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), and completion of variable and function names (see Autoload by Prefix below).
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
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
loaddefs-generate, which constructs calls to
and arranges to execute them when Emacs is built.
This function defines the function (or macro) named function so as to load automatically from filename. The string filename specifies the file to load to get the real definition of function.
If filename does not contain either a directory name, or the
.elc, this function insists on adding one
of these suffixes, and it will not load from a file whose name is just
filename with no added suffix. (The variable
load-suffixes specifies the exact required suffixes.)
The argument docstring is the documentation string for the
function. Specifying the documentation string in the call to
autoload makes it possible to look at the documentation without
loading the function’s real definition. Normally, this should be
identical to the documentation string in the function definition
itself. If it isn’t, the function definition’s documentation string
takes effect when it is loaded.
If interactive is non-
nil, that says function can be
called interactively. This lets completion in M-x work without
loading function’s real definition. The complete interactive
specification is not given here; it’s not needed unless the user
actually calls function, and when that happens, it’s time to load
the real definition.
If interactive is a list, it is interpreted as a list of modes this command is applicable for.
You can autoload macros and keymaps as well as ordinary functions.
Specify type as
macro if function is really a macro.
Specify type as
keymap if function is really a
keymap. Various parts of Emacs need to know this information without
loading the real definition.
An autoloaded keymap loads automatically during key lookup when a prefix
key’s binding is the symbol function. Autoloading does not occur
for other kinds of access to the keymap. In particular, it does not
happen when a Lisp program gets the keymap from the value of a variable
keymap-set; not even if the variable name is the same
If function already has a non-void function definition that is not
an autoload object, this function does nothing and returns
Otherwise, it constructs an autoload object (see Autoload Type),
and stores it as the function definition for function. The
autoload object has this form:
(autoload filename docstring interactive type)
(symbol-function 'run-prolog) ⇒ (autoload "prolog" 169681 t nil)
In this case,
"prolog" is the name of the file to load, 169681
refers to the documentation string in the
emacs/etc/DOC file (see Documentation Basics),
t means the function is interactive, and
nil that it is
not a macro or a keymap.
This function returns non-
nil if object is an autoload
object. For example, to check if
run-prolog is defined as an
autoloaded function, evaluate
(autoloadp (symbol-function 'run-prolog))
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
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 function
writes a corresponding
autoload call into loaddefs.el.
(The string that serves as the autoload cookie and the name of the
file generated by
loaddefs-generate can be changed from the
above defaults, see below.)
Building Emacs loads loaddefs.el and thus calls
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
cl-defmacro (see Argument Lists in Common Lisp Extensions),
define-overloadable-function (see the commentary in
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
loaddefs-generate 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 'doctor "doctor" "\ Switch to *doctor* buffer and start giving psychotherapy. \(fn)" t nil)
While the loaddefs.el isn’t for editing, we try to keep it
somewhat readable for people. For instance, control characters in
defvar values are escaped, and we insert a backslash and
newline immediately following the double-quote of the doc string to
keep the line length down. ‘(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 value of this constant is a regexp that matches autoload cookies.
loaddefs-generate copies the Lisp form that follows the
cookie into the autoload file it generates. This will match comments
like ‘;;;###autoload’ and ‘;;;###calc-autoload’.
The value of this variable names an Emacs Lisp file where the autoload calls should go. The default value is loaddefs.el, but you can override that, e.g., in the local variables section of a .el file (see File Local Variables). The autoload file is assumed to contain a trailer starting with a formfeed character.
The following function may be used to explicitly load the library specified by an autoload object:
This function performs the loading specified by autoload, which
should be an autoload object. The optional argument name, if
nil, should be a symbol whose function value is
autoload; in that case, the return value of this function is the
symbol’s new function value. If the value of the optional argument
macro, this function avoids loading a
function, only a macro.