$< in Ordinary Make Rules
make macro=value and Submakes
SHELL
make -k
VPATH and Make
This manual (21 September 2010) is for GNU Autoconf (version 2.68), a package for creating scripts to configure source code packages using templates and an M4 macro package.
Copyright © 1992, 1993, 1994, 1995, 1996, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, with the Front-Cover texts being “A GNU Manual,” and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled “GNU Free Documentation License.”(a) The FSF's Back-Cover Text is: “You have the freedom to copy and modify this GNU manual. Buying copies from the FSF supports it in developing GNU and promoting software freedom.”
--- The Detailed Node Listing ---
The GNU Build System
Making configure Scripts
Writing configure.ac
Initialization and Output Files
Substitutions in Makefiles
Configuration Header Files
Existing Tests
Common Behavior
Alternative Programs
Library Functions
Header Files
Declarations
Structures
Types
Compilers and Preprocessors
Writing Tests
Writing Test Programs
Results of Tests
Caching Results
Programming in M4
Programming in M4sh
M4 Quotation
Using autom4te
Programming in M4sugar
Writing Autoconf Macros
Dependencies Between Macros
Portable Shell Programming
Portable Make Programming
VPATH and Make
Portable C and C++ Programming
Manual Configuration
Site Configuration
Transforming Program Names When Installing
Running configure Scripts
Obsolete Constructs
Upgrading From Version 1
Upgrading From Version 2.13
Generating Test Suites with Autotest
Using an Autotest Test Suite
Frequent Autoconf Questions, with answers
History of Autoconf
Indices
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 Posix-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 Posix variants. There is no need to maintain files that list the features supported by each release of each variant of Posix.
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.
Those who do not understand Autoconf are condemned to reinvent it, poorly. The primary goal of Autoconf is making the user's life easier; making the maintainer's life easier is only a secondary goal. Put another way, the primary goal is not to make the generation of configure automatic for package maintainers (although patches along that front are welcome, since package maintainers form the user base of Autoconf); rather, the goal is to make configure painless, portable, and predictable for the end user of each autoconfiscated package. And to this degree, Autoconf is highly successful at its goal — most complaints to the Autoconf list are about difficulties in writing Autoconf input, and not in the behavior of the resulting configure. Even packages that don't use Autoconf will generally provide a configure script, and the most common complaint about these alternative home-grown scripts is that they fail to meet one or more of the GNU Coding Standars (see Configuration) that users have come to expect from Autoconf-generated configure scripts.
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 version 1.4.6 or later in order to generate the scripts. It uses features that some versions of M4, including GNU M4 1.3, do not have. Autoconf works better with GNU M4 version 1.4.14 or later, though this is not required.
See Autoconf 1, for information about upgrading from version 1. See History, for the story of Autoconf's development. See FAQ, 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. Past suggestions are archived.
Mail bug reports to the Autoconf Bugs mailing list. Past bug reports are archived.
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 git; see the Autoconf Summary for details, or view the actual repository. Anonymous CVS access is also available, see README for more details. Patches relative to the current git version can be sent for review to the Autoconf Patches mailing list, with discussion on prior patches archived; and all commits are posted in the read-only Autoconf Commit mailing list, which is also archived.
Because of its mission, the Autoconf package itself 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.
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.
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 its numerous limitations. Its lack of
support for automatic dependency tracking, recursive builds in
subdirectories, reliable timestamps (e.g., for network file systems), 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
builds the hello program, and make install installs it
in /usr/local/bin (or whatever prefix was given to
configure, if not /usr/local).
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...
GNU software has a well-deserved reputation for running on many different types of systems. While our primary goal is to write software for the GNU system, many users and developers have been introduced to us through the systems that they were already using.
Gnulib is a central location for common GNU code, intended to be shared among free software packages. Its components are typically shared at the source level, rather than being a library that gets built, installed, and linked against. The idea is to copy files from Gnulib into your own source tree. There is no distribution tarball; developers should just grab source modules from the repository. The source files are available online, under various licenses, mostly GNU GPL or GNU LGPL.
Gnulib modules typically contain C source code along with Autoconf
macros used to configure the source code. For example, the Gnulib
stdbool module implements a stdbool.h header that nearly
conforms to C99, even on old-fashioned hosts that lack stdbool.h.
This module contains a source file for the replacement header, along
with an Autoconf macro that arranges to use the replacement header on
old-fashioned systems.
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 Make rules with the variable suffixes of shared libraries, linking reliably with 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 without Automake, 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.
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 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 is a piece of cake.)
There are a number of places that you can go to for more information on the GNU build tools.
The project home pages for Autoconf, Automake, Gnulib, and Libtool.
See Automake, for more information on Automake.
The book GNU Autoconf, Automake and Libtool1 describes the complete GNU build environment. You can also find the entire book on-line.
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:
#define directives (see Configuration Headers);
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 can 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, when using just Autoconf:
your source files --> [autoscan*] --> [configure.scan] --> configure.ac
configure.ac --.
| .------> autoconf* -----> configure
[aclocal.m4] --+---+
| `-----> [autoheader*] --> [config.h.in]
[acsite.m4] ---'
Makefile.in
Additionally, if you use Automake, the following additional productions come into play:
[acinclude.m4] --.
|
[local macros] --+--> aclocal* --> aclocal.m4
|
configure.ac ----'
configure.ac --.
+--> automake* --> Makefile.in
Makefile.am ---'
Files used in configuring a software package:
.-------------> [config.cache]
configure* ------------+-------------> config.log
|
[config.h.in] -. v .-> [config.h] -.
+--> config.status* -+ +--> make*
Makefile.in ---' `-> Makefile ---'
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 Programming. 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 process 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.
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, even in 2008, where shells without any function support are far and few between, there are pitfalls to avoid when making use of them. Also, finding a Bourne shell that accepts shell functions is not trivial, even though there is almost always one on interesting porting targets.
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 attractive: all
sorts of optimizations could easily be implemented in the compiler and
many rigorous checks could be performed on the Autoconf program
(e.g., rejecting any non-portable construct). Alternatively, you can
extend an existing language, such as the sh (Bourne shell)
language.
Autoconf does the latter: it is a layer on top of sh. It was
therefore most convenient to implement autoconf as a macro
expander: a program that repeatedly performs macro expansions on
text input, replacing macro calls with macro bodies and producing a pure
sh script in the end. Instead of implementing a dedicated
Autoconf macro expander, it is natural to use an existing
general-purpose macro language, such as M4, and implement the extensions
as a set of M4 macros.
The Autoconf language differs from many other computer languages because it treats actual code the same as plain text. Whereas in C, for instance, data and instructions have 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 white space between the macro name and the open parenthesis.
AC_INIT ([oops], [1.0]) # incorrect
AC_INIT([hello], [1.0]) # good
Arguments should be enclosed within the quote characters ‘[’ and ‘]’, and be separated by commas. Any leading blanks or newlines in arguments are ignored, unless they are quoted. You should always quote an argument that might contain a macro name, comma, parenthesis, or a leading blank or newline. This rule applies recursively for every macro call, including macros called from other macros. For more details on quoting rules, see Programming in M4.
For instance:
AC_CHECK_HEADER([stdio.h],
[AC_DEFINE([HAVE_STDIO_H], [1],
[Define to 1 if you 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], 1,
[Define to 1 if you have <stdio.h>.])],
[AC_MSG_ERROR([sorry, can't do anything for you])])
because ‘1’ cannot contain a macro call. Here, the argument of
AC_MSG_ERROR must be quoted; otherwise, its comma would be
interpreted as an argument separator. Also, the second and third arguments
of ‘AC_CHECK_HEADER’ must be quoted, since they contain
macro calls. The three arguments ‘HAVE_STDIO_H’, ‘stdio.h’,
and ‘Define to 1 if you have <stdio.h>.’ do not need quoting, but
if you unwisely defined a macro with a name like ‘Define’ or
‘stdio’ then they would need quoting. Cautious Autoconf users
would keep the quotes, but many Autoconf users find such precautions
annoying, and would rewrite the example as follows:
AC_CHECK_HEADER(stdio.h,
[AC_DEFINE(HAVE_STDIO_H, 1,
[Define to 1 if you have <stdio.h>.])],
[AC_MSG_ERROR([sorry, can't do anything for you])])
This is safe, so long as you adopt good naming conventions and do not define macros with names like ‘HAVE_STDIO_H’, ‘stdio’, or ‘h’. Though it is also safe here to omit the quotes around ‘Define to 1 if you have <stdio.h>.’ this is not recommended, as message strings are more likely to inadvertently contain commas.
The following example is wrong and dangerous, as it is underquoted:
AC_CHECK_HEADER(stdio.h,
AC_DEFINE(HAVE_STDIO_H, 1,
Define to 1 if you 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. For example, these two approaches in configure.ac
(quoting just the potential problems, or quoting the entire line) will
protect your script in case autoconf ever adds a macro AC_DC:
echo "Hard rock was here! --[AC_DC]"
[echo "Hard rock was here! --AC_DC"]
which results in this text in configure:
echo "Hard rock was here! --AC_DC"
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, either around just the problematic portions, or over the entire argument:
AC_MSG_WARN([[AC_DC] stinks --Iron Maiden])
AC_MSG_WARN([[AC_DC stinks --Iron Maiden]])
However, the above example triggers a warning about a possibly unexpanded macro when running autoconf, because it collides with the namespace of macros reserved for the Autoconf language. To be really safe, you can use additional escaping (either a quadrigraph, or creative shell constructs) to silence that particular warning:
echo "Hard rock was here! --AC""_DC"
AC_MSG_WARN([[AC@&t@_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 is 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!
On the other hand, descriptions (e.g., the last parameter of
AC_DEFINE or AS_HELP_STRING) are not literals—they
are subject to line breaking, for example—and should not be double quoted.
Even if these descriptions are short and are not actually broken, double
quoting them yields weird results.
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.
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
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 probably needs 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 contains 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, one or more blanks, and the Autoconf macro to output if that symbol is encountered. Lines starting with ‘#’ are comments.
autoscan accepts the following options:
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:
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:
AC_DIAGNOSE, for a comprehensive list of categories. Special
values include:
Warnings about ‘syntax’ are enabled by default, and the environment variable WARNINGS, a comma separated list of categories, is honored as well. Passing -W category actually behaves as if you had passed --warnings syntax,$WARNINGS,category. To disable the defaults and WARNINGS, and then enable warnings about obsolete constructs, use -W none,obsolete.
Because autoconf uses autom4te behind the scenes, it
displays a back trace for errors, but not for warnings; if you want
them, just pass -W error. See autom4te Invocation, for some
examples.
The format is a regular string, with newlines if desired, and
several special escape codes. It defaults to ‘$f:$l:$n:$%’; see
autom4te Invocation, for details on the format.
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. 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 their 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
Installing the various components of the GNU Build System can be tedious: running autopoint for Gettext, automake for Makefile.in 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.
autoreconf runs autoconf, autoheader, aclocal, automake, libtoolize, and autopoint (when appropriate) repeatedly to update the GNU Build System in the specified directories and their subdirectories (see Subdirectories). By default, it only remakes those files that are older than their sources. The environment variables AUTOM4TE, AUTOCONF, AUTOHEADER, AUTOMAKE, ACLOCAL, AUTOPOINT, LIBTOOLIZE, M4, and MAKE may be used to override the invocation of the respective tools.
If you install a new version of some tool, you can make autoreconf remake all of the files by giving it the --force option.
See Automatic Remaking, for Make rules to automatically rebuild 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.
Gettext supplies the autopoint command to add translation
infrastructure to a source package. If you use autopoint,
your configure.ac should invoke both AM_GNU_GETTEXT and
AM_GNU_GETTEXT_VERSION(gettext-version). See Invoking the autopoint Program, for further details.
autoreconf accepts the following options:
If deemed appropriate, this option triggers calls to
‘automake --add-missing’,
‘libtoolize’, ‘autopoint’, etc.
AC_CONFIG_SUBDIRS).
Warnings about ‘syntax’ are enabled by default, and the environment variable WARNINGS, a comma separated list of categories, is honored as well. Passing -W category actually behaves as if you had passed --warnings syntax,$WARNINGS,category. To disable the defaults and WARNINGS, and then enable warnings about obsolete constructs, use -W none,obsolete.
If you want autoreconf to pass flags that are not listed here
on to aclocal, set ACLOCAL_AMFLAGS in your Makefile.am.
Due to a limitation in the Autoconf implementation these flags currently
must be set on a single line in Makefile.am, without any
backslash-newlines.
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 the initialization and the creation of output files.
Every configure script must call AC_INIT before doing
anything else that produces output. Calls to silent macros, such as
AC_DEFUN, may also occur prior to AC_INIT, although these
are generally used via aclocal.m4, since that is implicitly
included before the start of configure.ac. The only other
required macro is AC_OUTPUT (see Output).
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 former is meant for distribution tar ball names (e.g., ‘autoconf’). It defaults to package with ‘GNU ’ stripped, lower-cased, and all characters other than alphanumerics and underscores are changed to ‘-’. If provided, url should be the home page for the package.
The arguments of
AC_INITmust be static, i.e., there should not be any shell computation, quotes, or newlines, but they can be computed by M4. This is because the package information strings are expanded at M4 time into several contexts, and must give the same text at shell time whether used in single-quoted strings, double-quoted strings, quoted here-documents, or unquoted here-documents. It is permissible to usem4_esyscmdorm4_esyscmd_sfor computing a version string that changes with every commit to a version control system (in fact, Autoconf does just that, for all builds of the development tree made between releases).The following M4 macros (e.g.,
AC_PACKAGE_NAME), output variables (e.g.,PACKAGE_NAME), and preprocessor symbols (e.g.,PACKAGE_NAME), are defined byAC_INIT:
AC_PACKAGE_NAME,PACKAGE_NAME- Exactly package.
AC_PACKAGE_TARNAME,PACKAGE_TARNAME- Exactly tarname, possibly generated from package.
AC_PACKAGE_VERSION,PACKAGE_VERSION- Exactly version.
AC_PACKAGE_STRING,PACKAGE_STRING- Exactly ‘package version’.
AC_PACKAGE_BUGREPORT,PACKAGE_BUGREPORT- Exactly bug-report, if one was provided. Typically an email address, or URL to a bug management web page.
AC_PACKAGE_URL,PACKAGE_URL- Exactly url, if one was provided. If url was empty, but package begins with ‘GNU ’, then this defaults to ‘http://www.gnu.org/software/tarname/’, otherwise, no URL is assumed.
If your configure script does its own option processing, it
should inspect ‘$@’ or ‘$*’ immediately after calling
AC_INIT, because other Autoconf macros liberally use the
set command to process strings, and this has the side effect
of updating ‘$@’ and ‘$*’. However, we suggest that you use
standard macros like AC_ARG_ENABLE instead of attempting to
implement your own option processing. See Site Configuration.
The following optional macros can be used to help choose the minimum version of Autoconf that can successfully compile a given configure.ac.
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 exit with failure (exit status is 63). For example:
AC_PREREQ([2.68])This macro may be used before
AC_INIT.
This macro was introduced in Autoconf 2.62. It identifies the version of Autoconf that is currently parsing the input file, in a format suitable for
m4_version_compare(see m4_version_compare); in other words, for this release of Autoconf, its value is ‘2.68’. One potential use of this macro is for writing conditional fallbacks based on when a feature was added to Autoconf, rather than usingAC_PREREQto require the newer version of Autoconf. However, remember that the Autoconf philosophy favors feature checks over version checks.You should not expand this macro directly; use ‘m4_defn([AC_AUTOCONF_VERSION])’ instead. This is because some users might have a beta version of Autoconf installed, with arbitrary letters included in its version string. This means it is possible for the version string to contain the name of a defined macro, such that expanding
AC_AUTOCONF_VERSIONwould trigger the expansion of that macro during rescanning, and change the version string to be different than what you intended to check.
The following macros manage version numbers for configure scripts. Using them is optional.
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 shows up in both the head of configure and in ‘configure --version’.
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
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.
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_INSTALLdoes 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.The auxiliary directory is commonly named build-aux. If you need portability to DOS variants, do not name the auxiliary directory aux. See File System Conventions.
Declares that file is expected in the directory defined above. In Autoconf proper, this macro does nothing: its sole purpose is to be traced by third-party tools to produce a list of expected auxiliary files. For instance it is called by macros like
AC_PROG_INSTALL(see Particular Programs) orAC_CANONICAL_BUILD(see Canonicalizing) to register the auxiliary files they need.
Similarly, packages that use aclocal should declare where
local macros can be found using AC_CONFIG_MACRO_DIR.
Specify dir as the location of additional local Autoconf macros. This macro is intended for use by future versions of commands like autoreconf that trace macro calls. It should be called directly from configure.ac so that tools that install macros for aclocal can find the macros' declarations.
Note that if you use aclocal from Automake to generate aclocal.m4, you must also set
ACLOCAL_AMFLAGS = -Idir in your top-level Makefile.am. Due to a limitation in the Autoconf implementation of autoreconf, these include directives currently must be set on a single line in Makefile.am, without any backslash-newlines.
Every Autoconf script, e.g., configure.ac, should finish by
calling AC_OUTPUT. That is the macro that generates and runs
config.status, which in turn creates the makefiles and any
other files resulting from configuration. This is the only required
macro besides AC_INIT (see Input).
Generate config.status and launch it. Call this macro once, at the end of configure.ac.
config.status performs all the configuration actions: all the output files (see Configuration Files, macro
AC_CONFIG_FILES), header files (see Configuration Headers, macroAC_CONFIG_HEADERS), commands (see Configuration Commands, macroAC_CONFIG_COMMANDS), links (see Configuration Links, macroAC_CONFIG_LINKS), subdirectories to configure (see Subdirectories, macroAC_CONFIG_SUBDIRS) are honored.The location of your
AC_OUTPUTinvocation is the exact point where configuration actions are taken: any code afterwards is executed by configure once config.status was run. If you want to bind actions to config.status itself (independently of whether configure is being run), see Running Arbitrary Configuration Commands.
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 in 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.
If the Make command,
$MAKEif set or else ‘make’, predefines$(MAKE), define output variableSET_MAKEto be empty. Otherwise, defineSET_MAKEto a macro definition that sets$(MAKE), such as ‘MAKE=make’. CallsAC_SUBSTforSET_MAKE.
If you use this macro, place a line like this in each Makefile.in that runs MAKE on other directories:
@SET_MAKE@
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_ITEMS(tag..., [commands], [init-cmds])
where the arguments are:
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_ITEMS([$my_foos])
and use this instead:
... && AC_CONFIG_ITEMS([fooo])
... && AC_CONFIG_ITEMS([foooo])
The macros AC_CONFIG_FILES and AC_CONFIG_HEADERS use
special tag values: they may have the form ‘output’ or
‘output:inputs’. The file output is instantiated
from its templates, inputs (defaulting to ‘output.in’).
‘AC_CONFIG_FILES([Makefile:boiler/top.mk:boiler/bot.mk)]’, for example, asks for the creation of the file Makefile that contains 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 file names. In the latter
case they are first looked for in the build tree, and then in the source
tree. Input files should be text files, and a line length below 2000
bytes should be safe.
The variables 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:
srcdirac_top_srcdirac_top_build_prefixac_srcdirtmpThe 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_build_prefix='../../'
# Concatenation of $ac_top_build_prefix and srcdir
ac_top_srcdir='../../../package'
# Concatenation of $ac_top_srcdir and deep/dir
ac_srcdir='../../../package/deep/dir'
independently of ‘in/in.in’.
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 tag values, of course!
Be sure to read the previous section, Configuration Actions.
Make
AC_OUTPUTcreate each file by copying an input file (by default file.in), substituting the output variable values. This macro is one of the instantiating macros; see Configuration Actions. See Makefile Substitutions, for more information on using output variables. See Setting Output Variables, for more information on creating them. This macro creates the directory that the file is in if it doesn't exist. Usually, makefiles are created this way, but other files, such as .gdbinit, can be specified as well.Typical calls to
AC_CONFIG_FILESlook 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 DOS variants, or to prepend and/or append boilerplate to the file.
Each subdirectory in a distribution that contains something to be
compiled or installed should come with a file Makefile.in, from
which configure creates a file Makefile in that directory.
To create 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 makefiles.
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, many of which are
precious variables (see Setting Output Variables,
AC_ARG_VAR).
The preset variables which are available during config.status (see Configuration Actions) may also be used during configure tests. For example, it is permissible to reference ‘$srcdir’ when constructing a list of directories to pass via option -I during a compiler feature check. When used in this manner, coupled with the fact that configure is always run from the top build directory, it is sufficient to use just ‘$srcdir’ instead of ‘$top_srcdir’.
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 or linking programs to test for C features.If a compiler option affects only the behavior of the preprocessor (e.g., -Dname), it should be put into
CPPFLAGSinstead. If it affects only the linker (e.g., -Ldirectory), it should be put intoLDFLAGSinstead. If it affects only the compiler proper,CFLAGSis the natural home for it. If an option affects multiple phases of the compiler, though, matters get tricky. One approach to put such options directly intoCC, e.g.,CC='gcc -m64'. Another is to put them into bothCPPFLAGSandLDFLAGS, but not intoCFLAGS.However, remember that some Makefile variables are reserved by the GNU Coding Standards for the use of the “user”—the person building the package. For instance,
CFLAGSis one such variable.Sometimes package developers are tempted to set user variables such as
CFLAGSbecause it appears to make their job easier. However, the package itself should never set a user variable, particularly not to include switches that are required for proper compilation of the package. Since these variables are documented as being for the package builder, that person rightfully expects to be able to override any of these variables at build time. If the package developer needs to add switches without interfering with the user, the proper way to do that is to introduce an additional variable. Automake makes this easy by introducingAM_CFLAGS(see Flag Variables Ordering), but the concept is the same even if Automake is not used.
A comment saying that the file was generated automatically by configure and giving the name of the input file.
AC_OUTPUTadds 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.
Preprocessor options for the C, C++, Objective C, and Objective 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 preprocessing or compiling programs to test for C, C++, Objective C, and Objective C++ features.
This variable's contents should contain options like -I, -D, and -U that affect only the behavior of the preprocessor. Please see the explanation of
CFLAGSfor what you can do if an option affects other phases of the compiler as well.Currently, configure always links as part of a single invocation of the compiler that also preprocesses and compiles, so it uses this variable also when linking programs. However, it is unwise to depend on this behavior because the GNU Coding Standards do not require it and many packages do not use
CPPFLAGSwhen linking programs.See Special Chars in Variables, for limitations that
CPPFLAGSmight run into.
Debugging and optimization options for the C++ compiler. It acts like
CFLAGS, but for C++ instead of C.
-D options to pass to the C compiler. If
AC_CONFIG_HEADERSis 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.
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_Tis set to tab. You might not want to use it.
Debugging and optimization options for the Erlang compiler. If it is not set in the environment when configure runs, the default value is empty. configure uses this variable when compiling programs to test for Erlang features.
Debugging and optimization options for the Fortran compiler. If it is not set in the environment when configure runs, the default value is set when you call
AC_PROG_FC(or empty if you don't). configure uses this variable when compiling or linking programs to test for Fortran features.
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 or linking programs to test for Fortran 77 features.
Options for the linker. If it is not set in the environment when configure runs, the default value is empty. configure uses this variable when linking programs to test for C, C++, Objective C, Objective C++, and Fortran features.
This variable's contents should contain options like -s and -L that affect only the behavior of the linker. Please see the explanation of
CFLAGSfor what you can do if an option also affects other phases of the compiler.Don't use this variable to pass library names (-l) to the linker; use
LIBSinstead.
-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++, Objective C, Objective C++, and Fortran features.
Debugging and optimization options for the Objective C compiler. It acts like
CFLAGS, but for Objective C instead of C.
Debugging and optimization options for the Objective C++ compiler. It acts like
CXXFLAGS, but for Objective C++ instead of C++.
The relative name of the top level of the current build tree. In the top-level directory, this is the same as
builddir.
The relative name of the top level of the current build tree with final slash if nonemtpy. This is the same as
top_builddir, except that it contains zero or more runs of../, so it should not be appended with a slash for concatenation. This helps for make implementations that otherwise do not treat ./file and file as equal in the toplevel build directory.
The name of the top-level source code directory for the package. In the top-level directory, this is the same as
srcdir.
The following variables specify the directories for package installation, see Variables for Installation Directories, for more information. Each variable corresponds to an argument of configure; trailing slashes are stripped so that expressions such as ‘${prefix}/lib’ expand with only one slash between directory names. See the end of this section for details on when and how to use these variables.
The directory for installing idiosyncratic read-only architecture-independent data.
The root of the directory tree for read-only architecture-independent data files.
The installation prefix for architecture-dependent files. By default it's the same as
prefix. You should avoid installing anything directly toexec_prefix. However, the default value for directories containing architecture-dependent files should be relative toexec_prefix.
The directory for installing locale-dependent but architecture-independent data, such as message catalogs. This directory usually has a subdirectory per locale.
The common installation prefix for all files. If
exec_prefixis defined to a different value,prefixis used only for architecture-independent files.
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 ‘@datarootdir@’ is
replaced by ‘${prefix}/share’, not ‘/usr/local/share’, and
‘@datadir@’ is replaced by ‘${datarootdir}’.
This behavior is mandated by the GNU Coding Standards, so that when the user runs:
In order to support these features, it is essential that
datarootdir remains defined as ‘${prefix}/share’,
so that its value can be expanded based
on 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 hard-coding it in makefiles using
e.g., ‘AC_DEFINE_UNQUOTED([DATADIR], ["$datadir"], [Data directory.])’,
you should add
-DDATADIR='$(datadir)' to your makefile's definition of
CPPFLAGS (AM_CPPFLAGS if you are also using Automake).
Similarly, you should not rely on AC_CONFIG_FILES to replace
bindir and friends in your shell scripts and other files; instead,
let make manage their replacement. For instance Autoconf
ships templates of its shell scripts ending with ‘.in’, and uses a
makefile snippet similar to the following to build scripts like
autoheader and autom4te:
edit = sed \
-e 's|@bindir[@]|$(bindir)|g' \
-e 's|@pkgdatadir[@]|$(pkgdatadir)|g' \
-e 's|@prefix[@]|$(prefix)|g'
autoheader autom4te: Makefile
rm -f $@ $@.tmp
srcdir=''; \
test -f ./$@.in || srcdir=$(srcdir)/; \
$(edit) $${srcdir}$@.in >$@.tmp
chmod +x $@.tmp
chmod a-w $@.tmp
mv $@.tmp $@
autoheader: $(srcdir)/autoheader.in
autom4te: $(srcdir)/autom4te.in
Some details are noteworthy:
VPATH should
not contain shell metacharacters or white
space. See Special Chars in Variables.
edit uses values that depend on the configuration specific
values (prefix, etc.) and not only on VERSION and so forth,
the output depends on Makefile, not configure.ac.
autoconf autoheader: Makefile
.in:
rm -f $@ $@.tmp
$(edit) $< >$@.tmp
chmod +x $@.tmp
mv $@.tmp $@
See Single Suffix Rules, for details.
For the more specific installation of Erlang libraries, the following variables are defined:
The common parent directory of Erlang library installation directories. This variable is set by calling the
AC_ERLANG_SUBST_INSTALL_LIB_DIRmacro in configure.ac.
The installation directory for Erlang library library. This variable is set by using the ‘AC_ERLANG_SUBST_INSTALL_LIB_SUBDIR’ macro in configure.ac.
See Erlang Libraries, for details.
In Autoconf 2.60, the set of directory variables has changed, and the defaults of some variables have been adjusted (see Installation Directory Variables) to changes in the GNU Coding Standards. Notably, datadir, infodir, and mandir are now expressed in terms of datarootdir. If you are upgrading from an earlier Autoconf version, you may need to adjust your files to ensure that the directory variables are substituted correctly (see Defining Directories), and that a definition of datarootdir is in place. For example, in a Makefile.in, adding
datarootdir = @datarootdir@
is usually sufficient. If you use Automake to create Makefile.in, it will add this for you.
To help with the transition, Autoconf warns about files that seem to use
datarootdir without defining it. In some cases, it then expands
the value of $datarootdir in substitutions of the directory
variables. The following example shows such a warning:
$ cat configure.ac
AC_INIT
AC_CONFIG_FILES([Makefile])
AC_OUTPUT
$ cat Makefile.in
prefix = @prefix@
datadir = @datadir@
$ autoconf
$ configure
configure: creating ./config.status
config.status: creating Makefile
config.status: WARNING:
Makefile.in seems to ignore the --datarootdir setting
$ cat Makefile
prefix = /usr/local
datadir = ${prefix}/share
Usually one can easily change the file to accommodate both older and newer Autoconf releases:
$ cat Makefile.in
prefix = @prefix@
datarootdir = @datarootdir@
datadir = @datadir@
$ configure
configure: creating ./config.status
config.status: creating Makefile
$ cat Makefile
prefix = /usr/local
datarootdir = ${prefix}/share
datadir = ${datarootdir}
In some cases, however, the checks may not be able to detect that a suitable
initialization of datarootdir is in place, or they may fail to detect
that such an initialization is necessary in the output file. If, after
auditing your package, there are still spurious configure warnings about
datarootdir, you may add the line
AC_DEFUN([AC_DATAROOTDIR_CHECKED])
to your configure.ac to disable the warnings. This is an exception
to the usual rule that you should not define a macro whose name begins with
AC_ (see Macro Names).
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
can do this. Most other recent make programs can do this as
well, though they may have difficulties and it is often simpler to
recommend GNU make (see VPATH and Make). Older
make programs do not support VPATH; when using them, the
source code must be in the same directory as the object files.
If you are using GNU Automake, the remaining details in this
section are already covered for you, based on the contents of your
Makefile.am. But if you are using Autoconf in isolation, then
supporting VPATH requires the following in your
Makefile.in:
srcdir = @srcdir@
VPATH = @srcdir@
Do not set VPATH to the value of another variable (see Variables listed in VPATH.
configure substitutes 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 $< in explicit rules; they
expand it to an empty value.
Instead, Make command lines should always refer to source files by prefixing them with ‘$(srcdir)/’. For example:
time.info: time.texinfo
$(MAKEINFO) '$(srcdir)/time.texinfo'
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 are not changed if remaking them does not change their contents. This feature avoids unnecessary recompilation. You should include the file stamp-h.in in your package's distribution, so that make considers config.h.in up to date. Don't use touch (see Limitations of Usual Tools); instead, 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 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 ensures 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.
When a package contains more than a few tests that define 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. Though it can be called anywhere
between AC_INIT and AC_OUTPUT, it is customary to call
it 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).
To provide for VPATH builds, remember to pass the C compiler a -I. option (or -I..; whichever directory contains config.h). Even if you use ‘#include "config.h"’, the preprocessor searches only the directory of the currently read file, i.e., the source directory, not the build directory.
With the appropriate -I option, you can use ‘#include <config.h>’. Actually, it's a good habit to use it, because in the rare case when the source directory contains another config.h, the build directory should be searched first.
This macro is one of the instantiating macros; see Configuration Actions. Make
AC_OUTPUTcreate the file(s) in the blank-or-newline-separated list header containing C preprocessor#definestatements, and replace ‘@DEFS@’ in generated files with -DHAVE_CONFIG_H instead of the value ofDEFS. The usual name for header is config.h.If header already exists and its contents are identical to what
AC_OUTPUTwould put in it, it is left alone. Doing this allows making some changes in the 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. For example, you might need to make the input file name acceptable to DOS variants:
AC_CONFIG_HEADERS([config.h:config.hin])
This macro is defined as the name of the first declared config header and undefined if no config headers have been declared up to this point. A third-party macro may, for example, require use of a config header without invoking AC_CONFIG_HEADERS twice, like this:
AC_CONFIG_COMMANDS_PRE( [m4_ifndef([AH_HEADER], [AC_CONFIG_HEADERS([config.h])])])
See Configuration Actions, for more details on header.
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. The conf.h created by configure defines ‘HAVE_UNISTD_H’ to 1, if and only if the system has unistd.h.
/* Define as 1 if you have unistd.h. */
#undef HAVE_UNISTD_H
The format of the template file is stricter than what the C preprocessor is required to accept. A directive line should contain only whitespace, ‘#undef’, and ‘HAVE_UNISTD_H’. The use of ‘#define’ instead of ‘#undef’, or of comments on the same line as ‘#undef’, is strongly discouraged. Each hook should only be listed once. Other preprocessor lines, such as ‘#ifdef’ or ‘#include’, are copied verbatim from the template into the generated header.
Since it is a tedious task to keep a template header up to date, you may use autoheader to generate it, see autoheader Invocation.
During the instantiation of the header, each ‘#undef’ line in the template file for each symbol defined by ‘AC_DEFINE’ is changed to an appropriate ‘#define’. If the corresponding ‘AC_DEFINE’ has not been executed during the configure run, the ‘#undef’ line is commented out. (This is important, e.g., for ‘_POSIX_SOURCE’: on many systems, it can be implicitly defined by the compiler, and undefining it in the header would then break compilation of subsequent headers.)
Currently, all remaining ‘#undef’ lines in the header template are commented out, whether or not there was a corresponding ‘AC_DEFINE’ for the macro name; but this behavior is not guaranteed for future releases of Autoconf.
Generally speaking, since you should not use ‘#define’, and you cannot guarantee whether a ‘#undef’ directive in the header template will be converted to a ‘#define’ or commented out in the generated header file, the template file cannot be used for conditional definition effects. Consequently, if you need to use the construct
#ifdef THIS
# define THAT
#endif
you must place it outside of the template. If you absolutely need to hook it to the config header itself, please put the directives to a separate file, and ‘#include’ that file from the config header template. If you are using autoheader, you would probably use ‘AH_BOTTOM’ to append the ‘#include’ directive.
The autoheader program can create a template file of C
‘#define’ statements for configure to use.
It searches for the first invocation of AC_CONFIG_HEADERS in
configure sources to determine the name of the template.
(If the first call of AC_CONFIG_HEADERS specifies more than one
input file name, autoheader uses the first one.)
It is recommended that only one input file is used. If you want to append
a boilerplate code, it is preferable to use
‘AH_BOTTOM([#include <conf_post.h>])’.
File conf_post.h is not processed during the configuration then,
which make things clearer. Analogically, AH_TOP can be used to
prepend a boilerplate code.
In order to do its job, autoheader needs you to document all
of the symbols that you might use. Typically this is done via an
AC_DEFINE or AC_DEFINE_UNQUOTED call whose first argument
is a literal symbol and whose third argument describes the symbol
(see Defining Symbols). Alternatively, you can use
AH_TEMPLATE (see Autoheader Macros), or you can supply a
suitable input file for a subsequent configuration header file.
Symbols defined by Autoconf's builtin tests are already documented properly;
you need to document only 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 defined (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: the invocation of autoheader...
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:
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 template for a symbol is created
by autoheader from
the description argument to an AC_DEFINE;
see Defining Symbols.
For special needs, you can use the following macros.
Tell autoheader to generate a template for key. This macro generates standard templates just like
AC_DEFINEwhen 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.])generates 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
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 a symbol that can be defined via
AC_DEFINE.
Please note that text gets included “verbatim” to the template file, not to the resulting config header, so it can easily get mangled when the template is processed. There is rarely a need for something other than
AH_BOTTOM([#include <custom.h>])
You can execute arbitrary commands 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.
Specify additional shell commands to run at the end of config.status, and shell commands to initialize any variables from configure. Associate the commands with tag. Since typically the cmds create a file, tag should naturally be the name of that file. If needed, the directory hosting tag is created. 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([timestamp], [date >timestamp])
The following two macros look similar, but in fact they are not of the same breed: they are executed directly by configure, so you cannot use config.status to rerun them.
Execute the cmds right before creating config.status.
This macro presents the last opportunity to call
AC_SUBST,AC_DEFINE, orAC_CONFIG_ITEMS macros.
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 a directory different from the source directory.
Make
AC_OUTPUTlink each of the existing files source to the corresponding link name dest. Makes a symbolic link if possible, otherwise a hard link if possible, otherwise a copy. 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.hto create the links.
In most situations, calling AC_OUTPUT is sufficient to produce
makefiles in subdirectories. However, configure scripts
that control more than one independent package can use
AC_CONFIG_SUBDIRS to run configure scripts for other
packages in subdirectories.
Make
AC_OUTPUTrun configure in each subdirectory dir in the given blank-or-newline-separated list. Each dir should be a literal, i.e., please do not use:if test "x$package_foo_enabled" = xyes; then my_subdirs="$my_subdirs foo" fi AC_CONFIG_SUBDIRS([$my_subdirs])because this prevents ‘./configure --help=recursive’ from displaying the options of the package
foo. Instead, you should write:if test "x$package_foo_enabled" = xyes; then AC_CONFIG_SUBDIRS([foo]) fiIf a given dir is not found at configure run time, a warning is reported; if the subdirectory is optional, write:
if test -d "$srcdir/foo"; then AC_CONFIG_SUBDIRS([foo]) fiIf 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 file systems. Likewise, if a dir contains configure.in but no configure, the Cygnus configure script found by
AC_CONFIG_AUX_DIRis 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 name for the cache file;
- adjusting a relative name for the source directory;
- propagating the current value of
$prefix, including if it was defaulted, and if the default values of the top level and of the subdirectory configure differ.This macro also sets the output variable
subdirsto the list of directories ‘dir ...’. Make rules can use this variable to determine which subdirectories to recurse into.This macro may be called multiple times.
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 other than
/usr/local by default. To accomplish that, use the
AC_PREFIX_DEFAULT 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.
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, else default the prefix as described above (/usr/local or
AC_PREFIX_DEFAULT). For example, if program isgccand the PATH contains /usr/local/gnu/bin/gcc, set the prefix to /usr/local/gnu.
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 the C preprocessor symbol name to the value 1”. See Defining Symbols, for how to get those symbol definitions into your program.
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.
All the generic macros that AC_DEFINE a symbol as a result of
their test transform their argument values 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*])
defines the symbol ‘HAVE_STRUCT__EXPENSIVEP’ if the check succeeds.
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:
#ifdef TIME_WITH_SYS_TIME
# include <sys/time.h>
# include <time.h>
#else
# ifdef HAVE_SYS_TIME_H
# include <sys/time.h>
# else
# include <time.h>
# endif
#endif
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 use the following macro to provide the default set of includes:
Expand to include-directives if defined, otherwise to:
#include <stdio.h> #ifdef HAVE_SYS_TYPES_H # include <sys/types.h> #endif #ifdef HAVE_SYS_STAT_H # include <sys/stat.h> #endif #ifdef STDC_HEADERS # include <stdlib.h> # include <stddef.h> #else # ifdef HAVE_STDLIB_H # include <stdlib.h> # endif #endif #ifdef HAVE_STRING_H # if !defined STDC_HEADERS && defined HAVE_MEMORY_H # include <memory.h> # endif # include <string.h> #endif #ifdef HAVE_STRINGS_H # include <strings.h> #endif #ifdef HAVE_INTTYPES_H # include <inttypes.h> #endif #ifdef HAVE_STDINT_H # include <stdint.h> #endif #ifdef HAVE_UNISTD_H # include <unistd.h> #endifIf the default includes are used, then check for the presence of these headers and their compatibility, i.e., you don't need to run
AC_HEADER_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_His defined, notHAVE_STRINGS_H.
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.
These macros check for particular programs—whether they exist, and in some cases whether they support certain features.
Check for
gawk,mawk,nawk, andawk, in that order, and set output variableAWKto the first one that is found. It triesgawkfirst because that is reported to be the best implementation. The result can be overridden by setting the variableAWKor the cache variableac_cv_prog_AWK.Using this macro is sufficient to avoid the pitfalls of traditional awk (see Limitations of Usual Tools).
Look for the best available
greporggrepthat accepts the longest input lines possible, and that supports multiple -e options. Set the output variableGREPto whatever is chosen. See Limitations of Usual Tools, for more information about portability problems with the grep command family. The result can be overridden by setting theGREPvariable and is cached in theac_cv_path_GREPvariable.
Check whether
$GREP -Eworks, or else look for the best availableegreporgegrepthat accepts the longest input lines possible. Set the output variableEGREPto whatever is chosen. The result can be overridden by setting theEGREPvariable and is cached in theac_cv_path_EGREPvariable.
Check whether
$GREP -Fworks, or else look for the best availablefgreporgfgrepthat accepts the longest input lines possible. Set the output variableFGREPto whatever is chosen. The result can be overridden by setting theFGREPvariable and is cached in theac_cv_path_FGREPvariable.
Set output variable
INSTALLto the name of a BSD-compatible install program, if one is found in the current PATH. Otherwise, setINSTALLto ‘dir/install-sh -c’, checking the directories specified toAC_CONFIG_AUX_DIR(or its default directories) to determine dir (see Output). Also set the variablesINSTALL_PROGRAMandINSTALL_SCRIPTto ‘${INSTALL}’ andINSTALL_DATAto ‘${INSTALL} -m 644’.‘@INSTALL@’ is special, as its value may vary for different configuration files.
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. Further, this macro requires install to be able to install multiple files into a target directory in a single invocation.
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; otherwise configure produces 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.The result of the test can be overridden by setting the variable
INSTALLor the cache variableac_cv_path_install.
Set output variable
MKDIR_Pto a program that ensures that for each argument, a directory named by this argument exists, creating it and its parent directories if needed, and without race conditions when two instances of the program attempt to make the same directory at nearly the same time.This macro uses the ‘mkdir -p’ command if possible. Otherwise, it falls back on invoking install-sh with the -d option, so your package should contain install-sh as described under
AC_PROG_INSTALL. An install-sh file that predates Autoconf 2.60 or Automake 1.10 is vulnerable to race conditions, so if you want to support parallel installs from different packages into the same directory you need to make sure you have an up-to-date install-sh. In particular, be careful about using ‘autoreconf -if’ if your Automake predates Automake 1.10.This macro is related to the
AS_MKDIR_Pmacro (see Programming in M4sh), but it sets an output variable intended for use in other files, whereasAS_MKDIR_Pis intended for use in scripts like configure. Also,AS_MKDIR_Pdoes not accept options, butMKDIR_Psupports the -m option, e.g., a makefile might invoke$(MKDIR_P) -m 0 dirto create an inaccessible directory, and conversely a makefile should use$(MKDIR_P) -- $(FOO)if FOO might yield a value that begins with ‘-’. Finally,AS_MKDIR_Pdoes not check for race condition vulnerability, whereasAC_PROG_MKDIR_Pdoes.‘@MKDIR_P@’ is special, as its value may vary for different configuration files.
The result of the test can be overridden by setting the variable
MKDIR_Por the cache variableac_cv_path_mkdir.
If
flexis found, set output variableLEXto ‘flex’ andLEXLIBto -lfl, if that library is in a standard place. Otherwise setLEXto ‘lex’ andLEXLIBto -ll, if found. If neither variant is available, setLEXto ‘:’; for packages that ship the generated file.yy.c alongside the source file.l, this default allows users without a lexer generator to still build the package even if the timestamp for file.l is inadvertantly changed.Define
YYTEXT_POINTERifyytextdefaults to ‘char *’ instead of to ‘char []’. Also set output variableLEX_OUTPUT_ROOTto the base of the file name that the lexer generates; usually lex.yy, but sometimes something else. These results vary according to whetherlexorflexis being used.You are encouraged to use Flex in your sources, since it is both more pleasant to use than plain Lex and the C source it produces is portable. In order to ensure portability, however, you must either provide a function
yywrapor, if you don't use it (e.g., your scanner has no ‘#include’-like feature), simply include a ‘%noyywrap’ statement in the scanner's source. Once this done, the scanner is portable (unless you felt free to use nonportable constructs) and does not depend on any library. In this case, and in this case only, it is suggested that you use this Autoconf snippet:AC_PROG_LEX if test "x$LEX" != xflex; then LEX="$SHELL $missing_dir/missing flex" AC_SUBST([LEX_OUTPUT_ROOT], [lex.yy]) AC_SUBST([LEXLIB], ['']) fiThe shell script missing can be found in the Automake distribution.
Remember that the user may have supplied an alternate location in LEX, so if Flex is required, it is better to check that the user provided something sufficient by parsing the output of ‘$LEX --version’ than by simply relying on
test "x$LEX" = xflex.To ensure backward compatibility, Automake's
AM_PROG_LEXinvokes (indirectly) this macro twice, which causes an annoying but benign “AC_PROG_LEXinvoked multiple times” warning. Future versions of Automake will fix this issue; meanwhile, just ignore this message.As part of running the test, this macro may delete any file in the configuration directory named lex.yy.c or lexyy.c.
The result of this test can be influenced by setting the variable
LEXor the cache variableac_cv_prog_LEX.
If ‘ln -s’ works on the current file system (the operating system and file system support symbolic links), set the output variable
LN_Sto ‘ln -s’; otherwise, if ‘ln’ works, setLN_Sto ‘ln’, and otherwise set it to ‘cp -p’.If you make a link in a directory other than the current directory, its meaning depends on whether ‘ln’ or ‘ln -s’ is used. To safely create links using ‘$(LN_S)’, either find out which form is used and adjust the arguments, or always invoke
lnin the directory where the link is to be created.In other words, it does not work to do:
$(LN_S) foo /x/barInstead, do:
(cd /x && $(LN_S) foo bar)
Set output variable
RANLIBto ‘ranlib’ ifranlibis found, and otherwise to ‘:’ (do nothing).
Set output variable
SEDto a Sed implementation that conforms to Posix and does not have arbitrary length limits. Report an error if no acceptable Sed is found. See Limitations of Usual Tools, for more information about portability problems with Sed.The result of this test can be overridden by setting the
SEDvariable and is cached in theac_cv_path_SEDvariable.
If
bisonis found, set output variableYACCto ‘bison -y’. Otherwise, ifbyaccis found, setYACCto ‘byacc’. Otherwise setYACCto ‘yacc’. The result of this test can be influenced by setting the variableYACCor the cache variableac_cv_prog_YACC.
These macros are used to find programs not covered by the “particular” test macros. If you need to check the behavior of a program as well as find out whether it is present, you have to write your own test for it (see Writing Tests). By default, these macros use the environment variable PATH. If you need to check for a program that might not be in the user's PATH, you can pass a modified path to use instead, like this:
AC_PATH_PROG([INETD], [inetd], [/usr/libexec/inetd],
[$PATH$PATH_SEPARATOR/usr/libexec$PATH_SEPARATOR]dnl
[/usr/sbin$PATH_SEPARATOR/usr/etc$PATH_SEPARATOR/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.
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_SUBSTfor variable. The result of this test can be overridden by setting the variable variable or the cache variableac_cv_prog_variable.
Check for each program in the blank-separated list progs-to-check-for existing in the path. If one 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_SUBSTfor variable. The result of this test can be overridden by setting the variable variable or the cache variableac_cv_prog_variable.
Like
AC_CHECK_PROG, but first looks for prog-to-check-for with a prefix of the target type as determined byAC_CANONICAL_TARGET, followed by a dash (see Canonicalizing). If the tool cannot be found with a prefix, and if the build and target types are equal, then it is also searched for without a prefix.As noted in Specifying Target Triplets, the target is rarely specified, because most of the time it is the same as the host: it is the type of system for which any compiler tool in the package produces code. What this macro looks for is, for example, a tool (assembler, linker, etc.) that the compiler driver (gcc for the GNU C Compiler) uses to produce objects, archives or executables.
Like
AC_CHECK_PROG, but first looks for prog-to-check-for with a prefix of the host type as specified by --host, followed by a dash. For example, if the user runs ‘configure --build=x86_64-gnu --host=i386-gnu’, then this call:AC_CHECK_TOOL([RANLIB], [ranlib], [:])sets
RANLIBto 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.When cross-compiling, this macro will issue a warning if no program prefixed with the host type could be found. For more information, see Specifying Target Triplets.
Like
AC_CHECK_TARGET_TOOL, each of the tools in the list progs-to-check-for are checked with a prefix of the target type as determined byAC_CANONICAL_TARGET, followed by a dash (see Canonicalizing). If none of the tools can be found with a prefix, and if the build and target types are equal, 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. CallsAC_SUBSTfor variable.
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 byAC_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. CallsAC_SUBSTfor variable.When cross-compiling, this macro will issue a warning if no program prefixed with the host type could be found. For more information, see Specifying Target Triplets.
Like
AC_CHECK_PROG, but set variable to the absolute name of prog-to-check-for if found. The result of this test can be overridden by setting the variable variable. A positive result of this test is cached in theac_cv_path_variable variable.
Like
AC_CHECK_PROGS, but if any of progs-to-check-for are found, set variable to the absolute name of the program found. The result of this test can be overridden by setting the variable variable. A positive result of this test is cached in theac_cv_path_variable variable.
This macro was introduced in Autoconf 2.62. If variable is not empty, then set the cache variable
ac_cv_path_variable to its value. Otherwise, check for each program in the blank-separated list progs-to-check-for existing in path. For each program found, execute feature-test withac_path_variable set to the absolute name of the candidate program. If no invocation of feature-test sets the shell variableac_cv_path_variable, then action-if-not-found is executed. feature-test will be run even whenac_cv_path_variable is set, to provide the ability to choose a better candidate found later in path; to accept the current setting and bypass all futher checks, feature-test can executeac_path_variable_found=:.Note that this macro has some subtle differences from
AC_CHECK_PROGS. It is designed to be run insideAC_CACHE_VAL, therefore, it should have no side effects. In particular, variable is not set to the final value ofac_cv_path_variable, nor isAC_SUBSTautomatically run. Also, on failure, any action can be performed, whereasAC_CHECK_PROGSonly performs variable=value-if-not-found.Here is an example, similar to what Autoconf uses in its own configure script. It will search for an implementation of m4 that supports the
indirbuiltin, even if it goes by the name gm4 or is not the first implementation on PATH.AC_CACHE_CHECK([for m4 that supports indir], [ac_cv_path_M4], [AC_PATH_PROGS_FEATURE_CHECK([M4], [m4 gm4], [[m4out=`echo 'changequote([,])indir([divnum])' | $ac_path_M4` test "x$m4out" = x0 \ && ac_cv_path_M4=$ac_path_M4 ac_path_M4_found=:]], [AC_MSG_ERROR([could not find m4 that supports indir])])]) AC_SUBST([M4], [$ac_cv_path_M4])
Like
AC_CHECK_TARGET_TOOL, but set variable to the absolute name of the program if it is found.
Like
AC_CHECK_TOOL, but set variable to the absolute name of the program if it is found.When cross-compiling, this macro will issue a warning if no program prefixed with the host type could be found. For more information, see Specifying Target Triplets.
You might also need to check for the existence of files. Before using these macros, ask yourself whether a runtime 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.
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. The result of this test is cached in the
ac_cv_file_file variable, with characters not suitable for a variable name mapped to underscores.
Executes
AC_CHECK_FILEonce for each file listed in files. Additionally, defines ‘HAVE_file’ (see Standard Symbols) for each file found. The results of each test are cached in theac_cv_file_file variable, with characters not suitable for a variable name mapped to underscores.
The following macros check for the presence of certain C, C++, or Fortran library archive files.
Test whether the library library is available by trying to link a test program that calls function function with the library. function should be a function provided by the library. Use 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 prepends -llibrary to
LIBSand defines ‘HAVE_LIBlibrary’ (in all capitals). This macro is intended to support buildingLIBSin a right-to-left (least-dependent to most-dependent) fashion such that library dependencies are satisfied as a natural side effect of consecutive tests. Linkers are sensitive to library ordering so the order in whichLIBSis 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 may fail to detect that library is present, because linking the test program can 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_CHECK_LIBrequires some care in usage, and should be avoided in some common cases. Many standard functions likegethostbynameappear in the standard C library on some hosts, and in special libraries likenslon other hosts. On some hosts the special libraries contain variant implementations that you may not want to use. These days it is normally better to useAC_SEARCH_LIBS([gethostbyname], [nsl])instead ofAC_CHECK_LIB([nsl], [gethostbyname]).The result of this test is cached in the
ac_cv_lib_library_function variable.
Search for a library defining function if it's not already available. This equates to calling ‘AC_LINK_IFELSE([AC_LANG_CALL([], [function])])’ first with no libraries, then for each library listed in search-libs.
Prepend -llibrary to
LIBSfor 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 fails to detect that function is present, because linking the test program always fails with unresolved symbols.
The result of this test is cached in the
ac_cv_search_function variable as ‘none required’ if function is already available, as ‘no’ if no library containing function was found, otherwise as the -llibrary option that needs to be prepended toLIBS.
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.
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 always requires additions. A much more complete list is maintained by the Gnulib project (see Gnulib), covering Current Posix Functions, Legacy Functions, and Glibc Functions. Please help us keep the gnulib list as complete as possible.
exitexit returned int.
This is because exit predates void, and there was a long
tradition of it returning int.
On current hosts, the problem more likely is that exit is not
declared, due to C++ problems of some sort or another. For this reason
we suggest that test programs not invoke exit, but return from
main instead.
freefree (NULL) does nothing, but
some old systems don't support this (e.g., NextStep).
isinfisnanisinf and isnan are
macros. On some systems just macros are available
(e.g., HP-UX and Solaris 10), on
some systems both macros and functions (e.g., glibc 2.3.2), and on some
systems only functions (e.g., IRIX 6 and Solaris 9). In some cases
these functions are declared in nonstandard headers like
<sunmath.h> and defined in non-default libraries like
-lm or -lsunmath.
The C99 isinf and isnan macros work correctly with
long double arguments, but pre-C99 systems that use functions
typically assume double arguments. On such a system,
isinf incorrectly returns true for a finite long double
argument that is outside the range of double.
The best workaround for these issues is to use gnulib modules
isinf and isnan (see Gnulib). But a lighter weight
solution involves code like the following.
#include <math.h>
#ifndef isnan
# define isnan(x) \
(sizeof (x) == sizeof (long double) ? isnan_ld (x) \
: sizeof (x) == sizeof (double) ? isnan_d (x) \
: isnan_f (x))
static inline int isnan_f (float x) { return x != x; }
static inline int isnan_d (double x) { return x != x; }
static inline int isnan_ld (long double x) { return x != x; }
#endif
#ifndef isinf
# define isinf(x) \
(sizeof (x) == sizeof (long double) ? isinf_ld (x) \
: sizeof (x) == sizeof (double) ? isinf_d (x) \
: isinf_f (x))
static inline int isinf_f (float x)
{ return !isnan (x) && isnan (x - x); }
static inline int isinf_d (double x)
{ return !isnan (x) && isnan (x - x); }
static inline int isinf_ld (long double x)
{ return !isnan (x) && isnan (x - x); }
#endif
Use AC_C_INLINE (see C Compiler) so that this code works on
compilers that lack the inline keyword. Some optimizing
compilers mishandle these definitions, but systems with that bug
typically have many other floating point corner-case compliance problems
anyway, so it's probably not worth worrying about.
mallocmalloc (0) is implementation
dependent. It can return either NULL or a new non-null pointer.
The latter is more common (e.g., the GNU C Library) but is by
no means universal. AC_FUNC_MALLOC
can be used to insist on non-NULL (see Particular Functions).
putenvsetenv to putenv; among other things,
putenv is not required of all Posix implementations, but
setenv is.
Posix specifies that putenv puts the given string directly in
environ, but some systems make a copy of it instead (e.g.,
glibc 2.0, or BSD). And when a copy is made, unsetenv might
not free it, causing a memory leak (e.g., FreeBSD 4).
On some systems putenv ("FOO") removes ‘FOO’ from the
environment, but this is not standard usage and it dumps core
on some systems (e.g., AIX).
On MinGW, a call putenv ("FOO=") removes ‘FOO’ from the
environment, rather than inserting it with an empty value.
reallocrealloc (NULL, size) is equivalent
to malloc (size), but some old systems don't support this (e.g.,
NextStep).
signal handlersignal takes a handler function with a return type of
void, but some old systems required int instead. Any
actual int value returned is not used; this is only a
difference in the function prototype demanded.
All systems we know of in current use return void. The
int was to support K&R C, where of course void is not
available. The obsolete macro AC_TYPE_SIGNAL
(see AC_TYPE_SIGNAL) can be used to establish the correct type in
all cases.
In most cases, it is more robust to use sigaction when it is
available, rather than signal.
snprintfsnprintf 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).
sprintfsprintf and vsprintf return the
number of bytes written. On some ancient systems (SunOS 4 for
instance) they return the buffer pointer instead, but these no
longer need to be worried about.
sscanfsscanf requires
that its
input string be 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.
strerror_rstrerror_r returns an int, but many
systems (e.g., GNU C Library version 2.2.4) provide a
different version returning a char *. AC_FUNC_STRERROR_R
can detect which is in use (see Particular Functions).
strnlen 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
sysconf_SC_PAGESIZE is standard, but some older systems (e.g., HP-UX
9) have _SC_PAGE_SIZE instead. This can be tested with
#ifdef.
unlinkunlink causes the given file to be
removed only after there are no more open file handles for it. Some
non-Posix hosts have trouble with this requirement, though,
and some DOS variants even corrupt the file system.
unsetenvunsetenv is not available, but a variable ‘FOO’
can be removed with a call putenv ("FOO="), as described under
putenv above.
va_copyva_copy for copying
va_list variables. It may be available in older environments
too, though possibly as __va_copy (e.g., gcc in strict
pre-C99 mode). These can be tested with #ifdef. A fallback to
memcpy (&dst, &src, sizeof (va_list)) gives maximum
portability.
va_listva_list is not necessarily just a pointer. It can be a
struct (e.g., gcc on Alpha), which means NULL is
not portable. Or it can be an array (e.g., 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 (e.g., vsnprintf in the GNU C Library
2.1).
>>>> right shift of a signed type replicates the
high bit, giving a so-called “arithmetic” shift. But care should be
taken since Standard C doesn't require that behavior. 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.
/These macros check for particular C functions—whether they exist, and in some cases how they respond when given certain arguments.
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 definesHAVE_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 variableALLOCAto ‘${LIBOBJDIR}alloca.o’ and definesC_ALLOCA(so programs can periodically call ‘alloca (0)’ to garbage collect). This variable is separate fromLIBOBJSso multiple programs can share the value ofALLOCAwithout needing to create an actual library, in case only some of them use the code inLIBOBJS. The ‘${LIBOBJDIR}’ prefix serves the same purpose as inLIBOBJS(see AC_LIBOBJ vs LIBOBJS).This macro does not try to get
allocafrom 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 containallocaor contain a buggy version. If you still want to use theiralloca, usearto extract alloca.o from them instead of compiling alloca.c.Source files that use
allocashould start with a piece of code like the following, to declare it properly.#ifdef STDC_HEADERS # include <stdlib.h> # include <stddef.h> #else # ifdef HAVE_STDLIB_H # include <stdlib.h> # endif #endif #ifdef HAVE_ALLOCA_H # include <alloca.h> #elif defined __GNUC__ # define alloca __builtin_alloca #elif defined _AIX # define alloca __alloca #elif defined _MSC_VER # include <malloc.h> # define alloca _alloca #else # ifndef HAVE_ALLOCA # ifdef __cplusplus extern "C" # endif void *alloca (size_t); # endif #endif
If the
chownfunction is available and works (in particular, it should accept -1 foruidandgid), defineHAVE_CHOWN. The result of this macro is cached in theac_cv_func_chown_worksvariable.
If the
closedirfunction does not return a meaningful value, defineCLOSEDIR_VOID. Otherwise, callers ought to check its return value for an error indicator.Currently this test is implemented by running a test program. When cross compiling the pessimistic assumption that
closedirdoes not return a meaningful value is made.The result of this macro is cached in the
ac_cv_func_closedir_voidvariable.This macro is obsolescent, as
closedirreturns a meaningful value on current systems. New programs need not use this macro.
If the
error_at_linefunction is not found, require anAC_LIBOBJreplacement of ‘error’.The result of this macro is cached in the
ac_cv_lib_error_at_linevariable.The
AC_FUNC_ERROR_AT_LINEmacro is obsolescent. New programs should use Gnulib'serrormodule. See Gnulib.
If the
fnmatchfunction conforms to Posix, defineHAVE_FNMATCH. Detect common implementation bugs, for example, the bugs in Solaris 2.4.Unlike the other specific
AC_FUNCmacros,AC_FUNC_FNMATCHdoes not replace a broken/missingfnmatch. This is for historical reasons. SeeAC_REPLACE_FNMATCHbelow.The result of this macro is cached in the
ac_cv_func_fnmatch_worksvariable.This macro is obsolescent. New programs should use Gnulib's
fnmatch-posixmodule. See Gnulib.
Behave like
AC_REPLACE_FNMATCH(replace) but also test whetherfnmatchsupports GNU extensions. Detect common implementation bugs, for example, the bugs in the GNU C Library 2.1.The result of this macro is cached in the
ac_cv_func_fnmatch_gnuvariable.This macro is obsolescent. New programs should use Gnulib's
fnmatch-gnumodule. See Gnulib.
This macro checks for the
forkandvforkfunctions. If a workingforkis found, defineHAVE_WORKING_FORK. This macro checks whetherforkis just a stub by trying to run it.If vfork.h is found, define
HAVE_VFORK_H. If a workingvforkis found, defineHAVE_WORKING_VFORK. Otherwise, definevforkto beforkfor backward compatibility with previous versions of autoconf. This macro checks for several known errors in implementations ofvforkand considers the system to not have a workingvforkif it detects any of them. It is not considered to be an implementation error if a child's invocation ofsignalmodifies the parent's signal handler, since child processes rarely change their signal handlers.Since this macro defines
vforkonly for backward compatibility with previous versions of autoconf you're encouraged to define it yourself in new code:#ifndef HAVE_WORKING_VFORK # define vfork fork #endif
If the
fseekofunction is available, defineHAVE_FSEEKO. Define_LARGEFILE_SOURCEif necessary to make the prototype visible on some systems (e.g., glibc 2.2). Otherwise linkage problems may occur when compiling withAC_SYS_LARGEFILEon largefile-sensitive systems whereoff_tdoes not default to a 64bit entity. All systems withfseekoalso supplyftello.
If the
getgroupsfunction is available and works (unlike on Ultrix 4.3, where ‘getgroups (0, 0)’ always fails), defineHAVE_GETGROUPS. SetGETGROUPS_LIBSto any libraries needed to get that function. This macro runsAC_TYPE_GETGROUPS.
Check how to get the system load averages. To perform its tests properly, this macro needs the file getloadavg.c; therefore, be sure to set the
AC_LIBOBJreplacement directory properly (see Generic Functions,AC_CONFIG_LIBOBJ_DIR).If the system has the
getloadavgfunction, defineHAVE_GETLOADAVG, and setGETLOADAVG_LIBSto any libraries necessary to get that function. Also addGETLOADAVG_LIBStoLIBS. Otherwise, require anAC_LIBOBJreplacement for ‘getloadavg’ with source code in dir/getloadavg.c, and possibly define several other C preprocessor macros and output variables:
- Define
C_GETLOADAVG.- Define
SVR4,DGUX,UMAX, orUMAX4_3if on those systems.- If nlist.h is found, define
HAVE_NLIST_H.- If ‘struct nlist’ has an ‘n_un.n_name’ member, define
HAVE_STRUCT_NLIST_N_UN_N_NAME. The obsolete symbolNLIST_NAME_UNIONis still defined, but do not depend upon it.- Programs may need to be installed set-group-ID (or set-user-ID) for
getloadavgto work. In this case, defineGETLOADAVG_PRIVILEGED, set the output variableNEED_SETGIDto ‘true’ (and otherwise to ‘false’), and setKMEM_GROUPto the name of the group that should own the installed program.The
AC_FUNC_GETLOADAVGmacro is obsolescent. New programs should use Gnulib'sgetloadavgmodule. See Gnulib.
Check for
getmntentin the standard C library, and then in the sun, seq, and gen libraries, for UNICOS, IRIX 4, PTX, and UnixWare, respectively. Then, ifgetmntentis available, defineHAVE_GETMNTENTand setac_cv_func_getmntenttoyes. Otherwise setac_cv_func_getmntenttono.The result of this macro can be overridden by setting the cache variable
ac_cv_search_getmntent.
Define
GETPGRP_VOIDif it is an error to pass 0 togetpgrp; this is the Posix behavior. On older BSD systems, you must pass 0 togetpgrp, as it takes an argument and behaves like Posix'sgetpgid.#ifdef GETPGRP_VOID pid = getpgrp (); #else pid = getpgrp (0); #endifThis macro does not check whether
getpgrpexists at all; if you need to work in that situation, first callAC_CHECK_FUNCforgetpgrp.The result of this macro is cached in the
ac_cv_func_getpgrp_voidvariable.This macro is obsolescent, as current systems have a
getpgrpwhose signature conforms to Posix. New programs need not use this macro.
If link is a symbolic link, then
lstatshould treat link/ the same as link/.. However, many olderlstatimplementations incorrectly ignore trailing slashes.It is safe to assume that if
lstatincorrectly ignores trailing slashes, then other symbolic-link-aware functions likeunlinkalso incorrectly ignore trailing slashes.If
lstatbehaves properly, defineLSTAT_FOLLOWS_SLASHED_SYMLINK, otherwise require anAC_LIBOBJreplacement oflstat.The result of this macro is cached in the
ac_cv_func_lstat_dereferences_slashed_symlinkvariable.The
AC_FUNC_LSTAT_FOLLOWS_SLASHED_SYMLINKmacro is obsolescent. New programs should use Gnulib'slstatmodule. See Gnulib.
If the
mallocfunction is compatible with the GNU C librarymalloc(i.e., ‘malloc (0)’ returns a valid pointer), defineHAVE_MALLOCto 1. Otherwise defineHAVE_MALLOCto 0, ask for anAC_LIBOBJreplacement for ‘malloc’, and definemalloctorpl_mallocso that the nativemallocis not used in the main project.Typically, the replacement file malloc.c should look like (note the ‘#undef malloc’):
#include <config.h> #undef malloc #include <sys/types.h> void *malloc (); /* Allocate an N-byte block of memory from the heap. If N is zero, allocate a 1-byte block. */ void * rpl_malloc (size_t n) { if (n == 0) n = 1; return malloc (n); }The result of this macro is cached in the
ac_cv_func_malloc_0_nonnullvariable.
Define
HAVE_MBRTOWCto 1 if the functionmbrtowcand the typembstate_tare properly declared.The result of this macro is cached in the
ac_cv_func_mbrtowcvariable.
If the
memcmpfunction 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 anAC_LIBOBJreplacement for ‘memcmp’.The result of this macro is cached in the
ac_cv_func_memcmp_workingvariable.This macro is obsolescent, as current systems have a working
memcmp. New programs need not use this macro.
If the
mktimefunction is not available, or does not work correctly, require anAC_LIBOBJreplacement for ‘mktime’. For the purposes of this test,mktimeshould conform to the Posix standard and should be the inverse oflocaltime.The result of this macro is cached in the
ac_cv_func_working_mktimevariable.The
AC_FUNC_MKTIMEmacro is obsolescent. New programs should use Gnulib'smktimemodule. See Gnulib.
If the
mmapfunction exists and works correctly, defineHAVE_MMAP. This checks only private fixed mapping of already-mapped memory.The result of this macro is cached in the
ac_cv_func_mmap_fixed_mappedvariable.
If the obstacks are found, define
HAVE_OBSTACK, else require anAC_LIBOBJreplacement for ‘obstack’.The result of this macro is cached in the
ac_cv_func_obstackvariable.
If the
reallocfunction is compatible with the GNU C libraryrealloc(i.e., ‘realloc (NULL, 0)’ returns a valid pointer), defineHAVE_REALLOCto 1. Otherwise defineHAVE_REALLOCto 0, ask for anAC_LIBOBJreplacement for ‘realloc’, and definerealloctorpl_reallocso that the nativereallocis not used in the main project. SeeAC_FUNC_MALLOCfor details.The result of this macro is cached in the
ac_cv_func_realloc_0_nonnullvariable.
Determines the correct type to be passed for each of the
selectfunction's arguments, and defines those types inSELECT_TYPE_ARG1,SELECT_TYPE_ARG234, andSELECT_TYPE_ARG5respectively.SELECT_TYPE_ARG1defaults to ‘int’,SELECT_TYPE_ARG234defaults to ‘int *’, andSELECT_TYPE_ARG5defaults to ‘struct timeval *’.This macro is obsolescent, as current systems have a
selectwhose signature conforms to Posix. New programs need not use this macro.
If
setpgrptakes no argument (the Posix version), defineSETPGRP_VOID. Otherwise, it is the BSD version, which takes two process IDs as arguments. This macro does not check whethersetpgrpexists at all; if you need to work in that situation, first callAC_CHECK_FUNCforsetpgrp.The result of this macro is cached in the
ac_cv_func_setpgrp_voidvariable.This macro is obsolescent, as current systems have a
setpgrpwhose signature conforms to Posix. New programs need not use this macro.
Determine whether
statorlstathave the bug that it succeeds when given the zero-length file name as argument. Thestatandlstatfrom SunOS 4.1.4 and the Hurd (as of 1998-11-01) do this.If it does, then define
HAVE_STAT_EMPTY_STRING_BUG(orHAVE_LSTAT_EMPTY_STRING_BUG) and ask for anAC_LIBOBJreplacement of it.The results of these macros are cached in the
ac_cv_func_stat_empty_string_bugand theac_cv_func_lstat_empty_string_bugvariables, respectively.These macros are obsolescent, as no current systems have the bug. New programs need not use these macros.
If the
strcollfunction exists and works correctly, defineHAVE_STRCOLL. This does a bit more than ‘AC_CHECK_FUNCS(strcoll)’, because some systems have incorrect definitions ofstrcollthat should not be used.The result of this macro is cached in the
ac_cv_func_strcoll_worksvariable.
If
strerror_ris available, defineHAVE_STRERROR_R, and if it is declared, defineHAVE_DECL_STRERROR_R. If it returns achar *message, defineSTRERROR_R_CHAR_P; otherwise it returns aninterror number. The Thread-Safe Functions option of Posix requiresstrerror_rto returnint, but many systems (including, for example, version 2.2.4 of the GNU C Library) return achar *value that is not necessarily equal to the buffer argument.The result of this macro is cached in the
ac_cv_func_strerror_r_char_pvariable.
Check for
strftimein the intl library, for SCO Unix. Then, ifstrftimeis available, defineHAVE_STRFTIME.This macro is obsolescent, as no current systems require the intl library for
strftime. New programs need not use this macro.
If the
strtodfunction does not exist or doesn't work correctly, ask for anAC_LIBOBJreplacement of ‘strtod’. In this case, because strtod.c is likely to need ‘pow’, set the output variablePOW_LIBto the extra library needed.This macro caches its result in the
ac_cv_func_strtodvariable and depends upon the result in theac_cv_func_powvariable.The
AC_FUNC_STRTODmacro is obsolescent. New programs should use Gnulib'sstrtodmodule. See Gnulib.
If the
strtoldfunction exists and conforms to C99, defineHAVE_STRTOLD.This macro caches its result in the
ac_cv_func_strtoldvariable.
If the
strnlenfunction is not available, or is buggy (like the one from AIX 4.3), require anAC_LIBOBJreplacement for it.This macro caches its result in the
ac_cv_func_strnlen_workingvariable.
If ‘utime (file, NULL)’ sets file's timestamp to the present, define
HAVE_UTIME_NULL.This macro caches its result in the
ac_cv_func_utime_nullvariable.This macro is obsolescent, as all current systems have a
utimethat behaves this way. New programs need not use this macro.
If
vprintfis found, defineHAVE_VPRINTF. Otherwise, if_doprntis found, defineHAVE_DOPRNT. (Ifvprintfis available, you may assume thatvfprintfandvsprintfare also available.)This macro is obsolescent, as all current systems have
vprintf. New programs need not use this macro.
If the
fnmatchfunction does not conform to Posix (seeAC_FUNC_FNMATCH), ask for itsAC_LIBOBJreplacement.The files fnmatch.c, fnmatch_loop.c, and fnmatch_.h in the
AC_LIBOBJreplacement directory are assumed to contain a copy of the source code of GNUfnmatch. If necessary, this source code is compiled as anAC_LIBOBJreplacement, and the fnmatch_.h file is linked to fnmatch.h so that it can be included in place of the system<fnmatch.h>.This macro caches its result in the
ac_cv_func_fnmatch_worksvariable.This macro is obsolescent, as it assumes the use of particular source files. New programs should use Gnulib's
fnmatch-posixmodule, which provides this macro along with the source files. See Gnulib.
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).
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_FUNCSinstead. This macro checks for functions with C linkage even whenAC_LANG(C++)has been called, since C is more standardized than C++. (see Language Choice, for more information about selecting the language for checks.)This macro caches its result in the
ac_cv_func_function variable.
For each function enumerated in the blank-or-newline-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.Results are cached for each function as in
AC_CHECK_FUNC.
For each function enumerated in the blank-or-newline-separated argument list, define
HAVE_function (in all capitals) if it is available. This is a once-only variant ofAC_CHECK_FUNCS. It generates the checking code at most once, so that configure is smaller and faster; but the checks cannot be conditionalized and are always done once, early during the configure run.
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.
Suitable replacements for many such problem functions are available from Gnulib (see Gnulib).
Specify that ‘function.c’ must be included in the executables to replace a missing or broken implementation of function.
Technically, it adds ‘function.$ac_objext’ to the output variable
LIBOBJSif it is not already in, and callsAC_LIBSOURCEfor ‘function.c’. You should not directly changeLIBOBJS, since this is not traceable.
Specify that file might be needed to compile the project. If you need to know what files might be needed by a configure.ac, you should trace
AC_LIBSOURCE. file must be a literal.This macro is called automatically from
AC_LIBOBJ, but you must call it explicitly if you pass a shell variable toAC_LIBOBJ. In that case, since shell variables cannot be traced statically, you must pass toAC_LIBSOURCEany possible files that the shell variable might causeAC_LIBOBJto need. For example, if you want to pass a variable$foo_or_bartoAC_LIBOBJthat holds either"foo"or"bar", you should do:AC_LIBSOURCE([foo.c]) AC_LIBSOURCE([bar.c]) AC_LIBOBJ([$foo_or_bar])There is usually a way to avoid this, however, and you are encouraged to simply call
AC_LIBOBJwith literal arguments.Note that this macro replaces the obsolete
AC_LIBOBJ_DECL, with slightly different semantics: the old macro took the function name, e.g.,foo, as its argument rather than the file name.
Like
AC_LIBSOURCE, but accepts one or more files in a comma-separated M4 list. Thus, the above example might be rewritten:AC_LIBSOURCES([foo.c, bar.c]) AC_LIBOBJ([$foo_or_bar])
Specify that
AC_LIBOBJreplacement files are to be found in directory, a name relative to the top level of the source tree. The replacement directory defaults to ., the top level directory, and the most typical value is lib, corresponding to ‘AC_CONFIG_LIBOBJ_DIR([lib])’.configure might need to know the replacement directory for the following reasons: (i) some checks use the replacement files, (ii) some macros bypass broken system headers by installing links to the replacement headers (iii) when used in conjunction with Automake, within each makefile, directory is used as a relative path from
$(top_srcdir)to each object named inLIBOBJSandLTLIBOBJS, etc.
It is common to merely check for the existence of a function, and ask for its
AC_LIBOBJ replacement if missing. The following macro is
a convenient shorthand.
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 ‘#ifndef HAVE_function’. If the system has the function, it probably declares it in a header file you should be including, so you shouldn't redeclare it lest your declaration conflict.
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.
This section documents some collected knowledge about common headers, and the problems they cause. By definition, this list always requires additions. A much more complete list is maintained by the Gnulib project (see Gnulib), covering Posix Headers and Glibc Headers. Please help us keep the gnulib list as complete as possible.
LLONG_MIN,
LLONG_MAX, and ULLONG_MAX, but many almost-C99
environments (e.g., default GCC 4.0.2 + glibc 2.4) do not
define them.
AC_CHECK_HEADERS([sys/socket.h])
AC_CHECK_HEADERS([net/if.h], [], [],
[#include <stdio.h>
#ifdef STDC_HEADERS
# include <stdlib.h>
# include <stddef.h>
#else
# ifdef HAVE_STDLIB_H
# include <stdlib.h>
# endif
#endif
#ifdef HAVE_SYS_SOCKET_H
# include <sys/socket.h>
#endif
])
AC_CHECK_HEADERS([sys/socket.h])
AC_CHECK_HEADERS([netinet/if_ether.h], [], [],
[#include <stdio.h>
#ifdef STDC_HEADERS
# include <stdlib.h>
# include <stddef.h>
#else
# ifdef HAVE_STDLIB_H
# include <stdlib.h>
# endif
#endif
#ifdef HAVE_SYS_SOCKET_H
# include <sys/socket.h>
#endif
])
AC_CHECK_HEADERS([X11/extensions/scrnsaver.h], [], [],
[[#include <X11/Xlib.h>
]])
These macros check for particular system header files—whether they exist, and in some cases whether they declare certain symbols.
Check whether to enable assertions in the style of assert.h. Assertions are enabled by default, but the user can override this by invoking configure with the --disable-assert option.
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_Hsys/ndir.h HAVE_SYS_NDIR_Hsys/dir.h HAVE_SYS_DIR_Hndir.h HAVE_NDIR_HThe directory-library declarations in your source code should look something like the following:
#include <sys/types.h> #ifdef HAVE_DIRENT_H # include <dirent.h> # define NAMLEN(dirent) strlen ((dirent)->d_name) #else # define dirent direct # define NAMLEN(dirent) ((dirent)->d_namlen) # ifdef HAVE_SYS_NDIR_H # include <sys/ndir.h> # endif # ifdef HAVE_SYS_DIR_H # include <sys/dir.h> # endif # ifdef HAVE_NDIR_H # include <ndir.h> # endif #endifUsing the above declarations, the program would declare variables to be of type
struct dirent, notstruct direct, and would access the length of a directory entry name by passing a pointer to astruct direntto theNAMLENmacro.This macro also checks for the SCO Xenix dir and x libraries.
This macro is obsolescent, as all current systems with directory libraries have
<dirent.h>. New programs need not use this macro.Also see
AC_STRUCT_DIRENT_D_INOandAC_STRUCT_DIRENT_D_TYPE(see Particular Structures).
If sys/types.h does not define
major,minor, andmakedev, but sys/mkdev.h does, defineMAJOR_IN_MKDEV; otherwise, if sys/sysmacros.h does, defineMAJOR_IN_SYSMACROS.
Checks for header resolv.h, checking for prerequisites first. To properly use resolv.h, your code should contain something like the following:
#ifdef HAVE_SYS_TYPES_H # include <sys/types.h> #endif #ifdef HAVE_NETINET_IN_H # include <netinet/in.h> /* inet_ functions / structs */ #endif #ifdef HAVE_ARPA_NAMESER_H # include <arpa/nameser.h> /* DNS HEADER struct */ #endif #ifdef HAVE_NETDB_H # include <netdb.h> #endif #include <resolv.h>
If the macros
S_ISDIR,S_ISREG, etc. defined in sys/stat.h do not work properly (returning false positives), defineSTAT_MACROS_BROKEN. This is the case on Tektronix UTekV, Amdahl UTS and Motorola System V/88.This macro is obsolescent, as no current systems have the bug. New programs need not use this macro.
If stdbool.h exists and conforms to C99, define
HAVE_STDBOOL_Hto 1; if the type_Boolis defined, defineHAVE__BOOLto 1. To fulfill the C99 requirements, your system.h could contain the following code:#ifdef HAVE_STDBOOL_H # include <stdbool.h> #else # ifndef HAVE__BOOL # ifdef __cplusplus typedef bool _Bool; # else # define _Bool signed char # endif # endif # define bool _Bool # define false 0 # define true 1 # define __bool_true_false_are_defined 1 #endifAlternatively you can use the ‘stdbool’ package of Gnulib (see Gnulib); it packages the above code into a replacement header and contains a few other bells and whistles.
This macro caches its result in the
ac_cv_header_stdbool_hvariable.
Define
STDC_HEADERSif the system has C header files conforming to ANSI C89 (ISO C90). 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 C89 header files. This macro also checks whether string.h declaresmemchr(and thus presumably the othermemfunctions), whether stdlib.h declarefree(and thus presumablymallocand other related functions), and whether the ctype.h macros work on characters with the high bit set, as the C standard requires.If you use this macro, your code can refer to
STDC_HEADERSto determine whether the system has conforming header files (and probably C library functions).This macro caches its result in the
ac_cv_header_stdcvariable.This macro is obsolescent, as current systems have conforming header files. New programs need not use this macro.
Nowadays string.h is part of the C standard and declares functions like
strcpy, and strings.h is standardized by Posix and declares BSD functions likebcopy; but historically, string functions were a major sticking point in this area. If you still want to worry about portability to ancient systems without standard 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 ancient systems contained a mix of functions from the C standard and from BSD; some were mostly standard but lacked ‘memmove’; some defined the BSD functions as macros in string.h or strings.h; some had only the BSD functions but string.h; some declared 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 had the standard versions of those then it probably had most of the others. If you put the following in configure.ac:# This example is obsolescent. # Nowadays you can omit these macro calls. AC_HEADER_STDC AC_CHECK_FUNCS([strchr memcpy])then, in your code, you can use declarations like this:
/* This example is obsolescent. Nowadays you can just #include <string.h>. */ #ifdef STDC_HEADERS # include <string.h> #else # ifndef HAVE_STRCHR # define strchr index # define strrchr rindex # endif char *strchr (), *strrchr (); # ifndef HAVE_MEMCPY # define memcpy(d, s, n) bcopy ((s), (d), (n)) # define memmove(d, s, n) bcopy ((s), (d), (n)) # endif #endifIf you use a function like
memchr,memset,strtok, orstrspn, which have no BSD equivalent, then macros don't suffice to port to ancient hosts; 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, takingmemchrfor example, put it in memchr.c and use ‘AC_REPLACE_FUNCS([memchr])’.
If sys/wait.h exists and is compatible with Posix, define
HAVE_SYS_WAIT_H. Incompatibility can occur if sys/wait.h does not exist, or if it uses the old BSDunion waitinstead ofintto store a status value. If sys/wait.h is not Posix compatible, then instead of including it, define the Posix macros with their usual interpretations. Here is an example:#include <sys/types.h> #ifdef HAVE_SYS_WAIT_H # include <sys/wait.h> #endif #ifndef WEXITSTATUS # define WEXITSTATUS(stat_val) ((unsigned int) (stat_val) >> 8) #endif #ifndef WIFEXITED # define WIFEXITED(stat_val) (((stat_val) & 255) == 0) #endifThis macro caches its result in the
ac_cv_header_sys_wait_hvariable.This macro is obsolescent, as current systems are compatible with Posix. New programs need not use this macro.
_POSIX_VERSION is defined when unistd.h is included on
Posix systems. If there is no unistd.h, it is definitely
not a Posix system. However, some non-Posix systems do
have unistd.h.
The way to check whether the system supports Posix is:
#ifdef HAVE_UNISTD_H
# include <sys/types.h>
# include <unistd.h>
#endif
#ifdef _POSIX_VERSION
/* Code for Posix systems. */
#endif
If a program may include both time.h and sys/time.h, define
TIME_WITH_SYS_TIME. On some ancient systems, sys/time.h included time.h, but time.h was not protected against multiple inclusion, so programs could not explicitly include both files. This macro is useful in programs that use, for example,struct timevalas well asstruct tm. It is best used in conjunction withHAVE_SYS_TIME_H, which can be checked for usingAC_CHECK_HEADERS([sys/time.h]).#ifdef TIME_WITH_SYS_TIME # include <sys/time.h> # include <time.h> #else # ifdef HAVE_SYS_TIME_H # include <sys/time.h> # else # include <time.h> # endif #endifThis macro caches its result in the
ac_cv_header_timevariable.This macro is obsolescent, as current systems can include both files when they exist. New programs need not use this macro.
If the use of
TIOCGWINSZrequires <sys/ioctl.h>, then defineGWINSZ_IN_SYS_IOCTL. OtherwiseTIOCGWINSZcan be found in <termios.h>.Use:
#ifdef HAVE_TERMIOS_H # include <termios.h> #endif #ifdef GWINSZ_IN_SYS_IOCTL # include <sys/ioctl.h> #endif
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).
If the system header file header-file is compilable, 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_HEADERSinstead.includes is decoded to determine the appropriate include directives. If omitted or empty, configure will check for both header existence (with the preprocessor) and usability (with the compiler), using
AC_INCLUDES_DEFAULTfor the compile test. If there is a discrepancy between the results, a warning is issued to the user, and the compiler results are favored (see Present But Cannot Be Compiled). In general, favoring the compiler results means that a header will be treated as not found even though the file exists, because you did not provide enough prerequisites.Providing a non-empty includes argument allows the code to provide any prerequisites prior to including the header under test; it is common to use the argument
AC_INCLUDES_DEFAULT(see Default Includes). With an explicit fourth argument, no preprocessor test is needed. As a special case, an includes of exactly ‘-’ triggers the older preprocessor check, which merely determines existence of the file in the preprocessor search path; this should only be used as a last resort (it is safer to determine the actual prerequisites and perform a compiler check, or else useAC_PREPROC_IFELSEto make it obvious that only a preprocessor check is desired).This macro caches its result in the
ac_cv_header_header-file variable, with characters not suitable for a variable name mapped to underscores.
For each given system header file header-file in the blank-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.includes is interpreted as in
AC_CHECK_HEADER, in order to choose the set of preprocessor directives supplied before the header under test.This macro caches its result in the
ac_cv_header_header-file variable, with characters not suitable for a variable name mapped to underscores.
Previous versions of Autoconf merely checked whether the header was
accepted by the preprocessor. This was changed because the old test was
inappropriate for typical uses. Headers are typically used to compile,
not merely to preprocess, and the old behavior sometimes accepted
headers that clashed at compile-time (see Present But Cannot Be Compiled). If you need to check whether a header is preprocessable,
you can use AC_PREPROC_IFELSE (see Running the Preprocessor).
Actually requiring a header to compile improves the robustness of the test, but it also requires that you make sure that headers that must be included before the header-file be part of the includes, (see Default Includes). If looking for bar.h, which requires that foo.h be included before if it exists, we suggest the following scheme:
AC_CHECK_HEADERS([foo.h]) AC_CHECK_HEADERS([bar.h], [], [], [#ifdef HAVE_FOO_H # include <foo.h> #endif ])
The following variant generates smaller, faster configure
files if you do not need the full power of AC_CHECK_HEADERS.
For each given system header file header-file in the blank-separated argument list that exists, define
HAVE_header-file (in all capitals). This is a once-only variant ofAC_CHECK_HEADERS. It generates the checking code at most once, so that configure is smaller and faster; but the checks cannot be conditionalized and are always done once, early during the configure run. Thus, this macro is only safe for checking headers that do not have prerequisites beyond whatAC_INCLUDES_DEFAULTprovides.
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_COMPILE_IFELSE (see Running the Compiler).
There are no specific macros for declarations.
These macros are used to find declarations not covered by the “particular” test macros.
If symbol (a function, variable, or constant) is not declared in includes and a declaration is needed, run the shell commands action-if-not-found, otherwise action-if-found. includes is a series of include directives, defaulting to
AC_INCLUDES_DEFAULT(see Default Includes), which are used prior to the declaration under test.This macro actually tests whether symbol is defined as a macro or can be used as an r-value, not whether it is really declared, because it is much safer to avoid introducing extra declarations when they are not needed. In order to facilitate use of C++ and overloaded function declarations, it is possible to specify function argument types in parentheses for types which can be zero-initialized:
AC_CHECK_DECL([basename(char *)])This macro caches its result in the
ac_cv_have_decl_symbol variable, with characters not suitable for a variable name mapped to underscores.
For each of the symbols (comma-separated list with optional function argument types for C++ overloads), 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.includes is a series of include directives, defaulting to
AC_INCLUDES_DEFAULT(see Default Includes), which are used prior to the declarations under test.This macro uses an M4 list as first argument:
AC_CHECK_DECLS([strdup]) AC_CHECK_DECLS([strlen]) AC_CHECK_DECLS([malloc, realloc, calloc, free]) AC_CHECK_DECLS([j0], [], [], [[#include <math.h>]]) AC_CHECK_DECLS([[basename(char *)], [dirname(char *)]])Unlike the other ‘AC_CHECK_*S’ macros, when a symbol is not declared,
HAVE_DECL_symbol is defined to ‘0’ instead of leavingHAVE_DECL_symbol undeclared. When you are sure that the check was performed, useHAVE_DECL_symbol in#if:#if !HAVE_DECL_SYMBOL extern char *symbol; #endifIf 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 void *malloc (size_t *s); #endifYou 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.
This macro caches its results in
ac_cv_have_decl_symbol variables, with characters not suitable for a variable name mapped to underscores.
For each of the symbols (comma-separated list), define
HAVE_DECL_symbol (in all capitals) to ‘1’ if symbol is declared in the default include files, otherwise to ‘0’. This is a once-only variant ofAC_CHECK_DECLS. It generates the checking code at most once, so that configure is smaller and faster; but the checks cannot be conditionalized and are always done once, early during the configure run.
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 macros
(see Generic Structures) or, for more complex tests, you may use
AC_COMPILE_IFELSE (see Running the Compiler).
The following macros check for certain structures or structure members.
Perform all the actions of
AC_HEADER_DIRENT(see Particular Headers). Then, ifstruct direntcontains ad_inomember, defineHAVE_STRUCT_DIRENT_D_INO.
HAVE_STRUCT_DIRENT_D_INOindicates only the presence ofd_ino, not whether its contents are always reliable. Traditionally, a zerod_inoindicated a deleted directory entry, though current systems hide this detail from the user and never return zerod_inovalues. Many current systems report an incorrectd_inofor a directory entry that is a mount point.
Perform all the actions of
AC_HEADER_DIRENT(see Particular Headers). Then, ifstruct direntcontains ad_typemember, defineHAVE_STRUCT_DIRENT_D_TYPE.
If
struct statcontains anst_blocksmember, defineHAVE_STRUCT_STAT_ST_BLOCKS. Otherwise, require anAC_LIBOBJreplacement of ‘fileblocks’. The former name,HAVE_ST_BLOCKSis to be avoided, as its support will cease in the future.This macro caches its result in the
ac_cv_member_struct_stat_st_blocksvariable.
If time.h does not define
struct tm, defineTM_IN_SYS_TIME, which means that including sys/time.h had better definestruct tm.This macro is obsolescent, as time.h defines
struct tmin current systems. New programs need not use this macro.
Figure out how to get the current timezone. If
struct tmhas atm_zonemember, defineHAVE_STRUCT_TM_TM_ZONE(and the obsoletedHAVE_TM_ZONE). Otherwise, if the external arraytznameis found, defineHAVE_TZNAME; if it is declared, defineHAVE_DECL_TZNAME.
These macros are used to find structure members not covered by the “particular” test macros.
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 submembers:
AC_CHECK_MEMBER(struct top.middle.bot)This macro caches its result in the
av_cv_member_aggregate_member variable, with characters not suitable for a variable name mapped to underscores.
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). If action-if-found is given, it is executed for each of the found members. If action-if-not-found is given, it is executed for each of the members that could not be found.includes is a series of include directives, defaulting to
AC_INCLUDES_DEFAULT(see Default Includes), which are used prior to the members under test.This macro uses M4 lists:
AC_CHECK_MEMBERS([struct stat.st_rdev, struct stat.st_blksize])
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.
These macros check for particular C types in sys/types.h, stdlib.h, stdint.h, inttypes.h and others, if they exist.
The Gnulib stdint module is an alternate way to define many of
these symbols; it is useful if you prefer your code to assume a
C99-or-better environment. See Gnulib.
Define
GETGROUPS_Tto be whichever ofgid_torintis the base type of the array argument togetgroups.This macro caches the base type in the
ac_cv_type_getgroupsvariable.
If stdint.h or inttypes.h does not define the type
int8_t, defineint8_tto a signed integer type that is exactly 8 bits wide and that uses two's complement representation, if such a type exists. If you are worried about porting to hosts that lack such a type, you can use the results of this macro in C89-or-later code as follows:#if HAVE_STDINT_H # include <stdint.h> #endif #if defined INT8_MAX || defined int8_t code using int8_t #else complicated alternative using >8-bit 'signed char' #endifThis macro caches the type in the
ac_cv_c_int8_tvariable.
If stdint.h or inttypes.h defines the type
intmax_t, defineHAVE_INTMAX_T. Otherwise, defineintmax_tto the widest signed integer type.
If stdint.h or inttypes.h defines the type
intptr_t, defineHAVE_INTPTR_T. Otherwise, defineintptr_tto a signed integer type wide enough to hold a pointer, if such a type exists.
If the C compiler supports a working
long doubletype, defineHAVE_LONG_DOUBLE. Thelong doubletype might have the same range and precision asdouble.This macro caches its result in the
ac_cv_type_long_doublevariable.This macro is obsolescent, as current C compilers support
long double. New programs need not use this macro.
If the C compiler supports a working
long doubletype with more range or precision than thedoubletype, defineHAVE_LONG_DOUBLE_WIDER.This macro caches its result in the
ac_cv_type_long_double_widervariable.
If the C compiler supports a working
long long inttype, defineHAVE_LONG_LONG_INT. However, this test does not testlong long intvalues in preprocessor#ifexpressions, because too many compilers mishandle such expressions. See Preprocessor Arithmetic.This macro caches its result in the
ac_cv_type_long_long_intvariable.
Define
HAVE_MBSTATE_Tif<wchar.h>declares thembstate_ttype. Also, definembstate_tto be a type if<wchar.h>does not declare it.This macro caches its result in the
ac_cv_type_mbstate_tvariable.
Define
mode_tto a suitable type, if standard headers do not define it.This macro caches its result in the
ac_cv_type_mode_tvariable.
Define
off_tto a suitable type, if standard headers do not define it.This macro caches its result in the
ac_cv_type_off_tvariable.
Define
pid_tto a suitable type, if standard headers do not define it.This macro caches its result in the
ac_cv_type_pid_tvariable.
Define
size_tto a suitable type, if standard headers do not define it.This macro caches its result in the
ac_cv_type_size_tvariable.
Define
ssize_tto a suitable type, if standard headers do not define it.This macro caches its result in the
ac_cv_type_ssize_tvariable.
Define
uid_tandgid_tto suitable types, if standard headers do not define them.This macro caches its result in the
ac_cv_type_uid_tvariable.
If stdint.h or inttypes.h does not define the type
uint8_t, defineuint8_tto an unsigned integer type that is exactly 8 bits wide, if such a type exists. This is likeAC_TYPE_INT8_T, except for unsigned integers.
If stdint.h or inttypes.h defines the type
uintmax_t, defineHAVE_UINTMAX_T. Otherwise, defineuintmax_tto the widest unsigned integer type.
If stdint.h or inttypes.h defines the type
uintptr_t, defineHAVE_UINTPTR_T. Otherwise, defineuintptr_tto an unsigned integer type wide enough to hold a pointer, if such a type exists.
If the C compiler supports a working
unsigned long long inttype, defineHAVE_UNSIGNED_LONG_LONG_INT. However, this test does not testunsigned long long intvalues in preprocessor#ifexpressions, because too many compilers mishandle such expressions. See Preprocessor Arithmetic.This macro caches its result in the
ac_cv_type_unsigned_long_long_intvariable.
These macros are used to check for types not covered by the “particular” test macros.
Check whether type is defined. It may be a compiler builtin type or defined by the includes. includes is a series of include directives, defaulting to
AC_INCLUDES_DEFAULT(see Default Includes), which are used prior to the type under test.In C, type must be a type-name, so that the expression ‘sizeof (type)’ is valid (but ‘sizeof ((type))’ is not). The same test is applied when compiling for C++, which means that in C++ type should be a type-id and should not be an anonymous ‘struct’ or ‘union’.
This macro caches its result in the
ac_cv_type_type variable, with ‘*’ mapped to ‘p’ and other characters not suitable for a variable name mapped to underscores.
For each type of the types that is defined, define
HAVE_type (in all capitals). Each type must follow the rules ofAC_CHECK_TYPE. 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 int, uintmax_t]) AC_CHECK_TYPES([float_t], [], [], [[#include <math.h>]])
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 heuristic, quite safe but not totally, is
implemented. In case of doubt, read the documentation of the former
AC_CHECK_TYPE, see Obsolete Macros.
All the tests for compilers (AC_PROG_CC, AC_PROG_CXX,
AC_PROG_F77) define the output variable EXEEXT based on
the output of the compiler, typically to the empty string if
Posix and ‘.exe’ if a DOS variant.
They also define the output variable OBJEXT based on the
output of the compiler, after .c files have been excluded, typically
to ‘o’ if Posix, ‘obj’ if a DOS variant.
If the compiler being used does not produce executables, the tests 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.
Some compilers exhibit different behaviors.
static int test_array[sizeof (int) == 4 ? 1 : -1];
To our knowledge, there is a single compiler that does not support this
trick: the HP C compilers (the real ones, not only the
“bundled”) on HP-UX 11.00.
They incorrectly reject the above program with the diagnostic
“Variable-length arrays cannot have static storage.”
This bug comes from HP compilers' mishandling of sizeof (int),
not from the ? 1 : -1, and
Autoconf works around this problem by casting sizeof (int) to
long int before comparing it.
Define
SIZEOF_type-or-expr (see Standard Symbols) to be the size in bytes of type-or-expr, which may be either a type or an expression returning a value that has a size. If the expression ‘sizeof (type-or-expr)’ is invalid, the result is 0. includes is a series of include directives, defaulting toAC_INCLUDES_DEFAULT(see Default Includes), which are used prior to the expression under test.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_Pto be 8 on DEC Alpha AXP systems.This macro caches its result in the
ac_cv_sizeof_type-or-expr variable, with ‘*’ mapped to ‘p’ and other characters not suitable for a variable name mapped to underscores.
Define
ALIGNOF_type (see Standard Symbols) to be the alignment in bytes of type. ‘type y;’ must be valid as a structure member declaration. If ‘type’ is unknown, the result is 0. If no includes are specified, the default includes are used (see Default Includes).This macro caches its result in the
ac_cv_alignof_type-or-expr variable, with ‘*’ mapped to ‘p’ and other characters not suitable for a variable name mapped to underscores.
Store into the shell variable var the value of the integer expression. The value should fit in an initializer in a C variable of type
signed long. To support cross compilation (in which case, the macro only works on hosts that use twos-complement arithmetic), it should be possible to evaluate the expression at compile-time. If no includes are specified, the default includes are used (see Default Includes).Execute action-if-fails if the value cannot be determined correctly.
Normally Autoconf ignores warnings generated by the compiler, linker, and preprocessor. If this macro is used, warnings count as fatal errors for the current language. This macro is useful when the results of configuration are used where warnings are unacceptable; for instance, if parts of a program are built with the GCC -Werror option. If the whole program is built using -Werror it is often simpler to put -Werror in the compiler flags (
CFLAGS, etc.).
OpenMP specifies extensions of C, C++, and Fortran that simplify optimization of shared memory parallelism, which is a common problem on multicore CPUs.
If the current language is C, the macro
AC_OPENMPsets the variableOPENMP_CFLAGSto the C compiler flags needed for supporting OpenMP.OPENMP_CFLAGSis set to empty if the compiler already supports OpenMP, if it has no way to activate OpenMP support, or if the user rejects OpenMP support by invoking ‘configure’ with the ‘--disable-openmp’ option.
OPENMP_CFLAGSneeds to be used when compiling programs, when preprocessing program source, and when linking programs. Therefore you need to add$(OPENMP_CFLAGS)to theCFLAGSof C programs that use OpenMP. If you preprocess OpenMP-specific C code, you also need to add$(OPENMP_CFLAGS)toCPPFLAGS. The presence of OpenMP support is revealed at compile time by the preprocessor macro_OPENMP.Linking a program with
OPENMP_CFLAGStypically adds one more shared library to the program's dependencies, so its use is recommended only on programs that actually require OpenMP.If the current language is C++,
AC_OPENMPsets the variableOPENMP_CXXFLAGS, suitably for the C++ compiler. The same remarks hold as for C.If the current language is Fortran 77 or Fortran,
AC_OPENMPsets the variableOPENMP_FFLAGSorOPENMP_FCFLAGS, respectively. Similar remarks as for C hold, except thatCPPFLAGSis not used for Fortran, and no preprocessor macro signals OpenMP support.For portability, it is best to avoid spaces between ‘#’ and ‘pragma omp’. That is, write ‘#pragma omp’, not ‘# pragma omp’. The Sun WorkShop 6.2 C compiler chokes on the latter.
The following macros provide ways to find and exercise a C Compiler. There are a few constructs that ought to be avoided, but do not deserve being checked for, since they can easily be worked around.
#ifdef __STDC__
/\
* A comment with backslash-newlines in it. %{ %} *\
\
/
char str[] = "\\
" A string with backslash-newlines in it %{ %} \\
"";
char apostrophe = '\\
\
'\
';
#endif
the compiler incorrectly fails with the diagnostics “Non-terminating
comment at end of file” and “Missing ‘#endif’ at end of file.”
Removing the lines with solitary backslashes solves the problem.
$ cc a.c b.c
a.c:
b.c:
This can cause problems if you observe the output of the compiler to
detect failures. Invoking ‘cc -c a.c && cc -c b.c && cc -o c a.o
b.o’ solves the issue.
#error failing#error "Unsupported word size"
it is more portable to use an invalid directive like #Unsupported
word size in Autoconf tests. In ordinary source code, #error is
OK, since installers with inadequate compilers like IRIX can simply
examine these compilers' diagnostic output.
#line support#line directives whose line
numbers are greater than 32767. Nothing in Posix
makes this invalid. That is why Autoconf stopped issuing
#line directives.
Determine a C compiler to use. If
CCis not already set in the environment, check forgccandcc, then for other C compilers. Set output variableCCto the name of the compiler found.This macro may, however, be invoked with an optional first argument which, if specified, must be a blank-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_CClike this:AC_PROG_CC([gcc cl cc])If the C compiler does not handle function prototypes correctly by default, try to add an option to output variable
CCto make it so. This macro tries various options that select standard-conformance modes on various systems.After calling this macro you can check whether the C compiler has been set to accept ANSI C89 (ISO C90); if not, the shell variable
ac_cv_prog_cc_c89is set to ‘no’. See alsoAC_C_PROTOTYPESbelow.If using the GNU C compiler, set shell variable
GCCto ‘yes’. If output variableCFLAGSwas not already set, set it to -g -O2 for the GNU C compiler (-O2 on systems where GCC does not accept -g), or -g for other compilers. If your package does not like this default, then it is acceptable to insert the line ‘: ${CFLAGS=""}’ afterAC_INITand beforeAC_PROG_CCto select an empty default instead.Many Autoconf macros use a compiler, and thus call ‘AC_REQUIRE([AC_PROG_CC])’ to ensure that the compiler has been determined before the body of the outermost
AC_DEFUNmacro. AlthoughAC_PROG_CCis safe to directly expand multiple times, it performs certain checks (such as the proper value of EXEEXT) only on the first invocation. Therefore, care must be used when invoking this macro from within another macro rather than at the top level (see Expanded Before Required).
If the C compiler does not accept the -c and -o options simultaneously, define
NO_MINUS_C_MINUS_O. This macro actually tests both the compiler found byAC_PROG_CC, and, if different, the firstccin the path. The test fails if one fails. This macro was created for GNU Make to choose the default C compilation rule.For the compiler compiler, this macro caches its result in the
ac_cv_prog_cc_compiler_c_ovariable.
Set output variable
CPPto a command that runs the C preprocessor. If ‘$CC -E’ doesn't work, /lib/cpp is used. It is only portable to runCPPon files with a .c extension.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. For most preprocessors, though, warnings do not cause include-file tests to fail unless
AC_PROG_CPP_WERRORis also specified.
This acts like
AC_PROG_CPP, except it treats warnings from the preprocessor as errors even if the preprocessor exit status indicates success. This is useful for avoiding headers that generate mandatory warnings, such as deprecation notices.
The following macros check for C compiler or machine architecture
features. To check for characteristics not listed here, use
AC_COMPILE_IFELSE (see Running the Compiler) or
AC_RUN_IFELSE (see Runtime).
If the C compiler cannot compile ISO Standard C (currently C99), try to add an option to output variable
CCto make it work. If the compiler does not support C99, fall back to supporting ANSI C89 (ISO C90).After calling this macro you can check whether the C compiler has been set to accept Standard C; if not, the shell variable
ac_cv_prog_cc_stdcis set to ‘no’.
If the C compiler is not in ANSI C89 (ISO C90) mode by default, try to add an option to output variable
CCto make it so. This macro tries various options that select ANSI C89 on some system or another, preferring extended functionality modes over strict conformance modes. It considers the compiler to be in ANSI C89 mode if it handles function prototypes correctly.After calling this macro you can check whether the C compiler has been set to accept ANSI C89; if not, the shell variable
ac_cv_prog_cc_c89is set to ‘no’.This macro is called automatically by
AC_PROG_CC.
If the C compiler is not in C99 mode by default, try to add an option to output variable
CCto make it so. This macro tries various options that select C99 on some system or another, preferring extended functionality modes over strict conformance modes. It considers the compiler to be in C99 mode if it handles_Bool,//comments, flexible array members,inline, signed and unsignedlong long int, mixed code and declarations, named initialization of structs,restrict,va_copy, varargs macros, variable declarations inforloops, and variable length arrays.After calling this macro you can check whether the C compiler has been set to accept C99; if not, the shell variable
ac_cv_prog_cc_c99is set to ‘no’.
Define ‘HAVE_C_BACKSLASH_A’ to 1 if the C compiler understands ‘\a’.
This macro is obsolescent, as current C compilers understand ‘\a’. New programs need not use this 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 least 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.
In some cases a single run of a compiler can generate code for multiple architectures. This can happen, for example, when generating Mac OS X universal binary files, which work on both PowerPC and Intel architectures. In this case, the different variants might be for different architectures whose endiannesses differ. If configure detects this, it executes action-if-universal instead of action-if-unknown.
The default for action-if-true is to define ‘WORDS_BIGENDIAN’. The default for action-if-false is to do nothing. The default for action-if-unknown is to abort configure and tell the installer how to bypass this test. And finally, the default for action-if-universal is to ensure that ‘WORDS_BIGENDIAN’ is defined if and only if a universal build is detected and the current code is big-endian; this default works only if autoheader is used (see autoheader Invocation).
If you use this macro without specifying action-if-universal, you should also use
AC_CONFIG_HEADERS; otherwise ‘WORDS_BIGENDIAN’ may be set incorrectly for Mac OS X universal binary files.
If the C compiler does not fully support the
constkeyword, defineconstto be empty. Some C compilers that do not define__STDC__do supportconst; some compilers that define__STDC__do not completely supportconst. Programs can simply useconstas if every C compiler supported it; for those that don't, the makefile or configuration header file defines 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++ treatconstdifferently. For example:const int foo;is valid in C but not in C++. These differences unfortunately cannot be papered over by defining
constto be empty.If autoconf detects this situation, it leaves
constalone, 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.This macro caches its result in the
ac_cv_c_constvariable.This macro is obsolescent, as current C compilers support
const. New programs need not use this macro.
If the C compiler recognizes a variant spelling for the
restrictkeyword (__restrict,__restrict__, or_Restrict), then definerestrictto that; this is more likely to do the right thing with compilers that support language variants where plainrestrictis not a keyword. Otherwise, if the C compiler recognizes therestrictkeyword, don't do anything. Otherwise, definerestrictto be empty. Thus, programs may simply userestrictas if every C compiler supported it; for those that do not, the makefile or configuration header defines it away.Although support in C++ for the
restrictkeyword is not required, several C++ compilers do accept the keyword. This macro works for them, too.This macro caches ‘no’ in the
ac_cv_c_restrictvariable ifrestrictis not supported, and a supported spelling otherwise.
If the C compiler does not understand the keyword
volatile, definevolatileto be empty. Programs can simply usevolatileas if every C compiler supported it; for those that do not, the makefile or configuration header defines 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 supportvolatile, you are at its mercy anyway. At least your program compiles, when it wouldn't before. See Volatile Objects, for more aboutvolatile.In general, the
volatilekeyword is a standard C feature, so you might expect thatvolatileis available only when__STDC__is defined. However, Ultrix 4.3's native compiler does support volatile, but does not define__STDC__.This macro is obsolescent, as current C compilers support
volatile. New programs need not use this macro.
If the C compiler supports the keyword
inline, do nothing. Otherwise defineinlineto__inline__or__inlineif it accepts one of those, otherwise defineinlineto be empty.
If the C type
charis unsigned, define__CHAR_UNSIGNED__, unless the C compiler predefines it.These days, using this macro is not necessary. The same information can be determined by this portable alternative, thus avoiding the use of preprocessor macros in the namespace reserved for the implementation.
#include <limits.h> #if CHAR_MIN == 0 # define CHAR_UNSIGNED 1 #endif
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) #yThis macro is obsolescent, as current C compilers support the stringizing operator. New programs need not use this macro.
If the C compiler supports flexible array members, define
FLEXIBLE_ARRAY_MEMBERto nothing; otherwise define it to 1. That way, a declaration like this:struct s { size_t n_vals; double val[FLEXIBLE_ARRAY_MEMBER]; };will let applications use the “struct hack” even with compilers that do not support flexible array members. To allocate and use such an object, you can use code like this:
size_t i; size_t n = compute_value_count (); struct s *p = malloc (offsetof (struct s, val) + n * sizeof (double)); p->n_vals = n; for (i = 0; i < n; i++) p->val[i] = compute_value (i);
If the C compiler supports variable-length arrays, define
HAVE_C_VARARRAYS. A variable-length array is an array of automatic storage duration whose length is determined at run time, when the array is declared.
If the C compiler supports GCC's
typeofsyntax either directly or through a different spelling of the keyword (e.g.,__typeof__), defineHAVE_TYPEOF. If the support is available only through a different spelling, definetypeofto that spelling.
If function prototypes are understood by the compiler (as determined by
AC_PROG_CC), definePROTOTYPESand__PROTOTYPES. Defining__PROTOTYPESis for the benefit of header files that cannot use macros that infringe on user name space.This macro is obsolescent, as current C compilers support prototypes. New programs need not use this macro.
Add -traditional to output variable
CCif using the GNU C compiler andioctldoes not work properly without -traditional. That usually happens when the fixed header files have not been installed on an old system.This macro is obsolescent, since current versions of the GNU C compiler fix the header files automatically when installed.
Determine a C++ compiler to use. Check whether the environment variable
CXXorCCC(in that order) is set; if so, then set output variableCXXto its value.Otherwise, if the macro is invoked without an argument, then search for a C++ compiler under the likely names (first
g++andc++then other names). If none of those checks succeed, then as a last resort setCXXtog++.This macro may, however, be invoked with an optional first argument which, if specified, must be a blank-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_CXXlike this:AC_PROG_CXX([gcc cl KCC CC cxx cc++ xlC aCC c++ g++])If using the GNU C++ compiler, set shell variable
GXXto ‘yes’. If output variableCXXFLAGSwas not already set, set it to -g -O2 for the GNU C++ compiler (-O2 on systems where G++ does not accept -g), or -g for other compilers. If your package does not like this default, then it is acceptable to insert the line ‘: ${CXXFLAGS=""}’ afterAC_INITand beforeAC_PROG_CXXto select an empty default instead.
Set output variable
CXXCPPto a command that runs the C++ preprocessor. If ‘$CXX -E’ doesn't work, /lib/cpp is used. It is portable to runCXXCPPonly on files with a .c, .C, .cc, or .cpp extension.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++.
Test whether the C++ compiler accepts the options -c and -o simultaneously, and define
CXX_NO_MINUS_C_MINUS_O, if it does not.
Determine an Objective C compiler to use. If
OBJCis not already set in the environment, check for Objective C compilers. Set output variableOBJCto the name of the compiler found.This macro may, however, be invoked with an optional first argument which, if specified, must be a blank-separated list of Objective C compilers to search for. This just gives the user an opportunity to specify an alternative search list for the Objective C compiler. For example, if you didn't like the default order, then you could invoke
AC_PROG_OBJClike this:AC_PROG_OBJC([gcc objcc objc])If using the GNU Objective C compiler, set shell variable
GOBJCto ‘yes’. If output variableOBJCFLAGSwas not already set, set it to -g -O2 for the GNU Objective C compiler (-O2 on systems where gcc does not accept -g), or -g for other compilers.
Set output variable
OBJCPPto a command that runs the Objective C preprocessor. If ‘$OBJC -E’ doesn't work, /lib/cpp is used.
Determine an Objective C++ compiler to use. If
OBJCXXis not already set in the environment, check for Objective C++ compilers. Set output variableOBJCXXto the name of the compiler found.This macro may, however, be invoked with an optional first argument which, if specified, must be a blank-separated list of Objective C++ compilers to search for. This just gives the user an opportunity to specify an alternative search list for the Objective C++ compiler. For example, if you didn't like the default order, then you could invoke
AC_PROG_OBJCXXlike this:AC_PROG_OBJCXX([gcc g++ objcc++ objcxx])If using the GNU Objective C++ compiler, set shell variable
GOBJCXXto ‘yes’. If output variableOBJCXXFLAGSwas not already set, set it to -g -O2 for the GNU Objective C++ compiler (-O2 on systems where gcc does not accept -g), or -g for other compilers.
Set output variable
OBJCXXCPPto a command that runs the Objective C++ preprocessor. If ‘$OBJCXX -E’ doesn't work, /lib/cpp is used.
Autoconf defines the following macros for determining paths to the essential Erlang/OTP programs:
Determine an Erlang compiler to use. If
ERLCis not already set in the environment, check for erlc. Set output variableERLCto the complete path of the compiler command found. In addition, ifERLCFLAGSis not set in the environment, set it to an empty value.The two optional arguments have the same meaning as the two last arguments of macro
AC_PROG_PATHfor looking for the erlc program. For example, to look for erlc only in the /usr/lib/erlang/bin directory:AC_ERLANG_PATH_ERLC([not found], [/usr/lib/erlang/bin])
A simplified variant of the
AC_ERLANG_PATH_ERLCmacro, that prints an error message and exits the configure script if the erlc program is not found.
Determine an Erlang interpreter to use. If
ERLis not already set in the environment, check for erl. Set output variableERLto the complete path of the interpreter command found.The two optional arguments have the same meaning as the two last arguments of macro
AC_PROG_PATHfor looking for the erl program. For example, to look for erl only in the /usr/lib/erlang/bin directory:AC_ERLANG_PATH_ERL([not found], [/usr/lib/erlang/bin])
A simplified variant of the
AC_ERLANG_PATH_ERLmacro, that prints an error message and exits the configure script if the erl program is not found.
The Autoconf Fortran support is divided into two categories: legacy
Fortran 77 macros (F77), and modern Fortran macros (FC).
The former are intended for traditional Fortran 77 code, and have output
variables like F77, FFLAGS, and FLIBS. The latter
are for newer programs that can (or must) compile under the newer
Fortran standards, and have output variables like FC,
FCFLAGS, and FCLIBS.
Except for the macros AC_FC_SRCEXT, AC_FC_FREEFORM,
AC_FC_FIXEDFORM, and AC_FC_LINE_LENGTH (see below), the
FC and F77 macros behave almost identically, and so they
are documented together in this section.
Determine a Fortran 77 compiler to use. If
F77is not already set in the environment, then check forg77andf77, and then some other names. Set the output variableF77to the name of the compiler found.This macro may, however, be invoked with an optional first argument which, if specified, must be a blank-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_F77like this:AC_PROG_F77([fl32 f77 fort77 xlf g77 f90 xlf90])If using
g77(the GNU Fortran 77 compiler), then set the shell variableG77to ‘yes’. If the output variableFFLAGSwas not already set in the environment, then set it to -g -02 forg77(or -O2 whereg77does not accept -g). Otherwise, setFFLAGSto -g for all other Fortran 77 compilers.
Determine a Fortran compiler to use. If
FCis not already set in the environment, thendialectis a hint to indicate what Fortran dialect to search for; the default is to search for the newest available dialect. Set the output variableFCto the name of the compiler found.By default, newer dialects are preferred over older dialects, but if
dialectis specified then older dialects are preferred starting with the specified dialect.dialectcan currently be one of Fortran 77, Fortran 90, or Fortran 95. However, this is only a hint of which compiler name to prefer (e.g.,f90orf95), and no attempt is made to guarantee that a particular language standard is actually supported. Thus, it is preferable that you avoid thedialectoption, and use AC_PROG_FC only for code compatible with the latest Fortran standard.This macro may, alternatively, be invoked with an optional first argument which, if specified, must be a blank-separated list of Fortran compilers to search for, just as in
AC_PROG_F77.If the output variable
FCFLAGSwas not already set in the environment, then set it to -g -02 for GNUg77(or -O2 whereg77does not accept -g). Otherwise, setFCFLAGSto -g for all other Fortran compilers.
Test whether the Fortran compiler accepts the options -c and -o simultaneously, and define
F77_NO_MINUS_C_MINUS_OorFC_NO_MINUS_C_MINUS_O, respectively, if it does not.
The following macros check for Fortran compiler characteristics.
To check for characteristics not listed here, use
AC_COMPILE_IFELSE (see Running the Compiler) or
AC_RUN_IFELSE (see Runtime), making sure to first set the
current language to Fortran 77 or Fortran via AC_LANG([Fortran 77])
or AC_LANG(Fortran) (see Language Choice).
Determine the linker flags (e.g., -L and -l) for the Fortran intrinsic and runtime libraries that are required to successfully link a Fortran program or shared library. The output variable
FLIBSorFCLIBSis set to these flags (which should be included afterLIBSwhen linking).This macro is intended to be used in those situations when it is necessary to mix, e.g., C++ and Fortran source code in a single program or shared library (see Mixing Fortran 77 With C and C++).
For example, if object files from a C++ and Fortran 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 intrinsic and runtime 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, this macro was created to determine these Fortran libraries.
The macros
AC_F77_DUMMY_MAINandAC_FC_DUMMY_MAINorAC_F77_MAINandAC_FC_MAINare probably also necessary to link C/C++ with Fortran; see below. Further, it is highly recommended that you useAC_CONFIG_HEADERS(see Configuration Headers) because the complex defines that the function wrapper macros create may not work with C/C++ compiler drivers.
With many compilers, the Fortran libraries detected by
AC_F77_LIBRARY_LDFLAGSorAC_FC_LIBRARY_LDFLAGSprovide their ownmainentry function that initializes things like Fortran I/O, and which then calls a user-provided entry function named (say)MAIN__to run the user's program. TheAC_F77_DUMMY_MAINandAC_FC_DUMMY_MAINorAC_F77_MAINandAC_FC_MAINmacros figure out how to deal with this interaction.When using Fortran for purely numerical functions (no I/O, etc.) often one prefers to provide one's own
mainand skip the Fortran library initializations. In this case, however, one may still need to provide a dummyMAIN__routine in order to prevent linking errors on some systems.AC_F77_DUMMY_MAINorAC_FC_DUMMY_MAINdetects whether any such routine is required for linking, and what its name is; the shell variableF77_DUMMY_MAINorFC_DUMMY_MAINholds this name,unknownwhen no solution was found, andnonewhen no such dummy main is needed.By default, action-if-found defines
F77_DUMMY_MAINorFC_DUMMY_MAINto 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(Replace
F77withFCfor Fortran instead of Fortran 77.)Note that this macro is called automatically from
AC_F77_WRAPPERSorAC_FC_WRAPPERS; there is generally no need to call it explicitly unless one wants to change the default actions.
As discussed above, many Fortran libraries allow you to provide an entry point called (say)
MAIN__instead of the usualmain, which is then called by amainfunction in the Fortran libraries that initializes things like Fortran I/O. TheAC_F77_MAINandAC_FC_MAINmacros detect whether it is possible to utilize such an alternate main function, and definesF77_MAINandFC_MAINto the name of the function. (If no alternate main function name is found,F77_MAINandFC_MAINare simply defined tomain.)Thus, when calling Fortran routines from C that perform things like I/O, one should use this macro and declare the "main" function like so:
#ifdef __cplusplus extern "C" #endif int F77_MAIN (int argc, char *argv[]);(Again, replace
F77withFCfor Fortran instead of Fortran 77.)
Defines C macros
F77_FUNC (name, NAME),FC_FUNC (name, NAME),F77_FUNC_(name, NAME), andFC_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 compiler.Fortran is case-insensitive, and in order to achieve this the Fortran compiler converts all identifiers into a canonical case and format. To call a Fortran subroutine from C or to write a C function that is callable from Fortran, the C program must explicitly use identifiers in the format expected by the Fortran compiler. In order to do this, one simply wraps all C identifiers in one of the macros provided by
AC_F77_WRAPPERSorAC_FC_WRAPPERS. For example, suppose you have the following Fortran 77 subroutine:subroutine foobar (x, y) double precision x, y y = 3.14159 * x return endYou 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_FUNCso 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++).(Replace
F77withFCfor Fortran instead of Fortran 77.)Although Autoconf tries to be intelligent about detecting the name-mangling scheme of the Fortran compiler, there may be Fortran compilers that it doesn't support yet. In this case, the above code generates a compile-time error, but some other behavior (e.g., disabling Fortran-related features) can be induced by checking whether
F77_FUNCorFC_FUNCis 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 identifier contains an underscore (e.g.,
foo_bar), you should useF77_FUNC_orFC_FUNC_instead ofF77_FUNCorFC_FUNC(with the same arguments). This is because some Fortran compilers mangle names differently if they contain an underscore.
Given an identifier name, set the shell variable shellvar to hold the mangled version name according to the rules of the Fortran linker (see also
AC_F77_WRAPPERSorAC_FC_WRAPPERS). shellvar is optional; if it is not supplied, the shell variable is 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++.
By default, the
FCmacros perform their tests using a .f extension for source-code files. Some compilers, however, only enable newer language features for appropriately named files, e.g., Fortran 90 features only for .f90 files. On the other hand, some other compilers expect all source files to end in .f and require special flags to support other file name extensions. TheAC_FC_SRCEXTmacro deals with both of these issues.The
AC_FC_SRCEXTtries to get theFCcompiler to accept files ending with the extension .ext (i.e., ext does not contain the dot). If any special compiler flags are needed for this, it stores them in the output variableFCFLAGS_ext. This extension and these flags are then used for all subsequentFCtests (untilAC_FC_SRCEXTis called again).For example, you would use
AC_FC_SRCEXT(f90)to employ the .f90 extension in future tests, and it would set theFCFLAGS_f90output variable with any extra flags that are needed to compile such files.The
FCFLAGS_ext can not be simply absorbed intoFCFLAGS, for two reasons based on the limitations of some compilers. First, only oneFCFLAGS_ext can be used at a time, so files with different extensions must be compiled separately. Second,FCFLAGS_ext must appear immediately before the source-code file name when compiling. So, continuing the example above, you might compile a foo.f90 file in your makefile with the command:foo.o: foo.f90 $(FC) -c $(FCFLAGS) $(FCFLAGS_f90) '$(srcdir)/foo.f90'If
AC_FC_SRCEXTsucceeds in compiling files with the ext extension, it calls action-if-success (defaults to nothing). If it fails, and cannot find a way to make theFCcompiler accept such files, it calls action-if-failure (defaults to exiting with an error message).
The
AC_FC_FREEFORMtries to ensure that the Fortran compiler ($FC) allows free-format source code (as opposed to the older fixed-format style from Fortran 77). If necessary, it may add some additional flags toFCFLAGS.This macro is most important if you are using the default .f extension, since many compilers interpret this extension as indicating fixed-format source unless an additional flag is supplied. If you specify a different extension with
AC_FC_SRCEXT, such as .f90, thenAC_FC_FREEFORMordinarily succeeds without modifyingFCFLAGS. For extensions which the compiler does not know about, the flag set by theAC_FC_SRCEXTmacro might let the compiler assume Fortran 77 by default, however.If
AC_FC_FREEFORMsucceeds in compiling free-form source, it calls action-if-success (defaults to nothing). If it fails, it calls action-if-failure (defaults to exiting with an error message).
The
AC_FC_FIXEDFORMtries to ensure that the Fortran compiler ($FC) allows the old fixed-format source code (as opposed to free-format style). If necessary, it may add some additional flags toFCFLAGS.This macro is needed for some compilers alias names like xlf95 which assume free-form source code by default, and in case you want to use fixed-form source with an extension like .f90 which many compilers interpret as free-form by default. If you specify a different extension with
AC_FC_SRCEXT, such as .f, thenAC_FC_FIXEDFORMordinarily succeeds without modifyingFCFLAGS.If
AC_FC_FIXEDFORMsucceeds in compiling fixed-form source, it calls action-if-success (defaults to nothing). If it fails, it calls action-if-failure (defaults to exiting with an error message).
The
AC_FC_LINE_LENGTHmacro tries to ensure that the Fortran compiler ($FC) accepts long source code lines. The length argument may be given as 80, 132, or unlimited, and defaults to 132. Note that line lengths above 254 columns are not portable, and some compilers do not accept more than 132 columns at least for fixed format source. If necessary, it may add some additional flags toFCFLAGS.If
AC_FC_LINE_LENGTHsucceeds in compiling fixed-form source, it calls action-if-success (defaults to nothing). If it fails, it calls action-if-failure (defaults to exiting with an error message).
The following macros check for operating system services or capabilities.
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(or an executable pointed to by theXMKMFenvironment variable) on a trivial Imakefile and examining the makefile that it produces. SettingXMKMFto ‘false’ disables this method.If this method fails to find the X Window System, configure looks for the files in several directories where they often reside. If either method is successful, set the shell variables
x_includesandx_librariesto 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_xto ‘yes’; otherwise set it to the empty string.
An enhanced version of
AC_PATH_X. It adds the C compiler flags that X needs to output variableX_CFLAGS, and the X linker flags toX_LIBS. DefineX_DISPLAY_MISSINGif 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 variableX_PRE_LIBS.
Check whether the system supports starting scripts with a line of the form ‘#!/bin/sh’ 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 is set to ‘yes’ if the system supports ‘#!’, ‘no’ if not.
Arrange for 64-bit file offsets, known as 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_BITSand_LARGE_FILESif 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_tis wider thanlong int, since this is common when large-file support is enabled. For example, it is not correct to print an arbitraryoff_tvalueXwithprintf ("%ld", (long int) X).The LFS introduced the
fseekoandftellofunctions to replace their C counterpartsfseekandftellthat do not useoff_t. Take care to useAC_FUNC_FSEEKOto make their prototypes available when using them and large-file support is enabled.
If the system supports file names longer than 14 characters, define
HAVE_LONG_FILE_NAMES.
Check to see if the Posix termios headers and functions are available on the system. If so, set the shell variable
ac_cv_sys_posix_termiosto ‘yes’. If not, set the variable to ‘no’.
The following macro makes it possible to use features of Posix that are extensions to C, as well as platform extensions not defined by Posix.
This macro was introduced in Autoconf 2.60. If possible, enable extensions to C or Posix on hosts that normally disable the extensions, typically due to standards-conformance namespace issues. This should be called before any macros that run the C compiler. The following preprocessor macros are defined where appropriate:
_GNU_SOURCE- Enable extensions on GNU/Linux.
__EXTENSIONS__- Enable general extensions on Solaris.
_POSIX_PTHREAD_SEMANTICS- Enable threading extensions on Solaris.
_TANDEM_SOURCE- Enable extensions for the HP NonStop platform.
_ALL_SOURCE- Enable extensions for AIX 3, and for Interix.
_POSIX_SOURCE- Enable Posix functions for Minix.
_POSIX_1_SOURCE- Enable additional Posix functions for Minix.
_MINIX- Identify Minix platform. This particular preprocessor macro is obsolescent, and may be removed in a future release of Autoconf.
The following macros check for an installation of Erlang/OTP, and for the presence of certain Erlang libraries. All those macros require the configuration of an Erlang interpreter and an Erlang compiler (see Erlang Compiler and Interpreter).
Set the output variable
ERLANG_ERTS_VERto the version of the Erlang runtime system (as returned by Erlang'serlang:system_info(version)function). The result of this test is cached if caching is enabled when running configure. TheERLANG_ERTS_VERvariable is not intended to be used for testing for features of specific ERTS versions, but to be used for substituting the ERTS version in Erlang/OTP release resource files (.relfiles), as shown below.
Set the output variable
ERLANG_ROOT_DIRto the path to the base directory in which Erlang/OTP is installed (as returned by Erlang'scode:root_dir/0function). The result of this test is cached if caching is enabled when running configure.
Set the output variable
ERLANG_LIB_DIRto the path of the library directory of Erlang/OTP (as returned by Erlang'scode:lib_dir/0function), which subdirectories each contain an installed Erlang/OTP library. The result of this test is cached if caching is enabled when running configure.
Test whether the Erlang/OTP library library is installed by calling Erlang's
code:lib_dir/1function. The result of this test is cached if caching is enabled when running configure. action-if-found is a list of shell commands to run if the library is installed; action-if-not-found is a list of shell commands to run if it is not. Additionally, if the library is installed, the output variable ‘ERLANG_LIB_DIR_library’ is set to the path to the library installation directory, and the output variable ‘ERLANG_LIB_VER_library’ is set to the version number that is part of the subdirectory name, if it is in the standard form (library-version). If the directory name does not have a version part, ‘ERLANG_LIB_VER_library’ is set to the empty string. If the library is not installed, ‘ERLANG_LIB_DIR_library’ and ‘ERLANG_LIB_VER_library’ are set to"not found". For example, to check if librarystdlibis installed:AC_ERLANG_CHECK_LIB([stdlib], [echo "stdlib version \"$ERLANG_LIB_VER_stdlib\"" echo "is installed in \"$ERLANG_LIB_DIR_stdlib\""], [AC_MSG_ERROR([stdlib was not found!])])The ‘ERLANG_LIB_VER_library’ variables (set by
AC_ERLANG_CHECK_LIB) and theERLANG_ERTS_VERvariable (set byAC_ERLANG_SUBST_ERTS_VER) are not intended to be used for testing for features of specific versions of libraries or of the Erlang runtime system. Those variables are intended to be substituted in Erlang release resource files (.relfiles). For instance, to generate a example.rel file for an application depending on thestdliblibrary, configure.ac could contain:AC_ERLANG_SUBST_ERTS_VER AC_ERLANG_CHECK_LIB([stdlib], [], [AC_MSG_ERROR([stdlib was not found!])]) AC_CONFIG_FILES([example.rel])The example.rel.in file used to generate example.rel should contain:
{release, {"@PACKAGE@", "@VERSION@"}, {erts, "@ERLANG_ERTS_VER@"}, [{stdlib, "@ERLANG_LIB_VER_stdlib@"}, {@PACKAGE@, "@VERSION@"}]}.
In addition to the above macros, which test installed Erlang libraries, the following macros determine the paths to the directories into which newly built Erlang libraries are to be installed:
Set the
ERLANG_INSTALL_LIB_DIRoutput variable to the directory into which every built Erlang library should be installed in a separate subdirectory. If this variable is not set in the environment when configure runs, its default value is${libdir}/erlang/lib.
Set the ‘ERLANG_INSTALL_LIB_DIR_library’ output variable to the directory into which the built Erlang library library version version should be installed. If this variable is not set in the environment when configure runs, its default value is ‘$ERLANG_INSTALL_LIB_DIR/library-version’, the value of the
ERLANG_INSTALL_LIB_DIRvariable being set by theAC_ERLANG_SUBST_INSTALL_LIB_DIRmacro.
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 compiler system of the current language (see Language Choice). 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 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.
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.
Do compilation tests using the compiler, preprocessor, and file extensions for the specified language.
Supported languages are:
- ‘C’
- Do compilation tests using
CCandCPPand use extension .c for test programs. Use compilation flags:CPPFLAGSwithCPP, and bothCPPFLAGSandCFLAGSwithCC.- ‘C++’
- Do compilation tests using
CXXandCXXCPPand use extension .C for test programs. Use compilation flags:CPPFLAGSwithCXXCPP, and bothCPPFLAGSandCXXFLAGSwithCXX.- ‘Fortran 77’
- Do compilation tests using
F77and use extension .f for test programs. Use compilation flags:FFLAGS.- ‘Fortran’
- Do compilation tests using
FCand use extension .f (or whatever has been set byAC_FC_SRCEXT) for test programs. Use compilation flags:FCFLAGS.- ‘Erlang’
- Compile and execute tests using
ERLCandERLand use extension .erl for test Erlang modules. Use compilation flags:ERLCFLAGS.- ‘Objective C’
- Do compilation tests using
OBJCandOBJCPPand use extension .m for test programs. Use compilation flags:CPPFLAGSwithOBJCPP, and bothCPPFLAGSandOBJCFLAGSwithOBJC.- ‘Objective C++’
- Do compilation tests using
OBJCXXandOBJCXXCPPand use extension .mm for test programs. Use compilation flags:CPPFLAGSwithOBJCXXCPP, and bothCPPFLAGSandOBJCXXFLAGSwithOBJCXX.
Remember the current language (as set by
AC_LANG) on a stack, and then select the language. Use this macro andAC_LANG_POPin macros that need to temporarily switch to a particular language.
Select the language that is saved on the top of the stack, as set by
AC_LANG_PUSH, and remove it from the stack.If given, language specifies the language we just quit. It is a good idea to specify it when it's known (which should be the case...), since Autoconf detects inconsistencies.
AC_LANG_PUSH([Fortran 77]) # Perform some tests on Fortran 77. # ... AC_LANG_POP([Fortran 77])
Check statically that the current language is language. You should use this in your language specific macros to avoid that they be called with an inappropriate language.
This macro runs only at autoconf time, and incurs no cost at configure time. Sadly enough and because Autoconf is a two layer language 2, the macros
AC_LANG_PUSHandAC_LANG_POPcannot be “optimizing”, therefore as much as possible you ought to avoid using them to wrap your code, rather, require from the user to run the macro with a correct current language, and check it withAC_LANG_ASSERT. And anyway, that may help the user understand she is running a Fortran macro while expecting a result about her Fortran 77 compiler...
Ensure that whichever preprocessor would currently be used for tests has been found. Calls
AC_REQUIRE(see Prerequisite Macros) with an argument of eitherAC_PROG_CPPorAC_PROG_CXXCPP, depending on which language is current.
Autoconf tests follow a common scheme: feed some program with some input, and most of the time, feed a compiler with some source file. This section is dedicated to these source samples.
The most important rule to follow when writing testing samples is:
This motto means that testing samples must be written with the same strictness as real programs are written. In particular, you should avoid “shortcuts” and simplifications.
Don't just play with the preprocessor if you want to prepare a compilation. For instance, using cpp to check whether a header is functional might let your configure accept a header which causes some compiler error. Do not hesitate to check a header with other headers included before, especially required headers.
Make sure the symbols you use are properly defined, i.e., refrain from simply declaring a function yourself instead of including the proper header.
Test programs should not write to standard output. They
should exit with status 0 if the test succeeds, and with status 1
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. Unless you arrange for exit to be declared, test
programs should return, not exit, from main,
because on many systems exit is not declared by default.
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_STDBOOL, then later on in
configure.ac you can have a test program that includes
stdbool.h conditionally:
#ifdef HAVE_STDBOOL_H
# include <stdbool.h>
#endif
Both #if HAVE_STDBOOL_H and #ifdef HAVE_STDBOOL_H will
work with any standard C compiler. Some developers prefer #if
because it is easier to read, while others prefer #ifdef because
it avoids diagnostics with picky compilers like GCC with the
-Wundef option.
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 -f -r conftest*’ after running test programs and if the script is interrupted.
These days it's safe to assume support for function prototypes (introduced in C89).
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"
#endif
void *valloc (size_t);
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, be 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).
Autoconf provides a set of macros that can be used to generate test source files. They are written to be language generic, i.e., they actually depend on the current language (see Language Choice) to “format” the output properly.
Save the source text in the current test source file: conftest.extension where the extension depends on the current language. As of Autoconf 2.63b, the source file also contains the results of all of the
AC_DEFINEperformed so far.Note that the source is evaluated exactly once, like regular Autoconf macro arguments, and therefore (i) you may pass a macro invocation, (ii) if not, be sure to double quote if needed.
This macro issues a warning during autoconf processing if source does not include an expansion of the macro
AC_LANG_DEFINES_PROVIDED(note that bothAC_LANG_SOURCEandAC_LANG_PROGRAMcall this macro, and thus avoid the warning).This macro is seldom called directly, but is used under the hood by more common macros such as
AC_COMPILE_IFELSEandAC_RUN_IFELSE.
This macro is called as a witness that the file conftest.extension appropriate for the current language is complete, including all previously determined results from
AC_DEFINE. This macro is seldom called directly, but exists if you have a compelling reason to write a conftest file without usingAC_LANG_SOURCE, yet still want to avoid a syntax warning fromAC_LANG_CONFTEST.
Expands into the source, with the definition of all the
AC_DEFINEperformed so far. This macro includes an expansion ofAC_LANG_DEFINES_PROVIDED.In many cases, you may find it more convenient to use the wrapper
AC_LANG_PROGRAM.
For instance, executing (observe the double quotation!):
AC_INIT([Hello], [1.0], [bug-hello@example.org], [],
[http://www.example.org/])
AC_DEFINE([HELLO_WORLD], ["Hello, World\n"],
[Greetings string.])
AC_LANG([C])
AC_LANG_CONFTEST(
[AC_LANG_SOURCE([[const char hw[] = "Hello, World\n";]])])
gcc -E -dD conftest.c
on a system with gcc installed, results in:
...
# 1 "conftest.c"
#define PACKAGE_NAME "Hello"
#define PACKAGE_TARNAME "hello"
#define PACKAGE_VERSION "1.0"
#define PACKAGE_STRING "Hello 1.0"
#define PACKAGE_BUGREPORT "bug-hello@example.org"
#define PACKAGE_URL "http://www.example.org/"
#define HELLO_WORLD "Hello, World\n"
const char hw[] = "Hello, World\n";
When the test language is Fortran or Erlang, the AC_DEFINE definitions
are not automatically translated into constants in the source code by this
macro.
Expands into a source file which consists of the prologue, and then body as body of the main function (e.g.,
mainin C). Since it usesAC_LANG_SOURCE, the features of the latter are available.
For instance:
AC_INIT([Hello], [1.0], [bug-hello@example.org], [],
[http://www.example.org/])
AC_DEFINE([HELLO_WORLD], ["Hello, World\n"],
[Greetings string.])
AC_LANG_CONFTEST(
[AC_LANG_PROGRAM([[const char hw[] = "Hello, World\n";]],
[[fputs (hw, stdout);]])])
gcc -E -dD conftest.c
on a system with gcc installed, results in:
...
# 1 "conftest.c"
#define PACKAGE_NAME "Hello"
#define PACKAGE_TARNAME "hello"
#define PACKAGE_VERSION "1.0"
#define PACKAGE_STRING "Hello 1.0"
#define PACKAGE_BUGREPORT "bug-hello@example.org"
#define PACKAGE_URL "http://www.example.org/"
#define HELLO_WORLD "Hello, World\n"
const char hw[] = "Hello, World\n";
int
main ()
{
fputs (hw, stdout);
;
return 0;
}
In Erlang tests, the created source file is that of an Erlang module called
conftest (conftest.erl). This module defines and exports
at least
one start/0 function, which is called to perform the test. The
prologue is optional code that is inserted between the module header and
the start/0 function definition. body is the body of the
start/0 function without the final period (see Runtime, about
constraints on this function's behavior).
For instance:
AC_INIT([Hello], [1.0], [bug-hello@example.org])
AC_LANG(Erlang)
AC_LANG_CONFTEST(
[AC_LANG_PROGRAM([[-define(HELLO_WORLD, "Hello, world!").]],
[[io:format("~s~n", [?HELLO_WORLD])]])])
cat conftest.erl
results in:
-module(conftest).
-export([start/0]).
-define(HELLO_WORLD, "Hello, world!").
start() ->
io:format("~s~n", [?HELLO_WORLD])
.
Expands into a source file which consists of the prologue, and then a call to the function as body of the main function (e.g.,
mainin C). Since it usesAC_LANG_PROGRAM, the feature of the latter are available.This function will probably be replaced in the future by a version which would enable specifying the arguments. The use of this macro is not encouraged, as it violates strongly the typing system.
This macro cannot be used for Erlang tests.
Expands into a source file which uses the function in the body of the main function (e.g.,
mainin C). Since it usesAC_LANG_PROGRAM, the features of the latter are available.As
AC_LANG_CALL, this macro is documented only for completeness. It is considered to be severely broken, and in the future will be removed in favor of actual function calls (with properly typed arguments).This macro cannot be used for Erlang tests.
Sometimes one might need to run the preprocessor on some source file. Usually it is a bad idea, as you typically need to compile your project, not merely run the preprocessor on it; therefore you certainly want to run the compiler, not the preprocessor. Resist the temptation of following the easiest path.
Nevertheless, if you need to run the preprocessor, then use
AC_PREPROC_IFELSE.
The macros described in this section cannot be used for tests in Erlang or Fortran, since those languages require no preprocessor.
Run the preprocessor of the current language (see Language Choice) on the input, run the shell commands action-if-true on success, action-if-false otherwise. The input can be made by
AC_LANG_PROGRAMand friends.This macro uses
CPPFLAGS, but notCFLAGS, because -g, -O, etc. are not valid options to many C preprocessors.It is customary to report unexpected failures with
AC_MSG_FAILURE. If needed, action-if-true can further access the preprocessed output in the file conftest.i.
For instance:
AC_INIT([Hello], [1.0], [bug-hello@example.org])
AC_DEFINE([HELLO_WORLD], ["Hello, World\n"],
[Greetings string.])
AC_PREPROC_IFELSE(
[AC_LANG_PROGRAM([[const char hw[] = "Hello, World\n";]],
[[fputs (hw, stdout);]])],
[AC_MSG_RESULT([OK])],
[AC_MSG_FAILURE([unexpected preprocessor failure])])
results in:
checking for gcc... gcc
checking for C compiler default output file name... a.out
checking whether the C compiler works... yes
checking whether we are cross compiling... no
checking for suffix of executables...
checking for suffix of object files... o
checking whether we are using the GNU C compiler... yes
checking whether gcc accepts -g... yes
checking for gcc option to accept ISO C89... none needed
checking how to run the C preprocessor... gcc -E
OK
The macro
AC_TRY_CPP (see Obsolete Macros) used to play the
role of AC_PREPROC_IFELSE, but double quotes its argument, making
it impossible to use it to elaborate sources. You are encouraged to
get rid of your old use of the macro AC_TRY_CPP in favor of
AC_PREPROC_IFELSE, but, in the first place, are you sure you need
to run the preprocessor and not the compiler?
If the output of running the preprocessor on the system header file header-file matches the extended regular expression pattern, execute shell commands action-if-found, otherwise execute action-if-not-found.
program is the text of a C or C++ program, on which shell variable, back quote, and backslash substitutions are performed. If the output of running the preprocessor on program matches the extended regular expression pattern, execute shell commands action-if-found, otherwise execute action-if-not-found.
To check for a syntax feature of the current language's (see Language Choice) compiler, such as whether it recognizes a certain keyword, or
simply to try some library feature, use AC_COMPILE_IFELSE to try
to compile a small program that uses that feature.
Run the compiler and compilation flags of the current language (see Language Choice) on the input, run the shell commands action-if-true on success, action-if-false otherwise. The input can be made by
AC_LANG_PROGRAMand friends.It is customary to report unexpected failures with
AC_MSG_FAILURE. This macro does not try to link; useAC_LINK_IFELSEif you need to do that (see Running the Linker). If needed, action-if-true can further access the just-compiled object file conftest.$OBJEXT.This macro uses
AC_REQUIREfor the compiler associated with the current language, which means that if the compiler has not yet been determined, the compiler determination will be made prior to the body of the outermustAC_DEFUNmacro that triggered this macro to expand (see Expanded Before Required).
For tests in Erlang, the input must be the source code of a module named
conftest. AC_COMPILE_IFELSE generates a conftest.beam
file that can be interpreted by the Erlang virtual machine (ERL). It is
recommended to use AC_LANG_PROGRAM to specify the test program,
to ensure that the Erlang module has the right name.
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, but accuracy is more
important than speed.
AC_LINK_IFELSE 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.
Run the compiler (and compilation flags) and the linker of the current language (see Language Choice) on the input, run the shell commands action-if-true on success, action-if-false otherwise. The input can be made by
AC_LANG_PROGRAMand friends. If needed, action-if-true can further access the just-linked program file conftest$EXEEXT.
LDFLAGSandLIBSare used for linking, in addition to the current compilation flags.It is customary to report unexpected failures with
AC_MSG_FAILURE. This macro does not try to execute the program; useAC_RUN_IFELSEif you need to do that (see Runtime).
The AC_LINK_IFELSE macro cannot be used for Erlang tests, since Erlang
programs are interpreted and do not require linking.
Sometimes you need to find out how a system performs at runtime, such as whether a given function has a certain capability or bug. If you can, make such checks when your program runs instead of when it is configured. You can check for things like the machine's endianness when your program initializes itself.
If you really need to test for a runtime behavior while configuring,
you can write a test program to determine the result, and compile and
run it using AC_RUN_IFELSE. Avoid running test programs if
possible, because this prevents people from configuring your package for
cross-compiling.
If program compiles and links successfully and returns an exit status of 0 when executed, run shell commands action-if-true. Otherwise, run shell commands action-if-false.
The input can be made by
AC_LANG_PROGRAMand friends.LDFLAGSandLIBSare used for linking, in addition to the compilation flags of the current language (see Language Choice). Additionally, action-if-true can run ./conftest$EXEEXT for further testing.If the 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 failing exit status is available in the shell variable ‘$?’. This exit status might be that of a failed compilation, or it might be that of a failed program execution.
It is customary to report unexpected failures with
AC_MSG_FAILURE.
Try to provide a pessimistic default value to use when cross-compiling
makes runtime tests impossible. You do this by passing the optional
last argument to AC_RUN_IFELSE. autoconf prints a
warning message when creating configure each time it
encounters a call to AC_RUN_IFELSE with no
action-if-cross-compiling argument given. You may ignore the
warning, though users cannot configure your package for
cross-compiling. A few of the macros distributed with Autoconf produce
this warning message.
To configure for cross-compiling you can also choose a value for those parameters based on the canonical system name (see Manual Configuration). Alternatively, set up a test results cache file with the correct values for the host system (see Caching Results).
To provide a default for calls of AC_RUN_IFELSE that are embedded
in other macros, including a few of the ones that come with Autoconf,
you can test whether the shell variable cross_compiling is set to
‘yes’, and then use an alternate method to get the results instead
of calling the macros.
It is also permissible to temporarily assign to cross_compiling
in order to force tests to behave as though they are in a
cross-compilation environment, particularly since this provides a way to
test your action-if-cross-compiling even when you are not using a
cross-compiler.
# We temporarily set cross-compile mode to force AC_COMPUTE_INT
# to use the slow link-only method
save_cross_compiling=$cross_compiling
cross_compiling=yes
AC_COMPUTE_INT([...])
cross_compiling=$save_cross_compiling
A C or C++ runtime test should be portable. See Portable C and C++.
Erlang tests must exit themselves the Erlang VM by calling the halt/1
function: the given status code is used to determine the success of the test
(status is 0) or its failure (status is different than 0), as
explained above. It must be noted that data output through the standard output
(e.g., using io:format/2) may be truncated when halting the VM.
Therefore, if a test must output configuration information, it is recommended
to create and to output data into the temporary file named conftest.out,
using the functions of module file. The conftest.out file is
automatically deleted by the AC_RUN_IFELSE macro. For instance, a
simplified implementation of Autoconf's AC_ERLANG_SUBST_LIB_DIR
macro is:
AC_INIT([LibdirTest], [1.0], [bug-libdirtest@example.org])
AC_ERLANG_NEED_ERL
AC_LANG(Erlang)
AC_RUN_IFELSE(
[AC_LANG_PROGRAM([], [dnl
file:write_file("conftest.out", code:lib_dir()),
halt(0)])],
[echo "code:lib_dir() returned: `cat conftest.out`"],
[AC_MSG_FAILURE([test Erlang program execution failed])])
This section aims at presenting some systems and pointers to documentation. It may help you addressing particular problems reported by users.
Posix-conforming systems are derived from the Unix operating system.
The Rosetta Stone for Unix contains a table correlating the features of various Posix-conforming systems. Unix History is a simplified diagram of how many Unix systems were derived from each other.
The Heirloom Project provides some variants of traditional implementations of Unix utilities.
That's all dependent on whether the file system is a UFS (case
sensitive) or HFS+ (case preserving). By default Apple wants you to
install the OS on HFS+. Unfortunately, there are some pieces of
software which really need to be built on UFS. We may want to rebuild
Darwin to have both UFS and HFS+ available (and put the /local/build
tree on the UFS).
Some operations are accomplished in several possible ways, depending on the OS 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. Note that
since the value of fstype is under our control, we don't have to
use the longer ‘test "x$fstype" = xno’.
AC_MSG_CHECKING([how to get file system type])
fstype=no
# The order of these tests is important.
AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <sys/statvfs.h>
#include <sys/fstyp.h>]])],
[AC_DEFINE([FSTYPE_STATVFS], [1],
[Define if statvfs exists.])
fstype=SVR4])
if test $fstype = no; then
AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <sys/statfs.h>
#include <sys/fstyp.h>]])],
[AC_DEFINE([FSTYPE_USG_STATFS], [1],
[Define if USG statfs.])
fstype=SVR3])
fi
if test $fstype = no; then
AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <sys/statfs.h>
#include <sys/vmount.h>]])]),
[AC_DEFINE([FSTYPE_AIX_STATFS], [1],
[Define if AIX statfs.])
fstype=AIX])
fi
# (more cases omitted here)
AC_MSG_RESULT([$fstype])
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.
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], [1],
[Define if vprintf exists.])])
if test "x$ac_cv_func_vprintf" != xyes; then
AC_CHECK_FUNC([_doprnt], [AC_DEFINE([HAVE_DOPRNT], [1],
[Define if _doprnt exists.])])
fi
If AC_CONFIG_HEADERS has been called, then instead of creating
DEFS, AC_OUTPUT creates a header file by substituting the
correct values into #define statements in a template file.
See Configuration Headers, for more information about this kind of
output.
Define variable to value (verbatim), by defining a C preprocessor macro for variable. variable should be a C identifier, optionally suffixed by a parenthesized argument list to define a C preprocessor macro with arguments. The macro argument list, if present, should be a comma-separated list of C identifiers, possibly terminated by an ellipsis ‘...’ if C99 syntax is employed. variable should not contain comments, white space, trigraphs, backslash-newlines, universal character names, or non-ASCII characters.
value may contain backslash-escaped newlines, which will be preserved if you use
AC_CONFIG_HEADERSbut flattened if passed via@DEFS@(with no effect on the compilation, since the preprocessor sees only one line in the first place). value should not contain raw newlines. If you are not usingAC_CONFIG_HEADERS, value should not contain any ‘#’ characters, as make tends to eat them. To use a shell variable, useAC_DEFINE_UNQUOTEDinstead.description is only useful if you are using
AC_CONFIG_HEADERS. In this case, description is put into the generated config.h.in as the comment before the macro define. The following example defines the C preprocessor variableEQUATIONto be the string constant ‘"$a > $b"’:AC_DEFINE([EQUATION], ["$a > $b"], [Equation string.])If neither value nor description are given, then value defaults to 1 instead of to the empty string. This is for backwards compatibility with older versions of Autoconf, but this usage is obsolescent and may be withdrawn in future versions of Autoconf.
If the variable is a literal string, it is passed to
m4_pattern_allow(see Forbidden Patterns).If multiple
AC_DEFINEstatements are executed for the same variable name (not counting any parenthesized argument list), the last one wins.
Like
AC_DEFINE, but three shell expansions are performed—once—on variable and value: variable expansion (‘$’), command substitution (‘`’), and backslash escaping (‘\’), as if in an unquoted here-document. Single and double quote characters in the value have no special meaning. Use this macro instead ofAC_DEFINEwhen variable or value is a shell variable. Examples:AC_DEFINE_UNQUOTED([config_machfile], ["$machfile"], [Configuration machine file.]) AC_DEFINE_UNQUOTED([GETGROUPS_T], [$ac_cv_type_getgroups], [getgroups return type.]) AC_DEFINE_UNQUOTED([$ac_tr_hdr], [1], [Translated header name.])
Due to a 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 blanks or
newlines. That is, do this:
AC_CHECK_HEADER([elf.h],
[AC_DEFINE([SVR4], [1], [System V Release 4]) LIBS="-lelf $LIBS"])
or this:
AC_CHECK_HEADER([elf.h],
[AC_DEFINE([SVR4], [1], [System V Release 4])
LIBS="-lelf $LIBS"])
instead of this:
AC_CHECK_HEADER([elf.h],
[AC_DEFINE([SVR4], [1], [System V Release 4]); LIBS="-lelf $LIBS"])
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.
Create an output variable from a shell variable. Make
AC_OUTPUTsubstitute the variable variable into output files (typically one or more makefiles). This means thatAC_OUTPUTreplaces instances of ‘@variable@’ in input files with the value that the shell variable variable has whenAC_OUTPUTis called. The value can contain any non-NULcharacter, including newline. If you are using Automake 1.11 or newer, for newlines in values you might want to consider usingAM_SUBST_NOTMAKEto prevent automake from adding a line variable= @variable@to the Makefile.in files (see Automake).Variable occurrences should not overlap: e.g., an input file should not contain ‘@var1@var2@’ if var1 and var2 are variable names. The substituted value is not rescanned for more output variables; occurrences of ‘@variable@’ in the value are inserted literally into the output file. (The algorithm uses the special marker
|#_!!_#|internally, so neither the substituted value nor the output file may contain|#_!!_#|.)If value is given, in addition assign it to variable.
The string variable is passed to
m4_pattern_allow(see Forbidden Patterns).
Another way to create an output variable from a shell variable. Make
AC_OUTPUTinsert (without substitutions) the contents of the file named by shell variable variable into output files. This means thatAC_OUTPUTreplaces instances of ‘@variable@’ in output files (such as Makefile.in) with the contents of the file that the shell variable variable names whenAC_OUTPUTis called. Set the variable to /dev/null for cases that do not have a file to insert. This substitution occurs only when the ‘@variable@’ is on a line by itself, optionally surrounded by spaces and tabs. The substitution replaces the whole line, including the spaces, tabs, and the terminating newline.This macro is useful for inserting makefile fragments containing special dependencies or other make directives for particular host or target types into makefiles. For example, configure.ac could contain:
AC_SUBST_FILE([host_frag]) host_frag=$srcdir/conf/sun4.mhand then a Makefile.in could contain:
@host_frag@The string variable is passed to
m4_pattern_allow(see Forbidden Patterns).
Running configure in varying 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 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.
Environment variables that affect this situation, such as ‘CC’
above, are called precious variables, and can be declared as such
by AC_ARG_VAR.
Declare variable is a precious variable, and include its description in the variable section of ‘./configure --help’.
Being precious means that
- variable is substituted via
AC_SUBST.- The value of variable when configure was launched is saved in the cache, including if it was not specified on the command line but via the environment. Indeed, while configure can notice the definition of
CCin ‘./configure CC=bizarre-cc’, it is impossible to notice it in ‘CC=bizarre-cc ./configure’, which, unfortunately, is what most users do.We emphasize that it is the initial value of variable which is saved, not that found during the execution of configure. Indeed, specifying ‘./configure FOO=foo’ and letting ‘./configure’ guess that
FOOisfoocan be two different things.- 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 overand similarly if the variable is unset, or if its content is changed. If the content has white space changes only, then the error is degraded to a warning only, but the old value is reused.
- 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 var=raboof --silent $ ./config.status --recheck running CONFIG_SHELL=/bin/sh /bin/sh ./configure var=raboof \ CC=/usr/bin/cc --no-create --no-recursion
Many output variables are intended to be evaluated both by make and by the shell. Some characters are expanded differently in these two contexts, so to avoid confusion these variables' values should not contain any of the following characters:
" # $ & ' ( ) * ; < > ? [ \ ^ ` |
Also, these variables' values should neither contain newlines, nor start with ‘~’, nor contain white space or ‘:’ immediately followed by ‘~’. The values can contain nonempty sequences of white space characters like tabs and spaces, but each such sequence might arbitrarily be replaced by a single space during substitution.
These restrictions apply both to the values that configure
computes, and to the values set directly by the user. For example, the
following invocations of configure are problematic, since they
attempt to use special characters within CPPFLAGS and white space
within $(srcdir):
CPPFLAGS='-DOUCH="&\"#$*?"' '../My Source/ouch-1.0/configure'
'../My Source/ouch-1.0/configure' CPPFLAGS='-DOUCH="&\"#$*?"'
To avoid checking for the same features repeatedly in various configure scripts (or in repeated runs of one script), configure can optionally save the results of many checks in a cache file (see Cache Files). If a configure script runs with caching enabled and finds a cache file, it reads the results of previous runs from the cache and avoids rerunning those checks. As a result, configure can then run much faster than if it had to perform all of the checks every time.
Ensure that the results of the check identified by cache-id are available. If the results of the check were in the cache file that was read, and configure was not given the --quiet or --silent option, print a message saying that the result was cached; otherwise, run the shell commands commands-to-set-it. If the shell commands are run to determine the value, the value is saved in the cache file just before configure creates its output files. See Cache Variable Names, for how to choose the name of the cache-id variable.
The commands-to-set-it must have no side effects except for setting the variable cache-id, see below.
A wrapper for
AC_CACHE_VALthat takes care of printing the messages. This macro provides a convenient shorthand for the most common way to use these macros. It callsAC_MSG_CHECKINGfor message, thenAC_CACHE_VALwith the cache-id and commands arguments, andAC_MSG_RESULTwith cache-id.The commands-to-set-it must have no side effects except for setting the variable cache-id, see below.
It is common to find buggy macros using AC_CACHE_VAL or
AC_CACHE_CHECK, because people are tempted to call
AC_DEFINE in the commands-to-set-it. Instead, the code that
follows the call to AC_CACHE_VAL should call
AC_DEFINE, by examining the value of the cache variable. For
instance, the following macro is broken:
AC_DEFUN([AC_SHELL_TRUE],
[AC_CACHE_CHECK([whether true(1) works], [my_cv_shell_true_works],
[my_cv_shell_true_works=no
(true) 2>/dev/null && my_cv_shell_true_works=yes
if test "x$my_cv_shell_true_works" = xyes; 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], [my_cv_shell_true_works],
[my_cv_shell_true_works=no
(true) 2>/dev/null && my_cv_shell_true_works=yes])
if test "x$my_cv_shell_true_works" = xyes; 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.
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:
_cv_The values assigned to cache variables may not contain newlines. Usually, their values are Boolean (‘yes’ or ‘no’) or the names of files or functions; so this is not an important restriction. Cache Variable Index for an index of cache variables with documented semantics.
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, or override documented cache variables on the configure command line.
By default, configure uses no cache file, 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 gradually accumulates 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.
If configure is interrupted at the right time when it updates a cache file outside of the build directory where the configure script is run, it may leave behind a temporary file named after the cache file with digits following it. You may safely delete such a file.
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
reduces the amount of time it takes to rerun the configure script with
(hopefully) the error that caused the previous abort corrected.
Loads values from existing cache file, or creates a new cache file if a cache file is not found. Called automatically from
AC_INIT.
Flushes all cached values to the cache file. Called automatically from
AC_OUTPUT, but it can be quite useful to callAC_CACHE_SAVEat key points in configure.ac.
For instance:
... AC_INIT, etc. ... # Checks for programs. AC_PROG_CC AC_PROG_AWK ... 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. ...
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. They direct output to the appropriate file descriptor (see File Descriptor Macros). 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 the callers change automatically.
To diagnose static issues, i.e., when autoconf is run, see Diagnostic Macros.
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_RESULTto 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.
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 toAC_MSG_CHECKING.This macro prints nothing if configure is run with the --quiet or --silent option.
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.
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 (‘$?’ by default, except that ‘0’ is converted to ‘1’). error-description should be something like ‘invalid value $HOME for \$HOME’.
The error-description should start with a lower-case letter, and “cannot” is preferred to “can't”.
This
AC_MSG_ERRORwrapper notifies the user of an error that prevents configure from completing and that additional details are provided in config.log. This is typically used when abnormal results are found during a compilation.
Notify the configure user of a possible problem. This macro prints the message to the standard error output; configure continues running afterward, so macros that call
AC_MSG_WARNshould provide a default (back-up) behavior for the situations they warn about. problem-description should be something like ‘ln -s seems to make hard links’.
Autoconf is written on top of two layers: M4sugar, which provides convenient macros for pure M4 programming, and M4sh, which provides macros dedicated to shell script generation.
As of this version of Autoconf, these two layers still contain experimental macros, whose interface might change in the future. As a matter of fact, anything that is not documented must not be used.
The most common problem with existing macros is an improper quotation. This section, which users of Autoconf can skip, but which macro writers must read, first justifies the quotation scheme that was chosen for Autoconf and then ends with a rule of thumb. Understanding the former helps one to follow the latter.
To fully understand where proper quotation is important, you first need to know what the special characters are in Autoconf: ‘#’ introduces a comment inside which no macro expansion is performed, ‘,’ separates arguments, ‘[’ and ‘]’ are the quotes themselves3, ‘(’ and ‘)’ (which M4 tries to match by pairs), and finally ‘$’ inside a macro definition.
In order to understand the delicate case of macro calls, we first have to present some obvious failures. Below they are “obvious-ified”, but when 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 might try hopelessly to use her macro
array:
define([array], [int tab[10];])
array
⇒int tab10;
[array]
⇒array
How can you correctly output the intended results4?
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, assuming
car is not already defined, it is equivalent to write:
define(car, $1)
But, while it is acceptable for a configure.ac to avoid unnecessary quotes, it is bad practice for Autoconf macros which must both be more robust and also advocate perfect style.
At the top level, there are only two possibilities: either you quote or you don't:
car(foo, bar, baz)
⇒foo
[car(foo, bar, baz)]
⇒car(foo, bar, baz)
Let's pay attention to the special characters:
car(#)
error-->EOF in argument list
The closing parenthesis is hidden in the comment; with a hypothetical quoting, the top level understood it this way:
car([#)]
Proper quotation, of course, fixes the problem:
car([#])
⇒#
Here are more examples:
car(foo, bar)
⇒foo
car([foo, bar])
⇒foo, bar
car((foo, bar))
⇒(foo, bar)
car([(foo], [bar)])
⇒(foo
define([a], [b])
⇒
car(a)
⇒b
car([a])
⇒b
car([[a]])
⇒a
car([[[a]]])
⇒[a]
When M4 encounters ‘$’ within a macro definition, followed immediately by a character it recognizes (‘0’...‘9’, ‘#’, ‘@’, or ‘*’), it will perform M4 parameter expansion. This happens regardless of how many layers of quotes the parameter expansion is nested within, or even if it occurs in text that will be rescanned as a comment.
define([none], [$1])
⇒
define([one], [[$1]])
⇒
define([two], [[[$1]]])
⇒
define([comment], [# $1])
⇒
define([active], [ACTIVE])
⇒
none([active])
⇒ACTIVE
one([active])
⇒active
two([active])
⇒[active]
comment([active])
⇒# active
On the other hand, since autoconf generates shell code, you often want to output shell variable expansion, rather than performing M4 parameter expansion. To do this, you must use M4 quoting to separate the ‘$’ from the next character in the definition of your macro. If the macro definition occurs in single-quoted text, then insert another level of quoting; if the usage is already inside a double-quoted string, then split it into concatenated strings.
define([single], [a single-quoted $[]1 definition])
⇒
define([double], [[a double-quoted $][1 definition]])
⇒
single
⇒a single-quoted $1 definition
double
⇒a double-quoted $1 definition
Posix states that M4 implementations are free to provide implementation extensions when ‘${’ is encountered in a macro definition. Autoconf reserves the longer sequence ‘${{’ for use with planned extensions that will be available in the future GNU M4 2.0, but guarantees that all other instances of ‘${’ will be output literally. Therefore, this idiom can also be used to output shell code parameter references:
define([first], [${1}])first
⇒${1}
Posix also states that ‘$11’ should expand to the first parameter concatenated with a literal ‘1’, although some versions of GNU M4 expand the eleventh parameter instead. For portability, you should only use single-digit M4 parameter expansion.
With this in mind, we can explore the cases where macros invoke 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 result of qar is always
a literal string, the only time a user can use nested macros is if she
relies on an unquoted macro call:
qar(active)
⇒ACT
qar([active])
⇒active
leaving no way for her 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.
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 one's
needs. For instance, by default M4 uses ‘`’ and ‘'’ as
quotes, but in the context of shell programming (and actually of most
programming languages), that's about the worst choice one can make:
because of strings and back-quoted expressions in shell code (such as
‘'this'’ and ‘`that`’), and because of literal characters in usual
programming languages (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 choose 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 nth argument, ‘$’ by
default, the parentheses 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 out, 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 are lost, as the current syntax is probably not the one they were implemented with.
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.
Additionally, there are a few m4sugar macros (such as m4_split
and m4_expand) which internally use special markers in addition
to the regular quoting characters. If the arguments to these macros
contain the literal strings ‘-=<{(’ or ‘)}>=-’, the macros
might behave incorrectly.
You can work around these problems by using one of the following quadrigraphs:
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, appears as ‘^[’ in the output.
The empty quadrigraph can be used:
Trailing spaces are smashed by autom4te. This is a feature.
For instance ‘@<@&t@:@’ produces ‘@<:@’. For a more contrived example:
m4_define([a], [A])m4_define([b], [B])m4_define([c], [C])dnl
m4_split([a )}>=- b -=<{( c])
⇒[a], [], [B], [], [c]
m4_split([a )}@&t@>=- b -=<@&t@{( c])
⇒[a], [)}>=-], [b], [-=<{(], [c]
For instance you might want to mention AC_FOO in a comment, while
still being sure that autom4te still catches 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.
One of the pitfalls of portable shell programming is that case statements require unbalanced parentheses (see Limitations of Shell Builtins). With syntax highlighting editors, the presence of unbalanced ‘)’ can interfere with editors that perform syntax highlighting of macro contents based on finding the matching ‘(’. Another concern is how much editing must be done when transferring code snippets between shell scripts and macro definitions. But most importantly, the presence of unbalanced parentheses can introduce expansion bugs.
For an example, here is an underquoted attempt to use the macro
my_case, which happens to expand to a portable case
statement:
AC_DEFUN([my_case],
[case $file_name in
*.c) echo "C source code";;
esac])
AS_IF(:, my_case)
In the above example, the AS_IF call underquotes its arguments.
As a result, the unbalanced ‘)’ generated by the premature
expansion of my_case results in expanding AS_IF with a
truncated parameter, and the expansion is syntactically invalid:
if :; then
case $file_name in
*.c
fi echo "C source code";;
esac)
If nothing else, this should emphasize the importance of the quoting
arguments to macro calls. On the other hand, there are several
variations for defining my_case to be more robust, even when used
without proper quoting, each with some benefits and some drawbacks.
AC_DEFUN([my_case],
[case $file_name in #(
*.c) echo "C source code";;
esac])
This version provides balanced parentheses to several editors, and can be copied and pasted into a terminal as is. Unfortunately, it is still unbalanced as an Autoconf argument, since ‘#(’ is an M4 comment that masks the normal properties of ‘(’.
AC_DEFUN([my_case],
[case $file_name in @%:@(
*.c) echo "C source code";;
esac])
This version provides balanced parentheses to even more editors, and can be used as a balanced Autoconf argument. Unfortunately, it requires some editing before it can be copied and pasted into a terminal, and the use of the quadrigraph ‘@%:@’ for ‘#’ reduces readability.
AC_DEFUN([my_case],
[case $file_name in
*.c[)] echo "C source code";;
esac])
This version quotes the ‘)’, so that it can be used as a balanced Autoconf argument. As written, this is not balanced to an editor, but it can be coupled with ‘[#(]’ to meet that need, too. However, it still requires some edits before it can be copied and pasted into a terminal.
AC_DEFUN([my_case],
[[case $file_name in #(
*.c) echo "C source code";;
esac]])
Since the entire macro is double-quoted, there is no problem with using this as an Autoconf argument; and since the double-quoting is over the entire statement, this code can be easily copied and pasted into a terminal. However, the double quoting prevents the expansion of any macros inside the case statement, which may cause its own set of problems.
AS_CASE
AC_DEFUN([my_case],
[AS_CASE([$file_name],
[*.c], [echo "C source code"])])
This version avoids the balancing issue altogether, by relying on
AS_CASE (see Common Shell Constructs); it also allows for the
expansion of AC_REQUIRE to occur prior to the entire case
statement, rather than within a branch of the case statement that might
not be taken. However, the abstraction comes with a penalty that it is
no longer a quick copy, paste, and edit to get back to shell code.
To conclude, the quotation rule of thumb is:
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 might 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])
Please note that the macro AC_TRY_LINK is obsolete, so you really
should be using AC_LINK_IFELSE instead.
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 may contain
unexpanded macros. The autoconf program checks for this problem
by looking for the string ‘AC_’ in configure. However, this
heuristic does not work in general: for example, it does not catch
overquoting in AC_DEFINE descriptions.
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: autom4te5.
autom4te is a preprocessor that is like m4. It supports M4 extensions designed for use in tools like Autoconf.
The command line arguments are modeled after M4's:
autom4te options files
where the files are directly passed to m4. By default, GNU M4 is found during configuration, but the environment variable M4 can be set to tell autom4te where to look. 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:
Of course, it supports the Autoconf common subset of options:
As an extension of m4, it includes the following options:
AC_DIAGNOSE, for a comprehensive list of categories. Special
values include:
Warnings about ‘syntax’ are enabled by default, and the environment variable WARNINGS, a comma separated list of categories, is honored. ‘autom4te -W category’ actually behaves as if you had run:
autom4te --warnings=syntax,$WARNINGS,category
For example, if you want to disable defaults and WARNINGS of autom4te, but enable the warnings about obsolete constructs, you would use -W none,obsolete.
autom4te displays a back trace for errors, but not for
warnings; if you want them, just pass -W error.
.m4f is
replaced by file.m4. This helps tracing the macros which
are executed only when the files are frozen, typically
m4_define. For instance, running:
autom4te --melt 1.m4 2.m4f 3.m4 4.m4f input.m4
is roughly equivalent to running:
m4 1.m4 2.m4 3.m4 4.m4 input.m4
while
autom4te 1.m4 2.m4f 3.m4 4.m4f input.m4
is equivalent to:
m4 --reload-state=4.m4f input.m4
autom4te 1.m4 2.m4 3.m4 --freeze --output=3.m4f
corresponds to
m4 1.m4 2.m4 3.m4 --freeze-state=3.m4f
As another additional feature over m4, autom4te caches its results. GNU M4 is able to produce a regular output and traces at the same time. Traces are heavily used in the GNU Build System: autoheader uses them to build config.h.in, autoreconf to determine what GNU Build System components are used, automake to “parse” configure.ac etc. To avoid recomputation, traces are cached while performing regular expansion, and conversely. This cache is (actually, the caches are) stored in the directory autom4te.cache. It can safely be removed at any moment (especially if for some reason autom4te considers it trashed).
Because traces are so important to the GNU Build System, autom4te provides high level tracing features as compared to M4, and helps exploiting the cache:
The format is a regular string, with newlines if desired, and several special escape codes. It defaults to ‘$f:$l:$n:$%’. It can use the following special escapes:
The escape ‘$%’ produces single-line trace outputs (unless you put newlines in the ‘separator’), while ‘$@’ and ‘$*’ do not.
See autoconf Invocation, for examples of trace uses.
Finally, autom4te introduces the concept of Autom4te libraries. They consists in a powerful yet extremely simple feature: sets of combined command line arguments:
M4sugarM4shAutotestAutoconf-without-aclocal-m4AutoconfAutoconf-without-aclocal-m4 and
additionally reads aclocal.m4.
As an example, if Autoconf is installed in its default location, /usr/local, the command ‘autom4te -l m4sugar foo.m4’ is strictly equivalent to the command:
autom4te --prepend-include /usr/local/share/autoconf \
m4sugar/m4sugar.m4f --warnings syntax foo.m4
Recursive expansion applies here: the command ‘autom4te -l m4sh foo.m4’ is the same as ‘autom4te --language M4sugar m4sugar/m4sh.m4f foo.m4’, i.e.:
autom4te --prepend-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.
One can customize autom4te via ~/.autom4te.cfg (i.e., as found in the user home directory), and ./.autom4te.cfg (i.e., as found in the directory from which autom4te is run). The order is first reading autom4te.cfg, then ~/.autom4te.cfg, then ./.autom4te.cfg, and finally the command line arguments.
In these text files, comments are introduced with #, and empty
lines are ignored. Customization is performed on a per-language basis,
wrapped in between a ‘begin-language: "language"’,
‘end-language: "language"’ pair.
Customizing a language stands for appending options (see autom4te Invocation) to the current definition of the language. Options, and more generally arguments, are introduced by ‘args: arguments’. You may use the traditional shell syntax to quote the arguments.
As an example, to disable Autoconf caches (autom4te.cache) globally, include the following lines in ~/.autom4te.cfg:
## ------------------ ## ## User Preferences. ## ## ------------------ ## begin-language: "Autoconf-without-aclocal-m4" args: --no-cache end-language: "Autoconf-without-aclocal-m4"
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”.
M4sugar reserves the macro namespace ‘^_m4_’ for internal use, and the macro namespace ‘^m4_’ for M4sugar macros. You should not define your own macros into these namespaces.
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.
The list of macros unchanged from M4, except for their name, is:
Some M4 macros are redefined, and are slightly incompatible with their native equivalent.
All M4 macros starting with ‘__’ retain their original name: for example, no
m4__file__is defined.
This is not technically a macro, but a feature of Autom4te. The sequence
__oline__can be used similarly to the other m4sugar location macros, but rather than expanding to the location of the input file, it is translated to the line number where it appears in the output file after all other M4 expansions.
This macro corresponds to
patsubst. The namem4_patsubstis kept for future versions of M4sugar, once GNU M4 2.0 is released and supports extended regular expression syntax.
This macro corresponds to
regexp. The namem4_regexpis kept for future versions of M4sugar, once GNU M4 2.0 is released and supports extended regular expression syntax.
These macros aren't directly builtins, but are closely related to
m4_pushdefandm4_defn.m4_copyandm4_renameensure that dest is undefined, whilem4_copy_forceandm4_rename_forceoverwrite any existing definition. All four macros then proceed to copy the entire pushdef stack of definitions of source over to dest.m4_copyandm4_copy_forcepreserve the source (including in the special case where source is undefined), whilem4_renameandm4_rename_forceundefine the original macro name (making it an error to rename an undefined source).Note that attempting to invoke a renamed macro might not work, since the macro may have a dependence on helper macros accessed via composition of ‘$0’ but that were not also renamed; likewise, other macros may have a hard-coded dependence on source and could break if source has been deleted. On the other hand, it is always safe to rename a macro to temporarily move it out of the way, then rename it back later to restore original semantics.
This macro fails if macro is not defined, even when using older versions of M4 that did not warn. See
m4_undefine. Unfortunately, in order to support these older versions of M4, there are some situations involving unbalanced quotes where concatenating multiple macros together will work in newer M4 but not in m4sugar; use quadrigraphs to work around this.
M4sugar relies heavily on diversions, so rather than behaving as a primitive,
m4_divertbehaves like:m4_divert_pop()m4_divert_push([diversion])See Diversion support, for more details about the use of the diversion stack. In particular, this implies that diversion should be a named diversion rather than a raw number. But be aware that it is seldom necessary to explicitly change the diversion stack, and that when done incorrectly, it can lead to syntactically invalid scripts.
m4_dumpdefis like the M4 builtin, except that this version requires at least one argument, output always goes to standard error rather than the current debug file, no sorting is done on multiple arguments, and an error is issued if any name is undefined.m4_dumpdefsis a convenience macro that callsm4_dumpdeffor all of them4_pushdefstack of definitions, starting with the current, and silently does nothing if name is undefined.Unfortunately, due to a limitation in M4 1.4.x, any macro defined as a builtin is output as the empty string. This behavior is rectified by using M4 1.6 or newer. However, this behavior difference means that
m4_dumpdefshould only be used while developing m4sugar macros, and never in the final published form of a macro.
Like
m4_esyscmd, this macro expands to the result of running command in a shell. The difference is that any trailing newlines are removed, so that the output behaves more like shell command substitution.
This macro corresponds to
ifelse. string-1 and string-2 are compared literally, so usually one of the two arguments is passed unquoted. See Conditional constructs, for more conditional idioms.
Like the M4 builtins, but warn against multiple inclusions of file.
Posix requires
maketempto replace the trailing ‘X’ characters in template with the process id, without regards to the existence of a file by that name, but this a security hole. When this was pointed out to the Posix folks, they agreed to invent a new macromkstempthat always creates a uniquely named file, but not all versions of GNU M4 support the new macro. In M4sugar,m4_maketempandm4_mkstempare synonyms for each other, and both have the secure semantics regardless of which macro the underlying M4 provides.
This macro fails if macro is not defined, even when using older versions of M4 that did not warn. See
m4_undefine.
This macro fails if macro is not defined, even when using older versions of M4 that did not warn. Use
m4_ifdef([macro], [m4_undefine([macro])])if you are not sure whether macro is defined.
Unlike the M4 builtin, at least one diversion must be specified. Also, since the M4sugar diversion stack prefers named diversions, the use of
m4_undivertto include files is risky. See Diversion support, for more details about the use of the diversion stack. But be aware that it is seldom necessary to explicitly change the diversion stack, and that when done incorrectly, it can lead to syntactically invalid scripts.
These macros correspond to
m4wrap. Posix requires arguments of multiple wrap calls to be reprocessed at EOF in the same order as the original calls (first-in, first-out). GNU M4 versions through 1.4.10, however, reprocess them in reverse order (last-in, first-out). Both orders are useful, therefore, you can rely onm4_wrapto provide FIFO semantics andm4_wrap_lifofor LIFO semantics, regardless of the underlying GNU M4 version.Unlike the GNU M4 builtin, these macros only recognize one argument, and avoid token pasting between consecutive invocations. On the other hand, nested calls to
m4_wrapfrom within wrapped text work just as in the builtin.
When macros statically diagnose abnormal situations, benign or fatal, they should report them using these macros. For issuing dynamic issues, i.e., when configure is run, see Printing Messages.
Assert that the arithmetic expression evaluates to non-zero. Otherwise, issue a fatal error, and exit autom4te with exit-status.
Similar to the builtin
m4_errprint, except that a newline is guaranteed after message.
Report a severe error message prefixed with the current location, and have autom4te die.
Report message as a warning (or as an error if requested by the user) if warnings of the category are turned on. If the message is emitted, it is prefixed with the current location, and followed by a call trace of all macros defined via
AC_DEFUNused to get to the current expansion. You are encouraged to use standard categories, which currently include:
- ‘all’
- messages that don't fall into one of the following categories. 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.
M4sugar makes heavy use of diversions under the hood, because it is often the case that text that must appear early in the output is not discovered until late in the input. Additionally, some of the topological sorting algorithms used in resolving macro dependencies use diversions. However, most macros should not need to change diversions directly, but rather rely on higher-level M4sugar macros to manage diversions transparently. If you change diversions improperly, you risk generating a syntactically invalid script, because an incorrect diversion will violate assumptions made by many macros about whether prerequisite text has been previously output. In short, if you manually change the diversion, you should not expect any macros provided by the Autoconf package to work until you have restored the diversion stack back to its original state.
In the rare case that it is necessary to write a macro that explicitly
outputs text to a different diversion, it is important to be aware of an
M4 limitation regarding diversions: text only goes to a diversion if it
is not part of argument collection. Therefore, any macro that changes
the current diversion cannot be used as an unquoted argument to another
macro, but must be expanded at the top level. The macro
m4_expand will diagnose any attempt to change diversions, since
it is generally useful only as an argument to another macro. The
following example shows what happens when diversion manipulation is
attempted within macro arguments:
m4_do([normal text]
m4_divert_push([KILL])unwanted[]m4_divert_pop([KILL])
[m4_divert_push([KILL])discarded[]m4_divert_pop([KILL])])dnl
⇒normal text
⇒unwanted
Notice that the unquoted text unwanted is output, even though it
was processed while the current diversion was KILL, because it
was collected as part of the argument to m4_do. However, the
text discarded disappeared as desired, because the diversion
changes were single-quoted, and were not expanded until the top-level
rescan of the output of m4_do.
To make diversion management easier, M4sugar uses the concept of named
diversions. Rather than using diversion numbers directly, it is nicer
to associate a name with each diversion. The diversion number associated
with a particular diversion name is an implementation detail, and a
syntax warning is issued if a diversion number is used instead of a
name. In general, you should not output text
to a named diversion until after calling the appropriate initialization
routine for your language (m4_init, AS_INIT,
AT_INIT, ...), although there are some exceptions documented
below.
M4sugar defines two named diversions.
KILLGROWAC_REQUIRE.
M4sh adds several more named diversions.
BINSHHEADER-REVISIONAC_REVISION.
HEADER-COMMENTHEADER-COPYRIGHTAC_COPYRIGHT.
M4SH-SANITIZEM4SH-INITBODYAutotest inherits diversions from M4sh, and changes the default
diversion from BODY back to KILL. It also adds several
more named diversions, with the following subset designed for developer
use.
PREPARE_TESTSAT_INIT.
Autoconf inherits diversions from M4sh, and adds the following named diversions which developers can utilize.
DEFAULTSHELP_ENABLEAC_PRESERVE_HELP_ORDER was used, then text placed in this
diversion will be included as part of a quoted here-doc providing all of
the --help output of configure related to options
created by AC_ARG_WITH and AC_ARG_ENABLE.
INIT_PREPAREFor now, the remaining named diversions of Autoconf, Autoheader, and Autotest are not documented. In other words, intentionally outputting text into an undocumented diversion is subject to breakage in a future release of Autoconf.
Permanently discard any text that has been diverted into diversion.
Similar to
m4_divert_text, except that content is only output to diversion if this is the first time thatm4_divert_oncehas been called with its particular arguments.
If provided, check that the current diversion is indeed diversion. Then change to the diversion located earlier on the stack, giving an error if an attempt is made to pop beyond the initial m4sugar diversion of
KILL.
Remember the former diversion on the diversion stack, and output subsequent text into diversion. M4sugar maintains a diversion stack, and issues an error if there is not a matching pop for every push.
Output content and a newline into diversion, without affecting the current diversion. Shorthand for:
m4_divert_push([diversion])content m4_divert_pop([diversion])dnlOne use of
m4_divert_textis to develop two related macros, where macro ‘MY_A’ does the work, but adjusts what work is performed based on whether the optional macro ‘MY_B’ has also been expanded. Of course, it is possible to useAC_BEFOREwithinMY_Ato require that ‘MY_B’ occurs first, if it occurs at all. But this imposes an ordering restriction on the user; it would be nicer if macros ‘MY_A’ and ‘MY_B’ can be invoked in either order. The trick is to let ‘MY_B’ leave a breadcrumb in an early diversion, which ‘MY_A’ can then use to determine whether ‘MY_B’ has been expanded.AC_DEFUN([MY_A], [# various actions if test -n "$b_was_used"; then # extra action fi]) AC_DEFUN([MY_B], [AC_REQUIRE([MY_A])dnl m4_divert_text([INIT_PREPARE], [b_was_used=true])])
Initialize the M4sugar environment, setting up the default named diversion to be
KILL.
The following macros provide additional conditional constructs as
convenience wrappers around m4_if.
The string string is repeatedly compared against a series of regex arguments; if a match is found, the expansion is the corresponding value, otherwise, the macro moves on to the next regex. If no regex match, then the result is the optional default, or nothing.
The string string is altered by regex-1 and subst-1, as if by:
m4_bpatsubst([[string]], [regex], [subst])The result of the substitution is then passed through the next set of regex and subst, and so forth. An empty subst implies deletion of any matched portions in the current string. Note that this macro over-quotes string; this behavior is intentional, so that the result of each step of the recursion remains as a quoted string. However, it means that anchors (‘^’ and ‘$’ in the regex will line up with the extra quotations, and not the characters of the original string. The overquoting is removed after the final substitution.
Test string against multiple value possibilities, resulting in the first if-value for a match, or in the optional default. This is shorthand for:
m4_if([string], [value-1], [if-value-1], [string], [value-2], [if-value-2], ..., [default])
This macro was introduced in Autoconf 2.62. Similar to
m4_if, except that each test is expanded only when it is encountered. This is useful for short-circuiting expensive tests; whilem4_ifrequires all its strings to be expanded up front before doing comparisons,m4_condonly expands a test when all earlier tests have failed.For an example, these two sequences give the same result, but in the case where ‘$1’ does not contain a backslash, the
m4_condversion only expandsm4_indexonce, instead of five times, for faster computation if this is a common case for ‘$1’. Notice that every third argument is unquoted form4_if, and quoted form4_cond:m4_if(m4_index([$1], [\]), [-1], [$2], m4_eval(m4_index([$1], [\\]) >= 0), [1], [$2], m4_eval(m4_index([$1], [\$]) >= 0), [1], [$2], m4_eval(m4_index([$1], [\`]) >= 0), [1], [$3], m4_eval(m4_index([$1], [\"]) >= 0), [1], [$3], [$2]) m4_cond([m4_index([$1], [\])], [-1], [$2], [m4_eval(m4_index([$1], [\\]) >= 0)], [1], [$2], [m4_eval(m4_index([$1], [\$]) >= 0)], [1], [$2], [m4_eval(m4_index([$1], [\`]) >= 0)], [1], [$3], [m4_eval(m4_index([$1], [\"]) >= 0)], [1], [$3], [$2])
If expr-1 contains text, use it. Otherwise, select expr-2.
m4_defaultexpands the result, whilem4_default_quoteddoes not. Useful for providing a fixed default if the expression that results in expr-1 would otherwise be empty. The difference betweenm4_defaultandm4_default_nblankis whether an argument consisting of just blanks (space, tab, newline) is significant. When using the expanding versions, note that an argument may contain text but still expand to an empty string.m4_define([active], [ACTIVE])dnl m4_define([empty], [])dnl m4_define([demo1], [m4_default([$1], [$2])])dnl m4_define([demo2], [m4_default_quoted([$1], [$2])])dnl m4_define([demo3], [m4_default_nblank([$1], [$2])])dnl m4_define([demo4], [m4_default_nblank_quoted([$1], [$2])])dnl demo1([active], [default]) ⇒ACTIVE demo1([], [active]) ⇒ACTIVE demo1([empty], [text]) ⇒ -demo1([ ], [active])- ⇒- - demo2([active], [default]) ⇒active demo2([], [active]) ⇒active demo2([empty], [text]) ⇒empty -demo2([ ], [active])- ⇒- - demo3([active], [default]) ⇒ACTIVE demo3([], [active]) ⇒ACTIVE demo3([empty], [text]) ⇒ -demo3([ ], [active])- ⇒-ACTIVE- demo4([active], [default]) ⇒active demo4([], [active]) ⇒active demo4([empty], [text]) ⇒empty -demo4([ ], [active])- ⇒-active-
If macro does not already have a definition, then define it to default-definition.
If cond is empty or consists only of blanks (space, tab, newline), then expand if-blank; otherwise, expand if-text. Two variants exist, in order to make it easier to select the correct logical sense when using only two parameters. Note that this is more efficient than the equivalent behavior of:
m4_ifval(m4_normalize([cond]), if-text, if-blank)
m4_ifdef([macro], [if-defined], [if-not-defined])
If macro is undefined, or is defined as the empty string, expand to if-false. Otherwise, expands to if-true. Similar to:
m4_ifval(m4_defn([macro]), [if-true], [if-false])except that it is not an error if macro is undefined.
Expands to if-true if cond is not empty, otherwise to if-false. This is shorthand for:
m4_if([cond], [], [if-true], [if-false])
Similar to
m4_ifval, except guarantee that a newline is present after any non-empty expansion. Often followed bydnl.
The following macros are useful in implementing recursive algorithms in
M4, including loop operations. An M4 list is formed by quoting a list
of quoted elements; generally the lists are comma-separated, although
m4_foreach_w is whitespace-separated. For example, the list
‘[[a], [b,c]]’ contains two elements: ‘[a]’ and ‘[b,c]’.
It is common to see lists with unquoted elements when those elements are
not likely to be macro names, as in ‘[fputc_unlocked,
fgetc_unlocked]’.
Although not generally recommended, it is possible for quoted lists to have side effects; all side effects are expanded only once, and prior to visiting any list element. On the other hand, the fact that unquoted macros are expanded exactly once means that macros without side effects can be used to generate lists. For example,
m4_foreach([i], [[1], [2], [3]m4_errprintn([hi])], [i])
error-->hi
⇒123
m4_define([list], [[1], [2], [3]])
⇒
m4_foreach([i], [list], [i])
⇒123
Extracts argument n (larger than 0) from the remaining arguments. If there are too few arguments, the empty string is used. For any n besides 1, this is more efficient than the similar ‘m4_car(m4_shiftn([n], [], [arg...]))’.
Expands to the quoted first arg. Can be used with
m4_cdrto recursively iterate through a list. Generally, when using quoted lists of quoted elements,m4_carshould be called without any extra quotes.
Expands to a quoted list of all but the first arg, or the empty string if there was only one argument. Generally, when using quoted lists of quoted elements,
m4_cdrshould be called without any extra quotes.For example, this is a simple implementation of
m4_map; note how each iteration checks for the end of recursion, then merely applies the first argument to the first element of the list, then repeats with the rest of the list. (The actual implementation in M4sugar is a bit more involved, to gain some speed and share code withm4_map_sep, and also to avoid expanding side effects in ‘$2’ twice).m4_define([m4_map], [m4_ifval([$2], [m4_apply([$1], m4_car($2))[]$0([$1], m4_cdr($2))])])dnl m4_map([ m4_eval], [[[1]], [[1+1]], [[10],[16]]]) ⇒ 1 2 a
Loop over the numeric values between first and last including bounds by increments of step. For each iteration, expand expression with the numeric value assigned to var. If step is omitted, it defaults to ‘1’ or ‘-1’ depending on the order of the limits. If given, step has to match this order. The number of iterations is determined independently from definition of var; iteration cannot be short-circuited or lengthened by modifying var from within expression.
Loop over the comma-separated M4 list list, assigning each value to var, and expand expression. The following example outputs two lines:
m4_foreach([myvar], [[foo], [bar, baz]], [echo myvar ])dnl ⇒echo foo ⇒echo bar, bazNote that for some forms of expression, it may be faster to use
m4_map_args.
Loop over the white-space-separated list list, assigning each value to var, and expand expression. If var is only referenced once in expression, it is more efficient to use
m4_map_args_w.The deprecated macro
AC_FOREACHis an alias ofm4_foreach_w.
Loop over the comma separated quoted list of argument descriptions in list, and invoke macro with the arguments. An argument description is in turn a comma-separated quoted list of quoted elements, suitable for
m4_apply. The macrosm4_mapandm4_map_sepignore empty argument descriptions, whilem4_mapallandm4_mapall_sepinvoke macro with no arguments. The macrosm4_map_sepandm4_mapall_sepadditionally expand separator between invocations of macro.Note that separator is expanded, unlike in
m4_join. When separating output with commas, this means that the map result can be used as a series of arguments, by using a single-quoted comma as separator, or as a single string, by using a double-quoted comma.m4_map([m4_count], []) ⇒ m4_map([ m4_count], [[], [[1]], [[1], [2]]]) ⇒ 1 2 m4_mapall([ m4_count], [[], [[1]], [[1], [2]]]) ⇒ 0 1 2 m4_map_sep([m4_eval], [,], [[[1+2]], [[10], [16]]]) ⇒3,a m4_map_sep([m4_echo], [,], [[[a]], [[b]]]) ⇒a,b m4_count(m4_map_sep([m4_echo], [,], [[[a]], [[b]]])) ⇒2 m4_map_sep([m4_echo], [[,]], [[[a]], [[b]]]) ⇒a,b m4_count(m4_map_sep([m4_echo], [[,]], [[[a]], [[b]]])) ⇒1
Repeatedly invoke macro with each successive arg as its only argument. In the following example, three solutions are presented with the same expansion; the solution using
m4_map_argsis the most efficient.m4_define([active], [ACTIVE])dnl m4_foreach([var], [[plain], [active]], [ m4_echo(m4_defn([var]))]) ⇒ plain active m4_map([ m4_echo], [[[plain]], [[active]]]) ⇒ plain active m4_map_args([ m4_echo], [plain], [active]) ⇒ plain activeIn cases where it is useful to operate on additional parameters besides the list elements, the macro
m4_currycan be used in macro to supply the argument currying necessary to generate the desired argument list. In the following example,list_add_nis more efficient thanlist_add_x. On the other hand, usingm4_map_args_sepcan be even more efficient.m4_define([list], [[1], [2], [3]])dnl m4_define([add], [m4_eval(([$1]) + ([$2]))])dnl dnl list_add_n(N, ARG...) dnl Output a list consisting of each ARG added to N m4_define([list_add_n], [m4_shift(m4_map_args([,m4_curry([add], [$1])], m4_shift($@)))])dnl list_add_n([1], list) ⇒2,3,4 list_add_n([2], list) ⇒3,4,5 m4_define([list_add_x], [m4_shift(m4_foreach([var], m4_dquote(m4_shift($@)), [,add([$1],m4_defn([var]))]))])dnl list_add_x([1], list) ⇒2,3,4
For every pair of arguments arg, invoke macro with two arguments. If there is an odd number of arguments, invoke macro-end, which defaults to macro, with the remaining argument.
m4_map_args_pair([, m4_reverse], [], [1], [2], [3]) ⇒, 2, 1, 3 m4_map_args_pair([, m4_reverse], [, m4_dquote], [1], [2], [3]) ⇒, 2, 1, [3] m4_map_args_pair([, m4_reverse], [, m4_dquote], [1], [2], [3], [4]) ⇒, 2, 1, 4, 3
Expand the sequence pre
[arg]post for each argument, additionally expanding sep between arguments. One common use of this macro is constructing a macro call, where the opening and closing parentheses are split between pre and post; in particular,m4_map_args([macro], [arg])is equivalent tom4_map_args_sep([macro(], [)], [], [arg]). This macro provides the most efficient means for iterating over an arbitrary list of arguments, particularly when repeatedly constructing a macro call with more arguments than arg.
Expand the sequence pre
[word]post for each word in the whitespace-separated string, additionally expanding sep between words. This macro provides the most efficient means for iterating over a whitespace-separated string. In particular,m4_map_args_w([string], [action(], [)])is more efficient thanm4_foreach_w([var], [string], [action(m4_defn([var]))]).
m4_shiftnperforms count iterations ofm4_shift, along with validation that enough arguments were passed in to match the shift count, and that the count is positive.m4_shift2andm4_shift3are specializations ofm4_shiftn, introduced in Autoconf 2.62, and are more efficient for two and three shifts, respectively.
For each of the
m4_pushdefdefinitions of macro, expand action with the single argument of a definition of macro.m4_stack_foreachstarts with the oldest definition, whilem4_stack_foreach_lifostarts with the current definition. action should not push or pop definitions of macro, nor is there any guarantee that the current definition of macro matches the argument that was passed to action. The macrom4_currycan be used if action needs more than one argument, although in that case it is more efficient to use m4_stack_foreach_sep.Due to technical limitations, there are a few low-level m4sugar functions, such as
m4_pushdef, that cannot be used as the macro argument.m4_pushdef([a], [1])m4_pushdef([a], [2])dnl m4_stack_foreach([a], [ m4_incr]) ⇒ 2 3 m4_stack_foreach_lifo([a], [ m4_curry([m4_substr], [abcd])]) ⇒ cd bcd
Expand the sequence pre
[definition]post for eachm4_pushdefdefinition of macro, additionally expanding sep between definitions.m4_stack_foreach_sepvisits the oldest definition first, whilem4_stack_foreach_sep_lifovisits the current definition first. This macro provides the most efficient means for iterating over a pushdef stack. In particular,m4_stack_foreach([macro], [action])is short form4_stack_foreach_sep([macro], [action(], [)]).
The following macros give some control over the order of the evaluation by adding or removing levels of quotes.
Apply the elements of the quoted, comma-separated list as the arguments to macro. If list is empty, invoke macro without arguments. Note the difference between
m4_indir, which expects its first argument to be a macro name but can use names that are otherwise invalid, andm4_apply, where macro can contain other text, but must end in a valid macro name.m4_apply([m4_count], []) ⇒0 m4_apply([m4_count], [[]]) ⇒1 m4_apply([m4_count], [[1], [2]]) ⇒2 m4_apply([m4_join], [[|], [1], [2]]) ⇒1|2
This macro returns the decimal count of the number of arguments it was passed.
This macro performs argument currying. The expansion of this macro is another macro name that expects exactly one argument; that argument is then appended to the arg list, and then macro is expanded with the resulting argument list.
m4_curry([m4_curry], [m4_reverse], [1])([2])([3]) ⇒3, 2, 1Unfortunately, due to a limitation in M4 1.4.x, it is not possible to pass the definition of a builtin macro as the argument to the output of
m4_curry; the empty string is used instead of the builtin token. This behavior is rectified by using M4 1.6 or newer.
This macro loops over its arguments and expands each arg in sequence. Its main use is for readability; it allows the use of indentation and fewer
dnlto result in the same expansion. This macro guarantees that no expansion will be concatenated with subsequent text; to achieve full concatenation, usem4_unquote(m4_join([],arg...)).m4_define([ab],[1])m4_define([bc],[2])m4_define([abc],[3])dnl m4_do([a],[b])c ⇒abc m4_unquote(m4_join([],[a],[b]))c ⇒3 m4_define([a],[A])m4_define([b],[B])m4_define([c],[C])dnl m4_define([AB],[4])m4_define([BC],[5])m4_define([ABC],[6])dnl m4_do([a],[b])c ⇒ABC m4_unquote(m4_join([],[a],[b]))c ⇒3
Return the arguments as a quoted list of quoted arguments. Conveniently, if there is just one arg, this effectively adds a level of quoting.
Return the arguments as a series of double-quoted arguments. Whereas
m4_dquotereturns a single argument,m4_dquote_eltreturns as many arguments as it was passed.
Return the arguments, with the same level of quoting. Other than discarding whitespace after unquoted commas, this macro is a no-op.
Return the expansion of arg as a quoted string. Whereas
m4_quoteis designed to collect expanded text into a single argument,m4_expandis designed to perform one level of expansion on quoted text. One distinction is in the treatment of whitespace following a comma in the original arg. Any time multiple arguments are collected into one withm4_quote, the M4 argument collection rules discard the whitespace. However, withm4_expand, whitespace is preserved, even after the expansion of macros contained in arg. Additionally,m4_expandis able to expand text that would involve an unterminated comment, whereas expanding that same text as the argument tom4_quoteruns into difficulty in finding the end of the argument. Since manipulating diversions during argument collection is inherently unsafe,m4_expandissues an error if arg attempts to change the current diversion (see Diversion support).m4_define([active], [ACT, IVE])dnl m4_define([active2], [[ACT, IVE]])dnl m4_quote(active, active) ⇒ACT,IVE,ACT,IVE m4_expand([active, active]) ⇒ACT, IVE, ACT, IVE m4_quote(active2, active2) ⇒ACT, IVE,ACT, IVE m4_expand([active2, active2]) ⇒ACT, IVE, ACT, IVE m4_expand([# m4_echo]) ⇒# m4_echo m4_quote(# m4_echo) ) ⇒# m4_echo) ⇒Note that
m4_expandcannot handle an arg that expands to literal unbalanced quotes, but that quadrigraphs can be used when unbalanced output is necessary. Likewise, unbalanced parentheses should be supplied with double quoting or a quadrigraph.m4_define([pattern], [[!@<:@]])dnl m4_define([bar], [BAR])dnl m4_expand([case $foo in m4_defn([pattern])@:}@ bar ;; *[)] blah ;; esac]) ⇒case $foo in ⇒ [![]) BAR ;; ⇒ *) blah ;; ⇒esac
This macro was introduced in Autoconf 2.62. Expands to nothing, ignoring all of its arguments. By itself, this isn't very useful. However, it can be used to conditionally ignore an arbitrary number of arguments, by deciding which macro name to apply to a list of arguments.
dnl foo outputs a message only if [debug] is defined. m4_define([foo], [m4_ifdef([debug],[AC_MSG_NOTICE],[m4_ignore])([debug message])])Note that for earlier versions of Autoconf, the macro
__gnu__can serve the same purpose, although it is less readable.
This macro exists to aid debugging of M4sugar algorithms. Its net effect is similar to
m4_dquote—it produces a quoted list of quoted arguments, for each arg. The difference is that this version uses a comma-newline separator instead of just comma, to improve readability of the list; with the result that it is less efficient thanm4_dquote.m4_define([zero],[0])m4_define([one],[1])m4_define([two],[2])dnl m4_dquote(zero, [one], [[two]]) ⇒[0],[one],[[two]] m4_make_list(zero, [one], [[two]]) ⇒[0], ⇒[one], ⇒[[two]] m4_foreach([number], m4_dquote(zero, [one], [[two]]), [ number]) ⇒ 0 1 two m4_foreach([number], m4_make_list(zero, [one], [[two]]), [ number]) ⇒ 0 1 two
Return the arguments as a single entity, i.e., wrap them into a pair of quotes. This effectively collapses multiple arguments into one, although it loses whitespace after unquoted commas in the process.
Outputs each argument with the same level of quoting, but in reverse order, and with space following each comma for readability.
m4_define([active], [ACT,IVE]) ⇒ m4_reverse(active, [active]) ⇒active, IVE, ACT
This macro was introduced in Autoconf 2.62. Expand each argument, separated by commas. For a single arg, this effectively removes a layer of quoting, and
m4_unquote([arg])is more efficient than the equivalentm4_do([arg]). For multiple arguments, this results in an unquoted list of expansions. This is commonly used withm4_split, in order to convert a single quoted list into a series of quoted elements.
The following example aims at emphasizing the difference between several
scenarios: not using these macros, using m4_defn, using
m4_quote, using m4_dquote, and using m4_expand.
$ cat example.m4
dnl Overquote, so that quotes are visible.
m4_define([show], [$[]1 = [$1], $[]@ = [$@]])
m4_define([a], [A])
m4_define([mkargs], [1, 2[,] 3])
m4_define([arg1], [[$1]])
m4_divert([0])dnl
show(a, b)
show([a, b])
show(m4_quote(a, b))
show(m4_dquote(a, b))
show(m4_expand([a, b]))
arg1(mkargs)
arg1([mkargs])
arg1(m4_defn([mkargs]))
arg1(m4_quote(mkargs))
arg1(m4_dquote(mkargs))
arg1(m4_expand([mkargs]))
$ autom4te -l m4sugar example.m4
$1 = A, $@ = [A],[b]
$1 = a, b, $@ = [a, b]
$1 = A,b, $@ = [A,b]
$1 = [A],[b], $@ = [[A],[b]]
$1 = A, b, $@ = [A, b]
1
mkargs
1, 2[,] 3
1,2, 3
[1],[2, 3]
1, 2, 3
The following macros may be used to manipulate strings in M4. Many of the macros in this section intentionally result in quoted strings as output, rather than subjecting the arguments to further expansions. As a result, if you are manipulating text that contains active M4 characters, the arguments are passed with single quoting rather than double.
Redefine macro-name to its former contents with separator and string added at the end. If macro-name was undefined before (but not if it was defined but empty), then no separator is added. As of Autoconf 2.62, neither string nor separator are expanded during this macro; instead, they are expanded when macro-name is invoked.
m4_appendcan be used to grow strings, andm4_append_uniqto grow strings without duplicating substrings. Additionally,m4_append_uniqtakes two optional parameters as of Autoconf 2.62; if-uniq is expanded if string was appended, and if-duplicate is expanded if string was already present. Also,m4_append_uniqwarns if separator is not empty, but occurs within string, since that can lead to duplicates.Note that
m4_appendcan scale linearly in the length of the final string, depending on the quality of the underlying M4 implementation, whilem4_append_uniqhas an inherent quadratic scaling factor. If an algorithm can tolerate duplicates in the final string, use the former for speed. If duplicates must be avoided, consider usingm4_set_addinstead (see Set manipulation Macros).m4_define([active], [ACTIVE])dnl m4_append([sentence], [This is an])dnl m4_append([sentence], [ active ])dnl m4_append([sentence], [symbol.])dnl sentence ⇒This is an ACTIVE symbol. m4_undefine([active])dnl ⇒This is an active symbol. m4_append_uniq([list], [one], [, ], [new], [existing]) ⇒new m4_append_uniq([list], [one], [, ], [new], [existing]) ⇒existing m4_append_uniq([list], [two], [, ], [new], [existing]) ⇒new m4_append_uniq([list], [three], [, ], [new], [existing]) ⇒new m4_append_uniq([list], [two], [, ], [new], [existing]) ⇒existing list ⇒one, two, three m4_dquote(list) ⇒[one],[two],[three] m4_append([list2], [one], [[, ]])dnl m4_append_uniq([list2], [two], [[, ]])dnl m4_append([list2], [three], [[, ]])dnl list2 ⇒one, two, three m4_dquote(list2) ⇒[one, two, three]
This macro was introduced in Autoconf 2.62. It is similar to
m4_append_uniq, but treats strings as a whitespace separated list of words to append, and only appends unique words. macro-name is updated with a single space between new words.m4_append_uniq_w([numbers], [1 1 2])dnl m4_append_uniq_w([numbers], [ 2 3 ])dnl numbers ⇒1 2 3
Output string in quotes, but without a trailing newline. The macro
m4_chompis slightly faster, and removes at most one newline; the macrom4_chomp_allremoves all consecutive trailing newlines. Unlikem4_flatten, embedded newlines are left intact, and backslash does not influence the result.
This macro produces a quoted string containing the pairwise combination of every element of the quoted, comma-separated prefix-list, and every element from the suffix arguments. Each pairwise combination is joined with infix in the middle, and successive pairs are joined by separator. No expansion occurs on any of the arguments. No output occurs if either the prefix or suffix list is empty, but the lists can contain empty elements.
m4_define([a], [oops])dnl m4_combine([, ], [[a], [b], [c]], [-], [1], [2], [3]) ⇒a-1, a-2, a-3, b-1, b-2, b-3, c-1, c-2, c-3 m4_combine([, ], [[a], [b]], [-]) ⇒ m4_combine([, ], [[a], [b]], [-], []) ⇒a-, b- m4_combine([, ], [], [-], [1], [2]) ⇒ m4_combine([, ], [[]], [-], [1], [2]) ⇒-1, -2
Convert all instances of ‘[’, ‘]’, ‘#’, and ‘$’ within string into their respective quadrigraphs. The result is still a quoted string.
Flatten string into a single line. Delete all backslash-newline pairs, and replace all remaining newlines with a space. The result is still a quoted string.
Concatenate each arg, separated by separator.
joinalluses every argument, whilejoinomits empty arguments so that there are no back-to-back separators in the output. The result is a quoted string.m4_define([active], [ACTIVE])dnl m4_join([|], [one], [], [active], [two]) ⇒one|active|two m4_joinall([|], [one], [], [active], [two]) ⇒one||active|twoNote that if all you intend to do is join args with commas between them, to form a quoted list suitable for
m4_foreach, it is more efficient to usem4_dquote.
This macro was introduced in Autoconf 2.62, and expands to a newline, followed by any text. It is primarily useful for maintaining macro formatting, and ensuring that M4 does not discard leading whitespace during argument collection.
Remove leading and trailing spaces and tabs, sequences of backslash-then-newline, and replace multiple spaces, tabs, and newlines with a single space. This is a combination of
m4_flattenandm4_strip. To determine if string consists only of bytes that would be removed bym4_normalize, you can usem4_ifblank.
Backslash-escape all characters in string that are active in regexps.
Split string into an M4 list of elements quoted by ‘[’ and ‘]’, while keeping white space at the beginning and at the end. If regexp is given, use it instead of ‘[\t ]+’ for splitting. If string is empty, the result is an empty list.
Strip whitespace from string. Sequences of spaces and tabs are reduced to a single space, then leading and trailing spaces are removed. The result is still a quoted string. Note that this does not interfere with newlines; if you want newlines stripped as well, consider
m4_flatten, or do it all at once withm4_normalize. To quickly test if string has only whitespace, usem4_ifblank.
Add a text box around message, using frame as the border character above and below the message. The frame argument must be a single byte, and does not support quadrigraphs. The frame correctly accounts for the subsequent expansion of message. For example:
m4_define([macro], [abc])dnl m4_text_box([macro]) ⇒## --- ## ⇒## abc ## ⇒## --- ##The message must contain balanced quotes and parentheses, although quadrigraphs can be used to work around this.
Break string into a series of whitespace-separated words, then output those words separated by spaces, and wrapping lines any time the output would exceed width columns. If given, prefix1 begins the first line, and prefix begins all wrapped lines. If prefix1 is longer than prefix, then the first line consists of just prefix1. If prefix is longer than prefix1, padding is inserted so that the first word of string begins at the same indentation as all wrapped lines. Note that using literal tab characters in any of the arguments will interfere with the calculation of width. No expansions occur on prefix, prefix1, or the words of string, although quadrigraphs are recognized.
For some examples:
m4_text_wrap([Short string */], [ ], [/* ], [20]) ⇒/* Short string */ m4_text_wrap([Much longer string */], [ ], [/* ], [20]) ⇒/* Much longer ⇒ string */ m4_text_wrap([Short doc.], [ ], [ --short ], [30]) ⇒ --short Short doc. m4_text_wrap([Short doc.], [ ], [ --too-wide ], [30]) ⇒ --too-wide ⇒ Short doc. m4_text_wrap([Super long documentation.], [ ], [ --too-wide ], 30) ⇒ --too-wide ⇒ Super long ⇒ documentation.
Return string with letters converted to upper or lower case, respectively.
The following macros facilitate integer arithmetic operations.
Where a parameter is documented as taking an arithmetic expression, you
can use anything that can be parsed by m4_eval.
Compare the arithmetic expressions expr-1 and expr-2, and expand to ‘-1’ if expr-1 is smaller, ‘0’ if they are equal, and ‘1’ if expr-1 is larger.
Compare the two M4 lists consisting of comma-separated arithmetic expressions, left to right. Expand to ‘-1’ for the first element pairing where the value from list-1 is smaller, ‘1’ where the value from list-2 is smaller, or ‘0’ if both lists have the same values. If one list is shorter than the other, the remaining elements of the longer list are compared against zero.
m4_list_cmp([1, 0], [1]) ⇒0 m4_list_cmp([1, [1 * 0]], [1, 0]) ⇒0 m4_list_cmp([1, 2], [1, 0]) ⇒1 m4_list_cmp([1, [1+1], 3],[1, 2]) ⇒1 m4_list_cmp([1, 2, -3], [1, 2]) ⇒-1 m4_list_cmp([1, 0], [1, 2]) ⇒-1 m4_list_cmp([1], [1, 2]) ⇒-1
This macro was introduced in Autoconf 2.62. Expand to the decimal value of the maximum arithmetic expression among all the arguments.
This macro was introduced in Autoconf 2.62. Expand to the decimal value of the minimum arithmetic expression among all the arguments.
Expand to ‘-1’ if the arithmetic expression expr is negative, ‘1’ if it is positive, and ‘0’ if it is zero.
This macro was introduced in Autoconf 2.53, but had a number of usability limitations that were not lifted until Autoconf 2.62. Compare the version strings version-1 and version-2, and expand to ‘-1’ if version-1 is smaller, ‘0’ if they are the same, or ‘1’ version-2 is smaller. Version strings must be a list of elements separated by ‘.’, ‘,’ or ‘-’, where each element is a number along with optional case-insensitive letters designating beta releases. The comparison stops at the leftmost element that contains a difference, although a 0 element compares equal to a missing element.
It is permissible to include commit identifiers in version, such as an abbreviated SHA1 of the commit, provided there is still a monotonically increasing prefix to allow for accurate version-based comparisons. For example, this paragraph was written when the development snapshot of autoconf claimed to be at version ‘2.61a-248-dc51’, or 248 commits after the 2.61a release, with an abbreviated commit identification of ‘dc51’.
m4_version_compare([1.1], [2.0]) ⇒-1 m4_version_compare([2.0b], [2.0a]) ⇒1 m4_version_compare([1.1.1], [1.1.1a]) ⇒-1 m4_version_compare([1.2], [1.1.1a]) ⇒1 m4_version_compare([1.0], [1]) ⇒0 m4_version_compare([1.1pre], [1.1PRE]) ⇒0 m4_version_compare([1.1a], [1,10]) ⇒-1 m4_version_compare([2.61a], [2.61a-248-dc51]) ⇒-1 m4_version_compare([2.61b], [2.61a-248-dc51]) ⇒1
Compares version against the version of Autoconf currently running. If the running version is at version or newer, expand if-new-enough, but if version is larger than the version currently executing, expand if-old, which defaults to printing an error message and exiting m4sugar with status 63. When given only one argument, this behaves like
AC_PREREQ(see Versioning). Remember that the autoconf philosophy favors feature checks over version checks.
Sometimes, it is necessary to track a set of data, where the order does
not matter and where there are no duplicates in the set. The following
macros facilitate set manipulations. Each set is an opaque object,
which can only be accessed via these basic operations. The underlying
implementation guarantees linear scaling for set creation, which is more
efficient than using the quadratic m4_append_uniq. Both set
names and values can be arbitrary strings, except for unbalanced quotes.
This implementation ties up memory for removed elements until the next
operation that must traverse all the elements of a set; and although
that may slow down some operations until the memory for removed elements
is pruned, it still guarantees linear performance.
Adds the string value as a member of set set. Expand if-uniq if the element was added, or if-dup if it was previously in the set. Operates in amortized constant time, so that set creation scales linearly.
Adds each value to the set set. This is slightly more efficient than repeatedly invoking
m4_set_add.
Expands if-present if the string value is a member of set, otherwise if-absent.
m4_set_contains([a], [1], [yes], [no]) ⇒no m4_set_add([a], [1], [added], [dup]) ⇒added m4_set_add([a], [1], [added], [dup]) ⇒dup m4_set_contains([a], [1], [yes], [no]) ⇒yes m4_set_remove([a], [1], [removed], [missing]) ⇒removed m4_set_contains([a], [1], [yes], [no]) ⇒no m4_set_remove([a], [1], [removed], [missing]) ⇒missing
Expands to a single string consisting of all the members of the set set, each separated by sep, which is not expanded.
m4_set_contentsleaves the elements in set but reclaims any memory occupied by removed elements, whilem4_set_dumpis a faster one-shot action that also deletes the set. No provision is made for disambiguating members that contain a non-empty sep as a substring; usem4_set_emptyto distinguish between an empty set and the set containing only the empty string. The order of the output is unspecified; in the current implementation, part of the speed ofm4_set_dumpresults from using a different output order thanm4_set_contents. These macros scale linearly in the size of the set before memory pruning, andm4_set_contents([set], [sep])is faster thanm4_joinall([sep]m4_set_listc([set])).m4_set_add_all([a], [1], [2], [3]) ⇒ m4_set_contents([a], [-]) ⇒1-2-3 m4_joinall([-]m4_set_listc([a])) ⇒1-2-3 m4_set_dump([a], [-]) ⇒3-2-1 m4_set_contents([a]) ⇒ m4_set_add([a], []) ⇒ m4_set_contents([a], [-]) ⇒
Delete all elements and memory associated with set. This is linear in the set size, and faster than removing one element at a time.
Compute the relation between seta and setb, and output the result as a list of quoted arguments without duplicates and with a leading comma. Set difference selects the elements in seta but not setb, intersection selects only elements in both sets, and union selects elements in either set. These actions are linear in the sum of the set sizes. The leading comma is necessary to distinguish between no elements and the empty string as the only element.
m4_set_add_all([a], [1], [2], [3]) ⇒ m4_set_add_all([b], [3], [], [4]) ⇒ m4_set_difference([a], [b]) ⇒,1,2 m4_set_difference([b], [a]) ⇒,,4 m4_set_intersection([a], [b]) ⇒,3 m4_set_union([a], [b]) ⇒,1,2,3,,4
Expand if-empty if the set set has no elements, otherwise expand if-elements. This macro operates in constant time. Using this macro can help disambiguate output from
m4_set_contentsorm4_set_list.
For each element in the set set, expand action with the macro variable defined as the set element. Behavior is unspecified if action recursively lists the contents of set (although listing other sets is acceptable), or if it modifies the set in any way other than removing the element currently contained in variable. This macro is faster than the corresponding
m4_foreach([variable], m4_indir([m4_dquote]m4_set_listc([set])), [action]), althoughm4_set_mapmight be faster still.m4_set_add_all([a]m4_for([i], [1], [5], [], [,i])) ⇒ m4_set_contents([a]) ⇒12345 m4_set_foreach([a], [i], [m4_if(m4_eval(i&1), [0], [m4_set_remove([a], i, [i])])]) ⇒24 m4_set_contents([a]) ⇒135
Produce a list of arguments, where each argument is a quoted element from the set set. The variant
m4_set_listcis unambiguous, by adding a leading comma if there are any set elements, whereas the variantm4_set_listcannot distinguish between an empty set and a set containing only the empty string. These can be directly used in macros that take multiple arguments, such asm4_joinorm4_set_add_all, or wrapped bym4_dquotefor macros that take a quoted list, such asm4_maporm4_foreach. Any memory occupied by removed elements is reclaimed during these macros.m4_set_add_all([a], [1], [2], [3]) ⇒ m4_set_list([a]) ⇒1,2,3 m4_set_list([b]) ⇒ m4_set_listc([b]) ⇒ m4_count(m4_set_list([b])) ⇒1 m4_set_empty([b], [0], [m4_count(m4_set_list([b]))]) ⇒0 m4_set_add([b], []) ⇒ m4_set_list([b]) ⇒ m4_set_listc([b]) ⇒, m4_count(m4_set_list([b])) ⇒1 m4_set_empty([b], [0], [m4_count(m4_set_list([b]))]) ⇒1
For each element in the set set, expand action with a single argument of the set element. Behavior is unspecified if action recursively lists the contents of set (although listing other sets is acceptable), or if it modifies the set in any way other than removing the element passed as an argument. This macro is faster than either corresponding counterpart of
m4_map_args([action]m4_set_listc([set]))orm4_set_foreach([set], [var], [action(m4_defn([var]))]). It is possible to usem4_curryif more than one argument is needed for action, although it is more efficient to usem4_set_map_sepin that case.
For each element in the set set, expand pre
[element]post, additionally expanding sep between elements. Behavior is unspecified if the expansion recursively lists the contents of set (although listing other sets is acceptable), or if it modifies the set in any way other than removing the element visited by the expansion. This macro provides the most efficient means for non-destructively visiting the elements of a set; in particular,m4_set_map([set], [action])is equivalent tom4_set_map_sep([set], [action(], [)]).
If value is an element in the set set, then remove it and expand if-present. Otherwise expand if-absent. This macro operates in constant time so that multiple removals will scale linearly rather than quadratically; but when used outside of
m4_set_foreachorm4_set_map, it leaves memory occupied until the set is later compacted bym4_set_contentsorm4_set_list. Several other set operations are then less efficient between the time of element removal and subsequent memory compaction, but still maintain their guaranteed scaling performance.
Expand to the size of the set set. This implementation operates in constant time, and is thus more efficient than
m4_eval(m4_count(m4_set_listc([set])) - 1).
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 overquotation).
M4sugar forbids all the tokens matching ‘^_?m4_’ and ‘^dnl$’. Additional layers, such as M4sh and Autoconf, add additional forbidden patterns to the list.
Declare that no token matching pattern must be found in the output. Comments are not checked; this can be a problem if, for instance, you have some macro left unexpanded after an ‘#include’. No consensus is currently found in the Autoconf community, as some people consider it should be valid to name macros in comments (which doesn't make sense to the authors of this documentation: input, such as macros, should be documented by ‘dnl’ comments; reserving ‘#’-comments to document the output).
Of course, you might encounter exceptions to these generic rules, for instance you might have to refer to ‘$m4_flags’.
Any token matching pattern is allowed, including if it matches an
m4_pattern_forbidpattern.
At times, it is desirable to see what was happening inside m4, to see
why output was not matching expectations. However, post-processing done
by autom4te means that directly using the m4 builtin
m4_traceon is likely to interfere with operation. Also, frequent
diversion changes and the concept of forbidden tokens make it difficult
to use m4_defn to generate inline comments in the final output.
There are a couple of tools to help with this. One is the use of the --trace option provided by autom4te (as well as each of the programs that wrap autom4te, such as autoconf), in order to inspect when a macro is called and with which arguments. For example, when this paragraph was written, the autoconf version could be found by:
$ autoconf --trace=AC_INIT
configure.ac:23:AC_INIT:GNU Autoconf:2.63b.95-3963:bug-autoconf@gnu.org
$ autoconf --trace='AC_INIT:version is $2'
version is 2.63b.95-3963
Another trick is to print out the expansion of various m4 expressions to
standard error or to an independent file, with no further m4 expansion,
and without interfering with diversion changes or the post-processing
done to standard output. m4_errprintn shows a given expression
on standard error. For example, if you want to see the expansion of an
autoconf primitive or of one of your autoconf macros, you can do it like
this:
$ cat <<\EOF > configure.ac
AC_INIT
m4_errprintn([The definition of AC_DEFINE_UNQUOTED:])
m4_errprintn(m4_defn([AC_DEFINE_UNQUOTED]))
AC_OUTPUT
EOF
$ autoconf
error-->The definition of AC_DEFINE_UNQUOTED:
error-->_AC_DEFINE_Q([], $@)
M4sh, pronounced “mash”, is aiming at producing portable Bourne shell scripts. This name was coined by Lars J. Aas, who notes that, according to the Webster's Revised Unabridged Dictionary (1913):
Mash \Mash\, n. [Akin to G. meisch, maisch, meische, maische, mash, wash, and prob. to AS. miscian to mix. See “Mix”.]
- A mass of mixed ingredients reduced to a soft pulpy state by beating or pressure...
- A mixture of meal or bran and water fed to animals.
- A mess; trouble. [Obs.] –Beau. & Fl.
M4sh reserves the M4 macro namespace ‘^_AS_’ for internal use, and the namespace ‘^AS_’ for M4sh macros. It also reserves the shell and environment variable namespace ‘^as_’, and the here-document delimiter namespace ‘^_AS[A-Z]’ in the output file. You should not define your own macros or output shell code that conflicts with these namespaces.
M4sh provides portable alternatives for some common shell constructs that unfortunately are not portable in practice.
Expand into shell code that will output text surrounded by a box with char in the top and bottom border. text should not contain a newline, but may contain shell expansions valid for unquoted here-documents. char defaults to ‘-’, but can be any character except ‘/’, ‘'’, ‘"’, ‘\’, ‘&’, or ‘`’. This is useful for outputting a comment box into log files to separate distinct phases of script operation.
Expand into a shell ‘case’ statement, where word is matched against one or more patterns. if-matched is run if the corresponding pattern matched word, else default is run. Avoids several portability issues (see Limitations of Shell Builtins).
Output the directory portion of file-name. For example, if
$fileis ‘/one/two/three’, the commanddir=`AS_DIRNAME(["$file"])`setsdirto ‘/one/two’.This interface may be improved in the future to avoid forks and losing trailing newlines.
Emits word to the standard output, followed by a newline. word must be a single shell word (typically a quoted string). The bytes of word are output as-is, even if it starts with "-" or contains "\". Redirections can be placed outside the macro invocation. This is much more portable than using echo (see Limitations of Shell Builtins).
Emits word to the standard output, without a following newline. word must be a single shell word (typically a quoted string) and, for portability, should not include more than one newline. The bytes of word are output as-is, even if it starts with "-" or contains "\". Redirections can be placed outside the macro invocation.
Expands to string, with any characters in chars escaped with a backslash (‘\’). chars should be at most four bytes long, and only contain characters from the set ‘`\"$’; however, characters may be safely listed more than once in chars for the sake of syntax highlighting editors. The current implementation expands string after adding escapes; if string contains macro calls that in turn expand to text needing shell quoting, you can use
AS_ESCAPE(m4_dquote(m4_expand([string]))).The default for chars (‘\"$`’) is the set of characters needing escapes when string will be used literally within double quotes. One common variant is the set of characters to protect when string will be used literally within back-ticks or an unquoted here-document (‘\$`’). Another common variant is ‘""’, which can be used to form a double-quoted string containing the same expansions that would have occurred if string were expanded in an unquoted here-document; however, when using this variant, care must be taken that string does not use double quotes within complex variable expansions (such as ‘${foo-`echo "hi"`}’) that would be broken with improper escapes.
This macro is often used with
AS_ECHO. For an example, observe the output generated by the shell code generated from this snippet:foo=bar AS_ECHO(["AS_ESCAPE(["$foo" = ])AS_ESCAPE(["$foo"], [""])"]) ⇒"$foo" = "bar" m4_define([macro], [a, [\b]]) AS_ECHO(["AS_ESCAPE([[macro]])"]) ⇒macro AS_ECHO(["AS_ESCAPE([macro])"]) ⇒a, b AS_ECHO(["AS_ESCAPE(m4_dquote(m4_expand([macro])))"]) ⇒a, \bTo escape a string that will be placed within single quotes, use:
m4_bpatsubst([[string]], ['], ['\\''])
Emit code to exit the shell with status, defaulting to ‘$?’. This macro works around shells that see the exit status of the command prior to
exitinside a ‘trap 0’ handler (see Limitations of Shell Builtins).
Run shell code test1. If test1 exits with a zero status then run shell code run-if-true1, else examine further tests. If no test exits with a zero status, run shell code run-if-false, with simplifications if either run-if-true1 or run-if-false is empty. For example,
AS_IF([test "x$foo" = xyes], [HANDLE_FOO([yes])], [test "x$foo" != xno], [HANDLE_FOO([maybe])], [echo foo not specified])ensures any required macros of
HANDLE_FOOare expanded before the first test.
Make the directory file-name, including intervening directories as necessary. This is equivalent to ‘mkdir -p -- file-name’, except that it is portable to older versions of mkdir that lack support for the -p option or for the -- delimiter (see Limitations of Usual Tools). Also,
AS_MKDIR_Psucceeds if file-name is a symbolic link to an existing directory, even though Posix is unclear whether ‘mkdir -p’ should succeed in that case. If creation of file-name fails, exit the script.Also see the
AC_PROG_MKDIR_Pmacro (see Particular Programs).
Emit shell code to set the value of ‘$?’ to status, as efficiently as possible. However, this is not guaranteed to abort a shell running with
set -e(see Limitations of Shell Builtins). This should also be used at the end of a complex shell function instead of ‘return’ (see Shell Functions) to avoid a DJGPP shell bug.
Transform expression into a valid right-hand side for a C
#define. For example:# This outputs "#define HAVE_CHAR_P 1". # Notice the m4 quoting around #, to prevent an m4 comment type="char *" echo "[#]define AS_TR_CPP([HAVE_$type]) 1"
Transform expression into shell code that generates a valid shell variable name. The result is literal when possible at m4 time, but must be used with
evalif expression causes shell indirections. For example:# This outputs "Have it!". header="sys/some file.h" eval AS_TR_SH([HAVE_$header])=yes if test "x$HAVE_sys_some_file_h" = xyes; then echo "Have it!"; fi
Set the polymorphic shell variable var to dir/file, but optimizing the common cases (dir or file is ‘.’, file is absolute, etc.).
Unsets the shell variable var, working around bugs in older shells (see Limitations of Shell Builtins). var can be a literal or indirect variable name.
Compare two strings version-1 and version-2, possibly containing shell variables, as version strings, and expand action-if-less, action-if-equal, or action-if-greater depending upon the result. The algorithm to compare is similar to the one used by strverscmp in glibc (see String/Array Comparison).
Often, it is convenient to write a macro that will emit shell code operating on a shell variable. The simplest case is when the variable name is known. But a more powerful idiom is writing shell code that can work through an indirection, where another variable or command substitution produces the name of the variable to actually manipulate. M4sh supports the notion of polymorphic shell variables, making it easy to write a macro that can deal with either literal or indirect variable names and output shell code appropriate for both use cases. Behavior is undefined if expansion of an indirect variable does not result in a literal variable name.
If the expansion of expression is definitely a shell literal, expand if-literal. If the expansion of expression looks like it might contain shell indirections (such as
$varor`expr`), then if-not is expanded. Sometimes, it is possible to output optimized code if expression consists only of shell variable expansions (such as${var}), in which case if-simple-ref can be provided; but defaulting to if-not should always be safe.AS_LITERAL_WORD_IFonly expands if-literal if expression looks like a single shell word, containing no whitespace; whileAS_LITERAL_IFallows whitespace in expression.In order to reduce the time spent recognizing whether an expression qualifies as a literal or a simple indirection, the implementation is somewhat conservative: expression must be a single shell word (possibly after stripping whitespace), consisting only of bytes that would have the same meaning whether unquoted or enclosed in double quotes (for example, ‘a.b’ results in if-literal, even though it is not a valid shell variable name; while both ‘'a'’ and ‘[$]’ result in if-not, because they behave differently than ‘"'a'"’ and ‘"[$]"’). This macro can be used in contexts for recognizing portable file names (such as in the implementation of
AC_LIBSOURCE), or coupled with some transliterations for forming valid variable names (such as in the implementation ofAS_TR_SH, which uses an additionalm4_translitto convert ‘.’ to ‘_’).This example shows how to read the contents of the shell variable
bar, exercising all three arguments toAS_LITERAL_IF. It results in a script that will output the line ‘hello’ three times.AC_DEFUN([MY_ACTION], [AS_LITERAL_IF([$1], [echo "$$1"], [AS_VAR_COPY([var], [$1]) echo "$var"], [eval 'echo "$'"$1"\"])]) foo=bar bar=hello MY_ACTION([bar]) MY_ACTION([`echo bar`]) MY_ACTION([$foo])
Emit shell code to append the shell expansion of text to the end of the current contents of the polymorphic shell variable var, taking advantage of shells that provide the ‘+=’ extension for more efficient scaling.
For situations where the final contents of var are relatively short (less than 256 bytes), it is more efficient to use the simpler code sequence of var
=${var}text (or its polymorphic equivalent ofAS_VAR_COPY([t], [var])andAS_VAR_SET([var], ["$t"text])). But in the case when the script will be repeatedly appending text intovar, issues of scaling start to become apparent. A naive implementation requires execution time linear to the length of the current contents of var as well as the length of text for a single append, for an overall quadratic scaling with multiple appends. This macro takes advantage of shells which provide the extension var+=text, which can provide amortized constant time for a single append, for an overall linear scaling with multiple appends. Note that unlikeAS_VAR_SET, this macro requires that text be quoted properly to avoid field splitting and file name expansion.
Emit shell code to compute the arithmetic expansion of expression, assigning the result as the contents of the polymorphic shell variable var. The code takes advantage of shells that provide ‘$(())’ for fewer forks, but uses expr as a fallback. Therefore, the syntax for a valid expression is rather limited: all operators must occur as separate shell arguments and with proper quoting, there is no portable equality operator, all variables containing numeric values must be expanded prior to the computation, all numeric values must be provided in decimal without leading zeroes, and the first shell argument should not be a negative number. In the following example, this snippet will print ‘(2+3)*4 == 20’.
bar=3 AS_VAR_ARITH([foo], [\( 2 + $bar \) \* 4]) echo "(2+$bar)*4 == $foo"
Emit shell code to assign the contents of the polymorphic shell variable source to the polymorphic shell variable dest. For example, executing this M4sh snippet will output ‘bar hi’:
foo=bar bar=hi AS_VAR_COPY([a], [foo]) AS_VAR_COPY([b], [$foo]) echo "$a $b"When it is necessary to access the contents of an indirect variable inside a shell double-quoted context, the recommended idiom is to first copy the contents into a temporary literal shell variable.
for header in stdint_h inttypes_h ; do AS_VAR_COPY([var], [ac_cv_header_$header]) echo "$header detected: $var" done
Output a shell conditional statement. If the contents of the polymorphic shell variable var match the string word, execute if-equal; otherwise execute if-not-equal. word must be a single shell word (typically a quoted string). Avoids shell bugs if an interrupt signal arrives while a command substitution in var is being expanded.
A common M4sh idiom involves composing shell variable names from an m4 argument (for example, writing a macro that uses a cache variable). value can be an arbitrary string, which will be transliterated into a valid shell name by
AS_TR_SH. In order to access the composed variable name based on value, it is easier to declare a temporary m4 macro m4-name withAS_VAR_PUSHDEF, then use that macro as the argument to subsequentAS_VARmacros as a polymorphic variable name, and finally free the temporary macro withAS_VAR_POPDEF. These macros are often followed withdnl, to avoid excess newlines in the output.Here is an involved example, that shows the power of writing macros that can handle composed shell variable names:
m4_define([MY_CHECK_HEADER], [AS_VAR_PUSHDEF([my_Header], [ac_cv_header_$1])dnl AS_VAR_IF([my_Header], [yes], [echo "header $1 detected"])dnl AS_VAR_POPDEF([my_Header])dnl ]) MY_CHECK_HEADER([stdint.h]) for header in inttypes.h stdlib.h ; do MY_CHECK_HEADER([$header]) doneIn the above example,
MY_CHECK_HEADERcan operate on polymorphic variable names. In the first invocation, the m4 argument isstdint.h, which transliterates into a literalstdint_h. As a result, the temporary macromy_Headerexpands to the literal shell name ‘ac_cv_header_stdint_h’. In the second invocation, the m4 argument toMY_CHECK_HEADERis$header, and the temporary macromy_Headerexpands to the indirect shell name ‘$as_my_Header’. During the shell execution of the for loop, when ‘$header’ contains ‘inttypes.h’, then ‘$as_my_Header’ contains ‘ac_cv_header_inttypes_h’. If this script is then run on a platform where all three headers have been previously detected, the output of the script will include:header stdint.h detected header inttypes.h detected header stdlib.h detected
Emit shell code to assign the contents of the polymorphic shell variable var to the shell expansion of value. value is not subject to field splitting or file name expansion, so if command substitution is used, it may be done with ‘`""`’ rather than using an intermediate variable (see Shell Substitutions). However, value does undergo rescanning for additional macro names; behavior is unspecified if late expansion results in any shell meta-characters.
Emit a shell conditional statement, which executes if-set if the polymorphic shell variable
varis set to any value, and if-undef otherwise.
Emit a shell statement that results in a successful exit status only if the polymorphic shell variable
varis set.
Set up the shell to be more compatible with the Bourne shell as standardized by Posix, if possible. This may involve setting environment variables, or setting options, or similar implementation-specific actions. This macro is deprecated, since
AS_INITalready invokes it.
Initialize the M4sh environment. This macro calls
m4_init, then outputs the#! /bin/shline, a notice about where the output was generated from, and code to sanitize the environment for the rest of the script. Among other initializations, this sets SHELL to the shell chosen to run the script (see CONFIG_SHELL), and LC_ALL to ensure the C locale. Finally, it changes the current diversion toBODY.AS_INITis called automatically byAC_INITandAT_INIT, so shell code in configure, config.status, and testsuite all benefit from a sanitized shell environment.
Emit shell code to start the creation of a subsidiary shell script in file, including changing file to be executable. This macro populates the child script with information learned from the parent (thus, the emitted code is equivalent in effect, but more efficient, than the code output by
AS_INIT,AS_BOURNE_COMPATIBLE, andAS_SHELL_SANITIZE). If present, comment is output near the beginning of the child, prior to the shell initialization code, and is subject to parameter expansion, command substitution, and backslash quote removal. The parent script should check the exit status after this macro, in case file could not be properly created (for example, if the disk was full). If successfully created, the parent script can then proceed to append additional M4sh constructs into the child script.Note that the child script starts life without a log file open, so if the parent script uses logging (see AS_MESSAGE_LOG_FD), you must temporarily disable any attempts to use the log file until after emitting code to open a log within the child. On the other hand, if the parent script has
AS_MESSAGE_FDredirected somewhere besides ‘1’, then the child script already has code that copies stdout to that descriptor. Currently, the suggested idiom for writing a M4sh shell script from within another script is:AS_INIT_GENERATED([file], [[# My child script. ]]) || { AS_ECHO(["Failed to create child script"]); AS_EXIT; } m4_pushdef([AS_MESSAGE_LOG_FD])dnl cat >> "file" <<\__EOF__ # Code to initialize AS_MESSAGE_LOG_FD m4_popdef([AS_MESSAGE_LOG_FD])dnl # Additional code __EOF__This, however, may change in the future as the M4sh interface is stabilized further.
Also, be aware that use of LINENO within the child script may report line numbers relative to their location in the parent script, even when using
AS_LINENO_PREPARE, if the parent script was unable to locate a shell with working LINENO support.
Find a shell that supports the special variable LINENO, which contains the number of the currently executing line. This macro is automatically invoked by
AC_INITin configure scripts.
Set up variable as_me to be the basename of the currently executing script. This macro is automatically invoked by
AC_INITin configure scripts.
Create, as safely as possible, a temporary sub-directory within dir with a name starting with prefix. prefix should be 2-4 characters, to make it slightly easier to identify the owner of the directory. If dir is omitted, then the value of TMPDIR will be used (defaulting to ‘/tmp’). On success, the name of the newly created directory is stored in the shell variable
tmp. On error, the script is aborted.Typically, this macro is coupled with some exit traps to delete the created directory and its contents on exit or interrupt. However, there is a slight window between when the directory is created and when the name is actually known to the shell, so an interrupt at the right moment might leave the temporary directory behind. Hence it is important to use a prefix that makes it easier to determine if a leftover temporary directory from an interrupted script is safe to delete.
The use of the output variable ‘$tmp’ rather than something in the ‘as_’ namespace is historical; it has the unfortunate consequence that reusing this otherwise common name for any other purpose inside your script has the potential to break any cleanup traps designed to remove the temporary directory.
Initialize the shell suitably for configure scripts. This has the effect of
AS_BOURNE_COMPATIBLE, and sets some other environment variables for predictable results from configuration tests. For example, it sets LC_ALL to change to the default C locale. See Special Shell Variables. This macro is deprecated, sinceAS_INITalready invokes it.
The following macros define file descriptors used to output messages (or input values) from configure scripts. For example:
echo "$wombats found" >&AS_MESSAGE_LOG_FD
echo 'Enter desired kangaroo count:' >&AS_MESSAGE_FD
read kangaroos <&AS_ORIGINAL_STDIN_FD`
However doing so is seldom needed, because Autoconf provides higher level macros as described below.
The file descriptor for ‘checking for...’ messages and results. By default,
AS_INITsets this to ‘1’ for standalone M4sh clients. However,AC_INITshuffles things around to another file descriptor, in order to allow the -q option of configure to choose whether messages should go to the script's standard output or be discarded.If you want to display some messages, consider using one of the printing macros (see Printing Messages) instead. Copies of messages output via these macros are also recorded in config.log.
This must either be empty, or expand to a file descriptor for log messages. By default,
AS_INITsets this macro to the empty string for standalone M4sh clients, thus disabling logging. However,AC_INITshuffles things around so that both configure and config.status use config.log for log messages. Macros that run tools, likeAC_COMPILE_IFELSE(see Running the Compiler), redirect all output to this descriptor. You may want to do so if you develop such a low-level macro.
This must expand to a file descriptor for the original standard input. By default,
AS_INITsets this macro to ‘0’ for standalone M4sh clients. However,AC_INITshuffles things around for safety.When configure runs, it may accidentally execute an interactive command that has the same name as the non-interactive meant to be used or checked. If the standard input was the terminal, such interactive programs would cause configure to stop, pending some user input. Therefore configure redirects its standard input from /dev/null during its initialization. This is not normally a problem, since configure normally does not need user input.
In the extreme case where your configure script really needs to obtain some values from the original standard input, you can read them explicitly from
AS_ORIGINAL_STDIN_FD.
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.
Autoconf macros are defined using the
AC_DEFUNmacro, which is similar to the M4 builtinm4_definemacro; this creates a macro named name and with body as its expansion. In addition to defining a macro,AC_DEFUNadds to it some code that is used to constrain the order in which macros are called, while avoiding redundant output (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.
Most macros fall in one of two general categories. The first category
includes macros which take arguments, in order to generate output
parameterized by those arguments. Macros in this category are designed
to be directly expanded, often multiple times, and should not be used as
the argument to AC_REQUIRE. The other category includes macros
which are shorthand for a fixed block of text, and therefore do not take
arguments. For this category of macros, directly expanding the macro
multiple times results in redundant output, so it is more common to use
the macro as the argument to AC_REQUIRE, or to declare the macro
with AC_DEFUN_ONCE (see One-Shot 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],
[{ AS_MESSAGE([error: $1], [2])
exit m4_default([$2], [1]); }])
Comments about the macro should be left in the header comment. Most other comments make their way into configure, so just keep using ‘#’ to introduce comments.
If you have some 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.
Public third-party macros need to use AC_DEFUN, and not
m4_define, in order to be found by aclocal
(see Extending aclocal).
Additionally, if it is ever determined that a macro should be made
obsolete, it is easy to convert from AC_DEFUN to AU_DEFUN
in order to have autoupdate assist the user in choosing a
better alternative, but there is no corresponding way to make
m4_define issue an upgrade notice (see AU_DEFUN).
There is another subtle, but important, difference between using
m4_define and AC_DEFUN: only the former is unaffected by
AC_REQUIRE. When writing a file, it is always safe to replace a
block of text with a m4_define macro that will expand to the same
text. But replacing a block of text with an AC_DEFUN macro with
the same content does not necessarily give the same results, because it
changes the location where any embedded but unsatisfied
AC_REQUIRE invocations within the block will be expanded. For an
example of this, see Expanded Before Required.
All of the public Autoconf macros have all-uppercase names in the namespace ‘^AC_’ to prevent them from accidentally conflicting with other text; Autoconf also reserves the namespace ‘^_AC_’ for internal macros. All shell variables that they use for internal purposes have mostly-lowercase names starting with ‘ac_’. Autoconf also uses here-document delimiters in the namespace ‘^_AC[A-Z]’. During configure, files produced by Autoconf make heavy use of the file system namespace ‘^conf’.
Since Autoconf is built on top of M4sugar (see Programming in M4sugar) and M4sh (see Programming in M4sh), you must also be aware of those namespaces (‘^_?\(m4\|AS\)_’). And since configure.ac is also designed to be scanned by Autoheader, Autoscan, Autoupdate, and Automake, you should be aware of the ‘^_?A[HNUM]_’ namespaces. In general, you should not use the namespace of a package that does not own the macro or shell code you are writing.
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. Historically, people have not always followed the
rule of using a namespace appropriate for their package, and this has
made it difficult for determining the origin of a macro (and where to
report bugs about that macro), as well as difficult for the true
namespace owner to add new macros without interference from pre-existing
uses of third-party macros. Perhaps the best example of this confusion
is the AM_GNU_GETTEXT macro, which belongs, not to Automake, but
to Gettext.
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 the namespace initials (such as ‘AC_’) usually tells the category of the 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.
CDECLFUNCGROUPHEADERLIBPROGMEMBERSYSTYPEVARAfter 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_PROG_CC_STDC checks whether the
C compiler supports ISO Standard C.
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.
When macros statically diagnose abnormal situations, benign or fatal, it is possible to make autoconf detect the problem, and refuse to create configure in the case of an error. The macros in this section are considered obsolescent, and new code should use M4sugar macros for this purpose, see Diagnostic Macros.
On the other hand, it is possible to want to detect errors when configure is run, which are dependent on the environment of the user rather than the maintainer. For dynamic diagnostics, see Printing Messages.
Report message as a warning (or as an error if requested by the user) if warnings of the category are turned on. This macro is obsolescent; you are encouraged to use:
m4_warn([category], [message])instead. See m4_warn, for more details, including valid category names.
Report message as a syntax warning. This macro is obsolescent; you are encouraged to use:
m4_warn([syntax], [message])instead. See m4_warn, for more details, as well as better finer-grained categories of warnings (not all problems have to do with syntax).
Report a severe error message, and have autoconf die. This macro is obsolescent; you are encouraged to use:
m4_fatal([message])instead. See m4_fatal, for more details.
When the user runs ‘autoconf -W error’, warnings from
m4_warn (including those issued through AC_DIAGNOSE and
AC_WARNING) are reported as errors, see autoconf Invocation.
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.
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.
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_DEFUNor else contain a call toAC_PROVIDEto indicate that it has been called.
AC_REQUIREmust be used inside a macro defined byAC_DEFUN; it must not be called from the top level. Also, it does not make sense to require a macro that takes parameters.
AC_REQUIRE is often misunderstood. It really implements
dependencies between macros in the sense that if one macro depends upon
another, the latter is expanded before the body of the
former. To be more precise, the required macro is expanded before
the outermost defined macro in the current expansion stack.
In particular, ‘AC_REQUIRE([FOO])’ is not replaced with the body of
FOO. For instance, this definition of macros:
AC_DEFUN([TRAVOLTA],
[test "$body_temperature_in_celsius" -gt "38" &&
dance_floor=occupied])
AC_DEFUN([NEWTON_JOHN],
[test "x$hair_style" = xcurly &&
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([Dance Manager], [1.0], [bug-dance@example.org])
RESERVE_DANCE_FLOOR
if test "x$dance_floor" = xoccupied; then
AC_MSG_ERROR([cannot pick up here, let's move])
fi
does not leave you with a better chance to meet a kindred soul at other times than Saturday night since it expands into:
test "$body_temperature_in_Celsius" -gt "38" &&
dance_floor=occupied
test "x$hair_style" = xcurly &&
dance_floor=occupied
fi
if date | grep '^Sat.*pm' >/dev/null 2>&1; then
fi
This behavior was chosen on purpose: (i) it prevents messages in required macros from interrupting the messages in the requiring macros; (ii) it avoids bad surprises when shell conditionals are used, as in:
if ...; then
AC_REQUIRE([SOME_CHECK])
fi
...
SOME_CHECK
However, this implementation can lead to another class of problems. Consider the case where an outer macro first expands, then indirectly requires, an inner macro:
AC_DEFUN([TESTA], [[echo in A
if test -n "$SEEN_A" ; then echo duplicate ; fi
SEEN_A=:]])
AC_DEFUN([TESTB], [AC_REQUIRE([TESTA])[echo in B
if test -z "$SEEN_A" ; then echo bug ; fi]])
AC_DEFUN([TESTC], [AC_REQUIRE([TESTB])[echo in C]])
AC_DEFUN([OUTER], [[echo in OUTER]
TESTA
TESTC])
OUTER
Prior to Autoconf 2.64, the implementation of AC_REQUIRE
recognized that TESTB needed to be hoisted prior to the expansion
of OUTER, but because TESTA had already been directly
expanded, it failed to hoist TESTA. Therefore, the expansion of
TESTB occurs prior to its prerequisites, leading to the following
output:
in B
bug
in OUTER
in A
in C
Newer Autoconf is smart enough to recognize this situation, and hoists
TESTA even though it has already been expanded, but issues a
syntax warning in the process. This is because the hoisted expansion of
TESTA defeats the purpose of using AC_REQUIRE to avoid
redundant code, and causes its own set of problems if the hoisted macro
is not idempotent:
in A
in B
in OUTER
in A
duplicate
in C
The bug is not in Autoconf, but in the macro definitions. If you ever
pass a particular macro name to AC_REQUIRE, then you are implying
that the macro only needs to be expanded once. But to enforce this,
either the macro must be declared with AC_DEFUN_ONCE (although
this only helps in Autoconf 2.64 or newer), or all
uses of that macro should be through AC_REQUIRE; directly
expanding the macro defeats the point of using AC_REQUIRE to
eliminate redundant expansion. In the example, this rule of thumb was
violated because TESTB requires TESTA while OUTER
directly expands it. One way of fixing the bug is to factor
TESTA into two macros, the portion designed for direct and
repeated use (here, named TESTA), and the portion designed for
one-shot output and used only inside AC_REQUIRE (here, named
TESTA_PREREQ). Then, by fixing all clients to use the correct
calling convention according to their needs:
AC_DEFUN([TESTA], [AC_REQUIRE([TESTA_PREREQ])[echo in A]])
AC_DEFUN([TESTA_PREREQ], [[echo in A_PREREQ
if test -n "$SEEN_A" ; then echo duplicate ; fi
SEEN_A=:]])
AC_DEFUN([TESTB], [AC_REQUIRE([TESTA_PREREQ])[echo in B
if test -z "$SEEN_A" ; then echo bug ; fi]])
AC_DEFUN([TESTC], [AC_REQUIRE([TESTB])[echo in C]])
AC_DEFUN([OUTER], [[echo in OUTER]
TESTA
TESTC])
OUTER
the resulting output will then obey all dependency rules and avoid any syntax warnings, whether the script is built with old or new Autoconf versions:
in A_PREREQ
in B
in OUTER
in A
in C
The helper macros AS_IF and AS_CASE may be used to
enforce expansion of required macros outside of shell conditional
constructs. You are furthermore encouraged, although not required, to
put all AC_REQUIRE calls
at the beginning of a macro. You can use dnl to avoid the empty
lines they leave.
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.
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 usingAC_DEFUNor else contain a call toAC_PROVIDEto indicate that it has been called.
Some macros should be called only once, either because calling them
multiple time is unsafe, or because it is bad style. For instance
Autoconf ensures that AC_CANONICAL_BUILD and cousins
(see Canonicalizing) are evaluated only once, because it makes no
sense to run these expensive checks more than once. Such one-shot
macros can be defined using AC_DEFUN_ONCE.
Declare macro macro-name like
AC_DEFUNwould (see Macro Definitions), but add additional logic that guarantees that only the first use of the macro (whether by direct expansion orAC_REQUIRE) causes an expansion of macro-body; the expansion will occur before the start of any enclosing macro defined byAC_DEFUN. Subsequent expansions are silently ignored. Generally, it does not make sense for macro-body to use parameters such as$1.
Prior to Autoconf 2.64, a macro defined by AC_DEFUN_ONCE would
emit a warning if it was directly expanded a second time, so for
portability, it is better to use AC_REQUIRE than direct
invocation of macro-name inside a macro defined by AC_DEFUN
(see Prerequisite 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 replace the old macro calls with their modern implementation.
Autoconf provides a simple means to obsolete a macro.
Define old-macro as implementation. The only difference with
AC_DEFUNis that the user is warned that old-macro is now obsolete.If she then uses autoupdate, the call to old-macro is replaced by the modern implementation. message should include information on what to do after running autoupdate; autoupdate prints it as a warning, and includes it in the updated configure.ac file.
The details of this macro are hairy: if autoconf encounters an
AU_DEFUNed macro, all macros inside its second argument are expanded as usual. However, when autoupdate is run, only M4 and M4sugar macros are expanded here, while all other macros are disabled and appear literally in the updated configure.ac.
Used if the old-name is to be replaced by a call to new-macro with the same parameters. This happens for example if the macro was renamed.
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 the Autoconf Macro Archive, or by 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 symbol macros 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).
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 blanks and newlines 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_RUN_IFELSE 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 an 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]’, ‘$[@]’, etc. Do
not escape when it is unnecessary. Common examples of useless quotation
are ‘[$]$1’ (write ‘$$1’), ‘[$]var’ (use ‘$var’),
etc. If you add portability issues to the picture, you'll prefer
‘${1+"$[@]"}’ to ‘"[$]@"’, and you'll prefer do something
better than hacking Autoconf :-).
When using sed, don't use -e except for indenting
purposes. With the s and y commands, the preferred
separator is ‘/’ unless ‘/’ itself might appear in the pattern
or replacement, in which case you should use ‘|’, or optionally
‘,’ if you know the pattern and replacement cannot contain a file
name. If none of these characters will do, choose a printable character
that cannot appear in the pattern or replacement. Characters from the
set ‘"#$&'()*;<=>?`|~’ are good choices if the pattern or
replacement might contain a file name, since they have special meaning
to the shell and are less likely to occur in file names.
See Macro Definitions, for details on how to define a macro. If a
macro doesn't use AC_REQUIRE, is expected to never be the object
of an AC_REQUIRE directive, and macros required by other macros
inside arguments do not need to be expanded before this macro, then
use m4_define. In case of doubt, use AC_DEFUN.
Also take into account that public third-party macros need to use
AC_DEFUN in order to be found by aclocal
(see Extending aclocal).
All the AC_REQUIRE statements should be at the beginning of the
macro, and each statement should be followed by dnl.
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 $x_includes])
fi[]dnl
])# AC_PATH_X
If the macro is long, try to split it into logical chunks. Typically,
macros that check for a bug in a function and prepare its
AC_LIBOBJ replacement should have an auxiliary macro to perform
this setup. Do not hesitate to introduce auxiliary macros to factor
your code.
In order to highlight the recommended coding style, here is a macro written the old way:
dnl Check for EMX on OS/2.
dnl _AC_EMXOS2
AC_DEFUN(_AC_EMXOS2,
[AC_CACHE_CHECK(for EMX OS/2 environment, ac_cv_emxos2,
[AC_COMPILE_IFELSE([AC_LANG_PROGRAM(, return __EMX__;)],
ac_cv_emxos2=yes, ac_cv_emxos2=no)])
test "x$ac_cv_emxos2" = xyes && EMXOS2=yes])
and the new way:
# _AC_EMXOS2
# ----------
# Check for EMX on OS/2.
m4_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 "x$ac_cv_emxos2" = xyes && EMXOS2=yes[]dnl
])# _AC_EMXOS2
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, and many features added to the original System7 shell are now supported on all interesting porting targets. However, 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 users' existing 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.
For this reason, part of the job of M4sh (see Programming in M4sh)
is to find such a shell. But to prevent trouble, if you're not using
M4sh you should not take advantage of features that were added after Unix
version 7, circa 1977 (see Systemology); you should not use aliases,
negated character classes, or even unset. # comments,
while not in Unix version 7, were retrofitted in the original Bourne
shell and can be assumed to be part of the least common denominator.
On the other hand, if you're using M4sh you can assume that the shell has the features that were added in SVR2 (circa 1984), including shell functions, return, unset, and I/O redirection for builtins. For more information, refer to http://www.in-ulm.de/~mascheck/bourne/. However, some pitfalls have to be avoided for portable use of these constructs; these will be documented in the rest of this chapter. See in particular Shell Functions and Limitations of Shell Builtins.
Some ancient systems have quite small limits on the length of the ‘#!’ line; for instance, 32 bytes (not including the newline) on SunOS 4. However, these ancient systems are no longer of practical concern.
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.
There are other sources of documentation about shells. The specification for the Posix Shell Command Language, though more generous than the restrictive shell subset described above, is fairly portable nowadays. Also please see the Shell FAQs.
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. The the Shell difference FAQ includes a small history of Posix shells, and a comparison between several of them.
Below we describe some of the members of the Bourne shell family.
To be compatible with Ash 0.2:
foo=
false
$foo
echo "Do not use it: $?"
false
eval 'echo "Do not use it: $?"'
cat ${FOO=`bar`}
BASH_VERSION is set. To require
Posix compatibility, run ‘set -o posix’. See Bash Posix Mode, for details.
Solaris systems have three variants: /usr/bin/ksh is ‘ksh88’; it is standard on Solaris 2.0 and later. /usr/xpg4/bin/sh is a Posix-compliant variant of ‘ksh88’; it is standard on Solaris 9 and later. /usr/dt/bin/dtksh is ‘ksh93’. Variants that are not standard may be parts of optional packages. There is no extra charge for these packages, but they are not part of a minimal OS install and therefore some installations may not have it.
Starting with Tru64 Version 4.0, the Korn shell /usr/bin/ksh
is also available as /usr/bin/posix/sh. If the environment
variable BIN_SH is set to xpg4, subsidiary invocations of
the standard shell conform to Posix.
KSH_VERSION, except if invoked as
/bin/sh on OpenBSD, and similarly to Bash you can require
Posix compatibility by running ‘set -o posix’. Unfortunately, with
pdksh 5.2.14 (the latest stable version as of January 2007)
Posix mode is buggy and causes pdksh to depart from Posix in
at least one respect, see Shell Substitutions.
ZSH_VERSION is set. By default zsh is not
compatible with the Bourne shell: you must execute ‘emulate sh’,
and for zsh versions before 3.1.6-dev-18 you must also
set NULLCMD to ‘:’. See Compatibility, for details.
The default Mac OS X sh was originally Zsh; it was changed to Bash in Mac OS X 10.2.
Don't rely on ‘\’ being preserved just because it has no special meaning together with the next symbol. In the native 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 sh
$ cat <<EOF
> \" \\
> EOF
" \
and with Bash:
bash-2.04$ cat <<EOF
> \" \\
> EOF
\" \
Some shells mishandle large here-documents: for example,
Solaris 10 dtksh and the UnixWare 7.1.1 Posix shell, which are
derived from Korn shell version M-12/28/93d, mishandle braced variable
expansion that crosses a 1024- or 4096-byte buffer boundary
within a here-document. Only the part of the variable name after the boundary
is used. For example, ${variable} could be replaced by the expansion
of ${ble}. If the end of the variable name is aligned with the block
boundary, the shell reports an error, as if you used ${}.
Instead of ${variable-default}, the shell may expand
${riable-default}, or even ${fault}. This bug can often
be worked around by omitting the braces: $variable. The bug was
fixed in
‘ksh93g’ (1998-04-30) but as of 2006 many operating systems were
still shipping older versions with the bug.
Empty here-documents are not portable either; with the following code, zsh up to at least version 4.3.10 creates a file with a single newline, whereas other shells create an empty file:
cat >file <<EOF
EOF
Many shells (including the Bourne shell) implement here-documents inefficiently. In particular, some shells can be extremely inefficient when a single statement contains many here-documents. 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 can take the shell 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 runtime, and we end up not
executing the macro at all.
Be careful with the use of ‘<<-’ to unindent here-documents. The behavior is only portable for stripping leading <TAB>s, and things can silently break if an overzealous editor converts to using leading spaces (not all shells are nice enough to warn about unterminated here-documents).
$ printf 'cat <<-x\n\t1\n\t 2\n\tx\n' | bash && echo done
1
2
done
$ printf 'cat <<-x\n 1\n 2\n x\n' | bash-3.2 && echo done
1
2
x
done
Most shells, if not all (including Bash, Zsh, Ash), output traces on stderr, even for subshells. This might result in undesirable 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
One workaround is to grep out uninteresting lines, hoping not to remove good ones.
If you intend to redirect both standard error and standard output, redirect standard output first. This works better with HP-UX, since its shell mishandles tracing if standard error is redirected first:
$ sh -x -c ': 2>err >out'
+ :
+ 2> err $ cat err
1> out
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.
On the other hand, some shells, such as Solaris or FreeBSD /bin/sh, warn about missing programs before performing redirections. Therefore, to silently check whether a program exists, it is necessary to perform redirections on a subshell:
$ /bin/sh -c 'nosuch 2>/dev/null'
nosuch: not found
$ /bin/sh -c '(nosuch) 2>/dev/null'
$ bash -c 'nosuch 2>/dev/null'
FreeBSD 6.2 sh may mix the trace output lines from the statements in a shell pipeline.
It is worth noting that Zsh (but not Ash nor Bash) makes it possible in assignments though: ‘foo=`cd /zorglub` 2>/dev/null’.
Some shells, like ash, don't recognize bi-directional redirection (‘<>’). And even on shells that recognize it, it is not portable to use on fifos: Posix does not require read-write support for named pipes, and Cygwin does not support it:
$ mkfifo fifo
$ exec 5<>fifo
$ echo hi >&5
bash: echo: write error: Communication error on send
When catering to old systems, don't redirect the same file descriptor several times, 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. However, this bug is probably not of practical concern to modern platforms.
Solaris 10 sh will try to optimize away a : command in a loop after the first iteration, even if it is redirected:
$ for i in 1 2 3 ; do : >x$i; done
$ ls
x1
As a workaround, echo or eval can be used.
Don't rely on file descriptors 0, 1, and 2 remaining closed in a subsidiary program. If any of these descriptors is closed, the operating system may open an unspecified file for the descriptor in the new process image. Posix says this may be done only if the subsidiary program is set-user-ID or set-group-ID, but HP-UX 11.23 does it even for ordinary programs.
Don't rely on open file descriptors being open in child processes. In ksh, file descriptors above 2 which are opened using ‘exec n>file’ are closed by a subsequent ‘exec’ (such as that involved in the fork-and-exec which runs a program or script). Thus, using sh, we have:
$ cat ./descrips
#!/bin/sh -
echo hello >&5
$ exec 5>t
$ ./descrips
$ cat t
hello
$
But using ksh:
$ exec 5>t
$ ./descrips
hello
$ cat t
$
Within the process which runs the ‘descrips’ script, file descriptor 5 is closed.
Don't rely on duplicating a closed file descriptor to cause an error. With Solaris /bin/sh, when the redirection fails, the output goes to the original file descriptor.
$ bash -c 'echo hi >&3' 3>&-; echo $?
bash: 3: Bad file descriptor
1
$ /bin/sh -c 'echo hi >&3' 3>&-; echo $?
hi
0
Fortunately, an attempt to close an already closed file descriptor will portably succeed. Likewise, it is safe to use either style of ‘n<&-’ or ‘n>&-’ for closing a file descriptor, even if it doesn't match the read/write mode that the file descriptor was opened with.
DOS variants cannot rename or remove open files, such as in ‘mv foo bar >foo’ or ‘rm foo >foo’, even though this is perfectly portable among Posix hosts.
A few ancient systems reserved some file descriptors. By convention, file descriptor 3 was opened to /dev/tty when you logged into Eighth Edition (1985) through Tenth Edition Unix (1989). File descriptor 4 had a special use on the Stardent/Kubota Titan (circa 1990), though we don't now remember what it was. Both these systems are obsolete, so it's now safe to treat file descriptors 3 and 4 like any other file descriptors.
On the other hand, you can't portably use multi-digit file descriptors. Solaris ksh doesn't understand any file descriptor larger than ‘9’:
$ bash -c 'exec 10>&-'; echo $?
0
$ ksh -c 'exec 9>&-'; echo $?
0
$ ksh -c 'exec 10>&-'; echo $?
ksh[1]: exec: 10: not found
127
Autoconf uses shell-script processing extensively, so the file names that it processes should not contain characters that are special to the shell. Special characters include space, tab, newline, NUL, and the following:
" # $ & ' ( ) * ; < = > ? [ \ ` |
Also, file names should not begin with ‘~’ or ‘-’, and should contain neither ‘-’ immediately after ‘/’ nor ‘~’ immediately after ‘:’. On Posix-like platforms, directory names should not contain ‘:’, as this runs afoul of ‘:’ used as the path separator.
These restrictions apply not only to the files that you distribute, but also to the absolute file names of your source, build, and destination directories.
On some Posix-like platforms, ‘!’ and ‘^’ are special too, so they should be avoided.
Posix lets implementations treat leading // specially, but requires leading /// and beyond to be equivalent to /. Most Unix variants treat // like /. However, some treat // as a “super-root” that can provide access to files that are not otherwise reachable from /. The super-root tradition began with Apollo Domain/OS, which died out long ago, but unfortunately Cygwin has revived it.
While autoconf and friends are usually run on some Posix variety, they can be used on other systems, most notably DOS variants. This impacts several assumptions regarding file names.
For example, the following code:
case $foo_dir in
/*) # Absolute
;;
*)
foo_dir=$dots$foo_dir ;;
esac
fails to properly detect absolute file names on those systems, because they can use a drivespec, and usually use a backslash as directory separator. If you want to be portable to DOS variants (at the price of rejecting valid but oddball Posix file names like a:\b), you can check for absolute file names like this:
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 Shell 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, you can use the
PATH_SEPARATOR output variable instead. configure sets this
to the appropriate value for the build system (‘:’ or ‘;’) when it
starts up.
File names need extra care as well. While DOS variants that are Posixy enough to run autoconf (such as DJGPP) are usually 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 under Microsoft Windows variants, while LFN identifies problems that exist even under Microsoft Windows variants.
This is perfectly OK on Posix variants:
AC_CONFIG_HEADERS([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_HEADERS([config.h:config.hin])
AC_CONFIG_FILES([source.c:source.cin foo.bar:foobar.in])
AC_OUTPUT
The 8+3 limit is not usually a problem under Microsoft Windows, as it
uses numeric
tails in the short version of file names to make them unique. However, a
registry setting can turn this behavior 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.
Nowadays portable patterns can use negated character classes like ‘[!-aeiou]’. The older syntax ‘[^-aeiou]’ is supported by some shells but not others; hence portable scripts should never use ‘^’ as the first character of a bracket pattern.
Outside the C locale, patterns like ‘[a-z]’ are problematic since they may match characters that are not lower-case letters.
Contrary to a persistent urban legend, the Bourne shell does not
systematically split variables and back-quoted 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 "`..."..."...`",
for example Solaris 10 ksh:
$ foo="`echo " bar" | sed 's, ,,'`"
ksh: : cannot execute
ksh: bar | sed 's, ,,': cannot execute
Posix does not specify behavior for this sequence. On the other hand,
behavior for "`...\"...\"...`" is specified by Posix,
but in practice, not all shells understand it the same way: pdksh 5.2.14
prints spurious quotes when in Posix mode:
$ echo "`echo \"hello\"`"
hello
$ set -o posix
$ echo "`echo \"hello\"`"
"hello"
There is just no portable way to use double-quoted strings inside double-quoted back-quoted expressions (pfew!).
Bash 4.1 has a bug where quoted empty strings adjacent to unquoted parameter expansions are elided during word splitting. Meanwhile, zsh does not perform word splitting except when in Bourne compatibility mode. In the example below, the correct behavior is to have five arguments to the function, and exactly two spaces on either side of the middle ‘-’, since word splitting collapses multiple spaces in ‘$f’ but leaves empty arguments intact.
$ bash -c 'n() { echo "$#$@"; }; f=" - "; n - ""$f"" -'
3- - -
$ ksh -c 'n() { echo "$#$@"; }; f=" - "; n - ""$f"" -'
5- - -
$ zsh -c 'n() { echo "$#$@"; }; f=" - "; n - ""$f"" -'
3- - -
$ zsh -c 'emulate sh;
> n() { echo "$#$@"; }; f=" - "; n - ""$f"" -'
5- - -
You can work around this by doing manual word splitting, such as using ‘"$str" $list’ rather than ‘"$str"$list’.
There are also portability pitfalls with particular expansions:
$@The traditional way to work around this portability problem is to use ‘${1+"$@"}’. Unfortunately this method does not work with Zsh (3.x and 4.x), which is used on Mac OS X. When emulating the Bourne shell, Zsh performs word splitting on ‘${1+"$@"}’:
zsh $ emulate sh
zsh $ for i in "$@"; do echo $i; done
Hello World
!
zsh $ for i in ${1+"$@"}; do echo $i; done
Hello
World
!
Zsh handles plain ‘"$@"’ properly, but we can't use plain ‘"$@"’ because of the portability problems mentioned above. One workaround relies on Zsh's “global aliases” to convert ‘${1+"$@"}’ into ‘"$@"’ by itself:
test "${ZSH_VERSION+set}" = set && alias -g '${1+"$@"}'='"$@"'
Zsh only recognizes this alias when a shell word matches it exactly; ‘"foo"${1+"$@"}’ remains subject to word splitting. Since this case always yields at least one shell word, use plain ‘"$@"’.
A more conservative workaround is to avoid ‘"$@"’ if it is possible that there may be no positional arguments. For example, instead of:
cat conftest.c "$@"
you can use this instead:
case $# in
0) cat conftest.c;;
*) cat conftest.c "$@";;
esac
Autoconf macros often use the set command to update
‘$@’, so if you are writing shell code intended for
configure you should not assume that the value of ‘$@’
persists for any length of time.
${10}shift. The 7th Edition shell reported an error if given
${10}, and
Solaris 10 /bin/sh still acts that way:
$ set 1 2 3 4 5 6 7 8 9 10
$ echo ${10}
bad substitution
${var:-value}sh, don't accept the
colon for any shell substitution, and complain and die.
Similarly for ${var:=value}, ${var:?value}, etc.
However, all shells that support functions allow the use of colon in
shell substitution, and since m4sh requires functions, you can portably
use null variable substitution patterns in configure scripts.
${var+value} $ /bin/sh -c 'echo ${a-b c}'
/bin/sh: bad substitution
$ /bin/sh -c 'echo ${a-'\''b c'\''}'
b c
$ /bin/sh -c 'echo "${a-b c}"'
b c
$ /bin/sh -c 'cat <<EOF
${a-b c}
EOF
b c
According to Posix, if an expansion occurs inside double quotes, then the use of unquoted double quotes within value is unspecified, and any single quotes become literal characters; in that case, escaping must be done with backslash. Likewise, the use of unquoted here-documents is a case where double quotes have unspecified results:
$ /bin/sh -c 'echo "${a-"b c"}"'
/bin/sh: bad substitution
$ ksh -c 'echo "${a-"b c"}"'
b c
$ bash -c 'echo "${a-"b c"}"'
b c
$ /bin/sh -c 'a=; echo ${a+'\''b c'\''}'
b c
$ /bin/sh -c 'a=; echo "${a+'\''b c'\''}"'
'b c'
$ /bin/sh -c 'a=; echo "${a+\"b c\"}"'
"b c"
$ /bin/sh -c 'a=; echo "${a+b c}"'
b c
$ /bin/sh -c 'cat <<EOF
${a-"b c"}
EOF'
"b c"
$ /bin/sh -c 'cat <<EOF
${a-'b c'}
EOF'
'b c'
$ bash -c 'cat <<EOF
${a-"b c"}
EOF'
b c
$ bash -c 'cat <<EOF
${a-'b c'}
EOF'
'b c'
Perhaps the easiest way to work around quoting issues in a manner portable to all shells is to place the results in a temporary variable, then use ‘$t’ as the value, rather than trying to inline the expression needing quoting.
$ /bin/sh -c 't="a b\"'\''}\\"; echo "${a-$t}"'
b c"'}\
$ ksh -c 't="a b\"'\''}\\"; echo "${a-$t}"'
b c"'}\
$ bash -c 't="a b\"'\''}\\"; echo "${a-$t}"'
b c"'}\
${var=value} $ time bash -c ': "${a=/usr/bin/*}"; echo "$a"'
/usr/bin/*
real 0m0.005s
user 0m0.002s
sys 0m0.003s
$ time bash -c ': ${a=/usr/bin/*}; echo "$a"'
/usr/bin/*
real 0m0.039s
user 0m0.026s
sys 0m0.009s
$ time bash -c 'a=/usr/bin/*; : ${a=noglob}; echo "$a"'
/usr/bin/*
real 0m0.031s
user 0m0.020s
sys 0m0.010s
$ time bash -c 'a=/usr/bin/*; : "${a=noglob}"; echo "$a"'
/usr/bin/*
real 0m0.006s
user 0m0.002s
sys 0m0.003s
As with ‘+’ and ‘-’, you must use quotes when using ‘=’ if the value contains more than one shell word; either single quotes for just the value, or double quotes around the entire expansion:
$ : ${var1='Some words'}
$ : "${var2=like this}"
$ echo $var1 $var2
Some words like this
otherwise some shells, such as Solaris /bin/sh or on Digital Unix V 5.0, die because of a “bad substitution”. Meanwhile, Posix requires that with ‘=’, quote removal happens prior to the assignment, and the expansion be the final contents of var without quoting (and thus subject to field splitting), in contrast to the behavior with ‘-’ passing the quoting through to the final expansion. However, bash 4.1 does not obey this rule.
$ ksh -c 'echo ${var-a\ \ b}'
a b
$ ksh -c 'echo ${var=a\ \ b}'
a b
$ bash -c 'echo ${var=a\ \ b}'
a b
Finally, Posix states that when mixing ‘${a=b}’ with regular commands, it is unspecified whether the assignments affect the parent shell environment. It is best to perform assignments independently from commands, to avoid the problems demonstrated in this example:
$ bash -c 'x= y=${x:=b} sh -c "echo +\$x+\$y+";echo -$x-'
+b+b+
-b-
$ /bin/sh -c 'x= y=${x:=b} sh -c "echo +\$x+\$y+";echo -$x-'
++b+
--
$ ksh -c 'x= y=${x:=b} sh -c "echo +\$x+\$y+";echo -$x-'
+b+b+
--
${var=value} $ 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, or when using a temporary variable holding the
problematic string.
${var=expanded-value} default="yu,yaa"
: ${var="$default"}
sets var to ‘M-yM-uM-,M-yM-aM-a’, i.e., the 8th bit of each char is set. You don'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}
${#var}${var%word}${var%%word}${var#word}${var##word}Also, pdksh 5.2.14 mishandles some word forms. For
example if ‘$1’ is ‘a/b’ and ‘$2’ is ‘a’, then
‘${1#$2}’ should yield ‘/b’, but with pdksh it
yields the empty string.
`commands`While in general it makes no sense, do not substitute a single builtin with side effects, because Ash 0.2, trying to optimize, does not fork a subshell 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 -z "`cd /`" && pwd
/
The result of ‘foo=`exit 1`’ is left as an exercise to the reader.
The MSYS shell leaves a stray byte in the expansion of a double-quoted command substitution of a native program, if the end of the substitution is not aligned with the end of the double quote. This may be worked around by inserting another pair of quotes:
$ echo "`printf 'foo\r\n'` bar" > broken
$ echo "`printf 'foo\r\n'`"" bar" | cmp - broken
- broken differ: char 4, line 1
Upon interrupt or SIGTERM, some shells may abort a command substitution, replace it with a null string, and wrongly evaluate the enclosing command before entering the trap or ending the script. This can lead to spurious errors:
$ sh -c 'if test `sleep 5; echo hi` = hi; then echo yes; fi'
$ ^C
sh: test: hi: unexpected operator/operand
You can avoid this by assigning the command substitution to a temporary variable:
$ sh -c 'res=`sleep 5; echo hi`
if test "x$res" = xhi; then echo yes; fi'
$ ^C
$(commands)`commands`.
This construct can be nested while this is impossible to do portably with back quotes. Unfortunately it is not yet universally supported. Most notably, even recent releases of Solaris don't support it:
$ showrev -c /bin/sh | grep version
Command version: SunOS 5.10 Generic 121005-03 Oct 2006
$ 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)
$(echo blah)
If you do use ‘$(commands)’, make sure that the commands do not start with a parenthesis, as that would cause confusion with a different notation ‘$((expression))’ that in modern shells is an arithmetic expression not a command. To avoid the confusion, insert a space between the two opening parentheses.
Avoid commands that contain unbalanced parentheses in here-documents, comments, or case statement patterns, as many shells mishandle them. For example, Bash 3.1, ‘ksh88’, pdksh 5.2.14, and Zsh 4.2.6 all mishandle the following valid command:
echo $(case x in x) echo hello;; esac)
$((expression))Among shells that do support ‘$(( ))’, not all of them obey the Posix rule that octal and hexadecimal constants must be recognized:
$ bash -c 'echo $(( 010 + 0x10 ))'
24
$ zsh -c 'echo $(( 010 + 0x10 ))'
26
$ zsh -c 'emulate sh; echo $(( 010 + 0x10 ))'
24
$ pdksh -c 'echo $(( 010 + 0x10 ))'
pdksh: 010 + 0x10 : bad number `0x10'
$ pdksh -c 'echo $(( 010 ))'
10
When it is available, using arithmetic expansion provides a noticeable
speedup in script execution; but testing for support requires
eval to avoid syntax errors. The following construct is used
by AS_VAR_ARITH to provide arithmetic computation when all
arguments are provided in decimal and without a leading zero, and all
operators are properly quoted and appear as distinct arguments:
if ( eval 'test $(( 1 + 1 )) = 2' ) 2>/dev/null; then
eval 'func_arith ()
{
func_arith_result=$(( $* ))
}'
else
func_arith ()
{
func_arith_result=`expr "$@"`
}
fi
func_arith 1 + 1
foo=$func_arith_result
^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 Solaris /bin/sh, but ‘2’ with Bash. You must use ‘;’ to enforce the order: ‘foo=1; foo=2; echo $foo’.
Don't rely on the following to find subdir/program:
PATH=subdir$PATH_SEPARATOR$PATH program
as this does not work with Zsh 3.0.6. Use something like this instead:
(PATH=subdir$PATH_SEPARATOR$PATH; export PATH; exec program)
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 $?
0
To assign default values, follow this algorithm:
: "${var='my literal'}"
: ${var="$default"}
var=${var="$default"}
test "${var+set}" = set || var="has a '}'"
In most cases ‘var=${var="$default"}’ is fine, but in case of doubt, just use the last form. See Shell Substitutions, items ‘${var:-value}’ and ‘${var=value}’ for the rationale.
Beware of two opening parentheses in a row, as many shell implementations treat them specially. Posix requires that the command ‘((cat))’ must behave like ‘(cat)’, but many shells, including Bash and the Korn shell, treat ‘((cat))’ as an arithmetic expression equivalent to ‘let "cat"’, and may or may not report an error when they detect that ‘cat’ is not a number. As another example, ‘pdksh’ 5.2.14 misparses the following code:
if ((true) || false); then
echo ok
fi
To work around this problem, insert a space between the two opening parentheses. There is a similar problem and workaround with ‘$((’; see Shell Substitutions.
Unpatched Tru64 5.1 sh omits the last slash of command-line arguments that contain two trailing slashes:
$ echo / // /// //// .// //.
/ / // /// ./ //.
$ x=//
$ eval "echo \$x"
/
$ set -x
$ echo abc | tr -t ab //
+ echo abc
+ tr -t ab /
/bc
Unpatched Tru64 4.0 sh adds a slash after ‘"$var"’ if the variable is empty and the second double-quote is followed by a word that begins and ends with slash:
$ sh -xc 'p=; echo "$p"/ouch/'
p=
+ echo //ouch/
//ouch/
However, our understanding is that patches are available, so perhaps it's not worth worrying about working around these horrendous bugs.
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; M4sh takes care of this and provides fallback values, whenever needed, to cater for a very old /bin/sh that does not support unset. (see Portable Shell Programming).
As a general rule, shell variable names containing a lower-case letter
are safe; you can define and use these variables without worrying about
their effect on the underlying system, and without worrying about
whether the shell changes them unexpectedly. (The exception is the
shell variable status, as described below.)
Here is a list of names that are known to cause trouble. This list is
not exhaustive, but you should be safe if you avoid the name
status and names containing only upper-case letters and
underscores.
? $ bash -c 'false; $empty; echo $?'
0
$ zsh -c 'false; $empty; echo $?'
1
_BIN_SHxpg4, subsidiary invocations of
the standard shell conform to Posix.
CDPATHcd with a relative file name that did not start
with ‘./’ or ‘../’. Posix
1003.1-2001 says that if a nonempty directory name from CDPATH
is used successfully, cd prints the resulting absolute
file name. Unfortunately this output can break idioms like
‘abs=`cd src && pwd`’ because abs receives the name twice.
Also, many shells do not conform to this part of Posix; for
example, zsh prints the result only if a directory name
other than . was chosen from CDPATH.
In practice the shells that have this problem also support unset, so you can work around the problem as follows:
(unset CDPATH) >/dev/null 2>&1 && unset CDPATH
You can also avoid output by ensuring that your directory name is absolute or anchored at ‘./’, as in ‘abs=`cd ./src && pwd`’.
Configure scripts use M4sh, which automatically unsets CDPATH if
possible, so you need not worry about this problem in those scripts.
CLICOLOR_FORCEDUALCASEENVMAILMAILPATHPS1PS2PS4 (unset ENV) >/dev/null 2>&1 && unset ENV MAIL MAILPATH
PS1='$ '
PS2='> '
PS4='+ '
(actually, there is some complication due to bugs in unset;
see see Limitations of Shell Builtins).
FPATHGREP_OPTIONSIFSDon't set the first character of IFS to backslash. Indeed, Bourne shells use the first character (backslash) when joining the components in ‘"$@"’ and some shells then reinterpret (!) the backslash escapes, so you can end up with backspace and other strange characters.
The proper value for IFS (in regular code, not when performing splits) is ‘<SPC><TAB><RET>’. The first character is especially important, as it is used to join the arguments in ‘$*’; however, note that traditional shells, but also bash-2.04, fail to adhere to this and join with a space anyway.
M4sh guarantees that IFS will have the default value at the beginning of a script, and many macros within autoconf rely on this setting. It is okay to use blocks of shell code that temporarily change the value of IFS in order to split on another character, but remember to restore it before expanding further macros.
Unsetting IFS instead of resetting it to the default sequence
is not suggested, since code that tries to save and restore the
variable's value will incorrectly reset it to an empty value, thus
disabling field splitting:
unset IFS
# default separators used for field splitting
save_IFS=$IFS
IFS=:
# ...
IFS=$save_IFS
# no field splitting performed
LANGLC_ALLLC_COLLATELC_CTYPELC_MESSAGESLC_MONETARYLC_NUMERICLC_TIMELANGUAGELC_ADDRESSLC_IDENTIFICATIONLC_MEASUREMENTLC_NAMELC_PAPERLC_TELEPHONELINENOLINENO.
Its value is the line number of the beginning of the current command.
M4sh, and hence Autoconf, attempts to execute configure with
a shell that supports LINENO. If no such shell is available, it
attempts to implement LINENO with a Sed prepass that replaces each
instance of the string $LINENO (not followed by an alphanumeric
character) with the line's number. In M4sh scripts you should execute
AS_LINENO_PREPARE so that these workarounds are included in
your script; configure scripts do this automatically in AC_INIT.
You should not rely on LINENO within eval or shell
functions, as the behavior differs in practice. The presence of a
quoted newline within simple commands can alter which line number is
used as the starting point for $LINENO substitutions within that
command. Also, the possibility of the Sed prepass means that you should
not rely on $LINENO when quoted, when in here-documents, or when
line continuations are used. Subshells should be OK, though. In the
following example, lines 1, 9, and 14 are portable, but the other
instances of $LINENO do not have deterministic values:
$ cat lineno
echo 1. $LINENO
echo "2. $LINENO
3. $LINENO"
cat <<EOF
5. $LINENO
6. $LINENO
7. \$LINENO
EOF
( echo 9. $LINENO )
eval 'echo 10. $LINENO'
eval 'echo 11. $LINENO
echo 12. $LINENO'
echo 13. '$LINENO'
echo 14. $LINENO '
15.' $LINENO
f () { echo $1 $LINENO;
echo $1 $LINENO }
f 18.
echo 19. \
$LINENO
$ bash-3.2 ./lineno
1. 1
2. 3
3. 3
5. 4
6. 4
7. $LINENO
9. 9
10. 10
11. 12
12. 13
13. $LINENO
14. 14
15. 14
18. 16
18. 17
19. 19
$ zsh-4.3.4 ./lineno
1. 1
2. 2
3. 2
5. 4
6. 4
7. $LINENO
9. 9
10. 1
11. 1
12. 2
13. $LINENO
14. 14
15. 14
18. 0
18. 1
19. 19
$ pdksh-5.2.14 ./lineno
1. 1
2. 2
3. 2
5. 4
6. 4
7. $LINENO
9. 9
10. 0
11. 0
12. 0
13. $LINENO
14. 14
15. 14
18. 16
18. 17
19. 19
$ sed '=' <lineno |
> sed '
> N
> s,$,-,
> t loop
> :loop
> s,^\([0-9]*\)\(.*\)[$]LINENO\([^a-zA-Z0-9_]\),\1\2\1\3,
> t loop
> s,-$,,
> s,^[0-9]*\n,,
> ' |
> sh
1. 1
2. 2
3. 3
5. 5
6. 6
7. \7
9. 9
10. 10
11. 11
12. 12
13. 13
14. 14
15. 15
18. 16
18. 17
19. 20
In particular, note that config.status (and any other subsidiary
script created by AS_INIT_GENERATED) might report line numbers
relative to the parent script as a result of the potential Sed pass.
NULLCMDoptionsemulate sh, so it should not be used.
PATH_SEPARATORPATH_SEPARATOR.
POSIXLY_CORRECT $ bash --posix -c 'set -o | grep posix
> unset POSIXLY_CORRECT
> set -o | grep posix'
posix on
posix off
PWDRANDOMRANDOM, a variable that returns a different
integer each time it is 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. It is common
practice to use $RANDOM as part of a file name, but code
shouldn't rely on $RANDOM expanding to a nonempty string.
statuszsh (at least 3.1.6),
hence read-only. Do not use it.
Nowadays, it is difficult to find a shell that does not support shell functions at all. However, some differences should be expected.
When declaring a shell function, you must include whitespace between the ‘)’ after the function name and the start of the compound expression, to avoid upsetting ksh. While it is possible to use any compound command, most scripts use ‘{...}’.
$ /bin/sh -c 'a(){ echo hi;}; a'
hi
$ ksh -c 'a(){ echo hi;}; a'
ksh: syntax error at line 1: `}' unexpected
$ ksh -c 'a() { echo hi;}; a'
hi
Inside a shell function, you should not rely on the error status of a
subshell if the last command of that subshell was exit or
trap, as this triggers bugs in zsh 4.x; while Autoconf tries to
find a shell that does not exhibit the bug, zsh might be the only shell
present on the user's machine.
Likewise, the state of ‘$?’ is not reliable when entering a shell function. This has the effect that using a function as the first command in a trap handler can cause problems.
$ bash -c 'foo() { echo $?; }; trap foo 0; (exit 2); exit 2'; echo $?
2
2
$ ash -c 'foo() { echo $?; }; trap foo 0; (exit 2); exit 2'; echo $?
0
2
DJGPP bash 2.04 has a bug in that return from a shell function which also used a command substitution causes a segmentation fault. To work around the issue, you can use return from a subshell, or ‘AS_SET_STATUS’ as last command in the execution flow of the function (see Common Shell Constructs).
Not all shells treat shell functions as simple commands impacted by ‘set -e’, for example with Solaris 10 bin/sh:
$ bash -c 'f() { return 1; }; set -e; f; echo oops
$ /bin/sh -c 'f() { return 1; }; set -e; f; echo oops
oops
Shell variables and functions may share the same namespace, for example with Solaris 10 /bin/sh:
$ f () { :; }; f=; f
f: not found
For this reason, Autoconf (actually M4sh, see Programming in M4sh) uses the prefix ‘as_fn_’ for its functions.
Handling of positional parameters and shell options varies among shells. For example, Korn shells reset and restore trace output (‘set -x’) and other options upon function entry and exit. Inside a function, IRIX sh sets ‘$0’ to the function name.
It is not portable to pass temporary environment variables to shell functions. Solaris /bin/sh does not see the variable. Meanwhile, not all shells follow the Posix rule that the assignment must affect the current environment in the same manner as special built-ins.
$ /bin/sh -c 'func() { echo $a;}; a=1 func; echo $a'
⇒
⇒
$ ash -c 'func() { echo $a;}; a=1 func; echo $a'
⇒1
⇒
$ bash -c 'set -o posix; func() { echo $a;}; a=1 func; echo $a'
⇒1
⇒1
Some ancient Bourne shell variants with function support did not reset ‘$i, i >= 0’, upon function exit, so effectively the arguments of the script were lost after the first function invocation. It is probably not worth worrying about these shells any more.
With AIX sh, a trap on 0 installed in a shell function triggers at function exit rather than at script exit. See Limitations of Shell Builtins.
No, no, we are serious: some shells do have limitations! :)
You should always keep in mind that any builtin or command may support
options, and therefore differ in behavior with arguments
starting with a dash. For instance, even 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. Many of these limitations
can be worked around using M4sh (see Programming in M4sh).
Not all shells gracefully handle syntax errors within a sourced file. On one extreme, some non-interactive shells abort the entire script. On the other, zsh 4.3.10 has a bug where it fails to react to the syntax error.
$ echo 'fi' > syntax
$ bash -c '. ./syntax; echo $?'
./syntax: line 1: syntax error near unexpected token `fi'
./syntax: line 1: `fi'
1
$ ash -c '. ./syntax; echo $?'
./syntax: 1: Syntax error: "fi" unexpected
$ zsh -c '. ./syntax; echo $?'
./syntax:1: parse error near `fi'
0
$ sh -c '! : | :'; echo $?
1
$ ash -c '! : | :'; echo $?
0
$ sh -c '! { :; }'; echo $?
1
$ ash -c '! { :; }'; echo $?
{: not found
Syntax error: "}" unexpected
2
Shell code like this:
if ! cmp file1 file2 >/dev/null 2>&1; then
echo files differ or trouble
fi
is therefore not portable in practice. Typically it is easy to rewrite such code, e.g.:
cmp file1 file2 >/dev/null 2>&1 ||
echo files differ or trouble
More generally, one can always rewrite ‘! command’ as:
if command; then (exit 1); else :; fi
$ bash -c '{ echo foo; } >/bad; echo $?'
bash: line 1: /bad: Permission denied
0
$ bash -c 'while :; do echo; done >/bad; echo $?'
bash: line 1: /bad: Permission denied
0
To work around the bug, prepend ‘:;’:
$ bash -c ':;{ echo foo; } >/bad; echo $?'
bash: line 1: /bad: Permission denied
1
Posix requires a syntax error if a brace list has no contents. However, not all shells obey this rule; and on shells where empty lists are permitted, the effect on ‘$?’ is inconsistent. To avoid problems, ensure that a brace list is never empty.
$ bash -c 'false; { }; echo $?' || echo $?
bash: line 1: syntax error near unexpected token `}'
bash: line 1: `false; { }; echo $?'
2
$ zsh -c 'false; { }; echo $?' || echo $?
1
$ pdksh -c 'false; { }; echo $?' || echo $?
0
You don't need the final ‘;;’, but you should use it.
Posix requires support for case patterns with opening
parentheses like this:
case $file_name in
(*.c) echo "C source code";;
esac
but the ( in this example is not portable to many Bourne
shell implementations, which is a pity for those of us using tools that
rely on balanced parentheses. For instance, with Solaris
/bin/sh:
$ case foo in (foo) echo foo;; esac
error-->syntax error: `(' unexpected
The leading ‘(’ can be omitted safely. Unfortunately, there are contexts where unbalanced parentheses cause other problems, such as when using a syntax-highlighting editor that searches for the balancing counterpart, or more importantly, when using a case statement as an underquoted argument to an Autoconf macro. See Balancing Parentheses, for tradeoffs involved in various styles of dealing with unbalanced ‘)’.
Zsh handles pattern fragments derived from parameter expansions or command substitutions as though quoted:
$ pat=\?; case aa in ?$pat) echo match;; esac
$ pat=\?; case a? in ?$pat) echo match;; esac
match
Because of a bug in its fnmatch, Bash fails to properly
handle backslashes in character classes:
bash-2.02$ case /tmp in [/\\]*) echo OK;; esac
bash-2.02$
This is extremely unfortunate, since you are likely to use this code to handle Posix or MS-DOS absolute file names. 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
Many Bourne shells cannot handle closing brackets in character classes correctly.
Some shells also have problems with backslash escaping in case you do not want to match the backslash: both a backslash and the escaped character match this pattern. To work around this, specify the character class in a variable, so that quote removal does not apply afterwards, and the special characters don't have to be backslash-escaped:
$ case '\' in [\<]) echo OK;; esac
OK
$ scanset='[<]'; case '\' in $scanset) echo OK;; esac
$
Even with this, Solaris ksh matches a backslash if the set contains any of the characters ‘|’, ‘&’, ‘(’, or ‘)’.
Conversely, Tru64 ksh (circa 2003) erroneously always matches a closing parenthesis if not specified in a character class:
$ case foo in *\)*) echo fail ;; esac
fail
$ case foo in *')'*) echo fail ;; esac
fail
Some shells, such as Ash 0.3.8, are confused by an empty
case/esac:
ash-0.3.8 $ case foo in esac;
error-->Syntax error: ";" unexpected (expecting ")")
Posix requires case to give an exit status of 0 if no cases
match. However, /bin/sh in Solaris 10 does not obey this
rule. Meanwhile, it is unclear whether a case that matches, but
contains no statements, must also change the exit status to 0. The M4sh
macro AS_CASE works around these inconsistencies.
$ bash -c 'case `false` in ?) ;; esac; echo $?'
0
$ /bin/sh -c 'case `false` in ?) ;; esac; echo $?'
255
Portable scripts should assume neither option is supported, and should
assume neither behavior is the default. This can be a bit tricky,
since the Posix default behavior means that, for example,
‘ls ..’ and ‘cd ..’ may refer to different directories if
the current logical directory is a symbolic link. It is safe to use
cd dir if dir contains no .. components.
Also, Autoconf-generated scripts check for this problem when computing
variables like ac_top_srcdir (see Configuration Actions),
so it is safe to cd to these variables.
Posix states that behavior is undefined if cd is given an explicit empty argument. Some shells do nothing, some change to the first entry in CDPATH, some change to HOME, and some exit the shell rather than returning an error. Unfortunately, this means that if ‘$var’ is empty, then ‘cd "$var"’ is less predictable than ‘cd $var’ (at least the latter is well-behaved in all shells at changing to HOME, although this is probably not what you wanted in a script). You should check that a directory name was supplied before trying to change locations.
See Special Shell Variables, for portability problems involving cd and the CDPATH environment variable. Also please see the discussion of the pwd command.
Do not use backslashes in the arguments, as there is no consensus on their handling. For ‘echo '\n' | wc -l’, the sh of Solaris outputs 2, but Bash and Zsh (in sh emulation mode) output 1. The problem is truly echo: all the shells understand ‘'\n'’ as the string composed of a backslash and an ‘n’. Within a command substitution, ‘echo 'string\c'’ will mess up the internal state of ksh88 on AIX 6.1 so that it will print the first character ‘s’ only, followed by a newline, and then entirely drop the output of the next echo in a command substitution.
Because of these problems, do not pass a string containing arbitrary characters to echo. For example, ‘echo "$foo"’ is safe only if you know that foo's value cannot contain backslashes and cannot start with ‘-’.
If this may not be true, printf is in general safer and easier to use than echo and echo -n. Thus, scripts where portability is not a major concern should use printf '%s\n' whenever echo could fail, and similarly use printf %s instead of echo -n. For portable shell scripts, instead, it is suggested to use a here-document like this:
cat <<EOF
$foo
EOF
Alternatively, M4sh provides AS_ECHO and AS_ECHO_N macros
which choose between various portable implementations: ‘echo’
or ‘print’ where they work, printf if it is available,
or else other creative tricks in order to work around the above problems.
You should also be wary of common bugs in eval implementations. In some shell implementations (e.g., older ash, OpenBSD 3.8 sh, pdksh v5.2.14 99/07/13.2, and zsh 4.2.5), the arguments of ‘eval’ are evaluated in a context where ‘$?’ is 0, so they exhibit behavior like this:
$ false; eval 'echo $?'
0
The correct behavior here is to output a nonzero value, but portable scripts should not rely on this.
You should not rely on LINENO within eval.
See Special Shell Variables.
Note that, even though these bugs are easily avoided, eval is tricky to use on arbitrary arguments. It is obviously unwise to use ‘eval $cmd’ if the string value of ‘cmd’ was derived from an untrustworthy source. But even if the string value is valid, ‘eval $cmd’ might not work as intended, since it causes field splitting and file name expansion to occur twice, once for the eval and once for the command itself. It is therefore safer to use ‘eval "$cmd"’. For example, if cmd has the value ‘cat test?.c’, ‘eval $cmd’ might expand to the equivalent of ‘cat test;.c’ if there happens to be a file named test;.c in the current directory; and this in turn mistakenly attempts to invoke cat on the file test and then execute the command .c. To avoid this problem, use ‘eval "$cmd"’ rather than ‘eval $cmd’.
However, suppose that you want to output the text of the evaluated
command just before executing it. Assuming the previous example,
‘echo "Executing: $cmd"’ outputs ‘Executing: cat test?.c’, but
this output doesn't show the user that ‘test;.c’ is the actual name
of the copied file. Conversely, ‘eval "echo Executing: $cmd"’
works on this example, but it fails with ‘cmd='cat foo >bar'’,
since it mistakenly replaces the contents of bar by the
string ‘cat foo’. No simple, general, and portable solution to
this problem is known.
$ sh -c 'exec cd /tmp'
sh: line 0: exec: cd: not found
All other built-ins that provide utilities specified by Posix must have a counterpart executable that exists on PATH, although Posix allows exec to use the built-in instead of the executable. For example, contrast bash 3.2 and pdksh 5.2.14:
$ bash -c 'pwd --version' | head -n1
bash: line 0: pwd: --: invalid option
pwd: usage: pwd [-LP]
$ bash -c 'exec pwd --version' | head -n1
pwd (GNU coreutils) 6.10
$ pdksh -c 'exec pwd --version' | head -n1
pdksh: pwd: --: unknown option
When it is desired to avoid a regular shell built-in, the workaround is to use some other forwarding command, such as env or nice, that will ensure a path search:
$ pdksh -c 'exec true --version' | head -n1
$ pdksh -c 'nice true --version' | head -n1
true (GNU coreutils) 6.10
$ pdksh -c 'env true --version' | head -n1
true (GNU coreutils) 6.10
$?;
unfortunately, some shells, such as the DJGPP port of Bash 2.04, just
perform ‘exit 0’.
bash-2.04$ 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 /bin/sh, an exit
trap ignores the exit command's argument. 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.
Alas, many shells, such as Solaris /bin/sh, IRIX 6.3, IRIX 5.2, AIX 4.1.5, and Digital Unix 4.0, forget to export the environment variables they receive. As a result, two variables coexist: 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 print alternately ‘foo’ and ‘bar’, although they should print only ‘foo’ and then a sequence of ‘bar’s.
Therefore you should export again each environment variable that you update; the export can occur before or after the assignment.
Posix is not clear on whether the export of an undefined variable causes the variable to be defined with the value of an empty string, or merely marks any future definition of a variable by that name for export. Various shells behave differently in this regard:
$ sh -c 'export foo; env | grep foo'
$ ash -c 'export foo; env | grep foo'
foo=
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 "$arg"
done
But keep in mind that Zsh, even in Bourne shell emulation mode, performs word splitting on ‘${1+"$@"}’; see Shell Substitutions, item ‘$@’, for more.
In Solaris /bin/sh, when the list of arguments of a for loop starts with unquoted tokens looking like variable assignments, the loop is not executed on those tokens:
$ /bin/sh -c 'for v in a=b c=d x e=f; do echo $v; done'
x
e=f
Thankfully, quoting the assignment-like tokens, or starting the list with other tokens (including unquoted variable expansion that results in an assignment-like result), avoids the problem, so it is easy to work around:
$ /bin/sh -c 'for v in "a=b"; do echo $v; done'
a=b
$ /bin/sh -c 'x=a=b; for v in $x c=d; do echo $v; done'
a=b
c=d
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
Or, especially if the else branch is short, you can use ||.
In M4sh, the AS_IF macro provides an easy way to write these kinds
of conditionals:
AS_IF([cmp -s file file.new], [], [mv file.new file])
This is especially useful in other M4 macros, where the then and else branches might be macro arguments.
Some very old shells did not reset the exit status from an if with no else:
$ if (exit 42); then true; fi; echo $?
42
whereas a proper shell should have printed ‘0’. But this is no longer a portability problem; any shell that supports functions gets it correct. However, it explains why some makefiles have lengthy constructs:
if test -f "$file"; then
install "$file" "$dest"
else
:
fi
printf %s -foo
Bash 2.03 mishandles an escape sequence that happens to evaluate to ‘%’:
$ printf '\045'
bash: printf: `%': missing format character
Large outputs may cause trouble. On Solaris 2.5.1 through 10, for example, /usr/bin/printf is buggy, so when using /bin/sh the command ‘printf %010000x 123’ normally dumps core.
Since printf is not always a shell builtin, there is a
potential speed penalty for using printf '%s\n' as a replacement
for an echo that does not interpret ‘\’ or leading
‘-’. With Solaris ksh, it is possible to use print
-r -- for this role instead.
For a discussion of portable alternatives to both printf
and echo, See Limitations of Shell Builtins.
Posix 1003.1-2001 requires that pwd must support the -L (“logical”) and -P (“physical”) options, with -L being the default. However, traditional shells do not support these options, and their pwd command has the -P behavior.
Portable scripts should assume neither option is supported, and should assume neither behavior is the default. Also, on many hosts ‘/bin/pwd’ is equivalent to ‘pwd -P’, but Posix does not require this behavior and portable scripts should not rely on it.
Typically it's best to use plain pwd. On modern hosts this outputs logical directory names, which have the following advantages:
Also please see the discussion of the cd command.
The set 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 parameter, even ‘-x’ for instance), but many traditional shells (e.g., Solaris 10 /bin/sh) simply stop 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
Avoid ‘set -’, e.g., ‘set - $my_list’. Posix no longer requires support for this command, and in traditional shells ‘set - $my_list’ resets the -v and -x options, which makes scripts harder to debug.
Some nonstandard shells do not recognize more than one option (e.g., ‘set -e -x’ assigns ‘-x’ to the command line). It is better to combine them:
set -ex
The option -e has historically been underspecified, with enough
ambiguities to cause numerous differences across various shell
implementations. Perhaps the best reference is
this link, recommending a change to Posix 2008 to match ksh88
behavior. Note that mixing set -e and shell functions is asking
for surprises:
set -e
doit()
{
rm file
echo one
}
doit || echo two
According to the recommendation, ‘one’ should always be output regardless of whether the rm failed, because it occurs within the body of the shell function ‘doit’ invoked on the left side of ‘||’, where the effects of ‘set -e’ are not enforced. Likewise, ‘two’ should never be printed, since the failure of rm does not abort the function, such that the status of ‘doit’ is 0.
The BSD shell has had several problems with the -e option. Older versions of the BSD shell (circa 1990) mishandled ‘&&’, ‘||’, ‘if’, and ‘case’ when -e was in effect, causing the shell to exit unexpectedly in some cases. This was particularly a problem with makefiles, and led to circumlocutions like ‘sh -c 'test -f file || touch file'’, where the seemingly-unnecessary ‘sh -c '...'’ wrapper works around the bug (see Failure in Make Rules).
Even relatively-recent versions of the BSD shell (e.g., OpenBSD 3.4) wrongly exit with -e if the last command within a compound statement fails and is guarded by an ‘&&’ only. For example:
#! /bin/sh
set -e
foo=''
test -n "$foo" && exit 1
echo one
if :; then
test -n "$foo" && exit 1
echo two
test -n "$foo" && exit 1
fi
echo three
does not print ‘three’. One workaround is to change the last instance of ‘test -n "$foo" && exit 1’ to be ‘if test -n "$foo"; then exit 1; fi’ instead. Another possibility is to warn BSD users not to use ‘sh -e’.
When ‘set -e’ is in effect, a failed command substitution in Solaris /bin/sh cannot be ignored, even with ‘||’.
$ /bin/sh -c 'set -e; foo=`false` || echo foo; echo bar'
$ bash -c 'set -e; foo=`false` || echo foo; echo bar'
foo
bar
Moreover, a command substitution, successful or not, causes this shell to exit from a failing outer command even in presence of an ‘&&’ list:
$ bash -c 'set -e; false `true` && echo notreached; echo ok'
ok
$ sh -c 'set -e; false `true` && echo notreached; echo ok'
$
Portable scripts should not use ‘set -e’ if trap is used to install an exit handler. This is because Tru64/OSF 5.1 sh sometimes enters the trap handler with the exit status of the command prior to the one that triggered the errexit handler:
$ sh -ec 'trap '\''echo $?'\'' 0; false'
0
$ sh -c 'set -e; trap '\''echo $?'\'' 0; false'
1
Thus, when writing a script in M4sh, rather than trying to rely on ‘set -e’, it is better to append ‘|| AS_EXIT’ to any statement where it is desirable to abort on failure.
Job control is not provided by all shells, so the use of ‘set -m’ or ‘set -b’ must be done with care. When using zsh in native mode, asynchronous notification (‘set -b’) is enabled by default, and using ‘emulate sh’ to switch to Posix mode does not clear this setting (although asynchronous notification has no impact unless job monitoring is also enabled). Also, zsh 4.3.10 and earlier have a bug where job control can be manipulated in interactive shells, but not in subshells or scripts. Furthermore, some shells, like pdksh, fail to treat subshells as interactive, even though the parent shell was.
$ echo $ZSH_VERSION
4.3.10
$ set -m; echo $?
0
$ zsh -c 'set -m; echo $?'
set: can't change option: -m
$ (set -m); echo $?
set: can't change option: -m
1
$ pdksh -ci 'echo $-; (echo $-)'
cim
c
Don't use ‘shift 2’ etc.; while it in the SVR1 shell (1983),
it is also absent in many pre-Posix shells.
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.
The -a, -o, ‘(’, and ‘)’ operands are not present in all implementations, and have been marked obsolete by Posix 2008. This is because there are inherent ambiguities in using them. For example, ‘test "$1" -a "$2"’ looks like a binary operator to check whether two strings are both non-empty, but if ‘$1’ is the literal ‘!’, then some implementations of test treat it as a negation of the unary operator -a.
Thus, portable uses of test should never have more than four arguments, and scripts should use shell constructs like ‘&&’ and ‘||’ instead. If you combine ‘&&’ and ‘||’ in the same statement, keep in mind that they have equal precedence, so it is often better to parenthesize even when this is redundant. For example:
# Not portable:
test "X$a" = "X$b" -a \
'(' "X$c" != "X$d" -o "X$e" = "X$f" ')'
# Portable:
test "X$a" = "X$b" &&
{ test "X$c" != "X$d" || test "X$e" = "X$f"; }
test does not process options like most other commands do; for example, it does not recognize the -- argument as marking the end of options.
It is safe to use ‘!’ as a test operator. For example,
‘if test ! -d foo; ...’ is portable even though ‘if ! test
-d foo; ...’ is not.
/bin/sh support only -h.
Posix also says that ‘test ! "string"’, ‘test -n "string"’ and ‘test -z "string"’ work with any string, but many shells (such as Solaris, AIX 3.2, UNICOS 10.0.0.6, Digital Unix 4, etc.) get confused if string looks like an operator:
$ test -n =
test: argument expected
$ test ! -n
test: argument expected
$ test -z ")"; echo $?
0
Similarly, Posix says that both ‘test "string1" = "string2"’ and ‘test "string1" != "string2"’ work for any pairs of strings, but in practice this is not true for troublesome strings that look like operators or parentheses, or that begin with ‘-’.
It is best to protect such strings with a leading ‘X’, e.g., ‘test "Xstring" != X’ rather than ‘test -n "string"’ or ‘test ! "string"’.
It is common 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 be avoided by using:
case $ac_feature in
*[!-a-zA-Z0-9_]*) action;;
esac
If the pattern is a complicated regular expression that cannot be expressed as a shell pattern, use something like this instead:
expr "X$ac_feature" : 'X.*[^-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.
Posix says that ‘trap - 1 2 13 15’ resets the traps for the specified signals to their default values, but many common shells (e.g., Solaris /bin/sh) misinterpret this and attempt to execute a “command” named - when the specified conditions arise. Posix 2008 also added a requirement to support ‘trap 1 2 13 15’ to reset traps, as this is supported by a larger set of shells, but there are still shells like dash that mistakenly try to execute 1 instead of resetting the traps. Therefore, there is no portable workaround, except for ‘trap - 0’, for which ‘trap '' 0’ is a portable substitute.
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 /bin/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. In M4sh, this is covered by using
AS_EXIT.
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.
Several shells fail to execute an exit trap that is defined inside a subshell, when the last command of that subshell is not a builtin. A workaround is to use ‘exit $?’ as the shell builtin.
$ bash -c '(trap "echo hi" 0; /bin/true)'
hi
$ /bin/sh -c '(trap "echo hi" 0; /bin/true)'
$ /bin/sh -c '(trap "echo hi" 0; /bin/true; exit $?)'
hi
Likewise, older implementations of bash failed to preserve ‘$?’ across an exit trap consisting of a single cleanup command.
$ bash -c 'trap "/bin/true" 0; exit 2'; echo $?
2
$ bash-2.05b -c 'trap "/bin/true" 0; exit 2'; echo $?
0
$ bash-2.05b -c 'trap ":; /bin/true" 0; exit 2'; echo $?
2
In a sense, yes, because if it doesn't exist, the shell will produce an exit status of failure, which is correct for false, but not for true.
Remember that even though ‘:’ ignores its arguments, it still takes time to compute those arguments. It is a good idea to use double quotes around any arguments to ‘:’ to avoid time spent in field splitting and file name expansion.
unset FOO fails
when FOO is not set. You can use
FOO=; unset FOO
if you are not sure that FOO is set.
A few ancient shells lack unset entirely. For some variables
such as PS1, you can use a neutralizing value instead:
PS1='$ '
Usually, shells that do not support unset need less effort to
make the environment sane, so for example is not a problem if you cannot
unset CDPATH on those shells. However, Bash 2.01 mishandles
unset MAIL in some cases and dumps core. So, you should do
something like
( (unset MAIL) || exit 1) >/dev/null 2>&1 && unset MAIL || :
See Special Shell Variables, for some neutralizing values. Also, see
Limitations of Builtins, for
the case of environment variables.
The small set of tools you can expect to find on any machine can still include some limitations you should be aware of.
$ 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!
Posix says that if a program contains only ‘BEGIN’ actions, and
contains no instances of getline, then the program merely
executes the actions without reading input. However, traditional Awk
implementations (such as Solaris 10 awk) read and discard
input in this case. Portable scripts can redirect input from
/dev/null to work around the problem. For example:
awk 'BEGIN {print "hello world"}' </dev/null
Posix says that in an ‘END’ action, ‘$NF’ (and presumably, ‘$1’) retain their value from the last record read, if no intervening ‘getline’ occurred. However, some implementations (such as Solaris 10 ‘/usr/bin/awk’, ‘nawk’, or Darwin ‘awk’) reset these variables. A workaround is to use an intermediate variable prior to the ‘END’ block. For example:
$ cat end.awk
{ tmp = $1 }
END { print "a", $1, $NF, "b", tmp }
$ echo 1 | awk -f end.awk
a b 1
$ echo 1 | gawk -f end.awk
a 1 1 b 1
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 implementations, such as HP-UX 11.0's native one, mishandle 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 implementations.
On ‘ia64-hp-hpux11.23’, Awk mishandles printf conversions
after %u:
$ awk 'BEGIN { printf "%u %d\n", 0, -1 }'
0 0
AIX version 5.2 has an arbitrary limit of 399 on the length of regular expressions and literal strings in an Awk program.
Traditional Awk implementations derived from Unix version 7, such as
Solaris /bin/awk, have many limitations and do not
conform to Posix. Nowadays AC_PROG_AWK (see Particular Programs) finds you an Awk that doesn't have these problems, but if
for some reason you prefer not to use AC_PROG_AWK you may need to
address them. For more detailed descriptions, see awk language history.
Traditional Awk does not support multidimensional arrays or user-defined functions.
Traditional Awk does not support the -v option. You can use
assignments after the program instead, e.g., $AWK '{print v
$1}' v=x; however, don't forget that such assignments are not
evaluated until they are encountered (e.g., after any BEGIN
action).
Traditional Awk does not support the keywords delete or do.
Traditional Awk does not support the expressions
a?b:c, !a, a^b,
or a^=b.
Traditional Awk does not support the predefined CONVFMT or
ENVIRON variables.
Traditional Awk supports only the predefined functions exp, index,
int, length, log, split, sprintf,
sqrt, and substr.
Traditional Awk getline is not at all compatible with Posix;
avoid it.
Traditional Awk has for (i in a) ... but no other uses of the
in keyword. For example, it lacks if (i in a) ....
In code portable to both traditional and modern Awk, FS must be a
string containing just one ordinary character, and similarly for the
field-separator argument to split.
Traditional Awk has a limit of 99 fields in a record. Since some Awk
implementations, like Tru64's, split the input even if you don't refer
to any field in the script, to circumvent this problem, set ‘FS’
to an unusual character and use split.
Traditional Awk has a limit of at most 99 bytes in a number formatted by
OFMT; for example, OFMT="%.300e"; print 0.1; typically
dumps core.
The original version of Awk had a limit of at most 99 bytes per
split field, 99 bytes per substr substring, and 99 bytes
per run of non-special characters in a printf format, but these
bugs have been fixed on all practical hosts that we know of.
HP-UX 11.00 and IRIX 6.5 Awk require that input files have a line length
of at most 3070 bytes.
AC_PROG_CC_C_O.
When a compilation such as ‘cc -o foo foo.c’ 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’ appears to succeed, but in fact does nothing.
The default executable, produced by ‘cc foo.c’, can be
The C compiler's traditional name is cc, but other names like
gcc are common. Posix 1003.1-2001 specifies the
name c99, but older Posix editions specified
c89 and anyway these standard names are rarely used in
practice. Typically the C compiler is invoked from makefiles that use
‘$(CC)’, so the value of the ‘CC’ make variable selects the
compiler name.
cp -r
reads from pipes instead of replicating them.
Some cp implementations (e.g., BSD/OS 4.2) do not allow trailing slashes at the end of nonexistent destination directories. To avoid this problem, omit the trailing slashes. For example, use ‘cp -R source /tmp/newdir’ rather than ‘cp -R source /tmp/newdir/’ if /tmp/newdir does not exist.
The ancient SunOS 4 cp does not support -f, although its mv does.
Traditionally, file timestamps had 1-second resolution, and ‘cp
-p’ copied the timestamps exactly. However, many modern file systems
have timestamps with 1-nanosecond resolution. Unfortunately, ‘cp
-p’ implementations truncate timestamps when copying files, so this
can result in the destination file appearing to be older than the
source. The exact amount of truncation depends on the resolution of
the system calls that cp uses; traditionally this was
utime, which has 1-second resolution, but some newer
cp implementations use utimes, which has
1-microsecond resolution. These newer implementations include GNU
Core Utilities 5.0.91 or later, and Solaris 8 (sparc) patch 109933-02 or
later. Unfortunately as of January 2006 there is still no system
call to set timestamps to the full nanosecond resolution.
Bob Proulx notes that ‘cp -p’ always tries to copy ownerships. But whether it actually does copy ownerships or not is a system dependent policy decision implemented by the kernel. If the kernel allows it then it happens. If the kernel does not allow it then it does not happen. It is not something cp itself has control over.
In Unix System V any user can chown files to any other user, and System V also has a non-sticky /tmp. That probably derives from the heritage of System V in a business environment without hostile users. BSD changed this to be a more secure model where only root can chown files and a sticky /tmp is used. That undoubtedly derives from the heritage of BSD in a campus environment.
GNU/Linux and Solaris by default follow BSD, but
can be configured to allow a System V style chown. On the
other hand, HP-UX follows System V, but can
be configured to use the modern security model and disallow
chown. Since it is an administrator-configurable parameter
you can't use the name of the kernel as an indicator of the behavior.
$ uname -a
OSF1 medusa.sis.pasteur.fr V5.1 732 alpha
$ date "+%s"
%s
Some implementations, such as Tru64's, fail when comparing to
/dev/null. Use an empty file instead.
AS_DIRNAME (see Programming in M4sh). For example:
dir=`dirname "$file"` # This is not portable.
dir=`AS_DIRNAME(["$file"])` # This is more portable.
grep -E. Also, some traditional implementations do
not work on long input lines. To work around these problems, invoke
AC_PROG_EGREP and then use $EGREP.
Portable extended regular expressions should use ‘\’ only to escape characters in the string ‘$()*+.?[\^{|’. For example, ‘\}’ is not portable, even though it typically matches ‘}’.
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
$EGREP also suffers the limitations of grep
(see Limitations of Usual Tools).
No expr keyword starts with ‘X’, so use ‘expr X"word" : 'Xregex'’ to keep expr from misinterpreting word.
Don't use length, substr, match and index.
expr '' \| ''
Posix 1003.2-1992 returns the empty string for this case, but traditional Unix returns ‘0’ (Solaris is one such example). In Posix 1003.1-2001, the specification was changed to match traditional Unix's behavior (which is bizarre, but it's too late to fix this). Please note that the same problem does arise when the empty string results from a computation, as in:
expr bar : foo \| foo : bar
Avoid this portability problem by avoiding the empty string.
Portable expr regular expressions should not begin with ‘^’. Patterns are automatically anchored so leading ‘^’ is not needed anyway.
On the other hand, the behavior of the ‘$’ anchor is not portable on multi-line strings. Posix is ambiguous whether the anchor applies to each line, as was done in older versions of GNU Coreutils, or whether it applies only to the end of the overall string, as in Coreutils 6.0 and most other implementations.
$ baz='foo
> bar'
$ expr "X$baz" : 'X\(foo\)$'
$ expr-5.97 "X$baz" : 'X\(foo\)$'
foo
The Posix standard is ambiguous as to whether ‘expr 'a' : '\(b\)'’ outputs ‘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’.
One might think that a way 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.
Some ancient 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. Nowadays this is of practical importance only for the rare installer who mistakenly puts /usr/ucb before /usr/bin in PATH.
On Mac OS X 10.4, expr mishandles the pattern ‘[^-]’ in some cases. For example, the command
expr Xpowerpc-apple-darwin8.1.0 : 'X[^-]*-[^-]*-\(.*\)'
outputs ‘apple-darwin8.1.0’ rather than the correct ‘darwin8.1.0’. This particular case can be worked around by substituting ‘[^--]’ for ‘[^-]’.
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/'
outputs ‘a’ on most hosts, but ‘aa’ on QNX 4.25. A simple workaround consists of testing expr and using a variable set to expr or to false according to the result.
Tru64 expr incorrectly treats the result as a number, if it can be interpreted that way:
$ expr 00001 : '.*\(...\)'
1
On HP-UX 11, expr only supports a single sub-expression.
$ expr 'Xfoo' : 'X\(f\(oo\)*\)$'
expr: More than one '\(' was used.
grep -F. Also, some traditional implementations do
not work on long input lines. To work around these problems, invoke
AC_PROG_FGREP and then use $FGREP.
Tru64/OSF 5.1 fgrep does not match an empty pattern.
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’.
Some of the options required by Posix are not portable in practice.
Don't use ‘grep -q’ to suppress output, because many grep
implementations (e.g., Solaris) do not support -q.
Don't use ‘grep -s’ to suppress output either, because Posix
says -s does not suppress output, only some error messages;
also, the -s option of traditional grep behaved
like -q does in most modern implementations. 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.
The QNX4 implementation fails to count lines with grep -c '$',
but works with grep -c '^'. Other alternatives for counting
lines are to use sed -n '$=' or wc -l.
Some traditional grep implementations do not work on long
input lines. On AIX the default grep silently truncates long
lines on the input before matching.
Also, many implementations do not support multiple regexps
with -e: they either reject -e entirely (e.g., Solaris)
or honor only the last pattern (e.g., IRIX 6.5 and NeXT). To
work around these problems, invoke AC_PROG_GREP and then use
$GREP.
Another possible workaround for the multiple -e problem is to separate the patterns by newlines, for example:
grep 'foo
bar' in.txt
except that this fails with traditional grep implementations and with OpenBSD 3.8 grep.
Traditional grep implementations (e.g., Solaris) do not
support the -E or -F options. To work around these
problems, invoke AC_PROG_EGREP and then use $EGREP, and
similarly for AC_PROG_FGREP and $FGREP. Even if you are
willing to require support for Posix grep, your script should
not use both -E and -F, since Posix does not allow
this combination.
Portable grep regular expressions should use ‘\’ only to escape characters in the string ‘$()*.0123456789[\^{}’. For example, alternation, ‘\|’, is common but Posix does not require its support in basic regular expressions, so it should be avoided in portable scripts. Solaris and HP-UX grep do not support it. Similarly, the following escape sequences should also be avoided: ‘\<’, ‘\>’, ‘\+’, ‘\?’, ‘\`’, ‘\'’, ‘\B’, ‘\b’, ‘\S’, ‘\s’, ‘\W’, and ‘\w’.
Posix does not specify the behavior of grep on binary files. An example where this matters is using BSD grep to search text that includes embedded ANSI escape sequences for colored output to terminals (‘\033[m’ is the sequence to restore normal output); the behavior depends on whether input is seekable:
$ printf 'esc\033[mape\n' > sample
$ grep . sample
Binary file sample matches
$ cat sample | grep .
escape
cat >file <<'EOF'
1 x
2 y
EOF
cat file | join file -
Use ‘join - file’ instead.
For versions of the DJGPP before 2.04,
ln emulates symbolic links
to executables by generating a stub that in turn calls the real
program. This feature also works with nonexistent files like in the
Posix spec. So ‘ln -s file link’ generates link.exe,
which attempts 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 support
for symbolic links.
On ancient hosts, ‘ls foo’ sent the diagnostic ‘foo not found’ to standard output if foo did not exist. Hence a shell command like ‘sources=`ls *.c 2>/dev/null`’ did not always work, since it was equivalent to ‘sources='*.c not found'’ in the absence of ‘.c’ files. This is no longer a practical problem, since current ls implementations send diagnostics to standard error.
The behavior of ls on a directory that is being concurrently
modified is not always predictable, because of a data race where cached
information returned by readdir does not match the current
directory state. In fact, MacOS 10.5 has an intermittent bug where
readdir, and thus ls, sometimes lists a file more than
once if other files were added or removed from the directory immediately
prior to the ls call. Since ls already sorts its
output, the duplicate entries can be avoided by piping the results
through uniq.
AS_MKDIR_P(file-name) (see Programming in M4sh)
or AC_PROG_MKDIR_P (see Particular Programs).
Combining the -m and -p options, as in ‘mkdir -m go-w -p dir’, often leads to trouble. FreeBSD mkdir incorrectly attempts to change the permissions of dir even if it already exists. HP-UX 11.23 and IRIX 6.5 mkdir often assign the wrong permissions to any newly-created parents of dir.
Posix does not clearly specify whether ‘mkdir -p foo’ should succeed when foo is a symbolic link to an already-existing directory. The GNU Core Utilities 5.1.0 mkdir succeeds, but Solaris mkdir fails.
Traditional mkdir -p implementations suffer from race conditions.
For example, if you invoke mkdir -p a/b and mkdir -p a/c
at the same time, both processes might detect that a is missing,
one might create a, then the other might try to create a
and fail with a File exists diagnostic. The GNU Core
Utilities (‘fileutils’ version 4.1), FreeBSD 5.0,
NetBSD 2.0.2, and OpenBSD 2.4 are known to be
race-free when two processes invoke mkdir -p simultaneously, but
earlier versions are vulnerable. Solaris mkdir is still
vulnerable as of Solaris 10, and other traditional Unix systems are
probably vulnerable too. This possible race is harmful in parallel
builds when several Make rules call mkdir -p to
construct directories. You may use
install-sh -d as a safe replacement, provided this script is
recent enough; the copy shipped with Autoconf 2.60 and Automake 1.10 is
OK, but copies from older versions are vulnerable.
Here is sample code to create a new temporary directory ‘$dir’ safely:
# Create a temporary directory $dir in $TMPDIR (default /tmp).
# Use mktemp if possible; otherwise fall back on mkdir,
# with $RANDOM to make collisions less likely.
: "${TMPDIR:=/tmp}"
{
dir=`
(umask 077 && mktemp -d "$TMPDIR/fooXXXXXX") 2>/dev/null
` &&
test -d "$dir"
} || {
dir=$TMPDIR/foo$$-$RANDOM
(umask 077 && mkdir "$dir")
} || exit $?
Moving individual files between file systems is portable (it was in Unix version 6), 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.
On some systems moving files from /tmp can sometimes cause undesirable (but perfectly valid) warnings, even if you created these files. This is because /tmp belongs to a group that ordinary users are not members of, and files created in /tmp inherit the group of /tmp. When the file is copied, mv issues a diagnostic without failing:
$ touch /tmp/foo
$ mv /tmp/foo .
error-->mv: ./foo: set owner/group (was: 100/0): Operation not permitted
$ echo $?
0
$ ls foo
foo
This annoying behavior conforms to Posix, unfortunately.
Moving directories across mount points is not portable, use cp and rm.
DOS variants cannot rename or remove open files, and do not
support commands like ‘mv foo bar >foo’, even though this is
perfectly portable among Posix hosts.
This problem no longer exists in Mac OS X 10.4.3.
It is not portable to invoke rm without operands. For example, on many systems ‘rm -f -r’ (with no other arguments) silently succeeds without doing anything, but it fails with a diagnostic on NetBSD 2.0.2.
A file might not be removed even if its parent directory is writable and searchable. Many Posix hosts cannot remove a mount point, a named stream, a working directory, or a last link to a file that is being executed.
DOS variants cannot rename or remove open files, and do not
support commands like ‘rm foo >foo’, even though this is
perfectly portable among Posix hosts.
Avoid empty patterns within parentheses (i.e., ‘\(\)’). Posix does not require support for empty patterns, and Unicos 9 sed rejects them.
Unicos 9 sed loops endlessly on patterns like ‘.*\n.*’.
Sed scripts should not use branch labels longer than 7 characters and should not contain comments; AIX 5.3 sed rejects indented comments. HP-UX sed has a limit of 99 commands (not counting ‘:’ commands) and 48 labels, which can not be circumvented by using more than one script file. It can execute up to 19 reads with the ‘r’ command per cycle. Solaris /usr/ucb/sed rejects usages that exceed a limit of about 6000 bytes for the internal representation of commands.
Avoid redundant ‘;’, as some sed implementations, such as NetBSD 1.4.2's, incorrectly 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 not have unreasonably long lines, since some sed
implementations have an input buffer limited to 4000 bytes. Likewise,
not all sed implementations can handle embedded NUL or
a missing trailing newline.
Remember that ranges within a bracket expression of a regular expression are only well-defined in the ‘C’ (or ‘POSIX’) locale. Meanwhile, support for character classes like ‘[[:upper:]]’ is not yet universal, so if you cannot guarantee the setting of LC_ALL, it is better to spell out a range ‘[ABCDEFGHIJKLMNOPQRSTUVWXYZ]’ than to rely on ‘[A-Z]’.
Additionally, Posix states that regular expressions are only well-defined on characters. Unfortunately, there exist platforms such as MacOS X 10.5 where not all 8-bit byte values are valid characters, even though that platform has a single-byte ‘C’ locale. And Posix allows the existence of a multi-byte ‘C’ locale, although that does not yet appear to be a common implementation. At any rate, it means that not all bytes will be matched by the regular expression ‘.’:
$ printf '\200\n' | LC_ALL=C sed -n /./p | wc -l
0
$ printf '\200\n' | LC_ALL=en_US.ISO8859-1 sed -n /./p | wc -l
1
Portable sed regular expressions should use ‘\’ only to escape characters in the string ‘$()*.0123456789[\^n{}’. For exam