Saving
• auto-save-timeout: Auto-Saving
• auto-save-visited-file-name: Auto-Saving
• auto-window-vscroll: Vertical Scrolling
• autoload: Autoload
• autoload cookie: Autoload
• autoload errors: Autoload
• autoload object: What Is a Function
• autoload-do-load: Autoload
• autoloadp: Autoload
• automatic face assignment: Auto Faces
• automatically buffer-local: Intro to Buffer-Local
• back-to-indentation: Motion by Indent
• background-color, a frame parameter: Font and Color Parameters
• background-mode, a frame parameter: Font and Color Parameters
• backing store: Display Feature Testing
• backquote (list substitution): Backquote
• backslash in character constants: General Escape Syntax
• backslash in regular expressions: Regexp Backslash
• backslash in strings: Syntax for Strings
• backslash in symbols: Symbol Type
• backspace: Basic Char Syntax
• backtrace: Internals of Debugger
• backtrace from emacsclient's --eval: Error Debugging
• backtrace-debug: Internals of Debugger
• backtrace-frame: Internals of Debugger
• backtracking: Backtracking
• backtracking and POSIX regular expressions: POSIX Regexps
• backtracking and regular expressions: Regexp Special
• backup file: Backup Files
• backup files, rename or copy: Rename or Copy
• backup-buffer: Making Backups
• backup-by-copying: Rename or Copy
• backup-by-copying-when-linked: Rename or Copy
• backup-by-copying-when-mismatch: Rename or Copy
• backup-by-copying-when-privileged-mismatch: Rename or Copy
• backup-directory-alist: Making Backups
• backup-enable-predicate: Making Backups
• backup-file-name-p: Backup Names
• backup-inhibited: Making Backups
• backups and auto-saving: Backups and Auto-Saving
• backward-button: Button Buffer Commands
• backward-char: Character Motion
• backward-delete-char-untabify: Deletion
• backward-delete-char-untabify-method: Deletion
• backward-list: List Motion
• backward-prefix-chars: Motion and Syntax
• backward-sexp: List Motion
• backward-to-indentation: Motion by Indent
• backward-up-list: List Motion
• backward-word: Word Motion
• backward-word-strictly: Word Motion
• balance-windows: Resizing Windows
• balance-windows-area: Resizing Windows
• balanced parenthesis motion: List Motion
• balancing parentheses: Blinking
• balancing window sizes: Resizing Windows
• barf-if-buffer-read-only: Read Only Buffers
• base 64 encoding: Base 64
• base buffer: Indirect Buffers
• base coding system: Coding System Basics
• base direction of a paragraph: Bidirectional Display
• base for reading an integer: Integer Basics
• base location, package archive: Package Archives
• base remapping, faces: Face Remapping
• base64-decode-region: Base 64
• base64-decode-string: Base 64
• base64-encode-region: Base 64
• base64-encode-string: Base 64
• basic code (of input character): Keyboard Events
• basic faces: Basic Faces
• batch mode: Batch Mode
• batch-byte-compile: Compilation Functions
• baud, in serial connections: Serial Ports
• baud-rate: Terminal Output
• beep: Beeping
• before point, insertion: Insertion
• before-change-functions: Change Hooks
• before-hack-local-variables-hook: File Local Variables
• before-init-hook: Init File
• before-init-time: Startup Summary
• before-make-frame-hook: Creating Frames
• before-revert-hook: Reverting
• before-save-hook: Saving Buffers
• before-string (overlay property): Overlay Properties
• beginning of line: Text Lines
• beginning of line in regexp: Regexp Special
• beginning-of-buffer: Buffer End Motion
• beginning-of-defun: List Motion
• beginning-of-defun-function: List Motion
• beginning-of-line: Text Lines
• bell: Beeping
• bell character: Basic Char Syntax
• benchmark.el: Profiling
• benchmarking: Profiling
• bidi-display-reordering: Bidirectional Display
• bidi-find-overridden-directionality: Bidirectional Display
• bidi-paragraph-direction: Bidirectional Display
• bidi-string-mark-left-to-right: Bidirectional Display
• bidirectional class of characters: Character Properties
• bidirectional display: Bidirectional Display
• bidirectional reordering: Bidirectional Display
• big endian: Bindat Spec
• binary coding system: Coding System Basics
• binary I/O in batch mode: Input Functions
• bindat-get-field: Bindat Functions
• bindat-ip-to-string: Bindat Functions
• bindat-length: Bindat Functions
• bindat-pack: Bindat Functions
• bindat-unpack: Bindat Functions
• binding arguments: Argument List
• binding local variables: Local Variables
• binding of a key: Keymap Basics
• bitmap-spec-p: Face Attributes
• bitmaps, fringe: Fringe Bitmaps
• bitwise arithmetic: Bitwise Operations
• blink-cursor-alist: Cursor Parameters
• blink-matching-delay: Blinking
• blink-matching-open: Blinking
• blink-matching-paren: Blinking
• blink-matching-paren-distance: Blinking
• blink-paren-function: Blinking
• blinking parentheses: Blinking
• bobp: Near Point
• body height of a window: Window Sizes
• body of a window: Window Sizes
• body of function: Lambda Components
• body size of a window: Window Sizes
• body width of a window: Window Sizes
• bolp: Near Point
• bool-vector: Bool-Vectors
• bool-vector-count-consecutive: Bool-Vectors
• bool-vector-count-population: Bool-Vectors
• bool-vector-exclusive-or: Bool-Vectors
• bool-vector-intersection: Bool-Vectors
• bool-vector-not: Bool-Vectors
• bool-vector-p: Bool-Vectors
• bool-vector-set-difference: Bool-Vectors
• bool-vector-subsetp: Bool-Vectors
• bool-vector-union: Bool-Vectors
• Bool-vectors: Bool-Vectors
• boolean: nil and t
• booleanp: nil and t
• border-color, a frame parameter: Font and Color Parameters
• border-width, a frame parameter: Layout Parameters
• bottom dividers: Window Dividers
• bottom-divider-width, a frame parameter: Layout Parameters
• boundp: Void Variables
• box diagrams, for lists: Box Diagrams
• break: Debugger
• breakpoints (Edebug): Breakpoints
• bucket (in obarray): Creating Symbols
• buffer: Buffers
• buffer boundaries, indicating: Fringe Indicators
• buffer contents: Text
• buffer file name: Buffer File Name
• buffer gap: Buffer Gap
• buffer input stream: Input Streams
• buffer internals: Buffer Internals
• buffer list: Buffer List
• buffer modification: Buffer Modification
• buffer names: Buffer Names
• buffer output stream: Output Streams
• buffer portion as string: Buffer Contents
• buffer position: Positions
• buffer text notation: Buffer Text Notation
• buffer, read-only: Read Only Buffers
• buffer-access-fontified-property: Lazy Properties
• buffer-access-fontify-functions: Lazy Properties
• buffer-auto-save-file-format: Format Conversion Round-Trip
• buffer-auto-save-file-name: Auto-Saving
• buffer-backed-up: Making Backups
• buffer-base-buffer: Indirect Buffers
• buffer-chars-modified-tick: Buffer Modification
• buffer-disable-undo: Maintaining Undo
• buffer-display-count: Buffers and Windows
• buffer-display-table: Active Display Table
• buffer-display-time: Buffers and Windows
• buffer-enable-undo: Maintaining Undo
• buffer-end: Point
• buffer-file-coding-system: Encoding and I/O
• buffer-file-format: Format Conversion Round-Trip
• buffer-file-name: Buffer File Name
• buffer-file-number: Buffer File Name
• buffer-file-truename: Buffer File Name
• buffer-invisibility-spec: Invisible Text
• buffer-list: Buffer List
• buffer-list, a frame parameter: Buffer Parameters
• buffer-list-update-hook: Standard Hooks
• buffer-list-update-hook: Buffer List
• buffer-live-p: Killing Buffers
• buffer-local variables: Buffer-Local Variables
• buffer-local variables in modes: Major Mode Conventions
• buffer-local-value: Creating Buffer-Local
• buffer-local-variables: Creating Buffer-Local
• buffer-modified-p: Buffer Modification
• buffer-modified-tick: Buffer Modification
• buffer-name: Buffer Names
• buffer-name-history: Minibuffer History
• buffer-narrowed-p: Narrowing
• buffer-offer-save: Killing Buffers
• buffer-predicate, a frame parameter: Buffer Parameters
• buffer-quit-function: Standard Hooks
• buffer-read-only: Read Only Buffers
• buffer-save-without-query: Killing Buffers
• buffer-saved-size: Auto-Saving
• buffer-size: Point
• buffer-stale-function: Reverting
• buffer-string: Buffer Contents
• buffer-substring: Buffer Contents
• buffer-substring-filters: Buffer Contents
• buffer-substring-no-properties: Buffer Contents
• buffer-substring-with-bidi-context: Bidirectional Display
• buffer-swap-text: Swapping Text
• buffer-undo-list: Undo
• bufferp: Buffer Basics
• bufferpos-to-filepos: Text Representations
• buffers to display on frame: Buffer Parameters
• buffers without undo information: Buffer Names
• buffers, controlled in windows: Buffers and Windows
• buffers, creating: Creating Buffers
• buffers, killing: Killing Buffers
• bugs: Caveats
• bugs in this manual: Caveats
• building Emacs: Building Emacs
• building lists: Building Lists
• built-in function: What Is a Function
• bury-buffer: Buffer List
• butlast: List Elements
• button (button property): Button Properties
• button buffer commands: Button Buffer Commands
• button properties: Button Properties
• button types: Button Types
• button-activate: Manipulating Buttons
• button-at: Manipulating Buttons
• button-down event: Button-Down Events
• button-end: Manipulating Buttons
• button-face, customization keyword: Type Keywords
• button-get: Manipulating Buttons
• button-has-type-p: Manipulating Buttons
• button-label: Manipulating Buttons
• button-prefix, customization keyword: Type Keywords
• button-put: Manipulating Buttons
• button-start: Manipulating Buttons
• button-suffix, customization keyword: Type Keywords
• button-type: Manipulating Buttons
• button-type-get: Manipulating Buttons
• button-type-put: Manipulating Buttons
• button-type-subtype-p: Manipulating Buttons
• buttons in buffers: Buttons
• byte compilation: Byte Compilation
• byte compiler warnings, how to avoid: Warning Tips
• byte packing and unpacking: Byte Packing
• byte to string: Converting Representations
• byte-boolean-vars: Writing Emacs Primitives
• byte-boolean-vars: Variables with Restricted Values
• byte-code: Byte Compilation
• byte-code function: Byte-Code Objects
• byte-code object: Byte-Code Objects
• byte-code-function-p: What Is a Function
• byte-compile: Compilation Functions
• byte-compile-debug: Compilation Functions
• byte-compile-dynamic: Dynamic Loading
• byte-compile-dynamic-docstrings: Docs and Compilation
• byte-compile-file: Compilation Functions
• byte-compiling macros: Compiling Macros
• byte-compiling require: Named Features
• byte-recompile-directory: Compilation Functions
• byte-to-position: Text Representations
• byte-to-string: Converting Representations
• bytes: Strings and Characters
• bytesize, in serial connections: Serial Ports
• C programming language: C Dialect
• C-c: Prefix Keys
• C-g: Quitting
• C-h: Prefix Keys
• C-M-x: Instrumenting
• C-x: Prefix Keys
• C-x 4: Prefix Keys
• C-x 5: Prefix Keys
• C-x 6: Prefix Keys
• C-x <RET>: Prefix Keys
• C-x C-a C-m: Edebug Execution Modes
• C-x v: Prefix Keys
• C-x X =: Coverage Testing
• caar: List Elements
• cadr: List Elements
• calendrical computations: Time Calculations
• calendrical information: Time Conversion
• call stack: Internals of Debugger
• call-interactively: Interactive Call
• call-process: Synchronous Processes
• call-process, command-line arguments from minibuffer: Shell Arguments
• call-process-region: Synchronous Processes
• call-process-shell-command: Synchronous Processes
• called-interactively-p: Distinguish Interactive
• calling a function: Calling Functions
• cancel-change-group: Atomic Changes
• cancel-debug-on-entry: Function Debugging
• cancel-timer: Timers
• canonical character height: Frame Font
• canonical character width: Frame Font
• capitalization: Case Conversion
• capitalize: Case Conversion
• capitalize-region: Case Changes
• capitalize-word: Case Changes
• car: List Elements
• car-safe: List Elements
• case conversion in buffers: Case Changes
• case conversion in Lisp: Case Conversion
• case in replacements: Replacing Match
• case-fold-search: Searching and Case
• case-replace: Searching and Case
• case-table-p: Case Tables
• catch: Catch and Throw
• categories of characters: Categories
• category (overlay property): Overlay Properties
• category (text property): Special Properties
• category set: Categories
• category table: Categories
• category, regexp search for: Regexp Backslash
• category-docstring: Categories
• category-set-mnemonics: Categories
• category-table: Categories
• category-table-p: Categories
• cdar: List Elements
• cddr: List Elements
• cdr: List Elements
• cdr-safe: List Elements
• ceiling: Numeric Conversions
• centering point: Textual Scrolling
• change hooks: Change Hooks
• change hooks for a character: Special Properties
• change load-path at configure time: Building Emacs
• change-major-mode-after-body-hook: Mode Hooks
• change-major-mode-hook: Creating Buffer-Local
• changing key bindings: Changing Key Bindings
• changing text properties: Changing Properties
• changing to another buffer: Current Buffer
• changing window size: Resizing Windows
• char-after: Near Point
• char-before: Near Point
• char-category-set: Categories
• char-charset: Character Sets
• char-code-property-description: Character Properties
• char-displayable-p: Fontsets
• char-equal: Text Comparison
• char-or-string-p: Predicates for Strings
• char-property-alias-alist: Examining Properties
• char-script-table: Character Properties
• char-syntax: Syntax Table Functions
• char-table length: Sequence Functions
• char-table-extra-slot: Char-Tables
• char-table-p: Char-Tables
• char-table-parent: Char-Tables
• char-table-range: Char-Tables
• char-table-subtype: Char-Tables
• char-tables: Char-Tables
• char-to-string: String Conversion
• char-width: Size of Displayed Text
• char-width-table: Character Properties
• character alternative (in regexp): Regexp Special
• character arrays: Strings and Characters
• character case: Case Conversion
• character categories: Categories
• character classes in regexp: Char Classes
• character code conversion: Coding System Basics
• character codepoint: Text Representations
• character codes: Character Codes
• character insertion: Commands for Insertion
• character printing: Describing Characters
• character properties: Character Properties
• character set, searching: Scanning Charsets
• character sets: Character Sets
• character to string: String Conversion
• character translation tables: Translation of Characters
• character width on display: Size of Displayed Text
• characterp: Character Codes
• characters: Strings and Characters
• characters for interactive codes: Interactive Codes
• characters, multi-byte: Non-ASCII Characters
• characters, representation in buffers and strings: Text Representations
• charset: Character Sets
• charset, coding systems to encode: Lisp and Coding Systems
• charset, text property: Explicit Encoding
• charset-after: Scanning Charsets
• charset-list: Character Sets
• charset-plist: Character Sets
• charset-priority-list: Character Sets
• charsetp: Character Sets
• charsets supported by a coding system: Lisp and Coding Systems
• check-coding-system: Lisp and Coding Systems
• check-coding-systems-region: Lisp and Coding Systems
• checkdoc: Tips
• checkdoc-current-buffer: Tips
• checkdoc-file: Tips
• checkdoc-minor-mode: Documentation Tips
• checkdoc-package-keywords: Library Headers
• checkdoc-package-keywords-flag: Library Headers
• child process: Processes
• child window: Windows and Frames
• choice, customization types: Splicing into Lists
• circular list: Cons Cells
• circular structure, read syntax: Circular Objects
• cl: Lisp History
• CL note—allocate more storage: Garbage Collection
• CL note—case of letters: Symbol Type
• CL note—default optional arg: Argument List
• CL note—integers vrs eq: Comparison of Numbers
• CL note—interning existing symbol: Creating Symbols
• CL note—lack union, intersection: Sets And Lists
• CL note—no continuable errors: Signaling Errors
• CL note—no setf functions: Adding Generalized Variables
• CL note—only throw in Emacs: Catch and Throw
• CL note—rplaca vs setcar: Modifying Lists
• CL note—special forms compared: Special Forms
• CL note—symbol in obarrays: Creating Symbols
• cl-call-next-method: Generic Functions
• cl-defgeneric: Generic Functions
• cl-defmethod: Generic Functions
• cl-next-method-p: Generic Functions
• classification of file types: Kinds of Files
• classifying events: Classifying Events
• cleanup forms: Cleanups
• clear-abbrev-table: Abbrev Tables
• clear-image-cache: Image Cache
• clear-string: Modifying Strings
• clear-this-command-keys: Command Loop Info
• clear-visited-file-modtime: Modification Time
• click event: Click Events
• clickable buttons in buffers: Buttons
• clickable text: Clickable Text
• clipboard: Window System Selections
• clipboard support (for MS-Windows): Window System Selections
• clone-indirect-buffer: Indirect Buffers
• CLOS: Generic Functions
• closure: Closures
• closures, example of using: Lexical Binding
• clrhash: Hash Access
• coded character set: Character Sets
• codepoint, largest value: Character Codes
• codes, interactive, description of: Interactive Codes
• codespace: Text Representations
• coding conventions in Emacs Lisp: Coding Conventions
• coding standards: Tips
• coding system: Coding Systems
• coding system for operation: Specifying Coding Systems
• coding system, automatically determined: Default Coding Systems
• coding system, validity check: Lisp and Coding Systems
• coding systems for encoding a string: Lisp and Coding Systems
• coding systems for encoding region: Lisp and Coding Systems
• coding systems, priority: Specifying Coding Systems
• coding-system-aliases: Coding System Basics
• coding-system-change-eol-conversion: Lisp and Coding Systems
• coding-system-change-text-conversion: Lisp and Coding Systems
• coding-system-charset-list: Lisp and Coding Systems
• coding-system-eol-type: Lisp and Coding Systems
• coding-system-for-read: Specifying Coding Systems
• coding-system-for-write: Specifying Coding Systems
• coding-system-get: Coding System Basics
• coding-system-list: Lisp and Coding Systems
• coding-system-p: Lisp and Coding Systems
• coding-system-priority-list: Specifying Coding Systems
• coding-system-require-warning: Specifying Coding Systems
• collapse-delayed-warnings: Delayed Warnings
• color names: Color Names
• color-defined-p: Color Names
• color-gray-p: Color Names
• color-supported-p: Color Names
• color-values: Color Names
• colors on text terminals: Text Terminal Colors
• column width: Frame Font
• columns: Columns
• COM1: Serial Ports
• combine-after-change-calls: Change Hooks
• combine-and-quote-strings: Shell Arguments
• combining conditions: Combining Conditions
• command: What Is a Function
• command descriptions: A Sample Function Description
• command history: Command History
• command in keymap: Key Lookup
• command loop: Command Loop
• command loop variables: Command Loop Info
• command loop, recursive: Recursive Editing
• command-debug-status: Internals of Debugger
• command-error-function: Processing of Errors
• command-execute: Interactive Call
• command-history: Command History
• command-line: Command-Line Arguments
• command-line arguments: Command-Line Arguments
• command-line options: Command-Line Arguments
• command-line-args: Command-Line Arguments
• command-line-args-left: Command-Line Arguments
• command-line-functions: Command-Line Arguments
• command-line-processed: Command-Line Arguments
• command-remapping: Remapping Commands
• command-switch-alist: Command-Line Arguments
• commandp: Interactive Call
• commandp example: High-Level Completion
• commands, defining: Defining Commands
• comment style: Syntax Flags
• comment syntax: Syntax Class Table
• comment-end-can-be-escaped: Control Parsing
• commentary, in a Lisp library: Library Headers
• comments: Comments
• comments, Lisp convention for: Comment Tips
• Common Lisp: Lisp History
• compare-buffer-substrings: Comparing Text
• compare-strings: Text Comparison
• compare-window-configurations: Window Configurations
• comparing buffer text: Comparing Text
• comparing file modification time: Modification Time
• comparing numbers: Comparison of Numbers
• comparing time values: Time Calculations
• compilation (Emacs Lisp): Byte Compilation
• compilation functions: Compilation Functions
• compile-defun: Compilation Functions
• compile-time constant: Eval During Compile
• compiled function: Byte-Code Objects
• compiler errors: Compiler Errors
• complete key: Keymap Basics
• completing-read: Minibuffer Completion
• completing-read-function: Minibuffer Completion
• completion: Completion
• completion styles: Completion Variables
• completion table: Basic Completion
• completion table, modifying: Basic Completion
• completion tables, combining: Basic Completion
• completion, file name: File Name Completion
• completion-at-point: Completion in Buffers
• completion-at-point-functions: Completion in Buffers
• completion-auto-help: Completion Commands
• completion-boundaries: Basic Completion
• completion-category-overrides: Completion Variables
• completion-extra-properties: Completion Variables
• completion-ignore-case: Basic Completion
• completion-ignored-extensions: File Name Completion
• completion-in-region: Completion in Buffers
• completion-regexp-list: Basic Completion
• completion-styles: Completion Variables
• completion-styles-alist: Completion Variables
• completion-table-case-fold: Basic Completion
• completion-table-dynamic: Programmed Completion
• completion-table-in-turn: Basic Completion
• completion-table-merge: Basic Completion
• completion-table-subvert: Basic Completion
• completion-table-with-cache: Programmed Completion
• completion-table-with-predicate: Basic Completion
• completion-table-with-quoting: Basic Completion
• completion-table-with-terminator: Basic Completion
• complex arguments: Minibuffers
• complex command: Command History
• composite types (customization): Composite Types
• composition (text property): Special Properties
• composition property, and point display: Adjusting Point
• compute-motion: Screen Lines
• concat: Creating Strings
• concatenating bidirectional strings: Bidirectional Display
• concatenating lists: Rearrangement
• concatenating strings: Creating Strings
• cond: Conditionals
• condition name: Error Symbols
• condition-case: Handling Errors
• condition-case-unless-debug: Handling Errors
• conditional evaluation: Conditionals
• conditional selection of windows: Cyclic Window Ordering
• cons: Building Lists
• cons cells: Building Lists
• cons-cells-consed: Memory Usage
• consing: Building Lists
• consp: List-related Predicates
• constant variables: Defining Variables
• constant variables: Constant Variables
• constrain-to-field: Fields
• content directory, package: Packaging Basics
• continuation lines: Truncation
• continue-process: Signals to Processes
• control character key constants: Changing Key Bindings
• control character printing: Describing Characters
• control characters: Ctl-Char Syntax
• control characters in display: Usual Display
• control characters, reading: Quoted Character Input
• control structures: Control Structures
• Control-X-prefix: Prefix Keys
• controller part, model/view/controller: Abstract Display Example
• controlling terminal: Suspending Emacs
• controlling-tty-p: Suspending Emacs
• conventions for writing major modes: Major Mode Conventions
• conventions for writing minor modes: Minor Mode Conventions
• conversion of strings: String Conversion
• convert buffer position to file byte: Text Representations
• convert file byte to buffer position: Text Representations
• convert-standard-filename: Standard File Names
• converting file names from/to MS-Windows syntax: File Names
• converting numbers: Numeric Conversions
• coordinate, relative to frame: Coordinates and Windows
• coordinates-in-window-p: Coordinates and Windows
• copy-abbrev-table: Abbrev Tables
• copy-alist: Association Lists
• copy-category-table: Categories
• copy-directory: Create/Delete Dirs
• copy-file: Changing Files
• copy-hash-table: Other Hash
• copy-keymap: Creating Keymaps
• copy-marker: Creating Markers
• copy-overlay: Managing Overlays
• copy-region-as-kill: Kill Functions
• copy-sequence: Sequence Functions
• copy-syntax-table: Syntax Table Functions
• copy-tree: Building Lists
• copying alists: Association Lists
• copying bidirectional text, preserve visual order: Bidirectional Display
• copying files: Changing Files
• copying lists: Building Lists
• copying sequences: Sequence Functions
• copying strings: Creating Strings
• copying vectors: Vector Functions
• copysign: Float Basics
• cos: Math Functions
• count-lines: Text Lines
• count-loop: A Sample Function Description
• count-screen-lines: Screen Lines
• count-words: Text Lines
• counting columns: Columns
• coverage testing: Test Coverage
• coverage testing (Edebug): Coverage Testing
• create subprocess: Subprocess Creation
• create-file-buffer: Subroutines of Visiting
• create-fontset-from-fontset-spec: Fontsets
• create-image: Defining Images
• create-lockfiles: File Locks
• creating buffers: Creating Buffers
• creating hash tables: Creating Hash
• creating keymaps: Creating Keymaps
• creating markers: Creating Markers
• creating strings: Creating Strings
• creating, copying and deleting directories: Create/Delete Dirs
• cryptographic hash: Checksum/Hash
• ctl-arrow: Usual Display
• ctl-x-4-map: Prefix Keys
• ctl-x-5-map: Prefix Keys
• ctl-x-map: Prefix Keys
• ctl-x-r-map: Standard Keymaps
• curly quotes: Documentation Tips
• curly quotes: Keys in Documentation
• curly quotes: Formatting Strings
• current binding: Local Variables
• current buffer: Current Buffer
• current buffer mark: The Mark
• current buffer point and mark (Edebug): Edebug Display Update
• current buffer position: Point
• current command: Command Loop Info
• current stack frame: Using Debugger
• current-active-maps: Active Keymaps
• current-bidi-paragraph-direction: Bidirectional Display
• current-buffer: Current Buffer
• current-case-table: Case Tables
• current-column: Columns
• current-fill-column: Margins
• current-frame-configuration: Frame Configurations
• current-global-map: Controlling Active Maps
• current-idle-time: Idle Timers
• current-indentation: Primitive Indent
• current-input-method: Input Methods
• current-input-mode: Input Modes
• current-justification: Filling
• current-kill: Low-Level Kill Ring
• current-left-margin: Margins
• current-local-map: Controlling Active Maps
• current-message: Displaying Messages
• current-minor-mode-maps: Controlling Active Maps
• current-prefix-arg: Prefix Command Arguments
• current-time: Time of Day
• current-time-string: Time of Day
• current-time-zone: Time Zone Rules
• current-window-configuration: Window Configurations
• current-word: Buffer Contents
• currying: Calling Functions
• cursor: Window Point
• cursor (text property): Special Properties
• cursor position for display properties and overlays: Special Properties
• cursor, and frame parameters: Cursor Parameters
• cursor, fringe: Fringe Cursors
• cursor-color, a frame parameter: Font and Color Parameters
• cursor-in-echo-area: Echo Area Customization
• cursor-in-non-selected-windows: Cursor Parameters
• cursor-intangible (text property): Special Properties
• cursor-intangible-mode: Special Properties
• cursor-sensor-functions (text property): Special Properties
• cursor-sensor-mode: Special Properties
• cursor-type: Cursor Parameters
• cursor-type, a frame parameter: Cursor Parameters
• curved quotes: Documentation Tips
• curved quotes: Keys in Documentation
• curved quotes: Formatting Strings
• cust-print: Printing in Edebug
• custom themes: Custom Themes
• custom-add-frequent-value: Variable Definitions
• custom-initialize-delay: Building Emacs
• custom-known-themes: Custom Themes
• custom-reevaluate-setting: Variable Definitions
• custom-set-faces: Applying Customizations
• custom-set-variables: Applying Customizations
• custom-theme-p: Custom Themes
• custom-theme-set-faces: Custom Themes
• custom-theme-set-variables: Custom Themes
• custom-unlispify-remove-prefixes: Group Definitions
• custom-variable-p: Variable Definitions
• customizable variables, how to define: Variable Definitions
• customization groups, defining: Group Definitions
• customization item: Customization
• customization keywords: Common Keywords
• customization types: Customization Types
• customization types, define new: Defining New Types
• customize-package-emacs-version-alist: Common Keywords
• cyclic ordering of windows: Cyclic Window Ordering
• cygwin-convert-file-name-from-windows: File Names
• cygwin-convert-file-name-to-windows: File Names
• data type: Lisp Data Types
• data-directory: Help Functions
• datagrams: Datagrams
• date-leap-year-p: Time Calculations
• date-to-time: Time Parsing
• deactivate-mark: The Mark
• deactivate-mark-hook: The Mark
• debug: Invoking the Debugger
• debug-ignored-errors: Error Debugging
• debug-on-entry: Function Debugging
• debug-on-error: Error Debugging
• debug-on-error use: Processing of Errors
• debug-on-event: Error Debugging
• debug-on-message: Error Debugging
• debug-on-next-call: Internals of Debugger
• debug-on-quit: Infinite Loops
• debug-on-signal: Error Debugging
• debugger: Internals of Debugger
• debugger command list: Debugger Commands
• debugger for Emacs Lisp: Debugger
• debugger, explicit entry: Explicit Debug
• debugger-bury-or-kill: Using Debugger
• debugging errors: Error Debugging
• debugging invalid Lisp syntax: Syntax Errors
• debugging lisp programs: Debugging
• debugging specific functions: Function Debugging
• declare: Declare Form
• declare-function: Declaring Functions
• declaring functions: Declaring Functions
• decode process output: Decoding Output
• decode-char: Character Sets
• decode-coding-inserted-region: Explicit Encoding
• decode-coding-region: Explicit Encoding
• decode-coding-string: Explicit Encoding
• decode-time: Time Conversion
• decoding file formats: Format Conversion
• decoding in coding systems: Explicit Encoding
• decrement field of register: Cons Cell Type
• dedicated window: Dedicated Windows
• def-edebug-spec: Instrumenting Macro Calls
• defalias: Defining Functions
• defalias-fset-function property: Defining Functions
• default argument string: Interactive Codes
• default character height: Frame Font
• default character size: Frame Font
• default character width: Frame Font
• default coding system: Default Coding Systems
• default coding system, functions to determine: Default Coding Systems
• default filter function of a process: Filter Functions
• default font: Frame Font
• default height of character: Frame Font
• default init file: Init File
• default key binding: Format of Keymaps
• default sentinel function of a process: Sentinels
• default value: Default Value
• default value of char-table: Char-Tables
• default width of character: Frame Font
• default-boundp: Default Value
• default-directory: File Name Expansion
• default-file-modes: Changing Files
• default-font-height: Low-Level Font
• default-font-width: Low-Level Font
• default-frame-alist: Initial Parameters
• default-input-method: Input Methods
• default-justification: Filling
• default-minibuffer-frame: Minibuffers and Frames
• default-process-coding-system: Default Coding Systems
• default-text-properties: Examining Properties
• default-toplevel-value: Default Value
• default-value: Default Value
• default.el: Startup Summary
• defconst: Defining Variables
• defcustom: Variable Definitions
• defface: Defining Faces
• defgroup: Group Definitions
• defimage: Defining Images
• define customization group: Group Definitions
• define customization options: Variable Definitions
• define hash comparisons: Defining Hash
• define image: Defining Images
• define new customization types: Defining New Types
• define-abbrev: Defining Abbrevs
• define-abbrev-table: Abbrev Tables
• define-advice: Advising Named Functions
• define-alternatives: Generic Commands
• define-button-type: Button Types
• define-category: Categories
• define-derived-mode: Derived Modes
• define-error: Error Symbols
• define-fringe-bitmap: Customizing Bitmaps
• define-generic-mode: Generic Modes
• define-globalized-minor-mode: Defining Minor Modes
• define-hash-table-test: Defining Hash
• define-inline: Defining Functions
• define-key: Changing Key Bindings
• define-key-after: Modifying Menus
• define-minor-mode: Defining Minor Modes
• define-obsolete-face-alias: Face Functions
• define-obsolete-function-alias: Obsolete Functions
• define-obsolete-variable-alias: Variable Aliases
• define-package: Multi-file Packages
• define-prefix-command: Prefix Keys
• defined-colors: Color Names
• defining a function: Defining Functions
• defining abbrevs: Defining Abbrevs
• defining commands: Defining Commands
• defining customization variables in C: Writing Emacs Primitives
• defining faces: Defining Faces
• defining Lisp variables in C: Writing Emacs Primitives
• defining macros: Defining Macros
• defining menus: Defining Menus
• defining tokens, SMIE: SMIE Lexer
• defining-kbd-macro: Keyboard Macros
• definitions of symbols: Definitions
• defmacro: Defining Macros
• defsubr, Lisp symbol for a primitive: Writing Emacs Primitives
• defsubst: Inline Functions
• deftheme: Custom Themes
• defun: Defining Functions
• DEFUN, C macro to define Lisp primitives: Writing Emacs Primitives
• defun-prompt-regexp: List Motion
• defvar: Defining Variables
• defvar-local: Creating Buffer-Local
• DEFVAR_INT, DEFVAR_LISP, DEFVAR_BOOL: Writing Emacs Primitives
• defvaralias: Variable Aliases
• delay-mode-hooks: Mode Hooks
• delayed warnings: Delayed Warnings
• delayed-warnings-hook: Standard Hooks
• delayed-warnings-hook: Delayed Warnings
• delayed-warnings-list: Delayed Warnings
• delete: Sets And Lists
• delete-and-extract-region: Deletion
• delete-auto-save-file-if-necessary: Auto-Saving
• delete-auto-save-files: Auto-Saving
• delete-backward-char: Deletion
• delete-blank-lines: User-Level Deletion
• delete-by-moving-to-trash: Create/Delete Dirs
• delete-by-moving-to-trash: Changing Files
• delete-char: Deletion
• delete-directory: Create/Delete Dirs
• delete-dups: Sets And Lists
• delete-exited-processes: Deleting Processes
• delete-field: Fields
• delete-file: Changing Files
• delete-frame: Deleting Frames
• delete-frame event: Misc Events
• delete-frame-functions: Deleting Frames
• delete-horizontal-space: User-Level Deletion
• delete-indentation: User-Level Deletion
• delete-minibuffer-contents: Minibuffer Contents
• delete-old-versions: Numbered Backups
• delete-other-windows: Deleting Windows
• delete-overlay: Managing Overlays
• delete-process: Deleting Processes
• delete-region: Deletion
• delete-selection, symbol property: The Mark
• delete-selection-helper: The Mark
• delete-selection-pre-hook: The Mark
• delete-terminal: Multiple Terminals
• delete-terminal-functions: Multiple Terminals
• delete-to-left-margin: Margins
• delete-trailing-whitespace: User-Level Deletion
• delete-window: Deleting Windows
• delete-windows-on: Deleting Windows
• deleting files: Changing Files
• deleting frames: Deleting Frames
• deleting list elements: Sets And Lists
• deleting previous char: Deletion
• deleting processes: Deleting Processes
• deleting text vs killing: Deletion
• deleting whitespace: User-Level Deletion
• deleting windows: Deleting Windows
• delq: Sets And Lists
• dependencies: Packaging Basics
• derived mode: Derived Modes
• derived-mode-p: Derived Modes
• describe characters and events: Describing Characters
• describe-bindings: Scanning Keymaps
• describe-buffer-case-table: Case Tables
• describe-categories: Categories
• describe-current-display-table: Display Tables
• describe-display-table: Display Tables
• describe-mode: Mode Help
• describe-prefix-bindings: Help Functions
• describe-syntax: Syntax Table Functions
• description for interactive codes: Interactive Codes
• description format: Format of Descriptions
• deserializing: Byte Packing
• desktop notifications: Desktop Notifications
• desktop save mode: Desktop Save Mode
• desktop-buffer-mode-handlers: Desktop Save Mode
• desktop-save-buffer: Desktop Save Mode
• destroy-fringe-bitmap: Customizing Bitmaps
• destructive list operations: Modifying Lists
• detect-coding-region: Lisp and Coding Systems
• detect-coding-string: Lisp and Coding Systems
• diagrams, boxed, for lists: Box Diagrams
• dialog boxes: Dialog Boxes
• digit-argument: Prefix Command Arguments
• ding: Beeping
• dir-locals-class-alist: Directory Local Variables
• dir-locals-directory-cache: Directory Local Variables
• dir-locals-file: Directory Local Variables
• dir-locals-set-class-variables: Directory Local Variables
• dir-locals-set-directory-class: Directory Local Variables
• directional overrides: Bidirectional Display
• directory file name: Directory Names
• directory local variables: Directory Local Variables
• directory name: Directory Names
• directory part (of file name): File Name Components
• directory-abbrev-alist: Directory Names
• directory-file-name: Directory Names
• directory-files: Contents of Directories
• directory-files-and-attributes: Contents of Directories
• directory-files-recursively: Contents of Directories
• directory-name-p: Directory Names
• directory-oriented functions: Contents of Directories
• dired-kept-versions: Numbered Backups
• disable-command: Disabling Commands
• disable-point-adjustment: Adjusting Point
• disable-theme: Custom Themes
• disabled: Disabling Commands
• disabled command: Disabling Commands
• disabled-command-function: Disabling Commands
• disabling multibyte: Disabling Multibyte
• disabling undo: Maintaining Undo
• disassemble: Disassembly
• disassembled byte-code: Disassembly
• discard-input: Event Input Misc
• discarding input: Event Input Misc
• dispatch of methods for generic function: Generic Functions
• display (overlay property): Overlay Properties
• display (text property): Display Property
• display action: Choosing Window
• display area: Frame Layout
• display feature testing: Display Feature Testing
• display margins: Display Margins
• display message in echo area: Displaying Messages
• display name on X: Multiple Terminals
• display properties, and bidi reordering of text: Bidirectional Display
• display property, and point display: Adjusting Point
• display specification: Display Property
• display table: Display Tables
• display, a frame parameter: Basic Parameters
• display, abstract: Abstract Display
• display, arbitrary objects: Abstract Display
• display-backing-store: Display Feature Testing
• display-buffer: Choosing Window
• display-buffer-alist: Choosing Window
• display-buffer-at-bottom: Display Action Functions
• display-buffer-base-action: Choosing Window
• display-buffer-below-selected: Display Action Functions
• display-buffer-fallback-action: Choosing Window
• display-buffer-in-previous-window: Display Action Functions
• display-buffer-no-window: Display Action Functions
• display-buffer-overriding-action: Choosing Window
• display-buffer-pop-up-frame: Display Action Functions
• display-buffer-pop-up-window: Display Action Functions
• display-buffer-reuse-window: Display Action Functions
• display-buffer-same-window: Display Action Functions
• display-buffer-use-some-frame: Display Action Functions
• display-buffer-use-some-window: Display Action Functions
• display-color-cells: Display Feature Testing
• display-color-p: Display Feature Testing
• display-completion-list: Completion Commands
• display-delayed-warnings: Delayed Warnings
• display-graphic-p: Display Feature Testing
• display-grayscale-p: Display Feature Testing
• display-images-p: Display Feature Testing
• display-message-or-buffer: Displaying Messages
• display-mm-dimensions-alist: Display Feature Testing
• display-mm-height: Display Feature Testing
• display-mm-width: Display Feature Testing
• display-monitor-attributes-list: Multiple Terminals
• display-mouse-p: Display Feature Testing
• display-pixel-height: Display Feature Testing
• display-pixel-width: Display Feature Testing
• display-planes: Display Feature Testing
• display-popup-menus-p: Display Feature Testing
• display-save-under: Display Feature Testing
• display-screens: Display Feature Testing
• display-selections-p: Display Feature Testing
• display-start position: Window Start and End
• display-supports-face-attributes-p: Display Feature Testing
• display-table-slot: Display Tables
• display-type, a frame parameter: Basic Parameters
• display-visual-class: Display Feature Testing
• display-warning: Warning Basics
• displaying a buffer: Switching Buffers
• displaying faces: Displaying Faces
• displays, multiple: Multiple Terminals
• distinguish interactive calls: Distinguish Interactive
• dnd-protocol-alist: Drag and Drop
• do-auto-save: Auto-Saving
• DOC (documentation) file: Documentation Basics
• doc, customization keyword: Type Keywords
• doc-directory: Accessing Documentation
• Document Object Model: Document Object Model
• documentation: Accessing Documentation
• documentation conventions: Documentation Basics
• documentation for major mode: Mode Help
• documentation notation: Evaluation Notation
• documentation of function: Function Documentation
• documentation strings: Documentation
• documentation strings, conventions and tips: Documentation Tips
• documentation, keys in: Keys in Documentation
• documentation-property: Accessing Documentation
• dolist: Iteration
• DOM: Document Object Model
• dom-node: Document Object Model
• dotimes: Iteration
• dotimes-with-progress-reporter: Progress
• dotted list: Cons Cells
• dotted lists (Edebug): Specification List
• dotted pair notation: Dotted Pair Notation
• double-click events: Repeat Events
• double-click-fuzz: Repeat Events
• double-click-time: Repeat Events
• double-quote in strings: Syntax for Strings
• down-list: List Motion
• downcase: Case Conversion
• downcase-region: Case Changes
• downcase-word: Case Changes
• downcasing in lookup-key: Key Sequence Input
• drag and drop: Drag and Drop
• drag event: Drag Events
• drag-n-drop event: Misc Events
• dribble file: Recording Input
• dump-emacs: Building Emacs
• dumping Emacs: Building Emacs
• dynamic binding: Variable Scoping
• dynamic extent: Variable Scoping
• dynamic libraries: Dynamic Libraries
• dynamic loading of documentation: Docs and Compilation
• dynamic loading of functions: Dynamic Loading
• dynamic modules: Dynamic Modules
• dynamic scope: Variable Scoping
• dynamic-library-alist: Dynamic Libraries
• eager macro expansion: How Programs Do Loading
• easy-menu-define: Easy Menu
• easy-mmode-define-minor-mode: Defining Minor Modes
• echo area: The Echo Area
• echo area customization: Echo Area Customization
• echo-area-clear-hook: Echo Area Customization
• echo-keystrokes: Echo Area Customization
• edebug: Source Breakpoints
• Edebug debugging facility: Edebug
• Edebug execution modes: Edebug Execution Modes
• Edebug specification list: Specification List
• edebug-all-defs: Edebug Options
• edebug-all-forms: Edebug Options
• edebug-continue-kbd-macro: Edebug Options
• edebug-defun: Instrumenting
• edebug-display-freq-count: Coverage Testing
• edebug-eval-macro-args: Instrumenting Macro Calls
• edebug-eval-top-level-form: Instrumenting
• edebug-global-break-condition: Edebug Options
• edebug-initial-mode: Edebug Options
• edebug-on-error: Edebug Options
• edebug-on-quit: Edebug Options
• edebug-print-circle: Printing in Edebug
• edebug-print-length: Printing in Edebug
• edebug-print-level: Printing in Edebug
• edebug-print-trace-after: Trace Buffer
• edebug-print-trace-before: Trace Buffer
• edebug-save-displayed-buffer-points: Edebug Options
• edebug-save-windows: Edebug Options
• edebug-set-global-break-condition: Global Break Condition
• edebug-set-initial-mode: Edebug Execution Modes
• edebug-setup-hook: Edebug Options
• edebug-sit-for-seconds: Edebug Execution Modes
• edebug-temp-display-freq-count: Coverage Testing
• edebug-test-coverage: Edebug Options
• edebug-trace: Edebug Options
• edebug-trace: Trace Buffer
• edebug-tracing: Trace Buffer
• edebug-unwrap-results: Edebug Options
• edge detection, images: Image Descriptors
• edit-and-eval-command: Object from Minibuffer
• editing types: Editing Types
• editor command loop: Command Loop
• eight-bit, a charset: Character Sets
• electric-future-map: A Sample Variable Description
• element (of list): Lists
• elements of sequences: Sequence Functions
• elp.el: Profiling
• elt: Sequence Functions
• Emacs event standard notation: Describing Characters
• Emacs process run time: Processor Run Time
• emacs, a charset: Character Sets
• emacs-build-time: Version Info
• emacs-init-time: Processor Run Time
• emacs-internal coding system: Coding System Basics
• emacs-lisp-docstring-fill-column: Documentation Tips
• emacs-major-version: Version Info
• emacs-minor-version: Version Info
• emacs-pid: System Environment
• emacs-save-session-functions: Session Management
• emacs-session-restore: Session Management
• emacs-startup-hook: Init File
• emacs-uptime: Processor Run Time
• emacs-version: Version Info
• emacs_module_init: Dynamic Modules
• emacsclient, getting a backtrace: Error Debugging
• EMACSLOADPATH environment variable: Library Search
• embedded widgets: Xwidgets
• empty lines, indicating: Fringe Indicators
• empty list: Box Diagrams
• empty overlay: Managing Overlays
• empty region: The Region
• emulation-mode-map-alists: Controlling Active Maps
• enable-command: Disabling Commands
• enable-dir-local-variables: Directory Local Variables
• enable-local-eval: File Local Variables
• enable-local-variables: File Local Variables
• enable-multibyte-characters: Disabling Multibyte
• enable-multibyte-characters: Text Representations
• enable-recursive-minibuffers: Recursive Mini
• enable-theme: Custom Themes
• encapsulation, ewoc: Abstract Display
• encode-char: Character Sets
• encode-coding-region: Explicit Encoding
• encode-coding-string: Explicit Encoding
• encode-time: Time Conversion
• encoding file formats: Format Conversion
• encoding in coding systems: Explicit Encoding
• encrypted network connections: Network
• end of line in regexp: Regexp Special
• end-of-buffer: Buffer End Motion
• end-of-defun: List Motion
• end-of-defun-function: List Motion
• end-of-file: Input Functions
• end-of-line: Text Lines
• end-of-line conversion: Coding System Basics
• endianness: Bindat Spec
• environment: Intro Eval
• environment variable access: System Environment
• environment variables, subprocesses: Subprocess Creation
• eobp: Near Point
• EOL conversion: Coding System Basics
• eol conversion of coding system: Lisp and Coding Systems
• eol type of coding system: Lisp and Coding Systems
• eolp: Near Point
• epoch: Time of Day
• eq: Equality Predicates
• eql: Comparison of Numbers
• equal: Equality Predicates
• equal-including-properties: Equality Predicates
• equality: Equality Predicates
• erase-buffer: Deletion
• error: Signaling Errors
• error cleanup: Cleanups
• error debugging: Error Debugging
• error description: Handling Errors
• error display: The Echo Area
• error handler: Handling Errors
• error in debug: Invoking the Debugger
• error message notation: Error Messages
• error name: Error Symbols
• error symbol: Error Symbols
• error-conditions: Error Symbols
• error-message-string: Handling Errors
• errors: Errors
• esc-map: Prefix Keys
• ESC-prefix: Prefix Keys
• escape (ASCII character): Basic Char Syntax
• escape characters: Output Variables
• escape characters in printing: Output Functions
• escape sequence: Basic Char Syntax
• eval: Eval
• eval during compilation: Eval During Compile
• eval, and debugging: Internals of Debugger
• eval-and-compile: Eval During Compile
• eval-buffer: Eval
• eval-buffer (Edebug): Instrumenting
• eval-current-buffer: Eval
• eval-current-buffer (Edebug): Instrumenting
• eval-defun (Edebug): Instrumenting
• eval-expression (Edebug): Instrumenting
• eval-expression-debug-on-error: Error Debugging
• eval-expression-print-length: Output Variables
• eval-expression-print-level: Output Variables
• eval-minibuffer: Object from Minibuffer
• eval-region: Eval
• eval-region (Edebug): Instrumenting
• eval-when-compile: Eval During Compile
• evaluated expression argument: Interactive Codes
• evaluation: Evaluation
• evaluation error: Local Variables
• evaluation list group: Eval List
• evaluation notation: Evaluation Notation
• evaluation of buffer contents: Eval
• evaluation of special forms: Special Forms
• evaporate (overlay property): Overlay Properties
• even-window-sizes: Choosing Window Options
• event printing: Describing Characters
• event translation: Event Mod
• event type: Classifying Events
• event, reading only one: Reading One Event
• event-basic-type: Classifying Events
• event-click-count: Repeat Events
• event-convert-list: Classifying Events
• event-end: Accessing Mouse
• event-modifiers: Classifying Events
• event-start: Accessing Mouse
• eventp: Input Events
• events: Input Events
• ewoc: Abstract Display
• ewoc-buffer: Abstract Display Functions
• ewoc-collect: Abstract Display Functions
• ewoc-create: Abstract Display Functions
• ewoc-data: Abstract Display Functions
• ewoc-delete: Abstract Display Functions
• ewoc-enter-after: Abstract Display Functions
• ewoc-enter-before: Abstract Display Functions
• ewoc-enter-first: Abstract Display Functions
• ewoc-enter-last: Abstract Display Functions
• ewoc-filter: Abstract Display Functions
• ewoc-get-hf: Abstract Display Functions
• ewoc-goto-next: Abstract Display Functions
• ewoc-goto-node: Abstract Display Functions
• ewoc-goto-prev: Abstract Display Functions
• ewoc-invalidate: Abstract Display Functions
• ewoc-locate: Abstract Display Functions
• ewoc-location: Abstract Display Functions
• ewoc-map: Abstract Display Functions
• ewoc-next: Abstract Display Functions
• ewoc-nth: Abstract Display Functions
• ewoc-prev: Abstract Display Functions
• ewoc-refresh: Abstract Display Functions
• ewoc-set-data: Abstract Display Functions
• ewoc-set-hf: Abstract Display Functions
• examining text properties: Examining Properties
• examining the interactive form: Using Interactive
• examining windows: Buffers and Windows
• examples of using interactive: Interactive Examples
• excess close parentheses: Excess Close
• excess open parentheses: Excess Open
• excursion: Excursions
• exec-directory: Subprocess Creation
• exec-path: Subprocess Creation
• exec-suffixes: Subprocess Creation
• executable-find: Locating Files
• execute program: Subprocess Creation
• execute with prefix argument: Interactive Call
• execute-extended-command: Interactive Call
• execute-kbd-macro: Keyboard Macros
• executing-kbd-macro: Keyboard Macros
• execution speed: Compilation Tips
• exit: Recursive Editing
• exit recursive editing: Recursive Editing
• exit-minibuffer: Minibuffer Commands
• exit-recursive-edit: Recursive Editing
• exiting Emacs: Getting Out
• exp: Math Functions
• expand-abbrev: Abbrev Expansion
• expand-file-name: File Name Expansion
• expanding abbrevs: Abbrev Expansion
• expansion of file names: File Name Expansion
• expansion of macros: Expansion
• explicit selective display: Selective Display
• expression: Intro Eval
• expt: Math Functions
• extended file attributes: Extended Attributes
• extended menu item: Extended Menu Items
• extended-command-history: Minibuffer History
• extent: Variable Scoping
• external border: Frame Layout
• external menu bar: Frame Layout
• external tool bar: Frame Layout
• extra slots of char-table: Char-Tables
• extra-keyboard-modifiers: Event Mod
• face (button property): Button Properties
• face (overlay property): Overlay Properties
• face (text property): Special Properties
• face alias: Face Functions
• face attributes: Face Attributes
• face attributes, access and modification: Attribute Functions
• face codes of text: Special Properties
• face merging: Displaying Faces
• face name: Faces
• face remapping: Face Remapping
• face spec: Defining Faces
• face-all-attributes: Attribute Functions
• face-attribute: Attribute Functions
• face-attribute-relative-p: Attribute Functions
• face-background: Attribute Functions
• face-bold-p: Attribute Functions
• face-differs-from-default-p: Face Functions
• face-documentation: Face Functions
• face-documentation: Accessing Documentation
• face-equal: Face Functions
• face-font: Attribute Functions
• face-font-family-alternatives: Font Selection
• face-font-registry-alternatives: Font Selection
• face-font-rescale-alist: Font Selection
• face-font-selection-order: Font Selection
• face-foreground: Attribute Functions
• face-id: Face Functions
• face-inverse-video-p: Attribute Functions
• face-italic-p: Attribute Functions
• face-list: Face Functions
• face-name-history: Minibuffer History
• face-remap-add-relative: Face Remapping
• face-remap-remove-relative: Face Remapping
• face-remap-reset-base: Face Remapping
• face-remap-set-base: Face Remapping
• face-remapping-alist: Face Remapping
• face-spec-set: Defining Faces
• face-stipple: Attribute Functions
• face-underline-p: Attribute Functions
• facemenu-keymap: Prefix Keys
• facep: Faces
• faces: Faces
• faces for font lock: Faces for Font Lock
• faces, automatic choice: Auto Faces
• false: nil and t
• fboundp: Function Cells
• fceiling: Rounding Operations
• feature-unload-function: Unloading
• featurep: Named Features
• features: Named Features
• fetch-bytecode: Dynamic Loading
• ffloor: Rounding Operations
• field (overlay property): Overlay Properties
• field (text property): Special Properties
• field width: Formatting Strings
• field-beginning: Fields
• field-end: Fields
• field-string: Fields
• field-string-no-properties: Fields
• fields: Fields
• fifo data structure: Rings
• file accessibility: Testing Accessibility
• file age: File Attributes
• file attributes: File Attributes
• file classification: Kinds of Files
• file contents, and default coding system: Default Coding Systems
• file format conversion: Format Conversion
• file handler: Magic File Names
• file hard link: Changing Files
• file local variables: Security Considerations
• file local variables: File Local Variables
• file locks: File Locks
• file mode specification error: Auto Major Mode
• file modes: Testing Accessibility
• file modes and MS-DOS: Testing Accessibility
• file modes, setting: Changing Files
• file modification time: File Attributes
• file name abbreviations: Directory Names
• file name completion subroutines: File Name Completion
• file name of buffer: Buffer File Name
• file name of directory: Directory Names
• file name, and default coding system: Default Coding Systems
• file names: File Names
• file names in directory: Contents of Directories
• file names, trailing whitespace: Information about Files
• file notifications: File Notifications
• file open error: Subroutines of Visiting
• file permissions: Testing Accessibility
• file permissions, setting: Changing Files
• file symbolic links: Kinds of Files
• file with multiple names: Changing Files
• file, information about: Information about Files
• file-accessible-directory-p: Testing Accessibility
• file-acl: Extended Attributes
• file-already-exists: Changing Files
• file-attributes: File Attributes
• file-chase-links: Truenames
• file-coding-system-alist: Default Coding Systems
• file-directory-p: Kinds of Files
• file-equal-p: Truenames
• file-error: How Programs Do Loading
• file-executable-p: Testing Accessibility
• file-exists-p: Testing Accessibility
• file-expand-wildcards: Contents of Directories
• file-extended-attributes: Extended Attributes
• file-in-directory-p: Truenames
• file-local-copy: Magic File Names
• file-local-variables-alist: File Local Variables
• file-locked: File Locks
• file-locked-p: File Locks
• file-modes: Testing Accessibility
• file-modes-symbolic-to-number: Changing Files
• file-name encoding, MS-Windows: Encoding and I/O
• file-name-absolute-p: Relative File Names
• file-name-all-completions: File Name Completion
• file-name-as-directory: Directory Names
• file-name-base: File Name Components
• file-name-coding-system: Encoding and I/O
• file-name-completion: File Name Completion
• file-name-directory: File Name Components
• file-name-extension: File Name Components
• file-name-handler-alist: Magic File Names
• file-name-history: Minibuffer History
• file-name-nondirectory: File Name Components
• file-name-sans-extension: File Name Components
• file-name-sans-versions: File Name Components
• file-newer-than-file-p: File Attributes
• file-newest-backup: Backup Names
• file-nlinks: File Attributes
• file-notify-add-watch: File Notifications
• file-notify-rm-watch: File Notifications
• file-notify-valid-p: File Notifications
• file-ownership-preserved-p: Testing Accessibility
• file-precious-flag: Saving Buffers
• file-readable-p: Testing Accessibility
• file-regular-p: Kinds of Files
• file-relative-name: Relative File Names
• file-remote-p: Magic File Names
• file-selinux-context: Extended Attributes
• file-supersession: Modification Time
• file-symlink-p: Kinds of Files
• file-truename: Truenames
• file-writable-p: Testing Accessibility
• filepos-to-bufferpos: Text Representations
• fill-column: Margins
• fill-context-prefix: Adaptive Fill
• fill-forward-paragraph-function: Filling
• fill-individual-paragraphs: Filling
• fill-individual-varying-indent: Filling
• fill-nobreak-predicate: Margins
• fill-paragraph: Filling
• fill-paragraph-function: Filling
• fill-prefix: Margins
• fill-region: Filling
• fill-region-as-paragraph: Filling
• fillarray: Array Functions
• filling text: Filling
• filling, automatic: Auto Filling
• filter function: Filter Functions
• filter multibyte flag, of process: Decoding Output
• filter-buffer-substring: Buffer Contents
• filter-buffer-substring-function: Buffer Contents
• filter-buffer-substring-functions: Buffer Contents
• filtering killed text: Kill Functions
• filtering sequences: Sequence Functions
• find file in path: Locating Files
• find library: Library Search
• find-auto-coding: Default Coding Systems
• find-backup-file-name: Backup Names
• find-buffer-visiting: Buffer File Name
• find-charset-region: Scanning Charsets
• find-charset-string: Scanning Charsets
• find-coding-systems-for-charsets: Lisp and Coding Systems
• find-coding-systems-region: Lisp and Coding Systems
• find-coding-systems-string: Lisp and Coding Systems
• find-file: Visiting Functions
• find-file-hook: Visiting Functions
• find-file-literally: Visiting Functions
• find-file-name-handler: Magic File Names
• find-file-noselect: Visiting Functions
• find-file-not-found-functions: Visiting Functions
• find-file-other-window: Visiting Functions
• find-file-read-only: Visiting Functions
• find-file-wildcards: Visiting Functions
• find-font: Low-Level Font
• find-image: Defining Images
• find-operation-coding-system: Default Coding Systems
• find-word-boundary-function-table: Word Motion
• finding files: Visiting Files
• finding windows: Cyclic Window Ordering
• first-change-hook: Change Hooks
• fit-frame-to-buffer: Resizing Windows
• fit-frame-to-buffer-margins: Resizing Windows
• fit-frame-to-buffer-sizes: Resizing Windows
• fit-window-to-buffer: Resizing Windows
• fit-window-to-buffer-horizontally: Resizing Windows
• fixed-size window: Window Sizes
• fixup-whitespace: User-Level Deletion
• flags in format specifications: Formatting Strings
• float: Numeric Conversions
• float-e: Math Functions
• float-output-format: Output Variables
• float-pi: Math Functions
• float-time: Time of Day
• floating-point functions: Math Functions
• floatp: Predicates on Numbers
• floats-consed: Memory Usage
• floor: Numeric Conversions
• flowcontrol, in serial connections: Serial Ports
• flushing input: Event Input Misc
• fmakunbound: Function Cells
• fn in function's documentation string: Autoload
• focus event: Focus Events
• focus-follows-mouse: Input Focus
• focus-in-hook: Standard Hooks
• focus-in-hook: Input Focus
• focus-out-hook: Standard Hooks
• focus-out-hook: Input Focus
• follow links: Clickable Text
• follow-link (button property): Button Properties
• follow-link (text or overlay property): Clickable Text
• following-char: Near Point
• font and color, frame parameters: Font and Color Parameters
• font entity: Low-Level Font
• font information for layout: Low-Level Font
• font lock faces: Faces for Font Lock
• Font Lock mode: Font Lock Mode
• font lookup: Font Lookup
• font object: Low-Level Font
• font property: Low-Level Font
• font registry: Low-Level Font
• font selection: Font Selection
• font spec: Low-Level Font
• font, a frame parameter: Font and Color Parameters
• font-at: Low-Level Font
• font-backend, a frame parameter: Font and Color Parameters
• font-face-attributes: Low-Level Font
• font-family-list: Face Attributes
• font-get: Low-Level Font
• font-info: Low-Level Font
• font-lock-add-keywords: Customizing Keywords
• font-lock-builtin-face: Faces for Font Lock
• font-lock-comment-delimiter-face: Faces for Font Lock
• font-lock-comment-face: Faces for Font Lock
• font-lock-constant-face: Faces for Font Lock
• font-lock-defaults: Font Lock Basics
• font-lock-doc-face: Faces for Font Lock
• font-lock-ensure &optional beg end: Font Lock Basics
• font-lock-ensure-function: Other Font Lock Variables
• font-lock-extend-after-change-region-function: Region to Refontify
• font-lock-extra-managed-props: Other Font Lock Variables
• font-lock-face (text property): Special Properties
• font-lock-flush &optional beg end: Font Lock Basics
• font-lock-flush-function: Other Font Lock Variables
• font-lock-fontify-buffer: Font Lock Basics
• font-lock-fontify-buffer-function: Other Font Lock Variables
• font-lock-fontify-region beg end &optional loudly: Font Lock Basics
• font-lock-fontify-region-function: Other Font Lock Variables
• font-lock-function-name-face: Faces for Font Lock
• font-lock-keyword-face: Faces for Font Lock
• font-lock-keywords: Search-based Fontification
• font-lock-keywords-case-fold-search: Search-based Fontification
• font-lock-keywords-only: Syntactic Font Lock
• font-lock-mark-block-function: Other Font Lock Variables
• font-lock-multiline: Font Lock Multiline
• font-lock-negation-char-face: Faces for Font Lock
• font-lock-preprocessor-face: Faces for Font Lock
• font-lock-remove-keywords: Customizing Keywords
• font-lock-string-face: Faces for Font Lock
• font-lock-syntactic-face-function: Syntactic Font Lock
• font-lock-syntax-table: Syntactic Font Lock
• font-lock-type-face: Faces for Font Lock
• font-lock-unfontify-buffer: Font Lock Basics
• font-lock-unfontify-buffer-function: Other Font Lock Variables
• font-lock-unfontify-region beg end: Font Lock Basics
• font-lock-unfontify-region-function: Other Font Lock Variables
• font-lock-variable-name-face: Faces for Font Lock
• font-lock-warning-face: Faces for Font Lock
• font-put: Low-Level Font
• font-spec: Low-Level Font
• font-xlfd-name: Low-Level Font
• fontification-functions: Auto Faces
• fontified (text property): Special Properties
• fontp: Low-Level Font
• fontset: Fontsets
• foo: A Sample Function Description
• for: Argument Evaluation
• force coding system for operation: Specifying Coding Systems
• force entry to debugger: Explicit Debug
• force-mode-line-update: Mode Line Basics
• force-window-update: Forcing Redisplay
• forcing redisplay: Forcing Redisplay
• foreground-color, a frame parameter: Font and Color Parameters
• form: Intro Eval
• format: Formatting Strings
• format definition: Format Conversion Round-Trip
• format of keymaps: Format of Keymaps
• format specification: Formatting Strings
• format, customization keyword: Type Keywords
• format-alist: Format Conversion Round-Trip
• format-find-file: Format Conversion Round-Trip
• format-insert-file: Format Conversion Round-Trip
• format-message: Formatting Strings
• format-mode-line: Emulating Mode Line
• format-network-address: Misc Network
• format-seconds: Time Parsing
• format-time-string: Time Parsing
• format-write-file: Format Conversion Round-Trip
• formatting strings: Formatting Strings
• formatting time values: Time Parsing
• formfeed: Basic Char Syntax
• forward-button: Button Buffer Commands
• forward-char: Character Motion
• forward-comment: Motion via Parsing
• forward-line: Text Lines
• forward-list: List Motion
• forward-sexp: List Motion
• forward-to-indentation: Motion by Indent
• forward-word: Word Motion
• forward-word-strictly: Word Motion
• frame: Frames
• frame configuration: Frame Configurations
• frame creation: Creating Frames
• frame geometry: Frame Geometry
• frame layout: Frame Layout
• frame layout parameters: Layout Parameters
• frame parameters: Frame Parameters
• frame parameters for windowed displays: Window Frame Parameters
• frame position: Position Parameters
• frame position: Size and Position
• frame position: Frame Geometry
• frame size: Size and Position
• frame size: Frame Geometry
• frame title: Frame Titles
• frame visibility: Visibility of Frames
• frame without a minibuffer: Minibuffers and Frames
• frame, which buffers to display: Buffer Parameters
• frame-alpha-lower-limit: Font and Color Parameters
• frame-auto-hide-function: Quitting Windows
• frame-char-height: Frame Font
• frame-char-width: Frame Font
• frame-current-scroll-bars: Scroll Bars
• frame-edges: Frame Layout
• frame-first-window: Windows and Frames
• frame-geometry: Frame Layout
• frame-height: Size and Position
• frame-inherited-parameters: Creating Frames
• frame-inhibit-implied-resize: Implied Frame Resizing
• frame-list: Finding All Frames
• frame-live-p: Deleting Frames
• frame-monitor-attributes: Multiple Terminals
• frame-parameter: Parameter Access
• frame-parameters: Parameter Access
• frame-pixel-height: Size and Position
• frame-pixel-width: Size and Position
• frame-pointer-visible-p: Mouse Position
• frame-position: Size and Position
• frame-relative coordinate: Coordinates and Windows
• frame-resize-pixelwise: Size and Position
• frame-root-window: Windows and Frames
• frame-scroll-bar-height: Scroll Bars
• frame-scroll-bar-width: Scroll Bars
• frame-selected-window: Selecting Windows
• frame-terminal: Frames
• frame-text-height: Size and Position
• frame-text-width: Size and Position
• frame-title-format: Frame Titles
• frame-visible-p: Visibility of Frames
• frame-width: Size and Position
• framep: Frames
• frames, scanning all: Finding All Frames
• free list: Garbage Collection
• free variable: Using Lexical Binding
• frequency counts: Coverage Testing
• frexp: Float Basics
• fringe bitmaps: Fringe Bitmaps
• fringe bitmaps, customizing: Customizing Bitmaps
• fringe cursors: Fringe Cursors
• fringe indicators: Fringe Indicators
• fringe-bitmaps-at-pos: Fringe Bitmaps
• fringe-cursor-alist: Fringe Cursors
• fringe-indicator-alist: Fringe Indicators
• fringes: Fringes
• fringes, and empty line indication: Fringe Indicators
• fringes-outside-margins: Fringe Size/Pos
• fround: Rounding Operations
• fset: Function Cells
• ftp-login: Cleanups
• ftruncate: Rounding Operations
• full keymap: Format of Keymaps
• full-height window: Window Sizes
• full-width window: Window Sizes
• fullboth frames: Size Parameters
• fullheight frames: Size Parameters
• fullscreen, a frame parameter: Size Parameters
• fullscreen-restore, a frame parameter: Size Parameters
• fullwidth frames: Size Parameters
• funcall: Calling Functions
• funcall, and debugging: Internals of Debugger
• funcall-interactively: Interactive Call
• function: Anonymous Functions
• function aliases: Defining Functions
• function call: Function Forms
• function call debugging: Function Debugging
• function cell: Symbol Components
• function cell in autoload: Autoload
• function declaration: Declaring Functions
• function definition: Function Names
• function descriptions: A Sample Function Description
• function form evaluation: Function Forms
• function input stream: Input Streams
• function invocation: Calling Functions
• function keys: Function Keys
• function name: Function Names
• function output stream: Output Streams
• function quoting: Anonymous Functions
• function safety: Function Safety
• function-documentation property: Documentation Basics
• function-get: Symbol Plists
• function-put: Symbol Plists
• functionals: Calling Functions
• functionp: What Is a Function
• functions in modes: Major Mode Conventions
• functions, making them interactive: Defining Commands
• fundamental-mode: Major Modes
• fundamental-mode-abbrev-table: Standard Abbrev Tables
• gamma correction: Font and Color Parameters
• gap-position: Buffer Gap
• gap-size: Buffer Gap
• garbage collection: Garbage Collection
• garbage collection protection: Writing Emacs Primitives
• garbage-collect: Garbage Collection
• garbage-collection-messages: Garbage Collection
• gc-cons-percentage: Garbage Collection
• gc-cons-threshold: Garbage Collection
• gc-elapsed: Garbage Collection
• gcs-done: Garbage Collection
• generalized variable: Generalized Variables
• generate-autoload-cookie: Autoload
• generate-new-buffer: Creating Buffers
• generate-new-buffer-name: Buffer Names
• generated-autoload-file: Autoload
• generators: Generators
• generic commands: Generic Commands
• generic functions: Generic Functions
• generic mode: Generic Modes
• geometry specification: Geometry
• get: Symbol Plists
• get, defcustom keyword: Variable Definitions
• get-buffer: Buffer Names
• get-buffer-create: Creating Buffers
• get-buffer-process: Process Buffers
• get-buffer-window: Buffers and Windows
• get-buffer-window-list: Buffers and Windows
• get-buffer-xwidgets: Xwidgets
• get-byte: Character Codes
• get-char-code-property: Character Properties
• get-char-property: Examining Properties
• get-char-property-and-overlay: Examining Properties
• get-charset-property: Character Sets
• get-device-terminal: Multiple Terminals
• get-file-buffer: Buffer File Name
• get-internal-run-time: Processor Run Time
• get-largest-window: Cyclic Window Ordering
• get-load-suffixes: Load Suffixes
• get-lru-window: Cyclic Window Ordering
• get-mru-window: Cyclic Window Ordering
• get-pos-property: Examining Properties
• get-process: Process Information
• get-register: Registers
• get-text-property: Examining Properties
• get-unused-category: Categories
• get-window-with-predicate: Cyclic Window Ordering
• getenv: System Environment
• gethash: Hash Access
• GID: User Identification
• global binding: Local Variables
• global break condition: Global Break Condition
• global keymap: Active Keymaps
• global variable: Global Variables
• global-abbrev-table: Standard Abbrev Tables
• global-buffers-menu-map: Standard Keymaps
• global-disable-point-adjustment: Adjusting Point
• global-key-binding: Functions for Key Lookup
• global-map: Controlling Active Maps
• global-mode-string: Mode Line Variables
• global-set-key: Key Binding Commands
• global-unset-key: Key Binding Commands
• glyph: Glyphs
• glyph code: Glyphs
• glyph-char: Glyphs
• glyph-face: Glyphs
• glyph-table: Glyphs
• glyphless characters: Glyphless Chars
• glyphless-char-display: Glyphless Chars
• glyphless-char-display-control: Glyphless Chars
• goto-char: Character Motion
• goto-map: Prefix Keys
• grammar, SMIE: SMIE Grammar
• graphical display: Frames
• graphical terminal: Frames
• group, customization keyword: Common Keywords
• group-gid: User Identification
• group-real-gid: User Identification
• gui-get-selection: Window System Selections
• gui-set-selection: Window System Selections
• gv-define-expander: Adding Generalized Variables
• gv-define-setter: Adding Generalized Variables
• gv-define-simple-setter: Adding Generalized Variables
• gv-letplace: Adding Generalized Variables
• hack-dir-local-variables: Directory Local Variables
• hack-dir-local-variables-non-file-buffer: Directory Local Variables
• hack-local-variables: File Local Variables
• hack-local-variables-hook: File Local Variables
• handle-shift-selection: The Mark
• handle-switch-frame: Input Focus
• handling errors: Handling Errors
• hardening: Security Considerations
• hash code: Defining Hash
• hash notation: Printed Representation
• hash table access: Hash Access
• hash tables: Hash Tables
• hash, cryptographic: Checksum/Hash
• hash-table-count: Other Hash
• hash-table-p: Other Hash
• hash-table-rehash-size: Other Hash
• hash-table-rehash-threshold: Other Hash
• hash-table-size: Other Hash
• hash-table-test: Other Hash
• hash-table-weakness: Other Hash
• hashing: Creating Symbols
• header comments: Library Headers
• header line (of a window): Header Lines
• header-line prefix key: Key Sequence Input
• header-line-format: Header Lines
• height of a line: Line Height
• height of a window: Window Sizes
• height spec: Line Height
• height, a frame parameter: Size Parameters
• help for major mode: Mode Help
• help functions: Help Functions
• help-buffer: Help Functions
• help-char: Help Functions
• help-command: Help Functions
• help-echo (overlay property): Overlay Properties
• help-echo (text property): Special Properties
• help-echo event: Misc Events
• help-echo, customization keyword: Type Keywords
• help-event-list: Help Functions
• help-form: Help Functions
• help-index (button property): Button Properties
• help-map: Help Functions
• help-setup-xref: Help Functions
• help-window-select: Help Functions
• Helper-describe-bindings: Help Functions
• Helper-help: Help Functions
• Helper-help-map: Help Functions
• hex numbers: Integer Basics
• hidden buffers: Buffer Names
• history list: Minibuffer History
• history of commands: Command History
• history-add-new-input: Minibuffer History
• history-delete-duplicates: Minibuffer History
• history-length: Minibuffer History
• HOME environment variable: Subprocess Creation
• hook variables, list of: Standard Hooks
• hooks: Hooks
• hooks for changing a character: Special Properties
• hooks for loading: Hooks for Loading
• hooks for motion of point: Special Properties
• hooks for text changes: Change Hooks
• hooks for window operations: Window Hooks
• horizontal combination: Windows and Frames
• horizontal position: Columns
• horizontal scrolling: Horizontal Scrolling
• horizontal-scroll-bar: Scroll Bars
• horizontal-scroll-bar prefix key: Key Sequence Input
• horizontal-scroll-bar-mode: Scroll Bars
• horizontal-scroll-bars, a frame parameter: Layout Parameters
• horizontal-scroll-bars-available-p: Scroll Bars
• how to visit files: Visiting Functions
• HTML DOM: Document Object Model
• hyper characters: Other Char Bits
• hyperlinks in documentation strings: Documentation Tips
• icon-left, a frame parameter: Position Parameters
• icon-name, a frame parameter: Management Parameters
• icon-title-format: Frame Titles
• icon-top, a frame parameter: Position Parameters
• icon-type, a frame parameter: Management Parameters
• iconified frame: Visibility of Frames
• iconify-frame: Visibility of Frames
• iconify-frame event: Misc Events
• identity: Calling Functions
• idle timers: Idle Timers
• idleness: Idle Timers
• IEEE floating point: Float Basics
• if: Conditionals
• ignore: Calling Functions
• ignore-errors: Handling Errors
• ignore-window-parameters: Window Parameters
• ignored-local-variables: File Local Variables
• image animation: Multi-Frame Images
• image cache: Image Cache
• image descriptor: Image Descriptors
• image formats: Image Formats
• image frames: Multi-Frame Images
• image maps: Image Descriptors
• image slice: Showing Images
• image types: Image Formats
• image-animate: Multi-Frame Images
• image-animate-timer: Multi-Frame Images
• image-cache-eviction-delay: Image Cache
• image-current-frame: Multi-Frame Images
• image-default-frame-delay: Multi-Frame Images
• image-flush: Image Cache
• image-format-suffixes: ImageMagick Images
• image-load-path: Defining Images
• image-load-path-for-library: Defining Images
• image-mask-p: Image Descriptors
• image-minimum-frame-delay: Multi-Frame Images
• image-multi-frame-p: Multi-Frame Images
• image-show-frame: Multi-Frame Images
• image-size: Showing Images
• image-type-available-p: Image Formats
• image-types: Image Formats
• ImageMagick images: ImageMagick Images
• imagemagick-enabled-types: ImageMagick Images
• imagemagick-types: ImageMagick Images
• imagemagick-types-inhibit: ImageMagick Images
• images in buffers: Images
• images, support for more formats: ImageMagick Images
• Imenu: Imenu
• imenu-add-to-menubar: Imenu
• imenu-case-fold-search: Imenu
• imenu-create-index-function: Imenu
• imenu-extract-index-name-function: Imenu
• imenu-generic-expression: Imenu
• imenu-prev-index-position-function: Imenu
• imenu-syntax-alist: Imenu
• implicit progn: Sequencing
• implied frame resizing: Implied Frame Resizing
• implied resizing of frame: Implied Frame Resizing
• inactive minibuffer: Intro to Minibuffers
• inc: Simple Macro
• indefinite extent: Variable Scoping
• indent-according-to-mode: Mode-Specific Indent
• indent-code-rigidly: Region Indent
• indent-for-tab-command: Mode-Specific Indent
• indent-line-function: Mode-Specific Indent
• indent-region: Region Indent
• indent-region-function: Region Indent
• indent-relative: Relative Indent
• indent-relative-maybe: Relative Indent
• indent-rigidly: Region Indent
• indent-tabs-mode: Primitive Indent
• indent-to: Primitive Indent
• indent-to-left-margin: Margins
• indentation: Indentation
• indentation rules, SMIE: SMIE Indentation
• indicate-buffer-boundaries: Fringe Indicators
• indicate-empty-lines: Fringe Indicators
• indicators, fringe: Fringe Indicators
• indirect buffers: Indirect Buffers
• indirect specifications: Specification List
• indirect-function: Function Indirection
• indirect-variable: Variable Aliases
• indirection for functions: Function Indirection
• infinite loops: Infinite Loops
• infinite recursion: Local Variables
• infinity: Float Basics
• inheritance, for faces: Face Attributes
• inheritance, keymap: Inheritance and Keymaps
• inheritance, syntax table: Syntax Basics
• inheritance, text property: Sticky Properties
• inhibit-default-init: Init File
• inhibit-eol-conversion: Specifying Coding Systems
• inhibit-field-text-motion: Word Motion
• inhibit-file-name-handlers: Magic File Names
• inhibit-file-name-operation: Magic File Names
• inhibit-iso-escape-detection: Lisp and Coding Systems
• inhibit-local-variables-regexps: Auto Major Mode
• inhibit-local-variables-regexps: File Local Variables
• inhibit-message: Displaying Messages
• inhibit-modification-hooks: Change Hooks
• inhibit-null-byte-detection: Lisp and Coding Systems
• inhibit-point-motion-hooks: Special Properties
• inhibit-quit: Quitting
• inhibit-read-only: Read Only Buffers
• inhibit-read-only (text property): Special Properties
• inhibit-splash-screen: Startup Summary
• inhibit-startup-echo-area-message: Startup Summary
• inhibit-startup-message: Startup Summary
• inhibit-startup-screen: Startup Summary
• inhibit-x-resources: Resources
• init file: Init File
• init-file-user: User Identification
• init.el: Init File
• initial-buffer-choice: Startup Summary
• initial-environment: System Environment
• initial-frame-alist: Initial Parameters
• initial-major-mode: Auto Major Mode
• initial-scratch-message: Startup Summary
• initial-window-system: Window Systems
• initial-window-system, and startup: Startup Summary
• initialization of Emacs: Startup Summary
• initialize, defcustom keyword: Variable Definitions
• inline completion: Completion in Buffers
• inline functions: Inline Functions
• inline-const-p: Defining Functions
• inline-const-val: Defining Functions
• inline-error: Defining Functions
• inline-letevals: Defining Functions
• inline-quote: Defining Functions
• inner edges: Frame Layout
• inner frame: Frame Layout
• inner height: Frame Layout
• inner width: Frame Layout
• innermost containing parentheses: Parser State
• input events: Input Events
• input focus: Input Focus
• input methods: Input Methods
• input modes: Input Modes
• input stream: Input Streams
• input-decode-map: Translation Keymaps
• input-method-alist: Input Methods
• input-method-function: Invoking the Input Method
• input-pending-p: Event Input Misc
• insert: Insertion
• insert-abbrev-table-description: Abbrev Tables
• insert-and-inherit: Sticky Properties
• insert-before-markers: Insertion
• insert-before-markers-and-inherit: Sticky Properties
• insert-behind-hooks (overlay property): Overlay Properties
• insert-behind-hooks (text property): Special Properties
• insert-buffer: Commands for Insertion
• insert-buffer-substring: Insertion
• insert-buffer-substring-as-yank: Yanking
• insert-buffer-substring-no-properties: Insertion
• insert-button: Making Buttons
• insert-char: Insertion
• insert-default-directory: Reading File Names
• insert-directory: Contents of Directories
• insert-directory-program: Contents of Directories
• insert-file-contents: Reading from Files
• insert-file-contents-literally: Reading from Files
• insert-for-yank: Yanking
• insert-image: Showing Images
• insert-in-front-hooks (overlay property): Overlay Properties
• insert-in-front-hooks (text property): Special Properties
• insert-register: Registers
• insert-sliced-image: Showing Images
• insert-text-button: Making Buttons
• inserting killed text: Yank Commands
• insertion before point: Insertion
• insertion of text: Insertion
• insertion type of a marker: Marker Insertion Types
• inside comment: Parser State
• inside string: Parser State
• installation-directory: System Environment
• instrumenting for Edebug: Instrumenting
• int-to-string: String Conversion
• intangible (overlay property): Overlay Properties
• intangible (text property): Special Properties
• integer to decimal: String Conversion
• integer to hexadecimal: Formatting Strings
• integer to octal: Formatting Strings
• integer to string: String Conversion
• integer types (C programming language): C Integer Types
• integer-or-marker-p: Predicates on Markers
• integerp: Predicates on Numbers
• integers: Numbers
• integers in specific radix: Integer Basics
• interactive: Using Interactive
• interactive call: Interactive Call
• interactive code description: Interactive Codes
• interactive completion: Interactive Codes
• interactive function: Defining Commands
• interactive spec, using: Using Interactive
• interactive specification in primitives: Writing Emacs Primitives
• interactive, examples of using: Interactive Examples
• interactive-form: Using Interactive
• interactive-form property: Defining Commands
• interactive-form, symbol property: Using Interactive
• interactive-only property: Defining Commands
• intern: Creating Symbols
• intern-soft: Creating Symbols
• internal menu bar: Frame Layout
• internal representation of characters: Text Representations
• internal tool bar: Frame Layout
• internal windows: Basic Windows
• internal-border-width, a frame parameter: Layout Parameters
• internals, of buffer: Buffer Internals
• internals, of process: Process Internals
• internals, of window: Window Internals
• interning: Creating Symbols
• interpreter: Evaluation
• interpreter-mode-alist: Auto Major Mode
• interprogram-cut-function: Low-Level Kill Ring
• interprogram-paste-function: Low-Level Kill Ring
• interrupt Lisp functions: Quitting
• interrupt-process: Signals to Processes
• intervals: Not Intervals
• intervals-consed: Memory Usage
• invalid prefix key error: Changing Key Bindings
• invalid-function: Function Indirection
• invalid-read-syntax: Printed Representation
• invalid-regexp: Regexp Backslash
• invert-face: Attribute Functions
• invisible (overlay property): Overlay Properties
• invisible (text property): Special Properties
• invisible frame: Visibility of Frames
• invisible text: Invisible Text
• invisible-p: Invisible Text
• invisible/intangible text, and point: Adjusting Point
• invocation-directory: System Environment
• invocation-name: System Environment
• invoking input method: Invoking the Input Method
• invoking lisp debugger: Invoking the Debugger
• is this call interactive: Distinguish Interactive
• isnan: Float Basics
• italic text: Face Attributes
• iter-close: Generators
• iter-defun: Generators
• iter-do: Generators
• iter-lambda: Generators
• iter-next: Generators
• iter-yield: Generators
• iter-yield-from: Generators
• iteration: Iteration
• jit-lock-register: Other Font Lock Variables
• jit-lock-unregister: Other Font Lock Variables
• joining lists: Rearrangement
• jumbled display of bidirectional text: Bidirectional Display
• just-one-space: User-Level Deletion
• justify-current-line: Filling
• kbd: Key Sequences
• kbd-macro-termination-hook: Keyboard Macros
• kept-new-versions: Numbered Backups
• kept-old-versions: Numbered Backups
• key: Key Sequences
• key binding: Keymap Basics
• key binding, conventions for: Key Binding Conventions
• key lookup: Key Lookup
• key sequence: Key Sequences
• key sequence error: Changing Key Bindings
• key sequence input: Key Sequence Input
• key substitution sequence: Keys in Documentation
• key translation function: Translation Keymaps
• key-binding: Active Keymaps
• key-description: Describing Characters
• key-translation-map: Translation Keymaps
• keyboard events: Keyboard Events
• keyboard events in strings: Strings of Events
• keyboard events, data in: Accessing Mouse
• keyboard input: Reading Input
• keyboard input decoding on X: Locales
• keyboard macro execution: Interactive Call
• keyboard macro termination: Beeping
• keyboard macro, terminating: Event Input Misc
• keyboard macros: Keyboard Macros
• keyboard macros (Edebug): Edebug Execution Modes
• keyboard-coding-system: Terminal I/O Encoding
• keyboard-quit: Quitting
• keyboard-translate: Event Mod
• keyboard-translate-table: Event Mod
• keymap: Keymaps
• keymap (button property): Button Properties
• keymap (overlay property): Overlay Properties
• keymap (text property): Special Properties
• keymap entry: Key Lookup
• keymap format: Format of Keymaps
• keymap in keymap: Key Lookup
• keymap inheritance: Inheritance and Keymaps
• keymap inheritance from multiple maps: Inheritance and Keymaps
• keymap of character: Special Properties
• keymap of character (and overlays): Overlay Properties
• keymap prompt string: Format of Keymaps
• keymap-parent: Inheritance and Keymaps
• keymap-prompt: Defining Menus
• keymapp: Format of Keymaps
• keymaps for translating events: Translation Keymaps
• keymaps in modes: Major Mode Conventions
• keymaps, scanning: Scanning Keymaps
• keymaps, standard: Standard Keymaps
• keys in documentation strings: Keys in Documentation
• keys, reserved: Key Binding Conventions
• keystroke: Key Sequences
• keyword symbol: Constant Variables
• keywordp: Constant Variables
• kill command repetition: Command Loop Info
• kill ring: The Kill Ring
• kill-all-local-variables: Creating Buffer-Local
• kill-append: Low-Level Kill Ring
• kill-buffer: Killing Buffers
• kill-buffer-hook: Killing Buffers
• kill-buffer-query-functions: Killing Buffers
• kill-emacs: Killing Emacs
• kill-emacs-hook: Killing Emacs
• kill-emacs-query-functions: Killing Emacs
• kill-local-variable: Creating Buffer-Local
• kill-new: Low-Level Kill Ring
• kill-process: Signals to Processes
• kill-read-only-ok: Kill Functions
• kill-region: Kill Functions
• kill-ring: Internals of Kill Ring
• kill-ring-max: Internals of Kill Ring
• kill-ring-yank-pointer: Internals of Kill Ring
• killing buffers: Killing Buffers
• killing Emacs: Killing Emacs
• kmacro-keymap: Standard Keymaps
• lambda: Anonymous Functions
• lambda expression: Lambda Expressions
• lambda in debug: Invoking the Debugger
• lambda in keymap: Key Lookup
• lambda list: Lambda Components
• lambda-list (Edebug): Specification List
• language-change event: Misc Events
• largest Lisp integer: Integer Basics
• largest window: Cyclic Window Ordering
• last: List Elements
• last-abbrev: Abbrev Expansion
• last-abbrev-location: Abbrev Expansion
• last-abbrev-text: Abbrev Expansion
• last-buffer: Buffer List
• last-coding-system-used: Encoding and I/O
• last-command: Command Loop Info
• last-command-event: Command Loop Info
• last-event-frame: Command Loop Info
• last-input-event: Event Input Misc
• last-kbd-macro: Keyboard Macros
• last-nonmenu-event: Command Loop Info
• last-prefix-arg: Prefix Command Arguments
• last-repeatable-command: Command Loop Info
• lax-plist-get: Plist Access
• lax-plist-put: Plist Access
• layout of frame: Frame Layout
• layout on display, and bidirectional text: Bidirectional Display
• layout parameters of frames: Layout Parameters
• lazy loading: Dynamic Loading
• lazy-completion-table: Basic Completion
• ldexp: Float Basics
• least recently used window: Cyclic Window Ordering
• left, a frame parameter: Position Parameters
• left-fringe, a frame parameter: Layout Parameters
• left-fringe-width: Fringe Size/Pos
• left-margin: Margins
• left-margin-width: Display Margins
• length: Sequence Functions
• let: Local Variables
• let*: Local Variables
• lexical binding: Variable Scoping
• lexical binding (Edebug): Edebug Eval
• lexical comparison of strings: Text Comparison
• lexical environment: Lexical Binding
• lexical scope: Variable Scoping
• lexical-binding: Using Lexical Binding
• library: Loading
• library compilation: Compilation Functions
• library header comments: Library Headers
• library search: Library Search
• libxml-parse-html-region: Parsing HTML/XML
• libxml-parse-xml-region: Parsing HTML/XML
• line end conversion: Coding System Basics
• line height: Line Height
• line height: Frame Font
• line number: Text Lines
• line truncation: Truncation
• line wrapping: Truncation
• line-beginning-position: Text Lines
• line-end-position: Text Lines
• line-height (text property): Line Height
• line-height (text property): Special Properties
• line-move-ignore-invisible: Invisible Text
• line-number-at-pos: Text Lines
• line-pixel-height: Size of Displayed Text
• line-prefix: Truncation
• line-spacing: Line Height
• line-spacing (text property): Line Height
• line-spacing (text property): Special Properties
• line-spacing, a frame parameter: Layout Parameters
• lines: Text Lines
• lines in region: Text Lines
• link, customization keyword: Common Keywords
• linked list: Cons Cell Type
• linking files: Changing Files
• Lisp debugger: Debugger
• Lisp expression motion: List Motion
• Lisp history: Lisp History
• Lisp library: Loading
• Lisp nesting error: Eval
• Lisp object: Lisp Data Types
• Lisp objects, stack-allocated: Stack-allocated Objects
• Lisp package: Packaging
• Lisp printer: Output Functions
• Lisp reader: Streams Intro
• lisp variables defined in C, restrictions: Variables with Restricted Values
• lisp-indent-function property: Indenting Macros
• lisp-mode-abbrev-table: Standard Abbrev Tables
• lisp-mode.el: Example Major Modes
• list: Building Lists
• list all coding systems: Lisp and Coding Systems
• list elements: List Elements
• list form evaluation: Classifying Lists
• list in keymap: Key Lookup
• list length: Sequence Functions
• list modification: List Variables
• list motion: List Motion
• list predicates: List-related Predicates
• list reverse: Sequence Functions
• list structure: Cons Cells
• list structure: Cons Cell Type
• list, replace element: Setcar
• list-buffers-directory: Buffer File Name
• list-charset-chars: Character Sets
• list-fonts: Low-Level Font
• list-load-path-shadows: Library Search
• list-processes: Process Information
• list-system-processes: System Processes
• listify-key-sequence: Event Input Misc
• listing all buffers: Buffer List
• listp: List-related Predicates
• lists: Lists
• lists and cons cells: Cons Cells
• lists as sets: Sets And Lists
• literal evaluation: Self-Evaluating Forms
• little endian: Bindat Spec
• live buffer: Killing Buffers
• live windows: Basic Windows
• ln: Changing Files
• load: How Programs Do Loading
• load error with require: Named Features
• load errors: How Programs Do Loading
• load, customization keyword: Common Keywords
• load-average: System Environment
• load-file: How Programs Do Loading
• load-file-name: How Programs Do Loading
• load-file-rep-suffixes: Load Suffixes
• load-history: Where Defined
• load-in-progress: How Programs Do Loading
• load-library: How Programs Do Loading
• load-path: Library Search
• load-prefer-newer: Load Suffixes
• load-read-function: How Programs Do Loading
• load-suffixes: Load Suffixes
• load-theme: Custom Themes
• loading: Loading
• loading hooks: Hooks for Loading
• loading, and non-ASCII characters: Loading Non-ASCII
• loadup.el: Building Emacs
• local binding: Local Variables
• local keymap: Active Keymaps
• local variables: Local Variables
• local variables, killed by major mode: Creating Buffer-Local
• local-abbrev-table: Standard Abbrev Tables
• local-function-key-map: Translation Keymaps
• local-key-binding: Functions for Key Lookup
• local-map (overlay property): Overlay Properties
• local-map (text property): Special Properties
• local-set-key: Key Binding Commands
• local-unset-key: Key Binding Commands
• local-variable-if-set-p: Creating Buffer-Local
• local-variable-p: Creating Buffer-Local
• locale: Locales
• locale-coding-system: Locales
• locale-dependent string comparison: Text Comparison
• locale-dependent string equivalence: Text Comparison
• locale-info: Locales
• locate file in path: Locating Files
• locate-file: Locating Files
• locate-library: Library Search
• locate-user-emacs-file: Standard File Names
• lock file: File Locks
• lock-buffer: File Locks
• log: Math Functions
• logand: Bitwise Operations
• logb: Float Basics
• logging echo-area messages: Logging Messages
• logical arithmetic: Bitwise Operations
• logical order: Bidirectional Display
• logical shift: Bitwise Operations
• logior: Bitwise Operations
• lognot: Bitwise Operations
• logxor: Bitwise Operations
• looking up abbrevs: Abbrev Expansion
• looking up fonts: Font Lookup
• looking-at: Regexp Search
• looking-at-p: Regexp Search
• looking-back: Regexp Search
• lookup tables: Hash Tables
• lookup-key: Functions for Key Lookup
• loops, infinite: Infinite Loops
• lower case: Case Conversion
• lower-frame: Raising and Lowering
• lowering a frame: Raising and Lowering
• LRO: Bidirectional Display
• lsh: Bitwise Operations
• lwarn: Warning Basics
• M-g: Prefix Keys
• M-o: Prefix Keys
• M-s: Prefix Keys
• M-x: Interactive Call
• Maclisp: Lisp History
• macro: What Is a Function
• macro argument evaluation: Argument Evaluation
• macro call: Expansion
• macro call evaluation: Macro Forms
• macro caveats: Problems with Macros
• macro compilation: Compilation Functions
• macro descriptions: A Sample Function Description
• macro expansion: Expansion
• macro, how to define: Defining Macros
• macroexpand: Expansion
• macroexpand-1: Expansion
• macroexpand-all: Expansion
• macrop: Simple Macro
• macros: Macros
• macros, at compile time: Eval During Compile
• magic autoload comment: Autoload
• magic file names: Magic File Names
• magic-fallback-mode-alist: Auto Major Mode
• magic-mode-alist: Auto Major Mode
• mail-host-address: System Environment
• major mode: Major Modes
• major mode command: Major Modes
• major mode conventions: Major Mode Conventions
• major mode hook: Major Mode Conventions
• major mode keymap: Active Keymaps
• major mode, automatic selection: Auto Major Mode
• major-mode: Major Modes
• make-abbrev-table: Abbrev Tables
• make-auto-save-file-name: Auto-Saving
• make-backup-file-name: Backup Names
• make-backup-file-name-function: Making Backups
• make-backup-files: Making Backups
• make-bool-vector: Bool-Vectors
• make-button: Making Buttons
• make-byte-code: Byte-Code Objects
• make-category-set: Categories
• make-category-table: Categories
• make-char-table: Char-Tables
• make-composed-keymap: Inheritance and Keymaps
• make-directory: Create/Delete Dirs
• make-display-table: Display Tables
• make-finalizer: Finalizer Type
• make-frame: Creating Frames
• make-frame-invisible: Visibility of Frames
• make-frame-on-display: Multiple Terminals
• make-frame-visible: Visibility of Frames
• make-frame-visible event: Misc Events
• make-glyph-code: Glyphs
• make-hash-table: Creating Hash
• make-help-screen: Help Functions
• make-indirect-buffer: Indirect Buffers
• make-keymap: Creating Keymaps
• make-list: Building Lists
• make-local-variable: Creating Buffer-Local
• make-marker: Creating Markers
• make-network-process: Network Processes
• make-obsolete: Obsolete Functions
• make-obsolete-variable: Variable Aliases
• make-overlay: Managing Overlays
• make-pipe-process: Asynchronous Processes
• make-process: Asynchronous Processes
• make-progress-reporter: Progress
• make-ring: Rings
• make-serial-process: Serial Ports
• make-sparse-keymap: Creating Keymaps
• make-string: Creating Strings
• make-symbol: Creating Symbols
• make-symbolic-link: Changing Files
• make-syntax-table: Syntax Table Functions
• make-temp-file: Unique File Names
• make-temp-name: Unique File Names
• make-text-button: Making Buttons
• make-translation-table: Translation of Characters
• make-translation-table-from-alist: Translation of Characters
• make-translation-table-from-vector: Translation of Characters
• make-variable-buffer-local: Creating Buffer-Local
• make-vector: Vector Functions
• make-xwidget: Xwidgets
• making backup files: Making Backups
• making buttons: Making Buttons
• makunbound: Void Variables
• malicious use of directional overrides: Bidirectional Display
• managing overlays: Managing Overlays
• manipulating buttons: Manipulating Buttons
• map-char-table: Char-Tables
• map-charset-chars: Character Sets
• map-keymap: Scanning Keymaps
• map-y-or-n-p: Multiple Queries
• mapatoms: Creating Symbols
• mapc: Mapping Functions
• mapcar: Mapping Functions
• mapconcat: Mapping Functions
• maphash: Hash Access
• mapping functions: Mapping Functions
• margins, display: Display Margins
• margins, filling: Margins
• mark: The Mark
• mark excursion: Excursions
• mark ring: The Mark
• mark, the: The Mark
• mark-active: The Mark
• mark-even-if-inactive: The Mark
• mark-marker: The Mark
• mark-ring: The Mark
• mark-ring-max: The Mark
• marker argument: Interactive Codes
• marker creation: Creating Markers
• marker garbage collection: Overview of Markers
• marker information: Information from Markers
• marker input stream: Input Streams
• marker output stream: Output Streams
• marker relocation: Overview of Markers
• marker, how to move position: Moving Markers
• marker-buffer: Information from Markers
• marker-insertion-type: Marker Insertion Types
• marker-position: Information from Markers
• markerp: Predicates on Markers
• markers: Markers
• markers as numbers: Overview of Markers
• markers, predicates for: Predicates on Markers
• match data: Match Data
• match, customization keyword: Type Keywords
• match-alternatives, customization keyword: Composite Types
• match-beginning: Simple Match Data
• match-data: Entire Match Data
• match-end: Simple Match Data
• match-string: Simple Match Data
• match-string-no-properties: Simple Match Data
• match-substitute-replacement: Replacing Match
• mathematical functions: Math Functions
• max: Comparison of Numbers
• max-char: Character Codes
• max-image-size: Showing Images
• max-lisp-eval-depth: Eval
• max-mini-window-height: Minibuffer Misc
• max-mini-window-height: Minibuffer Windows
• max-specpdl-size: Local Variables
• maximize-window: Resizing Windows
• maximized frames: Size Parameters
• maximizing windows: Resizing Windows
• maximum Lisp integer: Integer Basics
• maximum value of character codepoint: Character Codes
• md5: Checksum/Hash
• MD5 checksum: Checksum/Hash
• measuring resource usage: Profiling
• member: Sets And Lists
• member-ignore-case: Sets And Lists
• membership in a list: Sets And Lists
• memory allocation: Garbage Collection
• memory usage: Memory Usage
• memory usage: Profiling
• memory-full: Garbage Collection
• memory-info: Garbage Collection
• memory-limit: Garbage Collection
• memory-use-counts: Garbage Collection
• memq: Sets And Lists
• memql: Sets And Lists
• menu bar: Menu Bar
• menu bar keymaps: Standard Keymaps
• menu definition example: Menu Example
• menu item: Defining Menus
• menu keymaps: Menu Keymaps
• menu modification: Modifying Menus
• menu prompt string: Defining Menus
• menu separators: Menu Separators
• menu-bar prefix key: Key Sequence Input
• menu-bar-file-menu: Standard Keymaps
• menu-bar-final-items: Menu Bar
• menu-bar-help-menu: Standard Keymaps
• menu-bar-lines frame parameter: Layout Parameters
• menu-bar-options-menu: Standard Keymaps
• menu-bar-tools-menu: Standard Keymaps
• menu-bar-update-hook: Menu Bar
• menu-item: Extended Menu Items
• menu-prompt-more-char: Keyboard Menus
• menus, popup: Pop-Up Menus
• merge-face-attribute: Attribute Functions
• message: Displaying Messages
• message digest: Checksum/Hash
• message, finding what causes a particular message: Error Debugging
• message-box: Displaying Messages
• message-log-max: Logging Messages
• message-or-box: Displaying Messages
• message-truncate-lines: Echo Area Customization
• messages-buffer: Logging Messages
• meta character key constants: Changing Key Bindings
• meta character printing: Describing Characters
• meta characters: Meta-Char Syntax
• meta characters lookup: Format of Keymaps
• meta-prefix-char: Functions for Key Lookup
• min: Comparison of Numbers
• minibuffer: Minibuffers
• minibuffer completion: Minibuffer Completion
• minibuffer contents, accessing: Minibuffer Contents
• minibuffer history: Minibuffer History
• minibuffer input: Recursive Editing
• minibuffer input, and command-line arguments: Shell Arguments
• minibuffer input, reading lisp objects: Object from Minibuffer
• minibuffer input, reading text strings: Text from Minibuffer
• minibuffer window, and next-window: Cyclic Window Ordering
• minibuffer windows: Minibuffer Windows
• minibuffer, a frame parameter: Buffer Parameters
• minibuffer-allow-text-properties: Text from Minibuffer
• minibuffer-auto-raise: Raising and Lowering
• minibuffer-complete: Completion Commands
• minibuffer-complete-and-exit: Completion Commands
• minibuffer-complete-word: Completion Commands
• minibuffer-completion-confirm: Completion Commands
• minibuffer-completion-help: Completion Commands
• minibuffer-completion-predicate: Completion Commands
• minibuffer-completion-table: Completion Commands
• minibuffer-confirm-exit-commands: Completion Commands
• minibuffer-contents: Minibuffer Contents
• minibuffer-contents-no-properties: Minibuffer Contents
• minibuffer-depth: Recursive Mini
• minibuffer-exit-hook: Minibuffer Misc
• minibuffer-frame-alist: Initial Parameters
• minibuffer-help-form: Minibuffer Misc
• minibuffer-history: Minibuffer History
• minibuffer-inactive-mode: Minibuffer Misc
• minibuffer-less frame: Frame Layout
• minibuffer-local-completion-map: Completion Commands
• minibuffer-local-filename-completion-map: Completion Commands
• minibuffer-local-map: Text from Minibuffer
• minibuffer-local-must-match-map: Completion Commands
• minibuffer-local-ns-map: Text from Minibuffer
• minibuffer-local-shell-command-map: Reading File Names
• minibuffer-message: Minibuffer Misc
• minibuffer-message-timeout: Minibuffer Misc
• minibuffer-only frame: Initial Parameters
• minibuffer-only frame: Frame Layout
• minibuffer-prompt: Minibuffer Contents
• minibuffer-prompt-end: Minibuffer Contents
• minibuffer-prompt-width: Minibuffer Contents
• minibuffer-scroll-window: Minibuffer Misc
• minibuffer-selected-window: Minibuffer Misc
• minibuffer-setup-hook: Minibuffer Misc
• minibuffer-window: Minibuffer Windows
• minibuffer-window-active-p: Minibuffer Windows
• minibufferp: Minibuffer Misc
• minimize-window: Resizing Windows
• minimized frame: Visibility of Frames
• minimizing windows: Resizing Windows
• minimum Lisp integer: Integer Basics
• minor mode: Minor Modes
• minor mode conventions: Minor Mode Conventions
• minor-mode-alist: Mode Line Variables
• minor-mode-key-binding: Functions for Key Lookup
• minor-mode-list: Minor Modes
• minor-mode-map-alist: Controlling Active Maps
• minor-mode-overriding-map-alist: Controlling Active Maps
• mirroring of characters: Character Properties
• misc-objects-consed: Memory Usage
• mkdir: Create/Delete Dirs
• mod: Arithmetic Operations
• mode: Modes
• mode bits: Testing Accessibility
• mode help: Mode Help
• mode hook: Major Mode Conventions
• mode line: Mode Line Format
• mode line construct: Mode Line Data
• mode loading: Major Mode Conventions
• mode variable: Minor Mode Conventions
• mode-class (property): Major Mode Conventions
• mode-line prefix key: Key Sequence Input
• mode-line-buffer-identification: Mode Line Variables
• mode-line-client: Mode Line Variables
• mode-line-coding-system-map: Standard Keymaps
• mode-line-column-line-number-mode-map: Standard Keymaps
• mode-line-end-spaces: Mode Line Variables
• mode-line-format: Mode Line Top
• mode-line-frame-identification: Mode Line Variables
• mode-line-front-space: Mode Line Variables
• mode-line-input-method-map: Standard Keymaps
• mode-line-misc-info: Mode Line Variables
• mode-line-modes: Mode Line Variables
• mode-line-modified: Mode Line Variables
• mode-line-mule-info: Mode Line Variables
• mode-line-position: Mode Line Variables
• mode-line-process: Mode Line Variables
• mode-line-remote: Mode Line Variables
• mode-name: Mode Line Variables
• mode-specific-map: Prefix Keys
• model/view/controller: Abstract Display
• modification flag (of buffer): Buffer Modification
• modification of lists: Rearrangement
• modification time of buffer: Modification Time
• modification time of file: File Attributes
• modification-hooks (overlay property): Overlay Properties
• modification-hooks (text property): Special Properties
• modifier bits (of input character): Keyboard Events
• modifiers of events: Event Mod
• modify a list: List Variables
• modify-all-frames-parameters: Parameter Access
• modify-category-entry: Categories
• modify-frame-parameters: Parameter Access
• modify-syntax-entry: Syntax Table Functions
• modifying strings: Modifying Strings
• module-file-suffix: Dynamic Modules
• modulus: Arithmetic Operations
• momentary-string-display: Temporary Displays
• most recently selected windows: Selecting Windows
• most recently used window: Cyclic Window Ordering
• most-negative-fixnum: Integer Basics
• most-positive-fixnum: Integer Basics
• motion based on parsing: Motion via Parsing
• motion by chars, words, lines, lists: Motion
• motion event: Motion Events
• mouse click event: Click Events
• mouse drag event: Drag Events
• mouse events, data in: Accessing Mouse
• mouse events, in special parts of frame: Key Sequence Input
• mouse events, repeated: Repeat Events
• mouse motion events: Motion Events
• mouse pointer shape: Pointer Shape
• mouse position: Mouse Position
• mouse position list: Click Events
• mouse position list, accessing: Accessing Mouse
• mouse tracking: Mouse Tracking
• mouse, availability: Display Feature Testing
• mouse-1: Clickable Text
• mouse-1-click-follows-link: Clickable Text
• mouse-2: Key Binding Conventions
• mouse-absolute-pixel-position: Mouse Position
• mouse-action (button property): Button Properties
• mouse-appearance-menu-map: Standard Keymaps
• mouse-color, a frame parameter: Font and Color Parameters
• mouse-face (button property): Button Properties
• mouse-face (overlay property): Overlay Properties
• mouse-face (text property): Special Properties
• mouse-leave-buffer-hook: Standard Hooks
• mouse-movement-p: Classifying Events
• mouse-on-link-p: Clickable Text
• mouse-pixel-position: Mouse Position
• mouse-position: Mouse Position
• mouse-position-function: Mouse Position
• mouse-wheel-down-event: Misc Events
• mouse-wheel-up-event: Misc Events
• move to beginning or end of buffer: Buffer End Motion
• move-marker: Moving Markers
• move-overlay: Managing Overlays
• move-point-visually: Bidirectional Display
• move-to-column: Columns
• move-to-left-margin: Margins
• move-to-window-group-line: Screen Lines
• move-to-window-group-line-function: Screen Lines
• move-to-window-line: Screen Lines
• movemail: Subprocess Creation
• moving across syntax classes: Motion and Syntax
• moving markers: Moving Markers
• MS-DOS and file modes: Testing Accessibility
• MS-Windows file-name syntax: File Names
• mule-keymap: Prefix Keys
• multi-file package: Multi-file Packages
• multi-frame images: Multi-Frame Images
• multi-monitor: Multiple Terminals
• multi-query-replace-map: Search and Replace
• multi-tty: Multiple Terminals
• multibyte characters: Non-ASCII Characters
• multibyte text: Text Representations
• multibyte-char-to-unibyte: Converting Representations
• multibyte-string-p: Text Representations
• multibyte-syntax-as-symbol: Control Parsing
• multiline font lock: Multiline Font Lock
• multiple terminals: Multiple Terminals
• multiple windows: Basic Windows
• multiple X displays: Multiple Terminals
• multiple yes-or-no questions: Multiple Queries
• multiple-dispatch methods: Generic Functions
• multiple-frames: Frame Titles
• name, a frame parameter: Basic Parameters
• named function: Function Names
• naming backup files: Backup Names
• NaN: Float Basics
• narrow-map: Standard Keymaps
• narrow-to-page: Narrowing
• narrow-to-region: Narrowing
• narrowing: Narrowing
• native edges: Frame Layout
• native frame: Frame Layout
• native height: Frame Layout
• native position: Frame Layout
• native width: Frame Layout
• natnump: Predicates on Numbers
• natural numbers: Predicates on Numbers
• nbutlast: List Elements
• nconc: Rearrangement
• negative infinity: Float Basics
• negative-argument: Prefix Command Arguments
• network byte ordering: Bindat Spec
• network connection: Network
• network connection, encrypted: Network
• network servers: Network Servers
• network service name, and default coding system: Default Coding Systems
• network-coding-system-alist: Default Coding Systems
• network-interface-info: Misc Network
• network-interface-list: Misc Network
• new file message: Subroutines of Visiting
• newline: Commands for Insertion
• newline: Basic Char Syntax
• newline and Auto Fill mode: Commands for Insertion
• newline in print: Output Functions
• newline in strings: Syntax for Strings
• newline-and-indent: Mode-Specific Indent
• next input: Event Input Misc
• next-button: Button Buffer Commands
• next-char-property-change: Property Search
• next-complete-history-element: Minibuffer Commands
• next-frame: Finding All Frames
• next-history-element: Minibuffer Commands
• next-matching-history-element: Minibuffer Commands
• next-overlay-change: Finding Overlays
• next-property-change: Property Search
• next-screen-context-lines: Textual Scrolling
• next-single-char-property-change: Property Search
• next-single-property-change: Property Search
• next-window: Cyclic Window Ordering
• nil: nil and t
• nil as a list: Box Diagrams
• nil in keymap: Key Lookup
• nil input stream: Input Streams
• nil output stream: Output Streams
• nlistp: List-related Predicates
• no-byte-compile: Byte Compilation
• no-catch: Catch and Throw
• no-conversion coding system: Coding System Basics
• no-redraw-on-reenter: Refresh Screen
• no-self-insert property: Defining Abbrevs
• node, ewoc: Abstract Display
• non-ASCII characters: Non-ASCII Characters
• non-ASCII characters in loaded files: Loading Non-ASCII
• non-ASCII text in keybindings: Key Binding Commands
• non-capturing group: Regexp Backslash
• non-greedy repetition characters in regexp: Regexp Special
• nondirectory part (of file name): File Name Components
• noninteractive: Batch Mode
• nonlocal exits: Nonlocal Exits
• nonlocal exits, cleaning up: Cleanups
• nonprinting characters, reading: Quoted Character Input
• noreturn: Test Coverage
• normal hook: Hooks
• normal-auto-fill-function: Auto Filling
• normal-backup-enable-predicate: Making Backups
• normal-mode: Auto Major Mode
• not: Combining Conditions
• not-modified: Buffer Modification
• notation: Evaluation Notation
• notifications, on desktop: Desktop Notifications
• notifications-close-notification: Desktop Notifications
• notifications-get-capabilities: Desktop Notifications
• notifications-get-server-information: Desktop Notifications
• notifications-notify: Desktop Notifications
• nreverse: Sequence Functions
• nth: List Elements
• nthcdr: List Elements
• null: List-related Predicates
• null bytes, and decoding text: Lisp and Coding Systems
• num-input-keys: Key Sequence Input
• num-nonmacro-input-events: Reading One Event
• number comparison: Comparison of Numbers
• number conversions: Numeric Conversions
• number-or-marker-p: Predicates on Markers
• number-sequence: Building Lists
• number-to-string: String Conversion
• numbered backups: Numbered Backups
• numberp: Predicates on Numbers
• numbers: Numbers
• numeric prefix argument: Prefix Command Arguments
• numeric prefix argument usage: Interactive Codes
• numerical RGB color specification: Color Names
• obarray: Creating Symbols
• obarray in completion: Basic Completion
• object: Lisp Data Types
• object internals: Object Internals
• object to string: Output Functions
• obsolete functions: Obsolete Functions
• octal character code: General Escape Syntax
• octal character input: Quoted Character Input
• octal escapes: Usual Display
• octal numbers: Integer Basics
• old advices, porting: Porting old advice
• one-window-p: Cyclic Window Ordering
• only-global-abbrevs: Defining Abbrevs
• opacity, frame: Font and Color Parameters
• open-dribble-file: Recording Input
• open-network-stream: Network
• open-paren-in-column-0-is-defun-start: List Motion
• open-termscript: Terminal Output
• OpenType font: Low-Level Font
• operating system environment: System Environment
• operating system signal: Killing Emacs
• operations (property): Magic File Names
• optimize regexp: Regexp Functions
• option descriptions: A Sample Variable Description
• optional arguments: Argument List
• options on command line: Command-Line Arguments
• options, defcustom keyword: Variable Definitions
• or: Combining Conditions
• ordering of windows, cyclic: Cyclic Window Ordering
• other-buffer: Buffer List
• other-window: Cyclic Window Ordering
• other-window-scroll-buffer: Textual Scrolling
• outer edges: Frame Layout
• outer frame: Frame Layout
• outer height: Frame Layout
• outer position: Frame Layout
• outer width: Frame Layout
• outer-window-id, a frame parameter: Management Parameters
• output from processes: Output from Processes
• output stream: Output Streams
• output-controlling variables: Output Variables
• overall prompt string: Format of Keymaps
• overflow: Integer Basics
• overflow-newline-into-fringe: Fringe Cursors
• overlay properties: Overlay Properties
• overlay, empty: Managing Overlays
• overlay-arrow-position: Overlay Arrow
• overlay-arrow-string: Overlay Arrow
• overlay-arrow-variable-list: Overlay Arrow
• overlay-buffer: Managing Overlays
• overlay-end: Managing Overlays
• overlay-get: Overlay Properties
• overlay-properties: Overlay Properties
• overlay-put: Overlay Properties
• overlay-recenter: Managing Overlays
• overlay-start: Managing Overlays
• overlayp: Managing Overlays
• overlays: Overlays
• overlays, managing: Managing Overlays
• overlays, scalability: Overlays
• overlays, searching for: Finding Overlays
• overlays-at: Finding Overlays
• overlays-in: Finding Overlays
• overlined text: Face Attributes
• override spec (for a face): Defining Faces
• overriding bidirectional properties: Bidirectional Display
• overriding-local-map: Controlling Active Maps
• overriding-local-map-menu-flag: Controlling Active Maps
• overriding-terminal-local-map: Controlling Active Maps
• overwrite-mode: Commands for Insertion
• package: Packaging
• package archive: Package Archives
• package archive security: Package Archives
• package attributes: Packaging Basics
• package autoloads: Packaging Basics
• package dependencies: Packaging Basics
• package name: Packaging Basics
• package signing: Package Archives
• package version: Packaging Basics
• package-archive-upload-base: Package Archives
• package-archives: Package Archives
• package-initialize: Packaging Basics
• package-upload-buffer: Package Archives
• package-upload-file: Package Archives
• package-version, customization keyword: Common Keywords
• packing: Byte Packing
• padding: Formatting Strings
• page-delimiter: Standard Regexps
• paragraph-separate: Standard Regexps
• paragraph-start: Standard Regexps
• parameters of initial frame: Initial Parameters
• parent of char-table: Char-Tables
• parent process: Processes
• parent window: Windows and Frames
• parenthesis: Cons Cell Type
• parenthesis depth: Low-Level Parsing
• parenthesis matching: Blinking
• parenthesis mismatch, debugging: Syntax Errors
• parity, in serial connections: Serial Ports
• parse state for a position: Position Parse
• parse-colon-path: System Environment
• parse-partial-sexp: Low-Level Parsing
• parse-sexp-ignore-comments: Control Parsing
• parse-sexp-lookup-properties: Control Parsing
• parse-sexp-lookup-properties: Syntax Properties
• parser state: Parser State
• parsing buffer text: Syntax Tables
• parsing expressions: Parsing Expressions
• parsing html: Parsing HTML/XML
• parsing xml: Parsing HTML/XML
• parsing, control parameters: Control Parsing
• partial application of functions: Calling Functions
• partial-width windows: Truncation
• passwords, reading: Reading a Password
• PATH environment variable: Subprocess Creation
• path-separator: System Environment
• pattern matching: Pattern matching case statement
• PBM: Other Image Types
• pcase: Pattern matching case statement
• pcase-defmacro: Pattern matching case statement
• peculiar error: Error Symbols
• peeking at input: Event Input Misc
• percent symbol in mode line: Mode Line Data
• perform-replace: Search and Replace
• performance analysis: Coverage Testing
• permanent local variable: Creating Buffer-Local
• permissions, file: Changing Files
• permissions, file: Testing Accessibility
• phishing using directional overrides: Bidirectional Display
• piece of advice: Advising Functions
• pipe: Asynchronous Processes
• pixel height of a window: Window Sizes
• pixel width of a window: Window Sizes
• pixelwise, resizing windows: Resizing Windows
• place form: Generalized Variables
• play-sound: Sound Output
• play-sound-file: Sound Output
• play-sound-functions: Sound Output
• plist: Property Lists
• plist access: Plist Access
• plist vs. alist: Plists and Alists
• plist-get: Plist Access
• plist-member: Plist Access
• plist-put: Plist Access
• plugin_is_GPL_compatible: Dynamic Modules
• point: Point
• point excursion: Excursions
• point in window: Window Point
• point with narrowing: Point
• point-entered (text property): Special Properties
• point-left (text property): Special Properties
• point-marker: Creating Markers
• point-max: Point
• point-max-marker: Creating Markers
• point-min: Point
• point-min-marker: Creating Markers
• pointer (text property): Special Properties
• pointer shape: Pointer Shape
• pointers: Cons Cell Type
• polymorphism: Generic Functions
• pop: List Elements
• pop-mark: The Mark
• pop-to-buffer: Switching Buffers
• pop-up-frame-alist: Choosing Window Options
• pop-up-frame-function: Choosing Window Options
• pop-up-frames: Choosing Window Options
• pop-up-windows: Choosing Window Options
• port number, and default coding system: Default Coding Systems
• pos-visible-in-window-group-p: Window Start and End
• pos-visible-in-window-group-p-function: Window Start and End
• pos-visible-in-window-p: Window Start and End
• position (in buffer): Positions
• position argument: Interactive Codes
• position in window: Window Point
• position of frame: Size and Position
• position of frame: Frame Geometry
• position of mouse: Mouse Position
• position-bytes: Text Representations
• positive infinity: Float Basics
• posix-looking-at: POSIX Regexps
• posix-search-backward: POSIX Regexps
• posix-search-forward: POSIX Regexps
• posix-string-match: POSIX Regexps
• posn-actual-col-row: Accessing Mouse
• posn-area: Accessing Mouse
• posn-at-point: Accessing Mouse
• posn-at-x-y: Accessing Mouse
• posn-col-row: Accessing Mouse
• posn-image: Accessing Mouse
• posn-object: Accessing Mouse
• posn-object-width-height: Accessing Mouse
• posn-object-x-y: Accessing Mouse
• posn-point: Accessing Mouse
• posn-string: Accessing Mouse
• posn-timestamp: Accessing Mouse
• posn-window: Accessing Mouse
• posn-x-y: Accessing Mouse
• posnp: Accessing Mouse
• post-command-hook: Command Overview
• post-gc-hook: Garbage Collection
• post-self-insert-hook: Commands for Insertion
• postscript images: PostScript Images
• pp: Output Functions
• pre-command-hook: Command Overview
• pre-redisplay-function: Forcing Redisplay
• pre-redisplay-functions: Forcing Redisplay
• preceding-char: Near Point
• precision in format specifications: Formatting Strings
• predicates for lists: List-related Predicates
• predicates for markers: Predicates on Markers
• predicates for numbers: Predicates on Numbers
• predicates for strings: Predicates for Strings
• prefer-utf-8 coding system: Coding System Basics
• prefix argument: Prefix Command Arguments
• prefix argument unreading: Event Input Misc
• prefix command: Prefix Keys
• prefix key: Prefix Keys
• prefix, defgroup keyword: Group Definitions
• prefix-arg: Prefix Command Arguments
• prefix-command-echo-keystrokes-functions: Standard Hooks
• prefix-command-preserve-state-hook: Standard Hooks
• prefix-help-command: Help Functions
• prefix-numeric-value: Prefix Command Arguments
• preloaded Lisp files: Building Emacs
• preloaded-file-list: Building Emacs
• preloading additional functions and variables: Building Emacs
• prepare-change-group: Atomic Changes
• preserving window sizes: Preserving Window Sizes
• preventing backtracking: Specification List
• preventing prefix key: Key Lookup
• preventing quitting: Quitting
• previous complete subexpression: Parser State
• previous-button: Button Buffer Commands
• previous-char-property-change: Property Search
• previous-complete-history-element: Minibuffer Commands
• previous-frame: Finding All Frames
• previous-history-element: Minibuffer Commands
• previous-matching-history-element: Minibuffer Commands
• previous-overlay-change: Finding Overlays
• previous-property-change: Property Search
• previous-single-char-property-change: Property Search
• previous-single-property-change: Property Search
• previous-window: Cyclic Window Ordering
• primary selection: Window System Selections
• primitive: What Is a Function
• primitive function: Primitive Function Type
• primitive function internals: Writing Emacs Primitives
• primitive type: Lisp Data Types
• primitive-undo: Undo
• prin1: Output Functions
• prin1-to-string: Output Functions
• princ: Output Functions
• print: Output Functions
• print example: Output Streams
• print name cell: Symbol Components
• print-circle: Output Variables
• print-continuous-numbering: Output Variables
• print-escape-multibyte: Output Variables
• print-escape-newlines: Output Variables
• print-escape-nonascii: Output Variables
• print-gensym: Output Variables
• print-length: Output Variables
• print-level: Output Variables
• print-number-table: Output Variables
• print-quoted: Output Variables
• printable ASCII characters: Usual Display
• printable-chars: Character Properties
• printed representation: Printed Representation
• printed representation for characters: Basic Char Syntax
• printing: Streams Intro
• printing (Edebug): Printing in Edebug
• printing circular structures: Printing in Edebug
• printing limits: Output Variables
• printing notation: Printing Notation
• priority (overlay property): Overlay Properties
• priority order of coding systems: Specifying Coding Systems
• process: Processes
• process creation: Subprocess Creation
• process filter: Filter Functions
• process filter multibyte flag: Decoding Output
• process information: Process Information
• process input: Input to Processes
• process internals: Process Internals
• process output: Output from Processes
• process sentinel: Sentinels
• process signals: Signals to Processes
• process-adaptive-read-buffering: Output from Processes
• process-attributes: System Processes
• process-buffer: Process Buffers
• process-coding-system: Process Information
• process-coding-system-alist: Default Coding Systems
• process-command: Process Information
• process-connection-type: Asynchronous Processes
• process-contact: Process Information
• process-datagram-address: Datagrams
• process-environment: System Environment
• process-exit-status: Process Information
• process-file: Synchronous Processes
• process-file-shell-command: Synchronous Processes
• process-file-side-effects: Synchronous Processes
• process-filter: Filter Functions
• process-get: Process Information
• process-id: Process Information
• process-kill-buffer-query-function: Process Buffers
• process-lines: Synchronous Processes
• process-list: Process Information
• process-live-p: Process Information
• process-mark: Process Buffers
• process-name: Process Information
• process-plist: Process Information
• process-put: Process Information
• process-query-on-exit-flag: Query Before Exit
• process-running-child-p: Input to Processes
• process-send-eof: Input to Processes
• process-send-region: Input to Processes
• process-send-string: Input to Processes
• process-sentinel: Sentinels
• process-status: Process Information
• process-tty-name: Process Information
• process-type: Process Information
• processing of errors: Processing of Errors
• processor run time: Processor Run Time
• processp: Processes
• profile: Profiling
• profiling: Profiling
• prog-mode: Basic Major Modes
• prog-mode, and bidi-paragraph-direction: Bidirectional Display
• prog-mode-hook: Basic Major Modes
• prog1: Sequencing
• prog2: Sequencing
• progn: Sequencing
• program arguments: Subprocess Creation
• program directories: Subprocess Creation
• program name, and default coding system: Default Coding Systems
• programmed completion: Programmed Completion
• programming conventions: Programming Tips
• programming types: Programming Types
• progress reporting: Progress
• progress-reporter-done: Progress
• progress-reporter-force-update: Progress
• progress-reporter-update: Progress
• prompt for file name: Reading File Names
• prompt string (of menu): Defining Menus
• prompt string of keymap: Format of Keymaps
• properties of text: Text Properties
• propertize: Changing Properties
• property category of text character: Special Properties
• property list: Property Lists
• property list cell: Symbol Components
• property lists vs association lists: Plists and Alists
• protect C variables from garbage collection: Writing Emacs Primitives
• protected forms: Cleanups
• provide: Named Features
• provide-theme: Custom Themes
• providing features: Named Features
• pty: Asynchronous Processes
• pure storage: Pure Storage
• pure-bytes-used: Pure Storage
• purecopy: Pure Storage
• purify-flag: Pure Storage
• push: List Variables
• push-button: Button Buffer Commands
• push-mark: The Mark
• put: Symbol Plists
• put-char-code-property: Character Properties
• put-charset-property: Character Sets
• put-image: Showing Images
• put-text-property: Changing Properties
• puthash: Hash Access
• query-font: Low-Level Font
• query-replace-history: Minibuffer History
• query-replace-map: Search and Replace
• querying the user: Yes-or-No Queries
• question mark in character constant: Basic Char Syntax
• quietly-read-abbrev-file: Abbrev Files
• QUIT, use in Lisp primitives: Writing Emacs Primitives
• quit-flag: Quitting
• quit-process: Signals to Processes
• quit-restore-window: Quitting Windows
• quit-window: Quitting Windows
• quitting: Quitting
• quitting from infinite loop: Infinite Loops
• quote: Quoting
• quote character: Parser State
• quote special characters in regexp: Regexp Functions
• quoted character input: Quoted Character Input
• quoted-insert suppression: Changing Key Bindings
• quoting and unquoting command-line arguments: Shell Arguments
• quoting characters in printing: Output Functions
• quoting using apostrophe: Quoting
• radio, customization types: Composite Types
• radix for reading an integer: Integer Basics
• raise-frame: Raising and Lowering
• raising a frame: Raising and Lowering
• random: Random Numbers
• random numbers: Random Numbers
• rassoc: Association Lists
• rassq: Association Lists
• rassq-delete-all: Association Lists
• raw prefix argument: Prefix Command Arguments
• raw prefix argument usage: Interactive Codes
• raw syntax descriptor: Syntax Table Internals
• raw-text coding system: Coding System Basics
• re-builder: Regular Expressions
• re-search-backward: Regexp Search
• re-search-forward: Regexp Search
• read: Input Functions
• read command name: Interactive Call
• read file names: Reading File Names
• read input: Reading Input
• read syntax: Printed Representation
• read syntax for characters: Basic Char Syntax
• read-buffer: High-Level Completion
• read-buffer-completion-ignore-case: High-Level Completion
• read-buffer-function: High-Level Completion
• read-char: Reading One Event
• read-char-choice: Reading One Event
• read-char-exclusive: Reading One Event
• read-circle: Input Functions
• read-coding-system: User-Chosen Coding Systems
• read-color: High-Level Completion
• read-command: High-Level Completion
• read-directory-name: Reading File Names
• read-event: Reading One Event
• read-expression-history: Minibuffer History
• read-file-modes: Changing Files
• read-file-name: Reading File Names
• read-file-name-completion-ignore-case: Reading File Names
• read-file-name-function: Reading File Names
• read-from-minibuffer: Text from Minibuffer
• read-from-string: Input Functions
• read-input-method-name: Input Methods
• read-kbd-macro: Describing Characters
• read-key: Reading One Event
• read-key-sequence: Key Sequence Input
• read-key-sequence-vector: Key Sequence Input
• read-minibuffer: Object from Minibuffer
• read-no-blanks-input: Text from Minibuffer
• read-non-nil-coding-system: User-Chosen Coding Systems
• read-only (text property): Special Properties
• read-only buffer: Read Only Buffers
• read-only buffers in interactive: Using Interactive
• read-only character: Special Properties
• read-only-mode: Read Only Buffers
• read-passwd: Reading a Password
• read-quoted-char: Quoted Character Input
• read-quoted-char quitting: Quitting
• read-regexp: Text from Minibuffer
• read-regexp-defaults-function: Text from Minibuffer
• read-shell-command: Reading File Names
• read-string: Text from Minibuffer
• read-variable: High-Level Completion
• reading: Streams Intro
• reading a single event: Reading One Event
• reading from files: Reading from Files
• reading from minibuffer with completion: Minibuffer Completion
• reading interactive arguments: Interactive Codes
• reading numbers in hex, octal, and binary: Integer Basics
• reading order: Bidirectional Display
• reading symbols: Creating Symbols
• real-last-command: Command Loop Info
• rearrangement of lists: Rearrangement
• rebinding: Changing Key Bindings
• recent-auto-save-p: Auto-Saving
• recent-keys: Recording Input
• recenter: Textual Scrolling
• recenter-positions: Textual Scrolling
• recenter-redisplay: Textual Scrolling
• recenter-top-bottom: Textual Scrolling
• recenter-window-group: Textual Scrolling
• recenter-window-group-function: Textual Scrolling
• recombining windows: Recombining Windows
• record command history: Interactive Call
• recording input: Recording Input
• recursion: Iteration
• recursion-depth: Recursive Editing
• recursive command loop: Recursive Editing
• recursive editing level: Recursive Editing
• recursive evaluation: Intro Eval
• recursive minibuffers: Recursive Mini
• recursive-edit: Recursive Editing
• redirect-frame-focus: Input Focus
• redisplay: Forcing Redisplay
• redo: Undo
• redraw-display: Refresh Screen
• redraw-frame: Refresh Screen
• reducing sequences: Sequence Functions
• references, following: Key Binding Conventions
• refresh the screen: Refresh Screen
• regexp: Regular Expressions
• regexp alternative: Regexp Backslash
• regexp grouping: Regexp Backslash
• regexp searching: Regexp Search
• regexp syntax: Syntax of Regexps
• regexp, special characters in: Regexp Special
• regexp-history: Minibuffer History
• regexp-opt: Regexp Functions
• regexp-opt-charset: Regexp Functions
• regexp-opt-depth: Regexp Functions
• regexp-quote: Regexp Functions
• regexps used standardly in editing: Standard Regexps
• region: The Region
• region argument: Interactive Codes
• region-beginning: The Region
• region-end: The Region
• register preview: Registers
• register-alist: Registers
• register-read-with-preview: Registers
• registers: Registers
• regular expression: Regular Expressions
• regular expression searching: Regexp Search
• regular expressions, developing: Regular Expressions
• reindent-then-newline-and-indent: Mode-Specific Indent
• relative file name: Relative File Names
• relative remapping, faces: Face Remapping
• remainder: Arithmetic Operations
• remapping commands: Remapping Commands
• remhash: Hash Access
• remote-file-name-inhibit-cache: Magic File Names
• remove: Sets And Lists
• remove-from-invisibility-spec: Invisible Text
• remove-function: Core Advising Primitives
• remove-hook: Setting Hooks
• remove-images: Showing Images
• remove-list-of-text-properties: Changing Properties
• remove-overlays: Managing Overlays
• remove-text-properties: Changing Properties
• removing from sequences: Sequence Functions
• remq: Sets And Lists
• rename-auto-save-file: Auto-Saving
• rename-buffer: Buffer Names
• rename-file: Changing Files
• rendering html: Parsing HTML/XML
• reordering, of bidirectional text: Bidirectional Display
• reordering, of elements in lists: Rearrangement
• repeat events: Repeat Events
• repeated loading: Repeated Loading
• replace bindings: Changing Key Bindings
• replace characters: Substitution
• replace characters in region: Substitution
• replace list element: Setcar
• replace matched text: Replacing Match
• replace part of list: Setcdr
• replace-buffer-in-windows: Buffers and Windows
• replace-match: Replacing Match
• replace-re-search-function: Search and Replace
• replace-regexp-in-string: Search and Replace
• replace-search-function: Search and Replace
• replacement after search: Search and Replace
• replacing display specs: Replacing Specs
• require: Named Features
• require, customization keyword: Common Keywords
• require-final-newline: Saving Buffers
• requiring features: Named Features
• reserved keys: Key Binding Conventions
• resize window: Resizing Windows
• resize-mini-windows: Minibuffer Windows
• rest arguments: Argument List
• restore-buffer-modified-p: Buffer Modification
• restricted-sexp, customization types: Composite Types
• restriction (in a buffer): Narrowing
• resume (cf. no-redraw-on-reenter): Refresh Screen
• resume-tty: Suspending Emacs
• resume-tty-functions: Suspending Emacs
• rethrow a signal: Handling Errors
• return (ASCII character): Basic Char Syntax
• return value: What Is a Function
• reverse: Sequence Functions
• reversing a list: Sequence Functions
• reversing a string: Sequence Functions
• reversing a vector: Sequence Functions
• revert-buffer: Reverting
• revert-buffer-function: Reverting
• revert-buffer-in-progress-p: Reverting
• revert-buffer-insert-file-contents-function: Reverting
• revert-without-query: Reverting
• reverting buffers: Reverting
• rgb value: Color Names
• right dividers: Window Dividers
• right-divider-width, a frame parameter: Layout Parameters
• right-fringe, a frame parameter: Layout Parameters
• right-fringe-width: Fringe Size/Pos
• right-margin-width: Display Margins
• right-to-left text: Bidirectional Display
• ring data structure: Rings
• ring-bell-function: Beeping
• ring-copy: Rings
• ring-elements: Rings
• ring-empty-p: Rings
• ring-insert: Rings
• ring-insert-at-beginning: Rings
• ring-length: Rings
• ring-p: Rings
• ring-ref: Rings
• ring-remove: Rings
• ring-size: Rings
• risky, defcustom keyword: Variable Definitions
• risky-local-variable-p: File Local Variables
• RLO: Bidirectional Display
• rm: Changing Files
• root window: Windows and Frames
• round: Numeric Conversions
• rounding in conversions: Numeric Conversions
• rounding without conversion: Rounding Operations
• rplaca: Modifying Lists
• rplacd: Modifying Lists
• run time stack: Internals of Debugger
• run-at-time: Timers
• run-hook-with-args: Running Hooks
• run-hook-with-args-until-failure: Running Hooks
• run-hook-with-args-until-success: Running Hooks
• run-hooks: Running Hooks
• run-mode-hooks: Mode Hooks
• run-with-idle-timer: Idle Timers
• S-expression: Intro Eval
• safe local variable: File Local Variables
• safe, defcustom keyword: Variable Definitions
• safe-length: List Elements
• safe-local-eval-forms: File Local Variables
• safe-local-variable-p: File Local Variables
• safe-local-variable-values: File Local Variables
• safe-magic (property): Magic File Names
• safely encode a string: Lisp and Coding Systems
• safely encode characters in a charset: Lisp and Coding Systems
• safely encode region: Lisp and Coding Systems
• safety of functions: Function Safety
• same-window-buffer-names: Choosing Window Options
• same-window-p: Choosing Window Options
• same-window-regexps: Choosing Window Options
• save abbrevs in files: Abbrev Files
• save-abbrevs: Abbrev Files
• save-buffer: Saving Buffers
• save-buffer-coding-system: Encoding and I/O
• save-current-buffer: Current Buffer
• save-excursion: Excursions
• save-mark-and-excursion: Excursions
• save-match-data: Saving Match Data
• save-restriction: Narrowing
• save-selected-window: Selecting Windows
• save-some-buffers: Saving Buffers
• save-window-excursion: Window Configurations
• SaveUnder feature: Display Feature Testing
• saving buffers: Saving Buffers
• saving text properties: Format Conversion
• saving window information: Window Configurations
• scalability of overlays: Overlays
• scalable fonts: Font Selection
• scalable-fonts-allowed: Font Selection
• scan-lists: Motion via Parsing
• scan-sexps: Motion via Parsing
• scanning expressions: Parsing Expressions
• scanning for character sets: Scanning Charsets
• scanning keymaps: Scanning Keymaps
• scope: Variable Scoping
• scoping rule: Variable Scoping
• screen layout: Frame Configuration Type
• screen lines, moving by: Screen Lines
• screen of terminal: Basic Windows
• screen refresh: Refresh Screen
• screen-gamma, a frame parameter: Font and Color Parameters
• script symbols: Character Properties
• scroll bar events, data in: Accessing Scroll
• scroll bars: Scroll Bars
• scroll-bar-background, a frame parameter: Font and Color Parameters
• scroll-bar-event-ratio: Accessing Scroll
• scroll-bar-foreground, a frame parameter: Font and Color Parameters
• scroll-bar-height: Scroll Bars
• scroll-bar-height, a frame parameter: Layout Parameters
• scroll-bar-mode: Scroll Bars
• scroll-bar-scale: Accessing Scroll
• scroll-bar-width: Scroll Bars
• scroll-bar-width, a frame parameter: Layout Parameters
• scroll-command property: Textual Scrolling
• scroll-conservatively: Textual Scrolling
• scroll-down: Textual Scrolling
• scroll-down-aggressively: Textual Scrolling
• scroll-down-command: Textual Scrolling
• scroll-error-top-bottom: Textual Scrolling
• scroll-left: Horizontal Scrolling
• scroll-margin: Textual Scrolling
• scroll-other-window: Textual Scrolling
• scroll-preserve-screen-position: Textual Scrolling
• scroll-right: Horizontal Scrolling
• scroll-step: Textual Scrolling
• scroll-up: Textual Scrolling
• scroll-up-aggressively: Textual Scrolling
• scroll-up-command: Textual Scrolling
• scrolling textually: Textual Scrolling
• search-backward: String Search
• search-failed: String Search
• search-forward: String Search
• search-map: Prefix Keys
• search-spaces-regexp: Regexp Search
• searching: Searching and Matching
• searching active keymaps for keys: Searching Keymaps
• searching and case: Searching and Case
• searching and replacing: Search and Replace
• searching for overlays: Finding Overlays
• searching for regexp: Regexp Search
• searching text properties: Property Search
• secondary selection: Window System Selections
• seconds-to-time: Time of Day
• secure-hash: Checksum/Hash
• security: Security Considerations
• seed, for random number generation: Random Numbers
• select safe coding system: User-Chosen Coding Systems
• select-frame: Input Focus
• select-frame-set-input-focus: Input Focus
• select-safe-coding-system: User-Chosen Coding Systems
• select-safe-coding-system-accept-default-p: User-Chosen Coding Systems
• select-safe-coding-system-function: User-Chosen Coding Systems
• select-window: Selecting Windows
• selected window: Basic Windows
• selected-frame: Input Focus
• selected-window: Basic Windows
• selected-window-group: Basic Windows
• selected-window-group-function: Basic Windows
• selecting a buffer: Current Buffer
• selecting a font: Font Selection
• selecting a window: Selecting Windows
• selection (for window systems): Window System Selections
• selection-coding-system: Window System Selections
• selective-display: Selective Display
• selective-display-ellipses: Selective Display
• self-evaluating form: Self-Evaluating Forms
• self-insert-and-exit: Minibuffer Commands
• self-insert-command: Commands for Insertion
• self-insert-command override: Changing Key Bindings
• self-insert-command, minor modes: Keymaps and Minor Modes
• self-insertion: Commands for Insertion
• SELinux context: Extended Attributes
• send-string-to-terminal: Terminal Output
• sending signals: Signals to Processes
• sentence-end: Standard Regexps
• sentence-end-double-space: Filling
• sentence-end-without-period: Filling
• sentence-end-without-space: Filling
• sentinel (of process): Sentinels
• seq library: Sequence Functions
• seq-concatenate: Sequence Functions
• seq-contains: Sequence Functions
• seq-count: Sequence Functions
• seq-difference: Sequence Functions
• seq-do: Sequence Functions
• seq-doseq: Sequence Functions
• seq-drop: Sequence Functions
• seq-drop-while: Sequence Functions
• seq-elt: Sequence Functions
• seq-empty-p: Sequence Functions
• seq-every-p: Sequence Functions
• seq-filter: Sequence Functions
• seq-find: Sequence Functions
• seq-group-by: Sequence Functions
• seq-intersection: Sequence Functions
• seq-into: Sequence Functions
• seq-length: Sequence Functions
• seq-let: Sequence Functions
• seq-map: Sequence Functions
• seq-mapcat: Sequence Functions
• seq-mapn: Sequence Functions
• seq-max: Sequence Functions
• seq-min: Sequence Functions
• seq-partition: Sequence Functions
• seq-position: Sequence Functions
• seq-reduce: Sequence Functions
• seq-remove: Sequence Functions
• seq-some: Sequence Functions
• seq-sort: Sequence Functions
• seq-subseq: Sequence Functions
• seq-take: Sequence Functions
• seq-take-while: Sequence Functions
• seq-uniq: Sequence Functions
• seqp: Sequence Functions
• sequence: Sequences Arrays Vectors
• sequence destructuring: Sequence Functions
• sequence functions in seq: Sequence Functions
• sequence iteration: Sequence Functions
• sequence length: Sequence Functions
• sequence reverse: Sequence Functions
• sequencep: Sequence Functions
• sequencing: Sequencing
• sequential execution: Sequencing
• serial connections: Serial Ports
• serial-process-configure: Serial Ports
• serial-term: Serial Ports
• serializing: Byte Packing
• session file: Session Management
• session manager: Session Management
• set: Setting Variables
• set, defcustom keyword: Variable Definitions
• set-advertised-calling-convention: Obsolete Functions
• set-after, defcustom keyword: Variable Definitions
• set-auto-coding: Default Coding Systems
• set-auto-mode: Auto Major Mode
• set-binary-mode: Input Functions
• set-buffer: Current Buffer
• set-buffer-auto-saved: Auto-Saving
• set-buffer-major-mode: Auto Major Mode
• set-buffer-modified-p: Buffer Modification
• set-buffer-multibyte: Selecting a Representation
• set-case-syntax: Case Tables
• set-case-syntax-delims: Case Tables
• set-case-syntax-pair: Case Tables
• set-case-table: Case Tables
• set-category-table: Categories
• set-char-table-extra-slot: Char-Tables
• set-char-table-parent: Char-Tables
• set-char-table-range: Char-Tables
• set-charset-priority: Character Sets
• set-coding-system-priority: Specifying Coding Systems
• set-default: Default Value
• set-default-file-modes: Changing Files
• set-default-toplevel-value: Default Value
• set-display-table-slot: Display Tables
• set-face-attribute: Attribute Functions
• set-face-background: Attribute Functions
• set-face-bold: Attribute Functions
• set-face-font: Attribute Functions
• set-face-foreground: Attribute Functions
• set-face-inverse-video: Attribute Functions
• set-face-italic: Attribute Functions
• set-face-stipple: Attribute Functions
• set-face-underline: Attribute Functions
• set-file-acl: Changing Files
• set-file-extended-attributes: Changing Files
• set-file-modes: Changing Files
• set-file-selinux-context: Changing Files
• set-file-times: Changing Files
• set-fontset-font: Fontsets
• set-frame-configuration: Frame Configurations
• set-frame-font: Frame Font
• set-frame-height: Size and Position
• set-frame-parameter: Parameter Access
• set-frame-position: Size and Position
• set-frame-selected-window: Selecting Windows
• set-frame-size: Size and Position
• set-frame-width: Size and Position
• set-fringe-bitmap-face: Customizing Bitmaps
• set-input-method: Input Methods
• set-input-mode: Input Modes
• set-keyboard-coding-system: Terminal I/O Encoding
• set-keymap-parent: Inheritance and Keymaps
• set-left-margin: Margins
• set-mark: The Mark
• set-marker: Moving Markers
• set-marker-insertion-type: Marker Insertion Types
• set-match-data: Entire Match Data
• set-minibuffer-window: Minibuffer Windows
• set-mouse-absolute-pixel-position: Mouse Position
• set-mouse-pixel-position: Mouse Position
• set-mouse-position: Mouse Position
• set-network-process-option: Network Options
• set-process-buffer: Process Buffers
• set-process-coding-system: Process Information
• set-process-datagram-address: Datagrams
• set-process-filter: Filter Functions
• set-process-plist: Process Information
• set-process-query-on-exit-flag: Query Before Exit
• set-process-sentinel: Sentinels
• set-process-window-size: Process Buffers
• set-register: Registers
• set-right-margin: Margins
• set-standard-case-table: Case Tables
• set-syntax-table: Syntax Table Functions
• set-terminal-coding-system: Terminal I/O Encoding
• set-terminal-parameter: Terminal Parameters
• set-text-properties: Changing Properties
• set-transient-map: Controlling Active Maps
• set-visited-file-modtime: Modification Time
• set-visited-file-name: Buffer File Name
• set-window-buffer: Buffers and Windows
• set-window-combination-limit: Recombining Windows
• set-window-configuration: Window Configurations
• set-window-dedicated-p: Dedicated Windows
• set-window-display-table: Active Display Table
• set-window-fringes: Fringe Size/Pos
• set-window-group-start: Window Start and End
• set-window-group-start-function: Window Start and End
• set-window-hscroll: Horizontal Scrolling
• set-window-margins: Display Margins
• set-window-next-buffers: Window History
• set-window-parameter: Window Parameters
• set-window-point: Window Point
• set-window-prev-buffers: Window History
• set-window-scroll-bars: Scroll Bars
• set-window-start: Window Start and End
• set-window-vscroll: Vertical Scrolling
• set-xwidget-plist: Xwidgets
• set-xwidget-query-on-exit-flag: Xwidgets
• setcar: Setcar
• setcdr: Setcdr
• setenv: System Environment
• setf: Setting Generalized Variables
• setplist: Symbol Plists
• setq: Setting Variables
• setq-default: Default Value
• setq-local: Creating Buffer-Local
• sets: Sets And Lists
• setting modes of files: Changing Files
• setting-constant error: Constant Variables
• severity level: Warning Basics
• sexp: Intro Eval
• sexp motion: List Motion
• SHA hash: Checksum/Hash
• shadowed Lisp files: Library Search
• shadowing of variables: Local Variables
• shared structure, read syntax: Circular Objects
• shell command arguments: Shell Arguments
• shell-command-history: Minibuffer History
• shell-command-to-string: Synchronous Processes
• shell-quote-argument: Shell Arguments
• shift-selection, and interactive spec: Using Interactive
• shift-translation: Key Sequence Input
• show image: Showing Images
• show-help-function: Special Properties
• shr-insert-document: Parsing HTML/XML
• shrink-window-if-larger-than-buffer: Resizing Windows
• shy groups: Regexp Backslash
• sibling window: Windows and Frames
• side effect: Intro Eval
• SIGHUP: Killing Emacs
• SIGINT: Killing Emacs
• signal: Signaling Errors
• signal-process: Signals to Processes
• signaling errors: Signaling Errors
• signals: Signals to Processes
• SIGTERM: Killing Emacs
• SIGTSTP: Suspending Emacs
• sigusr1 event: Misc Events
• sigusr2 event: Misc Events
• simple package: Simple Packages
• sin: Math Functions
• single file package: Simple Packages
• single-function hook: Hooks
• single-key-description: Describing Characters
• sit-for: Waiting
• site-init.el: Building Emacs
• site-lisp directories: Library Search
• site-load.el: Building Emacs
• site-run-file: Init File
• site-start.el: Startup Summary
• size of frame: Frame Geometry
• size of image: Showing Images
• size of text on display: Size of Displayed Text
• size of window: Window Sizes
• skip-chars-backward: Skipping Characters
• skip-chars-forward: Skipping Characters
• skip-syntax-backward: Motion and Syntax
• skip-syntax-forward: Motion and Syntax
• skipping characters: Skipping Characters
• skipping characters of certain syntax: Motion and Syntax
• skipping comments: Control Parsing
• sleep-for: Waiting
• slice, image: Showing Images
• small-temporary-file-directory: Unique File Names
• smallest Lisp integer: Integer Basics
• SMIE: SMIE
• SMIE grammar: SMIE Grammar
• SMIE lexer: SMIE Lexer
• smie-bnf->prec2: Operator Precedence Grammars
• smie-close-block: SMIE setup
• smie-config: SMIE Customization
• smie-config-guess: SMIE Customization
• smie-config-local: SMIE Customization
• smie-config-save: SMIE Customization
• smie-config-set-indent: SMIE Customization
• smie-config-show-indent: SMIE Customization
• smie-down-list: SMIE setup
• smie-merge-prec2s: Operator Precedence Grammars
• smie-prec2->grammar: Operator Precedence Grammars
• smie-precs->prec2: Operator Precedence Grammars
• smie-rule-bolp: SMIE Indentation Helpers
• smie-rule-hanging-p: SMIE Indentation Helpers
• smie-rule-next-p: SMIE Indentation Helpers
• smie-rule-parent: SMIE Indentation Helpers
• smie-rule-parent-p: SMIE Indentation Helpers
• smie-rule-prev-p: SMIE Indentation Helpers
• smie-rule-separator: SMIE Indentation Helpers
• smie-rule-sibling-p: SMIE Indentation Helpers
• smie-setup: SMIE setup
• Snarf-documentation: Accessing Documentation
• sort: Sequence Functions
• sort-columns: Sorting
• sort-fields: Sorting
• sort-fold-case: Sorting
• sort-lines: Sorting
• sort-numeric-base: Sorting
• sort-numeric-fields: Sorting
• sort-pages: Sorting
• sort-paragraphs: Sorting
• sort-regexp-fields: Sorting
• sort-subr: Sorting
• sorting lists: Sequence Functions
• sorting sequences: Sequence Functions
• sorting text: Sorting
• sorting vectors: Sequence Functions
• sound: Sound Output
• source breakpoints: Source Breakpoints
• space (ASCII character): Basic Char Syntax
• space display spec, and bidirectional text: Bidirectional Display
• spaces, pixel specification: Pixel Specification
• spaces, specified height or width: Specified Space
• sparse keymap: Format of Keymaps
• special events: Special Events
• special form descriptions: A Sample Function Description
• special forms: Special Forms
• special forms for control structures: Control Structures
• special modes: Major Mode Conventions
• special variables: Using Lexical Binding
• special-event-map: Controlling Active Maps
• special-form-p: Special Forms
• special-mode: Basic Major Modes
• special-variable-p: Using Lexical Binding
• specify coding system: Specifying Coding Systems
• specify color: Color Names
• speedups: Compilation Tips
• splicing (with backquote): Backquote
• split-height-threshold: Choosing Window Options
• split-string: Creating Strings
• split-string-and-unquote: Shell Arguments
• split-string-default-separators: Creating Strings
• split-width-threshold: Choosing Window Options
• split-window: Splitting Windows
• split-window-below: Splitting Windows
• split-window-keep-point: Splitting Windows
• split-window-preferred-function: Choosing Window Options
• split-window-right: Splitting Windows
• split-window-sensibly: Choosing Window Options
• splitting windows: Splitting Windows
• sqrt: Math Functions
• stable sort: Sequence Functions
• stack allocated Lisp objects: Stack-allocated Objects
• standard abbrev tables: Standard Abbrev Tables
• standard colors for character terminals: Font and Color Parameters
• standard errors: Standard Errors
• standard hooks: Standard Hooks
• standard regexps used in editing: Standard Regexps
• standard syntax table: Syntax Basics
• standard-case-table: Case Tables
• standard-category-table: Categories
• standard-display-table: Active Display Table
• standard-input: Input Functions
• standard-output: Output Variables
• standard-syntax-table: Syntax Basics
• standard-translation-table-for-decode: Translation of Characters
• standard-translation-table-for-encode: Translation of Characters
• standards of coding style: Tips
• start-file-process: Asynchronous Processes
• start-file-process-shell-command: Asynchronous Processes
• start-process: Asynchronous Processes
• start-process, command-line arguments from minibuffer: Shell Arguments
• start-process-shell-command: Asynchronous Processes
• STARTTLS network connections: Network
• startup of Emacs: Startup Summary
• startup screen: Startup Summary
• startup.el: Startup Summary
• staticpro, protection from GC: Writing Emacs Primitives
• sticky text properties: Sticky Properties
• sticky, a frame parameter: Management Parameters
• stop points: Using Edebug
• stop-process: Signals to Processes
• stopbits, in serial connections: Serial Ports
• stopping an infinite loop: Infinite Loops
• stopping on events: Global Break Condition
• storage of vector-like Lisp objects: Garbage Collection
• store-match-data: Entire Match Data
• store-substring: Modifying Strings
• stream (for printing): Output Streams
• stream (for reading): Input Streams
• strike-through text: Face Attributes
• string: Creating Strings
• string creation: Creating Strings
• string equality: Text Comparison
• string in keymap: Key Lookup
• string input stream: Input Streams
• string length: Sequence Functions
• string modification: Modifying Strings
• string predicates: Predicates for Strings
• string reverse: Sequence Functions
• string search: String Search
• string to number: String Conversion
• string to object: Input Functions
• string, number of bytes: Text Representations
• string, writing a doc string: Documentation Basics
• string-as-multibyte: Selecting a Representation
• string-as-unibyte: Selecting a Representation
• string-bytes: Text Representations
• string-chars-consed: Memory Usage
• string-collate-equalp: Text Comparison
• string-collate-lessp: Text Comparison
• string-equal: Text Comparison
• string-greaterp: Text Comparison
• string-lessp: Text Comparison
• string-match: Regexp Search
• string-match-p: Regexp Search
• string-or-null-p: Predicates for Strings
• string-prefix-p: Text Comparison
• string-suffix-p: Text Comparison
• string-to-char: String Conversion
• string-to-int: String Conversion
• string-to-multibyte: Converting Representations
• string-to-number: String Conversion
• string-to-syntax: Syntax Table Internals
• string-to-unibyte: Converting Representations
• string-width: Size of Displayed Text
• string<: Text Comparison
• string=: Text Comparison
• stringp: Predicates for Strings
• strings: Strings and Characters
• strings with keyboard events: Strings of Events
• strings, formatting them: Formatting Strings
• strings-consed: Memory Usage
• submenu: Mouse Menus
• subprocess: Processes
• subr: What Is a Function
• subr-arity: What Is a Function
• subrp: What Is a Function
• subst-char-in-region: Substitution
• substitute characters: Substitution
• substitute-command-keys: Keys in Documentation
• substitute-in-file-name: File Name Expansion
• substitute-key-definition: Changing Key Bindings
• substituting keys in documentation: Keys in Documentation
• substring: Creating Strings
• substring-no-properties: Creating Strings
• subtype of char-table: Char-Tables
• suggestions: Caveats
• super characters: Other Char Bits
• suppress-keymap: Changing Key Bindings
• surrogate minibuffer frame: Minibuffers and Frames
• suspend (cf. no-redraw-on-reenter): Refresh Screen
• suspend evaluation: Recursive Editing
• suspend-emacs: Suspending Emacs
• suspend-frame: Suspending Emacs
• suspend-hook: Suspending Emacs
• suspend-resume-hook: Suspending Emacs
• suspend-tty: Suspending Emacs
• suspend-tty-functions: Suspending Emacs
• suspending Emacs: Suspending Emacs
• swap text between buffers: Swapping Text
• switch-to-buffer: Switching Buffers
• switch-to-buffer-in-dedicated-window: Switching Buffers
• switch-to-buffer-other-frame: Switching Buffers
• switch-to-buffer-other-window: Switching Buffers
• switch-to-buffer-preserve-window-point: Switching Buffers
• switch-to-next-buffer: Window History
• switch-to-prev-buffer: Window History
• switch-to-visible-buffer: Window History
• switches on command line: Command-Line Arguments
• switching to a buffer: Switching Buffers
• sxhash: Defining Hash
• symbol: Symbols
• symbol components: Symbol Components
• symbol equality: Creating Symbols
• symbol evaluation: Symbol Forms
• symbol function indirection: Function Indirection
• symbol in keymap: Key Lookup
• symbol name hashing: Creating Symbols
• symbol property: Symbol Properties
• symbol that evaluates to itself: Constant Variables
• symbol with constant value: Constant Variables
• symbol, where defined: Where Defined
• symbol-file: Where Defined
• symbol-function: Function Cells
• symbol-name: Creating Symbols
• symbol-plist: Symbol Plists
• symbol-value: Accessing Variables
• symbolp: Symbols
• symbols-consed: Memory Usage
• synchronous subprocess: Synchronous Processes
• syntactic font lock: Syntactic Font Lock
• syntax class: Syntax Descriptors
• syntax class table: Syntax Class Table
• syntax code: Syntax Table Internals
• syntax descriptor: Syntax Descriptors
• syntax entry, setting: Syntax Table Functions
• syntax error (Edebug): Backtracking
• syntax flags: Syntax Flags
• syntax for characters: Basic Char Syntax
• syntax of regular expressions: Syntax of Regexps
• syntax table: Syntax Tables
• syntax table example: Example Major Modes
• syntax table internals: Syntax Table Internals
• syntax tables in modes: Major Mode Conventions
• syntax-after: Syntax Table Internals
• syntax-class: Syntax Table Internals
• syntax-ppss: Position Parse
• syntax-ppss-flush-cache: Position Parse
• syntax-ppss-toplevel-pos: Parser State
• syntax-propertize-extend-region-functions: Syntax Properties
• syntax-propertize-function: Syntax Properties
• syntax-table: Syntax Table Functions
• syntax-table (text property): Syntax Properties
• syntax-table-p: Syntax Basics
• system abbrev: Abbrevs
• system processes: System Processes
• system type and name: System Environment
• system-configuration: System Environment
• system-groups: User Identification
• system-key-alist: X11 Keysyms
• system-messages-locale: Locales
• system-name: System Environment
• system-time-locale: Locales
• system-type: System Environment
• system-users: User Identification
• t: nil and t
• t input stream: Input Streams
• t output stream: Output Streams
• tab (ASCII character): Basic Char Syntax
• tab deletion: Deletion
• tab-always-indent: Mode-Specific Indent
• tab-stop-list: Indent Tabs
• tab-to-tab-stop: Indent Tabs
• tab-width: Usual Display
• tabs stops for indentation: Indent Tabs
• Tabulated List mode: Tabulated List Mode
• tabulated-list-entries: Tabulated List Mode
• tabulated-list-format: Tabulated List Mode
• tabulated-list-init-header: Tabulated List Mode
• tabulated-list-mode: Tabulated List Mode
• tabulated-list-print: Tabulated List Mode
• tabulated-list-printer: Tabulated List Mode
• tabulated-list-revert-hook: Tabulated List Mode
• tabulated-list-sort-key: Tabulated List Mode
• tag on run time stack: Catch and Throw
• tag, customization keyword: Common Keywords
• tan: Math Functions
• TCP: Network
• temacs: Building Emacs
• TEMP environment variable: Unique File Names
• temp-buffer-max-height: Temporary Displays
• temp-buffer-max-width: Temporary Displays
• temp-buffer-resize-mode: Temporary Displays
• temp-buffer-setup-hook: Temporary Displays
• temp-buffer-show-function: Temporary Displays
• temp-buffer-show-hook: Temporary Displays
• temp-buffer-window-setup-hook: Temporary Displays
• temp-buffer-window-show-hook: Temporary Displays
• temporary buffer display: Temporary Displays
• temporary display: Temporary Displays
• temporary files: Unique File Names
• temporary-file-directory: Unique File Names
• TERM environment variable: Terminal-Specific
• term-file-aliases: Terminal-Specific
• term-file-prefix: Terminal-Specific
• Termcap: Terminal-Specific
• terminal: Frames
• terminal input: Terminal Input
• terminal input modes: Input Modes
• terminal output: Terminal Output
• terminal parameters: Terminal Parameters
• terminal screen: Basic Windows
• terminal type: Terminal Type
• terminal-coding-system: Terminal I/O Encoding
• terminal-list: Multiple Terminals
• terminal-live-p: Frames
• terminal-local variables: Multiple Terminals
• terminal-name: Multiple Terminals
• terminal-parameter: Terminal Parameters
• terminal-parameters: Terminal Parameters
• terminal-specific initialization: Terminal-Specific
• termscript file: Terminal Output
• terpri: Output Functions
• test-completion: Basic Completion
• testcover-mark-all: Test Coverage
• testcover-next-mark: Test Coverage
• testcover-start: Test Coverage
• testing types: Type Predicates
• text: Text
• text area: Frame Layout
• text area of a window: Window Sizes
• text comparison: Text Comparison
• text conversion of coding system: Lisp and Coding Systems
• text deletion: Deletion
• text insertion: Insertion
• text near point: Near Point
• text parsing: Syntax Tables
• text properties: Text Properties
• text properties in files: Format Conversion
• text properties in the mode line: Properties in Mode
• text properties, changing: Changing Properties
• text properties, examining: Examining Properties
• text properties, read syntax: Text Props and Strings
• text properties, searching: Property Search
• text representation: Text Representations
• text terminal: Frames
• text-char-description: Describing Characters
• text-mode: Basic Major Modes
• text-mode-abbrev-table: Standard Abbrev Tables
• text-properties-at: Examining Properties
• text-property-any: Property Search
• text-property-default-nonsticky: Sticky Properties
• text-property-not-all: Property Search
• text-quoting-style: Keys in Documentation
• textual order: Control Structures
• textual scrolling: Textual Scrolling
• thing-at-point: Buffer Contents
• this-command: Command Loop Info
• this-command-keys: Command Loop Info
• this-command-keys-shift-translated: Key Sequence Input
• this-command-keys-vector: Command Loop Info
• this-original-command: Command Loop Info
• three-step-help: Help Functions
• throw: Catch and Throw
• throw example: Recursive Editing
• tiled windows: Basic Windows
• time calculations: Time Calculations
• time conversion: Time Conversion
• time formatting: Time Parsing
• time of day: Time of Day
• time parsing: Time Parsing
• time value: Time of Day
• time zone rule: Time Zone Rules
• time zone rules: Time Zone Rules
• time zone, current: Time Zone Rules
• time-add: Time Calculations
• time-less-p: Time Calculations
• time-subtract: Time Calculations
• time-to-day-in-year: Time Calculations
• time-to-days: Time Calculations
• timer-max-repeats: Timers
• timerp: Timers
• timers: Timers
• timestamp of a mouse event: Accessing Mouse
• timing programs: Profiling
• tips for writing Lisp: Tips
• title bar: Frame Layout
• title, a frame parameter: Basic Parameters
• TLS network connections: Network
• TMP environment variable: Unique File Names
• TMPDIR environment variable: Unique File Names
• toggle-enable-multibyte-characters: Disabling Multibyte
• tool bar: Tool Bar
• tool-bar-add-item: Tool Bar
• tool-bar-add-item-from-menu: Tool Bar
• tool-bar-border: Tool Bar
• tool-bar-button-margin: Tool Bar
• tool-bar-button-relief: Tool Bar
• tool-bar-lines frame parameter: Layout Parameters
• tool-bar-local-item-from-menu: Tool Bar
• tool-bar-map: Tool Bar
• tool-bar-position frame parameter: Layout Parameters
• tooltip face: Tooltips
• tooltip for help strings: Special Properties
• tooltip-event-buffer: Tooltips
• tooltip-frame-parameters: Tooltips
• tooltip-functions: Tooltips
• tooltip-help-tips: Tooltips
• tooltip-mode: Tooltips
• tooltips: Tooltips
• top frame: Raising and Lowering
• top, a frame parameter: Position Parameters
• top-level: Recursive Editing
• top-level default value: Default Value
• top-level form: Loading
• total height of a window: Window Sizes
• total pixel height of a window: Window Sizes
• total pixel width of a window: Window Sizes
• total width of a window: Window Sizes
• tq-close: Transaction Queues
• tq-create: Transaction Queues
• tq-enqueue: Transaction Queues
• trace buffer: Trace Buffer
• track-mouse: Mouse Tracking
• trailing blanks in file names: Information about Files
• transaction queue: Transaction Queues
• transcendental functions: Math Functions
• transient keymap: Controlling Active Maps
• transient-mark-mode: The Mark
• translate-region: Substitution
• translating input events: Event Mod
• translation keymap: Translation Keymaps
• translation tables: Translation of Characters
• translation-table-for-input: Translation of Characters
• transparency, frame: Font and Color Parameters
• transpose-regions: Transposition
• trash: Create/Delete Dirs
• trash: Changing Files
• tray notifications, MS-Windows: Desktop Notifications
• triple-click events: Repeat Events
• true: nil and t
• true list: Cons Cells
• truename (of file): Truenames
• truncate: Numeric Conversions
• truncate-lines: Truncation
• truncate-partial-width-windows: Truncation
• truncate-string-ellipsis: Size of Displayed Text
• truncate-string-to-width: Size of Displayed Text
• truth value: nil and t
• try-completion: Basic Completion
• tty-color-alist: Text Terminal Colors
• tty-color-approximate: Text Terminal Colors
• tty-color-clear: Text Terminal Colors
• tty-color-define: Text Terminal Colors
• tty-color-mode, a frame parameter: Font and Color Parameters
• tty-color-translate: Text Terminal Colors
• tty-erase-char: System Environment
• tty-setup-hook: Terminal-Specific
• tty-top-frame: Raising and Lowering
• turn multibyte support on or off: Disabling Multibyte
• two's complement: Integer Basics
• type: Lisp Data Types
• type (button property): Button Properties
• type checking: Type Predicates
• type checking internals: Writing Emacs Primitives
• type predicates: Type Predicates
• type, defcustom keyword: Customization Types
• type-of: Type Predicates
• typographic conventions: Some Terms
• TZ, environment variable: Time Zone Rules
• UBA: Bidirectional Display
• UDP: Network
• UID: User Identification
• umask: Changing Files
• unassigned character codepoints: Character Properties
• unbalanced parentheses: Syntax Errors
• unbinding keys: Key Binding Commands
• unbury-buffer: Buffer List
• undecided coding system: Coding System Basics
• undecided coding-system, when encoding: Explicit Encoding
• undefined: Functions for Key Lookup
• undefined in keymap: Key Lookup
• undefined key: Keymap Basics
• underline-minimum-offset: Face Attributes
• underlined text: Face Attributes
• undo avoidance: Substitution
• undo-ask-before-discard: Maintaining Undo
• undo-auto-amalgamate: Undo
• undo-auto-current-boundary-timer: Undo
• undo-boundary: Undo
• undo-in-progress: Undo
• undo-limit: Maintaining Undo
• undo-outer-limit: Maintaining Undo
• undo-strong-limit: Maintaining Undo
• unexec: Building Emacs
• unhandled-file-name-directory: Magic File Names
• unibyte buffers, and bidi reordering: Bidirectional Display
• unibyte text: Text Representations
• unibyte-char-to-multibyte: Converting Representations
• unibyte-string: Text Representations
• Unicode: Text Representations
• unicode bidirectional algorithm: Bidirectional Display
• unicode character escape: General Escape Syntax
• unicode general category: Character Properties
• unicode, a charset: Character Sets
• unicode-category-table: Character Properties
• unintern: Creating Symbols
• uninterned symbol: Creating Symbols
• unique file names: Unique File Names
• universal-argument: Prefix Command Arguments
• universal-argument-map: Standard Keymaps
• unless: Conditionals
• unload-feature: Unloading
• unload-feature-special-hooks: Unloading
• unloading packages: Unloading
• unloading packages, preparing for: Coding Conventions
• unlock-buffer: File Locks
• unnumbered group: Regexp Backslash
• unpacking: Byte Packing
• unread-command-events: Event Input Misc
• unsafep: Function Safety
• unsplittable, a frame parameter: Buffer Parameters
• unused lexical variable: Using Lexical Binding
• unwind-protect: Cleanups
• unwinding: Cleanups
• up-list: List Motion
• upcase: Case Conversion
• upcase-initials: Case Conversion
• upcase-region: Case Changes
• upcase-word: Case Changes
• update-directory-autoloads: Autoload
• update-file-autoloads: Autoload
• upper case: Case Conversion
• upper case key sequence: Key Sequence Input
• uptime of Emacs: Processor Run Time
• use time of window: Selecting Windows
• use-empty-active-region: The Region
• use-global-map: Controlling Active Maps
• use-hard-newlines: Filling
• use-local-map: Controlling Active Maps
• use-region-p: The Region
• user errors, signaling: Signaling Errors
• user groups: User Identification
• user identification: User Identification
• user options, how to define: Variable Definitions
• user signals: Misc Events
• user-defined error: Error Symbols
• user-emacs-directory: Init File
• user-error: Signaling Errors
• user-full-name: User Identification
• user-init-file: Init File
• user-login-name: User Identification
• user-mail-address: User Identification
• user-position, a frame parameter: Position Parameters
• user-ptr object: Dynamic Modules
• user-ptrp: Dynamic Modules
• user-real-login-name: User Identification
• user-real-uid: User Identification
• user-size, a frame parameter: Size Parameters
• user-uid: User Identification
• utf-8-emacs coding system: Coding System Basics
• valid windows: Basic Windows
• validity of coding system: Lisp and Coding Systems
• value cell: Symbol Components
• value of expression: Evaluation
• value of function: What Is a Function
• values: Eval
• variable: Variables
• variable aliases: Variable Aliases
• variable definition: Defining Variables
• variable descriptions: A Sample Variable Description
• variable limit error: Local Variables
• variable with constant value: Constant Variables
• variable, buffer-local: Buffer-Local Variables
• variable-documentation property: Documentation Basics
• variable-width spaces: Specified Space
• variant coding system: Coding System Basics
• vc-mode: Mode Line Variables
• vc-prefix-map: Prefix Keys
• vc-responsible-backend: Truenames
• vconcat: Vector Functions
• vector: Vector Functions
• vector (type): Vectors
• vector evaluation: Self-Evaluating Forms
• vector length: Sequence Functions
• vector reverse: Sequence Functions
• vector-cells-consed: Memory Usage
• vector-like objects, storage: Garbage Collection
• vectorp: Vector Functions
• verify-visited-file-modtime: Modification Time
• version number (in file name): File Name Components
• version, customization keyword: Common Keywords
• version-control: Numbered Backups
• vertical combination: Windows and Frames
• vertical fractional scrolling: Vertical Scrolling
• vertical scroll position: Vertical Scrolling
• vertical tab: Basic Char Syntax
• vertical-line prefix key: Key Sequence Input
• vertical-motion: Screen Lines
• vertical-scroll-bar: Scroll Bars
• vertical-scroll-bar prefix key: Key Sequence Input
• vertical-scroll-bars, a frame parameter: Layout Parameters
• view part, model/view/controller: Abstract Display
• view-register: Registers
• virtual buffers: Swapping Text
• visibility, a frame parameter: Management Parameters
• visible frame: Visibility of Frames
• visible-bell: Beeping
• visible-frame-list: Finding All Frames
• visited file: Buffer File Name
• visited file mode: Auto Major Mode
• visited-file-modtime: Modification Time
• visiting files: Visiting Files
• visiting files, functions for: Visiting Functions
• visual order: Bidirectional Display
• visual order, preserve when copying bidirectional text: Bidirectional Display
• visual-order cursor motion: Bidirectional Display
• void function: Function Indirection
• void function cell: Function Cells
• void variable: Void Variables
• void-function: Function Cells
• void-text-area-pointer: Pointer Shape
• void-variable error: Void Variables
• w32-collate-ignore-punctuation: Text Comparison
• w32-notification-close: Desktop Notifications
• w32-notification-notify: Desktop Notifications
• wait-for-wm, a frame parameter: Management Parameters
• waiting: Waiting
• waiting for command key input: Event Input Misc
• waiting-for-user-input-p: Sentinels
• walk-windows: Cyclic Window Ordering
• warn: Warning Basics
• warning options: Warning Options
• warning type: Warning Basics
• warning variables: Warning Variables
• warning-fill-prefix: Warning Variables
• warning-levels: Warning Variables
• warning-minimum-level: Warning Options
• warning-minimum-log-level: Warning Options
• warning-prefix-function: Warning Variables
• warning-series: Warning Variables
• warning-suppress-log-types: Warning Options
• warning-suppress-types: Warning Options
• warning-type-format: Warning Variables
• warnings: Warnings
• watch, for filesystem events: File Notifications
• webkit browser widget: Xwidgets
• wheel-down event: Misc Events
• wheel-up event: Misc Events
• when: Conditionals
• where was a symbol defined: Where Defined
• where-is-internal: Scanning Keymaps
• while: Iteration
• while-no-input: Event Input Misc
• whitespace: Basic Char Syntax
• wholenump: Predicates on Numbers
• widen: Narrowing
• widening: Narrowing
• width of a window: Window Sizes
• width, a frame parameter: Size Parameters
• window: Basic Windows
• window (overlay property): Overlay Properties
• window body: Window Sizes
• window body height: Window Sizes
• window body size: Window Sizes
• window body width: Window Sizes
• window combination: Windows and Frames
• window combination limit: Recombining Windows
• window configuration (Edebug): Edebug Display Update
• window configurations: Window Configurations
• window dividers: Window Dividers
• window end position: Window Start and End
• window excursions: Excursions
• window header line: Header Lines
• window height: Window Sizes
• window history: Window History
• window in direction: Windows and Frames
• window internals: Window Internals
• window layout in a frame: Window Configuration Type
• window layout, all frames: Frame Configuration Type
• window manager interaction, and frame parameters: Management Parameters
• window order by time of last use: Selecting Windows
• window ordering, cyclic: Cyclic Window Ordering
• window parameters: Window Parameters
• window pixel height: Window Sizes
• window pixel width: Window Sizes
• window point: Window Point
• window point internals: Window Internals
• window position: Coordinates and Windows
• window position: Window Point
• window position on display: Position Parameters
• window positions and window managers: Position Parameters
• window resizing: Resizing Windows
• window selected within a frame: Basic Windows
• window size: Window Sizes
• window size on display: Size Parameters
• window size, changing: Resizing Windows
• window splitting: Splitting Windows
• window start position: Window Start and End
• window state: Window Configurations
• window that satisfies a predicate: Cyclic Window Ordering
• window top line: Window Start and End
• window tree: Windows and Frames
• window use time: Selecting Windows
• window width: Window Sizes
• window-absolute-body-pixel-edges: Coordinates and Windows
• window-absolute-pixel-edges: Coordinates and Windows
• window-absolute-pixel-position: Coordinates and Windows
• window-adjust-process-window-size-function: Process Buffers
• window-at: Coordinates and Windows
• window-body-edges: Coordinates and Windows
• window-body-height: Window Sizes
• window-body-pixel-edges: Coordinates and Windows
• window-body-size: Window Sizes
• window-body-width: Window Sizes
• window-bottom-divider-width: Window Dividers
• window-buffer: Buffers and Windows
• window-child: Windows and Frames
• window-combination-limit: Recombining Windows
• window-combination-resize: Recombining Windows
• window-combined-p: Windows and Frames
• window-configuration-change-hook: Window Hooks
• window-configuration-frame: Window Configurations
• window-configuration-p: Window Configurations
• window-current-scroll-bars: Scroll Bars
• window-dedicated-p: Dedicated Windows
• window-display-table: Active Display Table
• window-edges: Coordinates and Windows
• window-end: Window Start and End
• window-font-height: Low-Level Font
• window-font-width: Low-Level Font
• window-frame: Windows and Frames
• window-fringes: Fringe Size/Pos
• window-full-height-p: Window Sizes
• window-full-width-p: Window Sizes
• window-group-end: Window Start and End
• window-group-end-function: Window Start and End
• window-group-start: Window Start and End
• window-group-start-function: Window Start and End
• window-header-line-height: Window Sizes
• window-header-line-height: Header Lines
• window-hscroll: Horizontal Scrolling
• window-id, a frame parameter: Management Parameters
• window-in-direction: Windows and Frames
• window-left-child: Windows and Frames
• window-line-height: Window Start and End
• window-list: Windows and Frames
• window-live-p: Basic Windows
• window-margins: Display Margins
• window-max-chars-per-line: Window Sizes
• window-min-height: Window Sizes
• window-min-size: Window Sizes
• window-min-width: Window Sizes
• window-minibuffer-p: Minibuffer Windows
• window-mode-line-height: Window Sizes
• window-next-buffers: Window History
• window-next-sibling: Windows and Frames
• window-parameter: Window Parameters
• window-parameters: Window Parameters
• window-parent: Windows and Frames
• window-persistent-parameters: Window Parameters
• window-pixel-edges: Coordinates and Windows
• window-pixel-height: Window Sizes
• window-pixel-width: Window Sizes
• window-point: Window Point
• window-point-insertion-type: Window Point
• window-preserve-size: Preserving Window Sizes
• window-preserved-size: Preserving Window Sizes
• window-prev-buffers: Window History
• window-prev-sibling: Windows and Frames
• window-resizable: Resizing Windows
• window-resize: Resizing Windows
• window-resize-pixelwise: Resizing Windows
• window-right-divider-width: Window Dividers
• window-scroll-bar-height: Scroll Bars
• window-scroll-bar-width: Scroll Bars
• window-scroll-bars: Scroll Bars
• window-scroll-functions: Window Hooks
• window-setup-hook: Init File
• window-size-change-functions: Window Hooks
• window-size-fixed: Preserving Window Sizes
• window-start: Window Start and End
• window-state-get: Window Configurations
• window-state-put: Window Configurations
• window-system: Window Systems
• window-system-initialization-alist: Startup Summary
• window-text-change-functions: Standard Hooks
• window-text-pixel-size: Size of Displayed Text
• window-top-child: Windows and Frames
• window-total-height: Window Sizes
• window-total-size: Window Sizes
• window-total-width: Window Sizes
• window-tree: Windows and Frames
• window-use-time: Selecting Windows
• window-valid-p: Basic Windows
• window-vscroll: Vertical Scrolling
• windowp: Basic Windows
• windows, controlling precisely: Buffers and Windows
• windows, recombining: Recombining Windows
• with-case-table: Case Tables
• with-coding-priority: Specifying Coding Systems
• with-current-buffer: Current Buffer
• with-current-buffer-window: Temporary Displays
• with-demoted-errors: Handling Errors
• with-displayed-buffer-window: Temporary Displays
• with-eval-after-load: Hooks for Loading
• with-file-modes: Changing Files
• with-help-window: Help Functions
• with-local-quit: Quitting
• with-no-warnings: Compiler Errors
• with-output-to-string: Output Functions
• with-output-to-temp-buffer: Temporary Displays
• with-selected-window: Selecting Windows
• with-silent-modifications: Changing Properties
• with-syntax-table: Syntax Table Functions
• with-temp-buffer: Current Buffer
• with-temp-buffer-window: Temporary Displays
• with-temp-file: Writing to Files
• with-temp-message: Displaying Messages
• with-timeout: Timers
• word-search-backward: String Search
• word-search-backward-lax: String Search
• word-search-forward: String Search
• word-search-forward-lax: String Search
• word-search-regexp: String Search
• words in region: Text Lines
• words-include-escapes: Word Motion
• wrap-prefix: Truncation
• write-abbrev-file: Abbrev Files
• write-char: Output Functions
• write-contents-functions: Saving Buffers
• write-file: Saving Buffers
• write-file-functions: Saving Buffers
• write-region: Writing to Files
• write-region-annotate-functions: Format Conversion Piecemeal
• write-region-post-annotation-function: Format Conversion Piecemeal
• writing a documentation string: Documentation Basics
• writing Emacs primitives: Writing Emacs Primitives
• writing to files: Writing to Files
• wrong-number-of-arguments: Argument List
• wrong-type-argument: Type Predicates
• X display names: Multiple Terminals
• X Window System: Window Systems
• x-alt-keysym: X11 Keysyms
• x-alternatives-map: Standard Keymaps
• x-bitmap-file-path: Face Attributes
• x-close-connection: Multiple Terminals
• x-color-defined-p: Color Names
• x-color-values: Color Names
• x-defined-colors: Color Names
• x-display-color-p: Display Feature Testing
• x-display-list: Multiple Terminals
• x-dnd-known-types: Drag and Drop
• x-dnd-test-function: Drag and Drop
• x-dnd-types-alist: Drag and Drop
• x-family-fonts: Font Lookup
• x-get-resource: Resources
• x-gtk-use-system-tooltips: Tooltips
• x-hyper-keysym: X11 Keysyms
• x-list-fonts: Font Lookup
• x-meta-keysym: X11 Keysyms
• x-open-connection: Multiple Terminals
• x-parse-geometry: Geometry
• x-pointer-shape: Pointer Shape
• x-popup-dialog: Dialog Boxes
• x-popup-menu: Pop-Up Menus
• x-resource-class: Resources
• x-resource-name: Resources
• x-sensitive-text-pointer-shape: Pointer Shape
• x-server-vendor: Display Feature Testing
• x-server-version: Display Feature Testing
• x-setup-function-keys: Standard Keymaps
• x-stretch-cursor: Cursor Parameters
• x-super-keysym: X11 Keysyms
• X11 keysyms: X11 Keysyms
• XBM: XBM Images
• XML DOM: Document Object Model
• XPM: XPM Images
• xwidget: Xwidgets
• xwidget-buffer: Xwidgets
• xwidget-info: Xwidgets
• xwidget-plist: Xwidgets
• xwidget-query-on-exit-flag: Xwidgets
• xwidget-resize: Xwidgets
• xwidget-size-request: Xwidgets
• xwidget-webkit-execute-script: Xwidgets
• xwidget-webkit-execute-script-rv: Xwidgets
• xwidget-webkit-get-title: Xwidgets
• xwidget-webkit-goto-uri: Xwidgets
• xwidgetp: Xwidgets
• y-or-n-p: Yes-or-No Queries
• y-or-n-p-with-timeout: Yes-or-No Queries
• yank: Yank Commands
• yank suppression: Changing Key Bindings
• yank-excluded-properties: Yanking
• yank-handled-properties: Yanking
• yank-pop: Yank Commands
• yank-undo-function: Yank Commands
• yanking and text properties: Yanking
• yes-or-no questions: Yes-or-No Queries
• yes-or-no-p: Yes-or-No Queries
• zerop: Predicates on Numbers
• zlib-available-p: Decompression
• zlib-decompress-region: Decompression
• ‘|’ in regexp: Regexp Backslash