Table of Contents ***************** Autoconf 1 Introduction 2 The GNU Build System 2.1 Automake 2.2 Gnulib 2.3 Libtool 2.4 Pointers 3 Making `configure' Scripts 3.1 Writing `configure.ac' 3.1.1 A Shell Script Compiler 3.1.2 The Autoconf Language 3.1.3 Standard `configure.ac' Layout 3.2 Using `autoscan' to Create `configure.ac' 3.3 Using `ifnames' to List Conditionals 3.4 Using `autoconf' to Create `configure' 3.5 Using `autoreconf' to Update `configure' Scripts 4 Initialization and Output Files 4.1 Initializing `configure' 4.2 Dealing with Autoconf versions 4.3 Notices in `configure' 4.4 Finding `configure' Input 4.5 Outputting Files 4.6 Performing Configuration Actions 4.7 Creating Configuration Files 4.8 Substitutions in Makefiles 4.8.1 Preset Output Variables 4.8.2 Installation Directory Variables 4.8.3 Changed Directory Variables 4.8.4 Build Directories 4.8.5 Automatic Remaking 4.9 Configuration Header Files 4.9.1 Configuration Header Templates 4.9.2 Using `autoheader' to Create `config.h.in' 4.9.3 Autoheader Macros 4.10 Running Arbitrary Configuration Commands 4.11 Creating Configuration Links 4.12 Configuring Other Packages in Subdirectories 4.13 Default Prefix 5 Existing Tests 5.1 Common Behavior 5.1.1 Standard Symbols 5.1.2 Default Includes 5.2 Alternative Programs 5.2.1 Particular Program Checks 5.2.2 Generic Program and File Checks 5.3 Files 5.4 Library Files 5.5 Library Functions 5.5.1 Portability of C Functions 5.5.2 Particular Function Checks 5.5.3 Generic Function Checks 5.6 Header Files 5.6.1 Portability of Headers 5.6.2 Particular Header Checks 5.6.3 Generic Header Checks 5.7 Declarations 5.7.1 Particular Declaration Checks 5.7.2 Generic Declaration Checks 5.8 Structures 5.8.1 Particular Structure Checks 5.8.2 Generic Structure Checks 5.9 Types 5.9.1 Particular Type Checks 5.9.2 Generic Type Checks 5.10 Compilers and Preprocessors 5.10.1 Specific Compiler Characteristics 5.10.2 Generic Compiler Characteristics 5.10.3 C Compiler Characteristics 5.10.4 C++ Compiler Characteristics 5.10.5 Objective C Compiler Characteristics 5.10.6 Objective C++ Compiler Characteristics 5.10.7 Erlang Compiler and Interpreter Characteristics 5.10.8 Fortran Compiler Characteristics 5.11 System Services 5.12 Posix Variants 5.13 Erlang Libraries 6 Writing Tests 6.1 Language Choice 6.2 Writing Test Programs 6.2.1 Guidelines for Test Programs 6.2.2 Test Functions 6.2.3 Generating Sources 6.3 Running the Preprocessor 6.4 Running the Compiler 6.5 Running the Linker 6.6 Checking Runtime Behavior 6.7 Systemology 6.8 Multiple Cases 7 Results of Tests 7.1 Defining C Preprocessor Symbols 7.2 Setting Output Variables 7.3 Special Characters in Output Variables 7.4 Caching Results 7.4.1 Cache Variable Names 7.4.2 Cache Files 7.4.3 Cache Checkpointing 7.5 Printing Messages 8 Programming in M4 8.1 M4 Quotation 8.1.1 Active Characters 8.1.2 One Macro Call 8.1.3 Quoting and Parameters 8.1.4 Quotation and Nested Macros 8.1.5 `changequote' is Evil 8.1.6 Quadrigraphs 8.1.7 Dealing with unbalanced parentheses 8.1.8 Quotation Rule Of Thumb 8.2 Using `autom4te' 8.2.1 Invoking `autom4te' 8.2.2 Customizing `autom4te' 8.3 Programming in M4sugar 8.3.1 Redefined M4 Macros 8.3.2 Diagnostic messages from M4sugar 8.3.3 Diversion support 8.3.4 Conditional constructs 8.3.5 Looping constructs 8.3.6 Evaluation Macros 8.3.7 String manipulation in M4 8.3.8 Arithmetic computation in M4 8.3.9 Set manipulation in M4 8.3.10 Forbidden Patterns 8.4 Debugging via autom4te 9 Programming in M4sh 9.1 Common Shell Constructs 9.2 Support for indirect variable names 9.3 Initialization Macros 9.4 File Descriptor Macros 10 Writing Autoconf Macros 10.1 Macro Definitions 10.2 Macro Names 10.3 Reporting Messages 10.4 Dependencies Between Macros 10.4.1 Prerequisite Macros 10.4.2 Suggested Ordering 10.4.3 One-Shot Macros 10.5 Obsoleting Macros 10.6 Coding Style 11 Portable Shell Programming 11.1 Shellology 11.2 Here-Documents 11.3 File Descriptors 11.4 File System Conventions 11.5 Shell Pattern Matching 11.6 Shell Substitutions 11.7 Assignments 11.8 Parentheses in Shell Scripts 11.9 Slashes in Shell Scripts 11.10 Special Shell Variables 11.11 Shell Functions 11.12 Limitations of Shell Builtins 11.13 Limitations of Usual Tools 12 Portable Make Programming 12.1 `$<' in Ordinary Make Rules 12.2 Failure in Make Rules 12.3 Special Characters in Make Macro Names 12.4 Backslash-Newline-Newline in Make Macro Values 12.5 Backslash-Newline in Make Comments 12.6 Long Lines in Makefiles 12.7 `make macro=value' and Submakes 12.8 The Make Macro MAKEFLAGS 12.9 The Make Macro `SHELL' 12.10 Parallel Make 12.11 Comments in Make Rules 12.12 The `obj/' Subdirectory and Make 12.13 Exit Status of `make -k' 12.14 `VPATH' and Make 12.14.1 Variables listed in `VPATH' 12.14.2 `VPATH' and Double-colon Rules 12.14.3 `$<' Not Supported in Explicit Rules 12.14.4 Automatic Rule Rewriting 12.14.5 Tru64 `make' Creates Prerequisite Directories Magically 12.14.6 Make Target Lookup 12.15 Single Suffix Rules and Separated Dependencies 12.16 Timestamp Resolution and Make 13 Portable C and C++ Programming 13.1 Varieties of Unportability 13.2 Integer Overflow 13.2.1 Basics of Integer Overflow 13.2.2 Examples of Code Assuming Wraparound Overflow 13.2.3 Optimizations That Break Wraparound Arithmetic 13.2.4 Practical Advice for Signed Overflow Issues 13.2.5 Signed Integer Division and Integer Overflow 13.3 Preprocessor Arithmetic 13.4 Properties of Null Pointers 13.5 Buffer Overruns and Subscript Errors 13.6 Volatile Objects 13.7 Floating Point Portability 13.8 Exiting Portably 14 Manual Configuration 14.1 Specifying target triplets 14.2 Getting the Canonical System Type 14.3 Using the System Type 15 Site Configuration 15.1 Controlling Help Output 15.2 Working With External Software 15.3 Choosing Package Options 15.4 Making Your Help Strings Look Pretty 15.5 Controlling Checking of `configure' Options 15.6 Configuring Site Details 15.7 Transforming Program Names When Installing 15.7.1 Transformation Options 15.7.2 Transformation Examples 15.7.3 Transformation Rules 15.8 Setting Site Defaults 16 Running `configure' Scripts 16.1 Basic Installation 16.2 Compilers and Options 16.3 Compiling For Multiple Architectures 16.4 Installation Names 16.5 Optional Features 16.6 Particular systems 16.7 Specifying the System Type 16.8 Sharing Defaults 16.9 Defining Variables 16.10 `configure' Invocation 17 config.status Invocation 18 Obsolete Constructs 18.1 Obsolete `config.status' Invocation 18.2 `acconfig.h' 18.3 Using `autoupdate' to Modernize `configure.ac' 18.4 Obsolete Macros 18.5 Upgrading From Version 1 18.5.1 Changed File Names 18.5.2 Changed Makefiles 18.5.3 Changed Macros 18.5.4 Changed Results 18.5.5 Changed Macro Writing 18.6 Upgrading From Version 2.13 18.6.1 Changed Quotation 18.6.2 New Macros 18.6.3 Hosts and Cross-Compilation 18.6.4 `AC_LIBOBJ' vs. `LIBOBJS' 18.6.5 `AC_ACT_IFELSE' vs. `AC_TRY_ACT' 19 Generating Test Suites with Autotest 19.1 Using an Autotest Test Suite 19.1.1 `testsuite' Scripts 19.1.2 Autotest Logs 19.2 Writing `testsuite.at' 19.3 Running `testsuite' Scripts 19.4 Making `testsuite' Scripts 20 Frequent Autoconf Questions, with answers 20.1 Distributing `configure' Scripts 20.2 Why Require GNU M4? 20.3 How Can I Bootstrap? 20.4 Why Not Imake? 20.5 How Do I `#define' Installation Directories? 20.6 What is `autom4te.cache'? 20.7 Header Present But Cannot Be Compiled 20.8 Expanded Before Required 20.9 Debugging `configure' scripts 21 History of Autoconf 21.1 Genesis 21.2 Exodus 21.3 Leviticus 21.4 Numbers 21.5 Deuteronomy Appendix A GNU Free Documentation License Appendix B Indices B.1 Environment Variable Index B.2 Output Variable Index B.3 Preprocessor Symbol Index B.4 Cache Variable Index B.5 Autoconf Macro Index B.6 M4 Macro Index B.7 Autotest Macro Index B.8 Program and Function Index B.9 Concept Index Autoconf ******** This manual (4 November 2009) is for GNU Autoconf (version 2.65), a package for creating scripts to configure source code packages using templates and an M4 macro package. Copyright (C) 1992, 1993, 1994, 1995, 1996, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009 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 no Invariant Sections, 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." 1 Introduction ************** A physicist, an engineer, and a computer scientist were discussing the nature of God. "Surely a Physicist," said the physicist, "because early in the Creation, God made Light; and you know, Maxwell's equations, the dual nature of electromagnetic waves, the relativistic consequences..." "An Engineer!," said the engineer, "because before making Light, God split the Chaos into Land and Water; it takes a hell of an engineer to handle that big amount of mud, and orderly separation of solids from liquids..." The computer scientist shouted: "And the Chaos, where do you think it was coming from, hmm?" --Anonymous Autoconf is a tool for producing shell scripts that automatically configure software source code packages to adapt to many kinds of Posix-like systems. The configuration scripts produced by Autoconf are independent of Autoconf when they are run, so their users do not need to have Autoconf. The configuration scripts produced by Autoconf require no manual user intervention when run; they do not normally even need an argument specifying the system type. Instead, they individually test for the presence of each feature that the software package they are for might need. (Before each check, they print a one-line message stating what they are checking for, so the user doesn't get too bored while waiting for the script to finish.) As a result, they deal well with systems that are hybrids or customized from the more common Posix variants. There is no need to maintain files that list the features supported by each release of each variant of Posix. For each software package that Autoconf is used with, it creates a configuration script from a template file that lists the system features that the package needs or can use. After the shell code to recognize and respond to a system feature has been written, Autoconf allows it to be shared by many software packages that can use (or need) that feature. If it later turns out that the shell code needs adjustment for some reason, it needs to be changed in only one place; all of the configuration scripts can be regenerated automatically to take advantage of the updated code. Those who do not understand Autoconf are condemned to reinvent it, poorly. The primary goal of Autoconf is making the _user's_ life easier; making the _maintainer's_ life easier is only a secondary goal. Put another way, the primary goal is not to make the generation of `configure' automatic for package maintainers (although patches along that front are welcome, since package maintainers form the user base of Autoconf); rather, the goal is to make `configure' painless, portable, and predictable for the end user of each "autoconfiscated" package. And to this degree, Autoconf is highly successful at its goal -- most complaints to the Autoconf list are about difficulties in writing Autoconf input, and not in the behavior of the resulting `configure'. Even packages that don't use Autoconf will generally provide a `configure' script, and the most common complaint about these alternative home-grown scripts is that they fail to meet one or more of the GNU Coding Standards that users have come to expect from Autoconf-generated `configure' scripts. The Metaconfig package is similar in purpose to Autoconf, but the scripts it produces require manual user intervention, which is quite inconvenient when configuring large source trees. Unlike Metaconfig scripts, Autoconf scripts can support cross-compiling, if some care is taken in writing them. Autoconf does not solve all problems related to making portable software packages--for a more complete solution, it should be used in concert with other GNU build tools like Automake and Libtool. These other tools take on jobs like the creation of a portable, recursive makefile with all of the standard targets, linking of shared libraries, and so on. *Note The GNU Build System::, for more information. Autoconf imposes some restrictions on the names of macros used with `#if' in C programs (*note Preprocessor Symbol Index::). Autoconf requires GNU M4 version 1.4.6 or later in order to generate the scripts. It uses features that some versions of M4, including GNU M4 1.3, do not have. Autoconf works better with GNU M4 version 1.4.13 or later, though this is not required. *Note Autoconf 1::, for information about upgrading from version 1. *Note History::, for the story of Autoconf's development. *Note FAQ::, for answers to some common questions about Autoconf. See the Autoconf web page (http://www.gnu.org/software/autoconf/) for up-to-date information, details on the mailing lists, pointers to a list of known bugs, etc. Mail suggestions to the Autoconf mailing list . Past suggestions are archived (http://lists.gnu.org/archive/html/autoconf/). Mail bug reports to the Autoconf Bugs mailing list . Past bug reports are archived (http://lists.gnu.org/archive/html/bug-autoconf/). If possible, first check that your bug is not already solved in current development versions, and that it has not been reported yet. Be sure to include all the needed information and a short `configure.ac' that demonstrates the problem. Autoconf's development tree is accessible via `git'; see the Autoconf Summary (http://savannah.gnu.org/projects/autoconf/) for details, or view the actual repository (http://git.sv.gnu.org/gitweb/?p=autoconf.git). Anonymous CVS access is also available, see `README' for more details. Patches relative to the current `git' version can be sent for review to the Autoconf Patches mailing list , with discussion on prior patches archived (http://lists.gnu.org/archive/html/autoconf-patches/); and all commits are posted in the read-only Autoconf Commit mailing list , which is also archived (http://lists.gnu.org/archive/html/autoconf-commit/). Because of its mission, the Autoconf package itself includes only a set of often-used macros that have already demonstrated their usefulness. Nevertheless, if you wish to share your macros, or find existing ones, see the Autoconf Macro Archive (http://autoconf-archive.cryp.to/), which is kindly run by Peter Simons . 2 The GNU Build System ********************** Autoconf solves an important problem--reliable discovery of system-specific build and runtime information--but this is only one piece of the puzzle for the development of portable software. To this end, the GNU project has developed a suite of integrated utilities to finish the job Autoconf started: the GNU build system, whose most important components are Autoconf, Automake, and Libtool. In this chapter, we introduce you to those tools, point you to sources of more information, and try to convince you to use the entire GNU build system for your software. 2.1 Automake ============ The ubiquity of `make' means that a makefile is almost the only viable way to distribute automatic build rules for software, but one quickly runs into its numerous limitations. Its lack of support for automatic dependency tracking, recursive builds in subdirectories, reliable timestamps (e.g., for network file systems), and so on, mean that developers must painfully (and often incorrectly) reinvent the wheel for each project. Portability is non-trivial, thanks to the quirks of `make' on many systems. On top of all this is the manual labor required to implement the many standard targets that users have come to expect (`make install', `make distclean', `make uninstall', etc.). Since you are, of course, using Autoconf, you also have to insert repetitive code in your `Makefile.in' to recognize `@CC@', `@CFLAGS@', and other substitutions provided by `configure'. Into this mess steps "Automake". Automake allows you to specify your build needs in a `Makefile.am' file with a vastly simpler and more powerful syntax than that of a plain makefile, and then generates a portable `Makefile.in' for use with Autoconf. For example, the `Makefile.am' to build and install a simple "Hello world" program might look like: bin_PROGRAMS = hello hello_SOURCES = hello.c The resulting `Makefile.in' (~400 lines) automatically supports all the standard targets, the substitutions provided by Autoconf, automatic dependency tracking, `VPATH' building, and so on. `make' builds the `hello' program, and `make install' installs it in `/usr/local/bin' (or whatever prefix was given to `configure', if not `/usr/local'). The benefits of Automake increase for larger packages (especially ones with subdirectories), but even for small programs the added convenience and portability can be substantial. And that's not all... 2.2 Gnulib ========== GNU software has a well-deserved reputation for running on many different types of systems. While our primary goal is to write software for the GNU system, many users and developers have been introduced to us through the systems that they were already using. Gnulib is a central location for common GNU code, intended to be shared among free software packages. Its components are typically shared at the source level, rather than being a library that gets built, installed, and linked against. The idea is to copy files from Gnulib into your own source tree. There is no distribution tarball; developers should just grab source modules from the repository. The source files are available online, under various licenses, mostly GNU GPL or GNU LGPL. Gnulib modules typically contain C source code along with Autoconf macros used to configure the source code. For example, the Gnulib `stdbool' module implements a `stdbool.h' header that nearly conforms to C99, even on old-fashioned hosts that lack `stdbool.h'. This module contains a source file for the replacement header, along with an Autoconf macro that arranges to use the replacement header on old-fashioned systems. 2.3 Libtool =========== Often, one wants to build not only programs, but libraries, so that other programs can benefit from the fruits of your labor. Ideally, one would like to produce _shared_ (dynamically linked) libraries, which can be used by multiple programs without duplication on disk or in memory and can be updated independently of the linked programs. Producing shared libraries portably, however, is the stuff of nightmares--each system has its own incompatible tools, compiler flags, and magic incantations. Fortunately, GNU provides a solution: "Libtool". Libtool handles all the requirements of building shared libraries for you, and at this time seems to be the _only_ way to do so with any portability. It also handles many other headaches, such as: the interaction of Make rules with the variable suffixes of shared libraries, linking reliably with shared libraries before they are installed by the superuser, and supplying a consistent versioning system (so that different versions of a library can be installed or upgraded without breaking binary compatibility). Although Libtool, like Autoconf, can be used without Automake, it is most simply utilized in conjunction with Automake--there, Libtool is used automatically whenever shared libraries are needed, and you need not know its syntax. 2.4 Pointers ============ Developers who are used to the simplicity of `make' for small projects on a single system might be daunted at the prospect of learning to use Automake and Autoconf. As your software is distributed to more and more users, however, you otherwise quickly find yourself putting lots of effort into reinventing the services that the GNU build tools provide, and making the same mistakes that they once made and overcame. (Besides, since you're already learning Autoconf, Automake is a piece of cake.) There are a number of places that you can go to for more information on the GNU build tools. - Web The project home pages for Autoconf (http://www.gnu.org/software/autoconf/), Automake (http://www.gnu.org/software/automake/), Gnulib (http://www.gnu.org/software/gnulib/), and Libtool (http://www.gnu.org/software/libtool/). - Automake Manual *Note Automake: (automake)Top, for more information on Automake. - Books The book `GNU Autoconf, Automake and Libtool'(1) describes the complete GNU build environment. You can also find the entire book on-line (http://sources.redhat.com/autobook/). ---------- Footnotes ---------- (1) `GNU Autoconf, Automake and Libtool', by G. V. Vaughan, B. Elliston, T. Tromey, and I. L. Taylor. SAMS (originally New Riders), 2000, ISBN 1578701902. 3 Making `configure' Scripts **************************** The configuration scripts that Autoconf produces are by convention called `configure'. When run, `configure' creates several files, replacing configuration parameters in them with appropriate values. The files that `configure' creates are: - one or more `Makefile' files, usually one in each subdirectory of the package (*note Makefile Substitutions::); - optionally, a C header file, the name of which is configurable, containing `#define' directives (*note Configuration Headers::); - a shell script called `config.status' that, when run, recreates the files listed above (*note config.status Invocation::); - an optional shell script normally called `config.cache' (created when using `configure --config-cache') that saves the results of running many of the tests (*note Cache Files::); - a file called `config.log' containing any messages produced by compilers, to help debugging if `configure' makes a mistake. To create a `configure' script with Autoconf, you need to write an Autoconf input file `configure.ac' (or `configure.in') and run `autoconf' on it. If you write your own feature tests to supplement those that come with Autoconf, you might also write files called `aclocal.m4' and `acsite.m4'. If you use a C header file to contain `#define' directives, you might also run `autoheader', and you can distribute the generated file `config.h.in' with the package. Here is a diagram showing how the files that can be used in configuration are produced. Programs that are executed are suffixed by `*'. Optional files are enclosed in square brackets (`[]'). `autoconf' and `autoheader' also read the installed Autoconf macro files (by reading `autoconf.m4'). Files used in preparing a software package for distribution: your source files --> [autoscan*] --> [configure.scan] --> configure.ac configure.ac --. | .------> autoconf* -----> configure [aclocal.m4] --+---+ | `-----> [autoheader*] --> [config.h.in] [acsite.m4] ---' Makefile.in -------------------------------> Makefile.in Files used in configuring a software package: .-------------> [config.cache] configure* ------------+-------------> config.log | [config.h.in] -. v .-> [config.h] -. +--> config.status* -+ +--> make* Makefile.in ---' `-> Makefile ---' 3.1 Writing `configure.ac' ========================== To produce a `configure' script for a software package, create a file called `configure.ac' that contains invocations of the Autoconf macros that test the system features your package needs or can use. Autoconf macros already exist to check for many features; see *note Existing Tests::, for their descriptions. For most other features, you can use Autoconf template macros to produce custom checks; see *note Writing Tests::, for information about them. For especially tricky or specialized features, `configure.ac' might need to contain some hand-crafted shell commands; see *note Portable Shell Programming: Portable Shell. The `autoscan' program can give you a good start in writing `configure.ac' (*note autoscan Invocation::, for more information). Previous versions of Autoconf promoted the name `configure.in', which is somewhat ambiguous (the tool needed to process this file is not described by its extension), and introduces a slight confusion with `config.h.in' and so on (for which `.in' means "to be processed by `configure'"). Using `configure.ac' is now preferred. 3.1.1 A Shell Script Compiler ----------------------------- Just as for any other computer language, in order to properly program `configure.ac' in Autoconf you must understand _what_ problem the language tries to address and _how_ it does so. The problem Autoconf addresses is that the world is a mess. After all, you are using Autoconf in order to have your package compile easily on all sorts of different systems, some of them being extremely hostile. Autoconf itself bears the price for these differences: `configure' must run on all those systems, and thus `configure' must limit itself to their lowest common denominator of features. Naturally, you might then think of shell scripts; who needs `autoconf'? A set of properly written shell functions is enough to make it easy to write `configure' scripts by hand. Sigh! Unfortunately, even in 2008, where shells without any function support are far and few between, there are pitfalls to avoid when making use of them. Also, finding a Bourne shell that accepts shell functions is not trivial, even though there is almost always one on interesting porting targets. So, what is really needed is some kind of compiler, `autoconf', that takes an Autoconf program, `configure.ac', and transforms it into a portable shell script, `configure'. How does `autoconf' perform this task? There are two obvious possibilities: creating a brand new language or extending an existing one. The former option is attractive: all sorts of optimizations could easily be implemented in the compiler and many rigorous checks could be performed on the Autoconf program (e.g., rejecting any non-portable construct). Alternatively, you can extend an existing language, such as the `sh' (Bourne shell) language. Autoconf does the latter: it is a layer on top of `sh'. It was therefore most convenient to implement `autoconf' as a macro expander: a program that repeatedly performs "macro expansions" on text input, replacing macro calls with macro bodies and producing a pure `sh' script in the end. Instead of implementing a dedicated Autoconf macro expander, it is natural to use an existing general-purpose macro language, such as M4, and implement the extensions as a set of M4 macros. 3.1.2 The Autoconf Language --------------------------- The Autoconf language differs from many other computer languages because it treats actual code the same as plain text. Whereas in C, for instance, data and instructions have different syntactic status, in Autoconf their status is rigorously the same. Therefore, we need a means to distinguish literal strings from text to be expanded: quotation. When calling macros that take arguments, there must not be any white space between the macro name and the open parenthesis. Arguments should be enclosed within the M4 quote characters `[' and `]', and be separated by commas. Any leading blanks or newlines in arguments are ignored, unless they are quoted. You should always quote an argument that might contain a macro name, comma, parenthesis, or a leading blank or newline. This rule applies recursively for every macro call, including macros called from other macros. For instance: AC_CHECK_HEADER([stdio.h], [AC_DEFINE([HAVE_STDIO_H], [1], [Define to 1 if you have .])], [AC_MSG_ERROR([Sorry, can't do anything for you])]) is quoted properly. You may safely simplify its quotation to: AC_CHECK_HEADER([stdio.h], [AC_DEFINE([HAVE_STDIO_H], 1, [Define to 1 if you have .])], [AC_MSG_ERROR([Sorry, can't do anything for you])]) because `1' cannot contain a macro call. Here, the argument of `AC_MSG_ERROR' must be quoted; otherwise, its comma would be interpreted as an argument separator. Also, the second and third arguments of `AC_CHECK_HEADER' must be quoted, since they contain macro calls. The three arguments `HAVE_STDIO_H', `stdio.h', and `Define to 1 if you have .' do not need quoting, but if you unwisely defined a macro with a name like `Define' or `stdio' then they would need quoting. Cautious Autoconf users would keep the quotes, but many Autoconf users find such precautions annoying, and would rewrite the example as follows: AC_CHECK_HEADER(stdio.h, [AC_DEFINE(HAVE_STDIO_H, 1, [Define to 1 if you have .])], [AC_MSG_ERROR([Sorry, can't do anything for you])]) This is safe, so long as you adopt good naming conventions and do not define macros with names like `HAVE_STDIO_H', `stdio', or `h'. Though it is also safe here to omit the quotes around `Define to 1 if you have .' this is not recommended, as message strings are more likely to inadvertently contain commas. The following example is wrong and dangerous, as it is underquoted: AC_CHECK_HEADER(stdio.h, AC_DEFINE(HAVE_STDIO_H, 1, Define to 1 if you have .), AC_MSG_ERROR([Sorry, can't do anything for you])) In other cases, you may have to use text that also resembles a macro call. You must quote that text even when it is not passed as a macro argument. For example, these two approaches in `configure.ac' (quoting just the potential problems, or quoting the entire line) will protect your script in case autoconf ever adds a macro `AC_DC': echo "Hard rock was here! --[AC_DC]" [echo "Hard rock was here! --AC_DC"] which results in this text in `configure': echo "Hard rock was here! --AC_DC" echo "Hard rock was here! --AC_DC" When you use the same text in a macro argument, you must therefore have an extra quotation level (since one is stripped away by the macro substitution). In general, then, it is a good idea to _use double quoting for all literal string arguments_, either around just the problematic portions, or over the entire argument: AC_MSG_WARN([[AC_DC] stinks --Iron Maiden]) AC_MSG_WARN([[AC_DC stinks --Iron Maiden]]) However, the above example triggers a warning about a possibly unexpanded macro when running `autoconf', because it collides with the namespace of macros reserved for the Autoconf language. To be really safe, you can use additional escaping (either a quadrigraph, or creative shell constructs) to silence that particular warning: echo "Hard rock was here! --AC""_DC" AC_MSG_WARN([[AC@&t@_DC stinks --Iron Maiden]]) You are now able to understand one of the constructs of Autoconf that has been continually misunderstood... The rule of thumb is that _whenever you expect macro expansion, expect quote expansion_; i.e., expect one level of quotes to be lost. For instance: AC_COMPILE_IFELSE([char b[10];], [], [AC_MSG_ERROR([you lose])]) is incorrect: here, the first argument of `AC_COMPILE_IFELSE' is `char b[10];' and is expanded once, which results in `char b10;'. (There was an idiom common in Autoconf's past to address this issue via the M4 `changequote' primitive, but do not use it!) Let's take a closer look: the author meant the first argument to be understood as a literal, and therefore it must be quoted twice: AC_COMPILE_IFELSE([[char b[10];]], [], [AC_MSG_ERROR([you lose])]) Voila`, you actually produce `char b[10];' this time! On the other hand, descriptions (e.g., the last parameter of `AC_DEFINE' or `AS_HELP_STRING') are not literals--they are subject to line breaking, for example--and should not be double quoted. Even if these descriptions are short and are not actually broken, double quoting them yields weird results. Some macros take optional arguments, which this documentation represents as [ARG] (not to be confused with the quote characters). You may just leave them empty, or use `[]' to make the emptiness of the argument explicit, or you may simply omit the trailing commas. The three lines below are equivalent: AC_CHECK_HEADERS([stdio.h], [], [], []) AC_CHECK_HEADERS([stdio.h],,,) AC_CHECK_HEADERS([stdio.h]) It is best to put each macro call on its own line in `configure.ac'. Most of the macros don't add extra newlines; they rely on the newline after the macro call to terminate the commands. This approach makes the generated `configure' script a little easier to read by not inserting lots of blank lines. It is generally safe to set shell variables on the same line as a macro call, because the shell allows assignments without intervening newlines. You can include comments in `configure.ac' files by starting them with the `#'. For example, it is helpful to begin `configure.ac' files with a line like this: # Process this file with autoconf to produce a configure script. 3.1.3 Standard `configure.ac' Layout ------------------------------------ The order in which `configure.ac' calls the Autoconf macros is not important, with a few exceptions. Every `configure.ac' must contain a call to `AC_INIT' before the checks, and a call to `AC_OUTPUT' at the end (*note Output::). Additionally, some macros rely on other macros having been called first, because they check previously set values of some variables to decide what to do. These macros are noted in the individual descriptions (*note Existing Tests::), and they also warn you when `configure' is created if they are called out of order. To encourage consistency, here is a suggested order for calling the Autoconf macros. Generally speaking, the things near the end of this list are those that could depend on things earlier in it. For example, library functions could be affected by types and libraries. Autoconf requirements `AC_INIT(PACKAGE, VERSION, BUG-REPORT-ADDRESS)' information on the package checks for programs checks for libraries checks for header files checks for types checks for structures checks for compiler characteristics checks for library functions checks for system services `AC_CONFIG_FILES([FILE...])' `AC_OUTPUT' 3.2 Using `autoscan' to Create `configure.ac' ============================================= The `autoscan' program can help you create and/or maintain a `configure.ac' file for a software package. `autoscan' examines source files in the directory tree rooted at a directory given as a command line argument, or the current directory if none is given. It searches the source files for common portability problems and creates a file `configure.scan' which is a preliminary `configure.ac' for that package, and checks a possibly existing `configure.ac' for completeness. When using `autoscan' to create a `configure.ac', you should manually examine `configure.scan' before renaming it to `configure.ac'; it probably needs some adjustments. Occasionally, `autoscan' outputs a macro in the wrong order relative to another macro, so that `autoconf' produces a warning; you need to move such macros manually. Also, if you want the package to use a configuration header file, you must add a call to `AC_CONFIG_HEADERS' (*note Configuration Headers::). You might also have to change or add some `#if' directives to your program in order to make it work with Autoconf (*note ifnames Invocation::, for information about a program that can help with that job). When using `autoscan' to maintain a `configure.ac', simply consider adding its suggestions. The file `autoscan.log' contains detailed information on why a macro is requested. `autoscan' uses several data files (installed along with Autoconf) to determine which macros to output when it finds particular symbols in a package's source files. These data files all have the same format: each line consists of a symbol, one or more blanks, and the Autoconf macro to output if that symbol is encountered. Lines starting with `#' are comments. `autoscan' accepts the following options: `--help' `-h' Print a summary of the command line options and exit. `--version' `-V' Print the version number of Autoconf and exit. `--verbose' `-v' Print the names of the files it examines and the potentially interesting symbols it finds in them. This output can be voluminous. `--debug' `-d' Don't remove temporary files. `--include=DIR' `-I DIR' Append DIR to the include path. Multiple invocations accumulate. `--prepend-include=DIR' `-B DIR' Prepend DIR to the include path. Multiple invocations accumulate. 3.3 Using `ifnames' to List Conditionals ======================================== `ifnames' can help you write `configure.ac' for a software package. It prints the identifiers that the package already uses in C preprocessor conditionals. If a package has already been set up to have some portability, `ifnames' can thus help you figure out what its `configure' needs to check for. It may help fill in some gaps in a `configure.ac' generated by `autoscan' (*note autoscan Invocation::). `ifnames' scans all of the C source files named on the command line (or the standard input, if none are given) and writes to the standard output a sorted list of all the identifiers that appear in those files in `#if', `#elif', `#ifdef', or `#ifndef' directives. It prints each identifier on a line, followed by a space-separated list of the files in which that identifier occurs. `ifnames' accepts the following options: `--help' `-h' Print a summary of the command line options and exit. `--version' `-V' Print the version number of Autoconf and exit. 3.4 Using `autoconf' to Create `configure' ========================================== To create `configure' from `configure.ac', run the `autoconf' program with no arguments. `autoconf' processes `configure.ac' with the M4 macro processor, using the Autoconf macros. If you give `autoconf' an argument, it reads that file instead of `configure.ac' and writes the configuration script to the standard output instead of to `configure'. If you give `autoconf' the argument `-', it reads from the standard input instead of `configure.ac' and writes the configuration script to the standard output. The Autoconf macros are defined in several files. Some of the files are distributed with Autoconf; `autoconf' reads them first. Then it looks for the optional file `acsite.m4' in the directory that contains the distributed Autoconf macro files, and for the optional file `aclocal.m4' in the current directory. Those files can contain your site's or the package's own Autoconf macro definitions (*note Writing Autoconf Macros::, for more information). If a macro is defined in more than one of the files that `autoconf' reads, the last definition it reads overrides the earlier ones. `autoconf' accepts the following options: `--help' `-h' Print a summary of the command line options and exit. `--version' `-V' Print the version number of Autoconf and exit. `--verbose' `-v' Report processing steps. `--debug' `-d' Don't remove the temporary files. `--force' `-f' Remake `configure' even if newer than its input files. `--include=DIR' `-I DIR' Append DIR to the include path. Multiple invocations accumulate. `--prepend-include=DIR' `-B DIR' Prepend DIR to the include path. Multiple invocations accumulate. `--output=FILE' `-o FILE' Save output (script or trace) to FILE. The file `-' stands for the standard output. `--warnings=CATEGORY' `-W CATEGORY' Report the warnings related to CATEGORY (which can actually be a comma separated list). *Note Reporting Messages::, macro `AC_DIAGNOSE', for a comprehensive list of categories. Special values include: `all' report all the warnings `none' report none `error' treats warnings as errors `no-CATEGORY' disable warnings falling into CATEGORY Warnings about `syntax' are enabled by default, and the environment variable `WARNINGS', a comma separated list of categories, is honored as well. Passing `-W CATEGORY' actually behaves as if you had passed `--warnings syntax,$WARNINGS,CATEGORY'. To disable the defaults and `WARNINGS', and then enable warnings about obsolete constructs, use `-W none,obsolete'. Because `autoconf' uses `autom4te' behind the scenes, it displays a back trace for errors, but not for warnings; if you want them, just pass `-W error'. *Note autom4te Invocation::, for some examples. `--trace=MACRO[:FORMAT]' `-t MACRO[:FORMAT]' Do not create the `configure' script, but list the calls to MACRO according to the FORMAT. Multiple `--trace' arguments can be used to list several macros. Multiple `--trace' arguments for a single macro are not cumulative; instead, you should just make FORMAT as long as needed. The FORMAT is a regular string, with newlines if desired, and several special escape codes. It defaults to `$f:$l:$n:$%'; see *note autom4te Invocation::, for details on the FORMAT. `--initialization' `-i' By default, `--trace' does not trace the initialization of the Autoconf macros (typically the `AC_DEFUN' definitions). This results in a noticeable speedup, but can be disabled by this option. It is often necessary to check the content of a `configure.ac' file, but parsing it yourself is extremely fragile and error-prone. It is suggested that you rely upon `--trace' to scan `configure.ac'. For instance, to find the list of variables that are substituted, use: $ autoconf -t AC_SUBST configure.ac:2:AC_SUBST:ECHO_C configure.ac:2:AC_SUBST:ECHO_N configure.ac:2:AC_SUBST:ECHO_T More traces deleted The example below highlights the difference between `$@', `$*', and `$%'. $ cat configure.ac AC_DEFINE(This, is, [an [example]]) $ autoconf -t 'AC_DEFINE:@: $@ *: $* %: $%' @: [This],[is],[an [example]] *: This,is,an [example] %: This:is:an [example] The FORMAT gives you a lot of freedom: $ autoconf -t 'AC_SUBST:$$ac_subst{"$1"} = "$f:$l";' $ac_subst{"ECHO_C"} = "configure.ac:2"; $ac_subst{"ECHO_N"} = "configure.ac:2"; $ac_subst{"ECHO_T"} = "configure.ac:2"; More traces deleted A long SEPARATOR can be used to improve the readability of complex structures, and to ease their parsing (for instance when no single character is suitable as a separator): $ autoconf -t 'AM_MISSING_PROG:${|:::::|}*' ACLOCAL|:::::|aclocal|:::::|$missing_dir AUTOCONF|:::::|autoconf|:::::|$missing_dir AUTOMAKE|:::::|automake|:::::|$missing_dir More traces deleted 3.5 Using `autoreconf' to Update `configure' Scripts ==================================================== Installing the various components of the GNU Build System can be tedious: running `autopoint' for Gettext, `automake' for `Makefile.in' etc. in each directory. It may be needed either because some tools such as `automake' have been updated on your system, or because some of the sources such as `configure.ac' have been updated, or finally, simply in order to install the GNU Build System in a fresh tree. `autoreconf' runs `autoconf', `autoheader', `aclocal', `automake', `libtoolize', and `autopoint' (when appropriate) repeatedly to update the GNU Build System in the specified directories and their subdirectories (*note Subdirectories::). By default, it only remakes those files that are older than their sources. The environment variables `AUTOCONF', `AUTOHEADER', `AUTOMAKE', `ACLOCAL', `AUTOPOINT', `LIBTOOLIZE', `M4', and `MAKE' may be used to override the invocation of the respective tools. If you install a new version of some tool, you can make `autoreconf' remake _all_ of the files by giving it the `--force' option. *Note Automatic Remaking::, for Make rules to automatically rebuild `configure' scripts when their source files change. That method handles the timestamps of configuration header templates properly, but does not pass `--autoconf-dir=DIR' or `--localdir=DIR'. Gettext supplies the `autopoint' command to add translation infrastructure to a source package. If you use `autopoint', your `configure.ac' should invoke both `AM_GNU_GETTEXT' and `AM_GNU_GETTEXT_VERSION(GETTEXT-VERSION)'. *Note Invoking the `autopoint' Program: (gettext)autopoint Invocation, for further details. `autoreconf' accepts the following options: `--help' `-h' Print a summary of the command line options and exit. `--version' `-V' Print the version number of Autoconf and exit. `--verbose' `-V' Print the name of each directory `autoreconf' examines and the commands it runs. If given two or more times, pass `--verbose' to subordinate tools that support it. `--debug' `-d' Don't remove the temporary files. `--force' `-f' Remake even `configure' scripts and configuration headers that are newer than their input files (`configure.ac' and, if present, `aclocal.m4'). `--install' `-i' Install the missing auxiliary files in the package. By default, files are copied; this can be changed with `--symlink'. If deemed appropriate, this option triggers calls to `automake --add-missing', `libtoolize', `autopoint', etc. `--no-recursive' Do not rebuild files in subdirectories to configure (see *note Subdirectories::, macro `AC_CONFIG_SUBDIRS'). `--symlink' `-s' When used with `--install', install symbolic links to the missing auxiliary files instead of copying them. `--make' `-m' When the directories were configured, update the configuration by running `./config.status --recheck && ./config.status', and then run `make'. `--include=DIR' `-I DIR' Append DIR to the include path. Multiple invocations accumulate. Passed on to `aclocal', `autoconf' and `autoheader' internally. `--prepend-include=DIR' `-B DIR' Prepend DIR to the include path. Multiple invocations accumulate. Passed on to `autoconf' and `autoheader' internally. `--warnings=CATEGORY' `-W CATEGORY' Report the warnings related to CATEGORY (which can actually be a comma separated list). `cross' related to cross compilation issues. `obsolete' report the uses of obsolete constructs. `portability' portability issues `syntax' dubious syntactic constructs. `all' report all the warnings `none' report none `error' treats warnings as errors `no-CATEGORY' disable warnings falling into CATEGORY Warnings about `syntax' are enabled by default, and the environment variable `WARNINGS', a comma separated list of categories, is honored as well. Passing `-W CATEGORY' actually behaves as if you had passed `--warnings syntax,$WARNINGS,CATEGORY'. To disable the defaults and `WARNINGS', and then enable warnings about obsolete constructs, use `-W none,obsolete'. If you want `autoreconf' to pass flags that are not listed here on to `aclocal', set `ACLOCAL_AMFLAGS' in your `Makefile.am'. Due to a limitation in the Autoconf implementation these flags currently must be set on a single line in `Makefile.am', without any backslash-newlines. 4 Initialization and Output Files ********************************* Autoconf-generated `configure' scripts need some information about how to initialize, such as how to find the package's source files and about the output files to produce. The following sections describe the initialization and the creation of output files. 4.1 Initializing `configure' ============================ Every `configure' script must call `AC_INIT' before doing anything else that produces output. Calls to silent macros, such as `AC_DEFUN', may also occur prior to `AC_INIT', although these are generally used via `aclocal.m4', since that is implicitly included before the start of `configure.ac'. The only other required macro is `AC_OUTPUT' (*note Output::). -- Macro: AC_INIT (PACKAGE, VERSION, [BUG-REPORT], [TARNAME], [URL]) Process any command-line arguments and perform various initializations and verifications. Set the name of the PACKAGE and its VERSION. These are typically used in `--version' support, including that of `configure'. The optional argument BUG-REPORT should be the email to which users should send bug reports. The package TARNAME differs from PACKAGE: the latter designates the full package name (e.g., `GNU Autoconf'), while the former is meant for distribution tar ball names (e.g., `autoconf'). It defaults to PACKAGE with `GNU ' stripped, lower-cased, and all characters other than alphanumerics and underscores are changed to `-'. If provided, URL should be the home page for the package. It is preferable that the arguments of `AC_INIT' be static, i.e., there should not be any shell computation, but they can be computed by M4. The following M4 macros (e.g., `AC_PACKAGE_NAME'), output variables (e.g., `PACKAGE_NAME'), and preprocessor symbols (e.g., `PACKAGE_NAME'), are defined by `AC_INIT': `AC_PACKAGE_NAME', `PACKAGE_NAME' Exactly PACKAGE. `AC_PACKAGE_TARNAME', `PACKAGE_TARNAME' Exactly TARNAME, possibly generated from PACKAGE. `AC_PACKAGE_VERSION', `PACKAGE_VERSION' Exactly VERSION. `AC_PACKAGE_STRING', `PACKAGE_STRING' Exactly `PACKAGE VERSION'. `AC_PACKAGE_BUGREPORT', `PACKAGE_BUGREPORT' Exactly BUG-REPORT, if one was provided. `AC_PACKAGE_URL', `PACKAGE_URL' Exactly URL, if one was provided. If URL was empty, but PACKAGE begins with `GNU ', then this defaults to `http://www.gnu.org/software/TARNAME/', otherwise, no URL is assumed. If your `configure' script does its own option processing, it should inspect `$@' or `$*' immediately after calling `AC_INIT', because other Autoconf macros liberally use the `set' command to process strings, and this has the side effect of updating `$@' and `$*'. However, we suggest that you use standard macros like `AC_ARG_ENABLE' instead of attempting to implement your own option processing. *Note Site Configuration::. 4.2 Dealing with Autoconf versions ================================== The following optional macros can be used to help choose the minimum version of Autoconf that can successfully compile a given `configure.ac'. -- Macro: AC_PREREQ (VERSION) Ensure that a recent enough version of Autoconf is being used. If the version of Autoconf being used to create `configure' is earlier than VERSION, print an error message to the standard error output and exit with failure (exit status is 63). For example: AC_PREREQ([2.65]) This macro may be used before `AC_INIT'. -- Macro: AC_AUTOCONF_VERSION This macro was introduced in Autoconf 2.62. It identifies the version of Autoconf that is currently parsing the input file, in a format suitable for `m4_version_compare' (*note m4_version_compare::); in other words, for this release of Autoconf, its value is `2.65'. One potential use of this macro is for writing conditional fallbacks based on when a feature was added to Autoconf, rather than using `AC_PREREQ' to require the newer version of Autoconf. However, remember that the Autoconf philosophy favors feature checks over version checks. You should not expand this macro directly; use `m4_defn([AC_AUTOCONF_VERSION])' instead. This is because some users might have a beta version of Autoconf installed, with arbitrary letters included in its version string. This means it is possible for the version string to contain the name of a defined macro, such that expanding `AC_AUTOCONF_VERSION' would trigger the expansion of that macro during rescanning, and change the version string to be different than what you intended to check. 4.3 Notices in `configure' ========================== The following macros manage version numbers for `configure' scripts. Using them is optional. -- Macro: AC_COPYRIGHT (COPYRIGHT-NOTICE) State that, in addition to the Free Software Foundation's copyright on the Autoconf macros, parts of your `configure' are covered by the COPYRIGHT-NOTICE. The COPYRIGHT-NOTICE shows up in both the head of `configure' and in `configure --version'. -- Macro: AC_REVISION (REVISION-INFO) Copy revision stamp REVISION-INFO into the `configure' script, with any dollar signs or double-quotes removed. This macro lets you put a revision stamp from `configure.ac' into `configure' without RCS or CVS changing it when you check in `configure'. That way, you can determine easily which revision of `configure.ac' a particular `configure' corresponds to. For example, this line in `configure.ac': AC_REVISION([$Revision: 1.30 $]) produces this in `configure': #!/bin/sh # From configure.ac Revision: 1.30 4.4 Finding `configure' Input ============================= -- Macro: AC_CONFIG_SRCDIR (UNIQUE-FILE-IN-SOURCE-DIR) UNIQUE-FILE-IN-SOURCE-DIR is some file that is in the package's source directory; `configure' checks for this file's existence to make sure that the directory that it is told contains the source code in fact does. Occasionally people accidentally specify the wrong directory with `--srcdir'; this is a safety check. *Note configure Invocation::, for more information. Packages that do manual configuration or use the `install' program might need to tell `configure' where to find some other shell scripts by calling `AC_CONFIG_AUX_DIR', though the default places it looks are correct for most cases. -- Macro: AC_CONFIG_AUX_DIR (DIR) Use the auxiliary build tools (e.g., `install-sh', `config.sub', `config.guess', Cygnus `configure', Automake and Libtool scripts, etc.) that are in directory DIR. These are auxiliary files used in configuration. DIR can be either absolute or relative to `SRCDIR'. The default is `SRCDIR' or `SRCDIR/..' or `SRCDIR/../..', whichever is the first that contains `install-sh'. The other files are not checked for, so that using `AC_PROG_INSTALL' does not automatically require distributing the other auxiliary files. It checks for `install.sh' also, but that name is obsolete because some `make' have a rule that creates `install' from it if there is no makefile. The auxiliary directory is commonly named `build-aux'. If you need portability to DOS variants, do not name the auxiliary directory `aux'. *Note File System Conventions::. -- Macro: AC_REQUIRE_AUX_FILE (FILE) Declares that FILE is expected in the directory defined above. In Autoconf proper, this macro does nothing: its sole purpose is to be traced by third-party tools to produce a list of expected auxiliary files. For instance it is called by macros like `AC_PROG_INSTALL' (*note Particular Programs::) or `AC_CANONICAL_BUILD' (*note Canonicalizing::) to register the auxiliary files they need. Similarly, packages that use `aclocal' should declare where local macros can be found using `AC_CONFIG_MACRO_DIR'. -- Macro: AC_CONFIG_MACRO_DIR (DIR) Specify DIR as the location of additional local Autoconf macros. This macro is intended for use by future versions of commands like `autoreconf' that trace macro calls. It should be called directly from `configure.ac' so that tools that install macros for `aclocal' can find the macros' declarations. Note that if you use `aclocal' from Automake to generate `aclocal.m4', you must also set `ACLOCAL_AMFLAGS = -I DIR' in your top-level `Makefile.am'. Due to a limitation in the Autoconf implementation of `autoreconf', these include directives currently must be set on a single line in `Makefile.am', without any backslash-newlines. 4.5 Outputting Files ==================== Every Autoconf script, e.g., `configure.ac', should finish by calling `AC_OUTPUT'. That is the macro that generates and runs `config.status', which in turn creates the makefiles and any other files resulting from configuration. This is the only required macro besides `AC_INIT' (*note Input::). -- Macro: AC_OUTPUT Generate `config.status' and launch it. Call this macro once, at the end of `configure.ac'. `config.status' performs all the configuration actions: all the output files (see *note Configuration Files::, macro `AC_CONFIG_FILES'), header files (see *note Configuration Headers::, macro `AC_CONFIG_HEADERS'), commands (see *note Configuration Commands::, macro `AC_CONFIG_COMMANDS'), links (see *note Configuration Links::, macro `AC_CONFIG_LINKS'), subdirectories to configure (see *note Subdirectories::, macro `AC_CONFIG_SUBDIRS') are honored. The location of your `AC_OUTPUT' invocation is the exact point where configuration actions are taken: any code afterwards is executed by `configure' once `config.status' was run. If you want to bind actions to `config.status' itself (independently of whether `configure' is being run), see *note Running Arbitrary Configuration Commands: Configuration Commands. Historically, the usage of `AC_OUTPUT' was somewhat different. *Note Obsolete Macros::, for a description of the arguments that `AC_OUTPUT' used to support. If you run `make' in subdirectories, you should run it using the `make' variable `MAKE'. Most versions of `make' set `MAKE' to the name of the `make' program plus any options it was given. (But many do not include in it the values of any variables set on the command line, so those are not passed on automatically.) Some old versions of `make' do not set this variable. The following macro allows you to use it even with those versions. -- Macro: AC_PROG_MAKE_SET If the Make command, `$MAKE' if set or else `make', predefines `$(MAKE)', define output variable `SET_MAKE' to be empty. Otherwise, define `SET_MAKE' to a macro definition that sets `$(MAKE)', such as `MAKE=make'. Calls `AC_SUBST' for `SET_MAKE'. If you use this macro, place a line like this in each `Makefile.in' that runs `MAKE' on other directories: @SET_MAKE@ 4.6 Performing Configuration Actions ==================================== `configure' is designed so that it appears to do everything itself, but there is actually a hidden slave: `config.status'. `configure' is in charge of examining your system, but it is `config.status' that actually takes the proper actions based on the results of `configure'. The most typical task of `config.status' is to _instantiate_ files. This section describes the common behavior of the four standard instantiating macros: `AC_CONFIG_FILES', `AC_CONFIG_HEADERS', `AC_CONFIG_COMMANDS' and `AC_CONFIG_LINKS'. They all have this prototype: AC_CONFIG_ITEMS(TAG..., [COMMANDS], [INIT-CMDS]) where the arguments are: TAG... A blank-or-newline-separated list of tags, which are typically the names of the files to instantiate. You are encouraged to use literals as TAGS. In particular, you should avoid ... && my_foos="$my_foos fooo" ... && my_foos="$my_foos foooo" AC_CONFIG_ITEMS([$my_foos]) and use this instead: ... && AC_CONFIG_ITEMS([fooo]) ... && AC_CONFIG_ITEMS([foooo]) The macros `AC_CONFIG_FILES' and `AC_CONFIG_HEADERS' use special TAG values: they may have the form `OUTPUT' or `OUTPUT:INPUTS'. The file OUTPUT is instantiated from its templates, INPUTS (defaulting to `OUTPUT.in'). `AC_CONFIG_FILES([Makefile:boiler/top.mk:boiler/bot.mk)]', for example, asks for the creation of the file `Makefile' that contains the expansion of the output variables in the concatenation of `boiler/top.mk' and `boiler/bot.mk'. The special value `-' might be used to denote the standard output when used in OUTPUT, or the standard input when used in the INPUTS. You most probably don't need to use this in `configure.ac', but it is convenient when using the command line interface of `./config.status', see *note config.status Invocation::, for more details. The INPUTS may be absolute or relative file names. In the latter case they are first looked for in the build tree, and then in the source tree. Input files should be text files, and a line length below 2000 bytes should be safe. COMMANDS Shell commands output literally into `config.status', and associated with a tag that the user can use to tell `config.status' which commands to run. The commands are run each time a TAG request is given to `config.status', typically each time the file `TAG' is created. The variables set during the execution of `configure' are _not_ available here: you first need to set them via the INIT-CMDS. Nonetheless the following variables are precomputed: `srcdir' The name of the top source directory, assuming that the working directory is the top build directory. This is what the `configure' option `--srcdir' sets. `ac_top_srcdir' The name of the top source directory, assuming that the working directory is the current build directory. `ac_top_build_prefix' The name of the top build directory, assuming that the working directory is the current build directory. It can be empty, or else ends with a slash, so that you may concatenate it. `ac_srcdir' The name of the corresponding source directory, assuming that the working directory is the current build directory. `tmp' The name of a temporary directory within the build tree, which you can use if you need to create additional temporary files. The directory is cleaned up when `config.status' is done or interrupted. Please use package-specific file name prefixes to avoid clashing with files that `config.status' may use internally. The "current" directory refers to the directory (or pseudo-directory) containing the input part of TAGS. For instance, running AC_CONFIG_COMMANDS([deep/dir/out:in/in.in], [...], [...]) with `--srcdir=../package' produces the following values: # Argument of --srcdir srcdir='../package' # Reversing deep/dir ac_top_build_prefix='../../' # Concatenation of $ac_top_build_prefix and srcdir ac_top_srcdir='../../../package' # Concatenation of $ac_top_srcdir and deep/dir ac_srcdir='../../../package/deep/dir' independently of `in/in.in'. INIT-CMDS Shell commands output _unquoted_ near the beginning of `config.status', and executed each time `config.status' runs (regardless of the tag). Because they are unquoted, for example, `$var' is output as the value of `var'. INIT-CMDS is typically used by `configure' to give `config.status' some variables it needs to run the COMMANDS. You should be extremely cautious in your variable names: all the INIT-CMDS share the same name space and may overwrite each other in unpredictable ways. Sorry... All these macros can be called multiple times, with different TAG values, of course! 4.7 Creating Configuration Files ================================ Be sure to read the previous section, *note Configuration Actions::. -- Macro: AC_CONFIG_FILES (FILE..., [CMDS], [INIT-CMDS]) Make `AC_OUTPUT' create each `FILE' by copying an input file (by default `FILE.in'), substituting the output variable values. This macro is one of the instantiating macros; see *note Configuration Actions::. *Note Makefile Substitutions::, for more information on using output variables. *Note Setting Output Variables::, for more information on creating them. This macro creates the directory that the file is in if it doesn't exist. Usually, makefiles are created this way, but other files, such as `.gdbinit', can be specified as well. Typical calls to `AC_CONFIG_FILES' look like this: AC_CONFIG_FILES([Makefile src/Makefile man/Makefile X/Imakefile]) AC_CONFIG_FILES([autoconf], [chmod +x autoconf]) You can override an input file name by appending to FILE a colon-separated list of input files. Examples: AC_CONFIG_FILES([Makefile:boiler/top.mk:boiler/bot.mk] [lib/Makefile:boiler/lib.mk]) Doing this allows you to keep your file names acceptable to DOS variants, or to prepend and/or append boilerplate to the file. 4.8 Substitutions in Makefiles ============================== Each subdirectory in a distribution that contains something to be compiled or installed should come with a file `Makefile.in', from which `configure' creates a file `Makefile' in that directory. To create `Makefile', `configure' performs a simple variable substitution, replacing occurrences of `@VARIABLE@' in `Makefile.in' with the value that `configure' has determined for that variable. Variables that are substituted into output files in this way are called "output variables". They are ordinary shell variables that are set in `configure'. To make `configure' substitute a particular variable into the output files, the macro `AC_SUBST' must be called with that variable name as an argument. Any occurrences of `@VARIABLE@' for other variables are left unchanged. *Note Setting Output Variables::, for more information on creating output variables with `AC_SUBST'. A software package that uses a `configure' script should be distributed with a file `Makefile.in', but no makefile; that way, the user has to properly configure the package for the local system before compiling it. *Note Makefile Conventions: (standards)Makefile Conventions, for more information on what to put in makefiles. 4.8.1 Preset Output Variables ----------------------------- Some output variables are preset by the Autoconf macros. Some of the Autoconf macros set additional output variables, which are mentioned in the descriptions for those macros. *Note Output Variable Index::, for a complete list of output variables. *Note Installation Directory Variables::, for the list of the preset ones related to installation directories. Below are listed the other preset ones, many of which are precious variables (*note Setting Output Variables::, `AC_ARG_VAR'). The preset variables which are available during `config.status' (*note Configuration Actions::) may also be used during `configure' tests. For example, it is permissible to reference `$srcdir' when constructing a list of directories to pass via option `-I' during a compiler feature check. When used in this manner, coupled with the fact that `configure' is always run from the top build directory, it is sufficient to use just `$srcdir' instead of `$top_srcdir'. -- Variable: CFLAGS Debugging and optimization options for the C compiler. If it is not set in the environment when `configure' runs, the default value is set when you call `AC_PROG_CC' (or empty if you don't). `configure' uses this variable when compiling or linking programs to test for C features. If a compiler option affects only the behavior of the preprocessor (e.g., `-D NAME'), it should be put into `CPPFLAGS' instead. If it affects only the linker (e.g., `-L DIRECTORY'), it should be put into `LDFLAGS' instead. If it affects only the compiler proper, `CFLAGS' is the natural home for it. If an option affects multiple phases of the compiler, though, matters get tricky. One approach to put such options directly into `CC', e.g., `CC='gcc -m64''. Another is to put them into both `CPPFLAGS' and `LDFLAGS', but not into `CFLAGS'. However, remember that some `Makefile' variables are reserved by the GNU Coding Standards for the use of the "user"--the person building the package. For instance, `CFLAGS' is one such variable. Sometimes package developers are tempted to set user variables such as `CFLAGS' because it appears to make their job easier. However, the package itself should never set a user variable, particularly not to include switches that are required for proper compilation of the package. Since these variables are documented as being for the package builder, that person rightfully expects to be able to override any of these variables at build time. If the package developer needs to add switches without interfering with the user, the proper way to do that is to introduce an additional variable. Automake makes this easy by introducing `AM_CFLAGS' (*note Flag Variables Ordering: (automake)Flag Variables Ordering.), but the concept is the same even if Automake is not used. -- Variable: configure_input A comment saying that the file was generated automatically by `configure' and giving the name of the input file. `AC_OUTPUT' adds a comment line containing this variable to the top of every makefile it creates. For other files, you should reference this variable in a comment at the top of each input file. For example, an input shell script should begin like this: #!/bin/sh # @configure_input@ The presence of that line also reminds people editing the file that it needs to be processed by `configure' in order to be used. -- Variable: CPPFLAGS Preprocessor options for the C, C++, Objective C, and Objective C++ preprocessors and compilers. If it is not set in the environment when `configure' runs, the default value is empty. `configure' uses this variable when preprocessing or compiling programs to test for C, C++, Objective C, and Objective C++ features. This variable's contents should contain options like `-I', `-D', and `-U' that affect only the behavior of the preprocessor. Please see the explanation of `CFLAGS' for what you can do if an option affects other phases of the compiler as well. Currently, `configure' always links as part of a single invocation of the compiler that also preprocesses and compiles, so it uses this variable also when linking programs. However, it is unwise to depend on this behavior because the GNU Coding Standards do not require it and many packages do not use `CPPFLAGS' when linking programs. *Note Special Chars in Variables::, for limitations that `CPPFLAGS' might run into. -- Variable: CXXFLAGS Debugging and optimization options for the C++ compiler. It acts like `CFLAGS', but for C++ instead of C. -- Variable: DEFS `-D' options to pass to the C compiler. If `AC_CONFIG_HEADERS' is called, `configure' replaces `@DEFS@' with `-DHAVE_CONFIG_H' instead (*note Configuration Headers::). This variable is not defined while `configure' is performing its tests, only when creating the output files. *Note Setting Output Variables::, for how to check the results of previous tests. -- Variable: ECHO_C -- Variable: ECHO_N -- Variable: ECHO_T How does one suppress the trailing newline from `echo' for question-answer message pairs? These variables provide a way: echo $ECHO_N "And the winner is... $ECHO_C" sleep 100000000000 echo "${ECHO_T}dead." Some old and uncommon `echo' implementations offer no means to achieve this, in which case `ECHO_T' is set to tab. You might not want to use it. -- Variable: ERLCFLAGS Debugging and optimization options for the Erlang compiler. If it is not set in the environment when `configure' runs, the default value is empty. `configure' uses this variable when compiling programs to test for Erlang features. -- Variable: FCFLAGS Debugging and optimization options for the Fortran compiler. If it is not set in the environment when `configure' runs, the default value is set when you call `AC_PROG_FC' (or empty if you don't). `configure' uses this variable when compiling or linking programs to test for Fortran features. -- Variable: FFLAGS Debugging and optimization options for the Fortran 77 compiler. If it is not set in the environment when `configure' runs, the default value is set when you call `AC_PROG_F77' (or empty if you don't). `configure' uses this variable when compiling or linking programs to test for Fortran 77 features. -- Variable: LDFLAGS Options for the linker. If it is not set in the environment when `configure' runs, the default value is empty. `configure' uses this variable when linking programs to test for C, C++, Objective C, Objective C++, and Fortran features. This variable's contents should contain options like `-s' and `-L' that affect only the behavior of the linker. Please see the explanation of `CFLAGS' for what you can do if an option also affects other phases of the compiler. Don't use this variable to pass library names (`-l') to the linker; use `LIBS' instead. -- Variable: LIBS `-l' options to pass to the linker. The default value is empty, but some Autoconf macros may prepend extra libraries to this variable if those libraries are found and provide necessary functions, see *note Libraries::. `configure' uses this variable when linking programs to test for C, C++, Objective C, Objective C++, and Fortran features. -- Variable: OBJCFLAGS Debugging and optimization options for the Objective C compiler. It acts like `CFLAGS', but for Objective C instead of C. -- Variable: OBJCXXFLAGS Debugging and optimization options for the Objective C++ compiler. It acts like `CXXFLAGS', but for Objective C++ instead of C++. -- Variable: builddir Rigorously equal to `.'. Added for symmetry only. -- Variable: abs_builddir Absolute name of `builddir'. -- Variable: top_builddir The relative name of the top level of the current build tree. In the top-level directory, this is the same as `builddir'. -- Variable: top_build_prefix The relative name of the top level of the current build tree with final slash if nonemtpy. This is the same as `top_builddir', except that it contains zero or more runs of `../', so it should not be appended with a slash for concatenation. This helps for `make' implementations that otherwise do not treat `./file' and `file' as equal in the toplevel build directory. -- Variable: abs_top_builddir Absolute name of `top_builddir'. -- Variable: srcdir The name of the directory that contains the source code for that makefile. -- Variable: abs_srcdir Absolute name of `srcdir'. -- Variable: top_srcdir The name of the top-level source code directory for the package. In the top-level directory, this is the same as `srcdir'. -- Variable: abs_top_srcdir Absolute name of `top_srcdir'. 4.8.2 Installation Directory Variables -------------------------------------- The following variables specify the directories for package installation, see *note Variables for Installation Directories: (standards)Directory Variables, for more information. Each variable corresponds to an argument of `configure'; trailing slashes are stripped so that expressions such as `${prefix}/lib' expand with only one slash between directory names. See the end of this section for details on when and how to use these variables. -- Variable: bindir The directory for installing executables that users run. -- Variable: datadir The directory for installing idiosyncratic read-only architecture-independent data. -- Variable: datarootdir The root of the directory tree for read-only architecture-independent data files. -- Variable: docdir The directory for installing documentation files (other than Info and man). -- Variable: dvidir The directory for installing documentation files in DVI format. -- Variable: exec_prefix The installation prefix for architecture-dependent files. By default it's the same as PREFIX. You should avoid installing anything directly to EXEC_PREFIX. However, the default value for directories containing architecture-dependent files should be relative to EXEC_PREFIX. -- Variable: htmldir The directory for installing HTML documentation. -- Variable: includedir The directory for installing C header files. -- Variable: infodir The directory for installing documentation in Info format. -- Variable: libdir The directory for installing object code libraries. -- Variable: libexecdir The directory for installing executables that other programs run. -- Variable: localedir The directory for installing locale-dependent but architecture-independent data, such as message catalogs. This directory usually has a subdirectory per locale. -- Variable: localstatedir The directory for installing modifiable single-machine data. -- Variable: mandir The top-level directory for installing documentation in man format. -- Variable: oldincludedir The directory for installing C header files for non-GCC compilers. -- Variable: pdfdir The directory for installing PDF documentation. -- Variable: prefix The common installation prefix for all files. If EXEC_PREFIX is defined to a different value, PREFIX is used only for architecture-independent files. -- Variable: psdir The directory for installing PostScript documentation. -- Variable: sbindir The directory for installing executables that system administrators run. -- Variable: sharedstatedir The directory for installing modifiable architecture-independent data. -- Variable: sysconfdir The directory for installing read-only single-machine data. Most of these variables have values that rely on `prefix' or `exec_prefix'. It is deliberate that the directory output variables keep them unexpanded: typically `@datarootdir@' is replaced by `${prefix}/share', not `/usr/local/share', and `@datadir@' is replaced by `${datarootdir}'. This behavior is mandated by the GNU Coding Standards, so that when the user runs: `make' she can still specify a different prefix from the one specified to `configure', in which case, if needed, the package should hard code dependencies corresponding to the make-specified prefix. `make install' she can specify a different installation location, in which case the package _must_ still depend on the location which was compiled in (i.e., never recompile when `make install' is run). This is an extremely important feature, as many people may decide to install all the files of a package grouped together, and then install links from the final locations to there. In order to support these features, it is essential that `datarootdir' remains defined as `${prefix}/share', so that its value can be expanded based on the current value of `prefix'. A corollary is that you should not use these variables except in makefiles. For instance, instead of trying to evaluate `datadir' in `configure' and hard-coding it in makefiles using e.g., `AC_DEFINE_UNQUOTED([DATADIR], ["$datadir"], [Data directory.])', you should add `-DDATADIR='$(datadir)'' to your makefile's definition of `CPPFLAGS' (`AM_CPPFLAGS' if you are also using Automake). Similarly, you should not rely on `AC_CONFIG_FILES' to replace `bindir' and friends in your shell scripts and other files; instead, let `make' manage their replacement. For instance Autoconf ships templates of its shell scripts ending with `.in', and uses a makefile snippet similar to the following to build scripts like `autoheader' and `autom4te': edit = sed \ -e 's|@bindir[@]|$(bindir)|g' \ -e 's|@pkgdatadir[@]|$(pkgdatadir)|g' \ -e 's|@prefix[@]|$(prefix)|g' autoheader autom4te: Makefile rm -f $@ $@.tmp srcdir=''; \ test -f ./$@.in || srcdir=$(srcdir)/; \ $(edit) $${srcdir}$@.in >$@.tmp chmod +x $@.tmp chmod a-w $@.tmp mv $@.tmp $@ autoheader: $(srcdir)/autoheader.in autom4te: $(srcdir)/autom4te.in Some details are noteworthy: `@bindir[@]' The brackets prevent `configure' from replacing `@bindir@' in the Sed expression itself. Brackets are preferable to a backslash here, since Posix says `\@' is not portable. `$(bindir)' Don't use `@bindir@'! Use the matching makefile variable instead. `$(pkgdatadir)' The example takes advantage of the variable `$(pkgdatadir)' provided by Automake; it is equivalent to `$(datadir)/$(PACKAGE)'. `/' Don't use `/' in the Sed expressions that replace file names since most likely the variables you use, such as `$(bindir)', contain `/'. Use a shell metacharacter instead, such as `|'. special characters File names, file name components, and the value of `VPATH' should not contain shell metacharacters or white space. *Note Special Chars in Variables::. dependency on `Makefile' Since `edit' uses values that depend on the configuration specific values (`prefix', etc.) and not only on `VERSION' and so forth, the output depends on `Makefile', not `configure.ac'. `$@' The main rule is generic, and uses `$@' extensively to avoid the need for multiple copies of the rule. Separated dependencies and single suffix rules You can't use them! The above snippet cannot be (portably) rewritten as: autoconf autoheader: Makefile .in: rm -f $@ $@.tmp $(edit) $< >$@.tmp chmod +x $@.tmp mv $@.tmp $@ *Note Single Suffix Rules::, for details. `$(srcdir)' Be sure to specify the name of the source directory, otherwise the package won't support separated builds. For the more specific installation of Erlang libraries, the following variables are defined: -- Variable: ERLANG_INSTALL_LIB_DIR The common parent directory of Erlang library installation directories. This variable is set by calling the `AC_ERLANG_SUBST_INSTALL_LIB_DIR' macro in `configure.ac'. -- Variable: ERLANG_INSTALL_LIB_DIR_LIBRARY The installation directory for Erlang library LIBRARY. This variable is set by using the `AC_ERLANG_SUBST_INSTALL_LIB_SUBDIR' macro in `configure.ac'. *Note Erlang Libraries::, for details. 4.8.3 Changed Directory Variables --------------------------------- In Autoconf 2.60, the set of directory variables has changed, and the defaults of some variables have been adjusted (*note Installation Directory Variables::) to changes in the GNU Coding Standards. Notably, `datadir', `infodir', and `mandir' are now expressed in terms of `datarootdir'. If you are upgrading from an earlier Autoconf version, you may need to adjust your files to ensure that the directory variables are substituted correctly (*note Defining Directories::), and that a definition of `datarootdir' is in place. For example, in a `Makefile.in', adding datarootdir = @datarootdir@ is usually sufficient. If you use Automake to create `Makefile.in', it will add this for you. To help with the transition, Autoconf warns about files that seem to use `datarootdir' without defining it. In some cases, it then expands the value of `$datarootdir' in substitutions of the directory variables. The following example shows such a warning: $ cat configure.ac AC_INIT AC_CONFIG_FILES([Makefile]) AC_OUTPUT $ cat Makefile.in prefix = @prefix@ datadir = @datadir@ $ autoconf $ configure configure: creating ./config.status config.status: creating Makefile config.status: WARNING: Makefile.in seems to ignore the --datarootdir setting $ cat Makefile prefix = /usr/local datadir = ${prefix}/share Usually one can easily change the file to accommodate both older and newer Autoconf releases: $ cat Makefile.in prefix = @prefix@ datarootdir = @datarootdir@ datadir = @datadir@ $ configure configure: creating ./config.status config.status: creating Makefile $ cat Makefile prefix = /usr/local datarootdir = ${prefix}/share datadir = ${datarootdir} In some cases, however, the checks may not be able to detect that a suitable initialization of `datarootdir' is in place, or they may fail to detect that such an initialization is necessary in the output file. If, after auditing your package, there are still spurious `configure' warnings about `datarootdir', you may add the line AC_DEFUN([AC_DATAROOTDIR_CHECKED]) to your `configure.ac' to disable the warnings. This is an exception to the usual rule that you should not define a macro whose name begins with `AC_' (*note Macro Names::). 4.8.4 Build Directories ----------------------- You can support compiling a software package for several architectures simultaneously from the same copy of the source code. The object files for each architecture are kept in their own directory. To support doing this, `make' uses the `VPATH' variable to find the files that are in the source directory. GNU Make can do this. Most other recent `make' programs can do this as well, though they may have difficulties and it is often simpler to recommend GNU `make' (*note VPATH and Make::). Older `make' programs do not support `VPATH'; when using them, the source code must be in the same directory as the object files. If you are using GNU Automake, the remaining details in this section are already covered for you, based on the contents of your `Makefile.am'. But if you are using Autoconf in isolation, then supporting `VPATH' requires the following in your `Makefile.in': srcdir = @srcdir@ VPATH = @srcdir@ Do not set `VPATH' to the value of another variable (*note Variables listed in VPATH::. `configure' substitutes the correct value for `srcdir' when it produces `Makefile'. Do not use the `make' variable `$<', which expands to the file name of the file in the source directory (found with `VPATH'), except in implicit rules. (An implicit rule is one such as `.c.o', which tells how to create a `.o' file from a `.c' file.) Some versions of `make' do not set `$<' in explicit rules; they expand it to an empty value. Instead, Make command lines should always refer to source files by prefixing them with `$(srcdir)/'. For example: time.info: time.texinfo $(MAKEINFO) '$(srcdir)/time.texinfo' 4.8.5 Automatic Remaking ------------------------ You can put rules like the following in the top-level `Makefile.in' for a package to automatically update the configuration information when you change the configuration files. This example includes all of the optional files, such as `aclocal.m4' and those related to configuration header files. Omit from the `Makefile.in' rules for any of these files that your package does not use. The `$(srcdir)/' prefix is included because of limitations in the `VPATH' mechanism. The `stamp-' files are necessary because the timestamps of `config.h.in' and `config.h' are not changed if remaking them does not change their contents. This feature avoids unnecessary recompilation. You should include the file `stamp-h.in' in your package's distribution, so that `make' considers `config.h.in' up to date. Don't use `touch' (*note Limitations of Usual Tools: touch.); instead, use `echo' (using `date' would cause needless differences, hence CVS conflicts, etc.). $(srcdir)/configure: configure.ac aclocal.m4 cd '$(srcdir)' && autoconf # autoheader might not change config.h.in, so touch a stamp file. $(srcdir)/config.h.in: stamp-h.in $(srcdir)/stamp-h.in: configure.ac aclocal.m4 cd '$(srcdir)' && autoheader echo timestamp > '$(srcdir)/stamp-h.in' config.h: stamp-h stamp-h: config.h.in config.status ./config.status Makefile: Makefile.in config.status ./config.status config.status: configure ./config.status --recheck (Be careful if you copy these lines directly into your makefile, as you need to convert the indented lines to start with the tab character.) In addition, you should use AC_CONFIG_FILES([stamp-h], [echo timestamp > stamp-h]) so `config.status' ensures that `config.h' is considered up to date. *Note Output::, for more information about `AC_OUTPUT'. *Note config.status Invocation::, for more examples of handling configuration-related dependencies. 4.9 Configuration Header Files ============================== When a package contains more than a few tests that define C preprocessor symbols, the command lines to pass `-D' options to the compiler can get quite long. This causes two problems. One is that the `make' output is hard to visually scan for errors. More seriously, the command lines can exceed the length limits of some operating systems. As an alternative to passing `-D' options to the compiler, `configure' scripts can create a C header file containing `#define' directives. The `AC_CONFIG_HEADERS' macro selects this kind of output. Though it can be called anywhere between `AC_INIT' and `AC_OUTPUT', it is customary to call it right after `AC_INIT'. The package should `#include' the configuration header file before any other header files, to prevent inconsistencies in declarations (for example, if it redefines `const'). To provide for VPATH builds, remember to pass the C compiler a `-I.' option (or `-I..'; whichever directory contains `config.h'). Even if you use `#include "config.h"', the preprocessor searches only the directory of the currently read file, i.e., the source directory, not the build directory. With the appropriate `-I' option, you can use `#include '. Actually, it's a good habit to use it, because in the rare case when the source directory contains another `config.h', the build directory should be searched first. -- Macro: AC_CONFIG_HEADERS (HEADER ..., [CMDS], [INIT-CMDS]) This macro is one of the instantiating macros; see *note Configuration Actions::. Make `AC_OUTPUT' create the file(s) in the blank-or-newline-separated list HEADER containing C preprocessor `#define' statements, and replace `@DEFS@' in generated files with `-DHAVE_CONFIG_H' instead of the value of `DEFS'. The usual name for HEADER is `config.h'. If HEADER already exists and its contents are identical to what `AC_OUTPUT' would put in it, it is left alone. Doing this allows making some changes in the configuration without needlessly causing object files that depend on the header file to be recompiled. Usually the input file is named `HEADER.in'; however, you can override the input file name by appending to HEADER a colon-separated list of input files. For example, you might need to make the input file name acceptable to DOS variants: AC_CONFIG_HEADERS([config.h:config.hin]) -- Macro: AH_HEADER This macro is defined as the name of the first declared config header and undefined if no config headers have been declared up to this point. A third-party macro may, for example, require use of a config header without invoking AC_CONFIG_HEADERS twice, like this: AC_CONFIG_COMMANDS_PRE( [m4_ifndef([AH_HEADER], [AC_CONFIG_HEADERS([config.h])])]) *Note Configuration Actions::, for more details on HEADER. 4.9.1 Configuration Header Templates ------------------------------------ Your distribution should contain a template file that looks as you want the final header file to look, including comments, with `#undef' statements which are used as hooks. For example, suppose your `configure.ac' makes these calls: AC_CONFIG_HEADERS([conf.h]) AC_CHECK_HEADERS([unistd.h]) Then you could have code like the following in `conf.h.in'. The `conf.h' created by `configure' defines `HAVE_UNISTD_H' to 1, if and only if the system has `unistd.h'. /* Define as 1 if you have unistd.h. */ #undef HAVE_UNISTD_H The format of the template file is stricter than what the C preprocessor is required to accept. A directive line should contain only whitespace, `#undef', and `HAVE_UNISTD_H'. The use of `#define' instead of `#undef', or of comments on the same line as `#undef', is strongly discouraged. Each hook should only be listed once. Other preprocessor lines, such as `#ifdef' or `#include', are copied verbatim from the template into the generated header. Since it is a tedious task to keep a template header up to date, you may use `autoheader' to generate it, see *note autoheader Invocation::. During the instantiation of the header, each `#undef' line in the template file for each symbol defined by `AC_DEFINE' is changed to an appropriate `#define'. If the corresponding `AC_DEFINE' has not been executed during the `configure' run, the `#undef' line is commented out. (This is important, e.g., for `_POSIX_SOURCE': on many systems, it can be implicitly defined by the compiler, and undefining it in the header would then break compilation of subsequent headers.) Currently, _all_ remaining `#undef' lines in the header template are commented out, whether or not there was a corresponding `AC_DEFINE' for the macro name; but this behavior is not guaranteed for future releases of Autoconf. Generally speaking, since you should not use `#define', and you cannot guarantee whether a `#undef' directive in the header template will be converted to a `#define' or commented out in the generated header file, the template file cannot be used for conditional definition effects. Consequently, if you need to use the construct #ifdef THIS # define THAT #endif you must place it outside of the template. If you absolutely need to hook it to the config header itself, please put the directives to a separate file, and `#include' that file from the config header template. If you are using `autoheader', you would probably use `AH_BOTTOM' to append the `#include' directive. 4.9.2 Using `autoheader' to Create `config.h.in' ------------------------------------------------ The `autoheader' program can create a template file of C `#define' statements for `configure' to use. It searches for the first invocation of `AC_CONFIG_HEADERS' in `configure' sources to determine the name of the template. (If the first call of `AC_CONFIG_HEADERS' specifies more than one input file name, `autoheader' uses the first one.) It is recommended that only one input file is used. If you want to append a boilerplate code, it is preferable to use `AH_BOTTOM([#include ])'. File `conf_post.h' is not processed during the configuration then, which make things clearer. Analogically, `AH_TOP' can be used to prepend a boilerplate code. In order to do its job, `autoheader' needs you to document all of the symbols that you might use. Typically this is done via an `AC_DEFINE' or `AC_DEFINE_UNQUOTED' call whose first argument is a literal symbol and whose third argument describes the symbol (*note Defining Symbols::). Alternatively, you can use `AH_TEMPLATE' (*note Autoheader Macros::), or you can supply a suitable input file for a subsequent configuration header file. Symbols defined by Autoconf's builtin tests are already documented properly; you need to document only those that you define yourself. You might wonder why `autoheader' is needed: after all, why would `configure' need to "patch" a `config.h.in' to produce a `config.h' instead of just creating `config.h' from scratch? Well, when everything rocks, the answer is just that we are wasting our time maintaining `autoheader': generating `config.h' directly is all that is needed. When things go wrong, however, you'll be thankful for the existence of `autoheader'. The fact that the symbols are documented is important in order to _check_ that `config.h' makes sense. The fact that there is a well-defined list of symbols that should be defined (or not) is also important for people who are porting packages to environments where `configure' cannot be run: they just have to _fill in the blanks_. But let's come back to the point: the invocation of `autoheader'... If you give `autoheader' an argument, it uses that file instead of `configure.ac' and writes the header file to the standard output instead of to `config.h.in'. If you give `autoheader' an argument of `-', it reads the standard input instead of `configure.ac' and writes the header file to the standard output. `autoheader' accepts the following options: `--help' `-h' Print a summary of the command line options and exit. `--version' `-V' Print the version number of Autoconf and exit. `--verbose' `-v' Report processing steps. `--debug' `-d' Don't remove the temporary files. `--force' `-f' Remake the template file even if newer than its input files. `--include=DIR' `-I DIR' Append DIR to the include path. Multiple invocations accumulate. `--prepend-include=DIR' `-B DIR' Prepend DIR to the include path. Multiple invocations accumulate. `--warnings=CATEGORY' `-W CATEGORY' Report the warnings related to CATEGORY (which can actually be a comma separated list). Current categories include: `obsolete' report the uses of obsolete constructs `all' report all the warnings `none' report none `error' treats warnings as errors `no-CATEGORY' disable warnings falling into CATEGORY 4.9.3 Autoheader Macros ----------------------- `autoheader' scans `configure.ac' and figures out which C preprocessor symbols it might define. It knows how to generate templates for symbols defined by `AC_CHECK_HEADERS', `AC_CHECK_FUNCS' etc., but if you `AC_DEFINE' any additional symbol, you must define a template for it. If there are missing templates, `autoheader' fails with an error message. The template for a SYMBOL is created by `autoheader' from the DESCRIPTION argument to an `AC_DEFINE'; see *note Defining Symbols::. For special needs, you can use the following macros. -- Macro: AH_TEMPLATE (KEY, DESCRIPTION) Tell `autoheader' to generate a template for KEY. This macro generates standard templates just like `AC_DEFINE' when a DESCRIPTION is given. For example: AH_TEMPLATE([CRAY_STACKSEG_END], [Define to one of _getb67, GETB67, getb67 for Cray-2 and Cray-YMP systems. This function is required for alloca.c support on those systems.]) generates the following template, with the description properly justified. /* Define to one of _getb67, GETB67, getb67 for Cray-2 and Cray-YMP systems. This function is required for alloca.c support on those systems. */ #undef CRAY_STACKSEG_END -- Macro: AH_VERBATIM (KEY, TEMPLATE) Tell `autoheader' to include the TEMPLATE as-is in the header template file. This TEMPLATE is associated with the KEY, which is used to sort all the different templates and guarantee their uniqueness. It should be a symbol that can be defined via `AC_DEFINE'. -- Macro: AH_TOP (TEXT) Include TEXT at the top of the header template file. -- Macro: AH_BOTTOM (TEXT) Include TEXT at the bottom of the header template file. Please note that TEXT gets included "verbatim" to the template file, not to the resulting config header, so it can easily get mangled when the template is processed. There is rarely a need for something other than AH_BOTTOM([#include ]) 4.10 Running Arbitrary Configuration Commands ============================================= You can execute arbitrary commands before, during, and after `config.status' is run. The three following macros accumulate the commands to run when they are called multiple times. `AC_CONFIG_COMMANDS' replaces the obsolete macro `AC_OUTPUT_COMMANDS'; see *note Obsolete Macros::, for details. -- Macro: AC_CONFIG_COMMANDS (TAG..., [CMDS], [INIT-CMDS]) Specify additional shell commands to run at the end of `config.status', and shell commands to initialize any variables from `configure'. Associate the commands with TAG. Since typically the CMDS create a file, TAG should naturally be the name of that file. If needed, the directory hosting TAG is created. This macro is one of the instantiating macros; see *note Configuration Actions::. Here is an unrealistic example: fubar=42 AC_CONFIG_COMMANDS([fubar], [echo this is extra $fubar, and so on.], [fubar=$fubar]) Here is a better one: AC_CONFIG_COMMANDS([timestamp], [date >timestamp]) The following two macros look similar, but in fact they are not of the same breed: they are executed directly by `configure', so you cannot use `config.status' to rerun them. -- Macro: AC_CONFIG_COMMANDS_PRE (CMDS) Execute the CMDS right before creating `config.status'. This macro presents the last opportunity to call `AC_SUBST', `AC_DEFINE', or `AC_CONFIG_ITEMS' macros. -- Macro: AC_CONFIG_COMMANDS_POST (CMDS) Execute the CMDS right after creating `config.status'. 4.11 Creating Configuration Links ================================= You may find it convenient to create links whose destinations depend upon results of tests. One can use `AC_CONFIG_COMMANDS' but the creation of relative symbolic links can be delicate when the package is built in a directory different from the source directory. -- Macro: AC_CONFIG_LINKS (DEST:SOURCE..., [CMDS], [INIT-CMDS]) Make `AC_OUTPUT' link each of the existing files SOURCE to the corresponding link name DEST. Makes a symbolic link if possible, otherwise a hard link if possible, otherwise a copy. The DEST and SOURCE names should be relative to the top level source or build directory. This macro is one of the instantiating macros; see *note Configuration Actions::. For example, this call: AC_CONFIG_LINKS([host.h:config/$machine.h object.h:config/$obj_format.h]) creates in the current directory `host.h' as a link to `SRCDIR/config/$machine.h', and `object.h' as a link to `SRCDIR/config/$obj_format.h'. The tempting value `.' for DEST is invalid: it makes it impossible for `config.status' to guess the links to establish. One can then run: ./config.status host.h object.h to create the links. 4.12 Configuring Other Packages in Subdirectories ================================================= In most situations, calling `AC_OUTPUT' is sufficient to produce makefiles in subdirectories. However, `configure' scripts that control more than one independent package can use `AC_CONFIG_SUBDIRS' to run `configure' scripts for other packages in subdirectories. -- Macro: AC_CONFIG_SUBDIRS (DIR ...) Make `AC_OUTPUT' run `configure' in each subdirectory DIR in the given blank-or-newline-separated list. Each DIR should be a literal, i.e., please do not use: if test "x$package_foo_enabled" = xyes; then $my_subdirs="$my_subdirs foo" fi AC_CONFIG_SUBDIRS([$my_subdirs]) because this prevents `./configure --help=recursive' from displaying the options of the package `foo'. Instead, you should write: if test "x$package_foo_enabled" = xyes; then AC_CONFIG_SUBDIRS([foo]) fi If a given DIR is not found, an error is reported: if the subdirectory is optional, write: if test -d "$srcdir/foo"; then AC_CONFIG_SUBDIRS([foo]) fi If a given DIR contains `configure.gnu', it is run instead of `configure'. This is for packages that might use a non-Autoconf script `Configure', which can't be called through a wrapper `configure' since it would be the same file on case-insensitive file systems. Likewise, if a DIR contains `configure.in' but no `configure', the Cygnus `configure' script found by `AC_CONFIG_AUX_DIR' is used. The subdirectory `configure' scripts are given the same command line options that were given to this `configure' script, with minor changes if needed, which include: - adjusting a relative name for the cache file; - adjusting a relative name for the source directory; - propagating the current value of `$prefix', including if it was defaulted, and if the default values of the top level and of the subdirectory `configure' differ. This macro also sets the output variable `subdirs' to the list of directories `DIR ...'. Make rules can use this variable to determine which subdirectories to recurse into. This macro may be called multiple times. 4.13 Default Prefix =================== By default, `configure' sets the prefix for files it installs to `/usr/local'. The user of `configure' can select a different prefix using the `--prefix' and `--exec-prefix' options. There are two ways to change the default: when creating `configure', and when running it. Some software packages might want to install in a directory other than `/usr/local' by default. To accomplish that, use the `AC_PREFIX_DEFAULT' macro. -- Macro: AC_PREFIX_DEFAULT (PREFIX) Set the default installation prefix to PREFIX instead of `/usr/local'. It may be convenient for users to have `configure' guess the installation prefix from the location of a related program that they have already installed. If you wish to do that, you can call `AC_PREFIX_PROGRAM'. -- Macro: AC_PREFIX_PROGRAM (PROGRAM) If the user did not specify an installation prefix (using the `--prefix' option), guess a value for it by looking for PROGRAM in `PATH', the way the shell does. If PROGRAM is found, set the prefix to the parent of the directory containing PROGRAM, else default the prefix as described above (`/usr/local' or `AC_PREFIX_DEFAULT'). For example, if PROGRAM is `gcc' and the `PATH' contains `/usr/local/gnu/bin/gcc', set the prefix to `/usr/local/gnu'. 5 Existing Tests **************** These macros test for particular system features that packages might need or want to use. If you need to test for a kind of feature that none of these macros check for, you can probably do it by calling primitive test macros with appropriate arguments (*note Writing Tests::). These tests print messages telling the user which feature they're checking for, and what they find. They cache their results for future `configure' runs (*note Caching Results::). Some of these macros set output variables. *Note Makefile Substitutions::, for how to get their values. The phrase "define NAME" is used below as a shorthand to mean "define the C preprocessor symbol NAME to the value 1". *Note Defining Symbols::, for how to get those symbol definitions into your program. 5.1 Common Behavior =================== Much effort has been expended to make Autoconf easy to learn. The most obvious way to reach this goal is simply to enforce standard interfaces and behaviors, avoiding exceptions as much as possible. Because of history and inertia, unfortunately, there are still too many exceptions in Autoconf; nevertheless, this section describes some of the common rules. 5.1.1 Standard Symbols ---------------------- All the generic macros that `AC_DEFINE' a symbol as a result of their test transform their ARGUMENT values to a standard alphabet. First, ARGUMENT is converted to upper case and any asterisks (`*') are each converted to `P'. Any remaining characters that are not alphanumeric are converted to underscores. For instance, AC_CHECK_TYPES([struct $Expensive*]) defines the symbol `HAVE_STRUCT__EXPENSIVEP' if the check succeeds. 5.1.2 Default Includes ---------------------- Several tests depend upon a set of header files. Since these headers are not universally available, tests actually have to provide a set of protected includes, such as: #ifdef TIME_WITH_SYS_TIME # include # include #else # ifdef HAVE_SYS_TIME_H # include # else # include # endif #endif Unless you know exactly what you are doing, you should avoid using unconditional includes, and check the existence of the headers you include beforehand (*note Header Files::). Most generic macros use the following macro to provide the default set of includes: -- Macro: AC_INCLUDES_DEFAULT ([INCLUDE-DIRECTIVES]) Expand to INCLUDE-DIRECTIVES if defined, otherwise to: #include #ifdef HAVE_SYS_TYPES_H # include #endif #ifdef HAVE_SYS_STAT_H # include #endif #ifdef STDC_HEADERS # include # include #else # ifdef HAVE_STDLIB_H # include # endif #endif #ifdef HAVE_STRING_H # if !defined STDC_HEADERS && defined HAVE_MEMORY_H # include # endif # include #endif #ifdef HAVE_STRINGS_H # include #endif #ifdef HAVE_INTTYPES_H # include #endif #ifdef HAVE_STDINT_H # include #endif #ifdef HAVE_UNISTD_H # include #endif If the default includes are used, then check for the presence of these headers and their compatibility, i.e., you don't need to run `AC_HEADER_STDC', nor check for `stdlib.h' etc. These headers are checked for in the same order as they are included. For instance, on some systems `string.h' and `strings.h' both exist, but conflict. Then `HAVE_STRING_H' is defined, not `HAVE_STRINGS_H'. 5.2 Alternative Programs ======================== These macros check for the presence or behavior of particular programs. They are used to choose between several alternative programs and to decide what to do once one has been chosen. If there is no macro specifically defined to check for a program you need, and you don't need to check for any special properties of it, then you can use one of the general program-check macros. 5.2.1 Particular Program Checks ------------------------------- These macros check for particular programs--whether they exist, and in some cases whether they support certain features. -- Macro: AC_PROG_AWK Check for `gawk', `mawk', `nawk', and `awk', in that order, and set output variable `AWK' to the first one that is found. It tries `gawk' first because that is reported to be the best implementation. The result can be overridden by setting the variable `AWK' or the cache variable `ac_cv_prog_AWK'. -- Macro: AC_PROG_GREP Look for the best available `grep' or `ggrep' that accepts the longest input lines possible, and that supports multiple `-e' options. Set the output variable `GREP' to whatever is chosen. *Note Limitations of Usual Tools: grep, for more information about portability problems with the `grep' command family. The result can be overridden by setting the `GREP' variable and is cached in the `ac_cv_path_GREP' variable. -- Macro: AC_PROG_EGREP Check whether `$GREP -E' works, or else look for the best available `egrep' or `gegrep' that accepts the longest input lines possible. Set the output variable `EGREP' to whatever is chosen. The result can be overridden by setting the `EGREP' variable and is cached in the `ac_cv_path_EGREP' variable. -- Macro: AC_PROG_FGREP Check whether `$GREP -F' works, or else look for the best available `fgrep' or `gfgrep' that accepts the longest input lines possible. Set the output variable `FGREP' to whatever is chosen. The result can be overridden by setting the `FGREP' variable and is cached in the `ac_cv_path_FGREP' variable. -- Macro: AC_PROG_INSTALL Set output variable `INSTALL' to the name of a BSD-compatible `install' program, if one is found in the current `PATH'. Otherwise, set `INSTALL' to `DIR/install-sh -c', checking the directories specified to `AC_CONFIG_AUX_DIR' (or its default directories) to determine DIR (*note Output::). Also set the variables `INSTALL_PROGRAM' and `INSTALL_SCRIPT' to `${INSTALL}' and `INSTALL_DATA' to `${INSTALL} -m 644'. `@INSTALL@' is special, as its value may vary for different configuration files. This macro screens out various instances of `install' known not to work. It prefers to find a C program rather than a shell script, for speed. Instead of `install-sh', it can also use `install.sh', but that name is obsolete because some `make' programs have a rule that creates `install' from it if there is no makefile. Further, this macro requires `install' to be able to install multiple files into a target directory in a single invocation. Autoconf comes with a copy of `install-sh' that you can use. If you use `AC_PROG_INSTALL', you must include either `install-sh' or `install.sh' in your distribution; otherwise `configure' produces an error message saying it can't find them--even if the system you're on has a good `install' program. This check is a safety measure to prevent you from accidentally leaving that file out, which would prevent your package from installing on systems that don't have a BSD-compatible `install' program. If you need to use your own installation program because it has features not found in standard `install' programs, there is no reason to use `AC_PROG_INSTALL'; just put the file name of your program into your `Makefile.in' files. The result of the test can be overridden by setting the variable `INSTALL' or the cache variable `ac_cv_path_install'. -- Macro: AC_PROG_MKDIR_P Set output variable `MKDIR_P' to a program that ensures that for each argument, a directory named by this argument exists, creating it and its parent directories if needed, and without race conditions when two instances of the program attempt to make the same directory at nearly the same time. This macro uses the `mkdir -p' command if possible. Otherwise, it falls back on invoking `install-sh' with the `-d' option, so your package should contain `install-sh' as described under `AC_PROG_INSTALL'. An `install-sh' file that predates Autoconf 2.60 or Automake 1.10 is vulnerable to race conditions, so if you want to support parallel installs from different packages into the same directory you need to make sure you have an up-to-date `install-sh'. In particular, be careful about using `autoreconf -if' if your Automake predates Automake 1.10. This macro is related to the `AS_MKDIR_P' macro (*note Programming in M4sh::), but it sets an output variable intended for use in other files, whereas `AS_MKDIR_P' is intended for use in scripts like `configure'. Also, `AS_MKDIR_P' does not accept options, but `MKDIR_P' supports the `-m' option, e.g., a makefile might invoke `$(MKDIR_P) -m 0 dir' to create an inaccessible directory, and conversely a makefile should use `$(MKDIR_P) -- $(FOO)' if FOO might yield a value that begins with `-'. Finally, `AS_MKDIR_P' does not check for race condition vulnerability, whereas `AC_PROG_MKDIR_P' does. `@MKDIR_P@' is special, as its value may vary for different configuration files. The result of the test can be overridden by setting the variable `MKDIR_P' or the cache variable `ac_cv_path_mkdir'. -- Macro: AC_PROG_LEX If `flex' is found, set output variable `LEX' to `flex' and `LEXLIB' to `-lfl', if that library is in a standard place. Otherwise set `LEX' to `lex' and `LEXLIB' to `-ll', if found. If neither variant is available, set `LEX' to `:'; for packages that ship the generated `file.yy.c' alongside the source `file.l', this default allows users without a lexer generator to still build the package even if the timestamp for `file.l' is inadvertantly changed. Define `YYTEXT_POINTER' if `yytext' defaults to `char *' instead of to `char []'. Also set output variable `LEX_OUTPUT_ROOT' to the base of the file name that the lexer generates; usually `lex.yy', but sometimes something else. These results vary according to whether `lex' or `flex' is being used. You are encouraged to use Flex in your sources, since it is both more pleasant to use than plain Lex and the C source it produces is portable. In order to ensure portability, however, you must either provide a function `yywrap' or, if you don't use it (e.g., your scanner has no `#include'-like feature), simply include a `%noyywrap' statement in the scanner's source. Once this done, the scanner is portable (unless _you_ felt free to use nonportable constructs) and does not depend on any library. In this case, and in this case only, it is suggested that you use this Autoconf snippet: AC_PROG_LEX if test "x$LEX" != xflex; then LEX="$SHELL $missing_dir/missing flex" AC_SUBST([LEX_OUTPUT_ROOT], [lex.yy]) AC_SUBST([LEXLIB], ['']) fi The shell script `missing' can be found in the Automake distribution. Remember that the user may have supplied an alternate location in `LEX', so if Flex is required, it is better to check that the user provided something sufficient by parsing the output of `$LEX --version' than by simply relying on `test "x$LEX" = xflex'. To ensure backward compatibility, Automake's `AM_PROG_LEX' invokes (indirectly) this macro twice, which causes an annoying but benign "`AC_PROG_LEX' invoked multiple times" warning. Future versions of Automake will fix this issue; meanwhile, just ignore this message. As part of running the test, this macro may delete any file in the configuration directory named `lex.yy.c' or `lexyy.c'. The result of this test can be influenced by setting the variable `LEX' or the cache variable `ac_cv_prog_LEX'. -- Macro: AC_PROG_LN_S If `ln -s' works on the current file system (the operating system and file system support symbolic links), set the output variable `LN_S' to `ln -s'; otherwise, if `ln' works, set `LN_S' to `ln', and otherwise set it to `cp -p'. If you make a link in a directory other than the current directory, its meaning depends on whether `ln' or `ln -s' is used. To safely create links using `$(LN_S)', either find out which form is used and adjust the arguments, or always invoke `ln' in the directory where the link is to be created. In other words, it does not work to do: $(LN_S) foo /x/bar Instead, do: (cd /x && $(LN_S) foo bar) -- Macro: AC_PROG_RANLIB Set output variable `RANLIB' to `ranlib' if `ranlib' is found, and otherwise to `:' (do nothing). -- Macro: AC_PROG_SED Set output variable `SED' to a Sed implementation that conforms to Posix and does not have arbitrary length limits. Report an error if no acceptable Sed is found. *Note Limitations of Usual Tools: sed, for more information about portability problems with Sed. The result of this test can be overridden by setting the `SED' variable and is cached in the `ac_cv_path_SED' variable. -- Macro: AC_PROG_YACC If `bison' is found, set output variable `YACC' to `bison -y'. Otherwise, if `byacc' is found, set `YACC' to `byacc'. Otherwise set `YACC' to `yacc'. The result of this test can be influenced by setting the variable `YACC' or the cache variable `ac_cv_prog_YACC'. 5.2.2 Generic Program and File Checks ------------------------------------- These macros are used to find programs not covered by the "particular" test macros. If you need to check the behavior of a program as well as find out whether it is present, you have to write your own test for it (*note Writing Tests::). By default, these macros use the environment variable `PATH'. If you need to check for a program that might not be in the user's `PATH', you can pass a modified path to use instead, like this: AC_PATH_PROG([INETD], [inetd], [/usr/libexec/inetd], [$PATH$PATH_SEPARATOR/usr/libexec$PATH_SEPARATOR]dnl [/usr/sbin$PATH_SEPARATOR/usr/etc$PATH_SEPARATOR/etc]) You are strongly encouraged to declare the VARIABLE passed to `AC_CHECK_PROG' etc. as precious, *Note Setting Output Variables::, `AC_ARG_VAR', for more details. -- Macro: AC_CHECK_PROG (VARIABLE, PROG-TO-CHECK-FOR, VALUE-IF-FOUND, [VALUE-IF-NOT-FOUND], [PATH = `$PATH'], [REJECT]) Check whether program PROG-TO-CHECK-FOR exists in PATH. If it is found, set VARIABLE to VALUE-IF-FOUND, otherwise to VALUE-IF-NOT-FOUND, if given. Always pass over REJECT (an absolute file name) even if it is the first found in the search path; in that case, set VARIABLE using the absolute file name of the PROG-TO-CHECK-FOR found that is not REJECT. If VARIABLE was already set, do nothing. Calls `AC_SUBST' for VARIABLE. The result of this test can be overridden by setting the VARIABLE variable or the cache variable `ac_cv_prog_VARIABLE'. -- Macro: AC_CHECK_PROGS (VARIABLE, PROGS-TO-CHECK-FOR, [VALUE-IF-NOT-FOUND], [PATH = `$PATH']) Check for each program in the blank-separated list PROGS-TO-CHECK-FOR existing in the PATH. If one is found, set VARIABLE to the name of that program. Otherwise, continue checking the next program in the list. If none of the programs in the list are found, set VARIABLE to VALUE-IF-NOT-FOUND; if VALUE-IF-NOT-FOUND is not specified, the value of VARIABLE is not changed. Calls `AC_SUBST' for VARIABLE. The result of this test can be overridden by setting the VARIABLE variable or the cache variable `ac_cv_prog_VARIABLE'. -- Macro: AC_CHECK_TARGET_TOOL (VARIABLE, PROG-TO-CHECK-FOR, [VALUE-IF-NOT-FOUND], [PATH = `$PATH']) Like `AC_CHECK_PROG', but first looks for PROG-TO-CHECK-FOR with a prefix of the target type as determined by `AC_CANONICAL_TARGET', followed by a dash (*note Canonicalizing::). If the tool cannot be found with a prefix, and if the build and target types are equal, then it is also searched for without a prefix. As noted in *note Specifying Target Triplets::, the target is rarely specified, because most of the time it is the same as the host: it is the type of system for which any compiler tool in the package produces code. What this macro looks for is, for example, _a tool (assembler, linker, etc.) that the compiler driver (`gcc' for the GNU C Compiler) uses to produce objects, archives or executables_. -- Macro: AC_CHECK_TOOL (VARIABLE, PROG-TO-CHECK-FOR, [VALUE-IF-NOT-FOUND], [PATH = `$PATH']) Like `AC_CHECK_PROG', but first looks for PROG-TO-CHECK-FOR with a prefix of the host type as specified by `--host', followed by a dash. For example, if the user runs `configure --build=x86_64-gnu --host=i386-gnu', then this call: AC_CHECK_TOOL([RANLIB], [ranlib], [:]) sets `RANLIB' to `i386-gnu-ranlib' if that program exists in PATH, or otherwise to `ranlib' if that program exists in PATH, or to `:' if neither program exists. When cross-compiling, this macro will issue a warning if no program prefixed with the host type could be found. For more information, see *note Specifying Target Triplets::. -- Macro: AC_CHECK_TARGET_TOOLS (VARIABLE, PROGS-TO-CHECK-FOR, [VALUE-IF-NOT-FOUND], [PATH = `$PATH']) Like `AC_CHECK_TARGET_TOOL', each of the tools in the list PROGS-TO-CHECK-FOR are checked with a prefix of the target type as determined by `AC_CANONICAL_TARGET', followed by a dash (*note Canonicalizing::). If none of the tools can be found with a prefix, and if the build and target types are equal, then the first one without a prefix is used. If a tool is found, set VARIABLE to the name of that program. If none of the tools in the list are found, set VARIABLE to VALUE-IF-NOT-FOUND; if VALUE-IF-NOT-FOUND is not specified, the value of VARIABLE is not changed. Calls `AC_SUBST' for VARIABLE. -- Macro: AC_CHECK_TOOLS (VARIABLE, PROGS-TO-CHECK-FOR, [VALUE-IF-NOT-FOUND], [PATH = `$PATH']) Like `AC_CHECK_TOOL', each of the tools in the list PROGS-TO-CHECK-FOR are checked with a prefix of the host type as determined by `AC_CANONICAL_HOST', followed by a dash (*note Canonicalizing::). If none of the tools can be found with a prefix, then the first one without a prefix is used. If a tool is found, set VARIABLE to the name of that program. If none of the tools in the list are found, set VARIABLE to VALUE-IF-NOT-FOUND; if VALUE-IF-NOT-FOUND is not specified, the value of VARIABLE is not changed. Calls `AC_SUBST' for VARIABLE. When cross-compiling, this macro will issue a warning if no program prefixed with the host type could be found. For more information, see *note Specifying Target Triplets::. -- Macro: AC_PATH_PROG (VARIABLE, PROG-TO-CHECK-FOR, [VALUE-IF-NOT-FOUND], [PATH = `$PATH']) Like `AC_CHECK_PROG', but set VARIABLE to the absolute name of PROG-TO-CHECK-FOR if found. The result of this test can be overridden by setting the VARIABLE variable. A positive result of this test is cached in the `ac_cv_path_VARIABLE' variable. -- Macro: AC_PATH_PROGS (VARIABLE, PROGS-TO-CHECK-FOR, [VALUE-IF-NOT-FOUND], [PATH = `$PATH']) Like `AC_CHECK_PROGS', but if any of PROGS-TO-CHECK-FOR are found, set VARIABLE to the absolute name of the program found. The result of this test can be overridden by setting the VARIABLE variable. A positive result of this test is cached in the `ac_cv_path_VARIABLE' variable. -- Macro: AC_PATH_PROGS_FEATURE_CHECK (VARIABLE, PROGS-TO-CHECK-FOR, FEATURE-TEST, [ACTION-IF-NOT-FOUND], [PATH = `$PATH']) This macro was introduced in Autoconf 2.62. If VARIABLE is not empty, then set the cache variable `ac_cv_path_VARIABLE' to its value. Otherwise, check for each program in the blank-separated list PROGS-TO-CHECK-FOR existing in PATH. For each program found, execute FEATURE-TEST with `ac_path_VARIABLE' set to the absolute name of the candidate program. If no invocation of FEATURE-TEST sets the shell variable `ac_cv_path_VARIABLE', then ACTION-IF-NOT-FOUND is executed. FEATURE-TEST will be run even when `ac_cv_path_VARIABLE' is set, to provide the ability to choose a better candidate found later in PATH; to accept the current setting and bypass all futher checks, FEATURE-TEST can execute `ac_path_VARIABLE_found=:'. Note that this macro has some subtle differences from `AC_CHECK_PROGS'. It is designed to be run inside `AC_CACHE_VAL', therefore, it should have no side effects. In particular, VARIABLE is not set to the final value of `ac_cv_path_VARIABLE', nor is `AC_SUBST' automatically run. Also, on failure, any action can be performed, whereas `AC_CHECK_PROGS' only performs `VARIABLE=VALUE-IF-NOT-FOUND'. Here is an example, similar to what Autoconf uses in its own configure script. It will search for an implementation of `m4' that supports the `indir' builtin, even if it goes by the name `gm4' or is not the first implementation on `PATH'. AC_CACHE_CHECK([for m4 that supports indir], [ac_cv_path_M4], [AC_PATH_PROGS_FEATURE_CHECK([M4], [m4 gm4], [[m4out=`echo 'changequote([,])indir([divnum])' | $ac_path_M4` test "x$m4out" = x0 \ && ac_cv_path_M4=$ac_path_M4 ac_path_M4_found=:]], [AC_MSG_ERROR([could not find m4 that supports indir])])]) AC_SUBST([M4], [$ac_cv_path_M4]) -- Macro: AC_PATH_TARGET_TOOL (VARIABLE, PROG-TO-CHECK-FOR, [VALUE-IF-NOT-FOUND], [PATH = `$PATH']) Like `AC_CHECK_TARGET_TOOL', but set VARIABLE to the absolute name of the program if it is found. -- Macro: AC_PATH_TOOL (VARIABLE, PROG-TO-CHECK-FOR, [VALUE-IF-NOT-FOUND], [PATH = `$PATH']) Like `AC_CHECK_TOOL', but set VARIABLE to the absolute name of the program if it is found. When cross-compiling, this macro will issue a warning if no program prefixed with the host type could be found. For more information, see *note Specifying Target Triplets::. 5.3 Files ========= You might also need to check for the existence of files. Before using these macros, ask yourself whether a runtime test might not be a better solution. Be aware that, like most Autoconf macros, they test a feature of the host machine, and therefore, they die when cross-compiling. -- Macro: AC_CHECK_FILE (FILE, [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND]) Check whether file FILE exists on the native system. If it is found, execute ACTION-IF-FOUND, otherwise do ACTION-IF-NOT-FOUND, if given. The result of this test is cached in the `ac_cv_file_FILE' variable, with characters not suitable for a variable name mapped to underscores. -- Macro: AC_CHECK_FILES (FILES, [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND]) Executes `AC_CHECK_FILE' once for each file listed in FILES. Additionally, defines `HAVE_FILE' (*note Standard Symbols::) for each file found. The results of each test are cached in the `ac_cv_file_FILE' variable, with characters not suitable for a variable name mapped to underscores. 5.4 Library Files ================= The following macros check for the presence of certain C, C++, or Fortran library archive files. -- Macro: AC_CHECK_LIB (LIBRARY, FUNCTION, [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND], [OTHER-LIBRARIES]) Test whether the library LIBRARY is available by trying to link a test program that calls function FUNCTION with the library. FUNCTION should be a function provided by the library. Use the base name of the library; e.g., to check for `-lmp', use `mp' as the LIBRARY argument. ACTION-IF-FOUND is a list of shell commands to run if the link with the library succeeds; ACTION-IF-NOT-FOUND is a list of shell commands to run if the link fails. If ACTION-IF-FOUND is not specified, the default action prepends `-lLIBRARY' to `LIBS' and defines `HAVE_LIBLIBRARY' (in all capitals). This macro is intended to support building `LIBS' in a right-to-left (least-dependent to most-dependent) fashion such that library dependencies are satisfied as a natural side effect of consecutive tests. Linkers are sensitive to library ordering so the order in which `LIBS' is generated is important to reliable detection of libraries. If linking with LIBRARY results in unresolved symbols that would be resolved by linking with additional libraries, give those libraries as the OTHER-LIBRARIES argument, separated by spaces: e.g., `-lXt -lX11'. Otherwise, this macro may fail to detect that LIBRARY is present, because linking the test program can fail with unresolved symbols. The OTHER-LIBRARIES argument should be limited to cases where it is desirable to test for one library in the presence of another that is not already in `LIBS'. `AC_CHECK_LIB' requires some care in usage, and should be avoided in some common cases. Many standard functions like `gethostbyname' appear in the standard C library on some hosts, and in special libraries like `nsl' on other hosts. On some hosts the special libraries contain variant implementations that you may not want to use. These days it is normally better to use `AC_SEARCH_LIBS([gethostbyname], [nsl])' instead of `AC_CHECK_LIB([nsl], [gethostbyname])'. The result of this test is cached in the `ac_cv_lib_LIBRARY_FUNCTION' variable. -- Macro: AC_SEARCH_LIBS (FUNCTION, SEARCH-LIBS, [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND], [OTHER-LIBRARIES]) Search for a library defining FUNCTION if it's not already available. This equates to calling `AC_LINK_IFELSE([AC_LANG_CALL([], [FUNCTION])])' first with no libraries, then for each library listed in SEARCH-LIBS. Add `-lLIBRARY' to `LIBS' for the first library found to contain FUNCTION, and run ACTION-IF-FOUND. If the function is not found, run ACTION-IF-NOT-FOUND. If linking with LIBRARY results in unresolved symbols that would be resolved by linking with additional libraries, give those libraries as the OTHER-LIBRARIES argument, separated by spaces: e.g., `-lXt -lX11'. Otherwise, this macro fails to detect that FUNCTION is present, because linking the test program always fails with unresolved symbols. The result of this test is cached in the `ac_cv_search_FUNCTION' variable as `none required' if FUNCTION is already available, as `no' if no library containing FUNCTION was found, otherwise as the `-lLIBRARY' option that needs to be added to `LIBS'. 5.5 Library Functions ===================== The following macros check for particular C library functions. If there is no macro specifically defined to check for a function you need, and you don't need to check for any special properties of it, then you can use one of the general function-check macros. 5.5.1 Portability of C Functions -------------------------------- Most usual functions can either be missing, or be buggy, or be limited on some architectures. This section tries to make an inventory of these portability issues. By definition, this list always requires additions. Please help us keeping it as complete as possible. `exit' On ancient hosts, `exit' returned `int'. This is because `exit' predates `void', and there was a long tradition of it returning `int'. On current hosts, the problem more likely is that `exit' is not declared, due to C++ problems of some sort or another. For this reason we suggest that test programs not invoke `exit', but return from `main' instead. `free' The C standard says a call `free (NULL)' does nothing, but some old systems don't support this (e.g., NextStep). `isinf' `isnan' The C99 standard says that `isinf' and `isnan' are macros. On some systems just macros are available (e.g., HP-UX and Solaris 10), on some systems both macros and functions (e.g., glibc 2.3.2), and on some systems only functions (e.g., IRIX 6 and Solaris 9). In some cases these functions are declared in nonstandard headers like `' and defined in non-default libraries like `-lm' or `-lsunmath'. The C99 `isinf' and `isnan' macros work correctly with `long double' arguments, but pre-C99 systems that use functions typically assume `double' arguments. On such a system, `isinf' incorrectly returns true for a finite `long double' argument that is outside the range of `double'. The best workaround for these issues is to use gnulib modules `isinf' and `isnan' (*note Gnulib::). But a lighter weight solution involves code like the following. #include #ifndef isnan # define isnan(x) \ (sizeof (x) == sizeof (long double) ? isnan_ld (x) \ : sizeof (x) == sizeof (double) ? isnan_d (x) \ : isnan_f (x)) static inline int isnan_f (float x) { return x != x; } static inline int isnan_d (double x) { return x != x; } static inline int isnan_ld (long double x) { return x != x; } #endif #ifndef isinf # define isinf(x) \ (sizeof (x) == sizeof (long double) ? isinf_ld (x) \ : sizeof (x) == sizeof (double) ? isinf_d (x) \ : isinf_f (x)) static inline int isinf_f (float x) { return !isnan (x) && isnan (x - x); } static inline int isinf_d (double x) { return !isnan (x) && isnan (x - x); } static inline int isinf_ld (long double x) { return !isnan (x) && isnan (x - x); } #endif Use `AC_C_INLINE' (*note C Compiler::) so that this code works on compilers that lack the `inline' keyword. Some optimizing compilers mishandle these definitions, but systems with that bug typically have many other floating point corner-case compliance problems anyway, so it's probably not worth worrying about. `malloc' The C standard says a call `malloc (0)' is implementation dependent. It can return either `NULL' or a new non-null pointer. The latter is more common (e.g., the GNU C Library) but is by no means universal. `AC_FUNC_MALLOC' can be used to insist on non-`NULL' (*note Particular Functions::). `putenv' Posix prefers `setenv' to `putenv'; among other things, `putenv' is not required of all Posix implementations, but `setenv' is. Posix specifies that `putenv' puts the given string directly in `environ', but some systems make a copy of it instead (e.g., glibc 2.0, or BSD). And when a copy is made, `unsetenv' might not free it, causing a memory leak (e.g., FreeBSD 4). On some systems `putenv ("FOO")' removes `FOO' from the environment, but this is not standard usage and it dumps core on some systems (e.g., AIX). On MinGW, a call `putenv ("FOO=")' removes `FOO' from the environment, rather than inserting it with an empty value. `realloc' The C standard says a call `realloc (NULL, size)' is equivalent to `malloc (size)', but some old systems don't support this (e.g., NextStep). `signal' handler Normally `signal' takes a handler function with a return type of `void', but some old systems required `int' instead. Any actual `int' value returned is not used; this is only a difference in the function prototype demanded. All systems we know of in current use return `void'. The `int' was to support K&R C, where of course `void' is not available. The obsolete macro `AC_TYPE_SIGNAL' (*note AC_TYPE_SIGNAL::) can be used to establish the correct type in all cases. In most cases, it is more robust to use `sigaction' when it is available, rather than `signal'. `snprintf' The C99 standard says that if the output array isn't big enough and if no other errors occur, `snprintf' and `vsnprintf' truncate the output and return the number of bytes that ought to have been produced. Some older systems return the truncated length (e.g., GNU C Library 2.0.x or IRIX 6.5), some a negative value (e.g., earlier GNU C Library versions), and some the buffer length without truncation (e.g., 32-bit Solaris 7). Also, some buggy older systems ignore the length and overrun the buffer (e.g., 64-bit Solaris 7). `sprintf' The C standard says `sprintf' and `vsprintf' return the number of bytes written. On some ancient systems (SunOS 4 for instance) they return the buffer pointer instead, but these no longer need to be worried about. `sscanf' On various old systems, e.g., HP-UX 9, `sscanf' requires that its input string be writable (though it doesn't actually change it). This can be a problem when using `gcc' since it normally puts constant strings in read-only memory (*note Incompatibilities of GCC: (gcc)Incompatibilities.). Apparently in some cases even having format strings read-only can be a problem. `strerror_r' Posix specifies that `strerror_r' returns an `int', but many systems (e.g., GNU C Library version 2.2.4) provide a different version returning a `char *'. `AC_FUNC_STRERROR_R' can detect which is in use (*note Particular Functions::). `strnlen' AIX 4.3 provides a broken version which produces the following results: strnlen ("foobar", 0) = 0 strnlen ("foobar", 1) = 3 strnlen ("foobar", 2) = 2 strnlen ("foobar", 3) = 1 strnlen ("foobar", 4) = 0 strnlen ("foobar", 5) = 6 strnlen ("foobar", 6) = 6 strnlen ("foobar", 7) = 6 strnlen ("foobar", 8) = 6 strnlen ("foobar", 9) = 6 `sysconf' `_SC_PAGESIZE' is standard, but some older systems (e.g., HP-UX 9) have `_SC_PAGE_SIZE' instead. This can be tested with `#ifdef'. `unlink' The Posix spec says that `unlink' causes the given file to be removed only after there are no more open file handles for it. Some non-Posix hosts have trouble with this requirement, though, and some DOS variants even corrupt the file system. `unsetenv' On MinGW, `unsetenv' is not available, but a variable `FOO' can be removed with a call `putenv ("FOO=")', as described under `putenv' above. `va_copy' The C99 standard provides `va_copy' for copying `va_list' variables. It may be available in older environments too, though possibly as `__va_copy' (e.g., `gcc' in strict pre-C99 mode). These can be tested with `#ifdef'. A fallback to `memcpy (&dst, &src, sizeof (va_list))' gives maximum portability. `va_list' `va_list' is not necessarily just a pointer. It can be a `struct' (e.g., `gcc' on Alpha), which means `NULL' is not portable. Or it can be an array (e.g., `gcc' in some PowerPC configurations), which means as a function parameter it can be effectively call-by-reference and library routines might modify the value back in the caller (e.g., `vsnprintf' in the GNU C Library 2.1). Signed `>>' Normally the C `>>' right shift of a signed type replicates the high bit, giving a so-called "arithmetic" shift. But care should be taken since Standard C doesn't require that behavior. On those few processors without a native arithmetic shift (for instance Cray vector systems) zero bits may be shifted in, the same as a shift of an unsigned type. Integer `/' C divides signed integers by truncating their quotient toward zero, yielding the same result as Fortran. However, before C99 the standard allowed C implementations to take the floor or ceiling of the quotient in some cases. Hardly any implementations took advantage of this freedom, though, and it's probably not worth worrying about this issue nowadays. 5.5.2 Particular Function Checks -------------------------------- These macros check for particular C functions--whether they exist, and in some cases how they respond when given certain arguments. -- Macro: AC_FUNC_ALLOCA Check how to get `alloca'. Tries to get a builtin version by checking for `alloca.h' or the predefined C preprocessor macros `__GNUC__' and `_AIX'. If this macro finds `alloca.h', it defines `HAVE_ALLOCA_H'. If those attempts fail, it looks for the function in the standard C library. If any of those methods succeed, it defines `HAVE_ALLOCA'. Otherwise, it sets the output variable `ALLOCA' to `${LIBOBJDIR}alloca.o' and defines `C_ALLOCA' (so programs can periodically call `alloca (0)' to garbage collect). This variable is separate from `LIBOBJS' so multiple programs can share the value of `ALLOCA' without needing to create an actual library, in case only some of them use the code in `LIBOBJS'. The `${LIBOBJDIR}' prefix serves the same purpose as in `LIBOBJS' (*note AC_LIBOBJ vs LIBOBJS::). This macro does not try to get `alloca' from the System V R3 `libPW' or the System V R4 `libucb' because those libraries contain some incompatible functions that cause trouble. Some versions do not even contain `alloca' or contain a buggy version. If you still want to use their `alloca', use `ar' to extract `alloca.o' from them instead of compiling `alloca.c'. Source files that use `alloca' should start with a piece of code like the following, to declare it properly. #ifdef HAVE_ALLOCA_H # include #elif defined __GNUC__ # define alloca __builtin_alloca #elif defined _AIX # define alloca __alloca #elif defined _MSC_VER # include # define alloca _alloca #else # include # ifdef __cplusplus extern "C" # endif void *alloca (size_t); #endif -- Macro: AC_FUNC_CHOWN If the `chown' function is available and works (in particular, it should accept `-1' for `uid' and `gid'), define `HAVE_CHOWN'. The result of this macro is cached in the `ac_cv_func_chown_works' variable. -- Macro: AC_FUNC_CLOSEDIR_VOID If the `closedir' function does not return a meaningful value, define `CLOSEDIR_VOID'. Otherwise, callers ought to check its return value for an error indicator. Currently this test is implemented by running a test program. When cross compiling the pessimistic assumption that `closedir' does not return a meaningful value is made. The result of this macro is cached in the `ac_cv_func_closedir_void' variable. This macro is obsolescent, as `closedir' returns a meaningful value on current systems. New programs need not use this macro. -- Macro: AC_FUNC_ERROR_AT_LINE If the `error_at_line' function is not found, require an `AC_LIBOBJ' replacement of `error'. The result of this macro is cached in the `ac_cv_lib_error_at_line' variable. -- Macro: AC_FUNC_FNMATCH If the `fnmatch' function conforms to Posix, define `HAVE_FNMATCH'. Detect common implementation bugs, for example, the bugs in Solaris 2.4. Unlike the other specific `AC_FUNC' macros, `AC_FUNC_FNMATCH' does not replace a broken/missing `fnmatch'. This is for historical reasons. See `AC_REPLACE_FNMATCH' below. The result of this macro is cached in the `ac_cv_func_fnmatch_works' variable. This macro is obsolescent. New programs should use Gnulib's `fnmatch-posix' module. *Note Gnulib::. -- Macro: AC_FUNC_FNMATCH_GNU Behave like `AC_REPLACE_FNMATCH' (_replace_) but also test whether `fnmatch' supports GNU extensions. Detect common implementation bugs, for example, the bugs in the GNU C Library 2.1. The result of this macro is cached in the `ac_cv_func_fnmatch_gnu' variable. This macro is obsolescent. New programs should use Gnulib's `fnmatch-gnu' module. *Note Gnulib::. -- Macro: AC_FUNC_FORK This macro checks for the `fork' and `vfork' functions. If a working `fork' is found, define `HAVE_WORKING_FORK'. This macro checks whether `fork' is just a stub by trying to run it. If `vfork.h' is found, define `HAVE_VFORK_H'. If a working `vfork' is found, define `HAVE_WORKING_VFORK'. Otherwise, define `vfork' to be `fork' for backward compatibility with previous versions of `autoconf'. This macro checks for several known errors in implementations of `vfork' and considers the system to not have a working `vfork' if it detects any of them. It is not considered to be an implementation error if a child's invocation of `signal' modifies the parent's signal handler, since child processes rarely change their signal handlers. Since this macro defines `vfork' only for backward compatibility with previous versions of `autoconf' you're encouraged to define it yourself in new code: #ifndef HAVE_WORKING_VFORK # define vfork fork #endif -- Macro: AC_FUNC_FSEEKO If the `fseeko' function is available, define `HAVE_FSEEKO'. Define `_LARGEFILE_SOURCE' if necessary to make the prototype visible on some systems (e.g., glibc 2.2). Otherwise linkage problems may occur when compiling with `AC_SYS_LARGEFILE' on largefile-sensitive systems where `off_t' does not default to a 64bit entity. All systems with `fseeko' also supply `ftello'. -- Macro: AC_FUNC_GETGROUPS If the `getgroups' function is available and works (unlike on Ultrix 4.3, where `getgroups (0, 0)' always fails), define `HAVE_GETGROUPS'. Set `GETGROUPS_LIBS' to any libraries needed to get that function. This macro runs `AC_TYPE_GETGROUPS'. -- Macro: AC_FUNC_GETLOADAVG Check how to get the system load averages. To perform its tests properly, this macro needs the file `getloadavg.c'; therefore, be sure to set the `AC_LIBOBJ' replacement directory properly (see *note Generic Functions::, `AC_CONFIG_LIBOBJ_DIR'). If the system has the `getloadavg' function, define `HAVE_GETLOADAVG', and set `GETLOADAVG_LIBS' to any libraries necessary to get that function. Also add `GETLOADAVG_LIBS' to `LIBS'. Otherwise, require an `AC_LIBOBJ' replacement for `getloadavg' with source code in `DIR/getloadavg.c', and possibly define several other C preprocessor macros and output variables: 1. Define `C_GETLOADAVG'. 2. Define `SVR4', `DGUX', `UMAX', or `UMAX4_3' if on those systems. 3. If `nlist.h' is found, define `HAVE_NLIST_H'. 4. If `struct nlist' has an `n_un.n_name' member, define `HAVE_STRUCT_NLIST_N_UN_N_NAME'. The obsolete symbol `NLIST_NAME_UNION' is still defined, but do not depend upon it. 5. Programs may need to be installed set-group-ID (or set-user-ID) for `getloadavg' to work. In this case, define `GETLOADAVG_PRIVILEGED', set the output variable `NEED_SETGID' to `true' (and otherwise to `false'), and set `KMEM_GROUP' to the name of the group that should own the installed program. The `AC_FUNC_GETLOADAVG' macro is obsolescent. New programs should use Gnulib's `getloadavg' module. *Note Gnulib::. -- Macro: AC_FUNC_GETMNTENT Check for `getmntent' in the standard C library, and then in the `sun', `seq', and `gen' libraries, for UNICOS, IRIX 4, PTX, and UnixWare, respectively. Then, if `getmntent' is available, define `HAVE_GETMNTENT' and set `ac_cv_func_getmntent' to `yes'. Otherwise set `ac_cv_func_getmntent' to `no'. The result of this macro can be overridden by setting the cache variable `ac_cv_search_getmntent'. -- Macro: AC_FUNC_GETPGRP Define `GETPGRP_VOID' if it is an error to pass 0 to `getpgrp'; this is the Posix behavior. On older BSD systems, you must pass 0 to `getpgrp', as it takes an argument and behaves like Posix's `getpgid'. #ifdef GETPGRP_VOID pid = getpgrp (); #else pid = getpgrp (0); #endif This macro does not check whether `getpgrp' exists at all; if you need to work in that situation, first call `AC_CHECK_FUNC' for `getpgrp'. The result of this macro is cached in the `ac_cv_func_getpgrp_void' variable. This macro is obsolescent, as current systems have a `getpgrp' whose signature conforms to Posix. New programs need not use this macro. -- Macro: AC_FUNC_LSTAT_FOLLOWS_SLASHED_SYMLINK If `link' is a symbolic link, then `lstat' should treat `link/' the same as `link/.'. However, many older `lstat' implementations incorrectly ignore trailing slashes. It is safe to assume that if `lstat' incorrectly ignores trailing slashes, then other symbolic-link-aware functions like `unlink' also incorrectly ignore trailing slashes. If `lstat' behaves properly, define `LSTAT_FOLLOWS_SLASHED_SYMLINK', otherwise require an `AC_LIBOBJ' replacement of `lstat'. The result of this macro is cached in the `ac_cv_func_lstat_dereferences_slashed_symlink' variable. -- Macro: AC_FUNC_MALLOC If the `malloc' function is compatible with the GNU C library `malloc' (i.e., `malloc (0)' returns a valid pointer), define `HAVE_MALLOC' to 1. Otherwise define `HAVE_MALLOC' to 0, ask for an `AC_LIBOBJ' replacement for `malloc', and define `malloc' to `rpl_malloc' so that the native `malloc' is not used in the main project. Typically, the replacement file `malloc.c' should look like (note the `#undef malloc'): #include #undef malloc #include void *malloc (); /* Allocate an N-byte block of memory from the heap. If N is zero, allocate a 1-byte block. */ void * rpl_malloc (size_t n) { if (n == 0) n = 1; return malloc (n); } The result of this macro is cached in the `ac_cv_func_malloc_0_nonnull' variable. -- Macro: AC_FUNC_MBRTOWC Define `HAVE_MBRTOWC' to 1 if the function `mbrtowc' and the type `mbstate_t' are properly declared. The result of this macro is cached in the `ac_cv_func_mbrtowc' variable. -- Macro: AC_FUNC_MEMCMP If the `memcmp' function is not available, or does not work on 8-bit data (like the one on SunOS 4.1.3), or fails when comparing 16 bytes or more and with at least one buffer not starting on a 4-byte boundary (such as the one on NeXT x86 OpenStep), require an `AC_LIBOBJ' replacement for `memcmp'. The result of this macro is cached in the `ac_cv_func_memcmp_working' variable. This macro is obsolescent, as current systems have a working `memcmp'. New programs need not use this macro. -- Macro: AC_FUNC_MKTIME If the `mktime' function is not available, or does not work correctly, require an `AC_LIBOBJ' replacement for `mktime'. For the purposes of this test, `mktime' should conform to the Posix standard and should be the inverse of `localtime'. The result of this macro is cached in the `ac_cv_func_working_mktime' variable. -- Macro: AC_FUNC_MMAP If the `mmap' function exists and works correctly, define `HAVE_MMAP'. This checks only private fixed mapping of already-mapped memory. The result of this macro is cached in the `ac_cv_func_mmap_fixed_mapped' variable. -- Macro: AC_FUNC_OBSTACK If the obstacks are found, define `HAVE_OBSTACK', else require an `AC_LIBOBJ' replacement for `obstack'. The result of this macro is cached in the `ac_cv_func_obstack' variable. -- Macro: AC_FUNC_REALLOC If the `realloc' function is compatible with the GNU C library `realloc' (i.e., `realloc (NULL, 0)' returns a valid pointer), define `HAVE_REALLOC' to 1. Otherwise define `HAVE_REALLOC' to 0, ask for an `AC_LIBOBJ' replacement for `realloc', and define `realloc' to `rpl_realloc' so that the native `realloc' is not used in the main project. See `AC_FUNC_MALLOC' for details. The result of this macro is cached in the `ac_cv_func_realloc_0_nonnull' variable. -- Macro: AC_FUNC_SELECT_ARGTYPES Determines the correct type to be passed for each of the `select' function's arguments, and defines those types in `SELECT_TYPE_ARG1', `SELECT_TYPE_ARG234', and `SELECT_TYPE_ARG5' respectively. `SELECT_TYPE_ARG1' defaults to `int', `SELECT_TYPE_ARG234' defaults to `int *', and `SELECT_TYPE_ARG5' defaults to `struct timeval *'. This macro is obsolescent, as current systems have a `select' whose signature conforms to Posix. New programs need not use this macro. -- Macro: AC_FUNC_SETPGRP If `setpgrp' takes no argument (the Posix version), define `SETPGRP_VOID'. Otherwise, it is the BSD version, which takes two process IDs as arguments. This macro does not check whether `setpgrp' exists at all; if you need to work in that situation, first call `AC_CHECK_FUNC' for `setpgrp'. The result of this macro is cached in the `ac_cv_func_setpgrp_void' variable. This macro is obsolescent, as current systems have a `setpgrp' whose signature conforms to Posix. New programs need not use this macro. -- Macro: AC_FUNC_STAT -- Macro: AC_FUNC_LSTAT Determine whether `stat' or `lstat' have the bug that it succeeds when given the zero-length file name as argument. The `stat' and `lstat' from SunOS 4.1.4 and the Hurd (as of 1998-11-01) do this. If it does, then define `HAVE_STAT_EMPTY_STRING_BUG' (or `HAVE_LSTAT_EMPTY_STRING_BUG') and ask for an `AC_LIBOBJ' replacement of it. The results of these macros are cached in the `ac_cv_func_stat_empty_string_bug' and the `ac_cv_func_lstat_empty_string_bug' variables, respectively. These macros are obsolescent, as no current systems have the bug. New programs need not use these macros. -- Macro: AC_FUNC_STRCOLL If the `strcoll' function exists and works correctly, define `HAVE_STRCOLL'. This does a bit more than `AC_CHECK_FUNCS(strcoll)', because some systems have incorrect definitions of `strcoll' that should not be used. The result of this macro is cached in the `ac_cv_func_strcoll_works' variable. -- Macro: AC_FUNC_STRERROR_R If `strerror_r' is available, define `HAVE_STRERROR_R', and if it is declared, define `HAVE_DECL_STRERROR_R'. If it returns a `char *' message, define `STRERROR_R_CHAR_P'; otherwise it returns an `int' error number. The Thread-Safe Functions option of Posix requires `strerror_r' to return `int', but many systems (including, for example, version 2.2.4 of the GNU C Library) return a `char *' value that is not necessarily equal to the buffer argument. The result of this macro is cached in the `ac_cv_func_strerror_r_char_p' variable. -- Macro: AC_FUNC_STRFTIME Check for `strftime' in the `intl' library, for SCO Unix. Then, if `strftime' is available, define `HAVE_STRFTIME'. This macro is obsolescent, as no current systems require the `intl' library for `strftime'. New programs need not use this macro. -- Macro: AC_FUNC_STRTOD If the `strtod' function does not exist or doesn't work correctly, ask for an `AC_LIBOBJ' replacement of `strtod'. In this case, because `strtod.c' is likely to need `pow', set the output variable `POW_LIB' to the extra library needed. This macro caches its result in the `ac_cv_func_strtod' variable and depends upon the result in the `ac_cv_func_pow' variable. -- Macro: AC_FUNC_STRTOLD If the `strtold' function exists and conforms to C99, define `HAVE_STRTOLD'. This macro caches its result in the `ac_cv_func_strtold' variable. -- Macro: AC_FUNC_STRNLEN If the `strnlen' function is not available, or is buggy (like the one from AIX 4.3), require an `AC_LIBOBJ' replacement for it. This macro caches its result in the `ac_cv_func_strnlen_working' variable. -- Macro: AC_FUNC_UTIME_NULL If `utime (FILE, NULL)' sets FILE's timestamp to the present, define `HAVE_UTIME_NULL'. This macro caches its result in the `ac_cv_func_utime_null' variable. This macro is obsolescent, as all current systems have a `utime' that behaves this way. New programs need not use this macro. -- Macro: AC_FUNC_VPRINTF If `vprintf' is found, define `HAVE_VPRINTF'. Otherwise, if `_doprnt' is found, define `HAVE_DOPRNT'. (If `vprintf' is available, you may assume that `vfprintf' and `vsprintf' are also available.) This macro is obsolescent, as all current systems have `vprintf'. New programs need not use this macro. -- Macro: AC_REPLACE_FNMATCH If the `fnmatch' function does not conform to Posix (see `AC_FUNC_FNMATCH'), ask for its `AC_LIBOBJ' replacement. The files `fnmatch.c', `fnmatch_loop.c', and `fnmatch_.h' in the `AC_LIBOBJ' replacement directory are assumed to contain a copy of the source code of GNU `fnmatch'. If necessary, this source code is compiled as an `AC_LIBOBJ' replacement, and the `fnmatch_.h' file is linked to `fnmatch.h' so that it can be included in place of the system `'. This macro caches its result in the `ac_cv_func_fnmatch_works' variable. This macro is obsolescent, as it assumes the use of particular source files. New programs should use Gnulib's `fnmatch-posix' module, which provides this macro along with the source files. *Note Gnulib::. 5.5.3 Generic Function Checks ----------------------------- These macros are used to find functions not covered by the "particular" test macros. If the functions might be in libraries other than the default C library, first call `AC_CHECK_LIB' for those libraries. If you need to check the behavior of a function as well as find out whether it is present, you have to write your own test for it (*note Writing Tests::). -- Macro: AC_CHECK_FUNC (FUNCTION, [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND]) If C function FUNCTION is available, run shell commands ACTION-IF-FOUND, otherwise ACTION-IF-NOT-FOUND. If you just want to define a symbol if the function is available, consider using `AC_CHECK_FUNCS' instead. This macro checks for functions with C linkage even when `AC_LANG(C++)' has been called, since C is more standardized than C++. (*note Language Choice::, for more information about selecting the language for checks.) This macro caches its result in the `ac_cv_func_FUNCTION' variable. -- Macro: AC_CHECK_FUNCS (FUNCTION..., [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND]) For each FUNCTION enumerated in the blank-or-newline-separated argument list, define `HAVE_FUNCTION' (in all capitals) if it is available. If ACTION-IF-FOUND is given, it is additional shell code to execute when one of the functions is found. You can give it a value of `break' to break out of the loop on the first match. If ACTION-IF-NOT-FOUND is given, it is executed when one of the functions is not found. Results are cached for each FUNCTION as in `AC_CHECK_FUNC'. -- Macro: AC_CHECK_FUNCS_ONCE (FUNCTION...) For each FUNCTION enumerated in the blank-or-newline-separated argument list, define `HAVE_FUNCTION' (in all capitals) if it is available. This is a once-only variant of `AC_CHECK_FUNCS'. It generates the checking code at most once, so that `configure' is smaller and faster; but the checks cannot be conditionalized and are always done once, early during the `configure' run. Autoconf follows a philosophy that was formed over the years by those who have struggled for portability: isolate the portability issues in specific files, and then program as if you were in a Posix environment. Some functions may be missing or unfixable, and your package must be ready to replace them. Suitable replacements for many such problem functions are available from Gnulib (*note Gnulib::). -- Macro: AC_LIBOBJ (FUNCTION) Specify that `FUNCTION.c' must be included in the executables to replace a missing or broken implementation of FUNCTION. Technically, it adds `FUNCTION.$ac_objext' to the output variable `LIBOBJS' if it is not already in, and calls `AC_LIBSOURCE' for `FUNCTION.c'. You should not directly change `LIBOBJS', since this is not traceable. -- Macro: AC_LIBSOURCE (FILE) Specify that FILE might be needed to compile the project. If you need to know what files might be needed by a `configure.ac', you should trace `AC_LIBSOURCE'. FILE must be a literal. This macro is called automatically from `AC_LIBOBJ', but you must call it explicitly if you pass a shell variable to `AC_LIBOBJ'. In that case, since shell variables cannot be traced statically, you must pass to `AC_LIBSOURCE' any possible files that the shell variable might cause `AC_LIBOBJ' to need. For example, if you want to pass a variable `$foo_or_bar' to `AC_LIBOBJ' that holds either `"foo"' or `"bar"', you should do: AC_LIBSOURCE([foo.c]) AC_LIBSOURCE([bar.c]) AC_LIBOBJ([$foo_or_bar]) There is usually a way to avoid this, however, and you are encouraged to simply call `AC_LIBOBJ' with literal arguments. Note that this macro replaces the obsolete `AC_LIBOBJ_DECL', with slightly different semantics: the old macro took the function name, e.g., `foo', as its argument rather than the file name. -- Macro: AC_LIBSOURCES (FILES) Like `AC_LIBSOURCE', but accepts one or more FILES in a comma-separated M4 list. Thus, the above example might be rewritten: AC_LIBSOURCES([foo.c, bar.c]) AC_LIBOBJ([$foo_or_bar]) -- Macro: AC_CONFIG_LIBOBJ_DIR (DIRECTORY) Specify that `AC_LIBOBJ' replacement files are to be found in DIRECTORY, a name relative to the top level of the source tree. The replacement directory defaults to `.', the top level directory, and the most typical value is `lib', corresponding to `AC_CONFIG_LIBOBJ_DIR([lib])'. `configure' might need to know the replacement directory for the following reasons: (i) some checks use the replacement files, (ii) some macros bypass broken system headers by installing links to the replacement headers (iii) when used in conjunction with Automake, within each makefile, DIRECTORY is used as a relative path from `$(top_srcdir)' to each object named in `LIBOBJS' and `LTLIBOBJS', etc. It is common to merely check for the existence of a function, and ask for its `AC_LIBOBJ' replacement if missing. The following macro is a convenient shorthand. -- Macro: AC_REPLACE_FUNCS (FUNCTION...) Like `AC_CHECK_FUNCS', but uses `AC_LIBOBJ(FUNCTION)' as ACTION-IF-NOT-FOUND. You can declare your replacement function by enclosing the prototype in `#ifndef HAVE_FUNCTION'. If the system has the function, it probably declares it in a header file you should be including, so you shouldn't redeclare it lest your declaration conflict. 5.6 Header Files ================ The following macros check for the presence of certain C header files. If there is no macro specifically defined to check for a header file you need, and you don't need to check for any special properties of it, then you can use one of the general header-file check macros. 5.6.1 Portability of Headers ---------------------------- This section tries to collect knowledge about common headers, and the problems they cause. By definition, this list always requires additions. Please help us keeping it as complete as possible. `limits.h' C99 says that `limits.h' defines `LLONG_MIN', `LLONG_MAX', and `ULLONG_MAX', but many almost-C99 environments (e.g., default GCC 4.0.2 + glibc 2.4) do not define them. `inttypes.h' vs. `stdint.h' The C99 standard says that `inttypes.h' includes `stdint.h', so there's no need to include `stdint.h' separately in a standard environment. Some implementations have `inttypes.h' but not `stdint.h' (e.g., Solaris 7), but we don't know of any implementation that has `stdint.h' but not `inttypes.h'. `linux/irda.h' It requires `linux/types.h' and `sys/socket.h'. `linux/random.h' It requires `linux/types.h'. `net/if.h' On Darwin, this file requires that `sys/socket.h' be included beforehand. One should run: AC_CHECK_HEADERS([sys/socket.h]) AC_CHECK_HEADERS([net/if.h], [], [], [#include #ifdef STDC_HEADERS # include # include #else # ifdef HAVE_STDLIB_H # include # endif #endif #ifdef HAVE_SYS_SOCKET_H # include #endif ]) `netinet/if_ether.h' On Darwin, this file requires that `stdio.h' and `sys/socket.h' be included beforehand. One should run: AC_CHECK_HEADERS([sys/socket.h]) AC_CHECK_HEADERS([netinet/if_ether.h], [], [], [#include #ifdef STDC_HEADERS # include # include #else # ifdef HAVE_STDLIB_H # include # endif #endif #ifdef HAVE_SYS_SOCKET_H # include #endif ]) `stdint.h' See above, item `inttypes.h' vs. `stdint.h'. `stdlib.h' On many systems (e.g., Darwin), `stdio.h' is a prerequisite. `sys/mount.h' On FreeBSD 4.8 on ia32 and using gcc version 2.95.4, `sys/params.h' is a prerequisite. `sys/ptem.h' On Solaris 8, `sys/stream.h' is a prerequisite. `sys/socket.h' On Darwin, `stdlib.h' is a prerequisite. `sys/ucred.h' On Tru64 5.1, `sys/types.h' is a prerequisite. `X11/extensions/scrnsaver.h' Using XFree86, this header requires `X11/Xlib.h', which is probably so required that you might not even consider looking for it. AC_CHECK_HEADERS([X11/extensions/scrnsaver.h], [], [], [[#include ]]) 5.6.2 Particular Header Checks ------------------------------ These macros check for particular system header files--whether they exist, and in some cases whether they declare certain symbols. -- Macro: AC_HEADER_ASSERT Check whether to enable assertions in the style of `assert.h'. Assertions are enabled by default, but the user can override this by invoking `configure' with the `--disable-assert' option. -- Macro: AC_HEADER_DIRENT Check for the following header files. For the first one that is found and defines `DIR', define the listed C preprocessor macro: `dirent.h' `HAVE_DIRENT_H' `sys/ndir.h' `HAVE_SYS_NDIR_H' `sys/dir.h' `HAVE_SYS_DIR_H' `ndir.h' `HAVE_NDIR_H' The directory-library declarations in your source code should look something like the following: #include #ifdef HAVE_DIRENT_H # include # define NAMLEN(dirent) strlen ((dirent)->d_name) #else # define dirent direct # define NAMLEN(dirent) ((dirent)->d_namlen) # ifdef HAVE_SYS_NDIR_H # include # endif # ifdef HAVE_SYS_DIR_H # include # endif # ifdef HAVE_NDIR_H # include # endif #endif Using the above declarations, the program would declare variables to be of type `struct dirent', not `struct direct', and would access the length of a directory entry name by passing a pointer to a `struct dirent' to the `NAMLEN' macro. This macro also checks for the SCO Xenix `dir' and `x' libraries. This macro is obsolescent, as all current systems with directory libraries have `'. New programs need not use this macro. Also see `AC_STRUCT_DIRENT_D_INO' and `AC_STRUCT_DIRENT_D_TYPE' (*note Particular Structures::). -- Macro: AC_HEADER_MAJOR If `sys/types.h' does not define `major', `minor', and `makedev', but `sys/mkdev.h' does, define `MAJOR_IN_MKDEV'; otherwise, if `sys/sysmacros.h' does, define `MAJOR_IN_SYSMACROS'. -- Macro: AC_HEADER_RESOLV Checks for header `resolv.h', checking for prerequisites first. To properly use `resolv.h', your code should contain something like the following: #ifdef HAVE_SYS_TYPES_H # include #endif #ifdef HAVE_NETINET_IN_H # include /* inet_ functions / structs */ #endif #ifdef HAVE_ARPA_NAMESER_H # include /* DNS HEADER struct */ #endif #ifdef HAVE_NETDB_H # include #endif #include -- Macro: AC_HEADER_STAT If the macros `S_ISDIR', `S_ISREG', etc. defined in `sys/stat.h' do not work properly (returning false positives), define `STAT_MACROS_BROKEN'. This is the case on Tektronix UTekV, Amdahl UTS and Motorola System V/88. This macro is obsolescent, as no current systems have the bug. New programs need not use this macro. -- Macro: AC_HEADER_STDBOOL If `stdbool.h' exists and conforms to C99, define `HAVE_STDBOOL_H' to 1; if the type `_Bool' is defined, define `HAVE__BOOL' to 1. To fulfill the C99 requirements, your `system.h' could contain the following code: #ifdef HAVE_STDBOOL_H # include #else # ifndef HAVE__BOOL # ifdef __cplusplus typedef bool _Bool; # else # define _Bool signed char # endif # endif # define bool _Bool # define false 0 # define true 1 # define __bool_true_false_are_defined 1 #endif Alternatively you can use the `stdbool' package of Gnulib (*note Gnulib::); it packages the above code into a replacement header and contains a few other bells and whistles. This macro caches its result in the `ac_cv_header_stdbool_h' variable. -- Macro: AC_HEADER_STDC Define `STDC_HEADERS' if the system has C header files conforming to ANSI C89 (ISO C90). Specifically, this macro checks for `stdlib.h', `stdarg.h', `string.h', and `float.h'; if the system has those, it probably has the rest of the C89 header files. This macro also checks whether `string.h' declares `memchr' (and thus presumably the other `mem' functions), whether `stdlib.h' declare `free' (and thus presumably `malloc' and other related functions), and whether the `ctype.h' macros work on characters with the high bit set, as the C standard requires. If you use this macro, your code can refer to `STDC_HEADERS' to determine whether the system has conforming header files (and probably C library functions). This macro caches its result in the `ac_cv_header_stdc' variable. This macro is obsolescent, as current systems have conforming header files. New programs need not use this macro. Nowadays `string.h' is part of the C standard and declares functions like `strcpy', and `strings.h' is standardized by Posix and declares BSD functions like `bcopy'; but historically, string functions were a major sticking point in this area. If you still want to worry about portability to ancient systems without standard headers, there is so much variation that it is probably easier to declare the functions you use than to figure out exactly what the system header files declare. Some ancient systems contained a mix of functions from the C standard and from BSD; some were mostly standard but lacked `memmove'; some defined the BSD functions as macros in `string.h' or `strings.h'; some had only the BSD functions but `string.h'; some declared the memory functions in `memory.h', some in `string.h'; etc. It is probably sufficient to check for one string function and one memory function; if the library had the standard versions of those then it probably had most of the others. If you put the following in `configure.ac': # This example is obsolescent. # Nowadays you can omit these macro calls. AC_HEADER_STDC AC_CHECK_FUNCS([strchr memcpy]) then, in your code, you can use declarations like this: /* This example is obsolescent. Nowadays you can just #include . */ #ifdef STDC_HEADERS # include #else # ifndef HAVE_STRCHR # define strchr index # define strrchr rindex # endif char *strchr (), *strrchr (); # ifndef HAVE_MEMCPY # define memcpy(d, s, n) bcopy ((s), (d), (n)) # define memmove(d, s, n) bcopy ((s), (d), (n)) # endif #endif If you use a function like `memchr', `memset', `strtok', or `strspn', which have no BSD equivalent, then macros don't suffice to port to ancient hosts; you must provide an implementation of each function. An easy way to incorporate your implementations only when needed (since the ones in system C libraries may be hand optimized) is to, taking `memchr' for example, put it in `memchr.c' and use `AC_REPLACE_FUNCS([memchr])'. -- Macro: AC_HEADER_SYS_WAIT If `sys/wait.h' exists and is compatible with Posix, define `HAVE_SYS_WAIT_H'. Incompatibility can occur if `sys/wait.h' does not exist, or if it uses the old BSD `union wait' instead of `int' to store a status value. If `sys/wait.h' is not Posix compatible, then instead of including it, define the Posix macros with their usual interpretations. Here is an example: #include #ifdef HAVE_SYS_WAIT_H # include #endif #ifndef WEXITSTATUS # define WEXITSTATUS(stat_val) ((unsigned int) (stat_val) >> 8) #endif #ifndef WIFEXITED # define WIFEXITED(stat_val) (((stat_val) & 255) == 0) #endif This macro caches its result in the `ac_cv_header_sys_wait_h' variable. This macro is obsolescent, as current systems are compatible with Posix. New programs need not use this macro. `_POSIX_VERSION' is defined when `unistd.h' is included on Posix systems. If there is no `unistd.h', it is definitely not a Posix system. However, some non-Posix systems do have `unistd.h'. The way to check whether the system supports Posix is: #ifdef HAVE_UNISTD_H # include # include #endif #ifdef _POSIX_VERSION /* Code for Posix systems. */ #endif -- Macro: AC_HEADER_TIME If a program may include both `time.h' and `sys/time.h', define `TIME_WITH_SYS_TIME'. On some ancient systems, `sys/time.h' included `time.h', but `time.h' was not protected against multiple inclusion, so programs could not explicitly include both files. This macro is useful in programs that use, for example, `struct timeval' as well as `struct tm'. It is best used in conjunction with `HAVE_SYS_TIME_H', which can be checked for using `AC_CHECK_HEADERS([sys/time.h])'. #ifdef TIME_WITH_SYS_TIME # include # include #else # ifdef HAVE_SYS_TIME_H # include # else # include # endif #endif This macro caches its result in the `ac_cv_header_time' variable. This macro is obsolescent, as current systems can include both files when they exist. New programs need not use this macro. -- Macro: AC_HEADER_TIOCGWINSZ If the use of `TIOCGWINSZ' requires `', then define `GWINSZ_IN_SYS_IOCTL'. Otherwise `TIOCGWINSZ' can be found in `'. Use: #ifdef HAVE_TERMIOS_H # include #endif #ifdef GWINSZ_IN_SYS_IOCTL # include #endif 5.6.3 Generic Header Checks --------------------------- These macros are used to find system header files not covered by the "particular" test macros. If you need to check the contents of a header as well as find out whether it is present, you have to write your own test for it (*note Writing Tests::). -- Macro: AC_CHECK_HEADER (HEADER-FILE, [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND], [INCLUDES]) If the system header file HEADER-FILE is compilable, execute shell commands ACTION-IF-FOUND, otherwise execute ACTION-IF-NOT-FOUND. If you just want to define a symbol if the header file is available, consider using `AC_CHECK_HEADERS' instead. INCLUDES is decoded to determine the appropriate include directives. If omitted or empty, `configure' will check for both header existence (with the preprocessor) and usability (with the compiler), using `AC_INCLUDES_DEFAULT' for the compile test. If there is a discrepancy between the results, a warning is issued to the user, and the compiler results are favored (*note Present But Cannot Be Compiled::). In general, favoring the compiler results means that a header will be treated as not found even though the file exists, because you did not provide enough prerequisites. Providing a non-empty INCLUDES argument allows the code to provide any prerequisites prior to including the header under test; it is common to use the argument `AC_INCLUDES_DEFAULT' (*note Default Includes::). With an explicit fourth argument, no preprocessor test is needed. As a special case, an INCLUDES of exactly `-' triggers the older preprocessor check, which merely determines existence of the file in the preprocessor search path; this should only be used as a last resort (it is safer to determine the actual prerequisites and perform a compiler check, or else use `AC_PREPROC_IFELSE' to make it obvious that only a preprocessor check is desired). This macro caches its result in the `ac_cv_header_HEADER-FILE' variable, with characters not suitable for a variable name mapped to underscores. -- Macro: AC_CHECK_HEADERS (HEADER-FILE..., [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND], [INCLUDES]) For each given system header file HEADER-FILE in the blank-separated argument list that exists, define `HAVE_HEADER-FILE' (in all capitals). If ACTION-IF-FOUND is given, it is additional shell code to execute when one of the header files is found. You can give it a value of `break' to break out of the loop on the first match. If ACTION-IF-NOT-FOUND is given, it is executed when one of the header files is not found. INCLUDES is interpreted as in `AC_CHECK_HEADER', in order to choose the set of preprocessor directives supplied before the header under test. This macro caches its result in the `ac_cv_header_HEADER-FILE' variable, with characters not suitable for a variable name mapped to underscores. Previous versions of Autoconf merely checked whether the header was accepted by the preprocessor. This was changed because the old test was inappropriate for typical uses. Headers are typically used to compile, not merely to preprocess, and the old behavior sometimes accepted headers that clashed at compile-time (*note Present But Cannot Be Compiled::). If you need to check whether a header is preprocessable, you can use `AC_PREPROC_IFELSE' (*note Running the Preprocessor::). Actually requiring a header to compile improves the robustness of the test, but it also requires that you make sure that headers that must be included before the HEADER-FILE be part of the INCLUDES, (*note Default Includes::). If looking for `bar.h', which requires that `foo.h' be included before if it exists, we suggest the following scheme: AC_CHECK_HEADERS([foo.h]) AC_CHECK_HEADERS([bar.h], [], [], [#ifdef HAVE_FOO_H # include #endif ]) The following variant generates smaller, faster `configure' files if you do not need the full power of `AC_CHECK_HEADERS'. -- Macro: AC_CHECK_HEADERS_ONCE (HEADER-FILE...) For each given system header file HEADER-FILE in the blank-separated argument list that exists, define `HAVE_HEADER-FILE' (in all capitals). This is a once-only variant of `AC_CHECK_HEADERS'. It generates the checking code at most once, so that `configure' is smaller and faster; but the checks cannot be conditionalized and are always done once, early during the `configure' run. Thus, this macro is only safe for checking headers that do not have prerequisites beyond what `AC_INCLUDES_DEFAULT' provides. 5.7 Declarations ================ The following macros check for the declaration of variables and functions. If there is no macro specifically defined to check for a symbol you need, then you can use the general macros (*note Generic Declarations::) or, for more complex tests, you may use `AC_COMPILE_IFELSE' (*note Running the Compiler::). 5.7.1 Particular Declaration Checks ----------------------------------- There are no specific macros for declarations. 5.7.2 Generic Declaration Checks -------------------------------- These macros are used to find declarations not covered by the "particular" test macros. -- Macro: AC_CHECK_DECL (SYMBOL, [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND], [INCLUDES = `AC_INCLUDES_DEFAULT']) If SYMBOL (a function, variable, or constant) is not declared in INCLUDES and a declaration is needed, run the shell commands ACTION-IF-NOT-FOUND, otherwise ACTION-IF-FOUND. INCLUDES is a series of include directives, defaulting to `AC_INCLUDES_DEFAULT' (*note Default Includes::), which are used prior to the declaration under test. This macro actually tests whether SYMBOL is defined as a macro or can be used as an r-value, not whether it is really declared, because it is much safer to avoid introducing extra declarations when they are not needed. This macro caches its result in the `ac_cv_have_decl_SYMBOL' variable, with characters not suitable for a variable name mapped to underscores. -- Macro: AC_CHECK_DECLS (SYMBOLS, [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND], [INCLUDES = `AC_INCLUDES_DEFAULT']) For each of the SYMBOLS (_comma_-separated list), define `HAVE_DECL_SYMBOL' (in all capitals) to `1' if SYMBOL is declared, otherwise to `0'. If ACTION-IF-NOT-FOUND is given, it is additional shell code to execute when one of the function declarations is needed, otherwise ACTION-IF-FOUND is executed. INCLUDES is a series of include directives, defaulting to `AC_INCLUDES_DEFAULT' (*note Default Includes::), which are used prior to the declarations under test. This macro uses an M4 list as first argument: AC_CHECK_DECLS([strdup]) AC_CHECK_DECLS([strlen]) AC_CHECK_DECLS([malloc, realloc, calloc, free]) AC_CHECK_DECLS([j0], [], [], [[#include ]]) Unlike the other `AC_CHECK_*S' macros, when a SYMBOL is not declared, `HAVE_DECL_SYMBOL' is defined to `0' instead of leaving `HAVE_DECL_SYMBOL' undeclared. When you are _sure_ that the check was performed, use `HAVE_DECL_SYMBOL' in `#if': #if !HAVE_DECL_SYMBOL extern char *symbol; #endif If the test may have not been performed, however, because it is safer _not_ to declare a symbol than to use a declaration that conflicts with the system's one, you should use: #if defined HAVE_DECL_MALLOC && !HAVE_DECL_MALLOC void *malloc (size_t *s); #endif You fall into the second category only in extreme situations: either your files may be used without being configured, or they are used during the configuration. In most cases the traditional approach is enough. This macro caches its results in `ac_cv_have_decl_SYMBOL' variables, with characters not suitable for a variable name mapped to underscores. -- Macro: AC_CHECK_DECLS_ONCE (SYMBOLS) For each of the SYMBOLS (_comma_-separated list), define `HAVE_DECL_SYMBOL' (in all capitals) to `1' if SYMBOL is declared in the default include files, otherwise to `0'. This is a once-only variant of `AC_CHECK_DECLS'. It generates the checking code at most once, so that `configure' is smaller and faster; but the checks cannot be conditionalized and are always done once, early during the `configure' run. 5.8 Structures ============== The following macros check for the presence of certain members in C structures. If there is no macro specifically defined to check for a member you need, then you can use the general structure-member macros (*note Generic Structures::) or, for more complex tests, you may use `AC_COMPILE_IFELSE' (*note Running the Compiler::). 5.8.1 Particular Structure Checks --------------------------------- The following macros check for certain structures or structure members. -- Macro: AC_STRUCT_DIRENT_D_INO Perform all the actions of `AC_HEADER_DIRENT' (*note Particular Headers::). Then, if `struct dirent' contains a `d_ino' member, define `HAVE_STRUCT_DIRENT_D_INO'. `HAVE_STRUCT_DIRENT_D_INO' indicates only the presence of `d_ino', not whether its contents are always reliable. Traditionally, a zero `d_ino' indicated a deleted directory entry, though current systems hide this detail from the user and never return zero `d_ino' values. Many current systems report an incorrect `d_ino' for a directory entry that is a mount point. -- Macro: AC_STRUCT_DIRENT_D_TYPE Perform all the actions of `AC_HEADER_DIRENT' (*note Particular Headers::). Then, if `struct dirent' contains a `d_type' member, define `HAVE_STRUCT_DIRENT_D_TYPE'. -- Macro: AC_STRUCT_ST_BLOCKS If `struct stat' contains an `st_blocks' member, define `HAVE_STRUCT_STAT_ST_BLOCKS'. Otherwise, require an `AC_LIBOBJ' replacement of `fileblocks'. The former name, `HAVE_ST_BLOCKS' is to be avoided, as its support will cease in the future. This macro caches its result in the `ac_cv_member_struct_stat_st_blocks' variable. -- Macro: AC_STRUCT_TM If `time.h' does not define `struct tm', define `TM_IN_SYS_TIME', which means that including `sys/time.h' had better define `struct tm'. This macro is obsolescent, as `time.h' defines `struct tm' in current systems. New programs need not use this macro. -- Macro: AC_STRUCT_TIMEZONE Figure out how to get the current timezone. If `struct tm' has a `tm_zone' member, define `HAVE_STRUCT_TM_TM_ZONE' (and the obsoleted `HAVE_TM_ZONE'). Otherwise, if the external array `tzname' is found, define `HAVE_TZNAME'; if it is declared, define `HAVE_DECL_TZNAME'. 5.8.2 Generic Structure Checks ------------------------------ These macros are used to find structure members not covered by the "particular" test macros. -- Macro: AC_CHECK_MEMBER (AGGREGATE.MEMBER, [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND], [INCLUDES = `AC_INCLUDES_DEFAULT']) Check whether MEMBER is a member of the aggregate AGGREGATE. If no INCLUDES are specified, the default includes are used (*note Default Includes::). AC_CHECK_MEMBER([struct passwd.pw_gecos], [], [AC_MSG_ERROR([We need `passwd.pw_gecos'!])], [[#include ]]) You can use this macro for submembers: AC_CHECK_MEMBER(struct top.middle.bot) This macro caches its result in the `av_cv_member_AGGREGATE_MEMBER' variable, with characters not suitable for a variable name mapped to underscores. -- Macro: AC_CHECK_MEMBERS (MEMBERS, [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND], [INCLUDES = `AC_INCLUDES_DEFAULT']) Check for the existence of each `AGGREGATE.MEMBER' of MEMBERS using the previous macro. When MEMBER belongs to AGGREGATE, define `HAVE_AGGREGATE_MEMBER' (in all capitals, with spaces and dots replaced by underscores). If ACTION-IF-FOUND is given, it is executed for each of the found members. If ACTION-IF-NOT-FOUND is given, it is executed for each of the members that could not be found. INCLUDES is a series of include directives, defaulting to `AC_INCLUDES_DEFAULT' (*note Default Includes::), which are used prior to the members under test. This macro uses M4 lists: AC_CHECK_MEMBERS([struct stat.st_rdev, struct stat.st_blksize]) 5.9 Types ========= The following macros check for C types, either builtin or typedefs. If there is no macro specifically defined to check for a type you need, and you don't need to check for any special properties of it, then you can use a general type-check macro. 5.9.1 Particular Type Checks ---------------------------- These macros check for particular C types in `sys/types.h', `stdlib.h', `stdint.h', `inttypes.h' and others, if they exist. The Gnulib `stdint' module is an alternate way to define many of these symbols; it is useful if you prefer your code to assume a C99-or-better environment. *Note Gnulib::. -- Macro: AC_TYPE_GETGROUPS Define `GETGROUPS_T' to be whichever of `gid_t' or `int' is the base type of the array argument to `getgroups'. This macro caches the base type in the `ac_cv_type_getgroups' variable. -- Macro: AC_TYPE_INT8_T If `stdint.h' or `inttypes.h' does not define the type `int8_t', define `int8_t' to a signed integer type that is exactly 8 bits wide and that uses two's complement representation, if such a type exists. If you are worried about porting to hosts that lack such a type, you can use the results of this macro in C89-or-later code as follows: #if HAVE_STDINT_H # include #endif #if defined INT8_MAX || defined int8_t _code using int8_t_ #else _complicated alternative using >8-bit 'signed char'_ #endif This macro caches the type in the `ac_cv_c_int8_t' variable. -- Macro: AC_TYPE_INT16_T This is like `AC_TYPE_INT8_T', except for 16-bit integers. -- Macro: AC_TYPE_INT32_T This is like `AC_TYPE_INT8_T', except for 32-bit integers. -- Macro: AC_TYPE_INT64_T This is like `AC_TYPE_INT8_T', except for 64-bit integers. -- Macro: AC_TYPE_INTMAX_T If `stdint.h' or `inttypes.h' defines the type `intmax_t', define `HAVE_INTMAX_T'. Otherwise, define `intmax_t' to the widest signed integer type. -- Macro: AC_TYPE_INTPTR_T If `stdint.h' or `inttypes.h' defines the type `intptr_t', define `HAVE_INTPTR_T'. Otherwise, define `intptr_t' to a signed integer type wide enough to hold a pointer, if such a type exists. -- Macro: AC_TYPE_LONG_DOUBLE If the C compiler supports a working `long double' type, define `HAVE_LONG_DOUBLE'. The `long double' type might have the same range and precision as `double'. This macro caches its result in the `ac_cv_type_long_double' variable. This macro is obsolescent, as current C compilers support `long double'. New programs need not use this macro. -- Macro: AC_TYPE_LONG_DOUBLE_WIDER If the C compiler supports a working `long double' type with more range or precision than the `double' type, define `HAVE_LONG_DOUBLE_WIDER'. This macro caches its result in the `ac_cv_type_long_double_wider' variable. -- Macro: AC_TYPE_LONG_LONG_INT If the C compiler supports a working `long long int' type, define `HAVE_LONG_LONG_INT'. However, this test does not test `long long int' values in preprocessor `#if' expressions, because too many compilers mishandle such expressions. *Note Preprocessor Arithmetic::. This macro caches its result in the `ac_cv_type_long_long_int' variable. -- Macro: AC_TYPE_MBSTATE_T Define `HAVE_MBSTATE_T' if `' declares the `mbstate_t' type. Also, define `mbstate_t' to be a type if `' does not declare it. This macro caches its result in the `ac_cv_type_mbstate_t' variable. -- Macro: AC_TYPE_MODE_T Define `mode_t' to a suitable type, if standard headers do not define it. This macro caches its result in the `ac_cv_type_mode_t' variable. -- Macro: AC_TYPE_OFF_T Define `off_t' to a suitable type, if standard headers do not define it. This macro caches its result in the `ac_cv_type_off_t' variable. -- Macro: AC_TYPE_PID_T Define `pid_t' to a suitable type, if standard headers do not define it. This macro caches its result in the `ac_cv_type_pid_t' variable. -- Macro: AC_TYPE_SIZE_T Define `size_t' to a suitable type, if standard headers do not define it. This macro caches its result in the `ac_cv_type_size_t' variable. -- Macro: AC_TYPE_SSIZE_T Define `ssize_t' to a suitable type, if standard headers do not define it. This macro caches its result in the `ac_cv_type_ssize_t' variable. -- Macro: AC_TYPE_UID_T Define `uid_t' and `gid_t' to suitable types, if standard headers do not define them. This macro caches its result in the `ac_cv_type_uid_t' variable. -- Macro: AC_TYPE_UINT8_T If `stdint.h' or `inttypes.h' does not define the type `uint8_t', define `uint8_t' to an unsigned integer type that is exactly 8 bits wide, if such a type exists. This is like `AC_TYPE_INT8_T', except for unsigned integers. -- Macro: AC_TYPE_UINT16_T This is like `AC_TYPE_UINT8_T', except for 16-bit integers. -- Macro: AC_TYPE_UINT32_T This is like `AC_TYPE_UINT8_T', except for 32-bit integers. -- Macro: AC_TYPE_UINT64_T This is like `AC_TYPE_UINT8_T', except for 64-bit integers. -- Macro: AC_TYPE_UINTMAX_T If `stdint.h' or `inttypes.h' defines the type `uintmax_t', define `HAVE_UINTMAX_T'. Otherwise, define `uintmax_t' to the widest unsigned integer type. -- Macro: AC_TYPE_UINTPTR_T If `stdint.h' or `inttypes.h' defines the type `uintptr_t', define `HAVE_UINTPTR_T'. Otherwise, define `uintptr_t' to an unsigned integer type wide enough to hold a pointer, if such a type exists. -- Macro: AC_TYPE_UNSIGNED_LONG_LONG_INT If the C compiler supports a working `unsigned long long int' type, define `HAVE_UNSIGNED_LONG_LONG_INT'. However, this test does not test `unsigned long long int' values in preprocessor `#if' expressions, because too many compilers mishandle such expressions. *Note Preprocessor Arithmetic::. This macro caches its result in the `ac_cv_type_unsigned_long_long_int' variable. 5.9.2 Generic Type Checks ------------------------- These macros are used to check for types not covered by the "particular" test macros. -- Macro: AC_CHECK_TYPE (TYPE, [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND], [INCLUDES = `AC_INCLUDES_DEFAULT']) Check whether TYPE is defined. It may be a compiler builtin type or defined by the INCLUDES. INCLUDES is a series of include directives, defaulting to `AC_INCLUDES_DEFAULT' (*note Default Includes::), which are used prior to the type under test. In C, TYPE must be a type-name, so that the expression `sizeof (TYPE)' is valid (but `sizeof ((TYPE))' is not). The same test is applied when compiling for C++, which means that in C++ TYPE should be a type-id and should not be an anonymous `struct' or `union'. This macro caches its result in the `ac_cv_type_TYPE' variable, with `*' mapped to `p' and other characters not suitable for a variable name mapped to underscores. -- Macro: AC_CHECK_TYPES (TYPES, [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND], [INCLUDES = `AC_INCLUDES_DEFAULT']) For each TYPE of the TYPES that is defined, define `HAVE_TYPE' (in all capitals). Each TYPE must follow the rules of `AC_CHECK_TYPE'. If no INCLUDES are specified, the default includes are used (*note Default Includes::). If ACTION-IF-FOUND is given, it is additional shell code to execute when one of the types is found. If ACTION-IF-NOT-FOUND is given, it is executed when one of the types is not found. This macro uses M4 lists: AC_CHECK_TYPES([ptrdiff_t]) AC_CHECK_TYPES([unsigned long long int, uintmax_t]) AC_CHECK_TYPES([float_t], [], [], [[#include ]]) Autoconf, up to 2.13, used to provide to another version of `AC_CHECK_TYPE', broken by design. In order to keep backward compatibility, a simple heuristic, quite safe but not totally, is implemented. In case of doubt, read the documentation of the former `AC_CHECK_TYPE', see *note Obsolete Macros::. 5.10 Compilers and Preprocessors ================================ All the tests for compilers (`AC_PROG_CC', `AC_PROG_CXX', `AC_PROG_F77') define the output variable `EXEEXT' based on the output of the compiler, typically to the empty string if Posix and `.exe' if a DOS variant. They also define the output variable `OBJEXT' based on the output of the compiler, after `.c' files have been excluded, typically to `o' if Posix, `obj' if a DOS variant. If the compiler being used does not produce executables, the tests fail. If the executables can't be run, and cross-compilation is not enabled, they fail too. *Note Manual Configuration::, for more on support for cross compiling. 5.10.1 Specific Compiler Characteristics ---------------------------------------- Some compilers exhibit different behaviors. Static/Dynamic Expressions Autoconf relies on a trick to extract one bit of information from the C compiler: using negative array sizes. For instance the following excerpt of a C source demonstrates how to test whether `int' objects are 4 bytes wide: static int test_array[sizeof (int) == 4 ? 1 : -1]; To our knowledge, there is a single compiler that does not support this trick: the HP C compilers (the real ones, not only the "bundled") on HP-UX 11.00. They incorrectly reject the above program with the diagnostic "Variable-length arrays cannot have static storage." This bug comes from HP compilers' mishandling of `sizeof (int)', not from the `? 1 : -1', and Autoconf works around this problem by casting `sizeof (int)' to `long int' before comparing it. 5.10.2 Generic Compiler Characteristics --------------------------------------- -- Macro: AC_CHECK_SIZEOF (TYPE-OR-EXPR, [UNUSED], [INCLUDES = `AC_INCLUDES_DEFAULT']) Define `SIZEOF_TYPE-OR-EXPR' (*note Standard Symbols::) to be the size in bytes of TYPE-OR-EXPR, which may be either a type or an expression returning a value that has a size. If the expression `sizeof (TYPE-OR-EXPR)' is invalid, the result is 0. INCLUDES is a series of include directives, defaulting to `AC_INCLUDES_DEFAULT' (*note Default Includes::), which are used prior to the expression under test. This macro now works even when cross-compiling. The UNUSED argument was used when cross-compiling. For example, the call AC_CHECK_SIZEOF([int *]) defines `SIZEOF_INT_P' to be 8 on DEC Alpha AXP systems. This macro caches its result in the `ac_cv_sizeof_TYPE-OR-EXPR' variable, with `*' mapped to `p' and other characters not suitable for a variable name mapped to underscores. -- Macro: AC_CHECK_ALIGNOF (TYPE, [INCLUDES = `AC_INCLUDES_DEFAULT']) Define `ALIGNOF_TYPE' (*note Standard Symbols::) to be the alignment in bytes of TYPE. `TYPE y;' must be valid as a structure member declaration. If `type' is unknown, the result is 0. If no INCLUDES are specified, the default includes are used (*note Default Includes::). This macro caches its result in the `ac_cv_alignof_TYPE-OR-EXPR' variable, with `*' mapped to `p' and other characters not suitable for a variable name mapped to underscores. -- Macro: AC_COMPUTE_INT (VAR, EXPRESSION, [INCLUDES = `AC_INCLUDES_DEFAULT'], [ACTION-IF-FAILS]) Store into the shell variable VAR the value of the integer EXPRESSION. The value should fit in an initializer in a C variable of type `signed long'. To support cross compilation (in which case, the macro only works on hosts that use twos-complement arithmetic), it should be possible to evaluate the expression at compile-time. If no INCLUDES are specified, the default includes are used (*note Default Includes::). Execute ACTION-IF-FAILS if the value cannot be determined correctly. -- Macro: AC_LANG_WERROR Normally Autoconf ignores warnings generated by the compiler, linker, and preprocessor. If this macro is used, warnings count as fatal errors for the current language. This macro is useful when the results of configuration are used where warnings are unacceptable; for instance, if parts of a program are built with the GCC `-Werror' option. If the whole program is built using `-Werror' it is often simpler to put `-Werror' in the compiler flags (`CFLAGS', etc.). -- Macro: AC_OPENMP OpenMP (`http://www.openmp.org/') specifies extensions of C, C++, and Fortran that simplify optimization of shared memory parallelism, which is a common problem on multicore CPUs. If the current language is C, the macro `AC_OPENMP' sets the variable `OPENMP_CFLAGS' to the C compiler flags needed for supporting OpenMP. `OPENMP_CFLAGS' is set to empty if the compiler already supports OpenMP, if it has no way to activate OpenMP support, or if the user rejects OpenMP support by invoking `configure' with the `--disable-openmp' option. `OPENMP_CFLAGS' needs to be used when compiling programs, when preprocessing program source, and when linking programs. Therefore you need to add `$(OPENMP_CFLAGS)' to the `CFLAGS' of C programs that use OpenMP. If you preprocess OpenMP-specific C code, you also need to add `$(OPENMP_CFLAGS)' to `CPPFLAGS'. The presence of OpenMP support is revealed at compile time by the preprocessor macro `_OPENMP'. Linking a program with `OPENMP_CFLAGS' typically adds one more shared library to the program's dependencies, so its use is recommended only on programs that actually require OpenMP. If the current language is C++, `AC_OPENMP' sets the variable `OPENMP_CXXFLAGS', suitably for the C++ compiler. The same remarks hold as for C. If the current language is Fortran 77 or Fortran, `AC_OPENMP' sets the variable `OPENMP_FFLAGS' or `OPENMP_FCFLAGS', respectively. Similar remarks as for C hold, except that `CPPFLAGS' is not used for Fortran, and no preprocessor macro signals OpenMP support. For portability, it is best to avoid spaces between `#' and `pragma omp'. That is, write `#pragma omp', not `# pragma omp'. The Sun WorkShop 6.2 C compiler chokes on the latter. 5.10.3 C Compiler Characteristics --------------------------------- The following macros provide ways to find and exercise a C Compiler. There are a few constructs that ought to be avoided, but do not deserve being checked for, since they can easily be worked around. Don't use lines containing solitary backslashes They tickle a bug in the HP-UX C compiler (checked on HP-UX 10.20, 11.00, and 11i). When given the following source: #ifdef __STDC__ /\ * A comment with backslash-newlines in it. %{ %} *\ \ / char str[] = "\\ " A string with backslash-newlines in it %{ %} \\ ""; char apostrophe = '\\ \ '\ '; #endif the compiler incorrectly fails with the diagnostics "Non-terminating comment at end of file" and "Missing `#endif' at end of file." Removing the lines with solitary backslashes solves the problem. Don't compile several files at once if output matters to you Some compilers, such as HP's, report names of files being compiled when given more than one file operand. For instance: $ cc a.c b.c a.c: b.c: This can cause problems if you observe the output of the compiler to detect failures. Invoking `cc -c a.c && cc -c b.c && cc -o c a.o b.o' solves the issue. Don't rely on `#error' failing The IRIX C compiler does not fail when #error is preprocessed; it simply emits a diagnostic and continues, exiting successfully. So, instead of an error directive like `#error "Unsupported word size"' it is more portable to use an invalid directive like `#Unsupported word size' in Autoconf tests. In ordinary source code, `#error' is OK, since installers with inadequate compilers like IRIX can simply examine these compilers' diagnostic output. Don't rely on correct `#line' support On Solaris, `c89' (at least Sun C 5.3 through 5.8) diagnoses `#line' directives whose line numbers are greater than 32767. Nothing in Posix makes this invalid. That is why Autoconf stopped issuing `#line' directives. -- Macro: AC_PROG_CC ([COMPILER-SEARCH-LIST]) Determine a C compiler to use. If `CC' is not already set in the environment, check for `gcc' and `cc', then for other C compilers. Set output variable `CC' to the name of the compiler found. This macro may, however, be invoked with an optional first argument which, if specified, must be a blank-separated list of C compilers to search for. This just gives the user an opportunity to specify an alternative search list for the C compiler. For example, if you didn't like the default order, then you could invoke `AC_PROG_CC' like this: AC_PROG_CC([gcc cl cc]) If the C compiler does not handle function prototypes correctly by default, try to add an option to output variable `CC' to make it so. This macro tries various options that select standard-conformance modes on various systems. After calling this macro you can check whether the C compiler has been set to accept ANSI C89 (ISO C90); if not, the shell variable `ac_cv_prog_cc_c89' is set to `no'. See also `AC_C_PROTOTYPES' below. If using the GNU C compiler, set shell variable `GCC' to `yes'. If output variable `CFLAGS' was not already set, set it to `-g -O2' for the GNU C compiler (`-O2' on systems where GCC does not accept `-g'), or `-g' for other compilers. Many Autoconf macros use a compiler, and thus call `AC_REQUIRE([AC_PROG_CC])' to ensure that the compiler has been determined before the body of the outermost `AC_DEFUN' macro. Although `AC_PROG_CC' is safe to directly expand multiple times, it performs certain checks (such as the proper value of `EXEEXT') only on the first invocation. Therefore, care must be used when invoking this macro from within another macro rather than at the top level (*note Expanded Before Required::). -- Macro: AC_PROG_CC_C_O If the C compiler does not accept the `-c' and `-o' options simultaneously, define `NO_MINUS_C_MINUS_O'. This macro actually tests both the compiler found by `AC_PROG_CC', and, if different, the first `cc' in the path. The test fails if one fails. This macro was created for GNU Make to choose the default C compilation rule. For the compiler COMPILER, this macro caches its result in the `ac_cv_prog_cc_COMPILER_c_o' variable. -- Macro: AC_PROG_CPP Set output variable `CPP' to a command that runs the C preprocessor. If `$CC -E' doesn't work, `/lib/cpp' is used. It is only portable to run `CPP' on files with a `.c' extension. Some preprocessors don't indicate missing include files by the error status. For such preprocessors an internal variable is set that causes other macros to check the standard error from the preprocessor and consider the test failed if any warnings have been reported. For most preprocessors, though, warnings do not cause include-file tests to fail unless `AC_PROG_CPP_WERROR' is also specified. -- Macro: AC_PROG_CPP_WERROR This acts like `AC_PROG_CPP', except it treats warnings from the preprocessor as errors even if the preprocessor exit status indicates success. This is useful for avoiding headers that generate mandatory warnings, such as deprecation notices. The following macros check for C compiler or machine architecture features. To check for characteristics not listed here, use `AC_COMPILE_IFELSE' (*note Running the Compiler::) or `AC_RUN_IFELSE' (*note Runtime::). -- Macro: AC_PROG_CC_STDC If the C compiler cannot compile ISO Standard C (currently C99), try to add an option to output variable `CC' to make it work. If the compiler does not support C99, fall back to supporting ANSI C89 (ISO C90). After calling this macro you can check whether the C compiler has been set to accept Standard C; if not, the shell variable `ac_cv_prog_cc_stdc' is set to `no'. -- Macro: AC_PROG_CC_C89 If the C compiler is not in ANSI C89 (ISO C90) mode by default, try to add an option to output variable `CC' to make it so. This macro tries various options that select ANSI C89 on some system or another, preferring extended functionality modes over strict conformance modes. It considers the compiler to be in ANSI C89 mode if it handles function prototypes correctly. After calling this macro you can check whether the C compiler has been set to accept ANSI C89; if not, the shell variable `ac_cv_prog_cc_c89' is set to `no'. This macro is called automatically by `AC_PROG_CC'. -- Macro: AC_PROG_CC_C99 If the C compiler is not in C99 mode by default, try to add an option to output variable `CC' to make it so. This macro tries various options that select C99 on some system or another, preferring extended functionality modes over strict conformance modes. It considers the compiler to be in C99 mode if it handles `_Bool', `//' comments, flexible array members, `inline', signed and unsigned `long long int', mixed code and declarations, named initialization of structs, `restrict', `va_copy', varargs macros, variable declarations in `for' loops, and variable length arrays. After calling this macro you can check whether the C compiler has been set to accept C99; if not, the shell variable `ac_cv_prog_cc_c99' is set to `no'. -- Macro: AC_C_BACKSLASH_A Define `HAVE_C_BACKSLASH_A' to 1 if the C compiler understands `\a'. This macro is obsolescent, as current C compilers understand `\a'. New programs need not use this macro. -- Macro: AC_C_BIGENDIAN ([ACTION-IF-TRUE], [ACTION-IF-FALSE], [ACTION-IF-UNKNOWN], [ACTION-IF-UNIVERSAL]) If words are stored with the most significant byte first (like Motorola and SPARC CPUs), execute ACTION-IF-TRUE. If words are stored with the least significant byte first (like Intel and VAX CPUs), execute ACTION-IF-FALSE. This macro runs a test-case if endianness cannot be determined from the system header files. When cross-compiling, the test-case is not run but grep'ed for some magic values. ACTION-IF-UNKNOWN is executed if the latter case fails to determine the byte sex of the host system. In some cases a single run of a compiler can generate code for multiple architectures. This can happen, for example, when generating Mac OS X universal binary files, which work on both PowerPC and Intel architectures. In this case, the different variants might be for different architectures whose endiannesses differ. If `configure' detects this, it executes ACTION-IF-UNIVERSAL instead of ACTION-IF-UNKNOWN. The default for ACTION-IF-TRUE is to define `WORDS_BIGENDIAN'. The default for ACTION-IF-FALSE is to do nothing. The default for ACTION-IF-UNKNOWN is to abort configure and tell the installer how to bypass this test. And finally, the default for ACTION-IF-UNIVERSAL is to ensure that `WORDS_BIGENDIAN' is defined if and only if a universal build is detected and the current code is big-endian; this default works only if `autoheader' is used (*note autoheader Invocation::). If you use this macro without specifying ACTION-IF-UNIVERSAL, you should also use `AC_CONFIG_HEADERS'; otherwise `WORDS_BIGENDIAN' may be set incorrectly for Mac OS X universal binary files. -- Macro: AC_C_CONST If the C compiler does not fully support the `const' keyword, define `const' to be empty. Some C compilers that do not define `__STDC__' do support `const'; some compilers that define `__STDC__' do not completely support `const'. Programs can simply use `const' as if every C compiler supported it; for those that don't, the makefile or configuration header file defines it as empty. Occasionally installers use a C++ compiler to compile C code, typically because they lack a C compiler. This causes problems with `const', because C and C++ treat `const' differently. For example: const int foo; is valid in C but not in C++. These differences unfortunately cannot be papered over by defining `const' to be empty. If `autoconf' detects this situation, it leaves `const' alone, as this generally yields better results in practice. However, using a C++ compiler to compile C code is not recommended or supported, and installers who run into trouble in this area should get a C compiler like GCC to compile their C code. This macro caches its result in the `ac_cv_c_const' variable. This macro is obsolescent, as current C compilers support `const'. New programs need not use this macro. -- Macro: AC_C_RESTRICT If the C compiler recognizes a variant spelling for the `restrict' keyword (`__restrict', `__restrict__', or `_Restrict'), then define `restrict' to that; this is more likely to do the right thing with compilers that support language variants where plain `restrict' is not a keyword. Otherwise, if the C compiler recognizes the `restrict' keyword, don't do anything. Otherwise, define `restrict' to be empty. Thus, programs may simply use `restrict' as if every C compiler supported it; for those that do not, the makefile or configuration header defines it away. Although support in C++ for the `restrict' keyword is not required, several C++ compilers do accept the keyword. This macro works for them, too. This macro caches `no' in the `ac_cv_c_restrict' variable if `restrict' is not supported, and a supported spelling otherwise. -- Macro: AC_C_VOLATILE If the C compiler does not understand the keyword `volatile', define `volatile' to be empty. Programs can simply use `volatile' as if every C compiler supported it; for those that do not, the makefile or configuration header defines it as empty. If the correctness of your program depends on the semantics of `volatile', simply defining it to be empty does, in a sense, break your code. However, given that the compiler does not support `volatile', you are at its mercy anyway. At least your program compiles, when it wouldn't before. *Note Volatile Objects::, for more about `volatile'. In general, the `volatile' keyword is a standard C feature, so you might expect that `volatile' is available only when `__STDC__' is defined. However, Ultrix 4.3's native compiler does support volatile, but does not define `__STDC__'. This macro is obsolescent, as current C compilers support `volatile'. New programs need not use this macro. -- Macro: AC_C_INLINE If the C compiler supports the keyword `inline', do nothing. Otherwise define `inline' to `__inline__' or `__inline' if it accepts one of those, otherwise define `inline' to be empty. -- Macro: AC_C_CHAR_UNSIGNED If the C type `char' is unsigned, define `__CHAR_UNSIGNED__', unless the C compiler predefines it. These days, using this macro is not necessary. The same information can be determined by this portable alternative, thus avoiding the use of preprocessor macros in the namespace reserved for the implementation. #include #if CHAR_MIN == 0 # define CHAR_UNSIGNED 1 #endif -- Macro: AC_C_STRINGIZE If the C preprocessor supports the stringizing operator, define `HAVE_STRINGIZE'. The stringizing operator is `#' and is found in macros such as this: #define x(y) #y This macro is obsolescent, as current C compilers support the stringizing operator. New programs need not use this macro. -- Macro: AC_C_FLEXIBLE_ARRAY_MEMBER If the C compiler supports flexible array members, define `FLEXIBLE_ARRAY_MEMBER' to nothing; otherwise define it to 1. That way, a declaration like this: struct s { size_t n_vals; double val[FLEXIBLE_ARRAY_MEMBER]; }; will let applications use the "struct hack" even with compilers that do not support flexible array members. To allocate and use such an object, you can use code like this: size_t i; size_t n = compute_value_count (); struct s *p = malloc (offsetof (struct s, val) + n * sizeof (double)); p->n_vals = n; for (i = 0; i < n; i++) p->val[i] = compute_value (i); -- Macro: AC_C_VARARRAYS If the C compiler supports variable-length arrays, define `HAVE_C_VARARRAYS'. A variable-length array is an array of automatic storage duration whose length is determined at run time, when the array is declared. -- Macro: AC_C_TYPEOF If the C compiler supports GCC's `typeof' syntax either directly or through a different spelling of the keyword (e.g., `__typeof__'), define `HAVE_TYPEOF'. If the support is available only through a different spelling, define `typeof' to that spelling. -- Macro: AC_C_PROTOTYPES If function prototypes are understood by the compiler (as determined by `AC_PROG_CC'), define `PROTOTYPES' and `__PROTOTYPES'. Defining `__PROTOTYPES' is for the benefit of header files that cannot use macros that infringe on user name space. This macro is obsolescent, as current C compilers support prototypes. New programs need not use this macro. -- Macro: AC_PROG_GCC_TRADITIONAL Add `-traditional' to output variable `CC' if using the GNU C compiler and `ioctl' does not work properly without `-traditional'. That usually happens when the fixed header files have not been installed on an old system. This macro is obsolescent, since current versions of the GNU C compiler fix the header files automatically when installed. 5.10.4 C++ Compiler Characteristics ----------------------------------- -- Macro: AC_PROG_CXX ([COMPILER-SEARCH-LIST]) Determine a C++ compiler to use. Check whether the environment variable `CXX' or `CCC' (in that order) is set; if so, then set output variable `CXX' to its value. Otherwise, if the macro is invoked without an argument, then search for a C++ compiler under the likely names (first `g++' and `c++' then other names). If none of those checks succeed, then as a last resort set `CXX' to `g++'. This macro may, however, be invoked with an optional first argument which, if specified, must be a blank-separated list of C++ compilers to search for. This just gives the user an opportunity to specify an alternative search list for the C++ compiler. For example, if you didn't like the default order, then you could invoke `AC_PROG_CXX' like this: AC_PROG_CXX([gcc cl KCC CC cxx cc++ xlC aCC c++ g++]) If using the GNU C++ compiler, set shell variable `GXX' to `yes'. If output variable `CXXFLAGS' was not already set, set it to `-g -O2' for the GNU C++ compiler (`-O2' on systems where G++ does not accept `-g'), or `-g' for other compilers. -- Macro: AC_PROG_CXXCPP Set output variable `CXXCPP' to a command that runs the C++ preprocessor. If `$CXX -E' doesn't work, `/lib/cpp' is used. It is portable to run `CXXCPP' only on files with a `.c', `.C', `.cc', or `.cpp' extension. Some preprocessors don't indicate missing include files by the error status. For such preprocessors an internal variable is set that causes other macros to check the standard error from the preprocessor and consider the test failed if any warnings have been reported. However, it is not known whether such broken preprocessors exist for C++. -- Macro: AC_PROG_CXX_C_O Test whether the C++ compiler accepts the options `-c' and `-o' simultaneously, and define `CXX_NO_MINUS_C_MINUS_O', if it does not. 5.10.5 Objective C Compiler Characteristics ------------------------------------------- -- Macro: AC_PROG_OBJC ([COMPILER-SEARCH-LIST]) Determine an Objective C compiler to use. If `OBJC' is not already set in the environment, check for Objective C compilers. Set output variable `OBJC' to the name of the compiler found. This macro may, however, be invoked with an optional first argument which, if specified, must be a blank-separated list of Objective C compilers to search for. This just gives the user an opportunity to specify an alternative search list for the Objective C compiler. For example, if you didn't like the default order, then you could invoke `AC_PROG_OBJC' like this: AC_PROG_OBJC([gcc objcc objc]) If using the GNU Objective C compiler, set shell variable `GOBJC' to `yes'. If output variable `OBJCFLAGS' was not already set, set it to `-g -O2' for the GNU Objective C compiler (`-O2' on systems where `gcc' does not accept `-g'), or `-g' for other compilers. -- Macro: AC_PROG_OBJCPP Set output variable `OBJCPP' to a command that runs the Objective C preprocessor. If `$OBJC -E' doesn't work, `/lib/cpp' is used. 5.10.6 Objective C++ Compiler Characteristics --------------------------------------------- -- Macro: AC_PROG_OBJCXX ([COMPILER-SEARCH-LIST]) Determine an Objective C++ compiler to use. If `OBJCXX' is not already set in the environment, check for Objective C++ compilers. Set output variable `OBJCXX' to the name of the compiler found. This macro may, however, be invoked with an optional first argument which, if specified, must be a blank-separated list of Objective C++ compilers to search for. This just gives the user an opportunity to specify an alternative search list for the Objective C++ compiler. For example, if you didn't like the default order, then you could invoke `AC_PROG_OBJCXX' like this: AC_PROG_OBJCXX([gcc g++ objcc++ objcxx]) If using the GNU Objective C++ compiler, set shell variable `GOBJCXX' to `yes'. If output variable `OBJCXXFLAGS' was not already set, set it to `-g -O2' for the GNU Objective C++ compiler (`-O2' on systems where `gcc' does not accept `-g'), or `-g' for other compilers. -- Macro: AC_PROG_OBJCXXCPP Set output variable `OBJCXXCPP' to a command that runs the Objective C++ preprocessor. If `$OBJCXX -E' doesn't work, `/lib/cpp' is used. 5.10.7 Erlang Compiler and Interpreter Characteristics ------------------------------------------------------ Autoconf defines the following macros for determining paths to the essential Erlang/OTP programs: -- Macro: AC_ERLANG_PATH_ERLC ([VALUE-IF-NOT-FOUND], [PATH = `$PATH']) Determine an Erlang compiler to use. If `ERLC' is not already set in the environment, check for `erlc'. Set output variable `ERLC' to the complete path of the compiler command found. In addition, if `ERLCFLAGS' is not set in the environment, set it to an empty value. The two optional arguments have the same meaning as the two last arguments of macro `AC_PROG_PATH' for looking for the `erlc' program. For example, to look for `erlc' only in the `/usr/lib/erlang/bin' directory: AC_ERLANG_PATH_ERLC([not found], [/usr/lib/erlang/bin]) -- Macro: AC_ERLANG_NEED_ERLC ([PATH = `$PATH']) A simplified variant of the `AC_ERLANG_PATH_ERLC' macro, that prints an error message and exits the `configure' script if the `erlc' program is not found. -- Macro: AC_ERLANG_PATH_ERL ([VALUE-IF-NOT-FOUND], [PATH = `$PATH']) Determine an Erlang interpreter to use. If `ERL' is not already set in the environment, check for `erl'. Set output variable `ERL' to the complete path of the interpreter command found. The two optional arguments have the same meaning as the two last arguments of macro `AC_PROG_PATH' for looking for the `erl' program. For example, to look for `erl' only in the `/usr/lib/erlang/bin' directory: AC_ERLANG_PATH_ERL([not found], [/usr/lib/erlang/bin]) -- Macro: AC_ERLANG_NEED_ERL ([PATH = `$PATH']) A simplified variant of the `AC_ERLANG_PATH_ERL' macro, that prints an error message and exits the `configure' script if the `erl' program is not found. 5.10.8 Fortran Compiler Characteristics --------------------------------------- The Autoconf Fortran support is divided into two categories: legacy Fortran 77 macros (`F77'), and modern Fortran macros (`FC'). The former are intended for traditional Fortran 77 code, and have output variables like `F77', `FFLAGS', and `FLIBS'. The latter are for newer programs that can (or must) compile under the newer Fortran standards, and have output variables like `FC', `FCFLAGS', and `FCLIBS'. Except for two new macros `AC_FC_SRCEXT' and `AC_FC_FREEFORM' (see below), the `FC' and `F77' macros behave almost identically, and so they are documented together in this section. -- Macro: AC_PROG_F77 ([COMPILER-SEARCH-LIST]) Determine a Fortran 77 compiler to use. If `F77' is not already set in the environment, then check for `g77' and `f77', and then some other names. Set the output variable `F77' to the name of the compiler found. This macro may, however, be invoked with an optional first argument which, if specified, must be a blank-separated list of Fortran 77 compilers to search for. This just gives the user an opportunity to specify an alternative search list for the Fortran 77 compiler. For example, if you didn't like the default order, then you could invoke `AC_PROG_F77' like this: AC_PROG_F77([fl32 f77 fort77 xlf g77 f90 xlf90]) If using `g77' (the GNU Fortran 77 compiler), then set the shell variable `G77' to `yes'. If the output variable `FFLAGS' was not already set in the environment, then set it to `-g -02' for `g77' (or `-O2' where `g77' does not accept `-g'). Otherwise, set `FFLAGS' to `-g' for all other Fortran 77 compilers. -- Macro: AC_PROG_FC ([COMPILER-SEARCH-LIST], [DIALECT]) Determine a Fortran compiler to use. If `FC' is not already set in the environment, then `dialect' is a hint to indicate what Fortran dialect to search for; the default is to search for the newest available dialect. Set the output variable `FC' to the name of the compiler found. By default, newer dialects are preferred over older dialects, but if `dialect' is specified then older dialects are preferred starting with the specified dialect. `dialect' can currently be one of Fortran 77, Fortran 90, or Fortran 95. However, this is only a hint of which compiler _name_ to prefer (e.g., `f90' or `f95'), and no attempt is made to guarantee that a particular language standard is actually supported. Thus, it is preferable that you avoid the `dialect' option, and use AC_PROG_FC only for code compatible with the latest Fortran standard. This macro may, alternatively, be invoked with an optional first argument which, if specified, must be a blank-separated list of Fortran compilers to search for, just as in `AC_PROG_F77'. If the output variable `FCFLAGS' was not already set in the environment, then set it to `-g -02' for GNU `g77' (or `-O2' where `g77' does not accept `-g'). Otherwise, set `FCFLAGS' to `-g' for all other Fortran compilers. -- Macro: AC_PROG_F77_C_O -- Macro: AC_PROG_FC_C_O Test whether the Fortran compiler accepts the options `-c' and `-o' simultaneously, and define `F77_NO_MINUS_C_MINUS_O' or `FC_NO_MINUS_C_MINUS_O', respectively, if it does not. The following macros check for Fortran compiler characteristics. To check for characteristics not listed here, use `AC_COMPILE_IFELSE' (*note Running the Compiler::) or `AC_RUN_IFELSE' (*note Runtime::), making sure to first set the current language to Fortran 77 or Fortran via `AC_LANG([Fortran 77])' or `AC_LANG(Fortran)' (*note Language Choice::). -- Macro: AC_F77_LIBRARY_LDFLAGS -- Macro: AC_FC_LIBRARY_LDFLAGS Determine the linker flags (e.g., `-L' and `-l') for the "Fortran intrinsic and runtime libraries" that are required to successfully link a Fortran program or shared library. The output variable `FLIBS' or `FCLIBS' is set to these flags (which should be included after `LIBS' when linking). This macro is intended to be used in those situations when it is necessary to mix, e.g., C++ and Fortran source code in a single program or shared library (*note Mixing Fortran 77 With C and C++: (automake)Mixing Fortran 77 With C and C++.). For example, if object files from a C++ and Fortran compiler must be linked together, then the C++ compiler/linker must be used for linking (since special C++-ish things need to happen at link time like calling global constructors, instantiating templates, enabling exception support, etc.). However, the Fortran intrinsic and runtime libraries must be linked in as well, but the C++ compiler/linker doesn't know by default how to add these Fortran 77 libraries. Hence, this macro was created to determine these Fortran libraries. The macros `AC_F77_DUMMY_MAIN' and `AC_FC_DUMMY_MAIN' or `AC_F77_MAIN' and `AC_FC_MAIN' are probably also necessary to link C/C++ with Fortran; see below. -- Macro: AC_F77_DUMMY_MAIN ([ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND]) -- Macro: AC_FC_DUMMY_MAIN ([ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND]) With many compilers, the Fortran libraries detected by `AC_F77_LIBRARY_LDFLAGS' or `AC_FC_LIBRARY_LDFLAGS' provide their own `main' entry function that initializes things like Fortran I/O, and which then calls a user-provided entry function named (say) `MAIN__' to run the user's program. The `AC_F77_DUMMY_MAIN' and `AC_FC_DUMMY_MAIN' or `AC_F77_MAIN' and `AC_FC_MAIN' macros figure out how to deal with this interaction. When using Fortran for purely numerical functions (no I/O, etc.) often one prefers to provide one's own `main' and skip the Fortran library initializations. In this case, however, one may still need to provide a dummy `MAIN__' routine in order to prevent linking errors on some systems. `AC_F77_DUMMY_MAIN' or `AC_FC_DUMMY_MAIN' detects whether any such routine is _required_ for linking, and what its name is; the shell variable `F77_DUMMY_MAIN' or `FC_DUMMY_MAIN' holds this name, `unknown' when no solution was found, and `none' when no such dummy main is needed. By default, ACTION-IF-FOUND defines `F77_DUMMY_MAIN' or `FC_DUMMY_MAIN' to the name of this routine (e.g., `MAIN__') _if_ it is required. ACTION-IF-NOT-FOUND defaults to exiting with an error. In order to link with Fortran routines, the user's C/C++ program should then include the following code to define the dummy main if it is needed: #ifdef F77_DUMMY_MAIN # ifdef __cplusplus extern "C" # endif int F77_DUMMY_MAIN() { return 1; } #endif (Replace `F77' with `FC' for Fortran instead of Fortran 77.) Note that this macro is called automatically from `AC_F77_WRAPPERS' or `AC_FC_WRAPPERS'; there is generally no need to call it explicitly unless one wants to change the default actions. -- Macro: AC_F77_MAIN -- Macro: AC_FC_MAIN As discussed above, many Fortran libraries allow you to provide an entry point called (say) `MAIN__' instead of the usual `main', which is then called by a `main' function in the Fortran libraries that initializes things like Fortran I/O. The `AC_F77_MAIN' and `AC_FC_MAIN' macros detect whether it is _possible_ to utilize such an alternate main function, and defines `F77_MAIN' and `FC_MAIN' to the name of the function. (If no alternate main function name is found, `F77_MAIN' and `FC_MAIN' are simply defined to `main'.) Thus, when calling Fortran routines from C that perform things like I/O, one should use this macro and declare the "main" function like so: #ifdef __cplusplus extern "C" #endif int F77_MAIN(int argc, char *argv[]); (Again, replace `F77' with `FC' for Fortran instead of Fortran 77.) -- Macro: AC_F77_WRAPPERS -- Macro: AC_FC_WRAPPERS Defines C macros `F77_FUNC (name, NAME)', `FC_FUNC (name, NAME)', `F77_FUNC_(name, NAME)', and `FC_FUNC_(name, NAME)' to properly mangle the names of C/C++ identifiers, and identifiers with underscores, respectively, so that they match the name-mangling scheme used by the Fortran compiler. Fortran is case-insensitive, and in order to achieve this the Fortran compiler converts all identifiers into a canonical case and format. To call a Fortran subroutine from C or to write a C function that is callable from Fortran, the C program must explicitly use identifiers in the format expected by the Fortran compiler. In order to do this, one simply wraps all C identifiers in one of the macros provided by `AC_F77_WRAPPERS' or `AC_FC_WRAPPERS'. For example, suppose you have the following Fortran 77 subroutine: subroutine foobar (x, y) double precision x, y y = 3.14159 * x return end You would then declare its prototype in C or C++ as: #define FOOBAR_F77 F77_FUNC (foobar, FOOBAR) #ifdef __cplusplus extern "C" /* prevent C++ name mangling */ #endif void FOOBAR_F77(double *x, double *y); Note that we pass both the lowercase and uppercase versions of the function name to `F77_FUNC' so that it can select the right one. Note also that all parameters to Fortran 77 routines are passed as pointers (*note Mixing Fortran 77 With C and C++: (automake)Mixing Fortran 77 With C and C++.). (Replace `F77' with `FC' for Fortran instead of Fortran 77.) Although Autoconf tries to be intelligent about detecting the name-mangling scheme of the Fortran compiler, there may be Fortran compilers that it doesn't support yet. In this case, the above code generates a compile-time error, but some other behavior (e.g., disabling Fortran-related features) can be induced by checking whether `F77_FUNC' or `FC_FUNC' is defined. Now, to call that routine from a C program, we would do something like: { double x = 2.7183, y; FOOBAR_F77 (&x, &y); } If the Fortran identifier contains an underscore (e.g., `foo_bar'), you should use `F77_FUNC_' or `FC_FUNC_' instead of `F77_FUNC' or `FC_FUNC' (with the same arguments). This is because some Fortran compilers mangle names differently if they contain an underscore. -- Macro: AC_F77_FUNC (NAME, [SHELLVAR]) -- Macro: AC_FC_FUNC (NAME, [SHELLVAR]) Given an identifier NAME, set the shell variable SHELLVAR to hold the mangled version NAME according to the rules of the Fortran linker (see also `AC_F77_WRAPPERS' or `AC_FC_WRAPPERS'). SHELLVAR is optional; if it is not supplied, the shell variable is simply NAME. The purpose of this macro is to give the caller a way to access the name-mangling information other than through the C preprocessor as above, for example, to call Fortran routines from some language other than C/C++. -- Macro: AC_FC_SRCEXT (EXT, [ACTION-IF-SUCCESS], [ACTION-IF-FAILURE]) By default, the `FC' macros perform their tests using a `.f' extension for source-code files. Some compilers, however, only enable newer language features for appropriately named files, e.g., Fortran 90 features only for `.f90' files. On the other hand, some other compilers expect all source files to end in `.f' and require special flags to support other file name extensions. The `AC_FC_SRCEXT' macro deals with both of these issues. The `AC_FC_SRCEXT' tries to get the `FC' compiler to accept files ending with the extension .EXT (i.e., EXT does _not_ contain the dot). If any special compiler flags are needed for this, it stores them in the output variable `FCFLAGS_'EXT. This extension and these flags are then used for all subsequent `FC' tests (until `AC_FC_SRCEXT' is called again). For example, you would use `AC_FC_SRCEXT(f90)' to employ the `.f90' extension in future tests, and it would set the `FCFLAGS_f90' output variable with any extra flags that are needed to compile such files. The `FCFLAGS_'EXT can _not_ be simply absorbed into `FCFLAGS', for two reasons based on the limitations of some compilers. First, only one `FCFLAGS_'EXT can be used at a time, so files with different extensions must be compiled separately. Second, `FCFLAGS_'EXT must appear _immediately_ before the source-code file name when compiling. So, continuing the example above, you might compile a `foo.f90' file in your makefile with the command: foo.o: foo.f90 $(FC) -c $(FCFLAGS) $(FCFLAGS_f90) '$(srcdir)/foo.f90' If `AC_FC_SRCEXT' succeeds in compiling files with the EXT extension, it calls ACTION-IF-SUCCESS (defaults to nothing). If it fails, and cannot find a way to make the `FC' compiler accept such files, it calls ACTION-IF-FAILURE (defaults to exiting with an error message). -- Macro: AC_FC_FREEFORM ([ACTION-IF-SUCCESS], [ACTION-IF-FAILURE]) The `AC_FC_FREEFORM' tries to ensure that the Fortran compiler (`$FC') allows free-format source code (as opposed to the older fixed-format style from Fortran 77). If necessary, it may add some additional flags to `FCFLAGS'. This macro is most important if you are using the default `.f' extension, since many compilers interpret this extension as indicating fixed-format source unless an additional flag is supplied. If you specify a different extension with `AC_FC_SRCEXT', such as `.f90' or `.f95', then `AC_FC_FREEFORM' ordinarily succeeds without modifying `FCFLAGS'. If `AC_FC_FREEFORM' succeeds in compiling free-form source, it calls ACTION-IF-SUCCESS (defaults to nothing). If it fails, it calls ACTION-IF-FAILURE (defaults to exiting with an error message). 5.11 System Services ==================== The following macros check for operating system services or capabilities. -- Macro: AC_PATH_X Try to locate the X Window System include files and libraries. If the user gave the command line options `--x-includes=DIR' and `--x-libraries=DIR', use those directories. If either or both were not given, get the missing values by running `xmkmf' (or an executable pointed to by the `XMKMF' environment variable) on a trivial `Imakefile' and examining the makefile that it produces. Setting `XMKMF' to `false' disables this method. If this method fails to find the X Window System, `configure' looks for the files in several directories where they often reside. If either method is successful, set the shell variables `x_includes' and `x_libraries' to their locations, unless they are in directories the compiler searches by default. If both methods fail, or the user gave the command line option `--without-x', set the shell variable `no_x' to `yes'; otherwise set it to the empty string. -- Macro: AC_PATH_XTRA An enhanced version of `AC_PATH_X'. It adds the C compiler flags that X needs to output variable `X_CFLAGS', and the X linker flags to `X_LIBS'. Define `X_DISPLAY_MISSING' if X is not available. This macro also checks for special libraries that some systems need in order to compile X programs. It adds any that the system needs to output variable `X_EXTRA_LIBS'. And it checks for special X11R6 libraries that need to be linked with before `-lX11', and adds any found to the output variable `X_PRE_LIBS'. -- Macro: AC_SYS_INTERPRETER Check whether the system supports starting scripts with a line of the form `#!/bin/sh' to select the interpreter to use for the script. After running this macro, shell code in `configure.ac' can check the shell variable `interpval'; it is set to `yes' if the system supports `#!', `no' if not. -- Macro: AC_SYS_LARGEFILE Arrange for 64-bit file offsets, known as large-file support (http://www.unix-systems.org/version2/whatsnew/lfs20mar.html). On some hosts, one must use special compiler options to build programs that can access large files. Append any such options to the output variable `CC'. Define `_FILE_OFFSET_BITS' and `_LARGE_FILES' if necessary. Large-file support can be disabled by configuring with the `--disable-largefile' option. If you use this macro, check that your program works even when `off_t' is wider than `long int', since this is common when large-file support is enabled. For example, it is not correct to print an arbitrary `off_t' value `X' with `printf ("%ld", (long int) X)'. The LFS introduced the `fseeko' and `ftello' functions to replace their C counterparts `fseek' and `ftell' that do not use `off_t'. Take care to use `AC_FUNC_FSEEKO' to make their prototypes available when using them and large-file support is enabled. -- Macro: AC_SYS_LONG_FILE_NAMES If the system supports file names longer than 14 characters, define `HAVE_LONG_FILE_NAMES'. -- Macro: AC_SYS_POSIX_TERMIOS Check to see if the Posix termios headers and functions are available on the system. If so, set the shell variable `ac_cv_sys_posix_termios' to `yes'. If not, set the variable to `no'. 5.12 Posix Variants =================== The following macro makes it possible to use features of Posix that are extensions to C, as well as platform extensions not defined by Posix. -- Macro: AC_USE_SYSTEM_EXTENSIONS This macro was introduced in Autoconf 2.60. If possible, enable extensions to C or Posix on hosts that normally disable the extensions, typically due to standards-conformance namespace issues. This should be called before any macros that run the C compiler. The following preprocessor macros are defined where appropriate: `_GNU_SOURCE' Enable extensions on GNU/Linux. `__EXTENSIONS__' Enable general extensions on Solaris. `_POSIX_PTHREAD_SEMANTICS' Enable threading extensions on Solaris. `_TANDEM_SOURCE' Enable extensions for the HP NonStop platform. `_ALL_SOURCE' Enable extensions for AIX 3, and for Interix. `_POSIX_SOURCE' Enable Posix functions for Minix. `_POSIX_1_SOURCE' Enable additional Posix functions for Minix. `_MINIX' Identify Minix platform. This particular preprocessor macro is obsolescent, and may be removed in a future release of Autoconf. 5.13 Erlang Libraries ===================== The following macros check for an installation of Erlang/OTP, and for the presence of certain Erlang libraries. All those macros require the configuration of an Erlang interpreter and an Erlang compiler (*note Erlang Compiler and Interpreter::). -- Macro: AC_ERLANG_SUBST_ERTS_VER Set the output variable `ERLANG_ERTS_VER' to the version of the Erlang runtime system (as returned by Erlang's `erlang:system_info(version)' function). The result of this test is cached if caching is enabled when running `configure'. The `ERLANG_ERTS_VER' variable is not intended to be used for testing for features of specific ERTS versions, but to be used for substituting the ERTS version in Erlang/OTP release resource files (`.rel' files), as shown below. -- Macro: AC_ERLANG_SUBST_ROOT_DIR Set the output variable `ERLANG_ROOT_DIR' to the path to the base directory in which Erlang/OTP is installed (as returned by Erlang's `code:root_dir/0' function). The result of this test is cached if caching is enabled when running `configure'. -- Macro: AC_ERLANG_SUBST_LIB_DIR Set the output variable `ERLANG_LIB_DIR' to the path of the library directory of Erlang/OTP (as returned by Erlang's `code:lib_dir/0' function), which subdirectories each contain an installed Erlang/OTP library. The result of this test is cached if caching is enabled when running `configure'. -- Macro: AC_ERLANG_CHECK_LIB (LIBRARY, [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND]) Test whether the Erlang/OTP library LIBRARY is installed by calling Erlang's `code:lib_dir/1' function. The result of this test is cached if caching is enabled when running `configure'. ACTION-IF-FOUND is a list of shell commands to run if the library is installed; ACTION-IF-NOT-FOUND is a list of shell commands to run if it is not. Additionally, if the library is installed, the output variable `ERLANG_LIB_DIR_LIBRARY' is set to the path to the library installation directory, and the output variable `ERLANG_LIB_VER_LIBRARY' is set to the version number that is part of the subdirectory name, if it is in the standard form (`LIBRARY-VERSION'). If the directory name does not have a version part, `ERLANG_LIB_VER_LIBRARY' is set to the empty string. If the library is not installed, `ERLANG_LIB_DIR_LIBRARY' and `ERLANG_LIB_VER_LIBRARY' are set to `"not found"'. For example, to check if library `stdlib' is installed: AC_ERLANG_CHECK_LIB([stdlib], [echo "stdlib version \"$ERLANG_LIB_VER_stdlib\"" echo "is installed in \"$ERLANG_LIB_DIR_stdlib\""], [AC_MSG_ERROR([stdlib was not found!])]) The `ERLANG_LIB_VER_LIBRARY' variables (set by `AC_ERLANG_CHECK_LIB') and the `ERLANG_ERTS_VER' variable (set by `AC_ERLANG_SUBST_ERTS_VER') are not intended to be used for testing for features of specific versions of libraries or of the Erlang runtime system. Those variables are intended to be substituted in Erlang release resource files (`.rel' files). For instance, to generate a `example.rel' file for an application depending on the `stdlib' library, `configure.ac' could contain: AC_ERLANG_SUBST_ERTS_VER AC_ERLANG_CHECK_LIB([stdlib], [], [AC_MSG_ERROR([stdlib was not found!])]) AC_CONFIG_FILES([example.rel]) The `example.rel.in' file used to generate `example.rel' should contain: {release, {"@PACKAGE@", "@VERSION@"}, {erts, "@ERLANG_ERTS_VER@"}, [{stdlib, "@ERLANG_LIB_VER_stdlib@"}, {@PACKAGE@, "@VERSION@"}]}. In addition to the above macros, which test installed Erlang libraries, the following macros determine the paths to the directories into which newly built Erlang libraries are to be installed: -- Macro: AC_ERLANG_SUBST_INSTALL_LIB_DIR Set the `ERLANG_INSTALL_LIB_DIR' output variable to the directory into which every built Erlang library should be installed in a separate subdirectory. If this variable is not set in the environment when `configure' runs, its default value is `$ERLANG_LIB_DIR', which value is set by the `AC_ERLANG_SUBST_LIB_DIR' macro. -- Macro: AC_ERLANG_SUBST_INSTALL_LIB_SUBDIR (LIBRARY, VERSION) Set the `ERLANG_INSTALL_LIB_DIR_LIBRARY' output variable to the directory into which the built Erlang library LIBRARY version VERSION should be installed. If this variable is not set in the environment when `configure' runs, its default value is `$ERLANG_INSTALL_LIB_DIR/LIBRARY-VERSION', the value of the `ERLANG_INSTALL_LIB_DIR' variable being set by the `AC_ERLANG_SUBST_INSTALL_LIB_DIR' macro. 6 Writing Tests *************** If the existing feature tests don't do something you need, you have to write new ones. These macros are the building blocks. They provide ways for other macros to check whether various kinds of features are available and report the results. This chapter contains some suggestions and some of the reasons why the existing tests are written the way they are. You can also learn a lot about how to write Autoconf tests by looking at the existing ones. If something goes wrong in one or more of the Autoconf tests, this information can help you understand the assumptions behind them, which might help you figure out how to best solve the problem. These macros check the output of the compiler system of the current language (*note Language Choice::). They do not cache the results of their tests for future use (*note Caching Results::), because they don't know enough about the information they are checking for to generate a cache variable name. They also do not print any messages, for the same reason. The checks for particular kinds of features call these macros and do cache their results and print messages about what they're checking for. When you write a feature test that could be applicable to more than one software package, the best thing to do is encapsulate it in a new macro. *Note Writing Autoconf Macros::, for how to do that. 6.1 Language Choice =================== Autoconf-generated `configure' scripts check for the C compiler and its features by default. Packages that use other programming languages (maybe more than one, e.g., C and C++) need to test features of the compilers for the respective languages. The following macros determine which programming language is used in the subsequent tests in `configure.ac'. -- Macro: AC_LANG (LANGUAGE) Do compilation tests using the compiler, preprocessor, and file extensions for the specified LANGUAGE. Supported languages are: `C' Do compilation tests using `CC' and `CPP' and use extension `.c' for test programs. Use compilation flags: `CPPFLAGS' with `CPP', and both `CPPFLAGS' and `CFLAGS' with `CC'. `C++' Do compilation tests using `CXX' and `CXXCPP' and use extension `.C' for test programs. Use compilation flags: `CPPFLAGS' with `CXXCPP', and both `CPPFLAGS' and `CXXFLAGS' with `CXX'. `Fortran 77' Do compilation tests using `F77' and use extension `.f' for test programs. Use compilation flags: `FFLAGS'. `Fortran' Do compilation tests using `FC' and use extension `.f' (or whatever has been set by `AC_FC_SRCEXT') for test programs. Use compilation flags: `FCFLAGS'. `Erlang' Compile and execute tests using `ERLC' and `ERL' and use extension `.erl' for test Erlang modules. Use compilation flags: `ERLCFLAGS'. `Objective C' Do compilation tests using `OBJC' and `OBJCPP' and use extension `.m' for test programs. Use compilation flags: `CPPFLAGS' with `OBJCPP', and both `CPPFLAGS' and `OBJCFLAGS' with `OBJC'. `Objective C++' Do compilation tests using `OBJCXX' and `OBJCXXCPP' and use extension `.mm' for test programs. Use compilation flags: `CPPFLAGS' with `OBJCXXCPP', and both `CPPFLAGS' and `OBJCXXFLAGS' with `OBJCXX'. -- Macro: AC_LANG_PUSH (LANGUAGE) Remember the current language (as set by `AC_LANG') on a stack, and then select the LANGUAGE. Use this macro and `AC_LANG_POP' in macros that need to temporarily switch to a particular language. -- Macro: AC_LANG_POP ([LANGUAGE]) Select the language that is saved on the top of the stack, as set by `AC_LANG_PUSH', and remove it from the stack. If given, LANGUAGE specifies the language we just _quit_. It is a good idea to specify it when it's known (which should be the case...), since Autoconf detects inconsistencies. AC_LANG_PUSH([Fortran 77]) # Perform some tests on Fortran 77. # ... AC_LANG_POP([Fortran 77]) -- Macro: AC_LANG_ASSERT (LANGUAGE) Check statically that the current language is LANGUAGE. You should use this in your language specific macros to avoid that they be called with an inappropriate language. This macro runs only at `autoconf' time, and incurs no cost at `configure' time. Sadly enough and because Autoconf is a two layer language (1), the macros `AC_LANG_PUSH' and `AC_LANG_POP' cannot be "optimizing", therefore as much as possible you ought to avoid using them to wrap your code, rather, require from the user to run the macro with a correct current language, and check it with `AC_LANG_ASSERT'. And anyway, that may help the user understand she is running a Fortran macro while expecting a result about her Fortran 77 compiler... -- Macro: AC_REQUIRE_CPP Ensure that whichever preprocessor would currently be used for tests has been found. Calls `AC_REQUIRE' (*note Prerequisite Macros::) with an argument of either `AC_PROG_CPP' or `AC_PROG_CXXCPP', depending on which language is current. ---------- Footnotes ---------- (1) Because M4 is not aware of Sh code, especially conditionals, some optimizations that look nice statically may produce incorrect results at runtime. 6.2 Writing Test Programs ========================= Autoconf tests follow a common scheme: feed some program with some input, and most of the time, feed a compiler with some source file. This section is dedicated to these source samples. 6.2.1 Guidelines for Test Programs ---------------------------------- The most important rule to follow when writing testing samples is: _Look for realism._ This motto means that testing samples must be written with the same strictness as real programs are written. In particular, you should avoid "shortcuts" and simplifications. Don't just play with the preprocessor if you want to prepare a compilation. For instance, using `cpp' to check whether a header is functional might let your `configure' accept a header which causes some _compiler_ error. Do not hesitate to check a header with other headers included before, especially required headers. Make sure the symbols you use are properly defined, i.e., refrain from simply declaring a function yourself instead of including the proper header. Test programs should not write to standard output. They should exit with status 0 if the test succeeds, and with status 1 otherwise, so that success can be distinguished easily from a core dump or other failure; segmentation violations and other failures produce a nonzero exit status. Unless you arrange for `exit' to be declared, test programs should `return', not `exit', from `main', because on many systems `exit' is not declared by default. Test programs can use `#if' or `#ifdef' to check the values of preprocessor macros defined by tests that have already run. For example, if you call `AC_HEADER_STDBOOL', then later on in `configure.ac' you can have a test program that includes `stdbool.h' conditionally: #ifdef HAVE_STDBOOL_H # include #endif Both `#if HAVE_STDBOOL_H' and `#ifdef HAVE_STDBOOL_H' will work with any standard C compiler. Some developers prefer `#if' because it is easier to read, while others prefer `#ifdef' because it avoids diagnostics with picky compilers like GCC with the `-Wundef' option. If a test program needs to use or create a data file, give it a name that starts with `conftest', such as `conftest.data'. The `configure' script cleans up by running `rm -f -r conftest*' after running test programs and if the script is interrupted. 6.2.2 Test Functions -------------------- These days it's safe to assume support for function prototypes (introduced in C89). Functions that test programs declare should also be conditionalized for C++, which requires `extern "C"' prototypes. Make sure to not include any header files containing clashing prototypes. #ifdef __cplusplus extern "C" #endif void *valloc (size_t); If a test program calls a function with invalid parameters (just to see whether it exists), organize the program to ensure that it never invokes that function. You can do this by calling it in another function that is never invoked. You can't do it by putting it after a call to `exit', because GCC version 2 knows that `exit' never returns and optimizes out any code that follows it in the same block. If you include any header files, be sure to call the functions relevant to them with the correct number of arguments, even if they are just 0, to avoid compilation errors due to prototypes. GCC version 2 has internal prototypes for several functions that it automatically inlines; for example, `memcpy'. To avoid errors when checking for them, either pass them the correct number of arguments or redeclare them with a different return type (such as `char'). 6.2.3 Generating Sources ------------------------ Autoconf provides a set of macros that can be used to generate test source files. They are written to be language generic, i.e., they actually depend on the current language (*note Language Choice::) to "format" the output properly. -- Macro: AC_LANG_CONFTEST (SOURCE) Save the SOURCE text in the current test source file: `conftest.EXTENSION' where the EXTENSION depends on the current language. Note that the SOURCE is evaluated exactly once, like regular Autoconf macro arguments, and therefore (i) you may pass a macro invocation, (ii) if not, be sure to double quote if needed. -- Macro: AC_LANG_SOURCE (SOURCE) Expands into the SOURCE, with the definition of all the `AC_DEFINE' performed so far. For instance executing (observe the double quotation!): AC_INIT([Hello], [1.0], [bug-hello@example.org]) AC_DEFINE([HELLO_WORLD], ["Hello, World\n"], [Greetings string.]) AC_LANG(C) AC_LANG_CONFTEST( [AC_LANG_SOURCE([[const char hw[] = "Hello, World\n";]])]) gcc -E -dD -o - conftest.c results in: ... # 1 "conftest.c" #define PACKAGE_NAME "Hello" #define PACKAGE_TARNAME "hello" #define PACKAGE_VERSION "1.0" #define PACKAGE_STRING "Hello 1.0" #define PACKAGE_BUGREPORT "bug-hello@example.org" #define HELLO_WORLD "Hello, World\n" const char hw[] = "Hello, World\n"; When the test language is Fortran or Erlang, the `AC_DEFINE' definitions are not automatically translated into constants in the source code by this macro. -- Macro: AC_LANG_PROGRAM (PROLOGUE, BODY) Expands into a source file which consists of the PROLOGUE, and then BODY as body of the main function (e.g., `main' in C). Since it uses `AC_LANG_SOURCE', the features of the latter are available. For instance: AC_INIT([Hello], [1.0], [bug-hello@example.org]) AC_DEFINE([HELLO_WORLD], ["Hello, World\n"], [Greetings string.]) AC_LANG_CONFTEST( [AC_LANG_PROGRAM([[const char hw[] = "Hello, World\n";]], [[fputs (hw, stdout);]])]) gcc -E -dD -o - conftest.c results in: ... # 1 "conftest.c" #define PACKAGE_NAME "Hello" #define PACKAGE_TARNAME "hello" #define PACKAGE_VERSION "1.0" #define PACKAGE_STRING "Hello 1.0" #define PACKAGE_BUGREPORT "bug-hello@example.org" #define HELLO_WORLD "Hello, World\n" const char hw[] = "Hello, World\n"; int main () { fputs (hw, stdout); ; return 0; } In Erlang tests, the created source file is that of an Erlang module called `conftest' (`conftest.erl'). This module defines and exports at least one `start/0' function, which is called to perform the test. The PROLOGUE is optional code that is inserted between the module header and the `start/0' function definition. BODY is the body of the `start/0' function without the final period (*note Runtime::, about constraints on this function's behavior). For instance: AC_INIT([Hello], [1.0], [bug-hello@example.org]) AC_LANG(Erlang) AC_LANG_CONFTEST( [AC_LANG_PROGRAM([[-define(HELLO_WORLD, "Hello, world!").]], [[io:format("~s~n", [?HELLO_WORLD])]])]) cat conftest.erl results in: -module(conftest). -export([start/0]). -define(HELLO_WORLD, "Hello, world!"). start() -> io:format("~s~n", [?HELLO_WORLD]) . -- Macro: AC_LANG_CALL (PROLOGUE, FUNCTION) Expands into a source file which consists of the PROLOGUE, and then a call to the FUNCTION as body of the main function (e.g., `main' in C). Since it uses `AC_LANG_PROGRAM', the feature of the latter are available. This function will probably be replaced in the future by a version which would enable specifying the arguments. The use of this macro is not encouraged, as it violates strongly the typing system. This macro cannot be used for Erlang tests. -- Macro: AC_LANG_FUNC_LINK_TRY (FUNCTION) Expands into a source file which uses the FUNCTION in the body of the main function (e.g., `main' in C). Since it uses `AC_LANG_PROGRAM', the features of the latter are available. As `AC_LANG_CALL', this macro is documented only for completeness. It is considered to be severely broken, and in the future will be removed in favor of actual function calls (with properly typed arguments). This macro cannot be used for Erlang tests. 6.3 Running the Preprocessor ============================ Sometimes one might need to run the preprocessor on some source file. _Usually it is a bad idea_, as you typically need to _compile_ your project, not merely run the preprocessor on it; therefore you certainly want to run the compiler, not the preprocessor. Resist the temptation of following the easiest path. Nevertheless, if you need to run the preprocessor, then use `AC_PREPROC_IFELSE'. The macros described in this section cannot be used for tests in Erlang or Fortran, since those languages require no preprocessor. -- Macro: AC_PREPROC_IFELSE (INPUT, [ACTION-IF-TRUE], [ACTION-IF-FALSE]) Run the preprocessor of the current language (*note Language Choice::) on the INPUT, run the shell commands ACTION-IF-TRUE on success, ACTION-IF-FALSE otherwise. The INPUT can be made by `AC_LANG_PROGRAM' and friends. This macro uses `CPPFLAGS', but not `CFLAGS', because `-g', `-O', etc. are not valid options to many C preprocessors. It is customary to report unexpected failures with `AC_MSG_FAILURE'. For instance: AC_INIT([Hello], [1.0], [bug-hello@example.org]) AC_DEFINE([HELLO_WORLD], ["Hello, World\n"], [Greetings string.]) AC_PREPROC_IFELSE( [AC_LANG_PROGRAM([[const char hw[] = "Hello, World\n";]], [[fputs (hw, stdout);]])], [AC_MSG_RESULT([OK])], [AC_MSG_FAILURE([unexpected preprocessor failure])]) results in: checking for gcc... gcc checking for C compiler default output file name... a.out checking whether the C compiler works... yes checking whether we are cross compiling... no checking for suffix of executables... checking for suffix of object files... o checking whether we are using the GNU C compiler... yes checking whether gcc accepts -g... yes checking for gcc option to accept ISO C89... none needed checking how to run the C preprocessor... gcc -E OK The macro `AC_TRY_CPP' (*note Obsolete Macros::) used to play the role of `AC_PREPROC_IFELSE', but double quotes its argument, making it impossible to use it to elaborate sources. You are encouraged to get rid of your old use of the macro `AC_TRY_CPP' in favor of `AC_PREPROC_IFELSE', but, in the first place, are you sure you need to run the _preprocessor_ and not the compiler? -- Macro: AC_EGREP_HEADER (PATTERN, HEADER-FILE, ACTION-IF-FOUND, [ACTION-IF-NOT-FOUND]) If the output of running the preprocessor on the system header file HEADER-FILE matches the extended regular expression PATTERN, execute shell commands ACTION-IF-FOUND, otherwise execute ACTION-IF-NOT-FOUND. -- Macro: AC_EGREP_CPP (PATTERN, PROGRAM, [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND]) PROGRAM is the text of a C or C++ program, on which shell variable, back quote, and backslash substitutions are performed. If the output of running the preprocessor on PROGRAM matches the extended regular expression PATTERN, execute shell commands ACTION-IF-FOUND, otherwise execute ACTION-IF-NOT-FOUND. 6.4 Running the Compiler ======================== To check for a syntax feature of the current language's (*note Language Choice::) compiler, such as whether it recognizes a certain keyword, or simply to try some library feature, use `AC_COMPILE_IFELSE' to try to compile a small program that uses that feature. -- Macro: AC_COMPILE_IFELSE (INPUT, [ACTION-IF-TRUE], [ACTION-IF-FALSE]) Run the compiler and compilation flags of the current language (*note Language Choice::) on the INPUT, run the shell commands ACTION-IF-TRUE on success, ACTION-IF-FALSE otherwise. The INPUT can be made by `AC_LANG_PROGRAM' and friends. It is customary to report unexpected failures with `AC_MSG_FAILURE'. This macro does not try to link; use `AC_LINK_IFELSE' if you need to do that (*note Running the Linker::). This macro uses `AC_REQUIRE' for the compiler associated with the current language, which means that if the compiler has not yet been determined, the compiler determination will be made prior to the body of the outermust `AC_DEFUN' macro that triggered this macro to expand (*note Expanded Before Required::). For tests in Erlang, the INPUT must be the source code of a module named `conftest'. `AC_COMPILE_IFELSE' generates a `conftest.beam' file that can be interpreted by the Erlang virtual machine (`ERL'). It is recommended to use `AC_LANG_PROGRAM' to specify the test program, to ensure that the Erlang module has the right name. 6.5 Running the Linker ====================== To check for a library, a function, or a global variable, Autoconf `configure' scripts try to compile and link a small program that uses it. This is unlike Metaconfig, which by default uses `nm' or `ar' on the C library to try to figure out which functions are available. Trying to link with the function is usually a more reliable approach because it avoids dealing with the variations in the options and output formats of `nm' and `ar' and in the location of the standard libraries. It also allows configuring for cross-compilation or checking a function's runtime behavior if needed. On the other hand, it can be slower than scanning the libraries once, but accuracy is more important than speed. `AC_LINK_IFELSE' is used to compile test programs to test for functions and global variables. It is also used by `AC_CHECK_LIB' to check for libraries (*note Libraries::), by adding the library being checked for to `LIBS' temporarily and trying to link a small program. -- Macro: AC_LINK_IFELSE (INPUT, [ACTION-IF-TRUE], [ACTION-IF-FALSE]) Run the compiler (and compilation flags) and the linker of the current language (*note Language Choice::) on the INPUT, run the shell commands ACTION-IF-TRUE on success, ACTION-IF-FALSE otherwise. The INPUT can be made by `AC_LANG_PROGRAM' and friends. `LDFLAGS' and `LIBS' are used for linking, in addition to the current compilation flags. It is customary to report unexpected failures with `AC_MSG_FAILURE'. This macro does not try to execute the program; use `AC_RUN_IFELSE' if you need to do that (*note Runtime::). The `AC_LINK_IFELSE' macro cannot be used for Erlang tests, since Erlang programs are interpreted and do not require linking. 6.6 Checking Runtime Behavior ============================= Sometimes you need to find out how a system performs at runtime, such as whether a given function has a certain capability or bug. If you can, make such checks when your program runs instead of when it is configured. You can check for things like the machine's endianness when your program initializes itself. If you really need to test for a runtime behavior while configuring, you can write a test program to determine the result, and compile and run it using `AC_RUN_IFELSE'. Avoid running test programs if possible, because this prevents people from configuring your package for cross-compiling. -- Macro: AC_RUN_IFELSE (INPUT, [ACTION-IF-TRUE], [ACTION-IF-FALSE], [ACTION-IF-CROSS-COMPILING]) If PROGRAM compiles and links successfully and returns an exit status of 0 when executed, run shell commands ACTION-IF-TRUE. Otherwise, run shell commands ACTION-IF-FALSE. The INPUT can be made by `AC_LANG_PROGRAM' and friends. `LDFLAGS' and `LIBS' are used for linking, in addition to the compilation flags of the current language (*note Language Choice::). Additionally, ACTION-IF-TRUE can run `./conftest$EXEEXT' for further testing. If the compiler being used does not produce executables that run on the system where `configure' is being run, then the test program is not run. If the optional shell commands ACTION-IF-CROSS-COMPILING are given, they are run instead. Otherwise, `configure' prints an error message and exits. In the ACTION-IF-FALSE section, the failing exit status is available in the shell variable `$?'. This exit status might be that of a failed compilation, or it might be that of a failed program execution. It is customary to report unexpected failures with `AC_MSG_FAILURE'. Try to provide a pessimistic default value to use when cross-compiling makes runtime tests impossible. You do this by passing the optional last argument to `AC_RUN_IFELSE'. `autoconf' prints a warning message when creating `configure' each time it encounters a call to `AC_RUN_IFELSE' with no ACTION-IF-CROSS-COMPILING argument given. You may ignore the warning, though users cannot configure your package for cross-compiling. A few of the macros distributed with Autoconf produce this warning message. To configure for cross-compiling you can also choose a value for those parameters based on the canonical system name (*note Manual Configuration::). Alternatively, set up a test results cache file with the correct values for the host system (*note Caching Results::). To provide a default for calls of `AC_RUN_IFELSE' that are embedded in other macros, including a few of the ones that come with Autoconf, you can test whether the shell variable `cross_compiling' is set to `yes', and then use an alternate method to get the results instead of calling the macros. It is also permissible to temporarily assign to `cross_compiling' in order to force tests to behave as though they are in a cross-compilation environment, particularly since this provides a way to test your ACTION-IF-CROSS-COMPILING even when you are not using a cross-compiler. # We temporarily set cross-compile mode to force AC_COMPUTE_INT # to use the slow link-only method save_cross_compiling=$cross_compiling cross_compiling=yes AC_COMPUTE_INT([...]) cross_compiling=$save_cross_compiling A C or C++ runtime test should be portable. *Note Portable C and C++::. Erlang tests must exit themselves the Erlang VM by calling the `halt/1' function: the given status code is used to determine the success of the test (status is `0') or its failure (status is different than `0'), as explained above. It must be noted that data output through the standard output (e.g., using `io:format/2') may be truncated when halting the VM. Therefore, if a test must output configuration information, it is recommended to create and to output data into the temporary file named `conftest.out', using the functions of module `file'. The `conftest.out' file is automatically deleted by the `AC_RUN_IFELSE' macro. For instance, a simplified implementation of Autoconf's `AC_ERLANG_SUBST_LIB_DIR' macro is: AC_INIT([LibdirTest], [1.0], [bug-libdirtest@example.org]) AC_ERLANG_NEED_ERL AC_LANG(Erlang) AC_RUN_IFELSE( [AC_LANG_PROGRAM([], [dnl file:write_file("conftest.out", code:lib_dir()), halt(0)])], [echo "code:lib_dir() returned: `cat conftest.out`"], [AC_MSG_FAILURE([test Erlang program execution failed])]) 6.7 Systemology =============== This section aims at presenting some systems and pointers to documentation. It may help you addressing particular problems reported by users. Posix-conforming systems (http://www.opengroup.org/susv3) are derived from the Unix operating system (http://www.bell-labs.com/history/unix/). The Rosetta Stone for Unix (http://bhami.com/rosetta.html) contains a table correlating the features of various Posix-conforming systems. Unix History (http://www.levenez.com/unix/) is a simplified diagram of how many Unix systems were derived from each other. The Heirloom Project (http://heirloom.sourceforge.net/) provides some variants of traditional implementations of Unix utilities. Darwin Darwin is also known as Mac OS X. Beware that the file system _can_ be case-preserving, but case insensitive. This can cause nasty problems, since for instance the installation attempt for a package having an `INSTALL' file can result in `make install' report that nothing was to be done! That's all dependent on whether the file system is a UFS (case sensitive) or HFS+ (case preserving). By default Apple wants you to install the OS on HFS+. Unfortunately, there are some pieces of software which really need to be built on UFS. We may want to rebuild Darwin to have both UFS and HFS+ available (and put the /local/build tree on the UFS). QNX 4.25 QNX is a realtime operating system running on Intel architecture meant to be scalable from the small embedded systems to the hundred processor super-computer. It claims to be Posix certified. More information is available on the QNX home page (http://www.qnx.com/). Tru64 Documentation of several versions of Tru64 (http://h30097.www3.hp.com/docs/) is available in different formats. Unix version 7 Officially this was called the "Seventh Edition" of "the UNIX time-sharing system" but we use the more-common name "Unix version 7". Documentation is available in the Unix Seventh Edition Manual (http://plan9.bell-labs.com/7thEdMan/). Previous versions of Unix are called "Unix version 6", etc., but they were not as widely used. 6.8 Multiple Cases ================== Some operations are accomplished in several possible ways, depending on the OS variant. Checking for them essentially requires a "case statement". Autoconf does not directly provide one; however, it is easy to simulate by using a shell variable to keep track of whether a way to perform the operation has been found yet. Here is an example that uses the shell variable `fstype' to keep track of whether the remaining cases need to be checked. Note that since the value of `fstype' is under our control, we don't have to use the longer `test "x$fstype" = xno'. AC_MSG_CHECKING([how to get file system type]) fstype=no # The order of these tests is important. AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include #include ]])], [AC_DEFINE([FSTYPE_STATVFS], [1], [Define if statvfs exists.]) fstype=SVR4]) if test $fstype = no; then AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include #include ]])], [AC_DEFINE([FSTYPE_USG_STATFS], [1], [Define if USG statfs.]) fstype=SVR3]) fi if test $fstype = no; then AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include #include ]])]), [AC_DEFINE([FSTYPE_AIX_STATFS], [1], [Define if AIX statfs.]) fstype=AIX]) fi # (more cases omitted here) AC_MSG_RESULT([$fstype]) 7 Results of Tests ****************** Once `configure' has determined whether a feature exists, what can it do to record that information? There are four sorts of things it can do: define a C preprocessor symbol, set a variable in the output files, save the result in a cache file for future `configure' runs, and print a message letting the user know the result of the test. 7.1 Defining C Preprocessor Symbols =================================== A common action to take in response to a feature test is to define a C preprocessor symbol indicating the results of the test. That is done by calling `AC_DEFINE' or `AC_DEFINE_UNQUOTED'. By default, `AC_OUTPUT' places the symbols defined by these macros into the output variable `DEFS', which contains an option `-DSYMBOL=VALUE' for each symbol defined. Unlike in Autoconf version 1, there is no variable `DEFS' defined while `configure' is running. To check whether Autoconf macros have already defined a certain C preprocessor symbol, test the value of the appropriate cache variable, as in this example: AC_CHECK_FUNC([vprintf], [AC_DEFINE([HAVE_VPRINTF], [1], [Define if vprintf exists.])]) if test "x$ac_cv_func_vprintf" != xyes; then AC_CHECK_FUNC([_doprnt], [AC_DEFINE([HAVE_DOPRNT], [1], [Define if _doprnt exists.])]) fi If `AC_CONFIG_HEADERS' has been called, then instead of creating `DEFS', `AC_OUTPUT' creates a header file by substituting the correct values into `#define' statements in a template file. *Note Configuration Headers::, for more information about this kind of output. -- Macro: AC_DEFINE (VARIABLE, VALUE, [DESCRIPTION]) -- Macro: AC_DEFINE (VARIABLE) Define VARIABLE to VALUE (verbatim), by defining a C preprocessor macro for VARIABLE. VARIABLE should be a C identifier, optionally suffixed by a parenthesized argument list to define a C preprocessor macro with arguments. The macro argument list, if present, should be a comma-separated list of C identifiers, possibly terminated by an ellipsis `...' if C99 syntax is employed. VARIABLE should not contain comments, white space, trigraphs, backslash-newlines, universal character names, or non-ASCII characters. VALUE may contain backslash-escaped newlines, which will be preserved if you use `AC_CONFIG_HEADERS' but flattened if passed via `@DEFS@' (with no effect on the compilation, since the preprocessor sees only one line in the first place). VALUE should not contain raw newlines. If you are not using `AC_CONFIG_HEADERS', VALUE should not contain any `#' characters, as `make' tends to eat them. To use a shell variable, use `AC_DEFINE_UNQUOTED' instead. DESCRIPTION is only useful if you are using `AC_CONFIG_HEADERS'. In this case, DESCRIPTION is put into the generated `config.h.in' as the comment before the macro define. The following example defines the C preprocessor variable `EQUATION' to be the string constant `"$a > $b"': AC_DEFINE([EQUATION], ["$a > $b"], [Equation string.]) If neither VALUE nor DESCRIPTION are given, then VALUE defaults to 1 instead of to the empty string. This is for backwards compatibility with older versions of Autoconf, but this usage is obsolescent and may be withdrawn in future versions of Autoconf. If the VARIABLE is a literal string, it is passed to `m4_pattern_allow' (*note Forbidden Patterns::). If multiple `AC_DEFINE' statements are executed for the same VARIABLE name (not counting any parenthesized argument list), the last one wins. -- Macro: AC_DEFINE_UNQUOTED (VARIABLE, VALUE, [DESCRIPTION]) -- Macro: AC_DEFINE_UNQUOTED (VARIABLE) Like `AC_DEFINE', but three shell expansions are performed--once--on VARIABLE and VALUE: variable expansion (`$'), command substitution (``'), and backslash escaping (`\'), as if in an unquoted here-document. Single and double quote characters in the value have no special meaning. Use this macro instead of `AC_DEFINE' when VARIABLE or VALUE is a shell variable. Examples: AC_DEFINE_UNQUOTED([config_machfile], ["$machfile"], [Configuration machine file.]) AC_DEFINE_UNQUOTED([GETGROUPS_T], [$ac_cv_type_getgroups], [getgroups return type.]) AC_DEFINE_UNQUOTED([$ac_tr_hdr], [1], [Translated header name.]) Due to a syntactical bizarreness of the Bourne shell, do not use semicolons to separate `AC_DEFINE' or `AC_DEFINE_UNQUOTED' calls from other macro calls or shell code; that can cause syntax errors in the resulting `configure' script. Use either blanks or newlines. That is, do this: AC_CHECK_HEADER([elf.h], [AC_DEFINE([SVR4], [1], [System V Release 4]) LIBS="-lelf $LIBS"]) or this: AC_CHECK_HEADER([elf.h], [AC_DEFINE([SVR4], [1], [System V Release 4]) LIBS="-lelf $LIBS"]) instead of this: AC_CHECK_HEADER([elf.h], [AC_DEFINE([SVR4], [1], [System V Release 4]); LIBS="-lelf $LIBS"]) 7.2 Setting Output Variables ============================ Another way to record the results of tests is to set "output variables", which are shell variables whose values are substituted into files that `configure' outputs. The two macros below create new output variables. *Note Preset Output Variables::, for a list of output variables that are always available. -- Macro: AC_SUBST (VARIABLE, [VALUE]) Create an output variable from a shell variable. Make `AC_OUTPUT' substitute the variable VARIABLE into output files (typically one or more makefiles). This means that `AC_OUTPUT' replaces instances of `@VARIABLE@' in input files with the value that the shell variable VARIABLE has when `AC_OUTPUT' is called. The value can contain any non-`NUL' character, including newline. If you are using Automake 1.11 or newer, for newlines in values you might want to consider using `AM_SUBST_NOTMAKE' to prevent `automake' from adding a line `VARIABLE = @VARIABLE@' to the `Makefile.in' files (*note Automake: (automake)Optional.). Variable occurrences should not overlap: e.g., an input file should not contain `@VAR1@VAR2@' if VAR1 and VAR2 are variable names. The substituted value is not rescanned for more output variables; occurrences of `@VARIABLE@' in the value are inserted literally into the output file. (The algorithm uses the special marker `|#_!!_#|' internally, so neither the substituted value nor the output file may contain `|#_!!_#|'.) If VALUE is given, in addition assign it to VARIABLE. The string VARIABLE is passed to `m4_pattern_allow' (*note Forbidden Patterns::). -- Macro: AC_SUBST_FILE (VARIABLE) Another way to create an output variable from a shell variable. Make `AC_OUTPUT' insert (without substitutions) the contents of the file named by shell variable VARIABLE into output files. This means that `AC_OUTPUT' replaces instances of `@VARIABLE@' in output files (such as `Makefile.in') with the contents of the file that the shell variable VARIABLE names when `AC_OUTPUT' is called. Set the variable to `/dev/null' for cases that do not have a file to insert. This substitution occurs only when the `@VARIABLE@' is on a line by itself, optionally surrounded by spaces and tabs. The substitution replaces the whole line, including the spaces, tabs, and the terminating newline. This macro is useful for inserting makefile fragments containing special dependencies or other `make' directives for particular host or target types into makefiles. For example, `configure.ac' could contain: AC_SUBST_FILE([host_frag]) host_frag=$srcdir/conf/sun4.mh and then a `Makefile.in' could contain: @host_frag@ The string VARIABLE is passed to `m4_pattern_allow' (*note Forbidden Patterns::). Running `configure' in varying environments can be extremely dangerous. If for instance the user runs `CC=bizarre-cc ./configure', then the cache, `config.h', and many other output files depend upon `bizarre-cc' being the C compiler. If for some reason the user runs `./configure' again, or if it is run via `./config.status --recheck', (*Note Automatic Remaking::, and *note config.status Invocation::), then the configuration can be inconsistent, composed of results depending upon two different compilers. Environment variables that affect this situation, such as `CC' above, are called "precious variables", and can be declared as such by `AC_ARG_VAR'. -- Macro: AC_ARG_VAR (VARIABLE, DESCRIPTION) Declare VARIABLE is a precious variable, and include its DESCRIPTION in the variable section of `./configure --help'. Being precious means that - VARIABLE is substituted via `AC_SUBST'. - The value of VARIABLE when `configure' was launched is saved in the cache, including if it was not specified on the command line but via the environment. Indeed, while `configure' can notice the definition of `CC' in `./configure CC=bizarre-cc', it is impossible to notice it in `CC=bizarre-cc ./configure', which, unfortunately, is what most users do. We emphasize that it is the _initial_ value of VARIABLE which is saved, not that found during the execution of `configure'. Indeed, specifying `./configure FOO=foo' and letting `./configure' guess that `FOO' is `foo' can be two different things. - VARIABLE is checked for consistency between two `configure' runs. For instance: $ ./configure --silent --config-cache $ CC=cc ./configure --silent --config-cache configure: error: `CC' was not set in the previous run configure: error: changes in the environment can compromise \ the build configure: error: run `make distclean' and/or \ `rm config.cache' and start over and similarly if the variable is unset, or if its content is changed. If the content has white space changes only, then the error is degraded to a warning only, but the old value is reused. - VARIABLE is kept during automatic reconfiguration (*note config.status Invocation::) as if it had been passed as a command line argument, including when no cache is used: $ CC=/usr/bin/cc ./configure var=raboof --silent $ ./config.status --recheck running CONFIG_SHELL=/bin/sh /bin/sh ./configure var=raboof \ CC=/usr/bin/cc --no-create --no-recursion 7.3 Special Characters in Output Variables ========================================== Many output variables are intended to be evaluated both by `make' and by the shell. Some characters are expanded differently in these two contexts, so to avoid confusion these variables' values should not contain any of the following characters: " # $ & ' ( ) * ; < > ? [ \ ^ ` | Also, these variables' values should neither contain newlines, nor start with `~', nor contain white space or `:' immediately followed by `~'. The values can contain nonempty sequences of white space characters like tabs and spaces, but each such sequence might arbitrarily be replaced by a single space during substitution. These restrictions apply both to the values that `configure' computes, and to the values set directly by the user. For example, the following invocations of `configure' are problematic, since they attempt to use special characters within `CPPFLAGS' and white space within `$(srcdir)': CPPFLAGS='-DOUCH="&\"#$*?"' '../My Source/ouch-1.0/configure' '../My Source/ouch-1.0/configure' CPPFLAGS='-DOUCH="&\"#$*?"' 7.4 Caching Results =================== To avoid checking for the same features repeatedly in various `configure' scripts (or in repeated runs of one script), `configure' can optionally save the results of many checks in a "cache file" (*note Cache Files::). If a `configure' script runs with caching enabled and finds a cache file, it reads the results of previous runs from the cache and avoids rerunning those checks. As a result, `configure' can then run much faster than if it had to perform all of the checks every time. -- Macro: AC_CACHE_VAL (CACHE-ID, COMMANDS-TO-SET-IT) Ensure that the results of the check identified by CACHE-ID are available. If the results of the check were in the cache file that was read, and `configure' was not given the `--quiet' or `--silent' option, print a message saying that the result was cached; otherwise, run the shell commands COMMANDS-TO-SET-IT. If the shell commands are run to determine the value, the value is saved in the cache file just before `configure' creates its output files. *Note Cache Variable Names::, for how to choose the name of the CACHE-ID variable. The COMMANDS-TO-SET-IT _must have no side effects_ except for setting the variable CACHE-ID, see below. -- Macro: AC_CACHE_CHECK (MESSAGE, CACHE-ID, COMMANDS-TO-SET-IT) A wrapper for `AC_CACHE_VAL' that takes care of printing the messages. This macro provides a convenient shorthand for the most common way to use these macros. It calls `AC_MSG_CHECKING' for MESSAGE, then `AC_CACHE_VAL' with the CACHE-ID and COMMANDS arguments, and `AC_MSG_RESULT' with CACHE-ID. The COMMANDS-TO-SET-IT _must have no side effects_ except for setting the variable CACHE-ID, see below. It is common to find buggy macros using `AC_CACHE_VAL' or `AC_CACHE_CHECK', because people are tempted to call `AC_DEFINE' in the COMMANDS-TO-SET-IT. Instead, the code that _follows_ the call to `AC_CACHE_VAL' should call `AC_DEFINE', by examining the value of the cache variable. For instance, the following macro is broken: AC_DEFUN([AC_SHELL_TRUE], [AC_CACHE_CHECK([whether true(1) works], [my_cv_shell_true_works], [my_cv_shell_true_works=no (true) 2>/dev/null && my_cv_shell_true_works=yes if test "x$my_cv_shell_true_works" = xyes; then AC_DEFINE([TRUE_WORKS], [1], [Define if `true(1)' works properly.]) fi]) ]) This fails if the cache is enabled: the second time this macro is run, `TRUE_WORKS' _will not be defined_. The proper implementation is: AC_DEFUN([AC_SHELL_TRUE], [AC_CACHE_CHECK([whether true(1) works], [my_cv_shell_true_works], [my_cv_shell_true_works=no (true) 2>/dev/null && my_cv_shell_true_works=yes]) if test "x$my_cv_shell_true_works" = xyes; then AC_DEFINE([TRUE_WORKS], [1], [Define if `true(1)' works properly.]) fi ]) Also, COMMANDS-TO-SET-IT should not print any messages, for example with `AC_MSG_CHECKING'; do that before calling `AC_CACHE_VAL', so the messages are printed regardless of whether the results of the check are retrieved from the cache or determined by running the shell commands. 7.4.1 Cache Variable Names -------------------------- The names of cache variables should have the following format: PACKAGE-PREFIX_cv_VALUE-TYPE_SPECIFIC-VALUE_[ADDITIONAL-OPTIONS] for example, `ac_cv_header_stat_broken' or `ac_cv_prog_gcc_traditional'. The parts of the variable name are: PACKAGE-PREFIX An abbreviation for your package or organization; the same prefix you begin local Autoconf macros with, except lowercase by convention. For cache values used by the distributed Autoconf macros, this value is `ac'. `_cv_' Indicates that this shell variable is a cache value. This string _must_ be present in the variable name, including the leading underscore. VALUE-TYPE A convention for classifying cache values, to produce a rational naming system. The values used in Autoconf are listed in *note Macro Names::. SPECIFIC-VALUE Which member of the class of cache values this test applies to. For example, which function (`alloca'), program (`gcc'), or output variable (`INSTALL'). ADDITIONAL-OPTIONS Any particular behavior of the specific member that this test applies to. For example, `broken' or `set'. This part of the name may be omitted if it does not apply. The values assigned to cache variables may not contain newlines. Usually, their values are Boolean (`yes' or `no') or the names of files or functions; so this is not an important restriction. *note Cache Variable Index:: for an index of cache variables with documented semantics. 7.4.2 Cache Files ----------------- A cache file is a shell script that caches the results of configure tests run on one system so they can be shared between configure scripts and configure runs. It is not useful on other systems. If its contents are invalid for some reason, the user may delete or edit it. By default, `configure' uses no cache file, to avoid problems caused by accidental use of stale cache files. To enable caching, `configure' accepts `--config-cache' (or `-C') to cache results in the file `config.cache'. Alternatively, `--cache-file=FILE' specifies that FILE be the cache file. The cache file is created if it does not exist already. When `configure' calls `configure' scripts in subdirectories, it uses the `--cache-file' argument so that they share the same cache. *Note Subdirectories::, for information on configuring subdirectories with the `AC_CONFIG_SUBDIRS' macro. `config.status' only pays attention to the cache file if it is given the `--recheck' option, which makes it rerun `configure'. It is wrong to try to distribute cache files for particular system types. There is too much room for error in doing that, and too much administrative overhead in maintaining them. For any features that can't be guessed automatically, use the standard method of the canonical system type and linking files (*note Manual Configuration::). The site initialization script can specify a site-wide cache file to use, instead of the usual per-program cache. In this case, the cache file gradually accumulates information whenever someone runs a new `configure' script. (Running `configure' merges the new cache results with the existing cache file.) This may cause problems, however, if the system configuration (e.g., the installed libraries or compilers) changes and the stale cache file is not deleted. 7.4.3 Cache Checkpointing ------------------------- If your configure script, or a macro called from `configure.ac', happens to abort the configure process, it may be useful to checkpoint the cache a few times at key points using `AC_CACHE_SAVE'. Doing so reduces the amount of time it takes to rerun the configure script with (hopefully) the error that caused the previous abort corrected. -- Macro: AC_CACHE_LOAD Loads values from existing cache file, or creates a new cache file if a cache file is not found. Called automatically from `AC_INIT'. -- Macro: AC_CACHE_SAVE Flushes all cached values to the cache file. Called automatically from `AC_OUTPUT', but it can be quite useful to call `AC_CACHE_SAVE' at key points in `configure.ac'. For instance: ... AC_INIT, etc. ... # Checks for programs. AC_PROG_CC AC_PROG_AWK ... more program checks ... AC_CACHE_SAVE # Checks for libraries. AC_CHECK_LIB([nsl], [gethostbyname]) AC_CHECK_LIB([socket], [connect]) ... more lib checks ... AC_CACHE_SAVE # Might abort... AM_PATH_GTK([1.0.2], [], [AC_MSG_ERROR([GTK not in path])]) AM_PATH_GTKMM([0.9.5], [], [AC_MSG_ERROR([GTK not in path])]) ... AC_OUTPUT, etc. ... 7.5 Printing Messages ===================== `configure' scripts need to give users running them several kinds of information. The following macros print messages in ways appropriate for each kind. The arguments to all of them get enclosed in shell double quotes, so the shell performs variable and back-quote substitution on them. These macros are all wrappers around the `echo' shell command. They direct output to the appropriate file descriptor (*note File Descriptor Macros::). `configure' scripts should rarely need to run `echo' directly to print messages for the user. Using these macros makes it easy to change how and when each kind of message is printed; such changes need only be made to the macro definitions and all the callers change automatically. To diagnose static issues, i.e., when `autoconf' is run, see *note Diagnostic Macros::. -- Macro: AC_MSG_CHECKING (FEATURE-DESCRIPTION) Notify the user that `configure' is checking for a particular feature. This macro prints a message that starts with `checking ' and ends with `...' and no newline. It must be followed by a call to `AC_MSG_RESULT' to print the result of the check and the newline. The FEATURE-DESCRIPTION should be something like `whether the Fortran compiler accepts C++ comments' or `for c89'. This macro prints nothing if `configure' is run with the `--quiet' or `--silent' option. -- Macro: AC_MSG_RESULT (RESULT-DESCRIPTION) Notify the user of the results of a check. RESULT-DESCRIPTION is almost always the value of the cache variable for the check, typically `yes', `no', or a file name. This macro should follow a call to `AC_MSG_CHECKING', and the RESULT-DESCRIPTION should be the completion of the message printed by the call to `AC_MSG_CHECKING'. This macro prints nothing if `configure' is run with the `--quiet' or `--silent' option. -- Macro: AC_MSG_NOTICE (MESSAGE) Deliver the MESSAGE to the user. It is useful mainly to print a general description of the overall purpose of a group of feature checks, e.g., AC_MSG_NOTICE([checking if stack overflow is detectable]) This macro prints nothing if `configure' is run with the `--quiet' or `--silent' option. -- Macro: AC_MSG_ERROR (ERROR-DESCRIPTION, [EXIT-STATUS = `$?/1']) Notify the user of an error that prevents `configure' from completing. This macro prints an error message to the standard error output and exits `configure' with EXIT-STATUS (`$?' by default, except that `0' is converted to `1'). ERROR-DESCRIPTION should be something like `invalid value $HOME for \$HOME'. The ERROR-DESCRIPTION should start with a lower-case letter, and "cannot" is preferred to "can't". -- Macro: AC_MSG_FAILURE (ERROR-DESCRIPTION, [EXIT-STATUS]) This `AC_MSG_ERROR' wrapper notifies the user of an error that prevents `configure' from completing _and_ that additional details are provided in `config.log'. This is typically used when abnormal results are found during a compilation. -- Macro: AC_MSG_WARN (PROBLEM-DESCRIPTION) Notify the `configure' user of a possible problem. This macro prints the message to the standard error output; `configure' continues running afterward, so macros that call `AC_MSG_WARN' should provide a default (back-up) behavior for the situations they warn about. PROBLEM-DESCRIPTION should be something like `ln -s seems to make hard links'. 8 Programming in M4 ******************* Autoconf is written on top of two layers: "M4sugar", which provides convenient macros for pure M4 programming, and "M4sh", which provides macros dedicated to shell script generation. As of this version of Autoconf, these two layers still contain experimental macros, whose interface might change in the future. As a matter of fact, _anything that is not documented must not be used_. 8.1 M4 Quotation ================ The most common problem with existing macros is an improper quotation. This section, which users of Autoconf can skip, but which macro writers _must_ read, first justifies the quotation scheme that was chosen for Autoconf and then ends with a rule of thumb. Understanding the former helps one to follow the latter. 8.1.1 Active Characters ----------------------- To fully understand where proper quotation is important, you first need to know what the special characters are in Autoconf: `#' introduces a comment inside which no macro expansion is performed, `,' separates arguments, `[' and `]' are the quotes themselves, `(' and `)' (which M4 tries to match by pairs), and finally `$' inside a macro definition. In order to understand the delicate case of macro calls, we first have to present some obvious failures. Below they are "obvious-ified", but when you find them in real life, they are usually in disguise. Comments, introduced by a hash and running up to the newline, are opaque tokens to the top level: active characters are turned off, and there is no macro expansion: # define([def], ine) =># define([def], ine) Each time there can be a macro expansion, there is a quotation expansion, i.e., one level of quotes is stripped: int tab[10]; =>int tab10; [int tab[10];] =>int tab[10]; Without this in mind, the reader might try hopelessly to use her macro `array': define([array], [int tab[10];]) array =>int tab10; [array] =>array How can you correctly output the intended results(1)? ---------- Footnotes ---------- (1) Using `defn'. 8.1.2 One Macro Call -------------------- Let's proceed on the interaction between active characters and macros with this small macro, which just returns its first argument: define([car], [$1]) The two pairs of quotes above are not part of the arguments of `define'; rather, they are understood by the top level when it tries to find the arguments of `define'. Therefore, assuming `car' is not already defined, it is equivalent to write: define(car, $1) But, while it is acceptable for a `configure.ac' to avoid unnecessary quotes, it is bad practice for Autoconf macros which must both be more robust and also advocate perfect style. At the top level, there are only two possibilities: either you quote or you don't: car(foo, bar, baz) =>foo [car(foo, bar, baz)] =>car(foo, bar, baz) Let's pay attention to the special characters: car(#) error-->EOF in argument list The closing parenthesis is hidden in the comment; with a hypothetical quoting, the top level understood it this way: car([#)] Proper quotation, of course, fixes the problem: car([#]) =># Here are more examples: car(foo, bar) =>foo car([foo, bar]) =>foo, bar car((foo, bar)) =>(foo, bar) car([(foo], [bar)]) =>(foo define([a], [b]) => car(a) =>b car([a]) =>b car([[a]]) =>a car([[[a]]]) =>[a] 8.1.3 Quoting and Parameters ---------------------------- When M4 encounters `$' within a macro definition, followed immediately by a character it recognizes (`0'...`9', `#', `@', or `*'), it will perform M4 parameter expansion. This happens regardless of how many layers of quotes the parameter expansion is nested within, or even if it occurs in text that will be rescanned as a comment. define([none], [$1]) => define([one], [[$1]]) => define([two], [[[$1]]]) => define([comment], [# $1]) => define([active], [ACTIVE]) => none([active]) =>ACTIVE one([active]) =>active two([active]) =>[active] comment([active]) =># active On the other hand, since autoconf generates shell code, you often want to output shell variable expansion, rather than performing M4 parameter expansion. To do this, you must use M4 quoting to separate the `$' from the next character in the definition of your macro. If the macro definition occurs in single-quoted text, then insert another level of quoting; if the usage is already inside a double-quoted string, then split it into concatenated strings. define([single], [a single-quoted $[]1 definition]) => define([double], [[a double-quoted $][1 definition]]) => single =>a single-quoted $1 definition double =>a double-quoted $1 definition Posix states that M4 implementations are free to provide implementation extensions when `${' is encountered in a macro definition. Autoconf reserves the longer sequence `${{' for use with planned extensions that will be available in the future GNU M4 2.0, but guarantees that all other instances of `${' will be output literally. Therefore, this idiom can also be used to output shell code parameter references: define([first], [${1}])first =>${1} Posix also states that `$11' should expand to the first parameter concatenated with a literal `1', although some versions of GNU M4 expand the eleventh parameter instead. For portability, you should only use single-digit M4 parameter expansion. With this in mind, we can explore the cases where macros invoke macros... 8.1.4 Quotation and Nested Macros --------------------------------- The examples below use the following macros: define([car], [$1]) define([active], [ACT, IVE]) define([array], [int tab[10]]) Each additional embedded macro call introduces other possible interesting quotations: car(active) =>ACT car([active]) =>ACT, IVE car([[active]]) =>active In the first case, the top level looks for the arguments of `car', and finds `active'. Because M4 evaluates its arguments before applying the macro, `active' is expanded, which results in: car(ACT, IVE) =>ACT In the second case, the top level gives `active' as first and only argument of `car', which results in: active =>ACT, IVE i.e., the argument is evaluated _after_ the macro that invokes it. In the third case, `car' receives `[active]', which results in: [active] =>active exactly as we already saw above. The example above, applied to a more realistic example, gives: car(int tab[10];) =>int tab10; car([int tab[10];]) =>int tab10; car([[int tab[10];]]) =>int tab[10]; Huh? The first case is easily understood, but why is the second wrong, and the third right? To understand that, you must know that after M4 expands a macro, the resulting text is immediately subjected to macro expansion and quote removal. This means that the quote removal occurs twice--first before the argument is passed to the `car' macro, and second after the `car' macro expands to the first argument. As the author of the Autoconf macro `car', you then consider it to be incorrect that your users have to double-quote the arguments of `car', so you "fix" your macro. Let's call it `qar' for quoted car: define([qar], [[$1]]) and check that `qar' is properly fixed: qar([int tab[10];]) =>int tab[10]; Ahhh! That's much better. But note what you've done: now that the result of `qar' is always a literal string, the only time a user can use nested macros is if she relies on an _unquoted_ macro call: qar(active) =>ACT qar([active]) =>active leaving no way for her to reproduce what she used to do with `car': car([active]) =>ACT, IVE Worse yet: she wants to use a macro that produces a set of `cpp' macros: define([my_includes], [#include ]) car([my_includes]) =>#include qar(my_includes) error-->EOF in argument list This macro, `qar', because it double quotes its arguments, forces its users to leave their macro calls unquoted, which is dangerous. Commas and other active symbols are interpreted by M4 before they are given to the macro, often not in the way the users expect. Also, because `qar' behaves differently from the other macros, it's an exception that should be avoided in Autoconf. 8.1.5 `changequote' is Evil --------------------------- The temptation is often high to bypass proper quotation, in particular when it's late at night. Then, many experienced Autoconf hackers finally surrender to the dark side of the force and use the ultimate weapon: `changequote'. The M4 builtin `changequote' belongs to a set of primitives that allow one to adjust the syntax of the language to adjust it to one's needs. For instance, by default M4 uses ``' and `'' as quotes, but in the context of shell programming (and actually of most programming languages), that's about the worst choice one can make: because of strings and back-quoted expressions in shell code (such as `'this'' and ``that`'), and because of literal characters in usual programming languages (as in `'0''), there are many unbalanced ``' and `''. Proper M4 quotation then becomes a nightmare, if not impossible. In order to make M4 useful in such a context, its designers have equipped it with `changequote', which makes it possible to choose another pair of quotes. M4sugar, M4sh, Autoconf, and Autotest all have chosen to use `[' and `]'. Not especially because they are unlikely characters, but _because they are characters unlikely to be unbalanced_. There are other magic primitives, such as `changecom' to specify what syntactic forms are comments (it is common to see `changecom()' when M4 is used to produce HTML pages), `changeword' and `changesyntax' to change other syntactic details (such as the character to denote the Nth argument, `$' by default, the parentheses around arguments, etc.). These primitives are really meant to make M4 more useful for specific domains: they should be considered like command line options: `--quotes', `--comments', `--words', and `--syntax'. Nevertheless, they are implemented as M4 builtins, as it makes M4 libraries self contained (no need for additional options). There lies the problem... The problem is that it is then tempting to use them in the middle of an M4 script, as opposed to its initialization. This, if not carefully thought out, can lead to disastrous effects: _you are changing the language in the middle of the execution_. Changing and restoring the syntax is often not enough: if you happened to invoke macros in between, these macros are lost, as the current syntax is probably not the one they were implemented with. 8.1.6 Quadrigraphs ------------------ When writing an Autoconf macro you may occasionally need to generate special characters that are difficult to express with the standard Autoconf quoting rules. For example, you may need to output the regular expression `[^[]', which matches any character other than `['. This expression contains unbalanced brackets so it cannot be put easily into an M4 macro. Additionally, there are a few m4sugar macros (such as `m4_split' and `m4_expand') which internally use special markers in addition to the regular quoting characters. If the arguments to these macros contain the literal strings `-=<{(' or `)}>=-', the macros might behave incorrectly. You can work around these problems by using one of the following "quadrigraphs": `@<:@' `[' `@:>@' `]' `@S|@' `$' `@%:@' `#' `@{:@' `(' `@:}@' `)' `@&t@' Expands to nothing. Quadrigraphs are replaced at a late stage of the translation process, after `m4' is run, so they do not get in the way of M4 quoting. For example, the string `^@<:@', independently of its quotation, appears as `^[' in the output. The empty quadrigraph can be used: - to mark trailing spaces explicitly Trailing spaces are smashed by `autom4te'. This is a feature. - to produce quadrigraphs and other strings reserved by m4sugar For instance `@<@&t@:@' produces `@<:@'. For a more contrived example: m4_define([a], [A])m4_define([b], [B])m4_define([c], [C])dnl m4_split([a )}>=- b -=<{( c]) =>[a], [], [B], [], [c] m4_split([a )}@&t@>=- b -=<@&t@{( c]) =>[a], [)}>=-], [b], [-=<{(], [c] - to escape _occurrences_ of forbidden patterns For instance you might want to mention `AC_FOO' in a comment, while still being sure that `autom4te' still catches unexpanded `AC_*'. Then write `AC@&t@_FOO'. The name `@&t@' was suggested by Paul Eggert: I should give some credit to the `@&t@' pun. The `&' is my own invention, but the `t' came from the source code of the ALGOL68C compiler, written by Steve Bourne (of Bourne shell fame), and which used `mt' to denote the empty string. In C, it would have looked like something like: char const mt[] = ""; but of course the source code was written in Algol 68. I don't know where he got `mt' from: it could have been his own invention, and I suppose it could have been a common pun around the Cambridge University computer lab at the time. 8.1.7 Dealing with unbalanced parentheses ----------------------------------------- One of the pitfalls of portable shell programming is that `case' statements require unbalanced parentheses (*note Limitations of Shell Builtins: case.). With syntax highlighting editors, the presence of unbalanced `)' can interfere with editors that perform syntax highlighting of macro contents based on finding the matching `('. Another concern is how much editing must be done when transferring code snippets between shell scripts and macro definitions. But most importantly, the presence of unbalanced parentheses can introduce expansion bugs. For an example, here is an underquoted attempt to use the macro `my_case', which happens to expand to a portable `case' statement: AC_DEFUN([my_case], [case $file_name in *.c) echo "C source code";; esac]) AS_IF(:, my_case) In the above example, the `AS_IF' call underquotes its arguments. As a result, the unbalanced `)' generated by the premature expansion of `my_case' results in expanding `AS_IF' with a truncated parameter, and the expansion is syntactically invalid: if :; then case $file_name in *.c fi echo "C source code";; esac) If nothing else, this should emphasize the importance of the quoting arguments to macro calls. On the other hand, there are several variations for defining `my_case' to be more robust, even when used without proper quoting, each with some benefits and some drawbacks. Creative literal shell comment AC_DEFUN([my_case], [case $file_name in #( *.c) echo "C source code";; esac]) This version provides balanced parentheses to several editors, and can be copied and pasted into a terminal as is. Unfortunately, it is still unbalanced as an Autoconf argument, since `#(' is an M4 comment that masks the normal properties of `('. Quadrigraph shell comment AC_DEFUN([my_case], [case $file_name in @%:@( *.c) echo "C source code";; esac]) This version provides balanced parentheses to even more editors, and can be used as a balanced Autoconf argument. Unfortunately, it requires some editing before it can be copied and pasted into a terminal, and the use of the quadrigraph `@%:@' for `#' reduces readability. Quoting just the parenthesis AC_DEFUN([my_case], [case $file_name in *.c[)] echo "C source code";; esac]) This version quotes the `)', so that it can be used as a balanced Autoconf argument. As written, this is not balanced to an editor, but it can be coupled with `[#(]' to meet that need, too. However, it still requires some edits before it can be copied and pasted into a terminal. Double-quoting the entire statement AC_DEFUN([my_case], [[case $file_name in #( *.c) echo "C source code";; esac]]) Since the entire macro is double-quoted, there is no problem with using this as an Autoconf argument; and since the double-quoting is over the entire statement, this code can be easily copied and pasted into a terminal. However, the double quoting prevents the expansion of any macros inside the case statement, which may cause its own set of problems. Using `AS_CASE' AC_DEFUN([my_case], [AS_CASE([$file_name], [*.c], [echo "C source code"])]) This version avoids the balancing issue altogether, by relying on `AS_CASE' (*note Common Shell Constructs::); it also allows for the expansion of `AC_REQUIRE' to occur prior to the entire case statement, rather than within a branch of the case statement that might not be taken. However, the abstraction comes with a penalty that it is no longer a quick copy, paste, and edit to get back to shell code. 8.1.8 Quotation Rule Of Thumb ----------------------------- To conclude, the quotation rule of thumb is: _One pair of quotes per pair of parentheses._ Never over-quote, never under-quote, in particular in the definition of macros. In the few places where the macros need to use brackets (usually in C program text or regular expressions), properly quote _the arguments_! It is common to read Autoconf programs with snippets like: AC_TRY_LINK( changequote(<<, >>)dnl <<#include #ifndef tzname /* For SGI. */ extern char *tzname[]; /* RS6000 and others reject char **tzname. */ #endif>>, changequote([, ])dnl [atoi (*tzname);], ac_cv_var_tzname=yes, ac_cv_var_tzname=no) which is incredibly useless since `AC_TRY_LINK' is _already_ double quoting, so you just need: AC_TRY_LINK( [#include #ifndef tzname /* For SGI. */ extern char *tzname[]; /* RS6000 and others reject char **tzname. */ #endif], [atoi (*tzname);], [ac_cv_var_tzname=yes], [ac_cv_var_tzname=no]) The M4-fluent reader might note that these two examples are rigorously equivalent, since M4 swallows both the `changequote(<<, >>)' and `<<' `>>' when it "collects" the arguments: these quotes are not part of the arguments! Simplified, the example above is just doing this: changequote(<<, >>)dnl <<[]>> changequote([, ])dnl instead of simply: [[]] With macros that do not double quote their arguments (which is the rule), double-quote the (risky) literals: AC_LINK_IFELSE([AC_LANG_PROGRAM( [[#include #ifndef tzname /* For SGI. */ extern char *tzname[]; /* RS6000 and others reject char **tzname. */ #endif]], [atoi (*tzname);])], [ac_cv_var_tzname=yes], [ac_cv_var_tzname=no]) Please note that the macro `AC_TRY_LINK' is obsolete, so you really should be using `AC_LINK_IFELSE' instead. *Note Quadrigraphs::, for what to do if you run into a hopeless case where quoting does not suffice. When you create a `configure' script using newly written macros, examine it carefully to check whether you need to add more quotes in your macros. If one or more words have disappeared in the M4 output, you need more quotes. When in doubt, quote. However, it's also possible to put on too many layers of quotes. If this happens, the resulting `configure' script may contain unexpanded macros. The `autoconf' program checks for this problem by looking for the string `AC_' in `configure'. However, this heuristic does not work in general: for example, it does not catch overquoting in `AC_DEFINE' descriptions. 8.2 Using `autom4te' ==================== The Autoconf suite, including M4sugar, M4sh, and Autotest, in addition to Autoconf per se, heavily rely on M4. All these different uses revealed common needs factored into a layer over M4: `autom4te'(1). `autom4te' is a preprocessor that is like `m4'. It supports M4 extensions designed for use in tools like Autoconf. ---------- Footnotes ---------- (1) Yet another great name from Lars J. Aas. 8.2.1 Invoking `autom4te' ------------------------- The command line arguments are modeled after M4's: autom4te OPTIONS FILES where the FILES are directly passed to `m4'. By default, GNU M4 is found during configuration, but the environment variable `M4' can be set to tell `autom4te' where to look. In addition to the regular expansion, it handles the replacement of the quadrigraphs (*note Quadrigraphs::), and of `__oline__', the current line in the output. It supports an extended syntax for the FILES: `FILE.m4f' This file is an M4 frozen file. Note that _all the previous files are ignored_. See the option `--melt' for the rationale. `FILE?' If found in the library path, the FILE is included for expansion, otherwise it is ignored instead of triggering a failure. Of course, it supports the Autoconf common subset of options: `--help' `-h' Print a summary of the command line options and exit. `--version' `-V' Print the version number of Autoconf and exit. `--verbose' `-v' Report processing steps. `--debug' `-d' Don't remove the temporary files and be even more verbose. `--include=DIR' `-I DIR' Also look for input files in DIR. Multiple invocations accumulate. `--output=FILE' `-o FILE' Save output (script or trace) to FILE. The file `-' stands for the standard output. As an extension of `m4', it includes the following options: `--warnings=CATEGORY' `-W CATEGORY' Report the warnings related to CATEGORY (which can actually be a comma separated list). *Note Reporting Messages::, macro `AC_DIAGNOSE', for a comprehensive list of categories. Special values include: `all' report all the warnings `none' report none `error' treats warnings as errors `no-CATEGORY' disable warnings falling into CATEGORY Warnings about `syntax' are enabled by default, and the environment variable `WARNINGS', a comma separated list of categories, is honored. `autom4te -W CATEGORY' actually behaves as if you had run: autom4te --warnings=syntax,$WARNINGS,CATEGORY For example, if you want to disable defaults and `WARNINGS' of `autom4te', but enable the warnings about obsolete constructs, you would use `-W none,obsolete'. `autom4te' displays a back trace for errors, but not for warnings; if you want them, just pass `-W error'. `--melt' `-M' Do not use frozen files. Any argument `FILE.m4f' is replaced by `FILE.m4'. This helps tracing the macros which are executed only when the files are frozen, typically `m4_define'. For instance, running: autom4te --melt 1.m4 2.m4f 3.m4 4.m4f input.m4 is roughly equivalent to running: m4 1.m4 2.m4 3.m4 4.m4 input.m4 while autom4te 1.m4 2.m4f 3.m4 4.m4f input.m4 is equivalent to: m4 --reload-state=4.m4f input.m4 `--freeze' `-F' Produce a frozen state file. `autom4te' freezing is stricter than M4's: it must produce no warnings, and no output other than empty lines (a line with white space is _not_ empty) and comments (starting with `#'). Unlike `m4''s similarly-named option, this option takes no argument: autom4te 1.m4 2.m4 3.m4 --freeze --output=3.m4f corresponds to m4 1.m4 2.m4 3.m4 --freeze-state=3.m4f `--mode=OCTAL-MODE' `-m OCTAL-MODE' Set the mode of the non-traces output to OCTAL-MODE; by default `0666'. As another additional feature over `m4', `autom4te' caches its results. GNU M4 is able to produce a regular output and traces at the same time. Traces are heavily used in the GNU Build System: `autoheader' uses them to build `config.h.in', `autoreconf' to determine what GNU Build System components are used, `automake' to "parse" `configure.ac' etc. To avoid recomputation, traces are cached while performing regular expansion, and conversely. This cache is (actually, the caches are) stored in the directory `autom4te.cache'. _It can safely be removed_ at any moment (especially if for some reason `autom4te' considers it trashed). `--cache=DIRECTORY' `-C DIRECTORY' Specify the name of the directory where the result should be cached. Passing an empty value disables caching. Be sure to pass a relative file name, as for the time being, global caches are not supported. `--no-cache' Don't cache the results. `--force' `-f' If a cache is used, consider it obsolete (but update it anyway). Because traces are so important to the GNU Build System, `autom4te' provides high level tracing features as compared to M4, and helps exploiting the cache: `--trace=MACRO[:FORMAT]' `-t MACRO[:FORMAT]' Trace the invocations of MACRO according to the FORMAT. Multiple `--trace' arguments can be used to list several macros. Multiple `--trace' arguments for a single macro are not cumulative; instead, you should just make FORMAT as long as needed. The FORMAT is a regular string, with newlines if desired, and several special escape codes. It defaults to `$f:$l:$n:$%'. It can use the following special escapes: `$$' The character `$'. `$f' The file name from which MACRO is called. `$l' The line number from which MACRO is called. `$d' The depth of the MACRO call. This is an M4 technical detail that you probably don't want to know about. `$n' The name of the MACRO. `$NUM' The NUMth argument of the call to MACRO. `$@' `$SEP@' `${SEPARATOR}@' All the arguments passed to MACRO, separated by the character SEP or the string SEPARATOR (`,' by default). Each argument is quoted, i.e., enclosed in a pair of square brackets. `$*' `$SEP*' `${SEPARATOR}*' As above, but the arguments are not quoted. `$%' `$SEP%' `${SEPARATOR}%' As above, but the arguments are not quoted, all new line characters in the arguments are smashed, and the default separator is `:'. The escape `$%' produces single-line trace outputs (unless you put newlines in the `separator'), while `$@' and `$*' do not. *Note autoconf Invocation::, for examples of trace uses. `--preselect=MACRO' `-p MACRO' Cache the traces of MACRO, but do not enable traces. This is especially important to save CPU cycles in the future. For instance, when invoked, `autoconf' preselects all the macros that `autoheader', `automake', `autoreconf', etc., trace, so that running `m4' is not needed to trace them: the cache suffices. This results in a huge speed-up. Finally, `autom4te' introduces the concept of "Autom4te libraries". They consists in a powerful yet extremely simple feature: sets of combined command line arguments: `--language=LANGUAGE' `-l LANGUAGE' Use the LANGUAGE Autom4te library. Current languages include: `M4sugar' create M4sugar output. `M4sh' create M4sh executable shell scripts. `Autotest' create Autotest executable test suites. `Autoconf-without-aclocal-m4' create Autoconf executable configure scripts without reading `aclocal.m4'. `Autoconf' create Autoconf executable configure scripts. This language inherits all the characteristics of `Autoconf-without-aclocal-m4' and additionally reads `aclocal.m4'. `--prepend-include=DIR' `-B DIR' Prepend directory DIR to the search path. This is used to include the language-specific files before any third-party macros. As an example, if Autoconf is installed in its default location, `/usr/local', the command `autom4te -l m4sugar foo.m4' is strictly equivalent to the command: autom4te --prepend-include /usr/local/share/autoconf \ m4sugar/m4sugar.m4f --warnings syntax foo.m4 Recursive expansion applies here: the command `autom4te -l m4sh foo.m4' is the same as `autom4te --language M4sugar m4sugar/m4sh.m4f foo.m4', i.e.: autom4te --prepend-include /usr/local/share/autoconf \ m4sugar/m4sugar.m4f m4sugar/m4sh.m4f --mode 777 foo.m4 The definition of the languages is stored in `autom4te.cfg'. 8.2.2 Customizing `autom4te' ---------------------------- One can customize `autom4te' via `~/.autom4te.cfg' (i.e., as found in the user home directory), and `./.autom4te.cfg' (i.e., as found in the directory from which `autom4te' is run). The order is first reading `autom4te.cfg', then `~/.autom4te.cfg', then `./.autom4te.cfg', and finally the command line arguments. In these text files, comments are introduced with `#', and empty lines are ignored. Customization is performed on a per-language basis, wrapped in between a `begin-language: "LANGUAGE"', `end-language: "LANGUAGE"' pair. Customizing a language stands for appending options (*note autom4te Invocation::) to the current definition of the language. Options, and more generally arguments, are introduced by `args: ARGUMENTS'. You may use the traditional shell syntax to quote the ARGUMENTS. As an example, to disable Autoconf caches (`autom4te.cache') globally, include the following lines in `~/.autom4te.cfg': ## ------------------ ## ## User Preferences. ## ## ------------------ ## begin-language: "Autoconf-without-aclocal-m4" args: --no-cache end-language: "Autoconf-without-aclocal-m4" 8.3 Programming in M4sugar ========================== M4 by itself provides only a small, but sufficient, set of all-purpose macros. M4sugar introduces additional generic macros. Its name was coined by Lars J. Aas: "Readability And Greater Understanding Stands 4 M4sugar". M4sugar reserves the macro namespace `^_m4_' for internal use, and the macro namespace `^m4_' for M4sugar macros. You should not define your own macros into these namespaces. 8.3.1 Redefined M4 Macros ------------------------- With a few exceptions, all the M4 native macros are moved in the `m4_' pseudo-namespace, e.g., M4sugar renames `define' as `m4_define' etc. The list of macros unchanged from M4, except for their name, is: - m4_builtin - m4_changecom - m4_changequote - m4_debugfile - m4_debugmode - m4_decr - m4_define - m4_divnum - m4_errprint - m4_esyscmd - m4_eval - m4_format - m4_ifdef - m4_incr - m4_index - m4_indir - m4_len - m4_pushdef - m4_shift - m4_substr - m4_syscmd - m4_sysval - m4_traceoff - m4_traceon - m4_translit Some M4 macros are redefined, and are slightly incompatible with their native equivalent. -- Macro: __file__ -- Macro: __line__ All M4 macros starting with `__' retain their original name: for example, no `m4__file__' is defined. -- Macro: __oline__ This is not technically a macro, but a feature of Autom4te. The sequence `__oline__' can be used similarly to the other m4sugar location macros, but rather than expanding to the location of the input file, it is translated to the line number where it appears in the output file after all other M4 expansions. -- Macro: dnl This macro kept its original name: no `m4_dnl' is defined. -- Macro: m4_bpatsubst (STRING, REGEXP, [REPLACEMENT]) This macro corresponds to `patsubst'. The name `m4_patsubst' is kept for future versions of M4sugar, once GNU M4 2.0 is released and supports extended regular expression syntax. -- Macro: m4_bregexp (STRING, REGEXP, [REPLACEMENT]) This macro corresponds to `regexp'. The name `m4_regexp' is kept for future versions of M4sugar, once GNU M4 2.0 is released and supports extended regular expression syntax. -- Macro: m4_copy (SOURCE, DEST) -- Macro: m4_copy_force (SOURCE, DEST) -- Macro: m4_rename (SOURCE, DEST) -- Macro: m4_rename_force (SOURCE, DEST) These macros aren't directly builtins, but are closely related to `m4_pushdef' and `m4_defn'. `m4_copy' and `m4_rename' ensure that DEST is undefined, while `m4_copy_force' and `m4_rename_force' overwrite any existing definition. All four macros then proceed to copy the entire pushdef stack of definitions of SOURCE over to DEST. `m4_copy' and `m4_copy_force' preserve the source (including in the special case where SOURCE is undefined), while `m4_rename' and `m4_rename_force' undefine the original macro name (making it an error to rename an undefined SOURCE). Note that attempting to invoke a renamed macro might not work, since the macro may have a dependence on helper macros accessed via composition of `$0' but that were not also renamed; likewise, other macros may have a hard-coded dependence on SOURCE and could break if SOURCE has been deleted. On the other hand, it is always safe to rename a macro to temporarily move it out of the way, then rename it back later to restore original semantics. -- Macro: m4_defn (MACRO...) This macro fails if MACRO is not defined, even when using older versions of M4 that did not warn. See `m4_undefine'. Unfortunately, in order to support these older versions of M4, there are some situations involving unbalanced quotes where concatenating multiple macros together will work in newer M4 but not in m4sugar; use quadrigraphs to work around this. -- Macro: m4_divert (DIVERSION) M4sugar relies heavily on diversions, so rather than behaving as a primitive, `m4_divert' behaves like: m4_divert_pop()m4_divert_push([DIVERSION]) *Note Diversion support::, for more details about the use of the diversion stack. -- Macro: m4_dumpdef (NAME...) -- Macro: m4_dumpdefs (NAME...) `m4_dumpdef' is like the M4 builtin, except that this version requires at least one argument, output always goes to standard error rather than the current debug file, no sorting is done on multiple arguments, and an error is issued if any NAME is undefined. `m4_dumpdefs' is a convenience macro that calls `m4_dumpdef' for all of the `m4_pushdef' stack of definitions, starting with the current, and silently does nothing if NAME is undefined. Unfortunately, due to a limitation in M4 1.4.x, any macro defined as a builtin is output as the empty string. This behavior is rectified by using M4 1.6 or newer. However, this behavior difference means that `m4_dumpdef' should only be used while developing m4sugar macros, and never in the final published form of a macro. -- Macro: m4_esyscmd_s (COMMAND) Like `m4_esyscmd', this macro expands to the result of running COMMAND in a shell. The difference is that any trailing newlines are removed, so that the output behaves more like shell command substitution. -- Macro: m4_exit (EXIT-STATUS) This macro corresponds to `m4exit'. -- Macro: m4_if (COMMENT) -- Macro: m4_if (STRING-1, STRING-2, EQUAL, [NOT-EQUAL]) -- Macro: m4_if (STRING-1, STRING-2, EQUAL-1, STRING-3, STRING-4, EQUAL-2, ..., [NOT-EQUAL]) This macro corresponds to `ifelse'. STRING-1 and STRING-2 are compared literally, so usually one of the two arguments is passed unquoted. *Note Conditional constructs::, for more conditional idioms. -- Macro: m4_include (FILE) -- Macro: m4_sinclude (FILE) Like the M4 builtins, but warn against multiple inclusions of FILE. -- Macro: m4_mkstemp (TEMPLATE) -- Macro: m4_maketemp (TEMPLATE) Posix requires `maketemp' to replace the trailing `X' characters in TEMPLATE with the process id, without regards to the existence of a file by that name, but this a security hole. When this was pointed out to the Posix folks, they agreed to invent a new macro `mkstemp' that always creates a uniquely named file, but not all versions of GNU M4 support the new macro. In M4sugar, `m4_maketemp' and `m4_mkstemp' are synonyms for each other, and both have the secure semantics regardless of which macro the underlying M4 provides. -- Macro: m4_popdef (MACRO...) This macro fails if MACRO is not defined, even when using older versions of M4 that did not warn. See `m4_undefine'. -- Macro: m4_undefine (MACRO...) This macro fails if MACRO is not defined, even when using older versions of M4 that did not warn. Use m4_ifdef([MACRO], [m4_undefine([MACRO])]) if you are not sure whether MACRO is defined. -- Macro: m4_undivert (DIVERSION...) Unlike the M4 builtin, at least one DIVERSION must be specified. Also, since the M4sugar diversion stack prefers named diversions, the use of `m4_undivert' to include files is risky. *Note Diversion support::, for more details about the use of the diversion stack. -- Macro: m4_wrap (TEXT) -- Macro: m4_wrap_lifo (TEXT) These macros correspond to `m4wrap'. Posix requires arguments of multiple wrap calls to be reprocessed at EOF in the same order as the original calls (first-in, first-out). GNU M4 versions through 1.4.10, however, reprocess them in reverse order (last-in, first-out). Both orders are useful, therefore, you can rely on `m4_wrap' to provide FIFO semantics and `m4_wrap_lifo' for LIFO semantics, regardless of the underlying GNU M4 version. Unlike the GNU M4 builtin, these macros only recognize one argument, and avoid token pasting between consecutive invocations. On the other hand, nested calls to `m4_wrap' from within wrapped text work just as in the builtin. 8.3.2 Diagnostic messages from M4sugar -------------------------------------- When macros statically diagnose abnormal situations, benign or fatal, they should report them using these macros. For issuing dynamic issues, i.e., when `configure' is run, see *note Printing Messages::. -- Macro: m4_assert (EXPRESSION, [EXIT-STATUS = `1']) Assert that the arithmetic EXPRESSION evaluates to non-zero. Otherwise, issue a fatal error, and exit `autom4te' with EXIT-STATUS. -- Macro: m4_errprintn (MESSAGE) Similar to the builtin `m4_errprint', except that a newline is guaranteed after MESSAGE. -- Macro: m4_fatal (MESSAGE) Report a severe error MESSAGE prefixed with the current location, and have `autom4te' die. -- Macro: m4_location Useful as a prefix in a message line. Short for: __file__:__line__ -- Macro: m4_warn (CATEGORY, MESSAGE) Report MESSAGE as a warning (or as an error if requested by the user) if warnings of the CATEGORY are turned on. If the message is emitted, it is prefixed with the current location, and followed by a call trace of all macros defined via `AC_DEFUN' used to get to the current expansion. You are encouraged to use standard categories, which currently include: `all' messages that don't fall into one of the following categories. Use of an empty CATEGORY is equivalent. `cross' related to cross compilation issues. `obsolete' use of an obsolete construct. `syntax' dubious syntactic constructs, incorrectly ordered macro calls. 8.3.3 Diversion support ----------------------- M4sugar makes heavy use of diversions, because it is often the case that text that must appear early in the output is not discovered until late in the input. Additionally, some of the topological sorting algorithms used in resolving macro dependencies use diversions. However, most macros should not need to change diversions directly, but rather rely on higher-level M4sugar macros to manage diversions transparently. In the rare case that it is necessary to write a macro that explicitly outputs text to a different diversion, it is important to be aware of an M4 limitation regarding diversions: text only goes to a diversion if it is not part of argument collection. Therefore, any macro that changes the current diversion cannot be used as an unquoted argument to another macro, but must be expanded at the top level. The macro `m4_expand' will diagnose any attempt to change diversions, since it is generally useful only as an argument to another macro. The following example shows what happens when diversion manipulation is attempted within macro arguments: m4_do([normal text] m4_divert_push([KILL])unwanted[]m4_divert_pop([KILL]) [m4_divert_push([KILL])discarded[]m4_divert_pop([KILL])])dnl =>normal text =>unwanted Notice that the unquoted text `unwanted' is output, even though it was processed while the current diversion was `KILL', because it was collected as part of the argument to `m4_do'. However, the text `discarded' disappeared as desired, because the diversion changes were single-quoted, and were not expanded until the top-level rescan of the output of `m4_do'. To make diversion management easier, M4sugar uses the concept of named diversions. Rather than using diversion numbers directly, it is nicer to associate a name with each diversion; the diversion number associated with a particular diversion name is an implementation detail, so you should only use diversion names. In general, you should not output text to a named diversion until after calling the appropriate initialization routine for your language (`m4_init', `AS_INIT', `AT_INIT', ...), although there are some exceptions documented below. M4sugar defines two named diversions. `KILL' Text written to this diversion is discarded. This is the default diversion once M4sugar is initialized. `GROW' This diversion is used behind the scenes by topological sorting macros, such as `AC_REQUIRE'. M4sh adds several more named diversions. `BINSH' This diversion is reserved for the `#!' interpreter line. `HEADER-REVISION' This diversion holds text from `AC_REVISION'. `HEADER-COMMENT' This diversion holds comments about the purpose of a file. `HEADER-COPYRIGHT' This diversion is managed by `AC_COPYRIGHT'. `M4SH-SANITIZE' This diversion contains M4sh sanitization code, used to ensure M4sh is executing in a reasonable shell environment. `M4SH-INIT' This diversion contains M4sh initialization code, initializing variables that are required by other M4sh macros. `BODY' This diversion contains the body of the shell code, and is the default diversion once M4sh is initialized. Autotest inherits diversions from M4sh, and changes the default diversion from `BODY' back to `KILL'. It also adds several more named diversions, with the following subset designed for developer use. `PREPARE_TESTS' This diversion contains initialization sequences which are executed after `atconfig' and `atlocal', and after all command line arguments have been parsed, but prior to running any tests. It can be used to set up state that is required across all tests. This diversion will work even before `AT_INIT'. For now, the named diversions of Autoconf and Autoheader, and the remaining diversions of Autotest, are not documented. In other words, intentionally outputting text into an undocumented diversion is subject to breakage in a future release of Autoconf. -- Macro: m4_cleardivert (DIVERSION...) Permanently discard any text that has been diverted into DIVERSION. -- Macro: m4_divert_once (DIVERSION, [CONTENT]) Similar to `m4_divert_text', except that CONTENT is only output to DIVERSION if this is the first time that `m4_divert_once' has been called with its particular arguments. -- Macro: m4_divert_pop ([DIVERSION]) If provided, check that the current diversion is indeed DIVERSION. Then change to the diversion located earlier on the stack, giving an error if an attempt is made to pop beyond the initial m4sugar diversion of `KILL'. -- Macro: m4_divert_push (DIVERSION) Remember the former diversion on the diversion stack, and output subsequent text into DIVERSION. M4sugar maintains a diversion stack, and issues an error if there is not a matching pop for every push. -- Macro: m4_divert_text (DIVERSION, [CONTENT]) Output CONTENT and a newline into DIVERSION, without affecting the current diversion. Shorthand for: m4_divert_push([DIVERSION])CONTENT m4_divert_pop([DIVERSION])dnl -- Macro: m4_init Initialize the M4sugar environment, setting up the default named diversion to be `KILL'. 8.3.4 Conditional constructs ---------------------------- The following macros provide additional conditional constructs as convenience wrappers around `m4_if'. -- Macro: m4_bmatch (STRING, REGEX-1, VALUE-1, [REGEX-2], [VALUE-2], ..., [DEFAULT]) The string STRING is repeatedly compared against a series of REGEX arguments; if a match is found, the expansion is the corresponding VALUE, otherwise, the macro moves on to the next REGEX. If no REGEX match, then the result is the optional DEFAULT, or nothing. -- Macro: m4_bpatsubsts (STRING, REGEX-1, SUBST-1, [REGEX-2], [SUBST-2], ...) The string STRING is altered by REGEX-1 and SUBST-1, as if by: m4_bpatsubst([[STRING]], [REGEX], [SUBST]) The result of the substitution is then passed through the next set of REGEX and SUBST, and so forth. An empty SUBST implies deletion of any matched portions in the current string. Note that this macro over-quotes STRING; this behavior is intentional, so that the result of each step of the recursion remains as a quoted string. However, it means that anchors (`^' and `$' in the REGEX will line up with the extra quotations, and not the characters of the original string. The overquoting is removed after the final substitution. -- Macro: m4_case (STRING, VALUE-1, IF-VALUE-1, [VALUE-2], [IF-VALUE-2], ..., [DEFAULT]) Test STRING against multiple VALUE possibilities, resulting in the first IF-VALUE for a match, or in the optional DEFAULT. This is shorthand for: m4_if([STRING], [VALUE-1], [IF-VALUE-1], [STRING], [VALUE-2], [IF-VALUE-2], ..., [DEFAULT]) -- Macro: m4_cond (TEST-1, VALUE-1, IF-VALUE-1, [TEST-2], [VALUE-2], [IF-VALUE-2], ..., [DEFAULT]) This macro was introduced in Autoconf 2.62. Similar to `m4_if', except that each TEST is expanded only when it is encountered. This is useful for short-circuiting expensive tests; while `m4_if' requires all its strings to be expanded up front before doing comparisons, `m4_cond' only expands a TEST when all earlier tests have failed. For an example, these two sequences give the same result, but in the case where `$1' does not contain a backslash, the `m4_cond' version only expands `m4_index' once, instead of five times, for faster computation if this is a common case for `$1'. Notice that every third argument is unquoted for `m4_if', and quoted for `m4_cond': m4_if(m4_index([$1], [\]), [-1], [$2], m4_eval(m4_index([$1], [\\]) >= 0), [1], [$2], m4_eval(m4_index([$1], [\$]) >= 0), [1], [$2], m4_eval(m4_index([$1], [\`]) >= 0), [1], [$3], m4_eval(m4_index([$1], [\"]) >= 0), [1], [$3], [$2]) m4_cond([m4_index([$1], [\])], [-1], [$2], [m4_eval(m4_index([$1], [\\]) >= 0)], [1], [$2], [m4_eval(m4_index([$1], [\$]) >= 0)], [1], [$2], [m4_eval(m4_index([$1], [\`]) >= 0)], [1], [$3], [m4_eval(m4_index([$1], [\"]) >= 0)], [1], [$3], [$2]) -- Macro: m4_default (EXPR-1, EXPR-2) -- Macro: m4_default_quoted (EXPR-1, EXPR-2) -- Macro: m4_default_nblank (EXPR-1, [EXPR-2]) -- Macro: m4_default_nblank_quoted (EXPR-1, [EXPR-2]) If EXPR-1 contains text, use it. Otherwise, select EXPR-2. `m4_default' expands the result, while `m4_default_quoted' does not. Useful for providing a fixed default if the expression that results in EXPR-1 would otherwise be empty. The difference between `m4_default' and `m4_default_nblank' is whether an argument consisting of just blanks (space, tab, newline) is significant. When using the expanding versions, note that an argument may contain text but still expand to an empty string. m4_define([active], [ACTIVE])dnl m4_define([empty], [])dnl m4_define([demo1], [m4_default([$1], [$2])])dnl m4_define([demo2], [m4_default_quoted([$1], [$2])])dnl m4_define([demo3], [m4_default_nblank([$1], [$2])])dnl m4_define([demo4], [m4_default_nblank_quoted([$1], [$2])])dnl demo1([active], [default]) =>ACTIVE demo1([], [active]) =>ACTIVE demo1([empty], [text]) => -demo1([ ], [active])- =>- - demo2([active], [default]) =>active demo2([], [active]) =>active demo2([empty], [text]) =>empty -demo2([ ], [active])- =>- - demo3([active], [default]) =>ACTIVE demo3([], [active]) =>ACTIVE demo3([empty], [text]) => -demo3([ ], [active])- =>-ACTIVE- demo4([active], [default]) =>active demo4([], [active]) =>active demo4([empty], [text]) =>empty -demo4([ ], [active])- =>-active- -- Macro: m4_ifblank (COND, [IF-BLANK], [IF-TEXT]) -- Macro: m4_ifnblank (COND, [IF-TEXT], [IF-BLANK]) If COND is empty or consists only of blanks (space, tab, newline), then expand IF-BLANK; otherwise, expand IF-TEXT. Two variants exist, in order to make it easier to select the correct logical sense when using only two parameters. Note that this is more efficient than the equivalent behavior of: m4_ifval(m4_normalize([COND]), IF-TEXT, IF-COND) -- Macro: m4_ifndef (MACRO, IF-NOT-DEFINED, [IF-DEFINED]) This is shorthand for: m4_ifdef([MACRO], [IF-DEFINED], [IF-NOT-DEFINED]) -- Macro: m4_ifset (MACRO, [IF-TRUE], [IF-FALSE]) If MACRO is undefined, or is defined as the empty string, expand to IF-FALSE. Otherwise, expands to IF-TRUE. Similar to: m4_ifval(m4_defn([MACRO]), [IF-TRUE], [IF-FALSE]) except that it is not an error if MACRO is undefined. -- Macro: m4_ifval (COND, [IF-TRUE], [IF-FALSE]) Expands to IF-TRUE if COND is not empty, otherwise to IF-FALSE. This is shorthand for: m4_if([COND], [], [IF-TRUE], [IF-FALSE]) -- Macro: m4_ifvaln (COND, [IF-TRUE], [IF-FALSE]) Similar to `m4_ifval', except guarantee that a newline is present after any non-empty expansion. Often followed by `dnl'. -- Macro: m4_n (TEXT) Expand to TEXT, and add a newline if TEXT is not empty. Often followed by `dnl'. 8.3.5 Looping constructs ------------------------ The following macros are useful in implementing recursive algorithms in M4, including loop operations. An M4 list is formed by quoting a list of quoted elements; generally the lists are comma-separated, although `m4_foreach_w' is whitespace-separated. For example, the list `[[a], [b,c]]' contains two elements: `[a]' and `[b,c]'. It is common to see lists with unquoted elements when those elements are not likely to be macro names, as in `[fputc_unlocked, fgetc_unlocked]'. Although not generally recommended, it is possible for quoted lists to have side effects; all side effects are expanded only once, and prior to visiting any list element. On the other hand, the fact that unquoted macros are expanded exactly once means that macros without side effects can be used to generate lists. For example, m4_foreach([i], [[1], [2], [3]m4_errprintn([hi])], [i]) error-->hi =>123 m4_define([list], [[1], [2], [3]]) => m4_foreach([i], [list], [i]) =>123 -- Macro: m4_argn (N, [ARG]...) Extracts argument N (larger than 0) from the remaining arguments. If there are too few arguments, the empty string is used. For any N besides 1, this is more efficient than the similar `m4_car(m4_shiftn([N], [], [ARG...]))'. -- Macro: m4_car (ARG...) Expands to the quoted first ARG. Can be used with `m4_cdr' to recursively iterate through a list. Generally, when using quoted lists of quoted elements, `m4_car' should be called without any extra quotes. -- Macro: m4_cdr (ARG...) Expands to a quoted list of all but the first ARG, or the empty string if there was only one argument. Generally, when using quoted lists of quoted elements, `m4_cdr' should be called without any extra quotes. For example, this is a simple implementation of `m4_map'; note how each iteration checks for the end of recursion, then merely applies the first argument to the first element of the list, then repeats with the rest of the list. (The actual implementation in M4sugar is a bit more involved, to gain some speed and share code with `m4_map_sep', and also to avoid expanding side effects in `$2' twice). m4_define([m4_map], [m4_ifval([$2], [m4_apply([$1], m4_car($2))[]$0([$1], m4_cdr($2))])])dnl m4_map([ m4_eval], [[[1]], [[1+1]], [[10],[16]]]) => 1 2 a -- Macro: m4_for (VAR, FIRST, LAST, [STEP], EXPRESSION) Loop over the numeric values between FIRST and LAST including bounds by increments of STEP. For each iteration, expand EXPRESSION with the numeric value assigned to VAR. If STEP is omitted, it defaults to `1' or `-1' depending on the order of the limits. If given, STEP has to match this order. The number of iterations is determined independently from definition of VAR; iteration cannot be short-circuited or lengthened by modifying VAR from within EXPRESSION. -- Macro: m4_foreach (VAR, LIST, EXPRESSION) Loop over the comma-separated M4 list LIST, assigning each value to VAR, and expand EXPRESSION. The following example outputs two lines: m4_foreach([myvar], [[foo], [bar, baz]], [echo myvar ])dnl =>echo foo =>echo bar, baz Note that for some forms of EXPRESSION, it may be faster to use `m4_map_args'. -- Macro: m4_foreach_w (VAR, LIST, EXPRESSION) Loop over the white-space-separated list LIST, assigning each value to VAR, and expand EXPRESSION. If VAR is only referenced once in EXPRESSION, it is more efficient to use `m4_map_args_w'. The deprecated macro `AC_FOREACH' is an alias of `m4_foreach_w'. -- Macro: m4_map (MACRO, LIST) -- Macro: m4_mapall (MACRO, LIST) -- Macro: m4_map_sep (MACRO, SEPARATOR, LIST) -- Macro: m4_mapall_sep (MACRO, SEPARATOR, LIST) Loop over the comma separated quoted list of argument descriptions in LIST, and invoke MACRO with the arguments. An argument description is in turn a comma-separated quoted list of quoted elements, suitable for `m4_apply'. The macros `m4_map' and `m4_map_sep' ignore empty argument descriptions, while `m4_mapall' and `m4_mapall_sep' invoke MACRO with no arguments. The macros `m4_map_sep' and `m4_mapall_sep' additionally expand SEPARATOR between invocations of MACRO. Note that SEPARATOR is expanded, unlike in `m4_join'. When separating output with commas, this means that the map result can be used as a series of arguments, by using a single-quoted comma as SEPARATOR, or as a single string, by using a double-quoted comma. m4_map([m4_count], []) => m4_map([ m4_count], [[], [[1]], [[1], [2]]]) => 1 2 m4_mapall([ m4_count], [[], [[1]], [[1], [2]]]) => 0 1 2 m4_map_sep([m4_eval], [,], [[[1+2]], [[10], [16]]]) =>3,a m4_map_sep([m4_echo], [,], [[[a]], [[b]]]) =>a,b m4_count(m4_map_sep([m4_echo], [,], [[[a]], [[b]]])) =>2 m4_map_sep([m4_echo], [[,]], [[[a]], [[b]]]) =>a,b m4_count(m4_map_sep([m4_echo], [[,]], [[[a]], [[b]]])) =>1 -- Macro: m4_map_args (MACRO, ARG...) Repeatedly invoke MACRO with each successive ARG as its only argument. In the following example, three solutions are presented with the same expansion; the solution using `m4_map_args' is the most efficient. m4_define([active], [ACTIVE])dnl m4_foreach([var], [[plain], [active]], [ m4_echo(m4_defn([var]))]) => plain active m4_map([ m4_echo], [[[plain]], [[active]]]) => plain active m4_map_args([ m4_echo], [plain], [active]) => plain active In cases where it is useful to operate on additional parameters besides the list elements, the macro `m4_curry' can be used in MACRO to supply the argument currying necessary to generate the desired argument list. In the following example, `list_add_n' is more efficient than `list_add_x'. On the other hand, using `m4_map_args_sep' can be even more efficient. m4_define([list], [[1], [2], [3]])dnl m4_define([add], [m4_eval(([$1]) + ([$2]))])dnl dnl list_add_n(N, ARG...) dnl Output a list consisting of each ARG added to N m4_define([list_add_n], [m4_shift(m4_map_args([,m4_curry([add], [$1])], m4_shift($@)))])dnl list_add_n([1], list) =>2,3,4 list_add_n([2], list) =>3,4,5 m4_define([list_add_x], [m4_shift(m4_foreach([var], m4_dquote(m4_shift($@)), [,add([$1],m4_defn([var]))]))])dnl list_add_x([1], list) =>2,3,4 -- Macro: m4_map_args_pair (MACRO, [MACRO-END = `macro'], ARG...) For every pair of arguments ARG, invoke MACRO with two arguments. If there is an odd number of arguments, invoke MACRO-END, which defaults to MACRO, with the remaining argument. m4_map_args_pair([, m4_reverse], [], [1], [2], [3]) =>, 2, 1, 3 m4_map_args_pair([, m4_reverse], [, m4_dquote], [1], [2], [3]) =>, 2, 1, [3] m4_map_args_pair([, m4_reverse], [, m4_dquote], [1], [2], [3], [4]) =>, 2, 1, 4, 3 -- Macro: m4_map_args_sep ([PRE], [POST], [SEP], ARG...) Expand the sequence `PRE[ARG]POST' for each argument, additionally expanding SEP between arguments. One common use of this macro is constructing a macro call, where the opening and closing parentheses are split between PRE and POST; in particular, `m4_map_args([MACRO], [ARG])' is equivalent to `m4_map_args_sep([MACRO(], [)], [], [ARG])'. This macro provides the most efficient means for iterating over an arbitrary list of arguments, particularly when repeatedly constructing a macro call with more arguments than ARG. -- Macro: m4_map_args_w (STRING, [PRE], [POST], [SEP]) Expand the sequence `PRE[word]POST' for each word in the whitespace-separated STRING, additionally expanding SEP between words. This macro provides the most efficient means for iterating over a whitespace-separated string. In particular, `m4_map_args_w([STRING], [ACTION(], [)])' is more efficient than `m4_foreach_w([var], [STRING], [ACTION(m4_defn([var]))])'. -- Macro: m4_shiftn (COUNT, ...) -- Macro: m4_shift2 (...) -- Macro: m4_shift3 (...) `m4_shiftn' performs COUNT iterations of `m4_shift', along with validation that enough arguments were passed in to match the shift count, and that the count is positive. `m4_shift2' and `m4_shift3' are specializations of `m4_shiftn', introduced in Autoconf 2.62, and are more efficient for two and three shifts, respectively. -- Macro: m4_stack_foreach (MACRO, ACTION) -- Macro: m4_stack_foreach_lifo (MACRO, ACTION) For each of the `m4_pushdef' definitions of MACRO, expand ACTION with the single argument of a definition of MACRO. `m4_stack_foreach' starts with the oldest definition, while `m4_stack_foreach_lifo' starts with the current definition. ACTION should not push or pop definitions of MACRO, nor is there any guarantee that the current definition of MACRO matches the argument that was passed to ACTION. The macro `m4_curry' can be used if ACTION needs more than one argument, although in that case it is more efficient to use M4_STACK_FOREACH_SEP. Due to technical limitations, there are a few low-level m4sugar functions, such as `m4_pushdef', that cannot be used as the MACRO argument. m4_pushdef([a], [1])m4_pushdef([a], [2])dnl m4_stack_foreach([a], [ m4_incr]) => 2 3 m4_stack_foreach_lifo([a], [ m4_curry([m4_substr], [abcd])]) => cd bcd -- Macro: m4_stack_foreach_sep (MACRO, [PRE], [POST], [SEP]) -- Macro: m4_stack_foreach_sep_lifo (MACRO, [PRE], [POST], [SEP]) Expand the sequence `PRE[definition]POST' for each `m4_pushdef' definition of MACRO, additionally expanding SEP between definitions. `m4_stack_foreach_sep' visits the oldest definition first, while `m4_stack_foreach_sep_lifo' visits the current definition first. This macro provides the most efficient means for iterating over a pushdef stack. In particular, `m4_stack_foreach([MACRO], [ACTION])' is short for `m4_stack_foreach_sep([MACRO], [ACTION(], [)])'. 8.3.6 Evaluation Macros ----------------------- The following macros give some control over the order of the evaluation by adding or removing levels of quotes. -- Macro: m4_apply (MACRO, LIST) Apply the elements of the quoted, comma-separated LIST as the arguments to MACRO. If LIST is empty, invoke MACRO without arguments. Note the difference between `m4_indir', which expects its first argument to be a macro name but can use names that are otherwise invalid, and `m4_apply', where MACRO can contain other text, but must end in a valid macro name. m4_apply([m4_count], []) =>0 m4_apply([m4_count], [[]]) =>1 m4_apply([m4_count], [[1], [2]]) =>2 m4_apply([m4_join], [[|], [1], [2]]) =>1|2 -- Macro: m4_count (ARG, ...) This macro returns the decimal count of the number of arguments it was passed. -- Macro: m4_curry (MACRO, ARG...) This macro performs argument currying. The expansion of this macro is another macro name that expects exactly one argument; that argument is then appended to the ARG list, and then MACRO is expanded with the resulting argument list. m4_curry([m4_curry], [m4_reverse], [1])([2])([3]) =>3, 2, 1 Unfortunately, due to a limitation in M4 1.4.x, it is not possible to pass the definition of a builtin macro as the argument to the output of `m4_curry'; the empty string is used instead of the builtin token. This behavior is rectified by using M4 1.6 or newer. -- Macro: m4_do (ARG, ...) This macro loops over its arguments and expands each ARG in sequence. Its main use is for readability; it allows the use of indentation and fewer `dnl' to result in the same expansion. This macro guarantees that no expansion will be concatenated with subsequent text; to achieve full concatenation, use `m4_unquote(m4_join([], ARG...))'. m4_define([ab],[1])m4_define([bc],[2])m4_define([abc],[3])dnl m4_do([a],[b])c =>abc m4_unquote(m4_join([],[a],[b]))c =>3 m4_define([a],[A])m4_define([b],[B])m4_define([c],[C])dnl m4_define([AB],[4])m4_define([BC],[5])m4_define([ABC],[6])dnl m4_do([a],[b])c =>ABC m4_unquote(m4_join([],[a],[b]))c =>3 -- Macro: m4_dquote (ARG, ...) Return the arguments as a quoted list of quoted arguments. Conveniently, if there is just one ARG, this effectively adds a level of quoting. -- Macro: m4_dquote_elt (ARG, ...) Return the arguments as a series of double-quoted arguments. Whereas `m4_dquote' returns a single argument, `m4_dquote_elt' returns as many arguments as it was passed. -- Macro: m4_echo (ARG, ...) Return the arguments, with the same level of quoting. Other than discarding whitespace after unquoted commas, this macro is a no-op. -- Macro: m4_expand (ARG) Return the expansion of ARG as a quoted string. Whereas `m4_quote' is designed to collect expanded text into a single argument, `m4_expand' is designed to perform one level of expansion on quoted text. One distinction is in the treatment of whitespace following a comma in the original ARG. Any time multiple arguments are collected into one with `m4_quote', the M4 argument collection rules discard the whitespace. However, with `m4_expand', whitespace is preserved, even after the expansion of macros contained in ARG. Additionally, `m4_expand' is able to expand text that would involve an unterminated comment, whereas expanding that same text as the argument to `m4_quote' runs into difficulty in finding the end of the argument. Since manipulating diversions during argument collection is inherently unsafe, `m4_expand' issues an error if ARG attempts to change the current diversion (*note Diversion support::). m4_define([active], [ACT, IVE])dnl m4_define([active2], [[ACT, IVE]])dnl m4_quote(active, active) =>ACT,IVE,ACT,IVE m4_expand([active, active]) =>ACT, IVE, ACT, IVE m4_quote(active2, active2) =>ACT, IVE,ACT, IVE m4_expand([active2, active2]) =>ACT, IVE, ACT, IVE m4_expand([# m4_echo]) =># m4_echo m4_quote(# m4_echo) ) =># m4_echo) => Note that `m4_expand' cannot handle an ARG that expands to literal unbalanced quotes, but that quadrigraphs can be used when unbalanced output is necessary. Likewise, unbalanced parentheses should be supplied with double quoting or a quadrigraph. m4_define([pattern], [[!@<:@]])dnl m4_define([bar], [BAR])dnl m4_expand([case $foo in m4_defn([pattern])@:}@ bar ;; *[)] blah ;; esac]) =>case $foo in => [![]) BAR ;; => *) blah ;; =>esac -- Macro: m4_ignore (...) This macro was introduced in Autoconf 2.62. Expands to nothing, ignoring all of its arguments. By itself, this isn't very useful. However, it can be used to conditionally ignore an arbitrary number of arguments, by deciding which macro name to apply to a list of arguments. dnl foo outputs a message only if [debug] is defined. m4_define([foo], [m4_ifdef([debug],[AC_MSG_NOTICE],[m4_ignore])([debug message])]) Note that for earlier versions of Autoconf, the macro `__gnu__' can serve the same purpose, although it is less readable. -- Macro: m4_make_list (ARG, ...) This macro exists to aid debugging of M4sugar algorithms. Its net effect is similar to `m4_dquote'--it produces a quoted list of quoted arguments, for each ARG. The difference is that this version uses a comma-newline separator instead of just comma, to improve readability of the list; with the result that it is less efficient than `m4_dquote'. m4_define([zero],[0])m4_define([one],[1])m4_define([two],[2])dnl m4_dquote(zero, [one], [[two]]) =>[0],[one],[[two]] m4_make_list(zero, [one], [[two]]) =>[0], =>[one], =>[[two]] m4_foreach([number], m4_dquote(zero, [one], [[two]]), [ number]) => 0 1 two m4_foreach([number], m4_make_list(zero, [one], [[two]]), [ number]) => 0 1 two -- Macro: m4_quote (ARG, ...) Return the arguments as a single entity, i.e., wrap them into a pair of quotes. This effectively collapses multiple arguments into one, although it loses whitespace after unquoted commas in the process. -- Macro: m4_reverse (ARG, ...) Outputs each argument with the same level of quoting, but in reverse order, and with space following each comma for readability. m4_define([active], [ACT,IVE]) => m4_reverse(active, [active]) =>active, IVE, ACT -- Macro: m4_unquote (ARG, ...) This macro was introduced in Autoconf 2.62. Expand each argument, separated by commas. For a single ARG, this effectively removes a layer of quoting, and `m4_unquote([ARG])' is more efficient than the equivalent `m4_do([ARG])'. For multiple arguments, this results in an unquoted list of expansions. This is commonly used with `m4_split', in order to convert a single quoted list into a series of quoted elements. The following example aims at emphasizing the difference between several scenarios: not using these macros, using `m4_defn', using `m4_quote', using `m4_dquote', and using `m4_expand'. $ cat example.m4 dnl Overquote, so that quotes are visible. m4_define([show], [$[]1 = [$1], $[]@ = [$@]]) m4_define([a], [A]) m4_define([mkargs], [1, 2[,] 3]) m4_define([arg1], [[$1]]) m4_divert([0])dnl show(a, b) show([a, b]) show(m4_quote(a, b)) show(m4_dquote(a, b)) show(m4_expand([a, b])) arg1(mkargs) arg1([mkargs]) arg1(m4_defn([mkargs])) arg1(m4_quote(mkargs)) arg1(m4_dquote(mkargs)) arg1(m4_expand([mkargs])) $ autom4te -l m4sugar example.m4 $1 = A, $@ = [A],[b] $1 = a, b, $@ = [a, b] $1 = A,b, $@ = [A,b] $1 = [A],[b], $@ = [[A],[b]] $1 = A, b, $@ = [A, b] 1 mkargs 1, 2[,] 3 1,2, 3 [1],[2, 3] 1, 2, 3 8.3.7 String manipulation in M4 ------------------------------- The following macros may be used to manipulate strings in M4. Many of the macros in this section intentionally result in quoted strings as output, rather than subjecting the arguments to further expansions. As a result, if you are manipulating text that contains active M4 characters, the arguments are passed with single quoting rather than double. -- Macro: m4_append (MACRO-NAME, STRING, [SEPARATOR]) -- Macro: m4_append_uniq (MACRO-NAME, STRING, [SEPARATOR] [IF-UNIQ], [IF-DUPLICATE]) Redefine MACRO-NAME to its former contents with SEPARATOR and STRING added at the end. If MACRO-NAME was undefined before (but not if it was defined but empty), then no SEPARATOR is added. As of Autoconf 2.62, neither STRING nor SEPARATOR are expanded during this macro; instead, they are expanded when MACRO-NAME is invoked. `m4_append' can be used to grow strings, and `m4_append_uniq' to grow strings without duplicating substrings. Additionally, `m4_append_uniq' takes two optional parameters as of Autoconf 2.62; IF-UNIQ is expanded if STRING was appended, and IF-DUPLICATE is expanded if STRING was already present. Also, `m4_append_uniq' warns if SEPARATOR is not empty, but occurs within STRING, since that can lead to duplicates. Note that `m4_append' can scale linearly in the length of the final string, depending on the quality of the underlying M4 implementation, while `m4_append_uniq' has an inherent quadratic scaling factor. If an algorithm can tolerate duplicates in the final string, use the former for speed. If duplicates must be avoided, consider using `m4_set_add' instead (*note Set manipulation Macros::). m4_define([active], [ACTIVE])dnl m4_append([sentence], [This is an])dnl m4_append([sentence], [ active ])dnl m4_append([sentence], [symbol.])dnl sentence =>This is an ACTIVE symbol. m4_undefine([active])dnl =>This is an active symbol. m4_append_uniq([list], [one], [, ], [new], [existing]) =>new m4_append_uniq([list], [one], [, ], [new], [existing]) =>existing m4_append_uniq([list], [two], [, ], [new], [existing]) =>new m4_append_uniq([list], [three], [, ], [new], [existing]) =>new m4_append_uniq([list], [two], [, ], [new], [existing]) =>existing list =>one, two, three m4_dquote(list) =>[one],[two],[three] m4_append([list2], [one], [[, ]])dnl m4_append_uniq([list2], [two], [[, ]])dnl m4_append([list2], [three], [[, ]])dnl list2 =>one, two, three m4_dquote(list2) =>[one, two, three] -- Macro: m4_append_uniq_w (MACRO-NAME, STRINGS) This macro was introduced in Autoconf 2.62. It is similar to `m4_append_uniq', but treats STRINGS as a whitespace separated list of words to append, and only appends unique words. MACRO-NAME is updated with a single space between new words. m4_append_uniq_w([numbers], [1 1 2])dnl m4_append_uniq_w([numbers], [ 2 3 ])dnl numbers =>1 2 3 -- Macro: m4_chomp (STRING) -- Macro: m4_chomp_all (STRING) Output STRING in quotes, but without a trailing newline. The macro `m4_chomp' is slightly faster, and removes at most one newline; the macro `m4_chomp_all' removes all consecutive trailing newlines. Unlike `m4_flatten', embedded newlines are left intact, and backslash does not influence the result. -- Macro: m4_combine ([SEPARATOR], PREFIX-LIST, [INFIX], SUFFIX-1, [SUFFIX-2], ...) This macro produces a quoted string containing the pairwise combination of every element of the quoted, comma-separated PREFIX-LIST, and every element from the SUFFIX arguments. Each pairwise combination is joined with INFIX in the middle, and successive pairs are joined by SEPARATOR. No expansion occurs on any of the arguments. No output occurs if either the PREFIX or SUFFIX list is empty, but the lists can contain empty elements. m4_define([a], [oops])dnl m4_combine([, ], [[a], [b], [c]], [-], [1], [2], [3]) =>a-1, a-2, a-3, b-1, b-2, b-3, c-1, c-2, c-3 m4_combine([, ], [[a], [b]], [-]) => m4_combine([, ], [[a], [b]], [-], []) =>a-, b- m4_combine([, ], [], [-], [1], [2]) => m4_combine([, ], [[]], [-], [1], [2]) =>-1, -2 -- Macro: m4_escape (STRING) Convert all instances of `[', `]', `#', and `$' within STRING into their respective quadrigraphs. The result is still a quoted string. -- Macro: m4_flatten (STRING) Flatten STRING into a single line. Delete all backslash-newline pairs, and replace all remaining newlines with a space. The result is still a quoted string. -- Macro: m4_join ([SEPARATOR], ARGS...) -- Macro: m4_joinall ([SEPARATOR], ARGS...) Concatenate each ARG, separated by SEPARATOR. `joinall' uses every argument, while `join' omits empty arguments so that there are no back-to-back separators in the output. The result is a quoted string. m4_define([active], [ACTIVE])dnl m4_join([|], [one], [], [active], [two]) =>one|active|two m4_joinall([|], [one], [], [active], [two]) =>one||active|two Note that if all you intend to do is join ARGS with commas between them, to form a quoted list suitable for `m4_foreach', it is more efficient to use `m4_dquote'. -- Macro: m4_newline ([TEXT]) This macro was introduced in Autoconf 2.62, and expands to a newline, followed by any TEXT. It is primarily useful for maintaining macro formatting, and ensuring that M4 does not discard leading whitespace during argument collection. -- Macro: m4_normalize (STRING) Remove leading and trailing spaces and tabs, sequences of backslash-then-newline, and replace multiple spaces, tabs, and newlines with a single space. This is a combination of `m4_flatten' and `m4_strip'. To determine if STRING consists only of bytes that would be removed by `m4_normalize', you can use `m4_ifblank'. -- Macro: m4_re_escape (STRING) Backslash-escape all characters in STRING that are active in regexps. -- Macro: m4_split (STRING, [REGEXP = `[t ]+']) Split STRING into an M4 list of elements quoted by `[' and `]', while keeping white space at the beginning and at the end. If REGEXP is given, use it instead of `[\t ]+' for splitting. If STRING is empty, the result is an empty list. -- Macro: m4_strip (STRING) Strip whitespace from STRING. Sequences of spaces and tabs are reduced to a single space, then leading and trailing spaces are removed. The result is still a quoted string. Note that this does not interfere with newlines; if you want newlines stripped as well, consider `m4_flatten', or do it all at once with `m4_normalize'. To quickly test if STRING has only whitespace, use `m4_ifblank'. -- Macro: m4_text_box (MESSAGE, [FRAME = `-']) Add a text box around MESSAGE, using FRAME as the border character above and below the message. The frame correctly accounts for the subsequent expansion of MESSAGE. For example: m4_define([macro], [abc])dnl m4_text_box([macro]) =>## --- ## =>## abc ## =>## --- ## The MESSAGE must contain balanced quotes and parentheses, although quadrigraphs can be used to work around this. -- Macro: m4_text_wrap (STRING, [PREFIX], [PREFIX1 = `PREFIX'], [WIDTH = `79']) Break STRING into a series of whitespace-separated words, then output those words separated by spaces, and wrapping lines any time the output would exceed WIDTH columns. If given, PREFIX1 begins the first line, and PREFIX begins all wrapped lines. If PREFIX1 is longer than PREFIX, then the first line consists of just PREFIX1. If PREFIX is longer than PREFIX1, padding is inserted so that the first word of STRING begins at the same indentation as all wrapped lines. Note that using literal tab characters in any of the arguments will interfere with the calculation of width. No expansions occur on PREFIX, PREFIX1, or the words of STRING, although quadrigraphs are recognized. For some examples: m4_text_wrap([Short string */], [ ], [/* ], [20]) =>/* Short string */ m4_text_wrap([Much longer string */], [ ], [/* ], [20]) =>/* Much longer => string */ m4_text_wrap([Short doc.], [ ], [ --short ], [30]) => --short Short doc. m4_text_wrap([Short doc.], [ ], [ --too-wide ], [30]) => --too-wide => Short doc. m4_text_wrap([Super long documentation.], [ ], [ --too-wide ], 30) => --too-wide => Super long => documentation. -- Macro: m4_tolower (STRING) -- Macro: m4_toupper (STRING) Return STRING with letters converted to upper or lower case, respectively. 8.3.8 Arithmetic computation in M4 ---------------------------------- The following macros facilitate integer arithmetic operations. Where a parameter is documented as taking an arithmetic expression, you can use anything that can be parsed by `m4_eval'. -- Macro: m4_cmp (EXPR-1, EXPR-2) Compare the arithmetic expressions EXPR-1 and EXPR-2, and expand to `-1' if EXPR-1 is smaller, `0' if they are equal, and `1' if EXPR-1 is larger. -- Macro: m4_list_cmp (LIST-1, LIST-2) Compare the two M4 lists consisting of comma-separated arithmetic expressions, left to right. Expand to `-1' for the first element pairing where the value from LIST-1 is smaller, `1' where the value from LIST-2 is smaller, or `0' if both lists have the same values. If one list is shorter than the other, the remaining elements of the longer list are compared against zero. m4_list_cmp([1, 0], [1]) =>0 m4_list_cmp([1, [1 * 0]], [1, 0]) =>0 m4_list_cmp([1, 2], [1, 0]) =>1 m4_list_cmp([1, [1+1], 3],[1, 2]) =>1 m4_list_cmp([1, 2, -3], [1, 2]) =>-1 m4_list_cmp([1, 0], [1, 2]) =>-1 m4_list_cmp([1], [1, 2]) =>-1 -- Macro: m4_max (ARG, ...) This macro was introduced in Autoconf 2.62. Expand to the decimal value of the maximum arithmetic expression among all the arguments. -- Macro: m4_min (ARG, ...) This macro was introduced in Autoconf 2.62. Expand to the decimal value of the minimum arithmetic expression among all the arguments. -- Macro: m4_sign (EXPR) Expand to `-1' if the arithmetic expression EXPR is negative, `1' if it is positive, and `0' if it is zero. -- Macro: m4_version_compare (VERSION-1, VERSION-2) This macro was introduced in Autoconf 2.53, but had a number of usability limitations that were not lifted until Autoconf 2.62. Compare the version strings VERSION-1 and VERSION-2, and expand to `-1' if VERSION-1 is smaller, `0' if they are the same, or `1' VERSION-2 is smaller. Version strings must be a list of elements separated by `.', `,' or `-', where each element is a number along with optional case-insensitive letters designating beta releases. The comparison stops at the leftmost element that contains a difference, although a 0 element compares equal to a missing element. It is permissible to include commit identifiers in VERSION, such as an abbreviated SHA1 of the commit, provided there is still a monotonically increasing prefix to allow for accurate version-based comparisons. For example, this paragraph was written when the development snapshot of autoconf claimed to be at version `2.61a-248-dc51', or 248 commits after the 2.61a release, with an abbreviated commit identification of `dc51'. m4_version_compare([1.1], [2.0]) =>-1 m4_version_compare([2.0b], [2.0a]) =>1 m4_version_compare([1.1.1], [1.1.1a]) =>-1 m4_version_compare([1.2], [1.1.1a]) =>1 m4_version_compare([1.0], [1]) =>0 m4_version_compare([1.1pre], [1.1PRE]) =>0 m4_version_compare([1.1a], [1,10]) =>-1 m4_version_compare([2.61a], [2.61a-248-dc51]) =>-1 m4_version_compare([2.61b], [2.61a-248-dc51]) =>1 -- Macro: m4_version_prereq (VERSION, [IF-NEW-ENOUGH], [IF-OLD = `m4_fatal']) Compares VERSION against the version of Autoconf currently running. If the running version is at VERSION or newer, expand IF-NEW-ENOUGH, but if VERSION is larger than the version currently executing, expand IF-OLD, which defaults to printing an error message and exiting m4sugar with status 63. When given only one argument, this behaves like `AC_PREREQ' (*note Versioning::). Remember that the autoconf philosophy favors feature checks over version checks. 8.3.9 Set manipulation in M4 ---------------------------- Sometimes, it is necessary to track a set of data, where the order does not matter and where there are no duplicates in the set. The following macros facilitate set manipulations. Each set is an opaque object, which can only be accessed via these basic operations. The underlying implementation guarantees linear scaling for set creation, which is more efficient than using the quadratic `m4_append_uniq'. Both set names and values can be arbitrary strings, except for unbalanced quotes. This implementation ties up memory for removed elements until the next operation that must traverse all the elements of a set; and although that may slow down some operations until the memory for removed elements is pruned, it still guarantees linear performance. -- Macro: m4_set_add (SET, VALUE, [IF-UNIQ], [IF-DUP]) Adds the string VALUE as a member of set SET. Expand IF-UNIQ if the element was added, or IF-DUP if it was previously in the set. Operates in amortized constant time, so that set creation scales linearly. -- Macro: m4_set_add_all (SET, VALUE...) Adds each VALUE to the set SET. This is slightly more efficient than repeatedly invoking `m4_set_add'. -- Macro: m4_set_contains (SET, VALUE, [IF-PRESENT], [IF-ABSENT]) Expands IF-PRESENT if the string VALUE is a member of SET, otherwise IF-ABSENT. m4_set_contains([a], [1], [yes], [no]) =>no m4_set_add([a], [1], [added], [dup]) =>added m4_set_add([a], [1], [added], [dup]) =>dup m4_set_contains([a], [1], [yes], [no]) =>yes m4_set_remove([a], [1], [removed], [missing]) =>removed m4_set_contains([a], [1], [yes], [no]) =>no m4_set_remove([a], [1], [removed], [missing]) =>missing -- Macro: m4_set_contents (SET, [SEP]) -- Macro: m4_set_dump (SET, [SEP]) Expands to a single string consisting of all the members of the set SET, each separated by SEP, which is not expanded. `m4_set_contents' leaves the elements in SET but reclaims any memory occupied by removed elements, while `m4_set_dump' is a faster one-shot action that also deletes the set. No provision is made for disambiguating members that contain a non-empty SEP as a substring; use `m4_set_empty' to distinguish between an empty set and the set containing only the empty string. The order of the output is unspecified; in the current implementation, part of the speed of `m4_set_dump' results from using a different output order than `m4_set_contents'. These macros scale linearly in the size of the set before memory pruning, and `m4_set_contents([SET], [SEP])' is faster than `m4_joinall([SEP]m4_set_listc([SET]))'. m4_set_add_all([a], [1], [2], [3]) => m4_set_contents([a], [-]) =>1-2-3 m4_joinall([-]m4_set_listc([a])) =>1-2-3 m4_set_dump([a], [-]) =>3-2-1 m4_set_contents([a]) => m4_set_add([a], []) => m4_set_contents([a], [-]) => -- Macro: m4_set_delete (SET) Delete all elements and memory associated with SET. This is linear in the set size, and faster than removing one element at a time. -- Macro: m4_set_difference (SETA, SETB) -- Macro: m4_set_intersection (SETA, SETB) -- Macro: m4_set_union (SETA, SETB) Compute the relation between SETA and SETB, and output the result as a list of quoted arguments without duplicates and with a leading comma. Set difference selects the elements in SETA but not SETB, intersection selects only elements in both sets, and union selects elements in either set. These actions are linear in the sum of the set sizes. The leading comma is necessary to distinguish between no elements and the empty string as the only element. m4_set_add_all([a], [1], [2], [3]) => m4_set_add_all([b], [3], [], [4]) => m4_set_difference([a], [b]) =>,1,2 m4_set_difference([b], [a]) =>,,4 m4_set_intersection([a], [b]) =>,3 m4_set_union([a], [b]) =>,1,2,3,,4 -- Macro: m4_set_empty (SET, [IF-EMPTY], [IF-ELEMENTS]) Expand IF-EMPTY if the set SET has no elements, otherwise expand IF-ELEMENTS. This macro operates in constant time. Using this macro can help disambiguate output from `m4_set_contents' or `m4_set_list'. -- Macro: m4_set_foreach (SET, VARIABLE, ACTION) For each element in the set SET, expand ACTION with the macro VARIABLE defined as the set element. Behavior is unspecified if ACTION recursively lists the contents of SET (although listing other sets is acceptable), or if it modifies the set in any way other than removing the element currently contained in VARIABLE. This macro is faster than the corresponding `m4_foreach([VARIABLE], m4_indir([m4_dquote]m4_set_listc([SET])), [ACTION])', although `m4_set_map' might be faster still. m4_set_add_all([a]m4_for([i], [1], [5], [], [,i])) => m4_set_contents([a]) =>12345 m4_set_foreach([a], [i], [m4_if(m4_eval(i&1), [0], [m4_set_remove([a], i, [i])])]) =>24 m4_set_contents([a]) =>135 -- Macro: m4_set_list (SET) -- Macro: m4_set_listc (SET) Produce a list of arguments, where each argument is a quoted element from the set SET. The variant `m4_set_listc' is unambiguous, by adding a leading comma if there are any set elements, whereas the variant `m4_set_list' cannot distinguish between an empty set and a set containing only the empty string. These can be directly used in macros that take multiple arguments, such as `m4_join' or `m4_set_add_all', or wrapped by `m4_dquote' for macros that take a quoted list, such as `m4_map' or `m4_foreach'. Any memory occupied by removed elements is reclaimed during these macros. m4_set_add_all([a], [1], [2], [3]) => m4_set_list([a]) =>1,2,3 m4_set_list([b]) => m4_set_listc([b]) => m4_count(m4_set_list([b])) =>1 m4_set_empty([b], [0], [m4_count(m4_set_list([b]))]) =>0 m4_set_add([b], []) => m4_set_list([b]) => m4_set_listc([b]) =>, m4_count(m4_set_list([b])) =>1 m4_set_empty([b], [0], [m4_count(m4_set_list([b]))]) =>1 -- Macro: m4_set_map (SET, ACTION) For each element in the set SET, expand ACTION with a single argument of the set element. Behavior is unspecified if ACTION recursively lists the contents of SET (although listing other sets is acceptable), or if it modifies the set in any way other than removing the element passed as an argument. This macro is faster than either corresponding counterpart of `m4_map_args([ACTION]m4_set_listc([SET]))' or `m4_set_foreach([SET], [var], [ACTION(m4_defn([var]))])'. It is possible to use `m4_curry' if more than one argument is needed for ACTION, although it is more efficient to use `m4_set_map_sep' in that case. -- Macro: m4_set_map_sep (SET, [PRE], [POST], [SEP]) For each element in the set SET, expand `PRE[element]POST', additionally expanding SEP between elements. Behavior is unspecified if the expansion recursively lists the contents of SET (although listing other sets is acceptable), or if it modifies the set in any way other than removing the element visited by the expansion. This macro provides the most efficient means for non-destructively visiting the elements of a set; in particular, `m4_set_map([SET], [ACTION])' is equivalent to `m4_set_map_sep([SET], [ACTION(], [)])'. -- Macro: m4_set_remove (SET, VALUE, [IF-PRESENT], [IF-ABSENT]) If VALUE is an element in the set SET, then remove it and expand IF-PRESENT. Otherwise expand IF-ABSENT. This macro operates in constant time so that multiple removals will scale linearly rather than quadratically; but when used outside of `m4_set_foreach' or `m4_set_map', it leaves memory occupied until the set is later compacted by `m4_set_contents' or `m4_set_list'. Several other set operations are then less efficient between the time of element removal and subsequent memory compaction, but still maintain their guaranteed scaling performance. -- Macro: m4_set_size (SET) Expand to the size of the set SET. This implementation operates in constant time, and is thus more efficient than `m4_eval(m4_count(m4_set_listc([set])) - 1)'. 8.3.10 Forbidden Patterns ------------------------- M4sugar provides a means to define suspicious patterns, patterns describing tokens which should not be found in the output. For instance, if an Autoconf `configure' script includes tokens such as `AC_DEFINE', or `dnl', then most probably something went wrong (typically a macro was not evaluated because of overquotation). M4sugar forbids all the tokens matching `^_?m4_' and `^dnl$'. Additional layers, such as M4sh and Autoconf, add additional forbidden patterns to the list. -- Macro: m4_pattern_forbid (PATTERN) Declare that no token matching PATTERN must be found in the output. Comments are not checked; this can be a problem if, for instance, you have some macro left unexpanded after an `#include'. No consensus is currently found in the Autoconf community, as some people consider it should be valid to name macros in comments (which doesn't make sense to the authors of this documentation: input, such as macros, should be documented by `dnl' comments; reserving `#'-comments to document the output). Of course, you might encounter exceptions to these generic rules, for instance you might have to refer to `$m4_flags'. -- Macro: m4_pattern_allow (PATTERN) Any token matching PATTERN is allowed, including if it matches an `m4_pattern_forbid' pattern. 8.4 Debugging via autom4te ========================== At times, it is desirable to see what was happening inside m4, to see why output was not matching expectations. However, post-processing done by `autom4te' means that directly using the m4 builtin `m4_traceon' is likely to interfere with operation. Also, frequent diversion changes and the concept of forbidden tokens make it difficult to use `m4_defn' to generate inline comments in the final output. There are a couple of tools to help with this. One is the use of the `--trace' option provided by `autom4te' (as well as each of the programs that wrap `autom4te', such as `autoconf'), in order to inspect when a macro is called and with which arguments. For example, when this paragraph was written, the autoconf version could be found by: $ autoconf --trace=AC_INIT configure.ac:23:AC_INIT:GNU Autoconf:2.63b.95-3963:bug-autoconf@gnu.org $ autoconf --trace='AC_INIT:version is $2' version is 2.63b.95-3963 Another trick is to print out the expansion of various m4 expressions to standard error or to an independent file, with no further m4 expansion, and without interfering with diversion changes or the post-processing done to standard output. `m4_errprintn' shows a given expression on standard error. For example, if you want to see the expansion of an autoconf primitive or of one of your autoconf macros, you can do it like this: $ cat <<\EOF > configure.ac AC_INIT m4_errprintn([The definition of AC_DEFINE_UNQUOTED:]) m4_errprintn(m4_defn([AC_DEFINE_UNQUOTED])) AC_OUTPUT EOF $ autoconf error-->The definition of AC_DEFINE_UNQUOTED: error-->_AC_DEFINE_Q([], $@) 9 Programming in M4sh ********************* M4sh, pronounced "mash", is aiming at producing portable Bourne shell scripts. This name was coined by Lars J. Aas, who notes that, according to the Webster's Revised Unabridged Dictionary (1913): Mash \Mash\, n. [Akin to G. meisch, maisch, meische, maische, mash, wash, and prob. to AS. miscian to mix. See "Mix".] 1. A mass of mixed ingredients reduced to a soft pulpy state by beating or pressure... 2. A mixture of meal or bran and water fed to animals. 3. A mess; trouble. [Obs.] -Beau. & Fl. M4sh reserves the M4 macro namespace `^_AS_' for internal use, and the namespace `^AS_' for M4sh macros. It also reserves the shell and environment variable namespace `^as_', and the here-document delimiter namespace `^_AS[A-Z]' in the output file. You should not define your own macros or output shell code that conflicts with these namespaces. 9.1 Common Shell Constructs =========================== M4sh provides portable alternatives for some common shell constructs that unfortunately are not portable in practice. -- Macro: AS_CASE (WORD, [PATTERN1], [IF-MATCHED1], ..., [DEFAULT]) Expand into a shell `case' statement, where WORD is matched against one or more patterns. IF-MATCHED is run if the corresponding pattern matched WORD, else DEFAULT is run. Avoids several portability issues (*note Limitations of Shell Builtins: case.). -- Macro: AS_DIRNAME (FILE-NAME) Output the directory portion of FILE-NAME. For example, if `$file' is `/one/two/three', the command `dir=`AS_DIRNAME(["$file"])`' sets `dir' to `/one/two'. This interface may be improved in the future to avoid forks and losing trailing newlines. -- Macro: AS_ECHO (WORD) Emits WORD to the standard output, followed by a newline. WORD must be a single shell word (typically a quoted string). The bytes of WORD are output as-is, even if it starts with "-" or contains "\". Redirections can be placed outside the macro invocation. This is much more portable than using `echo' (*note Limitations of Shell Builtins: echo.). -- Macro: AS_ECHO_N (WORD) Emits WORD to the standard output, without a following newline. WORD must be a single shell word (typically a quoted string) and, for portability, should not include more than one newline. The bytes of WORD are output as-is, even if it starts with "-" or contains "\". Redirections can be placed outside the macro invocation. -- Macro: AS_ESCAPE (STRING, [CHARS = ``"$']) Expands to STRING, with any characters in CHARS escaped with a backslash (`\'). CHARS should be at most four bytes long, and only contain characters from the set ``\"$'; however, characters may be safely listed more than once in CHARS for the sake of syntax highlighting editors. The current implementation expands STRING after adding escapes; if STRING contains macro calls that in turn expand to text needing shell quoting, you can use `AS_ESCAPE(m4_dquote(m4_expand([string])))'. The default for CHARS (`\"$`') is the set of characters needing escapes when STRING will be used literally within double quotes. One common variant is the set of characters to protect when STRING will be used literally within back-ticks or an unquoted here-document (`\$`'). Another common variant is `""', which can be used to form a double-quoted string containing the same expansions that would have occurred if STRING were expanded in an unquoted here-document; however, when using this variant, care must be taken that STRING does not use double quotes within complex variable expansions (such as `${foo-`echo "hi"`}') that would be broken with improper escapes. This macro is often used with `AS_ECHO'. For an example, observe the output generated by the shell code generated from this snippet: foo=bar AS_ECHO(["AS_ESCAPE(["$foo" = ])AS_ESCAPE(["$foo"], [""])"]) =>"$foo" = "bar" m4_define([macro], [a, [\b]]) AS_ECHO(["AS_ESCAPE([[macro]])"]) =>macro AS_ECHO(["AS_ESCAPE([macro])"]) =>a, b AS_ECHO(["AS_ESCAPE(m4_dquote(m4_expand([macro])))"]) =>a, \b To escape a string that will be placed within single quotes, use: m4_bpatsubst([[STRING]], ['], ['\\'']) -- Macro: AS_EXIT ([STATUS = `$?']) Emit code to exit the shell with STATUS, defaulting to `$?'. This macro works around shells that see the exit status of the command prior to `exit' inside a `trap 0' handler (*note Limitations of Shell Builtins: trap.). -- Macro: AS_IF (TEST1, [RUN-IF-TRUE1], ..., [RUN-IF-FALSE]) Run shell code TEST1. If TEST1 exits with a zero status then run shell code RUN-IF-TRUE1, else examine further tests. If no test exits with a zero status, run shell code RUN-IF-FALSE, with simplifications if either RUN-IF-TRUE1 or RUN-IF-FALSE is empty. For example, AS_IF([test "x$foo" = xyes], [HANDLE_FOO([yes])], [test "x$foo" != xno], [HANDLE_FOO([maybe])], [echo foo not specified]) ensures any required macros of `HANDLE_FOO' are expanded before the first test. -- Macro: AS_MKDIR_P (FILE-NAME) Make the directory FILE-NAME, including intervening directories as necessary. This is equivalent to `mkdir -p -- FILE-NAME', except that it is portable to older versions of `mkdir' that lack support for the `-p' option or for the `--' delimiter (*note Limitations of Usual Tools: mkdir.). Also, `AS_MKDIR_P' succeeds if FILE-NAME is a symbolic link to an existing directory, even though Posix is unclear whether `mkdir -p' should succeed in that case. If creation of FILE-NAME fails, exit the script. Also see the `AC_PROG_MKDIR_P' macro (*note Particular Programs::). -- Macro: AS_SET_STATUS (STATUS) Emit shell code to set the value of `$?' to STATUS, as efficiently as possible. However, this is not guaranteed to abort a shell running with `set -e' (*note Limitations of Shell Builtins: set.). This should also be used at the end of a complex shell function instead of `return' (*note Shell Functions::) to avoid a DJGPP shell bug. -- Macro: AS_TR_CPP (EXPRESSION) Transform EXPRESSION into a valid right-hand side for a C `#define'. For example: # This outputs "#define HAVE_CHAR_P 1". # Notice the m4 quoting around #, to prevent an m4 comment type="char *" echo "[#]define AS_TR_CPP([HAVE_$type]) 1" -- Macro: AS_TR_SH (EXPRESSION) Transform EXPRESSION into a valid shell variable name. For example: # This outputs "Have it!". header="sys/some file.h" AS_TR_SH([HAVE_$header])=yes if test "x$HAVE_sys_some_file_h" = xyes; then echo "Have it!"; fi -- Macro: AS_SET_CATFILE (VAR, DIR, FILE) Set the shell variable VAR to DIR/FILE, but optimizing the common cases (DIR or FILE is `.', FILE is absolute, etc.). -- Macro: AS_UNSET (VAR) Unsets the shell variable VAR, working around bugs in older shells (*note Limitations of Shell Builtins: unset.). VAR can be a literal or indirect variable name. -- Macro: AS_VERSION_COMPARE (VERSION-1, VERSION-2, [ACTION-IF-LESS], [ACTION-IF-EQUAL], [ACTION-IF-GREATER]) Compare two strings VERSION-1 and VERSION-2, possibly containing shell variables, as version strings, and expand ACTION-IF-LESS, ACTION-IF-EQUAL, or ACTION-IF-GREATER depending upon the result. The algorithm to compare is similar to the one used by strverscmp in glibc (*note String/Array Comparison: (libc)String/Array Comparison.). 9.2 Support for indirect variable names ======================================= Often, it is convenient to write a macro that will emit shell code operating on a shell variable. The simplest case is when the variable name is known. But a more powerful idiom is writing shell code that can work through an indirection, where another variable or command substitution produces the name of the variable to actually manipulate. M4sh supports the notion of polymorphic shell variables, making it easy to write a macro that can deal with either literal or indirect variable names and output shell code appropriate for both use cases. Behavior is undefined if expansion of an indirect variable does not result in a literal variable name. -- Macro: AS_LITERAL_IF (EXPRESSION, [IF-LITERAL], [IF-NOT]) If the expansion of EXPRESSION is definitely a shell literal, expand IF-LITERAL. If the expansion of EXPRESSION looks like it might contain shell indirections (such as `$var' or ``expr`'), then IF-NOT is expanded. In order to reduce the time spent deciding whether an expression is literal, the implementation is somewhat conservative (for example, `'[$]'' is a single-quoted shell literal, but causes IF-NOT to be expanded). While this macro is often used for recognizing shell variable names, it can also be used in other contexts. AC_DEFUN([MY_ACTION], [AS_LITERAL_IF([$1], [echo "$1"], [AS_VAR_COPY([tmp], [$1]) echo "$tmp"])]) -- Macro: AS_VAR_APPEND (VAR, TEXT) Emit shell code to append the shell expansion of TEXT to the end of the current contents of the polymorphic shell variable VAR, taking advantage of shells that provide the `+=' extension for more efficient scaling. For situations where the final contents of VAR are relatively short (less than 256 bytes), it is more efficient to use the simpler code sequence of `VAR=${VAR}TEXT' (or its polymorphic equivalent of `AS_VAR_COPY([tmp], [VAR])' and `AS_VAR_SET([VAR], ["$tmp"TEXT])'). But in the case when the script will be repeatedly appending text into `var', issues of scaling start to become apparent. A naive implementation requires execution time linear to the length of the current contents of VAR as well as the length of TEXT for a single append, for an overall quadratic scaling with multiple appends. This macro takes advantage of shells which provide the extension `VAR+=TEXT', which can provide amortized constant time for a single append, for an overall linear scaling with multiple appends. Note that unlike `AS_VAR_SET', this macro requires that TEXT be quoted properly to avoid field splitting and file name expansion. -- Macro: AS_VAR_ARITH (VAR, EXPRESSION) Emit shell code to compute the arithmetic expansion of EXPRESSION, assigning the result as the contents of the polymorphic shell variable VAR. The code takes advantage of shells that provide `$(())' for fewer forks, but uses `expr' as a fallback. Therefore, the syntax for a valid EXPRESSION is rather limited: all operators must occur as separate shell arguments and with proper quoting, there is no portable equality operator, all variables containing numeric values must be expanded prior to the computation, all numeric values must be provided in decimal without leading zeroes, and the first shell argument should not be a negative number. In the following example, this snippet will print `(2+3)*4 == 20'. bar=3 AS_VAR_ARITH([foo], [\( 2 + $bar \) \* 4]) echo "(2+$bar)*4 == $foo" -- Macro: AS_VAR_COPY (DEST, SOURCE) Emit shell code to assign the contents of the polymorphic shell variable SOURCE to the polymorphic shell variable DEST. For example, executing this M4sh snippet will output `bar hi': foo=bar bar=hi AS_VAR_COPY([a], [foo]) AS_VAR_COPY([b], [$foo]) echo "$a $b" When it is necessary to access the contents of an indirect variable inside a shell double-quoted context, the recommended idiom is to first copy the contents into a temporary literal shell variable. for header in stdint_h inttypes_h ; do AS_VAR_COPY([var], [ac_cv_header_$header]) echo "$header detected: $var" done -- Macro: AS_VAR_IF (VAR, [VALUE], [IF-EQUAL], [IF-NOT-EQUAL]) Output a shell conditional statement. If the contents of the polymorphic shell variable VAR match the string VALUE, execute IF-EQUAL; otherwise execute IF-NOT-EQUAL. Avoids shell bugs if an interrupt signal arrives while a command substitution in VAR is being expanded. -- Macro: AS_VAR_PUSHDEF (M4-NAME, VALUE) -- Macro: AS_VAR_POPDEF (M4-NAME) A common M4sh idiom involves composing shell variable names from an m4 argument (for example, writing a macro that uses a cache variable). VALUE can be an arbitrary string, which will be transliterated into a valid shell name by `AS_TR_SH'. In order to access the composed variable name based on VALUE, it is easier to declare a temporary m4 macro M4-NAME with `AS_VAR_PUSHDEF', then use that macro as the argument to subsequent `AS_VAR' macros as a polymorphic variable name, and finally free the temporary macro with `AS_VAR_POPDEF'. These macros are often followed with `dnl', to avoid excess newlines in the output. Here is an involved example, that shows the power of writing macros that can handle composed shell variable names: m4_define([MY_CHECK_HEADER], [AS_VAR_PUSHDEF([my_Header], [ac_cv_header_$1])dnl AS_VAR_IF([my_Header], [yes], [echo "header $1 available"])dnl AS_VAR_POPDEF([my_Header])dnl ]) MY_CHECK_HEADER([stdint.h]) for header in inttypes.h stdlib.h ; do MY_CHECK_HEADER([$header]) done In the above example, `MY_CHECK_HEADER' can operate on polymorphic variable names. In the first invocation, the m4 argument is `stdint.h', which transliterates into a literal `stdint_h'. As a result, the temporary macro `my_Header' expands to the literal shell name `ac_cv_header_stdint_h'. In the second invocation, the m4 argument to `MY_CHECK_HEADER' is `$header', and the temporary macro `my_Header' expands to the indirect shell name `$as_my_Header'. During the shell execution of the for loop, when `$header' contains `inttypes.h', then `$as_my_Header' contains `ac_cv_header_inttypes_h'. If this script is then run on a platform where all three headers have been previously detected, the output of the script will include: header stdint.h detected header inttypes.h detected header stdlib.h detected -- Macro: AS_VAR_SET (VAR, [VALUE]) Emit shell code to assign the contents of the polymorphic shell variable VAR to the shell expansion of VALUE. VALUE is not subject to field splitting or file name expansion, so if command substitution is used, it may be done with ``""`' rather than using an intermediate variable (*note Shell Substitutions::). However, VALUE does undergo rescanning for additional macro names; behavior is unspecified if late expansion results in any shell meta-characters. -- Macro: AS_VAR_SET_IF (VAR, [IF-SET], [IF-UNDEF]) Emit a shell conditional statement, which executes IF-SET if the polymorphic shell variable `var' is set to any value, and IF-UNDEF otherwise. -- Macro: AS_VAR_TEST_SET (VAR) Emit a shell statement that results in a successful exit status only if the polymorphic shell variable `var' is set. 9.3 Initialization Macros ========================= -- Macro: AS_BOURNE_COMPATIBLE Set up the shell to be more compatible with the Bourne shell as standardized by Posix, if possible. This may involve setting environment variables, or setting options, or similar implementation-specific actions. This macro is deprecated, since `AS_INIT' already invokes it. -- Macro: AS_INIT Initialize the M4sh environment. This macro calls `m4_init', then outputs the `#! /bin/sh' line, a notice about where the output was generated from, and code to sanitize the environment for the rest of the script. Among other initializations, this sets `SHELL' to the shell chosen to run the script (*note CONFIG_SHELL::), and `LC_ALL' to ensure the C locale. Finally, it changes the current diversion to `BODY'. `AS_INIT' is called automatically by `AC_INIT' and `AT_INIT', so shell code in `configure', `config.status', and `testsuite' all benefit from a sanitized shell environment. -- Macro: AS_INIT_GENERATED (FILE, [COMMENT]) Emit shell code to start the creation of a subsidiary shell script in FILE, including changing FILE to be executable. This macro populates the child script with information learned from the parent (thus, the emitted code is equivalent in effect, but more efficient, than the code output by `AS_INIT', `AS_BOURNE_COMPATIBLE', and `AS_SHELL_SANITIZE'). If present, COMMENT is output near the beginning of the child, prior to the shell initialization code, and is subject to parameter expansion, command substitution, and backslash quote removal. The parent script should check the exit status after this macro, in case FILE could not be properly created (for example, if the disk was full). If successfully created, the parent script can then proceed to append additional M4sh constructs into the child script. Note that the child script starts life without a log file open, so if the parent script uses logging (*note AS_MESSAGE_LOG_FD::), you must temporarily disable any attempts to use the log file until after emitting code to open a log within the child. On the other hand, if the parent script has `AS_MESSAGE_FD' redirected somewhere besides `1', then the child script already has code that copies stdout to that descriptor. Currently, the suggested idiom for writing a M4sh shell script from within another script is: AS_INIT_GENERATED([FILE], [[# My child script. ]]) || { AS_ECHO(["Failed to create child script"]); AS_EXIT; } m4_pushdef([AS_MESSAGE_LOG_FD])dnl cat >> "FILE" <<\__EOF__ # Code to initialize AS_MESSAGE_LOG_FD m4_popdef([AS_MESSAGE_LOG_FD])dnl # Additional code __EOF__ This, however, may change in the future as the M4sh interface is stabilized further. Also, be aware that use of `LINENO' within the child script may report line numbers relative to their location in the parent script, even when using `AS_LINENO_PREPARE', if the parent script was unable to locate a shell with working `LINENO' support. -- Macro: AS_LINENO_PREPARE Find a shell that supports the special variable `LINENO', which contains the number of the currently executing line. This macro is automatically invoked by `AC_INIT' in configure scripts. -- Macro: AS_ME_PREPARE Set up variable `as_me' to be the basename of the currently executing script. This macro is automatically invoked by `AC_INIT' in configure scripts. -- Macro: AS_SHELL_SANITIZE Initialize the shell suitably for `configure' scripts. This has the effect of `AS_BOURNE_COMPATIBLE', and sets some other environment variables for predictable results from configuration tests. For example, it sets `LC_ALL' to change to the default C locale. *Note Special Shell Variables::. This macro is deprecated, since `AS_INIT' already invokes it. 9.4 File Descriptor Macros ========================== The following macros define file descriptors used to output messages (or input values) from `configure' scripts. For example: echo "$wombats found" >&AS_MESSAGE_LOG_FD echo 'Enter desired kangaroo count:' >&AS_MESSAGE_FD read kangaroos <&AS_ORIGINAL_STDIN_FD` However doing so is seldom needed, because Autoconf provides higher level macros as described below. -- Macro: AS_MESSAGE_FD The file descriptor for `checking for...' messages and results. By default, `AS_INIT' sets this to `1' for standalone M4sh clients. However, `AC_INIT' shuffles things around to another file descriptor, in order to allow the `-q' option of `configure' to choose whether messages should go to the script's standard output or be discarded. If you want to display some messages, consider using one of the printing macros (*note Printing Messages::) instead. Copies of messages output via these macros are also recorded in `config.log'. -- Macro: AS_MESSAGE_LOG_FD This must either be empty, or expand to a file descriptor for log messages. By default, `AS_INIT' sets this macro to the empty string for standalone M4sh clients, thus disabling logging. However, `AC_INIT' shuffles things around so that both `configure' and `config.status' use `config.log' for log messages. Macros that run tools, like `AC_COMPILE_IFELSE' (*note Running the Compiler::), redirect all output to this descriptor. You may want to do so if you develop such a low-level macro. -- Macro: AS_ORIGINAL_STDIN_FD This must expand to a file descriptor for the original standard input. By default, `AS_INIT' sets this macro to `0' for standalone M4sh clients. However, `AC_INIT' shuffles things around for safety. When `configure' runs, it may accidentally execute an interactive command that has the same name as the non-interactive meant to be used or checked. If the standard input was the terminal, such interactive programs would cause `configure' to stop, pending some user input. Therefore `configure' redirects its standard input from `/dev/null' during its initialization. This is not normally a problem, since `configure' normally does not need user input. In the extreme case where your `configure' script really needs to obtain some values from the original standard input, you can read them explicitly from `AS_ORIGINAL_STDIN_FD'. 10 Writing Autoconf Macros ************************** When you write a feature test that could be applicable to more than one software package, the best thing to do is encapsulate it in a new macro. Here are some instructions and guidelines for writing Autoconf macros. 10.1 Macro Definitions ====================== -- Macro: AC_DEFUN (NAME, [BODY]) Autoconf macros are defined using the `AC_DEFUN' macro, which is similar to the M4 builtin `m4_define' macro; this creates a macro named NAME and with BODY as its expansion. In addition to defining a macro, `AC_DEFUN' adds to it some code that is used to constrain the order in which macros are called, while avoiding redundant output (*note Prerequisite Macros::). An Autoconf macro definition looks like this: AC_DEFUN(MACRO-NAME, MACRO-BODY) You can refer to any arguments passed to the macro as `$1', `$2', etc. *Note How to define new macros: (m4.info)Definitions, for more complete information on writing M4 macros. Most macros fall in one of two general categories. The first category includes macros which take arguments, in order to generate output parameterized by those arguments. Macros in this category are designed to be directly expanded, often multiple times, and should not be used as the argument to `AC_REQUIRE'. The other category includes macros which are shorthand for a fixed block of text, and therefore do not take arguments. For this category of macros, directly expanding the macro multiple times results in redundant output, so it is more common to use the macro as the argument to `AC_REQUIRE', or to declare the macro with `AC_DEFUN_ONCE' (*note One-Shot Macros::). Be sure to properly quote both the MACRO-BODY _and_ the MACRO-NAME to avoid any problems if the macro happens to have been previously defined. Each macro should have a header comment that gives its prototype, and a brief description. When arguments have default values, display them in the prototype. For example: # AC_MSG_ERROR(ERROR, [EXIT-STATUS = 1]) # -------------------------------------- m4_define([AC_MSG_ERROR], [{ AS_MESSAGE([error: $1], [2]) exit m4_default([$2], [1]); }]) Comments about the macro should be left in the header comment. Most other comments make their way into `configure', so just keep using `#' to introduce comments. If you have some special comments about pure M4 code, comments that make no sense in `configure' and in the header comment, then use the builtin `dnl': it causes M4 to discard the text through the next newline. Keep in mind that `dnl' is rarely needed to introduce comments; `dnl' is more useful to get rid of the newlines following macros that produce no output, such as `AC_REQUIRE'. Public third-party macros need to use `AC_DEFUN', and not `m4_define', in order to be found by `aclocal' (*note Extending aclocal: (automake)Extending aclocal.). Additionally, if it is ever determined that a macro should be made obsolete, it is easy to convert from `AC_DEFUN' to `AU_DEFUN' in order to have `autoupdate' assist the user in choosing a better alternative, but there is no corresponding way to make `m4_define' issue an upgrade notice (*note AU_DEFUN::). There is another subtle, but important, difference between using `m4_define' and `AC_DEFUN': only the former is unaffected by `AC_REQUIRE'. When writing a file, it is always safe to replace a block of text with a `m4_define' macro that will expand to the same text. But replacing a block of text with an `AC_DEFUN' macro with the same content does not necessarily give the same results, because it changes the location where any embedded but unsatisfied `AC_REQUIRE' invocations within the block will be expanded. For an example of this, see *note Expanded Before Required::. 10.2 Macro Names ================ All of the public Autoconf macros have all-uppercase names in the namespace `^AC_' to prevent them from accidentally conflicting with other text; Autoconf also reserves the namespace `^_AC_' for internal macros. All shell variables that they use for internal purposes have mostly-lowercase names starting with `ac_'. Autoconf also uses here-document delimiters in the namespace `^_AC[A-Z]'. During `configure', files produced by Autoconf make heavy use of the file system namespace `^conf'. Since Autoconf is built on top of M4sugar (*note Programming in M4sugar::) and M4sh (*note Programming in M4sh::), you must also be aware of those namespaces (`^_?\(m4\|AS\)_'). And since `configure.ac' is also designed to be scanned by Autoheader, Autoscan, Autoupdate, and Automake, you should be aware of the `^_?A[HNUM]_' namespaces. In general, you _should not use_ the namespace of a package that does not own the macro or shell code you are writing. To ensure that your macros don't conflict with present or future Autoconf macros, you should prefix your own macro names and any shell variables they use with some other sequence. Possibilities include your initials, or an abbreviation for the name of your organization or software package. Historically, people have not always followed the rule of using a namespace appropriate for their package, and this has made it difficult for determining the origin of a macro (and where to report bugs about that macro), as well as difficult for the true namespace owner to add new macros without interference from pre-existing uses of third-party macros. Perhaps the best example of this confusion is the `AM_GNU_GETTEXT' macro, which belongs, not to Automake, but to Gettext. Most of the Autoconf macros' names follow a structured naming convention that indicates the kind of feature check by the name. The macro names consist of several words, separated by underscores, going from most general to most specific. The names of their cache variables use the same convention (*note Cache Variable Names::, for more information on them). The first word of the name after the namespace initials (such as `AC_') usually tells the category of the feature being tested. Here are the categories used in Autoconf for specific test macros, the kind of macro that you are more likely to write. They are also used for cache variables, in all-lowercase. Use them where applicable; where they're not, invent your own categories. `C' C language builtin features. `DECL' Declarations of C variables in header files. `FUNC' Functions in libraries. `GROUP' Posix group owners of files. `HEADER' Header files. `LIB' C libraries. `PROG' The base names of programs. `MEMBER' Members of aggregates. `SYS' Operating system features. `TYPE' C builtin or declared types. `VAR' C variables in libraries. After the category comes the name of the particular feature being tested. Any further words in the macro name indicate particular aspects of the feature. For example, `AC_PROG_CC_STDC' checks whether the C compiler supports ISO Standard C. An internal macro should have a name that starts with an underscore; Autoconf internals should therefore start with `_AC_'. Additionally, a macro that is an internal subroutine of another macro should have a name that starts with an underscore and the name of that other macro, followed by one or more words saying what the internal macro does. For example, `AC_PATH_X' has internal macros `_AC_PATH_X_XMKMF' and `_AC_PATH_X_DIRECT'. 10.3 Reporting Messages ======================= When macros statically diagnose abnormal situations, benign or fatal, it is possible to make `autoconf' detect the problem, and refuse to create `configure' in the case of an error. The macros in this section are considered obsolescent, and new code should use M4sugar macros for this purpose, see *note Diagnostic Macros::. On the other hand, it is possible to want to detect errors when `configure' is run, which are dependent on the environment of the user rather than the maintainer. For dynamic diagnostics, see *note Printing Messages::. -- Macro: AC_DIAGNOSE (CATEGORY, MESSAGE) Report MESSAGE as a warning (or as an error if requested by the user) if warnings of the CATEGORY are turned on. This macro is obsolescent; you are encouraged to use: m4_warn([CATEGORY], [MESSAGE]) instead. *Note m4_warn::, for more details, including valid CATEGORY names. -- Macro: AC_WARNING (MESSAGE) Report MESSAGE as a syntax warning. This macro is obsolescent; you are encouraged to use: m4_warn([syntax], [MESSAGE]) instead. *Note m4_warn::, for more details, as well as better finer-grained categories of warnings (not all problems have to do with syntax). -- Macro: AC_FATAL (MESSAGE) Report a severe error MESSAGE, and have `autoconf' die. This macro is obsolescent; you are encouraged to use: m4_fatal([MESSAGE]) instead. *Note m4_fatal::, for more details. When the user runs `autoconf -W error', warnings from `m4_warn' (including those issued through `AC_DIAGNOSE' and `AC_WARNING') are reported as errors, see *note autoconf Invocation::. 10.4 Dependencies Between Macros ================================ Some Autoconf macros depend on other macros having been called first in order to work correctly. Autoconf provides a way to ensure that certain macros are called if needed and a way to warn the user if macros are called in an order that might cause incorrect operation. 10.4.1 Prerequisite Macros -------------------------- A macro that you write might need to use values that have previously been computed by other macros. For example, `AC_DECL_YYTEXT' examines the output of `flex' or `lex', so it depends on `AC_PROG_LEX' having been called first to set the shell variable `LEX'. Rather than forcing the user of the macros to keep track of the dependencies between them, you can use the `AC_REQUIRE' macro to do it automatically. `AC_REQUIRE' can ensure that a macro is only called if it is needed, and only called once. -- Macro: AC_REQUIRE (MACRO-NAME) If the M4 macro MACRO-NAME has not already been called, call it (without any arguments). Make sure to quote MACRO-NAME with square brackets. MACRO-NAME must have been defined using `AC_DEFUN' or else contain a call to `AC_PROVIDE' to indicate that it has been called. `AC_REQUIRE' must be used inside a macro defined by `AC_DEFUN'; it must not be called from the top level. Also, it does not make sense to require a macro that takes parameters. `AC_REQUIRE' is often misunderstood. It really implements dependencies between macros in the sense that if one macro depends upon another, the latter is expanded _before_ the body of the former. To be more precise, the required macro is expanded before the outermost defined macro in the current expansion stack. In particular, `AC_REQUIRE([FOO])' is not replaced with the body of `FOO'. For instance, this definition of macros: AC_DEFUN([TRAVOLTA], [test "$body_temperature_in_celsius" -gt "38" && dance_floor=occupied]) AC_DEFUN([NEWTON_JOHN], [test "x$hair_style" = xcurly && dance_floor=occupied]) AC_DEFUN([RESERVE_DANCE_FLOOR], [if date | grep '^Sat.*pm' >/dev/null 2>&1; then AC_REQUIRE([TRAVOLTA]) AC_REQUIRE([NEWTON_JOHN]) fi]) with this `configure.ac' AC_INIT([Dance Manager], [1.0], [bug-dance@example.org]) RESERVE_DANCE_FLOOR if test "x$dance_floor" = xoccupied; then AC_MSG_ERROR([cannot pick up here, let's move]) fi does not leave you with a better chance to meet a kindred soul at other times than Saturday night since it expands into: test "$body_temperature_in_Celsius" -gt "38" && dance_floor=occupied test "x$hair_style" = xcurly && dance_floor=occupied fi if date | grep '^Sat.*pm' >/dev/null 2>&1; then fi This behavior was chosen on purpose: (i) it prevents messages in required macros from interrupting the messages in the requiring macros; (ii) it avoids bad surprises when shell conditionals are used, as in: if ...; then AC_REQUIRE([SOME_CHECK]) fi ... SOME_CHECK However, this implementation can lead to another class of problems. Consider the case where an outer macro first expands, then indirectly requires, an inner macro: AC_DEFUN([TESTA], [[echo in A if test -n "$SEEN_A" ; then echo duplicate ; fi SEEN_A=:]]) AC_DEFUN([TESTB], [AC_REQUIRE([TESTA])[echo in B if test -z "$SEEN_A" ; then echo bug ; fi]]) AC_DEFUN([TESTC], [AC_REQUIRE([TESTB])[echo in C]]) AC_DEFUN([OUTER], [[echo in OUTER] TESTA TESTC]) OUTER Prior to Autoconf 2.64, the implementation of `AC_REQUIRE' recognized that `TESTB' needed to be hoisted prior to the expansion of `OUTER', but because `TESTA' had already been directly expanded, it failed to hoist `TESTA'. Therefore, the expansion of `TESTB' occurs prior to its prerequisites, leading to the following output: in B bug in OUTER in A in C Newer Autoconf is smart enough to recognize this situation, and hoists `TESTA' even though it has already been expanded, but issues a syntax warning in the process. This is because the hoisted expansion of `TESTA' defeats the purpose of using `AC_REQUIRE' to avoid redundant code, and causes its own set of problems if the hoisted macro is not idempotent: in A in B in OUTER in A duplicate in C The bug is not in Autoconf, but in the macro definitions. If you ever pass a particular macro name to `AC_REQUIRE', then you are implying that the macro only needs to be expanded once. But to enforce this, either the macro must be declared with `AC_DEFUN_ONCE' (although this only helps in Autoconf 2.64 or newer), or all uses of that macro should be through `AC_REQUIRE'; directly expanding the macro defeats the point of using `AC_REQUIRE' to eliminate redundant expansion. In the example, this rule of thumb was violated because `TESTB' requires `TESTA' while `OUTER' directly expands it. One way of fixing the bug is to factor `TESTA' into two macros, the portion designed for direct and repeated use (here, named `TESTA'), and the portion designed for one-shot output and used only inside `AC_REQUIRE' (here, named `TESTA_PREREQ'). Then, by fixing all clients to use the correct calling convention according to their needs: AC_DEFUN([TESTA], [AC_REQUIRE([TESTA_PREREQ])[echo in A]]) AC_DEFUN([TESTA_PREREQ], [[echo in A_PREREQ if test -n "$SEEN_A" ; then echo duplicate ; fi SEEN_A=:]]) AC_DEFUN([TESTB], [AC_REQUIRE([TESTA_PREREQ])[echo in B if test -z "$SEEN_A" ; then echo bug ; fi]]) AC_DEFUN([TESTC], [AC_REQUIRE([TESTB])[echo in C]]) AC_DEFUN([OUTER], [[echo in OUTER] TESTA TESTC]) OUTER the resulting output will then obey all dependency rules and avoid any syntax warnings, whether the script is built with old or new Autoconf versions: in A_PREREQ in B in OUTER in A in C The helper macros `AS_IF' and `AS_CASE' may be used to enforce expansion of required macros outside of shell conditional constructs. You are furthermore encouraged, although not required, to put all `AC_REQUIRE' calls at the beginning of a macro. You can use `dnl' to avoid the empty lines they leave. 10.4.2 Suggested Ordering ------------------------- Some macros should be run before another macro if both are called, but neither _requires_ that the other be called. For example, a macro that changes the behavior of the C compiler should be called before any macros that run the C compiler. Many of these dependencies are noted in the documentation. Autoconf provides the `AC_BEFORE' macro to warn users when macros with this kind of dependency appear out of order in a `configure.ac' file. The warning occurs when creating `configure' from `configure.ac', not when running `configure'. For example, `AC_PROG_CPP' checks whether the C compiler can run the C preprocessor when given the `-E' option. It should therefore be called after any macros that change which C compiler is being used, such as `AC_PROG_CC'. So `AC_PROG_CC' contains: AC_BEFORE([$0], [AC_PROG_CPP])dnl This warns the user if a call to `AC_PROG_CPP' has already occurred when `AC_PROG_CC' is called. -- Macro: AC_BEFORE (THIS-MACRO-NAME, CALLED-MACRO-NAME) Make M4 print a warning message to the standard error output if CALLED-MACRO-NAME has already been called. THIS-MACRO-NAME should be the name of the macro that is calling `AC_BEFORE'. The macro CALLED-MACRO-NAME must have been defined using `AC_DEFUN' or else contain a call to `AC_PROVIDE' to indicate that it has been called. 10.4.3 One-Shot Macros ---------------------- Some macros should be called only once, either because calling them multiple time is unsafe, or because it is bad style. For instance Autoconf ensures that `AC_CANONICAL_BUILD' and cousins (*note Canonicalizing::) are evaluated only once, because it makes no sense to run these expensive checks more than once. Such one-shot macros can be defined using `AC_DEFUN_ONCE'. -- Macro: AC_DEFUN_ONCE (MACRO-NAME, MACRO-BODY) Declare macro MACRO-NAME like `AC_DEFUN' would (*note Macro Definitions::), but add additional logic that guarantees that only the first use of the macro (whether by direct expansion or `AC_REQUIRE') causes an expansion of MACRO-BODY; the expansion will occur before the start of any enclosing macro defined by `AC_DEFUN'. Subsequent expansions are silently ignored. Generally, it does not make sense for MACRO-BODY to use parameters such as `$1'. Prior to Autoconf 2.64, a macro defined by `AC_DEFUN_ONCE' would emit a warning if it was directly expanded a second time, so for portability, it is better to use `AC_REQUIRE' than direct invocation of MACRO-NAME inside a macro defined by `AC_DEFUN' (*note Prerequisite Macros::). 10.5 Obsoleting Macros ====================== Configuration and portability technology has evolved over the years. Often better ways of solving a particular problem are developed, or ad-hoc approaches are systematized. This process has occurred in many parts of Autoconf. One result is that some of the macros are now considered "obsolete"; they still work, but are no longer considered the best thing to do, hence they should be replaced with more modern macros. Ideally, `autoupdate' should replace the old macro calls with their modern implementation. Autoconf provides a simple means to obsolete a macro. -- Macro: AU_DEFUN (OLD-MACRO, IMPLEMENTATION, [MESSAGE]) Define OLD-MACRO as IMPLEMENTATION. The only difference with `AC_DEFUN' is that the user is warned that OLD-MACRO is now obsolete. If she then uses `autoupdate', the call to OLD-MACRO is replaced by the modern IMPLEMENTATION. MESSAGE should include information on what to do after running `autoupdate'; `autoupdate' prints it as a warning, and includes it in the updated `configure.ac' file. The details of this macro are hairy: if `autoconf' encounters an `AU_DEFUN'ed macro, all macros inside its second argument are expanded as usual. However, when `autoupdate' is run, only M4 and M4sugar macros are expanded here, while all other macros are disabled and appear literally in the updated `configure.ac'. -- Macro: AU_ALIAS (OLD-NAME, NEW-NAME) Used if the OLD-NAME is to be replaced by a call to NEW-MACRO with the same parameters. This happens for example if the macro was renamed. 10.6 Coding Style ================= The Autoconf macros follow a strict coding style. You are encouraged to follow this style, especially if you intend to distribute your macro, either by contributing it to Autoconf itself or the Autoconf archive (http://www.nongnu.org/autoconf-archive), or by other means. The first requirement is to pay great attention to the quotation. For more details, see *note Autoconf Language::, and *note M4 Quotation::. Do not try to invent new interfaces. It is likely that there is a macro in Autoconf that resembles the macro you are defining: try to stick to this existing interface (order of arguments, default values, etc.). We _are_ conscious that some of these interfaces are not perfect; nevertheless, when harmless, homogeneity should be preferred over creativity. Be careful about clashes both between M4 symbols and between shell variables. If you stick to the suggested M4 naming scheme (*note Macro Names::), you are unlikely to generate conflicts. Nevertheless, when you need to set a special value, _avoid using a regular macro name_; rather, use an "impossible" name. For instance, up to version 2.13, the macro `AC_SUBST' used to remember what SYMBOL macros were already defined by setting `AC_SUBST_SYMBOL', which is a regular macro name. But since there is a macro named `AC_SUBST_FILE', it was just impossible to `AC_SUBST(FILE)'! In this case, `AC_SUBST(SYMBOL)' or `_AC_SUBST(SYMBOL)' should have been used (yes, with the parentheses). No Autoconf macro should ever enter the user-variable name space; i.e., except for the variables that are the actual result of running the macro, all shell variables should start with `ac_'. In addition, small macros or any macro that is likely to be embedded in other macros should be careful not to use obvious names. Do not use `dnl' to introduce comments: most of the comments you are likely to write are either header comments which are not output anyway, or comments that should make their way into `configure'. There are exceptional cases where you do want to comment special M4 constructs, in which case `dnl' is right, but keep in mind that it is unlikely. M4 ignores the leading blanks and newlines before each argument. Use this feature to indent in such a way that arguments are (more or less) aligned with the opening parenthesis of the macro being called. For instance, instead of AC_CACHE_CHECK(for EMX OS/2 environment, ac_cv_emxos2, [AC_COMPILE_IFELSE([AC_LANG_PROGRAM(, [return __EMX__;])], [ac_cv_emxos2=yes], [ac_cv_emxos2=no])]) write AC_CACHE_CHECK([for EMX OS/2 environment], [ac_cv_emxos2], [AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [return __EMX__;])], [ac_cv_emxos2=yes], [ac_cv_emxos2=no])]) or even AC_CACHE_CHECK([for EMX OS/2 environment], [ac_cv_emxos2], [AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [return __EMX__;])], [ac_cv_emxos2=yes], [ac_cv_emxos2=no])]) When using `AC_RUN_IFELSE' or any macro that cannot work when cross-compiling, provide a pessimistic value (typically `no'). Feel free to use various tricks to prevent auxiliary tools, such as syntax-highlighting editors, from behaving improperly. For instance, instead of: m4_bpatsubst([$1], [$"]) use m4_bpatsubst([$1], [$""]) so that Emacsen do not open an endless "string" at the first quote. For the same reasons, avoid: test $[#] != 0 and use: test $[@%:@] != 0 Otherwise, the closing bracket would be hidden inside a `#'-comment, breaking the bracket-matching highlighting from Emacsen. Note the preferred style to escape from M4: `$[1]', `$[@]', etc. Do not escape when it is unnecessary. Common examples of useless quotation are `[$]$1' (write `$$1'), `[$]var' (use `$var'), etc. If you add portability issues to the picture, you'll prefer `${1+"$[@]"}' to `"[$]@"', and you'll prefer do something better than hacking Autoconf `:-)'. When using `sed', don't use `-e' except for indenting purposes. With the `s' and `y' commands, the preferred separator is `/' unless `/' itself might appear in the pattern or replacement, in which case you should use `|', or optionally `,' if you know the pattern and replacement cannot contain a file name. If none of these characters will do, choose a printable character that cannot appear in the pattern or replacement. Characters from the set `"#$&'()*;<=>?`|~' are good choices if the pattern or replacement might contain a file name, since they have special meaning to the shell and are less likely to occur in file names. *Note Macro Definitions::, for details on how to define a macro. If a macro doesn't use `AC_REQUIRE', is expected to never be the object of an `AC_REQUIRE' directive, and macros required by other macros inside arguments do not need to be expanded before this macro, then use `m4_define'. In case of doubt, use `AC_DEFUN'. Also take into account that public third-party macros need to use `AC_DEFUN' in order to be found by `aclocal' (*note Extending aclocal: (automake)Extending aclocal.). All the `AC_REQUIRE' statements should be at the beginning of the macro, and each statement should be followed by `dnl'. You should not rely on the number of arguments: instead of checking whether an argument is missing, test that it is not empty. It provides both a simpler and a more predictable interface to the user, and saves room for further arguments. Unless the macro is short, try to leave the closing `])' at the beginning of a line, followed by a comment that repeats the name of the macro being defined. This introduces an additional newline in `configure'; normally, that is not a problem, but if you want to remove it you can use `[]dnl' on the last line. You can similarly use `[]dnl' after a macro call to remove its newline. `[]dnl' is recommended instead of `dnl' to ensure that M4 does not interpret the `dnl' as being attached to the preceding text or macro output. For example, instead of: AC_DEFUN([AC_PATH_X], [AC_MSG_CHECKING([for X]) AC_REQUIRE_CPP() # ...omitted... AC_MSG_RESULT([libraries $x_libraries, headers $x_includes]) fi]) you would write: AC_DEFUN([AC_PATH_X], [AC_REQUIRE_CPP()[]dnl AC_MSG_CHECKING([for X]) # ...omitted... AC_MSG_RESULT([libraries $x_libraries, headers $x_includes]) fi[]dnl ])# AC_PATH_X If the macro is long, try to split it into logical chunks. Typically, macros that check for a bug in a function and prepare its `AC_LIBOBJ' replacement should have an auxiliary macro to perform this setup. Do not hesitate to introduce auxiliary macros to factor your code. In order to highlight the recommended coding style, here is a macro written the old way: dnl Check for EMX on OS/2. dnl _AC_EMXOS2 AC_DEFUN(_AC_EMXOS2, [AC_CACHE_CHECK(for EMX OS/2 environment, ac_cv_emxos2, [AC_COMPILE_IFELSE([AC_LANG_PROGRAM(, return __EMX__;)], ac_cv_emxos2=yes, ac_cv_emxos2=no)]) test "x$ac_cv_emxos2" = xyes && EMXOS2=yes]) and the new way: # _AC_EMXOS2 # ---------- # Check for EMX on OS/2. m4_define([_AC_EMXOS2], [AC_CACHE_CHECK([for EMX OS/2 environment], [ac_cv_emxos2], [AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [return __EMX__;])], [ac_cv_emxos2=yes], [ac_cv_emxos2=no])]) test "x$ac_cv_emxos2" = xyes && EMXOS2=yes[]dnl ])# _AC_EMXOS2 11 Portable Shell Programming ***************************** When writing your own checks, there are some shell-script programming techniques you should avoid in order to make your code portable. The Bourne shell and upward-compatible shells like the Korn shell and Bash have evolved over the years, and many features added to the original System7 shell are now supported on all interesting porting targets. However, the following discussion between Russ Allbery and Robert Lipe is worth reading: Russ Allbery: The GNU assumption that `/bin/sh' is the one and only shell leads to a permanent deadlock. Vendors don't want to break users' existing shell scripts, and there are some corner cases in the Bourne shell that are not completely compatible with a Posix shell. Thus, vendors who have taken this route will _never_ (OK..."never say never") replace the Bourne shell (as `/bin/sh') with a Posix shell. Robert Lipe: This is exactly the problem. While most (at least most System V's) do have a Bourne shell that accepts shell functions most vendor `/bin/sh' programs are not the Posix shell. So while most modern systems do have a shell _somewhere_ that meets the Posix standard, the challenge is to find it. For this reason, part of the job of M4sh (*note Programming in M4sh::) is to find such a shell. But to prevent trouble, if you're not using M4sh you should not take advantage of features that were added after Unix version 7, circa 1977 (*note Systemology::); you should not use aliases, negated character classes, or even `unset'. `#' comments, while not in Unix version 7, were retrofitted in the original Bourne shell and can be assumed to be part of the least common denominator. On the other hand, if you're using M4sh you can assume that the shell has the features that were added in SVR2 (circa 1984), including shell functions, `return', `unset', and I/O redirection for builtins. For more information, refer to `http://www.in-ulm.de/~mascheck/bourne/'. However, some pitfalls have to be avoided for portable use of these constructs; these will be documented in the rest of this chapter. See in particular *note Shell Functions:: and *note Limitations of Shell Builtins: Limitations of Builtins. Some ancient systems have quite small limits on the length of the `#!' line; for instance, 32 bytes (not including the newline) on SunOS 4. However, these ancient systems are no longer of practical concern. The set of external programs you should run in a `configure' script is fairly small. *Note Utilities in Makefiles: (standards)Utilities in Makefiles, for the list. This restriction allows users to start out with a fairly small set of programs and build the rest, avoiding too many interdependencies between packages. Some of these external utilities have a portable subset of features; see *note Limitations of Usual Tools::. There are other sources of documentation about shells. The specification for the Posix Shell Command Language (http://www.opengroup.org/susv3/utilities/xcu_chap02.html), though more generous than the restrictive shell subset described above, is fairly portable nowadays. Also please see the Shell FAQs (http://www.faqs.org/faqs/unix-faq/shell/). 11.1 Shellology =============== There are several families of shells, most prominently the Bourne family and the C shell family which are deeply incompatible. If you want to write portable shell scripts, avoid members of the C shell family. The the Shell difference FAQ (http://www.faqs.org/faqs/unix-faq/shell/shell-differences/) includes a small history of Posix shells, and a comparison between several of them. Below we describe some of the members of the Bourne shell family. Ash Ash is often used on GNU/Linux and BSD systems as a light-weight Bourne-compatible shell. Ash 0.2 has some bugs that are fixed in the 0.3.x series, but portable shell scripts should work around them, since version 0.2 is still shipped with many GNU/Linux distributions. To be compatible with Ash 0.2: - don't use `$?' after expanding empty or unset variables, or at the start of an `eval': foo= false $foo echo "Do not use it: $?" false eval 'echo "Do not use it: $?"' - don't use command substitution within variable expansion: cat ${FOO=`bar`} - beware that single builtin substitutions are not performed by a subshell, hence their effect applies to the current shell! *Note Shell Substitutions::, item "Command Substitution". Bash To detect whether you are running Bash, test whether `BASH_VERSION' is set. To require Posix compatibility, run `set -o posix'. *Note Bash Posix Mode: (bash)Bash POSIX Mode, for details. Bash 2.05 and later Versions 2.05 and later of Bash use a different format for the output of the `set' builtin, designed to make evaluating its output easier. However, this output is not compatible with earlier versions of Bash (or with many other shells, probably). So if you use Bash 2.05 or higher to execute `configure', you'll need to use Bash 2.05 for all other build tasks as well. Ksh The Korn shell is compatible with the Bourne family and it mostly conforms to Posix. It has two major variants commonly called `ksh88' and `ksh93', named after the years of initial release. It is usually called `ksh', but is called `sh' on some hosts if you set your path appropriately. Solaris systems have three variants: `/usr/bin/ksh' is `ksh88'; it is standard on Solaris 2.0 and later. `/usr/xpg4/bin/sh' is a Posix-compliant variant of `ksh88'; it is standard on Solaris 9 and later. `/usr/dt/bin/dtksh' is `ksh93'. Variants that are not standard may be parts of optional packages. There is no extra charge for these packages, but they are not part of a minimal OS install and therefore some installations may not have it. Starting with Tru64 Version 4.0, the Korn shell `/usr/bin/ksh' is also available as `/usr/bin/posix/sh'. If the environment variable `BIN_SH' is set to `xpg4', subsidiary invocations of the standard shell conform to Posix. Pdksh A public-domain clone of the Korn shell called `pdksh' is widely available: it has most of the `ksh88' features along with a few of its own. It usually sets `KSH_VERSION', except if invoked as `/bin/sh' on OpenBSD, and similarly to Bash you can require Posix compatibility by running `set -o posix'. Unfortunately, with `pdksh' 5.2.14 (the latest stable version as of January 2007) Posix mode is buggy and causes `pdksh' to depart from Posix in at least one respect: $ echo "`echo \"hello\"`" hello $ set -o posix $ echo "`echo \"hello\"`" "hello" The last line of output contains spurious quotes. This is yet another reason why portable shell code should not contain `"`...\"...\"...`"' constructs (*note Shell Substitutions::). Zsh To detect whether you are running `zsh', test whether `ZSH_VERSION' is set. By default `zsh' is _not_ compatible with the Bourne shell: you must execute `emulate sh', and for `zsh' versions before 3.1.6-dev-18 you must also set `NULLCMD' to `:'. *Note Compatibility: (zsh)Compatibility, for details. The default Mac OS X `sh' was originally Zsh; it was changed to Bash in Mac OS X 10.2. 11.2 Here-Documents =================== Don't rely on `\' being preserved just because it has no special meaning together with the next symbol. In the native `sh' on OpenBSD 2.7 `\"' expands to `"' in here-documents with unquoted delimiter. As a general rule, if `\\' expands to `\' use `\\' to get `\'. With OpenBSD 2.7's `sh' $ cat < \" \\ > EOF " \ and with Bash: bash-2.04$ cat < \" \\ > EOF \" \ Some shells mishandle large here-documents: for example, Solaris 10 `dtksh' and the UnixWare 7.1.1 Posix shell, which are derived from Korn shell version M-12/28/93d, mishandle braced variable expansion that crosses a 1024- or 4096-byte buffer boundary within a here-document. Only the part of the variable name after the boundary is used. For example, `${variable}' could be replaced by the expansion of `${ble}'. If the end of the variable name is aligned with the block boundary, the shell reports an error, as if you used `${}'. Instead of `${variable-default}', the shell may expand `${riable-default}', or even `${fault}'. This bug can often be worked around by omitting the braces: `$variable'. The bug was fixed in `ksh93g' (1998-04-30) but as of 2006 many operating systems were still shipping older versions with the bug. Many shells (including the Bourne shell) implement here-documents inefficiently. In particular, some shells can be extremely inefficient when a single statement contains many here-documents. For instance if your `configure.ac' includes something like: if ; then assume this and that else check this check that check something else ... on and on forever ... fi A shell parses the whole `if'/`fi' construct, creating temporary files for each here-document in it. Some shells create links for such here-documents on every `fork', so that the clean-up code they had installed correctly removes them. It is creating the links that can take the shell forever. Moving the tests out of the `if'/`fi', or creating multiple `if'/`fi' constructs, would improve the performance significantly. Anyway, this kind of construct is not exactly the typical use of Autoconf. In fact, it's even not recommended, because M4 macros can't look into shell conditionals, so we may fail to expand a macro when it was expanded before in a conditional path, and the condition turned out to be false at runtime, and we end up not executing the macro at all. 11.3 File Descriptors ===================== Most shells, if not all (including Bash, Zsh, Ash), output traces on stderr, even for subshells. This might result in undesirable content if you meant to capture the standard-error output of the inner command: $ ash -x -c '(eval "echo foo >&2") 2>stderr' $ cat stderr + eval echo foo >&2 + echo foo foo $ bash -x -c '(eval "echo foo >&2") 2>stderr' $ cat stderr + eval 'echo foo >&2' ++ echo foo foo $ zsh -x -c '(eval "echo foo >&2") 2>stderr' # Traces on startup files deleted here. $ cat stderr +zsh:1> eval echo foo >&2 +zsh:1> echo foo foo One workaround is to grep out uninteresting lines, hoping not to remove good ones. If you intend to redirect both standard error and standard output, redirect standard output first. This works better with HP-UX, since its shell mishandles tracing if standard error is redirected first: $ sh -x -c ': 2>err >out' + : + 2> err $ cat err 1> out Don't try to redirect the standard error of a command substitution. It must be done _inside_ the command substitution. When running `: `cd /zorglub` 2>/dev/null' expect the error message to escape, while `: `cd /zorglub 2>/dev/null`' works properly. It is worth noting that Zsh (but not Ash nor Bash) makes it possible in assignments though: `foo=`cd /zorglub` 2>/dev/null'. Some shells, like `ash', don't recognize bi-directional redirection (`<>'). And even on shells that recognize it, it is not portable to use on fifos: Posix does not require read-write support for named pipes, and Cygwin does not support it: $ mkfifo fifo $ exec 5<>fifo $ echo hi >&5 bash: echo: write error: Communication error on send When catering to old systems, don't redirect the same file descriptor several times, as you are doomed to failure under Ultrix. ULTRIX V4.4 (Rev. 69) System #31: Thu Aug 10 19:42:23 GMT 1995 UWS V4.4 (Rev. 11) $ eval 'echo matter >fullness' >void illegal io $ eval '(echo matter >fullness)' >void illegal io $ (eval '(echo matter >fullness)') >void Ambiguous output redirect. In each case the expected result is of course `fullness' containing `matter' and `void' being empty. However, this bug is probably not of practical concern to modern platforms. Solaris 10 `sh' will try to optimize away a `:' command in a loop after the first iteration, even if it is redirected: $ for i in 1 2 3 ; do : >x$i; done $ ls x1 As a workaround, `echo' or `eval' can be used. Don't rely on file descriptors 0, 1, and 2 remaining closed in a subsidiary program. If any of these descriptors is closed, the operating system may open an unspecified file for the descriptor in the new process image. Posix says this may be done only if the subsidiary program is set-user-ID or set-group-ID, but HP-UX 11.23 does it even for ordinary programs. Don't rely on open file descriptors being open in child processes. In `ksh', file descriptors above 2 which are opened using `exec N>file' are closed by a subsequent `exec' (such as that involved in the fork-and-exec which runs a program or script). Thus, using `sh', we have: $ cat ./descrips #!/bin/sh - echo hello >&5 $ exec 5>t $ ./descrips $ cat t hello $ But using ksh: $ exec 5>t $ ./descrips hello $ cat t $ Within the process which runs the `descrips' script, file descriptor 5 is closed. Don't rely on redirection to a closed file descriptor to cause an error. With Solaris `/bin/sh', when the redirection fails, the output goes to the original file descriptor. $ bash -c 'echo hi >&3' 3>&-; echo $? bash: 3: Bad file descriptor 1 $ /bin/sh -c 'echo hi >&3' 3>&-; echo $? hi 0 DOS variants cannot rename or remove open files, such as in `mv foo bar >foo' or `rm foo >foo', even though this is perfectly portable among Posix hosts. A few ancient systems reserved some file descriptors. By convention, file descriptor 3 was opened to `/dev/tty' when you logged into Eighth Edition (1985) through Tenth Edition Unix (1989). File descriptor 4 had a special use on the Stardent/Kubota Titan (circa 1990), though we don't now remember what it was. Both these systems are obsolete, so it's now safe to treat file descriptors 3 and 4 like any other file descriptors. 11.4 File System Conventions ============================ Autoconf uses shell-script processing extensively, so the file names that it processes should not contain characters that are special to the shell. Special characters include space, tab, newline, NUL, and the following: " # $ & ' ( ) * ; < = > ? [ \ ` | Also, file names should not begin with `~' or `-', and should contain neither `-' immediately after `/' nor `~' immediately after `:'. On Posix-like platforms, directory names should not contain `:', as this runs afoul of `:' used as the path separator. These restrictions apply not only to the files that you distribute, but also to the absolute file names of your source, build, and destination directories. On some Posix-like platforms, `!' and `^' are special too, so they should be avoided. Posix lets implementations treat leading `//' specially, but requires leading `///' and beyond to be equivalent to `/'. Most Unix variants treat `//' like `/'. However, some treat `//' as a "super-root" that can provide access to files that are not otherwise reachable from `/'. The super-root tradition began with Apollo Domain/OS, which died out long ago, but unfortunately Cygwin has revived it. While `autoconf' and friends are usually run on some Posix variety, they can be used on other systems, most notably DOS variants. This impacts several assumptions regarding file names. For example, the following code: case $foo_dir in /*) # Absolute ;; *) foo_dir=$dots$foo_dir ;; esac fails to properly detect absolute file names on those systems, because they can use a drivespec, and usually use a backslash as directory separator. If you want to be portable to DOS variants (at the price of rejecting valid but oddball Posix file names like `a:\b'), you can check for absolute file names like this: case $foo_dir in [\\/]* | ?:[\\/]* ) # Absolute ;; *) foo_dir=$dots$foo_dir ;; esac Make sure you quote the brackets if appropriate and keep the backslash as first character (*note Limitations of Shell Builtins: case.). Also, because the colon is used as part of a drivespec, these systems don't use it as path separator. When creating or accessing paths, you can use the `PATH_SEPARATOR' output variable instead. `configure' sets this to the appropriate value for the build system (`:' or `;') when it starts up. File names need extra care as well. While DOS variants that are Posixy enough to run `autoconf' (such as DJGPP) are usually able to handle long file names properly, there are still limitations that can seriously break packages. Several of these issues can be easily detected by the doschk (ftp://ftp.gnu.org/gnu/non-gnu/doschk/doschk-1.1.tar.gz) package. A short overview follows; problems are marked with SFN/LFN to indicate where they apply: SFN means the issues are only relevant to plain DOS, not to DOS under Microsoft Windows variants, while LFN identifies problems that exist even under Microsoft Windows variants. No multiple dots (SFN) DOS cannot handle multiple dots in file names. This is an especially important thing to remember when building a portable configure script, as `autoconf' uses a .in suffix for template files. This is perfectly OK on Posix variants: AC_CONFIG_HEADERS([config.h]) AC_CONFIG_FILES([source.c foo.bar]) AC_OUTPUT but it causes problems on DOS, as it requires `config.h.in', `source.c.in' and `foo.bar.in'. To make your package more portable to DOS-based environments, you should use this instead: AC_CONFIG_HEADERS([config.h:config.hin]) AC_CONFIG_FILES([source.c:source.cin foo.bar:foobar.in]) AC_OUTPUT No leading dot (SFN) DOS cannot handle file names that start with a dot. This is usually not important for `autoconf'. Case insensitivity (LFN) DOS is case insensitive, so you cannot, for example, have both a file called `INSTALL' and a directory called `install'. This also affects `make'; if there's a file called `INSTALL' in the directory, `make install' does nothing (unless the `install' target is marked as PHONY). The 8+3 limit (SFN) Because the DOS file system only stores the first 8 characters of the file name and the first 3 of the extension, those must be unique. That means that `foobar-part1.c', `foobar-part2.c' and `foobar-prettybird.c' all resolve to the same file name (`FOOBAR-P.C'). The same goes for `foo.bar' and `foo.bartender'. The 8+3 limit is not usually a problem under Microsoft Windows, as it uses numeric tails in the short version of file names to make them unique. However, a registry setting can turn this behavior off. While this makes it possible to share file trees containing long file names between SFN and LFN environments, it also means the above problem applies there as well. Invalid characters (LFN) Some characters are invalid in DOS file names, and should therefore be avoided. In a LFN environment, these are `/', `\', `?', `*', `:', `<', `>', `|' and `"'. In a SFN environment, other characters are also invalid. These include `+', `,', `[' and `]'. Invalid names (LFN) Some DOS file names are reserved, and cause problems if you try to use files with those names. These names include `CON', `AUX', `COM1', `COM2', `COM3', `COM4', `LPT1', `LPT2', `LPT3', `NUL', and `PRN'. File names are case insensitive, so even names like `aux/config.guess' are disallowed. 11.5 Shell Pattern Matching =========================== Nowadays portable patterns can use negated character classes like `[!-aeiou]'. The older syntax `[^-aeiou]' is supported by some shells but not others; hence portable scripts should never use `^' as the first character of a bracket pattern. Outside the C locale, patterns like `[a-z]' are problematic since they may match characters that are not lower-case letters. 11.6 Shell Substitutions ======================== Contrary to a persistent urban legend, the Bourne shell does not systematically split variables and back-quoted expressions, in particular on the right-hand side of assignments and in the argument of `case'. For instance, the following code: case "$given_srcdir" in .) top_srcdir="`echo "$dots" | sed 's|/$||'`" ;; *) top_srcdir="$dots$given_srcdir" ;; esac is more readable when written as: case $given_srcdir in .) top_srcdir=`echo "$dots" | sed 's|/$||'` ;; *) top_srcdir=$dots$given_srcdir ;; esac and in fact it is even _more_ portable: in the first case of the first attempt, the computation of `top_srcdir' is not portable, since not all shells properly understand `"`..."..."...`"'. Worse yet, not all shells understand `"`...\"...\"...`"' the same way. There is just no portable way to use double-quoted strings inside double-quoted back-quoted expressions (pfew!). `$@' One of the most famous shell-portability issues is related to `"$@"'. When there are no positional arguments, Posix says that `"$@"' is supposed to be equivalent to nothing, but the original Unix version 7 Bourne shell treated it as equivalent to `""' instead, and this behavior survives in later implementations like Digital Unix 5.0. The traditional way to work around this portability problem is to use `${1+"$@"}'. Unfortunately this method does not work with Zsh (3.x and 4.x), which is used on Mac OS X. When emulating the Bourne shell, Zsh performs word splitting on `${1+"$@"}': zsh $ emulate sh zsh $ for i in "$@"; do echo $i; done Hello World ! zsh $ for i in ${1+"$@"}; do echo $i; done Hello World ! Zsh handles plain `"$@"' properly, but we can't use plain `"$@"' because of the portability problems mentioned above. One workaround relies on Zsh's "global aliases" to convert `${1+"$@"}' into `"$@"' by itself: test "${ZSH_VERSION+set}" = set && alias -g '${1+"$@"}'='"$@"' Zsh only recognizes this alias when a shell word matches it exactly; `"foo"${1+"$@"}' remains subject to word splitting. Since this case always yields at least one shell word, use plain `"$@"'. A more conservative workaround is to avoid `"$@"' if it is possible that there may be no positional arguments. For example, instead of: cat conftest.c "$@" you can use this instead: case $# in 0) cat conftest.c;; *) cat conftest.c "$@";; esac Autoconf macros often use the `set' command to update `$@', so if you are writing shell code intended for `configure' you should not assume that the value of `$@' persists for any length of time. `${10}' The 10th, 11th, ... positional parameters can be accessed only after a `shift'. The 7th Edition shell reported an error if given `${10}', and Solaris 10 `/bin/sh' still acts that way: $ set 1 2 3 4 5 6 7 8 9 10 $ echo ${10} bad substitution `${VAR:-VALUE}' Old BSD shells, including the Ultrix `sh', don't accept the colon for any shell substitution, and complain and die. Similarly for ${VAR:=VALUE}, ${VAR:?VALUE}, etc. `${VAR=LITERAL}' Be sure to quote: : ${var='Some words'} otherwise some shells, such as on Digital Unix V 5.0, die because of a "bad substitution". Solaris `/bin/sh' has a frightening bug in its interpretation of this. Imagine you need set a variable to a string containing `}'. This `}' character confuses Solaris `/bin/sh' when the affected variable was already set. This bug can be exercised by running: $ unset foo $ foo=${foo='}'} $ echo $foo } $ foo=${foo='}' # no error; this hints to what the bug is $ echo $foo } $ foo=${foo='}'} $ echo $foo }} ^ ugh! It seems that `}' is interpreted as matching `${', even though it is enclosed in single quotes. The problem doesn't happen using double quotes. `${VAR=EXPANDED-VALUE}' On Ultrix, running default="yu,yaa" : ${var="$default"} sets VAR to `M-yM-uM-,M-yM-aM-a', i.e., the 8th bit of each char is set. You don't observe the phenomenon using a simple `echo $var' since apparently the shell resets the 8th bit when it expands $var. Here are two means to make this shell confess its sins: $ cat -v < broken $ echo "`printf 'foo\r\n'`"" bar" | cmp - broken - broken differ: char 4, line 1 Upon interrupt or SIGTERM, some shells may abort a command substitution, replace it with a null string, and wrongly evaluate the enclosing command before entering the trap or ending the script. This can lead to spurious errors: $ sh -c 'if test `sleep 5; echo hi` = hi; then echo yes; fi' $ ^C sh: test: hi: unexpected operator/operand You can avoid this by assigning the command substitution to a temporary variable: $ sh -c 'res=`sleep 5; echo hi` if test "x$res" = xhi; then echo yes; fi' $ ^C `$(COMMANDS)' This construct is meant to replace ``COMMANDS`', and it has most of the problems listed under ``COMMANDS`'. This construct can be nested while this is impossible to do portably with back quotes. Unfortunately it is not yet universally supported. Most notably, even recent releases of Solaris don't support it: $ showrev -c /bin/sh | grep version Command version: SunOS 5.10 Generic 121005-03 Oct 2006 $ echo $(echo blah) syntax error: `(' unexpected nor does IRIX 6.5's Bourne shell: $ uname -a IRIX firebird-image 6.5 07151432 IP22 $ echo $(echo blah) $(echo blah) If you do use `$(COMMANDS)', make sure that the commands do not start with a parenthesis, as that would cause confusion with a different notation `$((EXPRESSION))' that in modern shells is an arithmetic expression not a command. To avoid the confusion, insert a space between the two opening parentheses. Avoid COMMANDS that contain unbalanced parentheses in here-documents, comments, or case statement patterns, as many shells mishandle them. For example, Bash 3.1, `ksh88', `pdksh' 5.2.14, and Zsh 4.2.6 all mishandle the following valid command: echo $(case x in x) echo hello;; esac) `$((EXPRESSION))' Arithmetic expansion is not portable as some shells (most notably Solaris 10 `/bin/sh') don't support it. Among shells that do support `$(( ))', not all of them obey the Posix rule that octal and hexadecimal constants must be recognized: $ bash -c 'echo $(( 010 + 0x10 ))' 24 $ zsh -c 'echo $(( 010 + 0x10 ))' 26 $ zsh -c 'emulate sh; echo $(( 010 + 0x10 ))' 24 $ pdksh -c 'echo $(( 010 + 0x10 ))' pdksh: 010 + 0x10 : bad number `0x10' $ pdksh -c 'echo $(( 010 ))' 10 When it is available, using arithmetic expansion provides a noticeable speedup in script execution; but testing for support requires `eval' to avoid syntax errors. The following construct is used by `AS_VAR_ARITH' to provide arithmetic computation when all arguments are provided in decimal and without a leading zero, and all operators are properly quoted and appear as distinct arguments: if ( eval 'test $(( 1 + 1 )) = 2' ) 2>/dev/null; then eval 'func_arith () { func_arith_result=$(( $* )) }' else func_arith () { func_arith_result=`expr "$@"` } fi func_arith 1 + 1 foo=$func_arith_result `^' Always quote `^', otherwise traditional shells such as `/bin/sh' on Solaris 10 treat this like `|'. 11.7 Assignments ================ When setting several variables in a row, be aware that the order of the evaluation is undefined. For instance `foo=1 foo=2; echo $foo' gives `1' with Solaris `/bin/sh', but `2' with Bash. You must use `;' to enforce the order: `foo=1; foo=2; echo $foo'. Don't rely on the following to find `subdir/program': PATH=subdir$PATH_SEPARATOR$PATH program as this does not work with Zsh 3.0.6. Use something like this instead: (PATH=subdir$PATH_SEPARATOR$PATH; export PATH; exec program) Don't rely on the exit status of an assignment: Ash 0.2 does not change the status and propagates that of the last statement: $ false || foo=bar; echo $? 1 $ false || foo=`:`; echo $? 0 and to make things even worse, QNX 4.25 just sets the exit status to 0 in any case: $ foo=`exit 1`; echo $? 0 To assign default values, follow this algorithm: 1. If the default value is a literal and does not contain any closing brace, use: : ${var='my literal'} 2. If the default value contains no closing brace, has to be expanded, and the variable being initialized is not intended to be IFS-split (i.e., it's not a list), then use: : ${var="$default"} 3. If the default value contains no closing brace, has to be expanded, and the variable being initialized is intended to be IFS-split (i.e., it's a list), then use: var=${var="$default"} 4. If the default value contains a closing brace, then use: test "${var+set}" = set || var="has a '}'" In most cases `var=${var="$default"}' is fine, but in case of doubt, just use the last form. *Note Shell Substitutions::, items `${VAR:-VALUE}' and `${VAR=VALUE}' for the rationale. 11.8 Parentheses in Shell Scripts ================================= Beware of two opening parentheses in a row, as many shell implementations treat them specially. Posix requires that the command `((cat))' must behave like `(cat)', but many shells, including Bash and the Korn shell, treat `((cat))' as an arithmetic expression equivalent to `let "cat"', and may or may not report an error when they detect that `cat' is not a number. As another example, `pdksh' 5.2.14 misparses the following code: if ((true) || false); then echo ok fi To work around this problem, insert a space between the two opening parentheses. There is a similar problem and workaround with `$(('; see *note Shell Substitutions::. 11.9 Slashes in Shell Scripts ============================= Unpatched Tru64 5.1 `sh' omits the last slash of command-line arguments that contain two trailing slashes: $ echo / // /// //// .// //. / / // /// ./ //. $ x=// $ eval "echo \$x" / $ set -x $ echo abc | tr -t ab // + echo abc + tr -t ab / /bc Unpatched Tru64 4.0 `sh' adds a slash after `"$var"' if the variable is empty and the second double-quote is followed by a word that begins and ends with slash: $ sh -xc 'p=; echo "$p"/ouch/' p= + echo //ouch/ //ouch/ However, our understanding is that patches are available, so perhaps it's not worth worrying about working around these horrendous bugs. 11.10 Special Shell Variables ============================= Some shell variables should not be used, since they can have a deep influence on the behavior of the shell. In order to recover a sane behavior from the shell, some variables should be unset; M4sh takes care of this and provides fallback values, whenever needed, to cater for a very old `/bin/sh' that does not support `unset'. (*note Portable Shell Programming: Portable Shell.). As a general rule, shell variable names containing a lower-case letter are safe; you can define and use these variables without worrying about their effect on the underlying system, and without worrying about whether the shell changes them unexpectedly. (The exception is the shell variable `status', as described below.) Here is a list of names that are known to cause trouble. This list is not exhaustive, but you should be safe if you avoid the name `status' and names containing only upper-case letters and underscores. `?' Not all shells correctly reset `$?' after conditionals (*note Limitations of Shell Builtins: if.). Not all shells manage `$?' correctly in shell functions (*note Shell Functions::) or in traps (*note Limitations of Shell Builtins: trap.). Not all shells reset `$?' to zero after an empty command. $ bash -c 'false; $empty; echo $?' 0 $ zsh -c 'false; $empty; echo $?' 1 `_' Many shells reserve `$_' for various purposes, e.g., the name of the last command executed. `BIN_SH' In Tru64, if `BIN_SH' is set to `xpg4', subsidiary invocations of the standard shell conform to Posix. `CDPATH' When this variable is set it specifies a list of directories to search when invoking `cd' with a relative file name that did not start with `./' or `../'. Posix 1003.1-2001 says that if a nonempty directory name from `CDPATH' is used successfully, `cd' prints the resulting absolute file name. Unfortunately this output can break idioms like `abs=`cd src && pwd`' because `abs' receives the name twice. Also, many shells do not conform to this part of Posix; for example, `zsh' prints the result only if a directory name other than `.' was chosen from `CDPATH'. In practice the shells that have this problem also support `unset', so you can work around the problem as follows: (unset CDPATH) >/dev/null 2>&1 && unset CDPATH You can also avoid output by ensuring that your directory name is absolute or anchored at `./', as in `abs=`cd ./src && pwd`'. Configure scripts use M4sh, which automatically unsets `CDPATH' if possible, so you need not worry about this problem in those scripts. `CLICOLOR_FORCE' When this variable is set, some implementations of tools like `ls' attempt to add color to their output via terminal escape sequences, even when the output is not directed to a terminal, and can thus cause spurious failures in scripts. Configure scripts use M4sh, which automatically unsets this variable. `DUALCASE' In the MKS shell, case statements and file name generation are case-insensitive unless `DUALCASE' is nonzero. Autoconf-generated scripts export this variable when they start up. `ENV' `MAIL' `MAILPATH' `PS1' `PS2' `PS4' These variables should not matter for shell scripts, since they are supposed to affect only interactive shells. However, at least one shell (the pre-3.0 UWIN Korn shell) gets confused about whether it is interactive, which means that (for example) a `PS1' with a side effect can unexpectedly modify `$?'. To work around this bug, M4sh scripts (including `configure' scripts) do something like this: (unset ENV) >/dev/null 2>&1 && unset ENV MAIL MAILPATH PS1='$ ' PS2='> ' PS4='+ ' (actually, there is some complication due to bugs in `unset'; see *note Limitations of Shell Builtins: unset.). `FPATH' The Korn shell uses `FPATH' to find shell functions, so avoid `FPATH' in portable scripts. `FPATH' is consulted after `PATH', but you still need to be wary of tests that use `PATH' to find whether a command exists, since they might report the wrong result if `FPATH' is also set. `GREP_OPTIONS' When this variable is set, some implementations of `grep' honor these options, even if the options include direction to enable colored output via terminal escape sequences, and the result can cause spurious failures when the output is not directed to a terminal. Configure scripts use M4sh, which automatically unsets this variable. `IFS' Long ago, shell scripts inherited `IFS' from the environment, but this caused many problems so modern shells ignore any environment settings for `IFS'. Don't set the first character of `IFS' to backslash. Indeed, Bourne shells use the first character (backslash) when joining the components in `"$@"' and some shells then reinterpret (!) the backslash escapes, so you can end up with backspace and other strange characters. The proper value for `IFS' (in regular code, not when performing splits) is `'. The first character is especially important, as it is used to join the arguments in `$*'; however, note that traditional shells, but also bash-2.04, fail to adhere to this and join with a space anyway. `LANG' `LC_ALL' `LC_COLLATE' `LC_CTYPE' `LC_MESSAGES' `LC_MONETARY' `LC_NUMERIC' `LC_TIME' You should set all these variables to `C' because so much configuration code assumes the C locale and Posix requires that locale environment variables be set to `C' if the C locale is desired; `configure' scripts and M4sh do that for you. Export these variables after setting them. `LANGUAGE' `LANGUAGE' is not specified by Posix, but it is a GNU extension that overrides `LC_ALL' in some cases, so you (or M4sh) should set it too. `LC_ADDRESS' `LC_IDENTIFICATION' `LC_MEASUREMENT' `LC_NAME' `LC_PAPER' `LC_TELEPHONE' These locale environment variables are GNU extensions. They are treated like their Posix brethren (`LC_COLLATE', etc.) as described above. `LINENO' Most modern shells provide the current line number in `LINENO'. Its value is the line number of the beginning of the current command. M4sh, and hence Autoconf, attempts to execute `configure' with a shell that supports `LINENO'. If no such shell is available, it attempts to implement `LINENO' with a Sed prepass that replaces each instance of the string `$LINENO' (not followed by an alphanumeric character) with the line's number. In M4sh scripts you should execute `AS_LINENO_PREPARE' so that these workarounds are included in your script; configure scripts do this automatically in `AC_INIT'. You should not rely on `LINENO' within `eval' or shell functions, as the behavior differs in practice. The presence of a quoted newline within simple commands can alter which line number is used as the starting point for `$LINENO' substitutions within that command. Also, the possibility of the Sed prepass means that you should not rely on `$LINENO' when quoted, when in here-documents, or when line continuations are used. Subshells should be OK, though. In the following example, lines 1, 9, and 14 are portable, but the other instances of `$LINENO' do not have deterministic values: $ cat lineno echo 1. $LINENO echo "2. $LINENO 3. $LINENO" cat < N > s,$,-, > t loop > :loop > s,^\([0-9]*\)\(.*\)[$]LINENO\([^a-zA-Z0-9_]\),\1\2\1\3, > t loop > s,-$,, > s,^[0-9]*\n,, > ' | > sh 1. 1 2. 2 3. 3 5. 5 6. 6 7. \7 9. 9 10. 10 11. 11 12. 12 13. 13 14. 14 15. 15 18. 16 18. 17 19. 20 In particular, note that `config.status' (and any other subsidiary script created by `AS_INIT_GENERATED') might report line numbers relative to the parent script as a result of the potential Sed pass. `NULLCMD' When executing the command `>foo', `zsh' executes `$NULLCMD >foo' unless it is operating in Bourne shell compatibility mode and the `zsh' version is newer than 3.1.6-dev-18. If you are using an older `zsh' and forget to set `NULLCMD', your script might be suspended waiting for data on its standard input. `PATH_SEPARATOR' On DJGPP systems, the `PATH_SEPARATOR' environment variable can be set to either `:' or `;' to control the path separator Bash uses to set up certain environment variables (such as `PATH'). You can set this variable to `;' if you want `configure' to use `;' as a separator; this might be useful if you plan to use non-Posix shells to execute files. *Note File System Conventions::, for more information about `PATH_SEPARATOR'. `PWD' Posix 1003.1-2001 requires that `cd' and `pwd' must update the `PWD' environment variable to point to the logical name of the current directory, but traditional shells do not support this. This can cause confusion if one shell instance maintains `PWD' but a subsidiary and different shell does not know about `PWD' and executes `cd'; in this case `PWD' points to the wrong directory. Use ``pwd`' rather than `$PWD'. `RANDOM' Many shells provide `RANDOM', a variable that returns a different integer each time it is used. Most of the time, its value does not change when it is not used, but on IRIX 6.5 the value changes all the time. This can be observed by using `set'. It is common practice to use `$RANDOM' as part of a file name, but code shouldn't rely on `$RANDOM' expanding to a nonempty string. `status' This variable is an alias to `$?' for `zsh' (at least 3.1.6), hence read-only. Do not use it. 11.11 Shell Functions ===================== Nowadays, it is difficult to find a shell that does not support shell functions at all. However, some differences should be expected. Inside a shell function, you should not rely on the error status of a subshell if the last command of that subshell was `exit' or `trap', as this triggers bugs in zsh 4.x; while Autoconf tries to find a shell that does not exhibit the bug, zsh might be the only shell present on the user's machine. Likewise, the state of `$?' is not reliable when entering a shell function. This has the effect that using a function as the first command in a `trap' handler can cause problems. $ bash -c 'foo(){ echo $?; }; trap foo 0; (exit 2); exit 2'; echo $? 2 2 $ ash -c 'foo(){ echo $?; }; trap foo 0; (exit 2); exit 2'; echo $? 0 2 DJGPP bash 2.04 has a bug in that `return' from a shell function which also used a command substitution causes a segmentation fault. To work around the issue, you can use `return' from a subshell, or `AS_SET_STATUS' as last command in the execution flow of the function (*note Common Shell Constructs::). Not all shells treat shell functions as simple commands impacted by `set -e', for example with Solaris 10 `bin/sh': $ bash -c 'f(){ return 1; }; set -e; f; echo oops $ /bin/sh -c 'f(){ return 1; }; set -e; f; echo oops oops Shell variables and functions may share the same namespace, for example with Solaris 10 `/bin/sh': $ f () { :; }; f=; f f: not found For this reason, Autoconf (actually M4sh, *note Programming in M4sh::) uses the prefix `as_fn_' for its functions. Handling of positional parameters and shell options varies among shells. For example, Korn shells reset and restore trace output (`set -x') and other options upon function entry and exit. Inside a function, IRIX sh sets `$0' to the function name. It is not portable to pass temporary environment variables to shell functions. Solaris `/bin/sh' does not see the variable. Meanwhile, not all shells follow the Posix rule that the assignment must affect the current environment in the same manner as special built-ins. $ /bin/sh -c 'func(){ echo $a;}; a=1 func; echo $a' => => $ ash -c 'func(){ echo $a;}; a=1 func; echo $a' =>1 => $ bash -c 'set -o posix; func(){ echo $a;}; a=1 func; echo $a' =>1 =>1 Some ancient Bourne shell variants with function support did not reset `$I, I >= 0', upon function exit, so effectively the arguments of the script were lost after the first function invocation. It is probably not worth worrying about these shells any more. With AIX sh, a `trap' on 0 installed in a shell function triggers at function exit rather than at script exit, see *Note Limitations of Shell Builtins: trap. 11.12 Limitations of Shell Builtins =================================== No, no, we are serious: some shells do have limitations! :) You should always keep in mind that any builtin or command may support options, and therefore differ in behavior with arguments starting with a dash. For instance, even the innocent `echo "$word"' can give unexpected results when `word' starts with a dash. It is often possible to avoid this problem using `echo "x$word"', taking the `x' into account later in the pipe. Many of these limitations can be worked around using M4sh (*note Programming in M4sh::). `.' Use `.' only with regular files (use `test -f'). Bash 2.03, for instance, chokes on `. /dev/null'. Remember that `.' uses `PATH' if its argument contains no slashes. Also, some shells, including bash 3.2, implicitly append the current directory to this `PATH' search, even though Posix forbids it. So if you want to use `.' on a file `foo' in the current directory, you must use `. ./foo'. Not all shells gracefully handle syntax errors within a sourced file. On one extreme, some non-interactive shells abort the entire script. On the other, `zsh' 4.3.10 has a bug where it fails to react to the syntax error. $ echo 'fi' > syntax $ bash -c '. ./syntax; echo $?' ./syntax: line 1: syntax error near unexpected token `fi' ./syntax: line 1: `fi' 1 $ ash -c '. ./syntax; echo $?' ./syntax: 1: Syntax error: "fi" unexpected $ zsh -c '. ./syntax; echo $?' ./syntax:1: parse error near `fi' 0 `!' The Unix version 7 shell did not support negating the exit status of commands with `!', and this feature is still absent from some shells (e.g., Solaris `/bin/sh'). Other shells, such as FreeBSD `/bin/sh' or `ash', have bugs when using `!': $ sh -c '! : | :'; echo $? 1 $ ash -c '! : | :'; echo $? 0 $ sh -c '! { :; }'; echo $? 1 $ ash -c '! { :; }'; echo $? {: not found Syntax error: "}" unexpected 2 Shell code like this: if ! cmp file1 file2 >/dev/null 2>&1; then echo files differ or trouble fi is therefore not portable in practice. Typically it is easy to rewrite such code, e.g.: cmp file1 file2 >/dev/null 2>&1 || echo files differ or trouble More generally, one can always rewrite `! COMMAND' as: if COMMAND; then (exit 1); else :; fi `{...}' Bash 3.2 (and earlier versions) sometimes does not properly set `$?' when failing to write redirected output of a compound command. This problem is most commonly observed with `{...}'; it does not occur with `(...)'. For example: $ bash -c '{ echo foo; } >/bad; echo $?' bash: line 1: /bad: Permission denied 0 $ bash -c 'while :; do echo; done >/bad; echo $?' bash: line 1: /bad: Permission denied 0 To work around the bug, prepend `:;': $ bash -c ':;{ echo foo; } >/bad; echo $?' bash: line 1: /bad: Permission denied 1 Posix requires a syntax error if a brace list has no contents. However, not all shells obey this rule; and on shells where empty lists are permitted, the effect on `$?' is inconsistent. To avoid problems, ensure that a brace list is never empty. $ bash -c 'false; { }; echo $?' || echo $? bash: line 1: syntax error near unexpected token `}' bash: line 1: `false; { }; echo $?' 2 $ zsh -c 'false; { }; echo $?' || echo $? 1 $ pdksh -c 'false; { }; echo $?' || echo $? 0 `break' The use of `break 2' etc. is safe. `case' You don't need to quote the argument; no splitting is performed. You don't need the final `;;', but you should use it. Posix requires support for `case' patterns with opening parentheses like this: case $file_name in (*.c) echo "C source code";; esac but the `(' in this example is not portable to many Bourne shell implementations, which is a pity for those of us using tools that rely on balanced parentheses. For instance, with Solaris `/bin/sh': $ case foo in (foo) echo foo;; esac error-->syntax error: `(' unexpected The leading `(' can be omitted safely. Unfortunately, there are contexts where unbalanced parentheses cause other problems, such as when using a syntax-highlighting editor that searches for the balancing counterpart, or more importantly, when using a case statement as an underquoted argument to an Autoconf macro. *Note Balancing Parentheses::, for tradeoffs involved in various styles of dealing with unbalanced `)'. Zsh handles pattern fragments derived from parameter expansions or command substitutions as though quoted: $ pat=\?; case aa in ?$pat) echo match;; esac $ pat=\?; case a? in ?$pat) echo match;; esac match Because of a bug in its `fnmatch', Bash fails to properly handle backslashes in character classes: bash-2.02$ case /tmp in [/\\]*) echo OK;; esac bash-2.02$ This is extremely unfortunate, since you are likely to use this code to handle Posix or MS-DOS absolute file names. To work around this bug, always put the backslash first: bash-2.02$ case '\TMP' in [\\/]*) echo OK;; esac OK bash-2.02$ case /tmp in [\\/]*) echo OK;; esac OK Many Bourne shells cannot handle closing brackets in character classes correctly. Some shells also have problems with backslash escaping in case you do not want to match the backslash: both a backslash and the escaped character match this pattern. To work around this, specify the character class in a variable, so that quote removal does not apply afterwards, and the special characters don't have to be backslash-escaped: $ case '\' in [\<]) echo OK;; esac OK $ scanset='[<]'; case '\' in $scanset) echo OK;; esac $ Even with this, Solaris `ksh' matches a backslash if the set contains any of the characters `|', `&', `(', or `)'. Conversely, Tru64 `ksh' (circa 2003) erroneously always matches a closing parenthesis if not specified in a character class: $ case foo in *\)*) echo fail ;; esac fail $ case foo in *')'*) echo fail ;; esac fail Some shells, such as Ash 0.3.8, are confused by an empty `case'/`esac': ash-0.3.8 $ case foo in esac; error-->Syntax error: ";" unexpected (expecting ")") Posix requires `case' to give an exit status of 0 if no cases match. However, `/bin/sh' in Solaris 10 does not obey this rule. Meanwhile, it is unclear whether a case that matches, but contains no statements, must also change the exit status to 0. The M4sh macro `AS_CASE' works around these inconsistencies. $ bash -c 'case `false` in ?) ;; esac; echo $?' 0 $ /bin/sh -c 'case `false` in ?) ;; esac; echo $?' 255 `cd' Posix 1003.1-2001 requires that `cd' must support the `-L' ("logical") and `-P' ("physical") options, with `-L' being the default. However, traditional shells do not support these options, and their `cd' command has the `-P' behavior. Portable scripts should assume neither option is supported, and should assume neither behavior is the default. This can be a bit tricky, since the Posix default behavior means that, for example, `ls ..' and `cd ..' may refer to different directories if the current logical directory is a symbolic link. It is safe to use `cd DIR' if DIR contains no `..' components. Also, Autoconf-generated scripts check for this problem when computing variables like `ac_top_srcdir' (*note Configuration Actions::), so it is safe to `cd' to these variables. See *Note Special Shell Variables::, for portability problems involving `cd' and the `CDPATH' environment variable. Also please see the discussion of the `pwd' command. `echo' The simple `echo' is probably the most surprising source of portability troubles. It is not possible to use `echo' portably unless both options and escape sequences are omitted. Don't expect any option. Do not use backslashes in the arguments, as there is no consensus on their handling. For `echo '\n' | wc -l', the `sh' of Solaris outputs 2, but Bash and Zsh (in `sh' emulation mode) output 1. The problem is truly `echo': all the shells understand `'\n'' as the string composed of a backslash and an `n'. Within a command substitution, `echo 'string\c'' will mess up the internal state of ksh88 on AIX 6.1 so that it will print the first character `s' only, followed by a newline, and then entirely drop the output of the next echo in a command substitution. Because of these problems, do not pass a string containing arbitrary characters to `echo'. For example, `echo "$foo"' is safe only if you know that FOO's value cannot contain backslashes and cannot start with `-'. If this may not be true, `printf' is in general safer and easier to use than `echo' and `echo -n'. Thus, scripts where portability is not a major concern should use `printf '%s\n'' whenever `echo' could fail, and similarly use `printf %s' instead of `echo -n'. For portable shell scripts, instead, it is suggested to use a here-document like this: cat <