autoconf/doc/autoconf.texi

\input texinfo @c -*-texinfo-*-
@c %**start of header
@setfilename autoconf.info
@settitle Autoconf
@c For double-sided printing, uncomment:
@c @setchapternewpage odd
@c %**end of header

@set EDITION 1.95
@set VERSION 1.95
@set UPDATED July 1994

@iftex
@finalout
@end iftex

@ifinfo
@format
START-INFO-DIR-ENTRY
* autoconf: (autoconf).	The Autoconf configuration system.
END-INFO-DIR-ENTRY
@end format

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

Copyright (C) 1992, 1993, 1994 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.

@ignore
Permission is granted to process this file through TeX and print the
results, provided the printed document carries copying permission
notice identical to this one except for the removal of this paragraph
(this paragraph not being relevant to the printed manual).

@end ignore
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.
@end ifinfo

@titlepage
@title Autoconf
@subtitle Generating Automatic Configuration Scripts
@subtitle Edition @value{EDITION}, for Autoconf version @value{VERSION}
@subtitle @value{UPDATED}
@author by David MacKenzie, Roland McGrath, and Noah Friedman

@page
@vskip 0pt plus 1filll
Copyright @copyright{} 1992, 1993, 1994 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.
@end titlepage

@c Define a macro index that @@defmac doesn't write to.
@defcodeindex ma

@node Top, Introduction, , (dir)
@comment  node-name,  next,  previous,  up

@ifinfo
This file documents the GNU Autoconf package for creating scripts to
configure source code packages using templates and an @code{m4} macro
package.  This is edition @value{EDITION}, for Autoconf version @value{VERSION}.

@end ifinfo

@c The master menu, created with texinfo-master-menu, goes here.

@menu
* Introduction::                Autoconf's purpose, strengths, and weaknesses.
* Distributing::                Legal restrictions on Autoconf output.
* Making configure Scripts::    How to organize and produce Autoconf scripts.
* Specific Tests::              Macros that check for particular features.
* General Purpose Macros::      Macros that check for kinds of features.
* Manual Configuration::        Selecting features that can't be guessed.
* Writing Macros::              How to add your own macros to Autoconf.
* Caching Values::              Speeding up subsequent @code{configure} runs.
* Makefiles::                   Information Autoconf uses in @file{Makefile}s.
* Invoking configure::          How to use the Autoconf output.
* Invoking config.status::      Recreating a configuration.
* Site Default Values::         Providing local defaults for @code{configure}.
* Example::                     Sample Autoconf input files.
* Preprocessor Symbol Index::   Index of C preprocessor symbols defined.
* Macro Index::                 Index of Autoconf macros.

 --- The Detailed Node Listing ---

Making @code{configure} Scripts

* Writing configure.in::	What to put in an Autoconf input file.
* Invoking autoconf::		How to create configuration scripts.
* Invoking autoheader::		How to create configuration header files.
* Invoking autoscan::           Semi-automatic @file{configure.in} writing.
* Invoking ifnames::            Listing the conditionals in source code.
* Invoking autoreconf::         Remaking multiple @code{configure} scripts.
* Invoking shindent::           Making @code{configure} scripts more readable.

Specific Tests

* Alternative Programs::	Selecting between alternative programs.
* Header Files::		Header files that might be missing.
* Typedefs::			@code{typedef}s that might be missing.
* Library Functions::		C library functions that might be missing.
* Structures::			Structures or members that might be missing.
* Compiler Characteristics::	C compiler or machine architecture features.
* System Services::		Operating system services.
* UNIX Variants::		Special cases for specific UNIX variants.

General Purpose Macros

* Setup::			Controlling Autoconf operation.
* File Existence::              Checking whether particular files exist.
* C Features::                  Checking for features of the C system.
* Command Line::                Checking command line arguments.
* Setting Variables::		Setting shell and @code{make} variables.
* Printing Messages::           Notifying users of progress or problems.
* Language Choice::             Selecting which language to use for testing.
* Macro Ordering::		Enforcing ordering constraints.

Writing Macros

* Macro Format::		Basic format of an Autoconf macro.
* Quoting::			Protecting macros from unwanted expansion.
* Dependencies Between Macros::	What to do when macros depend on other macros.
* Checking for Files::		Finding whether a file exists.
* Checking for Symbols::	Finding whether a symbol is defined.
* Test Programs::		Writing programs to test for features.
* Multiple Cases::		Tests for several possible values.

Dependencies Between Macros

* Prerequisite Macros::		Ensuring required information.
* Suggested Ordering::		Warning about possible ordering problems.

Test Programs

* Guidelines::			General rules for writing test programs.
* Tricks::			Special ways to work around problems.

Makefiles

* Predefined Variables::	Heavily used @code{make} variables.
* VPATH Substitutions::		Compiling in a different directory.
* Automatic Remaking::		Makefile rules for configuring.

Running @code{configure} Scripts

* Basic Installation::          Instructions for typical cases.
* Build Directory::             Configuring in a different directory.
* Installation Directories::    Installing in different directories.
* System Type::                 Specifying the system type.
* Optional Features::           Selecting optional features.
* Compilers and Options::       Selecting compilers and optimization.

An Example

* Sample configure.in::		An example of a @file{configure} template.
* Sample Makefile.in::		An example of a @file{Makefile} template.
@end menu

@node Introduction, Distributing, Top, Top
@chapter Introduction

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 normally require no
manual user intervention when run; they do not even take an argument
specifying the system type.  Instead, they test for the presence of each
feature that the software package they are for might need individually.
(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 operating
system features that the package can use.  After the shell code to
recognize and respond to an operating 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 the configuration scripts can be regenerated
automatically to take advantage of the updated code.

Larry Wall's Metaconfig package is similar in purpose to Autoconf, but
is more general.  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.  They should avoid executing test
programs, since test programs compiled with a cross-compiler can not be
executed on the host system.  Also, they shouldn't do anything that
tests features of the host system instead of the target system.

Autoconf imposes some restrictions on the names of macros used with
@code{#ifdef} in C programs (@pxref{Preprocessor Symbol Index}).

Autoconf requires GNU @code{m4} in order to generate the scripts.  It
uses features that some UNIX versions of @code{m4} do not have.  It also
overflows internal limits of some versions of @code{m4}, including GNU
@code{m4} 1.0; so use a later version of GNU @code{m4}.

Autoconf does not work well with GNU C library releases before 1.06.
The GNU C library contains stubs (which always return an error) for
functions that are not available instead of omitting them from the
library.  As a result, Autoconf scripts are fooled into thinking that
those functions are available.  This problem does not exist with
releases 1.06 and later of the GNU C library, which define C
preprocessor macros that the Autoconf macros @code{AC_FUNC_CHECK} and
@code{AC_REPLACE_FUNCS} test, indicating that certain functions are
stubs (@pxref{C Features}, for more information on checking for
functions).

@ifinfo
Autoconf was written by David MacKenzie, with help from Franc,ois
@end ifinfo
@tex
Autoconf was written by David MacKenzie, with help from Fran\c cois
@end tex
Pinard, Karl Berry, Richard Pixley, Ian Lance Taylor, Roland McGrath,
Noah Friedman, and david d zuhn.  It was inspired by Brian Fox's
automatic configuration system for Bash, by Larry Wall's Metaconfig, and
by Richard Stallman, Richard Pixley, and John Gilmore's configuration
tools for the GNU compiler and object file utilities.

Mail suggestions and bug reports for Autoconf to
@code{bug-gnu-utils@@prep.ai.mit.edu}.  Please include the Autoconf version
number, which you can get by running @samp{autoconf --version}.

@node Distributing, Making configure Scripts, Introduction, Top
@chapter Distributing Autoconf Output

The configuration scripts that Autoconf produces are covered by the GNU
General Public License.  This is because they consist almost entirely of
parts of Autoconf itself, rearranged somewhat, and Autoconf is
distributed under the terms of the GPL.  As applied to Autoconf, the GPL
just means that you need to distribute @file{configure.in}, and
@file{aclocal.m4}, @file{acconfig.h}, and @file{@var{config}.h.top} and
@file{@var{config}.h.bot} if you use them, along with @file{configure}.

Programs that use Autoconf scripts to configure themselves do not
automatically come under the GPL.  Distributing an Autoconf
configuration script as part of a program is considered to be @emph{mere
aggregation} of that work with the Autoconf script.  Such programs are
not derivative works based on Autoconf; only their configuration scripts
are.  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.

@node Making configure Scripts, Specific Tests, Distributing, Top
@chapter Making @code{configure} Scripts

The configuration scripts that Autoconf produces are by convention
called @code{configure}.  When run, @code{configure} creates several
files, replacing configuration parameters in them with values
appropriate for the system being configured.  The files that
@code{configure} creates are:

@itemize @bullet
@item
one or more @file{Makefile} files (one in each subdirectory of the
package), from template @file{Makefile.in} files (@pxref{Makefiles});

@item
optionally, a C header file, the name of which is configurable,
containing @code{#define} directives (@pxref{Setup});

@item
a shell script called @file{config.status} that, when run, will recreate
the files listed above (@pxref{Invoking config.status});

@item
a shell script called @file{config.cache} that saves the results of
running many of the tests;

@item
a file called @file{config.log} containing any messages produced by
compilers, to help debugging if @code{configure} makes a mistake.
@end itemize

To create a @code{configure} script with Autoconf, you need to write an
Autoconf input file (@file{configure.in}) and run Autoconf on it to
produce the script.  If you write your own feature tests to supplement
those that come with Autoconf, you might also write a file called
@file{aclocal.m4}.  If you use a C header file to contain @code{#define}
directives, you might also write @file{config.h.top},
@file{config.h.bot}, and @file{acconfig.h}, and you will distribute the
Autoconf-generated file @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
@samp{*}.  Optional files are enclosed in square brackets (@samp{[]}).
@code{autoconf} and @code{autoheader} also read the installed files
@file{acgeneral.m4} and @file{acspecific.m4}, and also an installed
@file{aclocal.m4} if it exists.

@noindent
Files used in preparing a software package for distribution:
@example
@group
your source files --> [autoscan*] --> [configure.scan] --> configure.in

configure.in --.   .------> autoconf* -----> configure
               +---+ 
[aclocal.m4] --'   `---.
                       +--> [autoheader*] -> [config.h.in]
[acconfig.h] ----.     |
                 +-----'
[config.h.top] --+
[config.h.bot] --'

Makefile.in -------------------------------> Makefile.in
@end group
@end example

@noindent
Files used in configuring a software package:
@example
@group
configure* ------------.
                       |
[config.h.in] -.       v            .-> [config.h] -.
               +--> config.status* -+               +--> make*
Makefile.in ---'                    `-> Makefile ---'
@end group
@end example

@menu
* Writing configure.in::	What to put in an Autoconf input file.
* Invoking autoconf::		How to create configuration scripts.
* Invoking autoheader::		How to create configuration header files.
* Invoking autoscan::           Semi-automatic @file{configure.in} writing.
* Invoking ifnames::            Listing the conditionals in source code.
* Invoking autoreconf::         Remaking multiple @code{configure} scripts.
* Invoking shindent::           Making @code{configure} scripts more readable.
@end menu

@node Writing configure.in, Invoking autoconf, , Making configure Scripts
@section Writing @file{configure.in}

To produce a @code{configure} script for a software package, create a
file called @file{configure.in} 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
@ref{Specific Tests}, for their descriptions.  For most other
features, you can use Autoconf template macros to produce custom checks;
see @ref{General Purpose Macros}, for information about them.  For especially
tricky or specialized features, @file{configure.in} might need to
contain some hand-crafted shell commands.  @xref{Writing Macros}, for
guidelines on writing tests from scratch.

Other than that, the order in which @file{configure.in} calls the
Autoconf macros is generally not important, with a few exceptions.
Every @file{configure.in} must contain a call to @code{AC_INIT} before
the checks, and a call to @code{AC_OUTPUT} at the end (@pxref{Setup}).
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
(@pxref{Specific Tests}), and they also warn you if they are called out
of order.

To encourage consistency, here is a suggested order for calling the
Autoconf macros.  Note that there must not be any space between a macro
name and the open parentheses.

@display
@group
macros that control Autoconf operation
@code{AC_INIT(@var{file})}
checks for alternative programs
checks for UNIX variants that set C preprocessor variables
checks for header files
checks for typedefs
checks for library functions
checks for structures
checks for compiler characteristics
checks for system services
other checks for UNIX variants
@code{AC_OUTPUT(@r{[}@var{file@dots{}}@r{]})}
@end group
@end display

You can include comments in @file{configure.in} files by starting them
with the @code{m4} predefined macro @code{dnl}, which discards text up
through the next newline.  These comments do not appear in the generated
@code{configure} scripts.  For example, it is helpful to begin
@file{configure.in} files with a line like this:

@example
dnl Process this file with autoconf to produce a configure script.
@end example

@xref{Sample configure.in}, for an example of a real @file{configure.in}
script.

@node Invoking autoconf, Invoking autoheader, Writing configure.in, Making configure Scripts
@section Invoking @code{autoconf}

To create @code{configure} from @file{configure.in}, run the
@code{autoconf} program with no arguments.  @code{autoconf} processes
@file{configure.in} with the @code{m4} macro processor, using the
Autoconf macros.  If you give @code{autoconf} an argument, it reads that
file instead of @file{configure.in} and writes the configuration script
to the standard output instead of to @file{configure}.  If you give
@code{autoconf} the argument @samp{-}, it reads the standard input
instead of @file{configure.in} and writes the configuration script on
the standard output.

The Autoconf macros are defined in two or more files.  Two of the files
are distributed with Autoconf: @code{autoconf} first reads
@file{acgeneral.m4} (@pxref{General Purpose Macros}), then
@file{acspecific.m4} (@pxref{Specific Tests}).  After reading them,
@code{autoconf} looks for an optional file called @file{aclocal.m4},
first in the directory that contains other installed Autoconf macro
files, and then in the current directory.  If both files exist, it uses
both of them.  Those files can contain your site's own locally written
Autoconf macro definitions (@pxref{Writing Macros}, for more
information).  If a macro is defined in more than one of the files that
@code{autoconf} reads, the last definition it reads overrides the
earlier ones.

You can override the directory where @code{autoconf} looks for the
installed macro files by setting the @code{AC_MACRODIR} environment
variable to a different directory.  You can also give @code{autoconf}
the @samp{--macrodir} option, which overrides @code{AC_MACRODIR}.

@code{autoconf} also accepts the options @code{--version}, which prints
the Autoconf version number and exits, and @code{--help}, which prints a
summary of the command-line options and exits.

@node Invoking autoheader, Invoking autoscan, Invoking autoconf, Making configure Scripts
@section Invoking @code{autoheader}

You can use the @code{autoheader} program to create a template file of C
@samp{#define} statements for @code{configure} to use.  By default, the
file that @code{autoheader} creates is called @file{config.h.in}; if
@file{configure.in} invokes @code{AC_CONFIG_HEADER(@var{file})},
@code{autoheader} creates @file{@var{file}.in}.

@code{autoheader} scans @file{configure.in} and figures out which C
preprocessor symbols it might define.  It copies comments and
@code{#define} and @code{#undef} statements from a file called
@file{acconfig.h}, which comes with and is installed with Autoconf.  It
also uses a file called @file{acconfig.h} in the current directory, if
present; you must create that file to contain entries for any additional
symbols that you @code{AC_DEFINE}.  For symbols defined by
@code{AC_HAVE_HEADERS}, @code{AC_HAVE_FUNCS}, @code{AC_SIZEOF_TYPE},
or @code{AC_HAVE_LIBRARY},
@code{autoheader} generates comments and @code{#undef} statements itself
rather than copying them from a file, since the possible symbols are
effectively limitless.

The file that @code{autoheader} creates usually contains only
@code{#define} and @code{#undef} statements and their accompanying
comments.  However, if a file called @file{@var{file}.top} (typically
@file{config.h.top}) exists in the current directory, @code{autoheader}
copies that file to the beginning of its output.  Similarly, if
@file{@var{file}.bot} exists, it copies that file to the end of its output.

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

You can override the directory where @code{autoheader} looks for the
installed macro and @file{acconfig.h} files by setting the
@code{AC_MACRODIR} environment variable to a different directory.  You
can also give @code{autoheader} the @samp{--macrodir} option, which
overrides @code{AC_MACRODIR}.

@code{autoheader} also accepts the options @code{--version}, which prints
the Autoconf version number and exits, and @code{--help}, which prints a
summary of the command-line options and exits.

@node Invoking autoscan, Invoking ifnames, Invoking autoheader, Making configure Scripts
@section Invoking autoscan

The @code{autoscan} program can help you create a @file{configure.in}
file for a software package.  @code{autoscan} examines source files in
the 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 @file{configure.scan} which is a preliminary
@file{configure.in} for that package.  You should manually examine
@file{configure.scan} before renaming it to @file{configure.in}; it will
probably need some adjustments.  If you want the package to use a
configuration header file, you will have to add a call to
@code{AC_CONFIG_HEADER}.  You might have to change or add some
@code{#if} directives in order to make the package work with Autoconf
(@pxref{Invoking ifnames}, for information about a program that might
help with that job).

@code{autoscan} uses several data files, which are installed with the
distributed Autoconf macro files, to determine which macros to output
when it finds particular symbols in a package's source files.  These
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 @samp{#} are comments.

@code{autoscan} is only installed if you already have Perl installed.

@code{autoscan} accepts the following options:

@table @code
@item --help
Print a summary of the command line options and exit.

@item --macrodir=@var{dir}
Look for the data files in directory @var{dir} instead of the default
installation directory.

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

@item --version
Print the version number of @code{autoscan} and exit.
@end table

@node Invoking ifnames, Invoking autoreconf, Invoking autoscan, Making configure Scripts
@section Invoking ifnames

@code{ifnames} can help when writing a @file{configure.in} for a
software package.  It prints the identifiers used in C preprocessor
conditionals.  If a package has already been set up to have some
portability, this program can help you figure out what its
@code{configure} needs to check for.  It may fill in some gaps in a
@file{configure.in} generated by @code{autoscan} (@pxref{Invoking autoscan}).

@code{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 @code{#if}, @code{#elif}, or @code{#ifdef} directives.  It prints
each identifier on a line, followed by a space-separated list of the
files in which that identifier occurs.

@noindent
@code{ifnames} accepts the following options:

@table @code
@item --help
Print a summary of the command line options and exit.

@item --macrodir=@var{dir}
Look for the Autoconf macro files in directory @var{dir} instead of the
default installation directory.  Only used to get the version number.

@item --version
Print the version number of @code{ifnames} and exit.
@end table

@node Invoking autoreconf, Invoking shindent, Invoking ifnames, Making configure Scripts
@section Invoking autoreconf

If you have a lot of Autoconf-generated @code{configure} scripts and you
get a new version of Autoconf, the @code{autoreconf} program can be
handy.  It runs @code{autoconf} (and @code{autoheader}, where
appropriate) repeatedly to remake all of the Autoconf @code{configure}
scripts in a directory tree.

@noindent
@code{autoreconf} accepts the following options:

@table @code
@item --help
Print a summary of the command line options and exit.

@item --macrodir=@var{dir}
Look for the Autoconf macro files in directory @var{dir} instead of the
default installation directory.

@item --verbose
Print the name of each directory where @code{autoreconf} runs
@code{autoconf} (and @code{autoheader}, if that directory's
@file{configure.in} calls @code{AC_CONFIG_HEADER}).

@item --version
Print the version number of @code{autoreconf} and exit.
@end table

@node Invoking shindent, , Invoking autoreconf, Making configure Scripts
@section Invoking shindent

Autoconf-generated @code{configure} scripts can be somewhat hard to
read, because they @emph{look} machine-generated---mainly because they
do not use whitespace rationally.  Unfortunately, it is impossible to do
proper indentation using a macro processor like @code{m4}.  However, if
you have GNU @code{bash} installed, you can use the @code{shindent}
program, which uses @code{bash} to give a Bourne shell script readable
indentation. 

@code{shindent} indents each script named on the command line, backing
up the original version using the original file name with @samp{~} appended.
@code{shindent} places some restrictions on its input, due to the way it
uses @code{bash} to do the indentation:

@itemize @bullet
@item
It removes comments and blank lines between cases case statements.

@item
Doing odd things in case statements might cause odd problems.

@item
The script must not define any shell functions.

@item
If any string literals or here-documents contain lines that start
with four spaces, those spaces will be removed.

@item
The script must not be @code{shindent} itself.
@end itemize

@noindent
@code{shindent} accepts the following options:

@table @code
@item --help
Print a summary of the command line options and exit.

@item --macrodir=@var{dir}
Look for the Autoconf macro files in directory @var{dir} instead of the
default installation directory.  Only used to get the version number.

@item --version
Print the version number of @code{shindent} and exit.
@end table

@node Specific Tests, General Purpose Macros, Making configure Scripts, Top
@chapter Specific Tests

These macros test for particular operating system features that packages
might need or want to use.  If you need to test for a feature that none
of these macros check for, you can probably do it by calling one of the
general purpose test macros with appropriate arguments (@pxref{General
Purpose Macros}).

All of these macros that set @code{make} variables call @code{AC_SUBST}
on those variables (@pxref{Setting Variables}, for details about
@code{AC_SUBST}).  The phrase ``define @var{name}'' is used below as a
shorthand to mean either add @samp{-D@var{name}=1} to the @code{make}
variable @code{DEFS}, or put @samp{#define @var{name} 1} in the
configuration header file, depending on whether @code{AC_CONFIG_HEADER} has
been called.  @xref{Setting Variables}, for more information.

Within each section below, the macros are listed in alphabetical order.
The macros are generally named for the @code{make} variables or C
preprocessor macros that they define; those names are based largely on
what existing GNU programs use.  These macros are defined in the file
@file{acspecific.m4}.

@menu
* Alternative Programs::	Selecting between alternative programs.
* Header Files::		Header files that might be missing.
* Typedefs::			@code{typedef}s that might be missing.
* Library Functions::		C library functions that might be missing.
* Structures::			Structures or members that might be missing.
* Compiler Characteristics::	C compiler or machine architecture features.
* System Services::		Operating system services.
* UNIX Variants::		Special cases for specific UNIX variants.
@end menu

@node Alternative Programs, Header Files, , Specific Tests
@section Alternative Programs

The following macros check for the presence or behavior of particular
programs:

@defmac AC_GCC_TRADITIONAL
@maindex GCC_TRADITIONAL
Add @samp{-traditional} to @code{make} variable @code{CC} if using the
GNU C compiler and @code{ioctl} does not work properly without
@samp{-traditional}.  This macro calls @code{AC_PROG_CC} and
@code{AC_PROG_CPP} if they haven't been called already.
@end defmac

@defmac AC_LN_S
@maindex LN_S
If @samp{ln -s} works on the current filesystem (the operating system
and filesystem support symbolic links), set shell and @code{make}
variable @code{LN_S} to @samp{ln -s}, otherwise set it to @samp{ln}.
@end defmac

@defmac AC_MINUS_C_MINUS_O
@maindex MINUS_C_MINUS_O
@vindex NO_MINUS_C_MINUS_O
If the C compiler does not accept the @samp{-c} and @samp{-o} options
simultaneously, define @code{NO_MINUS_C_MINUS_O}.
@end defmac

@defmac AC_PROG_AWK
@maindex PROG_AWK
Check for @code{mawk}, @code{gawk}, @code{nawk}, and @code{awk}, in that
order, and set @code{make} variable @code{AWK} to the first one that it
finds.
@end defmac

@defmac AC_PROG_CC
@maindex PROG_CC
If @code{gcc} is found, set @code{make} variable @code{CC} to
@samp{gcc}, and set shell variable @code{GCC} to @samp{yes}.  Otherwise
set @code{GCC} to be empty.
@end defmac

@defmac AC_PROG_CPP
@maindex PROG_CPP
Set shell and @code{make} variable @code{CPP} to a command that runs the
C preprocessor.  If @samp{$CC -E} doesn't work, it uses @file{/lib/cpp}.
It is only portable to run @code{CPP} on files with a @file{.c}
extension.

If the current language is C (@pxref{Language Choice}), many of the
specific test macros use the value of @code{CPP} indirectly by calling
@code{AC_TEST_CPP}, @code{AC_HEADER_CHECK}, @code{AC_HEADER_EGREP}, or
@code{AC_PROGRAM_EGREP}.  Those macros call this macro first if it
hasn't been called already.  It calls @code{AC_PROG_CC} if it hasn't
been called already.
@end defmac

@defmac AC_PROG_CXX
@maindex PROG_CXX
Determine a C++ compiler to use.  Check if the environment variable
@var{CXX} or @var{CCC} (in that order) is set; if so, set @code{make}
variable @code{CXX} to its value.  Otherwise search for a C++ compiler
under likely names (@code{c++}, @code{g++}, @code{gcc}, and @code{CC}).
If none of those checks succeed, as a last resort set @code{CXX} to
@code{gcc}.  Also set shell variable @code{GXX} to @samp{yes} if it
found the GNU C++ compiler, empty otherwise.
@end defmac

@defmac AC_PROG_CXXCPP
@maindex PROG_CXXCPP
Set shell and @code{make} variable @code{CXXCPP} to a command that runs the
C++ preprocessor.  If @samp{$CXX -E} doesn't work, it uses @file{/lib/cpp}.
It is only portable to run @code{CXXCPP} on files with a @file{.c},
@file{.C}, or @file{.cc} extension.

If the current language is C++ (@pxref{Language Choice}), many of the
specific test macros use the value of @code{CXXCPP} indirectly by
calling @code{AC_TEST_CPP}, @code{AC_HEADER_CHECK},
@code{AC_HEADER_EGREP}, or @code{AC_PROGRAM_EGREP}.  Those macros call
this macro first if it hasn't been called already.  This macro calls
@code{AC_PROG_CXX} if it hasn't been called already.
@end defmac

@defmac AC_PROG_INSTALL
@maindex PROG_INSTALL
Set @code{make} variable @code{INSTALL} to @samp{install -c} if
@code{install} is found and is compatible with the BSD and GNU versions.
Otherwise, set @code{INSTALL} to @samp{@var{dir}/install.sh -c}, where
it checks the directories specified to @code{AC_CONFIG_AUX_DIR} (or its
default directories) to determine @var{dir} (@pxref{Setup}).

This macro screens out the false matches @file{/etc/install},
@file{/usr/sbin/install}, and other instances of @code{install} known
not to work.  It also sets the variable @code{INSTALL_PROGRAM} to
@samp{$@{INSTALL@}} and @code{INSTALL_DATA} to @samp{$@{INSTALL@} -m
644}.

If you need to use your own installation program because it has features
not found in standard @code{install} programs, there is no reason to use
@code{AC_PROG_INSTALL}; just put the pathname of your program into your
@file{Makefile.in} files.
@end defmac

@defmac AC_PROG_LEX
@maindex PROG_LEX
If @code{flex} is found, set @code{make} variable @code{LEX} to
@samp{flex} and @code{LEXLIB} to @samp{-lfl}, if that library is in a
standard place.  Otherwise set @code{LEX} to @samp{lex} and
@code{LEXLIB} to @samp{-ll}.
@end defmac

@defmac AC_PROG_RANLIB
@maindex PROG_RANLIB
Set @code{make} variable @code{RANLIB} to @samp{ranlib} if @code{ranlib}
is found, otherwise to @samp{:} (do nothing).
@end defmac

@defmac AC_PROG_YACC
@maindex PROG_YACC
If @code{bison} is found, set @code{make} variable @code{YACC} to
@samp{bison -y}.  Otherwise, if @code{byacc} is found, set @code{YACC}
to @samp{byacc}.  Otherwise set @code{YACC} to @samp{yacc}.
@end defmac

@defmac AC_RSH
@maindex RSH
@vindex RSH
@vindex NO_REMOTE
@vindex HAVE_NETDB_H
If a remote shell is available, set @code{make} variable @code{RSH} to
its pathname.  Otherwise, if @file{netdb.h} exists (implying the
@code{rexec} function), define @code{HAVE_NETDB_H}.  If either is true,
also put @samp{rtapelib.o} in @code{make} variable @code{RTAPELIB}.
Otherwise, define @code{NO_REMOTE}.
@end defmac

@defmac AC_SET_MAKE
@maindex SET_MAKE
If @code{make} predefines the variable @code{MAKE}, define @code{make}
variable @code{SET_MAKE} to be empty.  Otherwise, define @code{SET_MAKE}
to contain @samp{MAKE=make}.  Calls @code{AC_SUBST} for @code{SET_MAKE}.

In recent versions of @code{make}, the variable @code{MAKE} contains the
name of the @code{make} program plus options it was given.  It is used
when running @code{make} recursively in subdirectories.  But some old
versions of @code{make} don't set the @code{MAKE} variable.
This macro allows use of @code{MAKE} on all systems.

If you use this macro, simply place a line like this in your
@file{Makefile.in} file(s):

@example
@@SET_MAKE@@
@end example
@end defmac

@defmac AC_YYTEXT_POINTER
@maindex YYTEXT_POINTER
@vindex YYTEXT_POINTER
Define @code{YYTEXT_POINTER} if @code{yytext} is a @samp{char *} instead
of a @samp{char []}.  This depends on whether @code{lex} or @code{flex}
is being used.  This macro calls @code{AC_PROG_CPP} (or
@code{AC_PROG_CXXCPP} if C++ is the current language, @pxref{Language
Choice}) and @code{AC_PROG_LEX} if they haven't been called already.

This macro replaces @code{AC_DECLARE_YYTEXT}, which didn't work.
@end defmac

@node Header Files, Typedefs, Alternative Programs, Specific Tests
@section Header Files

The following macros check for the presence of certain C header files:

@defmac AC_DIR_HEADER
@maindex DIR_HEADER
@vindex DIRENT
@vindex SYSDIR
@vindex SYSNDIR
@vindex NDIR
@vindex VOID_CLOSEDIR
If the system has @file{dirent.h}, define @code{DIRENT}; otherwise, if
it has @file{sys/ndir.h}, define @code{SYSNDIR}; otherwise, if it has
@file{sys/dir.h}, define @code{SYSDIR}; otherwise, if it has
@file{ndir.h}, define @code{NDIR}.  Also, if the directory library
header file contains a declaration of the @code{closedir} function with
a @code{void} return type, define @code{VOID_CLOSEDIR}.

The directory library declarations in the source code should look
something like the following, which assumes that you have also called
@samp{AC_HAVE_HEADERS(unistd.h)}:

@example
@group
#ifdef HAVE_UNISTD_H
#include <sys/types.h>
#include <unistd.h>
#endif

/* unistd.h defines _POSIX_VERSION on POSIX.1 systems.  */
#if defined(DIRENT) || defined(_POSIX_VERSION)
#include <dirent.h>
#define NLENGTH(dirent) (strlen((dirent)->d_name))
#else /* not (DIRENT or _POSIX_VERSION) */
#define dirent direct
#define NLENGTH(dirent) ((dirent)->d_namlen)
#ifdef SYSNDIR
#include <sys/ndir.h>
#endif /* SYSNDIR */
#ifdef SYSDIR
#include <sys/dir.h>
#endif /* SYSDIR */
#ifdef NDIR
#include <ndir.h>
#endif /* NDIR */
#endif /* not (DIRENT or _POSIX_VERSION) */
@end group
@end example

Using the above declarations, the program would declare variables to be
type @code{struct dirent}, not @code{struct direct}, and would access
the length of a directory entry name by passing a pointer to a
@code{struct dirent} to the @code{NLENGTH} macro.
@end defmac

@defmac AC_MAJOR_HEADER
@maindex MAJOR_HEADER
@vindex MAJOR_IN_MKDEV
@vindex MAJOR_IN_SYSMACROS
If @file{sys/types.h} does not define @code{major}, @code{minor}, and
@code{makedev}, but @file{sys/mkdev.h} does, define
@code{MAJOR_IN_MKDEV}; otherwise, if @file{sys/sysmacros.h} does, define
@code{MAJOR_IN_SYSMACROS}.
@end defmac

@defmac AC_MEMORY_H
@maindex MEMORY_H
@vindex NEED_MEMORY_H
Define @code{NEED_MEMORY_H} if @code{memcpy}, @code{memcmp}, etc. are
not declared in @file{string.h} and @file{memory.h} exists.  This macro
is obsolete; instead, use @code{AC_HAVE_HEADERS(memory.h)}.  See the
example for @code{AC_STDC_HEADERS}.
@end defmac

@defmac AC_STDC_HEADERS
@maindex STDC_HEADERS
@vindex STDC_HEADERS
Define @code{STDC_HEADERS} if the system has ANSI C header files.
Specifically, this macro checks for @file{stdlib.h}, @file{stdarg.h},
@file{string.h}, and @file{float.h}; if the system has those, it
probably has the rest of the ANSI C header files.  This macro also
checks whether @file{string.h} declares @code{memchr} (and thus
presumably the other @code{mem} functions), whether @file{stdlib.h}
declare @code{free} (and thus presumably @code{malloc} and other related
functions), and whether the @file{ctype.h} macros work on characters
with the high bit set, as ANSI C requires.

Use @code{STDC_HEADERS} instead of @code{__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.

To check whether to use the System V/ANSI C string functions and header
file, you can put the following in @file{configure.in}:

@example
AC_STDC_HEADERS
AC_HAVE_HEADERS(string.h memory.h)
@end example

@noindent
Then, in the code, use a test like this:

@example
@group
#if STDC_HEADERS || HAVE_STRING_H
#include <string.h>
/* An ANSI string.h and pre-ANSI memory.h might conflict.  */
#if !STDC_HEADERS && HAVE_MEMORY_H
#include <memory.h>
#endif /* not STDC_HEADERS and HAVE_MEMORY_H */
#else /* not STDC_HEADERS and not HAVE_STRING_H */
#include <strings.h>
/* memory.h and strings.h conflict on some systems.  */
#ifndef strchr
#define strchr index
#endif
#ifndef strrchr
#define strrchr rindex
#endif
#ifndef memcpy
#define memcpy(d, s, n) bcopy ((s), (d), (n))
#endif
#ifndef memcmp
#define memcmp(s1, s2, n) bcmp ((s1), (s2), (n))
#endif
#endif /* not STDC_HEADERS and not HAVE_STRING_H */
@end group
@end example

@noindent
This example asssumes that your code uses the ANSI C/System V style
functions.  If you use the BSD style functions, replace the macro
definitions with ones that go in the other direction.  (Or, better,
modernize your code.)

This macro calls @code{AC_PROG_CPP} or @code{AC_PROG_CXXCPP} (depending
on which language is current, @pxref{Language Choice}), if it hasn't
been called already.
@end defmac

@defmac AC_UNISTD_H
@maindex UNISTD_H
@vindex HAVE_UNISTD_H
Define @code{HAVE_UNISTD_H} if the system has @file{unistd.h}.  This
macro is obsolete; instead, use @samp{AC_HAVE_HEADERS(unistd.h)}.

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

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

#ifdef _POSIX_VERSION
/* Code for POSIX.1 systems.  */
#endif
@end group
@end example

@vindex _POSIX_VERSION
@code{_POSIX_VERSION} is defined when @file{unistd.h} is included on
POSIX.1 systems.  If there is no @file{unistd.h}, it is definitely not a
POSIX.1 system.  However, some non-POSIX.1 systems do have @file{unistd.h}.
@end defmac

@defmac AC_USG
@maindex USG
@vindex USG
Define @code{USG} if the system does not have @file{strings.h},
@code{rindex}, @code{bzero}, etc.  This implies that it has
@file{string.h}, @code{strrchr}, @code{memset}, etc.

The symbol @code{USG} is obsolete.  Instead of this macro, use
@code{AC_HAVE_HEADERS(string.h)} and use @code{HAVE_STRING_H} in your
code.  See the example for @code{AC_STDC_HEADERS}.
@end defmac

@defmac AC_SYS_SIGLIST_DECLARED
@maindex SYS_SIGLIST_DECLARED
@vindex SYS_SIGLIST_DECLARED
Define @code{SYS_SIGLIST_DECLARED} if the variable @code{sys_siglist} is
declared in a system header file, either @file{signal.h} or
@file{unistd.h}.
@end defmac

@node Typedefs, Library Functions, Header Files, Specific Tests
@section Typedefs

The following macros check for predefined C types:

@defmac AC_GETGROUPS_T
@maindex GETGROUPS_T
@vindex GETGROUPS_T
Define @code{GETGROUPS_T} to be whichever of @code{gid_t} or @code{int}
is the base type of the array argument to @code{getgroups}.
@end defmac

@defmac AC_MODE_T
@maindex MODE_T
@vindex mode_t
If @code{mode_t} is not defined in @file{sys/types.h}, define
@code{mode_t} to be @code{int}.
@end defmac

@defmac AC_OFF_T
@maindex OFF_T
@vindex off_t
If @code{off_t} is not defined in @file{sys/types.h}, define
@code{off_t} to be @code{long}.
@end defmac

@defmac AC_PID_T
@maindex PID_T
@vindex pid_t
If @code{pid_t} is not defined in @file{sys/types.h}, define
@code{pid_t} to be @code{int}.
@end defmac

@defmac AC_RETSIGTYPE
@maindex RETSIGTYPE
@vindex RETSIGTYPE
If @file{signal.h} declares @code{signal} as returning a pointer to a
function returning @code{void}, define @code{RETSIGTYPE} to be
@code{void}; otherwise, define it to be @code{int}.

Define signal handlers as returning type @code{RETSIGTYPE}:

@example
@group
RETSIGTYPE
hup_handler ()
@{
@dots{}
@}
@end group
@end example
@end defmac

@defmac AC_SIZE_T
@maindex SIZE_T
@vindex size_t
If @code{size_t} is not defined in @file{sys/types.h}, define
@code{size_t} to be @code{unsigned}.
@end defmac

@defmac AC_UID_T
@maindex UID_T
@vindex uid_t
@vindex gid_t
If @code{uid_t} is not defined in @file{sys/types.h}, define
@code{uid_t} to be @code{int} and @code{gid_t} to be @code{int}.
@end defmac

@node Library Functions, Structures, Typedefs, Specific Tests
@section Library Functions

The following macros check for particular C library functions:

@defmac AC_ALLOCA
@maindex ALLOCA
@vindex C_ALLOCA
@vindex HAVE_ALLOCA_H
Check how to get @code{alloca}.  Tries to get a builtin version by
checking for @file{alloca.h} or the predefined C preprocessor macros
@code{__GNUC__} and @code{_AIX}.  If that fails, it looks for a function
in the standard C library.  If that fails, it sets the @code{make}
variable @code{ALLOCA} to @samp{alloca.o} and defines @code{C_ALLOCA}
(so programs can periodically call @samp{alloca(0)} to garbage collect).
This variable is separate from @code{LIBOBJS} so multiple programs can
share the value of @code{ALLOCA} without needing to create an actual
library, in case only some of them use the code in @code{LIBOBJS}.

If this macro finds @file{alloca.h}, it defines @code{HAVE_ALLOCA_H}.

This macro does not try to get @code{alloca} from the SVR3 @file{libPW}
or the SVR4 @file{libucb} because those libraries contain some
incompatible functions that cause trouble.  Some versions do not even
contain @code{alloca} or contain a buggy version.  If you still want to
use their @code{alloca}, use @code{ar} to extract @file{alloca.o} from
them instead of compiling @file{alloca.c}.

Source files that use @code{alloca} should start with a piece of code
like the following, to declare it properly.  Note that in some versions
of AIX, the declaration of @code{alloca} must precede everything else
except for comments and preprocessor directives.  The @code{#pragma}
directive is indented so that pre-ANSI C compilers will ignore it,
rather than choke on it.

@example
@group
/* AIX requires this to be the first thing in the file.  */
#ifdef __GNUC__
#define alloca __builtin_alloca
#else /* not __GNUC__ */
#if HAVE_ALLOCA_H
#include <alloca.h>
#else /* not HAVE_ALLOCA_H */
#ifdef _AIX
 #pragma alloca
#else /* not _AIX */
#ifndef alloca /* predefined by HP cc +Olibcalls */
char *alloca ();
#endif /* not defined(alloca) */
#endif /* not _AIX */
#endif /* not HAVE_ALLOCA_H */
#endif /* not __GNUC__ */
@end group
@end example
@end defmac

@defmac AC_GETLOADAVG
@maindex GETLOADAVG
@vindex SVR4
@vindex DGUX
@vindex UMAX
@vindex UMAX4_3
@vindex NLIST_STRUCT
@vindex NLIST_NAME_UNION
@vindex GETLODAVG_PRIVILEGED
@vindex NEED_SETGID
Check how to get the system load averages.  If the system has the
@code{getloadavg} function, this macro defines @code{HAVE_GETLOADAVG},
and adds to @code{LIBS} any libraries needed to get that function.

Otherwise, it adds @samp{getloadavg.o} to the @code{make} variable
@code{LIBOBJS}, and possibly defines several other C preprocessor
macros and @code{make} variables:

@enumerate
@item
It defines @code{SVR4}, @code{DGUX}, @code{UMAX}, or @code{UMAX4_3} if
on those systems.

@item
If it finds @file{nlist.h}, it defines @code{NLIST_STRUCT}.

@item
If @samp{struct nlist} has an @samp{n_un} member, it defines
@code{NLIST_NAME_UNION}.

@item
If compiling @file{getloadavg.c} defines @code{LDAV_PRIVILEGED},
programs need to be installed specially on this system for
@code{getloadavg} to work, and this macro defines
@code{GETLOADAVG_PRIVILEGED}.

@item
This macro always defines @code{NEED_SETGID}, for @code{make}.  The
value is @samp{true} if special installation is required, @samp{false}
if not.  If @code{NEED_SETGID} is @samp{true}, it sets @code{KMEM_GROUP}
to the name of the group that should own the installed program.
@end enumerate
@end defmac

@defmac AC_MMAP
@maindex MMAP
@vindex HAVE_MMAP
If the @code{mmap} function exists and works correctly, define
@code{HAVE_MMAP}.
@end defmac

@defmac AC_SETVBUF_REVERSED
@maindex SETVBUF_REVERSED
@vindex SETVBUF_REVERSED
If @code{setvbuf} takes the buffering type as its second argument and
the buffer pointer as the third, instead of the other way around, define
@code{SETVBUF_REVERSED}.  This is the case on System V before release 3.
@end defmac

@defmac AC_STRCOLL
@maindex STRCOLL
@vindex HAVE_STRCOLL
If the @code{strcoll} function exists and works correctly, define
@code{HAVE_STRCOLL}.  This does a bit more than
@samp{AC_HAVE_FUNCS(strcoll)}, because some systems have incorrect
definitions of @code{strcoll}, which should not be used.
@end defmac

@defmac AC_UTIME_NULL
@maindex UTIME_NULL
@vindex HAVE_UTIME_NULL
If @samp{utime(@var{file}, NULL)} sets @var{file}'s timestamp to
the present, define @code{HAVE_UTIME_NULL}.
@end defmac

@defmac AC_VFORK
@maindex VFORK
@vindex HAVE_VFORK_H
@vindex vfork
If @file{vfork.h} is found, define @code{HAVE_VFORK_H}.  If a working
@code{vfork} is not found, define @code{vfork} to be @code{fork}.  This
macro checks for several known errors in implementations of @code{vfork}
and considers the system to not have a working @code{vfork} if it
detects any of them.
@end defmac

@defmac AC_VPRINTF
@maindex VPRINTF
@vindex HAVE_VPRINTF
@vindex HAVE_DOPRNT
If @code{vprintf} is found, define @code{HAVE_VPRINTF}.  Otherwise, if
@code{_doprnt} is found, define @code{HAVE_DOPRNT}.  (If @code{vprintf}
is available, you may assume that @code{vfprintf} and @code{vsprintf}
are also available.)
@end defmac

@defmac AC_WAIT3
@maindex WAIT3
@vindex HAVE_WAIT3
If @code{wait3} is found and fills in the contents of its third argument
(a @samp{struct rusage *}), which HP-UX does not do, define
@code{HAVE_WAIT3}.
@end defmac

@node Structures, Compiler Characteristics, Library Functions, Specific Tests
@section Structures

The following macros check for certain structures or structure members:

@defmac AC_STAT_MACROS_BROKEN
@maindex STAT_MACROS_BROKEN
@maindex STAT_MACROS_BROKEN
If the macros @code{S_ISDIR}, @code{S_ISREG} et al. defined in
@file{sys/stat.h} do not work properly (returning false positives),
define @code{STAT_MACROS_BROKEN}.  This is the case on Tektronix UTekV,
Amdahl UTS and Motorola System V/88.
@end defmac

@defmac AC_ST_BLKSIZE
@maindex ST_BLKSIZE
@vindex HAVE_ST_BLKSIZE
If @code{struct stat} contains an @code{st_blksize} member, define
@code{HAVE_ST_BLKSIZE}.
@end defmac

@defmac AC_ST_BLOCKS
@maindex ST_BLOCKS
@vindex HAVE_ST_BLOCKS
If @code{struct stat} contains an @code{st_blocks} member, define
@code{HAVE_ST_BLOCKS}.  Otherwise, add @samp{fileblocks.o} to the
@code{make} variable @code{LIBOBJS}.
@end defmac

@defmac AC_ST_RDEV
@maindex ST_RDEV
@vindex HAVE_ST_RDEV
If @code{struct stat} contains an @code{st_rdev} member, define
@code{HAVE_ST_RDEV}.
@end defmac

@defmac AC_TIME_WITH_SYS_TIME
@maindex TIME_WITH_SYS_TIME
@vindex TIME_WITH_SYS_TIME
If a program may include both @file{time.h} and @file{sys/time.h},
define @code{TIME_WITH_SYS_TIME}.  On some older systems,
@file{sys/time.h} includes @file{time.h}, but @file{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, @code{struct timeval} or @code{struct timezone} as well as
@code{struct tm}.  It is best used in conjunction with
@code{HAVE_SYS_TIME_H}.

@example
#ifdef TIME_WITH_SYS_TIME
#include <sys/time.h>
#include <time.h>
#else
#ifdef HAVE_SYS_TIME_H
#include <sys/time.h>
#else
#include <time.h>
#endif
#endif
@end example
@end defmac

@defmac AC_STRUCT_TM
@maindex STRUCT_TM
@vindex TM_IN_SYS_TIME
If @file{time.h} does not define @code{struct tm}, define
@code{TM_IN_SYS_TIME}, which means that including @file{sys/time.h}
defines @code{struct tm}.
@end defmac

@defmac AC_TIMEZONE
@maindex TIMEZONE
@vindex HAVE_TM_ZONE
@vindex HAVE_TZNAME
Figure out how to get the current timezone.  If @code{struct tm} has a
@code{tm_zone} member, define @code{HAVE_TM_ZONE}.  Otherwise, if the
external array @code{tzname} is found, define @code{HAVE_TZNAME}.  This
macro calls @code{AC_STRUCT_TM} if it hasn't been called already.
@end defmac

@node Compiler Characteristics, System Services, Structures, Specific Tests
@section Compiler Characteristics

The following macros check for C compiler or machine architecture
features:

@defmac AC_ARG_ARRAY
@maindex ARG_ARRAY
@vindex NO_ARG_ARRAY
If the address of an argument to a C function can not be used like
the start of an array, define @code{NO_ARG_ARRAY}.  This ability allows
a sequence of arguments with the same type to be accessed as if they
were an array of values.
@end defmac

@defmac AC_CROSS_CHECK
@maindex CROSS_CHECK
If the C compiler being used does not produce executables that can run
on the system where @code{configure} is being run, set the shell
variable @code{cross_compiling} to @samp{yes}, otherwise @samp{no}.
This information can be used by @code{AC_TEST_RUN} to determine whether
to take a default action instead of trying to run a test program
(@pxref{C Features}).
@end defmac

@defmac AC_CHAR_UNSIGNED
@maindex CHAR_UNSIGNED
@vindex __CHAR_UNSIGNED__
If the C type @code{char} is unsigned, define @code{__CHAR_UNSIGNED__},
unless the C compiler predefines it.
@end defmac

@defmac AC_CONST
@maindex CONST
@vindex const
If the C compiler does not fully support the keyword @code{const},
define @code{const} to be empty.  Some C compilers that do not define
@code{__STDC__} do support @code{const}; some compilers that define
@code{__STDC__} do not completely support @code{const}.  Programs can
simply use @code{const} as if every C compiler supported it; for those
that don't, the @file{Makefile} or configuration header file will define
it as empty.  (If using a configuration header file, the program should
include it before any other header files, to prevent inconsistencies in
declarations.)
@end defmac

@defmac AC_INLINE
@maindex INLINE
@vindex inline
If the C compiler is a version of GCC that supports the keyword
@code{__inline} but not @code{inline} (such as some NeXT versions),
define @code{inline} to be @code{__inline}.  This macro calls
@code{AC_PROG_CC} if it hasn't been called already.
@end defmac

@defmac AC_INT_16_BITS
@maindex INT_16_BITS
@vindex INT_16_BITS
If the C type @code{int} is 16 bits wide, define @code{INT_16_BITS}.
This macro is obsolete; it is more general to use
@samp{AC_SIZEOF_TYPE(int)} instead (@pxref{C Features}).
@end defmac

@defmac AC_LONG_64_BITS
@maindex LONG_64_BITS
@vindex LONG_64_BITS
If the C type @code{long int} is 64 bits wide, define
@code{LONG_64_BITS}.  This macro is obsolete; it is more general to use
@samp{AC_SIZEOF_TYPE(long)} instead (@pxref{C Features}).
@end defmac

@defmac AC_LONG_DOUBLE
@maindex HAVE_LONG_DOUBLE
@vindex HAVE_LONG_DOUBLE
If the C compiler supports the @code{long double} type, define
@code{HAVE_LONG_DOUBLE}.  Some C compilers that do not define
@code{__STDC__} do support the @code{long double} type; some compilers
that define @code{__STDC__} do not support @code{long double}.
@end defmac

@defmac AC_WORDS_BIGENDIAN
@maindex WORDS_BIGENDIAN
@vindex WORDS_BIGENDIAN
If words are stored with the most significant byte first (like Motorola
and SPARC, but not Intel and VAX, CPUs), define @code{WORDS_BIGENDIAN}.
@end defmac

@node System Services, UNIX Variants, Compiler Characteristics, Specific Tests
@section System Services

The following macros check for operating system services:

@defmac AC_FIND_X
@maindex FIND_X
Try to locate the X Window System include files and libraries.  Try
first by running @code{xmkmf} on a trivial @file{Imakefile} and
examining the @file{Makefile} that it produces.  If that fails (such as
if @code{xmkmf} is not present), look for them in several directories
where they often reside.  If either method is successful, set the shell
variables @code{x_includes} and @code{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
@samp{--without-x}, set the shell variable @code{no_x} to @samp{true};
otherwise set it to the empty string.

The command line options @samp{--x-includes=@var{dir}} and
@samp{--x-libraries=@var{dir}} override the values chosen by this macro.
@end defmac

@defmac AC_FIND_XTRA
@maindex FIND_XTRA
An enhanced version of @code{AC_FIND_X}.  Put the C compiler flags that
X needs into @code{make} variable @code{X_CFLAGS}, and the X linker
flags into @code{X_LIBS}.  If X is not available, put
@samp{-DX_DISPLAY_MISSING} into @code{X_CFLAGS}.

Also check for special libraries that some systems need in order to
compile X programs.  Add any that the system needs to @code{make}
variable @code{X_EXTRA_LIBS}.  This macro calls @code{AC_FIND_X} and
@code{AC_ISC_POSIX} (@pxref{UNIX Variants}) if they have not already
been called.  Because of the macro dependencies, if you call this macro,
you should let it call @code{AC_FIND_X} rather than doing that yourself.
@end defmac

@defmac AC_HAVE_POUNDBANG (@var{action-if-supported} @r{[}, @var{action-if-not-supported}@r{]})
@maindex HAVE_POUNDBANG
Check whether the system supports starting shell scripts with a line of
the form @samp{#!/bin/csh} to select the shell to use.  If @samp{#!}
works, execute shell commands @var{action-if-supported}; if not, execute
@var{action-if-not-supported}.
@end defmac

@defmac AC_LONG_FILE_NAMES
@maindex LONG_FILE_NAMES
@vindex HAVE_LONG_FILE_NAMES
If the system supports file names longer than 14 characters, define
@code{HAVE_LONG_FILE_NAMES}.
@end defmac

@defmac AC_REMOTE_TAPE
@maindex REMOTE_TAPE
@vindex HAVE_SYS_MTIO_H
If BSD tape drive ioctls are available, define @code{HAVE_SYS_MTIO_H},
and if sockets are available add @code{rmt} to @code{make} variable
@code{PROGS}.
@end defmac

@defmac AC_RESTARTABLE_SYSCALLS
@maindex RESTARTABLE_SYSCALLS
@vindex HAVE_RESTARTABLE_SYSCALLS
If the system automatically restarts a system call that is interrupted
by a signal, define @code{HAVE_RESTARTABLE_SYSCALLS}.
@end defmac

@node UNIX Variants, , System Services, Specific Tests
@section 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 kludges; they should
be replaced someday with a more systematic approach, based either on the
functions they make available or the environments they provide.

@defmac AC_AIX
@maindex AIX
@vindex _ALL_SOURCE
If on AIX, define @code{_ALL_SOURCE}.  Allows the use of some BSD
functions.  Should be called before any macros that run the C compiler.
@end defmac

@defmac AC_DYNIX_SEQ
@maindex DYNIX_SEQ
If on DYNIX/ptx (Sequent UNIX), add @samp{-lseq} to @code{make}
variable @code{LIBS}.  Allows use of some BSD system calls and
@code{getmntent}.
@end defmac

@defmac AC_IRIX_SUN
@maindex IRIX_SUN
If on IRIX (Silicon Graphics UNIX), add @samp{-lsun} to @code{make}
variable @code{LIBS}.  Needed to get @code{getmntent}.  At sites using
Yellow Pages/NIS, it is also needed to get properly working
@code{gethostby*}, @code{getpw*}, @code{getgr*}, @code{getnetby*}, and
so on.
@c (According to Garrett.Wollman@uvm.edu.)
@end defmac

@defmac AC_ISC_POSIX
@maindex ISC_POSIX
@vindex _POSIX_SOURCE
If on a POSIXized ISC UNIX, define @code{_POSIX_SOURCE} and add
@samp{-posix} (for the GNU C compiler) or @samp{-Xp} (for other C
compilers) to @code{make} variable @code{CC}.  This allows the use of
POSIX facilities.  Must be called after @code{AC_PROG_CC} and before
any other macros that run the C compiler.
@end defmac

@defmac AC_MINIX
@maindex MINIX
@vindex _MINIX
@vindex _POSIX_SOURCE
@vindex _POSIX_1_SOURCE
If on Minix, define @code{_MINIX} and @code{_POSIX_SOURCE} and define
@code{_POSIX_1_SOURCE} to be 2.  This allows the use of POSIX
facilities.  Should be called before any macros that run the C compiler.
@end defmac

@defmac AC_SCO_INTL
@maindex SCO_INTL
If on SCO UNIX, add @samp{-lintl} to @code{make} variable @code{LIBS}.
Used to get @code{strftime}.  It must be called before checking for
@code{strftime}.
@end defmac

@defmac AC_XENIX_DIR
@maindex XENIX_DIR
@vindex VOID_CLOSEDIR
If on Xenix, define @code{VOID_CLOSEDIR} and add @samp{-lx} to
@code{make} variable @code{LIBS}.  Also, if @file{sys/ndir.h} is not
being used, add @samp{-ldir} to @code{LIBS}.  Needed when using the
directory reading functions.  This macro must be called after
@code{AC_DIR_HEADER}.
@end defmac

@node General Purpose Macros, Manual Configuration, Specific Tests, Top
@chapter General Purpose Macros

These macros provide ways for other macros to control the kind of output
that Autoconf produces or to check whether various kinds of features are
available.  They all take arguments.  When calling these macros, there
must not be any blank space between the macro name and the open
parentheses.
Arguments to these macros can be more than one line long if they are
enclosed within the @code{m4} quote characters @samp{[} and @samp{]}.

Many of these macros handle two
cases: what to do if the given condition is met, and what to do if the
condition is not met.  In some places you you might want to do something
if a condition is true but do nothing if it's false, or vice versa.  To
omit the true case, pass an empty value for the @var{action-if-found}
argument to the macro.  To omit the false case, omit the
@var{action-if-not-found} argument to the macro, including the comma
before it.

One shell programming construction that you should not use in the action
arguments to these macros is
@samp{@var{var}=$@{@var{var}:-@var{value}@}}.  Old BSD shells, including
the Ultrix @code{sh}, don't understand the colon, and complain and die.
If you omit the colon, it works fine:
@samp{@var{var}=$@{@var{var}-@var{value}@}}.  Using the form without the
colon has one small disadvantage.  Users can not select a default value
by giving a variable an empty value, e.g., @samp{CC= configure}.
Instead, they must unset the variable, e.g., @samp{unset CC; configure}.

Within each section below, the macros are listed in alphabetical order.
These macros are defined in the file @file{acgeneral.m4}.

@menu
* Setup::			Controlling Autoconf operation.
* File Existence::              Checking whether particular files exist.
* C Features::                  Checking for features of the C system.
* Command Line::                Checking command line arguments.
* Setting Variables::		Setting shell and @code{make} variables.
* Printing Messages::           Notifying users of progress or problems.
* Language Choice::             Selecting which language to use for testing.
* Macro Ordering::		Enforcing ordering constraints.
@end menu

@node Setup, File Existence, , General Purpose Macros
@section Controlling Autoconf Setup

The following macros control the kind of output that Autoconf produces.
@file{configure.in} should call them before @code{AC_INIT} or
@code{AC_PREPARE}.  Remember to put a @code{dnl} comment at the ends of
the lines containing their calls, to avoid inserting blank lines at the
beginning of @file{configure}.

@defmac AC_CONFIG_AUX_DIR(@var{dir})
@maindex CONFIG_AUX_DIR
Use the @file{install.sh}, @file{config.sub}, @file{config.guess}, and
Cygnus @file{configure} scripts that are in directory @var{dir}.  These
are auxiliary files used in configuration.  @var{dir} can be either
absolute or relative to @file{@var{srcdir}}.  The default is
@file{@var{srcdir}} or @file{@var{srcdir}/..} or
@file{@var{srcdir}/../..}, whichever is the first that contains
@file{install.sh}.  The other files are not checked for, so that using
@code{AC_PROG_INSTALL} does not automatically require distributing the
other auxiliary files.
@end defmac

@defmac AC_CONFIG_HEADER (@var{header-to-create} @dots{})
@maindex CONFIG_HEADER
@vindex HAVE_CONFIG_H
Make @code{AC_OUTPUT} create the file(s) in the whitespace-separated
list @var{header-to-create} containing C preprocessor @code{#define}
statements and replace @samp{@@DEFS@@} in generated files with
@samp{-DHAVE_CONFIG_H} instead of the value of @code{DEFS}.  The usual
name for @var{header-to-create} is @file{config.h}.

If @var{header-to-create} already exists and its contents are identical
to what @code{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.

Your distribution should contain a file @file{@var{header-to-create}.in}
that looks as you want the final header file to look, including
comments, with default values in the @code{#define} statements.  A
default value can be to @code{#undef} the variable instead of to define
it to a value, if your code tests for configuration options using
@code{#ifdef} instead of @code{#if}.

You can use the program @code{autoheader} to create
@file{@var{header-to-create}.in} (@pxref{Invoking autoheader}).
@end defmac

@defmac AC_CONFIG_SUBDIRS (@var{dir} @dots{})
@maindex CONFIG_SUBDIRS
Run @code{configure} in each subdirectory @var{dir} in the given
whitespace-separated list.  If a given @var{dir} is not found, no error
is reported, so a @code{configure} script can configure whichever parts
of a large source tree are present.  If a given @var{dir} contains
@file{configure.in} but no @file{configure}, the Cygnus @code{configure}
script found by @code{AC_CONFIG_AUXDIR} is used.  The subdirectory
@code{configure} scripts are given the same command line options that
were given to this @code{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 shell and @code{make} variable
@code{subdirs} to the list of directories @samp{@var{dir} @dots{}}.
@file{Makefile} rules can use this variable to determine which
subdirectories to recurse into.
@end defmac

@defmac AC_INIT (@var{unique-file-in-source-dir})
@maindex INIT
Process the command-line arguments and find the source code directory.
@var{unique-file-in-source-dir} is some file that is in the package's
source directory; @code{configure} checks for this file's existence to
make sure that the directory that it is told contains the source code in
fact does (@pxref{Invoking configure}, for more information).
@end defmac

@defmac AC_OUTPUT (@r{[}@var{file}@dots{}@r{] [,}@var{extra-cmds}@r{]})
@maindex OUTPUT
Create output files (typically one or more @file{Makefile}s) and
@file{config.status}.  If @code{AC_CONFIG_HEADER} has been called, also
create the header file that was named as its argument.  The argument is
a whitespace-separated list of files to create; if it is omitted, no
files are created.  @code{AC_OUTPUT} creates each file @file{@var{file}}
in the list by copying @file{@var{file}.in}, substituting the variable
values that have been selected by calling @code{AC_SUBST}.  It creates
the directory that each file is in if it doesn't exist (but not the
parents of that directory).  A plausible value for the argument to
@code{AC_OUTPUT} is @samp{Makefile src/Makefile man/Makefile
X/Imakefile}.

If you pass @var{extra-cmds}, those commands will be inserted into
@file{config.status} to be run after all its other processing.
@end defmac

@defmac AC_PREFIX_PROGRAM (@var{program})
@maindex PREFIX_PROGRAM
If the user did not specify an installation prefix (using the
@samp{--prefix} option), guess a value for it by looking for
@var{program} in @code{PATH}, the way the shell does.  If @var{program}
is found, set the prefix to the parent of the directory containing
@var{program}; otherwise leave the prefix specified in
@file{Makefile.in} unchanged.  For example, if @var{program} is
@code{gcc} and the @code{PATH} contains @file{/usr/local/gnu/bin/gcc},
set the prefix to @file{/usr/local/gnu}.
@end defmac

@defmac AC_PREPARE (@var{unique-file-in-source-dir})
@maindex PREPARE
Find the source code directory and set up shell variables necessary for
other Autoconf macros to work.  @var{unique-file-in-source-dir} is some
file that is in the package's source directory; @code{configure} checks
for this file's existence to make sure that the directory that it is
told contains the source code in fact does (@pxref{Invoking configure},
for more information).  @code{AC_PREPARE} is the last thing done by
@code{AC_INIT}.  Use @code{AC_PREPARE} instead of @code{AC_INIT} if you
want to do argument parsing yourself; never use both.
@end defmac

@defmac AC_PREREQ (@var{version})
@maindex PREREQ
Ensure that a recent enough version of Autoconf is being used.  If the
version of Autoconf being used to create @code{configure} is earlier
than @var{version} (e.g., @samp{1.8}), print an error message on the
standard error output and do not create @code{configure}.

This macro is useful if your @file{configure.in} relies on non-obvious
behavior that changed between Autoconf releases.  If it merely needs
recently added macros, then @code{AC_PREREQ} is less useful, because the
@code{autoconf} program already tells the user which macros are not
found.  The same thing happens if @file{configure.in} is processed by a
version of Autoconf older than when @code{AC_PREREQ} was added.
@end defmac

@defmac AC_REVISION (@var{revision-info})
@maindex REVISION
Copy revision stamp @var{revision-info} into the @file{configure}
script, with any dollar signs or double-quotes removed.  This macro lets
you put a revision stamp from @file{configure.in} into @file{configure}
without RCS or CVS changing it when you check in @file{configure}.  That
way, you can determine easily which revision of @file{configure.in} a
particular @file{configure} corresponds to.

It is a good idea to call this macro before @code{AC_INIT} so that the
revision number is near the top of both @file{configure.in} and
@file{configure}.  To support doing that, the @code{AC_REVISION} output
begins with @samp{#!/bin/sh}, like the normal start of a
@file{configure} script does.

For example, this line in @file{configure.in}:

@example
AC_REVISION($@asis{Revision: 1.30 }$)dnl
@end example

@noindent
produces this in @file{configure}:

@example
#!/bin/sh
# From configure.in Revision: 1.30 
@end example
@end defmac

@node File Existence, C Features, Setup, General Purpose Macros
@section Checking Whether Particular Files Exist

These macros check whether particular files exist.

@defmac AC_HAVE_LIBRARY (@var{library} @r{[}, @var{action-if-found} @r{[}, @var{action-if-not-found} @r{[}, @var{other-libraries}@r{]]]})
@maindex HAVE_LIBRARY
Create a test C program to see whether that program can be linked with
the library @var{library}.  @var{action-if-found} is a list of shell
commands to run if the link succeeds (which means that the library is
present); @var{action-if-not-found} is a list of shell commands to run
if the link fails.  If @var{action-if-found} and
@var{action-if-not-found} are not specified, the default action is to
add @samp{-lfoo} to @code{LIBS} and define @samp{HAVE_LIBfoo}
for library @samp{foo}.  @var{library} can be written as any of
@samp{foo}, @samp{-lfoo}, or @samp{libfoo.a}.  In all of those cases,
the compiler is passed @samp{-lfoo}.

If linking with @var{library} results in unresolved symbols, which would
be resolved by linking with additional libraries, give those libraries
as the @var{other-libraries} argument, separated by spaces: @samp{-lX11
-lXt}.  Otherwise this macro will fail to detect that @var{library} is
present, because linking the test program will always fail with
unresolved symbols.
@end defmac

@defmac AC_PREFIX (@var{program})
@maindex PREFIX
This macro is like @code{AC_PREFIX_PROGRAM}, except that it prevents
finding the site-wide defaults file correctly, so it is obsolete.
@end defmac

@defmac AC_PROGRAM_CHECK (@var{variable}, @var{prog-to-check-for}, @var{value-if-found} @r{[}, @var{value-if-not-found}@r{]})
@maindex PROGRAM_CHECK
Check whether program @var{prog-to-check-for} exists in @code{PATH}.  If
it is found, set @var{variable} to @var{value-if-found}, otherwise to
@var{value-if-not-found}, if given.  If @var{variable} was already set,
do nothing.  Calls @code{AC_SUBST} for @var{variable}.
@end defmac

@defmac AC_PROGRAM_PATH (@var{variable}, @var{prog-to-check-for} @r{[}, @var{value-if-not-found}@r{]})
@maindex PROGRAM_PATH
Similar to @code{AC_PROGRAM_CHECK}, but set @var{variable} to the entire
path of @var{prog-to-check-for} if found.  Otherwise, set @var{variable}
to @var{value-if-not-found}, if given.  If @var{variable} was already
set, do nothing.  Calls @code{AC_SUBST} for @var{variable}.
@end defmac

@defmac AC_PROGRAMS_CHECK (@var{variable}, @var{progs-to-check-for} @r{[}, @var{value-if-not-found}@r{]})
@maindex PROGRAMS_CHECK
Check for each program in the whitespace-separated list
@var{progs-to-check-for} exists in @code{PATH}.  If it is found, set
@var{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 @var{variable} to @var{value-if-not-found}; if
@var{value-if-not-found} is not specified, the value of @var{variable}
is not changed.  Calls @code{AC_SUBST} for @var{variable}.
@end defmac

@defmac AC_PROGRAMS_PATH (@var{variable}, @var{progs-to-check-for} @r{[}, @var{value-if-not-found}@r{]})
@maindex PROGRAMS_PATH
Like @code{AC_PROGRAMS_CHECK}, but if any of @var{progs-to-check-for}
are found, set @var{variable} to the entire pathname of the program
found.
@end defmac

@node C Features, Command Line, File Existence, General Purpose Macros
@section Checking For C Features

These macros check for features of the C compiler, libraries, and headers.

@defmac AC_CHECK_TYPE (@var{type}, @var{default})
@maindex CHECK_TYPE
If the type @var{type} is not defined in @file{sys/types.h}, define it
to be the C (or C++) builtin type @var{default}, e.g. @samp{short} or
@samp{unsigned}.
@end defmac

@defmac AC_COMPILE_CHECK (@var{echo-text}, @var{includes}, @var{function-body}, @var{action-if-found} @r{[}, @var{action-if-not-found}@r{]})
@maindex COMPILE_CHECK
This is an obsolete alias for @code{AC_TEST_LINK}, with the addition that it
prints @samp{checking for @var{echo-text}} to the standard output first.
@end defmac

@defmac AC_FUNC_CHECK (@var{function}, @var{action-if-found} @r{[}, @var{action-if-not-found}@r{]})
@maindex FUNC_CHECK
If @var{function} is available, run shell commands
@var{action-if-found}, otherwise @var{action-if-not-found}.  If the
functions might be in libraries other than the default C library, first
call @code{AC_HAVE_LIBRARY} for those libraries.  If you just want to
define a symbol if the function is available, consider using
@code{AC_HAVE_FUNCS} instead.
@end defmac

@defmac AC_HAVE_FUNCS (@var{function}@dots{})
@maindex HAVE_FUNCS
@vindex HAVE_@var{function}
For each given @var{function} in the whitespace-separated argument list
that is available, define @code{HAVE_@var{function}} (in all caps).
@xref{Specific Tests}, for a precise definition of ``define'' as it
is used here.  If the functions might be in libraries other than the
default C library, first call @code{AC_HAVE_LIBRARY} for those libraries.
@end defmac

@defmac AC_HAVE_HEADERS (@var{header-file}@dots{})
@maindex HAVE_HEADERS
@vindex HAVE_@var{header}
For each given @var{header-file} in the whitespace-separated argument
list that exists, define @code{HAVE_@var{header-file}} (in all caps).
@xref{Specific Tests}, for a precise definition of ``define'' as
it is used here.
@end defmac

@defmac AC_HEADER_CHECK (@var{header-file}, @var{action-if-found} @r{[}, @var{action-if-not-found}@r{]})
@maindex HEADER_CHECK
If @var{header-file} exists, execute shell commands
@var{action-if-found}, otherwise execute @var{action-if-not-found}.  If
you just want to define a symbol if the header file is available,
consider using @code{AC_HAVE_HEADERS} instead.
@end defmac

@defmac AC_HEADER_EGREP (@var{pattern}, @var{header-file}, @var{action-if-found} @r{[}, @var{action-if-not-found}@r{]})
@maindex HEADER_EGREP
If the output of running the preprocessor on @var{header-file}
contains the @code{egrep} regular expression @var{pattern}, execute
shell commands @var{action-if-found}, otherwise execute

This macro calls @code{AC_PROG_CPP} or @code{AC_PROG_CXXCPP} (depending
on which language is current, @pxref{Language Choice}), if it hasn't
been called already.
@var{action-if-not-found}.

You can not check whether preprocessor symbols are defined this way,
because they get expanded before @code{egrep} sees them.  But you can
almost always detect them by simply using @code{#ifdef} directives in
your programs.
@end defmac

@defmac AC_PROGRAM_EGREP (@var{pattern}, @var{program}, @var{action-if-found} @r{[}, @var{action-if-not-found}@r{]})
@maindex PROGRAM_EGREP
@var{program} is the text of a C or C++ program, on which shell variable and
backquote substitutions are performed.  If the output of running the
preprocessor on @var{program} contains the @code{egrep} regular
expression @var{pattern}, execute shell commands @var{action-if-found},
otherwise execute @var{action-if-not-found}.  (It is an unfortunate
oversight that we use the word @code{PROGRAM} in Autoconf macro names to
sometimes mean C or C++ source code and sometimes mean a UNIX command.)

This macro calls @code{AC_PROG_CPP} or @code{AC_PROG_CXXCPP} (depending
on which language is current, @pxref{Language Choice}), if it hasn't
been called already.
@end defmac

@defmac AC_REPLACE_FUNCS (@var{function-name}@dots{})
@maindex REPLACE_FUNCS
For each given @var{function-name} in the whitespace-separated argument
list that is not in the C library, add @samp{@var{function-name}.o} to
the value of the @code{make} variable @code{LIBOBJS}.  If the functions
might be in libraries other than the default C library, first call
@code{AC_HAVE_LIBRARY} for those libraries.
@end defmac

@defmac AC_SIZEOF_TYPE (@var{type})
@maindex SIZEOF_TYPE
Define @code{SIZEOF_@var{uctype}} to be the size in bytes of the C (or
C++) builtin type @var{type}, e.g. @samp{int} or @samp{char *}.  If
@samp{type} is unknown to the compiler, gets a size of 0.  @var{uctype}
is @var{type}, with lowercase converted to uppercase, spaces changed to
underscores, and asterisks changed to @samp{P}.  For example, the call
@example
AC_SIZEOF_TYPE(int *)
@end example
@noindent
defines @code{SIZEOF_INT_P} to be 8 on DEC Alpha AXP systems.
@end defmac

@defmac AC_TEST_CPP (@var{includes}, @var{action-if-true} @r{[}, @var{action-if-false}@r{]})
@maindex TEST_CPP
@var{includes} is C or C++ @code{#include} statements and declarations, on
which shell variable and backquote 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 @var{action-if-true}.  Otherwise run
shell commands @var{action-if-false}.

This macro calls @code{AC_PROG_CPP} or @code{AC_PROG_CXXCPP} (depending
on which language is current, @pxref{Language Choice}), if it hasn't
been called already.
@end defmac

@defmac AC_TEST_LINK (@var{includes}, @var{function-body}, @var{action-if-found} @r{[}, @var{action-if-not-found}@r{]})
@maindex TEST_LINK
Create a test C program to see whether a function whose body consists of
@var{function-body} can be compiled and linked; @var{includes} is any
@code{#include} statements needed by the code in @var{function-body}.
If the file compiles and links successfully, run shell commands
@var{action-if-found}, otherwise run @var{action-if-not-found}.
@end defmac

@defmac AC_TEST_PROGRAM (@var{program}, @var{action-if-true} @r{[}, @var{action-if-false} @r{[}, @var{action-if-cross-compiling}@r{]]})
@maindex TEST_PROGRAM
This is an obsolete alias for @code{AC_TEST_RUN}.
@end defmac

@defmac AC_TEST_RUN (@var{program}, @var{action-if-true} @r{[}, @var{action-if-false} @r{[}, @var{action-if-cross-compiling}@r{]]})
@maindex TEST_RUN
@var{program} is the text of a C program, on which shell variable and
backquote substitutions are performed.  If it compiles and links
successfully and returns an exit status of 0 when executed, run shell
commands @var{action-if-true}.  Otherwise run shell commands
@var{action-if-false}.

If the optional argument @var{action-if-cross-compiling} is given and
the C compiler being used does not produce executables that run on the
system where @code{configure} is being run, then the test program is not
run.  Instead, the shell commands @var{action-if-cross-compiling} are
run.  If that argument is given, this macro calls @code{AC_CROSS_CHECK}
if it has not already been called (@pxref{Compiler Characteristics}).
@end defmac

@node Command Line, Setting Variables, C Features, General Purpose Macros
@section Checking Command Line Arguments

These macros check whether the user gave @code{configure} various
command line arguments.  Like the general feature tests (@pxref{General
Purpose Macros}),  they may take an argument to use if the argument was
given and one for if it was not given.

@defmac AC_ARG_ENABLE (@var{feature}, @var{help-string}, @var{action-if-true} @r{[}, @var{action-if-false}@r{]})
@maindex ARG_ENABLE
If the user gave @code{configure} the option
@samp{--enable-@var{feature}} or @samp{--disable-@var{feature}}, run
shell commands @var{action-if-true}.  Otherwise run shell commands
@var{action-if-false}.  @var{help-string} is a description of
the option which looks like this:
@example
--enable-readline       support fancy command line editing
@end example
@noindent
@var{help-string} may be more than one line long, if more detail is
needed.  Just make sure the columns line up in @samp{configure --help}.

The name @var{feature} should consist only of alphanumeric characters
and dashes.  The @var{feature} indicates an optional user-level
facility.  This option allows users to choose which optional features to
build and install.  @samp{--enable-@var{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
@samp{=} and the argument.  Giving an argument of @samp{no} indicates
that the feature is @emph{not} available.  A feature with an argument
looks like @samp{--enable-debug=stabs}.

The argument is available to the shell commands @var{action-if-true} in
the shell variable @code{enableval}.  If no argument was given to
@samp{--enable-@var{feature}}, @code{enableval} is @samp{yes}.
@samp{--disable-@var{feature}} is equivalent to
@samp{--enable-@var{feature}=no}.  At present, arguments containing blanks
are not handled correctly; if you need an argument to contain a list,
require the items to be separated by commas instead.
@end defmac

@defmac AC_ARG_WITH (@var{package}, @var{help-string}, @var{action-if-true} @r{[}, @var{action-if-false}@r{]})
@maindex ARG_WITH
If the user gave @code{configure} the option @samp{--with-@var{package}}
or @samp{--without-@var{package}}, run shell commands
@var{action-if-true}.  Otherwise run shell commands
@var{action-if-false}.    @var{help-string} is a description of
the option which looks like this:
@example
--with-gnu-as           use the GNU assembler
@end example
@noindent
@var{help-string} may be more than one line long, if more detail is
needed.  Just make sure the columns line up in @samp{configure --help}.

The name @var{package} should consist only of alphanumeric characters
and dashes.  The @var{package} indicates another software package that
this program should work with.  For example, @samp{--with-gnu-ld} means
work with the GNU linker instead of some other linker.
@samp{--with-x11} means work with X11.

The user can give an argument by following the package name with
@samp{=} and the argument.  Giving an argument of @samp{no} is for
packages that would be used by default; it says to @emph{not} use the
package.  An argument that is neither @samp{yes} nor @samp{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.

The argument is available to the shell commands @var{action-if-true} in
the shell variable @code{withval}.  If no argument was given to
@samp{--with-@var{package}}, @code{withval} is @samp{yes}.
@samp{--without-@var{package}} is equivalent to
@samp{--with-@var{package}=no}.  At present, arguments containing blanks
are not handled correctly; if you need an argument to contain a list,
require the items to be separated by commas instead.  
@end defmac

@defmac AC_ENABLE (@var{feature}, @var{action-if-true} @r{[}, @var{action-if-false}@r{]})
@maindex ENABLE
This is an obsolete version of @code{AC_ARG_ENABLE} that does not
provide a help string.
@end defmac

@defmac AC_WITH (@var{package}, @var{action-if-true} @r{[}, @var{action-if-false}@r{]})
@maindex WITH
This is an obsolete version of @code{AC_ARG_WITH} that does not
provide a help string.
@end defmac

@node Setting Variables, Printing Messages, Command Line, General Purpose Macros
@section Setting Variables

These macros help other macros to define shell and @code{make}
variables.

@defmac AC_DEFINE (@var{variable} @r{[}, @var{value}@r{]})
@maindex DEFINE
Define C preprocessor variable @var{variable}.  If @var{value} is given,
set @var{variable} to that value, otherwise set it to 1.  To use a shell
variable as the value, use @code{AC_DEFINE_UNQUOTED} instead and precede
double quotes in the value with backslashes.

This macro adds to the shell variable @code{DEFS}.  @code{AC_OUTPUT}
later substitutes the values in @code{DEFS} into the file(s) that it
generates (typically @file{Makefile}).  Alternately, if
@code{AC_CONFIG_HEADER} has been called, @code{AC_OUTPUT} creates a
header file by substituting the correct values into @code{#define}
statements in a template file.

For example, suppose your @file{configure.in} calls
@code{AC_CONFIG_HEADER(conf.h)} and @code{AC_HAVE_HEADERS(unistd.h)}.
You could have code like this in @file{conf.h.in}:

@example
@group
/* Define as 1 if you have unistd.h.  */
#define HAVE_UNISTD_H 0
@end group
@end example

On systems that have @file{unistd.h}, @code{configure} will change the 0
to a 1.  On other systems, it will leave the line unchanged.
Alternately, if you prefer to use @code{#ifdef}, your @file{conf.h.in}
could have code like this:

@example
@group
/* Define if you have unistd.h.  */
#undef HAVE_UNISTD_H
@end group
@end example

On systems that have @file{unistd.h}, @code{configure} will change the
second line to read @samp{#define HAVE_UNISTD_H 1}.  On other systems,
it will comment that line out (in case the system predefines that symbol).

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

@example
AC_HEADER_CHECK(elf.h, AC_DEFINE(SVR4) LIBS="$LIBS -lelf")
@end example

@noindent
or this:

@example
AC_HEADER_CHECK(elf.h,
  AC_DEFINE(SVR4)
  LIBS="$LIBS -lelf")
@end example

@noindent
instead of this:

@example
AC_HEADER_CHECK(elf.h, AC_DEFINE(SVR4); LIBS="$LIBS -lelf")
@end example
@end defmac

@defmac AC_DEFINE_UNQUOTED (@var{variable} @r{[}, @var{value}@r{]})
@maindex DEFINE_UNQUOTED
Like @code{AC_DEFINE}, but it does nothing to quote @var{value} from
various shell and @code{sed} expansions it will undergo.  @var{value}
will be used in many different contexts requiring different quoting, and
it is up to you to make sure it works right.  Use this macro instead of
@code{AC_DEFINE} when @var{value} contains a shell variable.  For
example:

@example
AC_DEFINE_UNQUOTED(config_machfile,  $@{machfile@})
@end example
@end defmac

@defmac AC_SUBST (@var{variable})
@maindex SUBST
Substitute the variable @var{variable} when creating the output files
(typically one or more @file{Makefile}s).  This means replace instances
of @samp{@@@var{variable}@@}, e.g. in @file{Makefile.in}, with the
current value of the shell variable @var{variable}.  If this macro were
not called, the value of @var{variable} would not be set in the output
files, even though @code{configure} had figured out a value for it.

You can set or add to the value of @var{variable} in the usual shell
way.  For example, to add @samp{-ltermcap} to the value of the variable
@code{LIBS}:

@example
LIBS="$LIBS -ltermcap"
@end example
@end defmac

@defmac AC_SUBST_FILE (@var{variable}, @var{file})
@maindex SUBST_FILE
Substitute the contents of the file @file{@var{srcdir}/@var{file}} into
@file{Makefile} variable @var{variable} when creating the output files
(typically one or more @file{Makefile}s).  This macro is useful for
inserting @file{Makefile} fragments for particular host or tartet types
into @file{Makefile}s.
@end defmac

@node Printing Messages, Language Choice, Setting Variables, General Purpose Macros
@section Printing Messages

@code{configure} scripts need to give users running them several kinds
of information.  The following macros print messages in ways appropriate
for different kinds of information.  The arguments to all of them get
enclosed in shell double quotes, so the shell performs variable and
backquote substitution on them.

These macros are all wrappers around the @code{echo} shell command.
Other macros should rarely need to run @code{echo} directly to print
messages for the @code{configure} 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 change automatically.

@defmac AC_CHECKING (@var{feature-description})
@maindex CHECKING
Notify the user that @code{configure} is checking for a particular
feature.  This macro prints a message that starts with @samp{checking }.
It prints nothing if @code{configure} is run with the @samp{--silent} or
@samp{--quiet} option.  The @var{feature-description} should be
something like @samp{whether the Fortran compiler accepts C++ comments}
or @samp{for c89}.
@end defmac

@defmac AC_ERROR (@var{error-description})
@maindex ERROR
Notify the user of an error that prevents @code{configure} from
completing.  This macro prints an error message on the standard error
stream and exits @code{configure} with a nonzero status.
@var{error-description} should be something like @samp{invalid value
$HOME for \$HOME}.
@end defmac

@defmac AC_VERBOSE (@var{result-description})
@maindex VERBOSE
Notify the user of the results of a check.  This information is
only printed if @code{configure} is run with the @samp{--verbose}
option.  @var{result-description} should be something like @samp{setting
ADA to $ADA}.
@end defmac

@defmac AC_WARN (@var{problem-description})
@maindex WARN
Notify the @code{configure} user of a possible problem.  This macro
prints the message on the standard error stream; @code{configure}
continues running afterward, so macros that call @code{AC_WARN} should
provide a default (back-up) behavior for the situations they warn about.
@var{problem-description} should be something like @samp{ln -s seems to
make hard links}.
@end defmac

@node Language Choice, Macro Ordering, Printing Messages, General Purpose Macros
@section Language Choice

Packages that use both C and C++ need to test features of both
compilers.  Autoconf-generated @code{configure} scripts check for C
features by default.  The following macros determine which language's
compiler is used in tests that follow in @file{configure.in}.

@defmac AC_LANG_C
@maindex LANG_C
Do compilation tests using @code{CC} and @code{CPP} and use extension
@file{.c} for test programs.

This is the initial state.
@end defmac

@defmac AC_LANG_CPLUSPLUS
@maindex LANG_CPLUSPLUS
Do compilation tests using @code{CXX} and @code{CXXCPP} and use
extension @file{.C} for test programs.
@end defmac

@defmac AC_LANG_RESTORE
@maindex LANG_RESTORE
Select the language that is saved on the top of the stack, as set by
@code{AC_LANG_SAVE}, and remove it from the stack.  This macro is
equivalent to either @code{AC_LANG_C} or @code{AC_LANG_CPLUSPLUS},
whichever had been run most recently when @code{AC_LANG_SAVE} was last
called.

Do not call this macro more times than @code{AC_LANG_SAVE}.
@end defmac

@defmac AC_LANG_SAVE
@maindex LANG_SAVE
Remember the current language (as set by @code{AC_LANG_C} or
@code{AC_LANG_CPLUSPLUS}) on a stack.  Does not change which language is
current.  Use this macro and @code{AC_LANG_RESTORE} in macros that need
to temporarily switch to a particular language.
@end defmac

@defmac AC_REQUIRE_CPP
@maindex REQUIRE_CPP
Ensure that whichever preprocessor would currently be used for tests has
been found.  Calls @code{AC_REQUIRE} (@pxref{Macro Ordering}) with an
argument of either @code{AC_PROG_CPP} or @code{AC_PROG_CXXCPP},
depending on which language is current.
@end defmac

@node Macro Ordering, , Language Choice, General Purpose Macros
@section Macro Ordering

These macros provide ways for other macros to make sure that they are
called in the correct order.

@defmac AC_BEFORE (@var{this-macro-name}, @var{called-macro-name})
@maindex BEFORE
Make @code{m4} print a warning message on the standard error output if
@var{called-macro-name} has already been called.  @var{this-macro-name}
should be the name of the macro that is calling @code{AC_BEFORE}.  The
macro @var{called-macro-name} must contain a call to @code{AC_PROVIDE}
to indicate that it has been called.

This macro should be used when one macro makes changes that might affect
another macro, so that the other macro should probably not be called
first.  For example, @code{AC_PROG_CPP} checks whether the C compiler
can run the C preprocessor when given the @samp{-E} option.  It should
therefore be called after any macros that change which C compiler is
being used, such as @code{AC_PROG_CC}.  So @code{AC_PROG_CC} contains:

@example
AC_BEFORE([$0], [AC_PROG_CPP])
@end example

@noindent
This warns the user if a call to @code{AC_PROG_CPP} has already occurred
when @code{AC_PROG_CC} is called.
@end defmac

@defmac AC_OBSOLETE (@var{this-macro-name} @r{[}, @var{suggestion}@r{]})
@maindex OBSOLETE
Make @code{m4} print a message on the standard error output warning that
@var{this-macro-name} is obsolete, and giving the file and line number
where it was called.  @var{this-macro-name} should be the name of the
macro that is calling @code{AC_BEFORE}.  If @var{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 @var{this-macro-name}.

A sample call is:

@example
AC_OBSOLETE([$0], [; use AC_HAVE_HEADERS(unistd.h) instead])
@end example
@end defmac

@defmac AC_PROVIDE (@var{macro-name})
@maindex PROVIDE
Set a flag recording that @var{macro-name} has been called.  The
argument should be the name of the macro that is calling
@code{AC_PROVIDE}.  An easy way to get it is from the @code{m4} builtin
variable @code{$0}, like this:

@example
AC_PROVIDE([$0])
@end example
@end defmac

@defmac AC_REQUIRE (@var{macro-name})
@maindex REQUIRE
If the @code{m4} macro @var{macro-name} has not already been called,
call it (without any arguments).  Make sure to quote @var{macro-name}
with square brackets.  The body of @var{macro-name} must contain a call
to @code{AC_PROVIDE} to indicate that it has been called.

Macros that need some other macro to be called before they are called
can use @code{AC_REQUIRE} to ensure that it has been, in case the person
who made @file{configure.in} forgot or didn't know to do it.
@code{AC_REQUIRE} and @code{AC_PROVIDE} together can ensure that a macro
is only called if it is needed, and only called once.
@xref{Dependencies Between Macros}, for more information.
@end defmac

@node Manual Configuration, Writing Macros, General Purpose Macros, Top
@chapter Manual Configuration

Some kinds of features can't be guessed automatically by running test
programs.  For example, how to allocate a pty, or the details of the
object file format, or special options that need to be passed to the
compiler or linker to provide a POSIX or ANSI C environment.  It is
possible to check for such features using ad-hoc means, such as having
@code{configure} check the output of the @code{uname} program, or
looking for libraries that are unique to particular systems.  However,
Autoconf provides a uniform method for handling unguessable features.

Like other GNU @code{configure} scripts, Autoconf-generated
@code{configure} scripts can make decisions based on a canonical name
for the system type, which has the form:

@example
@var{cpu}-@var{company}-@var{system}
@end example

@code{configure} can usually guess the canonical name for the type of
system it's running on.  To do so it runs a script called
@code{config.guess}, which derives the name using the @code{uname}
command or symbols predefined by the C preprocessor.

Alternately, the user can specify the system type with command line
arguments to @code{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:

@table @code
@item --build=@var{build-type}
the type of system on which the package is being configured and
compiled;

@item --host=@var{host-type}
the type of system on which the package will run;

@item --target=@var{target-type}
the type of system for which any compiler tools in the package will
produce code.
@end table

@noindent
If the user gives @code{configure} a non-option argument, it is used as
the default for the host, target, and build system types if the user
does not specify them explicitly with options.  The target and build
types default to the host type if it is given and they are not.  Note
that if you are cross-compiling, you still have to specify the names of
the cross-tools you use, in particular the C compiler, on the
@code{configure} and @code{make} command lines, e.g.,

@example
CC=m68k-coff-gcc configure --target=m68k-coff; CC=m68k-coff-gcc make
@end example

@code{configure} recognizes short aliases for many system types; for
example, @samp{decstation} can be given on the command line instead of
@samp{mips-dec-ultrix4.2}.  @code{configure} runs a script called
@code{config.sub} to canonicalize system type aliases.

@defmac AC_CANONICAL_SYSTEM
@maindex CANONICAL_SYSTEM
Set shell and @code{make} variables to the names of the canonical system
types.  If the user did not specify one or more of those values on the
command line, run @code{config.guess} to determine them.  Run
@code{config.sub} to canonicalize any aliases the user gave.  If you use
this macro, you must distribute those two shell scripts along with your
source code (@pxref{Setup}, for information about the
@code{AC_CONFIG_AUX_DIR} macro which you can use to control which
directory @code{configure} looks for those scripts in).  If you do not
run this macro, @code{configure} ignores any @samp{--host}, etc. options
given to it.  The variables set are:

@table @code
@item @code{build}, @code{host}, @code{target}
the canonical system names;

@item @code{build_alias}, @code{host_alias}, @code{target_alias}
the names the user specified, or the canonical names if
@code{config.guess} was used;

@item @code{build_cpu}, @code{build_vendor}, @code{build_os}
@itemx @code{host_cpu}, @code{host_vendor}, @code{host_os}
@itemx @code{target_cpu}, @code{target_vendor}, @code{target_os}
the individual parts of the canonical names (for convenience).
@end table
@end defmac

How do you use a canonical system type?  Usually, you use it in one or
more @code{case} statements in @file{configure.in} to select
system-specific C files.  Then link those files, which have names based
on the system name, to generic names, such as @file{host.h} or
@file{target.c}.  The @code{case} statement patterns can use shell
wildcards to group several cases together, like in this fragment:

@example
case "$target" in
i386-*-mach* | i386-*-gnu*) obj_format=aout emulation=mach bfd_gas=yes ;;
i960-*-bout) obj_format=bout ;;
esac
@end example

@defmac AC_LINK_FILES (@var{link} @dots{}, @var{file} @dots{})
@maindex LINK_FILES
Link each of the existing files @var{file} to the corresponding link
name @var{link}.  Makes a symbolic link if possible, otherwise a hard
link.  For example, this call:

@example
AC_LINK_FILES(config/sun3.h config/aout.h, host.h object.h)
@end example

@noindent
creates in the current directory @file{host.h}, which is a link to
@file{@var{srcdir}/config/sun3.h}, and @file{object.h}, which is a link
to @file{@var{srcdir}/config/aout.h}.
@end defmac

@node Writing Macros, Caching Values, Manual Configuration, Top
@chapter Writing Macros

If your package needs to test for some feature that none of the macros
supplied with Autoconf handles, you'll need to write one or more new
Autoconf macros.  Here are some suggestions and some of the rationale
behind why the existing macros are written the way they are.  You can
also learn a lot about how to write Autoconf macros by looking at the
existing ones.  If something goes wrong in one or more of the Autoconf
tests, this information can help you understand why they work the way
they do and the assumptions behind them, which might help you figure out
how to best solve the problem.

If you add macros that you think would be useful to other people, or
find problems with the distributed macros, please send electronic mail
to @file{bug-gnu-utils@@prep.ai.mit.edu}, so we can consider them for
future releases of Autoconf.  Please include the Autoconf version
number, which you can get by running @samp{autoconf --version}.

@menu
* Macro Format::		Basic format of an Autoconf macro.
* Quoting::			Protecting macros from unwanted expansion.
* Dependencies Between Macros::	What to do when macros depend on other macros.
* Checking for Files::		Finding whether a file exists.
* Checking for Symbols::	Finding whether a symbol is defined.
* Test Programs::		Writing programs to test for features.
* Multiple Cases::		Tests for several possible values.
@end menu

@node Macro Format, Quoting, , Writing Macros
@section Macro Format

Autoconf macros are defined as arguments to the @code{m4} builtin
command @code{define}.  Their overall structure looks like this:

@example
define(@var{macro-name}, [@var{macro-body}])dnl
@end example

@noindent
The square brackets here do not indicate optional text: they should
literally be present in the macro definition.

All of the Autoconf macros have names starting with @samp{AC_} to
prevent them from accidentally conflicting with other text.  All shell
variables that they use for internal purposes have names starting with
@samp{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.

The @code{m4} builtin @code{dnl} prevents a newline from being inserted
in the output where the macro is defined; without it, the generated
@code{configure} script would begin with dozens of blank lines.
@code{dnl} is also used to introduce comments in @code{m4}; it causes
@code{m4} to discard the rest of the input line.

You should quote the entire macro body with square brackets to avoid
macro expansion problems (@pxref{Quoting}).  You can refer to any
arguments passed to the macro as @samp{$1}, @samp{$2}, etc.

@xref{Definitions, , How to define new macros, m4.info, GNU m4}, for
more complete information on writing @code{m4} macros.

@node Quoting, Dependencies Between Macros, Macro Format, Writing Macros
@section Quoting

Macros that are called by other macros are evaluated by @code{m4}
several times; each evaluation might require another layer of quotes to
prevent unwanted expansions of macros or @code{m4} builtins, such as
@samp{define} and @samp{$1}.  Quotes are also required around macro
arguments that contain commas, since commas separate the arguments from
each other.  It's a good idea to quote any macro arguments that contain
newlines or calls to other macros, as well.

Autoconf (in @file{acgeneral.m4}) changes the @code{m4} quote characters
from the default @samp{`} and @samp{'} to @samp{[} and @samp{]}, because
many of the macros use @samp{`} and @samp{'}, mismatched.  However, in a
few places the macros need to use brackets.  In those places, they use
the @code{m4} builtin command @code{changequote} to temporarily disable
quoting before the code that uses brackets, like this:

@example
changequote(,)dnl
@end example

@noindent
Then they turn quoting back on again with another call to
@code{changequote}:

@example
changequote([,])dnl
@end example

When you create a @code{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 @code{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 @code{configure} script will contain
unexpanded macros.  The @code{autoconf} program checks for this problem
by doing @samp{grep AC_ configure}.

@node Dependencies Between Macros, Checking for Files, Quoting, Writing Macros
@section 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.

@menu
* Prerequisite Macros::		Ensuring required information.
* Suggested Ordering::		Warning about possible ordering problems.
@end menu

@node Prerequisite Macros, Suggested Ordering, , Dependencies Between Macros
@subsection Prerequisite Macros

A macro that you write might need to use values that have previously
been computed by other macros.  For example, if you write a new macro
that uses the C preprocessor, it depends on @code{AC_PROG_CPP} having
been called first to set the shell variable @code{CPP}
(@pxref{Alternative Programs}).

Rather than forcing the user of the macros to keep track of all of the
dependencies between them, you can use the macros @code{AC_PROVIDE} and
@code{AC_REQUIRE} to do it automatically.  @xref{Macro Ordering}, for more
information on their syntax.

The new macro that runs the C preprocessor should contain, somewhere
before @code{CPP} is used, the statement

@example
AC_REQUIRE([AC_PROG_CPP])
@end example

@noindent
and the macro @code{AC_PROG_CPP} should contain the statement (anywhere
in its body)

@example
AC_PROVIDE([$0])
@end example

@noindent
Then, when the new macro is run, it will invoke @code{AC_PROG_CPP} if
and only if @code{AC_PROG_CPP} has not already been run.

@node Suggested Ordering, , Prerequisite Macros, Dependencies Between Macros
@subsection Suggested Ordering

Some macros should be run before another macro if both are called, but
neither requires the other to be called.  For example, a macro like
@code{AC_AIX} that changes the behavior of the C compiler (@pxref{UNIX
Variants}) should be called before any macros that run the C compiler.
Many of these dependencies are noted in the documentation.

Autoconf provides a way to warn users when macros with this kind of
dependency appear out of order in a @file{configure.in} file.  The
warning occurs when creating @file{configure} from @file{configure.in},
not when running @file{configure}.  It is not a fatal error;
@file{configure} is created as usual.

The @code{AC_BEFORE} macro causes @code{m4} to print a warning message
on the standard error output when a macro is used before another macro
which might change its behavior.  The macro which should come first
should contain a call to @code{AC_BEFORE} and the macro which should
come later should contain a call to @code{AC_PROVIDE}.

For example, @code{AC_AIX} contains

@example
AC_BEFORE([$0], [AC_TEST_LINK])
@end example

@noindent
and @code{AC_TEST_LINK} contains

@example
AC_PROVIDE([$0])
@end example

@noindent
As a result, if @code{AC_AIX} is called after @code{AC_TEST_LINK},
it will note that @code{AC_TEST_LINK} has already been called and
print a warning message.

@node Checking for Files, Checking for Symbols, Dependencies Between Macros, Writing Macros
@section Checking for Files

If you need to check whether a file other than a C header file exists,
use @samp{test -f @var{filename}}.  If you need to make multiple checks
using @code{test}, combine them with the shell operators @samp{&&} and
@samp{||} instead of using the @code{test} operators @samp{-a} and
@samp{-o}.  On System V, the precedence of @samp{-a} and @samp{-o} is
wrong relative to the unary operators; consequently, POSIX does not
specify them, so using them is nonportable.  If you combine @samp{&&}
and @samp{||} in the same statement, keep in mind that they have equal
precedence.

Do not use @samp{test -x}, because 4.3BSD does not have it.  Use
@samp{test -f} or @samp{test -r} instead.

@node Checking for Symbols, Test Programs, Checking for Files, Writing Macros
@section Checking for Symbols

If you need to check whether a symbol is defined in a C header file, you
can use @code{AC_HEADER_EGREP} if the symbol is not a C preprocessor
macro (@pxref{C Features}), or compile a small test program that includes
the file and references the symbol (@pxref{Test Programs}).  Don't
directly @code{grep} for the symbol in the file, because on some systems
it might be defined in another header file that the file you are
checking @samp{#include}s.

However, if you need to check for a particular UNIX variant which is
distinguished by having certain text in a certain file, then use
@code{grep} (or @code{egrep}).  But don't use @samp{grep -s} to suppress
output, because @samp{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 @code{grep} to
@file{/dev/null}.  Check the exit status of @code{grep} to determine
whether it found a match.

To check whether the Autoconf macros have already defined a certain C
preprocessor symbol, you can use a @code{case} statement like this:

@example
case "$DEFS" in
  *HAVE_FOO*) ;;
  *) LIBOBJS="$LIBOBJS foo.o" ;;
esac
@end example

@noindent
Make sure to enclose the variable name you are checking (usually
@code{DEFS}) in double quotes, because otherwise some old versions of
@code{bash} misinterpret the statement.

@node Test Programs, Multiple Cases, Checking for Symbols, Writing Macros
@section Test Programs

Autoconf checks for many features by compiling small test programs.  To
find out whether a library function is available, Autoconf tries to
compile a small program that uses it.  This is unlike Larry Wall's
Metaconfig, which uses @code{nm} or @code{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 and flexible approach because it
avoids dealing with the variations in the options and output formats of
@code{nm} and @code{ar} and in the location of the standard libraries.
It also allows @code{configure} to check aspects of the function's
runtime behavior if needed.  On the other hand, it is sometimes slower
than scanning the libraries.

If you need to check for a condition other than whether some symbol
exists on the system or has a certain value, then you can't use
@code{AC_TEST_LINK} (@pxref{C Features}).  You have to write a
test program by hand.  You can compile and run it using
@code{AC_TEST_RUN} (@pxref{C Features}).

Try to avoid writing test programs if possible, because using them
prevents people from configuring your package for cross-compiling.  If
it's really best that you test for a run-time behavior, try to provide a
default ``worst case'' value to use when cross-compiling makes run-time
tests impossible.  You do this by passing the optional last argument to
@code{AC_TEST_RUN}.

@menu
* Guidelines::			General rules for writing test programs.
* Tricks::			Special ways to work around problems.
@end menu

@node Guidelines, Tricks, , Test Programs
@subsection Guidelines for Test Programs

Test programs 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 @code{exit}, not @code{return}, from
@code{main}, because on some systems the argument to @code{return} in
@code{main} is ignored.  They should not write anything to the standard
output.

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

@example
@group
#if STDC_HEADERS
#include <stdlib.h>
#endif
@end group
@end example

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

@node Tricks, , Guidelines, Test Programs
@subsection Tricks for Test Programs

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
@code{exit}, because GCC version 2 knows that @code{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, @code{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 @code{char}).

@node Multiple Cases, , Test Programs, Writing Macros
@section 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 excerpted from the @file{configure.in} for GNU
@code{find}.  It uses the shell variable @code{fstype} to keep track of
whether the remaining cases need to be checked.  There are several more
cases which are not shown here but follow the same pattern.

@example
@group
echo checking how to get filesystem type
# SVR4.
AC_TEST_CPP([#include <sys/statvfs.h>
#include <sys/fstyp.h>], AC_DEFINE(FSTYPE_STATVFS) fstype=1)
if test -z "$fstype"; then
# SVR3.
AC_TEST_CPP([#include <sys/statfs.h>
#include <sys/fstyp.h>], AC_DEFINE(FSTYPE_USG_STATFS) fstype=1)
fi
if test -z "$fstype"; then
# AIX.
AC_TEST_CPP([#include <sys/statfs.h>
#include <sys/vmount.h>], AC_DEFINE(FSTYPE_AIX_STATFS) fstype=1)
fi
@end group
@end example

@node Caching Values, Makefiles, Writing Macros, Top
@chapter Caching Values

To avoid checking for the same features repeatedly in various
@code{configure} scripts (or repeated runs of one script),
@code{configure} saves the results of many of its checks in a @dfn{cache
file}.  If, when a @code{configure} script runs, it finds a cache file,
it reads from it the results from previous runs and avoids rerunning
those checks.  As a result, @code{configure} can run much faster than if
it had to perform all of the checks every time.

The 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 @file{./config.cache} as the cache file,
creating it if it does not exist already.  @code{configure} accepts the
@samp{--cache-file=@var{file}} option to use a different cache file;
that is what @code{configure} does when it calls @code{configure}
scripts in subdirectories, so they share the cache.  @xref{Setup}, for
information on configuring subdirectories with the
@code{AC_CONFIG_SUBDIRS} macro.  @file{config.status} only pays
attention to the cache file if it is given the @samp{--recheck} option,
which makes it rerun @code{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 (@pxref{Manual Configuration}).

The cache file on a particular system will gradually accumulate whenever
someone runs a @code{configure} script; it will be initially
nonexistent.  Running @code{configure} merges the new cache results with
the existing cache file.  The site initialization script can specify a
site-wide cache file to use instead of the default, to make it work
transparently (@pxref{Site Default Values}).

@defmac AC_CACHE_VAL (@var{cache-id}, @var{commands-to-set-it})
@maindex CACHE_VAL
Ensure that the results of the check identified by @var{cache-id} are
available.  If the results of the check were in the cache file that was
read, print a verbose message saying so; otherwise, run the shell
commands @var{commands-to-set-it}.  Those commands should have no side
effects except for setting the variable @var{cache-id}.  In particular,
they should not call @code{AC_DEFINE}; the code that follows the call to
@code{AC_CACHE_VAL} should do that, based on the cached value.  Also,
they should not print any messages, for example with @code{AC_CHECKING};
do that before calling @code{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.  If the shell
commands are run to determine the value, the value will be saved in the
cache file just before @code{configure} creates its output files.

@noindent
The names of cache variables should have this format:

@example
@var{package-prefix}_cv_@var{value-type}_@var{specific-value}@r{[}_@var{additional-options}@r{]}
@end example

@noindent
for example, @samp{ac_cv_header_stat_broken} or
@samp{ac_cv_prog_gcc_traditional}.  The parts of the variable name are:

@table @asis
@item @var{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
@samp{ac}. 

@item @code{_cv_}
Indicates that this shell variable is a cache value.

@item @var{value-type}
A convention for classifying cache values, to produce a rational naming
system.  The values used in Autoconf are for the following classes of
features; use them where applicable, otherwise invent your own categories:

@table @code
@item c
C language

@item func
C functions

@item group
Unix groups

@item header
C header files

@item lib
C libraries

@item path
the absolute path names of programs

@item prog
the base names of programs

@item struct
C structures

@item sys
operating systems

@item type
C types

@item var
C variables in libraries

@item x
X Window System
@end table

@item @var{specific-value}
Which member of the class of cache values this test applies to.
For example, which function (@samp{alloca}), program (@samp{gcc}), or
@code{make} variable (@samp{INSTALL}).

@item @var{additional-options}
Any particular behavior of the specific member that this test applies to.
For example, @samp{broken} or @samp{set}.  This part of the name may
be omitted if it does not apply.
@end table

Like their names, the values the may be assigned to cache variables have
a few restrictions.  The values may not contain single quotes or curly braces.
Usually, their values will be boolean (@samp{yes} or @samp{no}) or the
names of files or functions; so this is not an important restriction.
@end defmac

@node Makefiles, Invoking configure, Caching Values, Top
@chapter Makefiles

Each subdirectory in a distribution should come with a file
@file{Makefile.in}, from which @code{configure} will produce a
@file{Makefile} in that directory.  Most of the substitutions that
@code{configure} does are simple: for each configuration variable that
the package uses, it just replaces occurrences of
@samp{@@@var{variable}@@} with the value that @code{configure} has
determined for that variable.  Any occurrences of
@samp{@@@var{variable}@@} for variables that @code{configure} does not
know about are passed through unchanged.

There is no point in checking for the correct value to give a variable
that is never used.  Every variable that the @code{configure} script
might set a value for should appear in a @samp{@@@var{variable}@@} reference
in at least one @file{Makefile.in}.  If @code{AC_CONFIG_HEADER} is
called, @code{configure} replaces @samp{@@DEFS@@} with
@samp{-DHAVE_CONFIG_H}, since the contents of @code{DEFS} would be
redundant.

@xref{Makefile Conventions, , Makefile Conventions, standards.info, The
GNU Coding Standards}, for more information on what to put in Makefiles.
@xref{Sample Makefile.in}, for an example of a real @file{Makefile.in}.

@menu
* Predefined Variables::	Heavily used @code{make} variables.
* VPATH Substitutions::		Compiling in a different directory.
* Automatic Remaking::		Makefile rules for configuring.
@end menu

@node Predefined Variables, VPATH Substitutions, , Makefiles
@section Predefined Variables

Some @code{make} variables are predefined by the Autoconf macros.
@code{AC_SUBST} is called for them automatically (@pxref{Setting
Variables}), so in your @file{Makefile.in} files you can get their
values by enclosing their names in @samp{@@} characters.
@ifinfo
@xref{Makefiles}, for more information on @samp{@@} substitutions.
@end ifinfo
The variables that are defined by the general
purpose Autoconf macros are:

@defvar exec_prefix
The installation prefix for architecture-specific files.
@end defvar

@defvar prefix
The installation prefix for architecture-independent files.
@end defvar

@defvar srcdir
The directory that contains the source code for that @file{Makefile}.
@end defvar

@defvar top_srcdir
The top-level source code directory for the package.  In the top-level
directory, this is the same as @code{srcdir}.
@end defvar

@defvar DEFS
@samp{-D} options to pass to the C compiler.  If @code{AC_CONFIG_HEADER}
is called, @code{configure} replaces @samp{@@DEFS@@} with
@samp{-DHAVE_CONFIG_H}, since the contents of @code{DEFS} would be
redundant.
@end defvar

@defvar LIBS
@samp{-l} and @samp{-L} options to pass to the linker.
@end defvar

@defvar LIBOBJS
Names of object files (ending in @file{.o}).  Set by
@code{AC_REPLACE_FUNCS} (@pxref{C Features}).
@end defvar

@node VPATH Substitutions, Automatic Remaking, Predefined Variables, Makefiles
@section @code{VPATH} Substitutions

You might want to compile a software package in a different directory
from the one that contains the source code.  Doing this allows you to
compile the package for several architectures simultaneously from the
same copy of the source code and keep multiple sets of object files on
disk.

To support doing this, @code{make} uses the @code{VPATH} variable to
find the files that are in the source directory.  GNU @code{make} and
most other recent @code{make} programs can do this.  Older @code{make}
programs do not support @code{VPATH}; when using them, the source code
must be in the same directory as the object files.

To support @code{VPATH}, each @file{Makefile.in} should contain two
lines that look like:

@example
srcdir = @@srcdir@@
VPATH = @@srcdir@@
@end example

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

@code{configure} substitutes in the correct value for @code{srcdir} when
it produces @file{Makefile.in}.

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

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

@example
time.info: time.texinfo
        $(MAKEINFO) $(srcdir)/time.texinfo
@end example

@node Automatic Remaking, , VPATH Substitutions, Makefiles
@section Automatic Remaking

You can put rules like the following in the top-level @file{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 @file{aclocal.m4} and those related to
configuration header files.  Omit from the @file{Makefile.in} rules any
of these files that your package does not use.

The @file{stamp-} files are necessary because the timestamps of
@file{config.h.in} and @file{config.h} will not be changed if remaking
them does not change their contents.  This feature avoids unnecessary
recompilation.  You should include the file @file{stamp-h.in} your
package's distribution, so @code{make} will consider @file{config.h.in}
up to date.

@example
@group
configure: configure.in aclocal.m4
        cd $@{srcdir@} && autoconf

# autoheader might not change config.h.in
config.h.in: stamp-h.in
stamp-h.in: configure.in aclocal.m4 acconfig.h config.h.top config.h.bot
        cd $@{srcdir@} && autoheader
        touch $@{srcdir@}/stamp-h.in

# config.status might not change config.h
# Don't rerun config.status if we just configured.
config.h: stamp-h
stamp-h: config.h.in config.status
        test ! -f stamp-h || ./config.status
        touch stamp-h

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

config.status: configure
        ./config.status --recheck
@end group
@end example

@xref{Invoking config.status}, for more information on handling
configuration-related dependencies.

@node Invoking configure, Invoking config.status, Makefiles, Top
@chapter Running @code{configure} Scripts

A software package that uses a @code{configure} script
should be distributed with a file @file{Makefile.in}, but no
@file{Makefile}; that way, the user has to properly configure the
package for the local system before compiling it.
Below are instructions on how to configure a package that uses a
@code{configure} script, suitable for inclusion as an @file{INSTALL}
file in the package.

@menu
* Basic Installation::          Instructions for typical cases.
* Compilers and Options::       Selecting compilers and optimization.
* Build Directory::             Configuring in a different directory.
* Installation Directories::    Installing in different directories.
* System Type::                 Specifying the system type.
* Optional Features::           Selecting optional features.
@end menu

@include install.texi

@node Invoking config.status, Site Default Values, Invoking configure, Top
@chapter Recreating a Configuration

The @code{configure} script creates a file named @file{config.status}
which describes which configuration options were specified when the
package was last configured.  This file is a shell script which,
if run, will recreate the same configuration.

You can give @file{config.status} the @samp{--recheck} option to update
itself.  This option is useful if you change @code{configure}, so that
the results of some tests might be different from the previous run.  The
@samp{--recheck} option re-runs @code{configure} with the same arguments
you used before, plus the @samp{--no-create} option, which prevents
@code{configure} from running @file{config.status} and creating
@file{Makefile} and other files.  (This is so other @file{Makefile}
rules can run @file{config.status} when it changes; @pxref{Automatic
Remaking}, for an example).

@file{config.status} also accepts the options @samp{--help}, which
prints a summary of the options to @file{config.status}, and
@samp{--version}, which prints the version of Autoconf used to create
the @code{configure} script that generated @file{config.status}.

@file{config.status} checks several optional environment variables that
can alter its behavior:

@defvar CONFIG_SHELL
The shell with which to run @code{configure} for the @samp{--recheck}
option.  The default is @file{/bin/sh}.
@end defvar

@defvar CONFIG_STATUS
The file name to use for the shell script that records the
configuration.  The default is @file{./config.status}.  This variable is
useful when one package uses parts of another and the @file{configure}
scripts shouldn't be merged because they are maintained separately.
@end defvar

The following variables provide one way for separately distributed
packages to share the values computed by @code{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 variables allow a
@file{config.status} file to create files other than the ones that its
@file{configure.in} specifies, so it can be used for a different package.

@defvar CONFIG_FILES
The files in which to perform @samp{@@@var{variable}@@} substitutions.
The default is the arguments given to @code{AC_OUTPUT} in @file{configure.in}.
@end defvar

@defvar CONFIG_HEADERS
The files in which to substitute C @code{#define} statements.
The default is the arguments given to @code{AC_CONFIG_HEADER}; if that
macro was not called, @file{config.status} ignores this variable.
@end defvar

These variables also allow you to write @file{Makefile} rules that
regenerate only some of the files.  For example, in the dependencies
given above (@pxref{Automatic Remaking}), @file{config.status} is run
twice when @file{configure.in} has changed.  If that bothers you, you
can make each run only regenerate the files for that rule:

@example
@group
# config.status might not change config.h
config.h: stamp-h
stamp-h: config.h.in config.status
        CONFIG_FILES= CONFIG_HEADERS=config.h ./config.status
        touch stamp-h

Makefile: Makefile.in config.status
        CONFIG_FILES=Makefile CONFIG_HEADERS= ./config.status
@end group
@end example

@noindent
(If @file{configure.in} does not call @code{AC_CONFIG_HEADER}, there is
no need to set @code{CONFIG_HEADERS} in the @code{make} rules.)

@node Site Default Values, Example, Invoking config.status, Top
@chapter Site Default Values

Autoconf-generated @code{configure} scripts support site and system wide
initialization files.  You can create these files to provide default
values for some configuration values, both on all of your systems and
separately for each architecture.

@code{configure} reads the shell script
@file{@var{exec_prefix}/lib/config.site} if it exists, then
@file{@var{prefix}/lib/config.site} if it exists.  Thus, settings in
system dependent files override those in system independent ones in case
of conflict.  @code{configure} reads any cache file after it has read
any site files.  This way, the site file can define a default cache
file, to be shared between all Autoconf-generated @code{configure}
scripts run on that system.

Site files can be arbitrary shell scripts, but only certain kinds of
code are really appropriate to be in them.  Besides a cache file, they
are good places to set default values for other shell variables like
@code{CC}, if you need to give them unusual values: anything you would
normally do, repetitively, on the command line.  If you use non-default
values for @var{prefix} or @var{exec_prefix}, you can't set them in the
site file, since you have to specify them on the command line in order
for @code{configure} to find the site file!

In addition, it is possible to set some cache values in the site file.
If you are cross-compiling, 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
@file{@var{exec_prefix}/lib/config.site}.  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 @code{prefix} and @code{cache_file} have
their default values (as set near the top of @code{configure}) before
changing them.

@node Example, Preprocessor Symbol Index, Site Default Values, Top
@chapter An Example

Here are sample @file{configure.in} and @file{Makefile.in} files, to
give a real illustration of using Autoconf.  They are from the GNU
@code{cpio} package, which also includes the @code{mt} and @code{rmt}
programs.  This package does not use a configuration header file; it
passes @samp{-D} options to the C compiler on the command line.

@menu
* Sample configure.in::		An example of a @file{configure} template.
* Sample Makefile.in::		An example of a @file{Makefile} template.
@end menu

@node Sample configure.in, Sample Makefile.in, , Example
@section Sample @file{configure.in}

@noindent
Here is @file{configure.in} from GNU @code{cpio}.  The @code{dnl} macro
after @code{AC_SUBST} is suppresses an extra (though harmless) newline
in the generated @code{configure} script (because the @code{AC_SUBST}
macro does not produce any output where it is called).

@example
dnl Process this file with autoconf to produce a configure script.
AC_INIT(cpio.h)
PROGS="cpio"
AC_SUBST(PROGS)dnl
AC_PROG_CC
AC_PROG_CPP
AC_GCC_TRADITIONAL
AC_PROG_INSTALL
AC_AIX
AC_MINIX
AC_ISC_POSIX
AC_RETSIGTYPE
AC_MAJOR_HEADER
AC_REMOTE_TAPE
test -n "$have_mtio" && PROGS="$PROGS mt"
AC_RSH
AC_CONST
AC_UID_T
AC_STDC_HEADERS
AC_HAVE_HEADERS(string.h fcntl.h utime.h unistd.h sys/io/trioctl.h)
AC_REPLACE_FUNCS(fnmatch bcopy mkdir strdup)
AC_HAVE_FUNCS(strerror lchown)
AC_VPRINTF
AC_ALLOCA
AC_XENIX_DIR
AC_HAVE_LIBRARY(socket, [LIBS="$LIBS -lsocket"])
AC_HAVE_LIBRARY(nsl, [LIBS="$LIBS -lnsl"])
AC_OUTPUT(Makefile)
@end example

@node Sample Makefile.in, , Sample configure.in, Example
@section Sample @file{Makefile.in}

@noindent
Here is @file{Makefile.in} from GNU @code{cpio}, with some irrelevant
lines omitted, for brevity.

@example
srcdir = @@srcdir@@
VPATH = @@srcdir@@

CC = @@CC@@

INSTALL = @@INSTALL@@
INSTALL_PROGRAM = @@INSTALL_PROGRAM@@
INSTALL_DATA = @@INSTALL_DATA@@

DEFS = @@DEFS@@
LIBS = @@LIBS@@
RTAPELIB = @@RTAPELIB@@

CFLAGS = -g
LDFLAGS = -g

prefix = /usr/local
exec_prefix = $(prefix)
binprefix =
manprefix =

bindir = $(exec_prefix)/bin
libdir = $(exec_prefix)/lib
mandir = $(prefix)/man/man1
manext = 1

SHELL = /bin/sh

SRCS = copyin.c copyout.c copypass.c defer.c dstring.c global.c \
main.c tar.c util.c error.c getopt.c getopt1.c filemode.c version.c \
rtapelib.c dirname.c idcache.c makepath.c xmalloc.c stripslash.c \
userspec.c xstrdup.c bcopy.c fnmatch.c mkdir.c strdup.c
OBJS = copyin.o copyout.o copypass.o defer.o dstring.o global.o \
main.o tar.o util.o error.o getopt.o getopt1.o filemode.o version.o \
$(RTAPELIB) dirname.o idcache.o makepath.o xmalloc.o stripslash.o \
userspec.o xstrdup.o @@LIBOBJS@@ @@ALLOCA@@
# mt source files not shared with cpio.
MT_SRCS = mt.c argmatch.c
MT_OBJS = mt.o argmatch.o error.o getopt.o getopt1.o \
xmalloc.o version.o $(RTAPELIB) @@ALLOCA@@
HDRS = cpio.h cpiohdr.h tar.h tarhdr.h defer.h dstring.h extern.h filetypes.h \
system.h fnmatch.h getopt.h rmt.h 
DISTFILES = $(SRCS) $(HDRS) COPYING COPYING.LIB ChangeLog Makefile.in \
README NEWS INSTALL cpio.1 mt.1 makefile.pc makefile.os2 cpio.def \
configure configure.in mkinstalldirs $(MT_SRCS) rmt.c tcexparg.c alloca.c

all: @@PROGS@@

.c.o:
        $(CC) -c $(CPPFLAGS) $(DEFS) -I$(srcdir) $(CFLAGS) $<

install: installdirs all $(srcdir)/cpio.1 $(srcdir)/mt.1
        $(INSTALL_PROGRAM) cpio $(bindir)/$(binprefix)cpio
        test ! -f mt || $(INSTALL_PROGRAM) mt $(bindir)/$(binprefix)mt
        -test ! -f rmt || $(INSTALL_PROGRAM) rmt $(libdir)/rmt
        $(INSTALL_DATA) $(srcdir)/cpio.1 $(mandir)/$(manprefix)cpio.$(manext)
        test ! -f mt || \
        $(INSTALL_DATA) $(srcdir)/mt.1 $(mandir)/$(manprefix)mt.$(manext)

installdirs:
        $(srcdir)/mkinstalldirs $(bindir) $(libdir) $(mandir)

uninstall:
        cd $(bindir); rm -f $(binprefix)cpio $(binprefix)mt
        -rm -f $(libdir)/rmt
        cd $(mandir); rm -f $(manprefix)cpio.$(manext) $(manprefix)mt.$(manext)

check:
        @@echo No tests are supplied.

cpio: $(OBJS)
        $(CC) $(LDFLAGS) -o $@@ $(OBJS) $(LIBS)

rmt: rmt.o
        $(CC) $(LDFLAGS) -o $@@ rmt.o $(LIBS)

mt: $(MT_OBJS)
        $(CC) $(LDFLAGS) -o $@@ $(MT_OBJS) $(LIBS)

Makefile: Makefile.in config.status
        $(SHELL) config.status
config.status: configure
        $(SHELL) config.status --recheck
configure: configure.in
        cd $(srcdir); autoconf

TAGS: $(SRCS)
        etags $(SRCS)

clean:
        rm -f cpio rmt mt *.o core

mostlyclean: clean

distclean: clean
        rm -f Makefile config.status

realclean: distclean
        rm -f TAGS

dist: $(DISTFILES)
        echo cpio-`sed -e '/version_string/!d' \
        -e 's/[^0-9.]*\([0-9.]*\).*/\1/' -e q version.c` > .fname
        rm -rf `cat .fname`
        mkdir `cat .fname`
        -ln $(DISTFILES) `cat .fname`
        for file in $(DISTFILES); do \
          test -r `cat .fname`/$$file || cp -p $$file `cat .fname`; \
        done
        tar chzf `cat .fname`.tar.gz `cat .fname`
        rm -rf `cat .fname` .fname
@end example

@node Preprocessor Symbol Index, Macro Index, Example, Top
@unnumbered 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 @code{#if} directives.

@printindex vr

@node Macro Index, , Preprocessor Symbol Index, Top
@unnumbered 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 @samp{AC_}.

@printindex ma

@contents
@bye

@c Local variables:
@c compile-command: "makeinfo --no-split autoconf.texi"
@c End: