     [file version.texi not included]

Autoconf: Creating Automatic Configuration Scripts, by David MacKenzie.

This file documents the GNU Autoconf package for creating scripts to configure source code packages using templates and an m4 macro package.

Copyright 1992, 1993, 1994, 1995, 1996, 1998, 1999, 2000, 2001 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies.

Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that this permission notice may be stated in a translation approved by the Foundation.

This file documents the GNU Autoconf package for creating scripts to configure source code packages using templates and the GNU M4 macro package.  This is edition EDITION, for Autoconf version VERSION.

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 UNIX-like systems.  The configuration scripts produced by Autoconf are independent of Autoconf when they are run, so their users do not need to have Autoconf.

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

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

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

Autoconf does not solve all problems related to making portable software packages---for a more complete solution, it should be used in concert with other GNU build tools like Automake and Libtool.  These other tools take on jobs like the creation of a portable, recursive Makefile with all of the standard targets, linking of shared libraries, and so on.  See The GNU build system, for more information.

Autoconf imposes some restrictions on the names of macros used with #if in C programs (see Preprocessor Symbol Index).

Autoconf requires GNU M4 in order to generate the scripts.  It uses features that some UNIX versions of M4, including GNU M4 1.3, do not have.  You must use version 1.4 or later of GNU M4.

See Autoconf 1, for information about upgrading from version 1. See History, for the story of Autoconf's development. See Questions, for answers to some common questions about Autoconf.

See the Autoconf web page (Autoconf web page, http://www.gnu.org/software/autoconf/autoconf.html.) for up-to-date information, details on the mailing lists, pointers to a list of known bugs, etc.

Mail suggestions to the Autoconf mailing list.

Bug reports should be preferably submitted to the Autoconf Gnats database (Autoconf Gnats database, http://sources.redhat.com/cgi-bin/gnatsweb.pl?database=autoconf.), or sent to the Autoconf Bugs mailing list.  If possible, first check that your bug is not already solved in current development versions, and that it has not been reported yet.  Be sure to include all the needed information and a short configure.ac that demonstrates the problem.

Autoconf's development tree is accessible via CVS; see the Autoconf web page for details.  There is also a CVSweb
interface to the Autoconf development tree (CVSweb
interface to the Autoconf development tree, http://subversions.gnu.org/cgi-bin/cvsweb/autoconf/.).  Patches relative to the current CVS version can be sent for review to the Autoconf Patches mailing list.

Because of its mission, Autoconf includes only a set of often-used macros that have already demonstrated their usefulness.  Nevertheless, if you wish to share your macros, or find existing ones, see the Autoconf Macro
Archive (Autoconf Macro
Archive, http://www.gnu.org/software/ac-archive/.), which is kindly run by Peter Simons.

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.

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 make's numerous limitations.  Its lack of support for automatic dependency tracking, recursive builds in subdirectories, reliable timestamps (e.g. for network filesystems), and so on, mean that developers must painfully (and often incorrectly) reinvent the wheel for each project.  Portability is non-trivial, thanks to the quirks of make on many systems.  On top of all this is the manual labor required to implement the many standard targets that users have come to expect (make install, make distclean, make uninstall, etc.).  Since you are, of course, using Autoconf, you also have to insert repetitive code in your Makefile.in to recognize @CC@, @CFLAGS@, and other substitutions provided by configure.  Into this mess steps Automake.

Automake allows you to specify your build needs in a Makefile.am file with a vastly simpler and more powerful syntax than that of a plain Makefile, and then generates a portable Makefile.in for use with Autoconf.  For example, the Makefile.am to build and install a simple ``Hello world'' program might look like:

     bin_PROGRAMS = hello
     hello_SOURCES = hello.c

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

Automake may require that additional tools be present on the developer's machine.  For example, the Makefile.in that the developer works with may not be portable (e.g. it might use special features of your compiler to automatically generate dependency information).  Running make dist, however, produces a hello-1.0.tar.gz package (or whatever the program/version is) with a Makefile.in that will work on any system.

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...

Libtool
=======

Very often, one wants to build not only programs, but libraries, so that other programs can benefit from the fruits of your labor.  Ideally, one would like to produce shared (dynamically-linked) libraries, which can be used by multiple programs without duplication on disk or in memory and can be updated independently of the linked programs. Producing shared libraries portably, however, is the stuff of nightmares---each system has its own incompatible tools, compiler flags, and magic incantations.  Fortunately, GNU provides a solution: Libtool.

Libtool handles all the requirements of building shared libraries for you, and at this time seems to be the only way to do so with any portability.  It also handles many other headaches, such as: the interaction of Makefile rules with the variable suffixes of shared libraries, linking reliably to shared libraries before they are installed by the superuser, and supplying a consistent versioning system (so that different versions of a library can be installed or upgraded without breaking binary compatibility).  Although Libtool, like Autoconf, can be used on its own, it is most simply utilized in conjunction with Automake---there, Libtool is used automatically whenever shared libraries are needed, and you need not know its syntax.

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 will otherwise quickly find yourself putting lots of effort into reinventing the services that the GNU build tools provide, and making the same mistakes that they once made and overcame.  (Besides, since you're already learning Autoconf, Automake will be a piece of cake.)

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

  - Web

     The home pages for Autoconf (Autoconf, http://www.gnu.org/software/autoconf/.), Automake (Automake, http://www.gnu.org/software/automake/.), and Libtool (Libtool, http://www.gnu.org/software/libtool/.).

  - Automake Manual

     See Automake (GNU Automake), for more information on Automake.

  - Books

     The book GNU Autoconf, Automake and Libtool (GNU Autoconf, Automake and Libtool, by G. V. Vaughan, B. Elliston, T. Tromey, and I. L. Taylor. New Riders, 2000, ISBN 1578701902.) describes the complete GNU build environment.  You can also find the entire book on-line at ``The
     Goat Book'' home page (``The
     Goat Book'' home page, http://sources.redhat.com/autobook/.).

  - Tutorials and Examples

     The Autoconf Developer Page (Autoconf Developer Page, http://sources.redhat.com/autoconf/.) maintains links to a number of Autoconf/Automake tutorials online, and also links to the Autoconf Macro Archive (Autoconf Macro Archive, http://www.gnu.org/software/ac-archive/.).

Making configure Scripts
************************

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

  - one or more Makefile files, one in each subdirectory of the package (see Makefile Substitutions);

  - optionally, a C header file, the name of which is configurable, containing #define directives (see Configuration Headers);

  - a shell script called config.status that, when run, will recreate the files listed above (see config.status Invocation);

  - an optional shell script normally called config.cache (created when using configure --config-cache) that saves the results of running many of the tests (see Cache Files);

  - a file called config.log containing any messages produced by compilers, to help debugging if configure makes a mistake.

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

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

Files used in preparing a software package for distribution:

     your source files --> [autoscan*] --> [configure.scan] --> configure.ac

     configure.ac --.
                    |   .------> autoconf* -----> configure
     [aclocal.m4] --+---+
                    |   `-----> [autoheader*] --> [config.h.in]
     [acsite.m4] ---'

     Makefile.in -------------------------------> Makefile.in

Files used in configuring a software package:

                            .-------------> [config.cache]
     configure* ------------+-------------> config.log
                            |
     [config.h.in] -.       v            .-> [config.h] -.
                    +--> config.status* -+               +--> make*
     Makefile.in ---'                    `-> Makefile ---'

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 produce 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.

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.

So, what is really needed is some kind of compiler, autoconf, that takes an Autoconf program, configure.ac, and transforms it into a portable shell script, configure.

How does autoconf perform this task?

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

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

The Autoconf Language
---------------------

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

When calling macros that take arguments, there must not be any blank space between the macro name and the open parenthesis.  Arguments should be enclosed within the M4 quote characters [ and ], and be separated by commas.  Any leading spaces in arguments are ignored, unless they are quoted.  You may safely leave out the quotes when the argument is simple text, but always quote complex arguments such as other macro calls.  This rule applies recursively for every macro call, including macros called from other macros.

For instance:

     AC_CHECK_HEADER([stdio.h],
                     [AC_DEFINE([HAVE_STDIO_H])],
                     [AC_MSG_ERROR([Sorry, can't do anything for you])])

is quoted properly.  You may safely simplify its quotation to:

     AC_CHECK_HEADER(stdio.h,
                     [AC_DEFINE(HAVE_STDIO_H)],
                     [AC_MSG_ERROR([Sorry, can't do anything for you])])

Notice that the argument of AC_MSG_ERROR is still quoted; otherwise, its comma would have been interpreted as an argument separator.

The following example is wrong and dangerous, as it is underquoted:

     AC_CHECK_HEADER(stdio.h,
                     AC_DEFINE(HAVE_STDIO_H),
                     AC_MSG_ERROR([Sorry, can't do anything for you]))

In other cases, you may have to use text that also resembles a macro call.  You must quote that text even when it is not passed as a macro argument:

     echo "Hard rock was here!  --[AC_DC]"

which will result in

     echo "Hard rock was here!  --AC_DC"

When you use the same text in a macro argument, you must therefore have an extra quotation level (since one is stripped away by the macro substitution).  In general, then, it is a good idea to use double quoting for all literal string arguments:

     AC_MSG_WARN([[AC_DC stinks  --Iron Maiden]])

You are now able to understand one of the constructs of Autoconf that has been continually misunderstood...  The rule of thumb is that whenever you expect macro expansion, expect quote expansion; i.e., expect one level of quotes to be lost.  For instance:

     AC_COMPILE_IFELSE([char b[10];],, [AC_MSG_ERROR([you lose])])

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

     AC_COMPILE_IFELSE([[char b[10];]],, [AC_MSG_ERROR([you lose])])

Voilà, you actually produce char b[10]; this time!

The careful reader will notice that, according to these guidelines, the ``properly'' quoted AC_CHECK_HEADER example above is actually lacking three pairs of quotes!  Nevertheless, for the sake of readability, double quotation of literals is used only where needed in this manual.

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

     AC_CHECK_HEADERS(stdio.h, [], [], [])
     AC_CHECK_HEADERS(stdio.h,,,)
     AC_CHECK_HEADERS(stdio.h)

It is best to put each macro call on its own line in configure.ac.  Most of the macros don't add extra newlines; they rely on the newline after the macro call to terminate the commands. This approach makes the generated configure script a little easier to read by not inserting lots of blank lines.  It is generally safe to set shell variables on the same line as a macro call, because the shell allows assignments without intervening newlines.

You can include comments in configure.ac files by starting them with the #.  For example, it is helpful to begin configure.ac files with a line like this:

     # Process this file with autoconf to produce a configure script.

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.

     Autoconf requirements
     AC_INIT(PACKAGE, VERSION, BUG-REPORT-ADDRESS)
     information on the package
     checks for programs
     checks for libraries
     checks for header files
     checks for types
     checks for structures
     checks for compiler characteristics
     checks for library functions
     checks for system services
     AC_CONFIG_FILES([FILE...])
     AC_OUTPUT

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 will probably need some adjustments. Occasionally, autoscan outputs a macro in the wrong order relative to another macro, so that autoconf produces a warning; you need to move such macros manually.  Also, if you want the package to use a configuration header file, you must add a call to AC_CONFIG_HEADERS (see Configuration Headers).  You might also have to change or add some #if directives to your program in order to make it work with Autoconf (see ifnames Invocation, for information about a program that can help with that job).

When using autoscan to maintain a configure.ac, simply consider adding its suggestions.  The file autoscan.log will contain detailed information on why a macro is requested.

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

autoscan is only installed if you already have Perl installed. autoscan accepts the following options:

--help
-h
     Print a summary of the command line options and exit.

--version
-V
     Print the version number of Autoconf and exit.

--verbose
-v
     Print the names of the files it examines and the potentially interesting symbols it finds in them.  This output can be voluminous.

--autoconf-dir=DIR
-A DIR
     Override the location where the installed Autoconf data files are looked for.  You can also set the AC_MACRODIR environment variable to a directory; this option overrides the environment variable.

     This option is rarely needed and dangerous; it is only used when one plays with different versions of Autoconf simultaneously.

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.

ifnames accepts the following options:

--help
-h
     Print a summary of the command line options and exit.

--version
-V
     Print the version number of Autoconf and exit.

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.

autoconf accepts the following options:

--help
-h
     Print a summary of the command line options and exit.

--version
-V
     Print the version number of Autoconf and exit.

--verbose
-v
     Report processing steps.

--debug
-d
     Don't remove the temporary files.

--autoconf-dir=DIR
-A DIR
     Override the location where the installed Autoconf data files are looked for.  You can also set the AC_MACRODIR environment variable to a directory; this option overrides the environment variable.

     This option is rarely needed and dangerous; it is only used when one plays with different versions of Autoconf simultaneously.

--localdir=DIR
-l DIR
     Look for the package file aclocal.m4 in directory DIR instead of in the current directory.

--output=FILE
-o FILE
     Save output (script or trace) to FILE.  The file - stands for the standard output.

--warnings=CATEGORY
-W CATEGORY
     Report the warnings related to CATEGORY (which can actually be a comma separated list).  See Reporting Messages, macro AC_DIAGNOSE, for a comprehensive list of categories.  Special values include:

     all
          report all the warnings

     none
          report none

     error
          treats warnings as errors

     no-CATEGORY
          disable warnings falling into CATEGORY

     Warnings about syntax are enabled by default, and the environment variable WARNINGS, a comma separated list of categories, is honored. autoconf -W CATEGORY will actually behave as if you had run:

          autoconf --warnings=syntax,$WARNINGS,CATEGORY

     If you want to disable autoconf's defaults and WARNINGS, but (for example) enable the warnings about obsolete constructs, you would use -W none,obsolete.

     autoconf displays a back trace for errors, but not for warnings; if you want them, just pass -W error.  For instance, on this configure.ac:

          AC_DEFUN([INNER],
          [AC_TRY_RUN([true])])

          AC_DEFUN([OUTER],
          [INNER])

          AC_INIT
          OUTER

     you get:

          $ autoconf -Wcross
          configure.ac:8: warning: AC_TRY_RUN called without default \
          to allow cross compiling
          $ autoconf -Wcross,error
          configure.ac:8: error: AC_TRY_RUN called without default \
          to allow cross compiling
          acgeneral.m4:3044: AC_TRY_RUN is expanded from...
          configure.ac:2: INNER is expanded from...
          configure.ac:5: OUTER is expanded from...
          configure.ac:8: the top level

--trace=MACRO[:FORMAT]
-t MACRO[:FORMAT]
     Do not create the configure script, but list the calls to MACRO according to the FORMAT.  Multiple --trace arguments can be used to list several macros.  Multiple --trace arguments for a single macro are not cumulative; instead, you should just make FORMAT as long as needed.

     The FORMAT is a regular string, with newlines if desired, and several special escape codes.  It defaults to $f:$l:$n:$%; see below for details on the FORMAT.

--initialization
-i
     By default, --trace does not trace the initialization of the Autoconf macros (typically the AC_DEFUN definitions).  This results in a noticeable speedup, but can be disabled by this option.

It is often necessary to check the content of a configure.ac file, but parsing it yourself is extremely fragile and error-prone.  It is suggested that you rely upon --trace to scan configure.ac.

The FORMAT of --trace can use the following special escapes:

$$
     The character $.

$f
     The filename from which MACRO is called.

$l
     The line number from which MACRO is called.

$d
     The depth of the MACRO call.  This is an M4 technical detail that you probably don't want to know about.

$n
     The name of the MACRO.

$NUM
     The NUMth argument of the call to MACRO.

$@
$SEP@
${SEPARATOR}@
     All the arguments passed to MACRO, separated by the character SEP or the string SEPARATOR (, by default).  Each argument is quoted, i.e. enclosed in a pair of square brackets.

$*
$SEP*
${SEPARATOR}*
     As above, but the arguments are not quoted.

$%
$SEP%
${SEPARATOR}%
     As above, but the arguments are not quoted, all new line characters in the arguments are smashed, and the default separator is :.

     The escape $% produces single-line trace outputs (unless you put newlines in the separator), while $@ and $* do not.

For instance, to find the list of variables that are substituted, use:

     $ autoconf -t AC_SUBST
     configure.ac:2:AC_SUBST:ECHO_C
     configure.ac:2:AC_SUBST:ECHO_N
     configure.ac:2:AC_SUBST:ECHO_T
     More traces deleted

The example below highlights the difference between $@, $*, and $%.

     $ cat configure.ac
     AC_DEFINE(This, is, [an
     [example]])
     $ autoconf -t 'AC_DEFINE:@: $@
     *: $*
     $: $%'
     @: [This],[is],[an
     [example]]
     *: This,is,an
     [example]
     $: This:is:an [example]

The FORMAT gives you a lot of freedom:

     $ autoconf -t 'AC_SUBST:$$ac_subst{"$1"} = "$f:$l";'
     $ac_subst{"ECHO_C"} = "configure.ac:2";
     $ac_subst{"ECHO_N"} = "configure.ac:2";
     $ac_subst{"ECHO_T"} = "configure.ac:2";
     More traces deleted

A long SEPARATOR can be used to improve the readability of complex structures, and to ease its parsing (for instance when no single character is suitable as a separator)):

     $ autoconf -t 'AM_MISSING_PROG:${|:::::|}*'
     ACLOCAL|:::::|aclocal|:::::|$missing_dir
     AUTOCONF|:::::|autoconf|:::::|$missing_dir
     AUTOMAKE|:::::|automake|:::::|$missing_dir
     More traces deleted

Using autoreconf to Update configure Scripts
============================================

If you have a lot of Autoconf-generated configure scripts, the autoreconf program can save you some work.  It runs autoconf (and autoheader, where appropriate) repeatedly to remake the Autoconf configure scripts and configuration header templates in the directory tree rooted at the current directory.  By default, it only remakes those files that are older than their configure.ac or (if present) aclocal.m4.  Since autoheader does not change the timestamp of its output file if the file wouldn't be changing, this is not necessarily the minimum amount of work.  If you install a new version of Autoconf, you can make autoreconf remake all of the files by giving it the --force option.

If you give autoreconf the --autoconf-dir=DIR or --localdir=DIR options, it passes them down to autoconf and autoheader (with relative paths adjusted properly).

autoreconf does not support having, in the same directory tree, both directories that are parts of a larger package (sharing aclocal.m4 and acconfig.h) and directories that are independent packages (each with their own aclocal.m4 and acconfig.h).  It assumes that they are all part of the same package if you use --localdir, or that each directory is a separate package if you don't use it.  This restriction may be removed in the future.

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

autoreconf accepts the following options:

--help
-h
     Print a summary of the command line options and exit.

--version
-V
     Print the version number of Autoconf and exit.

--verbose
     Print the name of each directory where autoreconf runs autoconf (and autoheader, if appropriate).

--debug
-d
     Don't remove the temporary files.

--force
-f
     Remake even configure scripts and configuration headers that are newer than their input files (configure.ac and, if present, aclocal.m4).

--install
-i
     Copy missing auxiliary files.  This option is similar to the option --add-missing in automake.

--symlink
-s
     Instead of copying missing auxiliary files, install symbolic links.

--localdir=DIR
-l DIR
     Have autoconf and autoheader look for the package files aclocal.m4 and (autoheader only) acconfig.h (but not FILE.top and FILE.bot) in directory DIR instead of in the directory containing each configure.ac.

--autoconf-dir=DIR
-A DIR
     Override the location where the installed Autoconf data files are looked for.  You can also set the AC_MACRODIR environment variable to a directory; this option overrides the environment variable.

     This option is rarely needed and dangerous; it is only used when one plays with different versions of Autoconf simultaneously.

--m4dir=DIR
-M DIR
     Specify location of additional macro files (m4 by default).

Additionally, the following options are recognized and passed to automake:

--cygnus
     Assume program is part of Cygnus-style tree.

--foreign
     Set strictness to foreign.

--gnits
     Set strictness to gnits.

--gnu
     Set strictness to gnu.

--include-deps
     Include generated dependencies in Makefile.in.

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 initialization and the creation of output files.

Notices in configure
====================

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

 -- 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 do not create configure.  For example:

          AC_PREREQ(VERSION)

     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_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 will show 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

Finding configure Input
=======================

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-address])
     Process any command-line arguments and perform various initializations and verifications.  Set the name of the PACKAGE and its VERSION.  The optional argument BUG-REPORT-ADDRESS should be the email to which users should send bug reports.

 -- 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.

Outputting Files
================

Every Autoconf-generated configure script must finish by calling AC_OUTPUT.  It is the macro that generates config.status, which will create the Makefiles and any other files resulting from configuration.  The only other required macro is 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 will take 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.

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 on 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 make predefines the variable MAKE, define output variable SET_MAKE to be empty.  Otherwise, define SET_MAKE to contain MAKE=make.  Calls AC_SUBST for SET_MAKE.

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

     @SET_MAKE@

Taking Configuration Actions
============================

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

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

     AC_CONFIG_FOOS(TAG..., [COMMANDS], [INIT-CMDS])

where the arguments are:

TAG...
     A whitespace-separated list of tags, which are typically the names of the files to instantiate.

commands
     Shell commands output literally into config.status, and associated with a tag that the user can use to tell config.status which the commands to run.  The commands are run each time a TAG request is given to config.status; typically, each time the file TAG is created.

init-cmds
     Shell commands output unquoted near the beginning of config.status, and executed each time config.status runs (regardless of the tag).  Because they are unquoted, for example, $var will be output as the value of var.  INIT-CMDS is typically used by configure to give config.status some variables it needs to run the COMMANDS.

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

You are encouraged to use literals as TAGS.  In particular, you should avoid

     ... && my_foos="$my_foos fooo"
     ... && my_foos="$my_foos foooo"
     AC_CONFIG_FOOS($my_foos)

and use this instead:

     ... && AC_CONFIG_FOOS(fooo)
     ... && AC_CONFIG_FOOS(foooo)

The macro AC_CONFIG_FILES and AC_CONFIG_HEADERS use specials TAGs: they may have the form OUTPUT or OUTPUT:INPUTS. The file OUTPUT is instantiated from its templates, INPUTS if specified, defaulting to OUTPUT.in.

For instance AC_CONFIG_FILES(Makefile:boiler/top.mk:boiler/bot.mk) asks for the creation of Makefile that will be the expansion of the output variables in the concatenation of boiler/top.mk and boiler/bot.mk.

The special value - might be used to denote the standard output when used in OUTPUT, or the standard input when used in the INPUTS.  You most probably don't need to use this in configure.ac, but it is convenient when using the command line interface of ./config.status, see config.status Invocation, for more details.

The INPUTS may be absolute or relative filenames.  In the latter case they are first looked for in the build tree, and then in the source tree.

Creating Configuration Files
============================

Be sure to read the previous section, Configuration Actions.

 -- Macro: AC_CONFIG_FILES (FILE..., [cmds], [init-cmds])
     Make AC_OUTPUT create each FILE by copying an input file (by default FILE.in), substituting the output variable values. This macro is one of the instantiating macros, see 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 MS-DOS, or to prepend and/or append boilerplate to the file.

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 will create a Makefile in that directory. To create a Makefile, configure performs a simple variable substitution, replacing occurrences of @VARIABLE@ in Makefile.in with the value that configure has determined for that variable.  Variables that are substituted into output files in this way are called output variables.  They are ordinary shell variables that are set in configure.  To make configure substitute a particular variable into the output files, the macro AC_SUBST must be called with that variable name as an argument. Any occurrences of @VARIABLE@ for other variables are left unchanged.  See Setting Output Variables, for more information on creating output variables with AC_SUBST.

A software package that uses a configure script should be distributed with a file Makefile.in, but no Makefile; that way, the user has to properly configure the package for the local system before compiling it.

See Makefile Conventions (The GNU Coding Standards), for more information on what to put in Makefiles.

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 programs to test for C features.

 -- 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
     Header file search directory (-IDIR) and any other miscellaneous options for the C and C++ preprocessors and compilers.  If it is not set in the environment when configure runs, the default value is empty.  configure uses this variable when compiling or preprocessing programs to test for C and C++ features.

 -- Variable: CXXFLAGS
     Debugging and optimization options for the C++ compiler.  If it is not set in the environment when configure runs, the default value is set when you call AC_PROG_CXX (or empty if you don't). configure uses this variable when compiling programs to test for C++ features.

 -- 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: 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 programs to test for Fortran 77 features.

 -- Variable: LDFLAGS
     Stripping (-s), path (-L), and any other miscellaneous options for the linker.  Don't use this variable to pass library names (-l) to the linker, use LIBS instead.  If it is not set in the environment when configure runs, the default value is empty. configure uses this variable when linking programs to test for C, C++ and Fortran 77 features.

 -- 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 77 features.

 -- Variable: srcdir
     The directory that contains the source code for that Makefile.

 -- Variable: top_srcdir
     The top-level source code directory for the package.  In the top-level directory, this is the same as srcdir.

Installation Directory Variables
--------------------------------

The following variables specify the directories where the package will be installed, see Variables for Installation Directories (The GNU Coding Standards), for more information. See the end of this section for details on when and how to use these variables.

 -- Variable: bindir
     The directory for installing executables that users run.

 -- Variable: datadir
     The directory for installing read-only architecture-independent data.

 -- 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: 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: 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: 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: 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 on purpose that the directory output variables keep them unexpanded: typically @datadir@ will be replaced by ${prefix}/share, not /usr/local/share.

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

make
     she can still specify a different prefix from the one specified to configure, in which case, if needed, the package shall hard code dependencies to her late desires.

make install
     she can specify a different installation location, in which case the package must still depend on the location which was compiled in (i.e., never recompile when make install is run).  This is an extremely important feature, as many people may decide to install all the files of a package grouped together, and then install links from the final locations to there.

In order to support these features, it is essential that datadir remains being defined as ${prefix}/share to depend upon the current value of prefix.

A corollary is that you should not use these variables but in Makefiles. For instance, instead of trying to evaluate datadir in configure and hardcoding it in Makefiles using e.g. AC_DEFINE_UNQUOTED(DATADIR, "$datadir"), you should add -DDATADIR="$(datadir)" to your CPPFLAGS.

Similarly you should not rely on AC_OUTPUT_FILES to replace datadir and friends in your shell scripts and other files, rather let make manage their replacement.  For instance Autoconf ships templates of its shell scripts ending with .sh, and uses this Makefile snippet:

     .sh:
             rm -f $@ $@.tmp
             sed 's,@datadir\@,$(pkgdatadir),g' $< >$@.tmp
             chmod +x $@.tmp
             mv $@.tmp $@

Three things are noteworthy:

@datadir\@
     The backslash prevents configure from replacing @datadir@ in the sed expression itself.

$(pkgdatadir)
     Don't use @pkgdatadir@!  Use the matching makefile variable instead.

,
     Don't use / in the sed expression(s) since most probably the variables you use, such as $(pkgdatadir), will contain some.

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 and most other recent make programs can do this.  Older make programs do not support VPATH; when using them, the source code must be in the same directory as the object files.

To support VPATH, each Makefile.in should contain two lines that look like:

     srcdir = @srcdir@
     VPATH = @srcdir@

Do not set VPATH to the value of another variable, for example VPATH = $(srcdir), because some versions of make do not do variable substitutions on the value of VPATH.

configure substitutes in the correct value for srcdir when it produces Makefile.

Do not use the make variable $<, which expands to the file name of the file in the source directory (found with VPATH), except in implicit rules.  (An implicit rule is one such as .c.o, which tells how to create a .o file from a .c file.)  Some versions of make do not set $< in explicit rules; they expand it to an empty value.

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

     time.info: time.texinfo
             $(MAKEINFO) $(srcdir)/time.texinfo

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 will not be changed if remaking them does not change their contents.  This feature avoids unnecessary recompilation.  You should include the file stamp-h.in your package's distribution, so make will consider config.h.in up to date.  Don't use touch (see Limitations of Usual Tools), rather use echo (using date would cause needless differences, hence CVS conflicts etc.).

     $(srcdir)/configure: configure.ac aclocal.m4
             cd $(srcdir) && autoconf

     # autoheader might not change config.h.in, so touch a stamp file.
     $(srcdir)/config.h.in: stamp-h.in
     $(srcdir)/stamp-h.in: configure.ac aclocal.m4
             cd $(srcdir) && autoheader
             echo timestamp > $(srcdir)/stamp-h.in

     config.h: stamp-h
     stamp-h: config.h.in config.status
             ./config.status

     Makefile: Makefile.in config.status
             ./config.status

     config.status: configure
             ./config.status --recheck

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

In addition, you should use AC_CONFIG_FILES(stamp-h, echo timestamp > stamp-h) so config.status will ensure that config.h is considered up to date.  See Output, for more information about AC_OUTPUT.

See config.status Invocation, for more examples of handling configuration-related dependencies.

Configuration Header Files
==========================

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

The package should #include the configuration header file before any other header files, to prevent inconsistencies in declarations (for example, if it redefines const).  Use #include <config.h> instead of #include "config.h", and pass the C compiler a -I. option (or -I..; whichever directory contains config.h).  That way, even if the source directory is configured itself (perhaps to make a distribution), other build directories can also be configured without finding the config.h from the source directory.

 -- 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 whitespace-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 some changes in configuration without needlessly causing object files that depend on the header file to be recompiled.

     Usually the input file is named HEADER.in; however, you can override the input file name by appending to HEADER, a colon-separated list of input files.  Examples:

          AC_CONFIG_HEADERS(config.h:config.hin)
          AC_CONFIG_HEADERS(defines.h:defs.pre:defines.h.in:defs.post)

     Doing this allows you to keep your file names acceptable to MS-DOS, or to prepend and/or append boilerplate to the file.

See Configuration Actions, for more details on HEADER.

Configuration Header Templates
------------------------------

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

     AC_CONFIG_HEADERS(conf.h)
     AC_CHECK_HEADERS(unistd.h)

Then you could have code like the following in conf.h.in.  On systems that have unistd.h, configure will #define HAVE_UNISTD_H to 1.  On other systems, the whole line will be commented out (in case the system predefines that symbol).

     /* Define as 1 if you have unistd.h.  */
     #undef HAVE_UNISTD_H

You can then decode the configuration header using the preprocessor directives:

     #include <conf.h>

     #if HAVE_UNISTD_H
     # include <unistd.h>
     #else
     /* We are in trouble. */
     #endif

The use of old form templates, with #define instead of #undef is strongly discouraged.

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

Using autoheader to Create config.h.in
--------------------------------------

The autoheader program can create a template file of C #define statements for configure to use.  If configure.ac invokes AC_CONFIG_HEADERS(FILE), autoheader creates FILE.in; if multiple file arguments are given, the first one is used.  Otherwise, autoheader creates config.h.in.

In order to do its job, autoheader needs you to document all of the symbols that you might use; i.e., there must be at least one AC_DEFINE or one AC_DEFINE_UNQUOTED using its third argument for each symbol (see Defining Symbols).  An additional constraint is that the first argument of AC_DEFINE must be a literal.  Note that all symbols defined by Autoconf's built-in tests are already documented properly; you only need to document those that you define yourself.

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

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

But let's come back to the point: autoheader's invocation...

If you give autoheader an argument, it uses that file instead of configure.ac and writes the header file to the standard output instead of to config.h.in.  If you give autoheader an argument of -, it reads the standard input instead of configure.ac and writes the header file to the standard output.

autoheader accepts the following options:

--help
-h
     Print a summary of the command line options and exit.

--version
-V
     Print the version number of Autoconf and exit.

--debug
-d
     Don't remove the temporary files.

--verbose
-v
     Report processing steps.

--autoconf-dir=DIR
-A DIR
     Override the location where the installed Autoconf data files are looked for.  You can also set the AC_MACRODIR environment variable to a directory; this option overrides the environment variable.

     This option is rarely needed and dangerous; it is only used when one plays with different versions of Autoconf simultaneously.

--localdir=DIR
-l DIR
     Look for the package files aclocal.m4 and acconfig.h (but not FILE.top and FILE.bot) in directory DIR instead of in the current directory.

--warnings=CATEGORY
-W CATEGORY
     Report the warnings related to CATEGORY (which can actually be a comma separated list). Current categories include:

     obsolete
          report the uses of obsolete constructs

     all
          report all the warnings

     none
          report none

     error
          treats warnings as errors

     no-CATEGORY
          disable warnings falling into CATEGORY

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 simplest way to create a template for a SYMBOL is to supply the DESCRIPTION argument to an AC_DEFINE(SYMBOL); see Defining Symbols.  You may also use one of the following macros.

 -- 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 the symbol that can be AC_DEFINE'd.

     For example:

          AH_VERBATIM([_GNU_SOURCE],
          [/* Enable GNU extensions on systems that have them.  */
          #ifndef _GNU_SOURCE
          # define _GNU_SOURCE
          #endif])

 -- 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.])

     will generate the following template, with the description properly justified.

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

 -- 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.

Running Arbitrary Configuration Commands
========================================

You execute arbitrary commands either 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 to the TAG.  Since typically the CMDS create a file, TAG should naturally be the name of that file.  This macro is one of the instantiating macros, see Configuration Actions.

     Here is an unrealistic example:

          fubar=42
          AC_CONFIG_COMMANDS(fubar,
                             [echo this is extra $fubar, and so on.],
                             [fubar=$fubar])

     Here is a better one:

          AC_CONFIG_COMMANDS(time-stamp, [date >time-stamp])

 -- Macro: AC_CONFIG_COMMANDS_PRE (CMDS)
     Execute the CMDS right before creating config.status.  A typical use is computing values derived from variables built during the execution of configure:

          AC_CONFIG_COMMANDS_PRE(
          [LTLIBOBJS=`echo $LIBOBJS | sed 's/\.o/\.lo/g'`
          AC_SUBST(LTLIBOBJS)])

 -- Macro: AC_CONFIG_COMMANDS_POST (CMDS)
     Execute the CMDS right after creating config.status.

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 another directory than its sources.

 -- 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.  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.

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 whitespace-separated list.  Each DIR should be a literal, i.e., please do not use:

          if test "$package_foo_enabled" = yes; then
            $my_subdirs="$my_subdirs foo"
          fi
          AC_CONFIG_SUBDIRS($my_subdirs)

     because this prevents ./configure --help=recursive from displaying the options of the package foo.  Rather, you should write:

          if test "$package_foo_enabled" = yes then;
            AC_CONFIG_SUBDIRS(foo)
          fi

     If a given DIR is not found, no error is reported, so a configure script can configure whichever parts of a large source tree are present.  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 filesystems. Likewise, if a DIR contains configure.ac 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 (e.g., to adjust a relative path for the cache file or source directory).  This macro also sets the output variable subdirs to the list of directories DIR .... Makefile rules can use this variable to determine which subdirectories to recurse into.  This macro may be called multiple times.

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 besides /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; otherwise leave the prefix specified in Makefile.in unchanged.  For example, if PROGRAM is gcc and the PATH contains /usr/local/gnu/bin/gcc, set the prefix to /usr/local/gnu.

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 C preprocessor symbol NAME to the value 1''.  See Defining Symbols, for how to get those symbol definitions into your program.

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.

Standard Symbols
----------------

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

For instance,

     AC_CHECK_TYPES(struct $Expensive*)

will define the symbol HAVE_STRUCT__EXPENSIVEP if the check succeeds.

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:

     #if TIME_WITH_SYS_TIME
     # include <sys/time.h>
     # include <time.h>
     #else
     # if HAVE_SYS_TIME_H
     #  include <sys/time.h>
     # else
     #  include <time.h>
     # endif
     #endif

Unless you know exactly what you are doing, you should avoid using unconditional includes, and check the existence of the headers you include beforehand (see Header Files).

Most generic macros provide the following default set of includes:

     #include <stdio.h>
     #if HAVE_SYS_TYPES_H
     # include <sys/types.h>
     #endif
     #if HAVE_SYS_STAT_H
     # include <sys/stat.h>
     #endif
     #if STDC_HEADERS
     # include <stdlib.h>
     # include <stddef.h>
     #else
     # if HAVE_STDLIB_H
     #  include <stdlib.h>
     # endif
     #endif
     #if HAVE_STRING_H
     # if !STDC_HEADERS && HAVE_MEMORY_H
     #  include <memory.h>
     # endif
     # include <string.h>
     #endif
     #if HAVE_STRINGS_H
     # include <strings.h>
     #endif
     #if HAVE_INTTYPES_H
     # include <inttypes.h>
     #else
     # if HAVE_STDINT_H
     #  include <stdint.h>
     # endif
     #endif
     #if HAVE_UNISTD_H
     # include <unistd.h>
     #endif

If the default includes are used, then Autoconf will automatically check for the presence of these headers and their compatibility, i.e., you don't need to run AC_HEADERS_STDC, nor check for stdlib.h etc.

These headers are checked for in the same order as they are included. For instance, on some systems string.h and strings.h both exist, but conflict.  Then HAVE_STRING_H will be defined, but HAVE_STRINGS_H won't.

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.

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

 -- Macro: AC_PROG_INSTALL
     Set output variable INSTALL to the path 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.

     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.

     Autoconf comes with a copy of install-sh that you can use.  If you use AC_PROG_INSTALL, you must include either install-sh or install.sh in your distribution, or configure will produce an error message saying it can't find them---even if the system you're on has a 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_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 is a char * instead of a 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 will cause an annoying but benign ``AC_PROG_LEX invoked multiple times'' warning.  Future versions of Automake will fix this issue, meanwhile, just ignore this message.

 -- 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 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_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.

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:

     AC_PATH_PROG(INETD, inetd, /usr/libexec/inetd,
       $PATH:/usr/libexec:/usr/sbin:/usr/etc:etc)

You are strongly encouraged to declare the VARIABLE passed to AC_CHECK_PROG etc. as precious, see Setting Output Variables, AC_ARG_VAR, for more details.

 -- Macro: AC_CHECK_PROG (VARIABLE, PROG-TO-CHECK-FOR, VALUE-IF-FOUND, [value-if-not-found], [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])
     Check for each program in the whitespace-separated list PROGS-TO-CHECK-FOR exists on the PATH.  If it 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_TOOL (VARIABLE, PROG-TO-CHECK-FOR, [value-if-not-found], [path])
     Like AC_CHECK_PROG, but first looks for PROG-TO-CHECK-FOR with a prefix of the host type as determined by AC_CANONICAL_HOST, followed by a dash (see Canonicalizing). For example, if the user runs configure --host=i386-gnu, then this call:

          AC_CHECK_TOOL(RANLIB, ranlib, :)

     sets 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.

 -- Macro: AC_CHECK_TOOLS (VARIABLE, PROGS-TO-CHECK-FOR, [value-if-not-found], [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.

 -- Macro: AC_PATH_PROG (VARIABLE, PROG-TO-CHECK-FOR, [value-if-not-found], [path])
     Like AC_CHECK_PROG, but set VARIABLE to the entire path of PROG-TO-CHECK-FOR if found.

 -- Macro: AC_PATH_PROGS (VARIABLE, PROGS-TO-CHECK-FOR, [value-if-not-found], [path])
     Like AC_CHECK_PROGS, but if any of PROGS-TO-CHECK-FOR are found, set VARIABLE to the entire path of the program found.

 -- Macro: AC_PATH_TOOL (VARIABLE, PROG-TO-CHECK-FOR, [value-if-not-found], [path])
     Like AC_CHECK_TOOL, but set VARIABLE to the entire path of the program if it is found.

Files
=====

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

 -- 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.

Library Files
=============

The following macros check for the presence of certain C, C++ or Fortran 77 library archive files.

 -- Macro: AC_CHECK_LIB (LIBRARY, FUNCTION, [action-if-found], [action-if-not-found], [other-libraries])
     Depending on the current language(see Language Choice), try to ensure that the C, C++, or Fortran 77 function FUNCTION is available by checking whether a test program can be linked with the library LIBRARY to get the function.  LIBRARY is the base name of the library; e.g., to check for -lmp, use mp as the LIBRARY argument.

     ACTION-IF-FOUND is a list of shell commands to run if the link with the library succeeds; ACTION-IF-NOT-FOUND is a list of shell commands to run if the link fails.  If ACTION-IF-FOUND is not specified, the default action will prepend -lLIBRARY to LIBS and define HAVE_LIBLIBRARY (in all capitals). This macro is intended to support building of 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. Some linkers are very 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 will fail to detect that LIBRARY is present, because linking the test program will always fail with unresolved symbols. The OTHER-LIBRARIES argument should be limited to cases where it is desirable to test for one library in the presence of another that is not already in LIBS.

 -- 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_TRY_LINK_FUNC 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 will fail to detect that FUNCTION is present, because linking the test program will always fail with unresolved symbols.

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.

Portability of Classical 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 will always require additions, please help us keeping it as complete as possible

unlink
     The POSIX spec says that unlink causes the given files to be removed only after there are no more open file handles for it.  Not all OS's support this behaviour though.  So even on systems that provide unlink, you cannot portably assume it is OK to call it on files that are open.  For example, on Windows 9x and ME, such a call would fail; on DOS it could even lead to file system corruption, as the file might end up being written to after the OS has removed it.

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 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.

     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.  In some versions of AIX, the declaration of alloca must precede everything else except for comments and preprocessor directives.  The #pragma directive is indented so that pre-ANSI C compilers will ignore it, rather than choke on it.

          /* AIX requires this to be the first thing in the file.  */
          #ifndef __GNUC__
          # if HAVE_ALLOCA_H
          #  include <alloca.h>
          # else
          #  ifdef _AIX
           #pragma alloca
          #  else
          #   ifndef alloca /* predefined by HP cc +Olibcalls */
          char *alloca ();
          #   endif
          #  endif
          # endif
          #endif

 -- 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.

 -- 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 is available and works (unlike the one on Solaris 2.4), define HAVE_FNMATCH.

 -- 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:

          #if !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.

 -- 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.  If the system has the getloadavg function, define HAVE_GETLOADAVG, and set GETLOADAVG_LIBS to any libraries needed to get that function. Also add GETLOADAVG_LIBS to LIBS.

     Otherwise, require an AC_LIBOBJ replacement (getloadavg.c) of getloadavg, and possibly define several other C preprocessor macros and output variables:

      1. Define C_GETLOADAVG.

      2. Define SVR4, DGUX, UMAX, or UMAX4_3 if on those systems.

      3. If nlist.h is found, define NLIST_STRUCT.

      4. If struct nlist has an n_un.n_name member, define HAVE_STRUCT_NLIST_N_UN_N_NAME.  The obsolete symbol NLIST_NAME_UNION is still defined, but do not depend upon it.

      5. Programs may need to be installed setgid (or setuid) 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.

 -- Macro: AC_FUNC_GETMNTENT
     Check for getmntent in the sun, seq, and gen libraries, for Irix 4, PTX, and Unixware, respectively.  Then, if getmntent is available, define HAVE_GETMNTENT.

 -- Macro: AC_FUNC_GETPGRP
     If getpgrp takes no argument (the POSIX.1 version), define GETPGRP_VOID.  Otherwise, it is the BSD version, which takes a process ID as an argument.  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.

 -- 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 and 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 works correctly (malloc (0) returns a valid pointer), define HAVE_MALLOC.

 -- 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.

 -- Macro: AC_FUNC_MKTIME
     If the mktime function is not available, or does not work correctly, require an AC_LIBOBJ replacement for mktime.

 -- Macro: AC_FUNC_MMAP
     If the mmap function exists and works correctly, define HAVE_MMAP.  Only checks 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_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 *.

 -- Macro: AC_FUNC_SETPGRP
     If setpgrp takes no argument (the POSIX.1 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.

 -- 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 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.

 -- Macro: AC_FUNC_SETVBUF_REVERSED
     If setvbuf takes the buffering type as its second argument and the buffer pointer as the third, instead of the other way around, define SETVBUF_REVERSED.

 -- 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_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_STRERROR_R
     If strerror_r is available, define HAVE_STRERROR_R.  If its implementation correctly returns a char *, define HAVE_WORKING_STRERROR_R.  On at least DEC UNIX 4.0[A-D] and HP-UX B.10.20, strerror_r returns int.  Actually, this tests only whether it returns a scalar or an array, but that should be enough. This is used by the common error.c.

 -- Macro: AC_FUNC_STRFTIME
     Check for strftime in the intl library, for SCO UNIX. Then, if strftime is available, define HAVE_STRFTIME.

 -- Macro: AC_FUNC_UTIME_NULL
     If utime(FILE, NULL) sets FILE's timestamp to the present, define HAVE_UTIME_NULL.

 -- 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.)

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 in the whitespace-separated argument list, define HAVE_FUNCTION (in all capitals) if it is available. If ACTION-IF-FOUND is given, it is additional shell code to execute when one of the functions is found.  You can give it a value of break to break out of the loop on the first match.  If ACTION-IF-NOT-FOUND is given, it is executed when one of the functions is not found.

Autoconf follows a philosophy that was formed over the years by those who have struggled for portability: isolate the portability issues in specific files, and then program as if you were in a POSIX environment.  Some functions may be missing or unfixable, and your package must be ready to replace them.

Use the first three of the following macros to specify a function to be replaced, and the last one (AC_REPLACE_FUNCS) to check for and replace the function if needed.

 -- 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 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_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 #if !HAVE_FUNCTION.  If the system has the function, it probably declares it in a header file you should be including, so you shouldn't redeclare it lest your declaration conflict.

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.

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_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:

          #if HAVE_DIRENT_H
          # include <dirent.h>
          # define NAMLEN(dirent) strlen((dirent)->d_name)
          #else
          # define dirent direct
          # define NAMLEN(dirent) (dirent)->d_namlen
          # if HAVE_SYS_NDIR_H
          #  include <sys/ndir.h>
          # endif
          # if HAVE_SYS_DIR_H
          #  include <sys/dir.h>
          # endif
          # if HAVE_NDIR_H
          #  include <ndir.h>
          # endif
          #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.

 -- 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_STAT
     If the macros S_ISDIR, S_ISREG et al. 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.

 -- Macro: AC_HEADER_STDC
     Define STDC_HEADERS if the system has ANSI C header files. Specifically, this macro checks for stdlib.h, stdarg.h, string.h, and float.h; if the system has those, it probably has the rest of the ANSI C header files.  This macro also checks whether string.h 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 ANSI C requires.

     Use STDC_HEADERS instead of __STDC__ to determine whether the system has ANSI-compliant header files (and probably C library functions) because many systems that have GCC do not have ANSI C header files.

     On systems without ANSI C headers, there is so much variation that it is probably easier to declare the functions you use than to figure out exactly what the system header files declare.  Some systems contain a mix of functions ANSI and BSD; some are mostly ANSI but lack memmove; some define the BSD functions as macros in string.h or strings.h; some have only the BSD functions but string.h; some declare the memory functions in memory.h, some in string.h; etc.  It is probably sufficient to check for one string function and one memory function; if the library has the ANSI versions of those then it probably has most of the others.  If you put the following in configure.ac:

          AC_HEADER_STDC
          AC_CHECK_FUNCS(strchr memcpy)

     then, in your code, you can put declarations like this:

          #if STDC_HEADERS
          # include <string.h>
          #else
          # if !HAVE_STRCHR
          #  define strchr index
          #  define strrchr rindex
          # endif
          char *strchr (), *strrchr ();
          # if !HAVE_MEMCPY
          #  define memcpy(d, s, n) bcopy ((s), (d), (n))
          #  define memmove(d, s, n) bcopy ((s), (d), (n))
          # endif
          #endif

     If you use a function like memchr, memset, strtok, or strspn, which have no BSD equivalent, then macros won't suffice; you must provide an implementation of each function.  An easy way to incorporate your implementations only when needed (since the ones in system C libraries may be hand optimized) is to, 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.1, 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.1 compatible, then instead of including it, define the POSIX.1 macros with their usual interpretations.  Here is an example:

          #include <sys/types.h>
          #if HAVE_SYS_WAIT_H
          # include <sys/wait.h>
          #endif
          #ifndef WEXITSTATUS
          # define WEXITSTATUS(stat_val) ((unsigned)(stat_val) >> 8)
          #endif
          #ifndef WIFEXITED
          # define WIFEXITED(stat_val) (((stat_val) & 255) == 0)
          #endif

_POSIX_VERSION is defined when unistd.h is included on POSIX.1 systems.  If there is no unistd.h, it is definitely not a POSIX.1 system.  However, some non-POSIX.1 systems do have unistd.h.

The way to check if the system supports POSIX.1 is:

     #if HAVE_UNISTD_H
     # include <sys/types.h>
     # include <unistd.h>
     #endif

     #ifdef _POSIX_VERSION
     /* Code for POSIX.1 systems.  */
     #endif

 -- Macro: AC_HEADER_TIME
     If a program may include both time.h and sys/time.h, define TIME_WITH_SYS_TIME.  On some older systems, sys/time.h includes time.h, but time.h is not protected against multiple inclusion, so programs should not explicitly include both files.  This macro is useful in programs that use, for example, struct timeval or struct timezone 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).

          #if TIME_WITH_SYS_TIME
          # include <sys/time.h>
          # include <time.h>
          #else
          # if HAVE_SYS_TIME_H
          #  include <sys/time.h>
          # else
          #  include <time.h>
          # endif
          #endif

 -- 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:

          #if HAVE_TERMIOS_H
          # include <termios.h>
          #endif

          #if GWINSZ_IN_SYS_IOCTL
          # include <sys/ioctl.h>
          #endif

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])
     If the system header file HEADER-FILE is usable, 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.

     The meaning of ``usable'' depends upon the content of INCLUDES:

     if INCLUDES is empty
          check whether

               HEADER-FILE

          can be preprocessed without error.

     if INCLUDE is set
          Check whether

               INCLUDES
               #include <HEADER-FILE>

          can be compiled without error.  You may use AC_CHECK_HEADER (and AC_CHECK_HEADERS) to check whether two headers are compatible.

     You may pass any kind of dummy content for INCLUDES, such as a single space, a comment, to check whether HEADER-FILE compiles with success.

 -- Macro: AC_CHECK_HEADERS (HEADER-FILE..., [action-if-found], [action-if-not-found], [includes])
     For each given system header file HEADER-FILE in the whitespace-separated argument list that exists, define HAVE_HEADER-FILE (in all capitals).  If ACTION-IF-FOUND is given, it is additional shell code to execute when one of the header files is found.  You can give it a value of break to break out of the loop on the first match.  If ACTION-IF-NOT-FOUND is given, it is executed when one of the header files is not found.

     Be sure to read the documentation of AC_CHECK_HEADER to understand the influence of INCLUDES.

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_TRY_COMPILE (see Examining Syntax).

Particular Declaration Checks
-----------------------------

The following macros check for certain declarations.

 -- Macro: AC_DECL_SYS_SIGLIST
     Define SYS_SIGLIST_DECLARED if the variable sys_siglist is declared in a system header file, either signal.h or unistd.h.

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])
     If SYMBOL (a function or a variable) is not declared in INCLUDES and a declaration is needed, run the shell commands ACTION-IF-NOT-FOUND, otherwise ACTION-IF-FOUND.  If no INCLUDES are specified, the default includes are used (see Default Includes).

     This macro actually tests whether it is valid to use SYMBOL as an r-value, not if it is really declared, because it is much safer to avoid introducing extra declarations when they are not needed.

 -- Macro: AC_CHECK_DECLS (SYMBOLS, [action-if-found], [action-if-not-found], [includes])
     For each of the SYMBOLS (comma-separated list), define HAVE_DECL_SYMBOL (in all capitals) to 1 if SYMBOL is declared, otherwise to 0.  If ACTION-IF-NOT-FOUND is given, it is additional shell code to execute when one of the function declarations is needed, otherwise ACTION-IF-FOUND is executed.

     This macro uses an m4 list as first argument:

          AC_CHECK_DECLS(strdup)
          AC_CHECK_DECLS([strlen])
          AC_CHECK_DECLS([malloc, realloc, calloc, free])

     Unlike the other AC_CHECK_*S macros, when a SYMBOL is not declared, HAVE_DECL_SYMBOL is defined to 0 instead of leaving HAVE_DECL_SYMBOL undeclared.  When you are sure that the check was performed, use HAVE_DECL_SYMBOL just like any other result of Autoconf:

          #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
          char *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.

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 macro (see Generic Structures) or, for more complex tests, you may use AC_TRY_COMPILE (see Examining Syntax).

Particular Structure Checks
---------------------------

The following macros check for certain structures or structure members.

 -- Macro: AC_STRUCT_ST_BLKSIZE
     If struct stat contains an st_blksize member, define HAVE_STRUCT_STAT_ST_BLKSIZE.  The former name, HAVE_ST_BLKSIZE is to be avoided, as its support will cease in the future.  This macro is obsoleted, and should be replaced by

          AC_CHECK_MEMBERS([struct stat.st_blksize])

 -- 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_ST_RDEV
     If struct stat contains an st_rdev member, define HAVE_STRUCT_STAT_ST_RDEV.  The former name for this macro, HAVE_ST_RDEV, is to be avoided as it will cease to be supported in the future.  Actually, even the new macro is obsolete, and should be replaced by:

          AC_CHECK_MEMBERS([struct stat.st_rdev])

 -- 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.

 -- 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.

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])
     Check whether MEMBER is a member of the aggregate AGGREGATE. If no INCLUDES are specified, the default includes are used (see Default Includes).

          AC_CHECK_MEMBER(struct passwd.pw_gecos,,
                          [AC_MSG_ERROR([We need `passwd.pw_gecos'!])],
                          [#include <pwd.h>])

     You can use this macro for sub-members:

          AC_CHECK_MEMBER(struct top.middle.bot)

 -- Macro: AC_CHECK_MEMBERS (MEMBERS, [action-if-found], [action-if-not-found], [includes])
     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).

     This macro uses m4 lists:

          AC_CHECK_MEMBERS([struct stat.st_rdev, struct stat.st_blksize])

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.

Particular Type Checks
----------------------

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

 -- 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_MODE_T
     Equivalent to AC_CHECK_TYPE(mode_t, int).

 -- Macro: AC_TYPE_OFF_T
     Equivalent to AC_CHECK_TYPE(off_t, long).

 -- Macro: AC_TYPE_PID_T
     Equivalent to AC_CHECK_TYPE(pid_t, int).

 -- Macro: AC_TYPE_SIGNAL
     If signal.h declares signal as returning a pointer to a function returning void, define RETSIGTYPE to be void; otherwise, define it to be int.

     Define signal handlers as returning type RETSIGTYPE:

          RETSIGTYPE
          hup_handler ()
          {
          ...
          }

 -- Macro: AC_TYPE_SIZE_T
     Equivalent to AC_CHECK_TYPE(size_t, unsigned).

 -- Macro: AC_TYPE_UID_T
     If uid_t is not defined, define uid_t to be int and gid_t to be int.

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])
     Check whether TYPE is defined.  It may be a compiler builtin type or defined by the [includes] (see Default Includes).

 -- Macro: AC_CHECK_TYPES (TYPES, [action-if-found], [action-if-not-found], [includes])
     For each TYPE of the TYPES that is defined, define HAVE_TYPE (in all capitals).  If no INCLUDES are specified, the default includes are used (see Default Includes).  If ACTION-IF-FOUND is given, it is additional shell code to execute when one of the types is found.  If ACTION-IF-NOT-FOUND is given, it is executed when one of the types is not found.

     This macro uses m4 lists:

          AC_CHECK_TYPES(ptrdiff_t)
          AC_CHECK_TYPES([unsigned long long, uintmax_t])

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

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 Unix and .exe if Win32 or OS/2.

They also define the output variable OBJEXT based on the output of the compiler, after .c files have been excluded, typically to o if Unix, obj if Win32.

If the compiler being used does not produce executables, they 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.

Generic Compiler Characteristics
--------------------------------

 -- Macro: AC_CHECK_SIZEOF (TYPE, [unused], [includes])
     Define SIZEOF_TYPE (see Standard Symbols) to be the size in bytes of TYPE.  If type is unknown, it gets a size of 0.  If no INCLUDES are specified, the default includes are used (see Default Includes).  If you provide INCLUDE, make sure to include stdio.h which is required for this macro to run.

     This macro now works even when cross-compiling.  The UNUSED argument was used when cross-compiling.

     For example, the call

          AC_CHECK_SIZEOF(int *)

     defines SIZEOF_INT_P to be 8 on DEC Alpha AXP systems.

C Compiler Characteristics
--------------------------

 -- 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 space separated list of C compilers to search for.  This just gives the user an opportunity to specify an alternative search list for the C compiler.  For example, if you didn't like the default order, then you could invoke AC_PROG_CC like this:

          AC_PROG_CC(cl egcs gcc cc)

     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_CC_STDC
     If the C compiler is not in ANSI C mode by default, try to add an option to output variable CC to make it so.  This macro tries various options that select ANSI C on some system or another.  It considers the compiler to be in ANSI C mode if it handles function prototypes correctly.

     If you use this macro, you should check after calling it whether the C compiler has been set to accept ANSI C; if not, the shell variable ac_cv_prog_cc_stdc is set to no.  If you wrote your source code in ANSI C, you can make an un-ANSIfied copy of it by using the program ansi2knr, which comes with Automake.

 -- 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.

     If the current language is C (see Language Choice), many of the specific test macros use the value of CPP indirectly by calling AC_TRY_CPP, AC_CHECK_HEADER, AC_EGREP_HEADER, or AC_EGREP_CPP.

     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.

The following macros check for C compiler or machine architecture features.  To check for characteristics not listed here, use AC_TRY_COMPILE (see Examining Syntax) or AC_TRY_RUN (see Run Time)

 -- Macro: AC_C_BIGENDIAN
     If words are stored with the most significant byte first (like Motorola and SPARC, but not Intel and VAX, CPUs), define WORDS_BIGENDIAN.

 -- Macro: AC_C_CONST
     If the C compiler does not fully support the ANSI C qualifier const, 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 will define it as empty.

     Occasionally installers use a C++ compiler to compile C code, typically because they lack a C compiler.  This causes problems with const, because C and C++ 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.

 -- 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 will define it as empty.

     If the correctness of your program depends on the semantics of volatile, simply defining it to be empty does, in a sense, break your code.  However, given that the compiler does not support volatile, you are at its mercy anyway.  At least your program will compile, when it wouldn't before.

     In general, the volatile keyword is a feature of ANSI C, 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 defined __STDC__.

 -- 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.

 -- Macro: AC_C_LONG_DOUBLE
     If the C compiler supports the long double type, define HAVE_LONG_DOUBLE.  Some C compilers that do not define __STDC__ do support the long double type; some compilers that define __STDC__ do not support long double.

 -- 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

 -- Macro: AC_C_PROTOTYPES
     Check to see if function prototypes are understood by the compiler.  If so, define PROTOTYPES.  In the case the compiler does not handle prototypes, you should use ansi2knr, which comes with the Automake distribution, to unprotoize function definitions.  For function prototypes, you should first define PARAMS:

          #ifndef PARAMS
          # if PROTOTYPES
          #  define PARAMS(protos) protos
          # else /* no PROTOTYPES */
          #  define PARAMS(protos) ()
          # endif /* no PROTOTYPES */
          #endif

     then use it this way:

          size_t my_strlen PARAMS ((const char *));

 -- 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.  Since recent versions of the GNU C compiler fix the header files automatically when installed, this is becoming a less prevalent problem.

C++ Compiler Characteristics
----------------------------

 -- Macro: AC_PROG_CXX ([compiler-search-list])
     Determine a C++ compiler to use.  Check if 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 space separated list of C++ compilers to search for.  This just gives the user an opportunity to specify an alternative search list for the C++ compiler.  For example, if you didn't like the default order, then you could invoke AC_PROG_CXX like this:

          AC_PROG_CXX(cl KCC CC cxx cc++ xlC aCC c++ g++ egcs gcc)

     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 only portable to run CXXCPP on files with a .c, .C, or .cc extension.

     If the current language is C++ (see Language Choice), many of the specific test macros use the value of CXXCPP indirectly by calling AC_TRY_CPP, AC_CHECK_HEADER, AC_EGREP_HEADER, or AC_EGREP_CPP.

     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++.

Fortran 77 Compiler Characteristics
-----------------------------------

 -- 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 space separated list of Fortran 77 compilers to search for.  This just gives the user an opportunity to specify an alternative search list for the Fortran 77 compiler.  For example, if you didn't like the default order, then you could invoke AC_PROG_F77 like this:

          AC_PROG_F77(fl32 f77 fort77 xlf cf77 g77 f90 xlf90)

     If using g77 (the GNU Fortran 77 compiler), then AC_PROG_F77 will 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_F77_C_O
     Test if the Fortran 77 compiler accepts the options -c and -o simultaneously, and define F77_NO_MINUS_C_MINUS_O if it does not.

The following macros check for Fortran 77 compiler characteristics.  To check for characteristics not listed here, use AC_TRY_COMPILE (see Examining Syntax) or AC_TRY_RUN (see Run Time), making sure to first set the current language to Fortran 77 AC_LANG(Fortran 77) (see Language Choice).

 -- Macro: AC_F77_LIBRARY_LDFLAGS
     Determine the linker flags (e.g. -L and -l) for the Fortran 77 intrinsic and run-time libraries that are required to successfully link a Fortran 77 program or shared library.  The output variable FLIBS is set to these flags.

     This macro is intended to be used in those situations when it is necessary to mix, e.g. C++ and Fortran 77 source code into a single program or shared library (see Mixing Fortran 77 With C and C++ (GNU Automake)).

     For example, if object files from a C++ and Fortran 77 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 77 intrinsic and run-time libraries must be linked in as well, but the C++ compiler/linker doesn't know by default how to add these Fortran 77 libraries.  Hence, the macro AC_F77_LIBRARY_LDFLAGS was created to determine these Fortran 77 libraries.

     The macro AC_F77_DUMMY_MAIN or AC_F77_MAIN will probably also be necessary to link C/C++ with Fortran; see below.

 -- Macro: AC_F77_DUMMY_MAIN ([action-if-found], [action-if-not-found])
     With many compilers, the Fortran libraries detected by AC_F77_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 e.g. MAIN__ to run the user's program.  The AC_F77_DUMMY_MAIN or AC_F77_MAIN macro figures out how to deal with this interaction.

     When using Fortran for purely numerical functions (no I/O, etcetera), users often prefer to provide their 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 detects whether any such routine is required for linking, and what its name is; the shell variable F77_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 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

     Note that AC_F77_DUMMY_MAIN is called automatically from AC_F77_WRAPPERS; there is generally no need to call it explicitly unless one wants to change the default actions.

 -- Macro: AC_F77_MAIN
     As discussed above for AC_F77_DUMMY_MAIN, many Fortran libraries allow you to provide an entry point called e.g. 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 macro detects whether it is possible to utilize such an alternate main function, and defines F77_MAIN to the name of the function.  (If no alternate main function name is found, F77_MAIN is simply defined to main.)

     Thus, when calling Fortran routines from C that perform things like I/O, one should use this macro and name the "main" function F77_MAIN instead of main.

 -- Macro: AC_F77_WRAPPERS
     Defines C macros F77_FUNC(name,NAME) and F77_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 77 compiler.

     Fortran 77 is case-insensitive, and in order to achieve this the Fortran 77 compiler converts all identifiers into a canonical case and format. To call a Fortran 77 subroutine from C or to write a C function that is callable from Fortran 77, the C program must explicitly use identifiers in the format expected by the Fortran 77 compiler.  In order to do this, one simply wraps all C identifiers in one of the macros provided by AC_F77_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++ (GNU Automake)).

     Although Autoconf tries to be intelligent about detecting the name-mangling scheme of the Fortran 77 compiler, there may be Fortran 77 compilers that it doesn't support yet.  In this case, the above code will generate a compile-time error, but some other behavior (e.g. disabling Fortran-related features) can be induced by checking whether the F77_FUNC macro 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 77 identifier contains an underscore (e.g. foo_bar), you should use F77_FUNC_ instead of F77_FUNC (with the same arguments).  This is because some Fortran 77 compilers mangle names differently if they contain an underscore.

 -- Macro: AC_F77_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 77 linker (see also AC_F77_WRAPPERS).  SHELLVAR is optional; if it is not supplied, the shell variable will be simply NAME.  The purpose of this macro is to give the caller a way to access the name-mangling information other than through the C preprocessor as above; for example, to call Fortran routines from some language other than C/C++.

System Services
===============

The following macros check for operating system services or capabilities.

 -- Macro: AC_PATH_X
     Try to locate the X Window System include files and libraries.  If the user gave the command line options --x-includes=DIR and --x-libraries=DIR, use those directories.  If either or both were not given, get the missing values by running xmkmf on a trivial Imakefile and examining the Makefile that it produces.  If that fails (such as if xmkmf is not present), look for them 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/csh to select the interpreter to use for the script. After running this macro, shell code in configure.ac can check the shell variable interpval; it will be set to yes if the system supports #!, no if not.

 -- Macro: AC_SYS_LARGEFILE
     Arrange for large-file support (large-file support, http://www.sas.com/standards/large.file/x_open.20Mar96.html.).  On some hosts, one must use special compiler options to build programs that can access large files.  Append any such options to the output variable CC.  Define _FILE_OFFSET_BITS and _LARGE_FILES if necessary.

     Large-file support can be disabled by configuring with the --disable-largefile option.

     If you use this macro, check that your program works even when off_t is longer than long, 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) X).

 -- 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 POSIX termios headers and functions are available on the system.  If so, set the shell variable am_cv_sys_posix_termios to yes.  If not, set the variable to no.

UNIX Variants
=============

The following macros check for certain operating systems that need special treatment for some programs, due to exceptional oddities in their header files or libraries.  These macros are warts; they will be replaced by a more systematic approach, based on the functions they make available or the environments they provide.

 -- Macro: AC_AIX
     If on AIX, define _ALL_SOURCE.  Allows the use of some BSD functions.  Should be called before any macros that run the C compiler.

 -- Macro: AC_ISC_POSIX
     If on a POSIXized ISC UNIX, define _POSIX_SOURCE and add -posix (for the GNU C compiler) or -Xp (for other C compilers) to output variable CC.  This allows the use of POSIX facilities.  Must be called after AC_PROG_CC and before any other macros that run the C compiler.

 -- Macro: AC_MINIX
     If on Minix, define _MINIX and _POSIX_SOURCE and define _POSIX_1_SOURCE to be 2.  This allows the use of POSIX facilities.  Should be called before any macros that run the C compiler.

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 C compiler system.  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 C 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.

Examining Declarations
======================

The macro AC_TRY_CPP is used to check whether particular header files exist.  You can check for one at a time, or more than one if you need several header files to all exist for some purpose.

 -- Macro: AC_TRY_CPP (INCLUDES, [action-if-true], [action-if-false])
     INCLUDES is C or C++ #include statements and declarations, on which shell variable, back quote, and backslash substitutions are performed.  (Actually, it can be any C program, but other statements are probably not useful.)  If the preprocessor produces no error messages while processing it, run shell commands ACTION-IF-TRUE.  Otherwise run shell commands ACTION-IF-FALSE.

     This macro uses CPPFLAGS, but not CFLAGS, because -g, -O, etc. are not valid options to many C preprocessors.

Here is how to find out whether a header file contains a particular declaration, such as a typedef, a structure, a structure member, or a function.  Use AC_EGREP_HEADER instead of running grep directly on the header file; on some systems the symbol might be defined in another header file that the file you are checking #includes.

 -- 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 egrep regular expression PATTERN, execute shell commands ACTION-IF-FOUND, otherwise execute ACTION-IF-NOT-FOUND.

To check for C preprocessor symbols, either defined by header files or predefined by the C preprocessor, use AC_EGREP_CPP.  Here is an example of the latter:

     AC_EGREP_CPP(yes,
     [#ifdef _AIX
       yes
     #endif
     ], is_aix=yes, is_aix=no)

 -- 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 egrep regular expression PATTERN, execute shell commands ACTION-IF-FOUND, otherwise execute ACTION-IF-NOT-FOUND.

     This macro calls AC_PROG_CPP or AC_PROG_CXXCPP (depending on which language is current, see Language Choice), if it hasn't been called already.

Examining Syntax
================

To check for a syntax feature of the C, C++ or Fortran 77 compiler, such as whether it recognizes a certain keyword, use AC_TRY_COMPILE to try to compile a small program that uses that feature.  You can also use it to check for structures and structure members that are not present on all systems.

 -- Macro: AC_TRY_COMPILE (INCLUDES, FUNCTION-BODY, [action-if-found], [action-if-not-found])
     Create a C, C++ or Fortran 77 test program (depending on which language is current, see Language Choice), to see whether a function whose body consists of FUNCTION-BODY can be compiled.

     For C and C++, INCLUDES is any #include statements needed by the code in FUNCTION-BODY (INCLUDES will be ignored if the currently selected language is Fortran 77).  This macro also uses CFLAGS or CXXFLAGS if either C or C++ is the currently selected language, as well as CPPFLAGS, when compiling.  If Fortran 77 is the currently selected language then FFLAGS will be used when compiling.

     If the file compiles successfully, run shell commands ACTION-IF-FOUND, otherwise run ACTION-IF-NOT-FOUND.

     This macro does not try to link; use AC_TRY_LINK if you need to do that (see Examining Libraries).

Examining Libraries
===================

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.

A few systems have linkers that do not return a failure exit status when there are unresolved functions in the link.  This bug makes the configuration scripts produced by Autoconf unusable on those systems. However, some of them can be given options that make the exit status correct.  This is a problem that Autoconf does not currently handle automatically.  If users encounter this problem, they might be able to solve it by setting LDFLAGS in the environment to pass whatever options the linker needs (for example, -Wl,-dn on MIPS RISC/OS).

AC_TRY_LINK 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_TRY_LINK (INCLUDES, FUNCTION-BODY, [action-if-found], [action-if-not-found])
     Depending on the current language (see Language Choice), create a test program to see whether a function whose body consists of FUNCTION-BODY can be compiled and linked.

     For C and C++, INCLUDES is any #include statements needed by the code in FUNCTION-BODY (INCLUDES will be ignored if the currently selected language is Fortran 77).  This macro also uses CFLAGS or CXXFLAGS if either C or C++ is the currently selected language, as well as CPPFLAGS, when compiling.  If Fortran 77 is the currently selected language then FFLAGS will be used when compiling.  However, both LDFLAGS and LIBS will be used during linking in all cases.

     If the file compiles and links successfully, run shell commands ACTION-IF-FOUND, otherwise run ACTION-IF-NOT-FOUND.

 -- Macro: AC_TRY_LINK_FUNC (FUNCTION, [action-if-found], [action-if-not-found])
     Depending on the current language (see Language Choice), create a test program to see whether a program whose body consists of a prototype of and a call to FUNCTION can be compiled and linked.

     If the file compiles and links successfully, run shell commands ACTION-IF-FOUND, otherwise run ACTION-IF-NOT-FOUND.

Checking Run Time Behavior
==========================

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

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

Running Test Programs
---------------------

Use the following macro if you need to test run-time behavior of the system while configuring.

 -- Macro: AC_TRY_RUN (PROGRAM, [action-if-true], [action-if-false], [action-if-cross-compiling])
     PROGRAM is the text of a C program, on which shell variable and back quote substitutions are performed.  If it 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 exit status of the program is available in the shell variable $?.  This macro uses CFLAGS or CXXFLAGS, CPPFLAGS, LDFLAGS, and LIBS when compiling.

     If the C 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.

Try to provide a pessimistic default value to use when cross-compiling makes run-time tests impossible.  You do this by passing the optional last argument to AC_TRY_RUN.  autoconf prints a warning message when creating configure each time it encounters a call to AC_TRY_RUN with no ACTION-IF-CROSS-COMPILING argument given.  You may ignore the warning, though users will not be able to configure your package for cross-compiling.  A few of the macros distributed with Autoconf produce this warning message.

To configure for cross-compiling you can also choose a value for those parameters based on the canonical system name (see Manual Configuration).  Alternatively, set up a test results cache file with the correct values for the host system (see Caching Results).

To provide a default for calls of AC_TRY_RUN that are embedded in other macros, including a few of the ones that come with Autoconf, you can call AC_PROG_CC before running them.  Then, if the shell variable cross_compiling is set to yes, use an alternate method to get the results instead of calling the macros.

Guidelines for Test Programs
----------------------------

Test programs should not write anything to the standard output.  They should return 0 if the test succeeds, nonzero otherwise, so that success can be distinguished easily from a core dump or other failure; segmentation violations and other failures produce a nonzero exit status.  Test programs should exit, not return, from main, because on some systems (old Suns, at least) the argument to return in main is ignored.

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

     #if STDC_HEADERS
     # include <stdlib.h>
     #endif

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

Test Functions
--------------

Function declarations in test programs should have a prototype conditionalized for C++.  In practice, though, test programs rarely need functions that take arguments.

     #ifdef __cplusplus
     foo (int i)
     #else
     foo (i) int i;
     #endif

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

     #ifdef __cplusplus
     extern "C" void *malloc (size_t);
     #else
     char *malloc ();
     #endif

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

If you include any header files, make 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).

Systemology
===========

This section aims at presenting some systems and pointers to documentation.  It may help you addressing particular problems reported by users.

QNX 4.25
     QNX is a realtime operating system running on Intel architecture meant to be scalable from the small embedded systems to hundred processor super-computer.  It claims to be POSIX certified. More information is available on the QNX home page (QNX home page, www.qnx.com.), including the QNX
     man pages (QNX
     man pages, http://support.qnx.com/support/docs/qnx4/.).

Multiple Cases
==============

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

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

     AC_MSG_CHECKING([how to get file system type])
     fstype=no
     # The order of these tests is important.
     AC_TRY_CPP([#include <sys/statvfs.h>
     #include <sys/fstyp.h>],
                [AC_DEFINE(FSTYPE_STATVFS) fstype=SVR4])
     if test $fstype = no; then
       AC_TRY_CPP([#include <sys/statfs.h>
     #include <sys/fstyp.h>],
                  [AC_DEFINE(FSTYPE_USG_STATFS) fstype=SVR3])
     fi
     if test $fstype = no; then
       AC_TRY_CPP([#include <sys/statfs.h>
     #include <sys/vmount.h>],
                  [AC_DEFINE(FSTYPE_AIX_STATFS) fstype=AIX])
     fi
     # (more cases omitted here)
     AC_MSG_RESULT([$fstype])

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.

     C++
          Do compilation tests using CXX and CXXCPP and use extension .C for test programs.

     Fortran 77
          Do compilation tests using F77 and use extension .f for test programs.

 -- 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 will detect inconsistencies.

          AC_LANG_PUSH(Fortran 77)
          # Perform some tests on Fortran 77.
          # ...
          AC_LANG_POP(Fortran 77)

 -- 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.

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.

Defining C Preprocessor Symbols
===============================

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

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

     AC_CHECK_FUNC(vprintf, [AC_DEFINE(HAVE_VPRINTF)])
     if test "$ac_cv_func_vprintf" != yes; then
       AC_CHECK_FUNC(_doprnt, [AC_DEFINE(HAVE_DOPRNT)])
     fi

If AC_CONFIG_HEADERS has been called, then instead of creating DEFS, AC_OUTPUT creates a header file by substituting the correct values into #define statements in a template file. See Configuration Headers, for more information about this kind of output.

 -- Macro: AC_DEFINE (VARIABLE, [value], [description])
     Define C preprocessor variable VARIABLE.  If VALUE is given, set VARIABLE to that value (verbatim), otherwise set it to 1. VALUE should not contain literal newlines, and if you are not using AC_CONFIG_HEADERS it should not contain any # characters, as make tends to eat them.  To use a shell variable (which you need to do in order to define a value containing the M4 quote characters [ or ]), 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")

 -- Macro: AC_DEFINE_UNQUOTED (VARIABLE, [value], [description])
     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")
          AC_DEFINE_UNQUOTED(GETGROUPS_T, $ac_cv_type_getgroups)
          AC_DEFINE_UNQUOTED($ac_tr_hdr)

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

     AC_CHECK_HEADER(elf.h, [AC_DEFINE(SVR4) LIBS="$LIBS -lelf"])

or this:

     AC_CHECK_HEADER(elf.h,
      [AC_DEFINE(SVR4)
       LIBS="$LIBS -lelf"])

instead of this:

     AC_CHECK_HEADER(elf.h, [AC_DEFINE(SVR4); LIBS="$LIBS -lelf"])

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 will replace instances of @VARIABLE@ in input files with the value that the shell variable VARIABLE has when AC_OUTPUT is called.  This value of VARIABLE should not contain literal newlines.

     If VALUE is given, in addition assign it to variable.

 -- 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 will replace 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 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@

Running configure in different environments can be extremely dangerous.  If for instance the user runs CC=bizarre-cc ./configure, then the cache, config.h and many other output files will depend upon bizarre-cc being the C compiler.  If for some reason the user runs /configure again, or if it is run via ./config.status --recheck, (see Automatic Remaking, and see config.status Invocation), then the configuration can be inconsistent, composed of results depending upon two different compilers.

Such variables are named 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 AC_SUBST'd.

       - VARIABLE is kept in the cache including if it was not specified on the ./configure command line.  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.

       - 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.

       - VARIABLE is kept during automatic reconfiguration (see config.status Invocation) as if it had been passed as a command line argument, including when no cache is used:

               $ CC=/usr/bin/cc ./configure undeclared_var=raboof --silent
               $ ./config.status --recheck
               running /bin/sh ./configure undeclared_var=raboof --silent \
                 CC=/usr/bin/cc  --no-create --no-recursion

Caching Results
===============

To avoid checking for the same features repeatedly in various configure scripts (or in repeated runs of one script), configure can optionally save the results of many checks in a cache file (see Cache Files).  If a configure script runs with caching enabled and finds a cache file, it reads the results of previous runs from the cache and avoids rerunning those checks.  As a result, configure can then run much faster than if it had to perform all of the checks every time.

 -- Macro: AC_CACHE_VAL (CACHE-ID, COMMANDS-TO-SET-IT)
     Ensure that the results of the check identified by CACHE-ID are available.  If the results of the check were in the cache file that was read, and configure was not given the --quiet or --silent option, print a message saying that the result was cached; otherwise, run the shell commands COMMANDS-TO-SET-IT.  If the shell commands are run to determine the value, the value will be saved in the cache file just before configure creates its output files.  See Cache Variable Names, for how to choose the name of the CACHE-ID variable.

     The COMMANDS-TO-SET-IT must have no side effects except for setting the variable CACHE-ID, see below.

 -- Macro: AC_CACHE_CHECK (MESSAGE, CACHE-ID, COMMANDS-TO-SET-IT)
     A wrapper for AC_CACHE_VAL that takes care of printing the messages.  This macro provides a convenient shorthand for the most common way to use these macros.  It calls AC_MSG_CHECKING for MESSAGE, then AC_CACHE_VAL with the CACHE-ID and COMMANDS arguments, and AC_MSG_RESULT with CACHE-ID.

     The COMMANDS-TO-SET-IT must have no side effects except for setting the variable CACHE-ID, see below.

It is very common to find buggy macros using AC_CACHE_VAL or AC_CACHE_CHECK, because people are tempted to call AC_DEFINE in the COMMANDS-TO-SET-IT. Instead, the code that follows the call to AC_CACHE_VAL should call AC_DEFINE, by examining the value of the cache variable.  For instance, the following macro is broken:

     AC_DEFUN([AC_SHELL_TRUE],
     [AC_CACHE_CHECK([whether true(1) works], [ac_cv_shell_true_works],
                     [ac_cv_shell_true_works=no
                      true && ac_cv_shell_true_works=yes
                      if test $ac_cv_shell_true_works = yes; then
                        AC_DEFINE([TRUE_WORKS], 1
                                  [Define if `true(1)' works properly.])
                      fi])
     ])

This fails if the cache is enabled: the second time this macro is run, TRUE_WORKS will not be defined.  The proper implementation is:

     AC_DEFUN([AC_SHELL_TRUE],
     [AC_CACHE_CHECK([whether true(1) works], [ac_cv_shell_true_works],
                     [ac_cv_shell_true_works=no
                      true && ac_cv_shell_true_works=yes])
      if test $ac_cv_shell_true_works = yes; then
        AC_DEFINE([TRUE_WORKS], 1
                  [Define if `true(1)' works properly.])
      fi
     ])

Also, COMMANDS-TO-SET-IT should not print any messages, for example with AC_MSG_CHECKING; do that before calling AC_CACHE_VAL, so the messages are printed regardless of whether the results of the check are retrieved from the cache or determined by running the shell commands.

Cache Variable Names
--------------------

The names of cache variables should have the following format:

     PACKAGE-PREFIX_cv_VALUE-TYPE_SPECIFIC-VALUE_[additional-options]

for example, ac_cv_header_stat_broken or ac_cv_prog_gcc_traditional.  The parts of the variable name are:

PACKAGE-PREFIX
     An abbreviation for your package or organization; the same prefix you begin local Autoconf macros with, except lowercase by convention. For cache values used by the distributed Autoconf macros, this value is ac.

_cv_
     Indicates that this shell variable is a cache value. This string must be present in the variable name, including the leading underscore.

VALUE-TYPE
     A convention for classifying cache values, to produce a rational naming system.  The values used in Autoconf are listed in Macro Names.

SPECIFIC-VALUE
     Which member of the class of cache values this test applies to. For example, which function (alloca), program (gcc), or output variable (INSTALL).

ADDITIONAL-OPTIONS
     Any particular behavior of the specific member that this test applies to. For example, broken or set.  This part of the name may be omitted if it does not apply.

The values assigned to cache variables may not contain newlines. Usually, their values will be boolean (yes or no) or the names of files or functions; so this is not an important restriction.

Cache Files
-----------

A cache file is a shell script that caches the results of configure tests run on one system so they can be shared between configure scripts and configure runs.  It is not useful on other systems.  If its contents are invalid for some reason, the user may delete or edit it.

By default, configure uses no cache file (technically, it uses --cache-file=/dev/null), to avoid problems caused by accidental use of stale cache files.

To enable caching, configure accepts --config-cache (or -C) to cache results in the file config.cache. Alternatively, --cache-file=FILE specifies that FILE be the cache file.  The cache file is created if it does not exist already.  When configure calls configure scripts in subdirectories, it uses the --cache-file argument so that they share the same cache.  See Subdirectories, for information on configuring subdirectories with the AC_CONFIG_SUBDIRS macro.

config.status only pays attention to the cache file if it is given the --recheck option, which makes it rerun configure.

It is wrong to try to distribute cache files for particular system types. There is too much room for error in doing that, and too much administrative overhead in maintaining them.  For any features that can't be guessed automatically, use the standard method of the canonical system type and linking files (see Manual Configuration).

The site initialization script can specify a site-wide cache file to use, instead of the usual per-program cache.  In this case, the cache file will gradually accumulate information whenever someone runs a new configure script.  (Running configure merges the new cache results with the existing cache file.)  This may cause problems, however, if the system configuration (e.g. the installed libraries or compilers) changes and the stale cache file is not deleted.

Cache Checkpointing
-------------------

If your configure script, or a macro called from configure.ac, happens to abort the configure process, it may be useful to checkpoint the cache a few times at key points using AC_CACHE_SAVE.  Doing so will reduce the amount of time it takes to re-run the configure script with (hopefully) the error that caused the previous abort corrected.

 -- Macro: AC_CACHE_LOAD
     Loads values from existing cache file, or creates a new cache file if a cache file is not found.  Called automatically from AC_INIT.

 -- Macro: AC_CACHE_SAVE
     Flushes all cached values to the cache file.  Called automatically from AC_OUTPUT, but it can be quite useful to call AC_CACHE_SAVE at key points in configure.ac.

For instance:

      ... AC_INIT, etc. ...

     # Checks for programs.
     AC_PROG_CC
     AC_PROG_GCC_TRADITIONAL
      ... more program checks ...
     AC_CACHE_SAVE

     # Checks for libraries.
     AC_CHECK_LIB(nsl, gethostbyname)
     AC_CHECK_LIB(socket, connect)
      ... more lib checks ...
     AC_CACHE_SAVE

     # Might abort...
     AM_PATH_GTK(1.0.2,, (exit 1); exit)
     AM_PATH_GTKMM(0.9.5,, (exit 1); exit)

      ... AC_OUTPUT, etc. ...

Printing Messages
=================

configure scripts need to give users running them several kinds of information.  The following macros print messages in ways appropriate for each kind.  The arguments to all of them get enclosed in shell double quotes, so the shell performs variable and back-quote substitution on them.

These macros are all wrappers around the echo shell command. configure scripts should rarely need to run echo directly to print messages for the user.  Using these macros makes it easy to change how and when each kind of message is printed; such changes need only be made to the macro definitions and all of the callers will change automatically.

To diagnose static issues, i.e., when autoconf is run, see Reporting Messages.

 -- Macro: AC_MSG_CHECKING (FEATURE-DESCRIPTION)
     Notify the user that configure is checking for a particular feature.  This macro prints a message that starts with checking and ends with ... and no newline.  It must be followed by a call to AC_MSG_RESULT to print the result of the check and the newline.  The FEATURE-DESCRIPTION should be something like whether the Fortran compiler accepts C++ comments or for c89.

     This macro prints nothing if configure is run with the --quiet or --silent option.

 -- Macro: AC_MSG_RESULT (RESULT-DESCRIPTION)
     Notify the user of the results of a check.  RESULT-DESCRIPTION is almost always the value of the cache variable for the check, typically yes, no, or a file name.  This macro should follow a call to AC_MSG_CHECKING, and the RESULT-DESCRIPTION should be the completion of the message printed by the call to AC_MSG_CHECKING.

     This macro prints nothing if configure is run with the --quiet or --silent option.

 -- Macro: AC_MSG_NOTICE (MESSAGE)
     Deliver the MESSAGE to the user. It is useful mainly to print a general description of the overall purpose of a group of feature checks, e.g.,

          AC_MSG_NOTICE([checking if stack overflow is detectable])

     This macro prints nothing if configure is run with the --quiet or --silent option.

 -- Macro: AC_MSG_ERROR (ERROR-DESCRIPTION, [exit-status])
     Notify the user of an error that prevents configure from completing.  This macro prints an error message to the standard error output and exits configure with EXIT-STATUS (1 by default). ERROR-DESCRIPTION should be something like invalid value $HOME for \$HOME.

     The ERROR-DESCRIPTION should start with a lower-case letter, and ``cannot'' is preferred to ``can't''.

 -- Macro: AC_MSG_WARN (PROBLEM-DESCRIPTION)
     Notify the configure user of a possible problem.  This macro prints the message to the standard error output; configure continues running afterward, so macros that call AC_MSG_WARN should provide a default (back-up) behavior for the situations they warn about. PROBLEM-DESCRIPTION should be something like ln -s seems to make hard links.

Programming in M4
*****************

Autoconf is written on top of two layers: M4sugar, which provides convenient macros for pure M4 programming, and M4sh, which provides macros dedicated to shell script generation.

As of this version of Autoconf, these two layers are still experimental, and their interface might change in the future.  As a matter of fact, anything that is not documented must not be used.

M4 Quotation
============

The most common brokenness of existing macros is an improper quotation. This section, which users of Autoconf can skip, but which macro writers must read, first justifies the quotation scheme that was chosen for Autoconf and then ends with a rule of thumb.  Understanding the former helps one to follow the latter.

Active Characters
-----------------

To fully understand where proper quotation is important, you first need to know what are the special characters in Autoconf: # introduces a comment inside which no macro expansion is performed, , separates arguments, [ and ] are the quotes themselves, and finally ( and ) (which m4 tries to match by pairs).

In order to understand the delicate case of macro calls, we first have to present some obvious failures.  Below they are ``obvious-ified'', although you find them in real life, they are usually in disguise.

Comments, introduced by a hash and running up to the newline, are opaque tokens to the top level: active characters are turned off, and there is no macro expansion:

     # define([def], ine)
     =># define([def], ine)

Each time there can be a macro expansion, there is a quotation expansion; i.e., one level of quotes is stripped:

     int tab[10];
     =>int tab10;
     [int tab[10];]
     =>int tab[10];

Without this in mind, the reader will try hopelessly to use her macro array:

     define([array], [int tab[10];])
     array
     =>int tab10;
     [array]
     =>array

How can you correctly output the intended results (Using defn.)?

One Macro Call
--------------

Let's proceed on the interaction between active characters and macros with this small macro, which just returns its first argument:

     define([car], [$1])

The two pairs of quotes above are not part of the arguments of define; rather, they are understood by the top level when it tries to find the arguments of define.  Therefore, it is equivalent to write:

     define(car, $1)

But, while it is acceptable for a configure.ac to avoid unneeded quotes, it is bad practice for Autoconf macros which must both be more robust and also advocate perfect style.

At the top level, there are only two possible quotings: either you quote or you don't:

     car(foo, bar, baz)
     =>foo
     [car(foo, bar, baz)]
     =>car(foo, bar, baz)

Let's pay attention to the special characters:

     car(#)
     error-->EOF in argument list

The closing parenthesis is hidden in the comment; with a hypothetical quoting, the top level understood it this way:

     car([#)]

Proper quotation, of course, fixes the problem:

     car([#])
     =>#

The reader will easily understand the following examples:

     car(foo, bar)
     =>foo
     car([foo, bar])
     =>foo, bar
     car((foo, bar))
     =>(foo, bar)
     car([(foo], [bar)])
     =>(foo
     car([], [])
     =>
     car([[]], [[]])
     =>[]

With this in mind, we can explore the cases where macros invoke macros...

Quotation and Nested Macros
---------------------------

The examples below use the following macros:

     define([car], [$1])
     define([active], [ACT, IVE])
     define([array], [int tab[10]])

Each additional embedded macro call introduces other possible interesting quotations:

     car(active)
     =>ACT
     car([active])
     =>ACT, IVE
     car([[active]])
     =>active

In the first case, the top level looks for the arguments of car, and finds active.  Because m4 evaluates its arguments before applying the macro, active is expanded, which results in:

     car(ACT, IVE)
     =>ACT

In the second case, the top level gives active as first and only argument of car, which results in:

     active
     =>ACT, IVE

i.e., the argument is evaluated after the macro that invokes it. In the third case, car receives [active], which results in:

     [active]
     =>active

exactly as we already saw above.

The example above, applied to a more realistic example, gives:

     car(int tab[10];)
     =>int tab10;
     car([int tab[10];])
     =>int tab10;
     car([[int tab[10];]])
     =>int tab[10];

Huh?  The first case is easily understood, but why is the second wrong, and the third right?  To understand that, you must know that after m4 expands a macro, the resulting text is immediately subjected to macro expansion and quote removal.  This means that the quote removal occurs twice---first before the argument is passed to the car macro, and second after the car macro expands to the first argument.

As the author of the Autoconf macro car, you then consider it to be incorrect that your users have to double-quote the arguments of car, so you ``fix'' your macro.  Let's call it qar for quoted car:

     define([qar], [[$1]])

and check that qar is properly fixed:

     qar([int tab[10];])
     =>int tab[10];

Ahhh!  That's much better.

But note what you've done: now that the arguments are literal strings, if the user wants to use the results of expansions as arguments, she has to use an unquoted macro call:

     qar(active)
     =>ACT

where she wanted to reproduce what she used to do with car:

     car([active])
     =>ACT, IVE

Worse yet: she wants to use a macro that produces a set of cpp macros:

     define([my_includes], [#include <stdio.h>])
     car([my_includes])
     =>#include <stdio.h>
     qar(my_includes)
     error-->EOF in argument list

This macro, qar, because it double quotes its arguments, forces its users to leave their macro calls unquoted, which is dangerous. Commas and other active symbols are interpreted by m4 before they are given to the macro, often not in the way the users expect. Also, because qar behaves differently from the other macros, it's an exception that should be avoided in Autoconf.

Quadrigraphs
------------

When writing an autoconf macro you may occasionally need to generate special characters that are difficult to express with the standard autoconf quoting rules.  For example, you may need to output the regular expression [^[], which matches any character other than [. This expression contains unbalanced brackets so it cannot be put easily into an M4 macro.

You can work around this problem by using one of the following quadrigraphs:

@<:@
     [

@:>@
     ]

@S|@
     $

@%:@
     #

Quadrigraphs are replaced at a late stage of the translation process, after m4 is run, so they do not get in the way of M4 quoting. For example, the string [^@<:@], if properly quoted, will appear as [^[] in the configure script.

Quotation Rule Of Thumb
-----------------------

To conclude, the quotation rule of thumb is:

One pair of quotes per pair of parentheses.

Never over-quote, never under-quote, in particular in the definition of macros.  In the few places where the macros need to use brackets (usually in C program text or regular expressions), properly quote the arguments!

It is common to read Autoconf programs with snippets like:

     AC_TRY_LINK(
     changequote(<<, >>)dnl
     <<#include <time.h>
     #ifndef tzname /* For SGI.  */
     extern char *tzname[]; /* RS6000 and others reject char **tzname.  */
     #endif>>,
     changequote([, ])dnl
     [atoi (*tzname);], ac_cv_var_tzname=yes, ac_cv_var_tzname=no)

which is incredibly useless since AC_TRY_LINK is already double quoting, so you just need:

     AC_TRY_LINK(
     [#include <time.h>
     #ifndef tzname /* For SGI.  */
     extern char *tzname[]; /* RS6000 and others reject char **tzname.  */
     #endif],
                 [atoi (*tzname);],
                 [ac_cv_var_tzname=yes],
                 [ac_cv_var_tzname=no])

The M4-fluent reader will note that these two examples are rigorously equivalent, since m4 swallows both the changequote(<<, >>) and << >> when it collects the arguments: these quotes are not part of the arguments!

Simplified, the example above is just doing this:

     changequote(<<, >>)dnl
     <<[]>>
     changequote([, ])dnl

instead of simply:

     [[]]

With macros that do not double quote their arguments (which is the rule), double-quote the (risky) literals:

     AC_LINK_IFELSE([AC_LANG_PROGRAM(
     [[#include <time.h>
     #ifndef tzname /* For SGI.  */
     extern char *tzname[]; /* RS6000 and others reject char **tzname.  */
     #endif]],
                                     [atoi (*tzname);])],
                    [ac_cv_var_tzname=yes],
                    [ac_cv_var_tzname=no])

See Quadrigraphs, for what to do if you run into a hopeless case where quoting does not suffice.

When you create a configure script using newly written macros, examine it carefully to check whether you need to add more quotes in your macros.  If one or more words have disappeared in the m4 output, you need more quotes.  When in doubt, quote.

However, it's also possible to put on too many layers of quotes.  If this happens, the resulting configure script will contain unexpanded macros.  The autoconf program checks for this problem by doing grep AC_ configure.

Programming in M4sugar
======================

M4 by itself provides only a small, but sufficient, set of all-purpose macros.  M4sugar introduces additional generic macros.  Its name was coined by Lars J. Aas: ``Readability And Greater Understanding Stands 4 M4sugar''.

Redefined M4 Macros
-------------------

All the M4 native macros are moved in the m4_ pseudo-namespace, e.g., M4sugar renames define as m4_define etc.  There is one exception: dnl kept its original name, and no m4_dnl is defined.

M4sugar redefines some M4 macros, and made them slightly incompatible with their native equivalent.

 -- Macro: m4_defn (MACRO)
     Contrary to the M4 builtin, this macro fails if MACRO is not defined.  See m4_undefine.

 -- Macro: m4_undefine (MACRO)
     Contrary to the M4 builtin, this macro fails if MACRO is not defined.  Use

          m4_ifdef([MACRO], [m4_undefine([MACRO])])

     to recover the behavior of the builtin.

 -- Macro: m4_popdef (MACRO)
     Contrary to the M4 builtin, this macro fails if MACRO is not defined.  See m4_undefine.

Forbidden Patterns
------------------

M4sugar provides a means to define suspicious patterns, patterns describing tokens which should not be found in the output.  For instance, if an Autoconf configure script includes tokens such as AC_DEFINE, or dnl, then most probably something went wrong (typically a macro was not evaluated because of over quotation).

M4sugar forbids all the tokens matching ^m4_ and ^dnl$.

 -- Macro: m4_pattern_forbid (PATTERN)
     Declare no token matching PATTERN must be found in the output. Comments are not checked; this can be a problem if, for instance, you have some macro left unexpanded after an #include.  No consensus is currently found in the Autoconf community, as some people consider it should be valid to name macros in comments (which doesn't makes sense to the author of this documentation, as #-comments should document the output, not the input, documented vy dnl-comments).

Of course, you might encounter exceptions to these generic rules, for instance you might have to refer to $m4_flags.

 -- Macro: m4_pattern_allow (PATTERN)
     Any token matching PATTERN is allowed, including if it matches an m4_pattern_forbid pattern.

=================================================== Writing Autoconf Macros.

Writing Autoconf Macros
***********************

When you write a feature test that could be applicable to more than one software package, the best thing to do is encapsulate it in a new macro. Here are some instructions and guidelines for writing Autoconf macros.

Macro Definitions
=================

Autoconf macros are defined using the AC_DEFUN macro, which is similar to the M4 builtin define macro.  In addition to defining a macro, AC_DEFUN adds to it some code that is used to constrain the order in which macros are called (see Prerequisite Macros).

An Autoconf macro definition looks like this:

     AC_DEFUN(MACRO-NAME, MACRO-BODY)

You can refer to any arguments passed to the macro as $1, $2, etc.  See How to define new macros (GNU m4), for more complete information on writing M4 macros.

Be sure to properly quote both the MACRO-BODY and the MACRO-NAME to avoid any problems if the macro happens to have been previously defined.

Each macro should have a header comment that gives its prototype, and a brief description.  When arguments have default values, display them in the prototype.  For example:

     # AC_MSG_ERROR(ERROR, [EXIT-STATUS = 1])
     # --------------------------------------
     define([AC_MSG_ERROR],
     [{ _AC_ECHO([configure: error: $1], 2); exit m4_default([$2], 1); }])

Comments about the macro should be left in the header comment.  Most other comments will make their way into configure, so just keep using # to introduce comments.

If you have some very special comments about pure M4 code, comments that make no sense in configure and in the header comment, then use the builtin dnl: it causes m4 to discard the text through the next newline.

Keep in mind that dnl is rarely needed to introduce comments; dnl is more useful to get rid of the newlines following macros that produce no output, such as AC_REQUIRE.

Macro Names
===========

All of the Autoconf macros have all-uppercase names starting with AC_ to prevent them from accidentally conflicting with other text.  All shell variables that they use for internal purposes have mostly-lowercase names starting with ac_.  To ensure that your macros don't conflict with present or future Autoconf macros, you should prefix your own macro names and any shell variables they use with some other sequence.  Possibilities include your initials, or an abbreviation for the name of your organization or software package.

Most of the Autoconf macros' names follow a structured naming convention that indicates the kind of feature check by the name.  The macro names consist of several words, separated by underscores, going from most general to most specific.  The names of their cache variables use the same convention (see Cache Variable Names, for more information on them).

The first word of the name after AC_ usually tells the category of feature being tested.  Here are the categories used in Autoconf for specific test macros, the kind of macro that you are more likely to write.  They are also used for cache variables, in all-lowercase.  Use them where applicable; where they're not, invent your own categories.

C
     C language builtin features.

DECL
     Declarations of C variables in header files.

FUNC
     Functions in libraries.

GROUP
     UNIX group owners of files.

HEADER
     Header files.

LIB
     C libraries.

PATH
     The full path names to files, including programs.

PROG
     The base names of programs.

MEMBER
     Members of aggregates.

SYS
     Operating system features.

TYPE
     C builtin or declared types.

VAR
     C variables in libraries.

After the category comes the name of the particular feature being tested.  Any further words in the macro name indicate particular aspects of the feature.  For example, AC_FUNC_UTIME_NULL checks the behavior of the utime function when called with a NULL pointer.

An internal macro should have a name that starts with an underscore; Autoconf internals should therefore start with _AC_. Additionally, a macro that is an internal subroutine of another macro should have a name that starts with an underscore and the name of that other macro, followed by one or more words saying what the internal macro does.  For example, AC_PATH_X has internal macros _AC_PATH_X_XMKMF and _AC_PATH_X_DIRECT.

Reporting Messages
==================

When macros statically diagnose abnormal situations, benign or fatal, they should report them using these macros.  For dynamic issues, i.e., when configure is run, see Printing Messages.

 -- Macro: AC_DIAGNOSE (CATEGORY, MESSAGE)
     Report MESSAGE as a warning (or as an error if requested by the user) if it falls into the CATEGORY.  You are encouraged to use standard categories, which currently include:

     all
          messages that don't fall into one of the following category.  Use of an empty CATEGORY is equivalent.

     cross
          related to cross compilation issues.

     obsolete
          use of an obsolete construct.

     syntax
          dubious syntactic constructs, incorrectly ordered macro calls.

 -- Macro: AC_WARNING (MESSAGE)
     Equivalent to AC_DIAGNOSE([syntax], MESSAGE), but you are strongly encouraged to use a finer grained category.

 -- Macro: AC_FATAL (MESSAGE)
     Report a severe error MESSAGE, and have autoconf die.

When the user runs autoconf -W error, warnings from AC_DIAGNOSE and AC_WARNING are reported as error, see autoconf Invocation.

Dependencies Between Macros
===========================

Some Autoconf macros depend on other macros having been called first in order to work correctly.  Autoconf provides a way to ensure that certain macros are called if needed and a way to warn the user if macros are called in an order that might cause incorrect operation.

Prerequisite Macros
-------------------

A macro that you write might need to use values that have previously been computed by other macros.  For example, AC_DECL_YYTEXT examines the output of flex or lex, so it depends on AC_PROG_LEX having been called first to set the shell variable LEX.

Rather than forcing the user of the macros to keep track of the dependencies between them, you can use the AC_REQUIRE macro to do it automatically.  AC_REQUIRE can ensure that a macro is only called if it is needed, and only called once.

 -- Macro: AC_REQUIRE (MACRO-NAME)
     If the M4 macro MACRO-NAME has not already been called, call it (without any arguments).  Make sure to quote MACRO-NAME with square brackets.  MACRO-NAME must have been defined using AC_DEFUN or else contain a call to AC_PROVIDE to indicate that it has been called.

     AC_REQUIRE must be used inside an AC_DEFUN'd macro; it must not be called from the top level.

AC_REQUIRE is often misunderstood.  It really implements dependencies between macros in the sense that if one macro depends upon another, the latter will be expanded before the body of the former.  In particular, AC_REQUIRE(FOO) is not replaced with the body of FOO.  For instance, this definition of macros:

     AC_DEFUN([TRAVOLTA],
     [test "$body_temparature_in_celsius" -gt "38" &&
       dance_floor=occupied])
     AC_DEFUN([NEWTON_JOHN],
     [test "$hair_style" = "curly" &&
       dance_floor=occupied])

     AC_DEFUN([RESERVE_DANCE_FLOOR],
     [if date | grep '^Sat.*pm' >/dev/null 2>&1; then
       AC_REQUIRE([TRAVOLTA])
       AC_REQUIRE([NEWTON_JOHN])
     fi])

with this configure.ac

     AC_INIT
     RESERVE_DANCE_FLOOR
     if test "$dance_floor" = occupied; then
       AC_MSG_ERROR([cannot pick up here, let's move])
     fi

will not leave you with a better chance to meet a kindred soul at other times than Saturday night since it expands into:

     test "$body_temperature_in_Celsius" -gt "38" &&
       dance_floor=occupied
     test "$hair_style" = "curly" &&
       dance_floor=occupied
     fi
     if date | grep '^Sat.*pm' >/dev/null 2>&1; then

     fi

This behavior was chosen on purpose: (i) it prevents messages in required macros from interrupting the messages in the requiring macros; (ii) it avoids bad surprises when shell conditionals are used, as in:

     if ...; then
       AC_REQUIRE([SOME_CHECK])
     fi
     ...
     SOME_CHECK

You are encouraged to put all AC_REQUIREs at the beginning of a macro.  You can use dnl to avoid the empty lines they leave.

Suggested Ordering
------------------

Some macros should be run before another macro if both are called, but neither requires that the other be called.  For example, a macro that changes the behavior of the C compiler should be called before any macros that run the C compiler.  Many of these dependencies are noted in the documentation.

Autoconf provides the AC_BEFORE macro to warn users when macros with this kind of dependency appear out of order in a configure.ac file.  The warning occurs when creating configure from configure.ac, not when running configure.

For example, AC_PROG_CPP checks whether the C compiler can run the C preprocessor when given the -E option.  It should therefore be called after any macros that change which C compiler is being used, such as AC_PROG_CC.  So AC_PROG_CC contains:

     AC_BEFORE([$0], [AC_PROG_CPP])dnl

This warns the user if a call to AC_PROG_CPP has already occurred when AC_PROG_CC is called.

 -- Macro: AC_BEFORE (THIS-MACRO-NAME, CALLED-MACRO-NAME)
     Make m4 print a warning message to the standard error output if CALLED-MACRO-NAME has already been called.  THIS-MACRO-NAME should be the name of the macro that is calling AC_BEFORE.  The macro CALLED-MACRO-NAME must have been defined using AC_DEFUN or else contain a call to AC_PROVIDE to indicate that it has been called.

Obsoleting Macros
=================

Configuration and portability technology has evolved over the years. Often better ways of solving a particular problem are developed, or ad-hoc approaches are systematized.  This process has occurred in many parts of Autoconf.  One result is that some of the macros are now considered obsolete; they still work, but are no longer considered the best thing to do, hence they should be replaced with more modern macros.  Ideally, autoupdate should substitute the old macro calls with their modern implementation.

Autoconf provides a simple means to obsolete a macro.

 -- Macro: AU_DEFUN (OLD-MACRO, IMPLEMENTATION, [message])
     Define OLD-MACRO as IMPLEMENTATION.  The only difference with AC_DEFUN is that the user will be warned that OLD-MACRO is now obsolete.

     If she then uses autoupdate, the call to OLD-MACRO will be replaced by the modern IMPLEMENTATION.  The additional MESSAGE is then printed.

Coding Style
============

The Autoconf macros follow a strict coding style.  You are encouraged to follow this style, especially if you intend to distribute your macro, either by contributing it to Autoconf itself, or via other means.

The first requirement is to pay great attention to the quotation, for more details, see Autoconf Language, and M4 Quotation.

Do not try to invent new interfaces.  It is likely that there is a macro in Autoconf that resembles the macro you are defining: try to stick to this existing interface (order of arguments, default values, etc.).  We are conscious that some of these interfaces are not perfect; nevertheless, when harmless, homogeneity should be preferred over creativity.

Be careful about clashes both between M4 symbols and between shell variables.

If you stick to the suggested M4 naming scheme (see Macro Names), you are unlikely to generate conflicts.  Nevertheless, when you need to set a special value, avoid using a regular macro name; rather, use an ``impossible'' name.  For instance, up to version 2.13, the macro AC_SUBST used to remember what SYMBOLs were already defined by setting AC_SUBST_SYMBOL, which is a regular macro name. But since there is a macro named AC_SUBST_FILE, it was just impossible to AC_SUBST(FILE)!  In this case, AC_SUBST(SYMBOL) or _AC_SUBST(SYMBOL) should have been used (yes, with the parentheses)...or better yet, high-level macros such as AC_EXPAND_ONCE.

No Autoconf macro should ever enter the user-variable name space; i.e., except for the variables that are the actual result of running the macro, all shell variables should start with ac_.  In addition, small macros or any macro that is likely to be embedded in other macros should be careful not to use obvious names.

Do not use dnl to introduce comments: most of the comments you are likely to write are either header comments which are not output anyway, or comments that should make their way into configure. There are exceptional cases where you do want to comment special M4 constructs, in which case dnl is right, but keep in mind that it is unlikely.

M4 ignores the leading spaces before each argument, use this feature to indent in such a way that arguments are (more or less) aligned with the opening parenthesis of the macro being called.  For instance, instead of

     AC_CACHE_CHECK(for EMX OS/2 environment,
     ac_cv_emxos2,
     [AC_COMPILE_IFELSE([AC_LANG_PROGRAM(, [return __EMX__;])],
     [ac_cv_emxos2=yes], [ac_cv_emxos2=no])])

write

     AC_CACHE_CHECK([for EMX OS/2 environment], [ac_cv_emxos2],
     [AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [return __EMX__;])],
                        [ac_cv_emxos2=yes],
                        [ac_cv_emxos2=no])])

or even

     AC_CACHE_CHECK([for EMX OS/2 environment],
                    [ac_cv_emxos2],
                    [AC_COMPILE_IFELSE([AC_LANG_PROGRAM([],
                                                        [return __EMX__;])],
                                       [ac_cv_emxos2=yes],
                                       [ac_cv_emxos2=no])])

When using AC_TRY_RUN or any macro that cannot work when cross-compiling, provide a pessimistic value (typically no).

Feel free to use various tricks to prevent auxiliary tools, such as syntax-highlighting editors, from behaving improperly.  For instance, instead of:

     patsubst([$1], [$"])

use

     patsubst([$1], [$""])

so that Emacsen do not open a endless ``string'' at the first quote. For the same reasons, avoid:

     test $[#] != 0

and use:

     test $[@%:@] != 0

Otherwise, the closing bracket would be hidden inside a #-comment, breaking the bracket-matching highlighting from Emacsen.  Note the preferred style to escape from M4: $[1], $[@], etc.  Do not escape when it is unneeded.  Common examples of useless quotation are [$]$1 (write $$1), [$]var (use $var), etc.  If you add portability issues to the picture, you'll prefer ${1+"$[@]"} to "[$]@", and you'll prefer do something better than hacking Autoconf :-).

When using sed, don't use -e except for indenting purpose.  With the s command, the preferred separator is / unless / itself is used in the command, in which case you should use ,.

See Macro Definitions, for details on how to define a macro.  If a macro doesn't use AC_REQUIRE and it is expected to never be the object of an AC_REQUIRE directive, then use define.  In case of doubt, use AC_DEFUN.  All the AC_REQUIRE statements should be at the beginning of the macro, dnl'ed.

You should not rely on the number of arguments: instead of checking whether an argument is missing, test that it is not empty.  It provides both a simpler and a more predictable interface to the user, and saves room for further arguments.

Unless the macro is short, try to leave the closing ]) at the beginning of a line, followed by a comment that repeats the name of the macro being defined.  This introduces an additional newline in configure; normally, that is not a problem, but if you want to remove it you can use []dnl on the last line.  You can similarly use []dnl after a macro call to remove its newline.  []dnl is recommended instead of dnl to ensure that M4 does not interpret the dnl as being attached to the preceding text or macro output.  For example, instead of:

     AC_DEFUN([AC_PATH_X],
     [AC_MSG_CHECKING([for X])
     AC_REQUIRE_CPP()
     # ...omitted...
       AC_MSG_RESULT([libraries $x_libraries, headers $x_includes])
     fi])

you would write:

     AC_DEFUN([AC_PATH_X],
     [AC_REQUIRE_CPP()[]dnl
     AC_MSG_CHECKING([for X])
     # ...omitted...
       AC_MSG_RESULT([libraries $x_libraries, headers $x_includes])
     fi[]dnl
     ])# AC_PATH_X

If the macro is long, try to split it into logical chunks.  Typically, macros that check for a bug in a function and prepare its AC_LIBOBJ replacement should have an auxiliary macro to perform this setup.  Do not hesitate to introduce auxiliary macros to factor your code.

In order to highlight the recommended coding style, here is a macro written the old way:

     dnl Check for EMX on OS/2.
     dnl _AC_EMXOS2
     AC_DEFUN(_AC_EMXOS2,
     [AC_CACHE_CHECK(for EMX OS/2 environment, ac_cv_emxos2,
     [AC_COMPILE_IFELSE([AC_LANG_PROGRAM(, return __EMX__;)],
     ac_cv_emxos2=yes, ac_cv_emxos2=no)])
     test "$ac_cv_emxos2" = yes && EMXOS2=yes])

and the new way:

     # _AC_EMXOS2
     # ----------
     # Check for EMX on OS/2.
     define([_AC_EMXOS2],
     [AC_CACHE_CHECK([for EMX OS/2 environment], [ac_cv_emxos2],
     [AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [return __EMX__;])],
                        [ac_cv_emxos2=yes],
                        [ac_cv_emxos2=no])])
     test "$ac_cv_emxos2" = yes && EMXOS2=yes[]dnl
     ])# _AC_EMXOS2

Portable Shell Programming
**************************

When writing your own checks, there are some shell-script programming techniques you should avoid in order to make your code portable.  The Bourne shell and upward-compatible shells like the Korn shell and Bash have evolved over the years, but to prevent trouble, do not take advantage of features that were added after UNIX version 7, circa 1977.  You should not use shell functions, aliases, negated character classes, or other features that are not found in all Bourne-compatible shells; restrict yourself to the lowest common denominator.  Even unset is not supported by all shells!  Also, include a space after the exclamation point in interpreter specifications, like this:

     #! /usr/bin/perl

If you omit the space before the path, then 4.2BSD based systems (such as Sequent DYNIX) will ignore the line, because they interpret #! / as a 4-byte magic number.

The set of external programs you should run in a configure script is fairly small.  See Utilities in Makefiles (GNU Coding Standards), for the list.  This restriction allows users to start out with a fairly small set of programs and build the rest, avoiding too many interdependencies between packages.

Some of these external utilities have a portable subset of features; see Limitations of Usual Tools.

Shellology
==========

There are several families of shells, most prominently the Bourne family and the C shell family which are deeply incompatible.  If you want to write portable shell scripts, avoid members of the C shell family.

Below we describe some of the members of the Bourne shell family.

Ash
     ash is often used on GNU/Linux and BSD systems as a light-weight Bourne-compatible shell.  Ash 0.2 has some bugs that are fixed in the 0.3.x series, but portable shell scripts should workaround them, since version 0.2 is still shipped with many GNU/Linux distributions.

     To be compatible with Ash 0.2:

       - don't use $? after expanding empty or unset variables:

               foo=
               false
               $foo
               echo "Don't use it: $?"

       - don't use command substitution within variable expansion:

               cat ${FOO=`bar`}

       - beware that single builtin substitutions are not performed by a sub shell, hence their effect applies to the current shell!  See Shell Substitutions, item ``Command Substitution''.

Bash
     To detect whether you are running bash, test if BASH_VERSION is set.  To disable its extensions and require POSIX compatibility, run set -o posix. See Bash POSIX Mode (The GNU Bash Reference Manual), for details.

/usr/xpg4/bin/sh on Solaris
     The POSIX-compliant Bourne shell on a Solaris system is /usr/xpg4/bin/sh and is part of an extra optional package. There is no extra charge for this package, but it is also not part of a minimal OS install and therefore some folks may not have it.

Zsh
     To detect whether you are running zsh, test if ZSH_VERSION is set.  By default zsh is not compatible with the Bourne shell: you have to run emulate sh and set NULLCMD to :. See Compatibility (The Z Shell Manual), for details.

     Zsh 3.0.8 is the native /bin/sh on Mac OS X 10.0.3.

The following discussion between Russ Allbery and Robert Lipe is worth reading:

Russ Allbery:

     The GNU assumption that /bin/sh is the one and only shell leads to a permanent deadlock.  Vendors don't want to break user's existant shell scripts, and there are some corner cases in the Bourne shell that are not completely compatible with a POSIX shell.  Thus, vendors who have taken this route will never (OK...``never say never'') replace the Bourne shell (as /bin/sh) with a POSIX shell.

Robert Lipe:

     This is exactly the problem.  While most (at least most System V's) do have a bourne shell that accepts shell functions most vendor /bin/sh programs are not the POSIX shell.

     So while most modern systems do have a shell _somewhere_ that meets the POSIX standard, the challenge is to find it.

Here-Documents
==============

Don't rely on \ being preserved just because it has no special meaning together with the next symbol.  in the native /bin/sh on OpenBSD 2.7 \" expands to " in here-documents with unquoted delimiter.  As a general rule, if \\ expands to \ use \\ to get \.

With OpenBSD 2.7's /bin/sh

     $ cat <<EOF
     > \" \\
     > EOF
     " \

and with Bash:

     bash-2.04$ cat <<EOF
     > \" \\
     > EOF
     \" \

Many older shells (including the Bourne shell) implement here-documents inefficiently.  Users can generally speed things up by using a faster shell, e.g., by using the command bash ./configure rather than plain ./configure.

Some shells can be extremely inefficient when there are a lot of here-documents inside a single statement.  For instance if your configure.ac includes something like:

     if <cross_compiling>; then
       assume this and that
     else
       check this
       check that
       check something else
       ...
       on and on forever
       ...
     fi

A shell parses the whole if/fi construct, creating temporary files for each here document in it.  Some shells create links for such here-documents on every fork, so that the clean-up code they had installed correctly removes them.  It is creating the links that the shell can take forever.

Moving the tests out of the if/fi, or creating multiple if/fi constructs, would improve the performance significantly.  Anyway, this kind of construct is not exactly the typical use of Autoconf.  In fact, it's even not recommended, because M4 macros can't look into shell conditionals, so we may fail to expand a macro when it was expanded before in a conditional path, and the condition turned out to be false at run-time, and we end up not executing the macro at all.

File Descriptors
================

Some file descriptors shall not be used, since some systems, admittedly arcane, use them for special purpose:

3
     some systems may open it to /dev/tty.

4
     used on the Kubota Titan.

Don't redirect several times the same file descriptor, as you are doomed to failure under Ultrix.

     ULTRIX V4.4 (Rev. 69) System #31: Thu Aug 10 19:42:23 GMT 1995
     UWS V4.4 (Rev. 11)
     $ eval 'echo matter >fullness' >void
     illegal io
     $ eval '(echo matter >fullness)' >void
     illegal io
     $ (eval '(echo matter >fullness)') >void
     Ambiguous output redirect.

In each case the expected result is of course fullness containing matter and void being empty.

Don't try to redirect the standard error of a command substitution: it must be done inside the command substitution: when running : `cd /zorglub` 2>/dev/null expect the error message to escape, while : `cd /zorglub 2>/dev/null` works properly.

It is worth noting that Zsh (but not Ash nor Bash) makes it possible in assignments though: foo=`cd /zorglub` 2>/dev/null.

Most shells, if not all (including Bash, Zsh, Ash), output traces on stderr, even for sub-shells.  This might result in undesired content if you meant to capture the standard-error output of the inner command:

     $ ash -x -c '(eval "echo foo >&2") 2>stderr'
     $ cat stderr
     + eval echo foo >&2
     + echo foo
     foo
     $ bash -x -c '(eval "echo foo >&2") 2>stderr'
     $ cat stderr
     + eval 'echo foo >&2'
     ++ echo foo
     foo
     $ zsh -x -c '(eval "echo foo >&2") 2>stderr'
     # Traces on startup files deleted here.
     $ cat stderr
     +zsh:1> eval echo foo >&2
     +zsh:1> echo foo
     foo

You'll appreciate the various levels of detail...

One workaround is to grep out uninteresting lines, hoping not to remove good ones...

File System Conventions
=======================

While autoconf and friends will usually be run on some Unix variety, it can and will be used on other systems, most notably DOS variants.  This impacts several assumptions regarding file and path names.

For example, the following code:

     case $foo_dir in
       /*) # Absolute
          ;;
       *)
          foo_dir=$dots$foo_dir ;;
     esac

will fail to properly detect absolute paths on those systems, because they can use a drivespec, and will usually use a backslash as directory separator.  The canonical way to check for absolute paths is:

     case $foo_dir in
       [\\/]* | ?:[\\/]* ) # Absolute
          ;;
       *)
          foo_dir=$dots$foo_dir ;;
     esac

Make sure you quote the brackets if appropriate and keep the backslash as first character (see Limitations of Builtins).

Also, because the colon is used as part of a drivespec, these systems don't use it as path separator.  When creating or accessing paths, use $ac_path_separator instead (or the PATH_SEPARATOR output variable).  autoconf sets this to the appropriate value (: or ;) when it starts up.

File names need extra care as well.  While DOS-based environments that are Unixy enough to run autoconf (such as DJGPP) will usually be able to handle long file names properly, there are still limitations that can seriously break packages.  Several of these issues can be easily detected by the doschk (doschk, ftp://ftp.gnu.org/gnu/non-gnu/doschk/doschk-1.1.tar.gz.) package.

A short overview follows; problems are marked with SFN/LFN to indicate where they apply: SFN means the issues are only relevant to plain DOS, not to DOS boxes under Windows, while LFN identifies problems that exist even under Windows.

No multiple dots (SFN)
     DOS cannot handle multiple dots in filenames.  This is an especially important thing to remember when building a portable configure script, as autoconf uses a .in suffix for template files.

     This is perfectly OK on Unices:

          AC_CONFIG_HEADER(config.h)
          AC_CONFIG_FILES([source.c foo.bar])
          AC_OUTPUT

     but it causes problems on DOS, as it requires config.h.in, source.c.in and foo.bar.in.  To make your package more portable to DOS-based environments, you should use this instead:

          AC_CONFIG_HEADER(config.h:config.hin)
          AC_CONFIG_FILES([source.c:source.cin foo.bar:foobar.in])
          AC_OUTPUT

No leading dot (SFN)
     DOS cannot handle filenames that start with a dot.  This is usually not a very important issue for autoconf.

Case insensitivity (LFN)
     DOS is case insensitive, so you cannot, for example, have both a file called INSTALL and a directory called install.  This also affects make; if there's a file called INSTALL in the directory, make install will do nothing (unless the install target is marked as PHONY).

The 8+3 limit (SFN)
     Because the DOS file system only stores the first 8 characters of the filename and the first 3 of the extension, those must be unique. That means that foobar-part1.c, foobar-part2.c and foobar-prettybird.c all resolve to the same filename (FOOBAR-P.C).  The same goes for foo.bar and foo.bartender.

     Note: This is not usually a problem under Windows, as it uses numeric tails in the short version of filenames to make them unique.  However, a registry setting can turn this behaviour off.  While this makes it possible to share file trees containing long file names between SFN and LFN environments, it also means the above problem applies there as well.

Invalid characters
     Some characters are invalid in DOS filenames, and should therefore be avoided. In a LFN environment, these are /, \, ?, *, :, <, >, | and ". In a SFN environment, other characters are also invalid.  These include +, ,, [ and ].

Shell Substitutions
===================

Contrary to a persistent urban legend, the Bourne shell does not systematically split variables and backquoted expressions, in particular on the right-hand side of assignments and in the argument of case. For instance, the following code:

     case "$given_srcdir" in
     .)  top_srcdir="`echo "$dots" | sed 's,/$,,'`"
     *)  top_srcdir="$dots$given_srcdir" ;;
     esac

is more readable when written as:

     case $given_srcdir in
     .)  top_srcdir=`echo "$dots" | sed 's,/$,,'`
     *)  top_srcdir=$dots$given_srcdir ;;
     esac

and in fact it is even more portable: in the first case of the first attempt, the computation of top_srcdir is not portable, since not all shells properly understand "`..."..."...`". Worse yet, not all shells understand "`...\"...\"...`" the same way.  There is just no portable way to use double-quoted strings inside double-quoted backquoted expressions (pfew!).

$@
     One of the most famous shell-portability issues is related to "$@": when there are no positional arguments, it is supposed to be equivalent to nothing.  But some shells, for instance under Digital Unix 4.0 and 5.0, will then replace it with an empty argument.  To be portable, use ${1+"$@"}.

${VAR:-VALUE}
     Old BSD shells, including the Ultrix sh, don't accept the colon for any shell substitution, and complain and die.

${VAR=LITERAL}
     Be sure to quote:

          : ${var='Some words'}

     otherwise some shells, such as on Digital Unix V 5.0, will die because of a ``bad substitution''.

     Solaris' /bin/sh has a frightening bug in its interpretation of this.  Imagine you need set a variable to a string containing }.  This } character confuses Solaris' /bin/sh when the affected variable was already set.  This bug can be exercised by running:

          $ unset foo
          $ foo=${foo='}'}
          $ echo $foo
          }
          $ foo=${foo='}'   # no error; this hints to what the bug is
          $ echo $foo
          }
          $ foo=${foo='}'}
          $ echo $foo
          }}
           ^ ugh!

     It seems that } is interpreted as matching ${, even though it is enclosed in single quotes.  The problem doesn't happen using double quotes.

${VAR=EXPANDED-VALUE}
     On Ultrix, running

          default="yu,yaa"
          : ${var="$default"}

     will set VAR to M-yM-uM-,M-yM-aM-a, i.e., the 8th bit of each char will be set. You won't observe the phenomenon using a simple echo $var since apparently the shell resets the 8th bit when it expands $var.  Here are two means to make this shell confess its sins:

          $ cat -v <<EOF
          $var
          EOF

     and

          $ set | grep '^var=' | cat -v

     One classic incarnation of this bug is:

          default="a b c"
          : ${list="$default"}
          for c in $list; do
            echo $c
          done

     You'll get a b c on a single line.  Why?  Because there are no spaces in $list: there are M- , i.e., spaces with the 8th bit set, hence no IFS splitting is performed!!!

     One piece of good news is that Ultrix works fine with : ${list=$default}; i.e., if you don't quote.  The bad news is then that QNX 4.25 then sets LIST to the last item of DEFAULT!

     The portable way out consists in using a double assignment, to switch the 8th bit twice on Ultrix:

          list=${list="$default"}

     ...but beware of the } bug from Solaris (see above).  For safety, use:

          test "${var+set}" = set || var={VALUE}

`COMMANDS`
     While in general it makes no sense, do not substitute a single builtin with side effects as Ash 0.2, trying to optimize, does not fork a sub-shell to perform the command.

     For instance, if you wanted to check that cd is silent, do not use test -z "`cd /`" because the following can happen:

          $ pwd
          /tmp
          $ test -n "`cd /`" && pwd
          /

     The result of foo=`exit 1` is left as an exercise to the reader.

$(COMMANDS)
     This construct is meant to replace `COMMANDS`; they can be nested while this is impossible to do portably with back quotes. Unfortunately it is not yet widely supported.  Most notably, even recent releases of Solaris don't support it:

          $ showrev -c /bin/sh | grep version
          Command version: SunOS 5.8 Generic 109324-02 February 2001
          $ echo $(echo blah)
          syntax error: `(' unexpected

     nor does IRIX 6.5's Bourne shell:

          $ uname -a
          IRIX firebird-image 6.5 07151432 IP22
          $ echo $(echo blah)
          $(echo blah)

Assignments
===========

When setting several variables in a row, be aware that the order of the evaluation is undefined.  For instance foo=1 foo=2; echo $foo gives 1 with sh on Solaris, but 2 with Bash.  You must use ; to enforce the order: foo=1; foo=2; echo $foo.

Don't rely on the exit status of an assignment: Ash 0.2 does not change the status and propagates that of the last statement:

     $ false || foo=bar; echo $?
     1
     $ false || foo=`:`; echo $?
     0

and to make things even worse, QNX 4.25 just sets the exit status to 0 in any case:

     $ foo=`exit 1`; echo $?
     0

To assign default values, follow this algorithm:

 1. If the default value is a literal and does not contain any closing brace, use:

          : ${var='my literal'}

 2. If the default value contains no closing brace, has to be expanded, and the variable being initialized will never be IFS-split (i.e., it's not a list), then use:

          : ${var="$default"}

 3. If the default value contains no closing brace, has to be expanded, and the variable being initialized will be IFS-split (i.e., it's a list), then use:

          var=${var="$default"}

 4. If the default value contains a closing brace, then use:

          test "${var+set}" = set || var='${indirection}'

In most cases var=${var="$default"} is fine, but in case of doubt, just use the latter.  See Shell Substitutions, items ${VAR:-VALUE} and ${VAR=VALUE} for the rationale.

Special Shell Variables
=======================

Some shell variables should not be used, since they can have a deep influence on the behavior of the shell.  In order to recover a sane behavior from the shell, some variables should be unset, but unset is not portable (see Limitations of Builtins) and a fallback value is needed.  We list these values below.

CDPATH
     When this variable is set cd is verbose, so idioms such as abs=`cd $rel && pwd` break because abs receives the path twice.

     Setting CDPATH to the empty value is not enough for most shells. A simple colon is enough except for zsh, which prefers a leading dot:

          zsh-3.1.6 % mkdir foo && (CDPATH=: cd foo)
          /tmp/foo
          zsh-3.1.6 % (CDPATH=:. cd foo)
          /tmp/foo
          zsh-3.1.6 % (CDPATH=.: cd foo)
          zsh-3.1.6 %

     (of course we could just unset CDPATH, since it also behaves properly if set to the empty string).

     Life wouldn't be so much fun if bash and zsh had the same behavior:

          bash-2.02 % (CDPATH=:. cd foo)
          bash-2.02 % (CDPATH=.: cd foo)
          /tmp/foo

     Therefore, a portable solution to neutralize CDPATH is

          CDPATH=${ZSH_VERSION+.}:

     Note that since zsh supports unset, you may unset CDPATH using : as a fallback, see Limitations of Builtins.

IFS
     Don't set the first character of IFS to backslash.  Indeed, Bourne shells use the first character (backslash) when joining the components in "$@" and some shells then re-interpret (!) the backslash escapes, so you can end up with backspace and other strange characters.

LANG
LC_ALL
LC_TIME
LC_CTYPE
LANGUAGE
LC_COLLATE
LC_NUMERIC
LC_MESSAGES

     These must not be set unconditionally because not all systems understand e.g. LANG=C (notably SCO).  Fixing LC_MESSAGES prevents Solaris sh from translating var values in set!  Non-C LC_CTYPE values break the ctype check.  Fixing LC_COLLATE makes scripts more portable in some cases.  For example, it causes the regular expression [a-z] to match only lower-case letters on ASCII platforms.  However, [a-z] does not work in general even when LC_COLLATE is fixed; for example, it does not work for EBCDIC platforms.  For maximum portability, you should use regular expressions like [abcdefghijklmnopqrstuvwxyz] that list characters explicitly instead of relying on ranges.

     If one of these variables is set, you should try to unset it, using C as a fall back value. see Limitations of Builtins, builtin unset, for more details.

NULLCMD
     When executing the command >foo, zsh executes $NULLCMD >foo.  The Bourne shell considers NULLCMD is :, while zsh, even in Bourne shell compatibility mode, sets NULLCMD to cat.  If you forgot to set NULLCMD, your script might be suspended waiting for data on its standard input.

status
     This variable is an alias to $? for zsh (at least 3.1.6), hence read-only.  Do not use it.

PATH_SEPARATOR
     On DJGPP systems, the PATH_SEPARATOR variable can be set to either : or ; to control the path separator bash uses to set up certain environment variables (such as PATH). Since this only works inside bash, you want autoconf to detect the regular DOS path separator ;, so it can be safely substituted in files that may not support ; as path separator. So either unset this variable or set it to ;.

RANDOM
     Many shells provide RANDOM, a variable that returns a different integer when used.  Most of the time, its value does not change when it is not used, but on IRIX 6.5 the value changes all the time.  This can be observed by using set.

Limitations of Shell Builtins
=============================

No, no, we are serious: some shells do have limitations! :)

You should always keep in mind that any built-in or command may support options, and therefore have a very different behavior with arguments starting with a dash.  For instance, the innocent echo "$word" can give unexpected results when word starts with a dash.  It is often possible to avoid this problem using echo "x$word", taking the x into account later in the pipe.

!
     You can't use !, you'll have to rewrite your code.

break
     The use of break 2, etcetera, is safe.

case
     You don't need to quote the argument; no splitting is performed.

     You don't need the final ;;, but you should use it.

     Because of a bug in its fnmatch, bash fails to properly handle backslashes in character classes:

          bash-2.02$ case /tmp in [/\\]*) echo OK;; esac
          bash-2.02$

     This is extremely unfortunate, since you are likely to use this code to handle UNIX or MS-DOS absolute paths.  To work around this bug, always put the backslash first:

          bash-2.02$ case '\TMP' in [\\/]*) echo OK;; esac
          OK
          bash-2.02$ case /tmp in [\\/]*) echo OK;; esac
          OK

echo
     The simple echo is probably the most surprising source of portability troubles.  It is not possible to use echo portably unless both options and escape sequences are omitted.  New applications which are not aiming at portability should use printf instead of echo.

     Don't expect any option.  See Preset Output Variables, ECHO_N etc. for a means to simulate -c.

     Do not use backslashes in the arguments, as there is no consensus on their handling.  On echo '\n' | wc -l, the sh of Digital Unix 4.0, MIPS RISC/OS 4.52, answer 2, but the Solaris' sh, Bash and Zsh (in sh emulation mode) report 1. Please note that the problem is truly echo: all the shells understand '\n' as the string composed of a backslash and an n.

     Because of these problems, do not pass a string containing arbitrary characters to echo.  For example, echo "$foo" is safe if you know that FOO's value cannot contain backslashes and cannot start with -, but otherwise you should use a here-document like this:

          cat <<EOF
          $foo
          EOF

exit
     The default value of exit is supposed to be $?; unfortunately, some shells, such as the DJGPP port of Bash 2.04, just perform exit 0.

          bash-2.04$ foo=`exit 1` || echo fail
          fail
          bash-2.04$ foo=`(exit 1)` || echo fail
          fail
          bash-2.04$ foo=`(exit 1); exit` || echo fail
          bash-2.04$

     Using exit $? restores the expected behavior.

     Some shell scripts, such as those generated by autoconf, use a trap to clean up before exiting.  If the last shell command exited with nonzero status, the trap also exits with nonzero status so that the invoker can tell that an error occurred.

     Unfortunately, in some shells, such as Solaris 8 sh, an exit trap ignores the exit command's status.  In these shells, a trap cannot determine whether it was invoked by plain exit or by exit 1.  Instead of calling exit directly, use the AC_MSG_ERROR macro that has a workaround for this problem.

export
     The builtin export dubs environment variable a shell variable.  Each update of exported variables corresponds to an update of the environment variables.  Conversely, each environment variable received by the shell when it is launched should be imported as a shell variable marked as exported.

     Alas, many shells, such as Solaris 2.5, IRIX 6.3, IRIX 5.2, AIX 4.1.5 and DU 4.0, forget to export the environment variables they receive.  As a result, two variables are coexisting: the environment variable and the shell variable.  The following code demonstrates this failure:

          #! /bin/sh
          echo $FOO
          FOO=bar
          echo $FOO
          exec /bin/sh $0

     when run with FOO=foo in the environment, these shells will print alternately foo and bar, although it should only print foo and then a sequence of bars.

     Therefore you should export again each environment variable that you update.

false
     Don't expect false to exit with status 1: in the native Bourne shell of Solaris 8, it exits with status 255.

for
     To loop over positional arguments, use:

          for arg
          do
            echo "$arg"
          done

     You may not leave the do on the same line as for, since some shells improperly grok:

          for arg; do
            echo "$arg"
          done

     If you want to explicitly refer to the positional arguments, given the $@ bug (see Shell Substitutions), use:

          for arg in ${1+"$@"}; do
            echo "$arg"
          done

if
     Using ! is not portable.  Instead of:

          if ! cmp -s file file.new; then
            mv file.new file
          fi

     use:

          if cmp -s file file.new; then :; else
            mv file.new file
          fi

     There are shells that do not reset the exit status from an if:

          $ if (exit 42); then true; fi; echo $?
          42

     whereas a proper shell should have printed 0.  This is especially bad in Makefiles since it produces false failures.  This is why properly written Makefiles, such as Automake's, have such hairy constructs:

          if test -f "$file"; then
            install "$file" "$dest"
          else
            :
          fi

set
     This builtin faces the usual problem with arguments starting with a dash.  Modern shells such as Bash or Zsh understand -- to specify the end of the options (any argument after -- is a parameters, even -x for instance), but most shells simply stop the option processing as soon as a non-option argument is found.  Therefore, use dummy or simply x to end the option processing, and use shift to pop it out:

          set x $my_list; shift

shift
     Not only is shifting a bad idea when there is nothing left to shift, but in addition it is not portable: the shell of MIPS RISC/OS 4.52 refuses to do it.

test
     The test program is the way to perform many file and string tests.  It is often invoked by the alternate name [, but using that name in Autoconf code is asking for trouble since it is an M4 quote character.

     If you need to make multiple checks using test, combine them with the shell operators && and || instead of using the test operators -a and -o.  On System V, the precedence of -a and -o is wrong relative to the unary operators; consequently, POSIX does not specify them, so using them is nonportable.  If you combine && and || in the same statement, keep in mind that they have equal precedence.

     You may use ! with test, but not with if: test ! -r foo || exit 1.

test (files)
     To enable configure scripts to support cross-compilation, they shouldn't do anything that tests features of the build system instead of the host system.  But occasionally you may find it necessary to check whether some arbitrary file exists.  To do so, use test -f or test -r.  Do not use test -x, because 4.3BSD does not have it.  Do not use test -e either, because Solaris 2.5 does not have it.

test (strings)
     Avoid test "STRING", in particular if STRING might start with a dash, since test might interpret its argument as an option (e.g., STRING = "-n").

     Contrary to a common belief, test -n STRING and test -z STRING are portable, nevertheless many shells (such as Solaris 2.5, AIX 3.2, UNICOS 10.0.0.6, Digital Unix 4 etc.) have bizarre precedence and may be confused if STRING looks like an operator:

          $ test -n =
          test: argument expected

     If there are risks, use test "xSTRING" = x or test "xSTRING" != x instead.

     It is frequent to find variations of the following idiom:

          test -n "`echo $ac_feature | sed 's/[-a-zA-Z0-9_]//g'`" &&
            ACTION

     to take an action when a token matches a given pattern.  Such constructs should always be avoided by using:

          echo "$ac_feature" | grep '[^-a-zA-Z0-9_]' >/dev/null 2>&1 &&
            ACTION

     Use case where possible since it is faster, being a shell builtin:

          case $ac_feature in
            *[!-a-zA-Z0-9_]*) ACTION;;
          esac

     Alas, negated character classes are probably not portable, although no shell is known to not support the POSIX.2 syntax [!...] (when in interactive mode, zsh is confused by the [!...] syntax and looks for an event in its history because of !).  Many shells do not support the alternative syntax [^...] (Solaris, Digital Unix, etc.).

     One solution can be:

          expr "$ac_feature" : '.*[^-a-zA-Z0-9_]' >/dev/null &&
            ACTION

     or better yet

          expr "x$ac_feature" : '.*[^-a-zA-Z0-9_]' >/dev/null &&
            ACTION

     expr "XFOO" : "XBAR" is more robust than echo "XFOO" | grep "^XBAR", because it avoids problems when FOO contains backslashes.

trap
     It is safe to trap at least the signals 1, 2, 13 and 15.  You can also trap 0, i.e., have the trap run when the script ends (either via an explicit exit, or the end of the script).

     Although POSIX is not absolutely clear on this point, it is widely admitted that when entering the trap $? should be set to the exit status of the last command run before the trap.  The ambiguity can be summarized as: ``when the trap is launched by an exit, what is the last command run: that before exit, or exit itself?''

     Bash considers exit to be the last command, while Zsh and Solaris 8 sh consider that when the trap is run it is still in the exit, hence it is the previous exit status that the trap receives:

          $ cat trap.sh
          trap 'echo $?' 0
          (exit 42); exit 0
          $ zsh trap.sh
          42
          $ bash trap.sh
          0

     The portable solution is then simple: when you want to exit 42, run (exit 42); exit 42, the first exit being used to set the exit status to 42 for Zsh, and the second to trigger the trap and pass 42 as exit status for Bash.

     The shell in FreeBSD 4.0 has the following bug: $? is reset to 0 by empty lines if the code is inside trap.

          $ trap 'false

          echo $?' 0
          $ exit
          0

     Fortunately, this bug only affects trap.

true
     Don't worry: as far as we know true is portable. Nevertheless, it's not always a builtin (e.g., Bash 1.x), and the portable shell community tends to prefer using :.  This has a funny side effect: when asked whether false is more portable than true Alexandre Oliva answered:

          In a sense, yes, because if it doesn't exist, the shell will produce an exit status of failure, which is correct for false, but not for true.

unset
     You cannot assume the support of unset, nevertheless, because it is extremely useful to disable embarrassing variables such as CDPATH or LANG, you can test for its existence and use it provided you give a neutralizing value when unset is not supported:

          if (unset FOO) >/dev/null 2>&1; then
            unset=unset
          else
            unset=false
          fi
          $unset CDPATH || CDPATH=:

     See Special Shell Variables, for some neutralizing values. Also, see Limitations of Builtins, documentation of export, for the case of environment variables.

Limitations of Usual Tools
==========================

The small set of tools you can expect to find on any machine can still include some limitations you should be aware of.

awk
     Don't leave white spaces before the parentheses in user functions calls, GNU awk will reject it:

          $ gawk 'function die () { print "Aaaaarg!"  }
                  BEGIN { die () }'
          gawk: cmd. line:2:         BEGIN { die () }
          gawk: cmd. line:2:                      ^ parse error
          $ gawk 'function die () { print "Aaaaarg!"  }
                  BEGIN { die() }'
          Aaaaarg!

     If you want your program to be deterministic, don't depend on for on arrays:

          $ cat for.awk
          END {
            arr["foo"] = 1
            arr["bar"] = 1
            for (i in arr)
              print i
          }
          $ gawk -f for.awk </dev/null
          foo
          bar
          $ nawk -f for.awk </dev/null
          bar
          foo

     Some AWK, such as HPUX 11.0's native one, have regex engines fragile to inner anchors:

          $ echo xfoo | $AWK '/foo|^bar/ { print }'
          $ echo bar | $AWK '/foo|^bar/ { print }'
          bar
          $ echo xfoo | $AWK '/^bar|foo/ { print }'
          xfoo
          $ echo bar | $AWK '/^bar|foo/ { print }'
          bar

     Either do not depend on such patterns (i.e., use /^(.*foo|bar)/, or use a simple test to reject such AWK.

cat
     Don't rely on any option.  The option -v, which displays non-printing characters, seems portable, though.

cc
     When a compilation such as cc foo.c -o foo fails, some compilers (such as CDS on Reliant UNIX) leave a foo.o.

cmp
     cmp performs a raw data comparison of two files, while diff compares two text files.  Therefore, if you might compare DOS files, even if only checking whether two files are different, use diff to avoid spurious differences due to differences of newline encoding.

cp
     SunOS cp does not support -f, although its mv does.  It's possible to deduce why mv and cp are different with respect to -f.  mv prompts by default before overwriting a read-only file.  cp does not.  Therefore, mv requires a -f option, but cp does not.  mv and cp behave differently with respect to read-only files because the simplest form of cp cannot overwrite a read-only file, but the simplest form of mv can.  This is because cp opens the target for write access, whereas mv simply calls link (or, in newer systems, rename).

diff
     Option -u is nonportable.

     Some implementations, such as Tru64's, fail when comparing to /dev/null.  Use an empty file instead.

dirname
     Not all hosts have dirname, but it is reasonably easy to emulate, e.g.:

          dir=`expr "x$file" : 'x\(.*\)/[^/]*' \|
                    '.'      : '.'

     But there are a few subtilities, e.g., under UN*X, should //1 give /?  Paul Eggert answers:

          No, under some older flavors of Unix, leading // is a special path name: it refers to a ``super-root'' and is used to access other machines' files.  Leading ///, ////, etc. are equivalent to /; but leading // is special.  I think this tradition started with Apollo Domain/OS, an OS that is still in use on some older hosts.

          POSIX.2 allows but does not require the special treatment for //. It says that the behavior of dirname on path names of the form //([^/]+/*)?  is implementation defined.  In these cases, GNU dirname returns /, but it's more portable to return // as this works even on those older flavors of Unix.

          I have heard rumors that this special treatment of // may be dropped in future versions of POSIX, but for now it's still the standard.

egrep
     The empty alternative is not portable, use ? instead. For instance with Digital Unix v5.0:

          > printf "foo\n|foo\n" | egrep '^(|foo|bar)$'
          |foo
          > printf "bar\nbar|\n" | egrep '^(foo|bar|)$'
          bar|
          > printf "foo\nfoo|\n|bar\nbar\n" | egrep '^(foo||bar)$'
          foo
          |bar

     egrep also suffers the limitations of grep.

expr
     No expr keyword starts with x, so use expr x"WORD" : 'xREGEX' to keep expr from misinterpreting WORD.

     Don't use length, substr, match and index.

expr (|)
     You can use |.  Although POSIX does require that expr '' return the empty string, it does not specify the result when you | together the empty string (or zero) with the empty string.  For example:

          expr '' \| ''

     GNU/Linux and POSIX.2-1992 return the empty string for this case, but traditional Unix returns 0 (Solaris is one such example).  In the latest POSIX draft, the specification has been changed to match traditional Unix's behavior (which is bizarre, but it's too late to fix this).  Please note that the same problem does arise when the empty string results from a computation, as in:

          expr bar : foo \| foo : bar

     Avoid this portability problem by avoiding the empty string.

expr (:)
     Don't use \?, \+ and \| in patterns, they are not supported on Solaris.

     The POSIX.2-1992 standard is ambiguous as to whether expr a : b (and expr 'a' : '\(b\)') output 0 or the empty string. In practice, it outputs the empty string on most platforms, but portable scripts should not assume this.  For instance, the QNX 4.25 native expr returns 0.

     You may believe that one means to get a uniform behavior would be to use the empty string as a default value:

          expr a : b \| ''

     unfortunately this behaves exactly as the original expression, see the expr (:) entry for more information.

     Older expr implementations (e.g. SunOS 4 expr and Solaris 8 /usr/ucb/expr) have a silly length limit that causes expr to fail if the matched substring is longer than 120 bytes.  In this case, you might want to fall back on echo|sed if expr fails.

     Don't leave, there is some more!

     The QNX 4.25 expr, in addition of preferring 0 to the empty string, has a funny behavior in its exit status: it's always 1 when parentheses are used!

          $ val=`expr 'a' : 'a'`; echo "$?: $val"
          0: 1
          $ val=`expr 'a' : 'b'`; echo "$?: $val"
          1: 0

          $ val=`expr 'a' : '\(a\)'`; echo "?: $val"
          1: a
          $ val=`expr 'a' : '\(b\)'`; echo "?: $val"
          1: 0

     In practice this can be a big problem if you are ready to catch failures of expr programs with some other method (such as using sed), since you may get twice the result.  For instance

          $ expr 'a' : '\(a\)' || echo 'a' | sed 's/^\(a\)$/\1/'

     will output a on most hosts, but aa on QNX 4.25.  A simple work around consists in testing expr and use a variable set to expr or to false according to the result.

find
     The option -maxdepth seems to be GNU specific. Tru64 v5.1, NetBSD 1.5 and Solaris 2.5 find commands do not understand it.

grep
     Don't use grep -s to suppress output, because grep -s on System V does not suppress output, only error messages.  Instead, redirect the standard output and standard error (in case the file doesn't exist) of grep to /dev/null.  Check the exit status of grep to determine whether it found a match.

     Don't use multiple regexps with -e, as some grep will only honor the last pattern (eg., IRIX 6.5 and Solaris 2.5.1).  Anyway, Stardent Vistra SVR4 grep lacks -e...  Instead, use alternation and egrep.

ln
     Don't rely on ln having a -f option.  Symbolic links are not available on old systems, use ln as a fall back.

     For versions of the DJGPP before 2.04, ln emulates soft links for executables by generating a stub that in turn calls the real program.  This feature also works with nonexistent files like in the Unix spec. So ln -s file link will generate link.exe, which will attempt to call file.exe if run. But this feature only works for executables, so cp -p is used instead for these systems.  DJGPP versions 2.04 and later have full symlink support.

mv
     The only portable options are -f and -i.

     Moving individual files between file systems is portable (it was in V6), but it is not always atomic: when doing mv new existing, there's a critical section where neither the old nor the new version of existing actually exists.

     Moving directories across mount points is not portable, use cp and rm.

sed
     Patterns should not include the separator (unless escaped), even as part of a character class.  In conformance with POSIX, the Cray sed will reject s/[^/]*$//: use s,[^/]*$,,.

     Sed scripts should not use branch labels longer than 8 characters and should not contain comments.

     Don't include extra ;, as some sed, such as NetBSD 1.4.2's, try to interpret the second as a command:

          $ echo a | sed 's/x/x/;;s/x/x/'
          sed: 1: "s/x/x/;;s/x/x/": invalid command code ;

     Input should have reasonably long lines, since some sed have an input buffer limited to 4000 bytes.

     Alternation, \|, is common but not portable. Anchors (^ and $) inside groups are not portable.

     Nested groups are extremely portable, but there is at least one sed (System V/68 Base Operating System R3V7.1) that does not support it.

     Of course the option -e is portable, but it is not needed.  No valid Sed program can start with a dash, so it does not help disambiguating.  Its sole usefulness is helping enforcing indenting as in:

          sed -e INSTRUCTION-1 \
              -e INSTRUCTION-2

     as opposed to

          sed INSTRUCTION-1;INSTRUCTION-2

     Contrary to yet another urban legend, you may portably use & in the replacement part of the s command to mean ``what was matched''.

sed (t)
     Some old systems have sed that ``forget'' to reset their t flag when starting a new cycle.  For instance on MIPS RISC/OS, and on IRIX 5.3, if you run the following sed script (the line numbers are not actual part of the texts):

          s/keep me/kept/g  # a
          t end             # b
          s/.*/deleted/g    # c
          : end             # d

     on

          delete me         # 1
          delete me         # 2
          keep me           # 3
          delete me         # 4

     you get

          deleted
          delete me
          kept
          deleted

     instead of

          deleted
          deleted
          kept
          deleted

     Why? When processing 1, a matches, therefore sets the t flag, b jumps to d, and the output is produced.  When processing line 2, the t flag is still set (this is the bug).  Line a fails to match, but sed is not supposed to clear the t flag when a substitution fails.  Line b sees that the flag is set, therefore it clears it, and jumps to d, hence you get delete me instead of deleted.  When processing 3 t is clear, a matches, so the flag is set, hence b clears the flags and jumps.  Finally, since the flag is clear, 4 is processed properly.

     There are two things one should remind about t in sed. Firstly, always remember that t jumps if some substitution succeeded, not only the immediately preceding substitution, therefore, always use a fake t clear; : clear to reset the t flag where indeed.

     Secondly, you cannot rely on sed to clear the flag at each new cycle.

     One portable implementation of the script above is:

          t clear
          : clear
          s/keep me/kept/g
          t end
          s/.*/deleted/g
          : end

touch
     On some old BSD systems, touch or any command that results in an empty file does not update the timestamps, so use a command like echo as a workaround.

     GNU touch 3.16r (and presumably all before that) fails to work on SunOS 4.1.3 when the empty file is on an NFS-mounted 4.2 volume.

Limitations of Make
===================

Make itself suffers a great number of limitations, only a few of which being listed here.  First of all, remember that since commands are executed by the shell, all its weaknesses are inherited...

Leading underscore in macro names
     Some Make don't support leading underscores in macro names, such as on NEWS-OS 4.2R.

          $ cat Makefile
          _am_include = #
          _am_quote =
          all:; @echo this is test

          % make
          Make: Must be a separator on rules line 2.  Stop.

          $ cat Makefile2
          am_include = #
          am_quote =
          all:; @echo this is test

          $ make -f Makefile2
          this is test

VPATH
     Don't use it!  For instance any assignment to VPATH causes Sun make to only execute the first set of double-colon rules.

Manual Configuration
********************

A few kinds of features can't be guessed automatically by running test programs.  For example, the details of the object-file format, or special options that need to be passed to the compiler or linker.  You can check for such features using ad-hoc means, such as having configure check the output of the uname program, or looking for libraries that are unique to particular systems.  However, Autoconf provides a uniform method for handling unguessable features.

Specifying the System Type
==========================

Like other GNU configure scripts, Autoconf-generated configure scripts can make decisions based on a canonical name for the system type, which has the form: CPU-VENDOR-OS, where OS can be SYSTEM or KERNEL-SYSTEM

configure can usually guess the canonical name for the type of system it's running on.  To do so it runs a script called config.guess, which infers the name using the uname command or symbols predefined by the C preprocessor.

Alternately, the user can specify the system type with command line arguments to configure.  Doing so is necessary when cross-compiling.  In the most complex case of cross-compiling, three
system types are involved.  The options to specify them are (For
backward compatibility, configure will accept a system type as an
option by itself.  Such an option will override the defaults for build,
host and target system types.  The following configure statement will
configure a cross toolchain that will run on NetBSD/alpha but generate
code for GNU Hurd/sparc, which is also the build platform.)

     ./configure --host=alpha-netbsd sparc-gnu

:

--build=BUILD-TYPE
     the type of system on which the package is being configured and compiled.

--host=HOST-TYPE
     the type of system on which the package will run.

--target=TARGET-TYPE
     the type of system for which any compiler tools in the package will produce code (rarely needed).  By default, it is the same as host.

They all default to the result of running config.guess, unless you specify either --build or --host.  In this case, the default becomes the system type you specified.  If you specify both, and they're different, configure will enter cross compilation mode, so it won't run any tests that require execution.

Hint: if you mean to override the result of config.guess, prefer --build over --host.  In the future, --host will not override the name of the build system type.  Also, if you specify --host, but not --build, when configure performs the first compiler test it will try to run an executable produced by the compiler.  If the execution fails, it will enter cross-compilation mode. Note, however, that it won't guess the build-system type, since this may require running test programs.  Moreover, by the time the compiler test is performed, it may be too late to modify the build-system type: other tests may have already been performed.  Therefore, whenever you specify --host, be sure to specify --build too.

     ./configure --build=i686-pc-linux-gnu --host=m68k-coff

will enter cross-compilation mode, but configure will fail if it can't run the code generated by the specified compiler if you configure as follows:

     ./configure CC=m68k-coff-gcc

configure recognizes short aliases for many system types; for example, decstation can be used instead of mips-dec-ultrix4.2.  configure runs a script called config.sub to canonicalize system type aliases.

Getting the Canonical System Type
=================================

The following macros make the system type available to configure scripts.

The variables build_alias, host_alias, and target_alias are always exactly the arguments of --build, --host, and --target; in particular, they are left empty if the user did not use them, even if the corresponding AC_CANONICAL macro was run.  Any configure script may use these variables anywhere.  These are the variables that should be used when in interaction with the user.

If you need to recognize some special environments based on their system type, run the following macros to get canonical system names.  These variables are not set before the macro call.

If you use these macros, you must distribute config.guess and config.sub along with your source code.  See Output, for information about the AC_CONFIG_AUX_DIR macro which you can use to control in which directory configure looks for those scripts.

 -- Macro: AC_CANONICAL_BUILD
     Compute the canonical build-system type variable, build, and its three individual parts build_cpu, build_vendor, and build_os.

     If --build was specified, then build is the canonicalization of build_alias by config.sub, otherwise it is determined by the shell script config.guess.

 -- Macro: AC_CANONICAL_HOST
     Compute the canonical host-system type variable, host, and its three individual parts host_cpu, host_vendor, and host_os.

     If --host was specified, then host is the canonicalization of host_alias by config.sub, otherwise it defaults to build.

     For temporary backward-compatibility, when --host is specified by --build isn't, the build system will be assumed to be the same as --host, and build_alias will be set to that value.  Eventually, this historically incorrect behavior will go away.

 -- Macro: AC_CANONICAL_TARGET
     Compute the canonical target-system type variable, target, and its three individual parts target_cpu, target_vendor, and target_os.

     If --target was specified, then target is the canonicalization of target_alias by config.sub, otherwise it defaults to host.

Using the System Type
=====================

How do you use a canonical system type?  Usually, you use it in one or more case statements in configure.ac to select system-specific C files.  Then, using AC_CONFIG_LINKS, link those files which have names based on the system name, to generic names, such as host.h or target.c (see Configuration Links).  The case statement patterns can use shell wild cards to group several cases together, like in this fragment:

     case "$target" in
     i386-*-mach* | i386-*-gnu*)
                  obj_format=aout emulation=mach bfd_gas=yes ;;
     i960-*-bout) obj_format=bout ;;
     esac

and in configure.ac, use:

     AC_CONFIG_LINKS(host.h:config/$machine.h
                     object.h:config/$obj_format.h)

You can also use the host system type to find cross-compilation tools. See Generic Programs, for information about the AC_CHECK_TOOL macro which does that.

Site Configuration
******************

configure scripts support several kinds of local configuration decisions.  There are ways for users to specify where external software packages are, include or exclude optional features, install programs under modified names, and set default values for configure options.

Working With External Software
==============================

Some packages require, or can optionally use, other software packages that are already installed.  The user can give configure command line options to specify which such external software to use. The options have one of these forms:

     --with-PACKAGE=[arg]
     --without-PACKAGE

For example, --with-gnu-ld means work with the GNU linker instead of some other linker.  --with-x means work with The X Window System.

The user can give an argument by following the package name with = and the argument.  Giving an argument of no is for packages that are used by default; it says to not use the package.  An argument that is neither yes nor no could include a name or number of a version of the other package, to specify more precisely which other package this program is supposed to work with.  If no argument is given, it defaults to yes. --without-PACKAGE is equivalent to --with-PACKAGE=no.

configure scripts do not complain about --with-PACKAGE options that they do not support.  This behavior permits configuring a source tree containing multiple packages with a top-level configure script when the packages support different options, without spurious error messages about options that some of the packages support.  An unfortunate side effect is that option spelling errors are not diagnosed.  No better approach to this problem has been suggested so far.

For each external software package that may be used, configure.ac should call AC_ARG_WITH to detect whether the configure user asked to use it.  Whether each package is used or not by default, and which arguments are valid, is up to you.

 -- Macro: AC_ARG_WITH (PACKAGE, HELP-STRING, [action-if-given], [action-if-not-given])
     If the user gave configure the option --with-PACKAGE or --without-PACKAGE, run shell commands ACTION-IF-GIVEN.  If neither option was given, run shell commands ACTION-IF-NOT-GIVEN.  The name PACKAGE indicates another software package that this program should work with.  It should consist only of alphanumeric characters and dashes.

     The option's argument is available to the shell commands ACTION-IF-GIVEN in the shell variable withval, which is actually just the value of the shell variable with_PACKAGE, with any - characters changed into _.  You may use that variable instead, if you wish.

     The argument HELP-STRING is a description of the option that looks like this:

            --with-readline         support fancy command line editing

     HELP-STRING may be more than one line long, if more detail is needed.  Just make sure the columns line up in configure --help. Avoid tabs in the help string.  You'll need to enclose it in [ and ] in order to produce the leading spaces.

     You should format your HELP-STRING with the macro AC_HELP_STRING (see Pretty Help Strings).

 -- Macro: AC_WITH (PACKAGE, ACTION-IF-GIVEN, [action-if-not-given])
     This is an obsolete version of AC_ARG_WITH that does not support providing a help string.

Choosing Package Options
========================

If a software package has optional compile-time features, the user can give configure command line options to specify whether to compile them.  The options have one of these forms:

     --enable-FEATURE=[arg]
     --disable-FEATURE

These options allow users to choose which optional features to build and install.  --enable-FEATURE options should never make a feature behave differently or cause one feature to replace another. They should only cause parts of the program to be built rather than left out.

The user can give an argument by following the feature name with = and the argument.  Giving an argument of no requests that the feature not be made available.  A feature with an argument looks like --enable-debug=stabs.  If no argument is given, it defaults to yes.  --disable-FEATURE is equivalent to --enable-FEATURE=no.

configure scripts do not complain about --enable-FEATURE options that they do not support. This behavior permits configuring a source tree containing multiple packages with a top-level configure script when the packages support different options, without spurious error messages about options that some of the packages support. An unfortunate side effect is that option spelling errors are not diagnosed. No better approach to this problem has been suggested so far.

For each optional feature, configure.ac should call AC_ARG_ENABLE to detect whether the configure user asked to include it.  Whether each feature is included or not by default, and which arguments are valid, is up to you.

 -- Macro: AC_ARG_ENABLE (FEATURE, HELP-STRING, [action-if-given], [action-if-not-given])
     If the user gave configure the option --enable-FEATURE or --disable-FEATURE, run shell commands ACTION-IF-GIVEN.  If neither option was given, run shell commands ACTION-IF-NOT-GIVEN.  The name FEATURE indicates an optional user-level facility.  It should consist only of alphanumeric characters and dashes.

     The option's argument is available to the shell commands ACTION-IF-GIVEN in the shell variable enableval, which is actually just the value of the shell variable enable_FEATURE, with any - characters changed into _.  You may use that variable instead, if you wish.  The HELP-STRING argument is like that of AC_ARG_WITH (see External Software).

     You should format your HELP-STRING with the macro AC_HELP_STRING (see Pretty Help Strings).

 -- Macro: AC_ENABLE (FEATURE, ACTION-IF-GIVEN, [action-if-not-given])
     This is an obsolete version of AC_ARG_ENABLE that does not support providing a help string.

Making Your Help Strings Look Pretty
====================================

Properly formatting the help strings which are used in AC_ARG_WITH (see External Software) and AC_ARG_ENABLE (see Package Options) can be challenging.  Specifically, you want your own help strings to line up in the appropriate columns of configure --help just like the standard Autoconf help strings do.  This is the purpose of the AC_HELP_STRING macro.

 -- Macro: AC_HELP_STRING (LEFT-HAND-SIDE, RIGHT-HAND-SIDE)

     Expands into an help string that looks pretty when the user executes configure --help.  It is typically used in AC_ARG_WITH (see External Software) or AC_ARG_ENABLE (see Package Options).  The following example will make this clearer.

          AC_DEFUN(TEST_MACRO,
          [AC_ARG_WITH(foo,
                       AC_HELP_STRING([--with-foo],
                                      [use foo (default is NO)]),
                       ac_cv_use_foo=$withval, ac_cv_use_foo=no),
          AC_CACHE_CHECK(whether to use foo,
                         ac_cv_use_foo, ac_cv_use_foo=no)])

     Please note that the call to AC_HELP_STRING is unquoted. Then the last few lines of configure --help will appear like this:

          --enable and --with options recognized:
            --with-foo              use foo (default is NO)

     The AC_HELP_STRING macro is particularly helpful when the LEFT-HAND-SIDE and/or RIGHT-HAND-SIDE are composed of macro arguments, as shown in the following example.

          AC_DEFUN(MY_ARG_WITH,
          [AC_ARG_WITH([$1],
                       AC_HELP_STRING([--with-$1], [use $1 (default is $2)]),
                       ac_cv_use_$1=$withval, ac_cv_use_$1=no),
          AC_CACHE_CHECK(whether to use $1, ac_cv_use_$1, ac_cv_use_$1=$2)])

Configuring Site Details
========================

Some software packages require complex site-specific information.  Some examples are host names to use for certain services, company names, and email addresses to contact.  Since some configuration scripts generated by Metaconfig ask for such information interactively, people sometimes wonder how to get that information in Autoconf-generated configuration scripts, which aren't interactive.

Such site configuration information should be put in a file that is edited only by users, not by programs.  The location of the file can either be based on the prefix variable, or be a standard location such as the user's home directory.  It could even be specified by an environment variable.  The programs should examine that file at run time, rather than at compile time.  Run time configuration is more convenient for users and makes the configuration process simpler than getting the information while configuring.  See Variables for Installation Directories (GNU Coding Standards), for more information on where to put data files.

Transforming Program Names When Installing
==========================================

Autoconf supports changing the names of programs when installing them. In order to use these transformations, configure.ac must call the macro AC_ARG_PROGRAM.

 -- Macro: AC_ARG_PROGRAM
     Place in output variable program_transform_name a sequence of sed commands for changing the names of installed programs.

     If any of the options described below are given to configure, program names are transformed accordingly.  Otherwise, if AC_CANONICAL_TARGET has been called and a --target value is given that differs from the host type (specified with --host), the target type followed by a dash is used as a prefix.  Otherwise, no program name transformation is done.

Transformation Options
----------------------

You can specify name transformations by giving configure these command line options:

--program-prefix=PREFIX
     prepend PREFIX to the names;

--program-suffix=SUFFIX
     append SUFFIX to the names;

--program-transform-name=EXPRESSION
     perform sed substitution EXPRESSION on the names.

Transformation Examples
-----------------------

These transformations are useful with programs that can be part of a cross-compilation development environment.  For example, a cross-assembler running on a Sun 4 configured with --target=i960-vxworks is normally installed as i960-vxworks-as, rather than as, which could be confused with a native Sun 4 assembler.

You can force a program name to begin with g, if you don't want GNU programs installed on your system to shadow other programs with the same name.  For example, if you configure GNU diff with --program-prefix=g, then when you run make install it is installed as /usr/local/bin/gdiff.

As a more sophisticated example, you could use

     --program-transform-name='s/^/g/; s/^gg/g/; s/^gless/less/'

to prepend g to most of the program names in a source tree, excepting those like gdb that already have one and those like less and lesskey that aren't GNU programs.  (That is assuming that you have a source tree containing those programs that is set up to use this feature.)

One way to install multiple versions of some programs simultaneously is to append a version number to the name of one or both.  For example, if you want to keep Autoconf version 1 around for awhile, you can configure Autoconf version 2 using --program-suffix=2 to install the programs as /usr/local/bin/autoconf2, /usr/local/bin/autoheader2, etc.  Nevertheless, pay attention that only the binaries are renamed, therefore you'd have problems with the library files which might overlap.

Transformation Rules
--------------------

Here is how to use the variable program_transform_name in a Makefile.in:

     transform = @program_transform_name@
     install: all
             $(INSTALL_PROGRAM) myprog $(bindir)/`echo myprog | \
                                                    sed '$(transform)'`

     uninstall:
             rm -f $(bindir)/`echo myprog | sed '$(transform)'`

If you have more than one program to install, you can do it in a loop:

     PROGRAMS = cp ls rm
     install:
             for p in $(PROGRAMS); do \
               $(INSTALL_PROGRAM) $$p $(bindir)/`echo $$p | \
                                                   sed '$(transform)'`; \
             done

     uninstall:
             for p in $(PROGRAMS); do \
               rm -f $(bindir)/`echo $$p | sed '$(transform)'`; \
             done

It is guaranteed that program_transform_name is never empty, and that there are no useless separators.  Therefore you may safely embed program_transform_name within a sed program using ;:

     transform = @program_transform_name@
     transform_exe = s/$(EXEEXT)$$//;$(transform);s/$$/$(EXEEXT)/

Whether to do the transformations on documentation files (Texinfo or man) is a tricky question; there seems to be no perfect answer, due to the several reasons for name transforming.  Documentation is not usually particular to a specific architecture, and Texinfo files do not conflict with system documentation.  But they might conflict with earlier versions of the same files, and man pages sometimes do conflict with system documentation.  As a compromise, it is probably best to do name transformations on man pages but not on Texinfo manuals.

Setting Site Defaults
=====================

Autoconf-generated configure scripts allow your site to provide default values for some configuration values.  You do this by creating site- and system-wide initialization files.

If the environment variable CONFIG_SITE is set, configure uses its value as the name of a shell script to read.  Otherwise, it reads the shell script PREFIX/share/config.site if it exists, then PREFIX/etc/config.site if it exists.  Thus, settings in machine-specific files override those in machine-independent ones in case of conflict.

Site files can be arbitrary shell scripts, but only certain kinds of code are really appropriate to be in them.  Because configure reads any cache file after it has read any site files, a site file can define a default cache file to be shared between all Autoconf-generated configure scripts run on that system (see Cache Files).  If you set a default cache file in a site file, it is a good idea to also set the output variable CC in that site file, because the cache file is only valid for a particular compiler, but many systems have several available.

You can examine or override the value set by a command line option to configure in a site file; options set shell variables that have the same names as the options, with any dashes turned into underscores. The exceptions are that --without- and --disable- options are like giving the corresponding --with- or --enable- option and the value no.  Thus, --cache-file=localcache sets the variable cache_file to the value localcache; --enable-warnings=no or --disable-warnings sets the variable enable_warnings to the value no; --prefix=/usr sets the variable prefix to the value /usr; etc.

Site files are also good places to set default values for other output variables, such as CFLAGS, if you need to give them non-default values: anything you would normally do, repetitively, on the command line.  If you use non-default values for PREFIX or EXEC_PREFIX (wherever you locate the site file), you can set them in the site file if you specify it with the CONFIG_SITE environment variable.

You can set some cache values in the site file itself.  Doing this is useful if you are cross-compiling, so it is impossible to check features that require running a test program.  You could ``prime the cache'' by setting those values correctly for that system in PREFIX/etc/config.site.  To find out the names of the cache variables you need to set, look for shell variables with _cv_ in their names in the affected configure scripts, or in the Autoconf M4 source code for those macros.

The cache file is careful to not override any variables set in the site files.  Similarly, you should not override command-line options in the site files.  Your code should check that variables such as prefix and cache_file have their default values (as set near the top of configure) before changing them.

Here is a sample file /usr/share/local/gnu/share/config.site.  The command configure --prefix=/usr/share/local/gnu would read this file (if CONFIG_SITE is not set to a different file).

     # config.site for configure
     #
     # Change some defaults.
     test "$prefix" = NONE && prefix=/usr/share/local/gnu
     test "$exec_prefix" = NONE && exec_prefix=/usr/local/gnu
     test "$sharedstatedir" = '$prefix/com' && sharedstatedir=/var
     test "$localstatedir" = '$prefix/var' && localstatedir=/var

     # Give Autoconf 2.x generated configure scripts a shared default
     # cache file for feature test results, architecture-specific.
     if test "$cache_file" = /dev/null; then
       cache_file="$prefix/var/config.cache"
       # A cache file is only valid for one C compiler.
       CC=gcc
     fi

Running configure Scripts
*************************

Below are instructions on how to configure a package that uses a configure script, suitable for inclusion as an INSTALL file in the package.  A plain-text version of INSTALL which you may use comes with Autoconf.

     [file install.texi not included]

Recreating a Configuration
**************************

The configure script creates a file named config.status, which actually configures, instantiates, the template files.  It also records the configuration options that were specified when the package was last configured in case reconfiguring is needed.

Synopsis:

     ./config.status OPTION... [FILE...]

It configures the FILES, if none are specified, all the templates are instantiated.  The files must be specified without their dependencies, as in

     ./config.status foobar

not

     ./config.status foobar:foo.in:bar.in

The supported OPTIONs are:

--help
-h
     Print a summary of the command line options, the list of the template files and exit.

--version
-V
     Print the version number of Autoconf and exit.

--debug
-d
     Don't remove the temporary files.

--file=FILE[:TEMPLATE]
     Require that FILE be instantiated as if AC_CONFIG_FILES(FILE:TEMPLATE) was used.  Both FILE and TEMPLATE may be - in which case the standard output and/or standard input, respectively, is used.  If a TEMPLATE filename is relative, it is first looked for in the build tree, and then in the source tree. See Configuration Actions, for more details.

     This option and the following ones provide one way for separately distributed packages to share the values computed by configure. Doing so can be useful if some of the packages need a superset of the features that one of them, perhaps a common library, does.  These options allow a config.status file to create files other than the ones that its configure.ac specifies, so it can be used for a different package.

--header=FILE[:TEMPLATE]
     Same as --file above, but with AC_CONFIG_HEADERS.

--recheck
     Ask config.status to update itself and exit (no instantiation). This option is useful if you change configure, so that the results of some tests might be different from the previous run.  The --recheck option re-runs configure with the same arguments you used before, plus the --no-create option, which prevents configure from running config.status and creating Makefile and other files, and the --no-recursion option, which prevents configure from running other configure scripts in subdirectories.  (This is so other Makefile rules can run config.status when it changes; see Automatic Remaking, for an example).

config.status checks several optional environment variables that can alter its behavior:

 -- Variable: CONFIG_SHELL
     The shell with which to run configure for the --recheck option.  It must be Bourne-compatible.  The default is /bin/sh.

 -- Variable: CONFIG_STATUS
     The file name to use for the shell script that records the configuration.  The default is ./config.status.  This variable is useful when one package uses parts of another and the configure scripts shouldn't be merged because they are maintained separately.

You can use ./config.status in your Makefiles.  For example, in the dependencies given above (see Automatic Remaking), config.status is run twice when configure.ac has changed. If that bothers you, you can make each run only regenerate the files for that rule:

     config.h: stamp-h
     stamp-h: config.h.in config.status
             ./config.status config.h
             echo > stamp-h

     Makefile: Makefile.in config.status
             ./config.status Makefile

The calling convention of config.status has changed, see Obsolete config.status Use, for details.

Obsolete Constructs
*******************

Autoconf changes, and throughout the years some constructs are obsoleted. Most of the changes involve the macros, but the tools themselves, or even some concepts, are now considered obsolete.

You may completely skip this chapter if you are new to Autoconf, its intention is mainly to help maintainers updating their packages by understanding how to move to more modern constructs.

Obsolete config.status Invocation
=================================

config.status now supports arguments to specify the files to instantiate, see config.status Invocation, for more details. Before, environment variables had to be used.

 -- Variable: CONFIG_COMMANDS
     The tags of the commands to execute.  The default is the arguments given to AC_OUTPUT and AC_CONFIG_COMMANDS in configure.ac.

 -- Variable: CONFIG_FILES
     The files in which to perform @VARIABLE@ substitutions. The default is the arguments given to AC_OUTPUT and AC_CONFIG_FILES in configure.ac.

 -- Variable: CONFIG_HEADERS
     The files in which to substitute C #define statements.  The default is the arguments given to AC_CONFIG_HEADERS; if that macro was not called, config.status ignores this variable.

 -- Variable: CONFIG_LINKS
     The symbolic links to establish.  The default is the arguments given to AC_CONFIG_LINKS; if that macro was not called, config.status ignores this variable.

In config.status Invocation, using this old interface, the example would be:

     config.h: stamp-h
     stamp-h: config.h.in config.status
             CONFIG_COMMANDS= CONFIG_LINKS= CONFIG_FILES= \
               CONFIG_HEADERS=config.h ./config.status
             echo > stamp-h

     Makefile: Makefile.in config.status
             CONFIG_COMMANDS= CONFIG_LINKS= CONFIG_HEADERS= \
               CONFIG_FILES=Makefile ./config.status

(If configure.ac does not call AC_CONFIG_HEADERS, there is no need to set CONFIG_HEADERS in the make rules, equally for CONFIG_COMMANDS etc.)

acconfig.h
==========

In order to produce config.h.in, autoheader needs to build or to find templates for each symbol.  Modern releases of Autoconf use AH_VERBATIM and AH_TEMPLATE (see Autoheader Macros), but in older releases a file, acconfig.h, contained the list of needed templates.  autoheader copies comments and #define and #undef statements from acconfig.h in the current directory, if present.  This file used to be mandatory if you AC_DEFINE any additional symbols.

Modern releases of Autoconf also provide AH_TOP and AH_BOTTOM if you need to prepend/append some information to config.h.in.  Ancient versions of Autoconf had a similar feature: if ./acconfig.h contains the string @TOP@, autoheader copies the lines before the line containing @TOP@ into the top of the file that it generates.  Similarly, if ./acconfig.h contains the string @BOTTOM@, autoheader copies the lines after that line to the end of the file it generates.  Either or both of those strings may be omitted.  An even older alternate way to produce the same effect in jurasik versions of Autoconf is to create the files FILE.top (typically config.h.top) and/or FILE.bot in the current directory.  If they exist, autoheader copies them to the beginning and end, respectively, of its output.

In former versions of Autoconf, the files used in preparing a software package for distribution were:

     configure.ac --.   .------> autoconf* -----> configure
                    +---+
     [aclocal.m4] --+   `---.
     [acsite.m4] ---'       |
                            +--> [autoheader*] -> [config.h.in]
     [acconfig.h] ----.     |
                      +-----'
     [config.h.top] --+
     [config.h.bot] --'

Use only the AH_ macros, configure.ac should be self-contained, and should not depend upon acconfig.h etc.

Using autoupdate to Modernize configure.ac
==========================================

The autoupdate program updates a configure.ac file that calls Autoconf macros by their old names to use the current macro names. In version 2 of Autoconf, most of the macros were renamed to use a more uniform and descriptive naming scheme.  See Macro Names, for a description of the new scheme.  Although the old names still work (see Obsolete Macros, for a list of the old macros and the corresponding new names), you can make your configure.ac files more readable and make it easier to use the current Autoconf documentation if you update them to use the new macro names.

If given no arguments, autoupdate updates configure.ac, backing up the original version with the suffix ~ (or the value of the environment variable SIMPLE_BACKUP_SUFFIX, if that is set).  If you give autoupdate an argument, it reads that file instead of configure.ac and writes the updated file to the standard output.

autoupdate accepts the following options:

--help
-h
     Print a summary of the command line options and exit.

--version
-V
     Print the version number of Autoconf and exit.

--verbose
-v
     Report processing steps.

--debug
-d
     Don't remove the temporary files.

--autoconf-dir=DIR
-A DIR
     Override the location where the installed Autoconf data files are looked for.  You can also set the AC_MACRODIR environment variable to a directory; this option overrides the environment variable.

     This option is rarely needed and dangerous; it is only used when one plays with different versions of Autoconf simultaneously.

--localdir=DIR
-l DIR
     Look for the package file aclocal.m4 in directory DIR instead of in the current directory.

Obsolete Macros
===============

Several macros are obsoleted in Autoconf, for various reasons (typically they failed to quote properly, couldn't be extended for more recent issues etc.).  They are still supported, but deprecated: their use should be avoided.

During the jump from Autoconf version 1 to version 2, most of the macros were renamed to use a more uniform and descriptive naming scheme, but their signature did not change.  See Macro Names, for a description of the new naming scheme.  Below, there is just the mapping from old names to new names for these macros, the reader is invited to refer to the definition of the new macro for the signature and the description.

 -- Macro: AC_ALLOCA
     AC_FUNC_ALLOCA

 -- Macro: AC_ARG_ARRAY
     removed because of limited usefulness

 -- Macro: AC_C_CROSS
     This macro is obsolete; it does nothing.

 -- Macro: AC_CANONICAL_SYSTEM
     Determine the system type and set output variables to the names of the canonical system types.  See Canonicalizing, for details about the variables this macro sets.

     The user is encouraged to use either AC_CANONICAL_BUILD, or AC_CANONICAL_HOST, or AC_CANONICAL_TARGET, depending on the needs.  Using AC_CANONICAL_TARGET is enough to run the two other macros.

 -- Macro: AC_CHAR_UNSIGNED
     AC_C_CHAR_UNSIGNED

 -- Macro: AC_CHECK_TYPE (TYPE, DEFAULT)
     Autoconf, up to 2.13, used to provide this version of AC_CHECK_TYPE, deprecated because of its flaws.  Firstly, although it is a member of the CHECK clan, singular sub-family, it does more than just checking.  Second, missing types are not typedef'd, they are #define'd, which can lead to incompatible code in the case of pointer types.

     This use of AC_CHECK_TYPE is obsolete and discouraged, see Generic Types, for the description of the current macro.

     If the type TYPE is not defined, define it to be the C (or C++) builtin type DEFAULT; e.g., short or unsigned.

     This macro is equivalent to:

          AC_CHECK_TYPE([TYPE],
                        [AC_DEFINE([TYPE], [DEFAULT],
                                   [Define to `DEFAULT' if <sys/types.h>
                                    does not define.])])

     In order to keep backward compatibility, the two versions of AC_CHECK_TYPE are implemented, selected by a simple heuristics:

      1. If there are three or four arguments, the modern version is used.

      2. If the second argument appears to be a C or C++ type, then the obsolete version is used.  This happens if the argument is a C or C++ builtin type or a C identifier ending in _t, optionally followed by one of [(*  and then by a string of zero or more characters taken from the set []()* _a-zA-Z0-9.

      3. If the second argument is spelled with the alphabet of valid C and C++ types, the user is warned and the modern version is used.

      4. Otherwise, the modern version is used.

     You are encouraged either to use a valid builtin type, or to use the equivalent modern code (see above), or better yet, to use AC_CHECK_TYPES together with

          #if !HAVE_LOFF_T
          typedef loff_t off_t;
          #endif

 -- Macro: AC_CHECKING (FEATURE-DESCRIPTION)
     Same as AC_MSG_NOTICE([checking FEATURE-DESCRIPTION...].

 -- Macro: AC_COMPILE_CHECK (ECHO-TEXT, INCLUDES, FUNCTION-BODY, ACTION-IF-FOUND, [action-if-not-found])
     This is an obsolete version of AC_TRY_LINK (see Examining Libraries), with the addition that it prints checking for ECHO-TEXT to the standard output first, if ECHO-TEXT is non-empty.  Use AC_MSG_CHECKING and AC_MSG_RESULT instead to print messages (see Printing Messages).

 -- Macro: AC_CONST
     AC_C_CONST

 -- Macro: AC_CROSS_CHECK
     Same as AC_C_CROSS, which is obsolete too, and does nothing :-).

 -- Macro: AC_CYGWIN
     Check for the Cygwin environment in which case the shell variable CYGWIN is set to yes.  Don't use this macro, the dignified means to check the nature of the host is using AC_CANONICAL_HOST.  As a matter of fact this macro is defined as:

          AC_REQUIRE([AC_CANONICAL_HOST])[]dnl
          case $host_os in
            *cygwin* ) CYGWIN=yes;;
                   * ) CYGWIN=no;;
          esac

     Beware that the variable CYGWIN has a very special meaning when running CygWin32, and should not be changed.  That's yet another reason not to use this macro.

 -- Macro: AC_DECL_YYTEXT
     Does nothing, now integrated in AC_PROG_LEX.

 -- Macro: AC_DIR_HEADER
     Like calling AC_FUNC_CLOSEDIR_VOID andAC_HEADER_DIRENT, but defines a different set of C preprocessor macros to indicate which header file is found:

     Header              Old Symbol       New Symbol
     dirent.h     DIRENT    HAVE_DIRENT_H
     sys/ndir.h   SYSNDIR   HAVE_SYS_NDIR_H
     sys/dir.h    SYSDIR    HAVE_SYS_DIR_H
     ndir.h       NDIR      HAVE_NDIR_H

 -- Macro: AC_DYNIX_SEQ
     If on Dynix/PTX (Sequent UNIX), add -lseq to output variable LIBS.  This macro used to be defined as

          AC_CHECK_LIB(seq, getmntent, LIBS="-lseq $LIBS")

     now it is just AC_FUNC_GETMNTENT.

 -- Macro: AC_EXEEXT
     Defined the output variable EXEEXT based on the output of the compiler, which is now done automatically.  Typically set to empty string if Unix and .exe if Win32 or OS/2.

 -- Macro: AC_EMXOS2
     Similar to AC_CYGWIN but checks for the EMX environment on OS/2 and sets EMXOS2.

 -- Macro: AC_ERROR
     AC_MSG_ERROR

 -- Macro: AC_FIND_X
     AC_PATH_X

 -- Macro: AC_FIND_XTRA
     AC_PATH_XTRA

 -- Macro: AC_FUNC_CHECK
     AC_CHECK_FUNC

 -- Macro: AC_FUNC_WAIT3
     If wait3 is found and fills in the contents of its third argument (a struct rusage *), which HP-UX does not do, define HAVE_WAIT3.

     These days portable programs should use waitpid, not wait3, as wait3 is being removed from the Open Group standards, and will not appear in the next revision of POSIX.

 -- Macro: AC_GCC_TRADITIONAL
     AC_PROG_GCC_TRADITIONAL

 -- Macro: AC_GETGROUPS_T
     AC_TYPE_GETGROUPS

 -- Macro: AC_GETLOADAVG
     AC_FUNC_GETLOADAVG

 -- Macro: AC_HAVE_FUNCS
     AC_CHECK_FUNCS

 -- Macro: AC_HAVE_HEADERS
     AC_CHECK_HEADERS

 -- Macro: AC_HAVE_LIBRARY (LIBRARY, [action-if-found], [action-if-not-found], [other-libraries])
     This macro is equivalent to calling AC_CHECK_LIB with a FUNCTION argument of main.  In addition, LIBRARY can be written as any of foo, -lfoo, or libfoo.a.  In all of those cases, the compiler is passed -lfoo.  However, LIBRARY cannot be a shell variable; it must be a literal name.

 -- Macro: AC_HAVE_POUNDBANG
     AC_SYS_INTERPRETER (different calling convention)

 -- Macro: AC_HEADER_CHECK
     AC_CHECK_HEADER

 -- Macro: AC_HEADER_EGREP
     AC_EGREP_HEADER

 -- Macro: AC_INIT (UNIQUE-FILE-IN-SOURCE-DIR)
     Formerly AC_INIT used to have a single argument, and was equivalent to:

          AC_INIT
          AC_CONFIG_SRCDIR(UNIQUE-FILE-IN-SOURCE-DIR)

 -- Macro: AC_INLINE
     AC_C_INLINE

 -- Macro: AC_INT_16_BITS
     If the C type int is 16 bits wide, define INT_16_BITS. Use AC_CHECK_SIZEOF(int) instead.

 -- Macro: AC_IRIX_SUN
     If on IRIX (Silicon Graphics UNIX), add -lsun to output LIBS.  If you were using it to get getmntent, use AC_FUNC_GETMNTENT instead.  If you used it for the NIS versions of the password and group functions, use AC_CHECK_LIB(sun, getpwnam).  Up to Autoconf 2.13, it used to be

          AC_CHECK_LIB(sun, getmntent, LIBS="-lsun $LIBS")

     now it is defined as

          AC_FUNC_GETMNTENT
          AC_CHECK_LIB(sun, getpwnam)

 -- Macro: AC_LANG_C
     Same as AC_LANG(C).

 -- Macro: AC_LANG_CPLUSPLUS
     Same as AC_LANG(C++).

 -- Macro: AC_LANG_FORTRAN77
     Same as AC_LANG(Fortran 77).

 -- Macro: AC_LANG_RESTORE
     Select the LANGUAGE that is saved on the top of the stack, as set by AC_LANG_SAVE, remove it from the stack, and call AC_LANG(LANGUAGE).

 -- Macro: AC_LANG_SAVE
     Remember the current language (as set by AC_LANG) on a stack. The current language does not change.  AC_LANG_PUSH is preferred.

 -- Macro: AC_LINK_FILES (SOURCE..., DEST...)
     This is an obsolete version of AC_CONFIG_LINKS.  An updated version of:

          AC_LINK_FILES(config/$machine.h config/$obj_format.h,
                        host.h            object.h)

     is:

          AC_CONFIG_LINKS(host.h:config/$machine.h
                          object.h:config/$obj_format.h)

 -- Macro: AC_LN_S
     AC_PROG_LN_S

 -- Macro: AC_LONG_64_BITS
     Define LONG_64_BITS if the C type long int is 64 bits wide. Use the generic macro AC_CHECK_SIZEOF([long int]) instead.

 -- Macro: AC_LONG_DOUBLE
     AC_C_LONG_DOUBLE

 -- Macro: AC_LONG_FILE_NAMES
     AC_SYS_LONG_FILE_NAMES

 -- Macro: AC_MAJOR_HEADER
     AC_HEADER_MAJOR

 -- Macro: AC_MEMORY_H
     Used to define NEED_MEMORY_H if the mem functions were defined in memory.h.  Today it is equivalent to AC_CHECK_HEADERS(memory.h).  Adjust your code to depend upon HAVE_MEMORY_H, not NEED_MEMORY_H, see Standard Symbols.

 -- Macro: AC_MINGW32
     Similar to AC_CYGWIN but checks for the MingW32 compiler environment and sets MINGW32.

 -- Macro: AC_MINUS_C_MINUS_O
     AC_PROG_CC_C_O

 -- Macro: AC_MMAP
     AC_FUNC_MMAP

 -- Macro: AC_MODE_T
     AC_TYPE_MODE_T

 -- Macro: AC_OBJEXT
     Defined the output variable OBJEXT based on the output of the compiler, after .c files have been excluded.  Typically set to o if Unix, obj if Win32.  Now the compiler checking macros handle this automatically.

 -- Macro: AC_OBSOLETE (THIS-MACRO-NAME, [suggestion])
     Make m4 print a message to the standard error output warning that THIS-MACRO-NAME is obsolete, and giving the file and line number where it was called.  THIS-MACRO-NAME should be the name of the macro that is calling AC_OBSOLETE.  If SUGGESTION is given, it is printed at the end of the warning message; for example, it can be a suggestion for what to use instead of THIS-MACRO-NAME.

     For instance

          AC_OBSOLETE([$0], [; use AC_CHECK_HEADERS(unistd.h) instead])dnl

     You are encouraged to use AU_DEFUN instead, since it gives better services to the user.

 -- Macro: AC_OFF_T
     AC_TYPE_OFF_T

 -- Macro: AC_OUTPUT ([file]..., [extra-cmds], [init-cmds])
     The use of AC_OUTPUT with argument is deprecated, this obsoleted interface is equivalent to:

          AC_CONFIG_FILES(FILE...)
          AC_CONFIG_COMMANDS([default],
                             EXTRA-CMDS, INIT-CMDS)
          AC_OUTPUT

 -- Macro: AC_OUTPUT_COMMANDS (EXTRA-CMDS, [init-cmds])
     Specify additional shell commands to run at the end of config.status, and shell commands to initialize any variables from configure.  This macro may be called multiple times.  It is obsolete, replaced by AC_CONFIG_COMMANDS.

     Here is an unrealistic example:

          fubar=27
          AC_OUTPUT_COMMANDS([echo this is extra $fubar, and so on.],
                             fubar=$fubar)
          AC_OUTPUT_COMMANDS([echo this is another, extra, bit],
                             [echo init bit])

     Aside from the fact that AC_CONFIG_COMMANDS requires an additional key, an important difference is that AC_OUTPUT_COMMANDS is quoting its arguments twice, while AC_CONFIG_COMMANDS.  This means that AC_CONFIG_COMMANDS can safely be given macro calls as arguments:

          AC_CONFIG_COMMANDS(foo, [my_FOO()])

     conversely, where one level of quoting was enough for literal strings with AC_OUTPUT_COMMANDS, you need two with AC_CONFIG_COMMANDS.  The following lines are equivalent:

          AC_OUTPUT_COMMANDS([echo "Square brackets: []"])
          AC_CONFIG_COMMANDS(default, [[echo "Square brackets: []"]])

 -- Macro: AC_PID_T
     AC_TYPE_PID_T

 -- Macro: AC_PREFIX
     AC_PREFIX_PROGRAM

 -- Macro: AC_PROGRAMS_CHECK
     AC_CHECK_PROGS

 -- Macro: AC_PROGRAMS_PATH
     AC_PATH_PROGS

 -- Macro: AC_PROGRAM_CHECK
     AC_CHECK_PROG

 -- Macro: AC_PROGRAM_EGREP
     AC_EGREP_CPP

 -- Macro: AC_PROGRAM_PATH
     AC_PATH_PROG

 -- Macro: AC_REMOTE_TAPE
     removed because of limited usefulness

 -- Macro: AC_RESTARTABLE_SYSCALLS
     AC_SYS_RESTARTABLE_SYSCALLS

 -- Macro: AC_RETSIGTYPE
     AC_TYPE_SIGNAL

 -- Macro: AC_RSH
     Removed because of limited usefulness.

 -- Macro: AC_SCO_INTL
     If on SCO UNIX, add -lintl to output variable LIBS.  This macro used to

          AC_CHECK_LIB(intl, strftime, LIBS="-lintl $LIBS")

     now it just calls AC_FUNC_STRFTIME instead.

 -- Macro: AC_SETVBUF_REVERSED
     AC_FUNC_SETVBUF_REVERSED

 -- Macro: AC_SET_MAKE
     AC_PROG_MAKE_SET

 -- Macro: AC_SIZEOF_TYPE
     AC_CHECK_SIZEOF

 -- Macro: AC_SIZE_T
     AC_TYPE_SIZE_T

 -- Macro: AC_STAT_MACROS_BROKEN
     AC_HEADER_STAT

 -- Macro: AC_STDC_HEADERS
     AC_HEADER_STDC

 -- Macro: AC_STRCOLL
     AC_FUNC_STRCOLL

 -- Macro: AC_ST_BLKSIZE
     AC_STRUCT_ST_BLKSIZE

 -- Macro: AC_ST_BLOCKS
     AC_STRUCT_ST_BLOCKS

 -- Macro: AC_ST_RDEV
     AC_STRUCT_ST_RDEV

 -- Macro: AC_SYS_RESTARTABLE_SYSCALLS
     If the system automatically restarts a system call that is interrupted by a signal, define HAVE_RESTARTABLE_SYSCALLS. This macro does not check if system calls are restarted in general--it tests whether a signal handler installed with signal (but not sigaction) causes system calls to be restarted. It does not test if system calls can be restarted when interrupted by signals that have no handler.

     These days portable programs should use sigaction with SA_RESTART if they want restartable system calls.  They should not rely on HAVE_RESTARTABLE_SYSCALLS, since nowadays whether a system call is restartable is a dynamic issue, not a configuration-time issue.

 -- Macro: AC_SYS_SIGLIST_DECLARED
     AC_DECL_SYS_SIGLIST

 -- Macro: AC_TEST_CPP
     AC_TRY_CPP

 -- Macro: AC_TEST_PROGRAM
     AC_TRY_RUN

 -- Macro: AC_TIMEZONE
     AC_STRUCT_TIMEZONE

 -- Macro: AC_TIME_WITH_SYS_TIME
     AC_HEADER_TIME

 -- Macro: AC_UID_T
     AC_TYPE_UID_T

 -- Macro: AC_UNISTD_H
     Same as AC_CHECK_HEADERS(unistd.h).

 -- Macro: AC_USG
     Define USG if the BSD string functions are defined in strings.h.  You should no longer depend upon USG, but on HAVE_STRING_H, see Standard Symbols.

 -- Macro: AC_UTIME_NULL
     AC_FUNC_UTIME_NULL

 -- Macro: AC_VALIDATE_CACHED_SYSTEM_TUPLE ([cmd])
     If the cache file is inconsistent with the current host, target and build system types, it used to execute CMD or print a default error message.

     This is now handled by default.

 -- Macro: AC_VERBOSE (RESULT-DESCRIPTION)
     AC_MSG_RESULT.

 -- Macro: AC_VFORK
     AC_FUNC_VFORK

 -- Macro: AC_VPRINTF
     AC_FUNC_VPRINTF

 -- Macro: AC_WAIT3
     AC_FUNC_WAIT3

 -- Macro: AC_WARN
     AC_MSG_WARN

 -- Macro: AC_WORDS_BIGENDIAN
     AC_C_BIGENDIAN

 -- Macro: AC_XENIX_DIR
     This macro used to add -lx to output variable LIBS if on Xenix.  Also, if dirent.h is being checked for, added -ldir to LIBS.  Now it is merely an alias of AC_HEADER_DIRENT instead, plus some code to detect whether running XENIX on which you should not depend:

          AC_MSG_CHECKING([for Xenix])
          AC_EGREP_CPP(yes,
          [#if defined M_XENIX && !defined M_UNIX
            yes
          #endif],
                       [AC_MSG_RESULT([yes]); XENIX=yes],
                       [AC_MSG_RESULT([no]); XENIX=])

 -- Macro: AC_YYTEXT_POINTER
     AC_DECL_YYTEXT

Upgrading From Version 1
========================

Autoconf version 2 is mostly backward compatible with version 1. However, it introduces better ways to do some things, and doesn't support some of the ugly things in version 1.  So, depending on how sophisticated your configure.ac files are, you might have to do some manual work in order to upgrade to version 2.  This chapter points out some problems to watch for when upgrading.  Also, perhaps your configure scripts could benefit from some of the new features in version 2; the changes are summarized in the file NEWS in the Autoconf distribution.

Changed File Names
------------------

If you have an aclocal.m4 installed with Autoconf (as opposed to in a particular package's source directory), you must rename it to acsite.m4.  See autoconf Invocation.

If you distribute install.sh with your package, rename it to install-sh so make builtin rules won't inadvertently create a file called install from it.  AC_PROG_INSTALL looks for the script under both names, but it is best to use the new name.

If you were using config.h.top, config.h.bot, or acconfig.h, you still can, but you will have less clutter if you use the AH_ macros.  See Autoheader Macros.

Changed Makefiles
-----------------

Add @CFLAGS@, @CPPFLAGS@, and @LDFLAGS@ in your Makefile.in files, so they can take advantage of the values of those variables in the environment when configure is run. Doing this isn't necessary, but it's a convenience for users.

Also add @configure_input@ in a comment to each input file for AC_OUTPUT, so that the output files will contain a comment saying they were produced by configure.  Automatically selecting the right comment syntax for all the kinds of files that people call AC_OUTPUT on became too much work.

Add config.log and config.cache to the list of files you remove in distclean targets.

If you have the following in Makefile.in:

     prefix = /usr/local
     exec_prefix = $(prefix)

you must change it to:

     prefix = @prefix@
     exec_prefix = @exec_prefix@

The old behavior of replacing those variables without @ characters around them has been removed.

Changed Macros
--------------

Many of the macros were renamed in Autoconf version 2.  You can still use the old names, but the new ones are clearer, and it's easier to find the documentation for them.  See Obsolete Macros, for a table showing the new names for the old macros.  Use the autoupdate program to convert your configure.ac to using the new macro names. See autoupdate Invocation.

Some macros have been superseded by similar ones that do the job better, but are not call-compatible.  If you get warnings about calling obsolete macros while running autoconf, you may safely ignore them, but your configure script will generally work better if you follow the advice it prints about what to replace the obsolete macros with.  In particular, the mechanism for reporting the results of tests has changed.  If you were using echo or AC_VERBOSE (perhaps via AC_COMPILE_CHECK), your configure script's output will look better if you switch to AC_MSG_CHECKING and AC_MSG_RESULT.  See Printing Messages.  Those macros work best in conjunction with cache variables.  See Caching Results.

Changed Results
---------------

If you were checking the results of previous tests by examining the shell variable DEFS, you need to switch to checking the values of the cache variables for those tests.  DEFS no longer exists while configure is running; it is only created when generating output files.  This difference from version 1 is because properly quoting the contents of that variable turned out to be too cumbersome and inefficient to do every time AC_DEFINE is called.  See Cache Variable Names.

For example, here is a configure.ac fragment written for Autoconf version 1:

     AC_HAVE_FUNCS(syslog)
     case "$DEFS" in
     *-DHAVE_SYSLOG*) ;;
     *) # syslog is not in the default libraries.  See if it's in some other.
       saved_LIBS="$LIBS"
       for lib in bsd socket inet; do
         AC_CHECKING(for syslog in -l$lib)
         LIBS="$saved_LIBS -l$lib"
         AC_HAVE_FUNCS(syslog)
         case "$DEFS" in
         *-DHAVE_SYSLOG*) break ;;
         *) ;;
         esac
         LIBS="$saved_LIBS"
       done ;;
     esac

Here is a way to write it for version 2:

     AC_CHECK_FUNCS(syslog)
     if test $ac_cv_func_syslog = no; then
       # syslog is not in the default libraries.  See if it's in some other.
       for lib in bsd socket inet; do
         AC_CHECK_LIB($lib, syslog, [AC_DEFINE(HAVE_SYSLOG)
           LIBS="$LIBS -l$lib"; break])
       done
     fi

If you were working around bugs in AC_DEFINE_UNQUOTED by adding backslashes before quotes, you need to remove them.  It now works predictably, and does not treat quotes (except back quotes) specially. See Setting Output Variables.

All of the boolean shell variables set by Autoconf macros now use yes for the true value.  Most of them use no for false, though for backward compatibility some use the empty string instead.  If you were relying on a shell variable being set to something like 1 or t for true, you need to change your tests.

Changed Macro Writing
---------------------

When defining your own macros, you should now use AC_DEFUN instead of define.  AC_DEFUN automatically calls AC_PROVIDE and ensures that macros called via AC_REQUIRE do not interrupt other macros, to prevent nested checking... messages on the screen.  There's no actual harm in continuing to use the older way, but it's less convenient and attractive.  See Macro Definitions.

You probably looked at the macros that came with Autoconf as a guide for how to do things.  It would be a good idea to take a look at the new versions of them, as the style is somewhat improved and they take advantage of some new features.

If you were doing tricky things with undocumented Autoconf internals (macros, variables, diversions), check whether you need to change anything to account for changes that have been made.  Perhaps you can even use an officially supported technique in version 2 instead of kludging.  Or perhaps not.

To speed up your locally written feature tests, add caching to them. See whether any of your tests are of general enough usefulness to encapsulate into macros that you can share.

Upgrading From Version 2.13
===========================

The introduction of the previous section (see Autoconf 1) perfectly suits this section...

     Autoconf version 2.50 is mostly backward compatible with version 2.13. However, it introduces better ways to do some things, and doesn't support some of the ugly things in version 2.13.  So, depending on how sophisticated your configure.ac files are, you might have to do some manual work in order to upgrade to version 2.50.  This chapter points out some problems to watch for when upgrading.  Also, perhaps your configure scripts could benefit from some of the new features in version 2.50; the changes are summarized in the file NEWS in the Autoconf distribution.

Changed Quotation
-----------------

The most important changes are invisible to you: the implementation of most macros have completely changed.  This allowed more factorization of the code, better error messages, a higher uniformity of the user's interface etc.  Unfortunately, as a side effect, some construct which used to (miraculously) work might break starting with Autoconf 2.50. The most common culprit is bad quotation.

For instance, in the following example, the message is not properly quoted:

     AC_INIT
     AC_CHECK_HEADERS(foo.h,,
     AC_MSG_ERROR(cannot find foo.h, bailing out))
     AC_OUTPUT

Autoconf 2.13 simply ignores it:

     $ autoconf-2.13; ./configure --silent
     creating cache ./config.cache
     configure: error: cannot find foo.h
     $

while Autoconf 2.50 will produce a broken configure:

     $ autoconf-2.50; ./configure --silent
     configure: error: cannot find foo.h
     ./configure: exit: bad non-numeric arg `bailing'
     ./configure: exit: bad non-numeric arg `bailing'
     $

The message needs to be quoted, and the AC_MSG_ERROR invocation too!

     AC_INIT
     AC_CHECK_HEADERS(foo.h,,
                      [AC_MSG_ERROR([cannot find foo.h, bailing out])])
     AC_OUTPUT

Many many (and many more) Autoconf macros were lacking proper quotation, including no less than... AC_DEFUN itself!

     $ cat configure.in
     AC_DEFUN([AC_PROG_INSTALL],
     [# My own much better version
     ])
     AC_INIT
     AC_PROG_INSTALL
     AC_OUTPUT
     $ autoconf-2.13
     autoconf: Undefined macros:
     ***BUG in Autoconf--please report*** AC_FD_MSG
     ***BUG in Autoconf--please report*** AC_EPI
     configure.in:1:AC_DEFUN([AC_PROG_INSTALL],
     configure.in:5:AC_PROG_INSTALL
     $ autoconf-2.50
     $

New Macros
----------

Because Autoconf has been dormant for years, Automake provided Autoconf-like macros for a while.  Autoconf 2.50 now provides better versions of these macros, integrated in the AC_ namespace, instead of AM_.  But in order to ease the upgrading via autoupdate, bindings to such AM_ macros are provided.

Unfortunately Automake did not quote the name of these macros! Therefore, when m4 find in aclocal.m4 something like AC_DEFUN(AM_TYPE_PTRDIFF_T, ...), AM_TYPE_PTRDIFF_T is expanded, replaced with its Autoconf definition.

Fortunately Autoconf catches pre-AC_INIT expansions, and will complain, in its own words:

     $ cat configure.in
     AC_INIT
     AM_TYPE_PTRDIFF_T
     $ aclocal-1.4
     $ autoconf
     ./aclocal.m4:17: error: m4_defn: undefined macro: _m4_divert_diversion
     actypes.m4:289: AM_TYPE_PTRDIFF_T is expanded from...
     ./aclocal.m4:17: the top level
     $

Future versions of Automake will simply no longer define most of these macros, and will properly quote the names of the remaining macros. But you don't have to wait for it to happen to do the right thing right now: do not depend upon macros from Automake as it is simply not its job to provide macros (but the one it requires by itself):

     $ cat configure.in
     AC_INIT
     AM_TYPE_PTRDIFF_T
     $ rm aclocal.m4
     $ autoupdate
     autoupdate: `configure.in' is updated
     $ cat configure.in
     AC_INIT
     AC_CHECK_TYPES([ptrdiff_t])
     $ aclocal-1.4
     $ autoconf
     $

Questions About Autoconf
************************

Several questions about Autoconf come up occasionally.  Here some of them are addressed.

Distributing configure Scripts
==============================

     What are the restrictions on distributing configure
     scripts that Autoconf generates?  How does that affect my
     programs that use them?

There are no restrictions on how the configuration scripts that Autoconf produces may be distributed or used.  In Autoconf version 1, they were covered by the GNU General Public License.  We still encourage software authors to distribute their work under terms like those of the GPL, but doing so is not required to use Autoconf.

Of the other files that might be used with configure, config.h.in is under whatever copyright you use for your configure.ac.  config.sub and config.guess have an exception to the GPL when they are used with an Autoconf-generated configure script, which permits you to distribute them under the same terms as the rest of your package.  install-sh is from the X Consortium and is not copyrighted.

Why Require GNU M4?
===================

     Why does Autoconf require GNU M4?

Many M4 implementations have hard-coded limitations on the size and number of macros that Autoconf exceeds.  They also lack several builtin macros that it would be difficult to get along without in a sophisticated application like Autoconf, including:

     builtin
     indir
     patsubst
     __file__
     __line__

Autoconf requires version 1.4 or above of GNU M4 because it uses frozen state files.

Since only software maintainers need to use Autoconf, and since GNU M4 is simple to configure and install, it seems reasonable to require GNU M4 to be installed also.  Many maintainers of GNU and other free software already have most of the GNU utilities installed, since they prefer them.

How Can I Bootstrap?
====================

     If Autoconf requires GNU M4 and GNU M4 has an Autoconf
     configure script, how do I bootstrap?  It seems like a chicken
     and egg problem!

This is a misunderstanding.  Although GNU M4 does come with a configure script produced by Autoconf, Autoconf is not required in order to run the script and install GNU M4.  Autoconf is only required if you want to change the M4 configure script, which few people have to do (mainly its maintainer).

Why Not Imake?
==============

     Why not use Imake instead of configure scripts?

Several people have written addressing this question, so I include adaptations of their explanations here.

The following answer is based on one written by Richard Pixley:

     Autoconf generated scripts frequently work on machines that it has never been set up to handle before.  That is, it does a good job of inferring a configuration for a new system.  Imake cannot do this.

     Imake uses a common database of host specific data.  For X11, this makes sense because the distribution is made as a collection of tools, by one central authority who has control over the database.

     GNU tools are not released this way.  Each GNU tool has a maintainer; these maintainers are scattered across the world.  Using a common database would be a maintenance nightmare.  Autoconf may appear to be this kind of database, but in fact it is not.  Instead of listing host dependencies, it lists program requirements.

     If you view the GNU suite as a collection of native tools, then the problems are similar.  But the GNU development tools can be configured as cross tools in almost any host+target permutation.  All of these configurations can be installed concurrently.  They can even be configured to share host independent files across hosts.  Imake doesn't address these issues.

     Imake templates are a form of standardization.  The GNU coding standards address the same issues without necessarily imposing the same restrictions.

Here is some further explanation, written by Per Bothner:

     One of the advantages of Imake is that it easy to generate large Makefiles using cpp's #include and macro mechanisms. However, cpp is not programmable: it has limited conditional facilities, and no looping.  And cpp cannot inspect its environment.

     All of these problems are solved by using sh instead of cpp.  The shell is fully programmable, has macro substitution, can execute (or source) other shell scripts, and can inspect its environment.

Paul Eggert elaborates more:

     With Autoconf, installers need not assume that Imake itself is already installed and working well.  This may not seem like much of an advantage to people who are accustomed to Imake.  But on many hosts Imake is not installed or the default installation is not working well, and requiring Imake to install a package hinders the acceptance of that package on those hosts.  For example, the Imake template and configuration files might not be installed properly on a host, or the Imake build procedure might wrongly assume that all source files are in one big directory tree, or the Imake configuration might assume one compiler whereas the package or the installer needs to use another, or there might be a version mismatch between the Imake expected by the package and the Imake supported by the host.  These problems are much rarer with Autoconf, where each package comes with its own independent configuration processor.

     Also, Imake often suffers from unexpected interactions between make and the installer's C preprocessor.  The fundamental problem here is that the C preprocessor was designed to preprocess C programs, not Makefiles.  This is much less of a problem with Autoconf, which uses the general-purpose preprocessor m4, and where the package's author (rather than the installer) does the preprocessing in a standard way.

Finally, Mark Eichin notes:

     Imake isn't all that extensible, either.  In order to add new features to Imake, you need to provide your own project template, and duplicate most of the features of the existing one.  This means that for a sophisticated project, using the vendor-provided Imake templates fails to provide any leverage---since they don't cover anything that your own project needs (unless it is an X11 program).

     On the other side, though:

     The one advantage that Imake has over configure: Imakefiles tend to be much shorter (likewise, less redundant) than Makefile.ins.  There is a fix to this, however---at least for the Kerberos V5 tree, we've modified things to call in common post.in and pre.in Makefile fragments for the entire tree.  This means that a lot of common things don't have to be duplicated, even though they normally are in configure setups.

History of Autoconf
*******************

You may be wondering, Why was Autoconf originally written?  How did it get into its present form?  (Why does it look like gorilla spit?)  If you're not wondering, then this chapter contains no information useful to you, and you might as well skip it.  If you are wondering, then let there be light...

Genesis
=======

In June 1991 I was maintaining many of the GNU utilities for the Free Software Foundation.  As they were ported to more platforms and more programs were added, the number of -D options that users had to select in the Makefile (around 20) became burdensome. Especially for me---I had to test each new release on a bunch of different systems.  So I wrote a little shell script to guess some of the correct settings for the fileutils package, and released it as part of fileutils 2.0.  That configure script worked well enough that the next month I adapted it (by hand) to create similar configure scripts for several other GNU utilities packages.  Brian Berliner also adapted one of my scripts for his CVS revision control system.

Later that summer, I learned that Richard Stallman and Richard Pixley were developing similar scripts to use in the GNU compiler tools; so I adapted my configure scripts to support their evolving interface: using the file name Makefile.in as the templates; adding +srcdir, the first option (of many); and creating config.status files.

Exodus
======

As I got feedback from users, I incorporated many improvements, using Emacs to search and replace, cut and paste, similar changes in each of the scripts.  As I adapted more GNU utilities packages to use configure scripts, updating them all by hand became impractical. Rich Murphey, the maintainer of the GNU graphics utilities, sent me mail saying that the configure scripts were great, and asking if I had a tool for generating them that I could send him.  No, I thought, but I should!  So I started to work out how to generate them.  And the journey from the slavery of hand-written configure scripts to the abundance and ease of Autoconf began.

Cygnus configure, which was being developed at around that time, is table driven; it is meant to deal mainly with a discrete number of system types with a small number of mainly unguessable features (such as details of the object file format).  The automatic configuration system that Brian Fox had developed for Bash takes a similar approach.  For general use, it seems to me a hopeless cause to try to maintain an up-to-date database of which features each variant of each operating system has.  It's easier and more reliable to check for most features on the fly---especially on hybrid systems that people have hacked on locally or that have patches from vendors installed.

I considered using an architecture similar to that of Cygnus configure, where there is a single configure script that reads pieces of configure.in when run.  But I didn't want to have to distribute all of the feature tests with every package, so I settled on having a different configure made from each configure.in by a preprocessor.  That approach also offered more control and flexibility.

I looked briefly into using the Metaconfig package, by Larry Wall, Harlan Stenn, and Raphael Manfredi, but I decided not to for several reasons.  The Configure scripts it produces are interactive, which I find quite inconvenient; I didn't like the ways it checked for some features (such as library functions); I didn't know that it was still being maintained, and the Configure scripts I had seen didn't work on many modern systems (such as System V R4 and NeXT); it wasn't very flexible in what it could do in response to a feature's presence or absence; I found it confusing to learn; and it was too big and complex for my needs (I didn't realize then how much Autoconf would eventually have to grow).

I considered using Perl to generate my style of configure scripts, but decided that M4 was better suited to the job of simple textual substitutions: it gets in the way less, because output is implicit.  Plus, everyone already has it.  (Initially I didn't rely on the GNU extensions to M4.)  Also, some of my friends at the University of Maryland had recently been putting M4 front ends on several programs, including tvtwm, and I was interested in trying out a new language.

Leviticus
=========

Since my configure scripts determine the system's capabilities automatically, with no interactive user intervention, I decided to call the program that generates them Autoconfig.  But with a version number tacked on, that name would be too long for old UNIX file systems, so I shortened it to Autoconf.

In the fall of 1991 I called together a group of fellow questers after the Holy Grail of portability (er, that is, alpha testers) to give me feedback as I encapsulated pieces of my handwritten scripts in M4 macros and continued to add features and improve the techniques used in the checks.  Prominent among the testers were François Pinard, who came up with the idea of making an autoconf shell script to run m4 and check for unresolved macro calls; Richard Pixley, who suggested running the compiler instead of searching the file system to find include files and symbols, for more accurate results; Karl Berry, who got Autoconf to configure TeX and added the macro index to the documentation; and Ian Lance Taylor, who added support for creating a C header file as an alternative to putting -D options in a Makefile, so he could use Autoconf for his UUCP package. The alpha testers cheerfully adjusted their files again and again as the names and calling conventions of the Autoconf macros changed from release to release.  They all contributed many specific checks, great ideas, and bug fixes.

Numbers
=======

In July 1992, after months of alpha testing, I released Autoconf 1.0, and converted many GNU packages to use it.  I was surprised by how positive the reaction to it was.  More people started using it than I could keep track of, including people working on software that wasn't part of the GNU Project (such as TCL, FSP, and Kerberos V5). Autoconf continued to improve rapidly, as many people using the configure scripts reported problems they encountered.

Autoconf turned out to be a good torture test for M4 implementations. UNIX m4 started to dump core because of the length of the macros that Autoconf defined, and several bugs showed up in GNU m4 as well.  Eventually, we realized that we needed to use some features that only GNU M4 has.  4.3BSD m4, in particular, has an impoverished set of builtin macros; the System V version is better, but still doesn't provide everything we need.

More development occurred as people put Autoconf under more stresses (and to uses I hadn't anticipated).  Karl Berry added checks for X11. david zuhn contributed C++ support.  François Pinard made it diagnose invalid arguments.  Jim Blandy bravely coerced it into configuring GNU Emacs, laying the groundwork for several later improvements. Roland McGrath got it to configure the GNU C Library, wrote the autoheader script to automate the creation of C header file templates, and added a --verbose option to configure. Noah Friedman added the --autoconf-dir option and AC_MACRODIR environment variable.  (He also coined the term autoconfiscate to mean ``adapt a software package to use Autoconf''.)  Roland and Noah improved the quoting protection in AC_DEFINE and fixed many bugs, especially when I got sick of dealing with portability problems from February through June, 1993.

Deuteronomy
===========

A long wish list for major features had accumulated, and the effect of several years of patching by various people had left some residual cruft.  In April 1994, while working for Cygnus Support, I began a major revision of Autoconf.  I added most of the features of the Cygnus configure that Autoconf had lacked, largely by adapting the relevant parts of Cygnus configure with the help of david zuhn and Ken Raeburn.  These features include support for using config.sub, config.guess, --host, and --target; making links to files; and running configure scripts in subdirectories.  Adding these features enabled Ken to convert GNU as, and Rob Savoye to convert DejaGNU, to using Autoconf.

I added more features in response to other peoples' requests.  Many people had asked for configure scripts to share the results of the checks between runs, because (particularly when configuring a large source tree, like Cygnus does) they were frustratingly slow.  Mike Haertel suggested adding site-specific initialization scripts.  People distributing software that had to unpack on MS-DOS asked for a way to override the .in extension on the file names, which produced file names like config.h.in containing two dots.  Jim Avera did an extensive examination of the problems with quoting in AC_DEFINE and AC_SUBST; his insights led to significant improvements. Richard Stallman asked that compiler output be sent to config.log instead of /dev/null, to help people debug the Emacs configure script.

I made some other changes because of my dissatisfaction with the quality of the program.  I made the messages showing results of the checks less ambiguous, always printing a result.  I regularized the names of the macros and cleaned up coding style inconsistencies.  I added some auxiliary utilities that I had developed to help convert source code packages to use Autoconf.  With the help of François Pinard, I made the macros not interrupt each others' messages.  (That feature revealed some performance bottlenecks in GNU m4, which he hastily corrected!)  I reorganized the documentation around problems people want to solve.  And I began a test suite, because experience had shown that Autoconf has a pronounced tendency to regress when we change it.

Again, several alpha testers gave invaluable feedback, especially François Pinard, Jim Meyering, Karl Berry, Rob Savoye, Ken Raeburn, and Mark Eichin.

Finally, version 2.0 was ready.  And there was much rejoicing.  (And I have free time again.  I think.  Yeah, right.)

Environment Variable Index
**************************

This is an alphabetical list of the environment variables that Autoconf checks.

Output Variable Index
*********************

This is an alphabetical list of the variables that Autoconf can substitute into files that it creates, typically one or more Makefiles.  See Setting Output Variables, for more information on how this is done.

Preprocessor Symbol Index
*************************

This is an alphabetical list of the C preprocessor symbols that the Autoconf macros define.  To work with Autoconf, C source code needs to use these names in #if directives.

Autoconf Macro Index
********************

This is an alphabetical list of the Autoconf macros.  To make the list easier to use, the macros are listed without their preceding AC_.

M4 Macro Index
**************

This is an alphabetical list of the M4, M4sugar, and M4sh macros.  To make the list easier to use, the macros are listed without their preceding m4_ or AS_.

Concept Index
*************

This is an alphabetical list of the files, tools, and concepts introduced in this document.