This manual is for GNU Autoconf (version 2.59, 5 November 2003), a package for creating scripts to configure source code packages using templates and an M4 macro package.
Copyright © 1992, 1993, 1994, 1995, 1996, 1998, 1999, 2000, 2001, 2002, 2003 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 1.1 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 freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.”
--- The Detailed Node Listing ---
The GNU Build System
Making configure Scripts
Writing configure.ac
Initialization and Output Files
Substitutions in Makefiles
Configuration Header Files
Existing Tests
Common Behavior
Alternative Programs
Library Functions
Header Files
Declarations
Structures
Types
Compilers and Preprocessors
Writing Tests
Writing Test Programs
Results of Tests
Caching Results
Programming in M4
M4 Quotation
Using autom4te
Programming in M4sugar
Writing Autoconf Macros
Dependencies Between Macros
Portable Shell Programming
Manual Configuration
Site Configuration
Transforming Program Names When Installing
Running configure Scripts
Obsolete Constructs
Upgrading From Version 1
Upgrading From Version 2.13
Generating Test Suites with Autotest
Using an Autotest Test Suite
Frequent Autoconf Questions, with answers
History of Autoconf
Copying This Manual
Indices
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 unix-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 unix variants. There is no need to maintain files that list the features supported by each release of each variant of unix.
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.
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. See The GNU Build System, for more information.
Autoconf imposes some restrictions on the names of macros used with
#if in C programs (see Preprocessor Symbol Index).
Autoconf requires GNU M4 in order to generate the scripts. It uses features that some unix versions of M4, including GNU M4 1.3, do not have. You must use version 1.4 or later of GNU M4.
See Autoconf 1, for information about upgrading from version 1. See History, for the story of Autoconf's development. See FAQ, for answers to some common questions about Autoconf.
See the Autoconf web page 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.
Bug reports should be preferably submitted to the Autoconf Gnats database, or sent to the Autoconf Bugs mailing list. 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 CVS; see the Autoconf web page for details. There is also a CVSweb interface to the Autoconf development tree. Patches relative to the current CVS version can be sent for review to the Autoconf Patches mailing list.
Because of its mission, Autoconf 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, which is kindly run by Peter Simons.
Autoconf solves an important problem—reliable discovery of system-specific build and run-time 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.
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 make's numerous limitations. Its lack of
support for automatic dependency tracking, recursive builds in
subdirectories, reliable timestamps (e.g., for network filesystems), 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 will
build the hello program, and make install will install 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....
Very 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 Makefile 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 on its own, 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.
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 will 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 will be a piece of cake.)
There are a number of places that you can go to for more information on the GNU build tools.
See Automake (GNU Automake), for more information on Automake.
The book GNU Autoconf, Automake and Libtool1 describes the complete GNU build environment. You can also find the entire book on-line at “The Goat Book” home page.
The Autoconf Developer Page maintains links to a number of Autoconf/Automake tutorials online, and also links to the Autoconf Macro Archive.
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:
#define directives (see Configuration Headers);
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 will 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 ---'
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 Existing Tests, for their descriptions. For most other features, you can use Autoconf template macros to produce custom checks; see Writing Tests, for information about them. For especially tricky or specialized features, configure.ac might need to contain some hand-crafted shell commands; see Portable Shell. The autoscan program can give you a good start in writing configure.ac (see 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.
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, shell functions do not belong to the least common denominator; therefore, where you would like to define a function and use it ten times, you would instead need to copy its body ten times.
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 very 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.
The Autoconf language is very different from many other computer languages because it treats actual code the same as plain text. Whereas in C, for instance, data and instructions have very 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 blank 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 spaces in arguments are ignored, unless they are quoted. You may safely leave out the quotes when the argument is simple text, but always quote complex arguments such as other macro calls. 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])],
[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)],
[AC_MSG_ERROR([Sorry, can't do anything for you])])
Notice that the argument of AC_MSG_ERROR is still quoted;
otherwise, its comma would have been interpreted as an argument separator.
The following example is wrong and dangerous, as it is underquoted:
AC_CHECK_HEADER(stdio.h,
AC_DEFINE(HAVE_STDIO_H),
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:
echo "Hard rock was here! --[AC_DC]"
which will result in
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:
AC_MSG_WARN([[AC_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 will be 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])])
Voilà, you actually produce `char b[10];' this time!
The careful reader will notice that, according to these guidelines, the
“properly” quoted AC_CHECK_HEADER example above is actually
lacking three pairs of quotes! Nevertheless, for the sake of readability,
double quotation of literals is used only where needed in this manual.
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.
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 (see 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 (see 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
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 will probably need 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 (see Configuration Headers). You might
also have to change or add some #if directives to your program in
order to make it work with Autoconf (see 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 will contain 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, whitespace, and the Autoconf macro to output if that symbol is encountered. Lines starting with `#' are comments.
autoscan accepts the following options:
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 (see 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:
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 (see 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:
AC_DIAGNOSE, for a comprehensive list of categories. Special
values include:
Warnings about `syntax' are enabled by default, and the environment
variable WARNINGS, a comma separated list of categories, is
honored. Passing `-W category' will actually behave as if
you had passed `--warnings=syntax,$WARNINGS,category'. If
you want to disable the defaults and WARNINGS, but (for example)
enable the warnings about obsolete constructs, you would use -W
none,obsolete.
Because autoconf uses autom4te behind the scenes, it
displays a back trace for errors, but not for warnings; if you want
them, just pass -W error. See autom4te Invocation, for some
examples.
The format is a regular string, with newlines if desired, and
several special escape codes. It defaults to `$f:$l:$n:$%'; see
autom4te Invocation, for details on the format.
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
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 (see Subdirectories). By default, it only remakes those files that are older than their sources.
If you install a new version of some tool, you can make autoreconf remake all of the files by giving it the --force option.
See Automatic Remaking, for Makefile rules to automatically remake 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.
autoreconf accepts the following options:
This option triggers calls to `automake --add-missing',
`libtoolize', `autopoint', etc.
Warnings about `syntax' are enabled by default, and the environment
variable WARNINGS, a comma separated list of categories, is
honored. Passing `-W category' will actually behave as if
you had passed `--warnings=syntax,$WARNINGS,category'. If
you want to disable the defaults and WARNINGS, but (for example)
enable the warnings about obsolete constructs, you would use -W
none,obsolete.
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.
Every configure script must call AC_INIT before doing
anything else. The only other required macro is AC_OUTPUT
(see Output).
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 `-'.
It is preferable that the arguments of
AC_INITbe 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 byAC_INIT:
The following macros manage version numbers for configure scripts. Using them is optional.
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(2.59)This macro is the only macro that may be used before
AC_INIT, but for consistency, you are invited not to do so.
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 will show up in both the head of configure and in `configure --version'.
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
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. See 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.
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_INSTALLdoes not automatically require distributing the other auxiliary files. It checks for install.sh also, but that name is obsolete because somemakehave a rule that creates install from it if there is no Makefile.
Similarly, packages that use aclocal should declare where
local macros can be found using AC_CONFIG_MACRO_DIR.
Future versions of autopoint, libtoolize, aclocal and autoreconf will use directory dir as the location of additional local Autoconf macros. Be sure to call this macro directly from configure.ac so that tools that install macros for aclocal can find the declaration before --trace can be called safely.
Every Autoconf script, e.g., configure.ac, should finish by
calling AC_OUTPUT. That is the macro that generates and runs
config.status, which will create the Makefiles and any
other files resulting from configuration. This is the only required
macro besides AC_INIT (see Input).
Generate config.status and launch it. Call this macro once, at the end of configure.ac.
config.status will perform all the configuration actions: all the output files (see Configuration Files, macro
AC_CONFIG_FILES), header files (see Configuration Headers, macroAC_CONFIG_HEADERS), commands (see Configuration Commands, macroAC_CONFIG_COMMANDS), links (see Configuration Links, macroAC_CONFIG_LINKS), subdirectories to configure (see Subdirectories, macroAC_CONFIG_SUBDIRS) are honored.The location of your
AC_OUTPUTinvocation is the exact point where configuration actions are taken: any code afterwards will be executed byconfigureonce config.status was run. If you want to bind actions to config.status itself (independently of whether configure is being run), see Running Arbitrary Configuration Commands.
Historically, the usage of AC_OUTPUT was somewhat different.
See 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.
If make predefines the Make variable
MAKE, define output variableSET_MAKEto be empty. Otherwise, defineSET_MAKEto contain `MAKE=make'. CallsAC_SUBSTforSET_MAKE.
If you use this macro, place a line like this in each Makefile.in
that runs MAKE on other directories:
@SET_MAKE@
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_FOOS(tag..., [commands], [init-cmds])
where the arguments are:
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_FOOS($my_foos)
and use this instead:
... && AC_CONFIG_FOOS(fooo)
... && AC_CONFIG_FOOS(foooo)
The macros AC_CONFIG_FILES and AC_CONFIG_HEADERS use
special tags: they may have the form `output' or
`output:inputs'. The file output is instantiated
from its templates, inputs (defaulting to `output.in').
For instance `AC_CONFIG_FILES(Makefile:boiler/top.mk:boiler/bot.mk)' asks for the creation of Makefile that will be 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 config.status Invocation, for more details.
The inputs may be absolute or relative filenames. In the latter
case they are first looked for in the build tree, and then in the source
tree.
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:
srcdirac_top_srcdirac_top_builddirac_srcdirThe 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_builddir='../../'
# Concatenation of $ac_top_builddir and srcdir
ac_top_srcdir='../../../package'
# Concatenation of $ac_top_srcdir and deep/dir
ac_srcdir='../../../package/deep/dir'
independently of `in/in.in'.
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 tags, of course!
Be sure to read the previous section, Configuration Actions.
Make
AC_OUTPUTcreate 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 Configuration Actions. See Makefile Substitutions, for more information on using output variables. See 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_FILESlook 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 MS-DOS, or to prepend and/or append boilerplate to the file.
Each subdirectory in a distribution that contains something to be
compiled or installed should come with a file Makefile.in, from
which configure will create a Makefile in that directory.
To create a 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. See 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.
See Makefile Conventions (The GNU Coding Standards), for more information on what to put in Makefiles.
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. See Output Variable Index, for a
complete list of output variables. See Installation Directory Variables, for the list of the preset ones related to installation
directories. Below are listed the other preset ones. They all are
precious variables (see Setting Output Variables,
AC_ARG_VAR).
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 programs to test for C features.
A comment saying that the file was generated automatically by configure and giving the name of the input file.
AC_OUTPUTadds 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.
Header file search directory (-Idir) and any other miscellaneous options for the C and 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 compiling or preprocessing programs to test for C and C++ features.
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_CXX(or empty if you don't). configure uses this variable when compiling programs to test for C++ features.
-D options to pass to the C compiler. If
AC_CONFIG_HEADERSis called, configure replaces `@DEFS@' with -DHAVE_CONFIG_H instead (see Configuration Headers). This variable is not defined while configure is performing its tests, only when creating the output files. See Setting Output Variables, for how to check the results of previous tests.
How does one suppress the trailing newline from
echofor 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
echoimplementations offer no means to achieve this, in which caseECHO_Tis set to tab. You might not want to use it.
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 programs to test for Fortran features.
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 programs to test for Fortran 77 features.
Stripping (-s), path (-L), and any other miscellaneous options for the linker. Don't use this variable to pass library names (-l) to the linker, use
LIBSinstead. 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++, and Fortran features.
-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 Libraries. configure uses this variable when linking programs to test for C, C++, and Fortran features.
The relative path to the top-level of the current build tree. In the top-level directory, this is the same as
builddir.
The relative path to the directory that contains the source code for that Makefile.
The relative path to the top-level source code directory for the package. In the top-level directory, this is the same as
srcdir.
The following variables specify the directories where the package will be installed, see Variables for Installation Directories (The GNU Coding Standards), for more information. See the end of this section for details on when and how to use these variables.
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.
The common installation prefix for all files. If exec_prefix is defined to a different value, prefix is used only for architecture-independent files.
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 `@datadir@' will be
replaced by `${prefix}/share', not `/usr/local/share'.
This behavior is mandated by the GNU coding standards, so that when the user runs:
In order to support these features, it is essential that datadir
remains being defined as `${prefix}/share' to depend upon 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")', you should add
`-DDATADIR="$(datadir)"' to your CPPFLAGS.
Similarly you should not rely on AC_OUTPUT_FILES to replace
datadir and friends in your shell scripts and other files, rather
let make manage their replacement. For instance Autoconf
ships templates of its shell scripts ending with `.in', and uses a
Makefile snippet similar to:
edit = sed \
-e 's,@datadir\@,$(pkgdatadir),g' \
-e 's,@prefix\@,$(prefix),g'
autoconf: Makefile $(srcdir)/autoconf.in
rm -f autoconf autoconf.tmp
$(edit) $(srcdir)/autoconf.in >autoconf.tmp
chmod +x autoconf.tmp
mv autoconf.tmp autoconf
autoheader: Makefile $(srcdir)/autoheader.in
rm -f autoheader autoheader.tmp
$(edit) $(srcdir)/autoconf.in >autoheader.tmp
chmod +x autoheader.tmp
mv autoheader.tmp autoheader
Some details are noteworthy:
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.
autoconf autoheader: Makefile
.in:
rm -f $@ $@.tmp
$(edit) $< >$@.tmp
chmod +x $@.tmp
mv $@.tmp $@
See Limitations of Make, for details.
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
and most other recent make programs can do this. Older
make programs do not support VPATH; when using them, the
source code must be in the same directory as the object files.
To support VPATH, each Makefile.in should contain two
lines that look like:
srcdir = @srcdir@
VPATH = @srcdir@
Do not set VPATH to the value of another variable, for example
`VPATH = $(srcdir)', because some versions of make do not do
variable substitutions on the value of 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, Makefile command lines should always refer to source files by prefixing them with `$(srcdir)/'. For example:
time.info: time.texinfo
$(MAKEINFO) $(srcdir)/time.texinfo
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 will not be changed if remaking them does not change their contents. This feature avoids unnecessary recompilation. You should include the file stamp-h.in your package's distribution, so make will consider config.h.in up to date. Don't use touch (see Limitations of Usual Tools), rather 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 will 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 will ensure that
config.h is considered up to date. See Output, for more
information about AC_OUTPUT.
See config.status Invocation, for more examples of handling configuration-related dependencies.
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. It should be called 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). Use `#include <config.h>'
instead of `#include "config.h"', and pass the C compiler a
-I. option (or -I..; whichever directory contains
config.h). That way, even if the source directory is configured
itself (perhaps to make a distribution), other build directories can
also be configured without finding the config.h from the source
directory.
This macro is one of the instantiating macros; see Configuration Actions. Make
AC_OUTPUTcreate the file(s) in the whitespace-separated list header containing C preprocessor#definestatements, and replace `@DEFS@' in generated files with -DHAVE_CONFIG_H instead of the value ofDEFS. The usual name for header is config.h.If header already exists and its contents are identical to what
AC_OUTPUTwould 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. Examples:
AC_CONFIG_HEADERS([config.h:config.hin]) AC_CONFIG_HEADERS([defines.h:defs.pre:defines.h.in:defs.post])Doing this allows you to keep your file names acceptable to MS-DOS, or to prepend and/or append boilerplate to the file.
See Configuration Actions, for more details on header.
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. On systems that have unistd.h, configure will `#define' `HAVE_UNISTD_H' to 1. On other systems, the whole line will be commented out (in case the system predefines that symbol).
/* Define as 1 if you have unistd.h. */
#undef HAVE_UNISTD_H
Pay attention that `#undef' is in the first column, and there is nothing behind `HAVE_UNISTD_H', not even white spaces. You can then decode the configuration header using the preprocessor directives:
#include <conf.h>
#if HAVE_UNISTD_H
# include <unistd.h>
#else
/* We are in trouble. */
#endif
The use of old form templates, with `#define' instead of `#undef' is strongly discouraged. Similarly with old templates with comments on the same line as the `#undef'. Anyway, putting comments in preprocessor macros has never been a good idea.
Since it is a tedious task to keep a template header up to date, you may use autoheader to generate it, see autoheader Invocation.
The autoheader program can create a template file of C
`#define' statements for configure to use. If
configure.ac invokes AC_CONFIG_HEADERS(file),
autoheader creates file.in; if multiple file
arguments are given, the first one is used. Otherwise,
autoheader creates config.h.in.
In order to do its job, autoheader needs you to document all
of the symbols that you might use; i.e., there must be at least one
AC_DEFINE or one AC_DEFINE_UNQUOTED call with a third
argument for each symbol (see Defining Symbols). An additional
constraint is that the first argument of AC_DEFINE must be a
literal. Note that all symbols defined by Autoconf's builtin tests are
already documented properly; you only need to document 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 #define'd (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: autoheader's invocation...
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:
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 simplest way to create a template for a symbol is to supply the description argument to an `AC_DEFINE(symbol)'; see Defining Symbols. You may also use one of the following macros.
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
AC_DEFINE'd.For example:
AH_VERBATIM([_GNU_SOURCE], [/* Enable GNU extensions on systems that have them. */ #ifndef _GNU_SOURCE # define _GNU_SOURCE #endif])
Tell autoheader to generate a template for key. This macro generates standard templates just like
AC_DEFINEwhen 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.])will generate 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
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 Obsolete Macros, for details.
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 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([time-stamp], [date >time-stamp])
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.
Make
AC_OUTPUTlink 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 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.hto create the links.
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.
Make
AC_OUTPUTrun configure in each subdirectory dir in the given whitespace-separated list. Each dir should be a literal, i.e., please do not use:if test "$package_foo_enabled" = yes; 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. Rather, you should write:if test "$package_foo_enabled" = yes; then AC_CONFIG_SUBDIRS(foo) fiIf 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) fiIf 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 filesystems. Likewise, if a dir contains configure.in but no configure, the Cygnus configure script found by
AC_CONFIG_AUX_DIRis 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 path for the cache file;
- adjusting a relative path 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
subdirsto the list of directories `dir ...'. Makefile rules can use this variable to determine which subdirectories to recurse into.This macro may be called multiple times.
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.
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.
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 orAC_PREFIX_DEFAULT). For example, if program isgccand thePATHcontains /usr/local/gnu/bin/gcc, set the prefix to /usr/local/gnu.
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 (see 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 (see Caching Results).
Some of these macros set output variables. See Makefile Substitutions, for how to get their values. The phrase “define name” is used below as a shorthand to mean “define C preprocessor symbol name to the value 1”. See Defining Symbols, for how to get those symbol definitions into your program.
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.
All the generic macros that AC_DEFINE a symbol as a result of
their test transform their arguments 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*)
will define the symbol `HAVE_STRUCT__EXPENSIVEP' if the check succeeds.
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:
#if TIME_WITH_SYS_TIME
# include <sys/time.h>
# include <time.h>
#else
# if HAVE_SYS_TIME_H
# include <sys/time.h>
# else
# include <time.h>
# 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 (see Header Files).
Most generic macros use the following macro to provide the default set of includes:
Expand to include-directives if defined, otherwise to:
#include <stdio.h> #if HAVE_SYS_TYPES_H # include <sys/types.h> #endif #if HAVE_SYS_STAT_H # include <sys/stat.h> #endif #if STDC_HEADERS # include <stdlib.h> # include <stddef.h> #else # if HAVE_STDLIB_H # include <stdlib.h> # endif #endif #if HAVE_STRING_H # if !STDC_HEADERS && HAVE_MEMORY_H # include <memory.h> # endif # include <string.h> #endif #if HAVE_STRINGS_H # include <strings.h> #endif #if HAVE_INTTYPES_H # include <inttypes.h> #else # if HAVE_STDINT_H # include <stdint.h> # endif #endif #if HAVE_UNISTD_H # include <unistd.h> #endifIf 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_HEADERS_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_Hwill be defined, butHAVE_STRINGS_Hwon't.
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.
These macros check for particular programs—whether they exist, and in some cases whether they support certain features.
Check for
gawk,mawk,nawk, andawk, in that order, and set output variableAWKto the first one that is found. It triesgawkfirst because that is reported to be the best implementation.
Check for
grep -Eandegrep, in that order, and set output variableEGREPto the first one that is found.
Check for
grep -Fandfgrep, in that order, and set output variableFGREPto the first one that is found.
Set output variable
INSTALLto the path of a BSD-compatibleinstallprogram, if one is found in the currentPATH. Otherwise, setINSTALLto `dir/install-sh -c', checking the directories specified toAC_CONFIG_AUX_DIR(or its default directories) to determine dir (see Output). Also set the variablesINSTALL_PROGRAMandINSTALL_SCRIPTto `${INSTALL}' andINSTALL_DATAto `${INSTALL} -m 644'.This macro screens out various instances of
installknown 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.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, or configure will produce an error message saying it can't find them—even if the system you're on has a goodinstallprogram. 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-compatibleinstallprogram.If you need to use your own installation program because it has features not found in standard
installprograms, there is no reason to useAC_PROG_INSTALL; just put the file name of your program into your Makefile.in files.
If
flexis found, set output variableLEXto `flex' andLEXLIBto -lfl, if that library is in a standard place. Otherwise setLEXto `lex' andLEXLIBto -ll.Define
YYTEXT_POINTERifyytextis a `char *' instead of a `char []'. Also set output variableLEX_OUTPUT_ROOTto the base of the file name that the lexer generates; usually lex.yy, but sometimes something else. These results vary according to whetherlexorflexis 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
yywrapor, 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 "$LEX" != flex; then LEX="$SHELL $missing_dir/missing flex" AC_SUBST(LEX_OUTPUT_ROOT, lex.yy) AC_SUBST(LEXLIB, '') fiThe shell script missing can be found in the Automake distribution.
To ensure backward compatibility, Automake's
AM_PROG_LEXinvokes (indirectly) this macro twice, which will cause an annoying but benign “AC_PROG_LEXinvoked multiple times” warning. Future versions of Automake will fix this issue; meanwhile, just ignore this message.
If `ln -s' works on the current file system (the operating system and file system support symbolic links), set the output variable
LN_Sto `ln -s'; otherwise, if `ln' works, setLN_Sto `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
lnin the directory where the link is to be created.In other words, it does not work to do:
$(LN_S) foo /x/barInstead, do:
(cd /x && $(LN_S) foo bar)
Set output variable
RANLIBto `ranlib' ifranlibis found, and otherwise to `:' (do nothing).
If
bisonis found, set output variableYACCto `bison -y'. Otherwise, ifbyaccis found, setYACCto `byacc'. Otherwise setYACCto `yacc'.
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
(see 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:/usr/libexec:/usr/sbin:/usr/etc:etc])
You are strongly encouraged to declare the variable passed to
AC_CHECK_PROG etc. as precious, See Setting Output Variables,
AC_ARG_VAR, for more details.
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. CallsAC_SUBSTfor variable.
Check for each program in the whitespace-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. CallsAC_SUBSTfor variable.
Like
AC_CHECK_PROG, but first looks for prog-to-check-for with a prefix of the host type as determined byAC_CANONICAL_HOST, followed by a dash (see Canonicalizing). For example, if the user runs `configure --host=i386-gnu', then this call:AC_CHECK_TOOL(RANLIB, ranlib, :)sets
RANLIBto i386-gnu-ranlib if that program exists inPATH, or otherwise to `ranlib' if that program exists inPATH, or to `:' if neither program exists.
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 byAC_CANONICAL_HOST, followed by a dash (see 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. CallsAC_SUBSTfor variable.
Like
AC_CHECK_PROG, but set variable to the entire path of prog-to-check-for if found.
Like
AC_CHECK_PROGS, but if any of progs-to-check-for are found, set variable to the entire path of the program found.
Like
AC_CHECK_TOOL, but set variable to the entire path of the program if it is found.
You might also need to check for the existence of files. Before using these macros, ask yourself whether a run-time 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.
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.
Executes
AC_CHECK_FILEonce for each file listed in files. Additionally, defines `HAVE_file' (see Standard Symbols) for each file found.
The following macros check for the presence of certain C, C++, or Fortran library archive files.
Depending on the current language(see Language Choice), try to ensure that the C, C++, or Fortran function function is available by checking whether a test program can be linked with the library library to get the function. library is 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 will prepend -llibrary to
LIBSand define `HAVE_LIBlibrary' (in all capitals). This macro is intended to support buildingLIBSin a right-to-left (least-dependent to most-dependent) fashion such that library dependencies are satisfied as a natural side-effect of consecutive tests. Some linkers are very sensitive to library ordering so the order in whichLIBSis 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 will fail to detect that library is present, because linking the test program will always 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.
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
LIBSfor 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 will fail to detect that function is present, because linking the test program will always fail with unresolved symbols.
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.
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 will always require additions. Please help us keeping it as complete as possible.
exitexit returns int?
This is because exit predates void, and there was a long
tradition of it returning int.
putenvputenv puts the given string directly in
environ, but some systems make a copy of it instead (eg.
glibc 2.0, or BSD). And when a copy is made, unsetenv might
not free it, causing a memory leak (eg. FreeBSD 4).
POSIX specifies that putenv("FOO") removes `FOO' from the
environment, but on some systems (eg. FreeBSD 4) this is not the
case and instead unsetenv must be used.
On MINGW, a call putenv("FOO=") removes `FOO' from the
environment, rather than inserting it with an empty value.
signal handlersignal 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 take void. Presumably
int was to support K&R C, where of course void is not
available. AC_TYPE_SIGNAL (see Particular Types) can be
used to establish the correct type in all cases.
snprintfsnprintf 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).
sprintfsprintf and vsprintf return the
number of bytes written, but on some old systems (SunOS 4 for
instance) they return the buffer pointer instead.
sscanfsscanf 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
(see Incompatibilities of GCC (Using and Porting the GNU Compiler Collection)). Apparently in some cases even
having format strings read-only can be a problem.
strnlen 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 (eg. HP-UX
9) have _SC_PAGE_SIZE instead. This can be tested with
#ifdef.
unlinkunlink causes the given file to be
removed only after there are no more open file handles for it. Not all
OS's support this behavior though. So even on systems that provide
unlink, you cannot portably assume it is OK to call it on files
that are open. For example, on Windows 9x and ME, such a call would fail;
on DOS it could even lead to file system corruption, as the file might end
up being written to after the OS has removed it.
unsetenvunsetenv is not available, but a variable `FOO'
can be removed with a call putenv("FOO="), as described under
putenv above.
va_copyva_copy for copying
va_list variables. It may be available in older environments
too, though possibly as __va_copy (e.g., gcc in strict
C89 mode). These can be tested with #ifdef. A fallback to
memcpy (&dst, &src, sizeof(va_list)) will give maximum
portability.
va_listva_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).
>>>> right shift of a signed type replicates the
high bit, giving a so-called “arithmetic” shift. But care should be
taken since the ISO C standard 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.
These macros check for particular C functions—whether they exist, and in some cases how they respond when given certain arguments.
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 definesHAVE_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 variableALLOCAto `alloca.o' and definesC_ALLOCA(so programs can periodically call `alloca(0)' to garbage collect). This variable is separate fromLIBOBJSso multiple programs can share the value ofALLOCAwithout needing to create an actual library, in case only some of them use the code inLIBOBJS.This macro does not try to get
allocafrom 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 containallocaor contain a buggy version. If you still want to use theiralloca, usearto extract alloca.o from them instead of compiling alloca.c.Source files that use
allocashould start with a piece of code like the following, to declare it properly. In some versions of AIX, the declaration ofallocamust precede everything else except for comments and preprocessor directives. The#pragmadirective is indented so that pre-ANSI C compilers will ignore it, rather than choke on it./* AIX requires this to be the first thing in the file. */ #ifndef __GNUC__ # if HAVE_ALLOCA_H # include <alloca.h> # else # ifdef _AIX #pragma alloca # else # ifndef alloca /* predefined by HP cc +Olibcalls */ char *alloca (); # endif # endif # endif #endif
If the
chownfunction is available and works (in particular, it should accept -1 foruidandgid), defineHAVE_CHOWN.
If the
closedirfunction does not return a meaningful value, defineCLOSEDIR_VOID. Otherwise, callers ought to check its return value for an error indicator.
If the
error_at_linefunction is not found, require anAC_LIBOBJreplacement of `error'.
If the
fnmatchfunction conforms to POSIX, defineHAVE_FNMATCH. Detect common implementation bugs, for example, the bugs in Solaris 2.4.Note that for historical reasons, contrary to the other specific
AC_FUNCmacros,AC_FUNC_FNMATCHdoes not replace a broken/missingfnmatch. SeeAC_REPLACE_FNMATCHbelow.
Behave like
AC_REPLACE_FNMATCH(replace) but also test whetherfnmatchsupports GNU extensions. Detect common implementation bugs, for example, the bugs in the GNU C Library 2.1.
This macro checks for the
forkandvforkfunctions. If a workingforkis found, defineHAVE_WORKING_FORK. This macro checks whetherforkis just a stub by trying to run it.If vfork.h is found, define
HAVE_VFORK_H. If a workingvforkis found, defineHAVE_WORKING_VFORK. Otherwise, definevforkto beforkfor backward compatibility with previous versions of autoconf. This macro checks for several known errors in implementations ofvforkand considers the system to not have a workingvforkif it detects any of them. It is not considered to be an implementation error if a child's invocation ofsignalmodifies the parent's signal handler, since child processes rarely change their signal handlers.Since this macro defines
vforkonly for backward compatibility with previous versions of autoconf you're encouraged to define it yourself in new code:#if !HAVE_WORKING_VFORK # define vfork fork #endif
If the
fseekofunction is available, defineHAVE_FSEEKO. Define_LARGEFILE_SOURCEif necessary to make the prototype visible on some systems (e.g. glibc 2.2). Otherwise linkage problems may occur when compiling withAC_SYS_LARGEFILEon largefile-sensitive systems whereoff_tdoes not default to a 64bit entity.
If the
getgroupsfunction is available and works (unlike on Ultrix 4.3, where `getgroups (0, 0)' always fails), defineHAVE_GETGROUPS. SetGETGROUPS_LIBSto any libraries needed to get that function. This macro runsAC_TYPE_GETGROUPS.
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_LIBOBJreplacement directory properly (see Generic Functions,AC_CONFIG_LIBOBJ_DIR).If the system has the
getloadavgfunction, defineHAVE_GETLOADAVG, and setGETLOADAVG_LIBSto any libraries needed to get that function. Also addGETLOADAVG_LIBStoLIBS. Otherwise, require anAC_LIBOBJreplacement for `getloadavg' with source code in dir/getloadavg.c, and possibly define several other C preprocessor macros and output variables:
- Define
C_GETLOADAVG.- Define
SVR4,DGUX,UMAX, orUMAX4_3if on those systems.- If nlist.h is found, define
HAVE_NLIST_H.- If `struct nlist' has an `n_un.n_name' member, define
HAVE_STRUCT_NLIST_N_UN_N_NAME. The obsolete symbolNLIST_NAME_UNIONis still defined, but do not depend upon it.- Programs may need to be installed setgid (or setuid) for
getloadavgto work. In this case, defineGETLOADAVG_PRIVILEGED, set the output variableNEED_SETGIDto `true' (and otherwise to `false'), and setKMEM_GROUPto the name of the group that should own the installed program.
Check for
getmntentin the sun, seq, and gen libraries, for irix 4, PTX, and Unixware, respectively. Then, ifgetmntentis available, defineHAVE_GETMNTENT.
Define
GETPGRP_VOIDif it is an error to pass 0 togetpgrp; this is the POSIX behavior. On older BSD systems, you must pass 0 togetpgrp, as it takes an argument and behaves like POSIX'sgetpgid.#if GETPGRP_VOID pid = getpgrp (); #else pid = getpgrp (0); #endifThis macro does not check whether
getpgrpexists at all; if you need to work in that situation, first callAC_CHECK_FUNCforgetpgrp.
If link is a symbolic link, then
lstatshould treat link/ the same as link/.. However, many olderlstatimplementations incorrectly ignore trailing slashes.It is safe to assume that if
lstatincorrectly ignores trailing slashes, then other symbolic-link-aware functions likeunlinkalso incorrectly ignore trailing slashes.If
lstatbehaves properly, defineLSTAT_FOLLOWS_SLASHED_SYMLINK, otherwise require anAC_LIBOBJreplacement oflstat.
If the
mallocfunction is compatible with the GNU C librarymalloc(i.e., `malloc (0)' returns a valid pointer), defineHAVE_MALLOCto 1. Otherwise defineHAVE_MALLOCto 0, ask for anAC_LIBOBJreplacement for `malloc', and definemalloctorpl_mallocso that the nativemallocis not used in the main project.Typically, the replacement file malloc.c should look like (note the `#undef malloc'):
#if HAVE_CONFIG_H # include <config.h> #endif #undef malloc #include <sys/types.h> 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); }
If the
memcmpfunction 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 anAC_LIBOBJreplacement for `memcmp'.
Define
HAVE_MBRTOWCto 1 if the functionmbrtowcand the typembstate_tare properly declared.
If the
mktimefunction is not available, or does not work correctly, require anAC_LIBOBJreplacement for `mktime'. For the purposes of this test,mktimeshould conform to the POSIX standard and should be the inverse oflocaltime.
If the
mmapfunction exists and works correctly, defineHAVE_MMAP. Only checks private fixed mapping of already-mapped memory.
If the obstacks are found, define
HAVE_OBSTACK, else require anAC_LIBOBJreplacement for `obstack'.
If the
reallocfunction is compatible with the GNU C libraryrealloc(i.e., `realloc (0, 0)' returns a valid pointer), defineHAVE_REALLOCto 1. Otherwise defineHAVE_REALLOCto 0, ask for anAC_LIBOBJreplacement for `realloc', and definerealloctorpl_reallocso that the nativereallocis not used in the main project. SeeAC_FUNC_MALLOCfor details.
Determines the correct type to be passed for each of the
selectfunction's arguments, and defines those types inSELECT_TYPE_ARG1,SELECT_TYPE_ARG234, andSELECT_TYPE_ARG5respectively.SELECT_TYPE_ARG1defaults to `int',SELECT_TYPE_ARG234defaults to `int *', andSELECT_TYPE_ARG5defaults to `struct timeval *'.
If
setpgrptakes no argument (the POSIX version), defineSETPGRP_VOID. Otherwise, it is the BSD version, which takes two process IDs as arguments. This macro does not check whethersetpgrpexists at all; if you need to work in that situation, first callAC_CHECK_FUNCforsetpgrp.
Determine whether
statorlstathave the bug that it succeeds when given the zero-length file name as argument. Thestatandlstatfrom SunOS 4.1.4 and the Hurd (as of 1998-11-01) do this.If it does, then define
HAVE_STAT_EMPTY_STRING_BUG(orHAVE_LSTAT_EMPTY_STRING_BUG) and ask for anAC_LIBOBJreplacement of it.
If
setvbuftakes the buffering type as its second argument and the buffer pointer as the third, instead of the other way around, defineSETVBUF_REVERSED.
If the
strcollfunction exists and works correctly, defineHAVE_STRCOLL. This does a bit more than `AC_CHECK_FUNCS(strcoll)', because some systems have incorrect definitions ofstrcollthat should not be used.
If the
strtodfunction does not exist or doesn't work correctly, ask for anAC_LIBOBJreplacement of `strtod'. In this case, because strtod.c is likely to need `pow', set the output variablePOW_LIBto the extra library needed.
If
strerror_ris available, defineHAVE_STRERROR_R, and if it is declared, defineHAVE_DECL_STRERROR_R. If it returns achar *message, defineSTRERROR_R_CHAR_P; otherwise it returns aninterror number. The Thread-Safe Functions option of POSIX requiresstrerror_rto returnint, but many systems (including, for example, version 2.2.4 of the GNU C Library) return achar *value that is not necessarily equal to the buffer argument.
Check for
strftimein the intl library, for SCO unix. Then, ifstrftimeis available, defineHAVE_STRFTIME.
If the
strnlenfunction is not available, or is buggy (like the one from AIX 4.3), require anAC_LIBOBJreplacement for it.
If `utime(file, NULL)' sets file's timestamp to the present, define
HAVE_UTIME_NULL.
If
vprintfis found, defineHAVE_VPRINTF. Otherwise, if_doprntis found, defineHAVE_DOPRNT. (Ifvprintfis available, you may assume thatvfprintfandvsprintfare also available.)
If the
fnmatchfunction does not conform to POSIX (seeAC_FUNC_FNMATCH), ask for itsAC_LIBOBJreplacement.The files fnmatch.c, fnmatch_loop.c, and fnmatch_.h in the
AC_LIBOBJreplacement directory are assumed to contain a copy of the source code of GNUfnmatch. If necessary, this source code is compiled as anAC_LIBOBJreplacement, and the fnmatch_.h file is linked to fnmatch.h so that it can be included in place of the system<fnmatch.h>.
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 (see Writing Tests).
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_FUNCSinstead. This macro checks for functions with C linkage even whenAC_LANG(C++)has been called, since C is more standardized than C++. (see Language Choice, for more information about selecting the language for checks.)
For each function in the whitespace-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.
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.
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
LIBOBJSif it is not already in, and callsAC_LIBSOURCEfor `function.c'. You should not directly changeLIBOBJS, since this is not traceable.
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 toAC_LIBOBJ. In that case, since shell variables cannot be traced statically, you must pass toAC_LIBSOURCEany possible files that the shell variable might causeAC_LIBOBJto need. For example, if you want to pass a variable$foo_or_bartoAC_LIBOBJthat 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_LIBOBJwith 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.
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)
Specify that
AC_LIBOBJreplacement files are to be found in directory, a relative path starting from 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, 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.
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 `#if !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.
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.
This section tries to collect knowledge about common headers, and the problems they cause. By definition, this list will always require additions. Please help us keeping it as complete as possible.
AC_CHECK_HEADERS([sys/socket.h])
AC_CHECK_HEADERS([net/if.h], [], [],
[#include <stdio.h>
#if STDC_HEADERS
# include <stdlib.h>
# include <stddef.h>
#else
# if HAVE_STDLIB_H
# include <stdlib.h>
# endif
#endif
#if HAVE_SYS_SOCKET_H
# include <sys/socket.h>
#endif
])
AC_CHECK_HEADERS([sys/socket.h])
AC_CHECK_HEADERS([netinet/if_ether.h], [], [],
[#include <stdio.h>
#if STDC_HEADERS
# include <stdlib.h>
# include <stddef.h>
#else
# if HAVE_STDLIB_H
# include <stdlib.h>
# endif
#endif
#if HAVE_SYS_SOCKET_H
# include <sys/socket.h>
#endif
])
AC_CHECK_HEADERS([X11/extensions/scrnsaver.h], [], [],
[[#include <X11/Xlib.h>
]])
These macros check for particular system header files—whether they exist, and in some cases whether they declare certain symbols.
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_Hsys/ndir.h HAVE_SYS_NDIR_Hsys/dir.h HAVE_SYS_DIR_Hndir.h HAVE_NDIR_HThe directory-library declarations in your source code should look something like the following:
#if HAVE_DIRENT_H # include <dirent.h> # define NAMLEN(dirent) strlen((dirent)->d_name) #else # define dirent direct # define NAMLEN(dirent) (dirent)->d_namlen # if HAVE_SYS_NDIR_H # include <sys/ndir.h> # endif # if HAVE_SYS_DIR_H # include <sys/dir.h> # endif # if HAVE_NDIR_H # include <ndir.h> # endif #endifUsing the above declarations, the program would declare variables to be of type
struct dirent, notstruct direct, and would access the length of a directory entry name by passing a pointer to astruct direntto theNAMLENmacro.This macro also checks for the SCO Xenix dir and x libraries.
If sys/types.h does not define
major,minor, andmakedev, but sys/mkdev.h does, defineMAJOR_IN_MKDEV; otherwise, if sys/sysmacros.h does, defineMAJOR_IN_SYSMACROS.
If the macros
S_ISDIR,S_ISREG, etc. defined in sys/stat.h do not work properly (returning false positives), defineSTAT_MACROS_BROKEN. This is the case on Tektronix UTekV, Amdahl UTS and Motorola System V/88.
If stdbool.h exists and is conformant to C99, define
HAVE_STDBOOL_Hto 1; if the type_Boolis defined, defineHAVE__BOOLto 1. To fulfill the C99 requirements, your system.h should contain the following code:#if HAVE_STDBOOL_H # include <stdbool.h> #else # if ! HAVE__BOOL # ifdef __cplusplus typedef bool _Bool; # else typedef unsigned char _Bool; # endif # endif # define bool _Bool # define false 0 # define true 1 # define __bool_true_false_are_defined 1 #endif
Define
STDC_HEADERSif the system has ANSI C header files. 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 ANSI C header files. This macro also checks whether string.h declaresmemchr(and thus presumably the othermemfunctions), whether stdlib.h declarefree(and thus presumablymallocand other related functions), and whether the ctype.h macros work on characters with the high bit set, as ANSI C requires.Use
STDC_HEADERSinstead of__STDC__to determine whether the system has ANSI-compliant header files (and probably C library functions) because many systems that have GCC do not have ANSI C header files.On systems without ANSI C 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 systems contain a mix of functions from ANSI and BSD; some are mostly ANSI but lack `memmove'; some define the BSD functions as macros in string.h or strings.h; some have only the BSD functions but string.h; some declare 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 has the ANSI versions of those then it probably has most of the others. If you put the following in configure.ac:
AC_HEADER_STDC AC_CHECK_FUNCS(strchr memcpy)then, in your code, you can use declarations like this:
#if STDC_HEADERS # include <string.h> #else # if !HAVE_STRCHR # define strchr index # define strrchr rindex # endif char *strchr (), *strrchr (); # if !HAVE_MEMCPY # define memcpy(d, s, n) bcopy ((s), (d), (n)) # define memmove(d, s, n) bcopy ((s), (d), (n)) # endif #endifIf you use a function like
memchr,memset,strtok, orstrspn, which have no BSD equivalent, then macros won't suffice; 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, takingmemchrfor example, put it in memchr.c and use `AC_REPLACE_FUNCS(memchr)'.
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 BSDunion waitinstead ofintto 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 <sys/types.h> #if HAVE_SYS_WAIT_H # include <sys/wait.h> #endif #ifndef WEXITSTATUS # define WEXITSTATUS(stat_val) ((unsigned)(stat_val) >> 8) #endif #ifndef WIFEXITED # define WIFEXITED(stat_val) (((stat_val) & 255) == 0) #endif
_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 if the system supports POSIX is:
#if HAVE_UNISTD_H
# include <sys/types.h>
# include <unistd.h>
#endif
#ifdef _POSIX_VERSION
/* Code for POSIX systems. */
#endif
If a program may include both time.h and sys/time.h, define
TIME_WITH_SYS_TIME. On some older systems, sys/time.h includes time.h, but time.h is not protected against multiple inclusion, so programs should not explicitly include both files. This macro is useful in programs that use, for example,struct timevalas well asstruct tm. It is best used in conjunction withHAVE_SYS_TIME_H, which can be checked for usingAC_CHECK_HEADERS(sys/time.h).#if TIME_WITH_SYS_TIME # include <sys/time.h> # include <time.h> #else # if HAVE_SYS_TIME_H # include <sys/time.h> # else # include <time.h> # endif #endif
If the use of
TIOCGWINSZrequires <sys/ioctl.h>, then defineGWINSZ_IN_SYS_IOCTL. OtherwiseTIOCGWINSZcan be found in <termios.h>.Use:
#if HAVE_TERMIOS_H # include <termios.h> #endif #if GWINSZ_IN_SYS_IOCTL # include <sys/ioctl.h> #endif
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 (see Writing Tests).
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_HEADERSinstead.For compatibility issues with older versions of Autoconf, please read below.
For each given system header file header-file in the whitespace-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.For compatibility issues with older versions of Autoconf, please read below.
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. If you need to check whether a
header is preprocessable, you can use AC_PREPROC_IFELSE
(see Running the Preprocessor).
This scheme, which improves the robustness of the test, also requires that you make sure that headers that must be included before the header-file be part of the includes, (see 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], [], [], [#if HAVE_FOO_H # include <foo.h> # endif ])
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 (see Generic Declarations) or, for more complex tests, you may use
AC_COMPILE_IFELSE (see Running the Compiler).
There are no specific macros for declarations.
These macros are used to find declarations not covered by the “particular” test macros.
If symbol (a function or a variable) is not declared in includes and a declaration is needed, run the shell commands action-if-not-found, otherwise action-if-found. If no includes are specified, the default includes are used (see Default Includes).
This macro actually tests whether it is valid to use symbol as an r-value, not if it is really declared, because it is much safer to avoid introducing extra declarations when they are not needed.
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.This macro uses an m4 list as first argument:
AC_CHECK_DECLS(strdup) AC_CHECK_DECLS([strlen]) AC_CHECK_DECLS([malloc, realloc, calloc, free])Unlike the other `AC_CHECK_*S' macros, when a symbol is not declared,
HAVE_DECL_symbol is defined to `0' instead of leavingHAVE_DECL_symbol undeclared. When you are sure that the check was performed, useHAVE_DECL_symbol just like any other result of Autoconf:#if !HAVE_DECL_SYMBOL extern char *symbol; #endifIf 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); #endifYou 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.
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
(see Generic Structures) or, for more complex tests, you may use
AC_COMPILE_IFELSE (see Running the Compiler).
The following macros check for certain structures or structure members.
If
struct statcontains anst_blksizemember, defineHAVE_STRUCT_STAT_ST_BLKSIZE. The former name,HAVE_ST_BLKSIZEis to be avoided, as its support will cease in the future. This macro is obsoleted, and should be replaced byAC_CHECK_MEMBERS([struct stat.st_blksize])
If
struct statcontains anst_blocksmember, defineHAVE_STRUCT_STAT_ST_BLOCKS. Otherwise, require anAC_LIBOBJreplacement of `fileblocks'. The former name,HAVE_ST_BLOCKSis to be avoided, as its support will cease in the future.
If
struct statcontains anst_rdevmember, defineHAVE_STRUCT_STAT_ST_RDEV. The former name for this macro,HAVE_ST_RDEV, is to be avoided as it will cease to be supported in the future. Actually, even the new macro is obsolete and should be replaced by:AC_CHECK_MEMBERS([struct stat.st_rdev])
If time.h does not define
struct tm, defineTM_IN_SYS_TIME, which means that including sys/time.h had better definestruct tm.
Figure out how to get the current timezone. If
struct tmhas atm_zonemember, defineHAVE_STRUCT_TM_TM_ZONE(and the obsoletedHAVE_TM_ZONE). Otherwise, if the external arraytznameis found, defineHAVE_TZNAME.
These macros are used to find structure members not covered by the “particular” test macros.
Check whether member is a member of the aggregate aggregate. If no includes are specified, the default includes are used (see Default Includes).
AC_CHECK_MEMBER(struct passwd.pw_gecos,, [AC_MSG_ERROR([We need `passwd.pw_gecos'!])], [#include <pwd.h>])You can use this macro for sub-members:
AC_CHECK_MEMBER(struct top.middle.bot)
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.This macro uses m4 lists:
AC_CHECK_MEMBERS([struct stat.st_rdev, struct stat.st_blksize])
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.
These macros check for particular C types in sys/types.h, stdlib.h and others, if they exist.
Define
GETGROUPS_Tto be whichever ofgid_torintis the base type of the array argument togetgroups.
Define
HAVE_MBSTATE_Tif<wchar.h>declares thembstate_ttype. Also, definembstate_tto be a type if<wchar.h>does not declare it.
If signal.h declares
signalas returning a pointer to a function returningvoid, defineRETSIGTYPEto bevoid; otherwise, define it to beint.Define signal handlers as returning type
RETSIGTYPE:RETSIGTYPE hup_handler () { ... }
These macros are used to check for types not covered by the “particular” test macros.
Check whether type is defined. It may be a compiler builtin type or defined by the includes (see Default Includes).
For each type of the types that is defined, define
HAVE_type (in all capitals). If no includes are specified, the default includes are used (see 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, uintmax_t])
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 heuristics, quite safe but not totally, is
implemented. In case of doubt, read the documentation of the former
AC_CHECK_TYPE, see Obsolete Macros.
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 Unix and
`.exe' if Win32 or OS/2.
They also define the output variable OBJEXT based on the
output of the compiler, after .c files have been excluded, typically
to `o' if Unix, `obj' if Win32.
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. See Manual Configuration, for more on support for cross compiling.
Some compilers exhibit different behaviors.
int
main (void)
{
static int test_array [sizeof (int) == 4 ? 1 : -1];
test_array [0] = 0
return 0;
}
To our knowledge, there is a single compiler that does not support this trick: the HP C compilers (the real one, not only the “bundled”) on HP-UX 11.00:
$ cc -c -Ae +O2 +Onolimit conftest.c
cc: "conftest.c": error 1879: Variable-length arrays cannot \
have static storage.
Autoconf works around this problem by casting sizeof (int) to
long before comparing it.
Define
SIZEOF_type (see Standard Symbols) to be the size in bytes of type. If `type' is unknown, it gets a size of 0. If no includes are specified, the default includes are used (see Default Includes). If you provide include, be sure to include stdio.h which is required for this macro to run.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_Pto be 8 on DEC Alpha AXP systems.
Normally Autoconf ignores warnings generated by the compiler, linker, and preprocessor. If this macro is used, warnings will be treated as fatal errors instead for the current language. This macro is useful when the results of configuration will be used where warnings are unacceptable; for instance, if parts of a program are built with the GCC `-Werror' option. If the whole program will be built using `-Werror' it is often simpler to put `-Werror' in the compiler flags (
CFLAGSetc.).
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.
#ifdef __STDC__
/\
* A comment with backslash-newlines in it. %{ %} *\
\
/
char str[] = "\\
" A string with backslash-newlines in it %{ %} \\
"";
char apostrophe = '\\
\
'\
';
#endif
yields
error-->cpp: "foo.c", line 13: error 4048: Non-terminating comment at end of file.
error-->cpp: "foo.c", line 13: error 4033: Missing #endif at end of file.
Removing the lines with solitary backslashes solves the problem.
$ 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 -o a.o; cc -c b.c -o b.o; cc
a.o b.o -o c' solves the issue.
#line support#line directives whose line
numbers are greater than 32767. In addition, nothing in posix
makes this invalid. That is the reason why Autoconf stopped issuing
#line directives.
Determine a C compiler to use. If
CCis not already set in the environment, check forgccandcc, then for other C compilers. Set output variableCCto the name of the compiler found.This macro may, however, be invoked with an optional first argument which, if specified, must be a space 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_CClike this:AC_PROG_CC(cl egcs gcc cc)If the C compiler is not in ANSI C mode by default, try to add an option to output variable
CCto make it so. This macro tries various options that select ANSI C on some system or another. It considers the compiler to be in ANSI C mode if it handles function prototypes correctly.After calling this macro you can check whether the C compiler has been set to accept ANSI C; if not, the shell variable
ac_cv_prog_cc_stdcis set to `no'. If you wrote your source code in ANSI C, you can make an un-ANSIfied copy of it by using the programansi2knr, which comes with Automake. See also underAC_C_PROTOTYPESbelow.If using the GNU C compiler, set shell variable
GCCto `yes'. If output variableCFLAGSwas 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.
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 byAC_PROG_CC, and, if different, the firstccin the path. The test fails if one fails. This macro was created for GNU Make to choose the default C compilation rule.
Set output variable
CPPto a command that runs the C preprocessor. If `$CC -E' doesn't work, /lib/cpp is used. It is only portable to runCPPon 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_WERRORis also specified.
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 (see Running the Compiler) or
AC_RUN_IFELSE (see Run Time).
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.
The default for action-if-true is to define `WORDS_BIGENDIAN'. The default for action-if-false is to do nothing. And finally, the default for action-if-unknown is to abort configure and tell the installer which variable he should preset to bypass this test.
If the C compiler does not fully support the ANSI C qualifier
const, defineconstto be empty. Some C compilers that do not define__STDC__do supportconst; some compilers that define__STDC__do not completely supportconst. Programs can simply useconstas if every C compiler supported it; for those that don't, the Makefile or configuration header file will define 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++ treatconstdifferently. For example:const int foo;is valid in C but not in C++. These differences unfortunately cannot be papered over by defining
constto be empty.If autoconf detects this situation, it leaves
constalone, 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.
If the C compiler recognizes the
restrictkeyword, don't do anything. If it recognizes only a variant spelling (__restrict,__restrict__, or_Restrict), then definerestrictto that. Otherwise, definerestrictto be empty. Thus, programs may simply userestrictas 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
restrictkeyword is not required, several C++ compilers do accept the keyword. This macro works for them, too.
If the C compiler does not understand the keyword
volatile, definevolatileto be empty. Programs can simply usevolatileas if every C compiler supported it; for those that do not, the Makefile or configuration header will define 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 supportvolatile, you are at its mercy anyway. At least your program will compile, when it wouldn't before.In general, the
volatilekeyword is a feature of ANSI C, so you might expect thatvolatileis available only when__STDC__is defined. However, Ultrix 4.3's native compiler does support volatile, but does not define__STDC__.
If the C compiler supports the keyword
inline, do nothing. Otherwise defineinlineto__inline__or__inlineif it accepts one of those, otherwise defineinlineto be empty.
If the C type
charis unsigned, define__CHAR_UNSIGNED__, unless the C compiler predefines it.
If the C compiler supports a working
long doubletype with more range or precision than thedoubletype, defineHAVE_LONG_DOUBLE.
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
If function prototypes are understood by the compiler (as determined by
AC_PROG_CC), definePROTOTYPESand__PROTOTYPES. In the case the compiler does not handle prototypes, you should useansi2knr, which comes with the Automake distribution, to unprotoize function definitions. For function prototypes, you should first definePARAMS:#ifndef PARAMS # if PROTOTYPES # define PARAMS(protos) protos # else /* no PROTOTYPES */ # define PARAMS(protos) () # endif /* no PROTOTYPES */ #endifthen use it this way:
size_t my_strlen PARAMS ((const char *));
This macro also defines __PROTOTYPES; this is for the benefit of
header files that cannot use macros that infringe on user name space.
Add -traditional to output variable
CCif using the GNU C compiler andioctldoes not work properly without -traditional. That usually happens when the fixed header files have not been installed on an old system. Since recent versions of the GNU C compiler fix the header files automatically when installed, this is becoming a less prevalent problem.
Determine a C++ compiler to use. Check if the environment variable
CXXorCCC(in that order) is set; if so, then set output variableCXXto its value.Otherwise, if the macro is invoked without an argument, then search for a C++ compiler under the likely names (first
g++andc++then other names). If none of those checks succeed, then as a last resort setCXXtog++.This macro may, however, be invoked with an optional first argument which, if specified, must be a space 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_CXXlike this:AC_PROG_CXX(cl KCC CC cxx cc++ xlC aCC c++ g++ egcs gcc)If using the GNU C++ compiler, set shell variable
GXXto `yes'. If output variableCXXFLAGSwas 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.
Set output variable
CXXCPPto a command that runs the C++ preprocessor. If `$CXX -E' doesn't work, /lib/cpp is used. It is only portable to runCXXCPPon files with a .c, .C, or .cc 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++.
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.
Determine a Fortran 77 compiler to use. If
F77is not already set in the environment, then check forg77andf77, and then some other names. Set the output variableF77to the name of the compiler found.This macro may, however, be invoked with an optional first argument which, if specified, must be a space 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_F77like this:AC_PROG_F77(fl32 f77 fort77 xlf g77 f90 xlf90)If using
g77(the GNU Fortran 77 compiler), thenAC_PROG_F77will set the shell variableG77to `yes'. If the output variableFFLAGSwas not already set in the environment, then set it to -g -02 forg77(or -O2 whereg77does not accept -g). Otherwise, setFFLAGSto -g for all other Fortran 77 compilers.
Determine a Fortran compiler to use. If
FCis not already set in the environment, thendialectis a hint to indicate what Fortran dialect to search for; the default is to search for the newest available dialect. Set the output variableFCto the name of the compiler found.By default, newer dialects are preferred over older dialects, but if
dialectis specified then older dialects are preferred starting with the specified dialect.dialectcan 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.f90orf95), and no attempt is made to guarantee that a particular language standard is actually supported. Thus, it is preferable that you avoid thedialectoption, 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 space separated list of Fortran compilers to search for, just as in
AC_PROG_F77.If the output variable
FCFLAGSwas not already set in the environment, then set it to -g -02 for GNUg77(or -O2 whereg77does not accept -g). Otherwise, setFCFLAGSto -g for all other Fortran compilers.
Test if the Fortran compiler accepts the options -c and -o simultaneously, and define
F77_NO_MINUS_C_MINUS_OorFC_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 (see Running the Compiler) or
AC_RUN_IFELSE (see Run Time), making sure to first set the
current language to Fortran 77 or Fortran via AC_LANG(Fortran 77)
or AC_LANG(Fortran) (see Language Choice).
Determine the linker flags (e.g., -L and -l) for the Fortran intrinsic and run-time libraries that are required to successfully link a Fortran program or shared library. The output variable
FLIBSorFCLIBSis set to these flags (which should be include afterLIBSwhen 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 (see Mixing Fortran 77 With C and C++ (GNU Automake)).
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 run-time 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/AC_FC_DUMMY_MAINorAC_F77_MAIN/AC_FC_MAINwill probably also be necessary to link C/C++ with Fortran; see below.
With many compilers, the Fortran libraries detected by
AC_F77_LIBRARY_LDFLAGSorAC_FC_LIBRARY_LDFLAGSprovide their ownmainentry 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. TheAC_F77_DUMMY_MAIN/AC_FC_DUMMY_MAINorAC_F77_MAIN/AC_FC_MAINmacro figures 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
mainand skip the Fortran library initializations. In this case, however, one may still need to provide a dummyMAIN__routine in order to prevent linking errors on some systems.AC_F77_DUMMY_MAINorAC_FC_DUMMY_MAINdetects whether any such routine is required for linking, and what its name is; the shell variableF77_DUMMY_MAINorFC_DUMMY_MAINholds this name,unknownwhen no solution was found, andnonewhen no such dummy main is needed.By default, action-if-found defines
F77_DUMMY_MAINorFC_DUMMY_MAINto 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
F77withFCfor Fortran instead of Fortran 77.)Note that this macro is called automatically from
AC_F77_WRAPPERSorAC_FC_WRAPPERS; there is generally no need to call it explicitly unless one wants to change the default actions.
As discussed above, many Fortran libraries allow you to provide an entry point called (say)
MAIN__instead of the usualmain, which is then called by amainfunction in the Fortran libraries that initializes things like Fortran I/O. TheAC_F77_MAIN/AC_FC_MAINmacro detects whether it is possible to utilize such an alternate main function, and definesF77_MAIN/FC_MAINto the name of the function. (If no alternate main function name is found,F77_MAIN/FC_MAINis simply defined tomain.)Thus, when calling Fortran routines from C that perform things like I/O, one should use this macro and name the "main" function
F77_MAIN/FC_MAINinstead ofmain.
Defines C macros
F77_FUNC(name,NAME)/FC_FUNC(name,NAME)andF77_FUNC_(name,NAME)/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_WRAPPERSorAC_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 endYou 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_FUNCso that it can select the right one. Note also that all parameters to Fortran 77 routines are passed as pointers (see Mixing Fortran 77 With C and C++ (GNU Automake)).(Replace
F77withFCfor 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 will generate a compile-time error, but some other behavior (e.g., disabling Fortran-related features) can be induced by checking whether the
F77_FUNC/FC_FUNCmacro 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 useF77_FUNC_/FC_FUNC_instead ofF77_FUNC/FC_FUNC(with the same arguments). This is because some Fortran compilers mangle names differently if they contain an underscore.
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_WRAPPERSorAC_FC_WRAPPERS). shellvar is optional; if it is not supplied, the shell variable will be 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++.
By default, the
FCmacros 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 filename extensions. TheAC_FC_SRCEXTmacro deals with both of these issues.The
AC_FC_SRCEXTtries to get theFCcompiler 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 variableFCFLAGS_ext. This extension and these flags are then used for all subsequentFCtests (untilAC_FC_SRCEXTis called again).For example, you would use
AC_FC_SRCEXT(f90)to employ the .f90 extension in future tests, and it would set aFCFLAGS_f90output variable with any extra flags that are needed to compile such files.The
FCFLAGS_ext can not be simply absorbed intoFCFLAGS, for two reasons based on the limitations of some compilers. First, only oneFCFLAGS_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 filename 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) foo.f90If
AC_FC_SRCEXTsucceeds 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 theFCcompiler accept such files, it calls [action-if-failure] (defaults to exiting with an error message).
The
AC_FC_FREEFORMtries 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 toFCFLAGS.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, thenAC_FC_FREEFORMwill ordinarily succeed without modifyingFCFLAGS.If
AC_FC_FREEFORMsucceeds 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).
The following macros check for operating system services or capabilities.
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
xmkmfon a trivial Imakefile and examining the Makefile that it produces. If that fails (such as ifxmkmfis not present), look for the files in several directories where they often reside. If either method is successful, set the shell variablesx_includesandx_librariesto 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_xto `yes'; otherwise set it to the empty string.
An enhanced version of
AC_PATH_X. It adds the C compiler flags that X needs to output variableX_CFLAGS, and the X linker flags toX_LIBS. DefineX_DISPLAY_MISSINGif 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 variableX_PRE_LIBS.
Check whether the system supports starting scripts with a line of the form `#! /bin/csh' 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 will be set to `yes' if the system supports `#!', `no' if not.
Arrange for large-file support. 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_BITSand_LARGE_FILESif 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_tis longer thanlong, since this is common when large-file support is enabled. For example, it is not correct to print an arbitraryoff_tvalueXwithprintf ("%ld", (long) X).The LFS introduced the
fseekoandftellofunctions to replace their C counterpartsfseekandftellthat do not useoff_t. Take care to useAC_FUNC_FSEEKOto make their prototypes available when using them and large-file support is enabled.
If the system supports file names longer than 14 characters, define
HAVE_LONG_FILE_NAMES.
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_termiosto `yes'. If not, set the variable to `no'.
The following macros check for certain operating systems that need special treatment for some programs, due to exceptional oddities in their header files or libraries. These macros are warts; they will be replaced by a more systematic approach, based on the functions they make available or the environments they provide.
If on AIX, define
_ALL_SOURCE. Allows the use of some BSD functions. Should be called before any macros that run the C compiler.
If using the GNU C library, define
_GNU_SOURCE. Allows the use of some GNU functions. Should be called before any macros that run the C compiler.
For interactive unix (ISC), add -lcposix to output variable
LIBSif necessary for POSIX facilities. Call this afterAC_PROG_CCand before any other macros that use POSIX interfaces. interactive unix is no longer sold, and Sun says that they will drop support for it on 2006-07-23, so this macro is becoming obsolescent.
If on Minix, define
_MINIXand_POSIX_SOURCEand define_POSIX_1_SOURCEto be 2. This allows the use of POSIX facilities. Should be called before any macros that run the C compiler.
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 (see Language Choice). They do not cache the results of their tests for future use (see 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. See Writing Autoconf Macros, for how to do that.
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.
Do compilation tests using the compiler, preprocessor, and file extensions for the specified language.
Supported languages are:
- `C'
- Do compilation tests using
CCandCPPand use extension .c for test programs. Use compilation flags:CPPFLAGSwithCPP, and bothCPPFLAGSandCFLAGSwithCC.- `C++'
- Do compilation tests using
CXXandCXXCPPand use extension .C for test programs. Use compilation flags:CPPFLAGSwithCXXPP, and bothCPPFLAGSandCXXFLAGSwithCXX.- `Fortran 77'
- Do compilation tests using
F77and use extension .f for test programs. Use compilation flags:FFLAGS.- `Fortran'
- Do compilation tests using
FCand use extension .f (or whatever has been set byAC_FC_SRCEXT) for test programs. Use compilation flags:FCFLAGS.
Remember the current language (as set by
AC_LANG) on a stack, and then select the language. Use this macro andAC_LANG_POPin macros that need to temporarily switch to a particular 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 will detect inconsistencies.
AC_LANG_PUSH(Fortran 77) # Perform some tests on Fortran 77. # ... AC_LANG_POP(Fortran 77)
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 2, the macros
AC_LANG_PUSH/AC_LANG_POPcannot 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 withAC_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...
Ensure that whichever preprocessor would currently be used for tests has been found. Calls
AC_REQUIRE(see Prerequisite Macros) with an argument of eitherAC_PROG_CPPorAC_PROG_CXXCPP, depending on which language is current.
Autoconf tests follow is common scheme: feeding some program with some input, and most of the time, feeding a compiler with some source file. This section is dedicated to these source samples.
The most important rule to follow when writing testing samples is:
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 if a header is functional might let your configure accept a header which will cause some compiler error. Do not hesitate checking header with other headers included before, especially required headers.
Make sure the symbols you use are properly defined, i.e., refrain for simply declaring a function yourself instead of including the proper header.
Test programs should not write anything to the standard output. They
should return 0 if the test succeeds, nonzero 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. Test programs should exit, not return, from
main, because on some systems (old Suns, at least) the argument
to return in main is ignored.
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_STDC, then later on in
configure.ac you can have a test program that includes an
ANSI C header file conditionally:
#if STDC_HEADERS
# include <stdlib.h>
#endif
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 -rf conftest*' after running test programs and if the script is interrupted.
Function declarations in test programs should have a prototype conditionalized for C++. In practice, though, test programs rarely need functions that take arguments.
#ifdef __cplusplus
foo (int i)
#else
foo (i) int i;
#endif
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" void *malloc (size_t);
#else
void *malloc ();
#endif
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).
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 (see Language Choice) to “format” the output properly.
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.
Expands into the source, with the definition of all the
AC_DEFINEperformed so far.
For instance executing (observe the double quotation!):
AC_INIT(Autoconf Documentation, 2.59, bug-autoconf@gnu.org)
AC_DEFINE([HELLO_WORLD], ["Hello, World\n"])
AC_LANG_CONFTEST(
[AC_LANG_SOURCE([[const char hw[] = "Hello, World\n";]])])
gcc -E -dD conftest.c -o -
results in:
# 1 "conftest.c"
# 1169 "configure"
# 1 "confdefs.h" 1
#define PACKAGE_NAME "Autoconf Documentation"
#define PACKAGE_TARNAME "autoconf-documentation"
#define PACKAGE_VERSION "2.59"
#define PACKAGE_STRING "Autoconf Documentation 2.59"
#define PACKAGE_BUGREPORT "bug-autoconf@gnu.org"
#define HELLO_WORLD "Hello, World\n"
# 1170 "configure" 2
const char hw[] = "Hello, World\n";
Expands into a source file which consists of the prologue, and then body as body of the main function (e.g.,
mainin C). Since it usesAC_LANG_SOURCE, the feature of the latter are available.
For instance:
AC_INIT(Autoconf Documentation, 2.59, bug-autoconf@gnu.org)
AC_DEFINE([HELLO_WORLD], ["Hello, World\n"])
AC_LANG_CONFTEST(
[AC_LANG_PROGRAM([[const char hw[] = "Hello, World\n";]],
[[fputs (hw, stdout);]])])
gcc -E -dD conftest.c -o -
results in:
# 1 "conftest.c"
# 1169 "configure"
# 1 "confdefs.h" 1
#define PACKAGE_NAME "Autoconf Documentation"
#define PACKAGE_TARNAME "autoconf-documentation"
#define PACKAGE_VERSION "2.59"
#define PACKAGE_STRING "Autoconf Documentation 2.59"
#define PACKAGE_BUGREPORT "bug-autoconf@gnu.org"
#define HELLO_WORLD "Hello, World\n"
# 1170 "configure" 2
const char hw[] = "Hello, World\n";
int
main ()
{
fputs (hw, stdout);
;
return 0;
}
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.,
mainin C). Since it usesAC_LANG_PROGRAMS, 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.
Expands into a source file which consists of a pseudo use of the function as body of the main function (e.g.,
mainin C): a simple (function pointer) assignment. Since it usesAC_LANG_PROGRAMS, the feature 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).
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 to the temptation of following the easiest path.
Nevertheless, if you need to run the preprocessor, then use
AC_PREPROC_IFELSE.
Run the preprocessor of the current language (see 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_PROGRAMand friends.This macro uses
CPPFLAGS, but notCFLAGS, 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(Autoconf Documentation, 2.59, bug-autoconf@gnu.org)
AC_DEFINE([HELLO_WORLD], ["Hello, World\n"])
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... 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 ANSI C... none needed
checking how to run the C preprocessor... gcc -E
OK
The macro
AC_TRY_CPP (see 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?
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.
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.
To check for a syntax feature of the current language's (see 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.
Run the compiler and compilation flags of the current language (see 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_PROGRAMand friends.It is customary to report unexpected failures with
AC_MSG_FAILURE. This macro does not try to link; useAC_LINK_IFELSEif you need to do that (see 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 run-time 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 (see Libraries), by adding the library being
checked for to LIBS temporarily and trying to link a small
program.
Run the compiler (and compilation flags) and the linker of the current language (see 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_PROGRAMand friends.
LDFLAGSandLIBSare 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; useAC_RUN_IFELSEif you need to do that (see Run Time).
Sometimes you need to find out how a system performs at run time, 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 run-time 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.
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_PROGRAMand friends.LDFLAGSandLIBSare used for linking, in addition to the compilation flags of the current language (see Language Choice).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 exit status of the program is available in the shell variable `$?', but be very careful to limit yourself to positive values smaller than 127; bigger values should be saved into a file by the program. Note also that you have simply no guarantee that this exit status is issued by the program, or by the failure of its compilation. In other words, use this feature if sadist only, it was reestablished because the Autoconf maintainers grew tired of receiving “bug reports”.
It is customary to report unexpected failures with
AC_MSG_FAILURE.
Try to provide a pessimistic default value to use when cross-compiling
makes run-time 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 will not be able to 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 (see Manual Configuration). Alternatively, set up a test results cache file with the correct values for the host system (see 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.
This section aims at presenting some systems and pointers to documentation. It may help you addressing particular problems reported by users.
The Rosetta Stone for Unix contains a lot of interesting crossed information on various Unices.
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).
Some operations are accomplished in several possible ways, depending on the unix 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.
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 <sys/statvfs.h>
#include <sys/fstyp.h>]])],
[AC_DEFINE(FSTYPE_STATVFS) fstype=SVR4])
if test $fstype = no; then
AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <sys/statfs.h>
#include <sys/fstyp.h>]])],
[AC_DEFINE(FSTYPE_USG_STATFS) fstype=SVR3])
fi
if test $fstype = no; then
AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <sys/statfs.h>
#include <sys/vmount.h>]])]),
[AC_DEFINE(FSTYPE_AIX_STATFS) fstype=AIX])
fi
# (more cases omitted here)
AC_MSG_RESULT([$fstype])
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.
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)])
if test "$ac_cv_func_vprintf" != yes; then
AC_CHECK_FUNC(_doprnt, [AC_DEFINE(HAVE_DOPRNT)])
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.
See Configuration Headers, for more information about this kind of
output.
Define the C preprocessor variable variable to value (verbatim). value should not contain literal newlines, and if you are not using
AC_CONFIG_HEADERSit should not contain any `#' characters, as make tends to eat them. To use a shell variable (which you need to do in order to define a value containing the M4 quote characters `[' or `]'), useAC_DEFINE_UNQUOTEDinstead. description is only useful if you are usingAC_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 variableEQUATIONto be the string constant `"$a > $b"':AC_DEFINE(EQUATION, "$a > $b")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.
Like
AC_DEFINE, but three shell expansions are performed—once—on variable and value: variable expansion (`$'), command substitution (``'), and backslash escaping (`\'). Single and double quote characters in the value have no special meaning. Use this macro instead ofAC_DEFINEwhen variable or value is a shell variable. Examples:AC_DEFINE_UNQUOTED(config_machfile, "$machfile") AC_DEFINE_UNQUOTED(GETGROUPS_T, $ac_cv_type_getgroups) AC_DEFINE_UNQUOTED($ac_tr_hdr)
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 spaces or
newlines. That is, do this:
AC_CHECK_HEADER(elf.h, [AC_DEFINE(SVR4) LIBS="$LIBS -lelf"])
or this:
AC_CHECK_HEADER(elf.h,
[AC_DEFINE(SVR4)
LIBS="$LIBS -lelf"])
instead of this:
AC_CHECK_HEADER(elf.h, [AC_DEFINE(SVR4); LIBS="$LIBS -lelf"])
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. See Preset Output Variables, for a list of output variables that are always available.
Create an output variable from a shell variable. Make
AC_OUTPUTsubstitute the variable variable into output files (typically one or more Makefiles). This means thatAC_OUTPUTwill replace instances of `@variable@' in input files with the value that the shell variable variable has whenAC_OUTPUTis called. This value of variable should not contain literal newlines.If value is given, in addition assign it to variable.
Another way to create an output variable from a shell variable. Make
AC_OUTPUTinsert (without substitutions) the contents of the file named by shell variable variable into output files. This means thatAC_OUTPUTwill replace instances of `@variable@' in output files (such as Makefile.in) with the contents of the file that the shell variable variable names whenAC_OUTPUTis called. Set the variable to /dev/null for cases that do not have a file to insert.This macro is useful for inserting Makefile fragments containing special dependencies or other
makedirectives for particular host or target types into Makefiles. For example, configure.ac could contain:AC_SUBST_FILE(host_frag) host_frag=$srcdir/conf/sun4.mhand then a Makefile.in could contain:
@host_frag@
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 will 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', (See Automatic Remaking, and see 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.
Declare variable is a precious variable, and include its description in the variable section of `./configure --help'.
Being precious means that
- variable is
AC_SUBST'd.- 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
CCin `./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
FOOisfoocan be two very different runs.- 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 overand similarly if the variable is unset, or if its content is changed.
- variable is kept during automatic reconfiguration (see 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 undeclared_var=raboof --silent $ ./config.status --recheck running /bin/sh ./configure undeclared_var=raboof --silent \ CC=/usr/bin/cc --no-create --no-recursion
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 (see 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.
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 will be saved in the cache file just before configure creates its output files. See 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.
A wrapper for
AC_CACHE_VALthat takes care of printing the messages. This macro provides a convenient shorthand for the most common way to use these macros. It callsAC_MSG_CHECKINGfor message, thenAC_CACHE_VALwith the cache-id and commands arguments, andAC_MSG_RESULTwith cache-id.The commands-to-set-it must have no side effects except for setting the variable cache-id, see below.
It is very 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], [ac_cv_shell_true_works],
[ac_cv_shell_true_works=no
true && ac_cv_shell_true_works=yes
if test $ac_cv_shell_true_works = yes; 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], [ac_cv_shell_true_works],
[ac_cv_shell_true_works=no
true && ac_cv_shell_true_works=yes])
if test $ac_cv_shell_true_works = yes; 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.
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:
_cv_The values assigned to cache variables may not contain newlines. Usually, their values will be Boolean (`yes' or `no') or the names of files or functions; so this is not an important restriction.
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 (technically, it uses --cache-file=/dev/null), 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. See 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 (see 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 will gradually accumulate 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.
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 will
reduce the amount of time it takes to re-run the configure script with
(hopefully) the error that caused the previous abort corrected.
Loads values from existing cache file, or creates a new cache file if a cache file is not found. Called automatically from
AC_INIT.
Flushes all cached values to the cache file. Called automatically from
AC_OUTPUT, but it can be quite useful to callAC_CACHE_SAVEat key points in configure.ac.
For instance:
... AC_INIT, etc. ... # Checks for programs. AC_PROG_CC AC_PROG_GCC_TRADITIONAL ... 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. ...
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.
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 of the callers will change
automatically.
To diagnose static issues, i.e., when autoconf is run, see Reporting Messages.
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_RESULTto 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.
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 toAC_MSG_CHECKING.This macro prints nothing if configure is run with the --quiet or --silent option.
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.
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 (1 by default). 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”.
This
AC_MSG_ERRORwrapper 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.
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_WARNshould 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'.
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 are still experimental, and their interface might change in the future. As a matter of fact, anything that is not documented must not be used.
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.
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 finally `(' and `)' (which M4 tries to match by pairs).
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 will try hopelessly to use her macro
array:
define([array], [int tab[10];])
array
=>int tab10;
[array]
=>array
How can you correctly output the intended results3?
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, 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([#])
=>#
The reader will easily understand the following examples:
car(foo, bar)
=>foo
car([foo, bar])
=>foo, bar
car((foo, bar))
=>(foo, bar)
car([(foo], [bar)])
=>(foo
car([], [])
=>
car([[]], [[]])
=>[]
With this in mind, we can explore the cases where macros invoke 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 arguments are literal strings, if the user wants to use the results of expansions as arguments, she has to use an unquoted macro call:
qar(active)
=>ACT
where she wanted 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 <stdio.h>])
car([my_includes])
=>#include <stdio.h>
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.
changequote is EvilThe 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`'), 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 n-th argument, `$' by
default, the parenthesis 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 will be lost, as the current syntax will probably not be the one they were implemented with.
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.
You can work around this problem by using one of the following quadrigraphs:
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, will appear as `^[' in the output.
The empty quadrigraph can be used:
Trailing spaces are smashed by autom4te. This is a feature.
For instance `@<@&t@:@' produces `@<:@'.
For instance you might want to mention AC_FOO in a comment, while
still being sure that autom4te will still catch 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.
To conclude, the quotation rule of thumb is:
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 <time.h>
#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 <time.h>
#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 will 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 <time.h>
#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])
See 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 will contain unexpanded macros. The autoconf program checks for this problem by doing `grep AC_ configure'.
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: autom4te4.
autom4te is a preprocessor that is like m4. It supports M4 extensions designed for use in tools like Autoconf.
The command line arguments are modeled after M4's:
autom4te options files
where the files are directly passed to m4. In addition to the regular expansion, it handles the replacement of the quadrigraphs (see Quadrigraphs), and of `__oline__', the current line in the output. It supports an extended syntax for the files:
Of course, it supports the Autoconf common subset of options:
As an extension of m4, it includes the following options:
AC_DIAGNOSE, for a comprehensive list of categories. Special
values include:
Warnings about `syntax' are enabled by default, and the environment
variable WARNINGS, a comma separated list of categories, is
honored. autom4te -W category will actually
behave as if you had run:
autom4te --warnings=syntax,$WARNINGS,category
If you want to disable autom4te's defaults and
WARNINGS, but (for example) 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. For instance, on this configure.ac:
AC_DEFUN([INNER],
[AC_RUN_IFELSE([AC_LANG_PROGRAM([exit (0)])])])
AC_DEFUN([OUTER],
[INNER])
AC_INIT
OUTER
you get:
$ autom4te -l autoconf -Wcross
configure.ac:8: warning: AC_RUN_IFELSE called without default \
to allow cross compiling
$ autom4te -l autoconf -Wcross,error -f
configure.ac:8: error: AC_RUN_IFELSE called without default \
to allow cross compiling
acgeneral.m4:3044: AC_RUN_IFELSE is expanded from...
configure.ac:2: INNER is expanded from...
configure.ac:5: OUTER is expanded from...
configure.ac:8: the top level
.m4f will be
replaced with 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
autom4te 1.m4 2.m4 3.m4 --freeze --output=3.m4f
corresponds to
m4 1.m4 2.m4 3.m4 --freeze-state=3.m4f
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 save the long runs of m4, 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 is trashed).
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:
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 escape `$%' produces single-line trace outputs (unless you put newlines in the `separator'), while `$@' and `$*' do not.
See autoconf Invocation, for examples of trace uses.
Finally, autom4te introduces the concept of Autom4te libraries. They consists in a powerful yet extremely simple feature: sets of combined command line arguments:
M4sugarM4shAutotestAutoconfAutoconf-without-aclocal-m4As an example, if Autoconf is installed in its default location, /usr/local, running `autom4te -l m4sugar foo.m4' is strictly equivalent to running `autom4te --prepend-include /usr/local/share/autoconf m4sugar/m4sugar.m4f --warnings syntax foo.m4'. Recursive expansion applies: running `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.
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 (see 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" args: --no-cache end-language: "Autoconf"
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”.
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.
Some M4 macros are redefined, and are slightly incompatible with their native equivalent.
Contrary to the M4 builtin, this macro fails if macro is not defined. See
m4_undefine.
Contrary to the M4 builtin, this macro fails if macro is not defined. Use
m4_ifdef([macro], [m4_undefine([macro])])to recover the behavior of the builtin.
This macro corresponds to
patsubst. The namem4_patsubstis kept for future versions of M4sh, on top of GNU M4 which will provide extended regular expression syntax viaepatsubst.
Contrary to the M4 builtin, this macro fails if macro is not defined. See
m4_undefine.
This macro corresponds to
regexp. The namem4_regexpis kept for future versions of M4sh, on top of GNU M4 which will provide extended regular expression syntax viaeregexp.
This macro corresponds to
m4wrap.You are encouraged to end text with `[]', so that there are no risks that two consecutive invocations of
m4_wrapresult in an unexpected pasting of tokens, as inm4_define([foo], [Foo]) m4_define([bar], [Bar]) m4_define([foobar], [FOOBAR]) m4_wrap([bar]) m4_wrap([foo]) =>FOOBAR
The following macros give some control over the order of the evaluation by adding or removing levels of quotes. They are meant for hard-core M4 programmers.
Return the arguments as a single entity, i.e., wrap them into a pair of quotes.
The following example aims at emphasizing the difference between (i), not
using these macros, (ii), using m4_quote, and (iii), using
m4_dquote.
$ cat example.m4
# Overquote, so that quotes are visible.
m4_define([show], [$[]1 = [$1], $[]@ = [$@]])
m4_divert(0)dnl
show(a, b)
show(m4_quote(a, b))
show(m4_dquote(a, b))
$ autom4te -l m4sugar example.m4
$1 = a, $@ = [a],[b]
$1 = a,b, $@ = [a,b]
$1 = [a],[b], $@ = [[a],[b]]
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$'.
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 makes sense to the author of this documentation, as `#'-comments should document the output, not the input, documented by `dnl' comments).
Of course, you might encounter exceptions to these generic rules, for instance you might have to refer to `$m4_flags'.
Any token matching pattern is allowed, including if it matches an
m4_pattern_forbidpattern.
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”.]
- A mass of mixed ingredients reduced to a soft pulpy state by beating or pressure....
- A mixture of meal or bran and water fed to animals.
- A mess; trouble. [Obs.] –Beau. & Fl.
For the time being, it is not mature enough to be widely used.
M4sh provides portable alternatives for some common shell constructs that unfortunately are not portable in practice.
Return the directory portion of pathname, using the algorithm required by POSIX. See Limitations of Usual Tools, for more details about what this returns and why it is more portable than the dirname command.
Run shell code TEST. If TEST exits with a zero status then run shell code RUN-IF-TRUE, else run shell code RUN-IF-FALSE, with simplifications if either RUN-IF-TRUE or RUN-IF-FALSE is empty.
Make the directory filename, including intervening directories as necessary. This is equivalent to `mkdir -p filename', except that it is portable to older versions of mkdir that lack support for the -p option.
Set the shell variable var to dir/file, but optimizing the common cases (dir or file is `.', file is absolute etc.).
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.
Autoconf macros are defined using the AC_DEFUN macro, which is
similar to the M4 builtin m4_define macro. 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 (see 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. See How to define new macros (GNU m4), for more complete information on writing M4 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],
[{ _AC_ECHO([configure: error: $1], 2); exit m4_default([$2], 1); }])
Comments about the macro should be left in the header comment. Most other comments will make their way into configure, so just keep using `#' to introduce comments.
If you have some very 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.
All of the Autoconf macros have all-uppercase names starting with `AC_' to prevent them from accidentally conflicting with other text. All shell variables that they use for internal purposes have mostly-lowercase names starting with `ac_'. 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.
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 (see Cache Variable Names, for more information on them).
The first word of the name after `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.
CDECLFUNCGROUPHEADERLIBPATHPROGMEMBERSYSTYPEVARAfter 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_FUNC_UTIME_NULL checks the
behavior of the utime function when called with a NULL
pointer.
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.
When macros statically diagnose abnormal situations, benign or fatal, they should report them using these macros. For dynamic issues, i.e., when configure is run, see Printing Messages.
Report message as a warning (or as an error if requested by the user) if warnings of the category are turned on. 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.
Equivalent to `AC_DIAGNOSE([syntax], message)', but you are strongly encouraged to use a finer grained category.
When the user runs `autoconf -W error', warnings from
AC_DIAGNOSE and AC_WARNING are reported as error, see
autoconf Invocation.
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.
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.
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_DEFUNor else contain a call toAC_PROVIDEto indicate that it has been called.
AC_REQUIREmust be used inside anAC_DEFUN'd macro; it must not be called from the top level.
AC_REQUIRE is often misunderstood. It really implements
dependencies between macros in the sense that if one macro depends upon
another, the latter will be expanded before the body of the
former. 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 "$hair_style" = "curly" &&
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
RESERVE_DANCE_FLOOR
if test "$dance_floor" = occupied; then
AC_MSG_ERROR([cannot pick up here, let's move])
fi
will 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 "$hair_style" = "curly" &&
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
You are encouraged to put all AC_REQUIREs at the beginning of a
macro. You can use dnl to avoid the empty lines they leave.
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.
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 usingAC_DEFUNor else contain a call toAC_PROVIDEto indicate that it has been called.
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.
Define old-macro as implementation. The only difference with
AC_DEFUNis that the user will be warned that old-macro is now obsolete.If she then uses autoupdate, the call to old-macro will be replaced by the modern implementation. The additional message is then printed.
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 via other means.
The first requirement is to pay great attention to the quotation. For more details, see Autoconf Language, and 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 (see 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 symbols 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)...or better yet, high-level
macros such as AC_EXPAND_ONCE.
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 spaces 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
purpose. With the s command, the preferred separator is `/'
unless `/' itself is used in the command, in which case you should
use `,'.
See Macro Definitions, for details on how to define a macro. If a
macro doesn't use AC_REQUIRE and it is expected to never be the
object of an AC_REQUIRE directive, then use m4_define. In
case of doubt, use AC_DEFUN. All the AC_REQUIRE
statements should be at the beginning of the macro, dnl'ed.
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 $x_includes])
fi[]dnl
])# AC_PATH_X
If the macro is long, try to split it into logical chunks. Typically,
macros that check for a bug in a function and prepare its
AC_LIBOBJ replacement should have an auxiliary macro to perform
this setup. Do not hesitate to introduce auxiliary macros to factor
your code.
In order to highlight the recommended coding style, here is a macro written the old way:
dnl Check for EMX on OS/2.
dnl _AC_EMXOS2
AC_DEFUN(_AC_EMXOS2,
[AC_CACHE_CHECK(for EMX OS/2 environment, ac_cv_emxos2,
[AC_COMPILE_IFELSE([AC_LANG_PROGRAM(, return __EMX__;)],
ac_cv_emxos2=yes, ac_cv_emxos2=no)])
test "$ac_cv_emxos2" = yes && 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 "$ac_cv_emxos2" = yes && EMXOS2=yes[]dnl
])# _AC_EMXOS2
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, but to prevent trouble, do not take advantage of features that were added after unix version 7, circa 1977 (see Systemology).
You should not use shell functions, aliases, negated character
classes, or other features that are not found in all Bourne-compatible
shells; restrict yourself to the lowest common denominator. Even
unset is not supported by all shells! Also, include a space
after the exclamation point in interpreter specifications, like this:
#! /usr/bin/perl
If you omit the space before the path, then 4.2BSD based systems (such as DYNIX) will ignore the line, because they interpret `#! /' as a 4-byte magic number. Some old systems have quite small limits on the length of the `#!' line too, for instance 32 bytes (not including the newline) on SunOS 4.
The set of external programs you should run in a configure script is fairly small. See Utilities in Makefiles (GNU Coding Standards), 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 Limitations of Usual Tools.
There are other sources of documentation about shells. See for instance the Shell FAQs.
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 includes a small history of Unix shells, and a comparison between several of them.
Below we describe some of the members of the Bourne shell family.
To be compatible with Ash 0.2:
foo=
false
$foo
echo "Don't use it: $?"
cat ${FOO=`bar`}
BASH_VERSION is set. To disable its extensions and require
POSIX compatibility, run `set -o posix'. See Bash POSIX Mode (The GNU Bash Reference Manual), for details.
ZSH_VERSION is set. By default zsh is not
compatible with the Bourne shell: you have to run `emulate sh' and
set NULLCMD to `:'. See Compatibility (The Z Shell Manual), for details.
Zsh 3.0.8 is the native /bin/sh on Mac OS X 10.0.3.
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.
Don't rely on `\' being preserved just because it has no special meaning together with the next symbol. In the native /bin/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 /bin/sh
$ cat <<EOF
> \" \\
> EOF
" \
and with Bash:
bash-2.04$ cat <<EOF
> \" \\
> EOF
\" \
Many older shells (including the Bourne shell) implement here-documents inefficiently. And some shells mishandle large here-documents: for example, Solaris 8 dtksh, which is derived from ksh M-12/28/93d, mishandles variable expansion that occurs on 1024-byte buffer boundaries within a here-document. Users can generally fix these problems by using a faster or more reliable shell, e.g., by using the command `bash ./configure' rather than plain `./configure'.
Some shells can be extremely inefficient when there are a lot of here-documents inside a single statement. For instance if your configure.ac includes something like:
if <cross_compiling>; 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 run-time, and we end up not
executing the macro at all.
Some file descriptors sh