Autoconf

This manual (9 September 2008) is for GNU Autoconf (version 2.63), a package for creating scripts to configure source code packages using templates and an M4 macro package.

1 Introduction

A physicist, an engineer, and a computer scientist were discussing the

nature of God. “Surely a Physicist,” said the physicist, “because

early in the Creation, God made Light; and you know, Maxwell's

equations, the dual nature of electromagnetic waves, the relativistic

consequences....” “An Engineer!,” said the engineer, “because

before making Light, God split the Chaos into Land and Water; it takes a

hell of an engineer to handle that big amount of mud, and orderly

separation of solids from liquids....” The computer scientist

shouted: “And the Chaos, where do you think it was coming from, hmm?”

—Anonymous

Autoconf is a tool for producing shell scripts that automatically configure software source code packages to adapt to many kinds of Posix-like systems. The configuration scripts produced by Autoconf are independent of Autoconf when they are run, so their users do not need to have Autoconf.

The configuration scripts produced by Autoconf require no manual user intervention when run; they do not normally even need an argument specifying the system type. Instead, they individually test for the presence of each feature that the software package they are for might need. (Before each check, they print a one-line message stating what they are checking for, so the user doesn't get too bored while waiting for the script to finish.) As a result, they deal well with systems that are hybrids or customized from the more common Posix variants. There is no need to maintain files that list the features supported by each release of each variant of Posix.

For each software package that Autoconf is used with, it creates a configuration script from a template file that lists the system features that the package needs or can use. After the shell code to recognize and respond to a system feature has been written, Autoconf allows it to be shared by many software packages that can use (or need) that feature. If it later turns out that the shell code needs adjustment for some reason, it needs to be changed in only one place; all of the configuration scripts can be regenerated automatically to take advantage of the updated code.

Those who do not understand Autoconf are condemned to reinvent it, poorly. The primary goal of Autoconf is making the user's life easier; making the maintainer's life easier is only a secondary goal. Put another way, the primary goal is not to make the generation of configure automatic for package maintainers (although patches along that front are welcome, since package maintainers form the user base of Autoconf); rather, the goal is to make configure painless, portable, and predictable for the end user of each autoconfiscated package. And to this degree, Autoconf is highly successful at its goal — most complaints to the Autoconf list are about difficulties in writing Autoconf input, and not in the behavior of the resulting configure. Even packages that don't use Autoconf will generally provide a configure script, and the most common complaint about these alternative home-grown scripts is that they fail to meet one or more of the GNU Coding Standards that users have come to expect from Autoconf-generated configure scripts.

The Metaconfig package is similar in purpose to Autoconf, but the scripts it produces require manual user intervention, which is quite inconvenient when configuring large source trees. Unlike Metaconfig scripts, Autoconf scripts can support cross-compiling, if some care is taken in writing them.

Autoconf does not solve all problems related to making portable software packages—for a more complete solution, it should be used in concert with other GNU build tools like Automake and Libtool. These other tools take on jobs like the creation of a portable, recursive makefile with all of the standard targets, linking of shared libraries, and so on. 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 version 1.4.5 or later in order to generate the scripts. It uses features that some versions of M4, including GNU M4 1.3, do not have. Autoconf works better with GNU M4 version 1.4.11 or later, though this is not required.

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.

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.

Because of its mission, the Autoconf package itself includes only a set of often-used macros that have already demonstrated their usefulness. Nevertheless, if you wish to share your macros, or find existing ones, see the Autoconf Macro Archive, which is kindly run by Peter Simons.

2 The GNU Build System

Autoconf solves an important problem—reliable discovery of system-specific build and runtime information—but this is only one piece of the puzzle for the development of portable software. To this end, the GNU project has developed a suite of integrated utilities to finish the job Autoconf started: the GNU build system, whose most important components are Autoconf, Automake, and Libtool. In this chapter, we introduce you to those tools, point you to sources of more information, and try to convince you to use the entire GNU build system for your software.

2.1 Automake

The ubiquity of make means that a makefile is almost the only viable way to distribute automatic build rules for software, but one quickly runs into its numerous limitations. Its lack of support for automatic dependency tracking, recursive builds in subdirectories, reliable timestamps (e.g., for network file systems), and so on, mean that developers must painfully (and often incorrectly) reinvent the wheel for each project. Portability is non-trivial, thanks to the quirks of make on many systems. On top of all this is the manual labor required to implement the many standard targets that users have come to expect (make install, make distclean, make uninstall, etc.). Since you are, of course, using Autoconf, you also have to insert repetitive code in your Makefile.in to recognize @CC@, @CFLAGS@, and other substitutions provided by configure. Into this mess steps Automake. Automake allows you to specify your build needs in a Makefile.am file with a vastly simpler and more powerful syntax than that of a plain makefile, and then generates a portable Makefile.in for use with Autoconf. For example, the Makefile.am to build and install a simple “Hello world” program might look like:

The resulting Makefile.in (~400 lines) automatically supports all the standard targets, the substitutions provided by Autoconf, automatic dependency tracking, VPATH building, and so on. make builds the hello program, and make install installs it in /usr/local/bin (or whatever prefix was given to configure, if not /usr/local).

The benefits of Automake increase for larger packages (especially ones with subdirectories), but even for small programs the added convenience and portability can be substantial. And that's not all....

2.2 Gnulib

GNU software has a well-deserved reputation for running on many different types of systems. While our primary goal is to write software for the GNU system, many users and developers have been introduced to us through the systems that they were already using.

Gnulib is a central location for common GNU code, intended to be shared among free software packages. Its components are typically shared at the source level, rather than being a library that gets built, installed, and linked against. The idea is to copy files from Gnulib into your own source tree. There is no distribution tarball; developers should just grab source modules from the repository. The source files are available online, under various licenses, mostly GNU GPL or GNU LGPL.

Gnulib modules typically contain C source code along with Autoconf macros used to configure the source code. For example, the Gnulib stdbool module implements a stdbool.h header that nearly conforms to C99, even on old-fashioned hosts that lack stdbool.h. This module contains a source file for the replacement header, along with an Autoconf macro that arranges to use the replacement header on old-fashioned systems.

2.3 Libtool

Often, one wants to build not only programs, but libraries, so that other programs can benefit from the fruits of your labor. Ideally, one would like to produce shared (dynamically linked) libraries, which can be used by multiple programs without duplication on disk or in memory and can be updated independently of the linked programs. Producing shared libraries portably, however, is the stuff of nightmares—each system has its own incompatible tools, compiler flags, and magic incantations. Fortunately, GNU provides a solution: Libtool. Libtool handles all the requirements of building shared libraries for you, and at this time seems to be the only way to do so with any portability. It also handles many other headaches, such as: the interaction of Make rules with the variable suffixes of shared libraries, linking reliably with shared libraries before they are installed by the superuser, and supplying a consistent versioning system (so that different versions of a library can be installed or upgraded without breaking binary compatibility). Although Libtool, like Autoconf, can be used without Automake, it is most simply utilized in conjunction with Automake—there, Libtool is used automatically whenever shared libraries are needed, and you need not know its syntax.

2.4 Pointers

Developers who are used to the simplicity of make for small projects on a single system might be daunted at the prospect of learning to use Automake and Autoconf. As your software is distributed to more and more users, however, you otherwise quickly find yourself putting lots of effort into reinventing the services that the GNU build tools provide, and making the same mistakes that they once made and overcame. (Besides, since you're already learning Autoconf, Automake is a piece of cake.)

There are a number of places that you can go to for more information on the GNU build tools.

3 Making configure Scripts

The configuration scripts that Autoconf produces are by convention called configure. When run, configure creates several files, replacing configuration parameters in them with appropriate values. The files that configure creates are:

To create a configure script with Autoconf, you need to write an Autoconf input file configure.ac (or configure.in) and run autoconf on it. If you write your own feature tests to supplement those that come with Autoconf, you might also write files called aclocal.m4 and acsite.m4. If you use a C header file to contain #define directives, you might also run autoheader, and you can distribute the generated file config.h.in with the package.

Here is a diagram showing how the files that can be used in configuration are produced. Programs that are executed are suffixed by `*'. Optional files are enclosed in square brackets (`[]'). autoconf and autoheader also read the installed Autoconf macro files (by reading autoconf.m4).

3.1 Writing configure.ac

To produce a configure script for a software package, create a file called configure.ac that contains invocations of the Autoconf macros that test the system features your package needs or can use. Autoconf macros already exist to check for many features; see 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.

3.1.1 A Shell Script Compiler

Just as for any other computer language, in order to properly program configure.ac in Autoconf you must understand what problem the language tries to address and how it does so.

The problem Autoconf addresses is that the world is a mess. After all, you are using Autoconf in order to have your package compile easily on all sorts of different systems, some of them being extremely hostile. Autoconf itself bears the price for these differences: configure must run on all those systems, and thus configure must limit itself to their lowest common denominator of features.

Naturally, you might then think of shell scripts; who needs autoconf? A set of properly written shell functions is enough to make it easy to write configure scripts by hand. Sigh! Unfortunately, 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. Even in 2007, where shells without any function support are far and few between, there are pitfalls to avoid when making use of them.

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.

There are two obvious possibilities: creating a brand new language or extending an existing one. The former option is attractive: all sorts of optimizations could easily be implemented in the compiler and many rigorous checks could be performed on the Autoconf program (e.g., rejecting any non-portable construct). Alternatively, you can extend an existing language, such as the sh (Bourne shell) language.

Autoconf does the latter: it is a layer on top of sh. It was therefore most convenient to implement autoconf as a macro expander: a program that repeatedly performs macro expansions on text input, replacing macro calls with macro bodies and producing a pure sh script in the end. Instead of implementing a dedicated Autoconf macro expander, it is natural to use an existing general-purpose macro language, such as M4, and implement the extensions as a set of M4 macros.

3.1.2 The Autoconf Language

The Autoconf language differs from many other computer languages because it treats actual code the same as plain text. Whereas in C, for instance, data and instructions have different syntactic status, in Autoconf their status is rigorously the same. Therefore, we need a means to distinguish literal strings from text to be expanded: quotation.

When calling macros that take arguments, there must not be any white space between the macro name and the open parenthesis. Arguments should be enclosed within the M4 quote characters `[' and `]', and be separated by commas. Any leading blanks or newlines in arguments are ignored, unless they are quoted. You should always quote an argument that might contain a macro name, comma, parenthesis, or a leading blank or newline. This rule applies recursively for every macro call, including macros called from other macros.

because `1' cannot contain a macro call. Here, the argument of AC_MSG_ERROR must be quoted; otherwise, its comma would be interpreted as an argument separator. Also, the second and third arguments of `AC_CHECK_HEADER' must be quoted, since they contain macro calls. The three arguments `HAVE_STDIO_H', `stdio.h', and `Define to 1 if you have <stdio.h>.' do not need quoting, but if you unwisely defined a macro with a name like `Define' or `stdio' then they would need quoting. Cautious Autoconf users would keep the quotes, but many Autoconf users find such precautions annoying, and would rewrite the example as follows:

This is safe, so long as you adopt good naming conventions and do not define macros with names like `HAVE_STDIO_H', `stdio', or `h'. Though it is also safe here to omit the quotes around `Define to 1 if you have <stdio.h>.' this is not recommended, as message strings are more likely to inadvertently contain commas.

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:

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:

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:

is incorrect: here, the first argument of AC_COMPILE_IFELSE is `char b[10];' and is expanded once, which results in `char b10;'. (There was an idiom common in Autoconf's past to address this issue via the M4 changequote primitive, but do not use it!) Let's take a closer look: the author meant the first argument to be understood as a literal, and therefore it must be quoted twice:

On the other hand, descriptions (e.g., the last parameter of AC_DEFINE or AS_HELP_STRING) are not literals—they are subject to line breaking, for example—and should not be double quoted. Even if these descriptions are short and are not actually broken, double quoting them yields weird results.

Some macros take optional arguments, which this documentation represents as [arg] (not to be confused with the quote characters). You may just leave them empty, or use `[]' to make the emptiness of the argument explicit, or you may simply omit the trailing commas. The three lines below are equivalent:

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:

3.1.3 Standard configure.ac Layout

The order in which configure.ac calls the Autoconf macros is not important, with a few exceptions. Every configure.ac must contain a call to AC_INIT before the checks, and a call to AC_OUTPUT at the end (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.

3.2 Using autoscan to Create configure.ac

The autoscan program can help you create and/or maintain a configure.ac file for a software package. autoscan examines source files in the directory tree rooted at a directory given as a command line argument, or the current directory if none is given. It searches the source files for common portability problems and creates a file configure.scan which is a preliminary configure.ac for that package, and checks a possibly existing configure.ac for completeness.

When using autoscan to create a configure.ac, you should manually examine configure.scan before renaming it to configure.ac; it probably needs some adjustments. Occasionally, autoscan outputs a macro in the wrong order relative to another macro, so that autoconf produces a warning; you need to move such macros manually. Also, if you want the package to use a configuration header file, you must add a call to AC_CONFIG_HEADERS (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 contains detailed information on why a macro is requested.

autoscan uses several data files (installed along with Autoconf) to determine which macros to output when it finds particular symbols in a package's source files. These data files all have the same format: each line consists of a symbol, one or more blanks, and the Autoconf macro to output if that symbol is encountered. Lines starting with `#' are comments.

3.3 Using ifnames to List Conditionals

ifnames can help you write configure.ac for a software package. It prints the identifiers that the package already uses in C preprocessor conditionals. If a package has already been set up to have some portability, ifnames can thus help you figure out what its configure needs to check for. It may help fill in some gaps in a configure.ac generated by autoscan (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.

3.4 Using autoconf to Create configure

To create configure from configure.ac, run the autoconf program with no arguments. autoconf processes configure.ac with the M4 macro processor, using the Autoconf macros. If you give autoconf an argument, it reads that file instead of configure.ac and writes the configuration script to the standard output instead of to configure. If you give autoconf the argument -, it reads from the standard input instead of configure.ac and writes the configuration script to the standard output.

The Autoconf macros are defined in several files. Some of the files are distributed with Autoconf; autoconf reads them first. Then it looks for the optional file acsite.m4 in the directory that contains the distributed Autoconf macro files, and for the optional file aclocal.m4 in the current directory. Those files can contain your site's or the package's own Autoconf macro definitions (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.

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:

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):

3.5 Using autoreconf to Update configure Scripts

Installing the various components of the GNU Build System can be tedious: running autopoint for Gettext, automake for Makefile.in etc. in each directory. It may be needed either because some tools such as automake have been updated on your system, or because some of the sources such as configure.ac have been updated, or finally, simply in order to install the GNU Build System in a fresh tree.

autoreconf runs autoconf, autoheader, aclocal, automake, libtoolize, and autopoint (when appropriate) repeatedly to update the GNU Build System in the specified directories and their subdirectories (see Subdirectories). By default, it only remakes those files that are older than their sources. The environment variables AUTOCONF, AUTOHEADER, AUTOMAKE, ACLOCAL, AUTOPOINT, LIBTOOLIZE, M4, and MAKE may be used to override the invocation of the respective tools.

If you install a new version of some tool, you can make autoreconf remake all of the files by giving it the --force option.

See Automatic Remaking, for Make rules to automatically rebuild configure scripts when their source files change. That method handles the timestamps of configuration header templates properly, but does not pass --autoconf-dir=dir or --localdir=dir.

Gettext supplies the autopoint command to add translation infrastructure to a source package. If you use autopoint, your configure.ac should invoke both AM_GNU_GETTEXT and AM_GNU_GETTEXT_VERSION(gettext-version). See Invoking the autopoint Program, for further details.

If you want autoreconf to pass flags that are not listed here on to aclocal, set ACLOCAL_AMFLAGS in your Makefile.am. Due to a limitation in the Autoconf implementation these flags currently must be set on a single line in Makefile.am, without any backslash-newlines.

4 Initialization and Output Files

Autoconf-generated configure scripts need some information about how to initialize, such as how to find the package's source files and about the output files to produce. The following sections describe the initialization and the creation of output files.

4.1 Initializing configure

Every configure script must call AC_INIT before doing anything else. The only other required macro is AC_OUTPUT (see Output).

— Macro: AC_INIT (package, version, [bug-report], [tarname])

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_INIT be static, i.e., there should not be any shell computation, but they can be computed by M4.
The following M4 macros (e.g., AC_PACKAGE_NAME), output variables (e.g., PACKAGE_NAME), and preprocessor symbols (e.g., PACKAGE_NAME), are defined by AC_INIT:

AC_PACKAGE_NAME, PACKAGE_NAME
Exactly package.
AC_PACKAGE_TARNAME, PACKAGE_TARNAME
Exactly tarname.
AC_PACKAGE_VERSION, PACKAGE_VERSION
Exactly version.
AC_PACKAGE_STRING, PACKAGE_STRING
Exactly `package version'.
AC_PACKAGE_BUGREPORT, PACKAGE_BUGREPORT
Exactly bug-report.

If your configure script does its own option processing, it should inspect `$@' or `$*' immediately after calling AC_INIT, because other Autoconf macros liberally use the set command to process strings, and this has the side effect of updating `$@' and `$*'. However, we suggest that you use standard macros like AC_ARG_ENABLE instead of attempting to implement your own option processing. See Site Configuration.

4.2 Dealing with Autoconf versions

The following optional macros can be used to help choose the minimum version of Autoconf that can successfully compile a given configure.ac.

— Macro: AC_PREREQ (version)

Ensure that a recent enough version of Autoconf is being used. If the version of Autoconf being used to create configure is earlier than version, print an error message to the standard error output and exit with failure (exit status is 63). For example:
          AC_PREREQ([2.63])
     
This macro is the only macro that may be used before AC_INIT, but for consistency, you are invited not to do so.

— Macro: AC_AUTOCONF_VERSION

This macro was introduced in Autoconf 2.62. It identifies the version of Autoconf that is currently parsing the input file, in a format suitable for m4_version_compare (see m4_version_compare); in other words, for this release of Autoconf, its value is `2.63'. One potential use of this macro is for writing conditional fallbacks based on when a feature was added to Autoconf, rather than using AC_PREREQ to require the newer version of Autoconf. However, remember that the Autoconf philosophy favors feature checks over version checks.
You should not expand this macro directly; use `m4_defn([AC_AUTOCONF_VERSION])' instead. This is because some users might have a beta version of Autoconf installed, with arbitrary letters included in its version string. This means it is possible for the version string to contain the name of a defined macro, such that expanding AC_AUTOCONF_VERSION would trigger the expansion of that macro during rescanning, and change the version string to be different than what you intended to check.

4.3 Notices in configure

The following macros manage version numbers for configure scripts. Using them is optional.

— Macro: AC_COPYRIGHT (copyright-notice)

State that, in addition to the Free Software Foundation's copyright on the Autoconf macros, parts of your configure are covered by the copyright-notice.
The copyright-notice shows up in both the head of configure and in `configure --version'.

— Macro: AC_REVISION (revision-info)

Copy revision stamp revision-info into the configure script, with any dollar signs or double-quotes removed. This macro lets you put a revision stamp from configure.ac into configure without RCS or CVS changing it when you check in configure. That way, you can determine easily which revision of configure.ac a particular configure corresponds to.
For example, this line in configure.ac:
          AC_REVISION([$Revision: 1.30 $])
     
produces this in configure:
          #!/bin/sh
          # From configure.ac Revision: 1.30
     

4.4 Finding configure Input

— Macro: AC_CONFIG_SRCDIR (unique-file-in-source-dir)

unique-file-in-source-dir is some file that is in the package's source directory; configure checks for this file's existence to make sure that the directory that it is told contains the source code in fact does. Occasionally people accidentally specify the wrong directory with --srcdir; this is a safety check. 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.

— Macro: AC_CONFIG_AUX_DIR (dir)

Use the auxiliary build tools (e.g., install-sh, config.sub, config.guess, Cygnus configure, Automake and Libtool scripts, etc.) that are in directory dir. These are auxiliary files used in configuration. dir can be either absolute or relative to srcdir. The default is srcdir or srcdir/.. or srcdir/../.., whichever is the first that contains install-sh. The other files are not checked for, so that using AC_PROG_INSTALL does not automatically require distributing the other auxiliary files. It checks for install.sh also, but that name is obsolete because some make have a rule that creates install from it if there is no makefile.
The auxiliary directory is commonly named build-aux. If you need portability to DOS variants, do not name the auxiliary directory aux. See File System Conventions.

— Macro: AC_REQUIRE_AUX_FILE (file)

Declares that file is expected in the directory defined above. In Autoconf proper, this macro does nothing: its sole purpose is to be traced by third-party tools to produce a list of expected auxiliary files. For instance it is called by macros like AC_PROG_INSTALL (see Particular Programs) or AC_CANONICAL_BUILD (see Canonicalizing) to register the auxiliary files they need.

Similarly, packages that use aclocal should declare where local macros can be found using AC_CONFIG_MACRO_DIR.

— Macro: AC_CONFIG_MACRO_DIR (dir)

Specify dir as the location of additional local Autoconf macros. This macro is intended for use by future versions of commands like autoreconf that trace macro calls. It should be called directly from configure.ac so that tools that install macros for aclocal can find the macros' declarations.
Note that if you use aclocal from Automake to generate aclocal.m4, you must also set ACLOCAL_AMFLAGS = -Idir in your top-level Makefile.am. Due to a limitation in the Autoconf implementation of autoreconf, these include directives currently must be set on a single line in Makefile.am, without any backslash-newlines.

4.5 Outputting Files

Every Autoconf script, e.g., configure.ac, should finish by calling AC_OUTPUT. That is the macro that generates and runs config.status, which in turn creates the makefiles and any other files resulting from configuration. This is the only required macro besides AC_INIT (see Input).

— Macro: AC_OUTPUT

Generate config.status and launch it. Call this macro once, at the end of configure.ac.
config.status performs all the configuration actions: all the output files (see Configuration Files, macro AC_CONFIG_FILES), header files (see Configuration Headers, macro AC_CONFIG_HEADERS), commands (see Configuration Commands, macro AC_CONFIG_COMMANDS), links (see Configuration Links, macro AC_CONFIG_LINKS), subdirectories to configure (see Subdirectories, macro AC_CONFIG_SUBDIRS) are honored.
The location of your AC_OUTPUT invocation is the exact point where configuration actions are taken: any code afterwards is executed by configure once config.status was run. If you want to bind actions to config.status itself (independently of whether configure is being run), see 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.

— Macro: AC_PROG_MAKE_SET

If the Make command, $MAKE if set or else `make', predefines $(MAKE), define output variable SET_MAKE to be empty. Otherwise, define SET_MAKE to a macro definition that sets $(MAKE), such as `MAKE=make'. Calls AC_SUBST for SET_MAKE.

If you use this macro, place a line like this in each Makefile.in that runs MAKE on other directories:

4.6 Performing Configuration Actions

configure is designed so that it appears to do everything itself, but there is actually a hidden slave: config.status. configure is in charge of examining your system, but it is config.status that actually takes the proper actions based on the results of configure. The most typical task of config.status is to instantiate files.

This section describes the common behavior of the four standard instantiating macros: AC_CONFIG_FILES, AC_CONFIG_HEADERS, AC_CONFIG_COMMANDS and AC_CONFIG_LINKS. They all have this prototype:

All these macros can be called multiple times, with different tag values, of course!

4.7 Creating Configuration Files

— Macro: AC_CONFIG_FILES (file..., [cmds], [init-cmds])

Make AC_OUTPUT create each file by copying an input file (by default file.in), substituting the output variable values. This macro is one of the instantiating macros; see 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_FILES look like this:
          AC_CONFIG_FILES([Makefile src/Makefile man/Makefile X/Imakefile])
          AC_CONFIG_FILES([autoconf], [chmod +x autoconf])
     
You can override an input file name by appending to file a colon-separated list of input files. Examples:
          AC_CONFIG_FILES([Makefile:boiler/top.mk:boiler/bot.mk]
                          [lib/Makefile:boiler/lib.mk])
     
Doing this allows you to keep your file names acceptable to DOS variants, or to prepend and/or append boilerplate to the file.

4.8 Substitutions in Makefiles

Each subdirectory in a distribution that contains something to be compiled or installed should come with a file Makefile.in, from which configure creates a file Makefile in that directory. To create Makefile, configure performs a simple variable substitution, replacing occurrences of `@variable@' in Makefile.in with the value that configure has determined for that variable. Variables that are substituted into output files in this way are called output variables. They are ordinary shell variables that are set in configure. To make configure substitute a particular variable into the output files, the macro AC_SUBST must be called with that variable name as an argument. Any occurrences of `@variable@' for other variables are left unchanged. 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.

4.8.1 Preset Output Variables

Some output variables are preset by the Autoconf macros. Some of the Autoconf macros set additional output variables, which are mentioned in the descriptions for those macros. 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).

— Variable: CFLAGS

Debugging and optimization options for the C compiler. If it is not set in the environment when configure runs, the default value is set when you call AC_PROG_CC (or empty if you don't). configure uses this variable when compiling or linking programs to test for C features.
If a compiler option affects only the behavior of the preprocessor (e.g., -D name), it should be put into CPPFLAGS instead. If it affects only the linker (e.g., -L directory), it should be put into LDFLAGS instead. If it affects only the compiler proper, CFLAGS is the natural home for it. If an option affects multiple phases of the compiler, though, matters get tricky. One approach to put such options directly into CC, e.g., CC='gcc -m64'. Another is to put them into both CPPFLAGS and LDFLAGS, but not into CFLAGS.

— Variable: configure_input

A comment saying that the file was generated automatically by configure and giving the name of the input file. AC_OUTPUT adds a comment line containing this variable to the top of every makefile it creates. For other files, you should reference this variable in a comment at the top of each input file. For example, an input shell script should begin like this:
          #!/bin/sh
          # @configure_input@
     
The presence of that line also reminds people editing the file that it needs to be processed by configure in order to be used.

— Variable: CPPFLAGS

Preprocessor options for the C, C++, and Objective C preprocessors and compilers. If it is not set in the environment when configure runs, the default value is empty. configure uses this variable when preprocessing or compiling programs to test for C, C++, and Objective C features.
This variable's contents should contain options like -I, -D, and -U that affect only the behavior of the preprocessor. Please see the explanation of CFLAGS for what you can do if an option affects other phases of the compiler as well.
Currently, configure always links as part of a single invocation of the compiler that also preprocesses and compiles, so it uses this variable also when linking programs. However, it is unwise to depend on this behavior because the GNU coding standards do not require it and many packages do not use CPPFLAGS when linking programs.
See Special Chars in Variables, for limitations that CPPFLAGS might run into.

— Variable: CXXFLAGS

Debugging and optimization options for the C++ compiler. It acts like CFLAGS, but for C++ instead of C.

— Variable: DEFS

-D options to pass to the C compiler. If AC_CONFIG_HEADERS is called, configure replaces `@DEFS@' with -DHAVE_CONFIG_H instead (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.

— Variable: ECHO_C
— Variable: ECHO_N
— Variable: ECHO_T

How does one suppress the trailing newline from echo for question-answer message pairs? These variables provide a way:
          echo $ECHO_N "And the winner is... $ECHO_C"
          sleep 100000000000
          echo "${ECHO_T}dead."
     
Some old and uncommon echo implementations offer no means to achieve this, in which case ECHO_T is set to tab. You might not want to use it.

— Variable: ERLCFLAGS

Debugging and optimization options for the Erlang compiler. If it is not set in the environment when configure runs, the default value is empty. configure uses this variable when compiling programs to test for Erlang features.

— Variable: FCFLAGS

Debugging and optimization options for the Fortran compiler. If it is not set in the environment when configure runs, the default value is set when you call AC_PROG_FC (or empty if you don't). configure uses this variable when compiling or linking programs to test for Fortran features.

— Variable: FFLAGS

Debugging and optimization options for the Fortran 77 compiler. If it is not set in the environment when configure runs, the default value is set when you call AC_PROG_F77 (or empty if you don't). configure uses this variable when compiling or linking programs to test for Fortran 77 features.

— Variable: LDFLAGS

Options for the linker. If it is not set in the environment when configure runs, the default value is empty. configure uses this variable when linking programs to test for C, C++, Objective C, and Fortran features.
This variable's contents should contain options like -s and -L that affect only the behavior of the linker. Please see the explanation of CFLAGS for what you can do if an option also affects other phases of the compiler.
Don't use this variable to pass library names (-l) to the linker; use LIBS instead.

— Variable: LIBS

-l options to pass to the linker. The default value is empty, but some Autoconf macros may prepend extra libraries to this variable if those libraries are found and provide necessary functions, see Libraries. configure uses this variable when linking programs to test for C, C++, and Fortran features.

— Variable: OBJCFLAGS

Debugging and optimization options for the Objective C compiler. It acts like CFLAGS, but for Objective C instead of C.

— Variable: builddir

Rigorously equal to `.'. Added for symmetry only.

— Variable: abs_builddir

Absolute name of builddir.

— Variable: top_builddir

The relative name of the top level of the current build tree. In the top-level directory, this is the same as builddir.

— Variable: top_build_prefix

The relative name of the top level of the current build tree with final slash if nonemtpy. This is the same as top_builddir, except that it contains of zero of more runs of ../, so it should not be appended with a slash for concatenation. This helps for make implementations that otherwise do not treat ./file and file as equal in the toplevel build directory.

— Variable: abs_top_builddir

Absolute name of top_builddir.

— Variable: srcdir

The name of the directory that contains the source code for that makefile.

— Variable: abs_srcdir

Absolute name of srcdir.

— Variable: top_srcdir

The name of the top-level source code directory for the package. In the top-level directory, this is the same as srcdir.

— Variable: abs_top_srcdir

Absolute name of top_srcdir.

4.8.2 Installation Directory Variables

The following variables specify the directories for package installation, see Variables for Installation Directories, for more information. Each variable corresponds to an argument of configure; trailing slashes are stripped so that expressions such as `${prefix}/lib' expand with only one slash between directory names. See the end of this section for details on when and how to use these variables.

— Variable: bindir

The directory for installing executables that users run.

— Variable: datadir

The directory for installing idiosyncratic read-only architecture-independent data.

— Variable: datarootdir

The root of the directory tree for read-only architecture-independent data files.

— Variable: docdir

The directory for installing documentation files (other than Info and man).

— Variable: dvidir

The directory for installing documentation files in DVI format.

— Variable: exec_prefix

The installation prefix for architecture-dependent files. By default it's the same as prefix. You should avoid installing anything directly to exec_prefix. However, the default value for directories containing architecture-dependent files should be relative to exec_prefix.

— Variable: htmldir

The directory for installing HTML documentation.

— Variable: includedir

The directory for installing C header files.

— Variable: infodir

The directory for installing documentation in Info format.

— Variable: libdir

The directory for installing object code libraries.

— Variable: libexecdir

The directory for installing executables that other programs run.

— Variable: localedir

The directory for installing locale-dependent but architecture-independent data, such as message catalogs. This directory usually has a subdirectory per locale.

— Variable: localstatedir

The directory for installing modifiable single-machine data.

— Variable: mandir

The top-level directory for installing documentation in man format.

— Variable: oldincludedir

The directory for installing C header files for non-GCC compilers.

— Variable: pdfdir

The directory for installing PDF documentation.

— Variable: prefix

The common installation prefix for all files. If exec_prefix is defined to a different value, prefix is used only for architecture-independent files.

— Variable: psdir

The directory for installing PostScript documentation.

— Variable: sbindir

The directory for installing executables that system administrators run.

— Variable: sharedstatedir

The directory for installing modifiable architecture-independent data.

— Variable: sysconfdir

The directory for installing read-only single-machine data.

Most of these variables have values that rely on prefix or exec_prefix. It is deliberate that the directory output variables keep them unexpanded: typically `@datarootdir@' is replaced by `${prefix}/share', not `/usr/local/share', and `@datadir@' is replaced by `${datarootdir}'.

This behavior is mandated by the GNU coding standards, so that when the user runs:

In order to support these features, it is essential that datarootdir 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], ["$datadir"], [Data directory.])', you should add -DDATADIR='$(datadir)' to your makefile's definition of CPPFLAGS (AM_CPPFLAGS if you are also using Automake).

Similarly, you should not rely on AC_CONFIG_FILES to replace datadir and friends in your shell scripts and other files; instead, let make manage their replacement. For instance Autoconf ships templates of its shell scripts ending with `.in', and uses a makefile snippet similar to the following to build scripts like autoheader and autom4te:

For the more specific installation of Erlang libraries, the following variables are defined:

— Variable: ERLANG_INSTALL_LIB_DIR

The common parent directory of Erlang library installation directories. This variable is set by calling the AC_ERLANG_SUBST_INSTALL_LIB_DIR macro in configure.ac.

— Variable: ERLANG_INSTALL_LIB_DIR_library

The installation directory for Erlang library library. This variable is set by calling the `AC_ERLANG_SUBST_INSTALL_LIB_SUBDIR(library, version' macro in configure.ac.

4.8.3 Changed Directory Variables

In Autoconf 2.60, the set of directory variables has changed, and the defaults of some variables have been adjusted (see Installation Directory Variables) to changes in the GNU Coding Standards. Notably, datadir, infodir, and mandir are now expressed in terms of datarootdir. If you are upgrading from an earlier Autoconf version, you may need to adjust your files to ensure that the directory variables are substituted correctly (see Defining Directories), and that a definition of datarootdir is in place. For example, in a Makefile.in, adding

is usually sufficient. If you use Automake to create Makefile.in, it will add this for you.

To help with the transition, Autoconf warns about files that seem to use datarootdir without defining it. In some cases, it then expands the value of $datarootdir in substitutions of the directory variables. The following example shows such a warning:

Usually one can easily change the file to accommodate both older and newer Autoconf releases:

In some cases, however, the checks may not be able to detect that a suitable initialization of datarootdir is in place, or they may fail to detect that such an initialization is necessary in the output file. If, after auditing your package, there are still spurious configure warnings about datarootdir, you may add the line

to your configure.ac to disable the warnings. This is an exception to the usual rule that you should not define a macro whose name begins with AC_ (see Macro Names).

4.8.4 Build Directories

You can support compiling a software package for several architectures simultaneously from the same copy of the source code. The object files for each architecture are kept in their own directory.

To support doing this, make uses the VPATH variable to find the files that are in the source directory. GNU Make can do this. Most other recent make programs can do this as well, though they may have difficulties and it is often simpler to recommend GNU make (see VPATH and Make). Older make programs do not support VPATH; when using them, the source code must be in the same directory as the object files.

If you are using GNU Automake, the remaining details in this section are already covered for you, based on the contents of your Makefile.am. But if you are using Autoconf in isolation, then supporting VPATH requires the following in your Makefile.in:

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, Make command lines should always refer to source files by prefixing them with `$(srcdir)/'. For example:

4.8.5 Automatic Remaking

You can put rules like the following in the top-level Makefile.in for a package to automatically update the configuration information when you change the configuration files. This example includes all of the optional files, such as aclocal.m4 and those related to configuration header files. Omit from the Makefile.in rules for any of these files that your package does not use.

The `$(srcdir)/' prefix is included because of limitations in the VPATH mechanism.

The stamp- files are necessary because the timestamps of config.h.in and config.h are not changed if remaking them does not change their contents. This feature avoids unnecessary recompilation. You should include the file stamp-h.in your package's distribution, so that make considers config.h.in up to date. Don't use touch (see Limitations of Usual Tools); instead, use echo (using date would cause needless differences, hence CVS conflicts, etc.).

(Be careful if you copy these lines directly into your makefile, as you need to convert the indented lines to start with the tab character.)

so config.status ensures that config.h is considered up to date. See Output, for more information about AC_OUTPUT.

4.9 Configuration Header Files

When a package contains more than a few tests that define C preprocessor symbols, the command lines to pass -D options to the compiler can get quite long. This causes two problems. One is that the make output is hard to visually scan for errors. More seriously, the command lines can exceed the length limits of some operating systems. As an alternative to passing -D options to the compiler, configure scripts can create a C header file containing `#define' directives. The AC_CONFIG_HEADERS macro selects this kind of output. Though it can be called anywhere between AC_INIT and AC_OUTPUT, it is customary to call it right after AC_INIT.

The package should `#include' the configuration header file before any other header files, to prevent inconsistencies in declarations (for example, if it redefines const).

To provide for VPATH builds, remember to pass the C compiler a -I. option (or -I..; whichever directory contains config.h). Even if you use `#include "config.h"', the preprocessor searches only the directory of the currently read file, i.e., the source directory, not the build directory.

With the appropriate -I option, you can use `#include <config.h>'. Actually, it's a good habit to use it, because in the rare case when the source directory contains another config.h, the build directory should be searched first.

— Macro: AC_CONFIG_HEADERS (header ..., [cmds], [init-cmds])

This macro is one of the instantiating macros; see Configuration Actions. Make AC_OUTPUT create the file(s) in the blank-or-newline-separated list header containing C preprocessor #define statements, and replace `@DEFS@' in generated files with -DHAVE_CONFIG_H instead of the value of DEFS. The usual name for header is config.h.
If header already exists and its contents are identical to what AC_OUTPUT would put in it, it is left alone. Doing this allows making some changes in the configuration without needlessly causing object files that depend on the header file to be recompiled.
Usually the input file is named header.in; however, you can override the input file name by appending to header a colon-separated list of input files. For example, you might need to make the input file name acceptable to DOS variants:
          AC_CONFIG_HEADERS([config.h:config.hin])
     

— Macro: AH_HEADER

This macro is defined as the name of the first declared config header and undefined if no config headers have been declared up to this point. A third-party macro may, for example, require use of a config header without invoking AC_CONFIG_HEADERS twice, like this:
          AC_CONFIG_COMMANDS_PRE(
                  [m4_ifndef([AH_HEADER], [AC_CONFIG_HEADERS([config.h])])])
     

4.9.1 Configuration Header Templates

Your distribution should contain a template file that looks as you want the final header file to look, including comments, with #undef statements which are used as hooks. For example, suppose your configure.ac makes these calls:

Then you could have code like the following in conf.h.in. The conf.h created by configure defines `HAVE_UNISTD_H' to 1, if and only if the system has unistd.h.

The format of the template file is stricter than what the C preprocessor is required to accept. A directive line should contain only whitespace, `#undef', and `HAVE_UNISTD_H'. The use of `#define' instead of `#undef', or of comments on the same line as `#undef', is strongly discouraged. Each hook should only be listed once. Other preprocessor lines, such as `#ifdef' or `#include', are copied verbatim from the template into the generated header.

Since it is a tedious task to keep a template header up to date, you may use autoheader to generate it, see autoheader Invocation.

During the instantiation of the header, each `#undef' line in the template file for each symbol defined by `AC_DEFINE' is changed to an appropriate `#define'. If the corresponding `AC_DEFINE' has not been executed during the configure run, the `#undef' line is commented out. (This is important, e.g., for `_POSIX_SOURCE': on many systems, it can be implicitly defined by the compiler, and undefining it in the header would then break compilation of subsequent headers.)

Currently, all remaining `#undef' lines in the header template are commented out, whether or not there was a corresponding `AC_DEFINE' for the macro name; but this behavior is not guaranteed for future releases of Autoconf.

Generally speaking, since you should not use `#define', and you cannot guarantee whether a `#undef' directive in the header template will be converted to a `#define' or commented out in the generated header file, the template file cannot be used for conditional definition effects. Consequently, if you need to use the construct

you must place it outside of the template. If you absolutely need to hook it to the config header itself, please put the directives to a separate file, and `#include' that file from the config header template. If you are using autoheader, you would probably use `AH_BOTTOM' to append the `#include' directive.

4.9.2 Using autoheader to Create config.h.in

The autoheader program can create a template file of C `#define' statements for configure to use. It searches for the first invocation of AC_CONFIG_HEADERS in configure sources to determine the name of the template. (If the first call of AC_CONFIG_HEADERS specifies more than one input file name, autoheader uses the first one.)

It is recommended that only one input file is used. If you want to append a boilerplate code, it is preferable to use `AH_BOTTOM([#include <conf_post.h>])'. File conf_post.h is not processed during the configuration then, which make things clearer. Analogically, AH_TOP can be used to prepend a boilerplate code.

In order to do its job, autoheader needs you to document all of the symbols that you might use. Typically this is done via an AC_DEFINE or AC_DEFINE_UNQUOTED call whose first argument is a literal symbol and whose third argument describes the symbol (see Defining Symbols). Alternatively, you can use AH_TEMPLATE (see Autoheader Macros), or you can supply a suitable input file for a subsequent configuration header file. Symbols defined by Autoconf's builtin tests are already documented properly; you need to document only those that you define yourself.

You might wonder why autoheader is needed: after all, why would configure need to “patch” a config.h.in to produce a config.h instead of just creating config.h from scratch? Well, when everything rocks, the answer is just that we are wasting our time maintaining autoheader: generating config.h directly is all that is needed. When things go wrong, however, you'll be thankful for the existence of autoheader.

The fact that the symbols are documented is important in order to check that config.h makes sense. The fact that there is a well-defined list of symbols that should be defined (or not) is also important for people who are porting packages to environments where configure cannot be run: they just have to fill in the blanks.

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.

4.9.3 Autoheader Macros

autoheader scans configure.ac and figures out which C preprocessor symbols it might define. It knows how to generate templates for symbols defined by AC_CHECK_HEADERS, AC_CHECK_FUNCS etc., but if you AC_DEFINE any additional symbol, you must define a template for it. If there are missing templates, autoheader fails with an error message.

The template for a symbol is created by autoheader from the description argument to an AC_DEFINE; see Defining Symbols.

— Macro: AH_TEMPLATE (key, description)

Tell autoheader to generate a template for key. This macro generates standard templates just like AC_DEFINE when a description is given.

For example:

          AH_TEMPLATE([CRAY_STACKSEG_END],
                      [Define to one of _getb67, GETB67, getb67
                       for Cray-2 and Cray-YMP systems.  This
                       function is required for alloca.c support
                       on those systems.])

generates the following template, with the description properly justified.

          /* Define to one of _getb67, GETB67, getb67 for Cray-2 and
             Cray-YMP systems.  This function is required for alloca.c
             support on those systems.  */
          #undef CRAY_STACKSEG_END

— Macro: AH_VERBATIM (key, template)

Tell autoheader to include the template as-is in the header template file. This template is associated with the key, which is used to sort all the different templates and guarantee their uniqueness. It should be a symbol that can be defined via AC_DEFINE.

— Macro: AH_TOP (text)

Include text at the top of the header template file.

— Macro: AH_BOTTOM (text)

Include text at the bottom of the header template file.

Please note that text gets included “verbatim” to the template file, not to the resulting config header, so it can easily get mangled when the template is processed. There is rarely a need for something other than

4.10 Running Arbitrary Configuration Commands

You can execute arbitrary commands before, during, and after config.status is run. The three following macros accumulate the commands to run when they are called multiple times. AC_CONFIG_COMMANDS replaces the obsolete macro AC_OUTPUT_COMMANDS; see Obsolete Macros, for details.

— Macro: AC_CONFIG_COMMANDS (tag..., [cmds], [init-cmds])

Specify additional shell commands to run at the end of config.status, and shell commands to initialize any variables from configure. Associate the commands with tag. Since typically the cmds create a file, tag should naturally be the name of that file. If needed, the directory hosting tag is created. This macro is one of the instantiating macros; see Configuration Actions.
Here is an unrealistic example:
          fubar=42
          AC_CONFIG_COMMANDS([fubar],
                             [echo this is extra $fubar, and so on.],
                             [fubar=$fubar])
     
Here is a better one:
          AC_CONFIG_COMMANDS([timestamp], [date >timestamp])
     

The following two macros look similar, but in fact they are not of the same breed: they are executed directly by configure, so you cannot use config.status to rerun them.

— Macro: AC_CONFIG_COMMANDS_PRE (cmds)

Execute the cmds right before creating config.status.
This macro presents the last opportunity to call AC_SUBST, AC_DEFINE, or AC_CONFIG_FOOS macros.

— Macro: AC_CONFIG_COMMANDS_POST (cmds)

Execute the cmds right after creating config.status.

4.11 Creating Configuration Links

You may find it convenient to create links whose destinations depend upon results of tests. One can use AC_CONFIG_COMMANDS but the creation of relative symbolic links can be delicate when the package is built in a directory different from the source directory.

— Macro: AC_CONFIG_LINKS (dest:source..., [cmds], [init-cmds])

Make AC_OUTPUT link each of the existing files source to the corresponding link name dest. Makes a symbolic link if possible, otherwise a hard link if possible, otherwise a copy. The dest and source names should be relative to the top level source or build directory. This macro is one of the instantiating macros; see Configuration Actions.
For example, this call:
          AC_CONFIG_LINKS([host.h:config/$machine.h
                          object.h:config/$obj_format.h])
     
creates in the current directory host.h as a link to srcdir/config/$machine.h, and object.h as a link to srcdir/config/$obj_format.h.
The tempting value `.' for dest is invalid: it makes it impossible for `config.status' to guess the links to establish.
One can then run:
          ./config.status host.h object.h
     
to create the links.

4.12 Configuring Other Packages in Subdirectories

In most situations, calling AC_OUTPUT is sufficient to produce makefiles in subdirectories. However, configure scripts that control more than one independent package can use AC_CONFIG_SUBDIRS to run configure scripts for other packages in subdirectories.

— Macro: AC_CONFIG_SUBDIRS (dir ...)

Make AC_OUTPUT run configure in each subdirectory dir in the given blank-or-newline-separated list. Each dir should be a literal, i.e., please do not use:
          if test "$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. Instead, you should write:
          if test "$package_foo_enabled" = yes; then
            AC_CONFIG_SUBDIRS([foo])
          fi
     
If a given dir is not found, an error is reported: if the subdirectory is optional, write:
          if test -d "$srcdir/foo"; then
            AC_CONFIG_SUBDIRS([foo])
          fi
     
If a given dir contains configure.gnu, it is run instead of configure. This is for packages that might use a non-Autoconf script Configure, which can't be called through a wrapper configure since it would be the same file on case-insensitive file systems. Likewise, if a dir contains configure.in but no configure, the Cygnus configure script found by AC_CONFIG_AUX_DIR is used.
The subdirectory configure scripts are given the same command line options that were given to this configure script, with minor changes if needed, which include:

adjusting a relative name for the cache file;
adjusting a relative name for the source directory;
propagating the current value of $prefix, including if it was defaulted, and if the default values of the top level and of the subdirectory configure differ.

This macro also sets the output variable subdirs to the list of directories `dir ...'. Make rules can use this variable to determine which subdirectories to recurse into.
This macro may be called multiple times.

4.13 Default Prefix

By default, configure sets the prefix for files it installs to /usr/local. The user of configure can select a different prefix using the --prefix and --exec-prefix options. There are two ways to change the default: when creating configure, and when running it.

Some software packages might want to install in a directory other than /usr/local by default. To accomplish that, use the AC_PREFIX_DEFAULT macro.

— Macro: AC_PREFIX_DEFAULT (prefix)

Set the default installation prefix to prefix instead of /usr/local.

It may be convenient for users to have configure guess the installation prefix from the location of a related program that they have already installed. If you wish to do that, you can call AC_PREFIX_PROGRAM.

— Macro: AC_PREFIX_PROGRAM (program)

If the user did not specify an installation prefix (using the --prefix option), guess a value for it by looking for program in PATH, the way the shell does. If program is found, set the prefix to the parent of the directory containing program, else default the prefix as described above (/usr/local or AC_PREFIX_DEFAULT). For example, if program is gcc and the PATH contains /usr/local/gnu/bin/gcc, set the prefix to /usr/local/gnu.

5 Existing Tests

These macros test for particular system features that packages might need or want to use. If you need to test for a kind of feature that none of these macros check for, you can probably do it by calling primitive test macros with appropriate arguments (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 the C preprocessor symbol name to the value 1”. See Defining Symbols, for how to get those symbol definitions into your program.

5.1 Common Behavior

Much effort has been expended to make Autoconf easy to learn. The most obvious way to reach this goal is simply to enforce standard interfaces and behaviors, avoiding exceptions as much as possible. Because of history and inertia, unfortunately, there are still too many exceptions in Autoconf; nevertheless, this section describes some of the common rules.

5.1.1 Standard Symbols

All the generic macros that AC_DEFINE a symbol as a result of their test transform their argument values to a standard alphabet. First, argument is converted to upper case and any asterisks (`*') are each converted to `P'. Any remaining characters that are not alphanumeric are converted to underscores.

5.1.2 Default Includes

Several tests depend upon a set of header files. Since these headers are not universally available, tests actually have to provide a set of protected includes, such as:

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:

— Macro: AC_INCLUDES_DEFAULT ([include-directives])

Expand to include-directives if defined, otherwise to:
          #include <stdio.h>
          #ifdef HAVE_SYS_TYPES_H
          # include <sys/types.h>
          #endif
          #ifdef HAVE_SYS_STAT_H
          # include <sys/stat.h>
          #endif
          #ifdef STDC_HEADERS
          # include <stdlib.h>
          # include <stddef.h>
          #else
          # ifdef HAVE_STDLIB_H
          #  include <stdlib.h>
          # endif
          #endif
          #ifdef HAVE_STRING_H
          # if !defined STDC_HEADERS && defined HAVE_MEMORY_H
          #  include <memory.h>
          # endif
          # include <string.h>
          #endif
          #ifdef HAVE_STRINGS_H
          # include <strings.h>
          #endif
          #ifdef HAVE_INTTYPES_H
          # include <inttypes.h>
          #endif
          #ifdef HAVE_STDINT_H
          # include <stdint.h>
          #endif
          #ifdef HAVE_UNISTD_H
          # include <unistd.h>
          #endif
     
If the default includes are used, then check for the presence of these headers and their compatibility, i.e., you don't need to run AC_HEADER_STDC, nor check for stdlib.h etc.
These headers are checked for in the same order as they are included. For instance, on some systems string.h and strings.h both exist, but conflict. Then HAVE_STRING_H is defined, not HAVE_STRINGS_H.

5.2 Alternative Programs

These macros check for the presence or behavior of particular programs. They are used to choose between several alternative programs and to decide what to do once one has been chosen. If there is no macro specifically defined to check for a program you need, and you don't need to check for any special properties of it, then you can use one of the general program-check macros.

5.2.1 Particular Program Checks

These macros check for particular programs—whether they exist, and in some cases whether they support certain features.

— Macro: AC_PROG_AWK

Check for gawk, mawk, nawk, and awk, in that order, and set output variable AWK to the first one that is found. It tries gawk first because that is reported to be the best implementation.

— Macro: AC_PROG_GREP

Look for the best available grep or ggrep that accepts the longest input lines possible, and that supports multiple -e options. Set the output variable GREP to whatever is chosen. See Limitations of Usual Tools, for more information about portability problems with the grep command family.

— Macro: AC_PROG_EGREP

Check whether $GREP -E works, or else look for the best available egrep or gegrep that accepts the longest input lines possible. Set the output variable EGREP to whatever is chosen.

— Macro: AC_PROG_FGREP

Check whether $GREP -F works, or else look for the best available fgrep or gfgrep that accepts the longest input lines possible. Set the output variable FGREP to whatever is chosen.

— Macro: AC_PROG_INSTALL

Set output variable INSTALL to the name of a BSD-compatible install program, if one is found in the current PATH. Otherwise, set INSTALL to `dir/install-sh -c', checking the directories specified to AC_CONFIG_AUX_DIR (or its default directories) to determine dir (see Output). Also set the variables INSTALL_PROGRAM and INSTALL_SCRIPT to `${INSTALL}' and INSTALL_DATA to `${INSTALL} -m 644'.
`@INSTALL@' is special, as its value may vary for different configuration files.
This macro screens out various instances of install known not to work. It prefers to find a C program rather than a shell script, for speed. Instead of install-sh, it can also use install.sh, but that name is obsolete because some make programs have a rule that creates install from it if there is no makefile. Further, this macro requires install to be able to install multiple files into a target directory in a single invocation.
Autoconf comes with a copy of install-sh that you can use. If you use AC_PROG_INSTALL, you must include either install-sh or install.sh in your distribution; otherwise configure produces an error message saying it can't find them—even if the system you're on has a good install program. This check is a safety measure to prevent you from accidentally leaving that file out, which would prevent your package from installing on systems that don't have a BSD-compatible install program.
If you need to use your own installation program because it has features not found in standard install programs, there is no reason to use AC_PROG_INSTALL; just put the file name of your program into your Makefile.in files.

— Macro: AC_PROG_MKDIR_P

Set output variable MKDIR_P to a program that ensures that for each argument, a directory named by this argument exists, creating it and its parent directories if needed, and without race conditions when two instances of the program attempt to make the same directory at nearly the same time.
This macro uses the `mkdir -p' command if possible. Otherwise, it falls back on invoking install-sh with the -d option, so your package should contain install-sh as described under AC_PROG_INSTALL. An install-sh file that predates Autoconf 2.60 or Automake 1.10 is vulnerable to race conditions, so if you want to support parallel installs from different packages into the same directory you need to make sure you have an up-to-date install-sh. In particular, be careful about using `autoreconf -if' if your Automake predates Automake 1.10.
This macro is related to the AS_MKDIR_P macro (see Programming in M4sh), but it sets an output variable intended for use in other files, whereas AS_MKDIR_P is intended for use in scripts like configure. Also, AS_MKDIR_P does not accept options, but MKDIR_P supports the -m option, e.g., a makefile might invoke $(MKDIR_P) -m 0 dir to create an inaccessible directory, and conversely a makefile should use $(MKDIR_P) -- $(FOO) if FOO might yield a value that begins with `-'. Finally, AS_MKDIR_P does not check for race condition vulnerability, whereas AC_PROG_MKDIR_P does.
`@MKDIR_P@' is special, as its value may vary for different configuration files.

— Macro: AC_PROG_LEX

If flex is found, set output variable LEX to `flex' and LEXLIB to -lfl, if that library is in a standard place. Otherwise set LEX to `lex' and LEXLIB to -ll.
Define YYTEXT_POINTER if yytext defaults to `char *' instead of to `char []'. Also set output variable LEX_OUTPUT_ROOT to the base of the file name that the lexer generates; usually lex.yy, but sometimes something else. These results vary according to whether lex or flex is being used.
You are encouraged to use Flex in your sources, since it is both more pleasant to use than plain Lex and the C source it produces is portable. In order to ensure portability, however, you must either provide a function yywrap or, if you don't use it (e.g., your scanner has no `#include'-like feature), simply include a `%noyywrap' statement in the scanner's source. Once this done, the scanner is portable (unless you felt free to use nonportable constructs) and does not depend on any library. In this case, and in this case only, it is suggested that you use this Autoconf snippet:
          AC_PROG_LEX
          if test "$LEX" != flex; then
            LEX="$SHELL $missing_dir/missing flex"
            AC_SUBST([LEX_OUTPUT_ROOT], [lex.yy])
            AC_SUBST([LEXLIB], [''])
          fi
     
The shell script missing can be found in the Automake distribution.
To ensure backward compatibility, Automake's AM_PROG_LEX invokes (indirectly) this macro twice, which causes an annoying but benign “AC_PROG_LEX invoked multiple times” warning. Future versions of Automake will fix this issue; meanwhile, just ignore this message.
As part of running the test, this macro may delete any file in the configuration directory named lex.yy.c or lexyy.c.

— Macro: AC_PROG_LN_S

If `ln -s' works on the current file system (the operating system and file system support symbolic links), set the output variable LN_S to `ln -s'; otherwise, if `ln' works, set LN_S to `ln', and otherwise set it to `cp -p'.
If you make a link in a directory other than the current directory, its meaning depends on whether `ln' or `ln -s' is used. To safely create links using `$(LN_S)', either find out which form is used and adjust the arguments, or always invoke ln in the directory where the link is to be created.
In other words, it does not work to do:
          $(LN_S) foo /x/bar
     
Instead, do:
          (cd /x && $(LN_S) foo bar)
     

— Macro: AC_PROG_RANLIB

Set output variable RANLIB to `ranlib' if ranlib is found, and otherwise to `:' (do nothing).

— Macro: AC_PROG_SED

Set output variable SED to a Sed implementation that conforms to Posix and does not have arbitrary length limits. Report an error if no acceptable Sed is found. See Limitations of Usual Tools, for more information about portability problems with Sed.

— Macro: AC_PROG_YACC

If bison is found, set output variable YACC to `bison -y'. Otherwise, if byacc is found, set YACC to `byacc'. Otherwise set YACC to `yacc'.

5.2.2 Generic Program and File Checks

These macros are used to find programs not covered by the “particular” test macros. If you need to check the behavior of a program as well as find out whether it is present, you have to write your own test for it (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:

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.

— Macro: AC_CHECK_PROG (variable, prog-to-check-for, value-if-found, [value-if-not-found], [path = `$PATH'], [reject])

Check whether program prog-to-check-for exists in path. If it is found, set variable to value-if-found, otherwise to value-if-not-found, if given. Always pass over reject (an absolute file name) even if it is the first found in the search path; in that case, set variable using the absolute file name of the prog-to-check-for found that is not reject. If variable was already set, do nothing. Calls AC_SUBST for variable.

— Macro: AC_CHECK_PROGS (variable, progs-to-check-for, [value-if-not-found], [path = `$PATH'])

Check for each program in the blank-separated list progs-to-check-for existing in the path. If one is found, set variable to the name of that program. Otherwise, continue checking the next program in the list. If none of the programs in the list are found, set variable to value-if-not-found; if value-if-not-found is not specified, the value of variable is not changed. Calls AC_SUBST for variable.

— Macro: AC_CHECK_TARGET_TOOL (variable, prog-to-check-for, [value-if-not-found], [path = `$PATH'])

Like AC_CHECK_PROG, but first looks for prog-to-check-for with a prefix of the target type as determined by AC_CANONICAL_TARGET, followed by a dash (see Canonicalizing). If the tool cannot be found with a prefix, and if the build and target types are equal, then it is also searched for without a prefix.
As noted in Specifying the system type, the target is rarely specified, because most of the time it is the same as the host: it is the type of system for which any compiler tool in the package produces code. What this macro looks for is, for example, a tool (assembler, linker, etc.) that the compiler driver (gcc for the GNU C Compiler) uses to produce objects, archives or executables.

— Macro: AC_CHECK_TOOL (variable, prog-to-check-for, [value-if-not-found], [path = `$PATH'])

Like AC_CHECK_PROG, but first looks for prog-to-check-for with a prefix of the host type as specified by --host, followed by a dash. For example, if the user runs `configure --build=x86_64-gnu --host=i386-gnu', then this call:
          AC_CHECK_TOOL([RANLIB], [ranlib], [:])
     
sets RANLIB to i386-gnu-ranlib if that program exists in path, or otherwise to `ranlib' if that program exists in path, or to `:' if neither program exists.
When cross-compiling, this macro will issue a warning if no program prefixed with the host type could be found. For more information, see Specifying the system type.

— Macro: AC_CHECK_TARGET_TOOLS (variable, progs-to-check-for, [value-if-not-found], [path = `$PATH'])

Like AC_CHECK_TARGET_TOOL, each of the tools in the list progs-to-check-for are checked with a prefix of the target type as determined by AC_CANONICAL_TARGET, followed by a dash (see Canonicalizing). If none of the tools can be found with a prefix, and if the build and target types are equal, then the first one without a prefix is used. If a tool is found, set variable to the name of that program. If none of the tools in the list are found, set variable to value-if-not-found; if value-if-not-found is not specified, the value of variable is not changed. Calls AC_SUBST for variable.

— Macro: AC_CHECK_TOOLS (variable, progs-to-check-for, [value-if-not-found], [path = `$PATH'])

Like AC_CHECK_TOOL, each of the tools in the list progs-to-check-for are checked with a prefix of the host type as determined by AC_CANONICAL_HOST, followed by a dash (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. Calls AC_SUBST for variable.
When cross-compiling, this macro will issue a warning if no program prefixed with the host type could be found. For more information, see Specifying the system type.

— Macro: AC_PATH_PROG (variable, prog-to-check-for, [value-if-not-found], [path = `$PATH'])

Like AC_CHECK_PROG, but set variable to the absolute name of prog-to-check-for if found.

— Macro: AC_PATH_PROGS (variable, progs-to-check-for, [value-if-not-found], [path = `$PATH'])

Like AC_CHECK_PROGS, but if any of progs-to-check-for are found, set variable to the absolute name of the program found.

— Macro: AC_PATH_PROGS_FEATURE_CHECK (variable, progs-to-check-for, feature-test, [action-if-not-found], [path = `$PATH'])

This macro was introduced in Autoconf 2.62. If variable is not empty, then set the cache variable $ac_cv_path_variable to its value. Otherwise, check for each program in the blank-separated list progs-to-check-for existing in path. For each program found, execute feature-test with $ac_path_variable set to the absolute name of the candidate program. If no invocation of feature-test sets the shell variable $ac_cv_path_variable, then action-if-not-found is executed. feature-test will be run even when ac_cv_path_variable is set, to provide the ability to choose a better candidate found later in path; to accept the current setting and bypass all futher checks, feature-test can execute ac_path_variable_found=:.
Note that this macro has some subtle differences from AC_CHECK_PROGS. It is designed to be run inside AC_CACHE_VAL, therefore, it should have no side effects. In particular, variable is not set to the final value of ac_cv_path_variable, nor is AC_SUBST automatically run. Also, on failure, any action can be performed, whereas AC_CHECK_PROGS only performs variable=value-if-not-found.
Here is an example, similar to what Autoconf uses in its own configure script. It will search for an implementation of m4 that supports the indir builtin, even if it goes by the name gm4 or is not the first implementation on PATH.
          AC_CACHE_CHECK([for m4 that supports indir], [ac_cv_path_M4],
            [AC_PATH_PROGS_FEATURE_CHECK([M4], [m4 gm4],
              [[m4out=`echo 'changequote([,])indir([divnum])' | $ac_path_M4`
                test "x$m4out" = x0 \
                && ac_cv_path_M4=$ac_path_M4 ac_path_M4_found=:]],
              [AC_MSG_ERROR([could not find m4 that supports indir])])])
          AC_SUBST([M4], [$ac_cv_path_M4])
     

— Macro: AC_PATH_TARGET_TOOL (variable, prog-to-check-for, [value-if-not-found], [path = `$PATH'])

Like AC_CHECK_TARGET_TOOL, but set variable to the absolute name of the program if it is found.

— Macro: AC_PATH_TOOL (variable, prog-to-check-for, [value-if-not-found], [path = `$PATH'])

Like AC_CHECK_TOOL, but set variable to the absolute name of the program if it is found.
When cross-compiling, this macro will issue a warning if no program prefixed with the host type could be found. For more information, see Specifying the system type.

5.3 Files

You might also need to check for the existence of files. Before using these macros, ask yourself whether a runtime test might not be a better solution. Be aware that, like most Autoconf macros, they test a feature of the host machine, and therefore, they die when cross-compiling.

— Macro: AC_CHECK_FILE (file, [action-if-found], [action-if-not-found])

Check whether file file exists on the native system. If it is found, execute action-if-found, otherwise do action-if-not-found, if given.

— Macro: AC_CHECK_FILES (files, [action-if-found], [action-if-not-found])

Executes AC_CHECK_FILE once for each file listed in files. Additionally, defines `HAVE_file' (see Standard Symbols) for each file found.

5.4 Library Files

The following macros check for the presence of certain C, C++, or Fortran library archive files.

— Macro: AC_CHECK_LIB (library, function, [action-if-found], [action-if-not-found], [other-libraries])

Test whether the library library is available by trying to link a test program that calls function function with the library. function should be a function provided by the library. Use the base name of the library; e.g., to check for -lmp, use `mp' as the library argument.
action-if-found is a list of shell commands to run if the link with the library succeeds; action-if-not-found is a list of shell commands to run if the link fails. If action-if-found is not specified, the default action prepends -llibrary to LIBS and defines `HAVE_LIBlibrary' (in all capitals). This macro is intended to support building LIBS in a right-to-left (least-dependent to most-dependent) fashion such that library dependencies are satisfied as a natural side effect of consecutive tests. Linkers are sensitive to library ordering so the order in which LIBS is generated is important to reliable detection of libraries.
If linking with library results in unresolved symbols that would be resolved by linking with additional libraries, give those libraries as the other-libraries argument, separated by spaces: e.g., -lXt -lX11. Otherwise, this macro fails to detect that library is present, because linking the test program always fails with unresolved symbols. The other-libraries argument should be limited to cases where it is desirable to test for one library in the presence of another that is not already in LIBS.
AC_CHECK_LIB requires some care in usage, and should be avoided in some common cases. Many standard functions like gethostbyname appear in the standard C library on some hosts, and in special libraries like nsl on other hosts. On some hosts the special libraries contain variant implementations that you may not want to use. These days it is normally better to use AC_SEARCH_LIBS([gethostbyname], [nsl]) instead of AC_CHECK_LIB([nsl], [gethostbyname]).

— Macro: AC_SEARCH_LIBS (function, search-libs, [action-if-found], [action-if-not-found], [other-libraries])

Search for a library defining function if it's not already available. This equates to calling `AC_LINK_IFELSE([AC_LANG_CALL([], [function])])' first with no libraries, then for each library listed in search-libs.
Add -llibrary to LIBS for the first library found to contain function, and run action-if-found. If the function is not found, run action-if-not-found.
If linking with library results in unresolved symbols that would be resolved by linking with additional libraries, give those libraries as the other-libraries argument, separated by spaces: e.g., -lXt -lX11. Otherwise, this macro fails to detect that function is present, because linking the test program always fails with unresolved symbols.

5.5 Library Functions

The following macros check for particular C library functions. If there is no macro specifically defined to check for a function you need, and you don't need to check for any special properties of it, then you can use one of the general function-check macros.

5.5.1 Portability of C Functions

Most usual functions can either be missing, or be buggy, or be limited on some architectures. This section tries to make an inventory of these portability issues. By definition, this list always requires additions. Please help us keeping it as complete as possible.

5.5.2 Particular Function Checks

These macros check for particular C functions—whether they exist, and in some cases how they respond when given certain arguments.

— Macro: AC_FUNC_ALLOCA

Check how to get alloca. Tries to get a builtin version by checking for alloca.h or the predefined C preprocessor macros __GNUC__ and _AIX. If this macro finds alloca.h, it defines HAVE_ALLOCA_H.
If those attempts fail, it looks for the function in the standard C library. If any of those methods succeed, it defines HAVE_ALLOCA. Otherwise, it sets the output variable ALLOCA to `${LIBOBJDIR}alloca.o' and defines C_ALLOCA (so programs can periodically call `alloca (0)' to garbage collect). This variable is separate from LIBOBJS so multiple programs can share the value of ALLOCA without needing to create an actual library, in case only some of them use the code in LIBOBJS. The `${LIBOBJDIR}' prefix serves the same purpose as in LIBOBJS (see AC_LIBOBJ vs LIBOBJS).
This macro does not try to get alloca from the System V R3 libPW or the System V R4 libucb because those libraries contain some incompatible functions that cause trouble. Some versions do not even contain alloca or contain a buggy version. If you still want to use their alloca, use ar to extract alloca.o from them instead of compiling alloca.c.
Source files that use alloca should start with a piece of code like the following, to declare it properly.
          #ifdef HAVE_ALLOCA_H
          # include <alloca.h>
          #elif defined __GNUC__
          # define alloca __builtin_alloca
          #elif defined _AIX
          # define alloca __alloca
          #elif defined _MSC_VER
          # include <malloc.h>
          # define alloca _alloca
          #else
          # include <stddef.h>
          # ifdef  __cplusplus
          extern "C"
          # endif
          void *alloca (size_t);
          #endif
     

— Macro: AC_FUNC_CHOWN

If the chown function is available and works (in particular, it should accept -1 for uid and gid), define HAVE_CHOWN.

— Macro: AC_FUNC_CLOSEDIR_VOID

If the closedir function does not return a meaningful value, define CLOSEDIR_VOID. Otherwise, callers ought to check its return value for an error indicator.
Currently this test is implemented by running a test program. When cross compiling the pessimistic assumption that closedir does not return a meaningful value is made.
This macro is obsolescent, as closedir returns a meaningful value on current systems. New programs need not use this macro.

— Macro: AC_FUNC_ERROR_AT_LINE

If the error_at_line function is not found, require an AC_LIBOBJ replacement of `error'.

— Macro: AC_FUNC_FNMATCH

If the fnmatch function conforms to Posix, define HAVE_FNMATCH. Detect common implementation bugs, for example, the bugs in Solaris 2.4.
Unlike the other specific AC_FUNC macros, AC_FUNC_FNMATCH does not replace a broken/missing fnmatch. This is for historical reasons. See AC_REPLACE_FNMATCH below.
This macro is obsolescent. New programs should use Gnulib's fnmatch-posix module. See Gnulib.

— Macro: AC_FUNC_FNMATCH_GNU

Behave like AC_REPLACE_FNMATCH (replace) but also test whether fnmatch supports GNU extensions. Detect common implementation bugs, for example, the bugs in the GNU C Library 2.1.
This macro is obsolescent. New programs should use Gnulib's fnmatch-gnu module. See Gnulib.

— Macro: AC_FUNC_FORK

This macro checks for the fork and vfork functions. If a working fork is found, define HAVE_WORKING_FORK. This macro checks whether fork is just a stub by trying to run it.
If vfork.h is found, define HAVE_VFORK_H. If a working vfork is found, define HAVE_WORKING_VFORK. Otherwise, define vfork to be fork for backward compatibility with previous versions of autoconf. This macro checks for several known errors in implementations of vfork and considers the system to not have a working vfork if it detects any of them. It is not considered to be an implementation error if a child's invocation of signal modifies the parent's signal handler, since child processes rarely change their signal handlers.
Since this macro defines vfork only for backward compatibility with previous versions of autoconf you're encouraged to define it yourself in new code:
          #ifndef HAVE_WORKING_VFORK
          # define vfork fork
          #endif
     

— Macro: AC_FUNC_FSEEKO

If the fseeko function is available, define HAVE_FSEEKO. Define _LARGEFILE_SOURCE if necessary to make the prototype visible on some systems (e.g., glibc 2.2). Otherwise linkage problems may occur when compiling with AC_SYS_LARGEFILE on largefile-sensitive systems where off_t does not default to a 64bit entity. All systems with fseeko also supply ftello.

— Macro: AC_FUNC_GETGROUPS

If the getgroups function is available and works (unlike on Ultrix 4.3, where `getgroups (0, 0)' always fails), define HAVE_GETGROUPS. Set GETGROUPS_LIBS to any libraries needed to get that function. This macro runs AC_TYPE_GETGROUPS.

— Macro: AC_FUNC_GETLOADAVG

Check how to get the system load averages. To perform its tests properly, this macro needs the file getloadavg.c; therefore, be sure to set the AC_LIBOBJ replacement directory properly (see Generic Functions, AC_CONFIG_LIBOBJ_DIR).
If the system has the getloadavg function, define HAVE_GETLOADAVG, and set GETLOADAVG_LIBS to any libraries necessary to get that function. Also add GETLOADAVG_LIBS to LIBS. Otherwise, require an AC_LIBOBJ replacement for `getloadavg' with source code in dir/getloadavg.c, and possibly define several other C preprocessor macros and output variables:

Define C_GETLOADAVG.
Define SVR4, DGUX, UMAX, or UMAX4_3 if 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 symbol NLIST_NAME_UNION is still defined, but do not depend upon it.
Programs may need to be installed set-group-ID (or set-user-ID) for getloadavg to work. In this case, define GETLOADAVG_PRIVILEGED, set the output variable NEED_SETGID to `true' (and otherwise to `false'), and set KMEM_GROUP to the name of the group that should own the installed program.

The AC_FUNC_GETLOADAVG macro is obsolescent. New programs should use Gnulib's getloadavg module. See Gnulib.

— Macro: AC_FUNC_GETMNTENT

Check for getmntent in the standard C library, and then in the sun, seq, and gen libraries, for unicos, irix 4, ptx, and UnixWare, respectively. Then, if getmntent is available, define HAVE_GETMNTENT.

— Macro: AC_FUNC_GETPGRP

Define GETPGRP_VOID if it is an error to pass 0 to getpgrp; this is the Posix behavior. On older BSD systems, you must pass 0 to getpgrp, as it takes an argument and behaves like Posix's getpgid.
          #ifdef GETPGRP_VOID
            pid = getpgrp ();
          #else
            pid = getpgrp (0);
          #endif
     
This macro does not check whether getpgrp exists at all; if you need to work in that situation, first call AC_CHECK_FUNC for getpgrp.
This macro is obsolescent, as current systems have a getpgrp whose signature conforms to Posix. New programs need not use this macro.

— Macro: AC_FUNC_LSTAT_FOLLOWS_SLASHED_SYMLINK

If link is a symbolic link, then lstat should treat link/ the same as link/.. However, many older lstat implementations incorrectly ignore trailing slashes.
It is safe to assume that if lstat incorrectly ignores trailing slashes, then other symbolic-link-aware functions like unlink also incorrectly ignore trailing slashes.
If lstat behaves properly, define LSTAT_FOLLOWS_SLASHED_SYMLINK, otherwise require an AC_LIBOBJ replacement of lstat.

— Macro: AC_FUNC_MALLOC

If the malloc function is compatible with the GNU C library malloc (i.e., `malloc (0)' returns a valid pointer), define HAVE_MALLOC to 1. Otherwise define HAVE_MALLOC to 0, ask for an AC_LIBOBJ replacement for `malloc', and define malloc to rpl_malloc so that the native malloc is not used in the main project.
Typically, the replacement file malloc.c should look like (note the `#undef malloc'):
     
     #include <config.h>
     #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);
     }

— Macro: AC_FUNC_MEMCMP

If the memcmp function is not available, or does not work on 8-bit data (like the one on SunOS 4.1.3), or fails when comparing 16 bytes or more and with at least one buffer not starting on a 4-byte boundary (such as the one on NeXT x86 OpenStep), require an AC_LIBOBJ replacement for `memcmp'.
This macro is obsolescent, as current systems have a working memcmp. New programs need not use this macro.

— Macro: AC_FUNC_MBRTOWC

Define HAVE_MBRTOWC to 1 if the function mbrtowc and the type mbstate_t are properly declared.

— Macro: AC_FUNC_MKTIME

If the mktime function is not available, or does not work correctly, require an AC_LIBOBJ replacement for `mktime'. For the purposes of this test, mktime should conform to the Posix standard and should be the inverse of localtime.

— Macro: AC_FUNC_MMAP

If the mmap function exists and works correctly, define HAVE_MMAP. This checks only private fixed mapping of already-mapped memory.

— Macro: AC_FUNC_OBSTACK

If the obstacks are found, define HAVE_OBSTACK, else require an AC_LIBOBJ replacement for `obstack'.

— Macro: AC_FUNC_REALLOC

If the realloc function is compatible with the GNU C library realloc (i.e., `realloc (NULL, 0)' returns a valid pointer), define HAVE_REALLOC to 1. Otherwise define HAVE_REALLOC to 0, ask for an AC_LIBOBJ replacement for `realloc', and define realloc to rpl_realloc so that the native realloc is not used in the main project. See AC_FUNC_MALLOC for details.

— Macro: AC_FUNC_SELECT_ARGTYPES

Determines the correct type to be passed for each of the select function's arguments, and defines those types in SELECT_TYPE_ARG1, SELECT_TYPE_ARG234, and SELECT_TYPE_ARG5 respectively. SELECT_TYPE_ARG1 defaults to `int', SELECT_TYPE_ARG234 defaults to `int *', and SELECT_TYPE_ARG5 defaults to `struct timeval *'.
This macro is obsolescent, as current systems have a select whose signature conforms to Posix. New programs need not use this macro.

— Macro: AC_FUNC_SETPGRP

If setpgrp takes no argument (the Posix version), define SETPGRP_VOID. Otherwise, it is the BSD version, which takes two process IDs as arguments. This macro does not check whether setpgrp exists at all; if you need to work in that situation, first call AC_CHECK_FUNC for setpgrp.
This macro is obsolescent, as current systems have a setpgrp whose signature conforms to Posix. New programs need not use this macro.

— Macro: AC_FUNC_STAT
— Macro: AC_FUNC_LSTAT

Determine whether stat or lstat have the bug that it succeeds when given the zero-length file name as argument. The stat and lstat from SunOS 4.1.4 and the Hurd (as of 1998-11-01) do this.
If it does, then define HAVE_STAT_EMPTY_STRING_BUG (or HAVE_LSTAT_EMPTY_STRING_BUG) and ask for an AC_LIBOBJ replacement of it.
These macros are obsolescent, as no current systems have the bug. New programs need not use these macros.

— Macro: AC_FUNC_STRCOLL

If the strcoll function exists and works correctly, define HAVE_STRCOLL. This does a bit more than `AC_CHECK_FUNCS(strcoll)', because some systems have incorrect definitions of strcoll that should not be used.

— Macro: AC_FUNC_STRERROR_R

If strerror_r is available, define HAVE_STRERROR_R, and if it is declared, define HAVE_DECL_STRERROR_R. If it returns a char * message, define STRERROR_R_CHAR_P; otherwise it returns an int error number. The Thread-Safe Functions option of Posix requires strerror_r to return int, but many systems (including, for example, version 2.2.4 of the GNU C Library) return a char * value that is not necessarily equal to the buffer argument.

— Macro: AC_FUNC_STRFTIME

Check for strftime in the intl library, for SCO Unix. Then, if strftime is available, define HAVE_STRFTIME.
This macro is obsolescent, as no current systems require the intl library for strftime. New programs need not use this macro.

— Macro: AC_FUNC_STRTOD

If the strtod function does not exist or doesn't work correctly, ask for an AC_LIBOBJ replacement of `strtod'. In this case, because strtod.c is likely to need `pow', set the output variable POW_LIB to the extra library needed.

— Macro: AC_FUNC_STRTOLD

If the strtold function exists and conforms to C99, define HAVE_STRTOLD.

— Macro: AC_FUNC_STRNLEN

If the strnlen function is not available, or is buggy (like the one from AIX 4.3), require an AC_LIBOBJ replacement for it.

— Macro: AC_FUNC_UTIME_NULL

If `utime (file, NULL)' sets file's timestamp to the present, define HAVE_UTIME_NULL.
This macro is obsolescent, as all current systems have a utime that behaves this way. New programs need not use this macro.

— Macro: AC_FUNC_VPRINTF

If vprintf is found, define HAVE_VPRINTF. Otherwise, if _doprnt is found, define HAVE_DOPRNT. (If vprintf is available, you may assume that vfprintf and vsprintf are also available.)
This macro is obsolescent, as all current systems have vprintf. New programs need not use this macro.

— Macro: AC_REPLACE_FNMATCH

If the fnmatch function does not conform to Posix (see AC_FUNC_FNMATCH), ask for its AC_LIBOBJ replacement.
The files fnmatch.c, fnmatch_loop.c, and fnmatch_.h in the AC_LIBOBJ replacement directory are assumed to contain a copy of the source code of GNU fnmatch. If necessary, this source code is compiled as an AC_LIBOBJ replacement, and the fnmatch_.h file is linked to fnmatch.h so that it can be included in place of the system <fnmatch.h>.
This macro is obsolescent, as it assumes the use of particular source files. New programs should use Gnulib's fnmatch-posix module, which provides this macro along with the source files. See Gnulib.

5.5.3 Generic Function Checks

These macros are used to find functions not covered by the “particular” test macros. If the functions might be in libraries other than the default C library, first call AC_CHECK_LIB for those libraries. If you need to check the behavior of a function as well as find out whether it is present, you have to write your own test for it (see Writing Tests).

— Macro: AC_CHECK_FUNC (function, [action-if-found], [action-if-not-found])

If C function function is available, run shell commands action-if-found, otherwise action-if-not-found. If you just want to define a symbol if the function is available, consider using AC_CHECK_FUNCS instead. This macro checks for functions with C linkage even when AC_LANG(C++) has been called, since C is more standardized than C++. (see Language Choice, for more information about selecting the language for checks.)

— Macro: AC_CHECK_FUNCS (function..., [action-if-found], [action-if-not-found])

For each function enumerated in the blank-or-newline-separated argument list, define HAVE_function (in all capitals) if it is available. If action-if-found is given, it is additional shell code to execute when one of the functions is found. You can give it a value of `break' to break out of the loop on the first match. If action-if-not-found is given, it is executed when one of the functions is not found.

— Macro: AC_CHECK_FUNCS_ONCE (function...)

For each function enumerated in the blank-or-newline-separated argument list, define HAVE_function (in all capitals) if it is available. This is a once-only variant of AC_CHECK_FUNCS. It generates the checking code at most once, so that configure is smaller and faster; but the checks cannot be conditionalized and are always done once, early during the configure run.

Suitable replacements for many such problem functions are available from Gnulib (see Gnulib).

— Macro: AC_LIBOBJ (function)

Specify that `function.c' must be included in the executables to replace a missing or broken implementation of function.
Technically, it adds `function.$ac_objext' to the output variable LIBOBJS if it is not already in, and calls AC_LIBSOURCE for `function.c'. You should not directly change LIBOBJS, since this is not traceable.

— Macro: AC_LIBSOURCE (file)

Specify that file might be needed to compile the project. If you need to know what files might be needed by a configure.ac, you should trace AC_LIBSOURCE. file must be a literal.
This macro is called automatically from AC_LIBOBJ, but you must call it explicitly if you pass a shell variable to AC_LIBOBJ. In that case, since shell variables cannot be traced statically, you must pass to AC_LIBSOURCE any possible files that the shell variable might cause AC_LIBOBJ to need. For example, if you want to pass a variable $foo_or_bar to AC_LIBOBJ that holds either "foo" or "bar", you should do:
          AC_LIBSOURCE([foo.c])
          AC_LIBSOURCE([bar.c])
          AC_LIBOBJ([$foo_or_bar])
     
There is usually a way to avoid this, however, and you are encouraged to simply call AC_LIBOBJ with literal arguments.
Note that this macro replaces the obsolete AC_LIBOBJ_DECL, with slightly different semantics: the old macro took the function name, e.g., foo, as its argument rather than the file name.

— Macro: AC_LIBSOURCES (files)

Like AC_LIBSOURCE, but accepts one or more files in a comma-separated M4 list. Thus, the above example might be rewritten:
          AC_LIBSOURCES([foo.c, bar.c])
          AC_LIBOBJ([$foo_or_bar])
     

— Macro: AC_CONFIG_LIBOBJ_DIR (directory)

Specify that AC_LIBOBJ replacement files are to be found in directory, a name relative to the top level of the source tree. The replacement directory defaults to ., the top level directory, and the most typical value is lib, corresponding to `AC_CONFIG_LIBOBJ_DIR([lib])'.
configure might need to know the replacement directory for the following reasons: (i) some checks use the replacement files, (ii) some macros bypass broken system headers by installing links to the replacement headers (iii) when used in conjunction with Automake, within each makefile, directory is used as a relative path from $(top_srcdir) to each object named in LIBOBJS and LTLIBOBJS, etc.

— Macro: AC_REPLACE_FUNCS (function...)

Like AC_CHECK_FUNCS, but uses `AC_LIBOBJ(function)' as action-if-not-found. You can declare your replacement function by enclosing the prototype in `#ifndef HAVE_function'. If the system has the function, it probably declares it in a header file you should be including, so you shouldn't redeclare it lest your declaration conflict.

5.6 Header Files

The following macros check for the presence of certain C header files. If there is no macro specifically defined to check for a header file you need, and you don't need to check for any special properties of it, then you can use one of the general header-file check macros.

5.6.1 Portability of Headers

This section tries to collect knowledge about common headers, and the problems they cause. By definition, this list always requires additions. Please help us keeping it as complete as possible.

5.6.2 Particular Header Checks

These macros check for particular system header files—whether they exist, and in some cases whether they declare certain symbols.

— Macro: AC_HEADER_ASSERT

Check whether to enable assertions in the style of assert.h. Assertions are enabled by default, but the user can override this by invoking configure with the --disable-assert option.

— Macro: AC_HEADER_DIRENT

Check for the following header files. For the first one that is found and defines `DIR', define the listed C preprocessor macro:
dirent.h HAVE_DIRENT_H
sys/ndir.h HAVE_SYS_NDIR_H
sys/dir.h HAVE_SYS_DIR_H
ndir.h HAVE_NDIR_H

The directory-library declarations in your source code should look something like the following:
          #include <sys/types.h>
          #ifdef HAVE_DIRENT_H
          # include <dirent.h>
          # define NAMLEN(dirent) strlen ((dirent)->d_name)
          #else
          # define dirent direct
          # define NAMLEN(dirent) ((dirent)->d_namlen)
          # ifdef HAVE_SYS_NDIR_H
          #  include <sys/ndir.h>
          # endif
          # ifdef HAVE_SYS_DIR_H
          #  include <sys/dir.h>
          # endif
          # ifdef HAVE_NDIR_H
          #  include <ndir.h>
          # endif
          #endif
     
Using the above declarations, the program would declare variables to be of type struct dirent, not struct direct, and would access the length of a directory entry name by passing a pointer to a struct dirent to the NAMLEN macro.
This macro also checks for the SCO Xenix dir and x libraries.
This macro is obsolescent, as all current systems with directory libraries have <dirent.h>. New programs need not use this macro.
Also see AC_STRUCT_DIRENT_D_INO and AC_STRUCT_DIRENT_D_TYPE (see Particular Structures).

— Macro: AC_HEADER_MAJOR

If sys/types.h does not define major, minor, and makedev, but sys/mkdev.h does, define MAJOR_IN_MKDEV; otherwise, if sys/sysmacros.h does, define MAJOR_IN_SYSMACROS.

— Macro: AC_HEADER_RESOLV

Checks for header resolv.h, checking for prerequisites first. To properly use resolv.h, your code should contain something like the following:

     
     #ifdef HAVE_SYS_TYPES_H
     #  include <sys/types.h>
     #endif
     #ifdef HAVE_NETINET_IN_H
     #  include <netinet/in.h>   /* inet_ functions / structs */
     #endif
     #ifdef HAVE_ARPA_NAMESER_H
     #  include <arpa/nameser.h> /* DNS HEADER struct */
     #endif
     #ifdef HAVE_NETDB_H
     #  include <netdb.h>
     #endif
     #include <resolv.h>

— Macro: AC_HEADER_STAT

If the macros S_ISDIR, S_ISREG, etc. defined in sys/stat.h do not work properly (returning false positives), define STAT_MACROS_BROKEN. This is the case on Tektronix UTekV, Amdahl UTS and Motorola System V/88.
This macro is obsolescent, as no current systems have the bug. New programs need not use this macro.

— Macro: AC_HEADER_STDBOOL

If stdbool.h exists and conforms to C99, define HAVE_STDBOOL_H to 1; if the type _Bool is defined, define HAVE__BOOL to 1. To fulfill the C99 requirements, your system.h could contain the following code:
     
     #ifdef HAVE_STDBOOL_H
     # include <stdbool.h>
     #else
     # ifndef HAVE__BOOL
     #  ifdef __cplusplus
     typedef bool _Bool;
     #  else
     #   define _Bool signed char
     #  endif
     # endif
     # define bool _Bool
     # define false 0
     # define true 1
     # define __bool_true_false_are_defined 1
     #endif
Alternatively you can use the `stdbool' package of Gnulib (see Gnulib); it packages the above code into a replacement header and contains a few other bells and whistles.

— Macro: AC_HEADER_STDC

Define STDC_HEADERS if the system has C header files conforming to ANSI C89 (ISO C90). Specifically, this macro checks for stdlib.h, stdarg.h, string.h, and float.h; if the system has those, it probably has the rest of the C89 header files. This macro also checks whether string.h declares memchr (and thus presumably the other mem functions), whether stdlib.h declare free (and thus presumably malloc and other related functions), and whether the ctype.h macros work on characters with the high bit set, as the C standard requires.
If you use this macro, your code can refer to STDC_HEADERS to determine whether the system has conforming header files (and probably C library functions).
This macro is obsolescent, as current systems have conforming header files. New programs need not use this macro.
Nowadays string.h is part of the C standard and declares functions like strcpy, and strings.h is standardized by Posix and declares BSD functions like bcopy; but historically, string functions were a major sticking point in this area. If you still want to worry about portability to ancient systems without standard headers, there is so much variation that it is probably easier to declare the functions you use than to figure out exactly what the system header files declare. Some ancient systems contained a mix of functions from the C standard and from BSD; some were mostly standard but lacked `memmove'; some defined the BSD functions as macros in string.h or strings.h; some had only the BSD functions but string.h; some declared the memory functions in memory.h, some in string.h; etc. It is probably sufficient to check for one string function and one memory function; if the library had the standard versions of those then it probably had most of the others. If you put the following in configure.ac:
          # This example is obsolescent.
          # Nowadays you can omit these macro calls.
          AC_HEADER_STDC
          AC_CHECK_FUNCS([strchr memcpy])
     
then, in your code, you can use declarations like this:
          /* This example is obsolescent.
             Nowadays you can just #include <string.h>.  */
          #ifdef STDC_HEADERS
          # include <string.h>
          #else
          # ifndef HAVE_STRCHR
          #  define strchr index
          #  define strrchr rindex
          # endif
          char *strchr (), *strrchr ();
          # ifndef HAVE_MEMCPY
          #  define memcpy(d, s, n) bcopy ((s), (d), (n))
          #  define memmove(d, s, n) bcopy ((s), (d), (n))
          # endif
          #endif
     
If you use a function like memchr, memset, strtok, or strspn, which have no BSD equivalent, then macros don't suffice to port to ancient hosts; you must provide an implementation of each function. An easy way to incorporate your implementations only when needed (since the ones in system C libraries may be hand optimized) is to, taking memchr for example, put it in memchr.c and use `AC_REPLACE_FUNCS([memchr])'.

— Macro: AC_HEADER_SYS_WAIT

If sys/wait.h exists and is compatible with Posix, define HAVE_SYS_WAIT_H. Incompatibility can occur if sys/wait.h does not exist, or if it uses the old BSD union wait instead of int to store a status value. If sys/wait.h is not Posix compatible, then instead of including it, define the Posix macros with their usual interpretations. Here is an example:
          #include <sys/types.h>
          #ifdef HAVE_SYS_WAIT_H
          # include <sys/wait.h>
          #endif
          #ifndef WEXITSTATUS
          # define WEXITSTATUS(stat_val) ((unsigned int) (stat_val) >> 8)
          #endif
          #ifndef WIFEXITED
          # define WIFEXITED(stat_val) (((stat_val) & 255) == 0)
          #endif
     
This macro is obsolescent, as current systems are compatible with Posix. New programs need not use this macro.

_POSIX_VERSION is defined when unistd.h is included on Posix systems. If there is no unistd.h, it is definitely not a Posix system. However, some non-Posix systems do have unistd.h.

— Macro: AC_HEADER_TIME

If a program may include both time.h and sys/time.h, define TIME_WITH_SYS_TIME. On some ancient systems, sys/time.h included time.h, but time.h was not protected against multiple inclusion, so programs could not explicitly include both files. This macro is useful in programs that use, for example, struct timeval as well as struct tm. It is best used in conjunction with HAVE_SYS_TIME_H, which can be checked for using AC_CHECK_HEADERS([sys/time.h]).
          #ifdef TIME_WITH_SYS_TIME
          # include <sys/time.h>
          # include <time.h>
          #else
          # ifdef HAVE_SYS_TIME_H
          #  include <sys/time.h>
          # else
          #  include <time.h>
          # endif
          #endif
     
This macro is obsolescent, as current systems can include both files when they exist. New programs need not use this macro.

— Macro: AC_HEADER_TIOCGWINSZ

If the use of TIOCGWINSZ requires <sys/ioctl.h>, then define GWINSZ_IN_SYS_IOCTL. Otherwise TIOCGWINSZ can be found in <termios.h>.
Use:
          #ifdef HAVE_TERMIOS_H
          # include <termios.h>
          #endif
          
          #ifdef GWINSZ_IN_SYS_IOCTL
          # include <sys/ioctl.h>
          #endif
     

5.6.3 Generic Header Checks

These macros are used to find system header files not covered by the “particular” test macros. If you need to check the contents of a header as well as find out whether it is present, you have to write your own test for it (see Writing Tests).

— Macro: AC_CHECK_HEADER (header-file, [action-if-found], [action-if-not-found], [includes = `AC_INCLUDES_DEFAULT'])

If the system header file header-file is compilable, execute shell commands action-if-found, otherwise execute action-if-not-found. If you just want to define a symbol if the header file is available, consider using AC_CHECK_HEADERS instead.
includes is a series of include directives, defaulting to AC_INCLUDES_DEFAULT (see Default Includes), which are used prior to the header under test.
For compatibility issues with older versions of Autoconf, please read below.

— Macro: AC_CHECK_HEADERS (header-file..., [action-if-found], [action-if-not-found], [includes = `AC_INCLUDES_DEFAULT'])

For each given system header file header-file in the blank-separated argument list that exists, define HAVE_header-file (in all capitals). If action-if-found is given, it is additional shell code to execute when one of the header files is found. You can give it a value of `break' to break out of the loop on the first match. If action-if-not-found is given, it is executed when one of the header files is not found.
includes is a series of include directives, defaulting to AC_INCLUDES_DEFAULT (see Default Includes), which are used prior to the headers under test.
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:

The following variant generates smaller, faster configure files if you do not need the full power of AC_CHECK_HEADERS.

— Macro: AC_CHECK_HEADERS_ONCE (header-file...)

For each given system header file header-file in the blank-separated argument list that exists, define HAVE_header-file (in all capitals). This is a once-only variant of AC_CHECK_HEADERS. It generates the checking code at most once, so that configure is smaller and faster; but the checks cannot be conditionalized and are always done once, early during the configure run.

5.7 Declarations

The following macros check for the declaration of variables and functions. If there is no macro specifically defined to check for a symbol you need, then you can use the general macros (see Generic Declarations) or, for more complex tests, you may use AC_COMPILE_IFELSE (see Running the Compiler).

5.7.1 Particular Declaration Checks

5.7.2 Generic Declaration Checks

These macros are used to find declarations not covered by the “particular” test macros.

— Macro: AC_CHECK_DECL (symbol, [action-if-found], [action-if-not-found], [includes = `AC_INCLUDES_DEFAULT'])

If symbol (a function, variable, or constant) is not declared in includes and a declaration is needed, run the shell commands action-if-not-found, otherwise action-if-found. includes is a series of include directives, defaulting to AC_INCLUDES_DEFAULT (see Default Includes), which are used prior to the declaration under test.
This macro actually tests whether symbol is defined as a macro or can be used as an r-value, not whether it is really declared, because it is much safer to avoid introducing extra declarations when they are not needed.

— Macro: AC_CHECK_DECLS (symbols, [action-if-found], [action-if-not-found], [includes = `AC_INCLUDES_DEFAULT'])

For each of the symbols (comma-separated list), define HAVE_DECL_symbol (in all capitals) to `1' if symbol is declared, otherwise to `0'. If action-if-not-found is given, it is additional shell code to execute when one of the function declarations is needed, otherwise action-if-found is executed.
includes is a series of include directives, defaulting to AC_INCLUDES_DEFAULT (see Default Includes), which are used prior to the declarations under test.
This macro uses an M4 list as first argument:
          AC_CHECK_DECLS([strdup])
          AC_CHECK_DECLS([strlen])
          AC_CHECK_DECLS([malloc, realloc, calloc, free])
          AC_CHECK_DECLS([j0], [], [], [[#include <math.h>]])
     
Unlike the other `AC_CHECK_*S' macros, when a symbol is not declared, HAVE_DECL_symbol is defined to `0' instead of leaving HAVE_DECL_symbol undeclared. When you are sure that the check was performed, use HAVE_DECL_symbol in #if:
          #if !HAVE_DECL_SYMBOL
          extern char *symbol;
          #endif
     
If the test may have not been performed, however, because it is safer not to declare a symbol than to use a declaration that conflicts with the system's one, you should use:
          #if defined HAVE_DECL_MALLOC && !HAVE_DECL_MALLOC
          void *malloc (size_t *s);
          #endif
     
You fall into the second category only in extreme situations: either your files may be used without being configured, or they are used during the configuration. In most cases the traditional approach is enough.

— Macro: AC_CHECK_DECLS_ONCE (symbols)

For each of the symbols (comma-separated list), define HAVE_DECL_symbol (in all capitals) to `1' if symbol is declared in the default include files, otherwise to `0'. This is a once-only variant of AC_CHECK_DECLS. It generates the checking code at most once, so that configure is smaller and faster; but the checks cannot be conditionalized and are always done once, early during the configure run.

5.8 Structures

The following macros check for the presence of certain members in C structures. If there is no macro specifically defined to check for a member you need, then you can use the general structure-member macros (see Generic Structures) or, for more complex tests, you may use AC_COMPILE_IFELSE (see Running the Compiler).

5.8.1 Particular Structure Checks

— Macro: AC_STRUCT_DIRENT_D_INO

Perform all the actions of AC_HEADER_DIRENT (see Particular Headers). Then, if struct dirent contains a d_ino member, define HAVE_STRUCT_DIRENT_D_INO.
HAVE_STRUCT_DIRENT_D_INO indicates only the presence of d_ino, not whether its contents are always reliable. Traditionally, a zero d_ino indicated a deleted directory entry, though current systems hide this detail from the user and never return zero d_ino values. Many current systems report an incorrect d_ino for a directory entry that is a mount point.

— Macro: AC_STRUCT_DIRENT_D_TYPE

Perform all the actions of AC_HEADER_DIRENT (see Particular Headers). Then, if struct dirent contains a d_type member, define HAVE_STRUCT_DIRENT_D_TYPE.

— Macro: AC_STRUCT_ST_BLOCKS

If struct stat contains an st_blocks member, define HAVE_STRUCT_STAT_ST_BLOCKS. Otherwise, require an AC_LIBOBJ replacement of `fileblocks'. The former name, HAVE_ST_BLOCKS is to be avoided, as its support will cease in the future.

— Macro: AC_STRUCT_TM

If time.h does not define struct tm, define TM_IN_SYS_TIME, which means that including sys/time.h had better define struct tm.
This macro is obsolescent, as time.h defines struct tm in current systems. New programs need not use this macro.

— Macro: AC_STRUCT_TIMEZONE

Figure out how to get the current timezone. If struct tm has a tm_zone member, define HAVE_STRUCT_TM_TM_ZONE (and the obsoleted HAVE_TM_ZONE). Otherwise, if the external array tzname is found, define HAVE_TZNAME; if it is declared, define HAVE_DECL_TZNAME.

5.8.2 Generic Structure Checks

These macros are used to find structure members not covered by the “particular” test macros.

— Macro: AC_CHECK_MEMBER (aggregate.member, [action-if-found], [action-if-not-found], [includes = `AC_INCLUDES_DEFAULT'])

Check whether member is a member of the aggregate aggregate. If no includes are specified, the default includes are used (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 submembers:
          AC_CHECK_MEMBER(struct top.middle.bot)
     

— Macro: AC_CHECK_MEMBERS (members, [action-if-found], [action-if-not-found], [includes = `AC_INCLUDES_DEFAULT'])

Check for the existence of each `aggregate.member' of members using the previous macro. When member belongs to aggregate, define HAVE_aggregate_member (in all capitals, with spaces and dots replaced by underscores). If action-if-found is given, it is executed for each of the found members. If action-if-not-found is given, it is executed for each of the members that could not be found.
includes is a series of include directives, defaulting to AC_INCLUDES_DEFAULT (see Default Includes), which are used prior to the members under test.
This macro uses M4 lists:
          AC_CHECK_MEMBERS([struct stat.st_rdev, struct stat.st_blksize])
     

5.9 Types

The following macros check for C types, either builtin or typedefs. If there is no macro specifically defined to check for a type you need, and you don't need to check for any special properties of it, then you can use a general type-check macro.

5.9.1 Particular Type Checks

These macros check for particular C types in sys/types.h, stdlib.h, stdint.h, inttypes.h and others, if they exist.

The Gnulib stdint module is an alternate way to define many of these symbols; it is useful if you prefer your code to assume a C99-or-better environment. See Gnulib.

— Macro: AC_TYPE_GETGROUPS

Define GETGROUPS_T to be whichever of gid_t or int is the base type of the array argument to getgroups.

— Macro: AC_TYPE_INT8_T

If stdint.h or inttypes.h does not define the type int8_t, define int8_t to a signed integer type that is exactly 8 bits wide and that uses two's complement representation, if such a type exists. If you are worried about porting to hosts that lack such a type, you can use the results of this macro in C89-or-later code as follows:
          #if HAVE_STDINT_H
          # include <stdint.h>
          #endif
          #if defined INT8_MAX || defined int8_t
           code using int8_t
          #else
           complicated alternative using >8-bit 'signed char'
          #endif
     

— Macro: AC_TYPE_INT16_T

This is like AC_TYPE_INT8_T, except for 16-bit integers.

— Macro: AC_TYPE_INT32_T

This is like AC_TYPE_INT8_T, except for 32-bit integers.

— Macro: AC_TYPE_INT64_T

This is like AC_TYPE_INT8_T, except for 64-bit integers.

— Macro: AC_TYPE_INTMAX_T

If stdint.h or inttypes.h defines the type intmax_t, define HAVE_INTMAX_T. Otherwise, define intmax_t to the widest signed integer type.

— Macro: AC_TYPE_INTPTR_T

If stdint.h or inttypes.h defines the type intptr_t, define HAVE_INTPTR_T. Otherwise, define intptr_t to a signed integer type wide enough to hold a pointer, if such a type exists.

— Macro: AC_TYPE_LONG_DOUBLE

If the C compiler supports a working long double type, define HAVE_LONG_DOUBLE. The long double type might have the same range and precision as double.
This macro is obsolescent, as current C compilers support long double. New programs need not use this macro.

— Macro: AC_TYPE_LONG_DOUBLE_WIDER

If the C compiler supports a working long double type with more range or precision than the double type, define HAVE_LONG_DOUBLE_WIDER.

— Macro: AC_TYPE_LONG_LONG_INT

If the C compiler supports a working long long int type, define HAVE_LONG_LONG_INT. However, this test does not test long long int values in preprocessor #if expressions, because too many compilers mishandle such expressions. See Preprocessor Arithmetic.

— Macro: AC_TYPE_MBSTATE_T

Define HAVE_MBSTATE_T if <wchar.h> declares the mbstate_t type. Also, define mbstate_t to be a type if <wchar.h> does not declare it.

— Macro: AC_TYPE_MODE_T

Define mode_t to a suitable type, if standard headers do not define it.

— Macro: AC_TYPE_OFF_T

Define off_t to a suitable type, if standard headers do not define it.

— Macro: AC_TYPE_PID_T

Define pid_t to a suitable type, if standard headers do not define it.

— Macro: AC_TYPE_SIZE_T

Define size_t to a suitable type, if standard headers do not define it.

— Macro: AC_TYPE_SSIZE_T

Define ssize_t to a suitable type, if standard headers do not define it.

— Macro: AC_TYPE_UID_T

Define uid_t and gid_t to suitable types, if standard headers do not define them.

— Macro: AC_TYPE_UINT8_T

If stdint.h or inttypes.h does not define the type uint8_t, define uint8_t to an unsigned integer type that is exactly 8 bits wide, if such a type exists. This is like AC_TYPE_INT8_T, except for unsigned integers.

— Macro: AC_TYPE_UINT16_T

This is like AC_TYPE_UINT8_T, except for 16-bit integers.

— Macro: AC_TYPE_UINT32_T

This is like AC_TYPE_UINT8_T, except for 32-bit integers.

— Macro: AC_TYPE_UINT64_T

This is like AC_TYPE_UINT8_T, except for 64-bit integers.

— Macro: AC_TYPE_UINTMAX_T

If stdint.h or inttypes.h defines the type uintmax_t, define HAVE_UINTMAX_T. Otherwise, define uintmax_t to the widest unsigned integer type.

— Macro: AC_TYPE_UINTPTR_T

If stdint.h or inttypes.h defines the type uintptr_t, define HAVE_UINTPTR_T. Otherwise, define uintptr_t to an unsigned integer type wide enough to hold a pointer, if such a type exists.

— Macro: AC_TYPE_UNSIGNED_LONG_LONG_INT

If the C compiler supports a working unsigned long long int type, define HAVE_UNSIGNED_LONG_LONG_INT. However, this test does not test unsigned long long int values in preprocessor #if expressions, because too many compilers mishandle such expressions. See Preprocessor Arithmetic.

5.9.2 Generic Type Checks

These macros are used to check for types not covered by the “particular” test macros.

— Macro: AC_CHECK_TYPE (type, [action-if-found], [action-if-not-found], [includes = `AC_INCLUDES_DEFAULT'])

Check whether type is defined. It may be a compiler builtin type or defined by the includes. includes is a series of include directives, defaulting to AC_INCLUDES_DEFAULT (see Default Includes), which are used prior to the type under test.
In C, type must be a type-name, so that the expression `sizeof (type)' is valid (but `sizeof ((type))' is not). The same test is applied when compiling for C++, which means that in C++ type should be a type-id and should not be an anonymous `struct' or `union'.

— Macro: AC_CHECK_TYPES (types, [action-if-found], [action-if-not-found], [includes = `AC_INCLUDES_DEFAULT'])

For each type of the types that is defined, define HAVE_type (in all capitals). Each type must follow the rules of AC_CHECK_TYPE. If no includes are specified, the default includes are used (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 int, uintmax_t])
          AC_CHECK_TYPES([float_t], [], [], [[#include <math.h>]])
     

Autoconf, up to 2.13, used to provide to another version of AC_CHECK_TYPE, broken by design. In order to keep backward compatibility, a simple heuristic, quite safe but not totally, is implemented. In case of doubt, read the documentation of the former AC_CHECK_TYPE, see Obsolete Macros.

5.10 Compilers and Preprocessors

All the tests for compilers (AC_PROG_CC, AC_PROG_CXX, AC_PROG_F77) define the output variable EXEEXT based on the output of the compiler, typically to the empty string if Posix and `.exe' if a DOS variant.

They also define the output variable OBJEXT based on the output of the compiler, after .c files have been excluded, typically to `o' if Posix, `obj' if a DOS variant.

If the compiler being used does not produce executables, the tests fail. If the executables can't be run, and cross-compilation is not enabled, they fail too. See Manual Configuration, for more on support for cross compiling.

5.10.1 Specific Compiler Characteristics

5.10.2 Generic Compiler Characteristics

— Macro: AC_CHECK_SIZEOF (type-or-expr, [unused], [includes = `AC_INCLUDES_DEFAULT'])

Define SIZEOF_type-or-expr (see Standard Symbols) to be the size in bytes of type-or-expr, which may be either a type or an expression returning a value that has a size. If the expression `sizeof (type-or-expr)' is invalid, the result is 0. includes is a series of include directives, defaulting to AC_INCLUDES_DEFAULT (see Default Includes), which are used prior to the expression under test.
This macro now works even when cross-compiling. The unused argument was used when cross-compiling.
For example, the call
          AC_CHECK_SIZEOF([int *])
     
defines SIZEOF_INT_P to be 8 on DEC Alpha AXP systems.

— Macro: AC_CHECK_ALIGNOF (type, [includes = `AC_INCLUDES_DEFAULT'])

Define ALIGNOF_type (see Standard Symbols) to be the alignment in bytes of type. `type y;' must be valid as a structure member declaration. If `type' is unknown, the result is 0. If no includes are specified, the default includes are used (see Default Includes).

— Macro: AC_COMPUTE_INT (var, expression, [includes = `AC_INCLUDES_DEFAULT'], [action-if-fails])

Store into the shell variable var the value of the integer expression. The value should fit in an initializer in a C variable of type signed long. To support cross compilation (in which case, the macro only works on hosts that use twos-complement arithmetic), it should be possible to evaluate the expression at compile-time. If no includes are specified, the default includes are used (see Default Includes).
Execute action-if-fails if the value cannot be determined correctly.

— Macro: AC_LANG_WERROR

Normally Autoconf ignores warnings generated by the compiler, linker, and preprocessor. If this macro is used, warnings count as fatal errors for the current language. This macro is useful when the results of configuration are used where warnings are unacceptable; for instance, if parts of a program are built with the GCC -Werror option. If the whole program is built using -Werror it is often simpler to put -Werror in the compiler flags (CFLAGS, etc.).

— Macro: AC_OPENMP

OpenMP (http://www.openmp.org/) specifies extensions of C, C++, and Fortran that simplify optimization of shared memory parallelism, which is a common problem on multicore CPUs.
If the current language is C, the macro AC_OPENMP sets the variable OPENMP_CFLAGS to the C compiler flags needed for supporting OpenMP. OPENMP_CFLAGS is set to empty if the compiler already supports OpenMP, if it has no way to activate OpenMP support, or if the user rejects OpenMP support by invoking `configure' with the `--disable-openmp' option.
OPENMP_CFLAGS needs to be used when compiling programs, when preprocessing program source, and when linking programs. Therefore you need to add $(OPENMP_CFLAGS) to the CFLAGS of C programs that use OpenMP. If you preprocess OpenMP-specific C code, you also need to add $(OPENMP_CFLAGS) to CPPFLAGS. The presence of OpenMP support is revealed at compile time by the preprocessor macro _OPENMP.
Linking a program with OPENMP_CFLAGS typically adds one more shared library to the program's dependencies, so its use is recommended only on programs that actually require OpenMP.
If the current language is C++, AC_OPENMP sets the variable OPENMP_CXXFLAGS, suitably for the C++ compiler. The same remarks hold as for C.
If the current language is Fortran 77 or Fortran, AC_OPENMP sets the variable OPENMP_FFLAGS or OPENMP_FCFLAGS, respectively. Similar remarks as for C hold, except that CPPFLAGS is not used for Fortran, and no preprocessor macro signals OpenMP support.

5.10.3 C Compiler Characteristics

The following macros provide ways to find and exercise a C Compiler. There are a few constructs that ought to be avoided, but do not deserve being checked for, since they can easily be worked around.

— Macro: AC_PROG_CC ([compiler-search-list])

Determine a C compiler to use. If CC is not already set in the environment, check for gcc and cc, then for other C compilers. Set output variable CC to the name of the compiler found.
This macro may, however, be invoked with an optional first argument which, if specified, must be a blank-separated list of C compilers to search for. This just gives the user an opportunity to specify an alternative search list for the C compiler. For example, if you didn't like the default order, then you could invoke AC_PROG_CC like this:
          AC_PROG_CC([gcc cl cc])
     
If the C compiler does not handle function prototypes correctly by default, try to add an option to output variable CC to make it so. This macro tries various options that select standard-conformance modes on various systems.
After calling this macro you can check whether the C compiler has been set to accept ANSI C89 (ISO C90); if not, the shell variable ac_cv_prog_cc_c89 is set to `no'. See also AC_C_PROTOTYPES below.
If using the GNU C compiler, set shell variable GCC to `yes'. If output variable CFLAGS was not already set, set it to -g -O2 for the GNU C compiler (-O2 on systems where GCC does not accept -g), or -g for other compilers.

— Macro: AC_PROG_CC_C_O

If the C compiler does not accept the -c and -o options simultaneously, define NO_MINUS_C_MINUS_O. This macro actually tests both the compiler found by AC_PROG_CC, and, if different, the first cc in the path. The test fails if one fails. This macro was created for GNU Make to choose the default C compilation rule.

— Macro: AC_PROG_CPP

Set output variable CPP to a command that runs the C preprocessor. If `$CC -E' doesn't work, /lib/cpp is used. It is only portable to run CPP on files with a .c extension.
Some preprocessors don't indicate missing include files by the error status. For such preprocessors an internal variable is set that causes other macros to check the standard error from the preprocessor and consider the test failed if any warnings have been reported. For most preprocessors, though, warnings do not cause include-file tests to fail unless AC_PROG_CPP_WERROR is also specified.

— Macro: AC_PROG_CPP_WERROR

This acts like AC_PROG_CPP, except it treats warnings from the preprocessor as errors even if the preprocessor exit status indicates success. This is useful for avoiding headers that generate mandatory warnings, such as deprecation notices.

The following macros check for C compiler or machine architecture features. To check for characteristics not listed here, use AC_COMPILE_IFELSE (see Running the Compiler) or AC_RUN_IFELSE (see Runtime).

— Macro: AC_PROG_CC_STDC

If the C compiler cannot compile ISO Standard C (currently C99), try to add an option to output variable CC to make it work. If the compiler does not support C99, fall back to supporting ANSI C89 (ISO C90).
After calling this macro you can check whether the C compiler has been set to accept Standard C; if not, the shell variable ac_cv_prog_cc_stdc is set to `no'.

— Macro: AC_PROG_CC_C89

If the C compiler is not in ANSI C89 (ISO C90) mode by default, try to add an option to output variable CC to make it so. This macro tries various options that select ANSI C89 on some system or another. It considers the compiler to be in ANSI C89 mode if it handles function prototypes correctly.
After calling this macro you can check whether the C compiler has been set to accept ANSI C89; if not, the shell variable ac_cv_prog_cc_c89 is set to `no'.
This macro is called automatically by AC_PROG_CC.

— Macro: AC_PROG_CC_C99

If the C compiler is not in C99 mode by default, try to add an option to output variable CC to make it so. This macro tries various options that select C99 on some system or another. It considers the compiler to be in C99 mode if it handles _Bool, // comments, flexible array members, inline, signed and unsigned long long int, mixed code and declarations, named initialization of structs, restrict, va_copy, varargs macros, variable declarations in for loops, and variable length arrays.
After calling this macro you can check whether the C compiler has been set to accept C99; if not, the shell variable ac_cv_prog_cc_c99 is set to `no'.

— Macro: AC_C_BACKSLASH_A

Define `HAVE_C_BACKSLASH_A' to 1 if the C compiler understands `\a'.
This macro is obsolescent, as current C compilers understand `\a'. New programs need not use this macro.

— Macro: AC_C_BIGENDIAN ([action-if-true], [action-if-false], [action-if-unknown], [action-if-universal])

If words are stored with the most significant byte first (like Motorola and SPARC CPUs), execute action-if-true. If words are stored with the least significant byte first (like Intel and VAX CPUs), execute action-if-false.
This macro runs a test-case if endianness cannot be determined from the system header files. When cross-compiling, the test-case is not run but grep'ed for some magic values. action-if-unknown is executed if the latter case fails to determine the byte sex of the host system.
In some cases a single run of a compiler can generate code for multiple architectures. This can happen, for example, when generating Mac OS X universal binary files, which work on both PowerPC and Intel architectures. In this case, the different variants might be for different architectures whose endiannesses differ. If configure detects this, it executes action-if-universal instead of action-if-unknown.
The default for action-if-true is to define `WORDS_BIGENDIAN'. The default for action-if-false is to do nothing. The default for action-if-unknown is to abort configure and tell the installer how to bypass this test. And finally, the default for action-if-universal is to ensure that `WORDS_BIGENDIAN' is defined if and only if a universal build is detected and the current code is big-endian; this default works only if autoheader is used (see autoheader Invocation).
If you use this macro without specifying action-if-universal, you should also use AC_CONFIG_HEADERS; otherwise `WORDS_BIGENDIAN' may be set incorrectly for Mac OS X universal binary files.

— Macro: AC_C_CONST

If the C compiler does not fully support the const keyword, define const to be empty. Some C compilers that do not define __STDC__ do support const; some compilers that define __STDC__ do not completely support const. Programs can simply use const as if every C compiler supported it; for those that don't, the makefile or configuration header file defines it as empty.
Occasionally installers use a C++ compiler to compile C code, typically because they lack a C compiler. This causes problems with const, because C and C++ treat const differently. For example:
          const int foo;
     
is valid in C but not in C++. These differences unfortunately cannot be papered over by defining const to be empty.
If autoconf detects this situation, it leaves const alone, as this generally yields better results in practice. However, using a C++ compiler to compile C code is not recommended or supported, and installers who run into trouble in this area should get a C compiler like GCC to compile their C code.
This macro is obsolescent, as current C compilers support const. New programs need not use this macro.

— Macro: AC_C_RESTRICT

If the C compiler recognizes a variant spelling for the restrict keyword (__restrict, __restrict__, or _Restrict), then define restrict to that; this is more likely to do the right thing with compilers that support language variants where plain restrict is not a keyword. Otherwise, if the C compiler recognizes the restrict keyword, don't do anything. Otherwise, define restrict to be empty. Thus, programs may simply use restrict as if every C compiler supported it; for those that do not, the makefile or configuration header defines it away.
Although support in C++ for the restrict keyword is not required, several C++ compilers do accept the keyword. This macro works for them, too.

— Macro: AC_C_VOLATILE

If the C compiler does not understand the keyword volatile, define volatile to be empty. Programs can simply use volatile as if every C compiler supported it; for those that do not, the makefile or configuration header defines it as empty.
If the correctness of your program depends on the semantics of volatile, simply defining it to be empty does, in a sense, break your code. However, given that the compiler does not support volatile, you are at its mercy anyway. At least your program compiles, when it wouldn't before. See Volatile Objects, for more about volatile.
In general, the volatile keyword is a standard C feature, so you might expect that volatile is available only when __STDC__ is defined. However, Ultrix 4.3's native compiler does support volatile, but does not define __STDC__.
This macro is obsolescent, as current C compilers support volatile. New programs need not use this macro.

— Macro: AC_C_INLINE

If the C compiler supports the keyword inline, do nothing. Otherwise define inline to __inline__ or __inline if it accepts one of those, otherwise define inline to be empty.

— Macro: AC_C_CHAR_UNSIGNED

If the C type char is unsigned, define __CHAR_UNSIGNED__, unless the C compiler predefines it.
These days, using this macro is not necessary. The same information can be determined by this portable alternative, thus avoiding the use of preprocessor macros in the namespace reserved for the implementation.
          #include <limits.h>
          #if CHAR_MIN == 0
          # define CHAR_UNSIGNED 1
          #endif
     

— Macro: AC_C_STRINGIZE

If the C preprocessor supports the stringizing operator, define HAVE_STRINGIZE. The stringizing operator is `#' and is found in macros such as this:
          #define x(y) #y
     
This macro is obsolescent, as current C compilers support the stringizing operator. New programs need not use this macro.

— Macro: AC_C_FLEXIBLE_ARRAY_MEMBER

If the C compiler supports flexible array members, define FLEXIBLE_ARRAY_MEMBER to nothing; otherwise define it to 1. That way, a declaration like this:
          struct s
            {
              size_t n_vals;
              double val[FLEXIBLE_ARRAY_MEMBER];
            };
     
will let applications use the “struct hack” even with compilers that do not support flexible array members. To allocate and use such an object, you can use code like this:
          size_t i;
          size_t n = compute_value_count ();
          struct s *p =
             malloc (offsetof (struct s, val)
                     + n * sizeof (double));
          p->n_vals = n;
          for (i = 0; i < n; i++)
            p->val[i] = compute_value (i);
     

— Macro: AC_C_VARARRAYS

If the C compiler supports variable-length arrays, define HAVE_C_VARARRAYS. A variable-length array is an array of automatic storage duration whose length is determined at run time, when the array is declared.

— Macro: AC_C_TYPEOF

If the C compiler supports GCC's typeof syntax either directly or through a different spelling of the keyword (e.g., __typeof__), define HAVE_TYPEOF. If the support is available only through a different spelling, define typeof to that spelling.

— Macro: AC_C_PROTOTYPES

If function prototypes are understood by the compiler (as determined by AC_PROG_CC), define PROTOTYPES and __PROTOTYPES. Defining __PROTOTYPES is for the benefit of header files that cannot use macros that infringe on user name space.
This macro is obsolescent, as current C compilers support prototypes. New programs need not use this macro.

— Macro: AC_PROG_GCC_TRADITIONAL

Add -traditional to output variable CC if using the GNU C compiler and ioctl does not work properly without -traditional. That usually happens when the fixed header files have not been installed on an old system.
This macro is obsolescent, since current versions of the GNU C compiler fix the header files automatically when installed.

5.10.4 C++ Compiler Characteristics

— Macro: AC_PROG_CXX ([compiler-search-list])

Determine a C++ compiler to use. Check whether the environment variable CXX or CCC (in that order) is set; if so, then set output variable CXX to its value.
Otherwise, if the macro is invoked without an argument, then search for a C++ compiler under the likely names (first g++ and c++ then other names). If none of those checks succeed, then as a last resort set CXX to g++.
This macro may, however, be invoked with an optional first argument which, if specified, must be a blank-separated list of C++ compilers to search for. This just gives the user an opportunity to specify an alternative search list for the C++ compiler. For example, if you didn't like the default order, then you could invoke AC_PROG_CXX like this:
          AC_PROG_CXX([gcc cl KCC CC cxx cc++ xlC aCC c++ g++])
     
If using the GNU C++ compiler, set shell variable GXX to `yes'. If output variable CXXFLAGS was not already set, set it to -g -O2 for the GNU C++ compiler (-O2 on systems where G++ does not accept -g), or -g for other compilers.

— Macro: AC_PROG_CXXCPP

Set output variable CXXCPP to a command that runs the C++ preprocessor. If `$CXX -E' doesn't work, /lib/cpp is used. It is portable to run CXXCPP only on files with a .c, .C, .cc, or .cpp extension.
Some preprocessors don't indicate missing include files by the error status. For such preprocessors an internal variable is set that causes other macros to check the standard error from the preprocessor and consider the test failed if any warnings have been reported. However, it is not known whether such broken preprocessors exist for C++.

— Macro: AC_PROG_CXX_C_O

Test whether the C++ compiler accepts the options -c and -o simultaneously, and define CXX_NO_MINUS_C_MINUS_O, if it does not.

5.10.5 Objective C Compiler Characteristics

— Macro: AC_PROG_OBJC ([compiler-search-list])

Determine an Objective C compiler to use. If OBJC is not already set in the environment, check for Objective C compilers. Set output variable OBJC to the name of the compiler found.
This macro may, however, be invoked with an optional first argument which, if specified, must be a blank-separated list of Objective C compilers to search for. This just gives the user an opportunity to specify an alternative search list for the Objective C compiler. For example, if you didn't like the default order, then you could invoke AC_PROG_OBJC like this:
          AC_PROG_OBJC([gcc objcc objc])
     
If using the GNU Objective C compiler, set shell variable GOBJC to `yes'. If output variable OBJCFLAGS was not already set, set it to -g -O2 for the GNU Objective C compiler (-O2 on systems where gcc does not accept -g), or -g for other compilers.

— Macro: AC_PROG_OBJCPP

Set output variable OBJCPP to a command that runs the Objective C preprocessor. If `$OBJC -E' doesn't work, /lib/cpp is used.

5.10.6 Erlang Compiler and Interpreter Characteristics

Autoconf defines the following macros for determining paths to the essential Erlang/OTP programs:

— Macro: AC_ERLANG_PATH_ERLC ([value-if-not-found], [path = `$PATH'])

Determine an Erlang compiler to use. If ERLC is not already set in the environment, check for erlc. Set output variable ERLC to the complete path of the compiler command found. In addition, if ERLCFLAGS is not set in the environment, set it to an empty value.
The two optional arguments have the same meaning as the two last arguments of macro AC_PROG_PATH for looking for the erlc program. For example, to look for erlc only in the /usr/lib/erlang/bin directory:
          AC_ERLANG_PATH_ERLC([not found], [/usr/lib/erlang/bin])
     

— Macro: AC_ERLANG_NEED_ERLC ([path = `$PATH'])

A simplified variant of the AC_ERLANG_PATH_ERLC macro, that prints an error message and exits the configure script if the erlc program is not found.

— Macro: AC_ERLANG_PATH_ERL ([value-if-not-found], [path = `$PATH'])

Determine an Erlang interpreter to use. If ERL is not already set in the environment, check for erl. Set output variable ERL to the complete path of the interpreter command found.
The two optional arguments have the same meaning as the two last arguments of macro AC_PROG_PATH for looking for the erl program. For example, to look for erl only in the /usr/lib/erlang/bin directory:
          AC_ERLANG_PATH_ERL([not found], [/usr/lib/erlang/bin])
     

— Macro: AC_ERLANG_NEED_ERL ([path = `$PATH'])

A simplified variant of the AC_ERLANG_PATH_ERL macro, that prints an error message and exits the configure script if the erl program is not found.

5.10.7 Fortran Compiler Characteristics

The Autoconf Fortran support is divided into two categories: legacy Fortran 77 macros (F77), and modern Fortran macros (FC). The former are intended for traditional Fortran 77 code, and have output variables like F77, FFLAGS, and FLIBS. The latter are for newer programs that can (or must) compile under the newer Fortran standards, and have output variables like FC, FCFLAGS, and FCLIBS.

Except for two new macros AC_FC_SRCEXT and AC_FC_FREEFORM (see below), the FC and F77 macros behave almost identically, and so they are documented together in this section.

— Macro: AC_PROG_F77 ([compiler-search-list])

Determine a Fortran 77 compiler to use. If F77 is not already set in the environment, then check for g77 and f77, and then some other names. Set the output variable F77 to the name of the compiler found.
This macro may, however, be invoked with an optional first argument which, if specified, must be a blank-separated list of Fortran 77 compilers to search for. This just gives the user an opportunity to specify an alternative search list for the Fortran 77 compiler. For example, if you didn't like the default order, then you could invoke AC_PROG_F77 like this:
          AC_PROG_F77([fl32 f77 fort77 xlf g77 f90 xlf90])
     
If using g77 (the GNU Fortran 77 compiler), then set the shell variable G77 to `yes'. If the output variable FFLAGS was not already set in the environment, then set it to -g -02 for g77 (or -O2 where g77 does not accept -g). Otherwise, set FFLAGS to -g for all other Fortran 77 compilers.

— Macro: AC_PROG_FC ([compiler-search-list], [dialect])

Determine a Fortran compiler to use. If FC is not already set in the environment, then dialect is a hint to indicate what Fortran dialect to search for; the default is to search for the newest available dialect. Set the output variable FC to the name of the compiler found.
By default, newer dialects are preferred over older dialects, but if dialect is specified then older dialects are preferred starting with the specified dialect. dialect can currently be one of Fortran 77, Fortran 90, or Fortran 95. However, this is only a hint of which compiler name to prefer (e.g., f90 or f95), and no attempt is made to guarantee that a particular language standard is actually supported. Thus, it is preferable that you avoid the dialect option, and use AC_PROG_FC only for code compatible with the latest Fortran standard.
This macro may, alternatively, be invoked with an optional first argument which, if specified, must be a blank-separated list of Fortran compilers to search for, just as in AC_PROG_F77.
If the output variable FCFLAGS was not already set in the environment, then set it to -g -02 for GNU g77 (or -O2 where g77 does not accept -g). Otherwise, set FCFLAGS to -g for all other Fortran compilers.

— Macro: AC_PROG_F77_C_O
— Macro: AC_PROG_FC_C_O

Test whether the Fortran compiler accepts the options -c and -o simultaneously, and define F77_NO_MINUS_C_MINUS_O or FC_NO_MINUS_C_MINUS_O, respectively, if it does not.

The following macros check for Fortran compiler characteristics. To check for characteristics not listed here, use AC_COMPILE_IFELSE (see Running the Compiler) or AC_RUN_IFELSE (see Runtime), 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).

— Macro: AC_F77_LIBRARY_LDFLAGS
— Macro: AC_FC_LIBRARY_LDFLAGS

Determine the linker flags (e.g., -L and -l) for the Fortran intrinsic and runtime libraries that are required to successfully link a Fortran program or shared library. The output variable FLIBS or FCLIBS is set to these flags (which should be included after LIBS when linking).
This macro is intended to be used in those situations when it is necessary to mix, e.g., C++ and Fortran source code in a single program or shared library (see Mixing Fortran 77 With C and C++).
For example, if object files from a C++ and Fortran compiler must be linked together, then the C++ compiler/linker must be used for linking (since special C++-ish things need to happen at link time like calling global constructors, instantiating templates, enabling exception support, etc.).
However, the Fortran intrinsic and runtime libraries must be linked in as well, but the C++ compiler/linker doesn't know by default how to add these Fortran 77 libraries. Hence, this macro was created to determine these Fortran libraries.
The macros AC_F77_DUMMY_MAIN and AC_FC_DUMMY_MAIN or AC_F77_MAIN and AC_FC_MAIN are probably also necessary to link C/C++ with Fortran; see below.

— Macro: AC_F77_DUMMY_MAIN ([action-if-found], [action-if-not-found])
— Macro: AC_FC_DUMMY_MAIN ([action-if-found], [action-if-not-found])

With many compilers, the Fortran libraries detected by AC_F77_LIBRARY_LDFLAGS or AC_FC_LIBRARY_LDFLAGS provide their own main entry function that initializes things like Fortran I/O, and which then calls a user-provided entry function named (say) MAIN__ to run the user's program. The AC_F77_DUMMY_MAIN and AC_FC_DUMMY_MAIN or AC_F77_MAIN and AC_FC_MAIN macros figure out how to deal with this interaction.
When using Fortran for purely numerical functions (no I/O, etc.) often one prefers to provide one's own main and skip the Fortran library initializations. In this case, however, one may still need to provide a dummy MAIN__ routine in order to prevent linking errors on some systems. AC_F77_DUMMY_MAIN or AC_FC_DUMMY_MAIN detects whether any such routine is required for linking, and what its name is; the shell variable F77_DUMMY_MAIN or FC_DUMMY_MAIN holds this name, unknown when no solution was found, and none when no such dummy main is needed.
By default, action-if-found defines F77_DUMMY_MAIN or FC_DUMMY_MAIN to the name of this routine (e.g., MAIN__) if it is required. action-if-not-found defaults to exiting with an error.
In order to link with Fortran routines, the user's C/C++ program should then include the following code to define the dummy main if it is needed:
          #ifdef F77_DUMMY_MAIN
          #  ifdef __cplusplus
               extern "C"
          #  endif
             int F77_DUMMY_MAIN() { return 1; }
          #endif
     
(Replace F77 with FC for Fortran instead of Fortran 77.)
Note that this macro is called automatically from AC_F77_WRAPPERS or AC_FC_WRAPPERS; there is generally no need to call it explicitly unless one wants to change the default actions.

— Macro: AC_F77_MAIN
— Macro: AC_FC_MAIN

As discussed above, many Fortran libraries allow you to provide an entry point called (say) MAIN__ instead of the usual main, which is then called by a main function in the Fortran libraries that initializes things like Fortran I/O. The AC_F77_MAIN and AC_FC_MAIN macros detect whether it is possible to utilize such an alternate main function, and defines F77_MAIN and FC_MAIN to the name of the function. (If no alternate main function name is found, F77_MAIN and FC_MAIN are simply defined to main.)
Thus, when calling Fortran routines from C that perform things like I/O, one should use this macro and declare the "main" function like so:
          #ifdef __cplusplus
            extern "C"
          #endif
          int F77_MAIN(int argc, char *argv[]);
     
(Again, replace F77 with FC for Fortran instead of Fortran 77.)

— Macro: AC_F77_WRAPPERS
— Macro: AC_FC_WRAPPERS

Defines C macros F77_FUNC (name, NAME), FC_FUNC (name, NAME), F77_FUNC_(name, NAME), and FC_FUNC_(name, NAME) to properly mangle the names of C/C++ identifiers, and identifiers with underscores, respectively, so that they match the name-mangling scheme used by the Fortran compiler.
Fortran is case-insensitive, and in order to achieve this the Fortran compiler converts all identifiers into a canonical case and format. To call a Fortran subroutine from C or to write a C function that is callable from Fortran, the C program must explicitly use identifiers in the format expected by the Fortran compiler. In order to do this, one simply wraps all C identifiers in one of the macros provided by AC_F77_WRAPPERS or AC_FC_WRAPPERS. For example, suppose you have the following Fortran 77 subroutine:
                subroutine foobar (x, y)
                double precision x, y
                y = 3.14159 * x
                return
                end
     
You would then declare its prototype in C or C++ as:
          #define FOOBAR_F77 F77_FUNC (foobar, FOOBAR)
          #ifdef __cplusplus
          extern "C"  /* prevent C++ name mangling */
          #endif
          void FOOBAR_F77(double *x, double *y);
     
Note that we pass both the lowercase and uppercase versions of the function name to F77_FUNC so that it can select the right one. Note also that all parameters to Fortran 77 routines are passed as pointers (see Mixing Fortran 77 With C and C++).
(Replace F77 with FC for Fortran instead of Fortran 77.)
Although Autoconf tries to be intelligent about detecting the name-mangling scheme of the Fortran compiler, there may be Fortran compilers that it doesn't support yet. In this case, the above code generates a compile-time error, but some other behavior (e.g., disabling Fortran-related features) can be induced by checking whether F77_FUNC or FC_FUNC is defined.
Now, to call that routine from a C program, we would do something like:
          {
              double x = 2.7183, y;
              FOOBAR_F77 (&x, &y);
          }
     
If the Fortran identifier contains an underscore (e.g., foo_bar), you should use F77_FUNC_ or FC_FUNC_ instead of F77_FUNC or FC_FUNC (with the same arguments). This is because some Fortran compilers mangle names differently if they contain an underscore.

— Macro: AC_F77_FUNC (name, [shellvar])
— Macro: AC_FC_FUNC (name, [shellvar])

Given an identifier name, set the shell variable shellvar to hold the mangled version name according to the rules of the Fortran linker (see also AC_F77_WRAPPERS or AC_FC_WRAPPERS). shellvar is optional; if it is not supplied, the shell variable is simply name. The purpose of this macro is to give the caller a way to access the name-mangling information other than through the C preprocessor as above, for example, to call Fortran routines from some language other than C/C++.

— Macro: AC_FC_SRCEXT (ext, [action-if-success], [action-if-failure])

By default, the FC macros perform their tests using a .f extension for source-code files. Some compilers, however, only enable newer language features for appropriately named files, e.g., Fortran 90 features only for .f90 files. On the other hand, some other compilers expect all source files to end in .f and require special flags to support other file name extensions. The AC_FC_SRCEXT macro deals with both of these issues.
The AC_FC_SRCEXT tries to get the FC compiler to accept files ending with the extension .ext (i.e., ext does not contain the dot). If any special compiler flags are needed for this, it stores them in the output variable FCFLAGS_ext. This extension and these flags are then used for all subsequent FC tests (until AC_FC_SRCEXT is called again).
For example, you would use AC_FC_SRCEXT(f90) to employ the .f90 extension in future tests, and it would set a FCFLAGS_f90 output variable with any extra flags that are needed to compile such files.
The FCFLAGS_ext can not be simply absorbed into FCFLAGS, for two reasons based on the limitations of some compilers. First, only one FCFLAGS_ext can be used at a time, so files with different extensions must be compiled separately. Second, FCFLAGS_ext must appear immediately before the source-code file name when compiling. So, continuing the example above, you might compile a foo.f90 file in your makefile with the command:
          foo.o: foo.f90
               $(FC) -c $(FCFLAGS) $(FCFLAGS_f90) '$(srcdir)/foo.f90'
     
If AC_FC_SRCEXT succeeds in compiling files with the ext extension, it calls action-if-success (defaults to nothing). If it fails, and cannot find a way to make the FC compiler accept such files, it calls action-if-failure (defaults to exiting with an error message).

— Macro: AC_FC_FREEFORM ([action-if-success], [action-if-failure])

The AC_FC_FREEFORM tries to ensure that the Fortran compiler ($FC) allows free-format source code (as opposed to the older fixed-format style from Fortran 77). If necessary, it may add some additional flags to FCFLAGS.
This macro is most important if you are using the default .f extension, since many compilers interpret this extension as indicating fixed-format source unless an additional flag is supplied. If you specify a different extension with AC_FC_SRCEXT, such as .f90 or .f95, then AC_FC_FREEFORM ordinarily succeeds without modifying FCFLAGS.
If AC_FC_FREEFORM succeeds in compiling free-form source, it calls action-if-success (defaults to nothing). If it fails, it calls action-if-failure (defaults to exiting with an error message).

5.11 System Services

— Macro: AC_PATH_X

Try to locate the X Window System include files and libraries. If the user gave the command line options --x-includes=dir and --x-libraries=dir, use those directories.
If either or both were not given, get the missing values by running xmkmf (or an executable pointed to by the XMKMF environment variable) on a trivial Imakefile and examining the makefile that it produces. Setting XMKMF to `false' disables this method.
If this method fails to find the X Window System, configure looks for the files in several directories where they often reside. If either method is successful, set the shell variables x_includes and x_libraries to their locations, unless they are in directories the compiler searches by default.
If both methods fail, or the user gave the command line option --without-x, set the shell variable no_x to `yes'; otherwise set it to the empty string.

— Macro: AC_PATH_XTRA

An enhanced version of AC_PATH_X. It adds the C compiler flags that X needs to output variable X_CFLAGS, and the X linker flags to X_LIBS. Define X_DISPLAY_MISSING if X is not available.
This macro also checks for special libraries that some systems need in order to compile X programs. It adds any that the system needs to output variable X_EXTRA_LIBS. And it checks for special X11R6 libraries that need to be linked with before -lX11, and adds any found to the output variable X_PRE_LIBS.

— Macro: AC_SYS_INTERPRETER

Check whether the system supports starting scripts with a line of the form `#!/bin/sh' to select the interpreter to use for the script. After running this macro, shell code in configure.ac can check the shell variable interpval; it is set to `yes' if the system supports `#!', `no' if not.

— Macro: AC_SYS_LARGEFILE

Arrange for 64-bit file offsets, known as large-file support. On some hosts, one must use special compiler options to build programs that can access large files. Append any such options to the output variable CC. Define _FILE_OFFSET_BITS and _LARGE_FILES if necessary.
Large-file support can be disabled by configuring with the --disable-largefile option.
If you use this macro, check that your program works even when off_t is wider than long int, since this is common when large-file support is enabled. For example, it is not correct to print an arbitrary off_t value X with printf ("%ld", (long int) X).
The LFS introduced the fseeko and ftello functions to replace their C counterparts fseek and ftell that do not use off_t. Take care to use AC_FUNC_FSEEKO to make their prototypes available when using them and large-file support is enabled.

— Macro: AC_SYS_LONG_FILE_NAMES

If the system supports file names longer than 14 characters, define HAVE_LONG_FILE_NAMES.

— Macro: AC_SYS_POSIX_TERMIOS

Check to see if the Posix termios headers and functions are available on the system. If so, set the shell variable ac_cv_sys_posix_termios to `yes'. If not, set the variable to `no'.

5.12 Posix Variants

The following macro makes it possible to use features of Posix that are extensions to C, as well as platform extensions not defined by Posix.

— Macro: AC_USE_SYSTEM_EXTENSIONS

This macro was introduced in Autoconf 2.60. If possible, enable extensions to C or Posix on hosts that normally disable the extensions, typically due to standards-conformance namespace issues. This should be called before any macros that run the C compiler. The following preprocessor macros are defined where appropriate:

_GNU_SOURCE
Enable extensions on GNU/Linux.
__EXTENSIONS__
Enable general extensions on Solaris.
_POSIX_PTHREAD_SEMANTICS
Enable threading extensions on Solaris.
_TANDEM_SOURCE
Enable extensions for the HP NonStop platform.
_ALL_SOURCE
Enable extensions for AIX 3, and for Interix.
_POSIX_SOURCE
Enable Posix functions for Minix.
_POSIX_1_SOURCE
Enable additional Posix functions for Minix.
_MINIX
Identify Minix platform. This particular preprocessor macro is obsolescent, and may be removed in a future release of Autoconf.

5.13 Erlang Libraries

The following macros check for an installation of Erlang/OTP, and for the presence of certain Erlang libraries. All those macros require the configuration of an Erlang interpreter and an Erlang compiler (see Erlang Compiler and Interpreter).

— Macro: AC_ERLANG_SUBST_ROOT_DIR

Set the output variable ERLANG_ROOT_DIR to the path to the base directory in which Erlang/OTP is installed (as returned by Erlang's code:root_dir/0 function). The result of this test is cached if caching is enabled when running configure.

— Macro: AC_ERLANG_SUBST_LIB_DIR

Set the output variable ERLANG_LIB_DIR to the path of the library directory of Erlang/OTP (as returned by Erlang's code:lib_dir/0 function), which subdirectories each contain an installed Erlang/OTP library. The result of this test is cached if caching is enabled when running configure.

— Macro: AC_ERLANG_CHECK_LIB (library, [action-if-found], [action-if-not-found])

Test whether the Erlang/OTP library library is installed by calling Erlang's code:lib_dir/1 function. The result of this test is cached if caching is enabled when running configure. action-if-found is a list of shell commands to run if the library is installed; action-if-not-found is a list of shell commands to run if it is not. Additionally, if the library is installed, the output variable `ERLANG_LIB_DIR_library' is set to the path to the library installation directory, and the output variable `ERLANG_LIB_VER_library' is set to the version number that is part of the subdirectory name, if it is in the standard form (library-version). If the directory name does not have a version part, `ERLANG_LIB_VER_library' is set to the empty string. If the library is not installed, `ERLANG_LIB_DIR_library' and `ERLANG_LIB_VER_library' are set to "not found". For example, to check if library stdlib is installed:
          AC_ERLANG_CHECK_LIB([stdlib],
            [echo "stdlib version \"$ERLANG_LIB_VER_stdlib\""
             echo "is installed in \"$ERLANG_LIB_DIR_stdlib\""],
            [AC_MSG_ERROR([stdlib was not found!])])
     

In addition to the above macros, which test installed Erlang libraries, the following macros determine the paths to the directories into which newly built Erlang libraries are to be installed:

— Macro: AC_ERLANG_SUBST_INSTALL_LIB_DIR

Set the ERLANG_INSTALL_LIB_DIR output variable to the directory into which every built Erlang library should be installed in a separate subdirectory. If this variable is not set in the environment when configure runs, its default value is $ERLANG_LIB_DIR, which value is set by the AC_ERLANG_SUBST_LIB_DIR macro.

— Macro: AC_ERLANG_SUBST_INSTALL_LIB_SUBDIR (library, version)

Set the `ERLANG_INSTALL_LIB_DIR_library' output variable to the directory into which the built Erlang library library version version should be installed. If this variable is not set in the environment when configure runs, its default value is `$ERLANG_INSTALL_LIB_DIR/library-version', the value of the ERLANG_INSTALL_LIB_DIR variable being set by the AC_ERLANG_SUBST_INSTALL_LIB_DIR macro.

6 Writing Tests

If the existing feature tests don't do something you need, you have to write new ones. These macros are the building blocks. They provide ways for other macros to check whether various kinds of features are available and report the results.

This chapter contains some suggestions and some of the reasons why the existing tests are written the way they are. You can also learn a lot about how to write Autoconf tests by looking at the existing ones. If something goes wrong in one or more of the Autoconf tests, this information can help you understand the assumptions behind them, which might help you figure out how to best solve the problem.

These macros check the output of the compiler system of the current language (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.

6.1 Language Choice

Autoconf-generated configure scripts check for the C compiler and its features by default. Packages that use other programming languages (maybe more than one, e.g., C and C++) need to test features of the compilers for the respective languages. The following macros determine which programming language is used in the subsequent tests in configure.ac.

— Macro: AC_LANG (language)

Do compilation tests using the compiler, preprocessor, and file extensions for the specified language.
Supported languages are:

`C'
Do compilation tests using CC and CPP and use extension .c for test programs. Use compilation flags: CPPFLAGS with CPP, and both CPPFLAGS and CFLAGS with CC.
`C++'
Do compilation tests using CXX and CXXCPP and use extension .C for test programs. Use compilation flags: CPPFLAGS with CXXCPP, and both CPPFLAGS and CXXFLAGS with CXX.
`Fortran 77'
Do compilation tests using F77 and use extension .f for test programs. Use compilation flags: FFLAGS.
`Fortran'
Do compilation tests using FC and use extension .f (or whatever has been set by AC_FC_SRCEXT) for test programs. Use compilation flags: FCFLAGS.
`Erlang'
Compile and execute tests using ERLC and ERL and use extension .erl for test Erlang modules. Use compilation flags: ERLCFLAGS.
`Objective C'
Do compilation tests using OBJC and OBJCPP and use extension .m for test programs. Use compilation flags: CPPFLAGS with OBJCPP, and both CPPFLAGS and OBJCFLAGS with OBJC.

— Macro: AC_LANG_PUSH (language)

Remember the current language (as set by AC_LANG) on a stack, and then select the language. Use this macro and AC_LANG_POP in macros that need to temporarily switch to a particular language.

— Macro: AC_LANG_POP ([language])

Select the language that is saved on the top of the stack, as set by AC_LANG_PUSH, and remove it from the stack.
If given, language specifies the language we just quit. It is a good idea to specify it when it's known (which should be the case...), since Autoconf detects inconsistencies.
          AC_LANG_PUSH([Fortran 77])
          # Perform some tests on Fortran 77.
          # ...
          AC_LANG_POP([Fortran 77])
     

— Macro: AC_LANG_ASSERT (language)

Check statically that the current language is language. You should use this in your language specific macros to avoid that they be called with an inappropriate language.
This macro runs only at autoconf time, and incurs no cost at configure time. Sadly enough and because Autoconf is a two layer language ², the macros AC_LANG_PUSH and AC_LANG_POP cannot be “optimizing”, therefore as much as possible you ought to avoid using them to wrap your code, rather, require from the user to run the macro with a correct current language, and check it with AC_LANG_ASSERT. And anyway, that may help the user understand she is running a Fortran macro while expecting a result about her Fortran 77 compiler....

— Macro: AC_REQUIRE_CPP

Ensure that whichever preprocessor would currently be used for tests has been found. Calls AC_REQUIRE (see Prerequisite Macros) with an argument of either AC_PROG_CPP or AC_PROG_CXXCPP, depending on which language is current.

6.2 Writing Test Programs

Autoconf tests follow a common scheme: feed some program with some input, and most of the time, feed a compiler with some source file. This section is dedicated to these source samples.

6.2.1 Guidelines for Test Programs

This motto means that testing samples must be written with the same strictness as real programs are written. In particular, you should avoid “shortcuts” and simplifications.

Don't just play with the preprocessor if you want to prepare a compilation. For instance, using cpp to check whether a header is functional might let your configure accept a header which causes some compiler error. Do not hesitate to check a header with other headers included before, especially required headers.

Make sure the symbols you use are properly defined, i.e., refrain for simply declaring a function yourself instead of including the proper header.

Test programs should not write to standard output. They should exit with status 0 if the test succeeds, and with status 1 otherwise, so that success can be distinguished easily from a core dump or other failure; segmentation violations and other failures produce a nonzero exit status. Unless you arrange for exit to be declared, test programs should return, not exit, from main, because on many systems exit is not declared by default.

Test programs can use #if or #ifdef to check the values of preprocessor macros defined by tests that have already run. For example, if you call AC_HEADER_STDBOOL, then later on in configure.ac you can have a test program that includes stdbool.h conditionally:

Both #if HAVE_STDBOOL_H and #ifdef HAVE_STDBOOL_H will work with any standard C compiler. Some developers prefer #if because it is easier to read, while others prefer #ifdef because it avoids diagnostics with picky compilers like GCC with the -Wundef option.

If a test program needs to use or create a data file, give it a name that starts with conftest, such as conftest.data. The configure script cleans up by running `rm -f -r conftest*' after running test programs and if the script is interrupted.

6.2.2 Test Functions

These days it's safe to assume support for function prototypes (introduced in C89).

Functions that test programs declare should also be conditionalized for C++, which requires `extern "C"' prototypes. Make sure to not include any header files containing clashing prototypes.

If a test program calls a function with invalid parameters (just to see whether it exists), organize the program to ensure that it never invokes that function. You can do this by calling it in another function that is never invoked. You can't do it by putting it after a call to exit, because GCC version 2 knows that exit never returns and optimizes out any code that follows it in the same block.

If you include any header files, be sure to call the functions relevant to them with the correct number of arguments, even if they are just 0, to avoid compilation errors due to prototypes. GCC version 2 has internal prototypes for several functions that it automatically inlines; for example, memcpy. To avoid errors when checking for them, either pass them the correct number of arguments or redeclare them with a different return type (such as char).

6.2.3 Generating Sources

Autoconf provides a set of macros that can be used to generate test source files. They are written to be language generic, i.e., they actually depend on the current language (see Language Choice) to “format” the output properly.

— Macro: AC_LANG_CONFTEST (source)

Save the source text in the current test source file: conftest.extension where the extension depends on the current language.
Note that the source is evaluated exactly once, like regular Autoconf macro arguments, and therefore (i) you may pass a macro invocation, (ii) if not, be sure to double quote if needed.

— Macro: AC_LANG_SOURCE (source)

Expands into the source, with the definition of all the AC_DEFINE performed so far.

When the test language is Fortran or Erlang, the AC_DEFINE definitions are not automatically translated into constants in the source code by this macro.

— Macro: AC_LANG_PROGRAM (prologue, body)

Expands into a source file which consists of the prologue, and then body as body of the main function (e.g., main in C). Since it uses AC_LANG_SOURCE, the features of the latter are available.

In Erlang tests, the created source file is that of an Erlang module called conftest (conftest.erl). This module defines and exports at least one start/0 function, which is called to perform the test. The prologue is optional code that is inserted between the module header and the start/0 function definition. body is the body of the start/0 function without the final period (see Runtime, about constraints on this function's behavior).

— Macro: AC_LANG_CALL (prologue, function)

Expands into a source file which consists of the prologue, and then a call to the function as body of the main function (e.g., main in C). Since it uses AC_LANG_PROGRAM, the feature of the latter are available.
This function will probably be replaced in the future by a version which would enable specifying the arguments. The use of this macro is not encouraged, as it violates strongly the typing system.
This macro cannot be used for Erlang tests.

— Macro: AC_LANG_FUNC_LINK_TRY (function)

Expands into a source file which uses the function in the body of the main function (e.g., main in C). Since it uses AC_LANG_PROGRAM, the features of the latter are available.
As AC_LANG_CALL, this macro is documented only for completeness. It is considered to be severely broken, and in the future will be removed in favor of actual function calls (with properly typed arguments).
This macro cannot be used for Erlang tests.

6.3 Running the Preprocessor

Sometimes one might need to run the preprocessor on some source file. Usually it is a bad idea, as you typically need to compile your project, not merely run the preprocessor on it; therefore you certainly want to run the compiler, not the preprocessor. Resist the temptation of following the easiest path.

Nevertheless, if you need to run the preprocessor, then use AC_PREPROC_IFELSE.

The macros described in this section cannot be used for tests in Erlang or Fortran, since those languages require no preprocessor.

— Macro: AC_PREPROC_IFELSE (input, [action-if-true], [action-if-false])

Run the preprocessor of the current language (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_PROGRAM and friends.
This macro uses CPPFLAGS, but not CFLAGS, because -g, -O, etc. are not valid options to many C preprocessors.
It is customary to report unexpected failures with AC_MSG_FAILURE.

— Macro: AC_EGREP_HEADER (pattern, header-file, action-if-found, [action-if-not-found])

If the output of running the preprocessor on the system header file header-file matches the extended regular expression pattern, execute shell commands action-if-found, otherwise execute action-if-not-found.

— Macro: AC_EGREP_CPP (pattern, program, [action-if-found], [action-if-not-found])

program is the text of a C or C++ program, on which shell variable, back quote, and backslash substitutions are performed. If the output of running the preprocessor on program matches the extended regular expression pattern, execute shell commands action-if-found, otherwise execute action-if-not-found.

6.4 Running the Compiler

To check for a syntax feature of the current language's (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.

— Macro: AC_COMPILE_IFELSE (input, [action-if-true], [action-if-false])

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_PROGRAM and friends.
It is customary to report unexpected failures with AC_MSG_FAILURE. This macro does not try to link; use AC_LINK_IFELSE if you need to do that (see Running the Linker).

For tests in Erlang, the input must be the source code of a module named conftest. AC_COMPILE_IFELSE generates a conftest.beam file that can be interpreted by the Erlang virtual machine (ERL). It is recommended to use AC_LANG_PROGRAM to specify the test program, to ensure that the Erlang module has the right name.

6.5 Running the Linker

To check for a library, a function, or a global variable, Autoconf configure scripts try to compile and link a small program that uses it. This is unlike Metaconfig, which by default uses nm or ar on the C library to try to figure out which functions are available. Trying to link with the function is usually a more reliable approach because it avoids dealing with the variations in the options and output formats of nm and ar and in the location of the standard libraries. It also allows configuring for cross-compilation or checking a function's runtime behavior if needed. On the other hand, it can be slower than scanning the libraries once, but accuracy is more important than speed.

AC_LINK_IFELSE is used to compile test programs to test for functions and global variables. It is also used by AC_CHECK_LIB to check for libraries (see Libraries), by adding the library being checked for to LIBS temporarily and trying to link a small program.

— Macro: AC_LINK_IFELSE (input, [action-if-true], [action-if-false])

Run the compiler (and compilation flags) and the linker of the current language (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_PROGRAM and friends.
LDFLAGS and LIBS are used for linking, in addition to the current compilation flags.
It is customary to report unexpected failures with AC_MSG_FAILURE. This macro does not try to execute the program; use AC_RUN_IFELSE if you need to do that (see Runtime).

The AC_LINK_IFELSE macro cannot be used for Erlang tests, since Erlang programs are interpreted and do not require linking.

6.6 Checking Runtime Behavior

Sometimes you need to find out how a system performs at runtime, such as whether a given function has a certain capability or bug. If you can, make such checks when your program runs instead of when it is configured. You can check for things like the machine's endianness when your program initializes itself.

If you really need to test for a runtime behavior while configuring, you can write a test program to determine the result, and compile and run it using AC_RUN_IFELSE. Avoid running test programs if possible, because this prevents people from configuring your package for cross-compiling.

— Macro: AC_RUN_IFELSE (input, [action-if-true], [action-if-false], [action-if-cross-compiling])

If program compiles and links successfully and returns an exit status of 0 when executed, run shell commands action-if-true. Otherwise, run shell commands action-if-false.
The input can be made by AC_LANG_PROGRAM and friends. LDFLAGS and LIBS are used for linking, in addition to the compilation flags of the current language (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 failing exit status is available in the shell variable `$?'. This exit status might be that of a failed compilation, or it might be that of a failed program execution.
It is customary to report unexpected failures with AC_MSG_FAILURE.

Try to provide a pessimistic default value to use when cross-compiling makes runtime tests impossible. You do this by passing the optional last argument to AC_RUN_IFELSE. autoconf prints a warning message when creating configure each time it encounters a call to AC_RUN_IFELSE with no action-if-cross-compiling argument given. You may ignore the warning, though users cannot configure your package for cross-compiling. A few of the macros distributed with Autoconf produce this warning message.

To configure for cross-compiling you can also choose a value for those parameters based on the canonical system name (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.

It is also permissible to temporarily assign to cross_compiling in order to force tests to behave as though they are in a cross-compilation environment, particularly since this provides a way to test your action-if-cross-compiling even when you are not using a cross-compiler.

Erlang tests must exit themselves the Erlang VM by calling the halt/1 function: the given status code is used to determine the success of the test (status is 0) or its failure (status is different than 0), as explained above. It must be noted that data output through the standard output (e.g., using io:format/2) may be truncated when halting the VM. Therefore, if a test must output configuration information, it is recommended to create and to output data into the temporary file named conftest.out, using the functions of module file. The conftest.out file is automatically deleted by the AC_RUN_IFELSE macro. For instance, a simplified implementation of Autoconf's AC_ERLANG_SUBST_LIB_DIR macro is:

6.7 Systemology

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 table correlating the features of various Posix-conforming systems. Unix History is a simplified diagram of how many Unix systems were derived from each other.

The Heirloom Project provides some variants of traditional implementations of Unix utilities.

6.8 Multiple Cases

Some operations are accomplished in several possible ways, depending on the OS variant. Checking for them essentially requires a “case statement”. Autoconf does not directly provide one; however, it is easy to simulate by using a shell variable to keep track of whether a way to perform the operation has been found yet.

Here is an example that uses the shell variable fstype to keep track of whether the remaining cases need to be checked.

7 Results of Tests

Once configure has determined whether a feature exists, what can it do to record that information? There are four sorts of things it can do: define a C preprocessor symbol, set a variable in the output files, save the result in a cache file for future configure runs, and print a message letting the user know the result of the test.

7.1 Defining C Preprocessor Symbols

A common action to take in response to a feature test is to define a C preprocessor symbol indicating the results of the test. That is done by calling AC_DEFINE or AC_DEFINE_UNQUOTED.

By default, AC_OUTPUT places the symbols defined by these macros into the output variable DEFS, which contains an option -Dsymbol=value for each symbol defined. Unlike in Autoconf version 1, there is no variable DEFS defined while configure is running. To check whether Autoconf macros have already defined a certain C preprocessor symbol, test the value of the appropriate cache variable, as in this example:

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.

— Macro: AC_DEFINE (variable, value, [description])
— Macro: AC_DEFINE (variable)

Define variable to value (verbatim), by defining a C preprocessor macro for variable. variable should be a C identifier, optionally suffixed by a parenthesized argument list to define a C preprocessor macro with arguments. The macro argument list, if present, should be a comma-separated list of C identifiers, possibly terminated by an ellipsis `...' if C99 syntax is employed. variable should not contain comments, white space, trigraphs, backslash-newlines, universal character names, or non-ASCII characters.
value may contain backslash-escaped newlines, which will be preserved if you use AC_CONFIG_HEADERS but flattened if passed via @DEFS@ (with no effect on the compilation, since the preprocessor sees only one line in the first place). value should not contain raw newlines. If you are not using AC_CONFIG_HEADERS, value should not contain any `#' characters, as make tends to eat them. To use a shell variable, use AC_DEFINE_UNQUOTED instead.
description is only useful if you are using AC_CONFIG_HEADERS. In this case, description is put into the generated config.h.in as the comment before the macro define. The following example defines the C preprocessor variable EQUATION to be the string constant `"$a > $b"':
          AC_DEFINE([EQUATION], ["$a > $b"],
            [Equation string.])
     
If neither value nor description are given, then value defaults to 1 instead of to the empty string. This is for backwards compatibility with older versions of Autoconf, but this usage is obsolescent and may be withdrawn in future versions of Autoconf.
If the variable is a literal string, it is passed to m4_pattern_allow (see Forbidden Patterns).
If multiple AC_DEFINE statements are executed for the same variable name (not counting any parenthesized argument list), the last one wins.

— Macro: AC_DEFINE_UNQUOTED (variable, value, [description])
— Macro: AC_DEFINE_UNQUOTED (variable)

Like AC_DEFINE, but three shell expansions are performed—once—on variable and value: variable expansion (`$'), command substitution (``'), and backslash escaping (`\'). Single and double quote characters in the value have no special meaning. Use this macro instead of AC_DEFINE when variable or value is a shell variable. Examples:
          AC_DEFINE_UNQUOTED([config_machfile], ["$machfile"],
            [Configuration machine file.])
          AC_DEFINE_UNQUOTED([GETGROUPS_T], [$ac_cv_type_getgroups],
            [getgroups return type.])
          AC_DEFINE_UNQUOTED([$ac_tr_hdr], [1],
            [Translated header name.])
     

Due to a syntactical bizarreness of the Bourne shell, do not use semicolons to separate AC_DEFINE or AC_DEFINE_UNQUOTED calls from other macro calls or shell code; that can cause syntax errors in the resulting configure script. Use either blanks or newlines. That is, do this:

7.2 Setting Output Variables

Another way to record the results of tests is to set output variables, which are shell variables whose values are substituted into files that configure outputs. The two macros below create new output variables. See Preset Output Variables, for a list of output variables that are always available.

— Macro: AC_SUBST (variable, [value])

Create an output variable from a shell variable. Make AC_OUTPUT substitute the variable variable into output files (typically one or more makefiles). This means that AC_OUTPUT replaces instances of `@variable@' in input files with the value that the shell variable variable has when AC_OUTPUT is called. The value can contain any non-NUL character, including newline. Variable occurrences should not overlap: e.g., an input file should not contain `@var1@var2@' if var1 and var2 are variable names. The substituted value is not rescanned for more output variables; occurrences of `@variable@' in the value are inserted literally into the output file. (The algorithm uses the special marker |#_!!_#| internally, so neither the substituted value nor the output file may contain |#_!!_#|.)
If value is given, in addition assign it to variable.
The string variable is passed to m4_pattern_allow (see Forbidden Patterns).

— Macro: AC_SUBST_FILE (variable)

Another way to create an output variable from a shell variable. Make AC_OUTPUT insert (without substitutions) the contents of the file named by shell variable variable into output files. This means that AC_OUTPUT replaces instances of `@variable@' in output files (such as Makefile.in) with the contents of the file that the shell variable variable names when AC_OUTPUT is called. Set the variable to /dev/null for cases that do not have a file to insert. This substitution occurs only when the `@variable@' is on a line by itself, optionally surrounded by spaces and tabs. The substitution replaces the whole line, including the spaces, tabs, and the terminating newline.
This macro is useful for inserting makefile fragments containing special dependencies or other make directives for particular host or target types into makefiles. For example, configure.ac could contain:
          AC_SUBST_FILE([host_frag])
          host_frag=$srcdir/conf/sun4.mh
     
and then a Makefile.in could contain:
          @host_frag@
     
The string variable is passed to m4_pattern_allow (see Forbidden Patterns).

Running configure in varying environments can be extremely dangerous. If for instance the user runs `CC=bizarre-cc ./configure', then the cache, config.h, and many other output files depend upon bizarre-cc being the C compiler. If for some reason the user runs ./configure again, or if it is run via `./config.status --recheck', (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.

— Macro: AC_ARG_VAR (variable, description)

Declare variable is a precious variable, and include its description in the variable section of `./configure --help'.
Being precious means that
variable is substituted via AC_SUBST.
The value of variable when configure was launched is saved in the cache, including if it was not specified on the command line but via the environment. Indeed, while configure can notice the definition of CC in `./configure CC=bizarre-cc', it is impossible to notice it in `CC=bizarre-cc ./configure', which, unfortunately, is what most users do.
We emphasize that it is the initial value of variable which is saved, not that found during the execution of configure. Indeed, specifying `./configure FOO=foo' and letting `./configure' guess that FOO is foo can be two different things.
variable is checked for consistency between two configure runs. For instance:
               $ ./configure --silent --config-cache
               $ CC=cc ./configure --silent --config-cache
               configure: error: `CC' was not set in the previous run
               configure: error: changes in the environment can compromise \
               the build
               configure: error: run `make distclean' and/or \
               `rm config.cache' and start over
          
and similarly if the variable is unset, or if its content is changed. If the content has white space changes only, then the error is degraded to a warning only, but the old value is reused.
variable is kept during automatic reconfiguration (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 var=raboof --silent
               $ ./config.status --recheck
               running CONFIG_SHELL=/bin/sh /bin/sh ./configure var=raboof \
                 CC=/usr/bin/cc  --no-create --no-recursion
          

7.3 Special Characters in Output Variables

Many output variables are intended to be evaluated both by make and by the shell. Some characters are expanded differently in these two contexts, so to avoid confusion these variables' values should not contain any of the following characters:

Also, these variables' values should neither contain newlines, nor start with `~', nor contain white space or `:' immediately followed by `~'. The values can contain nonempty sequences of white space characters like tabs and spaces, but each such sequence might arbitrarily be replaced by a single space during substitution.

These restrictions apply both to the values that configure computes, and to the values set directly by the user. For example, the following invocations of configure are problematic, since they attempt to use special characters within CPPFLAGS and white space within $(srcdir):

`dirent.h`	`HAVE_DIRENT_H`
`sys/ndir.h`	`HAVE_SYS_NDIR_H`
`sys/dir.h`	`HAVE_SYS_DIR_H`
`ndir.h`	`HAVE_NDIR_H`

Autoconf

Table of Contents

Autoconf

1 Introduction

2 The GNU Build System

2.1 Automake

2.2 Gnulib

2.3 Libtool

2.4 Pointers

3 Making configure Scripts

3.1 Writing configure.ac

3.1.1 A Shell Script Compiler

3.1.2 The Autoconf Language

3.1.3 Standard configure.ac Layout

3.2 Using autoscan to Create configure.ac

3.3 Using ifnames to List Conditionals

3.4 Using autoconf to Create configure

3.5 Using autoreconf to Update configure Scripts

4 Initialization and Output Files

4.1 Initializing configure

4.2 Dealing with Autoconf versions

4.3 Notices in configure

4.4 Finding configure Input

4.5 Outputting Files

4.6 Performing Configuration Actions

4.7 Creating Configuration Files

4.8 Substitutions in Makefiles

4.8.1 Preset Output Variables

4.8.2 Installation Directory Variables

4.8.3 Changed Directory Variables

4.8.4 Build Directories

4.8.5 Automatic Remaking

4.9 Configuration Header Files

4.9.1 Configuration Header Templates

4.9.2 Using autoheader to Create config.h.in

4.9.3 Autoheader Macros

4.10 Running Arbitrary Configuration Commands

4.11 Creating Configuration Links

4.12 Configuring Other Packages in Subdirectories

4.13 Default Prefix

5 Existing Tests

5.1 Common Behavior

5.1.1 Standard Symbols

5.1.2 Default Includes

5.2 Alternative Programs

5.2.1 Particular Program Checks

5.2.2 Generic Program and File Checks

5.3 Files

5.4 Library Files

5.5 Library Functions

5.5.1 Portability of C Functions

5.5.2 Particular Function Checks

5.5.3 Generic Function Checks

5.6 Header Files

5.6.1 Portability of Headers

5.6.2 Particular Header Checks

5.6.3 Generic Header Checks

5.7 Declarations

5.7.1 Particular Declaration Checks

5.7.2 Generic Declaration Checks

5.8 Structures

5.8.1 Particular Structure Checks

5.8.2 Generic Structure Checks

5.9 Types

5.9.1 Particular Type Checks

5.9.2 Generic Type Checks

5.10 Compilers and Preprocessors

5.10.1 Specific Compiler Characteristics

5.10.2 Generic Compiler Characteristics

5.10.3 C Compiler Characteristics

5.10.4 C++ Compiler Characteristics

5.10.5 Objective C Compiler Characteristics

5.10.6 Erlang Compiler and Interpreter Characteristics

5.10.7 Fortran Compiler Characteristics

5.11 System Services

5.12 Posix Variants

5.13 Erlang Libraries

6 Writing Tests

6.1 Language Choice

6.2 Writing Test Programs

3 Making `configure` Scripts

3.1 Writing `configure.ac`

3.1.3 Standard `configure.ac` Layout

3.2 Using `autoscan` to Create `configure.ac`

3.3 Using `ifnames` to List Conditionals

3.4 Using `autoconf` to Create `configure`

3.5 Using `autoreconf` to Update `configure` Scripts

4.1 Initializing `configure`

4.3 Notices in `configure`

4.4 Finding `configure` Input

4.9.2 Using `autoheader` to Create `config.h.in`