# Autoconf

Node:Top, Next:Introduction, Up:(dir)

## Autoconf

- Introduction: Autoconf's purpose, strengths, and weaknesses
- The GNU build system: A set of tools for portable software packages
- Making configure Scripts: How to organize and produce Autoconf scripts
- Setup: Initialization and output
- Existing Tests: Macros that check for particular features
- Writing Tests: How to write new feature checks
- Results: What to do with results from feature checks
- Programming in M4: Layers on top of which Autoconf is written
- Writing Autoconf Macros: Adding new macros to Autoconf
- Portable Shell: Shell script portability pitfalls
- Manual Configuration: Selecting features that can't be guessed
- Site Configuration: Local defaults for `configure`
- Running configure scripts: How to use the Autoconf output
- config.status Invocation: Recreating a configuration
- Obsolete Constructs: Kept for backward compatibility
- Using Autotest: Creating portable test suites
- Questions: Questions about Autoconf, with answers
- History: History of Autoconf
- Copying This Manual: How to make copies of this manual
- Indices: Indices of symbols, concepts, etc.

  --- The Detailed Node Listing ---

  The GNU build system

- Automake: Escaping Makefile hell
- Libtool: Building libraries portably
- Pointers: More info on the GNU build system

  Making `configure` Scripts

- Writing configure.ac: What to put in an Autoconf input file
- autoscan Invocation: Semi-automatic `configure.ac` writing
- ifnames Invocation: Listing the conditionals in source code
- autoconf Invocation: How to create configuration scripts
- autoreconf Invocation: Remaking multiple `configure` scripts

  Writing `configure.ac`

- Shell Script Compiler: Autoconf as solution of a problem
- Autoconf Language: Programming in Autoconf
- configure.ac Layout: Standard organization of configure.ac

  Initialization and Output Files

- Initializing configure: Option processing etc.
- Notices: Copyright, version numbers in `configure`
- Input: Where Autoconf should find files
- Output: Outputting results from the configuration
- Configuration Actions: Preparing the output based on results
- Configuration Files: Creating output files
- Makefile Substitutions: Using output variables in `Makefile`s
- Configuration Headers: Creating a configuration header file
- Configuration Commands: Running arbitrary instantiation commands
- Configuration Links: Links depending from the configuration
- Subdirectories: Configuring independent packages together
- Default Prefix: Changing the default installation prefix

  Substitutions in Makefiles

- Preset Output Variables: Output variables that are always set
- Installation Directory Variables: Other preset output variables
- Build Directories: Supporting multiple concurrent compiles
- Automatic Remaking: Makefile rules for configuring

  Configuration Header Files

- Header Templates: Input for the configuration headers
- autoheader Invocation: How to create configuration templates
- Autoheader Macros: How to specify CPP templates

  Existing Tests

- Common Behavior: Macros' standard schemes
- Alternative Programs: Selecting between alternative programs
- Files: Checking for the existence of files
- Libraries: Library archives that might be missing
- Library Functions: C library functions that might be missing
- Header Files: Header files that might be missing
- Declarations: Declarations that may be missing
- Structures: Structures or members that might be missing
- Types: Types that might be missing
- Compilers and Preprocessors: Checking for compiling programs
- System Services: Operating system services
- UNIX Variants: Special kludges for specific UNIX variants

  Common Behavior

- Standard Symbols: Symbols defined by the macros
- Default Includes: Includes used by the generic macros

  Alternative Programs

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

  Library Functions

- Function Portability: Pitfalls with usual functions
- Particular Functions: Special handling to find certain functions
- Generic Functions: How to find other functions

  Header Files

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

  Declarations

- Particular Declarations: Macros to check for certain declarations
- Generic Declarations: How to find other declarations

  Structures

- Particular Structures: Macros to check for certain structure members
- Generic Structures: How to find other structure members

  Types

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

  Compilers and Preprocessors

- Specific Compiler Characteristics: Some portability issues
- Generic Compiler Characteristics: Language independent tests
- C Compiler: Checking its characteristics
- C++ Compiler: Likewise
- Fortran 77 Compiler: Likewise

  Writing Tests

- Examining Declarations: Detecting header files and declarations
- Examining Syntax: Detecting language syntax features
- Examining Libraries: Detecting functions and global variables
- Run Time: Testing for run-time features
- Systemology: A zoology of operating systems
- Multiple Cases: Tests for several possible values
- Language Choice: Selecting which language to use for testing

  Checking Run Time Behavior

- Test Programs: Running test programs
- Guidelines: General rules for writing test programs
- Test Functions: Avoiding pitfalls in test programs

  Results of Tests

- Defining Symbols: Defining C preprocessor symbols
- Setting Output Variables: Replacing variables in output files
- Caching Results: Speeding up subsequent `configure` runs
- Printing Messages: Notifying `configure` users

  Caching Results

- Cache Variable Names: Shell variables used in caches
- Cache Files: Files `configure` uses for caching
- Cache Checkpointing: Loading and saving the cache file

  Programming in M4

- M4 Quotation: Protecting macros from unwanted expansion
- Invoking autom4te: The Autoconf executables backbone
- Programming in M4sugar: Convenient pure M4 macros
- Programming in M4sh: Common Shell Constructs

  M4 Quotation

- Active Characters: Characters that change the behavior of m4
- One Macro Call: Quotation and one macro call
- Quotation and Nested Macros: Macros calling macros
- Changequote is Evil: Worse than INTERCAL: M4 + changequote
- Quadrigraphs: Another way to escape special characters
- Quotation Rule Of Thumb: One parenthesis, one quote

  Programming in M4sugar

- Redefined M4 Macros: M4 builtins changed in M4sugar
- Evaluation Macros: More quotation and evaluation control
- Forbidden Patterns: Catching unexpanded macros

  Writing Autoconf Macros

- Macro Definitions: Basic format of an Autoconf macro
- Macro Names: What to call your new macros
- Reporting Messages: Notifying `autoconf` users
- Dependencies Between Macros: What to do when macros depend on other macros
- Obsoleting Macros: Warning about old ways of doing things
- Coding Style: Writing Autoconf macros à la Autoconf

  Dependencies Between Macros

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

  Portable Shell Programming

- Shellology: A zoology of shells
- Here-Documents: Quirks and tricks
- File Descriptors: FDs and redirections
- File System Conventions: File- and pathnames
- Shell Substitutions: Variable and command expansions
- Assignments: Varying side effects of assignments
- Special Shell Variables: Variables you should not change
- Limitations of Builtins: Portable use of not so portable /bin/sh
- Limitations of Usual Tools: Portable use of portable tools
- Limitations of Make: Portable Makefiles

  Manual Configuration

- Specifying Names: Specifying the system type
- Canonicalizing: Getting the canonical system type
- Using System Type: What to do with the system type

  Site Configuration

- External Software: Working with other optional software
- Package Options: Selecting optional features
- Pretty Help Strings: Formating help string
- Site Details: Configuring site details
- Transforming Names: Changing program names when installing
- Site Defaults: Giving `configure` local defaults

  Transforming Program Names When Installing

- Transformation Options: `configure` options to transform names
- Transformation Examples: Sample uses of transforming names
- Transformation Rules: `Makefile` uses of transforming names

  Running `configure` Scripts

- Basic Installation: Instructions for typical cases
- Compilers and Options: Selecting compilers and optimization
- Multiple Architectures: Compiling for multiple architectures at once
- Installation Names: Installing in different directories
- Optional Features: Selecting optional features
- System Type: Specifying the system type
- Sharing Defaults: Setting site-wide defaults for `configure`
- Defining Variables: Specifying the compiler etc.
- configure Invocation: Changing how `configure` runs

  Obsolete Constructs

- Obsolete config.status Use: Different calling convention
- acconfig.h: Additional entries in `config.h.in`
- autoupdate Invocation: Automatic update of `configure.ac`
- Obsolete Macros: Backward compatibility macros
- Autoconf 1: Tips for upgrading your files
- Autoconf 2.13: Some fresher tips

  Upgrading From Version 1

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

  Upgrading From Version 2.13

- Changed Quotation: Broken code which used to work
- New Macros: Interaction with foreign macros
- Hosts and Cross-Compilation: Bugward compatibility kludges
- AC_LIBOBJ vs. LIBOBJS:

  Generating Test Suites with Autotest

- Using an Autotest Test Suite: Autotest and the user
- Writing testsuite.at: Autotest macros
- testsuite Invocation: Running `testsuite` scripts
- Making testsuite Scripts: Using autom4te to create `testsuite`

  Using an Autotest Test Suite

- testsuite Scripts: The concepts of Autotest
- Autotest Logs: Their contents

  Questions About Autoconf

- Distributing: Distributing `configure` scripts
- Why GNU m4: Why not use the standard M4?
- Bootstrapping: Autoconf and GNU M4 require each other?
- Why Not Imake: Why GNU uses `configure` instead of Imake

  History of Autoconf

- Genesis: Prehistory and naming of `configure`
- Exodus: The plagues of M4 and Perl
- Leviticus: The priestly code of portability arrives
- Numbers: Growth and contributors
- Deuteronomy: Approaching the promises of easy configuration

  Copying This Manual

- GNU Free Documentation License: License for copying this manual

  Indices

- 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
- Autoconf Macro Index: Index of Autoconf macros
- M4 Macro Index: Index of M4, M4sugar, and M4sh macros
- Autotest Macro Index: Index of Autotest macros
- Program & Function Index: Index of those with portability problems
- Concept Index: General index

Node:Introduction, Next:The GNU build system, Previous:Top, Up:Top

## Introduction

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

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

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

equations, the dual nature of electromagnetic waves, the relativistic

consequences..." "An Engineer!," said the engineer, "because

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

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

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

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

--Anonymous

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

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

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

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

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

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

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

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

See the Autoconf web page for up-to-date information, details on the mailing lists, pointers to a list of known bugs, etc.

Mail suggestions to the Autoconf mailing list.

Bug reports should be preferably submitted to the Autoconf Gnats database, or sent to the Autoconf Bugs mailing list. If possible, first check that your bug is not already solved in current development versions, and that it has not been reported yet. Be sure to include all the needed information and a short `configure.ac` that demonstrates the problem.

Autoconf's development tree is accessible via CVS; see the Autoconf web page for details. There is also a CVSweb interface to the Autoconf development tree. Patches relative to the current CVS version can be sent for review to the Autoconf Patches mailing list.

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

Node:The GNU build system, Next:Making configure Scripts, Previous:Introduction, Up:Top

## The GNU build system

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

- Automake: Escaping Makefile hell
- Libtool: Building libraries portably
- Pointers: More info on the GNU build system

Node:Automake, Next:Libtool, Up:The GNU build system

### Automake

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

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

    bin_PROGRAMS = hello
    hello_SOURCES = hello.c

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

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

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

Node:Libtool, Next:Pointers, Previous:Automake, Up:The GNU build system

### Libtool

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

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

Node:Pointers, Previous:Libtool, Up:The GNU build system

### Pointers

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

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

- Web

  The home pages for Autoconf, Automake, and Libtool.

- Automake Manual

  See Automake, for more information on Automake.

- Books

  The book GNU Autoconf, Automake and Libtool1 describes the complete GNU build environment. You can also find the entire book on-line at "The Goat Book" home page.

- Tutorials and Examples

  The Autoconf Developer Page maintains links to a number of Autoconf/Automake tutorials online, and also links to the Autoconf Macro Archive.

Node:Making configure Scripts, Next:Setup, Previous:The GNU build system, Up:Top

## Making `configure` Scripts

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

- one or more `Makefile` files, one in each subdirectory of the package (see Makefile Substitutions);
- optionally, a C header file, the name of which is configurable, containing `#define` directives (see Configuration Headers);
- a shell script called `config.status` that, when run, will recreate the files listed above (see config.status Invocation);
- an optional shell script normally called `config.cache` (created when using `configure --config-cache`) that saves the results of running many of the tests (see Cache Files);
- a file called `config.log` containing any messages produced by compilers, to help debugging if `configure` makes a mistake.

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

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

Files used in preparing a software package for distribution:

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

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

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

Files used in configuring a software package:

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

- Writing configure.ac: What to put in an Autoconf input file
- autoscan Invocation: Semi-automatic `configure.ac` writing
- ifnames Invocation: Listing the conditionals in source code
- autoconf Invocation: How to create configuration scripts
- autoreconf Invocation: Remaking multiple `configure` scripts

Node:Writing configure.ac, Next:autoscan Invocation, Up:Making configure Scripts

### Writing `configure.ac`

To produce a `configure` script for a software package, create a file called `configure.ac` that contains invocations of the Autoconf macros that test the system features your package needs or can use. Autoconf macros already exist to check for many features; see Existing Tests, for their descriptions. For most other features, you can use Autoconf template macros to produce custom checks; see Writing Tests, for information about them. For especially tricky or specialized features, `configure.ac` might need to contain some hand-crafted shell commands; see Portable Shell. The `autoscan` program can give you a good start in writing `configure.ac` (see autoscan Invocation, for more information).

Previous versions of Autoconf promoted the name `configure.in`, which is somewhat ambiguous (the tool needed to produce this file is not described by its extension), and introduces a slight confusion with `config.h.in` and so on (for which `.in` means "to be processed by `configure`"). Using `configure.ac` is now preferred.

- Shell Script Compiler: Autoconf as solution of a problem
- Autoconf Language: Programming in Autoconf
- configure.ac Layout: Standard organization of configure.ac

Node:Shell Script Compiler, Next:Autoconf Language, Up:Writing configure.ac

#### A Shell Script Compiler

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

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

Naturally, you might then think of shell scripts; who needs `autoconf`? A set of properly written shell functions is enough to make it easy to write `configure` scripts by hand. Sigh! Unfortunately, shell functions do not belong to the least common denominator; therefore, where you would like to define a function and use it ten times, you would instead need to copy its body ten times.

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

How does `autoconf` perform this task?

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

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

Node:Autoconf Language, Next:configure.ac Layout, Previous:Shell Script Compiler, Up:Writing configure.ac

#### The Autoconf Language

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

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

For instance:

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

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

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

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

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

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

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

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

which will result in

    echo "Hard rock was here!  --AC_DC"

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

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

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

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

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

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

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

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

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

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

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

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

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

Node:configure.ac Layout, Previous:Autoconf Language, Up:Writing configure.ac

#### Standard `configure.ac` Layout

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

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

    Autoconf requirements
    AC_INIT(package, version, bug-report-address)
    information on the package
    checks for programs
    checks for libraries
    checks for header files
    checks for types
    checks for structures
    checks for compiler characteristics
    checks for library functions
    checks for system services
    AC_CONFIG_FILES([file...])
    AC_OUTPUT

Node:autoscan Invocation, Next:ifnames Invocation, Previous:Writing configure.ac, Up:Making configure Scripts

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

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

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

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

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

`autoscan` accepts the following options:

`--help`
`-h`
  Print a summary of the command line options and exit.
`--version`
`-V`
  Print the version number of Autoconf and exit.
`--verbose`
`-v`
  Print the names of the files it examines and the potentially interesting symbols it finds in them. This output can be voluminous.
`--include=dir`
`-I dir`
  Also look for input files in dir. Multiple invocations accumulate. Directories are browsed from last to first.

Node:ifnames Invocation, Next:autoconf Invocation, Previous:autoscan Invocation, Up:Making configure Scripts

### Using `ifnames` to List Conditionals

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

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

`ifnames` accepts the following options:

`--help`
`-h`
  Print a summary of the command line options and exit.
`--version`
`-V`
  Print the version number of Autoconf and exit.

Node:autoconf Invocation, Next:autoreconf Invocation, Previous:ifnames Invocation, Up:Making configure Scripts

### Using `autoconf` to Create `configure`

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

The Autoconf macros are defined in several files. Some of the files are distributed with Autoconf; `autoconf` reads them first. Then it looks for the optional file `acsite.m4` in the directory that contains the distributed Autoconf macro files, and for the optional file `aclocal.m4` in the current directory. Those files can contain your site's or the package's own Autoconf macro definitions (see Writing Autoconf Macros, for more information). If a macro is defined in more than one of the files that `autoconf` reads, the last definition it reads overrides the earlier ones.

`autoconf` accepts the following options:

`--help`
`-h`
  Print a summary of the command line options and exit.
`--version`
`-V`
  Print the version number of Autoconf and exit.
`--verbose`
`-v`
  Report processing steps.
`--debug`
`-d`
  Don't remove the temporary files.
`--force`
`-f`
  Remake `configure` even if newer than its input files.
`--include=dir`
`-I dir`
  Also look for input files in dir. Multiple invocations accumulate. Directories are browsed from last to first.
`--output=file`
`-o file`
  Save output (script or trace) to file. The file `-` stands for the standard output.
`--warnings=category`
`-W category`
  Report the warnings related to category (which can actually be a comma separated list). See Reporting Messages, macro `AC_DIAGNOSE`, for a comprehensive list of categories. Special values include:
  `all`
    report all the warnings
  `none`
    report none
  `error`
    treats warnings as errors
  `no-category`
    disable warnings falling into category

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

      autoconf --warnings=syntax,$WARNINGS,category

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

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

      AC_DEFUN([INNER],
      [AC_TRY_RUN([exit (0)])])

      AC_DEFUN([OUTER],
      [INNER])

      AC_INIT
      OUTER

  you get:

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

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

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

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

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

The format of `--trace` can use the following special escapes:

`$$`
  The character `$`.
`$f`
  The filename from which macro is called.
`$l`
  The line number from which macro is called.
`$d`
  The depth of the macro call. This is an M4 technical detail that you probably don't want to know about.
`$n`
  The name of the macro.
`$num`
  The numth argument of the call to macro.
`$@`
`$sep@`
`${separator}@`
  All the arguments passed to macro, separated by the character sep or the string separator (`,` by default). Each argument is quoted, i.e. enclosed in a pair of square brackets.
`$*`
`$sep*`
`${separator}*`
  As above, but the arguments are not quoted.
`$%`
`$sep%`
`${separator}%`
  As above, but the arguments are not quoted, all new line characters in the arguments are smashed, and the default separator is `:`.

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

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

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

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

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

The format gives you a lot of freedom:

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

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

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

Node:autoreconf Invocation, Previous:autoconf Invocation, Up:Making configure Scripts

### Using `autoreconf` to Update `configure` Scripts

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

It runs `autoconf`, `autoheader`, `aclocal`, `automake`, `libtoolize`, and `gettextize` (when appropriate) repeatedly to update the GNU Build System in specified directories, and their subdirectories (see Subdirectories). By default, it only remakes those files that are older than their sources.

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

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

`autoreconf` accepts the following options:

`--help`
`-h`
  Print a summary of the command line options and exit.
`--version`
`-V`
  Print the version number of Autoconf and exit.
`--verbose`
  Print the name of each directory where `autoreconf` runs `autoconf` (and `autoheader`, if appropriate).
`--debug`
`-d`
  Don't remove the temporary files.
`--force`
`-f`
  Remake even `configure` scripts and configuration headers that are newer than their input files (`configure.ac` and, if present, `aclocal.m4`).
`--install`
`-i`
  Copy missing auxiliary files. This option is similar to the option `--add-missing` in `automake`.
`--symlink`
`-s`
  Instead of copying missing auxiliary files, install symbolic links.
`--include=dir`
`-I dir`
  Also look for input files in dir. Multiple invocations accumulate. Directories are browsed from last to first.

Node:Setup, Next:Existing Tests, Previous:Making configure Scripts, Up:Top

## Initialization and Output Files

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

- Initializing configure: Option processing etc.
- Notices: Copyright, version numbers in `configure`
- Input: Where Autoconf should find files
- Output: Outputting results from the configuration
- Configuration Actions: Preparing the output based on results
- Configuration Files: Creating output files
- Makefile Substitutions: Using output variables in `Makefile`s
- Configuration Headers: Creating a configuration header file
- Configuration Commands: Running arbitrary instantiation commands
- Configuration Links: Links depending from the configuration
- Subdirectories: Configuring independent packages together
- Default Prefix: Changing the default installation prefix

Node:Initializing configure, Next:Notices, Up:Setup

### Initializing `configure`

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

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

Process any command-line arguments and perform various initializations and verifications. Set the name of the package and its version. These are typically used in `--version` support, including that of `configure`. The optional argument bug-report should be the email to which users should send bug reports. The package tarname differs from package: the latter designates the full package name (e.g., `GNU Autoconf`), while the latter is meant for distribution tar ball names (e.g., `autoconf`). It defaults to package once `GNU` strip, lower cased, and all non alphanumeric character mapped onto `-`. It is preferable that these arguments be static, i.e., there should not be any shell computation, but they can be computed by M4. The following M4 macros (e.g., `AC_PACKAGE_NAME`), output variables (e.g., `PACKAGE_NAME`), and preprocessor symbols (e.g., `PACKAGE_NAME`) are then defined: `AC_PACKAGE_NAME`, `PACKAGE_NAME` Exactly package. `AC_PACKAGE_TARNAME`, `PACKAGE_TARNAME` Exactly tarname. `AC_PACKAGE_VERSION`, `PACKAGE_VERSION` Exactly version. `AC_PACKAGE_STRING`, `PACKAGE_STRING` Exactly `package version`. `AC_PACKAGE_BUGREPORT`, `PACKAGE_BUGREPORT` Exactly bug-report.

Node:Notices, Next:Input, Previous:Initializing configure, Up:Setup

### Notices in `configure`

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

AC_PREREQ (version) | Macro

Ensure that a recent enough version of Autoconf is being used. If the version of Autoconf being used to create `configure` is earlier than version, print an error message to the standard error output and do not create `configure`. For example:
    AC_PREREQ(2.53)
This macro is the only macro that may be used before `AC_INIT`, but for consistency, you are invited not to do so.

AC_COPYRIGHT (copyright-notice) | Macro

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

AC_REVISION (revision-info) | Macro

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

Node:Input, Next:Output, Previous:Notices, Up:Setup

### Finding `configure` Input

AC_CONFIG_SRCDIR (unique-file-in-source-dir) | Macro

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

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

AC_CONFIG_AUX_DIR (dir) | Macro

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

Node:Output, Next:Configuration Actions, Previous:Input, Up:Setup

### Outputting Files

Every Autoconf script, e.g., `configure.ac`, should finish by calling `AC_OUTPUT`. It is the macro that generates `config.status`, which will create the `Makefile`s and any other files resulting from configuration. The only required macro is `AC_INIT` (see Input).

AC_OUTPUT | Macro

Generate `config.status` and launch it. Call this macro once, at the end of `configure.ac`. `config.status` will take all the configuration actions: all the output files (see Configuration Files, macro `AC_CONFIG_FILES`), header files (see Configuration Headers, macro `AC_CONFIG_HEADERS`), commands (see Configuration Commands, macro `AC_CONFIG_COMMANDS`), links (see Configuration Links, macro `AC_CONFIG_LINKS`), subdirectories to configure (see Subdirectories, macro `AC_CONFIG_SUBDIRS`) are honored.

Historically, the usage of `AC_OUTPUT` was somewhat different. See Obsolete Macros, for a description of the arguments that `AC_OUTPUT` used to support.

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

AC_PROG_MAKE_SET | Macro

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@

Node:Configuration Actions, Next:Configuration Files, Previous:Output, Up:Setup

### Taking Configuration Actions

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

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

    AC_CONFIG_FOOS(tag..., [commands], [init-cmds])

where the arguments are:

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

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

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

  and use this instead:

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

  The macros `AC_CONFIG_FILES` and `AC_CONFIG_HEADERS` use special tags: they may have the form `output` or `output:inputs`. The file output is instantiated from its templates, inputs (defaulting to `output.in`).

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

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

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

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

  The variable set during the execution of `configure` are not available here: you first need to set them via the init-cmds. Nonetheless the following variables are precomputed:

  `srcdir`
    The path from the top build directory to the top source directory. This is what `configure`'s option `--srcdir` sets.
  `ac_top_srcdir`
    The path from the current build directory to the top source directory.
  `ac_top_builddir`
    The path from the current build directory to the top build directory. It can be empty, or else ends with a slash, so that you may concatenate it.
  `ac_srcdir`
    The path from the current build directory to the corresponding source directory.

  The current directory refers to the directory (or pseudo-directory) containing the input part of tags. For instance, running

      AC_CONFIG_COMMANDS([deep/dir/out:in/in.in], [...], [...])

  with `--srcdir=../package` produces the following values:

      # Argument of --srcdir
      srcdir='../package'
      # Reversing deep/dir
      ac_top_builddir='../../'
      # Concatenation of $ac_top_builddir and srcdir
      ac_top_srcdir='../../../package'
      # Concatenation of $ac_top_srcdir and deep/dir
      ac_srcdir='../../../package/deep/dir'

  independently of `in/in.in`.

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

  You should be extremely cautious in your variable names: all the init-cmds share the same name space and may overwrite each other in unpredictable ways. Sorry...

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

Node:Configuration Files, Next:Makefile Substitutions, Previous:Configuration Actions, Up:Setup

### Creating Configuration Files

Be sure to read the previous section, Configuration Actions.

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

Make `AC_OUTPUT` create each `file` by copying an input file (by default `file.in`), substituting the output variable values. This macro is one of the instantiating macros, see Configuration Actions. See Makefile Substitutions, for more information on using output variables. See Setting Output Variables, for more information on creating them. This macro creates the directory that the file is in if it doesn't exist. Usually, `Makefile`s are created this way, but other files, such as `.gdbinit`, can be specified as well. Typical calls to `AC_CONFIG_FILES` look like this:
    AC_CONFIG_FILES([Makefile src/Makefile man/Makefile X/Imakefile])
    AC_CONFIG_FILES([autoconf], [chmod +x autoconf])
You can override an input file name by appending to file a colon-separated list of input files. Examples:
    AC_CONFIG_FILES([Makefile:boiler/top.mk:boiler/bot.mk]
                    [lib/Makefile:boiler/lib.mk])
Doing this allows you to keep your file names acceptable to MS-DOS, or to prepend and/or append boilerplate to the file.

Node:Makefile Substitutions, Next:Configuration Headers, Previous:Configuration Files, Up:Setup

### Substitutions in Makefiles

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

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

See Makefile Conventions, for more information on what to put in `Makefile`s.

- Preset Output Variables: Output variables that are always set
- Installation Directory Variables: Other preset output variables
- Build Directories: Supporting multiple concurrent compiles
- Automatic Remaking: Makefile rules for configuring

Node:Preset Output Variables, Next:Installation Directory Variables, Up:Makefile Substitutions

#### Preset Output Variables

Some output variables are preset by the Autoconf macros. Some of the Autoconf macros set additional output variables, which are mentioned in the descriptions for those macros. See Output Variable Index, for a complete list of output variables. See Installation Directory Variables, for the list of the preset ones related to installation directories. Below are listed the other preset ones. They all are precious variables (see Setting Output Variables, `AC_ARG_VAR`).

CFLAGS | Variable

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.

configure_input | Variable

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.

CPPFLAGS | Variable

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

CXXFLAGS | Variable

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.

DEFS | Variable

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

ECHO_C | Variable
ECHO_N | Variable
ECHO_T | Variable

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

FFLAGS | Variable

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.

LDFLAGS | Variable

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

LIBS | Variable

`-l` options to pass to the linker. The default value is empty, but some Autoconf macros may prepend extra libraries to this variable if those libraries are found and provide necessary functions, see Libraries. `configure` uses this variable when linking programs to test for C, C++ and Fortran 77 features.

builddir | Variable

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

abs_builddir | Variable

Absolute path of `builddir`.

top_builddir | Variable

The relative path to the top-level of the current build tree. In the top-level directory, this is the same as `srcbuild`.

abs_top_builddir | Variable

Absolute path of `top_builddir`.

srcdir | Variable

The relative path to the directory that contains the source code for that `Makefile`.

abs_srcdir | Variable

Absolute path of `srcdir`.

top_srcdir | Variable

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

abs_top_srcdir | Variable

Absolute path of `top_srcdir`.

Node:Installation Directory Variables, Next:Build Directories, Previous:Preset Output Variables, Up:Makefile Substitutions

#### Installation Directory Variables

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

bindir | Variable

The directory for installing executables that users run.

datadir | Variable

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

exec_prefix | Variable

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

includedir | Variable

The directory for installing C header files.

infodir | Variable

The directory for installing documentation in Info format.

libdir | Variable

The directory for installing object code libraries.

libexecdir | Variable

The directory for installing executables that other programs run.

localstatedir | Variable

The directory for installing modifiable single-machine data.

mandir | Variable

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

oldincludedir | Variable

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

prefix | Variable

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

sbindir | Variable

The directory for installing executables that system administrators run.

sharedstatedir | Variable

The directory for installing modifiable architecture-independent data.

sysconfdir | Variable

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

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

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

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

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

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

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

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

Three things are noteworthy:

`@datadir\@`
  The backslash prevents `configure` from replacing `@datadir@` in the sed expression itself.
`$(pkgdatadir)`
  Don't use `@pkgdatadir@`! Use the matching makefile variable instead.
`,`
  Don't use `/` in the sed expression(s) since most probably the variables you use, such as `$(pkgdatadir)`, will contain some.

Node:Build Directories, Next:Automatic Remaking, Previous:Installation Directory Variables, Up:Makefile Substitutions

#### Build Directories

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

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

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

    srcdir = @srcdir@
    VPATH = @srcdir@

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

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

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

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

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

Node:Automatic Remaking, Previous:Build Directories, Up:Makefile Substitutions

#### Automatic Remaking

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

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

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

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

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

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

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

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

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

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

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

Node:Configuration Headers, Next:Configuration Commands, Previous:Makefile Substitutions, Up:Setup

### Configuration Header Files

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

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

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

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

See Configuration Actions, for more details on header.

- Header Templates: Input for the configuration headers
- autoheader Invocation: How to create configuration templates
- Autoheader Macros: How to specify CPP templates

Node:Header Templates, Next:autoheader Invocation, Up:Configuration Headers

#### Configuration Header Templates

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

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

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

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

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

    #include <conf.h>

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

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

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

Node:autoheader Invocation, Next:Autoheader Macros, Previous:Header Templates, Up:Configuration Headers

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

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

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

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

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

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

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

`autoheader` accepts the following options:

`--help`
`-h`
  Print a summary of the command line options and exit.
`--version`
`-V`
  Print the version number of Autoconf and exit.
`--verbose`
`-v`
  Report processing steps.
`--debug`
`-d`
  Don't remove the temporary files.
`--force`
`-f`
  Remake the template file even if newer than its input files.
`--include=dir`
`-I dir`
  Also look for input files in dir. Multiple invocations accumulate. Directories are browsed from last to first.
`--warnings=category`
`-W category`
  Report the warnings related to category (which can actually be a comma separated list). Current categories include:
  `obsolete`
    report the uses of obsolete constructs
  `all`
    report all the warnings
  `none`
    report none
  `error`
    treats warnings as errors
  `no-category`
    disable warnings falling into category

Node:Autoheader Macros, Previous:autoheader Invocation, Up:Configuration Headers

#### Autoheader Macros

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

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

AH_VERBATIM (key, template) | Macro

Tell `autoheader` to include the template as-is in the header template file. This template is associated with the key, which is used to sort all the different templates and guarantee their uniqueness. It should be the symbol that can be `AC_DEFINE`'d. For example:
    AH_VERBATIM([_GNU_SOURCE],
    [/* Enable GNU extensions on systems that have them.  */
    #ifndef _GNU_SOURCE
    # define _GNU_SOURCE
    #endif])

AH_TEMPLATE (key, description) | Macro

Tell `autoheader` to generate a template for key. This macro generates standard templates just like `AC_DEFINE` when a description is given. For example:
    AH_TEMPLATE([CRAY_STACKSEG_END],
                [Define to one of _getb67, GETB67, getb67
                 for Cray-2 and Cray-YMP systems.  This
                 function is required for alloca.c support
                 on those systems.])
will generate the following template, with the description properly justified.
    /* Define to one of _getb67, GETB67, getb67 for Cray-2 and
       Cray-YMP systems. This function is required for alloca.c
       support on those systems. */
    #undef CRAY_STACKSEG_END

AH_TOP (text) | Macro

Include text at the top of the header template file.

AH_BOTTOM (text) | Macro

Include text at the bottom of the header template file.

Node:Configuration Commands, Next:Configuration Links, Previous:Configuration Headers, Up:Setup

### Running Arbitrary Configuration Commands

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

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

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

AC_CONFIG_COMMANDS_PRE (cmds) | Macro

Execute the cmds right before creating `config.status`. A typical use is computing values derived from variables built during the execution of `configure`:
    AC_CONFIG_COMMANDS_PRE(
    [LTLIBOBJS=`echo $LIBOBJS | sed 's/\.o/\.lo/g'`
    AC_SUBST(LTLIBOBJS)])

AC_CONFIG_COMMANDS_POST (cmds) | Macro

Execute the cmds right after creating `config.status`.

Node:Configuration Links, Next:Subdirectories, Previous:Configuration Commands, Up:Setup

### Creating Configuration Links

You may find it convenient to create links whose destinations depend upon results of tests. One can use `AC_CONFIG_COMMANDS` but the creation of relative symbolic links can be delicate when the package is built in another directory than its sources.

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

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

Node:Subdirectories, Next:Default Prefix, Previous:Configuration Links, Up:Setup

### 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.

AC_CONFIG_SUBDIRS (dir ...) | Macro

Make `AC_OUTPUT` run `configure` in each subdirectory dir in the given whitespace-separated list. Each dir should be a literal, i.e., please do not use:
    if test "$package_foo_enabled" = yes; then
      $my_subdirs="$my_subdirs foo"
    fi
    AC_CONFIG_SUBDIRS($my_subdirs)
because this prevents `./configure --help=recursive` from displaying the options of the package `foo`. Rather, you should write:
    if test "$package_foo_enabled" = yes; then
      AC_CONFIG_SUBDIRS(foo)
    fi
If a given dir is not found, an error is reported: if the subdirectory is optional, write:
    if test -d $srcdir/foo; then
      AC_CONFIG_SUBDIRS(foo)
    fi
If a given dir contains `configure.gnu`, it is run instead of `configure`. This is for packages that might use a non-autoconf script `Configure`, which can't be called through a wrapper `configure` since it would be the same file on case-insensitive filesystems. Likewise, if a dir contains `configure.ac` but no `configure`, the Cygnus `configure` script found by `AC_CONFIG_AUX_DIR` is used. The subdirectory `configure` scripts are given the same command line options that were given to this `configure` script, with minor changes if needed, which include: adjusting a relative path for the cache file; adjusting a relative path for the source directory; propagating the current value of `$prefix`, including if it was defaulted, and if default values of the top level and of sub directory `configure` differ. 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.

Node:Default Prefix, Previous:Subdirectories, Up:Setup

### 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.

AC_PREFIX_DEFAULT (prefix) | Macro

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`.

AC_PREFIX_PROGRAM (program) | Macro

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`.

Node:Existing Tests, Next:Writing Tests, Previous:Setup, Up:Top

## Existing Tests

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

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

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

- Common Behavior: Macros' standard schemes
- Alternative Programs: Selecting between alternative programs
- Files: Checking for the existence of files
- Libraries: Library archives that might be missing
- Library Functions: C library functions that might be missing
- Header Files: Header files that might be missing
- Declarations: Declarations that may be missing
- Structures: Structures or members that might be missing
- Types: Types that might be missing
- Compilers and Preprocessors: Checking for compiling programs
- System Services: Operating system services
- UNIX Variants: Special kludges for specific UNIX variants

Node:Common Behavior, Next:Alternative Programs, Up:Existing Tests

### Common Behavior

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

- Standard Symbols: Symbols defined by the macros
- Default Includes: Includes used by the generic macros

Node:Standard Symbols, Next:Default Includes, Up:Common Behavior

#### Standard Symbols

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

For instance,

    AC_CHECK_TYPES(struct $Expensive*)

will define the symbol `HAVE_STRUCT__EXPENSIVEP` if the check succeeds.

Node:Default Includes, Previous:Standard Symbols, Up:Common Behavior

#### Default Includes

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

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

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

Most generic macros provide the following default set of includes:

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

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

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

Node:Alternative Programs, Next:Files, Previous:Common Behavior, Up:Existing Tests

### Alternative Programs

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

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

Node:Particular Programs, Next:Generic Programs, Up:Alternative Programs

#### Particular Program Checks

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

AC_PROG_AWK | Macro

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

AC_PROG_INSTALL | Macro

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

AC_PROG_LEX | Macro

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

AC_PROG_LN_S | Macro

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

AC_PROG_RANLIB | Macro

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

AC_PROG_YACC | Macro

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`.

Node:Generic Programs, Previous:Particular Programs, Up:Alternative Programs

#### Generic Program and File Checks

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

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

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

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

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.

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

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

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

Like `AC_CHECK_PROG`, but first looks for prog-to-check-for with a prefix of the host type as determined by `AC_CANONICAL_HOST`, followed by a dash (see Canonicalizing). For example, if the user runs `configure --host=i386-gnu`, then this call:
    AC_CHECK_TOOL(RANLIB, ranlib, :)
sets `RANLIB` to `i386-gnu-ranlib` if that program exists in `PATH`, or otherwise to `ranlib` if that program exists in `PATH`, or to `:` if neither program exists.

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

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

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

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

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

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

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

Like `AC_CHECK_TOOL`, but set variable to the entire path of the program if it is found.

Node:Files, Next:Libraries, Previous:Alternative Programs, Up:Existing Tests

### Files

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

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

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.

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

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

Node:Libraries, Next:Library Functions, Previous:Files, Up:Existing Tests

### Library Files

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

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

Depending on the current language(see Language Choice), try to ensure that the C, C++, or Fortran 77 function function is available by checking whether a test program can be linked with the library library to get the function. library is the base name of the library; e.g., to check for `-lmp`, use `mp` as the library argument. action-if-found is a list of shell commands to run if the link with the library succeeds; action-if-not-found is a list of shell commands to run if the link fails. If action-if-found is not specified, the default action will prepend `-llibrary` to `LIBS` and define `HAVE_LIBlibrary` (in all capitals). This macro is intended to support building of `LIBS` in a right-to-left (least-dependent to most-dependent) fashion such that library dependencies are satisfied as a natural side-effect of consecutive tests. Some linkers are very sensitive to library ordering so the order in which `LIBS` is generated is important to reliable detection of libraries. If linking with library results in unresolved symbols that would be resolved by linking with additional libraries, give those libraries as the other-libraries argument, separated by spaces: e.g. `-lXt -lX11`. Otherwise, this macro will fail to detect that library is present, because linking the test program will always fail with unresolved symbols. The other-libraries argument should be limited to cases where it is desirable to test for one library in the presence of another that is not already in `LIBS`.

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

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

Node:Library Functions, Next:Header Files, Previous:Libraries, Up:Existing Tests

### 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.

- Function Portability: Pitfalls with usual functions
- Particular Functions: Special handling to find certain functions
- Generic Functions: How to find other functions

Node:Function Portability, Next:Particular Functions, Up:Library Functions

#### Portability of C Functions

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

`snprintf`
  The ISO C99 standard says that if the output array isn't big enough and if no other errors occur, `snprintf` and `vsnprintf` truncate the output and return the number of bytes that ought to have been produced. Some older systems return the truncated length (e.g., GNU C Library 2.0.x or IRIX 6.5), some a negative value (e.g., earlier GNU C Library versions), and some the buffer length without truncation (e.g., 32-bit Solaris 7). Also, some buggy older systems ignore the length and overrun the buffer (e.g., 64-bit Solaris 7).
`sprintf`
  The ISO C standard says `sprintf` and `vsprintf` return the number of bytes written, but on some old systems (SunOS 4 for instance) they return the buffer pointer instead.
`sscanf`
  On various old systems, e.g. HP-UX 9, `sscanf` requires that its input string is writable (though it doesn't actually change it). This can be a problem when using `gcc` since it normally puts constant strings in read-only memory (see Incompatibilities of GCC). Apparently in some cases even having format strings read-only can be a problem.
`strnlen`
  AIX 4.3 provides a broken version which produces funny results:

      strnlen ("foobar", 0) = 0
      strnlen ("foobar", 1) = 3
      strnlen ("foobar", 2) = 2
      strnlen ("foobar", 3) = 1
      strnlen ("foobar", 4) = 0
      strnlen ("foobar", 5) = 6
      strnlen ("foobar", 6) = 6
      strnlen ("foobar", 7) = 6
      strnlen ("foobar", 8) = 6
      strnlen ("foobar", 9) = 6

`unlink`
  The POSIX spec says that `unlink` causes the given files to be removed only after there are no more open file handles for it. Not all OS's support this behaviour though. So even on systems that provide `unlink`, you cannot portably assume it is OK to call it on files that are open. For example, on Windows 9x and ME, such a call would fail; on DOS it could even lead to file system corruption, as the file might end up being written to after the OS has removed it.
`va_copy`
  The ISO C99 standard provides `va_copy` for copying `va_list` variables. It may be available in older environments too, though possibly as `__va_copy` (eg. `gcc` in strict C89 mode). These can be tested with `#ifdef`. A fallback to `memcpy (&dst, &src, sizeof(va_list))` will give maximum portability.
`va_list`
  `va_list` is not necessarily just a pointer. It can be a `struct` (eg. `gcc` on Alpha), which means `NULL` is not portable. Or it can be an array (eg. `gcc` in some PowerPC configurations), which means as a function parameter it can be effectively call-by-reference and library routines might modify the value back in the caller (eg. `vsnprintf` in the GNU C Library 2.1).
Signed `>>`
  Normally the C `>>` right shift of a signed type replicates the high bit, giving a so-called "arithmetic" shift. But care should be taken since the ISO C standard doesn't require that behaviour. On those few processors without a native arithmetic shift (for instance Cray vector systems) zero bits may be shifted in, the same as a shift of an unsigned type.

Node:Particular Functions, Next:Generic Functions, Previous:Function Portability, Up:Library Functions

#### Particular Function Checks

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

AC_FUNC_ALLOCA | Macro

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

AC_FUNC_CHOWN | Macro

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

AC_FUNC_CLOSEDIR_VOID | Macro

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.

AC_FUNC_ERROR_AT_LINE | Macro

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

AC_FUNC_FNMATCH | Macro

If the `fnmatch` function is available and works (unlike the one on Solaris 2.4), define `HAVE_FNMATCH`.

AC_FUNC_FORK | Macro

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

AC_FUNC_FSEEKO | Macro

If the `fseeko` function is available, define `HAVE_FSEEKO`. Define `_LARGEFILE_SOURCE` if necessary.

AC_FUNC_GETGROUPS | Macro

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

AC_FUNC_GETLOADAVG | Macro

Check how to get the system load averages. If the system has the `getloadavg` function, define `HAVE_GETLOADAVG`, and set `GETLOADAVG_LIBS` to any libraries needed to get that function. Also add `GETLOADAVG_LIBS` to `LIBS`. Otherwise, require an `AC_LIBOBJ` replacement (`getloadavg.c`) of `getloadavg`, and possibly define several other C preprocessor macros and output variables: Define `C_GETLOADAVG`. Define `SVR4`, `DGUX`, `UMAX`, or `UMAX4_3` if on those systems. If `nlist.h` is found, define `NLIST_STRUCT`. If `struct nlist` has an `n_un.n_name` member, define `HAVE_STRUCT_NLIST_N_UN_N_NAME`. The obsolete symbol `NLIST_NAME_UNION` is still defined, but do not depend upon it. Programs may need to be installed setgid (or setuid) for `getloadavg` to work. In this case, define `GETLOADAVG_PRIVILEGED`, set the output variable `NEED_SETGID` to `true` (and otherwise to `false`), and set `KMEM_GROUP` to the name of the group that should own the installed program.

AC_FUNC_GETMNTENT | Macro

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`.

AC_FUNC_GETPGRP | Macro

Define `GETPGRP_VOID` if it is an error to pass 0 to `getpgrp`; this is the POSIX.1 behavior. On older BSD systems, you must pass 0 to `getpgrp`, as it takes an argument and behaves like POSIX.1's `getpgid`.
    #if GETPGRP_VOID
      pid = getpgrp ();
    #else
      pid = getpgrp (0);
    #endif
This macro does not check whether `getpgrp` exists at all; if you need to work in that situation, first call `AC_CHECK_FUNC` for `getpgrp`.

AC_FUNC_LSTAT_FOLLOWS_SLASHED_SYMLINK | Macro

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

AC_FUNC_MALLOC | Macro

If the `malloc` works correctly (`malloc (0)` returns a valid pointer), define `HAVE_MALLOC`.

AC_FUNC_MEMCMP | Macro

If the `memcmp` function is not available, or does not work on 8-bit data (like the one on SunOS 4.1.3), or fails when comparing 16 bytes or more and with at least one buffer not starting on a 4-byte boundary (such as the one on NeXT x86 OpenStep), require an `AC_LIBOBJ` replacement for `memcmp`.

AC_FUNC_MKTIME | Macro

If the `mktime` function is not available, or does not work correctly, require an `AC_LIBOBJ` replacement for `mktime`.

AC_FUNC_MMAP | Macro

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

AC_FUNC_OBSTACK | Macro

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

AC_FUNC_SELECT_ARGTYPES | Macro

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

AC_FUNC_SETPGRP | Macro

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

AC_FUNC_STAT | Macro
AC_FUNC_LSTAT | Macro

Determine whether `stat` or `lstat` have the bug that it succeeds when given the zero-length file name argument. The `stat` and `lstat` from SunOS 4.1.4 and the Hurd (as of 1998-11-01) do this. If it does, then define `HAVE_STAT_EMPTY_STRING_BUG` (or `HAVE_LSTAT_EMPTY_STRING_BUG`) and ask for an `AC_LIBOBJ` replacement of it.

AC_FUNC_SETVBUF_REVERSED | Macro

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`.

AC_FUNC_STRCOLL | Macro

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

AC_FUNC_STRTOD | Macro

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

AC_FUNC_STRERROR_R | Macro

If `strerror_r` is available, define `HAVE_STRERROR_R`, and if it is declared, define `HAVE_DECL_STRERROR_R`. If it returns a `char *` message, define `STRERROR_R_CHAR_P`; otherwise it returns an `int` error number. The Thread-Safe Functions option of POSIX-200X requires `strerror_r` to return `int`, but many systems (including, for example, version 2.2.4 of the GNU C Library) return a `char *` value that is not necessarily equal to the buffer argument.

AC_FUNC_STRFTIME | Macro

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

AC_FUNC_STRNLEN | Macro

Check for a working `strnlen`, and ask for its replacement. Some architectures are know to provide broken versions of `strnlen`, such as AIX 4.3.

AC_FUNC_UTIME_NULL | Macro

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

AC_FUNC_VPRINTF | Macro

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.)

Node:Generic Functions, Previous:Particular Functions, Up:Library Functions

#### Generic Function Checks

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

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

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

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

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

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

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

AC_LIBOBJ (function) | Macro

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

AC_LIBSOURCE (file) | Macro

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

AC_LIBSOURCES (files) | Macro

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

AC_REPLACE_FUNCS (function...) | Macro

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

Node:Header Files, Next:Declarations, Previous:Library Functions, Up:Existing Tests

### Header Files

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

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

Node:Particular Headers, Next:Generic Headers, Up:Header Files

#### Particular Header Checks

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

AC_HEADER_DIRENT | Macro

Check for the following header files. For the first one that is found and defines `DIR`, define the listed C preprocessor macro:
`dirent.h` | `HAVE_DIRENT_H`
`sys/ndir.h` | `HAVE_SYS_NDIR_H`
`sys/dir.h` | `HAVE_SYS_DIR_H`
`ndir.h` | `HAVE_NDIR_H`
The directory-library declarations in your source code should look something like the following:
    #if HAVE_DIRENT_H
    # include <dirent.h>
    # define NAMLEN(dirent) strlen((dirent)->d_name)
    #else
    # define dirent direct
    # define NAMLEN(dirent) (dirent)->d_namlen
    # if HAVE_SYS_NDIR_H
    #  include <sys/ndir.h>
    # endif
    # if HAVE_SYS_DIR_H
    #  include <sys/dir.h>
    # endif
    # if HAVE_NDIR_H
    #  include <ndir.h>
    # endif
    #endif
Using the above declarations, the program would declare variables to be of type `struct dirent`, not `struct direct`, and would access the length of a directory entry name by passing a pointer to a `struct dirent` to the `NAMLEN` macro. This macro also checks for the SCO Xenix `dir` and `x` libraries.

AC_HEADER_MAJOR | Macro

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`.

AC_HEADER_STAT | Macro

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.

AC_HEADER_STDC | Macro

Define `STDC_HEADERS` if the system has ANSI C header files. Specifically, this macro checks for `stdlib.h`, `stdarg.h`, `string.h`, and `float.h`; if the system has those, it probably has the rest of the ANSI C header files. This macro also checks whether `string.h` declares `memchr` (and thus presumably the other `mem` functions), whether `stdlib.h` declare `free` (and thus presumably `malloc` and other related functions), and whether the `ctype.h` macros work on characters with the high bit set, as ANSI C requires. Use `STDC_HEADERS` instead of `__STDC__` to determine whether the system has ANSI-compliant header files (and probably C library functions) because many systems that have GCC do not have ANSI C header files. On systems without ANSI C headers, there is so much variation that it is probably easier to declare the functions you use than to figure out exactly what the system header files declare. Some systems contain a mix of functions ANSI and BSD; some are mostly ANSI but lack `memmove`; some define the BSD functions as macros in `string.h` or `strings.h`; some have only the BSD functions but `string.h`; some declare the memory functions in `memory.h`, some in `string.h`; etc. It is probably sufficient to check for one string function and one memory function; if the library has the ANSI versions of those then it probably has most of the others. If you put the following in `configure.ac`:
    AC_HEADER_STDC
    AC_CHECK_FUNCS(strchr memcpy)
then, in your code, you can put declarations like this:
    #if STDC_HEADERS
    # include <string.h>
    #else
    # if !HAVE_STRCHR
    #  define strchr index
    #  define strrchr rindex
    # endif
    char *strchr (), *strrchr ();
    # if !HAVE_MEMCPY
    #  define memcpy(d, s, n) bcopy ((s), (d), (n))
    #  define memmove(d, s, n) bcopy ((s), (d), (n))
    # endif
    #endif
If you use a function like `memchr`, `memset`, `strtok`, or `strspn`, which have no BSD equivalent, then macros won't suffice; you must provide an implementation of each function. An easy way to incorporate your implementations only when needed (since the ones in system C libraries may be hand optimized) is to, taking `memchr` for example, put it in `memchr.c` and use `AC_REPLACE_FUNCS(memchr)`.

AC_HEADER_SYS_WAIT | Macro

If `sys/wait.h` exists and is compatible with POSIX.1, define `HAVE_SYS_WAIT_H`. Incompatibility can occur if `sys/wait.h` does not exist, or if it uses the old BSD `union wait` instead of `int` to store a status value. If `sys/wait.h` is not POSIX.1 compatible, then instead of including it, define the POSIX.1 macros with their usual interpretations. Here is an example:
    #include <sys/types.h>
    #if HAVE_SYS_WAIT_H
    # include <sys/wait.h>
    #endif
    #ifndef WEXITSTATUS
    # define WEXITSTATUS(stat_val) ((unsigned)(stat_val) >> 8)
    #endif
    #ifndef WIFEXITED
    # define WIFEXITED(stat_val) (((stat_val) & 255) == 0)
    #endif

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

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

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

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

AC_HEADER_TIME | Macro

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

AC_HEADER_TIOCGWINSZ | Macro

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

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

Node:Generic Headers, Previous:Particular Headers, Up:Header Files

#### Generic Header Checks

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

AC_CHECK_HEADER (header-file, [action-if-found], [action-if-not-found], [includes = `default-includes`]) | Macro

  If the system header file header-file is usable, execute shell commands action-if-found, otherwise execute action-if-not-found. If you just want to define a symbol if the header file is available, consider using `AC_CHECK_HEADERS` instead. The meaning of "usable" depends upon the content of includes: if includes is empty check whether
      header-file
  can be preprocessed without error. if include is set Check whether
      includes
      #include <header-file>
can be compiled without error. You may use `AC_CHECK_HEADER` (and `AC_CHECK_HEADERS`) to check whether two headers are compatible. You may pass any kind of dummy content for includes, such as a single space, a comment, to check whether header-file compiles with success.

AC_CHECK_HEADERS (header-file..., [action-if-found], [action-if-not-found], [includes = `default-includes`]) | Macro

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

Node:Declarations, Next:Structures, Previous:Header Files, Up:Existing Tests

### Declarations

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

- Particular Declarations: Macros to check for certain declarations
- Generic Declarations: How to find other declarations

Node:Particular Declarations, Next:Generic Declarations, Up:Declarations

#### Particular Declaration Checks

The following macros check for certain declarations.

AC_DECL_SYS_SIGLIST | Macro

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

Node:Generic Declarations, Previous:Particular Declarations, Up:Declarations

#### Generic Declaration Checks

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

AC_CHECK_DECL (symbol, [action-if-found], [action-if-not-found], [includes = `default-includes`]) | Macro

If symbol (a function or a variable) is not declared in includes and a declaration is needed, run the shell commands action-if-not-found, otherwise action-if-found. If no includes are specified, the default includes are used (see Default Includes). This macro actually tests whether it is valid to use symbol as an r-value, not if it is really declared, because it is much safer to avoid introducing extra declarations when they are not needed.

AC_CHECK_DECLS (symbols, [action-if-found], [action-if-not-found], [includes = `default-includes`]) | Macro

For each of the symbols (comma-separated list), define `HAVE_DECL_symbol` (in all capitals) to `1` if symbol is declared, otherwise to `0`. If action-if-not-found is given, it is additional shell code to execute when one of the function declarations is needed, otherwise action-if-found is executed. This macro uses an m4 list as first argument:
    AC_CHECK_DECLS(strdup)
    AC_CHECK_DECLS([strlen])
    AC_CHECK_DECLS([malloc, realloc, calloc, free])
Unlike the other `AC_CHECK_*S` macros, when a symbol is not declared, `HAVE_DECL_symbol` is defined to `0` instead of leaving `HAVE_DECL_symbol` undeclared. When you are sure that the check was performed, use `HAVE_DECL_symbol` just like any other result of Autoconf:
    #if !HAVE_DECL_SYMBOL
    extern char *symbol;
    #endif
If the test may have not been performed, however, because it is safer not to declare a symbol than to use a declaration that conflicts with the system's one, you should use:
    #if defined HAVE_DECL_MALLOC && !HAVE_DECL_MALLOC
    char *malloc (size_t *s);
    #endif
You fall into the second category only in extreme situations: either your files may be used without being configured, or they are used during the configuration. In most cases the traditional approach is enough.

Node:Structures, Next:Types, Previous:Declarations, Up:Existing Tests

### Structures

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

- Particular Structures: Macros to check for certain structure members
- Generic Structures: How to find other structure members

Node:Particular Structures, Next:Generic Structures, Up:Structures

#### Particular Structure Checks

The following macros check for certain structures or structure members.

AC_STRUCT_ST_BLKSIZE | Macro

If `struct stat` contains an `st_blksize` member, define `HAVE_STRUCT_STAT_ST_BLKSIZE`. The former name, `HAVE_ST_BLKSIZE` is to be avoided, as its support will cease in the future. This macro is obsoleted, and should be replaced by
    AC_CHECK_MEMBERS([struct stat.st_blksize])

AC_STRUCT_ST_BLOCKS | Macro

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

AC_STRUCT_ST_RDEV | Macro

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

AC_STRUCT_TM | Macro

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`.

AC_STRUCT_TIMEZONE | Macro

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

Node:Generic Structures, Previous:Particular Structures, Up:Structures

#### Generic Structure Checks

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

AC_CHECK_MEMBER (aggregate.member, [action-if-found], [action-if-not-found], [includes = `default-includes`]) | Macro

Check whether member is a member of the aggregate aggregate. If no includes are specified, the default includes are used (see Default Includes).
    AC_CHECK_MEMBER(struct passwd.pw_gecos,,
                    [AC_MSG_ERROR([We need `passwd.pw_gecos'!])],
                    [#include <pwd.h>])
You can use this macro for sub-members:
    AC_CHECK_MEMBER(struct top.middle.bot)

AC_CHECK_MEMBERS (members, [action-if-found], [action-if-not-found], [includes = `default-includes`]) | Macro

Check for the existence of each `aggregate.member` of members using the previous macro. When member belongs to aggregate, define `HAVE_aggregate_member` (in all capitals, with spaces and dots replaced by underscores). This macro uses m4 lists:
    AC_CHECK_MEMBERS([struct stat.st_rdev, struct stat.st_blksize])

Node:Types, Next:Compilers and Preprocessors, Previous:Structures, Up:Existing Tests

### Types

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

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

Node:Particular Types, Next:Generic Types, Up:Types

#### Particular Type Checks

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

AC_TYPE_GETGROUPS | Macro

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

AC_TYPE_MODE_T | Macro

Equivalent to `AC_CHECK_TYPE(mode_t, int)`.

AC_TYPE_OFF_T | Macro

Equivalent to `AC_CHECK_TYPE(off_t, long)`.

AC_TYPE_PID_T | Macro

Equivalent to `AC_CHECK_TYPE(pid_t, int)`.

AC_TYPE_SIGNAL | Macro

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 ()
    {
    ...
    }

AC_TYPE_SIZE_T | Macro

Equivalent to `AC_CHECK_TYPE(size_t, unsigned)`.

AC_TYPE_UID_T | Macro

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

Node:Generic Types, Previous:Particular Types, Up:Types

#### Generic Type Checks

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

AC_CHECK_TYPE (type, [action-if-found], [action-if-not-found], [includes = `default-includes`]) | Macro

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

AC_CHECK_TYPES (types, [action-if-found], [action-if-not-found], [includes = `default-includes`]) | Macro

For each type of the types that is defined, define `HAVE_type` (in all capitals). If no includes are specified, the default includes are used (see Default Includes). If action-if-found is given, it is additional shell code to execute when one of the types is found. If action-if-not-found is given, it is executed when one of the types is not found. This macro uses m4 lists:
    AC_CHECK_TYPES(ptrdiff_t)
    AC_CHECK_TYPES([unsigned long long, uintmax_t])

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

Node:Compilers and Preprocessors, Next:System Services, Previous:Types, Up:Existing Tests

### Compilers and Preprocessors

All the tests for compilers (`AC_PROG_CC`, `AC_PROG_CXX`, `AC_PROG_F77`) define the output variable `EXEEXT` based on the output of the 1compiler, typically to the empty string if Unix and `.exe` if Win32 or OS/2.

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

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

- Specific Compiler Characteristics: Some portability issues
- Generic Compiler Characteristics: Language independent tests
- C Compiler: Checking its characteristics
- C++ Compiler: Likewise
- Fortran 77 Compiler: Likewise

Node:Specific Compiler Characteristics, Next:Generic Compiler Characteristics, Up:Compilers and Preprocessors

#### Specific Compiler Characteristics

Some compilers exhibit different behaviors.

Static/Dynamic Expressions
  Autoconf relies on a trick to extract one bit of information from the C compiler: using negative array sizes. For instance the following excerpt of a C source demonstrates how to test whether `int`s are 4 bytes long:

      int
      main (void)
      {
        static int test_array [sizeof (int) == 4 ? 1 : -1];
        test_array [0] = 0
        return 0;
      }

  To our knowledge, there is a single compiler that does not support this trick: the HP C compilers (the real one, not only the "bundled") on HP-UX 11.00:

      $ cc -c -Ae +O2 +Onolimit conftest.c
      cc: "conftest.c": error 1879: Variable-length arrays cannot \
          have static storage.

  Autoconf works around this problem by casting `sizeof (int)` to `long` before comparing it.

Node:Generic Compiler Characteristics, Next:C Compiler, Previous:Specific Compiler Characteristics, Up:Compilers and Preprocessors

#### Generic Compiler Characteristics

AC_CHECK_SIZEOF (type, [unused], [includes = `default-includes`]) | Macro

Define `SIZEOF_type` (see Standard Symbols) to be the size in bytes of type. If `type` is unknown, it gets a size of 0. If no includes are specified, the default includes are used (see Default Includes). If you provide include, make sure to include `stdio.h` which is required for this macro to run. This macro now works even when cross-compiling. The unused argument was used when cross-compiling. For example, the call
    AC_CHECK_SIZEOF(int *)
defines `SIZEOF_INT_P` to be 8 on DEC Alpha AXP systems.

Node:C Compiler, Next:C++ Compiler, Previous:Generic Compiler Characteristics, Up:Compilers and Preprocessors

#### C Compiler Characteristics

AC_PROG_CC ([compiler-search-list]) | Macro

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

AC_PROG_CC_C_O | Macro

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

AC_PROG_CC_STDC | Macro

If the C compiler is not in ANSI C mode by default, try to add an option to output variable `CC` to make it so. This macro tries various options that select ANSI C on some system or another. It considers the compiler to be in ANSI C mode if it handles function prototypes correctly. If you use this macro, you should check after calling it whether the C compiler has been set to accept ANSI C; if not, the shell variable `ac_cv_prog_cc_stdc` is set to `no`. If you wrote your source code in ANSI C, you can make an un-ANSIfied copy of it by using the program `ansi2knr`, which comes with Automake.

AC_PROG_CPP | Macro

Set output variable `CPP` to a command that runs the C preprocessor. If `$CC -E` doesn't work, `/lib/cpp` is used. It is only portable to run `CPP` on files with a `.c` extension. If the current language is C (see Language Choice), many of the specific test macros use the value of `CPP` indirectly by calling `AC_TRY_CPP`, `AC_CHECK_HEADER`, `AC_EGREP_HEADER`, or `AC_EGREP_CPP`. Some preprocessors don't indicate missing include files by the error status. For such preprocessors an internal variable is set that causes other macros to check the standard error from the preprocessor and consider the test failed if any warnings have been reported.

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

AC_C_BIGENDIAN ([action-if-true], [action-if-false], [action-if-unknown]) | Macro

If words are stored with the most significant byte first (like Motorola and SPARC CPUs), execute action-if-true. If words are stored with the less significant byte first (like Intel and VAX CPUs), execute action-if-false. This macro runs a test-case if endianness cannot be determined from the system header files. When cross-compiling the test-case is not run but grep'ed for some magic values. action-if-unknown is executed if the latter case fails to determine the byte sex of the host system. The default for action-if-true is to define `WORDS_BIGENDIAN`. The default for action-if-false is to do nothing. And finally, the default for action-if-unknown is to abort configure and tell the installer which variable he should preset to bypass this test.

AC_C_CONST | Macro

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

AC_C_VOLATILE | Macro

If the C compiler does not understand the keyword `volatile`, define `volatile` to be empty. Programs can simply use `volatile` as if every C compiler supported it; for those that do not, the `Makefile` or configuration header will define it as empty. If the correctness of your program depends on the semantics of `volatile`, simply defining it to be empty does, in a sense, break your code. However, given that the compiler does not support `volatile`, you are at its mercy anyway. At least your program will compile, when it wouldn't before. In general, the `volatile` keyword is a feature of ANSI C, so you might expect that `volatile` is available only when `__STDC__` is defined. However, Ultrix 4.3's native compiler does support volatile, but does not defined `__STDC__`.

AC_C_INLINE | Macro

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.

AC_C_CHAR_UNSIGNED | Macro

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

AC_C_LONG_DOUBLE | Macro

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

AC_C_STRINGIZE | Macro

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

AC_C_PROTOTYPES | Macro

Check to see if function prototypes are understood by the compiler. If so, define `PROTOTYPES` and `__PROTOTYPES`. In the case the compiler does not handle prototypes, you should use `ansi2knr`, which comes with the Automake distribution, to unprotoize function definitions. For function prototypes, you should first define `PARAMS`:
    #ifndef PARAMS
    # if PROTOTYPES
    #  define PARAMS(protos) protos
    # else /* no PROTOTYPES */
    #  define PARAMS(protos) ()
    # endif /* no PROTOTYPES */
    #endif
then use it this way:
    size_t my_strlen PARAMS ((const char *));

This macro also defines `__PROTOTYPES`; this is for the benefit of header files that cannot use macros that infringe on user name space.

AC_PROG_GCC_TRADITIONAL | Macro

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.

Node:C++ Compiler, Next:Fortran 77 Compiler, Previous:C Compiler, Up:Compilers and Preprocessors

#### C++ Compiler Characteristics

AC_PROG_CXX ([compiler-search-list]) | Macro

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

AC_PROG_CXXCPP | Macro

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

Node:Fortran 77 Compiler, Previous:C++ Compiler, Up:Compilers and Preprocessors

#### Fortran 77 Compiler Characteristics

AC_PROG_F77 ([compiler-search-list]) | Macro

Determine a Fortran 77 compiler to use. If `F77` is not already set in the environment, then check for `g77` and `f77`, and then some other names. Set the output variable `F77` to the name of the compiler found. This macro may, however, be invoked with an optional first argument which, if specified, must be a space separated list of Fortran 77 compilers to search for. This just gives the user an opportunity to specify an alternative search list for the Fortran 77 compiler. For example, if you didn't like the default order, then you could invoke `AC_PROG_F77` like this:
    AC_PROG_F77(fl32 f77 fort77 xlf cf77 g77 f90 xlf90)
If using `g77` (the GNU Fortran 77 compiler), then `AC_PROG_F77` will set the shell variable `G77` to `yes`. If the output variable `FFLAGS` was not already set in the environment, then set it to `-g -02` for `g77` (or `-O2` where `g77` does not accept `-g`). Otherwise, set `FFLAGS` to `-g` for all other Fortran 77 compilers.

AC_PROG_F77_C_O | Macro

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

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

AC_F77_LIBRARY_LDFLAGS | Macro

Determine the linker flags (e.g. `-L` and `-l`) for the Fortran 77 intrinsic and run-time libraries that are required to successfully link a Fortran 77 program or shared library. The output variable `FLIBS` is set to these flags. This macro is intended to be used in those situations when it is necessary to mix, e.g. C++ and Fortran 77 source code into a single program or shared library (see Mixing Fortran 77 With C and C++). For example, if object files from a C++ and Fortran 77 compiler must be linked together, then the C++ compiler/linker must be used for linking (since special C++-ish things need to happen at link time like calling global constructors, instantiating templates, enabling exception support, etc.). However, the Fortran 77 intrinsic and run-time libraries must be linked in as well, but the C++ compiler/linker doesn't know by default how to add these Fortran 77 libraries. Hence, the macro `AC_F77_LIBRARY_LDFLAGS` was created to determine these Fortran 77 libraries. The macro `AC_F77_DUMMY_MAIN` or `AC_F77_MAIN` will probably also be necessary to link C/C++ with Fortran; see below.

AC_F77_DUMMY_MAIN ([action-if-found], [action-if-not-found]) | Macro

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

AC_F77_MAIN | Macro

As discussed above for `AC_F77_DUMMY_MAIN`, many Fortran libraries allow you to provide an entry point called e.g. `MAIN__` instead of the usual `main`, which is then called by a `main` function in the Fortran libraries that initializes things like Fortran I/O. The `AC_F77_MAIN` macro detects whether it is possible to utilize such an alternate main function, and defines `F77_MAIN` to the name of the function. (If no alternate main function name is found, `F77_MAIN` is simply defined to `main`.) Thus, when calling Fortran routines from C that perform things like I/O, one should use this macro and name the "main" function `F77_MAIN` instead of `main`.

AC_F77_WRAPPERS | Macro

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

AC_F77_FUNC (name, [shellvar]) | Macro

Given an identifier name, set the shell variable shellvar to hold the mangled version name according to the rules of the Fortran 77 linker (see also `AC_F77_WRAPPERS`). shellvar is optional; if it is not supplied, the shell variable will be simply name. The purpose of this macro is to give the caller a way to access the name-mangling information other than through the C preprocessor as above; for example, to call Fortran routines from some language other than C/C++.

Node:System Services, Next:UNIX Variants, Previous:Compilers and Preprocessors, Up:Existing Tests

### System Services

The following macros check for operating system services or capabilities.

AC_PATH_X | Macro

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.

AC_PATH_XTRA | Macro

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

AC_SYS_INTERPRETER | Macro

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

AC_SYS_LARGEFILE | Macro

Arrange for large-file support. On some hosts, one must use special compiler options to build programs that can access large files. Append any such options to the output variable `CC`. Define `_FILE_OFFSET_BITS` and `_LARGE_FILES` if necessary. Large-file support can be disabled by configuring with the `--disable-largefile` option. If you use this macro, check that your program works even when `off_t` is longer than `long`, since this is common when large-file support is enabled. For example, it is not correct to print an arbitrary `off_t` value `X` with `printf ("%ld", (long) X)`.

AC_SYS_LONG_FILE_NAMES | Macro

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

AC_SYS_POSIX_TERMIOS | Macro

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

Node:UNIX Variants, Previous:System Services, Up:Existing Tests

### 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.

AC_AIX | Macro

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

AC_ISC_POSIX | Macro

For INTERACTIVE UNIX (ISC), add `-lcposix` to output variable `LIBS` if necessary for POSIX facilities. Call this after `AC_PROG_CC` and before any other macros that use POSIX interfaces. INTERACTIVE UNIX is no longer sold, and Sun says that they will drop support for it on 2006-07-23, so this macro is becoming obsolescent.

AC_MINIX | Macro

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.

Node:Writing Tests, Next:Results, Previous:Existing Tests, Up:Top

## Writing Tests

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

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

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

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

- Examining Declarations: Detecting header files and declarations
- Examining Syntax: Detecting language syntax features
- Examining Libraries: Detecting functions and global variables
- Run Time: Testing for run-time features
- Systemology: A zoology of operating systems
- Multiple Cases: Tests for several possible values
- Language Choice: Selecting which language to use for testing

Node:Examining Declarations, Next:Examining Syntax, Up:Writing Tests

### 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.

AC_TRY_CPP (input, [action-if-true], [action-if-false]) | Macro

If the preprocessor produces no error messages while processing the input (typically includes), run shell commands action-if-true. Otherwise run shell commands action-if-false. Beware that input is double quoted. Shell variable, back quote, and backslash substitutions are performed on input. 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.

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

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)

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

program is the text of a C or C++ program, on which shell variable, back quote, and backslash substitutions are performed. If the output of running the preprocessor on program matches the `egrep` regular expression pattern, execute shell commands action-if-found, otherwise execute action-if-not-found. This macro calls `AC_PROG_CPP` or `AC_PROG_CXXCPP` (depending on which language is current, see Language Choice), if it hasn't been called already.

Node:Examining Syntax, Next:Examining Libraries, Previous:Examining Declarations, Up:Writing Tests

### 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.

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

Create a test program in the current language (see Language Choice) to see whether a function whose body consists of function-body can be compiled. If the file compiles successfully, run shell commands action-if-found, otherwise run action-if-not-found. This macro double quotes both includes and function-body. 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. This macro does not try to link; use `AC_TRY_LINK` if you need to do that (see Examining Libraries).

Node:Examining Libraries, Next:Run Time, Previous:Examining Syntax, Up:Writing Tests

### Examining Libraries

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

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

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

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

Depending on the current language (see Language Choice), create a test program to see whether a function whose body consists of function-body can be compiled and linked. If the file compiles and links successfully, run shell commands action-if-found, otherwise run action-if-not-found. This macro double quotes both includes and function-body. 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.

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

Depending on the current language (see Language Choice), create a test program to see whether a program whose body consists of a prototype of and a call to function can be compiled and linked. If the file compiles and links successfully, run shell commands action-if-found, otherwise run action-if-not-found.

Node:Run Time, Next:Systemology, Previous:Examining Libraries, Up:Writing Tests

### Checking Run Time Behavior

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

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

- Test Programs: Running test programs
- Guidelines: General rules for writing test programs
- Test Functions: Avoiding pitfalls in test programs

Node:Test Programs, Next:Guidelines, Up:Run Time

#### Running Test Programs

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

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

If program compiles and links successfully and returns an exit status of 0 when executed, run shell commands action-if-true. Otherwise, run shell commands action-if-false. This macro double quotes program, the text of a program in the current language (see Language Choice), on which shell variable and back quote substitutions are performed. 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. In the action-if-false section, the exit status of the program is available in the shell variable `$?`, but be very careful to limit yourself to positive values smaller than 127; bigger values shall be saved into a file by the program. Note also that you have simply no guarantee that this exit status is issued by the program, or by the failure of its compilation. In other words, use this feature if sadist only, it was reestablished because the Autoconf maintainers grew tired of receiving "bug reports".

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

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

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

Node:Guidelines, Next:Test Functions, Previous:Test Programs, Up:Run Time

#### Guidelines for Test Programs

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

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

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

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

Node:Test Functions, Previous:Guidelines, Up:Run Time

#### 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`).

Node:Systemology, Next:Multiple Cases, Previous:Run Time, Up:Writing Tests

### Systemology

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

QNX 4.25
  QNX is a realtime operating system running on Intel architecture meant to be scalable from the small embedded systems to hundred processor super-computer. It claims to be POSIX certified. More information is available on the QNX home page, including the QNX man pages.

Node:Multiple Cases, Next:Language Choice, Previous:Systemology, Up:Writing Tests

### Multiple Cases

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

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

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

Node:Language Choice, Previous:Multiple Cases, Up:Writing Tests

### Language Choice

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

AC_LANG (language) | Macro

Do compilation tests using the compiler, preprocessor and file extensions for the specified language. Supported languages are: `C` Do compilation tests using `CC` and `CPP` and use extension `.c` for test programs. `C++` Do compilation tests using `CXX` and `CXXCPP` and use extension `.C` for test programs. `Fortran 77` Do compilation tests using `F77` and use extension `.f` for test programs.

AC_LANG_PUSH (language) | Macro

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

AC_LANG_POP ([language]) | Macro

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

AC_REQUIRE_CPP | Macro

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

Node:Results, Next:Programming in M4, Previous:Writing Tests, Up:Top

## Results of Tests

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

- Defining Symbols: Defining C preprocessor symbols
- Setting Output Variables: Replacing variables in output files
- Caching Results: Speeding up subsequent `configure` runs
- Printing Messages: Notifying `configure` users

Node:Defining Symbols, Next:Setting Output Variables, Up:Results

### Defining C Preprocessor Symbols

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

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

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

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

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

Define C preprocessor variable variable. If value is given, set variable to that value (verbatim), otherwise set it to 1. value should not contain literal newlines, and if you are not using `AC_CONFIG_HEADERS` it should not contain any `#` characters, as `make` tends to eat them. To use a shell variable (which you need to do in order to define a value containing the M4 quote characters `[` or `]`), use `AC_DEFINE_UNQUOTED` instead. description is only useful if you are using `AC_CONFIG_HEADERS`. In this case, description is put into the generated `config.h.in` as the comment before the macro define. The following example defines the C preprocessor variable `EQUATION` to be the string constant `"$a > $b"`:
    AC_DEFINE(EQUATION, "$a > $b")

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

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"])

Node:Setting Output Variables, Next:Caching Results, Previous:Defining Symbols, Up:Results

### Setting Output Variables

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

AC_SUBST (variable, [value]) | Macro

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. This value of variable should not contain literal newlines. If value is given, in addition assign it to `variable`.

AC_SUBST_FILE (variable) | Macro

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.ac` could contain:
    AC_SUBST_FILE(host_frag)
    host_frag=$srcdir/conf/sun4.mh
and then a `Makefile.in` could contain:
    @host_frag@

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

Such variables are named precious variables, and can be declared as such by `AC_ARG_VAR`.

AC_ARG_VAR (variable, description) | Macro

- Declare variable is a precious variable, and include its description in the variable section of `./configure --help`. Being precious means that variable is `AC_SUBST`'d. variable is kept in the cache including if it was not specified on the `./configure` command line. Indeed, while `configure` can notice the definition of `CC` in `./configure CC=bizarre-cc`, it is impossible to notice it in `CC=bizarre-cc ./configure`, which, unfortunately, is what most users do. variable is checked for consistency between two `configure` runs. For instance:
      $ ./configure --silent --config-cache
      $ CC=cc ./configure --silent --config-cache
      configure: error: `CC' was not set in the previous run
      configure: error: changes in the environment can compromise \
      the build
      configure: error: run `make distclean' and/or \
      `rm config.cache' and start over
- and similarly if the variable is unset, or if its content is changed. variable is kept during automatic reconfiguration (see config.status Invocation) as if it had been passed as a command line argument, including when no cache is used:
      $ CC=/usr/bin/cc ./configure undeclared_var=raboof --silent
      $ ./config.status --recheck
      running /bin/sh ./configure undeclared_var=raboof --silent \
        CC=/usr/bin/cc  --no-create --no-recursion

Node:Caching Results, Next:Printing Messages, Previous:Setting Output Variables, Up:Results

### Caching Results

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

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

Ensure that the results of the check identified by cache-id are available. If the results of the check were in the cache file that was read, and `configure` was not given the `--quiet` or `--silent` option, print a message saying that the result was cached; otherwise, run the shell commands commands-to-set-it. If the shell commands are run to determine the value, the value will be saved in the cache file just before `configure` creates its output files. See Cache Variable Names, for how to choose the name of the cache-id variable. The commands-to-set-it must have no side effects except for setting the variable cache-id, see below.

AC_CACHE_CHECK (message, cache-id, commands-to-set-it) | Macro

A wrapper for `AC_CACHE_VAL` that takes care of printing the messages. This macro provides a convenient shorthand for the most common way to use these macros. It calls `AC_MSG_CHECKING` for message, then `AC_CACHE_VAL` with the cache-id and commands arguments, and `AC_MSG_RESULT` with cache-id. The commands-to-set-it must have no side effects except for setting the variable cache-id, see below.

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

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

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

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

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

- Cache Variable Names: Shell variables used in caches
- Cache Files: Files `configure` uses for caching
- Cache Checkpointing: Loading and saving the cache file

Node:Cache Variable Names, Next:Cache Files, Up:Caching Results

#### Cache Variable Names

The names of cache variables should have the following format:

    package-prefix_cv_value-type_specific-value_[additional-options]

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

package-prefix
  An abbreviation for your package or organization; the same prefix you begin local Autoconf macros with, except lowercase by convention. For cache values used by the distributed Autoconf macros, this value is `ac`.
`_cv_`
  Indicates that this shell variable is a cache value. This string must be present in the variable name, including the leading underscore.
value-type
  A convention for classifying cache values, to produce a rational naming system. The values used in Autoconf are listed in Macro Names.
specific-value
  Which member of the class of cache values this test applies to. For example, which function (`alloca`), program (`gcc`), or output variable (`INSTALL`).
additional-options
  Any particular behavior of the specific member that this test applies to. For example, `broken` or `set`. This part of the name may be omitted if it does not apply.

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

Node:Cache Files, Next:Cache Checkpointing, Previous:Cache Variable Names, Up:Caching Results

#### Cache Files

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

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

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

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

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

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

Node:Cache Checkpointing, Previous:Cache Files, Up:Caching Results

#### Cache Checkpointing

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

AC_CACHE_LOAD | Macro

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

AC_CACHE_SAVE | Macro

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

For instance:

     ... AC_INIT, etc. ...
    # Checks for programs.
    AC_PROG_CC
    AC_PROG_GCC_TRADITIONAL
     ... more program checks ...
    AC_CACHE_SAVE

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

    # Might abort...
    AM_PATH_GTK(1.0.2,, [AC_MSG_ERROR([GTK not in path])])
    AM_PATH_GTKMM(0.9.5,, [AC_MSG_ERROR([GTK not in path])])
     ... AC_OUTPUT, etc. ...

Node:Printing Messages, Previous:Caching Results, Up:Results

### Printing Messages

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

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

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

AC_MSG_CHECKING (feature-description) | Macro

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.

AC_MSG_RESULT (result-description) | Macro

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.

AC_MSG_NOTICE (message) | Macro

Deliver the message to the user. It is useful mainly to print a general description of the overall purpose of a group of feature checks, e.g.,
    AC_MSG_NOTICE([checking if stack overflow is detectable])
This macro prints nothing if `configure` is run with the `--quiet` or `--silent` option.

AC_MSG_ERROR (error-description, [exit-status]) | Macro

Notify the user of an error that prevents `configure` from completing. This macro prints an error message to the standard error output and exits `configure` with exit-status (1 by default). error-description should be something like `invalid value $HOME for \$HOME`. The error-description should start with a lower-case letter, and "cannot" is preferred to "can't".

AC_MSG_WARN (problem-description) | Macro

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

Node:Programming in M4, Next:Writing Autoconf Macros, Previous:Results, Up:Top

## Programming in M4

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

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

- M4 Quotation: Protecting macros from unwanted expansion
- Invoking autom4te: The Autoconf executables backbone
- Programming in M4sugar: Convenient pure M4 macros
- Programming in M4sh: Common Shell Constructs

Node:M4 Quotation, Next:Invoking autom4te, Up:Programming in M4

### M4 Quotation

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

- Active Characters: Characters that change the behavior of m4
- One Macro Call: Quotation and one macro call
- Quotation and Nested Macros: Macros calling macros
- Changequote is Evil: Worse than INTERCAL: M4 + changequote
- Quadrigraphs: Another way to escape special characters
- Quotation Rule Of Thumb: One parenthesis, one quote

Node:Active Characters, Next:One Macro Call, Up:M4 Quotation

#### Active Characters

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

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

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

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

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

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

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

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

How can you correctly output the intended results2?

Node:One Macro Call, Next:Quotation and Nested Macros, Previous:Active Characters, Up:M4 Quotation

#### One Macro Call

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

    define([car], [$1])

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

    define(car, $1)

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

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

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

Let's pay attention to the special characters:

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

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

    car([#)]

Proper quotation, of course, fixes the problem:

    car([#])
    =>#

The reader will easily understand the following examples:

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

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

Node:Quotation and Nested Macros, Next:Changequote is Evil, Previous:One Macro Call, Up:M4 Quotation

#### Quotation and Nested Macros

The examples below use the following macros:

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

Each additional embedded macro call introduces other possible interesting quotations:

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

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

    car(ACT, IVE)
    =>ACT

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

    active
    =>ACT, IVE

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

    [active]
    =>active

exactly as we already saw above.

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

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

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

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

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

and check that `qar` is properly fixed:

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

Ahhh! That's much better.

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

    qar(active)
    =>ACT

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

    car([active])
    =>ACT, IVE

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

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

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

Node:Changequote is Evil, Next:Quadrigraphs, Previous:Quotation and Nested Macros, Up:M4 Quotation

#### `changequote` is Evil

The temptation is often high to bypass proper quotation, in particular when it's late at night. Then, many experienced Autoconf hackers finally surrender to the dark side of the force and use the ultimate weapon: `changequote`.

The M4 builtin `changequote` belongs to a set of primitives that allow one to adjust the syntax of the language to adjust it to her needs. For instance, by default M4 uses ``` and `'` as quotes, but in the context of shell programming (and actually of most programming languages), it's about the worst choice one can make: because of strings and back quoted expression in shell (such as `'this'` and ``that``), because of literal characters in usual programming language (as in `'0'`), there are many unbalanced ``` and `'`. Proper M4 quotation then becomes a nightmare, if not impossible. In order to make M4 useful in such a context, its designers have equipped it with `changequote`, which makes it possible to chose another pair of quotes. M4sugar, M4sh, Autoconf, and Autotest all have chosen to use `[` and `]`. Not especially because they are unlikely characters, but because they are characters unlikely to be unbalanced.

There are other magic primitives, such as `changecom` to specify what syntactic forms are comments (it is common to see `changecom(<!--, -->)` when M4 is used to produce HTML pages), `changeword` and `changesyntax` to change other syntactic details (such as the character to denote the n-th argument, `$` by default, the parenthesis around arguments etc.).

These primitives are really meant to make M4 more useful for specific domains: they should be considered like command line options: `--quotes`, `--comments`, `--words`, and `--syntax`. Nevertheless, they are implemented as M4 builtins, as it makes M4 libraries self contained (no need for additional options).

There lies the problem...

The problem is that it is then tempting to use them in the middle of an M4 script, as opposed to its initialization. This, if not carefully thought, can lead to disastrous effects: you are changing the language in the middle of the execution. Changing and restoring the syntax is often not enough: if you happened to invoke macros in between, these macros will be lost, as the current syntax will probably not be the one they were implemented with.

Node:Quadrigraphs, Next:Quotation Rule Of Thumb, Previous:Changequote is Evil, Up:M4 Quotation

#### Quadrigraphs

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

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

`@<:@`
  `[`
`@:>@`
  `]`
`@S|@`
  `$`
`@%:@`
  `#`
`@&t@`
  Expands to nothing.

Quadrigraphs are replaced at a late stage of the translation process, after `m4` is run, so they do not get in the way of M4 quoting. For example, the string `^@<:@`, independently of its quotation, will appear as `^[` in the output.

The empty quadrigraph can be used:

- to mark explicitly trailing spaces

  Trailing spaces are smashed by `autom4te`. This is a feature.

- to produce other quadrigraphs

  For instance `@<@&t@:@` produces `@<:@`.

- to escape occurrences of forbidden patterns

  For instance you might want to mention `AC_FOO` is a comment, while still being sure that `autom4te` will still catch unexpanded `AC_*`. Then write `AC@&t@_FOO`.

The name `@&t@` was suggested by Paul Eggert:

> I should give some credit to the `@&t@` pun. The `&` is my own invention, but the `t` came from the source code of the ALGOL68C compiler, written by Steve Bourne (of Bourne shell fame), and which used `mt` to denote the empty string. In C, it would have looked like something like:

>     char const mt[] = "";

> but of course the source code was written in Algol 68.

> I don't know where he got `mt` from: it could have been his own invention, and I suppose it could have been a common pun around the Cambridge University computer lab at the time.

Node:Quotation Rule Of Thumb, Previous:Quadrigraphs, Up:M4 Quotation

#### Quotation Rule Of Thumb

To conclude, the quotation rule of thumb is:

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

It is common to read Autoconf programs with snippets like:

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

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

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

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

Simplified, the example above is just doing this:

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

instead of simply:

    [[]]

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

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

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

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

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

Node:Invoking autom4te, Next:Programming in M4sugar, Previous:M4 Quotation, Up:Programming in M4

### Invoking `autom4te`

The Autoconf suite, including M4sugar, M4sh, and Autotest in addition to Autoconf per se, heavily rely on M4. All these different uses revealed common needs factored into a layer over `m4`: `autom4te`3.

`autom4te` should basically considered as a replacement of `m4` itself. In particular, its handling of command line arguments is modeled after M4's:

    autom4te options files

where the files are directly passed to `m4`. In addition to the regular expansion, it handles the replacement of the quadrigraphs (see Quadrigraphs), and of `__oline__`, the current line in the output. It supports an extended syntax for the files:

`file.m4f`
  This file is an M4 frozen file. Note that all the previous files are ignored. See the option `--melt` for the rationale.
`file?`
  If found in the library path, the file is included for expansion, otherwise it is ignored instead of triggering a failure.

Of course, it supports the Autoconf common subset of options:

`--help`
`-h`
  Print a summary of the command line options and exit.
`--version`
`-V`
  Print the version number of Autoconf and exit.
`--verbose`
`-v`
  Report processing steps.
`--debug`
`-d`
  Don't remove the temporary files and be even more verbose.
`--include=dir`
`-I dir`
  Also look for input files in dir. Multiple invocations accumulate. Contrary to M4 but in agreement with common sense, directories are browsed from last to first.
`--output=file`
`-o file`
  Save output (script or trace) to file. The file `-` stands for the standard output.

As an extension of `m4`, it includes the following options:

`--warnings=category`
`-W category`
  Report the warnings related to category (which can actually be a comma separated list). See Reporting Messages, macro `AC_DIAGNOSE`, for a comprehensive list of categories. Special values include:
  `all`
    report all the warnings
  `none`
    report none
  `error`
    treats warnings as errors
  `no-category`
    disable warnings falling into category

  Warnings about `syntax` are enabled by default, and the environment variable `WARNINGS`, a comma separated list of categories, is honored. `autom4te -W category` will actually behave as if you had run:

      autom4te --warnings=syntax,$WARNINGS,category

  If you want to disable `autom4te`'s defaults and `WARNINGS`, but (for example) enable the warnings about obsolete constructs, you would use `-W none,obsolete`.

  `autom4te` displays a back trace for errors, but not for warnings; if you want them, just pass `-W error`. For instance, on this `configure.ac`:

      AC_DEFUN([INNER],
      [AC_TRY_RUN([exit (0)])])

      AC_DEFUN([OUTER],
      [INNER])

      AC_INIT
      OUTER

  you get:

      $ autom4te -l autoconf -Wcross
      configure.ac:8: warning: AC_TRY_RUN called without default \
      to allow cross compiling
      $ autom4te -l autoconf -Wcross,error
      configure.ac:8: error: AC_TRY_RUN called without default \
      to allow cross compiling
      acgeneral.m4:3044: AC_TRY_RUN is expanded from...
      configure.ac:2: INNER is expanded from...
      configure.ac:5: OUTER is expanded from...
      configure.ac:8: the top level

`--melt`
`-m`
  Do not use frozen files. Any argument `file.m4f` will be replaced with `file.m4`. This helps tracing the macros which are executed only when the files are frozen, typically `m4_define`. For instance, running:

      autom4te --melt 1.m4 2.m4f 3.m4 4.m4f input.m4

  is roughly equivalent to running:

      m4 1.m4 2.m4 3.m4 4.m4 input.m4

  while

      autom4te 1.m4 2.m4f 3.m4 4.m4f input.m4

  is equivalent to:

      m4 --reload-state=4.m4f input.m4

`--freeze`
`-f`
  Produce a frozen state file. `autom4te` freezing is stricter than M4's: it must produce no warnings, and no output other than empty lines (a line with white spaces is not empty) and comments (starting with `#`). Please, note that contrary to `m4`, this options takes no argument:

      autom4te 1.m4 2.m4 3.m4 --freeze --output=3.m4f

  corresponds to

      m4 1.m4 2.m4 3.m4 --freeze-state=3.m4f

`--mode=octal-mode`
`-m octal-mode`
  Set the mode of the non traces output to octal-mode. By default, `0666`.

As another additional feature over `m4`, `autom4te` caches its results. GNU M4 is able to produce a regular output and traces at the same time. Traces are heavily used in the GNU Build System: `autoheader` uses them to build `config.h.in`, `autoreconf` to determine what GNU Build System components are used, `automake` to "parse" `configure.ac` etc. To save the long runs of `m4`, traces are cached while performing regular expansion, and conversely. This cache is (actually, the caches are) stored in the directory `autom4te.cache`. It can safely be removed at any moment (especially if for some reason `autom4te` considers it is trashed).

`--force`
`-f`
  Do not consider the cache (but update it anyway).

Because traces are so important to the GNU Build System, `autom4te` provides high level tracing features as compared to M4, and helps exploiting the cache:

`--trace=macro[:format]`
`-t macro[:format]`
  Trace the invocations to macro according to the format. Multiple `--trace` arguments can be used to list several macros. Multiple `--trace` arguments for a single macro are not cumulative; instead, you should just make format as long as needed.

  The format is a regular string, with newlines if desired, and several special escape codes. It defaults to `$f:$l:$n:$%`. It can use the following special escapes:

  `$$`
    The character `$`.
  `$f`
    The filename from which macro is called.
  `$l`
    The line number from which macro is called.
  `$d`
    The depth of the macro call. This is an M4 technical detail that you probably don't want to know about.
  `$n`
    The name of the macro.
  `$num`
    The numth argument of the call to macro.
  `$@`
  `$sep@`
  `${separator}@`
    All the arguments passed to macro, separated by the character sep or the string separator (`,` by default). Each argument is quoted, i.e. enclosed in a pair of square brackets.
  `$*`
  `$sep*`
  `${separator}*`
    As above, but the arguments are not quoted.
  `$%`
  `$sep%`
  `${separator}%`
    As above, but the arguments are not quoted, all new line characters in the arguments are smashed, and the default separator is `:`.

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

  See autoconf Invocation, for examples of trace uses.

`--preselect=macro`
`-p macro`
  Cache the traces of macro, but do not enable traces. This is especially important to save cpu cycles in the future. For instance, when invoked, `autoconf` preselects all the macros that `autoheader`, `automake`, `autoreconf` etc. will trace, so that running `m4` is not needed to trace them: the cache suffices. This results in a huge speed-up.

Finally, `autom4te` introduces the concept of Autom4te libraries. They consists in a powerful yet extremely simple feature: sets of combined command line arguments:

`--language=language`
`-l =language`
  Use the language Autom4te library. Current languages include:
  `M4sugar`
    create M4sugar output.
  `M4sh`
    create M4sh executable shell scripts.
  `Autotest`
    create Autotest executable test suites.
  `Autoconf`
    create Autoconf executable configure scripts.

As an example, if Autoconf is installed in its default location, `/usr/local`, running `autom4te -l m4sugar foo.m4` is strictly equivalent to running `autom4te --include /usr/local/share/autoconf m4sugar/m4sugar.m4f --warning syntax foo.m4`. Recursive expansion applies: running `autom4te -l m4sh foo.m4`, is the same as `autom4te --language M4sugar m4sugar/m4sh.m4f foo.m4`, i.e., `autom4te --include /usr/local/share/autoconf m4sugar/m4sugar.m4f m4sugar/m4sh.m4f --mode 777 foo.m4`. The definition of the languages is stored in `autom4te.cfg`.

Node:Programming in M4sugar, Next:Programming in M4sh, Previous:Invoking autom4te, Up:Programming in M4

### Programming in M4sugar

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

- Redefined M4 Macros: M4 builtins changed in M4sugar
- Evaluation Macros: More quotation and evaluation control
- Forbidden Patterns: Catching unexpanded macros

Node:Redefined M4 Macros, Next:Evaluation Macros, Up:Programming in M4sugar

#### Redefined M4 Macros

With a few exceptions, all the M4 native macros are moved in the `m4_` pseudo-namespace, e.g., M4sugar renames `define` as `m4_define` etc.

Some M4 macros are redefined, and are slightly incompatible with their native equivalent.

dnl | Macro

This macro kept its original name: no `m4_dnl` is defined.

m4_defn (macro) | Macro

Contrary to the M4 builtin, this macro fails if macro is not defined. See `m4_undefine`.

m4_exit (exit-status) | Macro

This macro corresponds to `m4exit`.

m4_if (comment) | Macro
m4_if (string-1, string-2, equal, [not-equal]) | Macro
m4_if (string-1, string-2, equal, ...) | Macro

This macro corresponds to `ifelse`.

m4_undefine (macro) | Macro

Contrary to the M4 builtin, this macro fails if macro is not defined. Use
    m4_ifdef([macro], [m4_undefine([macro])])
to recover the behavior of the builtin.

m4_bpatsubst (string, regexp, [replacement]) | Macro

This macro corresponds to `patsubst`. The name `m4_patsubst` is kept for future versions of M4sh, on top of GNU M4 which will provide extended regular expression syntax via `epatsubst`.

m4_popdef (macro) | Macro

Contrary to the M4 builtin, this macro fails if macro is not defined. See `m4_undefine`.

m4_bregexp (string, regexp, [replacement]) | Macro

This macro corresponds to `regexp`. The name `m4_regexp` is kept for future versions of M4sh, on top of GNU M4 which will provide extended regular expression syntax via `eregexp`.

m4_wrap (text) | Macro

This macro corresponds to `m4wrap`. You are encouraged to end text with `[]`, so that there are no risks that two consecutive invocations of `m4_wrap` result in an unexpected pasting of tokens, as in
    m4_define([foo], [Foo])
    m4_define([bar], [Bar])
    m4_define([foobar], [FOOBAR])
    m4_wrap([bar])
    m4_wrap([foo])
    =>FOOBAR

Node:Evaluation Macros, Next:Forbidden Patterns, Previous:Redefined M4 Macros, Up:Programming in M4sugar

#### Evaluation Macros

The following macros give some control over the order of the evaluation by adding or removing levels of quotes. They are meant for hard core M4 programmers.

m4_dquote (arg1, ...) | Macro

Return the arguments as a quoted list of quoted arguments.

m4_quote (arg1, ...) | Macro

Return the arguments as a single entity, i.e., wrap them into a pair of quotes.

The following example aims at emphasing the difference between (i), not using these macros, (ii), using `m4_quote`, and (iii), using `m4_dquote`.

    $ cat example.m4
    # Over quote, so that quotes are visible.
    m4_define([show], [$[]1 = [$1], $[]@ = [$@]])
    m4_divert(0)dnl
    show(a, b)
    show(m4_quote(a, b))
    show(m4_dquote(a, b))
    $ autom4te -l m4sugar example.m4
    $1 = a, $@ = [a],[b]
    $1 = a,b, $@ = [a,b]
    $1 = [a],[b], $@ = [[a],[b]]

Node:Forbidden Patterns, Previous:Evaluation Macros, Up:Programming in M4sugar

#### Forbidden Patterns

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

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

m4_pattern_forbid (pattern) | Macro

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

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

m4_pattern_allow (pattern) | Macro

Any token matching pattern is allowed, including if it matches an `m4_pattern_forbid` pattern.

Node:Programming in M4sh, Previous:Programming in M4sugar, Up:Programming in M4

### Programming in M4sh

M4sh provides portable alternatives for some common shell constructs that unfortunately are not portable in practice.

AS_DIRNAME (pathname) | Macro

Return the directory portion of pathname, using the algorithm required by POSIX. See Limitations of Usual Tools, for more details about what this returns and why it is more portable than the `dirname` command.

Node:Writing Autoconf Macros, Next:Portable Shell, Previous:Programming in M4, Up:Top

## Writing Autoconf Macros

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

- Macro Definitions: Basic format of an Autoconf macro
- Macro Names: What to call your new macros
- Reporting Messages: Notifying `autoconf` users
- Dependencies Between Macros: What to do when macros depend on other macros
- Obsoleting Macros: Warning about old ways of doing things
- Coding Style: Writing Autoconf macros à la Autoconf

Node:Macro Definitions, Next:Macro Names, Up:Writing Autoconf Macros

### Macro Definitions

Autoconf macros are defined using the `AC_DEFUN` macro, which is similar to the M4 builtin `m4_define` macro. In addition to defining a macro, `AC_DEFUN` adds to it some code that is used to constrain the order in which macros are called (see Prerequisite Macros).

An Autoconf macro definition looks like this:

    AC_DEFUN(macro-name, macro-body)

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

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

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

    # AC_MSG_ERROR(ERROR, [EXIT-STATUS = 1])
    # --------------------------------------
    m4_define([AC_MSG_ERROR],
    [{ _AC_ECHO([configure: error: $1], 2); exit m4_default([$2], 1); }])

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

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

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

Node:Macro Names, Next:Reporting Messages, Previous:Macro Definitions, Up:Writing Autoconf Macros

### Macro Names

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

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

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

`C`
  C language builtin features.
`DECL`
  Declarations of C variables in header files.
`FUNC`
  Functions in libraries.
`GROUP`
  UNIX group owners of files.
`HEADER`
  Header files.
`LIB`
  C libraries.
`PATH`
  The full path names to files, including programs.
`PROG`
  The base names of programs.
`MEMBER`
  Members of aggregates.
`SYS`
  Operating system features.
`TYPE`
  C builtin or declared types.
`VAR`
  C variables in libraries.

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

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

Node:Reporting Messages, Next:Dependencies Between Macros, Previous:Macro Names, Up:Writing Autoconf Macros

### Reporting Messages

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

AC_DIAGNOSE (category, message) | Macro

Report message as a warning (or as an error if requested by the user) if it falls into the category. You are encouraged to use standard categories, which currently include: `all` messages that don't fall into one of the following category. Use of an empty category is equivalent. `cross` related to cross compilation issues. `obsolete` use of an obsolete construct. `syntax` dubious syntactic constructs, incorrectly ordered macro calls.

AC_WARNING (message) | Macro

Equivalent to `AC_DIAGNOSE([syntax], message)`, but you are strongly encouraged to use a finer grained category.

AC_FATAL (message) | Macro

Report a severe error message, and have `autoconf` die.

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

Node:Dependencies Between Macros, Next:Obsoleting Macros, Previous:Reporting Messages, Up:Writing Autoconf Macros

### Dependencies Between Macros

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

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

Node:Prerequisite Macros, Next:Suggested Ordering, Up:Dependencies Between Macros

#### 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.

AC_REQUIRE (macro-name) | Macro

If the M4 macro macro-name has not already been called, call it (without any arguments). Make sure to quote macro-name with square brackets. macro-name must have been defined using `AC_DEFUN` or else contain a call to `AC_PROVIDE` to indicate that it has been called. `AC_REQUIRE` must be used inside an `AC_DEFUN`'d macro; it must not be called from the top level.

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

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

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

with this `configure.ac`

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

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

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

    fi

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

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

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

Node:Suggested Ordering, Previous:Prerequisite Macros, Up:Dependencies Between Macros

#### Suggested Ordering

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

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

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

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

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

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

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

Node:Obsoleting Macros, Next:Coding Style, Previous:Dependencies Between Macros, Up:Writing Autoconf Macros

### Obsoleting Macros

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

Autoconf provides a simple means to obsolete a macro.

AU_DEFUN (old-macro, implementation, [message]) | Macro

Define old-macro as implementation. The only difference with `AC_DEFUN` is that the user will be warned that old-macro is now obsolete. If she then uses `autoupdate`, the call to old-macro will be replaced by the modern implementation. The additional message is then printed.

Node:Coding Style, Previous:Obsoleting Macros, Up:Writing Autoconf Macros

### Coding Style

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

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

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

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

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

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

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

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

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

write

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

or even

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

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

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

    m4_bpatsubst([$1], [$"])

use

    m4_bpatsubst([$1], [$""])

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

    test $[#] != 0

and use:

    test $[@%:@] != 0

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

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

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

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

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

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

you would write:

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

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

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

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

and the new way:

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

Node:Portable Shell, Next:Manual Configuration, Previous:Writing Autoconf Macros, Up:Top

## Portable Shell Programming

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

    #! /usr/bin/perl

If you omit the space before the path, then 4.2BSD based systems (such as Sequent DYNIX) will ignore the line, because they interpret `#! /` as a 4-byte magic number. Some old systems have quite small limits on the length of the `#!` line too, for instance 32 bytes (not including the newline) on SunOS 4.

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

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

- Shellology: A zoology of shells
- Here-Documents: Quirks and tricks
- File Descriptors: FDs and redirections
- File System Conventions: File- and pathnames
- Shell Substitutions: Variable and command expansions
- Assignments: Varying side effects of assignments
- Special Shell Variables: Variables you should not change
- Limitations of Builtins: Portable use of not so portable /bin/sh
- Limitations of Usual Tools: Portable use of portable tools
- Limitations of Make: Portable Makefiles

Node:Shellology, Next:Here-Documents, Up:Portable Shell

### Shellology

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

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

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

  To be compatible with Ash 0.2:

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

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

  - don't use command substitution within variable expansion:

        cat ${FOO=`bar`}

  - beware that single builtin substitutions are not performed by a sub shell, hence their effect applies to the current shell! See Shell Substitutions, item "Command Substitution".
Bash
  To detect whether you are running `bash`, test if `BASH_VERSION` is set. To disable its extensions and require POSIX compatibility, run `set -o posix`. See Bash POSIX Mode, for details.
Bash 2.05 and later
  Versions 2.05 and later of `bash` use a different format for the output of the `set` builtin, designed to make evaluating this output easier. However, this output is not compatible with earlier versions of `bash` (or with many other shells, probably). So if you use `bash` 2.05 or higher to execute `configure`, you'll need to use `bash` 2.05 for all other build tasks as well.
`/usr/xpg4/bin/sh` on Solaris
  The POSIX-compliant Bourne shell on a Solaris system is `/usr/xpg4/bin/sh` and is part of an extra optional package. There is no extra charge for this package, but it is also not part of a minimal OS install and therefore some folks may not have it.
Zsh
  To detect whether you are running `zsh`, test if `ZSH_VERSION` is set. By default `zsh` is not compatible with the Bourne shell: you have to run `emulate sh` and set `NULLCMD` to `:`. See Compatibility, for details.

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

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

Russ Allbery:

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

Robert Lipe:

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

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

Node:Here-Documents, Next:File Descriptors, Previous:Shellology, Up:Portable Shell

### Here-Documents

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

With OpenBSD 2.7's `/bin/sh`

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

and with Bash:

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

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

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

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

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

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

Node:File Descriptors, Next:File System Conventions, Previous:Here-Documents, Up:Portable Shell

### File Descriptors

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

    3 --- some systems may open it to /dev/tty.
    4 --- used on the Kubota Titan.

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

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

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

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

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

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

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

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

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

Don't try to move/delete open files, such as in `exec >foo; mv foo bar`, see See Limitations of Builtins, `mv` for more details.

Node:File System Conventions, Next:Shell Substitutions, Previous:File Descriptors, Up:Portable Shell

### File System Conventions

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

For example, the following code:

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

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

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

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

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

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

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

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

  This is perfectly OK on Unices:

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

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

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

No leading dot (SFN)
  DOS cannot handle filenames that start with a dot. This is usually not a very important issue for `autoconf`.
Case insensitivity (LFN)
  DOS is case insensitive, so you cannot, for example, have both a file called `INSTALL` and a directory called `install`. This also affects `make`; if there's a file called `INSTALL` in the directory, `make install` will do nothing (unless the `install` target is marked as PHONY).
The 8+3 limit (SFN)
  Because the DOS file system only stores the first 8 characters of the filename and the first 3 of the extension, those must be unique. That means that `foobar-part1.c`, `foobar-part2.c` and `foobar-prettybird.c` all resolve to the same filename (`FOOBAR-P.C`). The same goes for `foo.bar` and `foo.bartender`.

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

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

Node:Shell Substitutions, Next:Assignments, Previous:File System Conventions, Up:Portable Shell

### Shell Substitutions

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

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

is more readable when written as:

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

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

`$@`
  One of the most famous shell-portability issues is related to `"$@"`: when there are no positional arguments, it is supposed to be equivalent to nothing. But some shells, for instance under Digital Unix 4.0 and 5.0, will then replace it with an empty argument. To be portable, use `${1+"$@"}`.
`${var:-value}`
  Old BSD shells, including the Ultrix `sh`, don't accept the colon for any shell substitution, and complain and die.
`${var=literal}`
  Be sure to quote:

      : ${var='Some words'}

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

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

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

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

`${var=expanded-value}`
  On Ultrix, running

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

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

      $ cat -v <<EOF
      $var
      EOF

  and

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

  One classic incarnation of this bug is:

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

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

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

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

      list=${list="$default"}

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

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

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

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

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

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

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

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

  nor does IRIX 6.5's Bourne shell:

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

Node:Assignments, Next:Special Shell Variables, Previous:Shell Substitutions, Up:Portable Shell

### Assignments

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

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

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

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

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

To assign default values, follow this algorithm:

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

       : ${var='my literal'}

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

       : ${var="$default"}

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

       var=${var="$default"}

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

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

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

Node:Special Shell Variables, Next:Limitations of Builtins, Previous:Assignments, Up:Portable Shell

### Special Shell Variables

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

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

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

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

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

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

      bash-2.02$ mkdir foo && (CDPATH=: cd foo)
      bash-2.02$ (CDPATH=:. cd foo)
      bash-2.02$ (CDPATH=.: cd foo)
      /tmp/foo

  Of course, even better style would be to use `PATH_SEPARATOR` instead of a `:`. Therefore, a portable solution to neutralize `CDPATH` is

      CDPATH=${ZSH_VERSION+.}$PATH_SEPARATOR

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

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

  Autoconf-generated scripts normally set all these variables to `C` because so much configuration code assumes the C locale and POSIX requires that `LC_ALL` be set to `C` if the C locale is desired. However, some older, nonstandard systems (notably SCO) break if `LC_ALL` is set to `C`, so when running on these systems Autoconf-generated scripts first try to unset the variables instead.

`LANGUAGE`

  `LANGUAGE` is not specified by POSIX, but it is a GNU extension that overrides `LC_ALL` in some cases, so Autoconf-generated scripts set it too.

`LINENO`
  Most modern shells provide the current line number in `LINENO`. Its value is the line number of the beginning of the current command. Autoconf attempts to execute `configure` with a modern shell. If no such shell is available, it attempts to implement `LINENO` with a Sed prepass that replaces the each instance of the string `$LINENO` (not followed by an alphanumeric character) with the line's number.

  You should not rely on `LINENO` within `eval`, as the behavior differs in practice. Also, the possibility of the Sed prepass means that you should not rely on `$LINENO` when quoted, when in here-documents, or when in long commands that cross line boundaries. Subshells should be OK, though. In the following example, lines 1, 6, and 9 are portable, but the other instances of `LINENO` are not:

      $ cat lineno
      echo 1. $LINENO
      cat <<EOF
      3. $LINENO
      4. $LINENO
      EOF
      ( echo 6. $LINENO )
      eval 'echo 7. $LINENO'
      echo 8. '$LINENO'
      echo 9. $LINENO '
      10.' $LINENO
      $ bash-2.05 lineno
      1. 1
      3. 2
      4. 2
      6. 6
      7. 1
      8. $LINENO
      9. 9
      10. 9
      $ zsh-3.0.6 lineno
      1. 1
      3. 2
      4. 2
      6. 6
      7. 7
      8. $LINENO
      9. 9
      10. 9
      $ pdksh-5.2.14 lineno
      1. 1
      3. 2
      4. 2
      6. 6
      7. 0
      8. $LINENO
      9. 9
      10. 9
      $ sed '=' <lineno |
      >   sed '
      >     N
      >     s,$,-,
      >     : loop
      >     s,^\([0-9]*\)\(.*\)[$]LINENO\([^a-zA-Z0-9_]\),\1\2\1\3,
      >     t loop
      >     s,-$,,
      >     s,^[0-9]*\n,,
      >   ' |
      >   sh
      1. 1
      3. 3
      4. 4
      6. 6
      7. 7
      8. 8
      9. 9
      10. 10

`NULLCMD`
  When executing the command `>foo`, `zsh` executes `$NULLCMD >foo`. The Bourne shell considers `NULLCMD` is `:`, while `zsh`, even in Bourne shell compatibility mode, sets `NULLCMD` to `cat`. If you forgot to set `NULLCMD`, your script might be suspended waiting for data on its standard input.
`status`
  This variable is an alias to `$?` for `zsh` (at least 3.1.6), hence read-only. Do not use it.
`PATH_SEPARATOR`
  If it is not set, `configure` will detect the appropriate path separator for the build system and set the `PATH_SEPARATOR` output variable accordingly.

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

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

Node:Limitations of Builtins, Next:Limitations of Usual Tools, Previous:Special Shell Variables, Up:Portable Shell

### Limitations of Shell Builtins

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

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

`.`
  Use `.` only with regular files (use `test -f`). Bash 2.03, for instance, chokes on `. /dev/null`. Also, remember that `.` uses `PATH` if its argument contains no slashes, so if you want to use `.` on a file `foo` in the current directory, you must use `. ./foo`.
`!`
  You can't use `!`, you'll have to rewrite your code.
`break`
  The use of `break 2`, etcetera, is safe.
`case`
  You don't need to quote the argument; no splitting is performed.

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

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

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

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

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

  Some shells, such as Ash 0.3.8, are confused by empty `case`/`esac`:

      ash-0.3.8 $ case foo in esac;
      error-->Syntax error: ";" unexpected (expecting ")")

  Many shells still do not support parenthesized cases, which is a pity for those of us using tools that rely on balanced parentheses. For instance, Solaris 2.8's Bourne shell:

      $ case foo in (foo) echo foo;; esac
      error-->syntax error: `(' unexpected

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

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

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

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

      cat <<EOF
      $foo
      EOF

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

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

  Using `exit $?` restores the expected behavior.

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

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

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

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

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

  when run with `FOO=foo` in the environment, these shells will print alternately `foo` and `bar`, although it should only print `foo` and then a sequence of `bar`s.

  Therefore you should `export` again each environment variable that you update.

`false`
  Don't expect `false` to exit with status 1: in the native Bourne shell of Solaris 8, it exits with status 255.
`for`
  To loop over positional arguments, use:

      for arg
      do
        echo "$arg"
      done

  You may not leave the `do` on the same line as `for`, since some shells improperly grok:

      for arg; do
        echo "$arg"
      done

  If you want to explicitly refer to the positional arguments, given the `$@` bug (see Shell Substitutions), use:

      for arg in ${1+"$@"}; do
        echo "$arg"
      done

`if`
  Using `!` is not portable. Instead of:

      if ! cmp -s file file.new; then
        mv file.new file
      fi

  use:

      if cmp -s file file.new; then :; else
        mv file.new file
      fi

  There are shells that do not reset the exit status from an `if`:

      $ if (exit 42); then true; fi; echo $?
      42

  whereas a proper shell should have printed `0`. This is especially bad in Makefiles since it produces false failures. This is why properly written Makefiles, such as Automake's, have such hairy constructs:

      if test -f "$file"; then
        install "$file" "$dest"
      else
        :
      fi

`set`
  This builtin faces the usual problem with arguments starting with a dash. Modern shells such as Bash or Zsh understand `--` to specify the end of the options (any argument after `--` is a parameters, even `-x` for instance), but most shells simply stop the option processing as soon as a non-option argument is found. Therefore, use `dummy` or simply `x` to end the option processing, and use `shift` to pop it out:

      set x $my_list; shift

`shift`
  Not only is `shift`ing a bad idea when there is nothing left to shift, but in addition it is not portable: the shell of MIPS RISC/OS 4.52 refuses to do it.
`source`
  This command is not portable, as POSIX does not require it; use `.` instead.
`test`
  The `test` program is the way to perform many file and string tests. It is often invoked by the alternate name `[`, but using that name in Autoconf code is asking for trouble since it is an M4 quote character.

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

  You may use `!` with `test`, but not with `if`: `test ! -r foo || exit 1`.

`test` (files)
  To enable `configure` scripts to support cross-compilation, they shouldn't do anything that tests features of the build system instead of the host system. But occasionally you may find it necessary to check whether some arbitrary file exists. To do so, use `test -f` or `test -r`. Do not use `test -x`, because 4.3BSD does not have it. Do not use `test -e` either, because Solaris 2.5 does not have it.
`test` (strings)
  Avoid `test "string"`, in particular if string might start with a dash, since `test` might interpret its argument as an option (e.g., `string = "-n"`).

  Contrary to a common belief, `test -n string` and `test -z string` are portable, nevertheless many shells (such as Solaris 2.5, AIX 3.2, UNICOS 10.0.0.6, Digital Unix 4 etc.) have bizarre precedence and may be confused if string looks like an operator:

      $ test -n =
      test: argument expected

  If there are risks, use `test "xstring" = x` or `test "xstring" != x` instead.

  It is frequent to find variations of the following idiom:

      test -n "`echo $ac_feature | sed 's/[-a-zA-Z0-9_]//g'`" &&
        action

  to take an action when a token matches a given pattern. Such constructs should always be avoided by using:

      echo "$ac_feature" | grep '[^-a-zA-Z0-9_]' >/dev/null 2>&1 &&
        action

  Use `case` where possible since it is faster, being a shell builtin:

      case $ac_feature in
        *[!-a-zA-Z0-9_]*) action;;
      esac

  Alas, negated character classes are probably not portable, although no shell is known to not support the POSIX.2 syntax `[!...]` (when in interactive mode, `zsh` is confused by the `[!...]` syntax and looks for an event in its history because of `!`). Many shells do not support the alternative syntax `[^...]` (Solaris, Digital Unix, etc.).

  One solution can be:

      expr "$ac_feature" : '.*[^-a-zA-Z0-9_]' >/dev/null &&
        action

  or better yet

      expr "x$ac_feature" : '.*[^-a-zA-Z0-9_]' >/dev/null &&
        action

  `expr "Xfoo" : "Xbar"` is more robust than `echo "Xfoo" | grep "^Xbar"`, because it avoids problems when `foo` contains backslashes.

`trap`
  It is safe to trap at least the signals 1, 2, 13 and 15. You can also trap 0, i.e., have the `trap` run when the script ends (either via an explicit `exit`, or the end of the script).

  Although POSIX is not absolutely clear on this point, it is widely admitted that when entering the trap `$?` should be set to the exit status of the last command run before the trap. The ambiguity can be summarized as: "when the trap is launched by an `exit`, what is the last command run: that before `exit`, or `exit` itself?"

  Bash considers `exit` to be the last command, while Zsh and Solaris 8 `sh` consider that when the trap is run it is still in the `exit`, hence it is the previous exit status that the trap receives:

      $ cat trap.sh
      trap 'echo $?' 0
      (exit 42); exit 0
      $ zsh trap.sh
      42
      $ bash trap.sh
      0

  The portable solution is then simple: when you want to `exit 42`, run `(exit 42); exit 42`, the first `exit` being used to set the exit status to 42 for Zsh, and the second to trigger the trap and pass 42 as exit status for Bash.

  The shell in FreeBSD 4.0 has the following bug: `$?` is reset to 0 by empty lines if the code is inside `trap`.

      $ trap 'false

      echo $?' 0
      $ exit
      0

  Fortunately, this bug only affects `trap`.

`true`
  Don't worry: as far as we know `true` is portable. Nevertheless, it's not always a builtin (e.g., Bash 1.x), and the portable shell community tends to prefer using `:`. This has a funny side effect: when asked whether `false` is more portable than `true` Alexandre Oliva answered:

  > In a sense, yes, because if it doesn't exist, the shell will produce an exit status of failure, which is correct for `false`, but not for `true`.

`unset`
  You cannot assume the support of `unset`, nevertheless, because it is extremely useful to disable embarrassing variables such as `CDPATH`, you can test for its existence and use it provided you give a neutralizing value when `unset` is not supported:

      if (unset FOO) >/dev/null 2>&1; then
        unset=unset
      else
        unset=false
      fi
      $unset CDPATH || CDPATH=:

  See Special Shell Variables, for some neutralizing values. Also, see Limitations of Builtins, documentation of `export`, for the case of environment variables.

Node:Limitations of Usual Tools, Next:Limitations of Make, Previous:Limitations of Builtins, Up:Portable Shell

### Limitations of Usual Tools

The small set of tools you can expect to find on any machine can still include some limitations you should be aware of.

`awk`
  Don't leave white spaces before the parentheses in user functions calls, GNU awk will reject it:

      $ gawk 'function die () { print "Aaaaarg!"  }
              BEGIN { die () }'
      gawk: cmd. line:2:         BEGIN { die () }
      gawk: cmd. line:2:                      ^ parse error
      $ gawk 'function die () { print "Aaaaarg!"  }
              BEGIN { die() }'
      Aaaaarg!

  If you want your program to be deterministic, don't depend on `for` on arrays:

      $ cat for.awk
      END {
        arr["foo"] = 1
        arr["bar"] = 1
        for (i in arr)
          print i
      }
      $ gawk -f for.awk </dev/null
      foo
      bar
      $ nawk -f for.awk </dev/null
      bar
      foo

  Some AWK, such as HPUX 11.0's native one, have regex engines fragile to inner anchors:

      $ echo xfoo | $AWK '/foo|^bar/ { print }'
      $ echo bar | $AWK '/foo|^bar/ { print }'
      bar
      $ echo xfoo | $AWK '/^bar|foo/ { print }'
      xfoo
      $ echo bar | $AWK '/^bar|foo/ { print }'
      bar

  Either do not depend on such patterns (i.e., use `/^(.*foo|bar)/`, or use a simple test to reject such AWK.

`cat`
  Don't rely on any option. The option `-v`, which displays non-printing characters, seems portable, though.
`cc`
  When a compilation such as `cc foo.c -o foo` fails, some compilers (such as CDS on Reliant UNIX) leave a `foo.o`.

  HP-UX `cc` doesn't accept `.S` files to preprocess and assemble. `cc -c foo.S` will appear to succeed, but in fact does nothing.

`cmp`
  `cmp` performs a raw data comparison of two files, while `diff` compares two text files. Therefore, if you might compare DOS files, even if only checking whether two files are different, use `diff` to avoid spurious differences due to differences of newline encoding.
`cp`
  SunOS `cp` does not support `-f`, although its `mv` does. It's possible to deduce why `mv` and `cp` are different with respect to `-f`. `mv` prompts by default before overwriting a read-only file. `cp` does not. Therefore, `mv` requires a `-f` option, but `cp` does not. `mv` and `cp` behave differently with respect to read-only files because the simplest form of `cp` cannot overwrite a read-only file, but the simplest form of `mv` can. This is because `cp` opens the target for write access, whereas `mv` simply calls `link` (or, in newer systems, `rename`).
`date`
  Some versions of `date` do not recognize special % directives, and unfortunately, instead of complaining, they just pass them through, and exit with success:

      $ uname -a
      OSF1 medusa.sis.pasteur.fr V5.1 732 alpha
      $ date "+%s"
      %s

`diff`
  Option `-u` is nonportable.

  Some implementations, such as Tru64's, fail when comparing to `/dev/null`. Use an empty file instead.

`dirname`
  Not all hosts have a working `dirname`, and you should instead use `AS_DIRNAME` (see Programming in M4sh). For example:

      dir=`dirname "$file"`       # This is not portable.
      dir=`AS_DIRNAME(["$file"])` # This is more portable.

  This handles a few subtleties in the standard way required by POSIX. For example, under UN*X, should `dirname //1` give `/`? Paul Eggert answers:

  > No, under some older flavors of Unix, leading `//` is a special path name: it refers to a "super-root" and is used to access other machines' files. Leading `///`, `////`, etc. are equivalent to `/`; but leading `//` is special. I think this tradition started with Apollo Domain/OS, an OS that is still in use on some older hosts.

  > POSIX allows but does not require the special treatment for `//`. It says that the behavior of dirname on path names of the form `//([^/]+/*)?` is implementation defined. In these cases, GNU `dirname` returns `/`, but it's more portable to return `//` as this works even on those older flavors of Unix.

`egrep`
  The empty alternative is not portable, use `?` instead. For instance with Digital Unix v5.0:

      > printf "foo\n|foo\n" | egrep '^(|foo|bar)$'
      |foo
      > printf "bar\nbar|\n" | egrep '^(foo|bar|)$'
      bar|
      > printf "foo\nfoo|\n|bar\nbar\n" | egrep '^(foo||bar)$'
      foo
      |bar

  `egrep` also suffers the limitations of `grep`.

`expr`
  No `expr` keyword starts with `x`, so use `expr x"word" : 'xregex'` to keep `expr` from misinterpreting word.

  Don't use `length`, `substr`, `match` and `index`.

`expr` (`|`)
  You can use `|`. Although POSIX does require that `expr ''` return the empty string, it does not specify the result when you `|` together the empty string (or zero) with the empty string. For example:

      expr '' \| ''

  GNU/Linux and POSIX.2-1992 return the empty string for this case, but traditional Unix returns `0` (Solaris is one such example). In the latest POSIX draft, the specification has been changed to match traditional Unix's behavior (which is bizarre, but it's too late to fix this). Please note that the same problem does arise when the empty string results from a computation, as in:

      expr bar : foo \| foo : bar

  Avoid this portability problem by avoiding the empty string.

`expr` (`:`)
  Don't use `\?`, `\+` and `\|` in patterns, they are not supported on Solaris.

  The POSIX.2-1992 standard is ambiguous as to whether `expr a : b` (and `expr 'a' : '\(b\)'`) output `0` or the empty string. In practice, it outputs the empty string on most platforms, but portable scripts should not assume this. For instance, the QNX 4.25 native `expr` returns `0`.

  You may believe that one means to get a uniform behavior would be to use the empty string as a default value:

      expr a : b \| ''

  unfortunately this behaves exactly as the original expression, see the ``expr` (`:`)` entry for more information.

  Older `expr` implementations (e.g. SunOS 4 `expr` and Solaris 8 `/usr/ucb/expr`) have a silly length limit that causes `expr` to fail if the matched substring is longer than 120 bytes. In this case, you might want to fall back on `echo|sed` if `expr` fails.

  Don't leave, there is some more!

  The QNX 4.25 `expr`, in addition of preferring `0` to the empty string, has a funny behavior in its exit status: it's always 1 when parentheses are used!

      $ val=`expr 'a' : 'a'`; echo "$?: $val"
      0: 1
      $ val=`expr 'a' : 'b'`; echo "$?: $val"
      1: 0

      $ val=`expr 'a' : '\(a\)'`; echo "?: $val"
      1: a
      $ val=`expr 'a' : '\(b\)'`; echo "?: $val"
      1: 0

  In practice this can be a big problem if you are ready to catch failures of `expr` programs with some other method (such as using `sed`), since you may get twice the result. For instance

      $ expr 'a' : '\(a\)' || echo 'a' | sed 's/^\(a\)$/\1/'

  will output `a` on most hosts, but `aa` on QNX 4.25. A simple work around consists in testing `expr` and use a variable set to `expr` or to `false` according to the result.

`find`
  The option `-maxdepth` seems to be GNU specific. Tru64 v5.1, NetBSD 1.5 and Solaris 2.5 `find` commands do not understand it.

  The replacement of `{}` is guaranteed only if the argument is exactly {}, not if it's only a part of an argument. For instance on DU, and HP-UX 10.20 and HP-UX 11:

      $ touch foo
      $ find . -name foo -exec echo "{}-{}" \;
      {}-{}

  while GNU `find` reports `./foo-./foo`.

`grep`
  Don't use `grep -s` to suppress output, because `grep -s` on System V does not suppress output, only error messages. Instead, redirect the standard output and standard error (in case the file doesn't exist) of `grep` to `/dev/null`. Check the exit status of `grep` to determine whether it found a match.

  Don't use multiple regexps with `-e`, as some `grep` will only honor the last pattern (eg., IRIX 6.5 and Solaris 2.5.1). Anyway, Stardent Vistra SVR4 `grep` lacks `-e`... Instead, use alternation and `egrep`.

`ln`
  Don't rely on `ln` having a `-f` option. Symbolic links are not available on old systems, use `ln` as a fall back.

  For versions of the DJGPP before 2.04, `ln` emulates soft links for executables by generating a stub that in turn calls the real program. This feature also works with nonexistent files like in the Unix spec. So `ln -s file link` will generate `link.exe`, which will attempt to call `file.exe` if run. But this feature only works for executables, so `cp -p` is used instead for these systems. DJGPP versions 2.04 and later have full symlink support.

`mv`
  The only portable options are `-f` and `-i`.

  Moving individual files between file systems is portable (it was in V6), but it is not always atomic: when doing `mv new existing`, there's a critical section where neither the old nor the new version of `existing` actually exists.

  Moving directories across mount points is not portable, use `cp` and `rm`.

  Moving/Deleting open files isn't portable. The following can't be done on DOS/WIN32:

      exec > foo
      mv foo bar

  nor can

      exec > foo
      rm -f foo

`sed`
  Patterns should not include the separator (unless escaped), even as part of a character class. In conformance with POSIX, the Cray `sed` will reject `s/[^/]*$//`: use `s,[^/]*$,,`.

  Sed scripts should not use branch labels longer than 8 characters and should not contain comments.

  Don't include extra `;`, as some `sed`, such as NetBSD 1.4.2's, try to interpret the second as a command:

      $ echo a | sed 's/x/x/;;s/x/x/'
      sed: 1: "s/x/x/;;s/x/x/": invalid command code ;

  Input should have reasonably long lines, since some `sed` have an input buffer limited to 4000 bytes.

  Alternation, `\|`, is common but POSIX.2 does not require its support, so it should be avoided in portable scripts. Solaris 8 `sed` does not support alternation; e.g. `sed '/a\|b/d'` deletes only lines that contain the literal string `a|b`.

  Anchors (`^` and `$`) inside groups are not portable.

  Nested parenthesization in patterns (e.g., `\(\(a*\)b*)\)`) is quite portable to modern hosts, but is not supported by some older `sed` implementations like SVR3.

  Of course the option `-e` is portable, but it is not needed. No valid Sed program can start with a dash, so it does not help disambiguating. Its sole usefulness is helping enforcing indenting as in:

      sed -e instruction-1 \
          -e instruction-2

  as opposed to

      sed instruction-1;instruction-2

  Contrary to yet another urban legend, you may portably use `&` in the replacement part of the `s` command to mean "what was matched". All descendents of Bell Lab's V7 `sed` (at least; we don't have first hand experience with older `sed`s) have supported it.

`sed` (`t`)
  Some old systems have `sed` that "forget" to reset their `t` flag when starting a new cycle. For instance on MIPS RISC/OS, and on IRIX 5.3, if you run the following `sed` script (the line numbers are not actual part of the texts):

      s/keep me/kept/g  # a
      t end             # b
      s/.*/deleted/g    # c
      : end             # d

  on

      delete me         # 1
      delete me         # 2
      keep me           # 3
      delete me         # 4

  you get

      deleted
      delete me
      kept
      deleted

  instead of

      deleted
      deleted
      kept
      deleted

  Why? When processing 1, a matches, therefore sets the t flag, b jumps to d, and the output is produced. When processing line 2, the t flag is still set (this is the bug). Line a fails to match, but `sed` is not supposed to clear the t flag when a substitution fails. Line b sees that the flag is set, therefore it clears it, and jumps to d, hence you get `delete me` instead of `deleted`. When processing 3 t is clear, a matches, so the flag is set, hence b clears the flags and jumps. Finally, since the flag is clear, 4 is processed properly.

  There are two things one should remind about `t` in `sed`. Firstly, always remember that `t` jumps if some substitution succeeded, not only the immediately preceding substitution, therefore, always use a fake `t clear; : clear` to reset the t flag where indeed.

  Secondly, you cannot rely on `sed` to clear the flag at each new cycle.

  One portable implementation of the script above is:

      t clear
      : clear
      s/keep me/kept/g
      t end
      s/.*/deleted/g
      : end

`touch`
  On some old BSD systems, `touch` or any command that results in an empty file does not update the timestamps, so use a command like `echo` as a workaround.

  GNU `touch` 3.16r (and presumably all before that) fails to work on SunOS 4.1.3 when the empty file is on an NFS-mounted 4.2 volume.

Node:Limitations of Make, Previous:Limitations of Usual Tools, Up:Portable Shell

### Limitations of Make

Make itself suffers a great number of limitations, only a few of which being listed here. First of all, remember that since commands are executed by the shell, all its weaknesses are inherited...

`$<`
  POSIX says that the `$<` construct in makefiles can be used only in inference rules and in the `.DEFAULT` rule; its meaning in ordinary rules is unspecified. Solaris 8's `make` for instance will replace it with the argument.
Leading underscore in macro names
  Some Make don't support leading underscores in macro names, such as on NEWS-OS 4.2R.

      $ cat Makefile
      _am_include = #
      _am_quote =
      all:; @echo this is test
      $ make
      Make: Must be a separator on rules line 2.  Stop.
      $ cat Makefile2
      am_include = #
      am_quote =
      all:; @echo this is test
      $ make -f Makefile2
      this is test

`VPATH`
  Don't use it! For instance any assignment to `VPATH` causes Sun `make` to only execute the first set of double-colon rules.

Node:Manual Configuration, Next:Site Configuration, Previous:Portable Shell, Up:Top

## Manual Configuration

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

- Specifying Names: Specifying the system type
- Canonicalizing: Getting the canonical system type
- Using System Type: What to do with the system type

Node:Specifying Names, Next:Canonicalizing, Up:Manual Configuration

### Specifying the System Type

Like other GNU `configure` scripts, Autoconf-generated `configure` scripts can make decisions based on a canonical name for the system type, which has the form: `cpu-vendor-os`, where os can be `system` or `kernel-system`

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

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

`--build=build-type`
  the type of system on which the package is being configured and compiled. It defaults to the result of running `config.guess`.
`--host=host-type`
  the type of system on which the package will run. By default it is the same as the build machine. Specifying it enables the cross-compilation mode.
`--target=target-type`
  the type of system for which any compiler tools in the package will produce code (rarely needed). By default, it is the same as host.

If you mean to override the result of `config.guess`, use `--build`, not `--host`, since the latter enables cross-compilation. For historical reasons, passing `--host` also changes the build type. Therefore, whenever you specify `--host`, be sure to specify `--build` too. This will be fixed in the future.

    ./configure --build=i686-pc-linux-gnu --host=m68k-coff

will enter cross-compilation mode, but `configure` will fail if it can't run the code generated by the specified compiler if you configure as follows:

    ./configure CC=m68k-coff-gcc

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

This section deliberately omits the description of the obsolete interface, see Hosts and Cross-Compilation.

Node:Canonicalizing, Next:Using System Type, Previous:Specifying Names, Up:Manual Configuration

### Getting the Canonical System Type

The following macros make the system type available to `configure` scripts.

The variables `build_alias`, `host_alias`, and `target_alias` are always exactly the arguments of `--build`, `--host`, and `--target`; in particular, they are left empty if the user did not use them, even if the corresponding `AC_CANONICAL` macro was run. Any configure script may use these variables anywhere. These are the variables that should be used when in interaction with the user.

If you need to recognize some special environments based on their system type, run the following macros to get canonical system names. These variables are not set before the macro call.

If you use these macros, you must distribute `config.guess` and `config.sub` along with your source code. See Output, for information about the `AC_CONFIG_AUX_DIR` macro which you can use to control in which directory `configure` looks for those scripts.

AC_CANONICAL_BUILD | Macro

Compute the canonical build-system type variable, `build`, and its three individual parts `build_cpu`, `build_vendor`, and `build_os`. If `--build` was specified, then `build` is the canonicalization of `build_alias` by `config.sub`, otherwise it is determined by the shell script `config.guess`.

AC_CANONICAL_HOST | Macro

Compute the canonical host-system type variable, `host`, and its three individual parts `host_cpu`, `host_vendor`, and `host_os`. If `--host` was specified, then `host` is the canonicalization of `host_alias` by `config.sub`, otherwise it defaults to `build`.

AC_CANONICAL_TARGET | Macro

Compute the canonical target-system type variable, `target`, and its three individual parts `target_cpu`, `target_vendor`, and `target_os`. If `--target` was specified, then `target` is the canonicalization of `target_alias` by `config.sub`, otherwise it defaults to `host`.

Note that there can be artifacts due to the backward compatibility code. See Hosts and Cross-Compilation, for more.

Node:Using System Type, Previous:Canonicalizing, Up:Manual Configuration

### Using the System Type

How do you use a canonical system type? Usually, you use it in one or more `case` statements in `configure.ac` to select system-specific C files. Then, using `AC_CONFIG_LINKS`, link those files which have names based on the system name, to generic names, such as `host.h` or `target.c` (see Configuration Links). The `case` statement patterns can use shell wild cards to group several cases together, like in this fragment:

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

and later in `configure.ac`, use:

    AC_CONFIG_LINKS(host.h:config/$machine.h
                    object.h:config/$obj_format.h)

Note that the above example uses `$target` because it's taken from a tool which can be built on some architecture (`$build`), run on another (`$host`), but yet handle data for a third architecture (`$target`). Such tools are usually part of a compiler suite, they generate code for a specific `$target`.

However `$target` should be meaningless for most packages. If you want to base a decision on the system where your program will be run, make sure you use the `$host` variable, as in the following excerpt:

    case $host in
      *-*-msdos* | *-*-go32* | *-*-mingw32* | *-*-cygwin* | *-*-windows*)
        MUMBLE_INIT="mumble.ini"
        ;;
      *)
        MUMBLE_INIT=".mumbleinit"
        ;;
    esac
    AC_SUBST([MUMBLE_INIT])

You can also use the host system type to find cross-compilation tools. See Generic Programs, for information about the `AC_CHECK_TOOL` macro which does that.

Node:Site Configuration, Next:Running configure scripts, Previous:Manual Configuration, Up:Top

## Site Configuration

`configure` scripts support several kinds of local configuration decisions. There are ways for users to specify where external software packages are, include or exclude optional features, install programs under modified names, and set default values for `configure` options.

- External Software: Working with other optional software
- Package Options: Selecting optional features
- Pretty Help Strings: Formating help string
- Site Details: Configuring site details
- Transforming Names: Changing program names when installing
- Site Defaults: Giving `configure` local defaults

Node:External Software, Next:Package Options, Up:Site Configuration

### Working With External Software

Some packages require, or can optionally use, other software packages that are already installed. The user can give `configure` command line options to specify which such external software to use. The options have one of these forms:

    --with-package[=arg]
    --without-package

For example, `--with-gnu-ld` means work with the GNU linker instead of some other linker. `--with-x` means work with The X Window System.

The user can give an argument by following the package name with `=` and the argument. Giving an argument of `no` is for packages that are used by default; it says to not use the package. An argument that is neither `yes` nor `no` could include a name or number of a version of the other package, to specify more precisely which other package this program is supposed to work with. If no argument is given, it defaults to `yes`. `--without-package` is equivalent to `--with-package=no`.

`configure` scripts do not complain about `--with-package` options that they do not support. This behavior permits configuring a source tree containing multiple packages with a top-level `configure` script when the packages support different options, without spurious error messages about options that some of the packages support. An unfortunate side effect is that option spelling errors are not diagnosed. No better approach to this problem has been suggested so far.

For each external software package that may be used, `configure.ac` should call `AC_ARG_WITH` to detect whether the `configure` user asked to use it. Whether each package is used or not by default, and which arguments are valid, is up to you.

AC_ARG_WITH (package, help-string, [action-if-given], [action-if-not-given]) | Macro

If the user gave `configure` the option `--with-package` or `--without-package`, run shell commands action-if-given. If neither option was given, run shell commands action-if-not-given. The name package indicates another software package that this program should work with. It should consist only of alphanumeric characters and dashes. The option's argument is available to the shell commands action-if-given in the shell variable `withval`, which is actually just the value of the shell variable `with_package`, with any `-` characters changed into `_`. You may use that variable instead, if you wish. The argument help-string is a description of the option that looks like this:
      --with-readline         support fancy command line editing
help-string may be more than one line long, if more detail is needed. Just make sure the columns line up in `configure --help`. Avoid tabs in the help string. You'll need to enclose it in `[` and `]` in order to produce the leading spaces. You should format your help-string with the macro `AC_HELP_STRING` (see Pretty Help Strings).

AC_WITH (package, action-if-given, [action-if-not-given]) | Macro

This is an obsolete version of `AC_ARG_WITH` that does not support providing a help string.

Node:Package Options, Next:Pretty Help Strings, Previous:External Software, Up:Site Configuration

### Choosing Package Options

If a software package has optional compile-time features, the user can give `configure` command line options to specify whether to compile them. The options have one of these forms:

    --enable-feature[=arg]
    --disable-feature

These options allow users to choose which optional features to build and install. `--enable-feature` options should never make a feature behave differently or cause one feature to replace another. They should only cause parts of the program to be built rather than left out.

The user can give an argument by following the feature name with `=` and the argument. Giving an argument of `no` requests that the feature not be made available. A feature with an argument looks like `--enable-debug=stabs`. If no argument is given, it defaults to `yes`. `--disable-feature` is equivalent to `--enable-feature=no`.

`configure` scripts do not complain about `--enable-feature` options that they do not support. This behavior permits configuring a source tree containing multiple packages with a top-level `configure` script when the packages support different options, without spurious error messages about options that some of the packages support. An unfortunate side effect is that option spelling errors are not diagnosed. No better approach to this problem has been suggested so far.

For each optional feature, `configure.ac` should call `AC_ARG_ENABLE` to detect whether the `configure` user asked to include it. Whether each feature is included or not by default, and which arguments are valid, is up to you.

AC_ARG_ENABLE (feature, help-string, [action-if-given], [action-if-not-given]) | Macro

If the user gave `configure` the option `--enable-feature` or `--disable-feature`, run shell commands action-if-given. If neither option was given, run shell commands action-if-not-given. The name feature indicates an optional user-level facility. It should consist only of alphanumeric characters and dashes. The option's argument is available to the shell commands action-if-given in the shell variable `enableval`, which is actually just the value of the shell variable `enable_feature`, with any `-` characters changed into `_`. You may use that variable instead, if you wish. The help-string argument is like that of `AC_ARG_WITH` (see External Software). You should format your help-string with the macro `AC_HELP_STRING` (see Pretty Help Strings).

AC_ENABLE (feature, action-if-given, [action-if-not-given]) | Macro

This is an obsolete version of `AC_ARG_ENABLE` that does not support providing a help string.

Node:Pretty Help Strings, Next:Site Details, Previous:Package Options, Up:Site Configuration

### Making Your Help Strings Look Pretty

Properly formatting the `help strings` which are used in `AC_ARG_WITH` (see External Software) and `AC_ARG_ENABLE` (see Package Options) can be challenging. Specifically, you want your own `help strings` to line up in the appropriate columns of `configure --help` just like the standard Autoconf `help strings` do. This is the purpose of the `AC_HELP_STRING` macro.

AC_HELP_STRING (left-hand-side, right-hand-side) | Macro

Expands into an help string that looks pretty when the user executes `configure --help`. It is typically used in `AC_ARG_WITH` (see External Software) or `AC_ARG_ENABLE` (see Package Options). The following example will make this clearer.
    AC_DEFUN(TEST_MACRO,
    [AC_ARG_WITH(foo,
                 AC_HELP_STRING([--with-foo],
                                [use foo (default is NO)]),
                 ac_cv_use_foo=$withval, ac_cv_use_foo=no),
    AC_CACHE_CHECK(whether to use foo,
                   ac_cv_use_foo, ac_cv_use_foo=no)])
Please note that the call to `AC_HELP_STRING` is unquoted. Then the last few lines of `configure --help` will appear like this:
    --enable and --with options recognized:
      --with-foo              use foo (default is NO)
The `AC_HELP_STRING` macro is particularly helpful when the left-hand-side and/or right-hand-side are composed of macro arguments, as shown in the following example.
    AC_DEFUN(MY_ARG_WITH,
    [AC_ARG_WITH([$1],
                 AC_HELP_STRING([--with-$1], [use $1 (default is $2)]),
                 ac_cv_use_$1=$withval, ac_cv_use_$1=no),
    AC_CACHE_CHECK(whether to use $1, ac_cv_use_$1, ac_cv_use_$1=$2)])

Node:Site Details, Next:Transforming Names, Previous:Pretty Help Strings, Up:Site Configuration

### Configuring Site Details

Some software packages require complex site-specific information. Some examples are host names to use for certain services, company names, and email addresses to contact. Since some configuration scripts generated by Metaconfig ask for such information interactively, people sometimes wonder how to get that information in Autoconf-generated configuration scripts, which aren't interactive.

Such site configuration information should be put in a file that is edited only by users, not by programs. The location of the file can either be based on the `prefix` variable, or be a standard location such as the user's home directory. It could even be specified by an environment variable. The programs should examine that file at run time, rather than at compile time. Run time configuration is more convenient for users and makes the configuration process simpler than getting the information while configuring. See Variables for Installation Directories, for more information on where to put data files.

Node:Transforming Names, Next:Site Defaults, Previous:Site Details, Up:Site Configuration

### Transforming Program Names When Installing

Autoconf supports changing the names of programs when installing them. In order to use these transformations, `configure.ac` must call the macro `AC_ARG_PROGRAM`.

AC_ARG_PROGRAM | Macro

Place in output variable `program_transform_name` a sequence of `sed` commands for changing the names of installed programs. If any of the options described below are given to `configure`, program names are transformed accordingly. Otherwise, if `AC_CANONICAL_TARGET` has been called and a `--target` value is given, the target type followed by a dash is used as a prefix. Otherwise, no program name transformation is done.

- Transformation Options: `configure` options to transform names
- Transformation Examples: Sample uses of transforming names
- Transformation Rules: `Makefile` uses of transforming names

Node:Transformation Options, Next:Transformation Examples, Up:Transforming Names

#### Transformation Options

You can specify name transformations by giving `configure` these command line options:

`--program-prefix=prefix`
  prepend prefix to the names;
`--program-suffix=suffix`
  append suffix to the names;
`--program-transform-name=expression`
  perform `sed` substitution expression on the names.

Node:Transformation Examples, Next:Transformation Rules, Previous:Transformation Options, Up:Transforming Names

#### Transformation Examples

These transformations are useful with programs that can be part of a cross-compilation development environment. For example, a cross-assembler running on a Sun 4 configured with `--target=i960-vxworks` is normally installed as `i960-vxworks-as`, rather than `as`, which could be confused with a native Sun 4 assembler.

You can force a program name to begin with `g`, if you don't want GNU programs installed on your system to shadow other programs with the same name. For example, if you configure GNU `diff` with `--program-prefix=g`, then when you run `make install` it is installed as `/usr/local/bin/gdiff`.

As a more sophisticated example, you could use

    --program-transform-name='s/^/g/; s/^gg/g/; s/^gless/less/'

to prepend `g` to most of the program names in a source tree, excepting those like `gdb` that already have one and those like `less` and `lesskey` that aren't GNU programs. (That is assuming that you have a source tree containing those programs that is set up to use this feature.)

One way to install multiple versions of some programs simultaneously is to append a version number to the name of one or both. For example, if you want to keep Autoconf version 1 around for awhile, you can configure Autoconf version 2 using `--program-suffix=2` to install the programs as `/usr/local/bin/autoconf2`, `/usr/local/bin/autoheader2`, etc. Nevertheless, pay attention that only the binaries are renamed, therefore you'd have problems with the library files which might overlap.

Node:Transformation Rules, Previous:Transformation Examples, Up:Transforming Names

#### Transformation Rules

Here is how to use the variable `program_transform_name` in a `Makefile.in`:

    PROGRAMS = cp ls rm
    transform = @program_transform_name@
    install:
            for p in $(PROGRAMS); do \
              $(INSTALL_PROGRAM) $$p $(DESTDIR)$(bindir)/`echo $$p | \
                                                  sed '$(transform)'`; \
            done

    uninstall:
            for p in $(PROGRAMS); do \
              rm -f $(DESTDIR)$(bindir)/`echo $$p | sed '$(transform)'`; \
            done

It is guaranteed that `program_transform_name` is never empty, and that there are no useless separators. Therefore you may safely embed `program_transform_name` within a sed program using `;`:

    transform = @program_transform_name@
    transform_exe = s/$(EXEEXT)$$//;$(transform);s/$$/$(EXEEXT)/

Whether to do the transformations on documentation files (Texinfo or `man`) is a tricky question; there seems to be no perfect answer, due to the several reasons for name transforming. Documentation is not usually particular to a specific architecture, and Texinfo files do not conflict with system documentation. But they might conflict with earlier versions of the same files, and `man` pages sometimes do conflict with system documentation. As a compromise, it is probably best to do name transformations on `man` pages but not on Texinfo manuals.

Node:Site Defaults, Previous:Transforming Names, Up:Site Configuration

### Setting Site Defaults

Autoconf-generated `configure` scripts allow your site to provide default values for some configuration values. You do this by creating site- and system-wide initialization files.

If the environment variable `CONFIG_SITE` is set, `configure` uses its value as the name of a shell script to read. Otherwise, it reads the shell script `prefix/share/config.site` if it exists, then `prefix/etc/config.site` if it exists. Thus, settings in machine-specific files override those in machine-independent ones in case of conflict.

Site files can be arbitrary shell scripts, but only certain kinds of code are really appropriate to be in them. Because `configure` reads any cache file after it has read any site files, a site file can define a default cache file to be shared between all Autoconf-generated `configure` scripts run on that system (see Cache Files). If you set a default cache file in a site file, it is a good idea to also set the output variable `CC` in that site file, because the cache file is only valid for a particular compiler, but many systems have several available.

You can examine or override the value set by a command line option to `configure` in a site file; options set shell variables that have the same names as the options, with any dashes turned into underscores. The exceptions are that `--without-` and `--disable-` options are like giving the corresponding `--with-` or `--enable-` option and the value `no`. Thus, `--cache-file=localcache` sets the variable `cache_file` to the value `localcache`; `--enable-warnings=no` or `--disable-warnings` sets the variable `enable_warnings` to the value `no`; `--prefix=/usr` sets the variable `prefix` to the value `/usr`; etc.

Site files are also good places to set default values for other output variables, such as `CFLAGS`, if you need to give them non-default values: anything you would normally do, repetitively, on the command line. If you use non-default values for prefix or exec_prefix (wherever you locate the site file), you can set them in the site file if you specify it with the `CONFIG_SITE` environment variable.

You can set some cache values in the site file itself. Doing this is useful if you are cross-compiling, so it is impossible to check features that require running a test program. You could "prime the cache" by setting those values correctly for that system in `prefix/etc/config.site`. To find out the names of the cache variables you need to set, look for shell variables with `_cv_` in their names in the affected `configure` scripts, or in the Autoconf M4 source code for those macros.

The cache file is careful to not override any variables set in the site files. Similarly, you should not override command-line options in the site files. Your code should check that variables such as `prefix` and `cache_file` have their default values (as set near the top of `configure`) before changing them.

Here is a sample file `/usr/share/local/gnu/share/config.site`. The command `configure --prefix=/usr/share/local/gnu` would read this file (if `CONFIG_SITE` is not set to a different file).

    # config.site for configure
    #
    # Change some defaults.
    test "$prefix" = NONE && prefix=/usr/share/local/gnu
    test "$exec_prefix" = NONE && exec_prefix=/usr/local/gnu
    test "$sharedstatedir" = '$prefix/com' && sharedstatedir=/var
    test "$localstatedir" = '$prefix/var' && localstatedir=/var

    # Give Autoconf 2.x generated configure scripts a shared default
    # cache file for feature test results, architecture-specific.
    if test "$cache_file" = /dev/null; then
      cache_file="$prefix/var/config.cache"
      # A cache file is only valid for one C compiler.
      CC=gcc
    fi

Node:Running configure scripts, Next:config.status Invocation, Previous:Site Configuration, Up:Top

## Running `configure` Scripts

Below are instructions on how to configure a package that uses a `configure` script, suitable for inclusion as an `INSTALL` file in the package. A plain-text version of `INSTALL` which you may use comes with Autoconf.

- Basic Installation: Instructions for typical cases
- Compilers and Options: Selecting compilers and optimization
- Multiple Architectures: Compiling for multiple architectures at once
- Installation Names: Installing in different directories
- Optional Features: Selecting optional features
- System Type: Specifying the system type
- Sharing Defaults: Setting site-wide defaults for `configure`
- Defining Variables: Specifying the compiler etc.
- configure Invocation: Changing how `configure` runs

Node:Basic Installation, Next:Compilers and Options, Up:Running configure scripts

### Basic Installation

These are generic installation instructions.

The `configure` shell script attempts to guess correct values for various system-dependent variables used during compilation. It uses those values to create a `Makefile` in each directory of the package. It may also create one or more `.h` files containing system-dependent definitions. Finally, it creates a shell script `config.status` that you can run in the future to recreate the current configuration, and a file `config.log` containing compiler output (useful mainly for debugging `configure`).

It can also use an optional file (typically called `config.cache` and enabled with `--cache-file=config.cache` or simply `-C`) that saves the results of its tests to speed up reconfiguring. (Caching is disabled by default to prevent problems with accidental use of stale cache files.)

If you need to do unusual things to compile the package, please try to figure out how `configure` could check whether to do them, and mail diffs or instructions to the address given in the `README` so they can be considered for the next release. If you are using the cache, and at some point `config.cache` contains results you don't want to keep, you may remove or edit it.

The file `configure.ac` (or `configure.in`) is used to create `configure` by a program called `autoconf`. You only need `configure.ac` if you want to change it or regenerate `configure` using a newer version of `autoconf`.

The simplest way to compile this package is:

1. `cd` to the directory containing the package's source code and type `./configure` to configure the package for your system. If you're using `csh` on an old version of System V, you might need to type `sh ./configure` instead to prevent `csh` from trying to execute `configure` itself.

   Running `configure` takes awhile. While running, it prints some messages telling which features it is checking for.

2. Type `make` to compile the package.
3. Optionally, type `make check` to run any self-tests that come with the package.
4. Type `make install` to install the programs and any data files and documentation.
5. You can remove the program binaries and object files from the source code directory by typing `make clean`. To also remove the files that `configure` created (so you can compile the package for a different kind of computer), type `make distclean`. There is also a `make maintainer-clean` target, but that is intended mainly for the package's developers. If you use it, you may have to get all sorts of other programs in order to regenerate files that came with the distribution.

Node:Compilers and Options, Next:Multiple Architectures, Previous:Basic Installation, Up:Running configure scripts

### Compilers and Options

Some systems require unusual options for compilation or linking that the `configure` script does not know about. Run `./configure --help` for details on some of the pertinent environment variables.

You can give `configure` initial values for variables by setting them in the environment. You can do that on the command line like this:

    ./configure CC=c89 CFLAGS=-O2 LIBS=-lposix

See Defining Variables, for more details.

Node:Multiple Architectures, Next:Installation Names, Previous:Compilers and Options, Up:Running configure scripts

### Compiling For Multiple Architectures

You can compile the package for more than one kind of computer at the same time, by placing the object files for each architecture in their own directory. To do this, you must use a version of `make` that supports the `VPATH` variable, such as GNU `make`. `cd` to the directory where you want the object files and executables to go and run the `configure` script. `configure` automatically checks for the source code in the directory that `configure` is in and in `..`.

If you have to use a `make` that does not support the `VPATH` variable, you have to compile the package for one architecture at a time in the source code directory. After you have installed the package for one architecture, use `make distclean` before reconfiguring for another architecture.

Node:Installation Names, Next:Optional Features, Previous:Multiple Architectures, Up:Running configure scripts

### Installation Names

By default, `make install` will install the package's files in `/usr/local/bin`, `/usr/local/man`, etc. You can specify an installation prefix other than `/usr/local` by giving `configure` the option `--prefix=path`.

You can specify separate installation prefixes for architecture-specific files and architecture-independent files. If you give `configure` the option `--exec-prefix=path`, the package will use path as the prefix for installing programs and libraries. Documentation and other data files will still use the regular prefix.

In addition, if you use an unusual directory layout you can give options like `--bindir=path` to specify different values for particular kinds of files. Run `configure --help` for a list of the directories you can set and what kinds of files go in them.

If the package supports it, you can cause programs to be installed with an extra prefix or suffix on their names by giving `configure` the option `--program-prefix=PREFIX` or `--program-suffix=SUFFIX`.

Node:Optional Features, Next:System Type, Previous:Installation Names, Up:Running configure scripts

### Optional Features

Some packages pay attention to `--enable-feature` options to `configure`, where feature indicates an optional part of the package. They may also pay attention to `--with-package` options, where package is something like `gnu-as` or `x` (for the X Window System). The `README` should mention any `--enable-` and `--with-` options that the package recognizes.

For packages that use the X Window System, `configure` can usually find the X include and library files automatically, but if it doesn't, you can use the `configure` options `--x-includes=dir` and `--x-libraries=dir` to specify their locations.

Node:System Type, Next:Sharing Defaults, Previous:Optional Features, Up:Running configure scripts

### Specifying the System Type

There may be some features `configure` cannot figure out automatically, but needs to determine by the type of machine the package will run on. Usually, assuming the package is built to be run on the same architectures, `configure` can figure that out, but if it prints a message saying it cannot guess the machine type, give it the `--build=type` option. type can either be a short name for the system type, such as `sun4`, or a canonical name which has the form:

    cpu-company-system

where system can have one of these forms:

    os kernel-os

See the file `config.sub` for the possible values of each field. If `config.sub` isn't included in this package, then this package doesn't need to know the machine type.

If you are building compiler tools for cross-compiling, you should use the `--target=type` option to select the type of system they will produce code for.

If you want to use a cross compiler, that generates code for a platform different from the build platform, you should specify the host platform (i.e., that on which the generated programs will eventually be run) with `--host=type`.

Node:Sharing Defaults, Next:Defining Variables, Previous:System Type, Up:Running configure scripts

### Sharing Defaults

If you want to set default values for `configure` scripts to share, you can create a site shell script called `config.site` that gives default values for variables like `CC`, `cache_file`, and `prefix`. `configure` looks for `prefix/share/config.site` if it exists, then `prefix/etc/config.site` if it exists. Or, you can set the `CONFIG_SITE` environment variable to the location of the site script. A warning: not all `configure` scripts look for a site script.

Node:Defining Variables, Next:configure Invocation, Previous:Sharing Defaults, Up:Running configure scripts

### Defining Variables

Variables not defined in a site shell script can be set in the environment passed to `configure`. However, some packages may run configure again during the build, and the customized values of these variables may be lost. In order to avoid this problem, you should set them in the `configure` command line, using `VAR=value`. For example:

    ./configure CC=/usr/local2/bin/gcc

will cause the specified gcc to be used as the C compiler (unless it is overridden in the site shell script).

Node:configure Invocation, Previous:Defining Variables, Up:Running configure scripts

### `configure` Invocation

`configure` recognizes the following options to control how it operates.

`--help`
`-h`
  Print a summary of the options to `configure`, and exit.
`--version`
`-V`
  Print the version of Autoconf used to generate the `configure` script, and exit.
`--cache-file=file`
  Enable the cache: use and save the results of the tests in file, traditionally `config.cache`. file defaults to `/dev/null` to disable caching.
`--config-cache`
`-C`
  Alias for `--cache-file=config.cache`.
`--quiet`
`--silent`
`-q`
  Do not print messages saying which checks are being made. To suppress all normal output, redirect it to `/dev/null` (any error messages will still be shown).
`--srcdir=dir`
  Look for the package's source code in directory dir. Usually `configure` can determine that directory automatically.

`configure` also accepts some other, not widely useful, options. Run `configure --help` for more details.

Node:config.status Invocation, Next:Obsolete Constructs, Previous:Running configure scripts, Up:Top

## Recreating a Configuration

The `configure` script creates a file named `config.status`, which actually configures, instantiates, the template files. It also records the configuration options that were specified when the package was last configured in case reconfiguring is needed.

Synopsis:

    ./config.status option... [file...]

It configures the files, if none are specified, all the templates are instantiated. The files must be specified without their dependencies, as in

    ./config.status foobar

not

    ./config.status foobar:foo.in:bar.in

The supported options are:

`--help`
`-h`
  Print a summary of the command line options, the list of the template files and exit.
`--version`
`-V`
  Print the version number of Autoconf and exit.
`--debug`
`-d`
  Don't remove the temporary files.
`--file=file[:template]`
  Require that file be instantiated as if `AC_CONFIG_FILES(file:template)` was used. Both file and template may be `-` in which case the standard output and/or standard input, respectively, is used. If a template filename is relative, it is first looked for in the build tree, and then in the source tree. See Configuration Actions, for more details.

  This option and the following ones provide one way for separately distributed packages to share the values computed by `configure`. Doing so can be useful if some of the packages need a superset of the features that one of them, perhaps a common library, does. These options allow a `config.status` file to create files other than the ones that its `configure.ac` specifies, so it can be used for a different package.

`--header=file[:template]`
  Same as `--file` above, but with `AC_CONFIG_HEADERS`.
`--recheck`
  Ask `config.status` to update itself and exit (no instantiation). This option is useful if you change `configure`, so that the results of some tests might be different from the previous run. The `--recheck` option re-runs `configure` with the same arguments you used before, plus the `--no-create` option, which prevents `configure` from running `config.status` and creating `Makefile` and other files, and the `--no-recursion` option, which prevents `configure` from running other `configure` scripts in subdirectories. (This is so other `Makefile` rules can run `config.status` when it changes; see Automatic Remaking, for an example).

`config.status` checks several optional environment variables that can alter its behavior:

CONFIG_SHELL | Variable

The shell with which to run `configure` for the `--recheck` option. It must be Bourne-compatible. The default is a shell that supports `LINENO` if available, and `/bin/sh` otherwise.

CONFIG_STATUS | Variable

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

You can use `./config.status` in your Makefiles. For example, in the dependencies given above (see Automatic Remaking), `config.status` is run twice when `configure.ac` has changed. If that bothers you, you can make each run only regenerate the files for that rule:

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

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

The calling convention of `config.status` has changed, see Obsolete config.status Use, for details.

Node:Obsolete Constructs, Next:Using Autotest, Previous:config.status Invocation, Up:Top

## Obsolete Constructs

Autoconf changes, and throughout the years some constructs are obsoleted. Most of the changes involve the macros, but the tools themselves, or even some concepts, are now considered obsolete.

You may completely skip this chapter if you are new to Autoconf, its intention is mainly to help maintainers updating their packages by understanding how to move to more modern constructs.

- Obsolete config.status Use: Different calling convention
- acconfig.h: Additional entries in `config.h.in`
- autoupdate Invocation: Automatic update of `configure.ac`
- Obsolete Macros: Backward compatibility macros
- Autoconf 1: Tips for upgrading your files
- Autoconf 2.13: Some fresher tips

Node:Obsolete config.status Use, Next:acconfig.h, Up:Obsolete Constructs

### Obsolete `config.status` Invocation

`config.status` now supports arguments to specify the files to instantiate, see config.status Invocation, for more details. Before, environment variables had to be used.

CONFIG_COMMANDS | Variable

The tags of the commands to execute. The default is the arguments given to `AC_OUTPUT` and `AC_CONFIG_COMMANDS` in `configure.ac`.

CONFIG_FILES | Variable

The files in which to perform `@variable@` substitutions. The default is the arguments given to `AC_OUTPUT` and `AC_CONFIG_FILES` in `configure.ac`.

CONFIG_HEADERS | Variable

The files in which to substitute C `#define` statements. The default is the arguments given to `AC_CONFIG_HEADERS`; if that macro was not called, `config.status` ignores this variable.

CONFIG_LINKS | Variable

The symbolic links to establish. The default is the arguments given to `AC_CONFIG_LINKS`; if that macro was not called, `config.status` ignores this variable.

In config.status Invocation, using this old interface, the example would be:

    config.h: stamp-h
    stamp-h: config.h.in config.status
            CONFIG_COMMANDS= CONFIG_LINKS= CONFIG_FILES= \
              CONFIG_HEADERS=config.h ./config.status
            echo > stamp-h

    Makefile: Makefile.in config.status
            CONFIG_COMMANDS= CONFIG_LINKS= CONFIG_HEADERS= \
              CONFIG_FILES=Makefile ./config.status

(If `configure.ac` does not call `AC_CONFIG_HEADERS`, there is no need to set `CONFIG_HEADERS` in the `make` rules, equally for `CONFIG_COMMANDS` etc.)

Node:acconfig.h, Next:autoupdate Invocation, Previous:Obsolete config.status Use, Up:Obsolete Constructs

### `acconfig.h`

In order to produce `config.h.in`, `autoheader` needs to build or to find templates for each symbol. Modern releases of Autoconf use `AH_VERBATIM` and `AH_TEMPLATE` (see Autoheader Macros), but in older releases a file, `acconfig.h`, contained the list of needed templates. `autoheader` copies comments and `#define` and `#undef` statements from `acconfig.h` in the current directory, if present. This file used to be mandatory if you `AC_DEFINE` any additional symbols.

Modern releases of Autoconf also provide `AH_TOP` and `AH_BOTTOM` if you need to prepend/append some information to `config.h.in`. Ancient versions of Autoconf had a similar feature: if `./acconfig.h` contains the string `@TOP@`, `autoheader` copies the lines before the line containing `@TOP@` into the top of the file that it generates. Similarly, if `./acconfig.h` contains the string `@BOTTOM@`, `autoheader` copies the lines after that line to the end of the file it generates. Either or both of those strings may be omitted. An even older alternate way to produce the same effect in jurasik versions of Autoconf is to create the files `file.top` (typically `config.h.top`) and/or `file.bot` in the current directory. If they exist, `autoheader` copies them to the beginning and end, respectively, of its output.

In former versions of Autoconf, the files used in preparing a software package for distribution were:

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

Use only the `AH_` macros, `configure.ac` should be self-contained, and should not depend upon `acconfig.h` etc.

Node:autoupdate Invocation, Next:Obsolete Macros, Previous:acconfig.h, Up:Obsolete Constructs

### Using `autoupdate` to Modernize `configure.ac`

The `autoupdate` program updates a `configure.ac` file that calls Autoconf macros by their old names to use the current macro names. In version 2 of Autoconf, most of the macros were renamed to use a more uniform and descriptive naming scheme. See Macro Names, for a description of the new scheme. Although the old names still work (see Obsolete Macros, for a list of the old macros and the corresponding new names), you can make your `configure.ac` files more readable and make it easier to use the current Autoconf documentation if you update them to use the new macro names.

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

`autoupdate` accepts the following options:

`--help`
`-h`
  Print a summary of the command line options and exit.
`--version`
`-V`
  Print the version number of Autoconf and exit.
`--verbose`
`-v`
  Report processing steps.
`--debug`
`-d`
  Don't remove the temporary files.
`--force`
`-f`
  Force the update even if the file has not changed. Disregard the cache.
`--include=dir`
`-I dir`
  Also look for input files in dir. Multiple invocations accumulate. Directories are browsed from last to first.

Node:Obsolete Macros, Next:Autoconf 1, Previous:autoupdate Invocation, Up:Obsolete Constructs

### Obsolete Macros

Several macros are obsoleted in Autoconf, for various reasons (typically they failed to quote properly, couldn't be extended for more recent issues etc.). They are still supported, but deprecated: their use should be avoided.

During the jump from Autoconf version 1 to version 2, most of the macros were renamed to use a more uniform and descriptive naming scheme, but their signature did not change. See Macro Names, for a description of the new naming scheme. Below, there is just the mapping from old names to new names for these macros, the reader is invited to refer to the definition of the new macro for the signature and the description.

AC_ALLOCA | Macro

`AC_FUNC_ALLOCA`

AC_ARG_ARRAY | Macro

removed because of limited usefulness

AC_C_CROSS | Macro

This macro is obsolete; it does nothing.

AC_CANONICAL_SYSTEM | Macro

Determine the system type and set output variables to the names of the canonical system types. See Canonicalizing, for details about the variables this macro sets. The user is encouraged to use either `AC_CANONICAL_BUILD`, or `AC_CANONICAL_HOST`, or `AC_CANONICAL_TARGET`, depending on the needs. Using `AC_CANONICAL_TARGET` is enough to run the two other macros.

AC_CHAR_UNSIGNED | Macro

`AC_C_CHAR_UNSIGNED`

AC_CHECK_TYPE (type, default) | Macro

Autoconf, up to 2.13, used to provide this version of `AC_CHECK_TYPE`, deprecated because of its flaws. Firstly, although it is a member of the `CHECK` clan, singular sub-family, it does more than just checking. Second, missing types are not `typedef`'d, they are `#define`'d, which can lead to incompatible code in the case of pointer types. This use of `AC_CHECK_TYPE` is obsolete and discouraged, see Generic Types, for the description of the current macro. If the type type is not defined, define it to be the C (or C++) builtin type default; e.g., `short` or `unsigned`. This macro is equivalent to:
    AC_CHECK_TYPE([type],
                  [AC_DEFINE([type], [default],
                             [Define to `default' if <sys/types.h>
                              does not define.])])
In order to keep backward compatibility, the two versions of `AC_CHECK_TYPE` are implemented, selected by a simple heuristics: If there are three or four arguments, the modern version is used. If the second argument appears to be a C or C++ type, then the obsolete version is used. This happens if the argument is a C or C++ builtin type or a C identifier ending in `_t`, optionally followed by one of `[(*` and then by a string of zero or more characters taken from the set `[]()* _a-zA-Z0-9`. If the second argument is spelled with the alphabet of valid C and C++ types, the user is warned and the modern version is used. Otherwise, the modern version is used. You are encouraged either to use a valid builtin type, or to use the equivalent modern code (see above), or better yet, to use `AC_CHECK_TYPES` together with
    #if !HAVE_LOFF_T
    typedef loff_t off_t;
    #endif

AC_CHECKING (feature-description) | Macro

Same as `AC_MSG_NOTICE([checking feature-description...]`.

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

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

AC_CONST | Macro

`AC_C_CONST`

AC_CROSS_CHECK | Macro

Same as `AC_C_CROSS`, which is obsolete too, and does nothing `:-)`.

AC_CYGWIN | Macro

Check for the Cygwin environment in which case the shell variable `CYGWIN` is set to `yes`. Don't use this macro, the dignified means to check the nature of the host is using `AC_CANONICAL_HOST`. As a matter of fact this macro is defined as:
    AC_REQUIRE([AC_CANONICAL_HOST])[]dnl
    case $host_os in
      *cygwin* ) CYGWIN=yes;;
             * ) CYGWIN=no;;
    esac
Beware that the variable `CYGWIN` has a very special meaning when running CygWin32, and should not be changed. That's yet another reason not to use this macro.

AC_DECL_YYTEXT | Macro

Does nothing, now integrated in `AC_PROG_LEX`.

AC_DIR_HEADER | Macro

Like calling `AC_FUNC_CLOSEDIR_VOID` and`AC_HEADER_DIRENT`, but defines a different set of C preprocessor macros to indicate which header file is found:
Header | Old Symbol | New Symbol
`dirent.h` | `DIRENT` | `HAVE_DIRENT_H`
`sys/ndir.h` | `SYSNDIR` | `HAVE_SYS_NDIR_H`
`sys/dir.h` | `SYSDIR` | `HAVE_SYS_DIR_H`
`ndir.h` | `NDIR` | `HAVE_NDIR_H`

AC_DYNIX_SEQ | Macro

If on Dynix/PTX (Sequent UNIX), add `-lseq` to output variable `LIBS`. This macro used to be defined as
    AC_CHECK_LIB(seq, getmntent, LIBS="-lseq $LIBS")
now it is just `AC_FUNC_GETMNTENT`.

AC_EXEEXT | Macro

Defined the output variable `EXEEXT` based on the output of the compiler, which is now done automatically. Typically set to empty string if Unix and `.exe` if Win32 or OS/2.

AC_EMXOS2 | Macro

Similar to `AC_CYGWIN` but checks for the EMX environment on OS/2 and sets `EMXOS2`.

AC_ERROR | Macro

`AC_MSG_ERROR`

AC_FIND_X | Macro

`AC_PATH_X`

AC_FIND_XTRA | Macro

`AC_PATH_XTRA`

AC_FUNC_CHECK | Macro

`AC_CHECK_FUNC`

AC_FUNC_WAIT3 | Macro

If `wait3` is found and fills in the contents of its third argument (a `struct rusage *`), which HP-UX does not do, define `HAVE_WAIT3`. These days portable programs should use `waitpid`, not `wait3`, as `wait3` is being removed from the Open Group standards, and will not appear in the next revision of POSIX.

AC_GCC_TRADITIONAL | Macro

`AC_PROG_GCC_TRADITIONAL`

AC_GETGROUPS_T | Macro

`AC_TYPE_GETGROUPS`

AC_GETLOADAVG | Macro

`AC_FUNC_GETLOADAVG`

AC_HAVE_FUNCS | Macro

`AC_CHECK_FUNCS`

AC_HAVE_HEADERS | Macro

`AC_CHECK_HEADERS`

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

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

AC_HAVE_POUNDBANG | Macro

`AC_SYS_INTERPRETER` (different calling convention)

AC_HEADER_CHECK | Macro

`AC_CHECK_HEADER`

AC_HEADER_EGREP | Macro

`AC_EGREP_HEADER`

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

Formerly `AC_INIT` used to have a single argument, and was equivalent to:
    AC_INIT
    AC_CONFIG_SRCDIR(unique-file-in-source-dir)

AC_INLINE | Macro

`AC_C_INLINE`

AC_INT_16_BITS | Macro

If the C type `int` is 16 bits wide, define `INT_16_BITS`. Use `AC_CHECK_SIZEOF(int)` instead.

AC_IRIX_SUN | Macro

If on IRIX (Silicon Graphics UNIX), add `-lsun` to output `LIBS`. If you were using it to get `getmntent`, use `AC_FUNC_GETMNTENT` instead. If you used it for the NIS versions of the password and group functions, use `AC_CHECK_LIB(sun, getpwnam)`. Up to Autoconf 2.13, it used to be
    AC_CHECK_LIB(sun, getmntent, LIBS="-lsun $LIBS")
now it is defined as
    AC_FUNC_GETMNTENT
    AC_CHECK_LIB(sun, getpwnam)

AC_LANG_C | Macro

Same as `AC_LANG(C)`.

AC_LANG_CPLUSPLUS | Macro

Same as `AC_LANG(C++)`.

AC_LANG_FORTRAN77 | Macro

Same as `AC_LANG(Fortran 77)`.

AC_LANG_RESTORE | Macro

Select the language that is saved on the top of the stack, as set by `AC_LANG_SAVE`, remove it from the stack, and call `AC_LANG(language)`.

AC_LANG_SAVE | Macro

Remember the current language (as set by `AC_LANG`) on a stack. The current language does not change. `AC_LANG_PUSH` is preferred.

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

This is an obsolete version of `AC_CONFIG_LINKS`. An updated version of:
    AC_LINK_FILES(config/$machine.h config/$obj_format.h,
                  host.h            object.h)
is:
    AC_CONFIG_LINKS(host.h:config/$machine.h
                    object.h:config/$obj_format.h)

AC_LN_S | Macro

`AC_PROG_LN_S`

AC_LONG_64_BITS | Macro

Define `LONG_64_BITS` if the C type `long int` is 64 bits wide. Use the generic macro `AC_CHECK_SIZEOF([long int])` instead.

AC_LONG_DOUBLE | Macro

`AC_C_LONG_DOUBLE`

AC_LONG_FILE_NAMES | Macro

`AC_SYS_LONG_FILE_NAMES`

AC_MAJOR_HEADER | Macro

`AC_HEADER_MAJOR`

AC_MEMORY_H | Macro

Used to define `NEED_MEMORY_H` if the `mem` functions were defined in `memory.h`. Today it is equivalent to `AC_CHECK_HEADERS(memory.h)`. Adjust your code to depend upon `HAVE_MEMORY_H`, not `NEED_MEMORY_H`, see See Standard Symbols.

AC_MINGW32 | Macro

Similar to `AC_CYGWIN` but checks for the MingW32 compiler environment and sets `MINGW32`.

AC_MINUS_C_MINUS_O | Macro

`AC_PROG_CC_C_O`

AC_MMAP | Macro

`AC_FUNC_MMAP`

AC_MODE_T | Macro

`AC_TYPE_MODE_T`

AC_OBJEXT | Macro

Defined the output variable `OBJEXT` based on the output of the compiler, after .c files have been excluded. Typically set to `o` if Unix, `obj` if Win32. Now the compiler checking macros handle this automatically.

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

Make `m4` print a message to the standard error output warning that this-macro-name is obsolete, and giving the file and line number where it was called. this-macro-name should be the name of the macro that is calling `AC_OBSOLETE`. If suggestion is given, it is printed at the end of the warning message; for example, it can be a suggestion for what to use instead of this-macro-name. For instance
    AC_OBSOLETE([$0], [; use AC_CHECK_HEADERS(unistd.h) instead])dnl
You are encouraged to use `AU_DEFUN` instead, since it gives better services to the user.

AC_OFF_T | Macro

`AC_TYPE_OFF_T`

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

The use of `AC_OUTPUT` with argument is deprecated, this obsoleted interface is equivalent to:
    AC_CONFIG_FILES(file...)
    AC_CONFIG_COMMANDS([default],
                       extra-cmds, init-cmds)
    AC_OUTPUT

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

Specify additional shell commands to run at the end of `config.status`, and shell commands to initialize any variables from `configure`. This macro may be called multiple times. It is obsolete, replaced by `AC_CONFIG_COMMANDS`. Here is an unrealistic example:
    fubar=27
    AC_OUTPUT_COMMANDS([echo this is extra $fubar, and so on.],
                       [fubar=$fubar])
    AC_OUTPUT_COMMANDS([echo this is another, extra, bit],
                       [echo init bit])
Aside from the fact that `AC_CONFIG_COMMANDS` requires an additional key, an important difference is that `AC_OUTPUT_COMMANDS` is quoting its arguments twice, while `AC_CONFIG_COMMANDS`. This means that `AC_CONFIG_COMMANDS` can safely be given macro calls as arguments:
    AC_CONFIG_COMMANDS(foo, [my_FOO()])
conversely, where one level of quoting was enough for literal strings with `AC_OUTPUT_COMMANDS`, you need two with `AC_CONFIG_COMMANDS`. The following lines are equivalent:
    AC_OUTPUT_COMMANDS([echo "Square brackets: []"])
    AC_CONFIG_COMMANDS([default], [[echo "Square brackets: []"]])

AC_PID_T | Macro

`AC_TYPE_PID_T`

AC_PREFIX | Macro

`AC_PREFIX_PROGRAM`

AC_PROGRAMS_CHECK | Macro

`AC_CHECK_PROGS`

AC_PROGRAMS_PATH | Macro

`AC_PATH_PROGS`

AC_PROGRAM_CHECK | Macro

`AC_CHECK_PROG`

AC_PROGRAM_EGREP | Macro

`AC_EGREP_CPP`

AC_PROGRAM_PATH | Macro

`AC_PATH_PROG`

AC_REMOTE_TAPE | Macro

removed because of limited usefulness

AC_RESTARTABLE_SYSCALLS | Macro

`AC_SYS_RESTARTABLE_SYSCALLS`

AC_RETSIGTYPE | Macro

`AC_TYPE_SIGNAL`

AC_RSH | Macro

Removed because of limited usefulness.

AC_SCO_INTL | Macro

If on SCO UNIX, add `-lintl` to output variable `LIBS`. This macro used to
    AC_CHECK_LIB(intl, strftime, LIBS="-lintl $LIBS")
now it just calls `AC_FUNC_STRFTIME` instead.

AC_SETVBUF_REVERSED | Macro

`AC_FUNC_SETVBUF_REVERSED`

AC_SET_MAKE | Macro

`AC_PROG_MAKE_SET`

AC_SIZEOF_TYPE | Macro

`AC_CHECK_SIZEOF`

AC_SIZE_T | Macro

`AC_TYPE_SIZE_T`

AC_STAT_MACROS_BROKEN | Macro

`AC_HEADER_STAT`

AC_STDC_HEADERS | Macro

`AC_HEADER_STDC`

AC_STRCOLL | Macro

`AC_FUNC_STRCOLL`

AC_ST_BLKSIZE | Macro

`AC_STRUCT_ST_BLKSIZE`

AC_ST_BLOCKS | Macro

`AC_STRUCT_ST_BLOCKS`

AC_ST_RDEV | Macro

`AC_STRUCT_ST_RDEV`

AC_SYS_RESTARTABLE_SYSCALLS | Macro

If the system automatically restarts a system call that is interrupted by a signal, define `HAVE_RESTARTABLE_SYSCALLS`. This macro does not check if system calls are restarted in general-it tests whether a signal handler installed with `signal` (but not `sigaction`) causes system calls to be restarted. It does not test if system calls can be restarted when interrupted by signals that have no handler. These days portable programs should use `sigaction` with `SA_RESTART` if they want restartable system calls. They should not rely on `HAVE_RESTARTABLE_SYSCALLS`, since nowadays whether a system call is restartable is a dynamic issue, not a configuration-time issue.

AC_SYS_SIGLIST_DECLARED | Macro

`AC_DECL_SYS_SIGLIST`

AC_TEST_CPP | Macro

`AC_TRY_CPP`

AC_TEST_PROGRAM | Macro

`AC_TRY_RUN`

AC_TIMEZONE | Macro

`AC_STRUCT_TIMEZONE`

AC_TIME_WITH_SYS_TIME | Macro

`AC_HEADER_TIME`

AC_UID_T | Macro

`AC_TYPE_UID_T`

AC_UNISTD_H | Macro

Same as `AC_CHECK_HEADERS(unistd.h)`.

AC_USG | Macro

Define `USG` if the BSD string functions are defined in `strings.h`. You should no longer depend upon `USG`, but on `HAVE_STRING_H`, see See Standard Symbols.

AC_UTIME_NULL | Macro

`AC_FUNC_UTIME_NULL`

AC_VALIDATE_CACHED_SYSTEM_TUPLE ([cmd]) | Macro

If the cache file is inconsistent with the current host, target and build system types, it used to execute cmd or print a default error message. This is now handled by default.

AC_VERBOSE (result-description) | Macro

`AC_MSG_RESULT`.

AC_VFORK | Macro

`AC_FUNC_VFORK`

AC_VPRINTF | Macro

`AC_FUNC_VPRINTF`

AC_WAIT3 | Macro

`AC_FUNC_WAIT3`

AC_WARN | Macro

`AC_MSG_WARN`

AC_WORDS_BIGENDIAN | Macro

`AC_C_BIGENDIAN`

AC_XENIX_DIR | Macro

This macro used to add `-lx` to output variable `LIBS` if on Xenix. Also, if `dirent.h` is being checked for, added `-ldir` to `LIBS`. Now it is merely an alias of `AC_HEADER_DIRENT` instead, plus some code to detect whether running XENIX on which you should not depend:
    AC_MSG_CHECKING([for Xenix])
    AC_EGREP_CPP(yes,
    [#if defined M_XENIX && !defined M_UNIX
      yes
    #endif],
                 [AC_MSG_RESULT([yes]); XENIX=yes],
                 [AC_MSG_RESULT([no]); XENIX=])

AC_YYTEXT_POINTER | Macro

`AC_DECL_YYTEXT`

Node:Autoconf 1, Next:Autoconf 2.13, Previous:Obsolete Macros, Up:Obsolete Constructs

### Upgrading From Version 1

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

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

Node:Changed File Names, Next:Changed Makefiles, Up:Autoconf 1

#### Changed File Names

If you have an `aclocal.m4` installed with Autoconf (as opposed to in a particular package's source directory), you must rename it to `acsite.m4`. See autoconf Invocation.

If you distribute `install.sh` with your package, rename it to `install-sh` so `make` builtin rules won't inadvertently create a file called `install` from it. `AC_PROG_INSTALL` looks for the script under both names, but it is best to use the new name.

If you were using `config.h.top`, `config.h.bot`, or `acconfig.h`, you still can, but you will have less clutter if you use the `AH_` macros. See Autoheader Macros.

Node:Changed Makefiles, Next:Changed Macros, Previous:Changed File Names, Up:Autoconf 1

#### Changed Makefiles

Add `@CFLAGS@`, `@CPPFLAGS@`, and `@LDFLAGS@` in your `Makefile.in` files, so they can take advantage of the values of those variables in the environment when `configure` is run. Doing this isn't necessary, but it's a convenience for users.

Also add `@configure_input@` in a comment to each input file for `AC_OUTPUT`, so that the output files will contain a comment saying they were produced by `configure`. Automatically selecting the right comment syntax for all the kinds of files that people call `AC_OUTPUT` on became too much work.

Add `config.log` and `config.cache` to the list of files you remove in `distclean` targets.

If you have the following in `Makefile.in`:

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

you must change it to:

    prefix = @prefix@
    exec_prefix = @exec_prefix@

The old behavior of replacing those variables without `@` characters around them has been removed.

Node:Changed Macros, Next:Changed Results, Previous:Changed Makefiles, Up:Autoconf 1

#### Changed Macros

Many of the macros were renamed in Autoconf version 2. You can still use the old names, but the new ones are clearer, and it's easier to find the documentation for them. See Obsolete Macros, for a table showing the new names for the old macros. Use the `autoupdate` program to convert your `configure.ac` to using the new macro names. See autoupdate Invocation.

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

Node:Changed Results, Next:Changed Macro Writing, Previous:Changed Macros, Up:Autoconf 1

#### Changed Results

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

For example, here is a `configure.ac` fragment written for Autoconf version 1:

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

Here is a way to write it for version 2:

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

If you were working around bugs in `AC_DEFINE_UNQUOTED` by adding backslashes before quotes, you need to remove them. It now works predictably, and does not treat quotes (except back quotes) specially. See Setting Output Variables.

All of the boolean shell variables set by Autoconf macros now use `yes` for the true value. Most of them use `no` for false, though for backward compatibility some use the empty string instead. If you were relying on a shell variable being set to something like 1 or `t` for true, you need to change your tests.

Node:Changed Macro Writing, Previous:Changed Results, Up:Autoconf 1

#### Changed Macro Writing

When defining your own macros, you should now use `AC_DEFUN` instead of `define`. `AC_DEFUN` automatically calls `AC_PROVIDE` and ensures that macros called via `AC_REQUIRE` do not interrupt other macros, to prevent nested `checking...` messages on the screen. There's no actual harm in continuing to use the older way, but it's less convenient and attractive. See Macro Definitions.

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

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

To speed up your locally written feature tests, add caching to them. See whether any of your tests are of general enough usefulness to encapsulate into macros that you can share.

Node:Autoconf 2.13, Previous:Autoconf 1, Up:Obsolete Constructs

### Upgrading From Version 2.13

The introduction of the previous section (see Autoconf 1) perfectly suits this section...

> Autoconf version 2.50 is mostly backward compatible with version 2.13. However, it introduces better ways to do some things, and doesn't support some of the ugly things in version 2.13. So, depending on how sophisticated your `configure.ac` files are, you might have to do some manual work in order to upgrade to version 2.50. This chapter points out some problems to watch for when upgrading. Also, perhaps your `configure` scripts could benefit from some of the new features in version 2.50; the changes are summarized in the file `NEWS` in the Autoconf distribution.

- Changed Quotation: Broken code which used to work
- New Macros: Interaction with foreign macros
- Hosts and Cross-Compilation: Bugward compatibility kludges
- AC_LIBOBJ vs. LIBOBJS: LIBOBJS is a forbidden token

Node:Changed Quotation, Next:New Macros, Up:Autoconf 2.13

#### Changed Quotation

The most important changes are invisible to you: the implementation of most macros have completely changed. This allowed more factorization of the code, better error messages, a higher uniformity of the user's interface etc. Unfortunately, as a side effect, some construct which used to (miraculously) work might break starting with Autoconf 2.50. The most common culprit is bad quotation.

For instance, in the following example, the message is not properly quoted:

    AC_INIT
    AC_CHECK_HEADERS(foo.h,,
    AC_MSG_ERROR(cannot find foo.h, bailing out))
    AC_OUTPUT

Autoconf 2.13 simply ignores it:

    $ autoconf-2.13; ./configure --silent
    creating cache ./config.cache
    configure: error: cannot find foo.h
    $

while Autoconf 2.50 will produce a broken `configure`:

    $ autoconf-2.50; ./configure --silent
    configure: error: cannot find foo.h
    ./configure: exit: bad non-numeric arg `bailing'
    ./configure: exit: bad non-numeric arg `bailing'
    $

The message needs to be quoted, and the `AC_MSG_ERROR` invocation too!

    AC_INIT
    AC_CHECK_HEADERS(foo.h,,
                     [AC_MSG_ERROR([cannot find foo.h, bailing out])])
    AC_OUTPUT

Many many (and many more) Autoconf macros were lacking proper quotation, including no less than... `AC_DEFUN` itself!

    $ cat configure.in
    AC_DEFUN([AC_PROG_INSTALL],
    [# My own much better version
    ])
    AC_INIT
    AC_PROG_INSTALL
    AC_OUTPUT
    $ autoconf-2.13
    autoconf: Undefined macros:
    ***BUG in Autoconf--please report*** AC_FD_MSG
    ***BUG in Autoconf--please report*** AC_EPI
    configure.in:1:AC_DEFUN([AC_PROG_INSTALL],
    configure.in:5:AC_PROG_INSTALL
    $ autoconf-2.50
    $

Node:New Macros, Next:Hosts and Cross-Compilation, Previous:Changed Quotation, Up:Autoconf 2.13

#### New Macros

Because Autoconf has been dormant for years, Automake provided Autoconf-like macros for a while. Autoconf 2.50 now provides better versions of these macros, integrated in the `AC_` namespace, instead of `AM_`. But in order to ease the upgrading via `autoupdate`, bindings to such `AM_` macros are provided.

Unfortunately Automake did not quote the name of these macros! Therefore, when `m4` finds something like `AC_DEFUN(AM_TYPE_PTRDIFF_T, ...)` in `aclocal.m4`, `AM_TYPE_PTRDIFF_T` is expanded, replaced with its Autoconf definition.

Fortunately Autoconf catches pre-`AC_INIT` expansions, and will complain, in its own words:

    $ cat configure.in
    AC_INIT
    AM_TYPE_PTRDIFF_T
    $ aclocal-1.4
    $ autoconf
    ./aclocal.m4:17: error: m4_defn: undefined macro: _m4_divert_diversion
    actypes.m4:289: AM_TYPE_PTRDIFF_T is expanded from...
    ./aclocal.m4:17: the top level
    $

Future versions of Automake will simply no longer define most of these macros, and will properly quote the names of the remaining macros. But you don't have to wait for it to happen to do the right thing right now: do not depend upon macros from Automake as it is simply not its job to provide macros (but the one it requires by itself):

    $ cat configure.in
    AC_INIT
    AM_TYPE_PTRDIFF_T
    $ rm aclocal.m4
    $ autoupdate
    autoupdate: `configure.in' is updated
    $ cat configure.in
    AC_INIT
    AC_CHECK_TYPES([ptrdiff_t])
    $ aclocal-1.4
    $ autoconf
    $

Node:Hosts and Cross-Compilation, Next:AC_LIBOBJ vs. LIBOBJS, Previous:New Macros, Up:Autoconf 2.13

#### Hosts and Cross-Compilation

Based on the experience of compiler writers, and after long public debates, many aspects of the cross-compilation chain have changed:

- the relationship between the build, host, and target architecture types,
- the command line interface for specifying them to `configure`,
- the variables defined in `configure`,
- the enabling of cross-compilation mode.

The relationship between build, host, and target have been cleaned up: the chain of default is now simply: target defaults to host, host to build, and build to the result of `config.guess`. Nevertheless, in order to ease the transition from 2.13 to 2.50, the following transition scheme is implemented. Do not rely on it, as it will be completely disabled in a couple of releases (we cannot keep it, as it proves to cause more problems than to cure).

They all default to the result of running `config.guess`, unless you specify either `--build` or `--host`. In this case, the default becomes the system type you specified. If you specify both, and they're different, `configure` will enter cross compilation mode, so it won't run any tests that require execution.

Hint: if you mean to override the result of `config.guess`, prefer `--build` over `--host`. In the future, `--host` will not override the name of the build system type. Whenever you specify `--host`, be sure to specify `--build` too.

For backward compatibility, `configure` will accept a system type as an option by itself. Such an option will override the defaults for build, host and target system types. The following configure statement will configure a cross toolchain that will run on NetBSD/alpha but generate code for GNU Hurd/sparc, which is also the build platform.

    ./configure --host=alpha-netbsd sparc-gnu

In Autoconf, the variables `build`, `host`, and `target` had a different semantics before and after the invocation of `AC_CANONICAL_BUILD` etc. Now, the argument of `--build` is strictly copied into `build_alias`, and is left empty otherwise. After the `AC_CANONICAL_BUILD`, `build` is set to the canonicalized build type. To ease the transition, before, its contents is the same as that of `build_alias`. Do not rely on this broken feature.

For consistency with the backward compatibility scheme exposed above, when `--host` is specified by `--build` isn't, the build system will be assumed to be the same as `--host`, and `build_alias` will be set to that value. Eventually, this historically incorrect behavior will go away.

The former scheme to enable cross-compilation proved to cause more harm than good, in particular, it used to be triggered too easily, leaving regular end users puzzled in front of cryptic error messages. `configure` could even enter cross-compilation mode, only because the compiler was not functional. This is mainly because `configure` used to try to detect cross-compilation, instead of waiting for an explicit flag from the user.

Now, `configure` enters cross-compilation mode iff `--host` is passed.

That's the short documentation. To ease the transition between 2.13 and its successors, a more complicated scheme is implemented. Do not rely on the following, as it will be removed in a near future.

If you specify `--host`, but not `--build`, when `configure` performs the first compiler test it will try to run an executable produced by the compiler. If the execution fails, it will enter cross-compilation mode. This is fragile. Moreover, by the time the compiler test is performed, it may be too late to modify the build-system type: other tests may have already been performed. Therefore, whenever you specify `--host`, be sure to specify `--build` too.

    ./configure --build=i686-pc-linux-gnu --host=m68k-coff

will enter cross-compilation mode. The former interface, which consisted in setting the compiler to a cross-compiler without informing `configure` is obsolete. For instance, `configure` will fail if it can't run the code generated by the specified compiler if you configure as follows:

    ./configure CC=m68k-coff-gcc

Node:AC_LIBOBJ vs. LIBOBJS, Previous:Hosts and Cross-Compilation, Up:Autoconf 2.13

#### `AC_LIBOBJ` vs. `LIBOBJS`

Up to Autoconf 2.13, the replacement of functions was triggered via the variable `LIBOBJS`. Since Autoconf 2.50, the macro `AC_LIBOBJ` should be used instead (see Generic Functions). Starting at Autoconf 2.53, the use of `LIBOBJS` is an error.

This change is mandated by the unification of the GNU Build System components. In particular, the various fragile techniques used to parse a `configure.ac` are all replaced with the use of traces. As a consequence, any action must be traceable, which obsoletes critical variable assignments. Fortunately, `LIBOBJS` was the only problem.

At the time this documentation is written, Automake does not rely on traces yet, but this is planed for a near future. Nevertheless, to ease the transition, and to guarantee this future Automake release will be able to use Autoconf 2.53, using `LIBOBJS` directly will make `autoconf` fail. But note that the output, `configure`, is correct and fully functional: you have some delay to adjust your source.

There are two typical uses of `LIBOBJS`: asking for a replacement function, and adjusting `LIBOBJS` for Automake and/or Libtool.

As for function replacement, the fix is immediate: use `AC_LIBOBJ`. For instance:

    LIBOBJS="$LIBOBJS fnmatch.o"
    LIBOBJS="$LIBOBJS malloc.$ac_objext"

should be replaced with:

    AC_LIBOBJ([fnmatch])
    AC_LIBOBJ([malloc])

When asked for automatic de-ANSI-fication, Automake needs `LIBOBJS`'ed filenames to have `$U` appended to the base names. Libtool requires the definition of `LTLIBOBJS`, which suffixes are mapped to `.lo`. Although Autoconf provides them with means to free the user to do that by herself, by the time of this writing, none do. Therefore, it is common to see `configure.ac` end with:

    # This is necessary so that .o files in LIBOBJS are also built via
    # the ANSI2KNR-filtering rules.
    LIBOBJS=`echo "$LIBOBJS" | sed 's/\.o /\$U.o /g;s/\.o$/\$U.o/'`
    LTLIBOBJS=`echo "$LIBOBJS" | sed 's/\.o/\.lo/g'`
    AC_SUBST(LTLIBOBJS)

First, note that this code is wrong, because `.o` is not the only possible extension4! Because the token `LIBOBJS` is now forbidden, you will have to replace this snippet with:

    # This is necessary so that .o files in LIBOBJS are also built via
    # the ANSI2KNR-filtering rules.
    LIB@&t@OBJS=`echo "$LIB@&t@OBJS" |
                 sed 's,\.[[^.]]* ,$U&,g;s,\.[[^.]]*$,$U&,'`
    LTLIBOBJS=`echo "$LIB@&t@OBJS" |
               sed 's,\.[[^.]]* ,.lo ,g;s,\.[[^.]]*$,.lo,'`
    AC_SUBST(LTLIBOBJS)

Unfortunately, `autoupdate` cannot help here, since... this is not a macro! Of course, first make sure your release of Automake and/or Libtool still requires these.

Node:Using Autotest, Next:Questions, Previous:Obsolete Constructs, Up:Top

## Generating Test Suites with Autotest

    Note: This section describes an experimental feature which will
    be part of Autoconf in a forthcoming release.  Although we believe
    Autotest is stabilizing, this documentation describes an interface which
    might change in the future: do not depend upon Autotest without
    subscribing to the Autoconf mailing lists.

It is paradoxical that portable projects depend on nonportable tools to run their test suite. Autoconf by itself is the paragon of this problem: although it aims at perfectly portability, up to 2.13, its test suite was using DejaGNU, a rich and complex testing framework, but which is far from being standard on Unix systems. Worse yet, it was likely to be missing on the most fragile platforms, the very platforms that are most likely to torture Autoconf and exhibit deficiencies.

To circumvent this problem many package maintainers have developed their own testing framework, based on simple shell scripts whose sole output are their exit status: the test succeeded, or failed. In addition, most of these tests share some common patterns, what results in lots of duplicated code, tedious maintenance etc.

Following exactly the same reasoning that yielded to the inception of Autoconf, Autotest provides a test suite generation frame work, based on M4 macros, building a portable shell script. The suite itself is equipped with automatic logging and tracing facilities which greatly diminish the interaction with bug reporters, and simple timing reports.

Autoconf itself has been using Autotest for years, and we do attest that it has considerably improved the strength of the test suite, and the quality of bug reports. Other projects are known to use some generation of Autotest, such as Bison, Free Recode, Free Wdiff, GNU Tar, each of them having different needs, what slowly polishes Autotest as a general testing framework.

Nonetheless, compared to DejaGNU, Autotest is inadequate for interactive tool testing, which is probably its main limitation.

- Using an Autotest Test Suite: Autotest and the user
- Writing testsuite.at: Autotest macros
- testsuite Invocation: Running `testsuite` scripts
- Making testsuite Scripts: Using autom4te to create `testsuite`

Node:Using an Autotest Test Suite, Next:Writing testsuite.at, Up:Using Autotest

### Using an Autotest Test Suite

- testsuite Scripts: The concepts of Autotest
- Autotest Logs: Their contents

Node:testsuite Scripts, Next:Autotest Logs, Up:Using an Autotest Test Suite

#### `testsuite` Scripts

Generating testing or validation suites using Autotest is rather easy. The whole validation suite is held in a file to be processed through `autom4te`, itself using GNU `m4` under the scene, to produce a stand-alone Bourne shell script which then gets distributed. Neither `autom4te` nor GNU `m4` are not needed anymore at the installer end.

Each test of the validation suite should be part of some test group. A test group is a sequence of interwoven tests that ought to be executed together, usually because one test in the group creates data files than a later test in the same group needs to read. Complex test groups make later debugging more tedious. It is much better keeping keep only a few tests per test group, and if you can put only one test per test group, this is just ideal.

For all but the simplest packages, some file such as `testsuite.at` does not fully hold all test sources, as these are often easier to maintain in separate files. Each of these separate files holds a single test group, or a sequence of test groups all addressing some common functionality in the package. In such cases, file `testsuite.at` only initializes the whole validation suite, and sometimes do elementary health checking, before listing include statements for all other test files. The special file `package.m4`, containing the identification of the package, is automatically included if found.

The validation scripts that Autotest produces are by convention called `testsuite`. When run, `testsuite` executes each test group in turn, producing only one summary line per test to say if that particular test succeeded or failed. At end of all tests, summarizing counters get printed. If any test failed, one debugging script gets automatically generated for each test group which failed. These debugging scripts are named `testsuite.nn`, where nn is the sequence number of the test group. In the ideal situation, none of the tests fail, and consequently, no debugging script is generated out of validation.

The automatic generation of debugging scripts for failed test has the purpose of easing the chase for bugs.

It often happens in practice that individual tests in the validation suite need to get information coming out of the configuration process. Some of this information, common for all validation suites, is provided through the file `atconfig`, automatically created by `AC_CONFIG_TESTDIR`. For configuration informations which your testing environment specifically needs, you might prepare an optional file named `atlocal.in`, instantiated by `AC_CONFIG_FILES`. The configuration process produces `atconfig` and `atlocal` out of these two input files, and these two produced files are automatically read by the `testsuite` script.

Here is a diagram showing the relationship between files.

Files used in preparing a software package for distribution:

    subfile-1.at ->.
        ...         \
    subfile-i.at ---->-- testsuite.at -->.
        ...         /                     \
    subfile-n.at ->'                       >-- autom4te* -->testsuite
                                          /
                          [package.m4] ->'

Files used in configuring a software package:

                                         .--> atconfig
                                        /
    [atlocal.in] -->  config.status* --<
                                        \
                                         `--> [atlocal]

Files created during the test suite execution:

    atconfig -->.                    .--> testsuite.log
                 \                  /
                  >-- testsuite* --<
                 /                  \
    [atlocal] ->'                    `--> [testsuite.nn*]

Node:Autotest Logs, Previous:testsuite Scripts, Up:Using an Autotest Test Suite

#### Autotest Logs

When run, the test suite creates a log file named after itself, e.g., a test suite named `testsuite` creates `testsuite.log`. It contains a lot of information, usually more than maintainers actually need, but therefore most of the time it contains all that is needed:

command line arguments
  A very bad Unix habit which is unfortunately wide spread consists of setting environment variables before the command, such as in `CC=my-home-grown-cc ./testsuite`. This results in the test suite not knowing this change, hence (i) it can't report it to you, and (ii) it cannot preserve the value of `CC` for subsequent runs5. Autoconf faced exactly the same problem, and solved it by asking users to pass the variable definitions as command line arguments. Autotest requires this rule too, but has no means to enforce it; the log then contains a trace of the variables the user changed.
`ChangeLog` excerpts
  The topmost lines of all the `ChangeLog`s found in the source hierarchy. This is especially useful when bugs are reported against development versions of the package, since the version string does not provide sufficient information to know the exact state of the sources the user compiled. Of course this relies on the use of a `ChangeLog`.
build machine
  Running a test suite in a cross-compile environment is not an easy task, since it would mean having the test suite run on a machine build, while running programs on a machine host. It is much simpler to run both the test suite and the programs on host, but then, from the point of view of the test suite, there remains a single environment, host = build. The log contains relevant information on the state of the build machine, including some important environment variables.
tested programs
  The absolute path and answers to `--version` of the tested programs (see Writing testsuite.at, `AT_TESTED`).
configuration log
  The contents of `config.log`, as created by `configure`, are appended. It contains the configuration flags and a detailed report on the configuration itself.

Node:Writing testsuite.at, Next:testsuite Invocation, Previous:Using an Autotest Test Suite, Up:Using Autotest

### Writing `testsuite.at`

The `testsuite.at` is a Bourne shell script making use of special Autotest M4 macros. It often contains a call to `AT_INIT` nears its beginning followed by one call to `m4_include` per source file for tests. Each such included file, or the remainder of `testsuite.at` if include files are not used, contain a sequence of test groups. Each test group begins with one call to `AT_SETUP`, it contains an arbitrary number of shell commands or calls to `AT_CHECK`, and it completes with one call to `AT_CLEANUP`.

AT_INIT ([name]) | Macro

Initialize Autotest. Giving a name to the test suite is encouraged if your package includes several test suites. In any case, the test suite always displays the package name and version. It also inherits the package bug report address.

AT_TESTED (executables) | Macro

Log the path and answer to `--version` of each program in space-separated list executables. Several invocations register new executables, in other words, don't fear registering one program several times.

Autotest test suites rely on the `PATH` to find the tested program. This saves from generating the absolute paths to the various tools, and makes it possible to test installed programs. Therefore, knowing what programs are being exercised is crucial to understand some problems in the test suite itself, or its occasional misuses. It is a good idea to also subscribe foreign programs you depend upon, to ease incompatibility diagnostics.

AT_SETUP (test-group-name) | Macro

This macro starts a group of related tests, all to be executed in the same subshell. It accepts a single argument, which holds a few words (no more than about 30 or 40 characters) quickly describing the purpose of the test group being started.

AT_KEYWORDS (keywords) | Macro

Associate the space-separated list of keywords to the enclosing test group. This makes it possible to run "slices" of the test suite. For instance if some of your test groups exercise some `foo` feature, then using `AT_KEYWORDS(foo)` lets you run `./testsuite -k foo` to run exclusively these test groups. The title of the test group is automatically recorded to `AT_KEYWORDS`. Several invocations within a test group accumulate new keywords. In other words, don't fear registering several times the same keyword in a test group.

AT_CLEANUP | Macro

End the current test group.

AT_DATA (file, contents) | Macro

Initialize an input data file with given contents. Of course, the contents have to be properly quoted between square brackets to protect against included commas or spurious `m4` expansion. The contents ought to end with an end of line.

AT_CHECK (commands, [status = ``0``], [stdout], [stderr]) | Macro

Execute a test by performing given shell commands. These commands should normally exit with status, while producing expected stdout and stderr contents. If commands exit with status 77, then the whole test group is skipped. The commands must not redirect the standard output, nor the standard error. If status, or stdout, or stderr is `ignore`, then the corresponding value is not checked. The special value `expout` for stdout means the expected output of the commands is the content of the file `expout`. If stdout is `stdout`, then the standard output of the commands is available for further tests in the file `stdout`. Similarly for stderr with `expout` and `stderr`.

Node:testsuite Invocation, Next:Making testsuite Scripts, Previous:Writing testsuite.at, Up:Using Autotest

### Running `testsuite` Scripts

Autotest test suites support the following arguments:

`--help`
`-h`
  Display the list of options and exit successfully.
`--version`
`-V`
  Display the version of the test suite and exit successfully.
`--clean`
`-c`
  Remove all the files the test suite might have created and exit. Meant for `clean` Makefile targets.
`--list`
`-l`
  List all the tests (or only the selection), including their possible keywords.

By default all the tests are performed (or described with `--list`) in the default environment first silently, then verbosely, but the environment, set of tests, and verbosity level can be tuned:

`variable=value`
  Set the environment variable to value. Do not run `FOO=foo ./testsuite` as debugging scripts would then run in a different environment.

  The variable `AUTOTEST_PATH` specifies the testing path to prepend to `PATH`. It handles specially relative paths (not starting with `/`): they are considered to be relative to the top level of the package being built. All the directories are made absolute, first starting from the top level build tree, then from the source tree. For instance `./testsuite AUTOTEST_PATH=tests:bin` for a `/src/foo-1.0` source package built in `/tmp/foo` results in `/tmp/foo/tests:/tmp/foo/bin` and then `/src/foo-1.0/tests:/src/foo-1.0/bin` being prepended to `PATH`.

`number`
`number-number`
`number-`
`-number`
  Add the corresponding test groups, with obvious semantics, to the selection.
`--keywords=keywords`
`-k keywords`
  Add to the selection the test groups which title or keywords (arguments to `AT_SETUP` or `AT_KEYWORDS`) match all the keywords of the comma separated list keywords.

  Running `./testsuite -k autoupdate,FUNC` will select all the tests tagged with `autoupdate` and `FUNC` (as in `AC_CHECK_FUNC`, `AC_FUNC_FNMATCH` etc.) while `./testsuite -k autoupdate -k FUNC` runs all the tests tagged with `autoupdate` or `FUNC`.

`--errexit`
`-e`
  If any test fails, immediately abort testing. It implies `--debug`: post test group clean up, debugging script generation, and logging are inhibited. This option is meant for the full test suite, it is not really useful for generated debugging scripts.
`--verbose`
`-v`
  Force more verbosity in the detailed output of what is being done. This is the default for debugging scripts.
`--debug`
`-d`
  Do not remove the files after a test group was performed --but they are still removed before, therefore using this option is sane when running several test groups. Do not create debugging scripts. Do not log (in order to preserve supposedly existing full log file). This is the default for debugging scripts.
`--trace`
`-x`
  Trigger shell tracing of the test groups.

Node:Making testsuite Scripts, Previous:testsuite Invocation, Up:Using Autotest

### Making `testsuite` Scripts

For putting Autotest into movement, you need some configuration and Makefile machinery. We recommend, at least if your package uses deep or shallow hierarchies, that you use `tests/` as the name of the directory holding all your tests and their `Makefile`. Here is a check list of things to do.

- Make sure to create the file `package.m4`, which defines the identity of the package. It must define `AT_PACKAGE_STRING`, the full signature of the package, and `AT_PACKAGE_BUGREPORT`, the address to which bug reports should be sent. For sake of completeness, we suggest that you also define `AT_PACKAGE_NAME`, `AT_PACKAGE_TARNAME`, and `AT_PACKAGE_VERSION`. See Initializing configure, for a description of these variables. We suggest the following Makefile excerpt:

      $(srcdir)/package.m4: $(top_srcdir)/configure.ac
              {                                      \
                echo '# Signature of the current package.'; \
                echo 'm4_define([AT_PACKAGE_NAME],      [@PACKAGE_NAME@])'; \
                echo 'm4_define([AT_PACKAGE_TARNAME],   [@PACKAGE_TARNAME@])'; \
                echo 'm4_define([AT_PACKAGE_VERSION],   [@PACKAGE_VERSION@])'; \
                echo 'm4_define([AT_PACKAGE_STRING],    [@PACKAGE_STRING@])'; \
                echo 'm4_define([AT_PACKAGE_BUGREPORT], [@PACKAGE_BUGREPORT@])'; \
              } >$(srcdir)/package.m4

  Be sure to distribute `package.m4` and to put it into the source hierarchy: the test suite ought to be shipped!

- Use the `AT_CONFIG` macro from within file `configure.ac`. This macro accepts one argument, which is the directory, relative to the test directory, where the executables are prepared.
- Still within `configure.ac`, ensure that some `AC_CONFIG_FILES` command includes substitution for `tests/atconfig` and also, as appropriate, `tests/atlocal`.
- The `tests/Makefile.in` should be modified so the validation in your package is triggered by `make check`. An example is provided below.

With Automake, here is a minimal example about how to link `make check` with a validation suite.

    EXTRA_DIST = testsuite.at testsuite
    TESTSUITE = $(srcdir)/testsuite
    check-local: atconfig atlocal $(TESTSUITE)
            $(SHELL) $(TESTSUITE)

    AUTOTEST = $(AUTOM4TE) --language=autotest
    $(TESTSUITE): $(srcdir)/testsuite.at
            $(AUTOTEST) -I $(srcdir) $.at -o $.tmp
            mv $.tmp $

You might want to list explicitly the dependencies, i.e., the list of the files `testsuite.at` includes.

With strict Autoconf, you might need to add lines inspired from the following:

    subdir = tests

    atconfig: $(top_builddir)/config.status
    	cd $(top_builddir) && \
               $(SHELL) ./config.status $(subdir)/$
    atlocal: $(srcdir)/atlocal.in $(top_builddir)/config.status
    	cd $(top_builddir) && \
               $(SHELL) ./config.status $(subdir)/$

and manage to have `atconfig.in` and `$(EXTRA_DIST)` distributed.

Node:Questions, Next:History, Previous:Using Autotest, Up:Top

## Questions About Autoconf

Several questions about Autoconf come up occasionally. Here some of them are addressed.

- Distributing: Distributing `configure` scripts
- Why GNU m4: Why not use the standard M4?
- Bootstrapping: Autoconf and GNU M4 require each other?
- Why Not Imake: Why GNU uses `configure` instead of Imake

Node:Distributing, Next:Why GNU m4, Up:Questions

### Distributing `configure` Scripts

    What are the restrictions on distributing configure
    scripts that Autoconf generates?  How does that affect my
    programs that use them?

There are no restrictions on how the configuration scripts that Autoconf produces may be distributed or used. In Autoconf version 1, they were covered by the GNU General Public License. We still encourage software authors to distribute their work under terms like those of the GPL, but doing so is not required to use Autoconf.

Of the other files that might be used with `configure`, `config.h.in` is under whatever copyright you use for your `configure.ac`. `config.sub` and `config.guess` have an exception to the GPL when they are used with an Autoconf-generated `configure` script, which permits you to distribute them under the same terms as the rest of your package. `install-sh` is from the X Consortium and is not copyrighted.

Node:Why GNU m4, Next:Bootstrapping, Previous:Distributing, Up:Questions

### Why Require GNU M4?

    Why does Autoconf require GNU M4?

Many M4 implementations have hard-coded limitations on the size and number of macros that Autoconf exceeds. They also lack several builtin macros that it would be difficult to get along without in a sophisticated application like Autoconf, including:

    m4_builtin
    m4_indir
    m4_bpatsubst
    __file__
    __line__

Autoconf requires version 1.4 or above of GNU M4 because it uses frozen state files.

Since only software maintainers need to use Autoconf, and since GNU M4 is simple to configure and install, it seems reasonable to require GNU M4 to be installed also. Many maintainers of GNU and other free software already have most of the GNU utilities installed, since they prefer them.

Node:Bootstrapping, Next:Why Not Imake, Previous:Why GNU m4, Up:Questions

### How Can I Bootstrap?

    If Autoconf requires GNU M4 and GNU M4 has an Autoconf
    configure script, how do I bootstrap?  It seems like a chicken
    and egg problem!

This is a misunderstanding. Although GNU M4 does come with a `configure` script produced by Autoconf, Autoconf is not required in order to run the script and install GNU M4. Autoconf is only required if you want to change the M4 `configure` script, which few people have to do (mainly its maintainer).

Node:Why Not Imake, Previous:Bootstrapping, Up:Questions

### Why Not Imake?

    Why not use Imake instead of configure scripts?

Several people have written addressing this question, so I include adaptations of their explanations here.

The following answer is based on one written by Richard Pixley:

> Autoconf generated scripts frequently work on machines that it has never been set up to handle before. That is, it does a good job of inferring a configuration for a new system. Imake cannot do this.

> Imake uses a common database of host specific data. For X11, this makes sense because the distribution is made as a collection of tools, by one central authority who has control over the database.

> GNU tools are not released this way. Each GNU tool has a maintainer; these maintainers are scattered across the world. Using a common database would be a maintenance nightmare. Autoconf may appear to be this kind of database, but in fact it is not. Instead of listing host dependencies, it lists program requirements.

> If you view the GNU suite as a collection of native tools, then the problems are similar. But the GNU development tools can be configured as cross tools in almost any host+target permutation. All of these configurations can be installed concurrently. They can even be configured to share host independent files across hosts. Imake doesn't address these issues.

> Imake templates are a form of standardization. The GNU coding standards address the same issues without necessarily imposing the same restrictions.

Here is some further explanation, written by Per Bothner:

> One of the advantages of Imake is that it easy to generate large Makefiles using `cpp`'s `#include` and macro mechanisms. However, `cpp` is not programmable: it has limited conditional facilities, and no looping. And `cpp` cannot inspect its environment.

> All of these problems are solved by using `sh` instead of `cpp`. The shell is fully programmable, has macro substitution, can execute (or source) other shell scripts, and can inspect its environment.

Paul Eggert elaborates more:

> With Autoconf, installers need not assume that Imake itself is already installed and working well. This may not seem like much of an advantage to people who are accustomed to Imake. But on many hosts Imake is not installed or the default installation is not working well, and requiring Imake to install a package hinders the acceptance of that package on those hosts. For example, the Imake template and configuration files might not be installed properly on a host, or the Imake build procedure might wrongly assume that all source files are in one big directory tree, or the Imake configuration might assume one compiler whereas the package or the installer needs to use another, or there might be a version mismatch between the Imake expected by the package and the Imake supported by the host. These problems are much rarer with Autoconf, where each package comes with its own independent configuration processor.

> Also, Imake often suffers from unexpected interactions between `make` and the installer's C preprocessor. The fundamental problem here is that the C preprocessor was designed to preprocess C programs, not `Makefile`s. This is much less of a problem with Autoconf, which uses the general-purpose preprocessor `m4`, and where the package's author (rather than the installer) does the preprocessing in a standard way.

Finally, Mark Eichin notes:

> Imake isn't all that extensible, either. In order to add new features to Imake, you need to provide your own project template, and duplicate most of the features of the existing one. This means that for a sophisticated project, using the vendor-provided Imake templates fails to provide any leverage--since they don't cover anything that your own project needs (unless it is an X11 program).

> On the other side, though:

> The one advantage that Imake has over `configure`: `Imakefile`s tend to be much shorter (likewise, less redundant) than `Makefile.in`s. There is a fix to this, however--at least for the Kerberos V5 tree, we've modified things to call in common `post.in` and `pre.in` `Makefile` fragments for the entire tree. This means that a lot of common things don't have to be duplicated, even though they normally are in `configure` setups.

Node:History, Next:Copying This Manual, Previous:Questions, Up:Top

## History of Autoconf

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

- Genesis: Prehistory and naming of `configure`
- Exodus: The plagues of M4 and Perl
- Leviticus: The priestly code of portability arrives
- Numbers: Growth and contributors
- Deuteronomy: Approaching the promises of easy configuration

Node:Genesis, Next:Exodus, Up:History

### Genesis

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

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

Node:Exodus, Next:Leviticus, Previous:Genesis, Up:History

### Exodus

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

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

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

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

I considered using Perl to generate my style of `configure` scripts, but decided that M4 was better suited to the job of simple textual substitutions: it gets in the way less, because output is implicit. Plus, everyone already has it. (Initially I didn't rely on the GNU extensions to M4.) Also, some of my friends at the University of Maryland had recently been putting M4 front ends on several programs, including `tvtwm`, and I was interested in trying out a new language.

Node:Leviticus, Next:Numbers, Previous:Exodus, Up:History

### Leviticus

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

In the fall of 1991 I called together a group of fellow questers after the Holy Grail of portability (er, that is, alpha testers) to give me feedback as I encapsulated pieces of my handwritten scripts in M4 macros and continued to add features and improve the techniques used in the checks. Prominent among the testers were François Pinard, who came up with the idea of making an `autoconf` shell script to run `m4` and check for unresolved macro calls; Richard Pixley, who suggested running the compiler instead of searching the file system to find include files and symbols, for more accurate results; Karl Berry, who got Autoconf to configure TeX and added the macro index to the documentation; and Ian Lance Taylor, who added support for creating a C header file as an alternative to putting `-D` options in a `Makefile`, so he could use Autoconf for his UUCP package. The alpha testers cheerfully adjusted their files again and again as the names and calling conventions of the Autoconf macros changed from release to release. They all contributed many specific checks, great ideas, and bug fixes.

Node:Numbers, Next:Deuteronomy, Previous:Leviticus, Up:History

### Numbers

In July 1992, after months of alpha testing, I released Autoconf 1.0, and converted many GNU packages to use it. I was surprised by how positive the reaction to it was. More people started using it than I could keep track of, including people working on software that wasn't part of the GNU Project (such as TCL, FSP, and Kerberos V5). Autoconf continued to improve rapidly, as many people using the `configure` scripts reported problems they encountered.

Autoconf turned out to be a good torture test for M4 implementations. UNIX `m4` started to dump core because of the length of the macros that Autoconf defined, and several bugs showed up in GNU `m4` as well. Eventually, we realized that we needed to use some features that only GNU M4 has. 4.3BSD `m4`, in particular, has an impoverished set of builtin macros; the System V version is better, but still doesn't provide everything we need.

More development occurred as people put Autoconf under more stresses (and to uses I hadn't anticipated). Karl Berry added checks for X11. david zuhn contributed C++ support. François Pinard made it diagnose invalid arguments. Jim Blandy bravely coerced it into configuring GNU Emacs, laying the groundwork for several later improvements. Roland McGrath got it to configure the GNU C Library, wrote the `autoheader` script to automate the creation of C header file templates, and added a `--verbose` option to `configure`. Noah Friedman added the `--autoconf-dir` option and `AC_MACRODIR` environment variable. (He also coined the term autoconfiscate to mean "adapt a software package to use Autoconf".) Roland and Noah improved the quoting protection in `AC_DEFINE` and fixed many bugs, especially when I got sick of dealing with portability problems from February through June, 1993.

Node:Deuteronomy, Previous:Numbers, Up:History

### Deuteronomy

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

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

I made some other changes because of my dissatisfaction with the quality of the program. I made the messages showing results of the checks less ambiguous, always printing a result. I regularized the names of the macros and cleaned up coding style inconsistencies. I added some auxiliary utilities that I had developed to help convert source code packages to use Autoconf. With the help of François Pinard, I made the macros not interrupt each others' messages. (That feature revealed some performance bottlenecks in GNU `m4`, which he hastily corrected!) I reorganized the documentation around problems people want to solve. And I began a test suite, because experience had shown that Autoconf has a pronounced tendency to regress when we change it.

Again, several alpha testers gave invaluable feedback, especially François Pinard, Jim Meyering, Karl Berry, Rob Savoye, Ken Raeburn, and Mark Eichin.

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

Node:Copying This Manual, Next:Indices, Previous:History, Up:Top

## Copying This Manual

- GNU Free Documentation License: License for copying this manual

Node:GNU Free Documentation License, Up:Copying This Manual

### GNU Free Documentation License

Version 1.1, March 2000

    Copyright © 2000 Free Software Foundation, Inc.
    59 Temple Place, Suite 330, Boston, MA  02111-1307, USA

    Everyone is permitted to copy and distribute verbatim copies
    of this license document, but changing it is not allowed.

0. PREAMBLE

   The purpose of this License is to make a manual, textbook, or other written document free in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.

   This License is a kind of "copyleft", which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

   We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.

1. APPLICABILITY AND DEFINITIONS

   This License applies to any manual or other work that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. The "Document", below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as "you".

   A "Modified Version" of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language.

   A "Secondary Section" is a named appendix or a front-matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document's overall subject (or to related matters) and contains nothing that could fall directly within that overall subject. (For example, if the Document is in part a textbook of mathematics, a Secondary Section may not explain any mathematics.) The relationship could be a matter of historical connection with the subject or with related matters, or of legal, commercial, philosophical, ethical or political position regarding them.

   The "Invariant Sections" are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License.

   The "Cover Texts" are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in the notice that says that the Document is released under this License.

   A "Transparent" copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, whose contents can be viewed and edited directly and straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file format whose markup has been designed to thwart or discourage subsequent modification by readers is not Transparent. A copy that is not "Transparent" is called "Opaque".

   Examples of suitable formats for Transparent copies include plain ASCII without markup, Texinfo input format, LaTeX input format, SGML or XML using a publicly available DTD, and standard-conforming simple HTML designed for human modification. Opaque formats include PostScript, PDF, proprietary formats that can be read and edited only by proprietary word processors, SGML or XML for which the DTD and/or processing tools are not generally available, and the machine-generated HTML produced by some word processors for output purposes only.

   The "Title Page" means, for a printed book, the title page itself, plus such following pages as are needed to hold, legibly, the material this License requires to appear in the title page. For works in formats which do not have any title page as such, "Title Page" means the text near the most prominent appearance of the work's title, preceding the beginning of the body of the text.

2. VERBATIM COPYING

   You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow the conditions in section 3.

   You may also lend copies, under the same conditions stated above, and you may publicly display copies.

3. COPYING IN QUANTITY

   If you publish printed copies of the Document numbering more than 100, and the Document's license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects.

   If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages.

   If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a publicly-accessible computer-network location containing a complete Transparent copy of the Document, free of added material, which the general network-using public has access to download anonymously at no charge using public-standard network protocols. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public.

   It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document.

4. MODIFICATIONS

   You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:

   1. Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of previous versions (which should, if there were any, be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission.
   2. List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of its principal authors, if it has less than five).
   3. State on the Title page the name of the publisher of the Modified Version, as the publisher.
   4. Preserve all the copyright notices of the Document.
   5. Add an appropriate copyright notice for your modifications adjacent to the other copyright notices.
   6. Include, immediately after the copyright notices, a license notice giving the public permission to use the Modified Version under the terms of this License, in the form shown in the Addendum below.
   7. Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document's license notice.
   8. Include an unaltered copy of this License.
   9. Preserve the section entitled "History", and its title, and add to it an item stating at least the title, year, new authors, and publisher of the Modified Version as given on the Title Page. If there is no section entitled "History" in the Document, create one stating the title, year, authors, and publisher of the Document as given on its Title Page, then add an item describing the Modified Version as stated in the previous sentence.
   10. Preserve the network location, if any, given in the Document for public access to a Transparent copy of the Document, and likewise the network locations given in the Document for previous versions it was based on. These may be placed in the "History" section. You may omit a network location for a work that was published at least four years before the Document itself, or if the original publisher of the version it refers to gives permission.
   11. In any section entitled "Acknowledgments" or "Dedications", preserve the section's title, and preserve in the section all the substance and tone of each of the contributor acknowledgments and/or dedications given therein.
   12. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles.
   13. Delete any section entitled "Endorsements". Such a section may not be included in the Modified Version.
   14. Do not retitle any existing section as "Endorsements" or to conflict in title with any Invariant Section.

   If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version's license notice. These titles must be distinct from any other section titles.

   You may add a section entitled "Endorsements", provided it contains nothing but endorsements of your Modified Version by various parties--for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.

   You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.

   The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version.

5. COMBINING DOCUMENTS

   You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice.

   The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.

   In the combination, you must combine any sections entitled "History" in the various original documents, forming one section entitled "History"; likewise combine any sections entitled "Acknowledgments", and any sections entitled "Dedications". You must delete all sections entitled "Endorsements."

6. COLLECTIONS OF DOCUMENTS

   You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.

   You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.

7. AGGREGATION WITH INDEPENDENT WORKS

   A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, does not as a whole count as a Modified Version of the Document, provided no compilation copyright is claimed for the compilation. Such a compilation is called an "aggregate", and this License does not apply to the other self-contained works thus compiled with the Document, on account of their being thus compiled, if they are not themselves derivative works of the Document.

   If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one quarter of the entire aggregate, the Document's Cover Texts may be placed on covers that surround only the Document within the aggregate. Otherwise they must appear on covers around the whole aggregate.

8. TRANSLATION

   Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License provided that you also include the original English version of this License. In case of a disagreement between the translation and the original English version of this License, the original English version will prevail.

9. TERMINATION

   You may not copy, modify, sublicense, or distribute the Document except as expressly provided for under this License. Any other attempt to copy, modify, sublicense or distribute the Document is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.

10. FUTURE REVISIONS OF THIS LICENSE

    The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.

    Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License "or any later version" applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation.

#### ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

      Copyright (C)  year  your name.
      Permission is granted to copy, distribute and/or modify this document
      under the terms of the GNU Free Documentation License, Version 1.1
      or any later version published by the Free Software Foundation;
      with the Invariant Sections being list their titles, with the
      Front-Cover Texts being list, and with the Back-Cover Texts being list.
      A copy of the license is included in the section entitled ``GNU
      Free Documentation License''.

If you have no Invariant Sections, write "with no Invariant Sections" instead of saying which ones are invariant. If you have no Front-Cover Texts, write "no Front-Cover Texts" instead of "Front-Cover Texts being list"; likewise for Back-Cover Texts.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.

Node:Indices, Previous:Copying This Manual, Up:Top

## Indices

- 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
- Autoconf Macro Index: Index of Autoconf macros
- M4 Macro Index: Index of M4, M4sugar, and M4sh macros
- Autotest Macro Index: Index of Autotest macros
- Program & Function Index: Index of those with portability problems
- Concept Index: General index

Node:Environment Variable Index, Next:Output Variable Index, Up:Indices

### Environment Variable Index

This is an alphabetical list of the environment variables that Autoconf checks.

- `CDPATH`: Special Shell Variables
- `CONFIG_COMMANDS`: Obsolete config.status Use
- `CONFIG_FILES`: Obsolete config.status Use
- `CONFIG_HEADERS`: Obsolete config.status Use
- `CONFIG_LINKS`: Obsolete config.status Use
- `CONFIG_SHELL`: config.status Invocation
- `CONFIG_SITE`: Site Defaults
- `CONFIG_STATUS`: config.status Invocation
- `IFS`: Special Shell Variables
- `LANG`: Special Shell Variables
- `LANGUAGE`: Special Shell Variables
- `LC_ALL`: Special Shell Variables
- `LC_COLLATE`: Special Shell Variables
- `LC_CTYPE`: Special Shell Variables
- `LC_MESSAGES`: Special Shell Variables
- `LC_NUMERIC`: Special Shell Variables
- `LC_TIME`: Special Shell Variables
- `LINENO`: Special Shell Variables
- `NULLCMD`: Special Shell Variables
- `PATH_SEPARATOR`: Special Shell Variables
- `RANDOM`: Special Shell Variables
- `SIMPLE_BACKUP_SUFFIX`: autoupdate Invocation
- `status`: Special Shell Variables
- `WARNINGS`: Invoking autom4te, autoheader Invocation, autoconf Invocation

Node:Output Variable Index, Next:Preprocessor Symbol Index, Previous:Environment Variable Index, Up:Indices

### Output Variable Index

This is an alphabetical list of the variables that Autoconf can substitute into files that it creates, typically one or more `Makefile`s. See Setting Output Variables, for more information on how this is done.

- `abs_builddir`: Preset Output Variables
- `abs_srcdir`: Preset Output Variables
- `abs_top_builddir`: Preset Output Variables
- `abs_top_srcdir`: Preset Output Variables
- `ALLOCA`: Particular Functions
- `AWK`: Particular Programs
- `bindir`: Installation Directory Variables
- `build`: Canonicalizing
- `build_alias`: Canonicalizing
- `build_cpu`: Canonicalizing
- `build_os`: Canonicalizing
- `build_vendor`: Canonicalizing
- `builddir`: Preset Output Variables
- `CC`: System Services, C Compiler
- `CFLAGS`: C Compiler, Preset Output Variables
- `configure_input`: Preset Output Variables
- `CPP`: C Compiler
- `CPPFLAGS`: Preset Output Variables
- `cross_compiling`: Specifying Names
- `CXX`: C++ Compiler
- `CXXCPP`: C++ Compiler
- `CXXFLAGS`: C++ Compiler, Preset Output Variables
- `datadir`: Installation Directory Variables
- `DEFS`: Preset Output Variables
- `ECHO_C`: Preset Output Variables
- `ECHO_N`: Preset Output Variables
- `ECHO_T`: Preset Output Variables
- `exec_prefix`: Installation Directory Variables
- `EXEEXT`: Obsolete Macros, Compilers and Preprocessors
- `F77`: Fortran 77 Compiler
- `FFLAGS`: Fortran 77 Compiler, Preset Output Variables
- `FLIBS`: Fortran 77 Compiler
- `GETGROUPS_LIBS`: Particular Functions
- `GETLOADAVG_LIBS`: Particular Functions
- `host`: Canonicalizing
- `host_alias`: Canonicalizing
- `host_cpu`: Canonicalizing
- `host_os`: Canonicalizing
- `host_vendor`: Canonicalizing
- `includedir`: Installation Directory Variables
- `infodir`: Installation Directory Variables
- `INSTALL`: Particular Programs
- `INSTALL_DATA`: Particular Programs
- `INSTALL_PROGRAM`: Particular Programs
- `INSTALL_SCRIPT`: Particular Programs
- `KMEM_GROUP`: Particular Functions
- `LDFLAGS`: Preset Output Variables
- `LEX`: Particular Programs
- `LEX_OUTPUT_ROOT`: Particular Programs
- `LEXLIB`: Particular Programs
- `libdir`: Installation Directory Variables
- `libexecdir`: Installation Directory Variables
- `LIBOBJS`: Particular Structures, Generic Functions, Particular Functions
- `LIBS`: Obsolete Macros, UNIX Variants, Preset Output Variables
- `LN_S`: Particular Programs
- `localstatedir`: Installation Directory Variables
- `mandir`: Installation Directory Variables
- `NEED_SETGID`: Particular Functions
- `OBJEXT`: Obsolete Macros, Compilers and Preprocessors
- `oldincludedir`: Installation Directory Variables
- `PACKAGE_BUGREPORT`: Initializing configure
- `PACKAGE_NAME`: Initializing configure
- `PACKAGE_STRING`: Initializing configure
- `PACKAGE_TARNAME`: Initializing configure
- `PACKAGE_VERSION`: Initializing configure
- `POW_LIB`: Particular Functions
- `prefix`: Installation Directory Variables
- `program_transform_name`: Transforming Names
- `RANLIB`: Particular Programs
- `sbindir`: Installation Directory Variables
- `SET_MAKE`: Output
- `sharedstatedir`: Installation Directory Variables
- `srcdir`: Preset Output Variables
- `subdirs`: Subdirectories
- `sysconfdir`: Installation Directory Variables
- `target`: Canonicalizing
- `target_alias`: Canonicalizing
- `target_cpu`: Canonicalizing
- `target_os`: Canonicalizing
- `target_vendor`: Canonicalizing
- `top_builddir`: Preset Output Variables
- `top_srcdir`: Preset Output Variables
- `X_CFLAGS`: System Services
- `X_EXTRA_LIBS`: System Services
- `X_LIBS`: System Services
- `X_PRE_LIBS`: System Services
- `YACC`: Particular Programs

Node:Preprocessor Symbol Index, Next:Autoconf Macro Index, Previous:Output Variable Index, Up:Indices

### Preprocessor Symbol Index

This is an alphabetical list of the C preprocessor symbols that the Autoconf macros define. To work with Autoconf, C source code needs to use these names in `#if` directives.

- `__CHAR_UNSIGNED__`: C Compiler
- `__PROTOTYPES`: C Compiler
- `_ALL_SOURCE`: UNIX Variants
- `_FILE_OFFSET_BITS`: System Services
- `_LARGE_FILES`: System Services
- `_LARGEFILE_SOURCE`: Particular Functions
- `_MINIX`: UNIX Variants
- `_POSIX_1_SOURCE`: UNIX Variants
- `_POSIX_SOURCE`: UNIX Variants
- `_POSIX_VERSION`: Particular Headers
- `C_ALLOCA`: Particular Functions
- `C_GETLOADAVG`: Particular Functions
- `CLOSEDIR_VOID`: Particular Functions
- `const`: C Compiler
- `DGUX`: Particular Functions
- `DIRENT`: Obsolete Macros
- `F77_DUMMY_MAIN`: Fortran 77 Compiler
- `F77_FUNC`: Fortran 77 Compiler
- `F77_FUNC_`: Fortran 77 Compiler
- `F77_MAIN`: Fortran 77 Compiler
- `F77_NO_MINUS_C_MINUS_O`: Fortran 77 Compiler
- `GETGROUPS_T`: Particular Types
- `GETLODAVG_PRIVILEGED`: Particular Functions
- `GETPGRP_VOID`: Particular Functions
- `gid_t`: Particular Types
- `GWINSZ_IN_SYS_IOCTL`: Particular Headers
- `HAVE_ALLOCA_H`: Particular Functions
- `HAVE_CONFIG_H`: Configuration Headers
- `HAVE_DECL_STRERROR_R`: Particular Functions
- `HAVE_DECL_symbol`: Generic Declarations
- `HAVE_DIRENT_H`: Particular Headers
- `HAVE_DOPRNT`: Particular Functions
- `HAVE_function`: Generic Functions
- `HAVE_GETMNTENT`: Particular Functions
- `HAVE_header`: Generic Headers
- `HAVE_LONG_DOUBLE`: C Compiler
- `HAVE_LONG_FILE_NAMES`: System Services
- `HAVE_LSTAT_EMPTY_STRING_BUG`: Particular Functions
- `HAVE_MMAP`: Particular Functions
- `HAVE_NDIR_H`: Particular Headers
- `HAVE_OBSTACK`: Particular Functions
- `HAVE_RESTARTABLE_SYSCALLS`: Obsolete Macros
- `HAVE_ST_BLKSIZE`: Particular Structures
- `HAVE_ST_BLOCKS`: Particular Structures
- `HAVE_ST_RDEV`: Particular Structures
- `HAVE_STAT_EMPTY_STRING_BUG`: Particular Functions
- `HAVE_STRCOLL`: Particular Functions
- `HAVE_STRERROR_R`: Particular Functions
- `HAVE_STRFTIME`: Particular Functions
- `HAVE_STRINGIZE`: C Compiler
- `HAVE_STRNLEN`: Particular Functions
- `HAVE_STRUCT_STAT_ST_BLKSIZE`: Particular Structures
- `HAVE_STRUCT_STAT_ST_BLOCKS`: Particular Structures
- `HAVE_STRUCT_STAT_ST_RDEV`: Particular Structures
- `HAVE_SYS_DIR_H`: Particular Headers
- `HAVE_SYS_NDIR_H`: Particular Headers
- `HAVE_SYS_WAIT_H`: Particular Headers
- `HAVE_TM_ZONE`: Particular Structures
- `HAVE_TZNAME`: Particular Structures
- `HAVE_UTIME_NULL`: Particular Functions
- `HAVE_VFORK_H`: Particular Functions
- `HAVE_VPRINTF`: Particular Functions
- `HAVE_WAIT3`: Obsolete Macros
- `HAVE_WORKING_FORK`: Particular Functions
- `HAVE_WORKING_VFORK`: Particular Functions
- `inline`: C Compiler
- `INT_16_BITS`: Obsolete Macros
- `LONG_64_BITS`: Obsolete Macros
- `LSTAT_FOLLOWS_SLASHED_SYMLINK`: Particular Functions
- `MAJOR_IN_MKDEV`: Particular Headers
- `MAJOR_IN_SYSMACROS`: Particular Headers
- `mode_t`: Particular Types
- `NDIR`: Obsolete Macros
- `NEED_MEMORY_H`: Obsolete Macros
- `NEED_SETGID`: Particular Functions
- `NLIST_NAME_UNION`: Particular Functions
- `NLIST_STRUCT`: Particular Functions
- `NO_MINUS_C_MINUS_O`: C Compiler
- `off_t`: Particular Types
- `PACKAGE_BUGREPORT`: Initializing configure
- `PACKAGE_NAME`: Initializing configure
- `PACKAGE_STRING`: Initializing configure
- `PACKAGE_TARNAME`: Initializing configure
- `PACKAGE_VERSION`: Initializing configure
- `PARAMS`: C Compiler
- `pid_t`: Particular Types
- `PROTOTYPES`: C Compiler
- `RETSIGTYPE`: Particular Types
- `SELECT_TYPE_ARG1`: Particular Functions
- `SELECT_TYPE_ARG234`: Particular Functions
- `SELECT_TYPE_ARG5`: Particular Functions
- `SETPGRP_VOID`: Particular Functions
- `SETVBUF_REVERSED`: Particular Functions
- `size_t`: Particular Types
- `STDC_HEADERS`: Particular Headers
- `STRERROR_R_CHAR_P`: Particular Functions
- `SVR4`: Particular Functions
- `SYS_SIGLIST_DECLARED`: Particular Declarations
- `SYSDIR`: Obsolete Macros
- `SYSNDIR`: Obsolete Macros
- `TIME_WITH_SYS_TIME`: Particular Headers
- `TM_IN_SYS_TIME`: Particular Structures
- `uid_t`: Particular Types
- `UMAX`: Particular Functions
- `UMAX4_3`: Particular Functions
- `USG`: Obsolete Macros
- `vfork`: Particular Functions
- `volatile`: C Compiler
- `WORDS_BIGENDIAN`: C Compiler
- `X_DISPLAY_MISSING`: System Services
- `YYTEXT_POINTER`: Particular Programs

Node:Autoconf Macro Index, Next:M4 Macro Index, Previous:Preprocessor Symbol Index, Up:Indices

### Autoconf Macro Index

This is an alphabetical list of the Autoconf macros. To make the list easier to use, the macros are listed without their preceding `AC_`.

- `AH_BOTTOM`: Autoheader Macros
- `AH_TEMPLATE`: Autoheader Macros
- `AH_TOP`: Autoheader Macros
- `AH_VERBATIM`: Autoheader Macros
- `AIX`: UNIX Variants
- `ALLOCA`: Obsolete Macros
- `ARG_ARRAY`: Obsolete Macros
- `ARG_ENABLE`: Package Options
- `ARG_PROGRAM`: Transforming Names
- `ARG_VAR`: Setting Output Variables
- `ARG_WITH`: External Software
- `AU_DEFUN`: Obsoleting Macros
- `BEFORE`: Suggested Ordering
- `BOTTOM`: Autoheader Macros
- `C_BIGENDIAN`: C Compiler
- `C_CHAR_UNSIGNED`: C Compiler
- `C_CONST`: C Compiler
- `C_CROSS`: Obsolete Macros
- `C_INLINE`: C Compiler
- `C_LONG_DOUBLE`: C Compiler
- `C_PROTOTYPES`: C Compiler
- `C_STRINGIZE`: C Compiler
- `C_VOLATILE`: C Compiler
- `CACHE_CHECK`: Caching Results
- `CACHE_LOAD`: Cache Checkpointing
- `CACHE_SAVE`: Cache Checkpointing
- `CACHE_VAL`: Caching Results
- `CANONICAL_BUILD`: Canonicalizing
- `CANONICAL_HOST`: Canonicalizing
- `CANONICAL_SYSTEM`: Obsolete Macros
- `CANONICAL_TARGET`: Canonicalizing
- `CHAR_UNSIGNED`: Obsolete Macros
- `CHECK_DECL`: Generic Declarations
- `CHECK_DECLS`: Generic Declarations
- `CHECK_FILE`: Files
- `CHECK_FILES`: Files
- `CHECK_FUNC`: Generic Functions
- `CHECK_FUNCS`: Generic Functions
- `CHECK_HEADER`: Generic Headers
- `CHECK_HEADERS`: Generic Headers
- `CHECK_LIB`: Libraries
- `CHECK_MEMBER`: Generic Structures
- `CHECK_MEMBERS`: Generic Structures
- `CHECK_PROG`: Generic Programs
- `CHECK_PROGS`: Generic Programs
- `CHECK_SIZEOF`: Generic Compiler Characteristics
- `CHECK_TOOL`: Generic Programs
- `CHECK_TOOLS`: Generic Programs
- `CHECK_TYPE`: Obsolete Macros, Generic Types
- `CHECK_TYPES`: Generic Types
- `CHECKING`: Obsolete Macros
- `COMPILE_CHECK`: Obsolete Macros
- `CONFIG_AUX_DIR`: Input
- `CONFIG_COMMANDS`: Configuration Commands
- `CONFIG_FILES`: Configuration Files
- `CONFIG_HEADERS`: Configuration Headers
- `CONFIG_LINKS`: Configuration Links
- `CONFIG_SRCDIR`: Input
- `CONFIG_SUBDIRS`: Subdirectories
- `CONST`: Obsolete Macros
- `COPYRIGHT`: Notices
- `CROSS_CHECK`: Obsolete Macros
- `CYGWIN`: Obsolete Macros
- `DECL_SYS_SIGLIST`: Particular Declarations
- `DECL_YYTEXT`: Obsolete Macros
- `DEFINE`: Defining Symbols
- `DEFINE_UNQUOTED`: Defining Symbols
- `DEFUN`: Obsoleting Macros, Macro Definitions
- `DIAGNOSE`: Reporting Messages
- `DIR_HEADER`: Obsolete Macros
- `DYNIX_SEQ`: Obsolete Macros
- `EGREP_CPP`: Examining Declarations
- `EGREP_HEADER`: Examining Declarations
- `EMXOS2`: Obsolete Macros
- `ENABLE`: Package Options
- `ERROR`: Obsolete Macros
- `EXEEXT`: Obsolete Macros
- `F77_DUMMY_MAIN`: Fortran 77 Compiler
- `F77_FUNC`: Fortran 77 Compiler
- `F77_LIBRARY_LDFLAGS`: Fortran 77 Compiler
- `F77_MAIN`: Fortran 77 Compiler
- `F77_WRAPPERS`: Fortran 77 Compiler
- `FATAL`: Reporting Messages
- `FIND_X`: Obsolete Macros
- `FIND_XTRA`: Obsolete Macros
- `FUNC_ALLOCA`: Particular Functions
- `FUNC_CHECK`: Obsolete Macros
- `FUNC_CHOWN`: Particular Functions
- `FUNC_CLOSEDIR_VOID`: Particular Functions
- `FUNC_ERROR_AT_LINE`: Particular Functions
- `FUNC_FNMATCH`: Particular Functions
- `FUNC_FORK`: Particular Functions
- `FUNC_FSEEKO`: Particular Functions
- `FUNC_GETGROUPS`: Particular Functions
- `FUNC_GETLOADAVG`: Particular Functions
- `FUNC_GETMNTENT`: Particular Functions
- `FUNC_GETPGRP`: Particular Functions
- `FUNC_LSTAT`: Particular Functions
- `FUNC_LSTAT_FOLLOWS_SLASHED_SYMLINK`: Particular Functions
- `FUNC_MALLOC`: Particular Functions
- `FUNC_MEMCMP`: Particular Functions
- `FUNC_MKTIME`: Particular Functions
- `FUNC_MMAP`: Particular Functions
- `FUNC_OBSTACK`: Particular Functions
- `FUNC_SELECT_ARGTYPES`: Particular Functions
- `FUNC_SETPGRP`: Particular Functions
- `FUNC_SETVBUF_REVERSED`: Particular Functions
- `FUNC_STAT`: Particular Functions
- `FUNC_STRCOLL`: Particular Functions
- `FUNC_STRERROR_R`: Particular Functions
- `FUNC_STRFTIME`: Particular Functions
- `FUNC_STRNLEN`: Particular Functions
- `FUNC_STRTOD`: Particular Functions
- `FUNC_UTIME_NULL`: Particular Functions
- `FUNC_VPRINTF`: Particular Functions
- `FUNC_WAIT3`: Obsolete Macros
- `GCC_TRADITIONAL`: Obsolete Macros
- `GETGROUPS_T`: Obsolete Macros
- `GETLOADAVG`: Obsolete Macros
- `HAVE_FUNCS`: Obsolete Macros
- `HAVE_HEADERS`: Obsolete Macros
- `HAVE_LIBRARY`: Obsolete Macros
- `HAVE_POUNDBANG`: Obsolete Macros
- `HEADER_CHECK`: Obsolete Macros
- `HEADER_DIRENT`: Particular Headers
- `HEADER_EGREP`: Obsolete Macros
- `HEADER_MAJOR`: Particular Headers
- `HEADER_STAT`: Particular Headers
- `HEADER_STDC`: Particular Headers
- `HEADER_SYS_WAIT`: Particular Headers
- `HEADER_TIME`: Particular Headers
- `HEADER_TIOCGWINSZ`: Particular Headers
- `HELP_STRING`: Pretty Help Strings
- `INIT`: Obsolete Macros, Initializing configure
- `INLINE`: Obsolete Macros
- `INT_16_BITS`: Obsolete Macros
- `IRIX_SUN`: Obsolete Macros
- `ISC_POSIX`: UNIX Variants
- `LANG_C`: Obsolete Macros
- `LANG_CPLUSPLUS`: Obsolete Macros
- `LANG_FORTRAN77`: Obsolete Macros
- `LANG_POP`: Language Choice
- `LANG_PUSH`: Language Choice
- `LANG_RESTORE`: Obsolete Macros
- `LANG_SAVE`: Obsolete Macros
- `LIBOBJ`: Generic Functions
- `LIBSOURCE`: Generic Functions
- `LIBSOURCES`: Generic Functions
- `LINK_FILES`: Obsolete Macros
- `LN_S`: Obsolete Macros
- `LONG_64_BITS`: Obsolete Macros
- `LONG_DOUBLE`: Obsolete Macros
- `LONG_FILE_NAMES`: Obsolete Macros
- `MAJOR_HEADER`: Obsolete Macros
- `MEMORY_H`: Obsolete Macros
- `MINGW32`: Obsolete Macros
- `MINIX`: UNIX Variants
- `MINUS_C_MINUS_O`: Obsolete Macros
- `MMAP`: Obsolete Macros
- `MODE_T`: Obsolete Macros
- `MSG_CHECKING`: Printing Messages
- `MSG_ERROR`: Printing Messages
- `MSG_NOTICE`: Printing Messages
- `MSG_RESULT`: Printing Messages
- `MSG_WARN`: Printing Messages
- `OBJEXT`: Obsolete Macros
- `OBSOLETE`: Obsolete Macros
- `OFF_T`: Obsolete Macros
- `OUTPUT`: Obsolete Macros, Output
- `OUTPUT_COMMANDS`: Obsolete Macros
- `OUTPUT_COMMANDS_POST`: Configuration Commands
- `OUTPUT_COMMANDS_PRE`: Configuration Commands
- `PACKAGE_BUGREPORT`: Initializing configure
- `PACKAGE_NAME`: Initializing configure
- `PACKAGE_STRING`: Initializing configure
- `PACKAGE_TARNAME`: Initializing configure
- `PACKAGE_VERSION`: Initializing configure
- `PATH_PROG`: Generic Programs
- `PATH_PROGS`: Generic Programs
- `PATH_TOOL`: Generic Programs
- `PATH_X`: System Services
- `PATH_XTRA`: System Services
- `PID_T`: Obsolete Macros
- `PREFIX`: Obsolete Macros
- `PREFIX_DEFAULT`: Default Prefix
- `PREFIX_PROGRAM`: Default Prefix
- `PREREQ`: Notices
- `PROG_AWK`: Particular Programs
- `PROG_CC`: C Compiler
- `PROG_CC_C_O`: C Compiler
- `PROG_CC_STDC`: C Compiler
- `PROG_CPP`: C Compiler
- `PROG_CXX`: C++ Compiler
- `PROG_CXXCPP`: C++ Compiler
- `PROG_F77_C_O`: Fortran 77 Compiler
- `PROG_FORTRAN`: Fortran 77 Compiler
- `PROG_GCC_TRADITIONAL`: C Compiler
- `PROG_INSTALL`: Particular Programs
- `PROG_LEX`: Particular Programs
- `PROG_LN_S`: Particular Programs
- `PROG_MAKE_SET`: Output
- `PROG_RANLIB`: Particular Programs
- `PROG_YACC`: Particular Programs
- `PROGRAM_CHECK`: Obsolete Macros
- `PROGRAM_EGREP`: Obsolete Macros
- `PROGRAM_PATH`: Obsolete Macros
- `PROGRAMS_CHECK`: Obsolete Macros
- `PROGRAMS_PATH`: Obsolete Macros
- `REMOTE_TAPE`: Obsolete Macros
- `REPLACE_FUNCS`: Generic Functions
- `REQUIRE`: Prerequisite Macros
- `REQUIRE_CPP`: Language Choice
- `RESTARTABLE_SYSCALLS`: Obsolete Macros
- `RETSIGTYPE`: Obsolete Macros
- `REVISION`: Notices
- `RSH`: Obsolete Macros
- `SCO_INTL`: Obsolete Macros
- `SEARCH_LIBS`: Libraries
- `SET_MAKE`: Obsolete Macros
- `SETVBUF_REVERSED`: Obsolete Macros
- `SIZE_T`: Obsolete Macros
- `SIZEOF_TYPE`: Obsolete Macros
- `ST_BLKSIZE`: Obsolete Macros
- `ST_BLOCKS`: Obsolete Macros
- `ST_RDEV`: Obsolete Macros
- `STAT_MACROS_BROKEN`: Obsolete Macros, Particular Headers
- `STDC_HEADERS`: Obsolete Macros
- `STRCOLL`: Obsolete Macros
- `STRUCT_ST_BLKSIZE`: Particular Structures
- `STRUCT_ST_BLOCKS`: Particular Structures
- `STRUCT_ST_RDEV`: Particular Structures
- `STRUCT_TIMEZONE`: Particular Structures
- `STRUCT_TM`: Particular Structures
- `SUBST`: Setting Output Variables
- `SUBST_FILE`: Setting Output Variables
- `SYS_INTERPRETER`: System Services
- `SYS_LARGEFILE`: System Services
- `SYS_LONG_FILE_NAMES`: System Services
- `SYS_POSIX_TERMIOS`: System Services
- `SYS_RESTARTABLE_SYSCALLS`: Obsolete Macros
- `SYS_SIGLIST_DECLARED`: Obsolete Macros
- `TEMPLATE`: Autoheader Macros
- `TEST_CPP`: Obsolete Macros
- `TEST_PROGRAM`: Obsolete Macros
- `TIME_WITH_SYS_TIME`: Obsolete Macros
- `TIMEZONE`: Obsolete Macros
- `TOP`: Autoheader Macros
- `TRY_COMPILE`: Examining Syntax
- `TRY_CPP`: Examining Declarations
- `TRY_LINK`: Examining Libraries
- `TRY_LINK_FUNC`: Examining Libraries
- `TRY_RUN`: Test Programs
- `TYPE_GETGROUPS`: Particular Types
- `TYPE_MODE_T`: Particular Types
- `TYPE_OFF_T`: Particular Types
- `TYPE_PID_T`: Particular Types
- `TYPE_SIGNAL`: Particular Types
- `TYPE_SIZE_T`: Particular Types
- `TYPE_UID_T`: Particular Types
- `UID_T`: Obsolete Macros
- `UNISTD_H`: Obsolete Macros
- `USG`: Obsolete Macros
- `UTIME_NULL`: Obsolete Macros
- `VALIDATE_CACHED_SYSTEM_TUPLE`: Obsolete Macros
- `VERBATIM`: Autoheader Macros
- `VERBOSE`: Obsolete Macros
- `VFORK`: Obsolete Macros
- `VPRINTF`: Obsolete Macros
- `WAIT3`: Obsolete Macros
- `WARN`: Obsolete Macros
- `WARNING`: Reporting Messages
- `WITH`: External Software
- `WORDS_BIGENDIAN`: Obsolete Macros
- `XENIX_DIR`: Obsolete Macros
- `YYTEXT_POINTER`: Obsolete Macros

Node:M4 Macro Index, Next:Autotest Macro Index, Previous:Autoconf Macro Index, Up:Indices

### M4 Macro Index

This is an alphabetical list of the M4, M4sugar, and M4sh macros. To make the list easier to use, the macros are listed without their preceding `m4_` or `AS_`.

- `bpatsubst`: Redefined M4 Macros
- `bregexp`: Redefined M4 Macros
- `defn`: Redefined M4 Macros
- `DIRNAME`: Programming in M4sh
- `dnl`: Redefined M4 Macros
- `dquote`: Evaluation Macros
- `m4_exit`: Redefined M4 Macros
- `m4_if`: Redefined M4 Macros
- `m4_wrap`: Redefined M4 Macros
- `pattern_allow`: Forbidden Patterns
- `pattern_forbid`: Forbidden Patterns
- `quote`: Evaluation Macros
- `undefine`: Redefined M4 Macros

Node:Autotest Macro Index, Next:Program & Function Index, Previous:M4 Macro Index, Up:Indices

### Autotest Macro Index

This is an alphabetical list of the Autotest macros. To make the list easier to use, the macros are listed without their preceding `AT_`.

- `CHECK`: Writing testsuite.at
- `CLEANUP`: Writing testsuite.at
- `DATA`: Writing testsuite.at
- `INIT`: Writing testsuite.at
- `KEYWORDS`: Writing testsuite.at
- `SETUP`: Writing testsuite.at
- `TESTED`: Writing testsuite.at

Node:Program & Function Index, Next:Concept Index, Previous:Autotest Macro Index, Up:Indices

### Program and Function Index

This is an alphabetical list of the programs and functions which portability is discussed in this document.

- `!`: Limitations of Builtins
- `.`: Limitations of Builtins
- `/usr/xpg4/bin/sh` on Solaris: Shellology
- `:`: Limitations of Builtins
- `alloca`: Particular Functions
- `awk`: Limitations of Usual Tools
- `break`: Limitations of Builtins
- `case`: Limitations of Builtins
- `cat`: Limitations of Usual Tools
- `chown`: Particular Functions
- `closedir`: Particular Functions
- `cmp`: Limitations of Usual Tools
- `cp`: Limitations of Usual Tools
- `date`: Limitations of Usual Tools
- `diff`: Limitations of Usual Tools
- `dirname`: Limitations of Usual Tools
- `echo`: Limitations of Builtins
- `egrep`: Limitations of Usual Tools
- `error_at_line`: Particular Functions
- `exit`: Limitations of Builtins
- `export`: Limitations of Builtins
- `expr`: Limitations of Usual Tools
- `expr` (`|`): Limitations of Usual Tools
- `false`: Limitations of Builtins
- `fnmatch`: Particular Functions
- `for`: Limitations of Builtins
- `fork`: Particular Functions
- `fseeko`: Particular Functions
- `getgroups`: Particular Functions
- `getloadavg`: Particular Functions
- `getmntent`: Particular Functions
- `getpgid`: Particular Functions
- `getpgrp`: Particular Functions
- `grep`: Limitations of Usual Tools
- `if`: Limitations of Builtins
- `ln`: Limitations of Usual Tools
- `lstat`: Particular Functions
- `malloc`: Particular Functions
- `memcmp`: Particular Functions
- `mktime`: Particular Functions
- `mmap`: Particular Functions
- `mv`: Limitations of Usual Tools
- `sed`: Limitations of Usual Tools
- `sed` (`t`): Limitations of Usual Tools
- `select`: Particular Functions
- `set`: Limitations of Builtins
- `setpgrp`: Particular Functions
- `setvbuf`: Particular Functions
- `shift`: Limitations of Builtins
- `snprintf`: Function Portability
- `source`: Limitations of Builtins
- `sprintf`: Function Portability
- `sscanf`: Function Portability
- `stat`: Particular Functions
- `strcoll`: Particular Functions
- `strerror_r`: Particular Functions
- `strftime`: Particular Functions
- `strnlen`: Particular Functions, Function Portability
- `strtod`: Particular Functions
- `test`: Limitations of Builtins
- `touch`: Limitations of Usual Tools
- `trap`: Limitations of Builtins
- `true`: Limitations of Builtins
- `unlink`: Function Portability
- `unset`: Limitations of Builtins
- `utime`: Particular Functions
- `va_copy`: Function Portability
- `va_list`: Function Portability
- `vfork`: Particular Functions
- `vprintf`: Particular Functions
- `vsnprintf`: Function Portability
- `vsprintf`: Function Portability

Node:Concept Index, Previous:Program & Function Index, Up:Indices

### Concept Index

This is an alphabetical list of the files, tools, and concepts introduced in this document.

- `"$@"`: Shell Substitutions
- $(commands): Shell Substitutions
- ${var:-value}: Shell Substitutions
- ${var=expanded-value}: Shell Substitutions
- ${var=literal}: Shell Substitutions
- `@%:@`: Quadrigraphs
- `@&t@`: Quadrigraphs
- `@:>@`: Quadrigraphs
- `@<:@`: Quadrigraphs
- `@S|@`: Quadrigraphs
- `commands`: Shell Substitutions
- `acconfig.h`: acconfig.h
- `aclocal.m4`: Making configure Scripts
- Ash: Shellology
- `autoconf`: autoconf Invocation
- `autoheader`: autoheader Invocation
- Autom4te Library: Invoking autom4te
- `autom4te.cache`: Invoking autom4te
- `autom4te.cfg`: Invoking autom4te
- Automake: Automake
- `autoreconf`: autoreconf Invocation
- `autoscan`: autoscan Invocation
- Autotest: Using Autotest
- `AUTOTEST_PATH`: testsuite Invocation
- `autoupdate`: autoupdate Invocation
- Back trace: Invoking autom4te, autoconf Invocation
- Bash: Shellology
- Bash 2.05 and later: Shellology
- Cache: Caching Results
- Cache variable: Cache Variable Names
- Cache, enabling: configure Invocation
- Command Substitution: Shell Substitutions
- `config.h`: Configuration Headers
- `config.h.bot`: acconfig.h
- `config.h.in`: Header Templates
- `config.h.top`: acconfig.h
- `config.status`: config.status Invocation
- `config.sub`: Specifying Names
- Configuration Header: Configuration Headers
- Configuration Header Template: Header Templates
- `configure`: Running configure scripts, Making configure Scripts
- `configure.ac`: Making configure Scripts
- `configure.in`: Making configure Scripts
- Copyright Notice: Notices
- Declaration, checking: Declarations
- `dnl`: Coding Style, Macro Definitions
- Endianness: C Compiler
- FDL, GNU Free Documentation License: GNU Free Documentation License
- File, checking: Files
- Function, checking: Particular Functions
- Header, checking: Header Files
- `ifnames`: ifnames Invocation
- Includes, default: Default Includes
- Instantiation: Output
- Language: Language Choice
- Library, checking: Libraries
- Libtool: Libtool
- Links: Configuration Links
- M4sugar: Programming in M4sugar
- Macro invocation stack: Invoking autom4te, autoconf Invocation
- Messages, from `autoconf`: Reporting Messages
- Messages, from `configure`: Printing Messages
- Moving open files: Limitations of Usual Tools
- obstack: Particular Functions
- `package.m4`: Making testsuite Scripts
- POSIX termios headers: System Services
- Previous Variable: Setting Output Variables
- Programs, checking: Alternative Programs
- QNX 4.25: Systemology
- quadrigraphs: Quadrigraphs
- quotation: M4 Quotation, Autoconf Language
- Revision: Notices
- Structure, checking: Structures
- Symbolic links: Limitations of Usual Tools
- termios POSIX headers: System Services
- test group: testsuite Scripts
- `testsuite`: testsuite Invocation, testsuite Scripts
- `undefined macro: _m4_divert_diversion`: New Macros
- Variable, Precious: Setting Output Variables
- Version: Notices
- `VPATH`: Limitations of Make
- Zsh: Shellology

## Table of Contents

- Autoconf
- Introduction
- The GNU build system
  - Automake
  - Libtool
  - Pointers
- Making `configure` Scripts
  - Writing `configure.ac`
    - A Shell Script Compiler
    - The Autoconf Language
    - Standard `configure.ac` Layout
  - Using `autoscan` to Create `configure.ac`
  - Using `ifnames` to List Conditionals
  - Using `autoconf` to Create `configure`
  - Using `autoreconf` to Update `configure` Scripts
- Initialization and Output Files
  - Initializing `configure`
  - Notices in `configure`
  - Finding `configure` Input
  - Outputting Files
  - Taking Configuration Actions
  - Creating Configuration Files
  - Substitutions in Makefiles
    - Preset Output Variables
    - Installation Directory Variables
    - Build Directories
    - Automatic Remaking
  - Configuration Header Files
    - Configuration Header Templates
    - Using `autoheader` to Create `config.h.in`
    - Autoheader Macros
  - Running Arbitrary Configuration Commands
  - Creating Configuration Links
  - Configuring Other Packages in Subdirectories
  - Default Prefix
- Existing Tests
  - Common Behavior
    - Standard Symbols
    - Default Includes
  - Alternative Programs
    - Particular Program Checks
    - Generic Program and File Checks
  - Files
  - Library Files
  - Library Functions
    - Portability of C Functions
    - Particular Function Checks
    - Generic Function Checks
  - Header Files
    - Particular Header Checks
    - Generic Header Checks
  - Declarations
    - Particular Declaration Checks
    - Generic Declaration Checks
  - Structures
    - Particular Structure Checks
    - Generic Structure Checks
  - Types
    - Particular Type Checks
    - Generic Type Checks
  - Compilers and Preprocessors
    - Specific Compiler Characteristics
    - Generic Compiler Characteristics
    - C Compiler Characteristics
    - C++ Compiler Characteristics
    - Fortran 77 Compiler Characteristics
  - System Services
  - UNIX Variants
- Writing Tests
  - Examining Declarations
  - Examining Syntax
  - Examining Libraries
  - Checking Run Time Behavior
    - Running Test Programs
    - Guidelines for Test Programs
    - Test Functions
  - Systemology
  - Multiple Cases
  - Language Choice
- Results of Tests
  - Defining C Preprocessor Symbols
  - Setting Output Variables
  - Caching Results
    - Cache Variable Names
    - Cache Files
    - Cache Checkpointing
  - Printing Messages
- Programming in M4
  - M4 Quotation
    - Active Characters
    - One Macro Call
    - Quotation and Nested Macros
    - `changequote` is Evil
    - Quadrigraphs
    - Quotation Rule Of Thumb
  - Invoking `autom4te`
  - Programming in M4sugar
    - Redefined M4 Macros
    - Evaluation Macros
    - Forbidden Patterns
  - Programming in M4sh
- Writing Autoconf Macros
  - Macro Definitions
  - Macro Names
  - Reporting Messages
  - Dependencies Between Macros
    - Prerequisite Macros
    - Suggested Ordering
  - Obsoleting Macros
  - Coding Style
- Portable Shell Programming
  - Shellology
  - Here-Documents
  - File Descriptors
  - File System Conventions
  - Shell Substitutions
  - Assignments
  - Special Shell Variables
  - Limitations of Shell Builtins
  - Limitations of Usual Tools
  - Limitations of Make
- Manual Configuration
  - Specifying the System Type
  - Getting the Canonical System Type
  - Using the System Type
- Site Configuration
  - Working With External Software
  - Choosing Package Options
  - Making Your Help Strings Look Pretty
  - Configuring Site Details
  - Transforming Program Names When Installing
    - Transformation Options
    - Transformation Examples
    - Transformation Rules
  - Setting Site Defaults
- Running `configure` Scripts
  - Basic Installation
  - Compilers and Options
  - Compiling For Multiple Architectures
  - Installation Names
  - Optional Features
  - Specifying the System Type
  - Sharing Defaults
  - Defining Variables
  - `configure` Invocation
- Recreating a Configuration
- Obsolete Constructs
  - Obsolete `config.status` Invocation
  - `acconfig.h`
  - Using `autoupdate` to Modernize `configure.ac`
  - Obsolete Macros
  - Upgrading From Version 1
    - Changed File Names
    - Changed Makefiles
    - Changed Macros
    - Changed Results
    - Changed Macro Writing
  - Upgrading From Version 2.13
    - Changed Quotation
    - New Macros
    - Hosts and Cross-Compilation
    - `AC_LIBOBJ` vs. `LIBOBJS`
- Generating Test Suites with Autotest
  - Using an Autotest Test Suite
    - `testsuite` Scripts
    - Autotest Logs
  - Writing `testsuite.at`
  - Running `testsuite` Scripts
  - Making `testsuite` Scripts
- Questions About Autoconf
  - Distributing `configure` Scripts
  - Why Require GNU M4?
  - How Can I Bootstrap?
  - Why Not Imake?
- History of Autoconf
  - Genesis
  - Exodus
  - Leviticus
  - Numbers
  - Deuteronomy
- Copying This Manual
  - GNU Free Documentation License
    - ADDENDUM: How to use this License for your documents
- Indices
  - Environment Variable Index
  - Output Variable Index
  - Preprocessor Symbol Index
  - Autoconf Macro Index
  - M4 Macro Index
  - Autotest Macro Index
  - Program and Function Index
  - Concept Index

#### Footnotes

1. GNU Autoconf, Automake and Libtool, by G. V. Vaughan, B. Elliston, T. Tromey, and I. L. Taylor. New Riders, 2000, ISBN 1578701902.

2. Using `defn`.

3. Yet another great name for Lars J. Aas.

4. Yet another reason why assigning `LIBOBJS` directly is discouraged.

5. When a failure occurs, the test suite is rerun, verbosely, and the user is asked to "play" with this failure to provide better information. It is important to keep the same environment between the first run, and bug-tracking runs.