Autoconf

This file documents the GNU Autoconf package for creating scripts to configure source code packages using templates and an m4 macro package. This is edition 2.13, for Autoconf version 2.13.

1. Introduction		Autoconf's purpose, strengths, and weaknesses.
2. Making configure Scripts		How to organize and produce Autoconf scripts.
3. Initialization and Output Files		Initialization and output.
4. Existing Tests		Macros that check for particular features.
5. Writing Tests		How to write new feature checks.
6. Results of Tests		What to do with results from feature checks.
7. Writing Macros		Adding new macros to Autoconf.
8. Manual Configuration		Selecting features that can't be guessed.
9. Site Configuration		Local defaults for configure.
10. Running configure Scripts		How to use the Autoconf output.
11. Recreating a Configuration		Recreating a configuration.
12. Questions About Autoconf		Questions about Autoconf, with answers.
13. Upgrading From Version 1		Tips for upgrading from version 1.
14. History of Autoconf
15. Old Macro Names		Backward compatibility macros.
Environment Variable Index		Index of environment variables used.
Output Variable Index		Index of variables set in output files.
Preprocessor Symbol Index		Index of C preprocessor symbols defined.
Macro Index		Index of Autoconf macros.

 -- The Detailed Node Listing ---

Making configure Scripts

2.1 Writing `configure.in'		What to put in an Autoconf input file.
2.2 Using autoscan to Create `configure.in'		Semi-automatic `configure.in' writing.
2.3 Using ifnames to List Conditionals		Listing the conditionals in source code.
2.4 Using autoconf to Create configure		How to create configuration scripts.
2.5 Using autoreconf to Update configure Scripts		Remaking multiple configure scripts.

Initialization and Output Files

3.1 Finding configure Input		Where Autoconf should find files.
3.2 Creating Output Files		Creating output files.
3.3 Substitutions in Makefiles		Using output variables in `Makefile's.
3.4 Configuration Header Files		Creating a configuration header file.
3.5 Configuring Other Packages in Subdirectories		Configuring independent packages together.
3.6 Default Prefix		Changing the default installation prefix.
3.7 Version Numbers in configure		Version numbers in configure.

Substitutions in Makefiles

3.3.1 Preset Output Variables		Output variables that are always set.
3.3.2 Build Directories		Supporting multiple concurrent compiles.
3.3.3 Automatic Remaking		Makefile rules for configuring.

Configuration Header Files

3.4.1 Configuration Header Templates		Input for the configuration headers.
3.4.2 Using autoheader to Create `config.h.in'		How to create configuration templates.

Existing Tests

4.1 Alternative Programs		Selecting between alternative programs.
4.2 Library Files		Library archives that might be missing.
4.3 Library Functions		C library functions that might be missing.
4.4 Header Files		Header files that might be missing.
4.5 Structures		Structures or members that might be missing.
4.6 Typedefs		typedefs that might be missing.
4.7 C Compiler Characteristics
4.8 Fortran 77 Compiler Characteristics
4.9 System Services		Operating system services.
4.10 UNIX Variants		Special kludges for specific UNIX variants.

Alternative Programs

4.1.1 Particular Program Checks		Special handling to find certain programs.
4.1.2 Generic Program and File Checks		How to find other programs.

Library Functions

4.3.1 Particular Function Checks		Special handling to find certain functions.
4.3.2 Generic Function Checks		How to find other functions.

Header Files

4.4.1 Particular Header Checks		Special handling to find certain headers.
4.4.2 Generic Header Checks		How to find other headers.

Typedefs

4.6.1 Particular Typedef Checks		Special handling to find certain types.
4.6.2 Generic Typedef Checks		How to find other types.

Writing Tests

5.1 Examining Declarations		Detecting header files and declarations.
5.2 Examining Syntax		Detecting language syntax features.
5.3 Examining Libraries		Detecting functions and global variables.
5.4 Checking Run Time Behavior		Testing for run-time features.
5.5 Portable Shell Programming		Shell script portability pitfalls.
5.6 Testing Values and Files		Checking strings and files.
5.7 Multiple Cases		Tests for several possible values.
5.8 Language Choice		Selecting which language to use for testing.

Checking Run Time Behavior

5.4.1 Running Test Programs		Running test programs.
5.4.2 Guidelines for Test Programs		General rules for writing test programs.
5.4.3 Test Functions		Avoiding pitfalls in test programs.

Results of Tests

6.1 Defining C Preprocessor Symbols		Defining C preprocessor symbols.
6.2 Setting Output Variables		Replacing variables in output files.
6.3 Caching Results		Speeding up subsequent configure runs.
6.4 Printing Messages		Notifying users of progress or problems.

Caching Results

6.3.1 Cache Variable Names		Shell variables used in caches.
6.3.2 Cache Files		Files configure uses for caching.

Writing Macros

7.1 Macro Definitions		Basic format of an Autoconf macro.
7.2 Macro Names		What to call your new macros.
7.3 Quoting		Protecting macros from unwanted expansion.
7.4 Dependencies Between Macros		What to do when macros depend on other macros.

Dependencies Between Macros

7.4.1 Prerequisite Macros		Ensuring required information.
7.4.2 Suggested Ordering		Warning about possible ordering problems.
7.4.3 Obsolete Macros		Warning about old ways of doing things.

Manual Configuration

8.1 Specifying the System Type		Specifying the system type.
8.2 Getting the Canonical System Type		Getting the canonical system type.
8.3 System Type Variables		Variables containing the system type.
8.4 Using the System Type		What to do with the system type.

Site Configuration

9.1 Working With External Software		Working with other optional software.
9.2 Choosing Package Options		Selecting optional features.
9.3 Configuring Site Details		Configuring site details.
9.4 Transforming Program Names When Installing		Changing program names when installing.
9.5 Setting Site Defaults		Giving configure local defaults.

Transforming Program Names When Installing

9.4.1 Transformation Options		configure options to transform names.
9.4.2 Transformation Examples		Sample uses of transforming names.
9.4.3 Transformation Rules		`Makefile' uses of transforming names.

Running configure Scripts

10.1 Basic Installation		Instructions for typical cases.
10.2 Compilers and Options		Selecting compilers and optimization.
10.3 Compiling For Multiple Architectures		Compiling for multiple architectures at once.
10.4 Installation Names		Installing in different directories.
10.5 Optional Features		Selecting optional features.
10.6 Specifying the System Type		Specifying the system type.
10.7 Sharing Defaults		Setting site-wide defaults for configure.
10.8 Operation Controls		Changing how configure runs.

Questions About Autoconf

12.1 Distributing configure Scripts		Distributing configure scripts.
12.2 Why Require GNU m4?		Why not use the standard m4?
12.3 How Can I Bootstrap?		Autoconf and GNU m4 require each other?
12.4 Why Not Imake?		Why GNU uses configure instead of Imake.

Upgrading From Version 1

13.1 Changed File Names		Files you might rename.
13.2 Changed Makefiles		New things to put in `Makefile.in'.
13.3 Changed Macros		Macro calls you might replace.
13.4 Using autoupdate to Modernize configure		Replacing old macro names in configure.in.
13.5 Changed Results		Changes in how to check test results.
13.6 Changed Macro Writing		Better ways to write your own macros.

History of Autoconf

14.1 Genesis		Prehistory and naming of configure.
14.2 Exodus		The plagues of m4 and Perl.
14.3 Leviticus		The priestly code of portability arrives.
14.4 Numbers		Growth and contributors.
14.5 Deuteronomy		Approaching the promises of easy configuration.

1. Introduction

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

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

The configuration scripts produced by Autoconf require no manual user intervention when run; they do not normally even need an argument specifying the system type. Instead, they 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 system features that the package needs or can use. After the shell code to recognize and respond to a system feature has been written, Autoconf allows it to be shared by many software packages that can use (or need) that feature. If it later turns out that the shell code needs adjustment for some reason, it needs to be changed in only one place; all of the configuration scripts can be regenerated automatically to take advantage of the updated code.

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

There are several jobs related to making portable software packages that Autoconf currently does not do. Among these are automatically creating `Makefile' files with all of the standard targets, and supplying replacements for standard library functions and header files on systems that lack them. Work is in progress to add those features in the future.

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

Autoconf requires GNU m4 in order to generate the scripts. It uses features that some UNIX versions of m4 do not have. It also overflows internal limits of some versions of m4, including GNU m4 1.0. You must use version 1.1 or later of GNU m4. Using version 1.3 or later will be much faster than 1.1 or 1.2.

See section 13. Upgrading From Version 1, for information about upgrading from version 1. See section 14. History of Autoconf, for the story of Autoconf's development. See section 12. Questions About Autoconf, for answers to some common questions about Autoconf.

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

2. Making configure Scripts

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

• one or more `Makefile' files, one in each subdirectory of the package (see section 3.3 Substitutions in Makefiles);

• optionally, a C header file, the name of which is configurable, containing #define directives (see section 3.4 Configuration Header Files);

• a shell script called `config.status' that, when run, will recreate the files listed above (see section 11. Recreating a Configuration);

• a shell script called `config.cache' that saves the results of running many of the tests (see section 6.3.2 Cache Files);

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

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

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

Files used in preparing a software package for distribution:

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

Files used in configuring a software package:

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

2.1 Writing `configure.in'		What to put in an Autoconf input file.
2.2 Using autoscan to Create `configure.in'		Semi-automatic `configure.in' writing.
2.3 Using ifnames to List Conditionals		Listing the conditionals in source code.
2.4 Using autoconf to Create configure		How to create configuration scripts.
2.5 Using autoreconf to Update configure Scripts		Remaking multiple configure scripts.

2.1 Writing `configure.in'

To produce a configure script for a software package, create a file called `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 4. Existing Tests, for their descriptions. For most other features, you can use Autoconf template macros to produce custom checks; see 5. Writing Tests, for information about them. For especially tricky or specialized features, `configure.in' might need to contain some hand-crafted shell commands. The autoscan program can give you a good start in writing `configure.in' (see section 2.2 Using autoscan to Create `configure.in', for more information).

The order in which `configure.in' calls the Autoconf macros is not important, with a few exceptions. Every `configure.in' must contain a call to AC_INIT before the checks, and a call to AC_OUTPUT at the end (see section 3.2 Creating Output Files). Additionally, some macros rely on other macros having been called first, because they check previously set values of some variables to decide what to do. These macros are noted in the individual descriptions (see section 4. Existing Tests), and they also warn you when creating configure if they are called out of order.

To encourage consistency, here is a suggested order for calling the Autoconf macros. Generally speaking, the things near the end of this list could depend on things earlier in it. For example, library functions could be affected by typedefs and libraries.

AC_INIT(file) checks for programs checks for libraries checks for header files checks for typedefs checks for structures checks for compiler characteristics checks for library functions checks for system services AC_OUTPUT([file...])

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

When calling macros that take arguments, there must not be any blank space between the macro name and the open parenthesis. Arguments can be more than one line long if they are enclosed within the m4 quote characters `[' and `]'. If you have a long line such as a list of file names, you can generally use a backslash at the end of a line to continue it logically on the next line (this is implemented by the shell, not by anything special that Autoconf does).

Some 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 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 action-if-found argument to the macro. To omit the false case, omit the action-if-not-found argument to the macro, including the comma before it.

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

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

2.2 Using autoscan to Create `configure.in'

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

You should manually examine `configure.scan' before renaming it to `configure.in'; it will probably need some adjustments. Occasionally autoscan outputs a macro in the wrong order relative to another macro, so that autoconf produces a warning; you need to move such macros manually. Also, if you want the package to use a configuration header file, you must add a call to AC_CONFIG_HEADER (see section 3.4 Configuration Header Files). You might also have to change or add some #if directives to your program in order to make it work with Autoconf (see section 2.3 Using ifnames to List Conditionals, for information about a program that can help with that job).

autoscan uses several data files, which are installed along 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 `#' are comments.

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

--help

Print a summary of the command line options and exit.

--macrodir=dir

Look for the data files in directory dir instead of the default installation directory. You can also set the AC_MACRODIR environment variable to a directory; this option overrides the environment variable.

--verbose

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

--version

Print the version number of Autoconf and exit.

2.3 Using ifnames to List Conditionals

ifnames can help when writing a `configure.in' for a software package. It prints the identifiers that the package already uses in C preprocessor conditionals. If a package has already been set up to have some portability, this program can help you figure out what its configure needs to check for. It may help fill in some gaps in a `configure.in' generated by autoscan (see section 2.2 Using autoscan to Create `configure.in').

ifnames scans all of the C source files named on the command line (or the standard input, if none are given) and writes to the standard output a sorted list of all the identifiers that appear in those files in #if, #elif, #ifdef, or #ifndef directives. It prints each identifier on a line, followed by a space-separated list of the files in which that identifier occurs.

ifnames accepts the following options:

--help

-h

Print a summary of the command line options and exit.

--macrodir=dir

-m dir

Look for the Autoconf macro files in directory dir instead of the default installation directory. Only used to get the version number. You can also set the AC_MACRODIR environment variable to a directory; this option overrides the environment variable.

--version

Print the version number of Autoconf and exit.

2.4 Using autoconf to Create configure

To create configure from `configure.in', run the autoconf program with no arguments. autoconf processes `configure.in' with the m4 macro processor, using the Autoconf macros. If you give autoconf an argument, it reads that file instead of `configure.in' and writes the configuration script to the standard output instead of to configure. If you give autoconf the argument `-', it reads the standard input instead of `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; autoconf reads them first. Then it looks for the optional file `acsite.m4' in the directory that contains the distributed Autoconf macro files, and for the optional file `aclocal.m4' in the current directory. Those files can contain your site's or the package's own Autoconf macro definitions (see section 7. Writing Macros, for more information). If a macro is defined in more than one of the files that autoconf reads, the last definition it reads overrides the earlier ones.

autoconf accepts the following options:

--help

-h

Print a summary of the command line options and exit.

--localdir=dir

-l dir

Look for the package file `aclocal.m4' in directory dir instead of in the current directory.

--macrodir=dir

-m dir

Look for the installed macro files in directory dir. You can also set the AC_MACRODIR environment variable to a directory; this option overrides the environment variable.

--version

Print the version number of Autoconf and exit.

2.5 Using autoreconf to Update configure Scripts

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

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

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

See section 3.3.3 Automatic Remaking, for `Makefile' rules to automatically remake configure scripts when their source files change. That method handles the timestamps of configuration header templates properly, but does not pass `--macrodir=dir' or `--localdir=dir'.

autoreconf accepts the following options:

--help

-h

Print a summary of the command line options and exit.

--force

-f

Remake even `configure' scripts and configuration headers that are newer than their input files (`configure.in' and, if present, `aclocal.m4').

--localdir=dir

-l dir

Have autoconf and autoheader look for the package files `aclocal.m4' and (autoheader only) `acconfig.h' (but not `file.top' and `file.bot') in directory dir instead of in the directory containing each `configure.in'.

--macrodir=dir

-m dir

Look for the Autoconf macro files in directory dir instead of the default installation directory. You can also set the AC_MACRODIR environment variable to a directory; this option overrides the environment variable.

--verbose

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

--version

Print the version number of Autoconf and exit.

3. Initialization and Output Files

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

3.1 Finding configure Input		Where Autoconf should find files.
3.2 Creating Output Files		Creating output files.
3.3 Substitutions in Makefiles		Using output variables in `Makefile's.
3.4 Configuration Header Files		Creating a configuration header file.
3.5 Configuring Other Packages in Subdirectories		Configuring independent packages together.
3.6 Default Prefix		Changing the default installation prefix.
3.7 Version Numbers in configure		Version numbers in configure.

3.1 Finding configure Input

Every configure script must call AC_INIT before doing anything else. The only other required macro is AC_OUTPUT (see section 3.2 Creating Output Files).

Macro: AC_INIT (unique-file-in-source-dir)

Process any command-line arguments and find the source code directory. unique-file-in-source-dir is some file that is in the package's source directory; configure checks for this file's existence to make sure that the directory that it is told contains the source code in fact does. Occasionally people accidentally specify the wrong directory with `--srcdir'; this is a safety check. See section 10. Running configure Scripts, for more information.

Packages that do manual configuration or use the install program might need to tell configure where to find some other shell scripts by calling AC_CONFIG_AUX_DIR, though the default places it looks are correct for most cases.

Macro: AC_CONFIG_AUX_DIR(dir)

Use the `install-sh', `config.sub', `config.guess', and Cygnus configure scripts that are in directory dir. These are auxiliary files used in configuration. dir can be either absolute or relative to `srcdir'. The default is `srcdir' or `srcdir/..' or `srcdir/../..', whichever is the first that contains `install-sh'. The other files are not checked for, so that using AC_PROG_INSTALL does not automatically require distributing the other auxiliary files. It checks for `install.sh' also, but that name is obsolete because some make programs have a rule that creates `install' from it if there is no `Makefile'.

3.2 Creating Output Files

Every Autoconf-generated configure script must finish by calling AC_OUTPUT. It is the macro that creates the `Makefile's and optional other files resulting from configuration. The only other required macro is AC_INIT (see section 3.1 Finding configure Input).

Macro: AC_OUTPUT ([file... [, extra-cmds [, init-cmds]]])

Create output files. Call this macro once, at the end of `configure.in'. The file... argument is a whitespace-separated list of output files; it may be empty. This macro creates each file `file' by copying an input file (by default named `file.in'), substituting the output variable values. See section 3.3 Substitutions in Makefiles, for more information on using output variables. See section 6.2 Setting Output Variables, for more information on creating them. This macro creates the directory that the file is in if it doesn't exist (but not the parents of that directory). Usually, `Makefile's are created this way, but other files, such as `.gdbinit', can be specified as well.

If AC_CONFIG_HEADER, AC_LINK_FILES, or AC_CONFIG_SUBDIRS has been called, this macro also creates the files named as their arguments.

A typical call to AC_OUTPUT looks like this:

AC_OUTPUT(Makefile src/Makefile man/Makefile X/Imakefile)

You can override an input file name by appending to file a colon-separated list of input files. Examples:

AC_OUTPUT(Makefile:templates/top.mk lib/Makefile:templates/lib.mk) AC_OUTPUT(Makefile:templates/vars.mk:Makefile.in:templates/rules.mk)

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

If you pass extra-cmds, those commands will be inserted into `config.status' to be run after all its other processing. If init-cmds are given, they are inserted just before extra-cmds, with shell variable, command, and backslash substitutions performed on them in configure. You can use init-cmds to pass variables from configure to the extra-cmds. If AC_OUTPUT_COMMANDS has been called, the commands given to it are run just before the commands passed to this macro.

Macro: AC_OUTPUT_COMMANDS (extra-cmds [, init-cmds])

Specify additional shell commands to run at the end of `config.status', and shell commands to initialize any variables from configure. This macro may be called multiple times. Here is an unrealistic example:

fubar=27 AC_OUTPUT_COMMANDS([echo this is extra $fubar, and so on.], fubar=$fubar) AC_OUTPUT_COMMANDS([echo this is another, extra, bit], [echo init bit])

If you run make on subdirectories, you should run it using the make variable MAKE. Most versions of make set MAKE to the name of the make program plus any options it was given. (But many do not include in it the values of any variables set on the command line, so those are not passed on automatically.) Some old versions of make do not set this variable. The following macro allows you to use it even with those versions.

Macro: AC_PROG_MAKE_SET

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

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

@SET_MAKE@

3.3 Substitutions in Makefiles

Each subdirectory in a distribution that contains something to be compiled or installed should come with a file `Makefile.in', from which configure will create a `Makefile' in that directory. To create a `Makefile', configure performs a simple variable substitution, replacing occurrences of `@variable@' in `Makefile.in' with the value that configure has determined for that variable. Variables that are substituted into output files in this way are called output variables. They are ordinary shell variables that are set in configure. To make configure substitute a particular variable into the output files, the macro AC_SUBST must be called with that variable name as an argument. Any occurrences of `@variable@' for other variables are left unchanged. See section 6.2 Setting Output Variables, for more information on creating output variables with AC_SUBST.

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

See section `Makefile Conventions' in The GNU Coding Standards, for more information on what to put in `Makefile's.

3.3.1 Preset Output Variables		Output variables that are always set.
3.3.2 Build Directories		Supporting multiple concurrent compiles.
3.3.3 Automatic Remaking		Makefile rules for configuring.

3.3.1 Preset Output Variables

Some output variables are preset by the Autoconf macros. Some of the Autoconf macros set additional output variables, which are mentioned in the descriptions for those macros. See section Output Variable Index, for a complete list of output variables. Here is what each of the preset ones contains. See section `Variables for Installation Directories' in The GNU Coding Standards, for more information about the variables with names that end in `dir'.

Variable: bindir

The directory for installing executables that users run.

Variable: configure_input

A comment saying that the file was generated automatically by configure and giving the name of the input file. AC_OUTPUT adds a comment line containing this variable to the top of every `Makefile' it creates. For other files, you should reference this variable in a comment at the top of each input file. For example, an input shell script should begin like this:

#! /bin/sh # @configure_input@

The presence of that line also reminds people editing the file that it needs to be processed by configure in order to be used.

Variable: datadir

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

Variable: exec_prefix

The installation prefix for architecture-dependent files.

Variable: includedir

The directory for installing C header files.

Variable: infodir

The directory for installing documentation in Info format.

Variable: libdir

The directory for installing object code libraries.

Variable: libexecdir

The directory for installing executables that other programs run.

Variable: localstatedir

The directory for installing modifiable single-machine data.

Variable: mandir

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

Variable: oldincludedir

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

Variable: prefix

The installation prefix for architecture-independent files.

Variable: sbindir

The directory for installing executables that system administrators run.

Variable: sharedstatedir

The directory for installing modifiable architecture-independent data.

Variable: srcdir

The directory that contains the source code for that `Makefile'.

Variable: sysconfdir

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

Variable: top_srcdir

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

Variable: CFLAGS

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

Variable: CPPFLAGS

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

Variable: CXXFLAGS

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

Variable: FFLAGS

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

Variable: DEFS

`-D' options to pass to the C compiler. If AC_CONFIG_HEADER is called, configure replaces `@DEFS@' with `-DHAVE_CONFIG_H' instead (see section 3.4 Configuration Header Files). This variable is not defined while configure is performing its tests, only when creating the output files. See section 6.2 Setting Output Variables, for how to check the results of previous tests.

Variable: LDFLAGS

Stripping (`-s') and any other miscellaneous options for the linker. If it is not set in the environment when configure runs, the default value is empty. configure uses this variable when linking programs to test for C features.

Variable: LIBS

`-l' and `-L' options to pass to the linker.

3.3.2 Build Directories

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

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

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

srcdir = @srcdir@ VPATH = @srcdir@

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

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

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

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

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

3.3.3 Automatic Remaking

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

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

The `stamp-' files are necessary because the timestamps of `config.h.in' and `config.h' will not be changed if remaking them does not change their contents. This feature avoids unnecessary recompilation. You should include the file `stamp-h.in' your package's distribution, so make will consider `config.h.in' up to date. On some old BSD systems, touch or any command that results in an empty file does not update the timestamps, so use a command like echo as a workaround.

${srcdir}/configure: configure.in aclocal.m4 cd ${srcdir} && autoconf # autoheader might not change config.h.in, so touch a stamp file. ${srcdir}/config.h.in: stamp-h.in ${srcdir}/stamp-h.in: configure.in aclocal.m4 acconfig.h \ config.h.top config.h.bot cd ${srcdir} && autoheader echo timestamp > ${srcdir}/stamp-h.in config.h: stamp-h stamp-h: config.h.in config.status ./config.status Makefile: Makefile.in config.status ./config.status config.status: configure ./config.status --recheck

In addition, you should pass `echo timestamp > stamp-h' in the extra-cmds argument to AC_OUTPUT, so `config.status' will ensure that `config.h' is considered up to date. See section 3.2 Creating Output Files, for more information about AC_OUTPUT.

See section 11. Recreating a Configuration, for more examples of handling configuration-related dependencies.

3.4 Configuration Header Files

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

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

Macro: AC_CONFIG_HEADER (header-to-create ...)

Make AC_OUTPUT create the file(s) in the whitespace-separated list header-to-create containing C preprocessor #define statements, and replace `@DEFS@' in generated files with `-DHAVE_CONFIG_H' instead of the value of DEFS. The usual name for header-to-create is `config.h'.

If header-to-create already exists and its contents are identical to what AC_OUTPUT would put in it, it is left alone. Doing this allows some changes in configuration without needlessly causing object files that depend on the header file to be recompiled.

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

AC_CONFIG_HEADER(defines.h:defines.hin) AC_CONFIG_HEADER(defines.h:defs.pre:defines.h.in:defs.post)

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

3.4.1 Configuration Header Templates		Input for the configuration headers.
3.4.2 Using autoheader to Create `config.h.in'		How to create configuration templates.

3.4.1 Configuration Header Templates

Your distribution should contain a template file that looks as you want the final header file to look, including comments, with default values in the #define statements. For example, suppose your `configure.in' makes these calls:

AC_CONFIG_HEADER(conf.h) AC_CHECK_HEADERS(unistd.h)

Then you could have code like the following in `conf.h.in'. On systems that have `unistd.h', configure will change the 0 to a 1. On other systems, it will leave the line unchanged.

/* Define as 1 if you have unistd.h. */ #define HAVE_UNISTD_H 0

Alternately, if your code tests for configuration options using #ifdef instead of #if, a default value can be to #undef the variable instead of to define it to a value. On systems that have `unistd.h', configure will change the second line to read `#define HAVE_UNISTD_H 1'. On other systems, it will comment that line out (in case the system predefines that symbol).

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

3.4.2 Using autoheader to Create `config.h.in'

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

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

autoheader scans `configure.in' and figures out which C preprocessor symbols it might define. It copies comments and #define and #undef statements from a file called `acconfig.h', which comes with and is installed with Autoconf. It also uses a file called `acconfig.h' in the current directory, if present. If you AC_DEFINE any additional symbols, you must create that file with entries for them. For symbols defined by AC_CHECK_HEADERS, AC_CHECK_FUNCS, AC_CHECK_SIZEOF, or AC_CHECK_LIB, autoheader generates comments and #undef statements itself rather than copying them from a file, since the possible symbols are effectively limitless.

The file that autoheader creates contains mainly #define and #undef statements and their accompanying comments. If `./acconfig.h' contains the string `@TOP@', autoheader copies the lines before the line containing `@TOP@' into the top of the file that it generates. Similarly, if `./acconfig.h' contains the string `@BOTTOM@', autoheader copies the lines after that line to the end of the file it generates. Either or both of those strings may be omitted.

An alternate way to produce the same effect is to create the files `file.top' (typically `config.h.top') and/or `file.bot' in the current directory. If they exist, autoheader copies them to the beginning and end, respectively, of its output. 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. But if you use the `--localdir=dir' option to use an `acconfig.h' in another directory, they give you a way to put custom boilerplate in each individual `config.h.in'.

autoheader accepts the following options:

--help

-h

Print a summary of the command line options and exit.

--localdir=dir

-l dir

Look for the package files `aclocal.m4' and `acconfig.h' (but not `file.top' and `file.bot') in directory dir instead of in the current directory.

--macrodir=dir

-m dir

Look for the installed macro files and `acconfig.h' in directory dir. You can also set the AC_MACRODIR environment variable to a directory; this option overrides the environment variable.

--version

Print the version number of Autoconf and exit.

3.5 Configuring Other Packages in Subdirectories

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

Macro: AC_CONFIG_SUBDIRS (dir ...)

Make AC_OUTPUT run configure in each subdirectory dir in the given whitespace-separated list. If a given dir is not found, no error is reported, so a configure script can configure whichever parts of a large source tree are present. If a given dir contains `configure.in' but no configure, the Cygnus configure script found by AC_CONFIG_AUXDIR is used.

The subdirectory configure scripts are given the same command line options that were given to this configure script, with minor changes if needed (e.g., to adjust a relative path for the cache file or source directory). This macro also sets the output variable subdirs to the list of directories `dir ...'. `Makefile' rules can use this variable to determine which subdirectories to recurse into. This macro may be called multiple times.

3.6 Default Prefix

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

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

Macro: AC_PREFIX_DEFAULT (prefix)

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

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

Macro: AC_PREFIX_PROGRAM (program)

If the user did not specify an installation prefix (using the `--prefix' option), guess a value for it by looking for program in PATH, the way the shell does. If program is found, set the prefix to the parent of the directory containing program; otherwise leave the prefix specified in `Makefile.in' unchanged. For example, if program is gcc and the PATH contains `/usr/local/gnu/bin/gcc', set the prefix to `/usr/local/gnu'.

3.7 Version Numbers in configure

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

Macro: AC_PREREQ (version)

Ensure that a recent enough version of Autoconf is being used. If the version of Autoconf being used to create configure is earlier than version, print an error message on the standard error output and do not create configure. For example:

AC_PREREQ(1.8)

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

Macro: AC_REVISION (revision-info)

Copy revision stamp revision-info into the configure script, with any dollar signs or double-quotes removed. This macro lets you put a revision stamp from `configure.in' into configure without RCS or CVS changing it when you check in configure. That way, you can determine easily which revision of `configure.in' a particular configure corresponds to.

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

For example, this line in `configure.in':

AC_REVISION($Revision: 1.30 $)dnl

produces this in configure:

#! /bin/sh # From configure.in Revision: 1.30

4. Existing Tests

These macros test for particular system features that packages might need or want to use. If you need to test for a kind of feature that none of these macros check for, you can probably do it by calling primitive test macros with appropriate arguments (see section 5. Writing Tests).

These tests print messages telling the user which feature they're checking for, and what they find. They cache their results for future configure runs (see section 6.3 Caching Results).

Some of these macros set output variables. See section 3.3 Substitutions in Makefiles, for how to get their values. The phrase "define name" is used below as a shorthand to mean "define C preprocessor symbol name to the value 1". See section 6.1 Defining C Preprocessor Symbols, for how to get those symbol definitions into your program.

4.1 Alternative Programs		Selecting between alternative programs.
4.2 Library Files		Library archives that might be missing.
4.3 Library Functions		C library functions that might be missing.
4.4 Header Files		Header files that might be missing.
4.5 Structures		Structures or members that might be missing.
4.6 Typedefs		typedefs that might be missing.
4.7 C Compiler Characteristics
4.8 Fortran 77 Compiler Characteristics
4.9 System Services		Operating system services.
4.10 UNIX Variants		Special kludges for specific UNIX variants.

4.1 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.

4.1.1 Particular Program Checks		Special handling to find certain programs.
4.1.2 Generic Program and File Checks		How to find other programs.

4.1.1 Particular Program Checks

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

Macro: AC_DECL_YYTEXT

Define YYTEXT_POINTER if yytext is a `char *' instead of a `char []'. Also set output variable LEX_OUTPUT_ROOT to the base of the file name that the lexer generates; usually `lex.yy', but sometimes something else. These results vary according to whether lex or flex is being used.

Macro: AC_PROG_AWK

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

Macro: AC_PROG_CC

Determine a C compiler to use. If CC is not already set in the environment, check for gcc, and use cc if that's not found. Set output variable CC to the name of the compiler found.

If using the GNU C compiler, set shell variable GCC to `yes', empty otherwise. If output variable CFLAGS was not already set, set it to `-g -O2' for the GNU C compiler (`-O2' on systems where GCC does not accept `-g'), or `-g' for other compilers.

If the C compiler being used does not produce executables that can run on the system where configure is being run, set the shell variable cross_compiling to `yes', otherwise `no'. In other words, this tests whether the build system type is different from the host system type (the target system type is irrelevant to this test). See section 8. Manual Configuration, for more on support for cross compiling.

Macro: AC_PROG_CC_C_O

If the C compiler does not accept the `-c' and `-o' options simultaneously, define NO_MINUS_C_MINUS_O.

Macro: AC_PROG_CPP

Set output variable CPP to a command that runs the C preprocessor. If `$CC -E' doesn't work, it uses `/lib/cpp'. It is only portable to run CPP on files with a `.c' extension.

If the current language is C (see section 5.8 Language Choice), many of the specific test macros use the value of CPP indirectly by calling AC_TRY_CPP, AC_CHECK_HEADER, AC_EGREP_HEADER, or AC_EGREP_CPP.

Macro: AC_PROG_CXX

Determine a C++ compiler to use. Check if the environment variable CXX or CCC (in that order) is set; if so, set output variable CXX to its value. Otherwise search for a C++ compiler under likely names (c++, g++, gcc, CC, cxx, and cc++). If none of those checks succeed, as a last resort set CXX to gcc.

If using the GNU C++ compiler, set shell variable GXX to `yes', empty otherwise. If output variable CXXFLAGS was not already set, set it to `-g -O2' for the GNU C++ compiler (`-O2' on systems where G++ does not accept `-g'), or `-g' for other compilers.

If the C++ compiler being used does not produce executables that can run on the system where configure is being run, set the shell variable cross_compiling to `yes', otherwise `no'. In other words, this tests whether the build system type is different from the host system type (the target system type is irrelevant to this test). See section 8. Manual Configuration, for more on support for cross compiling.

Macro: AC_PROG_CXXCPP

Set output variable CXXCPP to a command that runs the C++ preprocessor. If `$CXX -E' doesn't work, it uses `/lib/cpp'. It is only portable to run CXXCPP on files with a `.c', `.C', or `.cc' extension.

If the current language is C++ (see section 5.8 Language Choice), many of the specific test macros use the value of CXXCPP indirectly by calling AC_TRY_CPP, AC_CHECK_HEADER, AC_EGREP_HEADER, or AC_EGREP_CPP.

Macro: AC_PROG_F77

Determine a Fortran 77 compiler to use. If F77 is not already set in the environment, check for g77, f77 and f2c, in that order. Set the output variable F77 to the name of the compiler found.

If using g77 (the GNU Fortran 77 compiler), then AC_PROG_F77 will set the shell variable G77 to `yes', and empty otherwise. If the output variable FFLAGS was not already set in the environment, then set it to `-g -02' for g77 (or `-O2' where g77 does not accept `-g'). Otherwise, set FFLAGS to `-g' for all other Fortran 77 compilers.

Macro: AC_PROG_F77_C_O

Test if the Fortran 77 compiler accepts the options `-c' and `-o' simultaneously, and define F77_NO_MINUS_C_MINUS_O if it does not.

Macro: AC_PROG_GCC_TRADITIONAL

Add `-traditional' to output variable CC if using the GNU C compiler and ioctl does not work properly without `-traditional'. That usually happens when the fixed header files have not been installed on an old system. Since recent versions of the GNU C compiler fix the header files automatically when installed, this is becoming a less prevalent problem.

Macro: AC_PROG_INSTALL

Set output variable INSTALL to the path of a BSD compatible install program, if one is found in the current PATH. Otherwise, set INSTALL to `dir/install-sh -c', checking the directories specified to AC_CONFIG_AUX_DIR (or its default directories) to determine dir (see section 3.2 Creating Output Files). Also set the variables INSTALL_PROGRAM and INSTALL_SCRIPT to `${INSTALL}' and INSTALL_DATA to `${INSTALL} -m 644'.

This macro screens out various instances of install known to not work. It prefers to find a C program rather than a shell script, for speed. Instead of `install-sh', it can also use `install.sh', but that name is obsolete because some make programs have a rule that creates `install' from it if there is no `Makefile'.

A copy of `install-sh' which you may use comes with Automake. If you use AC_PROG_INSTALL, you must include either `install-sh' or `install.sh' in your distribution, or configure will produce an error message saying it can't find them--even if the system you're on has a good install program. This check is a safety measure to prevent you from accidentally leaving that file out, which would prevent your package from installing on systems that don't have a BSD-compatible install program.

If you need to use your own installation program because it has features not found in standard install programs, there is no reason to use AC_PROG_INSTALL; just put the pathname of your program into your `Makefile.in' files.

Macro: AC_PROG_LEX

If flex is found, set output variable LEX to `flex' and LEXLIB to `-lfl', if that library is in a standard place. Otherwise set LEX to `lex' and LEXLIB to `-ll'.

Macro: AC_PROG_LN_S

If `ln -s' works on the current filesystem (the operating system and filesystem support symbolic links), set output variable LN_S to `ln -s', otherwise set it to `ln'.

If the link is put in a directory other than the current directory, its meaning depends on whether `ln' or `ln -s' is used. To safely create links using `$(LN_S)', either find out which form is used and adjust the arguments, or always invoke ln in the directory where the link is to be created.

In other words, it does not work to do

$(LN_S) foo /x/bar

Instead, do

(cd /x && $(LN_S) foo bar)

Macro: AC_PROG_RANLIB

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

Macro: AC_PROG_YACC

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

4.1.2 Generic Program and File Checks

These macros are used to find programs not covered by the particular test macros. If you need to check the behavior of a program as well as find out whether it is present, you have to write your own test for it (see section 5. Writing Tests). By default, these macros use the environment variable PATH. If you need to check for a program that might not be in the user's PATH, you can pass a modified path to use instead, like this:

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

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

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

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

Executes AC_CHECK_FILE once for each file listed in files. Additionally, defines `HAVEfile' for each file found, set to 1.

Macro: AC_CHECK_PROG (variable, prog-to-check-for, value-if-found [, value-if-not-found [, path, [ reject ]]])

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

Macro: AC_CHECK_PROGS (variable, progs-to-check-for [, value-if-not-found [, path]])

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

Macro: AC_CHECK_TOOL (variable, prog-to-check-for [, value-if-not-found [, path]])

Like AC_CHECK_PROG, but first looks for prog-to-check-for with a prefix of the host type as determined by AC_CANONICAL_HOST, followed by a dash (see section 8.2 Getting the Canonical System Type). For example, if the user runs `configure --host=i386-gnu', then this call:

AC_CHECK_TOOL(RANLIB, ranlib, :)

sets RANLIB to `i386-gnu-ranlib' if that program exists in PATH, or to `ranlib' if that program exists in PATH, or to `:' if neither program exists.

Macro: AC_PATH_PROG (variable, prog-to-check-for [, value-if-not-found [, path]])

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

Macro: AC_PATH_PROGS (variable, progs-to-check-for [, value-if-not-found [, path]])

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

4.2 Library Files

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

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

Depending on the current language(see section 5.8 Language Choice), try to ensure that the C, C++ or Fortran 77 function function is available by checking whether a test program can be linked with the library library to get the function. library is the base name of the library; e.g., to check for `-lmp', use `mp' as the library argument.

action-if-found is a list of shell commands to run if the link with the library succeeds; action-if-not-found is a list of shell commands to run if the link fails. If action-if-found is not specified, the default action will add `-llibrary' to LIBS and define `HAVE_LIBlibrary' (in all capitals).

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

Macro: AC_HAVE_LIBRARY (library, [, action-if-found [, action-if-not-found [, other-libraries]]])

This macro is equivalent to calling AC_CHECK_LIB with a function argument of main. In addition, library can be written as any of `foo', `-lfoo', or `libfoo.a'. In all of those cases, the compiler is passed `-lfoo'. However, library can not be a shell variable; it must be a literal name. This macro is considered obsolete.

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

Search for a library defining function, if it's not already available. This equates to calling AC_TRY_LINK_FUNC first with no libraries, then for each library listed in search-libs.

If the function is found, run action-if-found, otherwise run action-if-not-found.

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

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

This macro is equivalent to calling AC_TRY_LINK_FUNC once for each library listed in search-libs. Add `-llibrary' to LIBS for the first library found to contain function, and execute action-if-found. Otherwise execute action-if-not-found.

4.3 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.

4.3.1 Particular Function Checks		Special handling to find certain functions.
4.3.2 Generic Function Checks		How to find other functions.

4.3.1 Particular Function Checks

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

Macro: AC_FUNC_ALLOCA

Check how to get alloca. Tries to get a builtin version by checking for `alloca.h' or the predefined C preprocessor macros __GNUC__ and _AIX. If this macro finds `alloca.h', it defines HAVE_ALLOCA_H.

If those attempts fail, it looks for the function in the standard C library. If any of those methods succeed, it defines HAVE_ALLOCA. Otherwise, it sets the output variable ALLOCA to `alloca.o' and defines C_ALLOCA (so programs can periodically call `alloca(0)' to garbage collect). This variable is separate from LIBOBJS so multiple programs can share the value of ALLOCA without needing to create an actual library, in case only some of them use the code in LIBOBJS.

This macro does not try to get alloca from the System V R3 `libPW' or the System V R4 `libucb' because those libraries contain some incompatible functions that cause trouble. Some versions do not even contain alloca or contain a buggy version. If you still want to use their alloca, use ar to extract `alloca.o' from them instead of compiling `alloca.c'.

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

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

Macro: AC_FUNC_CLOSEDIR_VOID

If the closedir function does not return a meaningful value, define CLOSEDIR_VOID. Otherwise, callers ought to check its return value for an error indicator.

Macro: AC_FUNC_FNMATCH

If the fnmatch function is available and works (unlike the one on SunOS 5.4), define HAVE_FNMATCH.

Macro: AC_FUNC_GETLOADAVG

Check how to get the system load averages. If the system has the getloadavg function, this macro defines HAVE_GETLOADAVG, and adds to LIBS any libraries needed to get that function.

Otherwise, it adds `getloadavg.o' to the output variable LIBOBJS, and possibly defines several other C preprocessor macros and output variables:

1. It defines SVR4, DGUX, UMAX, or UMAX4_3 if on those systems.

2. If it finds `nlist.h', it defines NLIST_STRUCT.

3. If `struct nlist' has an `n_un' member, it defines NLIST_NAME_UNION.

4. If compiling `getloadavg.c' defines LDAV_PRIVILEGED, programs need to be installed specially on this system for getloadavg to work, and this macro defines GETLOADAVG_PRIVILEGED.

5. This macro sets the output variable NEED_SETGID. The value is `true' if special installation is required, `false' if not. If NEED_SETGID is `true', this macro sets KMEM_GROUP to the name of the group that should own the installed program.

Macro: AC_FUNC_GETMNTENT

Check for getmntent in the `sun', `seq', and `gen' libraries, for Irix 4, PTX, and Unixware, respectively. Then, if getmntent is available, define HAVE_GETMNTENT.

Macro: AC_FUNC_GETPGRP

If getpgrp takes no argument (the POSIX.1 version), define GETPGRP_VOID. Otherwise, it is the BSD version, which takes a process ID as an argument. This macro does not check whether getpgrp exists at all; if you need to work in that situation, first call AC_CHECK_FUNC for getpgrp.

Macro: AC_FUNC_MEMCMP

If the memcmp function is not available, or does not work on 8-bit data (like the one on SunOS 4.1.3), add `memcmp.o' to output variable LIBOBJS.

Macro: AC_FUNC_MMAP

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

Macro: AC_FUNC_SELECT_ARGTYPES

Determines the correct type to be passed to each of the select function's arguments, and defines those types in SELECT_TYPE_ARG1, SELECT_TYPE_ARG234, and SELECT_TYPE_ARG5 respectively. SELECT_TYPE_ARG1 defaults to `int', SELECT_TYPE_ARG234 defaults to `int *', and SELECT_TYPE_ARG5 defaults to `struct timeval *'.

Macro: AC_FUNC_SETPGRP

If setpgrp takes no argument (the POSIX.1 version), define SETPGRP_VOID. Otherwise, it is the BSD version, which takes two process ID as arguments. This macro does not check whether setpgrp exists at all; if you need to work in that situation, first call AC_CHECK_FUNC for setpgrp.

Macro: AC_FUNC_SETVBUF_REVERSED

If setvbuf takes the buffering type as its second argument and the buffer pointer as the third, instead of the other way around, define SETVBUF_REVERSED. This is the case on System V before release 3.

Macro: AC_FUNC_STRCOLL

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

Macro: AC_FUNC_STRFTIME

Check for strftime in the `intl' library, for SCO UNIX. Then, if strftime is available, define HAVE_STRFTIME.

Macro: AC_FUNC_UTIME_NULL

If `utime(file, NULL)' sets file's timestamp to the present, define HAVE_UTIME_NULL.

Macro: AC_FUNC_VFORK

If `vfork.h' is found, define HAVE_VFORK_H. If a working vfork is not found, define vfork to be fork. This macro checks for several known errors in implementations of vfork and considers the system to not have a working vfork if it detects any of them. It is not considered to be an implementation error if a child's invocation of signal modifies the parent's signal handler, since child processes rarely change their signal handlers.

Macro: AC_FUNC_VPRINTF

If vprintf is found, define HAVE_VPRINTF. Otherwise, if _doprnt is found, define HAVE_DOPRNT. (If vprintf is available, you may assume that vfprintf and vsprintf are also available.)

Macro: AC_FUNC_WAIT3

If wait3 is found and fills in the contents of its third argument (a `struct rusage *'), which HP-UX does not do, define HAVE_WAIT3.

4.3.2 Generic Function Checks

These macros are used to find functions not covered by the particular test macros. If the functions might be in libraries other than the default C library, first call AC_CHECK_LIB for those libraries. If you need to check the behavior of a function as well as find out whether it is present, you have to write your own test for it (see section 5. Writing Tests).

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

If C function function is available, run shell commands action-if-found, otherwise action-if-not-found. If you just want to define a symbol if the function is available, consider using AC_CHECK_FUNCS instead. This macro checks for functions with C linkage even when AC_LANG_CPLUSPLUS has been called, since C++ is more standardized than C is. (see section 5.8 Language Choice, for more information about selecting the language for checks.)

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

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

Macro: AC_REPLACE_FUNCS (function...)

Like calling AC_CHECK_FUNCS using an action-if-not-found that adds `function.o' to the value of the output variable LIBOBJS. You can declare a function for which your replacement version is used by enclosing the prototype in `#ifndef HAVE_function'. If the system has the function, it probably declares it in a header file you should be including, so you shouldn't redeclare it, lest your declaration conflict.

4.4 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.

4.4.1 Particular Header Checks		Special handling to find certain headers.
4.4.2 Generic Header Checks		How to find other headers.

4.4.1 Particular Header Checks

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

Macro: AC_DECL_SYS_SIGLIST

Define SYS_SIGLIST_DECLARED if the variable sys_siglist is declared in a system header file, either `signal.h' or `unistd.h'.

Macro: AC_DIR_HEADER

Like calling AC_HEADER_DIRENT and AC_FUNC_CLOSEDIR_VOID, 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:

`dirent.h'

DIRENT

`sys/ndir.h'

SYSNDIR

`sys/dir.h'

SYSDIR

`ndir.h'

NDIR

In addition, if the closedir function does not return a meaningful value, define VOID_CLOSEDIR.

Macro: AC_HEADER_DIRENT

Check for the following header files, and for the first one that is found and defines `DIR', define the listed C preprocessor macro:

`dirent.h'

HAVE_DIRENT_H

`sys/ndir.h'

HAVE_SYS_NDIR_H

`sys/dir.h'

HAVE_SYS_DIR_H

`ndir.h'

HAVE_NDIR_H

The directory library declarations in the source code should look something like the following:

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

Using the above declarations, the program would declare variables to be type struct dirent, not struct direct, and would access the length of a directory entry name by passing a pointer to a struct dirent to the NAMLEN macro.

This macro also checks for the SCO Xenix `dir' and `x' libraries.

Macro: AC_HEADER_MAJOR

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

Macro: AC_HEADER_STDC

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

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

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

AC_HEADER_STDC AC_CHECK_FUNCS(strchr memcpy)

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

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

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

Macro: AC_HEADER_SYS_WAIT

If `sys/wait.h' exists and is compatible with POSIX.1, define HAVE_SYS_WAIT_H. Incompatibility can occur if `sys/wait.h' does not exist, or if it uses the old BSD union wait instead of int to store a status value. If `sys/wait.h' is not POSIX.1 compatible, then instead of including it, define the POSIX.1 macros with their usual interpretations. Here is an example:

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

Macro: AC_MEMORY_H

Define NEED_MEMORY_H if memcpy, memcmp, etc. are not declared in `string.h' and `memory.h' exists. This macro is obsolete; instead, use AC_CHECK_HEADERS(memory.h). See the example for AC_HEADER_STDC.

Macro: AC_UNISTD_H

Define HAVE_UNISTD_H if the system has `unistd.h'. This macro is obsolete; instead, use `AC_CHECK_HEADERS(unistd.h)'.

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

#if HAVE_UNISTD_H # include <sys/types.h> # include <unistd.h> #endif #ifdef _POSIX_VERSION /* Code for POSIX.1 systems. */ #endif

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

Macro: AC_USG

Define USG if the system does not have `strings.h', rindex, bzero, etc. This implies that it has `string.h', strrchr, memset, etc.

The symbol USG is obsolete. Instead of this macro, see the example for AC_HEADER_STDC.

4.4.2 Generic Header Checks

These macros are used to find system header files not covered by the particular test macros. If you need to check the contents of a header as well as find out whether it is present, you have to write your own test for it (see section 5. Writing Tests).

Macro: AC_CHECK_HEADER (header-file, [action-if-found [, action-if-not-found]])

If the system header file header-file exists, execute shell commands action-if-found, otherwise execute action-if-not-found. If you just want to define a symbol if the header file is available, consider using AC_CHECK_HEADERS instead.

Macro: AC_CHECK_HEADERS (header-file... [, action-if-found [, action-if-not-found]])

For each given system header file header-file in the whitespace-separated argument list that exists, define HAVE_header-file (in all capitals). If action-if-found is given, it is additional shell code to execute when one of the header files is found. You can give it a value of `break' to break out of the loop on the first match. If action-if-not-found is given, it is executed when one of the header files is not found.

4.5 Structures

The following macros check for certain structures or structure members. To check structures not listed here, use AC_EGREP_CPP (see section 5.1 Examining Declarations) or AC_TRY_COMPILE (see section 5.2 Examining Syntax).

Macro: AC_HEADER_STAT

If the macros S_ISDIR, S_ISREG et al. defined in `sys/stat.h' do not work properly (returning false positives), define STAT_MACROS_BROKEN. This is the case on Tektronix UTekV, Amdahl UTS and Motorola System V/88.

Macro: AC_HEADER_TIME

If a program may include both `time.h' and `sys/time.h', define TIME_WITH_SYS_TIME. On some older systems, `sys/time.h' includes `time.h', but `time.h' is not protected against multiple inclusion, so programs should not explicitly include both files. This macro is useful in programs that use, for example, struct timeval or struct timezone as well as struct tm. It is best used in conjunction with HAVE_SYS_TIME_H, which can be checked for using AC_CHECK_HEADERS(sys/time.h).

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

Macro: AC_STRUCT_ST_BLKSIZE

If struct stat contains an st_blksize member, define HAVE_ST_BLKSIZE.

Macro: AC_STRUCT_ST_BLOCKS

If struct stat contains an st_blocks member, define HAVE_ST_BLOCKS. Otherwise, add `fileblocks.o' to the output variable LIBOBJS.

Macro: AC_STRUCT_ST_RDEV

If struct stat contains an st_rdev member, define HAVE_ST_RDEV.

Macro: AC_STRUCT_TM

If `time.h' does not define struct tm, define TM_IN_SYS_TIME, which means that including `sys/time.h' had better define struct tm.

Macro: AC_STRUCT_TIMEZONE

Figure out how to get the current timezone. If struct tm has a tm_zone member, define HAVE_TM_ZONE. Otherwise, if the external array tzname is found, define HAVE_TZNAME.

4.6 Typedefs

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

4.6.1 Particular Typedef Checks		Special handling to find certain types.
4.6.2 Generic Typedef Checks		How to find other types.

4.6.1 Particular Typedef Checks

These macros check for particular C typedefs in `sys/types.h' and `stdlib.h' (if it exists).

Macro: AC_TYPE_GETGROUPS

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

Macro: AC_TYPE_MODE_T

If mode_t is not defined, define mode_t to be int.

Macro: AC_TYPE_OFF_T

If off_t is not defined, define off_t to be long.

Macro: AC_TYPE_PID_T

If pid_t is not defined, define pid_t to be int.

Macro: AC_TYPE_SIGNAL

If `signal.h' declares signal as returning a pointer to a function returning void, define RETSIGTYPE to be void; otherwise, define it to be int.

Define signal handlers as returning type RETSIGTYPE:

RETSIGTYPE hup_handler () { ... }

Macro: AC_TYPE_SIZE_T

If size_t is not defined, define size_t to be unsigned.

Macro: AC_TYPE_UID_T

If uid_t is not defined, define uid_t to be int and gid_t to be int.

4.6.2 Generic Typedef Checks

This macro is used to check for typedefs not covered by the particular test macros.

Macro: AC_CHECK_TYPE (type, default)

If the type type is not defined in `sys/types.h', or `stdlib.h' or `stddef.h' if they exist, define it to be the C (or C++) builtin type default; e.g., `short' or `unsigned'.

4.7 C Compiler Characteristics

The following macros check for C compiler or machine architecture features. To check for characteristics not listed here, use AC_TRY_COMPILE (see section 5.2 Examining Syntax) or AC_TRY_RUN (see section 5.4 Checking Run Time Behavior)

Macro: AC_C_BIGENDIAN

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

Macro: AC_C_CONST

If the C compiler does not fully support the keyword const, define const to be empty. Some C compilers that do not define __STDC__ do support const; some compilers that define __STDC__ do not completely support const. Programs can simply use const as if every C compiler supported it; for those that don't, the `Makefile' or configuration header file will define it as empty.

Macro: AC_C_INLINE

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

Macro: AC_C_CHAR_UNSIGNED

If the C type char is unsigned, define __CHAR_UNSIGNED__, unless the C compiler predefines it.

Macro: AC_C_LONG_DOUBLE

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

Macro: AC_C_STRINGIZE

If the C preprocessor supports the stringizing operator, define HAVE_STRINGIZE. The stringizing operator is `#' and is found in macros such as this:

#define x(y) #y

Macro: AC_CHECK_SIZEOF (type [, cross-size])

Define SIZEOF_uctype to be the size in bytes of the C (or C++) builtin type type, e.g. `int' or `char *'. If `type' is unknown to the compiler, it gets a size of 0. uctype is type, with lowercase converted to uppercase, spaces changed to underscores, and asterisks changed to `P'. If cross-compiling, the value cross-size is used if given, otherwise configure exits with an error message.

For example, the call

AC_CHECK_SIZEOF(int *)

defines SIZEOF_INT_P to be 8 on DEC Alpha AXP systems.

Macro: AC_INT_16_BITS

If the C type int is 16 bits wide, define INT_16_BITS. This macro is obsolete; it is more general to use `AC_CHECK_SIZEOF(int)' instead.

Macro: AC_LONG_64_BITS

If the C type long int is 64 bits wide, define LONG_64_BITS. This macro is obsolete; it is more general to use `AC_CHECK_SIZEOF(long)' instead.

4.8 Fortran 77 Compiler Characteristics

The following macros check for Fortran 77 compiler characteristics. To check for characteristics not listed here, use AC_TRY_COMPILE (see section 5.2 Examining Syntax) or AC_TRY_RUN (see section 5.4 Checking Run Time Behavior), making sure to first set the current lanuage to Fortran 77 AC_LANG_FORTRAN77 (see section 5.8 Language Choice).

Macro: AC_F77_LIBRARY_LDFLAGS

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

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

For example, if object files from a C++ and Fortran 77 compiler must be linked together, then the C++ compiler/linker must be used for linking (since special C++-ish things need to happen at link time like calling global constructors, instantiating templates, enabling exception support, etc.).

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

4.9 System Services

The following macros check for operating system services or capabilities.

Macro: AC_CYGWIN

Checks for the Cygwin environment. If present, sets shell variable CYGWIN to `yes'. If not present, sets CYGWIN to the empty string.

Macro: AC_EXEEXT

Defines substitute variable EXEEXT based on the output of the compiler, after .c, .o, and .obj files have been excluded. Typically set to empty string if Unix, `.exe' or `.EXE' if Win32.

Macro: AC_OBJEXT

Defines substitute variable OBJEXT based on the output of the compiler, after .c files have been excluded. Typically set to `.o' if Unix, `.obj' if Win32.

Macro: AC_MINGW32

Checks for the MingW32 compiler environment. If present, sets shell variable MINGW32 to `yes'. If not present, sets MINGW32 to the empty string.

Macro: AC_PATH_X

Try to locate the X Window System include files and libraries. If the user gave the command line options `--x-includes=dir' and `--x-libraries=dir', use those directories. If either or both were not given, get the missing values by running xmkmf on a trivial `Imakefile' and examining the `Makefile' that it produces. If that fails (such as if xmkmf is not present), look for them in several directories where they often reside. If either method is successful, set the shell variables x_includes and x_libraries to their locations, unless they are in directories the compiler searches by default.

If both methods fail, or the user gave the command line option `--without-x', set the shell variable no_x to `yes'; otherwise set it to the empty string.

Macro: AC_PATH_XTRA

An enhanced version of AC_PATH_X. It adds the C compiler flags that X needs to output variable X_CFLAGS, and the X linker flags to X_LIBS. If X is not available, adds `-DX_DISPLAY_MISSING' to X_CFLAGS.

This macro also checks for special libraries that some systems need in order to compile X programs. It adds any that the system needs to output variable X_EXTRA_LIBS. And it checks for special X11R6 libraries that need to be linked with before `-lX11', and adds any found to the output variable X_PRE_LIBS.

Macro: AC_SYS_INTERPRETER

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

Macro: AC_SYS_LONG_FILE_NAMES

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

Macro: AC_SYS_RESTARTABLE_SYSCALLS

If the system automatically restarts a system call that is interrupted by a signal, define HAVE_RESTARTABLE_SYSCALLS.

4.10 UNIX Variants

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

Macro: AC_AIX

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

Macro: AC_DYNIX_SEQ

If on Dynix/PTX (Sequent UNIX), add `-lseq' to output variable LIBS. This macro is obsolete; instead, use AC_FUNC_GETMNTENT.

Macro: AC_IRIX_SUN

If on IRIX (Silicon Graphics UNIX), add `-lsun' to output variable LIBS. This macro is obsolete. If you were using it to get getmntent, use AC_FUNC_GETMNTENT instead. If you used it for the NIS versions of the password and group functions, use `AC_CHECK_LIB(sun, getpwnam)'.

Macro: AC_ISC_POSIX

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

Macro: AC_MINIX

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

Macro: AC_SCO_INTL

If on SCO UNIX, add `-lintl' to output variable LIBS. This macro is obsolete; instead, use AC_FUNC_STRFTIME.

Macro: AC_XENIX_DIR

If on Xenix, add `-lx' to output variable LIBS. Also, if `dirent.h' is being used, add `-ldir' to LIBS. This macro is obsolete; use AC_HEADER_DIRENT instead.

5. Writing Tests

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

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

These macros check the output of the C compiler system. They do not cache the results of their tests for future use (see section 6.3 Caching Results), because they don't know enough about the information they are checking for to generate a cache variable name. They also do not print any messages, for the same reason. The checks for particular kinds of C features call these macros and do cache their results and print messages about what they're checking for.

When you write a feature test that could be applicable to more than one software package, the best thing to do is encapsulate it in a new macro. See section 7. Writing Macros, for how to do that.

5.1 Examining Declarations		Detecting header files and declarations.
5.2 Examining Syntax		Detecting language syntax features.
5.3 Examining Libraries		Detecting functions and global variables.
5.4 Checking Run Time Behavior		Testing for run-time features.
5.5 Portable Shell Programming		Shell script portability pitfalls.
5.6 Testing Values and Files		Checking strings and files.
5.7 Multiple Cases		Tests for several possible values.
5.8 Language Choice		Selecting which language to use for testing.

5.1 Examining Declarations

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

Macro: AC_TRY_CPP (includes, [action-if-true [, action-if-false]])

includes is C or C++ #include statements and declarations, on which shell variable, 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 action-if-true. Otherwise run shell commands action-if-false.

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

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

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

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

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

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

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

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

This macro calls AC_PROG_CPP or AC_PROG_CXXCPP (depending on which language is current, see section 5.8 Language Choice), if it hasn't been called already.

5.2 Examining Syntax

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

Macro: AC_TRY_COMPILE (includes, function-body, [action-if-found [, action-if-not-found]])

Create a C, C++ or Fortran 77 test program (depending on which language is current, see section 5.8 Language Choice), to see whether a function whose body consists of function-body can be compiled.

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

If the file compiles successfully, run shell commands action-if-found, otherwise run action-if-not-found.

This macro does not try to link; use AC_TRY_LINK if you need to do that (see section 5.3 Examining Libraries).

5.3 Examining Libraries

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

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

AC_TRY_LINK is used to compile test programs to test for functions and global variables. It is also used by AC_CHECK_LIB to check for libraries (see section 4.2 Library Files), by adding the library being checked for to LIBS temporarily and trying to link a small program.

Macro: AC_TRY_LINK (includes, function-body, [action-if-found [, action-if-not-found]])

Depending on the current language (see section 5.8 Language Choice), create a test program to see whether a function whose body consists of function-body can be compiled and linked.

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

If the file compiles and links successfully, run shell commands action-if-found, otherwise run action-if-not-found.

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

Depending on the current language (see section 5.8 Language Choice), create a test program to see whether a program whose body consists of a prototype of and a call to function can be compiled and linked.

If the file compiles and links successfully, run shell commands action-if-found, otherwise run action-if-not-found.

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

Attempt to compile and link a small program that links with function. If the file compiles and links successfully, run shell commands action-if-found, otherwise run action-if-not-found.

Macro: AC_COMPILE_CHECK (echo-text, includes, function-body, action-if-found [, action-if-not-found])

This is an obsolete version of AC_TRY_LINK, with the addition that it prints `checking for echo-text' to the standard output first, if echo-text is non-empty. Use AC_MSG_CHECKING and AC_MSG_RESULT instead to print messages (see section 6.4 Printing Messages).

5.4 Checking Run Time Behavior

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

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

5.4.1 Running Test Programs		Running test programs.
5.4.2 Guidelines for Test Programs		General rules for writing test programs.
5.4.3 Test Functions		Avoiding pitfalls in test programs.

5.4.1 Running Test Programs

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

Macro: AC_TRY_RUN (program, [action-if-true [, action-if-false [, action-if-cross-compiling]]])

program is the text of a C program, on which shell variable and backquote substitutions are performed. If it compiles and links successfully and returns an exit status of 0 when executed, run shell commands action-if-true. Otherwise run shell commands action-if-false; the exit status of the program is available in the shell variable `$?'. This macro uses CFLAGS or CXXFLAGS, CPPFLAGS, LDFLAGS, and LIBS when compiling.

If the C compiler being used does not produce executables that run on the system where configure is being run, then the test program is not run. If the optional shell commands action-if-cross-compiling are given, they are run instead. Otherwise, configure prints an error message and exits.

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

To configure for cross-compiling you can also choose a value for those parameters based on the canonical system name (see section 8. Manual Configuration). Alternatively, set up a test results cache file with the correct values for the target system (see section 6.3 Caching Results).

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

Macro: AC_C_CROSS

This macro is obsolete; it does nothing.

5.4.2 Guidelines for Test Programs

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

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

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

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

5.4.3 Test Functions

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

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

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

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

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

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

5.5 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, negated character classes, or other features that are not found in all Bourne-compatible shells; restrict yourself to the lowest common denominator. Even unset is not supported by all shells! Also, include a space after the exclamation point in interpreter specifications, like this:

#! /usr/bin/perl

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

The set of external programs you should run in a configure script is fairly small. See section `Utilities in Makefiles' in 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 ln having a `-f' option or cat having any options. sed scripts should not contain comments or use branch labels longer than 8 characters. Don't use `grep -s' to suppress output, because `grep -s' on System V does not suppress output, only error messages. Instead, redirect the standard output and standard error (in case the file doesn't exist) of grep to `/dev/null'. Check the exit status of grep to determine whether it found a match.

5.6 Testing Values and Files

configure scripts need to test properties of many files and strings. Here are some portability problems to watch out for when doing those tests.

The test program is the way to perform many file and string tests. It is often invoked by the alternate name `[', but using that name in Autoconf code is asking for trouble since it is an m4 quote character.

If you need to make multiple checks using test, combine them with the shell operators `&&' and `||' instead of using the test operators `-a' and `-o'. On System V, the precedence of `-a' and `-o' is wrong relative to the unary operators; consequently, POSIX does not specify them, so using them is nonportable. If you combine `&&' and `||' in the same statement, keep in mind that they have equal precedence.

To enable configure scripts to support cross-compilation, they shouldn't do anything that tests features of the host system instead of the target system. But occasionally you may find it necessary to check whether some arbitrary file exists. To do so, use `test -f' or `test -r'. Do not use `test -x', because 4.3BSD does not have it.

Another nonportable shell programming construction is

var=${var:-value}

The intent is to set var to value only if it is not already set, but if var has any value, even the empty string, to leave it alone. Old BSD shells, including the Ultrix sh, don't accept the colon, and complain and die. A portable equivalent is

: ${var=value}

5.7 Multiple Cases

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

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

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

5.8 Language Choice

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

Macro: AC_LANG_C

Do compilation tests using CC and CPP and use extension `.c' for test programs. Set the shell variable cross_compiling to the value computed by AC_PROG_CC if it has been run, empty otherwise.

Macro: AC_LANG_CPLUSPLUS

Do compilation tests using CXX and CXXCPP and use extension `.C' for test programs. Set the shell variable cross_compiling to the value computed by AC_PROG_CXX if it has been run, empty otherwise.

Macro: AC_LANG_FORTRAN77

Do compilation tests using F77 and use extension `.f' for test programs. Set the shell variable cross_compiling to the value computed by AC_PROG_F77 if it has been run, empty otherwise.

Macro: AC_LANG_SAVE

Remember the current language (as set by AC_LANG_C, AC_LANG_CPLUSPLUS or AC_LANG_FORTRAN77) on a stack. Does not change which language is current. Use this macro and AC_LANG_RESTORE in macros that need to temporarily switch to a particular language.

Macro: AC_LANG_RESTORE

Select the language that is saved on the top of the stack, as set by AC_LANG_SAVE, and remove it from the stack. This macro is equivalent to either AC_LANG_C, AC_LANG_CPLUSPLUS or AC_LANG_FORTRAN77, whichever had been run most recently when AC_LANG_SAVE was last called.

Do not call this macro more times than AC_LANG_SAVE.

Macro: AC_REQUIRE_CPP

Ensure that whichever preprocessor would currently be used for tests has been found. Calls AC_REQUIRE (see section 7.4.1 Prerequisite Macros) with an argument of either AC_PROG_CPP or AC_PROG_CXXCPP, depending on which language is current.

6. Results of Tests

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

6.1 Defining C Preprocessor Symbols		Defining C preprocessor symbols.
6.2 Setting Output Variables		Replacing variables in output files.
6.3 Caching Results		Speeding up subsequent configure runs.
6.4 Printing Messages		Notifying users of progress or problems.

6.1 Defining C Preprocessor Symbols

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

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

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

If AC_CONFIG_HEADER has been called, then instead of creating DEFS, AC_OUTPUT creates a header file by substituting the correct values into #define statements in a template file. See section 3.4 Configuration Header Files, for more information about this kind of output.

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

Define C preprocessor variable variable. If value is given, set variable to that value (verbatim), otherwise set it to 1. value should not contain literal newlines, and if you are not using AC_CONFIG_HEADER it should not contain any `#' characters, as make tends to eat them. To use a shell variable (which you need to do in order to define a value containing the m4 quote characters `[' or `]'), use AC_DEFINE_UNQUOTED instead. description is only useful if you are using AC_CONFIG_HEADER. In this case, description is put into the generated `config.h.in' as the comment before the macro define; the macro need not be mentioned in `acconfig.h'. The following example defines the C preprocessor variable EQUATION to be the string constant `"$a > $b"':

AC_DEFINE(EQUATION, "$a > $b")

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

Like AC_DEFINE, but three shell expansions are performed--once--on variable and value: variable expansion (`$'), command substitution (``'), and backslash escaping (`\'). Single and double quote characters in the value have no special meaning. Use this macro instead of AC_DEFINE when variable or value is a shell variable. Examples:

AC_DEFINE_UNQUOTED(config_machfile, "${machfile}") AC_DEFINE_UNQUOTED(GETGROUPS_T, $ac_cv_type_getgroups) AC_DEFINE_UNQUOTED(${ac_tr_hdr})

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

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

or this:

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

instead of this:

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

6.2 Setting Output Variables

One way to record the results of tests is to set output variables, which are shell variables whose values are substituted into files that configure outputs. The two macros below create new output variables. See section 3.3.1 Preset Output Variables, for a list of output variables that are always available.

Macro: AC_SUBST (variable)

Create an output variable from a shell variable. Make AC_OUTPUT substitute the variable variable into output files (typically one or more `Makefile's). This means that AC_OUTPUT will replace instances of `@variable@' in input files with the value that the shell variable variable has when AC_OUTPUT is called. The value of variable should not contain literal newlines.

Macro: AC_SUBST_FILE (variable)

Another way to create an output variable from a shell variable. Make AC_OUTPUT insert (without substitutions) the contents of the file named by shell variable variable into output files. This means that AC_OUTPUT will replace instances of `@variable@' in output files (such as `Makefile.in') with the contents of the file that the shell variable variable names when AC_OUTPUT is called. Set the variable to `/dev/null' for cases that do not have a file to insert.

This macro is useful for inserting `Makefile' fragments containing special dependencies or other make directives for particular host or target types into `Makefile's. For example, `configure.in' could contain:

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

and then a `Makefile.in' could contain:

@host_frag@

6.3 Caching Results

To avoid checking for the same features repeatedly in various configure scripts (or repeated runs of one script), configure saves the results of many of its checks in a cache file. If, when a 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, configure can run much faster than if it had to perform all of the checks every time.

Macro: AC_CACHE_VAL (cache-id, commands-to-set-it)

Ensure that the results of the check identified by cache-id are available. If the results of the check were in the cache file that was read, and configure was not given the `--quiet' or `--silent' option, print a message saying that the result was cached; otherwise, run the shell commands commands-to-set-it. Those commands should have no side effects except for setting the variable cache-id. In particular, they should not call AC_DEFINE; the code that follows the call to AC_CACHE_VAL should do that, based on the cached value. Also, they should not print any messages, for example with AC_MSG_CHECKING; do that before calling AC_CACHE_VAL, so the messages are printed regardless of whether the results of the check are retrieved from the cache or determined by running the shell commands. If the shell commands are run to determine the value, the value will be saved in the cache file just before configure creates its output files. See section 6.3.1 Cache Variable Names, for how to choose the name of the cache-id variable.

Macro: AC_CACHE_CHECK (message, cache-id, commands)

A wrapper for AC_CACHE_VAL that takes care of printing the messages. This macro provides a convenient shorthand for the most common way to use these macros. It calls AC_MSG_CHECKING for message, then AC_CACHE_VAL with the cache-id and commands arguments, and AC_MSG_RESULT with cache-id.

Macro: AC_CACHE_LOAD

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

Macro: AC_CACHE_SAVE

Flushes all cached values to the cache file. Called automatically from AC_OUTPUT, but it can be quite useful to call AC_CACHE_SAVE at key points in configure.in. Doing so checkpoints the cache in case of an early configure script abort.

6.3.1 Cache Variable Names		Shell variables used in caches.
6.3.2 Cache Files		Files configure uses for caching.

6.3.1 Cache Variable Names

The names of cache variables should have the following format:

package-prefix_cv_value-type_specific-value[_additional-options]

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

package-prefix

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

_cv_

Indicates that this shell variable is a cache value.

value-type

A convention for classifying cache values, to produce a rational naming system. The values used in Autoconf are listed in 7.2 Macro Names.

specific-value

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

additional-options

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

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

6.3.2 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 `./config.cache' as the cache file, creating it if it does not exist already. configure accepts the `--cache-file=file' option to use a different cache file; that is what configure does when it calls configure scripts in subdirectories, so they share the cache. See section 3.5 Configuring Other Packages in Subdirectories, for information on configuring subdirectories with the AC_CONFIG_SUBDIRS macro.

Giving `--cache-file=/dev/null' disables caching, for debugging configure. `config.status' only pays attention to the cache file if it is given the `--recheck' option, which makes it rerun configure. If you are anticipating a long debugging period, you can also disable cache loading and saving for a configure script by redefining the cache macros at the start of `configure.in':

define([AC_CACHE_LOAD], )dnl define([AC_CACHE_SAVE], )dnl AC_INIT(whatever) ... rest of configure.in ...

It is wrong to try to distribute cache files for particular system types. There is too much room for error in doing that, and too much administrative overhead in maintaining them. For any features that can't be guessed automatically, use the standard method of the canonical system type and linking files (see section 8. Manual Configuration).

The cache file on a particular system will gradually accumulate whenever someone runs a configure script; it will be initially nonexistent. Running 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 (see section 9.5 Setting Site Defaults).

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

... AC_INIT, etc. ... dnl checks for programs AC_PROG_CC AC_PROG_GCC_TRADITIONAL ... more program checks ... AC_CACHE_SAVE dnl checks for libraries AC_CHECK_LIB(nsl, gethostbyname) AC_CHECK_LIB(socket, connect) ... more lib checks ... AC_CACHE_SAVE dnl Might abort... AM_PATH_GTK(1.0.2, , exit 1) AM_PATH_GTKMM(0.9.5, , exit 1)

6.4 Printing Messages

configure scripts need to give users running them several kinds of information. The following macros print messages in ways appropriate for each kind. The arguments to all of them get enclosed in shell double quotes, so the shell performs variable and backquote substitution on them. You can print a message containing a comma by quoting the message with the m4 quote characters:

AC_MSG_RESULT([never mind, I found the BASIC compiler])

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

Macro: AC_MSG_CHECKING (feature-description)

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

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

Macro: AC_MSG_RESULT (result-description)

Notify the user of the results of a check. result-description is almost always the value of the cache variable for the check, typically `yes', `no', or a file name. This macro should follow a call to AC_MSG_CHECKING, and the result-description should be the completion of the message printed by the call to AC_MSG_CHECKING.

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

Macro: AC_MSG_ERROR (error-description)

Notify the user of an error that prevents configure from completing. This macro prints an error message on the standard error output and exits configure with a nonzero status. error-description should be something like `invalid value $HOME for \$HOME'.

Macro: AC_MSG_WARN (problem-description)

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

The following two macros are an obsolete alternative to AC_MSG_CHECKING and AC_MSG_RESULT.

Macro: AC_CHECKING (feature-description)

This macro is similar to AC_MSG_CHECKING, except that it prints a newline after the feature-description. It is useful mainly to print a general description of the overall purpose of a group of feature checks, e.g.,

AC_CHECKING(if stack overflow is detectable)

Macro: AC_VERBOSE (result-description)

This macro is similar to AC_MSG_RESULT, except that it is meant to follow a call to AC_CHECKING instead of AC_MSG_CHECKING; it starts the message it prints with a tab. It is considered obsolete.

7. Writing Macros

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

7.1 Macro Definitions		Basic format of an Autoconf macro.
7.2 Macro Names		What to call your new macros.
7.3 Quoting		Protecting macros from unwanted expansion.
7.4 Dependencies Between Macros		What to do when macros depend on other macros.

7.1 Macro Definitions

Autoconf macros are defined using the AC_DEFUN macro, which is similar to the m4 builtin define macro. In addition to defining a macro, AC_DEFUN adds to it some code which is used to constrain the order in which macros are called (see section 7.4.1 Prerequisite Macros).

An Autoconf macro definition looks like this:

AC_DEFUN(macro-name, [macro-body])

The square brackets here do not indicate optional text: they should literally be present in the macro definition to avoid macro expansion problems (see section 7.3 Quoting). You can refer to any arguments passed to the macro as `$1', `$2', etc.

To introduce comments in m4, use the m4 builtin dnl; it causes m4 to discard the text through the next newline. It is not needed between macro definitions in `acsite.m4' and `aclocal.m4', because all output is discarded until AC_INIT is called.

See section `How to define new macros' in GNU m4, for more complete information on writing m4 macros.

7.2 Macro Names

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

Most of the Autoconf macros' names follow a structured naming convention that indicates the kind of feature check by the name. The macro names consist of several words, separated by underscores, going from most general to most specific. The names of their cache variables use the same convention (see section 6.3.1 Cache Variable Names, for more information on them).

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

C

C language builtin features.

DECL

Declarations of C variables in header files.

FUNC

Functions in libraries.

GROUP

UNIX group owners of files.

HEADER

Header files.

LIB

C libraries.

PATH

The full path names to files, including programs.

PROG

The base names of programs.

STRUCT

Definitions of C structures in header files.

SYS

Operating system features.

TYPE

C builtin or declared types.

VAR

C variables in libraries.

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

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, AC_PATH_X has internal macros AC_PATH_X_XMKMF and AC_PATH_X_DIRECT.

7.3 Quoting

Macros that are called by other macros are evaluated by m4 several times; each evaluation might require another layer of quotes to prevent unwanted expansions of macros or m4 builtins, such as `define' and `$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 m4 quote characters from the default ``' and `'' to `[' and `]', because many of the macros use ``' and `'', 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 m4 builtin command changequote to temporarily change the quote characters to `<<' and `>>'. (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:

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)

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

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

7.4 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.

7.4.1 Prerequisite Macros		Ensuring required information.
7.4.2 Suggested Ordering		Warning about possible ordering problems.
7.4.3 Obsolete Macros		Warning about old ways of doing things.

7.4.1 Prerequisite Macros

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

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

Macro: AC_REQUIRE (macro-name)

If the m4 macro macro-name has not already been called, call it (without any arguments). Make sure to quote macro-name with square brackets. macro-name must have been defined using AC_DEFUN or else contain a call to AC_PROVIDE to indicate that it has been called.

An alternative to using AC_DEFUN is to use define and call AC_PROVIDE. Because this technique does not prevent nested messages, it is considered obsolete.

Macro: AC_PROVIDE (this-macro-name)

Record the fact that this-macro-name has been called. this-macro-name should be the name of the macro that is calling AC_PROVIDE. An easy way to get it is from the m4 builtin variable $0, like this:

AC_PROVIDE([$0])

7.4.2 Suggested Ordering

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

Autoconf provides the AC_BEFORE macro to warn users when macros with this kind of dependency appear out of order in a `configure.in' file. The warning occurs when creating configure from `configure.in', not when running configure. For example, AC_PROG_CPP checks whether the C compiler can run the C preprocessor when given the `-E' option. It should therefore be called after any macros that change which C compiler is being used, such as AC_PROG_CC. So AC_PROG_CC contains:

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

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

Macro: AC_BEFORE (this-macro-name, called-macro-name)

Make m4 print a warning message on the standard error output if called-macro-name has already been called. this-macro-name should be the name of the macro that is calling AC_BEFORE. The macro called-macro-name must have been defined using AC_DEFUN or else contain a call to AC_PROVIDE to indicate that it has been called.

7.4.3 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 obsolete; they still work, but are no longer considered the best thing to do. Autoconf provides the AC_OBSOLETE macro to warn users producing configure scripts when they use obsolete macros, to encourage them to modernize. A sample call is:

AC_OBSOLETE([$0], [; use AC_CHECK_HEADERS(unistd.h) instead])dnl

Macro: AC_OBSOLETE (this-macro-name [, suggestion])

Make m4 print a message on the standard error output warning that this-macro-name is obsolete, and giving the file and line number where it was called. this-macro-name should be the name of the macro that is calling AC_OBSOLETE. If suggestion is given, it is printed at the end of the warning message; for example, it can be a suggestion for what to use instead of this-macro-name.

8. Manual Configuration

A few kinds of features can't be guessed automatically by running test programs. For example, the details of the object file format, or special options that need to be passed to the compiler or linker. You can check for such features using ad-hoc means, such as having configure check the output of the uname program, or looking for libraries that are unique to particular systems. However, Autoconf provides a uniform method for handling unguessable features.

8.1 Specifying the System Type		Specifying the system type.
8.2 Getting the Canonical System Type		Getting the canonical system type.
8.3 System Type Variables		Variables containing the system type.
8.4 Using the System Type		What to do with the system type.

8.1 Specifying the System Type

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

cpu-company-system

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

Alternately, the user can specify the system type with command line arguments to configure. Doing so is necessary when cross-compiling. In the most complex case of cross-compiling, three system types are involved. The options to specify them are:

--build=build-type

the type of system on which the package is being configured and compiled (rarely needed);

--host=host-type

the type of system on which the package will run;

--target=target-type

the type of system for which any compiler tools in the package will produce code.

If the user gives 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. 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 configure command line, e.g.,

CC=m68k-coff-gcc configure --target=m68k-coff

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

8.2 Getting the Canonical System Type

The following macros make the system type available to configure scripts. They run the shell script config.guess to determine any values for the host, target, and build types that they need and the user did not specify on the command line. They run config.sub to canonicalize any aliases the user gave. If you use these macros, you must distribute those two shell scripts along with your source code. See section 3.2 Creating Output Files, for information about the AC_CONFIG_AUX_DIR macro which you can use to control which directory configure looks for those scripts in. If you do not use either of these macros, configure ignores any `--host', `--target', and `--build' options given to it.

Macro: AC_CANONICAL_SYSTEM

Determine the system type and set output variables to the names of the canonical system types. See section 8.3 System Type Variables, for details about the variables this macro sets.

Macro: AC_CANONICAL_HOST

Perform only the subset of AC_CANONICAL_SYSTEM relevant to the host type. This is all that is needed for programs that are not part of a compiler toolchain.

Macro: AC_VALIDATE_CACHED_SYSTEM_TUPLE (cmd)

If the cache file is inconsistent with the current host, target and build system types, execute cmd or print a default error message.

8.3 System Type Variables

After calling AC_CANONICAL_SYSTEM, the following output variables contain the system type information. After AC_CANONICAL_HOST, only the host variables below are set.

build, host, target

the canonical system names;

build_alias, host_alias, target_alias

the names the user specified, or the canonical names if config.guess was used;

build_cpu, build_vendor, build_os

host_cpu, host_vendor, host_os

target_cpu, target_vendor, target_os

the individual parts of the canonical names (for convenience).

8.4 Using the System Type

How do you use a canonical system type? Usually, you use it in one or more case statements in `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 `host.h' or `target.c'. The case statement patterns can use shell wildcards to group several cases together, like in this fragment:

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

Macro: AC_LINK_FILES (source..., dest...)

Make AC_OUTPUT link each of the existing files source to the corresponding link name dest. Makes a symbolic link if possible, otherwise a hard link. The dest and source names should be relative to the top level source or build directory. This macro may be called multiple times.

For example, this call:

AC_LINK_FILES(config/${machine}.h config/${obj_format}.h, host.h object.h)

creates in the current directory `host.h', which is a link to `srcdir/config/${machine}.h', and `object.h', which is a link to `srcdir/config/${obj_format}.h'.