autoconf/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 1.107
@set VERSION 1.107
@set UPDATED August 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
@c I think I've rewritten all of Noah and Roland's contributions by now.

@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
@c Define a CPP variable index that @@defvar doesn't write to.
@defcodeindex cv
@c Define an environment variable index that @@defvar doesn't write to.
@defcodeindex ev

@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.
* Making configure Scripts::    How to organize and produce Autoconf scripts.
* Operation::                   Controlling Autoconf operation.
* Existing Tests::              Macros that check for particular features.
* Primitives::                  Macros used to build other checks.
* 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}.
* Concerns::                    Concerns about Autoconf, with answers.
* Upgrading::                   Tips for upgrading from version 1.
* History::                     History of Autoconf.
* Old Macro Names::		Backward compatibility macros.
* Environment Variable Index::  Index of environment variables used.
* 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 autoupdate::         Replacing old macro names in @code{configure.in}.

Controlling Autoconf Operation

* Input::                       Where Autoconf should find files.
* Output::			Controlling Autoconf output files.
* Command Line::                Checking command line arguments.
* Versions::                    Version numbers in producing @code{configure}.

Existing Tests

* Alternative Programs::	Selecting between alternative programs.
* Libraries::                   Library archives that might be missing.
* 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.

Alternative Programs

* Particular Programs::         Special handling to find certain programs.
* Generic Programs::            How to find other programs.

Header Files

* Particular Headers::          Special handling to find certain headers.
* Generic Headers::             How to find other headers.

Typedefs

* Particular Types::            Special handling to find certain types.
* Generic Types::               How to find other types.

Library Functions

* Particular Functions::        Special handling to find certain functions.
* Generic Functions::           How to find other functions.

Primitives For Building Tests

* C System Output::             Checking output of C compiler system.
* 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.

Manual Configuration

* Specifying Names::            Specifying the system type.
* Canonicalizing::              Getting the canonical system type.
* System Name Variables::       Variables containing the system type.
* Using System Type::           What to do with the system type.

Writing Macros

* Macro Format::		Basic format of an Autoconf macro.
* Macro Naming::                What to call your new macros.
* Quoting::			Protecting macros from unwanted expansion.
* Dependencies Between Macros::	What to do when macros depend on other macros.
* Portable Shell::              Shell script portability pitfalls.
* 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.
* Obsolete Macros::             Warning about old ways of doing things.

Test Programs

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

Caching Values

* Cache Files::	        	Files @code{configure} uses for caching.
* Cache Variables::             Shell variables used in caches.

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.
* 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.

Concerns About Autoconf

* Copyright::                   Legal restrictions on Autoconf output.
* Why GNU m4::                  Why not use the standard @code{m4}?
* Bootstrapping::               Autoconf and GNU @code{m4} require each other?
* Policy Decisions::            Configuring site policy.

Upgrading From Version 1

* Changed File Names::          Files you might rename.
* Changed Makefiles::           New things to put in @file{Makefile.in}.
* Changed Macros::              Macro calls you might replace.
* Changed Results::             Changes in how to check test results.
* Changed Macro Writing::       Better ways to write your own macros.

History of Autoconf

* Genesis::			Prehistory and naming of @code{configure}.
* Exodus::			The plagues of @code{m4} and Perl.
* Leviticus::			The priestly code of portability arrives.
* Numbers::			Growth and contributors.
* Deuteronomy::			Approaching the promises of easy configuration.
@end menu

@node Introduction, Making configure Scripts, Top, Top
@chapter Introduction

@display
A physicist, an engineer, and a computer scientist were
discussing the nature of God.  Surely a Physicist, said the
physicist, because early in the Creation, God made Light; and you
know, the Maxwell's equations, the dual nature of
electro-magnetic waves, the relativist consequences@dots{} An
Engineer!, said the engineer, because before making Light, God
split the Chaos into Land and Water; it takes a hell of an
engineer to handle that big amount of mud, and orderly separation
of solids from liquids@dots{} The computer scientist shouted: And
the Chaos, where do you think it was it coming from, hmm?

---Anonymous
@end display
@c (via Franc,ois Pinard)

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

Autoconf 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; you must use version 1.1 or later of GNU @code{m4}.

@xref{Concerns}, for answers to some common questions about Autoconf.
@xref{Upgrading}, for information about upgrading from version 1.
@xref{History}, for the story of Autoconf's development.

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 Making configure Scripts, Operation, Introduction, 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 (@pxref{Makefiles});

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

@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 @code{autoconf} on it.
If you write your own feature tests to supplement those that come with
Autoconf, you might also write files called @file{aclocal.m4} and
@file{acsite.m4}.  If you use a C header file to contain @code{#define}
directives, you might also write @file{acconfig.h}, @file{config.h.top},
and @file{config.h.bot}, 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 Autoconf
macro files (by reading @file{autoconf.m4}).

@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] --+   `---.
[acsite.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.cache
                       |
                       +-------------> config.log
                       |
[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 autoupdate::         Replacing old macro names in @code{configure.in}.
@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{Existing Tests}, for their descriptions.  For most other
features, you can use Autoconf template macros to produce custom checks;
see @ref{Primitives}, 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{Output}).
Additionally, some macros rely on other macros having been called first,
because they check previously set values of some variables to decide
what to do.  These macros are noted in the individual descriptions
(@pxref{Existing 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.

@display
@group
@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

When calling macros that take arguments, there must not be any blank
space between the macro name and the open parentheses.  Arguments can be
more than one line long if they are enclosed within the @code{m4} quote
characters @samp{[} and @samp{]}.  Several 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.

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

@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 several files.  Some of the files are
distributed with Autoconf; @code{autoconf} reads them first.  Then it
looks for the optional file @file{acsite.m4} in the directory that
contains the distributed Autoconf macro files, and for the optional file
@file{aclocal.m4} in the current directory.  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.

@evindex AC_MACRODIR
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_CHECK_HEADERS}, @code{AC_CHECK_FUNCS}, @code{AC_CHECK_SIZEOF},
or @code{AC_CHECK_LIB},
@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 contains mainly @code{#define}
and @code{#undef} statements and their accompanying comments.  If
@file{./acconfig.h} contains the string @samp{@@TOP@@},
@code{autoheader} copies the lines before the line containing
@samp{@@TOP@@} into the top of the file that it generates.  Similarly,
if @file{./acconfig.h} contains the string @samp{@@BOTTOM@@},
@code{autoheader} copies the lines after that line to the end of the
file it generates.  Either of those strings may be omitted.

An obsolete way to produce the same effect is to create the files
@file{@var{file}.top} (typically @file{config.h.top}) and/or
@file{@var{file}.bot} in the current directory.  If they exist,
@code{autoheader} copies them to the beginning and end, respectively, of
its output.  Their use is discouraged because they have file names that
contain two periods, and so can not be stored on MS-DOS; also, they are
two more files to clutter up the directory.

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.

@evindex AC_MACRODIR
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 @code{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 tree rooted at a directory given as a command line
argument, or the current directory if none is given.  It searches the
source files for common portability problems and creates a file
@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 to your program in
order to make it 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 @code{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 help 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 autoupdate, Invoking ifnames, Making configure Scripts
@section Invoking @code{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 autoupdate, , Invoking autoreconf, Making configure Scripts
@section Invoking @code{autoupdate}

The @code{autoupdate} program updates a @file{configure.in} file that
calls Autoconf macros by their old names to use the current macro names.
In version 2 of Autoconf, most of the macros were renamed to use a more
uniform and descriptive naming scheme.  @xref{Macro Naming}, for a
description of the new scheme.  Although the old names still work
(@pxref{Old Macro Names}, for a list of the old macro names and the
corresponding new names), you can make your @file{configure.in} files
more readable and make it easier to use the current Autoconf
documentation if you update them to use the new macro names.

@evindex SIMPLE_BACKUP_SUFFIX
If given no arguments, @code{autoupdate} updates @file{configure.in},
backing up the original version with the suffix @file{~} (or the value
of the environment variable @code{SIMPLE_BACKUP_SUFFIX}, if that is
set).  If you give @code{autoupdate} an argument, it reads that file
instead of @file{configure.in} and writes the updated file to the
standard output.

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

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

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

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

@node Operation, Existing Tests, Making configure Scripts, Top
@chapter Controlling Autoconf Operation

These macros control the operation of Autoconf: where it finds
files, which output files it produces, how it responds to certain
command line arguments.

@menu
* Input::                       Where Autoconf should find files.
* Output::			Controlling Autoconf output files.
* Command Line::                Checking command line arguments.
* Versions::                    Version numbers in producing @code{configure}.
@end menu

@node Input, Output, , Operation
@section Finding Autoconf Input

The following macros control where Autoconf looks to find files it needs.

@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_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

@node Output, Command Line, Input, Operation
@section Controlling Autoconf Output

The following macros control the kind of output that Autoconf produces.

@defmac AC_CONFIG_HEADER (@var{header-to-create} @dots{})
@maindex CONFIG_HEADER
@cvindex 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 an input file
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}.  Usually this input file is named
@file{@var{header-to-create}.in}; however, you can override the input file
name by appending it to @var{header-to-create}, separated by a colon.
For example,
@example
AC_CONFIG_HEADER(defines.h:defines.hin)
@end example
@noindent
Doing this allows you to keep your filenames acceptable to MS-DOS.

You can use the program @code{autoheader} to create the input file
(@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_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 an input file (by default named
@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}.

You can override an input file name by appending it to @var{file},
separated by a colon.  For example,
@example
AC_OUTPUT(Makefile:templates/top.mk lib/Makefile:templates/lib.mk)
@end example

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

@node Command Line, Versions, Output, Operation
@section Checking Command Line Arguments

These macros check whether the user gave @code{configure} various
command line arguments.  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-editing        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}.
Avoid tabs in the help string.  You'll need to enclose it in @samp{[}
and @samp{]} in order to produce the leading spaces.

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}.
Avoid tabs in the help string.  You'll need to enclose it in @samp{[}
and @samp{]} in order to produce the leading spaces.

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 Versions, , Command Line, Operation
@section Controlling Autoconf Versions

The following macros handle issues of version numbers in producing
@code{configure} scripts.

@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 Existing Tests, Primitives, Operation, Top
@chapter Existing Tests

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

All of these macros that set @code{make} variables call @code{AC_SUBST}
on those variables to insert them into the output @file{Makefile} files
(@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.

@menu
* Alternative Programs::	Selecting between alternative programs.
* Libraries::                   Library archives that might be missing.
* 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, Libraries, , Existing Tests
@section Alternative Programs

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

@menu
* Particular Programs::         Special handling to find certain programs.
* Generic Programs::            How to find other programs.
@end menu

@node Particular Programs, Generic Programs, , Alternative Programs
@subsection Checks For Particular Programs

These macros check for particular programs.

@defmac AC_DECL_YYTEXT
@maindex DECL_YYTEXT
@cvindex 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

@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
Determine a C compiler to use.  If @code{CC} is not already set in the
environment, check for @code{gcc}, and use @code{cc} if it's not found.
Set @code{make} variable @code{CC} to the name of the compiler found.
Set shell variable @code{GCC} to @samp{yes} if it found the GNU C
compiler, empty otherwise, and if @code{make} variable @code{CFLAGS} was
not already set, set it to @samp{-g -O} for the GNU C compiler or
@samp{-g} for other compilers.
@end defmac

@defmac AC_PROG_CC_C_O
@maindex PROG_CC_C_O
@cvindex 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_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_TRY_CPP}, @code{AC_CHECK_HEADER}, @code{AC_EGREP_HEADER}, or
@code{AC_EGREP_CPP}.  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
@code{CXX} or @code{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}, @code{CC}, and
@code{cxx}).  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, and if
@code{make} variable @code{CXXFLAGS} was not already set, set it to
@samp{-g -O} for the GNU C++ compiler or @samp{-g} for other compilers.
@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_TRY_CPP}, @code{AC_CHECK_HEADER},
@code{AC_EGREP_HEADER}, or @code{AC_EGREP_CPP}.  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_GCC_TRADITIONAL
@maindex PROG_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}.  That usually happens when the fixed header files
have not been installed on an old system.  Since recent versions of the
GNU C compiler fix the header files automatically when installed, this
is becoming a less prevalent problem.  This macro calls
@code{AC_PROG_CC} and @code{AC_PROG_CPP} if they haven't been called
already.
@end defmac

@defmac AC_PROG_INSTALL
@maindex PROG_INSTALL
Set @code{make} variable @code{INSTALL} to the path of a BSD compatible
@code{install} program, if one is found in the current @code{PATH}.
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{Output}).

This macro screens out various instances of @code{install} known to not
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_LN_S
@maindex PROG_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_PROG_MAKE_SET
@maindex PROG_MAKE_SET
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_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

@node Generic Programs, , Particular Programs, Alternative Programs
@subsection Generic Program Checking Macros

These macros are used to find programs not covered by the particular
test macros.

@defmac AC_CHECK_PROG (@var{variable}, @var{prog-to-check-for}, @var{value-if-found} @r{[}, @var{value-if-not-found}@r{]})
@maindex CHECK_PROG
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_CHECK_PROGS (@var{variable}, @var{progs-to-check-for} @r{[}, @var{value-if-not-found}@r{]})
@maindex CHECK_PROGS
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_PATH_PROG (@var{variable}, @var{prog-to-check-for} @r{[}, @var{value-if-not-found}@r{]})
@maindex PATH_PROG
Similar to @code{AC_CHECK_PROG}, 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_PATH_PROGS (@var{variable}, @var{progs-to-check-for} @r{[}, @var{value-if-not-found}@r{]})
@maindex PATH_PROGS
Like @code{AC_CHECK_PROGS}, but if any of @var{progs-to-check-for}
are found, set @var{variable} to the entire pathname of the program
found.
@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

@node Libraries, Header Files, Alternative Programs, Existing Tests
@section Library Files

The following macros check for the presence of certain C library archive
files.

@defmac AC_CHECK_LIB (@var{library}, @var{function}, @r{[}, @var{action-if-found} @r{[}, @var{action-if-not-found} @r{[}, @var{other-libraries}@r{]]]})
@maindex CHECK_LIB
Create a test C program to see whether that program can be linked with
the library @var{library}.  @var{function} should be the name of a
function that that library contains, to force the linker to try to link
with the library to resolve the name.  @var{library} must be the base
name of the library; e.g., to check for @samp{-lmp}, use @samp{mp} as
the @var{library} argument.

@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{-l@var{library}} to
@code{LIBS} and define @samp{HAVE_LIB@var{library}}.

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_HAVE_LIBRARY (@var{library}, @r{[}, @var{action-if-found} @r{[}, @var{action-if-not-found} @r{[}, @var{other-libraries}@r{]]]})
@maindex HAVE_LIBRARY
This macro is equivalent to calling @code{AC_CHECK_LIB} with a
@var{function} argument of @code{main}.  In addition, @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}.  However,
@var{library} can not be a shell variable; it must be a constant.

This macro is considered obsolete, because some linkers (e.g., Apollo's)
do not fail when asked to link with a nonexistent library if there are
no unresolved sybols.
@end defmac

@node Header Files, Typedefs, Libraries, Existing Tests
@section Header Files

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

@menu
* Particular Headers::          Special handling to find certain headers.
* Generic Headers::             How to find other headers.
@end menu

@node Particular Headers, Generic Headers, , Header Files
@subsection Checks For Particular Headers

These macros check for particular system header files.

@defmac AC_DECL_SYS_SIGLIST
@maindex DECL_SYS_SIGLIST
@cvindex 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

@defmac AC_DIR_HEADER
@maindex DIR_HEADER
@cvindex DIRENT
@cvindex SYSDIR
@cvindex SYSNDIR
@cvindex NDIR
@cvindex VOID_CLOSEDIR
Like @code{AC_HEADER_DIRENT}, but defines a different set of C
preprocessor macros to indicate which header file is found.  This macro
and the names it defines are considered obsolete.  The names it defines are:

@table @file
@item dirent.h
@code{DIRENT}

@item sys/ndir.h
@code{SYSNDIR}

@item sys/dir.h
@code{SYSDIR}

@item ndir.h
@code{NDIR}
@end table

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}.
@end defmac

@defmac AC_HEADER_DIRENT
@maindex HEADER_DIRENT
@cvindex HAVE_DIRENT_H
@cvindex HAVE_NDIR_H
@cvindex HAVE_SYS_DIR_H
@cvindex HAVE_SYS_NDIR_H
@cvindex CLOSEDIR_VOID
Check for the the following header files, and for the first one that is
found, define the listed C preprocessor macro:

@table @file
@item dirent.h
@code{HAVE_DIRENT_H}

@item sys/ndir.h
@code{HAVE_SYS_NDIR_H}

@item sys/dir.h
@code{HAVE_SYS_DIR_H}

@item ndir.h
@code{HAVE_NDIR_H}
@end table

Also, if the directory library header file contains a declaration of the
@code{closedir} function with a @code{void} return type, define
@code{CLOSEDIR_VOID}.

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

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

#ifdef HAVE_DIRENT_H
# include <dirent.h>
# define NAMLEN(dirent) (strlen((dirent)->d_name))
#else
# define dirent direct
# define NAMLEN(dirent) ((dirent)->d_namlen)
# ifdef HAVE_SYS_NDIR_H
#  include <sys/ndir.h>
# endif
# ifdef HAVE_SYS_DIR_H
#  include <sys/dir.h>
# endif
# ifdef HAVE_NDIR_H
#  include <ndir.h>
# endif
#endif
@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{NAMLEN} macro.
@end defmac

@defmac AC_HEADER_MAJOR
@maindex HEADER_MAJOR
@cvindex MAJOR_IN_MKDEV
@cvindex 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_HEADER_STDC
@maindex HEADER_STDC
@cvindex 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.

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

@example
AC_HEADER_STDC
AC_CHECK_FUNCS(strchr memcpy)
@end example

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

@example
@group
#if STDC_HEADERS
# include <string.h>
#else
# ifndef HAVE_STRCHR
#  define strchr index
#  define strrchr rindex
# endif
char *strchr (), *strrchr ();
# ifndef HAVE_MEMCPY
#  define memcpy(d, s, n) bcopy ((s), (d), (n))
#  define memmove(d, s, n) bcopy ((s), (d), (n))
#  define memcmp(s1, s2, n) bcmp ((s1), (s2), (n))
# endif
#endif
@end group
@end example

@noindent
If you use a function like @code{memchr}, @code{memset}, @code{strtok},
or @code{strspn}, which have no BSD equivalent, then macros won't
suffice; you must provide an implementation of each function.  An easy
way to incorporate your implementations only when needed (since the ones
in system C libraries may be hand optimized) is to, taking @code{memchr}
for example, put it in @file{memchr.c} and use
@samp{AC_REPLACE_FUNCS(memchr)}.

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_MEMORY_H
@maindex MEMORY_H
@cvindex 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_CHECK_HEADERS(memory.h)}.  See the
example for @code{AC_HEADER_STDC}.
@end defmac

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

@cvindex _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
@cvindex 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_CHECK_HEADERS(string.h)} and use @code{HAVE_STRING_H} in your
code.  See the example for @code{AC_HEADER_STDC}.
@end defmac

@node Generic Headers, , Particular Headers, Header Files
@subsection Generic Header Checking Macros

These macros are used to find system header files not covered by the
particular test macros.

@defmac AC_CHECK_HEADER (@var{header-file}, @var{action-if-found} @r{[}, @var{action-if-not-found}@r{]})
@maindex CHECK_HEADER
If the system header file @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_CHECK_HEADERS} instead.
@end defmac

@defmac AC_CHECK_HEADERS (@var{header-file}@dots{} @r{[}, @var{action}@r{]})
@maindex CHECK_HEADERS
@cvindex HAVE_@var{header}
For each given system header file @var{header-file} in the
whitespace-separated argument list that exists, define
@code{HAVE_@var{header-file}} (in all caps).  @xref{Existing Tests}, for
a precise definition of ``define'' as it is used here.
If @var{action} is given, it is additional shell code to execute
when one of the header files is found.  You can give an @var{action} of
@samp{break} to break out of the loop on the first match.
@end defmac

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

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

@menu
* Particular Types::            Special handling to find certain types.
* Generic Types::               How to find other types.
@end menu

@node Particular Types, Generic Types, , Typedefs
@subsection Checks For Particular Types

These macros check for particular C types.

@defmac AC_TYPE_GETGROUPS
@maindex TYPE_GETGROUPS
@cvindex 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_TYPE_MODE_T
@maindex TYPE_MODE_T
@cvindex 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_TYPE_OFF_T
@maindex TYPE_OFF_T
@cvindex 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_TYPE_PID_T
@maindex TYPE_PID_T
@cvindex 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_TYPE_SIGNAL
@maindex TYPE_SIGNAL
@cvindex 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_TYPE_SIZE_T
@maindex TYPE_SIZE_T
@cvindex 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_TYPE_UID_T
@maindex TYPE_UID_T
@cvindex uid_t
@cvindex 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 Generic Types, , Particular Types, Typedefs
@subsection Generic Type Checking Macros

These macros are used to find types not covered by the
particular test macros.

@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_CHECK_SIZEOF (@var{type})
@maindex CHECK_SIZEOF
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_CHECK_SIZEOF(int *)
@end example
@noindent
defines @code{SIZEOF_INT_P} to be 8 on DEC Alpha AXP systems.
@end defmac

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

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

@menu
* Particular Functions::        Special handling to find certain functions.
* Generic Functions::           How to find other functions.
@end menu

@node Particular Functions, Generic Functions, , Library Functions
@subsection Checks For Particular Functions

These macros check for particular C functions.

@defmac AC_FUNC_ALLOCA
@maindex FUNC_ALLOCA
@cvindex C_ALLOCA
@cvindex 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 any of those methods succeed, it defines
@code{HAVE_ALLOCA}.  Otherwise, 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 System V R3
@file{libPW} or the System V R4 @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
# if HAVE_ALLOCA_H
#  include <alloca.h>
# else
#  ifdef _AIX
 #pragma alloca
#  else
#   ifndef alloca /* predefined by HP cc +Olibcalls */
char *alloca ();
#   endif
#  endif
# endif
#endif
@end group
@end example
@end defmac

@defmac AC_FUNC_GETLOADAVG
@maindex FUNC_GETLOADAVG
@cvindex SVR4
@cvindex DGUX
@cvindex UMAX
@cvindex UMAX4_3
@cvindex NLIST_STRUCT
@cvindex NLIST_NAME_UNION
@cvindex GETLODAVG_PRIVILEGED
@cvindex 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_FUNC_MMAP
@maindex FUNC_MMAP
@cvindex HAVE_MMAP
If the @code{mmap} function exists and works correctly, define
@code{HAVE_MMAP}.
@end defmac

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

@defmac AC_FUNC_UTIME_NULL
@maindex FUNC_UTIME_NULL
@cvindex 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_FUNC_VFORK
@maindex FUNC_VFORK
@cvindex HAVE_VFORK_H
@cvindex 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_FUNC_VPRINTF
@maindex FUNC_VPRINTF
@cvindex HAVE_VPRINTF
@cvindex 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_FUNC_WAIT3
@maindex FUNC_WAIT3
@cvindex 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 Generic Functions, , Particular Functions, Library Functions
@subsection Generic Function Checking Macros

These macros are used to find functions not covered by the particular
test macros.

@defmac AC_CHECK_FUNC (@var{function}, @var{action-if-found} @r{[}, @var{action-if-not-found}@r{]})
@maindex CHECK_FUNC
If C function @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_CHECK_LIB} for those libraries.  If you just want to
define a symbol if the function is available, consider using
@code{AC_CHECK_FUNCS} instead.  This macro checks for functions with C
linkage even when @code{AC_LANG_CPLUSPLUS} has been called, since C++ is
more standardized than C is.  (@pxref{Language Choice}, for more
information about selecting the language for checks.)
@end defmac

@defmac AC_CHECK_FUNCS (@var{function}@dots{} @r{[}, @var{action}@r{]})
@maindex CHECK_FUNCS
@cvindex 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{Existing Tests}, for a precise definition of ``define'' as it
is used here.  If @var{action} is given, it is additional shell code to execute
when one of the functions is found.  You can give an @var{action} of
@samp{break} to break out of the loop on the first match.
If the functions might be in libraries other than the
default C library, first call @code{AC_CHECK_LIB} for those libraries.
@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_CHECK_LIB} for those libraries.
@end defmac

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

The following macros check for certain structures or structure members.
You can use @code{AC_TRY_LINK} (@pxref{C System Output}) to check
structures not listed here.

@defmac AC_HEADER_STAT
@maindex HEADER_STAT
@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_HEADER_TIME
@maindex HEADER_TIME
@cvindex 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_ST_BLKSIZE
@maindex STRUCT_ST_BLKSIZE
@cvindex HAVE_ST_BLKSIZE
If @code{struct stat} contains an @code{st_blksize} member, define
@code{HAVE_ST_BLKSIZE}.
@end defmac

@defmac AC_STRUCT_ST_BLOCKS
@maindex STRUCT_ST_BLOCKS
@cvindex 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_STRUCT_ST_RDEV
@maindex STRUCT_ST_RDEV
@cvindex HAVE_ST_RDEV
If @code{struct stat} contains an @code{st_rdev} member, define
@code{HAVE_ST_RDEV}.
@end defmac

@defmac AC_STRUCT_TM
@maindex STRUCT_TM
@cvindex 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_STRUCT_TIMEZONE
@maindex STRUCT_TIMEZONE
@cvindex HAVE_TM_ZONE
@cvindex 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, Existing Tests
@section Compiler Characteristics

The following macros check for C compiler or machine architecture
features.  You can use @code{AC_TRY_LINK} or @code{AC_TRY_RUN} (@pxref{C
System Output}) to check for characteristics not listed here.

@defmac AC_C_BIGENDIAN
@maindex C_BIGENDIAN
@cvindex 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

@defmac AC_C_CHAR_UNSIGNED
@maindex C_CHAR_UNSIGNED
@cvindex __CHAR_UNSIGNED__
If the C type @code{char} is unsigned, define @code{__CHAR_UNSIGNED__},
unless the C compiler predefines it.
@end defmac

@defmac AC_C_CONST
@maindex C_CONST
@cvindex 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_C_CROSS
@maindex C_CROSS
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_TRY_RUN} to determine whether
to take a default action instead of trying to run a test program
(@pxref{C System Output}).
@end defmac

@defmac AC_C_INLINE
@maindex C_INLINE
@cvindex 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
@cvindex 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_CHECK_SIZEOF(int)} instead (@pxref{Generic Types}).
@end defmac

@defmac AC_LONG_64_BITS
@maindex LONG_64_BITS
@cvindex 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_CHECK_SIZEOF(long)} instead (@pxref{Generic Types}).
@end defmac

@defmac AC_C_LONG_DOUBLE
@maindex C_LONG_DOUBLE
@cvindex 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

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

The following macros check for operating system services or capabilities.

@defmac AC_SYS_INTERPRETER
@maindex SYS_INTERPRETER
Check whether the system supports starting shell scripts with a line of
the form @samp{#!/bin/csh} to select the shell to use.  After running
this macro, shell code in @code{configure.in} can check the variable
@code{ac_cv_sys_interpreter}; it will be set to @samp{yes} if the system
supports @samp{#!}, @samp{no} if not.
@end defmac

@defmac AC_PATH_X
@maindex PATH_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{yes};
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_PATH_XTRA
@maindex PATH_XTRA
An enhanced version of @code{AC_PATH_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_PATH_X} and
@code{AC_OS_ISC} (@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_PATH_X} rather than doing that yourself.
@end defmac

@defmac AC_SYS_LONG_FILE_NAMES
@maindex SYS_LONG_FILE_NAMES
@cvindex HAVE_LONG_FILE_NAMES
If the system supports file names longer than 14 characters, define
@code{HAVE_LONG_FILE_NAMES}.
@end defmac

@defmac AC_SYS_RESTARTABLE_SYSCALLS
@maindex SYS_RESTARTABLE_SYSCALLS
@cvindex 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, Existing 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 warts; they should
be replaced by a more systematic approach, based either on the
functions they make available or the environments they provide.

@defmac AC_OS_AIX
@maindex OS_AIX
@cvindex _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_OS_DYNIX
@maindex OS_DYNIX
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_OS_IRIX
@maindex OS_IRIX
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_OS_ISC
@maindex OS_ISC
@cvindex _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_OS_MINIX
@maindex OS_MINIX
@cvindex _MINIX
@cvindex _POSIX_SOURCE
@cvindex _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_OS_SCO
@maindex OS_SCO
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_OS_XENIX
@maindex OS_XENIX
If on Xenix, 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 calls
@code{AC_DIR_HEADER} if it has not been called already.
@end defmac

@node Primitives, Manual Configuration, Existing Tests, Top
@chapter Primitives For Building Tests

These macros provide ways for other macros to check whether various
kinds of features are available and report the results.  Within each
section below, the macros are listed in alphabetical order.

@menu
* C System Output::             Checking output of C compiler system.
* 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.
@end menu

@node C System Output, Setting Variables, , Primitives
@section Checking C Compiler System Output

These macros check the output of the C compiler system.  They @emph{do
not} cache the results of their tests for future use (@pxref{Caching
Values}), because they don't know enough about the information they are
checking for to generate a cache variable name.  The checks for
particular kinds of C features call these macros and do cache their
results.

@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_TRY_LINK}, with the addition that it
prints @samp{checking for @var{echo-text}} to the standard output first,
if @var{echo-text} is non-empty.
@end defmac

@defmac AC_EGREP_CPP (@var{pattern}, @var{program}, @var{action-if-found} @r{[}, @var{action-if-not-found}@r{]})
@maindex EGREP_CPP
@var{program} is the text of a C or C++ program, on which shell
variable, backquote, and backslash 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}.

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_EGREP_HEADER (@var{pattern}, @var{header-file}, @var{action-if-found} @r{[}, @var{action-if-not-found}@r{]})
@maindex EGREP_HEADER
If the output of running the preprocessor on the system header file
@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_TRY_CPP (@var{includes}, @var{action-if-true} @r{[}, @var{action-if-false}@r{]})
@maindex TRY_CPP
@var{includes} is C or C++ @code{#include} statements and declarations,
on which shell variable, backquote, and backslash substitutions are
performed.  (Actually, it can be any C program, but other statements are
probably not useful.)  If the preprocessor produces no error messages
while processing it, run shell commands @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.  It uses @code{CPPFLAGS}, but not @code{CFLAGS},
because @samp{-g}, @samp{-O}, etc. are not valid options to many C
preprocessors.
@end defmac

@defmac AC_TRY_LINK (@var{includes}, @var{function-body}, @var{action-if-found} @r{[}, @var{action-if-not-found}@r{]})
@maindex TRY_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}.  This
macro uses @code{CFLAGS} or @code{CXXFLAGS}, @code{CPPFLAGS},
@code{LDFLAGS}, and @code{LIBS} when compiling.

A few systems have linkers that do not return a failure exit status when
there are unresolved functions in the link.  This bug makes the
configuration scripts produced by Autoconf unusable on those systems.
However, some of them can be given options that make the exit status
correct.  This is a problem that Autoconf does not currently address.
@end defmac

@defmac AC_TRY_RUN (@var{program}, @var{action-if-true} @r{[}, @var{action-if-false} @r{[}, @var{action-if-cross-compiling}@r{]]})
@maindex TRY_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}; the exit status of the program is available in
the shell variable @samp{$?}.  This macro uses @code{CFLAGS} or
@code{CXXFLAGS}, @code{CPPFLAGS}, @code{LDFLAGS}, and @code{LIBS} when
compiling.

If 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.  If the optional shell commands @var{action-if-cross-compiling}
are given, they are run instead and this macro calls @code{AC_C_CROSS}
if it has not already been called (@pxref{Compiler Characteristics}).
Otherwise, @code{configure} prints an error message and exits.

It is preferable to use @code{AC_TRY_LINK} instead of @code{AC_TRY_RUN},
when possible.  @xref{Test Programs}, for more information.
@end defmac

@node Setting Variables, Printing Messages, C System Output, Primitives
@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 (verbatim), otherwise set it to 1.
@var{value} should not contain literal newlines, and if you are not
using @code{AC_CONFIG_HEADER} it should not contain any @samp{#}
characters, as @code{make} tends to eat them.  To use a shell variable
(which you need to do in order to define a value containing the
@code{m4} quote characters @samp{[} or @samp{]}), use
@code{AC_DEFINE_UNQUOTED} instead.

@code{AC_OUTPUT} substitutes the values defined by this macro as the
variable @code{DEFS} in the file(s) that it generates (typically
@file{Makefile}).  Unlike in Autoconf version 1, there is no variable
@code{DEFS} defined while @code{configure} is running, so macros cannot
check its value for the results of previous tests.  This difference is
because properly quoting the contents of that variable turned out to be
too cumbersome and inefficient to do every time @code{AC_DEFINE} is
called.  Checking cache variable values is a cleaner way to get the
effect of checking the value of @code{DEFS}.

If @code{AC_CONFIG_HEADER} has been called, then instead of substituting
@code{DEFS}, @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_CHECK_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_CHECK_HEADER(elf.h, AC_DEFINE(SVR4) LIBS="$LIBS -lelf")
@end example

@noindent
or this:

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

@noindent
instead of this:

@example
AC_CHECK_HEADER(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 three shell expansions are performed once on
@var{value}: variable expansion (@samp{$}), command substitution
(@samp{`}), and backslash escaping (@samp{\}).  Use this macro instead
of @code{AC_DEFINE} when @var{value} is 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}.  The value of
@var{variable} should not contain literal newlines.  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})
@maindex SUBST_FILE
Substitute the contents of the file named by shell variable
@var{variable} 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 containing
special dependencies or other @code{make} directives for particular host
or target types into @file{Makefile}s.

For example, @file{configure.in} could contain:

@example
AC_SUBST_FILE(host_frag)dnl
host_frag=$srcdir/conf/sun4.mh
@end example

@noindent
and then a @file{Makefile.in} could contain:

@example
@@host_frag@@
@end example
@end defmac

@node Printing Messages, Language Choice, Setting Variables, Primitives
@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
This macro is similar to @code{AC_MSG_CHECKING}, except that it prints a
newline after the @var{feature-description}.  It is useful mainly to
print a general description of the overall purpose of a group of feature
checks, e.g.

@example
AC_CHECKING(if stack overflow is detectable)
@end example
@end defmac

@defmac AC_MSG_CHECKING (@var{feature-description})
@maindex MSG_CHECKING
Notify the user that @code{configure} is checking for a particular
feature.  This macro prints a message that starts with @samp{checking }
and ends with @samp{...} and no newline.  It must be followed by a call
to @code{AC_MSG_RESULT} to print the result of the check and the
newline.  The @var{feature-description} should be something like
@samp{whether the Fortran compiler accepts C++ comments} or @samp{for
c89}.

This macro prints nothing if @code{configure} is run with the
@samp{--quiet} or @samp{--silent} option.
@end defmac

@defmac AC_MSG_ERROR (@var{error-description})
@maindex MSG_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_MSG_RESULT (@var{result-description})
@maindex MSG_RESULT
Notify the user of the results of a check.  @var{result-description} is
almost always the value of the cache variable for the check, typically
@samp{yes}, @samp{no}, or a file name.  This macro should follow a call
to @code{AC_MSG_CHECKING}, and the @var{result-description} should be
the completion of the message printed by the call to
@code{AC_MSG_CHECKING}.

This macro prints nothing if @code{configure} is run with the
@samp{--quiet} or @samp{--silent} option.
@end defmac

@defmac AC_MSG_WARN (@var{problem-description})
@maindex MSG_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_MSG_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

@defmac AC_VERBOSE (@var{result-description})
@maindex VERBOSE
This macro is similar to @code{AC_MSG_RESULT}, except that it is meant
to follow a call to @code{AC_CHECKING} instead of
@code{AC_MSG_CHECKING}; it starts the message it prints with a tab.  It
is considered obsolete.
@end defmac

@node Language Choice, , Printing Messages, Primitives
@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{Prerequisite Macros}) with an
argument of either @code{AC_PROG_CPP} or @code{AC_PROG_CXXCPP},
depending on which language is current.
@end defmac

@node Manual Configuration, Writing Macros, Primitives, Top
@chapter Manual Configuration

Some kinds of features can't be guessed automatically by running test
programs.  For example, how to allocate a pseudo tty, 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.

@menu
* Specifying Names::            Specifying the system type.
* Canonicalizing::              Getting the canonical system type.
* System Name Variables::       Variables containing the system type.
* Using System Type::           What to do with the system type.
@end menu

@node Specifying Names, Canonicalizing, , Manual Configuration
@section Specifying the System Type

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 (rarely needed);

@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.

@node Canonicalizing, System Name Variables, Specifying Names, Manual Configuration
@section Getting the Canonical System Type 

The following macros make the name of the system type available in
@code{configure} scripts.

@defmac AC_CANONICAL_HOST
@maindex CANONICAL_HOST
Perform only the subset of @code{AC_CANONICAL_SYSTEM} relevant to the
host type.  This is all that is needed for programs that are not part of
a compiler toolchain.
@end defmac

@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{Output}, 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.
@end defmac

@node System Name Variables, Using System Type, Canonicalizing, Manual Configuration
@section System Name Variables

After calling @code{AC_CANONICAL_SYSTEM} or @code{AC_CANONICAL_HOST},
the the shell and @code{make} variables that contain the system type
information 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

@node Using System Type, , System Name Variables, Manual Configuration
@section Using the System Type

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.
* Macro Naming::                What to call your new macros.
* Quoting::			Protecting macros from unwanted expansion.
* Dependencies Between Macros::	What to do when macros depend on other macros.
* Portable Shell::              Shell script portability pitfalls.
* 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, Macro Naming, , Writing Macros
@section Macro Format

@maindex DEFUN
Autoconf macros are defined as arguments to the @code{AC_DEFUN} macro,
which is similar to the @code{m4} builtin @code{define} macro.  In
addition to defining the macro, @code{AC_DEFUN} causes the macro, when
it is called, to set a flag which is used to constrain the order in
which macros are called (@pxref{Prerequisite Macros}).

An Autoconf macro definition looks like this:

@example
AC_DEFUN(@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.

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

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

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

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

@table @code
@item C
C language builtin features.
@item DECL
Declarations of C variables in header files.
@item FUNC
Functions in libraries.
@item GROUP
Unix group owners of files.
@item HEADER
Header files.
@item LIB
C libraries.
@item OS
Quirks of particular operating systems.
@item PATH
The full path names to files, including programs.
@item PROG
The base names of programs.
@item STRUCT
Definitions of C structures in header files.
@item SYS
Operating system features.
@item TYPE
C builtin or declared types. 
@item VAR
C variables in libraries.
@end table

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

A macro that is an internal subroutine of another macro should have a
name that starts with the name of that other macro, followed by one or
more words saying what the internal macro does.  For example,
@code{AC_PATH_X} has internal macros @code{AC_PATH_X_XMKMF} and
@code{AC_PATH_X_DIRECT}.

@node Quoting, Dependencies Between Macros, Macro Naming, 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 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 (usually in C program text or
regular expressions).  In those places, they use the @code{m4} builtin
command @code{changequote} to temporarily change the quote characters to
@samp{<<} and @samp{>>}.  (Sometimes, if they don't need to quote
anything, they disable quoting entirely instead by setting the quote
characters to empty strings.)  Here is an example:

@example
AC_TRY_LINK(
changequote(<<, >>)dnl
<<#include <time.h>
#ifndef tzname /* For SGI.  */
extern char *tzname[]; /* RS6000 and others reject char **tzname.  */
#endif>>,
changequote([, ])dnl
[atoi(*tzname);], ac_cv_var_tzname=yes, ac_cv_var_tzname=no)
@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, Portable Shell, 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.
* Obsolete Macros::             Warning about old ways of doing things.
@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, @code{AC_PROG_YYTEXT}
examines the output of @code{flex} or @code{lex}, so it depends on
@code{AC_PROG_LEX} having been called first to set the shell variable
@code{LEX}.

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

@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.  @var{macro-name} must have been defined using
@code{AC_DEFUN} or else contain a call to @code{AC_PROVIDE} to indicate
that it has been called.
@end defmac

An obsolete alternative to using @code{AC_DEFUN} is to use @code{define}
and call @code{AC_PROVIDE}:

@defmac AC_PROVIDE (@var{macro-name})
@maindex PROVIDE
Set a flag recording that @var{macro-name} has been called.
@var{macro-name} 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

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

Some macros should be run before another macro if both are called, but
neither @emph{requires} that the other be called.  For example, a macro
like @code{AC_OS_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}.
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}.

For example, @code{AC_OS_AIX} contains

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

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

@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 have been defined using
@code{AC_DEFUN} or else 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

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

Configuration and portability technology has evolved over the years.
Often better ways of solving a particular problem are developed, or
ad-hoc approaches are systematized.  This process has occurred in many
parts of Autoconf.  One result is that some of the macros are now
considered @dfn{obsolete}; they still work, but are no longer considered
the best thing to do.  Autoconf has a way to warn users producing
@code{configure} scripts when they use obsolete macros, to encourage
them to modernize.

@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_CHECK_HEADERS(unistd.h) instead])
@end example
@end defmac

@node Portable Shell, Checking for Files, Dependencies Between Macros, Writing Macros
@section Portable Shell Programming

When writing your own checks, there are some shell script programming
techniques you should avoid in order to make your code portable.  

The Bourne shell and upward-compatible shells like Bash and the Korn
shell have evolved over the years, but to prevent trouble, do not take
advantage of features that were added after Unix version 7, circa 1977.
You should not use shell functions, aliases, or other features that are
not found in all Bourne-compatible shells; restrict yourself to the
lowest common denominator.  Even @code{unset} is not found in all shells!

One subtly nonportable shell programming construction is
@samp{@var{var}=$@{@var{var}:-@var{value}@}}.  Old BSD shells, including
the Ultrix @code{sh}, don't understand the colon, and complain and die.
If you omit the colon, it works everywhere:
@samp{@var{var}=$@{@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}.
If you want to distinguish an empty value from an unset one, you can do
it in the following way.  This example sets @code{CFLAGS} to @samp{-g}
only if @code{CFLAGS} has not been set; if it has been set to an empty
value, it is left unchanged:

@example
test "$@{CFLAGS+set@}" = set || CFLAGS='-g'
@end example

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

Some of these external utilities have a portable subset of features, as
well; for example, don't rely on @code{ln} having a @samp{-f} option or
@code{cat} having any options.

@node Checking for Files, Checking for Symbols, Portable Shell, 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_EGREP_HEADER} if the symbol is not a C preprocessor
macro (@pxref{C System Output}), 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, test the value of the appropriate cache variable,
as in this example:

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

@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_TRY_LINK} (@pxref{C System Output}).  You have to write a
test program by hand.  You can compile and run it using
@code{AC_TRY_RUN} (@pxref{C System Output}).

Try to avoid running 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_TRY_RUN}.  To configure for cross-compiling you can also set up
a test results cache file with the correct values for the target system
(@pxref{Caching Values}).  But that is a last resort.  Alternatively,
choose a value for those parameters based on the canonical system name
(@pxref{Manual Configuration}).

To enable @code{configure} scripts to support cross-compilation, they
shouldn't do anything that tests features of the host system instead of
the target system.

@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 not write anything to the standard output.  They
should return 0 if the test succeeds, nonzero otherwise, so that success
can be distinguished easily from a core dump or other failure;
segmentation violations and other failures produce a nonzero exit
status.  Test programs should @code{exit}, not @code{return}, from
@code{main}, because on some systems (old Suns, at least) the argument
to @code{return} in @code{main} is ignored.  Functions that take
arguments should have a prototype conditionalized for C++.  In practice,
test programs rarely need functions that take arguments.

@example
#ifdef __cplusplus
foo(int i)
#else
foo(i) int i;
#endif
@end example

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

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

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_HEADER_STDC}, 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_TRY_CPP([#include <sys/statvfs.h>
#include <sys/fstyp.h>], AC_DEFINE(FSTYPE_STATVFS) fstype=1)
if test -z "$fstype"; then
# SVR3.
AC_TRY_CPP([#include <sys/statfs.h>
#include <sys/fstyp.h>], AC_DEFINE(FSTYPE_USG_STATFS) fstype=1)
fi
if test -z "$fstype"; then
# AIX.
AC_TRY_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.

@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, and @code{configure} was not given the @samp{--quiet} or
@samp{--silent} option, print a message saying that the result was
cached; 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_MSG_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.  @xref{Cache
Variables}, for how to choose the name of the @var{cache-id} variable.
@end defmac

@menu
* Cache Files::	        	Files @code{configure} uses for caching.
* Cache Variables::             Shell variables used in caches.
@end menu

@node Cache Files, Cache Variables, , Caching Values
@section Cache Files

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

By default, configure uses @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.  Giving
@samp{--cache-file=/dev/null} disables caching, for debugging
@code{configure}.  @xref{Output}, 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, as long as the same C compiler is used every time
(@pxref{Site Default Values}).

@node Cache Variables, , Cache Files, Caching Values
@section Cache Variables

The names of cache variables should have the following 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 listed in @ref{Macro Naming}.

@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.

@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.

@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 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 CFLAGS
Debugging and optimization options for the C compiler.  If it is not set
in the environment when @code{configure} runs, the default value is set
when you call @code{AC_PROG_CC} (or empty if you don't).  @code{configure}
uses this variable when compiling programs to test for C features.
@end defvar

@defvar CPPFLAGS
Header file search directory (@samp{-I@var{dir}}) and any other
miscellaneous options for the C preprocessor and compiler.  If it is not
set in the environment when @code{configure} runs, the default value is
empty.  @code{configure} uses this variable when compiling or
preprocessing programs to test for C features.
@end defvar

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

@defvar LDFLAGS
Stripping (@samp{-s}) and any other miscellaneous options for the
linker.  If it is not set in the environment when @code{configure} runs,
the default value is empty.  @code{configure} uses this variable when
linking programs to test for C features.
@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{Generic Functions}).
@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.
# Use || so the command line always returns success.
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 prevent
@code{configure} from running @file{config.status} and creating
@file{Makefile} and other files, and the @samp{--norecursion} option,
which prevents @code{configure} from running other @code{configure}
scripts in subdirectories.  (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
@evindex 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
@evindex 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
@evindex 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
@evindex 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, Concerns, 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.

@evindex CONFIG_SITE
If the environment variable @code{CONFIG_SITE} is set and contains the
name of a readable file, @code{configure} reads that file as a shell
script.  Otherwise, it reads the shell script
@file{@var{prefix}/lib/config.site} if it exists, then
@file{@var{exec_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 selecting 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}
(wherever you locate the site file), you can set them in the site file
if you specify it with the @code{CONFIG_SITE} environment variable.

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.

Here is a sample file @file{/usr/share/local/gnu/lib/config.site}.
@samp{configure --prefix=/usr/share/local/gnu} would read this file.

@example
# config.site for configure
#
# Give Autoconf 2.x generated configure scripts a shared default
# cache file for feature test results, architecture-specific.
test $cache_file = ./config.cache &&
  cache_file=/usr/local/gnu/lib/config.cache
# Also, if you give --prefix, you get --exec-prefix automatically.
test $exec_prefix = NONE && exec_prefix=/usr/local/gnu
@end example

@node Concerns, Upgrading, Site Default Values, Top
@chapter Concerns About Autoconf

Several concerns about Autoconf come up regularly.  Here some of them
are addressed.

@menu
* Copyright::                   Legal restrictions on Autoconf output.
* Why GNU m4::                  Why not use the standard @code{m4}?
* Bootstrapping::               Autoconf and GNU @code{m4} require each other?
* Policy Decisions::            Configuring site policy.
@end menu

@node Copyright, Why GNU m4, , Concerns
@section Copyright on @code{configure} Scripts

@display
What are the restrictions on distributing @code{configure}
scripts that Autoconf generated?  How does that affect my
programs that use them?
@end display

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{acsite.m4}, @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 Why GNU m4, Bootstrapping, Copyright, Concerns
@section Why Require GNU @code{m4}?

@display
Why does Autoconf require GNU @code{m4}?
@end display

Many @code{m4} implementations have hard-coded limitations on the size
and number of macros.  They also lack regular expression capabilities
and other builtins that it would be difficult to get along without in a
sophisticated application like Autoconf.  Since only software
maintainers need to use Autoconf, and since GNU @code{m4} is simple to
configure and install, it does not seem like an unreasonable burden to
require GNU @code{m4} to be installed also.  Most maintainers of GNU and
other free software already have most of the GNU utilities installed,
since they prefer them.

@node Bootstrapping, Policy Decisions, Why GNU m4, Concerns
@section How Can I Bootstrap?

@display
If Autoconf requires GNU @code{m4} and GNU @code{m4} has an
Autoconf @code{configure} script, how do I bootstrap?  It seems
like a chicken and egg problem!
@end display

Although GNU @code{m4} does come with a @code{configure} script produced
by Autoconf, Autoconf is @emph{not} required in order to run the script
and install GNU @code{m4}.  Autoconf is only required if you want to
@emph{change} the @code{m4} @code{configure} script, which few people
have to do (mainly its maintainer).

@node Policy Decisions, , Bootstrapping, Concerns
@section Configuring Site Policy

@display
How do I configure in policy decisions, since the
@code{configure} scripts aren't interactive?
@end display

There are two ways to do it.  Simple decisions can be specified using
@samp{--with} and @samp{--enable} options to @code{configure}
(@pxref{Command Line}).

More sophisticated site configuration information---company names, email
addresses to contact, etc.---should be put in a file that is edited
@emph{only by users}.  The programs should examine that file at run
time, rather than at compile time.  That approach is more convenient for
users and makes the configuration process simpler than asking for the
information while configuring.

@node Upgrading, History, Concerns, Top
@chapter Upgrading From Version 1

Autoconf version 2 is mostly backward compatible with version 1.
However, it introduces better ways to do some things, and doesn't
support some of the ugly things in version 1.  So, depending on how
sophisticated your @file{configure.in} files are, you might have to do
some manual work in order to upgrade to version 2.  This chapter points
out some problems to watch for when upgrading.  Also, perhaps your
@code{configure} scripts could benefit from some of the new features in
version 2.

First, make sure you have GNU @code{m4} version 1.1 or higher installed.
Earlier versions have bugs that prevent them from working with Autoconf
version 2.

@menu
* Changed File Names::          Files you might rename.
* Changed Makefiles::           New things to put in @file{Makefile.in}.
* Changed Macros::              Macro calls you might replace.
* Changed Results::             Changes in how to check test results.
* Changed Macro Writing::       Better ways to write your own macros.
@end menu

@node Changed File Names, Changed Makefiles, , Upgrading
@section Changed File Names

If you have an @file{aclocal.m4} installed with Autoconf (as opposed to
in a particular package's source directory), you must rename it to
@file{acsite.m4}.  @xref{Invoking autoconf}.

If you were using @file{config.h.top} or @file{config.h.bot}, you still
can, but you will have less clutter if you merge them into
@file{acconfig.h}.  @xref{Invoking autoheader}.

@node Changed Makefiles, Changed Macros, Changed File Names, Upgrading
@section Changed Makefiles

Add @samp{@@CFLAGS@@}, @samp{@@CPPFLAGS@@}, and @samp{@@LDFLAGS@@} in
your @file{Makefile.in} files, so they can take advantage of the values
of those variables in the environment when @code{configure} was run.
This isn't necessary, but it's a convenience for users.

If you have the following in @file{Makefile.in}:

@example
prefix = /usr/local
exec_prefix = $@{prefix@}
@end example

@noindent
you must change it to:

@example
prefix = @@prefix@@
exec_prefix = @@exec_prefix@@
@end example

@noindent
The old feature of replacing those variables without @samp{@@}
characters around them has been removed.

@node Changed Macros, Changed Results, Changed Makefiles, Upgrading
@section Changed Macros

Many of the macros were renamed in Autoconf version 2.  You can still
use the old names, but the new ones are clearer, and it's easier to find
the documentation for them.  @xref{Old Macro Names}, for a table showing
the new names for the old macros.  Use the @code{autoupdate} program to
convert your @file{configure.in} to using the new macro names.
@xref{Invoking autoupdate}.

Some macros have been superseded by similar ones that do the job better,
but are not call-compatible.  If you get warnings about calling obsolete
macros while running @code{autoconf}, you may ignore them, but your
@code{configure} script will generally work better if you follow the
advice it prints about what to replace the obsolete macros with.  In
particular, the mechanism for reporting the results of tests has
changed.  If you were using @code{echo} or @code{AC_VERBOSE}, your
@code{configure} script's output will look better if you switch to
@code{AC_MSG_CHECKING} and @code{AC_MSG_RESULT}.  @xref{Printing
Messages}.  Those macros work best in conjunction with cache variables.
@xref{Caching Values}.

@node Changed Results, Changed Macro Writing, Changed Macros, Upgrading
@section Changed Results

If you were checking the results of previous tests by examining the
shell variable @code{DEFS}, you need to switch to checking the values of
the cache variables for those tests.  @code{DEFS} no longer exists while
@code{configure} is running; it is only created when generating output
files.  This difference from version 1 is because properly quoting the
contents of that variable turned out to be too cumbersome and
inefficient to do every time @code{AC_DEFINE} is called.  @xref{Cache
Variables}.

For example, here is a @file{configure.in} fragment written for Autoconf
version 1:

@example
AC_HAVE_FUNCS(syslog)
case "$DEFS" in
*-DHAVE_SYSLOG*) ;;
*) # syslog is not in the default libraries.  See if it's in some other.
  saved_LIBS="$LIBS"
  for lib in bsd socket inet; do
    AC_CHECKING(for syslog in -l$lib)
    LIBS="$saved_LIBS -l$lib"
    AC_HAVE_FUNCS(syslog)
    case "$DEFS" in
    *-DHAVE_SYSLOG*) break ;;
    *) ;;
    esac
    LIBS="$saved_LIBS"
  done ;;
esac
@end example

Here is a way to write it for version 2:

@example
AC_CHECK_FUNCS(syslog)
if test $ac_cv_func_syslog = no; then
  # syslog is not in the default libraries.  See if it's in some other.
  for lib in bsd socket inet; do
    AC_CHECK_LIB($lib, syslog, [AC_DEFINE(HAVE_SYSLOG)
      LIBS="$LIBS $lib"; break])
  done
fi
@end example

If you were working around bugs in @code{AC_DEFINE_UNQUOTED} by adding
backslashes before quotes, you probably need to remove them.  It now
works predictably, and does not treat quotes (except backquotes)
specially.  @xref{Setting Variables}.

@node Changed Macro Writing, , Changed Results, Upgrading
@section Changed Macro Writing

When defining your own macros, you may now use @code{AC_DEFUN} instead of
@code{define}, because @code{AC_DEFUN} automatically calls
@code{AC_PROVIDE}.  There's no harm in continuing to use the older way;
it's just less convenient.  @xref{Macro Format}.

You probably looked at the macros that came with Autoconf as a guide for
how to do things.  It would be a good idea to take a look at the new
versions of them, as the style is somewhat improved and they take
advantage of some new features.

If you were doing tricky things with undocumented Autoconf internals
(diversions, macros, variables), check whether you need to change
anything to account for changes that have been made.  Perhaps you can
even use an officially supported technique in version 2 instead of
kludging.  Or perhaps not.

To speed up your locally written feature tests, add caching to them.  If
you don't encapsulate your tests into macros, do so; don't be sloppy.
(I've been guilty myself.)  @xref{Caching Values}.

@node History, Old Macro Names, Upgrading, Top
@chapter History of Autoconf

You may be wondering, Why was Autoconf originally written?  How did it
get into its present form?  (Why does it look like gorilla spit?)  If
you're not wondering, then this chapter contains no information useful
to you, and you might as well skip it.  If you @emph{are} wondering,
then let there be light@dots{}

@menu
* Genesis::			Prehistory and naming of @code{configure}.
* Exodus::			The plagues of @code{m4} and Perl.
* Leviticus::			The priestly code of portability arrives.
* Numbers::			Growth and contributors.
* Deuteronomy::			Approaching the promises of easy configuration.
@end menu

@node Genesis, Exodus, , History
@section Genesis

In June 1991 I was maintaining many of the GNU utilities for the Free
Software Foundation.  As they were ported to more platforms and more
programs were added, the number of @samp{-D} options that users had to
select in the @file{Makefile} (around 20) became burdensome.  Especially
for me---I had to test each new release on a bunch of different systems.
So I wrote a little shell script to guess some of the correct settings
for the fileutils package, and released it as part of fileutils 2.0.
That @code{configure} script worked well enough that the next month I
adapted it (by hand) to create similar @code{configure} scripts for
several other GNU utilities packages.  Brian Berliner also adapted one
of my scripts for his CVS revision control system.

Later that summer, I learned that Richard Stallman and Richard Pixley
were developing similar scripts to use in the GNU compiler tools; so I
adapted my @code{configure} scripts to support their evolving interface:
using the file name @file{Makefile.in} as the templates; adding
@samp{+srcdir}, the first option (of many); and creating
@file{config.status} files.

@node Exodus, Leviticus, Genesis, History
@section Exodus

As I got feedback from users, I incorporated many improvements, using
Emacs to search and replace, cut and paste, similar changes in each of
the scripts.  As I adapted more GNU utilities packages to use
@code{configure} scripts, updating them all by hand became impractical.
Rich Murphey, the maintainer of the GNU graphics utilities, sent me mail
saying that the @code{configure} scripts were great, and asking if I had
a tool for generating them that I could send him.  No, I thought, but
I should!  So I started to work out how to generate them.  And the
journey from the bondage of hand-written @file{configure} scripts to the
ease of Autoconf began.

Cygnus @code{configure}, which was being developed at around that time,
is table driven; it is meant to deal mainly with a discrete number of
system types with a small number of mainly unguessable features (such as
details of the object file format).  The automatic configuration system
that Brian Fox had developed for Bash takes a similar approach.  For
general use, it seems to me a hopeless cause to try to maintain an up to
date database of which features each variant of each operating system
has.  It's easier and more reliable to check for most features on the
fly---especially on hybrid systems that people have hacked on locally.

I considered using an architecture similar to that of Cygnus
@code{configure}, where there is a single @code{configure} script that
reads pieces of @file{configure.in} when run.  But I didn't want to have
to distribute all of the feature tests with every package, so I settled
on having a different @code{configure} made from each
@file{configure.in} by a preprocessor.  That approach also offered more
control and flexibility.

I looked briefly into using Larry Wall's Metaconfig program, but I
decided not to for several reasons.  The @code{Configure} scripts it
produces are interactive, which I find quite inconvenient; I didn't like
the ways it checked for some features (such as library functions); it
was not being maintained, and its scripts didn't work on many modern
systems (such as System V R4 and NeXT); it wasn't very flexible in what
it could do in response to a feature's presence or absence; I found it
confusing to learn; and it was too big and complex for my needs (I
didn't realize then how much Autoconf would eventually have to grow).

I considered using Perl to generate my style of @code{configure} scripts,
but decided that @code{m4} was better suited to the job of simple
textual substitutions: it gets in the way less, because output is
implicit.  Plus, everyone already has it.  (Initially I didn't rely on
the GNU extensions to @code{m4}.)  Also, some of my friends at the
University of Maryland had recently been putting @code{m4} front ends on
several programs, including @code{tvtwm}, and I was interested in trying
out a new language.

@node Leviticus, Numbers, Exodus, History
@section Leviticus

Since my @code{configure} scripts determine the system's capabilities
automatically, with no interactive user intervention, I decided to call
the program that generates them Autoconfig.  But with a version number
tacked on, that name would be too long for System V R3 file systems, so
I shortened it to Autoconf.

In the fall of 1991 I called together a group of priests of portability
(er, alpha testers) to give me feedback as I encapsulated pieces of my
handwritten scripts in @code{m4} macros and continued to add features
and improve the techniques used in the checks.  Prominent among the
testers were
@ifinfo
Franc,ois
@end ifinfo
@tex
Fran\c cois
@end tex
Pinard, who came up with the idea of making an @file{autoconf} shell
script to run @code{m4} and check for unresolved macro calls; Karl
Berry, who championed standardizing the @code{HAVE_@var{feature}} form
for C preprocessor symbols; Rich Pixley, who suggested running the
compiler instead of searching the file system to find include files and
symbols, for more accurate results; and Ian Taylor, who added support
for creating a C header file as an alternative to putting @samp{-D}
options in a @file{Makefile}, so he could use Autoconf in his UUCP
package.  The alpha testers cheerfully adjusted their files again and
again as the names and calling conventions of the Autoconf macros
changed from release to release.  They all contributed many specific
checks, great ideas, and bug fixes.

@node Numbers, Deuteronomy, Leviticus, History
@section Numbers

In July 1992, after months of alpha testing, I released Autoconf 1.0,
and converted many GNU packages to use it.  I was surprised by how
positive the reaction to it was.  More people started using it than I
could keep track of, including people working on software that wasn't
part of the GNU Project (such as TCL, FSP, @TeX{}, and Kerberos v5).
Autoconf continued to improve rapidly, as many people using the
@code{configure} scripts reported problems they encountered.

Autoconf turned out to be a good torture test for @code{m4}
implementations.  Unix @code{m4} started to dump core because of the
length of the macros that Autoconf defined, and several bugs showed up
in GNU @code{m4} as well.  Eventually, we realized that we needed to use
some features that only GNU @code{m4} has.  4.3BSD @code{m4}, in
particular, has an impoverished set of builtin macros; the System V
version is better, but still doesn't provide everything we need.

More development occurred as people put Autoconf under more stresses
(and to uses I hadn't anticipated).  Karl Berry added checks for X11.
david zuhn contributed C++ support.
@ifinfo
Franc,ois
@end ifinfo
@tex
Fran\c cois
@end tex
Pinard made it diagnose invalid arguments.  Jim Blandy bravely coerced
it into configuring GNU Emacs, laying the groundwork for several later
improvements.  Roland McGrath got it to configure the GNU C Library,
wrote the @code{autoheader} script to automate the creation of C header
file templates, and added a @samp{--verbose} option to @code{configure}.
Noah Friedman added the @samp{--macrodir} option and @code{AC_MACRODIR}
environment variable.  Roland and Noah improved the quoting protection
in @code{AC_DEFINE} and fixed many bugs, especially when I got sick of
dealing with portability problems from February through June, 1993.

@node Deuteronomy, , Numbers, History
@section Deuteronomy

A long wish list for major features had accumulated, and the effect of
several years of patching by various people had left some residual
cruft.  In April 1994, while working for Cygnus Support, I began a major
revision of Autoconf.  I added most of the features of the Cygnus
@code{configure} that Autoconf had lacked, largely by adapting the
relevant parts of Cygnus @code{configure} with the help of david zuhn
and Ken Raeburn.  These features include support for using
@file{config.sub}, @file{config.guess}, @samp{--host}, and
@samp{--target}; making links to files; and running @code{configure}
scripts in subdirectories.  Adding these features enabled Ken to convert
GNU @code{as}, and Rob Savoye to convert DejaGNU, to using Autoconf.

I added more features in response to other peoples' requests.  Many
people had asked for @code{configure} scripts to share the results of
the checks between runs, because (particularly when configuring a large
source tree, like Cygnus does) they were frustratingly slow.  Mike
Haertel suggested adding site-specific initialization scripts.  People
distributing software that had to unpack on MS-DOS asked for a way to
override the @file{.in} extension on the file names, which produced file
names like @file{config.h.in} containing two dots.  Jim Avera did an
extensive examination of the problems with quoting in @code{AC_DEFINE}
and @code{AC_SUBST}; his insights led to significant improvements.
Richard Stallman asked that compiler output be sent to @file{config.log}
instead of @file{/dev/null}, to help people debug the Emacs
@code{configure} script.

I made some other changes because of my dissatisfaction with the quality
of the program.  I made the messages showing results of the checks less
ambiguous, always printing a result.  I regularized the names of the
macros and cleaned up coding style inconsistencies.  I added some
auxiliary utilities that I had developed to help convert source code
packages to use Autoconf.  And I began a testsuite, because experience
had shown that Autoconf has a pronounced tendency to regress when we
change it.

Again, several alpha testers gave invaluable feedback, especially
@ifinfo
Franc,ois
@end ifinfo
@tex
Fran\c cois
@end tex
Pinard, Jim Meyering, Karl Berry, Rob Savoye, and Ken Raeburn.

Finally, version 2.0 was ready.  And there was much rejoicing.
(And I have free time again.  I think.)

@node Old Macro Names, Environment Variable Index, History, Top
@chapter Old Macro Names

In version 2 of Autoconf, most of the macros were renamed to use a more
uniform and descriptive naming scheme.  Here are the old names of the
macros that were renamed, followed by the current names of those macros.
Although the old names are still accepted by the @code{autoconf} program
for backward compatibility, the old names are considered obsolete.
@xref{Macro Naming}, for a description of the new naming scheme.

@table @code
@item AC_AIX
@maindex AIX
@code{AC_OS_AIX}
@item AC_ALLOCA
@maindex ALLOCA
@code{AC_FUNC_ALLOCA}
@item AC_ARG_ARRAY
@maindex ARG_ARRAY
removed because of limited usefulness
@item AC_CHAR_UNSIGNED
@maindex CHAR_UNSIGNED
@code{AC_C_CHAR_UNSIGNED}
@item AC_CONST
@maindex CONST
@code{AC_C_CONST}
@item AC_CROSS_CHECK
@maindex CROSS_CHECK
@code{AC_C_CROSS}
@item AC_DYNIX_SEQ
@maindex DYNIX_SEQ
@code{AC_OS_DYNIX}
@item AC_ERROR
@maindex ERROR
@code{AC_MSG_ERROR}
@item AC_FIND_X
@maindex FIND_X
@code{AC_PATH_X}
@item AC_FIND_XTRA
@maindex FIND_XTRA
@code{AC_PATH_XTRA}
@item AC_FUNC_CHECK
@maindex FUNC_CHECK
@code{AC_CHECK_FUNC}
@item AC_GCC_TRADITIONAL
@maindex GCC_TRADITIONAL
@code{AC_PROG_GCC_TRADITIONAL}
@item AC_GETGROUPS_T
@maindex GETGROUPS_T
@code{AC_TYPE_GETGROUPS}
@item AC_GETLOADAVG
@maindex GETLOADAVG
@code{AC_FUNC_GETLOADAVG}
@item AC_HAVE_FUNCS
@maindex HAVE_FUNCS
@code{AC_CHECK_FUNCS}
@item AC_HAVE_HEADERS
@maindex HAVE_HEADERS
@code{AC_CHECK_HEADERS}
@item AC_HAVE_POUNDBANG
@maindex HAVE_POUNDBANG
@code{AC_SYS_INTERPRETER} (different calling convention)
@item AC_HEADER_CHECK
@maindex HEADER_CHECK
@code{AC_CHECK_HEADER}
@item AC_HEADER_EGREP
@maindex HEADER_EGREP
@code{AC_EGREP_HEADER}
@item AC_INLINE
@maindex INLINE
@code{AC_C_INLINE}
@item AC_IRIX_SUN
@maindex IRIX_SUN
@code{AC_OS_IRIX}
@item AC_ISC_POSIX
@maindex ISC_POSIX
@code{AC_OS_ISC}
@item AC_LN_S
@maindex LN_S
@code{AC_PROG_LN_S}
@item AC_LONG_DOUBLE
@maindex LONG_DOUBLE
@code{AC_C_LONG_DOUBLE}
@item AC_LONG_FILE_NAMES
@maindex LONG_FILE_NAMES
@code{AC_SYS_LONG_FILE_NAMES}
@item AC_MAJOR_HEADER
@maindex MAJOR_HEADER
@code{AC_HEADER_MAJOR}
@item AC_MINIX
@maindex MINIX
@code{AC_OS_MINIX}
@item AC_MINUS_C_MINUS_O
@maindex MINUS_C_MINUS_O
@code{AC_PROG_CC_C_O}
@item AC_MMAP
@maindex MMAP
@code{AC_FUNC_MMAP}
@item AC_MODE_T
@maindex MODE_T
@code{AC_TYPE_MODE_T}
@item AC_OFF_T
@maindex OFF_T
@code{AC_TYPE_OFF_T}
@item AC_PID_T
@maindex PID_T
@code{AC_TYPE_PID_T}
@item AC_PREFIX
@maindex PREFIX
@code{AC_PREFIX_PROGRAM}
@item AC_PROGRAMS_CHECK
@maindex PROGRAMS_CHECK
@code{AC_CHECK_PROGS}
@item AC_PROGRAMS_PATH
@maindex PROGRAMS_PATH
@code{AC_PATH_PROGS}
@item AC_PROGRAM_CHECK
@maindex PROGRAM_CHECK
@code{AC_CHECK_PROG}
@item AC_PROGRAM_EGREP
@maindex PROGRAM_EGREP
@code{AC_EGREP_CPP}
@item AC_PROGRAM_PATH
@maindex PROGRAM_PATH
@code{AC_PATH_PROG}
@item AC_REMOTE_TAPE
@maindex REMOTE_TAPE
removed because of limited usefulness
@item AC_RESTARTABLE_SYSCALLS
@maindex RESTARTABLE_SYSCALLS
@code{AC_SYS_RESTARTABLE_SYSCALLS}
@item AC_RETSIGTYPE
@maindex RETSIGTYPE
@code{AC_TYPE_SIGNAL}
@item AC_RSH
@maindex RSH
removed because of limited usefulness
@item AC_SCO_INTL
@maindex SCO_INTL
@code{AC_OS_SCO}
@item AC_SETVBUF_REVERSED
@maindex SETVBUF_REVERSED
@code{AC_FUNC_SETVBUF_REVERSED}
@item AC_SET_MAKE
@maindex SET_MAKE
@code{AC_PROG_MAKE_SET}
@item AC_SIZEOF_TYPE
@maindex SIZEOF_TYPE
@code{AC_CHECK_SIZEOF}
@item AC_SIZE_T
@maindex SIZE_T
@code{AC_TYPE_SIZE_T}
@item AC_STAT_MACROS_BROKEN
@maindex STAT_MACROS_BROKEN
@code{AC_HEADER_STAT}
@item AC_STDC_HEADERS
@maindex STDC_HEADERS
@code{AC_HEADER_STDC}
@item AC_STRCOLL
@maindex STRCOLL
@code{AC_FUNC_STRCOLL}
@item AC_ST_BLKSIZE
@maindex ST_BLKSIZE
@code{AC_STRUCT_ST_BLKSIZE}
@item AC_ST_BLOCKS
@maindex ST_BLOCKS
@code{AC_STRUCT_ST_BLOCKS}
@item AC_ST_RDEV
@maindex ST_RDEV
@code{AC_STRUCT_ST_RDEV}
@item AC_SYS_SIGLIST_DECLARED
@maindex SYS_SIGLIST_DECLARED
@code{AC_DECL_SYS_SIGLIST}
@item AC_TEST_CPP
@maindex TEST_CPP
@code{AC_TRY_CPP}
@item AC_TEST_PROGRAM
@maindex TEST_PROGRAM
@code{AC_TRY_RUN}
@item AC_TIMEZONE
@maindex TIMEZONE
@code{AC_STRUCT_TIMEZONE}
@item AC_TIME_WITH_SYS_TIME
@maindex TIME_WITH_SYS_TIME
@code{AC_HEADER_TIME}
@item AC_UID_T
@maindex UID_T
@code{AC_TYPE_UID_T}
@item AC_UTIME_NULL
@maindex UTIME_NULL
@code{AC_FUNC_UTIME_NULL}
@item AC_VFORK
@maindex VFORK
@code{AC_FUNC_VFORK}
@item AC_VPRINTF
@maindex VPRINTF
@code{AC_FUNC_VPRINTF}
@item AC_WAIT3
@maindex WAIT3
@code{AC_FUNC_WAIT3}
@item AC_WARN
@maindex WARN
@code{AC_MSG_WARN}
@item AC_WORDS_BIGENDIAN
@maindex WORDS_BIGENDIAN
@code{AC_C_BIGENDIAN}
@item AC_XENIX_DIR
@maindex XENIX_DIR
@code{AC_OS_XENIX}
@item AC_YYTEXT_POINTER
@maindex YYTEXT_POINTER
@code{AC_DECL_YYTEXT}
@end table

@node Environment Variable Index, Preprocessor Symbol Index, Old Macro Names, Top
@unnumbered Environment Variable Index

This is an alphabetical list of the environment variables that Autoconf
uses in some way.  Some it sets, others it only checks.

@printindex ev

@node Preprocessor Symbol Index, Macro Index, Environment Variable Index, 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 cv

@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 autoconf.texi"
@c End: