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1. Introduction Autoconf's purpose, strengths, and weaknesses 2. The GNU Build System A set of tools for portable software packages 3. Making configureScriptsHow to organize and produce Autoconf scripts 4. Initialization and Output Files Initialization and output 5. Existing Tests Macros that check for particular features 6. Writing Tests How to write new feature checks 7. Results of Tests What to do with results from feature checks 8. Programming in M4 Layers on top of which Autoconf is written 9. Writing Autoconf Macros Adding new macros to Autoconf 10. Portable Shell Programming Shell script portability pitfalls 11. Manual Configuration Selecting features that can't be guessed 12. Site Configuration Local defaults for configure13. Running configureScriptsHow to use the Autoconf output 14. Recreating a Configuration Recreating a configuration 15. Obsolete Constructs Kept for backward compatibility 16. Generating Test Suites with Autotest Creating portable test suites 17. Frequent Autoconf Questions, with answers 18. History of Autoconf A. Copying This Manual How to make copies of this manual B. Indices Indices of symbols, concepts, etc.
-- The Detailed Node Listing ---
The GNU Build System
2.1 Automake Escaping Makefile hell 2.2 Libtool Building libraries portably 2.3 Pointers More info on the GNU build system
MakingconfigureScripts
3.1 Writing `configure.ac' What to put in an Autoconf input file 3.2 Using autoscanto Create `configure.ac'Semi-automatic `configure.ac' writing 3.3 Using ifnamesto List ConditionalsListing the conditionals in source code 3.4 Using autoconfto CreateconfigureHow to create configuration scripts 3.5 Using autoreconfto UpdateconfigureScriptsRemaking multiple configurescripts
Writing `configure.ac'
3.1.1 A Shell Script Compiler Autoconf as solution of a problem 3.1.2 The Autoconf Language Programming in Autoconf 3.1.3 Standard `configure.ac' Layout Standard organization of `configure.ac'
Initialization and Output Files
4.1 Initializing configureOption processing etc. 4.2 Notices in configureCopyright, version numbers in configure4.3 Finding configureInputWhere Autoconf should find files 4.4 Outputting Files Outputting results from the configuration 4.5 Performing Configuration Actions Preparing the output based on results 4.6 Creating Configuration Files Creating output files 4.7 Substitutions in Makefiles Using output variables in `Makefile's 4.8 Configuration Header Files Creating a configuration header file 4.9 Running Arbitrary Configuration Commands Running arbitrary instantiation commands 4.10 Creating Configuration Links Links depending on the configuration 4.11 Configuring Other Packages in Subdirectories Configuring independent packages together 4.12 Default Prefix Changing the default installation prefix
Substitutions in Makefiles
4.7.1 Preset Output Variables Output variables that are always set 4.7.2 Installation Directory Variables Other preset output variables 4.7.3 Build Directories Supporting multiple concurrent compiles 4.7.4 Automatic Remaking Makefile rules for configuring
Configuration Header Files
4.8.1 Configuration Header Templates Input for the configuration headers 4.8.2 Using autoheaderto Create `config.h.in'How to create configuration templates 4.8.3 Autoheader Macros How to specify CPP templates
Existing Tests
5.1 Common Behavior Macros' standard schemes 5.2 Alternative Programs Selecting between alternative programs 5.3 Files Checking for the existence of files 5.4 Library Files Library archives that might be missing 5.5 Library Functions C library functions that might be missing 5.6 Header Files Header files that might be missing 5.7 Declarations Declarations that may be missing 5.8 Structures Structures or members that might be missing 5.9 Types Types that might be missing 5.10 Compilers and Preprocessors Checking for compiling programs 5.11 System Services Operating system services 5.12 UNIX Variants Special kludges for specific UNIX variants
Common Behavior
5.1.1 Standard Symbols Symbols defined by the macros 5.1.2 Default Includes Includes used by the generic macros
Alternative Programs
5.2.1 Particular Program Checks Special handling to find certain programs 5.2.2 Generic Program and File Checks How to find other programs
Library Functions
5.5.1 Portability of C Functions Pitfalls with usual functions 5.5.2 Particular Function Checks Special handling to find certain functions 5.5.3 Generic Function Checks How to find other functions
Header Files
5.6.1 Portability of Headers Collected knowledge on common headers 5.6.2 Particular Header Checks Special handling to find certain headers 5.6.3 Generic Header Checks How to find other headers
Declarations
5.7.1 Particular Declaration Checks Macros to check for certain declarations 5.7.2 Generic Declaration Checks How to find other declarations
Structures
5.8.1 Particular Structure Checks Macros to check for certain structure members 5.8.2 Generic Structure Checks How to find other structure members
Types
5.9.1 Particular Type Checks Special handling to find certain types 5.9.2 Generic Type Checks How to find other types
Compilers and Preprocessors
5.10.1 Specific Compiler Characteristics Some portability issues 5.10.2 Generic Compiler Characteristics Language independent tests 5.10.3 C Compiler Characteristics Checking its characteristics 5.10.4 C++ Compiler Characteristics Likewise 5.10.5 Fortran 77 Compiler Characteristics Likewise
Writing Tests
6.1 Language Choice Selecting which language to use for testing 6.2 Writing Test Programs Forging source files for compilers 6.3 Running the Preprocessor Detecting preprocessor symbols 6.4 Running the Compiler Detecting language or header features 6.5 Running the Linker Detecting library features 6.6 Checking Run Time Behavior Testing for run-time features 6.7 Systemology A zoology of operating systems 6.8 Multiple Cases Tests for several possible values
Writing Test Programs
6.2.1 Guidelines for Test Programs General rules for writing test programs 6.2.2 Test Functions Avoiding pitfalls in test programs 6.2.3 Generating Sources Source program boilerplate
Results of Tests
7.1 Defining C Preprocessor Symbols Defining C preprocessor symbols 7.2 Setting Output Variables Replacing variables in output files 7.3 Caching Results Speeding up subsequent configureruns7.4 Printing Messages Notifying configureusers
Caching Results
7.3.1 Cache Variable Names Shell variables used in caches 7.3.2 Cache Files Files configureuses for caching7.3.3 Cache Checkpointing Loading and saving the cache file
Programming in M4
8.1 M4 Quotation Protecting macros from unwanted expansion 8.2 Using autom4teThe Autoconf executables backbone 8.3 Programming in M4sugar Convenient pure M4 macros 8.4 Programming in M4sh Common shell Constructs
M4 Quotation
8.1.1 Active Characters Characters that change the behavior of M4 8.1.2 One Macro Call Quotation and one macro call 8.1.3 Quotation and Nested Macros Macros calling macros 8.1.4 changequoteis EvilWorse than INTERCAL: M4 + changequote 8.1.5 Quadrigraphs Another way to escape special characters 8.1.6 Quotation Rule Of Thumb One parenthesis, one quote
Usingautom4te
8.2.1 Invoking autom4teA GNU M4 wrapper 8.2.2 Customizing autom4teCustomizing the Autoconf package
Programming in M4sugar
8.3.1 Redefined M4 Macros M4 builtins changed in M4sugar 8.3.2 Evaluation Macros More quotation and evaluation control 8.3.3 Forbidden Patterns Catching unexpanded macros
Writing Autoconf Macros
9.1 Macro Definitions Basic format of an Autoconf macro 9.2 Macro Names What to call your new macros 9.3 Reporting Messages Notifying autoconfusers9.4 Dependencies Between Macros What to do when macros depend on other macros 9.5 Obsoleting Macros Warning about old ways of doing things 9.6 Coding Style Writing Autoconf macros à la Autoconf
Dependencies Between Macros
9.4.1 Prerequisite Macros Ensuring required information 9.4.2 Suggested Ordering Warning about possible ordering problems
Portable Shell Programming
10.1 Shellology A zoology of shells 10.2 Here-Documents Quirks and tricks 10.3 File Descriptors FDs and redirections 10.4 File System Conventions File- and pathnames 10.5 Shell Substitutions Variable and command expansions 10.6 Assignments Varying side effects of assignments 10.7 Special Shell Variables Variables you should not change 10.8 Limitations of Shell Builtins Portable use of not so portable /bin/sh 10.9 Limitations of Usual Tools Portable use of portable tools 10.10 Limitations of Make Portable Makefiles
Manual Configuration
11.1 Specifying the System Type Specifying the system type 11.2 Getting the Canonical System Type Getting the canonical system type 11.3 Using the System Type What to do with the system type
Site Configuration
12.1 Working With External Software Working with other optional software 12.2 Choosing Package Options Selecting optional features 12.3 Making Your Help Strings Look Pretty Formatting help string 12.4 Configuring Site Details Configuring site details 12.5 Transforming Program Names When Installing Changing program names when installing 12.6 Setting Site Defaults Giving configurelocal defaults
Transforming Program Names When Installing
12.5.1 Transformation Options configureoptions to transform names12.5.2 Transformation Examples Sample uses of transforming names 12.5.3 Transformation Rules `Makefile' uses of transforming names
RunningconfigureScripts
13.1 Basic Installation Instructions for typical cases 13.2 Compilers and Options Selecting compilers and optimization 13.3 Compiling For Multiple Architectures Compiling for multiple architectures at once 13.4 Installation Names Installing in different directories 13.5 Optional Features Selecting optional features 13.6 Specifying the System Type Specifying the system type 13.7 Sharing Defaults Setting site-wide defaults for configure13.8 Defining Variables Specifying the compiler etc. 13.9 configureInvocationChanging how configureruns
Obsolete Constructs
15.1 Obsolete `config.status' Invocation Different calling convention 15.2 `acconfig.h' Additional entries in `config.h.in' 15.3 Using autoupdateto Modernize `configure.ac'Automatic update of `configure.ac' 15.4 Obsolete Macros Backward compatibility macros 15.5 Upgrading From Version 1 Tips for upgrading your files 15.6 Upgrading From Version 2.13 Some fresher tips
Upgrading From Version 1
15.5.1 Changed File Names Files you might rename 15.5.2 Changed Makefiles New things to put in `Makefile.in' 15.5.3 Changed Macros Macro calls you might replace 15.5.4 Changed Results Changes in how to check test results 15.5.5 Changed Macro Writing Better ways to write your own macros
Upgrading From Version 2.13
15.6.1 Changed Quotation Broken code which used to work 15.6.2 New Macros Interaction with foreign macros 15.6.3 Hosts and Cross-Compilation Bugward compatibility kludges 15.6.4 AC_LIBOBJvs.LIBOBJSLIBOBJS is a forbidden token 15.6.5 AC_FOO_IFELSEvs.AC_TRY_FOOA more generic scheme for testing sources
Generating Test Suites with Autotest
16.1 Using an Autotest Test Suite Autotest and the user 16.2 Writing `testsuite.at' Autotest macros 16.3 Running testsuiteScriptsRunning testsuitescripts16.4 Making testsuiteScriptsUsing autom4te to create testsuite
Using an Autotest Test Suite
16.1.1 testsuiteScriptsThe concepts of Autotest 16.1.2 Autotest Logs Their contents
Frequent Autoconf Questions, with answers
17.1 Distributing configureScriptsDistributing configurescripts17.2 Why Require GNU M4? Why not use the standard M4? 17.3 How Can I Bootstrap? Autoconf and GNU M4 require each other? 17.4 Why Not Imake? Why GNU uses configureinstead of Imake17.5 How Do I #defineInstallation Directories?Passing datadirto program17.6 What is `autom4te.cache'? What is it? Can I remove it?
History of Autoconf
18.1 Genesis Prehistory and naming of configure18.2 Exodus The plagues of M4 and Perl 18.3 Leviticus The priestly code of portability arrives 18.4 Numbers Growth and contributors 18.5 Deuteronomy Approaching the promises of easy configuration
Copying This Manual
A.1 GNU Free Documentation License License for copying this manual
Indices
B.1 Environment Variable Index Index of environment variables used B.2 Output Variable Index Index of variables set in output files B.3 Preprocessor Symbol Index Index of C preprocessor symbols defined B.4 Autoconf Macro Index Index of Autoconf macros B.5 M4 Macro Index Index of M4, M4sugar, and M4sh macros B.6 Autotest Macro Index Index of Autotest macros B.7 Program and Function Index Index of those with portability problems B.8 Concept Index General index
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A physicist, an engineer, and a computer scientist were discussing the nature of God. ``Surely a Physicist,'' said the physicist, ``because early in the Creation, God made Light; and you know, Maxwell's equations, the dual nature of electromagnetic waves, the relativistic consequences...'' ``An Engineer!,'' said the engineer, ``because before making Light, God split the Chaos into Land and Water; it takes a hell of an engineer to handle that big amount of mud, and orderly separation of solids from liquids...'' The computer scientist shouted: ``And the Chaos, where do you think it was coming from, hmm?'' ---Anonymous
Autoconf is a tool for producing shell scripts that automatically configure software source code packages to adapt to many kinds of UNIX-like systems. The configuration scripts produced by Autoconf are independent of Autoconf when they are run, so their users do not need to have Autoconf.
The configuration scripts produced by Autoconf require no manual user intervention when run; they do not normally even need an argument specifying the system type. Instead, they individually test for the presence of each feature that the software package they are for might need. (Before each check, they print a one-line message stating what they are checking for, so the user doesn't get too bored while waiting for the script to finish.) As a result, they deal well with systems that are hybrids or customized from the more common UNIX variants. There is no need to maintain files that list the features supported by each release of each variant of UNIX.
For each software package that Autoconf is used with, it creates a configuration script from a template file that lists the system features that the package needs or can use. After the shell code to recognize and respond to a system feature has been written, Autoconf allows it to be shared by many software packages that can use (or need) that feature. If it later turns out that the shell code needs adjustment for some reason, it needs to be changed in only one place; all of the configuration scripts can be regenerated automatically to take advantage of the updated code.
The Metaconfig package is similar in purpose to Autoconf, but the scripts it produces require manual user intervention, which is quite inconvenient when configuring large source trees. Unlike Metaconfig scripts, Autoconf scripts can support cross-compiling, if some care is taken in writing them.
Autoconf does not solve all problems related to making portable software packages--for a more complete solution, it should be used in concert with other GNU build tools like Automake and Libtool. These other tools take on jobs like the creation of a portable, recursive `Makefile' with all of the standard targets, linking of shared libraries, and so on. See section 2. The GNU Build System, for more information.
Autoconf imposes some restrictions on the names of macros used with
#if in C programs (see section B.3 Preprocessor Symbol Index).
Autoconf requires GNU M4 in order to generate the scripts. It uses features that some UNIX versions of M4, including GNU M4 1.3, do not have. You must use version 1.4 or later of GNU M4.
See section 15.5 Upgrading From Version 1, for information about upgrading from version 1. See section 18. History of Autoconf, for the story of Autoconf's development. See section 17. Frequent Autoconf Questions, with answers, for answers to some common questions about Autoconf.
See the \title\{http://www.gnu.org/software/autoconf/autoconf.html, Autoconf web page} for up-to-date information, details on the mailing lists, pointers to a list of known bugs, etc.
Mail suggestions to the Autoconf mailing list.
Bug reports should be preferably submitted to the \title\{http://bugs.gnu.org/cgi-bin/gnatsweb.pl?database=autoconf, Autoconf Gnats database}, or sent to the Autoconf Bugs mailing list. If possible, first check that your bug is not already solved in current development versions, and that it has not been reported yet. Be sure to include all the needed information and a short `configure.ac' that demonstrates the problem.
Autoconf's development tree is accessible via CVS; see the Autoconf web page for details. There is also a CVSweb interface to the Autoconf development tree. Patches relative to the current CVS version can be sent for review to the Autoconf Patches mailing list.
Because of its mission, Autoconf includes only a set of often-used macros that have already demonstrated their usefulness. Nevertheless, if you wish to share your macros, or find existing ones, see the \title\{http://www.gnu.org/software/ac-archive/, Autoconf Macro Archive}, which is kindly run by Peter Simons.
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Autoconf solves an important problem--reliable discovery of system-specific build and run-time 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.
2.1 Automake Escaping Makefile hell 2.2 Libtool Building libraries portably 2.3 Pointers More info on the GNU build system
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The ubiquity of make means that a `Makefile' is almost the
only viable way to distribute automatic build rules for software, but
one quickly runs into make's numerous limitations. Its lack of
support for automatic dependency tracking, recursive builds in
subdirectories, reliable timestamps (e.g., for network filesystems), and
so on, mean that developers must painfully (and often incorrectly)
reinvent the wheel for each project. Portability is non-trivial, thanks
to the quirks of make on many systems. On top of all this is the
manual labor required to implement the many standard targets that users
have come to expect (make install, make distclean,
make uninstall, etc.). Since you are, of course, using Autoconf,
you also have to insert repetitive code in your Makefile.in to
recognize @CC@, @CFLAGS@, and other substitutions
provided by configure. Into this mess steps Automake.
Automake allows you to specify your build needs in a Makefile.am
file with a vastly simpler and more powerful syntax than that of a plain
Makefile, and then generates a portable Makefile.in for
use with Autoconf. For example, the Makefile.am to build and
install a simple "Hello world" program might look like:
bin_PROGRAMS = hello hello_SOURCES = hello.c |
The resulting Makefile.in (~400 lines) automatically supports all
the standard targets, the substitutions provided by Autoconf, automatic
dependency tracking, VPATH building, and so on. make will
build the hello program, and make install will install it
in `/usr/local/bin' (or whatever prefix was given to
configure, if not `/usr/local').
Automake may require that additional tools be present on the
developer's machine. For example, the Makefile.in that
the developer works with may not be portable (e.g., it might use special
features of your compiler to automatically generate dependency
information). Running make dist, however, produces a
`hello-1.0.tar.gz' package (or whatever the program/version is)
with a Makefile.in that will work on any system.
The benefits of Automake increase for larger packages (especially ones with subdirectories), but even for small programs the added convenience and portability can be substantial. And that's not all....
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Very often, one wants to build not only programs, but libraries, so that other programs can benefit from the fruits of your labor. Ideally, one would like to produce shared (dynamically linked) libraries, which can be used by multiple programs without duplication on disk or in memory and can be updated independently of the linked programs. Producing shared libraries portably, however, is the stuff of nightmares--each system has its own incompatible tools, compiler flags, and magic incantations. Fortunately, GNU provides a solution: Libtool.
Libtool handles all the requirements of building shared libraries for
you, and at this time seems to be the only way to do so with any
portability. It also handles many other headaches, such as: the
interaction of Makefile rules with the variable suffixes of
shared libraries, linking reliably 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 on its own, it is most simply utilized in
conjunction with Automake--there, Libtool is used automatically
whenever shared libraries are needed, and you need not know its syntax.
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Developers who are used to the simplicity of make for small
projects on a single system might be daunted at the prospect of
learning to use Automake and Autoconf. As your software is
distributed to more and more users, however, you will otherwise
quickly find yourself putting lots of effort into reinventing the
services that the GNU build tools provide, and making the
same mistakes that they once made and overcame. (Besides, since
you're already learning Autoconf, Automake will be a piece of cake.)
There are a number of places that you can go to for more information on the GNU build tools.
The home pages for Autoconf, Automake, and Libtool.
See section `Automake' in GNU Automake, for more information on Automake.
The book GNU Autoconf, Automake and Libtool(1) describes the complete GNU build environment. You can also find the entire book on-line at \title\{http://sources.redhat.com/autobook/,"The Goat Book" home page}.
The Autoconf Developer Page maintains links to a number of Autoconf/Automake tutorials online, and also links to the \title\{http://www.gnu.org/software/ac-archive/, Autoconf Macro Archive}.
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configure Scripts
The configuration scripts that Autoconf produces are by convention
called configure. When run, configure creates several
files, replacing configuration parameters in them with appropriate
values. The files that configure creates are:
#define directives (see section 4.8 Configuration Header Files);
configure makes a mistake.
To create a configure script with Autoconf, you need to write an
Autoconf input file `configure.ac' (or `configure.in') and run
autoconf on it. If you write your own feature tests to
supplement those that come with Autoconf, you might also write files
called `aclocal.m4' and `acsite.m4'. If you use a C header
file to contain #define directives, you might also run
autoheader, and you will distribute the generated file
`config.h.in' with the package.
Here is a diagram showing how the files that can be used in
configuration are produced. Programs that are executed are suffixed by
`*'. Optional files are enclosed in square brackets (`[]').
autoconf and autoheader also read the installed Autoconf
macro files (by reading `autoconf.m4').
Files used in preparing a software package for distribution:
your source files --> [autoscan*] --> [configure.scan] --> configure.ac
configure.ac --.
| .------> autoconf* -----> configure
[aclocal.m4] --+---+
| `-----> [autoheader*] --> [config.h.in]
[acsite.m4] ---'
Makefile.in -------------------------------> Makefile.in
|
Files used in configuring a software package:
.-------------> [config.cache]
configure* ------------+-------------> config.log
|
[config.h.in] -. v .-> [config.h] -.
+--> config.status* -+ +--> make*
Makefile.in ---' `-> Makefile ---'
|
3.1 Writing `configure.ac' What to put in an Autoconf input file 3.2 Using autoscanto Create `configure.ac'Semi-automatic `configure.ac' writing 3.3 Using ifnamesto List ConditionalsListing the conditionals in source code 3.4 Using autoconfto CreateconfigureHow to create configuration scripts 3.5 Using autoreconfto UpdateconfigureScriptsRemaking multiple configurescripts
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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
5. Existing Tests, for their descriptions. For most other features,
you can use Autoconf template macros to produce custom checks; see
6. Writing Tests, for information about them. For especially tricky
or specialized features, `configure.ac' might need to contain some
hand-crafted shell commands; see 10. Portable Shell Programming. The
autoscan program can give you a good start in writing
`configure.ac' (see section 3.2 Using autoscan to Create `configure.ac', 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.
3.1.1 A Shell Script Compiler Autoconf as solution of a problem 3.1.2 The Autoconf Language Programming in Autoconf 3.1.3 Standard `configure.ac' Layout Standard organization of `configure.ac'
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Just as for any other computer language, in order to properly program `configure.ac' in Autoconf you must understand what problem the language tries to address and how it does so.
The problem Autoconf addresses is that the world is a mess. After all,
you are using Autoconf in order to have your package compile easily on
all sorts of different systems, some of them being extremely hostile.
Autoconf itself bears the price for these differences: configure
must run on all those systems, and thus configure must limit itself
to their lowest common denominator of features.
Naturally, you might then think of shell scripts; who needs
autoconf? A set of properly written shell functions is enough to
make it easy to write configure scripts by hand. Sigh!
Unfortunately, shell functions do not belong to the least common
denominator; therefore, where you would like to define a function and
use it ten times, you would instead need to copy its body ten times.
So, what is really needed is some kind of compiler, autoconf,
that takes an Autoconf program, `configure.ac', and transforms it
into a portable shell script, configure.
How does autoconf perform this task?
There are two obvious possibilities: creating a brand new language or
extending an existing one. The former option is very attractive: all
sorts of optimizations could easily be implemented in the compiler and
many rigorous checks could be performed on the Autoconf program
(e.g., rejecting any non-portable construct). Alternatively, you can
extend an existing language, such as the sh (Bourne shell)
language.
Autoconf does the latter: it is a layer on top of sh. It was
therefore most convenient to implement autoconf as a macro
expander: a program that repeatedly performs macro expansions on
text input, replacing macro calls with macro bodies and producing a pure
sh script in the end. Instead of implementing a dedicated
Autoconf macro expander, it is natural to use an existing
general-purpose macro language, such as M4, and implement the extensions
as a set of M4 macros.
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The Autoconf language is very different from many other computer languages because it treats actual code the same as plain text. Whereas in C, for instance, data and instructions have very different syntactic status, in Autoconf their status is rigorously the same. Therefore, we need a means to distinguish literal strings from text to be expanded: quotation.
When calling macros that take arguments, there must not be any blank space between the macro name and the open parenthesis. Arguments should be enclosed within the M4 quote characters `[' and `]', and be separated by commas. Any leading spaces in arguments are ignored, unless they are quoted. You may safely leave out the quotes when the argument is simple text, but always quote complex arguments such as other macro calls. This rule applies recursively for every macro call, including macros called from other macros.
For instance:
AC_CHECK_HEADER([stdio.h],
[AC_DEFINE([HAVE_STDIO_H])],
[AC_MSG_ERROR([Sorry, can't do anything for you])])
|
is quoted properly. You may safely simplify its quotation to:
AC_CHECK_HEADER(stdio.h,
[AC_DEFINE(HAVE_STDIO_H)],
[AC_MSG_ERROR([Sorry, can't do anything for you])])
|
Notice that the argument of AC_MSG_ERROR is still quoted;
otherwise, its comma would have been interpreted as an argument separator.
The following example is wrong and dangerous, as it is underquoted:
AC_CHECK_HEADER(stdio.h,
AC_DEFINE(HAVE_STDIO_H),
AC_MSG_ERROR([Sorry, can't do anything for you]))
|
In other cases, you may have to use text that also resembles a macro call. You must quote that text even when it is not passed as a macro argument:
echo "Hard rock was here! --[AC_DC]" |
which will result in
echo "Hard rock was here! --AC_DC" |
When you use the same text in a macro argument, you must therefore have an extra quotation level (since one is stripped away by the macro substitution). In general, then, it is a good idea to use double quoting for all literal string arguments:
AC_MSG_WARN([[AC_DC stinks --Iron Maiden]]) |
You are now able to understand one of the constructs of Autoconf that has been continually misunderstood... The rule of thumb is that whenever you expect macro expansion, expect quote expansion; i.e., expect one level of quotes to be lost. For instance:
AC_COMPILE_IFELSE([char b[10];],, [AC_MSG_ERROR([you lose])]) |
is incorrect: here, the first argument of AC_COMPILE_IFELSE is
`char b[10];' and will be expanded once, which results in
`char b10;'. (There was an idiom common in Autoconf's past to
address this issue via the M4 changequote primitive, but do not
use it!) Let's take a closer look: the author meant the first argument
to be understood as a literal, and therefore it must be quoted twice:
AC_COMPILE_IFELSE([[char b[10];]],, [AC_MSG_ERROR([you lose])]) |
Voilà, you actually produce `char b[10];' this time!
The careful reader will notice that, according to these guidelines, the
"properly" quoted AC_CHECK_HEADER example above is actually
lacking three pairs of quotes! Nevertheless, for the sake of readability,
double quotation of literals is used only where needed in this manual.
Some macros take optional arguments, which this documentation represents as [arg] (not to be confused with the quote characters). You may just leave them empty, or use `[]' to make the emptiness of the argument explicit, or you may simply omit the trailing commas. The three lines below are equivalent:
AC_CHECK_HEADERS(stdio.h, [], [], []) AC_CHECK_HEADERS(stdio.h,,,) AC_CHECK_HEADERS(stdio.h) |
It is best to put each macro call on its own line in
`configure.ac'. Most of the macros don't add extra newlines; they
rely on the newline after the macro call to terminate the commands.
This approach makes the generated configure script a little
easier to read by not inserting lots of blank lines. It is generally
safe to set shell variables on the same line as a macro call, because
the shell allows assignments without intervening newlines.
You can include comments in `configure.ac' files by starting them with the `#'. For example, it is helpful to begin `configure.ac' files with a line like this:
# Process this file with autoconf to produce a configure script. |
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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 section 4.4 Outputting Files). Additionally, some macros
rely on other macros having been called first, because they check
previously set values of some variables to decide what to do. These
macros are noted in the individual descriptions (see section 5. 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 |
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autoscan to Create `configure.ac'
The autoscan program can help you create and/or maintain a
`configure.ac' file for a software package. autoscan
examines source files in the directory tree rooted at a directory given
as a command line argument, or the current directory if none is given.
It searches the source files for common portability problems and creates
a file `configure.scan' which is a preliminary `configure.ac'
for that package, and checks a possibly existing `configure.ac' for
completeness.
When using autoscan to create a `configure.ac', you
should manually examine `configure.scan' before renaming it to
`configure.ac'; it will probably need some adjustments.
Occasionally, autoscan outputs a macro in the wrong order
relative to another macro, so that autoconf produces a warning;
you need to move such macros manually. Also, if you want the package to
use a configuration header file, you must add a call to
AC_CONFIG_HEADERS (see section 4.8 Configuration Header Files). You might
also have to change or add some #if directives to your program in
order to make it work with Autoconf (see section 3.3 Using ifnames to List Conditionals, for
information about a program that can help with that job).
When using autoscan to maintain a `configure.ac', simply
consider adding its suggestions. The file `autoscan.log' will
contain detailed information on why a macro is requested.
autoscan uses several data files (installed along with Autoconf)
to determine which macros to output when it finds particular symbols in
a package's source files. These data files all have the same format:
each line consists of a symbol, whitespace, and the Autoconf macro to
output if that symbol is encountered. Lines starting with `#' are
comments.
autoscan accepts the following options:
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ifnames to List Conditionals
ifnames can help you write `configure.ac' for a software
package. It prints the identifiers that the package already uses in C
preprocessor conditionals. If a package has already been set up to have
some portability, ifnames can thus help you figure out what its
configure needs to check for. It may help fill in some gaps in a
`configure.ac' generated by autoscan (see section 3.2 Using autoscan to Create `configure.ac').
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:
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autoconf to Create configure
To create configure from `configure.ac', run the
autoconf program with no arguments. autoconf processes
`configure.ac' with the M4 macro processor, using the
Autoconf macros. If you give autoconf an argument, it reads that
file instead of `configure.ac' and writes the configuration script
to the standard output instead of to configure. If you give
autoconf the argument `-', it reads from the standard
input instead of `configure.ac' and writes the configuration script
to the standard output.
The Autoconf macros are defined in several files. Some of the files are
distributed with Autoconf; autoconf reads them first. Then it
looks for the optional file `acsite.m4' in the directory that
contains the distributed Autoconf macro files, and for the optional file
`aclocal.m4' in the current directory. Those files can contain
your site's or the package's own Autoconf macro definitions
(see section 9. 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. Passing `-W category' will actually behave as if
you had passed `--warnings=syntax,$WARNINGS,category'. If
you want to disable the defaults and WARNINGS, but (for example)
enable the warnings about obsolete constructs, you would use `-W
none,obsolete'.
Because autoconf uses autom4te behind the scenes, it
displays a back trace for errors, but not for warnings; if you want
them, just pass `-W error'. See section 8.2.1 Invoking autom4te, for some
examples.
configure script, but list the calls to
macro according to the format. Multiple `--trace'
arguments can be used to list several macros. Multiple `--trace'
arguments for a single macro are not cumulative; instead, you should
just make format as long as needed.
The format is a regular string, with newlines if desired, and
several special escape codes. It defaults to `$f:$l:$n:$%'; see
8.2.1 Invoking autom4te, 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
|
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autoreconf to Update configure Scripts
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 section 4.11 Configuring Other Packages in Subdirectories). By default, it only remakes
those files that are older than their sources.
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 section 4.7.4 Automatic Remaking, for `Makefile' rules to automatically
remake configure scripts when their source files change. That
method handles the timestamps of configuration header templates
properly, but does not pass `--autoconf-dir=dir' or
`--localdir=dir'.
autoreconf accepts the following options:
autoreconf runs
autoconf (and autoheader, if appropriate).
This option triggers calls to `automake --add-missing', `libtoolize', `autopoint', etc.
Warnings about `syntax' are enabled by default, and the environment
variable WARNINGS, a comma separated list of categories, is
honored. Passing `-W category' will actually behave as if
you had passed `--warnings=syntax,$WARNINGS,category'. If
you want to disable the defaults and WARNINGS, but (for example)
enable the warnings about obsolete constructs, you would use `-W
none,obsolete'.
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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.
4.1 Initializing configureOption processing etc. 4.2 Notices in configureCopyright, version numbers in configure4.3 Finding configureInputWhere Autoconf should find files 4.4 Outputting Files Outputting results from the configuration 4.5 Performing Configuration Actions Preparing the output based on results 4.6 Creating Configuration Files Creating output files 4.7 Substitutions in Makefiles Using output variables in `Makefile's 4.8 Configuration Header Files Creating a configuration header file 4.9 Running Arbitrary Configuration Commands Running arbitrary instantiation commands 4.10 Creating Configuration Links Links depending on the configuration 4.11 Configuring Other Packages in Subdirectories Configuring independent packages together 4.12 Default Prefix Changing the default installation prefix
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configure
Every configure script must call AC_INIT before doing
anything else. The only other required macro is AC_OUTPUT
(see section 4.4 Outputting Files).
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 `-'.
It is preferable that the arguments of AC_INIT be static, i.e.,
there should not be any shell computation, but they can be computed by
M4.
The following M4 macros (e.g., AC_PACKAGE_NAME), output variables
(e.g., PACKAGE_NAME), and preprocessor symbols (e.g.,
PACKAGE_NAME) are defined by AC_INIT:
AC_PACKAGE_NAME, PACKAGE_NAME
AC_PACKAGE_TARNAME, PACKAGE_TARNAME
AC_PACKAGE_VERSION, PACKAGE_VERSION
AC_PACKAGE_STRING, PACKAGE_STRING
AC_PACKAGE_BUGREPORT, PACKAGE_BUGREPORT
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configure
The following macros manage version numbers for configure
scripts. Using them is optional.
configure is earlier
than version, print an error message to the standard error output
and do not create configure. For example:
AC_PREREQ(2.57) |
This macro is the only macro that may be used before AC_INIT, but
for consistency, you are invited not to do so.
configure are covered by the
copyright-notice.
The copyright-notice will show up in both the head of
configure and in `configure --version'.
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 |
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configure Input configure checks for this file's existence to
make sure that the directory that it is told contains the source code in
fact does. Occasionally people accidentally specify the wrong directory
with `--srcdir'; this is a safety check. See section 13.9 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.
configure,
Automake and Libtool scripts etc.) that are in directory dir.
These are auxiliary files used in configuration. dir can be
either absolute or relative to `srcdir'. The default is
`srcdir' or `srcdir/..' or
`srcdir/../..', whichever is the first that contains
`install-sh'. The other files are not checked for, so that using
AC_PROG_INSTALL does not automatically require distributing the
other auxiliary files. It checks for `install.sh' also, but that
name is obsolete because some make have a rule that creates
`install' from it if there is no `Makefile'.
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Every Autoconf script, e.g., `configure.ac', should finish by
calling AC_OUTPUT. That is the macro that generates
`config.status', which will create the `Makefile's and any
other files resulting from configuration. This is the only required
macro besides AC_INIT (see section 4.3 Finding configure Input).
`config.status' will perform all the configuration actions: all the
output files (see 4.6 Creating Configuration Files, macro
AC_CONFIG_FILES), header files (see 4.8 Configuration Header Files,
macro AC_CONFIG_HEADERS), commands (see 4.9 Running Arbitrary Configuration Commands, macro AC_CONFIG_COMMANDS), links (see
4.10 Creating Configuration Links, macro AC_CONFIG_LINKS), subdirectories
to configure (see 4.11 Configuring Other Packages in Subdirectories, macro AC_CONFIG_SUBDIRS)
are honored.
Historically, the usage of AC_OUTPUT was somewhat different.
See section 15.4 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.
make predefines the Make variable MAKE, define
output variable SET_MAKE to be empty. Otherwise, define
SET_MAKE to contain `MAKE=make'. Calls AC_SUBST for
SET_MAKE.
If you use this macro, place a line like this in each `Makefile.in'
that runs MAKE on other directories:
@SET_MAKE@ |
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`configure' is designed so that it appears to do everything itself, but there is actually a hidden slave: `config.status'. `configure' is in charge of examining your system, but it is `config.status' that actually takes the proper actions based on the results of `configure'. The most typical task of `config.status' is to instantiate files.
This section describes the common behavior of the four standard
instantiating macros: AC_CONFIG_FILES, AC_CONFIG_HEADERS,
AC_CONFIG_COMMANDS and AC_CONFIG_LINKS. They all
have this prototype:
AC_CONFIG_FOOS(tag..., [commands], [init-cmds]) |
where the arguments are:
You are encouraged to use literals as tags. In particular, you should avoid
... && my_foos="$my_foos fooo" ... && my_foos="$my_foos foooo" AC_CONFIG_FOOS($my_foos) |
and use this instead:
... && AC_CONFIG_FOOS(fooo) ... && AC_CONFIG_FOOS(foooo) |
The macros AC_CONFIG_FILES and AC_CONFIG_HEADERS use
special tags: they may have the form `output' or
`output:inputs'. The file output is instantiated
from its templates, inputs (defaulting to `output.in').
For instance `AC_CONFIG_FILES(Makefile:boiler/top.mk:boiler/bot.mk)' asks for the creation of `Makefile' that will be the expansion of the output variables in the concatenation of `boiler/top.mk' and `boiler/bot.mk'.
The special value `-' might be used to denote the standard output when used in output, or the standard input when used in the inputs. You most probably don't need to use this in `configure.ac', but it is convenient when using the command line interface of `./config.status', see 14. Recreating a Configuration, for more details.
The inputs may be absolute or relative filenames. In the latter case they are first looked for in the build tree, and then in the source tree.
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:
srcdir
configure's option `--srcdir' sets.
ac_top_srcdir
ac_top_builddir
ac_srcdir
The current directory refers to the directory (or pseudo-directory) containing the input part of tags. For instance, running
AC_CONFIG_COMMANDS([deep/dir/out:in/in.in], [...], [...]) |
with `--srcdir=../package' produces the following values:
# Argument of --srcdir srcdir='../package' # Reversing deep/dir ac_top_builddir='../../' # Concatenation of $ac_top_builddir and srcdir ac_top_srcdir='../../../package' # Concatenation of $ac_top_srcdir and deep/dir ac_srcdir='../../../package/deep/dir' |
independently of `in/in.in'.
var. init-cmds
is typically used by `configure' to give `config.status' some
variables it needs to run the commands.
You should be extremely cautious in your variable names: all the init-cmds share the same name space and may overwrite each other in unpredictable ways. Sorry....
All these macros can be called multiple times, with different tags, of course!
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Be sure to read the previous section, 4.5 Performing Configuration Actions.
AC_OUTPUT create each `file' by copying an input
file (by default `file.in'), substituting the output variable
values.
This macro is one of the instantiating macros; see 4.5 Performing Configuration Actions. See section 4.7 Substitutions in Makefiles, for more information on using
output variables. See section 7.2 Setting Output Variables, for more information
on creating them. This macro creates the directory that the file is in
if it doesn't exist. Usually, `Makefile's are created this way,
but other files, such as `.gdbinit', can be specified as well.
Typical calls to AC_CONFIG_FILES look like this:
AC_CONFIG_FILES([Makefile src/Makefile man/Makefile X/Imakefile]) AC_CONFIG_FILES([autoconf], [chmod +x autoconf]) |
You can override an input file name by appending to file a colon-separated list of input files. Examples:
AC_CONFIG_FILES([Makefile:boiler/top.mk:boiler/bot.mk]
[lib/Makefile:boiler/lib.mk])
|
Doing this allows you to keep your file names acceptable to MS-DOS, or to prepend and/or append boilerplate to the file.
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Each subdirectory in a distribution that contains something to be
compiled or installed should come with a file `Makefile.in', from
which configure will create a `Makefile' in that directory.
To create a `Makefile', configure performs a simple variable
substitution, replacing occurrences of `@variable@' in
`Makefile.in' with the value that configure has determined
for that variable. Variables that are substituted into output files in
this way are called output variables. They are ordinary shell
variables that are set in configure. To make configure
substitute a particular variable into the output files, the macro
AC_SUBST must be called with that variable name as an argument.
Any occurrences of `@variable@' for other variables are
left unchanged. See section 7.2 Setting Output Variables, for more information
on creating output variables with AC_SUBST.
A software package that uses a configure script should be
distributed with a file `Makefile.in', but no `Makefile'; that
way, the user has to properly configure the package for the local system
before compiling it.
See section `Makefile Conventions' in The GNU Coding Standards, for more information on what to put in `Makefile's.
4.7.1 Preset Output Variables Output variables that are always set 4.7.2 Installation Directory Variables Other preset output variables 4.7.3 Build Directories Supporting multiple concurrent compiles 4.7.4 Automatic Remaking Makefile rules for configuring
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Some output variables are preset by the Autoconf macros. Some of the
Autoconf macros set additional output variables, which are mentioned in
the descriptions for those macros. See section B.2 Output Variable Index, for a
complete list of output variables. See section 4.7.2 Installation Directory Variables, for the list of the preset ones related to installation
directories. Below are listed the other preset ones. They all are
precious variables (see section 7.2 Setting Output Variables,
AC_ARG_VAR).
configure runs, the default value is set
when you call AC_PROG_CC (or empty if you don't). configure
uses this variable when compiling programs to test for C features.
configure and giving the name of the input file.
AC_OUTPUT adds a comment line containing this variable to the top
of every `Makefile' it creates. For other files, you should
reference this variable in a comment at the top of each input file. For
example, an input shell script should begin like this:
#! /bin/sh # @configure_input@ |
The presence of that line also reminds people editing the file that it
needs to be processed by configure in order to be used.
configure runs, the default
value is empty. configure uses this variable when compiling or
preprocessing programs to test for C and C++ features.
configure runs, the default value is
set when you call AC_PROG_CXX (or empty if you don't).
configure uses this variable when compiling programs to test for
C++ features.
AC_CONFIG_HEADERS
is called, configure replaces `@DEFS@' with
`-DHAVE_CONFIG_H' instead (see section 4.8 Configuration Header Files). This
variable is not defined while configure is performing its tests,
only when creating the output files. See section 7.2 Setting Output Variables, for
how to check the results of previous tests.
echo for
question-answer message pairs? These variables provide a way:
echo $ECHO_N "And the winner is... $ECHO_C"
sleep 100000000000
echo "${ECHO_T}dead."
|
Some old and uncommon echo implementations offer no means to
achieve this, in which case ECHO_T is set to tab. You might not
want to use it.
configure runs, the default
value is set when you call AC_PROG_F77 (or empty if you don't).
configure uses this variable when compiling programs to test for
Fortran 77 features.
LIBS instead. If it is not set
in the environment when configure runs, the default value is empty.
configure uses this variable when linking programs to test for
C, C++ and Fortran 77 features.
configure uses this variable when linking
programs to test for C, C++ and Fortran 77 features.
builddir.
builddir.
top_builddir.
srcdir.
srcdir.
top_srcdir.
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The following variables specify the directories where the package will be installed, see section `Variables for Installation Directories' in The GNU Coding Standards, for more information. See the end of this section for details on when and how to use these variables.
Most of these variables have values that rely on prefix or
exec_prefix. It is deliberate that the directory output
variables keep them unexpanded: typically `@datadir@' will be
replaced by `${prefix}/share', not `/usr/local/share'.
This behavior is mandated by the GNU coding standards, so that when the user runs:
configure, in which case, if needed, the package shall hard
code dependencies corresponding to the make-specified prefix.
In order to support these features, it is essential that datadir
remains being defined as `${prefix}/share' to depend upon the
current value of prefix.
A corollary is that you should not use these variables except in
Makefiles. For instance, instead of trying to evaluate datadir
in `configure' and hard-coding it in Makefiles using
e.g., `AC_DEFINE_UNQUOTED(DATADIR, "$datadir")', you should add
`-DDATADIR="$(datadir)"' to your CPPFLAGS.
Similarly you should not rely on AC_OUTPUT_FILES to replace
datadir and friends in your shell scripts and other files, rather
let make manage their replacement. For instance Autoconf
ships templates of its shell scripts ending with `.in', and uses a
Makefile snippet similar to:
edit = sed \
-e 's,@datadir\@,$(pkgdatadir),g' \
-e 's,@prefix\@,$(prefix),g'
autoconf: Makefile $(srcdir)/autoconf.in
rm -f autoconf autoconf.tmp
$(edit) $(srcdir)/autoconf.in >autoconf.tmp
chmod +x autoconf.tmp
mv autoconf.tmp autoconf
autoheader: Makefile $(srcdir)/autoheader.in
rm -f autoheader autoheader.tmp
$(edit) $(srcdir)/autoconf.in >autoheader.tmp
chmod +x autoheader.tmp
mv autoheader.tmp autoheader
|
Some details are noteworthy:
configure from replacing
`@datadir@' in the sed expression itself.
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 section 10.10 Limitations of Make, for details.
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You can support compiling a software package for several architectures simultaneously from the same copy of the source code. The object files for each architecture are kept in their own directory.
To support doing this, make uses the VPATH variable to
find the files that are in the source directory. GNU Make
and most other recent make programs can do this. Older
make programs do not support VPATH; when using them, the
source code must be in the same directory as the object files.
To support VPATH, each `Makefile.in' should contain two
lines that look like:
srcdir = @srcdir@ VPATH = @srcdir@ |
Do not set VPATH to the value of another variable, for example
`VPATH = $(srcdir)', because some versions of make do not do
variable substitutions on the value of VPATH.
configure substitutes the correct value for srcdir when
it produces `Makefile'.
Do not use the make variable $<, which expands to the
file name of the file in the source directory (found with VPATH),
except in implicit rules. (An implicit rule is one such as `.c.o',
which tells how to create a `.o' file from a `.c' file.) Some
versions of make do not set $< in explicit rules; they
expand it to an empty value.
Instead, `Makefile' command lines should always refer to source files by prefixing them with `$(srcdir)/'. For example:
time.info: time.texinfo
$(MAKEINFO) $(srcdir)/time.texinfo
|
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You can put rules like the following in the top-level `Makefile.in' for a package to automatically update the configuration information when you change the configuration files. This example includes all of the optional files, such as `aclocal.m4' and those related to configuration header files. Omit from the `Makefile.in' rules for any of these files that your package does not use.
The `$(srcdir)/' prefix is included because of limitations in the
VPATH mechanism.
The `stamp-' files are necessary because the timestamps of
`config.h.in' and `config.h' will not be changed if remaking
them does not change their contents. This feature avoids unnecessary
recompilation. You should include the file `stamp-h.in' your
package's distribution, so make will consider
`config.h.in' up to date. Don't use touch
(see section 10.9 Limitations of Usual Tools), rather use echo (using
date would cause needless differences, hence CVS
conflicts etc.).
$(srcdir)/configure: configure.ac aclocal.m4
cd $(srcdir) && autoconf
# autoheader might not change config.h.in, so touch a stamp file.
$(srcdir)/config.h.in: stamp-h.in
$(srcdir)/stamp-h.in: configure.ac aclocal.m4
cd $(srcdir) && autoheader
echo timestamp > $(srcdir)/stamp-h.in
config.h: stamp-h
stamp-h: config.h.in config.status
./config.status
Makefile: Makefile.in config.status
./config.status
config.status: configure
./config.status --recheck
|
(Be careful if you copy these lines directly into your Makefile, as you will need to convert the indented lines to start with the tab character.)
In addition, you should use `AC_CONFIG_FILES([stamp-h], [echo
timestamp > stamp-h])' so `config.status' will ensure that
`config.h' is considered up to date. See section 4.4 Outputting Files, for more
information about AC_OUTPUT.
See section 14. Recreating a Configuration, for more examples of handling configuration-related dependencies.
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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. It should be called right after
AC_INIT.
The package should `#include' the configuration header file before
any other header files, to prevent inconsistencies in declarations (for
example, if it redefines const). Use `#include <config.h>'
instead of `#include "config.h"', and pass the C compiler a
`-I.' option (or `-I..'; whichever directory contains
`config.h'). That way, even if the source directory is configured
itself (perhaps to make a distribution), other build directories can
also be configured without finding the `config.h' from the source
directory.
AC_OUTPUT create the file(s) in the
whitespace-separated list header containing C preprocessor
#define statements, and replace `@DEFS@' in generated
files with `-DHAVE_CONFIG_H' instead of the value of DEFS.
The usual name for header is `config.h'.
If header already exists and its contents are identical to what
AC_OUTPUT would put in it, it is left alone. Doing this allows
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. Examples:
AC_CONFIG_HEADERS([config.h:config.hin]) AC_CONFIG_HEADERS([defines.h:defs.pre:defines.h.in:defs.post]) |
Doing this allows you to keep your file names acceptable to MS-DOS, or to prepend and/or append boilerplate to the file.
See section 4.5 Performing Configuration Actions, for more details on header.
4.8.1 Configuration Header Templates Input for the configuration headers 4.8.2 Using autoheaderto Create `config.h.in'How to create configuration templates 4.8.3 Autoheader Macros How to specify CPP templates
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Your distribution should contain a template file that looks as you want
the final header file to look, including comments, with #undef
statements which are used as hooks. For example, suppose your
`configure.ac' makes these calls:
AC_CONFIG_HEADERS([conf.h]) AC_CHECK_HEADERS([unistd.h]) |
Then you could have code like the following in `conf.h.in'. On
systems that have `unistd.h', configure will `#define'
`HAVE_UNISTD_H' to 1. On other systems, the whole line will be
commented out (in case the system predefines that symbol).
/* Define as 1 if you have unistd.h. */ #undef HAVE_UNISTD_H |
Pay attention that `#undef' is in the first column, and there is nothing behind `HAVE_UNISTD_H', not even white spaces. You can then decode the configuration header using the preprocessor directives:
#include <conf.h> #if HAVE_UNISTD_H # include <unistd.h> #else /* We are in trouble. */ #endif |
The use of old form templates, with `#define' instead of `#undef' is strongly discouraged. Similarly with old templates with comments on the same line as the `#undef'. Anyway, putting comments in preprocessor macros has never been a good idea.
Since it is a tedious task to keep a template header up to date, you may
use autoheader to generate it, see 4.8.2 Using autoheader to Create `config.h.in'.
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autoheader to Create `config.h.in'
The autoheader program can create a template file of C
`#define' statements for configure to use. If
`configure.ac' invokes AC_CONFIG_HEADERS(file),
autoheader creates `file.in'; if multiple file
arguments are given, the first one is used. Otherwise,
autoheader creates `config.h.in'.
In order to do its job, autoheader needs you to document all
of the symbols that you might use; i.e., there must be at least one
AC_DEFINE or one AC_DEFINE_UNQUOTED call with a third
argument for each symbol (see section 7.1 Defining C Preprocessor Symbols). An additional
constraint is that the first argument of AC_DEFINE must be a
literal. Note that all symbols defined by Autoconf's builtin tests are
already documented properly; you only need to document those that you
define yourself.
You might wonder why autoheader is needed: after all, why
would configure need to "patch" a `config.h.in' to
produce a `config.h' instead of just creating `config.h' from
scratch? Well, when everything rocks, the answer is just that we are
wasting our time maintaining autoheader: generating
`config.h' directly is all that is needed. When things go wrong,
however, you'll be thankful for the existence of autoheader.
The fact that the symbols are documented is important in order to
check that `config.h' makes sense. The fact that there is a
well-defined list of symbols that should be #define'd (or not) is
also important for people who are porting packages to environments where
configure cannot be run: they just have to fill in the
blanks.
But let's come back to the point: autoheader's invocation...
If you give autoheader an argument, it uses that file instead
of `configure.ac' and writes the header file to the standard output
instead of to `config.h.in'. If you give autoheader an
argument of `-', it reads the standard input instead of
`configure.ac' and writes the header file to the standard output.
autoheader accepts the following options:
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autoheader scans `configure.ac' and figures out which C
preprocessor symbols it might define. It knows how to generate
templates for symbols defined by AC_CHECK_HEADERS,
AC_CHECK_FUNCS etc., but if you AC_DEFINE any additional
symbol, you must define a template for it. If there are missing
templates, autoheader fails with an error message.
The simplest way to create a template for a symbol is to supply the description argument to an `AC_DEFINE(symbol)'; see 7.1 Defining C Preprocessor Symbols. You may also use one of the following macros.
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 AC_DEFINE'd.
For example:
AH_VERBATIM([_GNU_SOURCE], [/* Enable GNU extensions on systems that have them. */ #ifndef _GNU_SOURCE # define _GNU_SOURCE #endif]) |
autoheader to generate a template for key. This macro
generates standard templates just like AC_DEFINE when a
description is given.
For example:
AH_TEMPLATE([CRAY_STACKSEG_END],
[Define to one of _getb67, GETB67, getb67
for Cray-2 and Cray-YMP systems. This
function is required for alloca.c support
on those systems.])
|
will generate the following template, with the description properly justified.
/* Define to one of _getb67, GETB67, getb67 for Cray-2 and Cray-YMP systems. This function is required for alloca.c support on those systems. */ #undef CRAY_STACKSEG_END |
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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 15.4 Obsolete Macros, for details.
configure. Associate the commands with tag. Since
typically the cmds create a file, tag should naturally be
the name of that file. This macro is one of the instantiating macros;
see 4.5 Performing Configuration Actions.
Here is an unrealistic example:
fubar=42
AC_CONFIG_COMMANDS([fubar],
[echo this is extra $fubar, and so on.],
[fubar=$fubar])
|
Here is a better one:
AC_CONFIG_COMMANDS([time-stamp], [date >time-stamp]) |
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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.
AC_OUTPUT link each of the existing files source to
the corresponding link name dest. Makes a symbolic link if
possible, otherwise a hard link 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 4.5 Performing Configuration Actions.
For example, this call:
AC_CONFIG_LINKS(host.h:config/$machine.h
object.h:config/$obj_format.h)
|
creates in the current directory `host.h' as a link to `srcdir/config/$machine.h', and `object.h' as a link to `srcdir/config/$obj_format.h'.
The tempting value `.' for dest is invalid: it makes it impossible for `config.status' to guess the links to establish.
One can then run:
./config.status host.h object.h |
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In most situations, calling AC_OUTPUT is sufficient to produce
`Makefile's in subdirectories. However, configure scripts
that control more than one independent package can use
AC_CONFIG_SUBDIRS to run configure scripts for other
packages in subdirectories.
AC_OUTPUT run configure in each subdirectory
dir in the given whitespace-separated list. Each dir should
be a literal, i.e., please do not use:
if test "$package_foo_enabled" = yes; then $my_subdirs="$my_subdirs foo" fi AC_CONFIG_SUBDIRS($my_subdirs) |
because this prevents `./configure --help=recursive' from
displaying the options of the package foo. Rather, you should
write:
if test "$package_foo_enabled" = yes; then AC_CONFIG_SUBDIRS(foo) fi |
If a given dir is not found, an error is reported: if the subdirectory is optional, write:
if test -d $srcdir/foo; then AC_CONFIG_SUBDIRS(foo) fi |
If a given dir contains configure.gnu, it is run instead
of configure. This is for packages that might use a
non-Autoconf script Configure, which can't be called through a
wrapper configure since it would be the same file on
case-insensitive filesystems. Likewise, if a dir contains
`configure.in' but no configure, the Cygnus
configure script found by AC_CONFIG_AUX_DIR is used.
The subdirectory configure scripts are given the same command
line options that were given to this configure script, with minor
changes if needed, which include:
$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 subdirs to the list of
directories `dir ...'. `Makefile' rules can use
this variable to determine which subdirectories to recurse into.
This macro may be called multiple times.
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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.
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.
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 is gcc and the PATH contains
`/usr/local/gnu/bin/gcc', set the prefix to `/usr/local/gnu'.
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These macros test for particular system features that packages might need or want to use. If you need to test for a kind of feature that none of these macros check for, you can probably do it by calling primitive test macros with appropriate arguments (see section 6. Writing Tests).
These tests print messages telling the user which feature they're
checking for, and what they find. They cache their results for future
configure runs (see section 7.3 Caching Results).
Some of these macros set output variables. See section 4.7 Substitutions in Makefiles, for how to get their values. The phrase "define name" is used below as a shorthand to mean "define C preprocessor symbol name to the value 1". See section 7.1 Defining C Preprocessor Symbols, for how to get those symbol definitions into your program.
5.1 Common Behavior Macros' standard schemes 5.2 Alternative Programs Selecting between alternative programs 5.3 Files Checking for the existence of files 5.4 Library Files Library archives that might be missing 5.5 Library Functions C library functions that might be missing 5.6 Header Files Header files that might be missing 5.7 Declarations Declarations that may be missing 5.8 Structures Structures or members that might be missing 5.9 Types Types that might be missing 5.10 Compilers and Preprocessors Checking for compiling programs 5.11 System Services Operating system services 5.12 UNIX Variants Special kludges for specific UNIX variants
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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.
5.1.1 Standard Symbols Symbols defined by the macros 5.1.2 Default Includes Includes used by the generic macros
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All the generic macros that AC_DEFINE a symbol as a result of
their test transform their arguments to a standard alphabet.
First, argument is converted to upper case and any asterisks
(`*') are each converted to `P'. Any remaining characters
that are not alphanumeric are converted to underscores.
For instance,
AC_CHECK_TYPES(struct $Expensive*) |
will define the symbol `HAVE_STRUCT__EXPENSIVEP' if the check succeeds.
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Several tests depend upon a set of header files. Since these headers are not universally available, tests actually have to provide a set of protected includes, such as:
#if TIME_WITH_SYS_TIME # include <sys/time.h> # include <time.h> #else # if HAVE_SYS_TIME_H # include <sys/time.h> # else # include <time.h> # endif #endif |
Unless you know exactly what you are doing, you should avoid using unconditional includes, and check the existence of the headers you include beforehand (see section 5.6 Header Files).
Most generic macros provide the following default set of includes:
#include <stdio.h> #if HAVE_SYS_TYPES_H # include <sys/types.h> #endif #if HAVE_SYS_STAT_H # include <sys/stat.h> #endif #if STDC_HEADERS # include <stdlib.h> # include <stddef.h> #else # if HAVE_STDLIB_H # include <stdlib.h> # endif #endif #if HAVE_STRING_H # if !STDC_HEADERS && HAVE_MEMORY_H # include <memory.h> # endif # include <string.h> #endif #if HAVE_STRINGS_H # include <strings.h> #endif #if HAVE_INTTYPES_H # include <inttypes.h> #else # if HAVE_STDINT_H # include <stdint.h> # endif #endif #if HAVE_UNISTD_H # include <unistd.h> #endif |
If the default includes are used, then Autoconf will automatically check
for the presence of these headers and their compatibility, i.e., you
don't need to run AC_HEADERS_STDC, nor check for `stdlib.h'
etc.
These headers are checked for in the same order as they are included.
For instance, on some systems `string.h' and `strings.h' both
exist, but conflict. Then HAVE_STRING_H will be defined, but
HAVE_STRINGS_H won't.
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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.
5.2.1 Particular Program Checks Special handling to find certain programs 5.2.2 Generic Program and File Checks How to find other programs
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These macros check for particular programs--whether they exist, and in some cases whether they support certain features.
gawk, mawk, nawk, and awk, in that
order, and set output variable AWK to the first one that is found.
It tries gawk first because that is reported to be the
best implementation.
grep -E and egrep, in that order, and set
output variable EGREP to the first one that is found.
grep -F and fgrep, in that order, and set
output variable FGREP to the first one that is found.
INSTALL to the path of a BSD-compatible
install program, if one is found in the current PATH.
Otherwise, set INSTALL to `dir/install-sh -c',
checking the directories specified to AC_CONFIG_AUX_DIR (or its
default directories) to determine dir (see section 4.4 Outputting Files). Also set
the variables INSTALL_PROGRAM and INSTALL_SCRIPT to
`${INSTALL}' and INSTALL_DATA to `${INSTALL} -m 644'.
This macro screens out various instances of install known not to
work. It prefers to find a C program rather than a shell script, for
speed. Instead of `install-sh', it can also use `install.sh',
but that name is obsolete because some make programs have a rule
that creates `install' from it if there is no `Makefile'.
Autoconf comes with a copy of `install-sh' that you can use. If
you use AC_PROG_INSTALL, you must include either
`install-sh' or `install.sh' in your distribution, or
configure will produce an error message saying it can't find
them--even if the system you're on has a good install program.
This check is a safety measure to prevent you from accidentally leaving
that file out, which would prevent your package from installing on
systems that don't have a BSD-compatible install program.
If you need to use your own installation program because it has features
not found in standard install programs, there is no reason to use
AC_PROG_INSTALL; just put the file name of your program into your
`Makefile.in' files.
flex is found, set output variable LEX to `flex'
and LEXLIB to `-lfl', if that library is in a standard
place. Otherwise set LEX to `lex' and LEXLIB to
`-ll'.
Define YYTEXT_POINTER if yytext is a `char *' instead
of a `char []'. Also set output variable LEX_OUTPUT_ROOT to
the base of the file name that the lexer generates; usually
`lex.yy', but sometimes something else. These results vary
according to whether lex or flex is being used.
You are encouraged to use Flex in your sources, since it is both more
pleasant to use than plain Lex and the C source it produces is portable.
In order to ensure portability, however, you must either provide a
function yywrap or, if you don't use it (e.g., your scanner has
no `#include'-like feature), simply include a `%noyywrap'
statement in the scanner's source. Once this done, the scanner is
portable (unless you felt free to use nonportable constructs) and
does not depend on any library. In this case, and in this case only, it
is suggested that you use this Autoconf snippet:
AC_PROG_LEX if test "$LEX" != flex; then LEX="$SHELL $missing_dir/missing flex" AC_SUBST(LEX_OUTPUT_ROOT, lex.yy) AC_SUBST(LEXLIB, '') fi |
The shell script missing can be found in the Automake
distribution.
To ensure backward compatibility, Automake's AM_PROG_LEX invokes
(indirectly) this macro twice, which will cause an annoying but benign
"AC_PROG_LEX invoked multiple times" warning. Future versions
of Automake will fix this issue; meanwhile, just ignore this message.
LN_S to `ln -s'; otherwise, if `ln' works, set
LN_S to `ln', and otherwise set it to `cp -p'.
If you make a link in a directory other than the current directory, its
meaning depends on whether `ln' or `ln -s' is used. To safely
create links using `$(LN_S)', either find out which form is used
and adjust the arguments, or always invoke ln in the directory
where the link is to be created.
In other words, it does not work to do:
$(LN_S) foo /x/bar |
Instead, do:
(cd /x && $(LN_S) foo bar) |
RANLIB to `ranlib' if ranlib
is found, and otherwise to `:' (do nothing).
bison is found, set output variable YACC to `bison
-y'. Otherwise, if byacc is found, set YACC to
`byacc'. Otherwise set YACC to `yacc'.
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These macros are used to find programs not covered by the "particular"
test macros. If you need to check the behavior of a program as well as
find out whether it is present, you have to write your own test for it
(see section 6. Writing Tests). By default, these macros use the environment
variable PATH. If you need to check for a program that might not
be in the user's PATH, you can pass a modified path to use
instead, like this:
AC_PATH_PROG([INETD], [inetd], [/usr/libexec/inetd],
[$PATH:/usr/libexec:/usr/sbin:/usr/etc:etc])
|
You are strongly encouraged to declare the variable passed to
AC_CHECK_PROG etc. as precious, See section 7.2 Setting Output Variables,
AC_ARG_VAR, for more details.
PATH. If
it is found, set variable to value-if-found, otherwise to
value-if-not-found, if given. Always pass over reject (an
absolute file name) even if it is the first found in the search path; in
that case, set variable using the absolute file name of the
prog-to-check-for found that is not reject. If
variable was already set, do nothing. Calls AC_SUBST for
variable.
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_SUBST for variable.
AC_CHECK_PROG, but first looks for prog-to-check-for
with a prefix of the host type as determined by
AC_CANONICAL_HOST, followed by a dash (see section 11.2 Getting the Canonical System Type).
For example, if the user runs `configure --host=i386-gnu', then
this call:
AC_CHECK_TOOL(RANLIB, ranlib, :) |
RANLIB to `i386-gnu-ranlib' if that program exists in
PATH, or otherwise to `ranlib' if that program exists in
PATH, or to `:' if neither program exists.
AC_CHECK_TOOL, each of the tools in the list
progs-to-check-for are checked with a prefix of the host type as
determined by AC_CANONICAL_HOST, followed by a dash
(see section 11.2 Getting the Canonical System Type). If none of the tools can be found with a
prefix, then the first one without a prefix is used. If a tool is found,
set variable to the name of that program. If none of the tools in
the list are found, set variable to value-if-not-found; if
value-if-not-found is not specified, the value of variable
is not changed. Calls AC_SUBST for variable.
AC_CHECK_PROG, but set variable to the entire
path of prog-to-check-for if found.
AC_CHECK_PROGS, but if any of progs-to-check-for
are found, set variable to the entire path of the program
found.
AC_CHECK_TOOL, but set variable to the entire
path of the program if it is found.
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You might also need to check for the existence of files. Before using these macros, ask yourself whether a run-time test might not be a better solution. Be aware that, like most Autoconf macros, they test a feature of the host machine, and therefore, they die when cross-compiling.
AC_CHECK_FILE once for each file listed in files.
Additionally, defines `HAVE_file' (see section 5.1.1 Standard Symbols)
for each file found.
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The following macros check for the presence of certain C, C++, or Fortran 77 library archive files.
action-if-found is a list of shell commands to run if the link
with the library succeeds; action-if-not-found is a list of shell
commands to run if the link fails. If action-if-found is not
specified, the default action will prepend `-llibrary' to
LIBS and define `HAVE_LIBlibrary' (in all
capitals). This macro is intended to support building LIBS in
a right-to-left (least-dependent to most-dependent) fashion such that
library dependencies are satisfied as a natural side-effect of
consecutive tests. Some linkers are very sensitive to library ordering
so the order in which LIBS is generated is important to reliable
detection of libraries.
If linking with library results in unresolved symbols that would
be resolved by linking with additional libraries, give those libraries
as the other-libraries argument, separated by spaces:
e.g., `-lXt -lX11'. Otherwise, this macro will fail to detect
that library is present, because linking the test program will
always fail with unresolved symbols. The other-libraries argument
should be limited to cases where it is desirable to test for one library
in the presence of another that is not already in LIBS.
Add `-llibrary' to LIBS for the first library found
to contain function, and run action-if-found. If the
function is not found, run action-if-not-found.
If linking with library results in unresolved symbols that would be resolved by linking with additional libraries, give those libraries as the other-libraries argument, separated by spaces: e.g., `-lXt -lX11'. Otherwise, this macro will fail to detect that function is present, because linking the test program will always fail with unresolved symbols.
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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.
5.5.1 Portability of C Functions Pitfalls with usual functions 5.5.2 Particular Function Checks Special handling to find certain functions 5.5.3 Generic Function Checks How to find other functions
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Most usual functions can either be missing, or be buggy, or be limited on some architectures. This section tries to make an inventory of these portability issues. By definition, this list will always require additions. Please help us keeping it as complete as possible.
exit
exit returns int?
This is because exit predates void, and there was a long
tradition of it returning int.
snprintf
snprintf and vsnprintf
truncate the output and return the number of bytes that ought to have
been produced. Some older systems return the truncated length (e.g.,
GNU C Library 2.0.x or IRIX 6.5), some a negative value
(e.g., earlier GNU C Library versions), and some the buffer
length without truncation (e.g., 32-bit Solaris 7). Also, some buggy
older systems ignore the length and overrun the buffer (e.g., 64-bit
Solaris 7).
sprintf
sprintf and vsprintf return the
number of bytes written, but on some old systems (SunOS 4 for
instance) they return the buffer pointer instead.
sscanf
sscanf 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 section `Incompatibilities' in Using and Porting the GNU Compiler Collection). Apparently in some cases even
having format strings read-only can be a problem.
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
|
unlink
unlink causes the given file to be
removed only after there are no more open file handles for it. Not all
OS's support this behavior though. So even on systems that provide
unlink, you cannot portably assume it is OK to call it on files
that are open. For example, on Windows 9x and ME, such a call would fail;
on DOS it could even lead to file system corruption, as the file might end
up being written to after the OS has removed it.
va_copy
va_copy for copying
va_list variables. It may be available in older environments
too, though possibly as __va_copy (e.g., gcc in strict
C89 mode). These can be tested with #ifdef. A fallback to
memcpy (&dst, &src, sizeof(va_list)) will give maximum
portability.
va_list
va_list is not necessarily just a pointer. It can be a
struct (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 the ISO C standard 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.
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These macros check for particular C functions--whether they exist, and in some cases how they respond when given certain arguments.
alloca. Tries to get a builtin version by
checking for `alloca.h' or the predefined C preprocessor macros
__GNUC__ and _AIX. If this macro finds `alloca.h',
it defines HAVE_ALLOCA_H.
If those attempts fail, it looks for the function in the standard C
library. If any of those methods succeed, it defines
HAVE_ALLOCA. Otherwise, it sets the output variable
ALLOCA to `alloca.o' and defines C_ALLOCA (so
programs can periodically call `alloca(0)' to garbage collect).
This variable is separate from LIBOBJS so multiple programs can
share the value of ALLOCA without needing to create an actual
library, in case only some of them use the code in LIBOBJS.
This macro does not try to get alloca from the System V R3
`libPW' or the System V R4 `libucb' because those libraries
contain some incompatible functions that cause trouble. Some versions
do not even contain alloca or contain a buggy version. If you
still want to use their alloca, use ar to extract
`alloca.o' from them instead of compiling `alloca.c'.
Source files that use alloca should start with a piece of code
like the following, to declare it properly. In some versions of AIX,
the declaration of alloca must precede everything else except for
comments and preprocessor directives. The #pragma directive is
indented so that pre-ANSI C compilers will ignore it, rather than
choke on it.
/* AIX requires this to be the first thing in the file. */ #ifndef __GNUC__ # if HAVE_ALLOCA_H # include <alloca.h> # else # ifdef _AIX #pragma alloca # else # ifndef alloca /* predefined by HP cc +Olibcalls */ char *alloca (); # endif # endif # endif #endif |
chown function is available and works (in particular, it
should accept `-1' for uid and gid), define
HAVE_CHOWN.
closedir function does not return a meaningful value,
define CLOSEDIR_VOID. Otherwise, callers ought to check its
return value for an error indicator.
error_at_line function is not found, require an
AC_LIBOBJ replacement of `error'.
fnmatch function conforms to POSIX, define
HAVE_FNMATCH. Detect common implementation bugs, for example,
the bugs in Solaris 2.4.
Note that for historical reasons, contrary to the other specific
AC_FUNC macros, AC_FUNC_FNMATCH does not replace a
broken/missing fnmatch. See AC_REPLACE_FNMATCH below.
AC_REPLACE_FNMATCH (replace) but also test
whether fnmatch supports GNU extensions. Detect common
implementation bugs, for example, the bugs in the GNU C
Library 2.1.
fork and vfork functions. If a
working fork is found, define HAVE_WORKING_FORK. This macro
checks whether fork is just a stub by trying to run it.
If `vfork.h' is found, define HAVE_VFORK_H. If a working
vfork is found, define HAVE_WORKING_VFORK. Otherwise,
define vfork to be fork for backward compatibility with
previous versions of autoconf. This macro checks for several known
errors in implementations of vfork and considers the system to not
have a working vfork if it detects any of them. It is not considered
to be an implementation error if a child's invocation of signal
modifies the parent's signal handler, since child processes rarely change
their signal handlers.
Since this macro defines vfork only for backward compatibility with
previous versions of autoconf you're encouraged to define it
yourself in new code:
#if !HAVE_WORKING_VFORK # define vfork fork #endif |
fseeko function is available, define HAVE_FSEEKO.
Define _LARGEFILE_SOURCE if necessary.
getgroups function is available and works (unlike on
Ultrix 4.3, where `getgroups (0, 0)' always fails), define
HAVE_GETGROUPS. Set GETGROUPS_LIBS to any libraries
needed to get that function. This macro runs AC_TYPE_GETGROUPS.
AC_LIBOBJ replacement directory properly (see
5.5.3 Generic Function Checks, AC_CONFIG_LIBOBJ_DIR).
If the system has the getloadavg function, define
HAVE_GETLOADAVG, and set GETLOADAVG_LIBS to any libraries
needed to get that function. Also add GETLOADAVG_LIBS to
LIBS. Otherwise, require an AC_LIBOBJ replacement for
`getloadavg' with source code in `dir/getloadavg.c', and
possibly define several other C preprocessor macros and output
variables:
C_GETLOADAVG.
SVR4, DGUX, UMAX, or UMAX4_3 if on
those systems.
HAVE_NLIST_H.
HAVE_STRUCT_NLIST_N_UN_N_NAME. The obsolete symbol
NLIST_NAME_UNION is still defined, but do not depend upon it.
getloadavg to work. In this case, define
GETLOADAVG_PRIVILEGED, set the output variable NEED_SETGID
to `true' (and otherwise to `false'), and set
KMEM_GROUP to the name of the group that should own the installed
program.
getmntent in the `sun', `seq', and `gen'
libraries, for IRIX 4, PTX, and Unixware, respectively. Then, if
getmntent is available, define HAVE_GETMNTENT.
GETPGRP_VOID if it is an error to pass 0 to
getpgrp; this is the POSIX behavior. On older BSD
systems, you must pass 0 to getpgrp, as it takes an argument and
behaves like POSIX's getpgid.
#if GETPGRP_VOID pid = getpgrp (); #else pid = getpgrp (0); #endif |
This macro does not check whether
getpgrp exists at all; if you need to work in that situation,
first call AC_CHECK_FUNC for getpgrp.
lstat should treat
`link/' the same as `link/.'. However, many older
lstat implementations incorrectly ignore trailing slashes.
It is safe to assume that if lstat incorrectly ignores
trailing slashes, then other symbolic-link-aware functions like
unlink also incorrectly ignore trailing slashes.
If lstat behaves properly, define
LSTAT_FOLLOWS_SLASHED_SYMLINK, otherwise require an
AC_LIBOBJ replacement of lstat.
malloc function is compatible with the GNU C
library malloc (i.e., `malloc (0)' returns a valid
pointer), define HAVE_MALLOC to 1. Otherwise define
HAVE_MALLOC to 0, ask for an AC_LIBOBJ replacement for
`malloc', and define malloc to rpl_malloc so that the
native malloc is not used in the main project.
Typically, the replacement file `malloc.c' should look like (note the `#undef malloc'):
@verbatim #if HAVE_CONFIG_H # include <config.h> #endif #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); }
memcmp function is not available, or does not work on
8-bit data (like the one on SunOS 4.1.3), or fails when comparing 16
bytes or more and with at least one buffer not starting on a 4-byte
boundary (such as the one on NeXT x86 OpenStep), require an
AC_LIBOBJ replacement for `memcmp'.
HAVE_MBRTOWC to 1 if the function mbrtowc and the
type mbstate_t are properly declared.
mktime function is not available, or does not work
correctly, require an AC_LIBOBJ replacement for `mktime'.
mmap function exists and works correctly, define
HAVE_MMAP. Only checks private fixed mapping of already-mapped
memory.
HAVE_OBSTACK, else require an
AC_LIBOBJ replacement for `obstack'.
realloc function is compatible with the GNU C
library realloc (i.e., `realloc (0, 0)' returns a
valid pointer), define HAVE_REALLOC to 1. Otherwise define
HAVE_REALLOC to 0, ask for an AC_LIBOBJ replacement for
`realloc', and define realloc to rpl_realloc so that
the native realloc is not used in the main project. See
AC_FUNC_MALLOC for details.
select function's arguments, and defines those types
in SELECT_TYPE_ARG1, SELECT_TYPE_ARG234, and
SELECT_TYPE_ARG5 respectively. SELECT_TYPE_ARG1 defaults
to `int', SELECT_TYPE_ARG234 defaults to `int *',
and SELECT_TYPE_ARG5 defaults to `struct timeval *'.
setpgrp takes no argument (the POSIX version), define
SETPGRP_VOID. Otherwise, it is the BSD version, which takes
two process IDs as arguments. This macro does not check whether
setpgrp exists at all; if you need to work in that situation,
first call AC_CHECK_FUNC for setpgrp.
stat or lstat have the bug that it
succeeds when given the zero-length file name as argument. The stat
and lstat from SunOS 4.1.4 and the Hurd (as of 1998-11-01) do
this.
If it does, then define HAVE_STAT_EMPTY_STRING_BUG (or
HAVE_LSTAT_EMPTY_STRING_BUG) and ask for an AC_LIBOBJ
replacement of it.
setvbuf takes the buffering type as its second argument and
the buffer pointer as the third, instead of the other way around, define
SETVBUF_REVERSED.
strcoll function exists and works correctly, define
HAVE_STRCOLL. This does a bit more than
`AC_CHECK_FUNCS(strcoll)', because some systems have incorrect
definitions of strcoll that should not be used.
strtod function does not exist or doesn't work correctly,
ask for an AC_LIBOBJ replacement of `strtod'. In this case,
because `strtod.c' is likely to need `pow', set the output
variable POW_LIB to the extra library needed.
strerror_r is available, define HAVE_STRERROR_R, and if
it is declared, define HAVE_DECL_STRERROR_R. If it returns a
char * message, define STRERROR_R_CHAR_P; otherwise it
returns an int error number. The Thread-Safe Functions option of
POSIX requires strerror_r to return int, but
many systems (including, for example, version 2.2.4 of the GNU C
Library) return a char * value that is not necessarily equal to
the buffer argument.
strftime in the `intl' library, for SCO UNIX.
Then, if strftime is available, define HAVE_STRFTIME.
strnlen function is not available, or is buggy (like the one
from AIX 4.3), require an AC_LIBOBJ replacement for it.
HAVE_UTIME_NULL.
vprintf is found, define HAVE_VPRINTF. Otherwise, if
_doprnt is found, define HAVE_DOPRNT. (If vprintf
is available, you may assume that vfprintf and vsprintf
are also available.)
fnmatch function does not conform to POSIX (see
AC_FUNC_FNMATCH), ask for its AC_LIBOBJ replacement.
The files `fnmatch.c', `fnmatch_loop.c', and `fnmatch_.h'
in the AC_LIBOBJ replacement directory are assumed to contain a
copy of the source code of GNU fnmatch. If necessary,
this source code is compiled as an AC_LIBOBJ replacement, and the
`fnmatch_.h' file is linked to `fnmatch.h' so that it can be
included in place of the system <fnmatch.h>.
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These macros are used to find functions not covered by the "particular"
test macros. If the functions might be in libraries other than the
default C library, first call AC_CHECK_LIB for those libraries.
If you need to check the behavior of a function as well as find out
whether it is present, you have to write your own test for
it (see section 6. Writing Tests).
AC_CHECK_FUNCS instead. This macro checks for functions with C
linkage even when AC_LANG(C++) has been called, since C is more
standardized than C++. (see section 6.1 Language Choice, for more information
about selecting the language for checks.)
HAVE_function (in all capitals) if it is available.
If action-if-found is given, it is additional shell code to
execute when one of the functions is found. You can give it a value of
`break' to break out of the loop on the first match. If
action-if-not-found is given, it is executed when one of the
functions is not found.
Autoconf follows a philosophy that was formed over the years by those who have struggled for portability: isolate the portability issues in specific files, and then program as if you were in a POSIX environment. Some functions may be missing or unfixable, and your package must be ready to replace them.
Technically, it adds `function.$ac_objext' to the output
variable LIBOBJS and calls AC_LIBSOURCE for
`function.c'. You should not directly change LIBOBJS,
since this is not traceable.
AC_LIBSOURCE. file must be a literal.
This macro is called automatically from AC_LIBOBJ, but you must
call it explicitly if you pass a shell variable to AC_LIBOBJ. In
that case, since shell variables cannot be traced statically, you must
pass to AC_LIBSOURCE any possible files that the shell variable
might cause AC_LIBOBJ to need. For example, if you want to pass
a variable $foo_or_bar to AC_LIBOBJ that holds either
"foo" or "bar", you should do:
AC_LIBSOURCE(foo.c) AC_LIBSOURCE(bar.c) AC_LIBOBJ($foo_or_bar) |
There is usually a way to avoid this, however, and you are encouraged to
simply call AC_LIBOBJ with literal arguments.
Note that this macro replaces the obsolete AC_LIBOBJ_DECL, with
slightly different semantics: the old macro took the function name,
e.g., foo, as its argument rather than the file name.
AC_LIBSOURCE, but accepts one or more files in a
comma-separated M4 list. Thus, the above example might be rewritten:
AC_LIBSOURCES([foo.c, bar.c]) AC_LIBOBJ($foo_or_bar) |
AC_LIBOBJ replacement files are to be found in
directory, a relative path starting from 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, 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.
AC_CHECK_FUNCS, but uses `AC_LIBOBJ(function)' as
action-if-not-found. You can declare your replacement function by
enclosing the prototype in `#if !HAVE_function'. If the
system has the function, it probably declares it in a header file you
should be including, so you shouldn't redeclare it lest your declaration
conflict.
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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.
5.6.1 Portability of Headers Collected knowledge on common headers 5.6.2 Particular Header Checks Special handling to find certain headers 5.6.3 Generic Header Checks How to find other headers
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This section tries to collect knowledge about common headers, and the problems they cause. By definition, this list will always require additions. Please help us keeping it as complete as possible.
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These macros check for particular system header files--whether they exist, and in some cases whether they declare certain symbols.
| `dirent.h' | HAVE_DIRENT_H |
| `sys/ndir.h' | HAVE_SYS_NDIR_H |
| `sys/dir.h' | HAVE_SYS_DIR_H |
| `ndir.h' | HAVE_NDIR_H |
The directory-library declarations in your source code should look something like the following:
#if HAVE_DIRENT_H # include <dirent.h> # define NAMLEN(dirent) strlen((dirent)->d_name) #else # define dirent direct # define NAMLEN(dirent) (dirent)->d_namlen # if HAVE_SYS_NDIR_H # include <sys/ndir.h> # endif # if HAVE_SYS_DIR_H # include <sys/dir.h> # endif # if HAVE_NDIR_H # include <ndir.h> # endif #endif |
Using the above declarations, the program would declare variables to be
of type struct dirent, not struct direct, and would access
the length of a directory entry name by passing a pointer to a
struct dirent to the NAMLEN macro.
This macro also checks for the SCO Xenix `dir' and `x' libraries.
major, minor, and
makedev, but `sys/mkdev.h' does, define
MAJOR_IN_MKDEV; otherwise, if `sys/sysmacros.h' does, define
MAJOR_IN_SYSMACROS.
S_ISDIR, S_ISREG, etc. defined in
`sys/stat.h' do not work properly (returning false positives),
define STAT_MACROS_BROKEN. This is the case on Tektronix UTekV,
Amdahl UTS and Motorola System V/88.
HAVE_STDBOOL_H to 1; if the type _Bool is defined, define
HAVE__BOOL to 1. To fulfill the C99 requirements, your
`system.h' should contain the following code:
@verbatim #if HAVE_STDBOOL_H # include <stdbool.h> #else # if ! HAVE__BOOL # ifdef __cplusplus typedef bool _Bool; # else typedef unsigned char _Bool; # endif # endif # define bool _Bool # define false 0 # define true 1 # define __bool_true_false_are_defined 1 #endif
STDC_HEADERS if the system has ANSI C header files.
Specifically, this macro checks for `stdlib.h', `stdarg.h',
`string.h', and `float.h'; if the system has those, it
probably has the rest of the ANSI C header files. This macro also
checks whether `string.h' declares memchr (and thus
presumably the other mem functions), whether `stdlib.h'
declare free (and thus presumably malloc and other related
functions), and whether the `ctype.h' macros work on characters
with the high bit set, as ANSI C requires.
Use STDC_HEADERS instead of __STDC__ to determine whether
the system has ANSI-compliant header files (and probably C library
functions) because many systems that have GCC do not have ANSI C
header files.
On systems without ANSI C headers, there is so much variation that it is probably easier to declare the functions you use than to figure out exactly what the system header files declare. Some systems contain a mix of functions from ANSI and BSD; some are mostly ANSI but lack `memmove'; some define the BSD functions as macros in `string.h' or `strings.h'; some have only the BSD functions but `string.h'; some declare the memory functions in `memory.h', some in `string.h'; etc. It is probably sufficient to check for one string function and one memory function; if the library has the ANSI versions of those then it probably has most of the others. If you put the following in `configure.ac':
AC_HEADER_STDC AC_CHECK_FUNCS(strchr memcpy) |
then, in your code, you can use declarations like this:
#if STDC_HEADERS # include <string.h> #else # if !HAVE_STRCHR # define strchr index # define strrchr rindex # endif char *strchr (), *strrchr (); # if !HAVE_MEMCPY # define memcpy(d, s, n) bcopy ((s), (d), (n)) # define memmove(d, s, n) bcopy ((s), (d), (n)) # endif #endif |
If you use a function like memchr, memset, strtok,
or strspn, which have no BSD equivalent, then macros won't
suffice; you must provide an implementation of each function. An easy
way to incorporate your implementations only when needed (since the ones
in system C libraries may be hand optimized) is to, taking memchr
for example, put it in `memchr.c' and use
`AC_REPLACE_FUNCS(memchr)'.
HAVE_SYS_WAIT_H. Incompatibility can occur if `sys/wait.h'
does not exist, or if it uses the old BSD union wait instead
of int to store a status value. If `sys/wait.h' is not
POSIX compatible, then instead of including it, define the
POSIX macros with their usual interpretations. Here is an
example:
#include <sys/types.h> #if HAVE_SYS_WAIT_H # include <sys/wait.h> #endif #ifndef WEXITSTATUS # define WEXITSTATUS(stat_val) ((unsigned)(stat_val) >> 8) #endif #ifndef WIFEXITED # define WIFEXITED(stat_val) (((stat_val) & 255) == 0) #endif |
_POSIX_VERSION is defined when `unistd.h' is included on
POSIX 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 if the system supports POSIX is:
#if HAVE_UNISTD_H # include <sys/types.h> # include <unistd.h> #endif #ifdef _POSIX_VERSION /* Code for POSIX systems. */ #endif |
TIME_WITH_SYS_TIME. On some older systems,
`sys/time.h' includes `time.h', but `time.h' is not
protected against multiple inclusion, so programs should not explicitly
include both files. This macro is useful in programs that use, for
example, struct timeval as well as
struct tm. It is best used in conjunction with
HAVE_SYS_TIME_H, which can be checked for using
AC_CHECK_HEADERS(sys/time.h).
#if TIME_WITH_SYS_TIME # include <sys/time.h> # include <time.h> #else # if HAVE_SYS_TIME_H # include <sys/time.h> # else # include <time.h> # endif #endif |
TIOCGWINSZ requires `<sys/ioctl.h>', then
define GWINSZ_IN_SYS_IOCTL. Otherwise TIOCGWINSZ can be
found in `<termios.h>'.
Use:
#if HAVE_TERMIOS_H # include <termios.h> #endif #if GWINSZ_IN_SYS_IOCTL # include <sys/ioctl.h> #endif |
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These macros are used to find system header files not covered by the "particular" test macros. If you need to check the contents of a header as well as find out whether it is present, you have to write your own test for it (see section 6. Writing Tests).
AC_CHECK_HEADERS
instead.
For compatibility issues with older versions of Autoconf, please read below.
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.
For compatibility issues with older versions of Autoconf, please read below.
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. If you need to check whether a
header is preprocessable, you can use AC_PREPROC_IFELSE
(see section 6.3 Running the Preprocessor).
This scheme, which improves the robustness of the test, also requires that you make sure that headers that must be included before the header-file be part of the includes, (see section 5.1.2 Default Includes). If looking for `bar.h', which requires that `foo.h' be included before if it exists, we suggest the following scheme:
@verbatim AC_CHECK_HEADERS([foo.h]) AC_CHECK_HEADERS([bar.h], [], [], [#if HAVE_FOO_H # include <foo.h> # endif ])
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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 section 5.7.2 Generic Declaration Checks) or, for more complex tests, you may use
AC_COMPILE_IFELSE (see section 6.4 Running the Compiler).
5.7.1 Particular Declaration Checks Macros to check for certain declarations 5.7.2 Generic Declaration Checks How to find other declarations
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There are no specific macros for declarations.
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These macros are used to find declarations not covered by the "particular" test macros.
This macro actually tests whether it is valid to use symbol as an r-value, not if it is really declared, because it is much safer to avoid introducing extra declarations when they are not needed.
HAVE_DECL_symbol (in all capitals) to `1' if
symbol is declared, otherwise to `0'. If
action-if-not-found is given, it is additional shell code to
execute when one of the function declarations is needed, otherwise
action-if-found is executed.
This macro uses an m4 list as first argument:
AC_CHECK_DECLS(strdup) AC_CHECK_DECLS([strlen]) AC_CHECK_DECLS([malloc, realloc, calloc, free]) |
Unlike the other `AC_CHECK_*S' macros, when a symbol is not
declared, HAVE_DECL_symbol is defined to `0' instead
of leaving HAVE_DECL_symbol undeclared. When you are
sure that the check was performed, use
HAVE_DECL_symbol just like any other result of Autoconf:
#if !HAVE_DECL_SYMBOL extern char *symbol; #endif |
If the test may have not been performed, however, because it is safer not to declare a symbol than to use a declaration that conflicts with the system's one, you should use:
#if defined HAVE_DECL_MALLOC && !HAVE_DECL_MALLOC void *malloc (size_t *s); #endif |
You fall into the second category only in extreme situations: either your files may be used without being configured, or they are used during the configuration. In most cases the traditional approach is enough.
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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 section 5.8.2 Generic Structure Checks) or, for more complex tests, you may use
AC_COMPILE_IFELSE (see section 6.4 Running the Compiler).
5.8.1 Particular Structure Checks Macros to check for certain structure members 5.8.2 Generic Structure Checks How to find other structure members
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The following macros check for certain structures or structure members.
struct stat contains an st_blksize member, define
HAVE_STRUCT_STAT_ST_BLKSIZE. The former name,
HAVE_ST_BLKSIZE is to be avoided, as its support will cease in
the future. This macro is obsoleted, and should be replaced by
AC_CHECK_MEMBERS([struct stat.st_blksize]) |
struct stat contains an st_blocks member, define
HAVE_STRUCT STAT_ST_BLOCKS. Otherwise, require an
AC_LIBOBJ replacement of `fileblocks'. The former name,
HAVE_ST_BLOCKS is to be avoided, as its support will cease in the
future.
struct stat contains an st_rdev member, define
HAVE_STRUCT_STAT_ST_RDEV. The former name for this macro,
HAVE_ST_RDEV, is to be avoided as it will cease to be supported
in the future. Actually, even the new macro is obsolete and should be
replaced by:
AC_CHECK_MEMBERS([struct stat.st_rdev]) |
struct tm, define
TM_IN_SYS_TIME, which means that including `sys/time.h'
had better define struct tm.
struct tm has a
tm_zone member, define HAVE_STRUCT_TM_TM_ZONE (and the
obsoleted HAVE_TM_ZONE). Otherwise, if the external array
tzname is found, define HAVE_TZNAME.
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These macros are used to find structure members not covered by the "particular" test macros.
AC_CHECK_MEMBER(struct passwd.pw_gecos,,
[AC_MSG_ERROR([We need `passwd.pw_gecos'!])],
[#include <pwd.h>])
|
You can use this macro for sub-members:
AC_CHECK_MEMBER(struct top.middle.bot) |
HAVE_aggregate_member (in all
capitals, with spaces and dots replaced by underscores).
This macro uses m4 lists:
AC_CHECK_MEMBERS([struct stat.st_rdev, struct stat.st_blksize]) |
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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.
5.9.1 Particular Type Checks Special handling to find certain types 5.9.2 Generic Type Checks How to find other types
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These macros check for particular C types in `sys/types.h', `stdlib.h' and others, if they exist.
GETGROUPS_T to be whichever of gid_t or int
is the base type of the array argument to getgroups.
HAVE_MBSTATE_T if <wchar.h> declares the
mbstate_t type. Also, define mbstate_t to be a type if
<wchar.h> does not declare it.
signal as returning a pointer to a
function returning void, define RETSIGTYPE to be
void; otherwise, define it to be int.
Define signal handlers as returning type RETSIGTYPE:
RETSIGTYPE
hup_handler ()
{
...
}
|
uid_t is not defined, define uid_t to be int and
gid_t to be int.
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These macros are used to check for types not covered by the "particular" test macros.
HAVE_type (in all capitals). If no includes are
specified, the default includes are used (see section 5.1.2 Default Includes). If
action-if-found is given, it is additional shell code to execute
when one of the types is found. If action-if-not-found is given,
it is executed when one of the types is not found.
This macro uses m4 lists:
AC_CHECK_TYPES(ptrdiff_t) AC_CHECK_TYPES([unsigned long long, uintmax_t]) |
Autoconf, up to 2.13, used to provide to another version of
AC_CHECK_TYPE, broken by design. In order to keep backward
compatibility, a simple heuristics, quite safe but not totally, is
implemented. In case of doubt, read the documentation of the former
AC_CHECK_TYPE, see 15.4 Obsolete Macros.
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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 Unix and
`.exe' if Win32 or OS/2.
They also define the output variable OBJEXT based on the
output of the compiler, after `.c' files have been excluded, typically
to `o' if Unix, `obj' if Win32.
If the compiler being used does not produce executables, the tests fail. If the executables can't be run, and cross-compilation is not enabled, they fail too. See section 11. Manual Configuration, for more on support for cross compiling.
5.10.1 Specific Compiler Characteristics Some portability issues 5.10.2 Generic Compiler Characteristics Language independent tests 5.10.3 C Compiler Characteristics Checking its characteristics 5.10.4 C++ Compiler Characteristics Likewise 5.10.5 Fortran 77 Compiler Characteristics Likewise
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Some compilers exhibit different behaviors.
int
main (void)
{
static int test_array [sizeof (int) == 4 ? 1 : -1];
test_array [0] = 0
return 0;
}
|
To our knowledge, there is a single compiler that does not support this trick: the HP C compilers (the real one, not only the "bundled") on HP-UX 11.00:
$ cc -c -Ae +O2 +Onolimit conftest.c
cc: "conftest.c": error 1879: Variable-length arrays cannot \
have static storage.
|
Autoconf works around this problem by casting sizeof (int) to
long before comparing it.
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SIZEOF_type (see section 5.1.1 Standard Symbols) to be the
size in bytes of type. If `type' is unknown, it gets a size
of 0. If no includes are specified, the default includes are used
(see section 5.1.2 Default Includes). If you provide include, be sure to
include `stdio.h' which is required for this macro to run.
This macro now works even when cross-compiling. The unused argument was used when cross-compiling.
For example, the call
AC_CHECK_SIZEOF(int *) |
defines SIZEOF_INT_P to be 8 on DEC Alpha AXP systems.
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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
|
yields
error-->cpp: "foo.c", line 13: error 4048: Non-terminating comment at end of file. error-->cpp: "foo.c", line 13: error 4033: 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 -o a.o; cc -c b.c -o b.o; cc a.o b.o -o c' solves the issue.
CC is not already set in the
environment, check for gcc and cc, then for other C
compilers. Set output variable CC to the name of the compiler
found.
This macro may, however, be invoked with an optional first argument
which, if specified, must be a space separated list of C compilers to
search for. This just gives the user an opportunity to specify an
alternative search list for the C compiler. For example, if you didn't
like the default order, then you could invoke AC_PROG_CC like
this:
AC_PROG_CC(cl egcs gcc cc) |
If the C compiler is not in ANSI C mode by default, try to add an
option to output variable CC to make it so. This macro tries
various options that select ANSI C on some system or another. It
considers the compiler to be in ANSI C mode if it handles function
prototypes correctly.
After calling this macro you can check whether the C compiler has been
set to accept ANSI C; if not, the shell variable
ac_cv_prog_cc_stdc is set to `no'. If you wrote your source
code in ANSI C, you can make an un-ANSIfied copy of it by
using the program ansi2knr, which comes with Automake. See also
under AC_C_PROTOTYPES below.
If using the GNU C compiler, set shell variable GCC to
`yes'. If output variable CFLAGS was not already set, set
it to `-g -O2' for the GNU C compiler (`-O2' on systems
where GCC does not accept `-g'), or `-g' for other compilers.
NO_MINUS_C_MINUS_O. This macro actually
tests both the compiler found by AC_PROG_CC, and, if different,
the first cc in the path. The test fails if one fails. This
macro was created for GNU Make to choose the default C compilation
rule.
CPP to a command that runs the
C preprocessor. If `$CC -E' doesn't work, `/lib/cpp' is used.
It is only portable to run CPP on files with a `.c'
extension.
Some preprocessors don't indicate missing include files by the error status. For such preprocessors an internal variable is set that causes other macros to check the standard error from the preprocessor and consider the test failed if any warnings have been reported.
The following macros check for C compiler or machine architecture
features. To check for characteristics not listed here, use
AC_COMPILE_IFELSE (see section 6.4 Running the Compiler) or
AC_RUN_IFELSE (see section 6.6 Checking Run Time Behavior).
This macro runs a test-case if endianness cannot be determined from the system header files. When cross-compiling, the test-case is not run but grep'ed for some magic values. action-if-unknown is executed if the latter case fails to determine the byte sex of the host system.
The default for action-if-true is to define `WORDS_BIGENDIAN'. The default for action-if-false is to do nothing. And finally, the default for action-if-unknown is to abort configure and tell the installer which variable he should preset to bypass this test.
const, define const to be empty. Some C compilers that do
not define __STDC__ do support const; some compilers that
define __STDC__ do not completely support const. Programs
can simply use const as if every C compiler supported it; for
those that don't, the `Makefile' or configuration header file will
define it as empty.
Occasionally installers use a C++ compiler to compile C code, typically
because they lack a C compiler. This causes problems with const,
because C and C++ treat const differently. For example:
const int foo; |
is valid in C but not in C++. These differences unfortunately cannot be
papered over by defining const to be empty.
If autoconf detects this situation, it leaves const alone,
as this generally yields better results in practice. However, using a
C++ compiler to compile C code is not recommended or supported, and
installers who run into trouble in this area should get a C compiler
like GCC to compile their C code.
volatile,
define volatile to be empty. Programs can simply use
volatile as if every C compiler supported it; for those that do
not, the `Makefile' or configuration header will define it as
empty.
If the correctness of your program depends on the semantics of
volatile, simply defining it to be empty does, in a sense, break
your code. However, given that the compiler does not support
volatile, you are at its mercy anyway. At least your
program will compile, when it wouldn't before.
In general, the volatile keyword is a feature of ANSI C, so
you might expect that volatile is available only when
__STDC__ is defined. However, Ultrix 4.3's native compiler does
support volatile, but does not defined __STDC__.
inline, do nothing.
Otherwise define inline to __inline__ or __inline
if it accepts one of those, otherwise define inline to be empty.
char is unsigned, define __CHAR_UNSIGNED__,
unless the C compiler predefines it.
long double type with more
range or precision than the double type, define
HAVE_LONG_DOUBLE.
HAVE_STRINGIZE. The stringizing operator is `#' and is
found in macros such as this:
#define x(y) #y |
AC_PROG_CC), define PROTOTYPES and __PROTOTYPES.
In the case the compiler does not handle
prototypes, you should use ansi2knr, which comes with the
Automake distribution, to unprotoize function definitions. For
function prototypes, you should first define PARAMS:
#ifndef PARAMS # if PROTOTYPES # define PARAMS(protos) protos # else /* no PROTOTYPES */ # define PARAMS(protos) () # endif /* no PROTOTYPES */ #endif |
then use it this way:
size_t my_strlen PARAMS ((const char *)); |
This macro also defines __PROTOTYPES; this is for the benefit of
header files that cannot use macros that infringe on user name space.
CC if using the
GNU C compiler and ioctl does not work properly without
`-traditional'. That usually happens when the fixed header files
have not been installed on an old system. Since recent versions of the
GNU C compiler fix the header files automatically when installed,
this is becoming a less prevalent problem.
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CXX or CCC (in that order) is set; if so, then set output
variable CXX to its value.
Otherwise, if the macro is invoked without an argument, then search for
a C++ compiler under the likely names (first g++ and c++
then other names). If none of those checks succeed, then as a last
resort set CXX to g++.
This macro may, however, be invoked with an optional first argument
which, if specified, must be a space separated list of C++ compilers to
search for. This just gives the user an opportunity to specify an
alternative search list for the C++ compiler. For example, if you
didn't like the default order, then you could invoke AC_PROG_CXX
like this:
AC_PROG_CXX(cl KCC CC cxx cc++ xlC aCC c++ g++ egcs gcc) |
If using the GNU C++ compiler, set shell variable GXX to
`yes'. If output variable CXXFLAGS was not already set, set
it to `-g -O2' for the GNU C++ compiler (`-O2' on
systems where G++ does not accept `-g'), or `-g' for other
compilers.
CXXCPP to a command that runs the C++
preprocessor. If `$CXX -E' doesn't work, `/lib/cpp' is used.
It is only portable to run CXXCPP on files with a `.c',
`.C', or `.cc' extension.
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++.
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F77 is not already
set in the environment, then check for g77 and f77, and
then some other names. Set the output variable F77 to the name
of the compiler found.
This macro may, however, be invoked with an optional first argument
which, if specified, must be a space separated list of Fortran 77
compilers to search for. This just gives the user an opportunity to
specify an alternative search list for the Fortran 77 compiler. For
example, if you didn't like the default order, then you could invoke
AC_PROG_F77 like this:
AC_PROG_F77(fl32 f77 fort77 xlf g77 f90 xlf90) |
If using g77 (the GNU Fortran 77 compiler), then
AC_PROG_F77 will set the shell variable G77 to `yes'.
If the output variable FFLAGS was not already set in the
environment, then set it to `-g -02' for g77 (or `-O2'
where g77 does not accept `-g'). Otherwise, set
FFLAGS to `-g' for all other Fortran 77 compilers.
F77_NO_MINUS_C_MINUS_O if it
does not.
The following macros check for Fortran 77 compiler characteristics. To
check for characteristics not listed here, use AC_COMPILE_IFELSE
(see section 6.4 Running the Compiler) or AC_RUN_IFELSE (see section 6.6 Checking Run Time Behavior), making sure to first set the current language to Fortran 77
AC_LANG(Fortran 77) (see section 6.1 Language Choice).
FLIBS is set to these flags.
This macro is intended to be used in those situations when it is necessary to mix, e.g., C++ and Fortran 77 source code into a single program or shared library (see section `Mixing Fortran 77 With C and C++' in GNU Automake).
For example, if object files from a C++ and Fortran 77 compiler must be linked together, then the C++ compiler/linker must be used for linking (since special C++-ish things need to happen at link time like calling global constructors, instantiating templates, enabling exception support, etc.).
However, the Fortran 77 intrinsic and run-time libraries must be linked
in as well, but the C++ compiler/linker doesn't know by default how to
add these Fortran 77 libraries. Hence, the macro
AC_F77_LIBRARY_LDFLAGS was created to determine these Fortran 77
libraries.
The macro AC_F77_DUMMY_MAIN or AC_F77_MAIN will probably
also be necessary to link C/C++ with Fortran; see below.
AC_F77_LIBRARY_LDFLAGS provide their own main entry
function that initializes things like Fortran I/O, and which then calls
a user-provided entry function named (say) MAIN__ to run the
user's program. The AC_F77_DUMMY_MAIN or AC_F77_MAIN
macro figures out how to deal with this interaction.
When using Fortran for purely numerical functions (no I/O, etc.) often one
prefers to provide one's own main and skip the Fortran library
initializations. In this case, however, one may still need to provide a
dummy MAIN__ routine in order to prevent linking errors on some
systems. AC_F77_DUMMY_MAIN detects whether any such routine is
required for linking, and what its name is; the shell variable
F77_DUMMY_MAIN holds this name, unknown when no solution
was found, and none when no such dummy main is needed.
By default, action-if-found defines F77_DUMMY_MAIN to the
name of this routine (e.g., MAIN__) if it is required.
[action-if-not-found] defaults to exiting with an error.
In order to link with Fortran routines, the user's C/C++ program should then include the following code to define the dummy main if it is needed:
#ifdef F77_DUMMY_MAIN
# ifdef __cplusplus
extern "C"
# endif
int F77_DUMMY_MAIN() { return 1; }
#endif
|
Note that AC_F77_DUMMY_MAIN is called automatically from
AC_F77_WRAPPERS; there is generally no need to call it explicitly
unless one wants to change the default actions.
AC_F77_DUMMY_MAIN, many Fortran libraries
allow you to provide an entry point called (say) MAIN__ instead of
the usual main, which is then called by a main function in
the Fortran libraries that initializes things like Fortran I/O. The
AC_F77_MAIN macro detects whether it is possible to
utilize such an alternate main function, and defines F77_MAIN to
the name of the function. (If no alternate main function name is found,
F77_MAIN is simply defined to main.)
Thus, when calling Fortran routines from C that perform things like I/O,
one should use this macro and name the "main" function F77_MAIN
instead of main.
F77_FUNC(name,NAME) and
F77_FUNC_(name,NAME) to properly mangle the names of C/C++
identifiers, and identifiers with underscores, respectively, so that
they match the name-mangling scheme used by the Fortran 77 compiler.
Fortran 77 is case-insensitive, and in order to achieve this the Fortran
77 compiler converts all identifiers into a canonical case and format.
To call a Fortran 77 subroutine from C or to write a C function that is
callable from Fortran 77, the C program must explicitly use identifiers
in the format expected by the Fortran 77 compiler. In order to do this,
one simply wraps all C identifiers in one of the macros provided by
AC_F77_WRAPPERS. For example, suppose you have the following
Fortran 77 subroutine:
subroutine foobar(x,y)
double precision x, y
y = 3.14159 * x
return
end
|
You would then declare its prototype in C or C++ as:
#define FOOBAR_F77 F77_FUNC(foobar,FOOBAR) #ifdef __cplusplus extern "C" /* prevent C++ name mangling */ #endif void FOOBAR_F77(double *x, double *y); |
Note that we pass both the lowercase and uppercase versions of the
function name to F77_FUNC so that it can select the right one.
Note also that all parameters to Fortran 77 routines are passed as
pointers (see section `Mixing Fortran 77 With C and C++' in GNU Automake).
Although Autoconf tries to be intelligent about detecting the
name-mangling scheme of the Fortran 77 compiler, there may be Fortran 77
compilers that it doesn't support yet. In this case, the above code
will generate a compile-time error, but some other behavior
(e.g., disabling Fortran-related features) can be induced by checking
whether the F77_FUNC macro is defined.
Now, to call that routine from a C program, we would do something like:
{
double x = 2.7183, y;
FOOBAR_F77(&x, &y);
}
|
If the Fortran 77 identifier contains an underscore
(e.g., foo_bar), you should use F77_FUNC_ instead of
F77_FUNC (with the same arguments). This is because some Fortran
77 compilers mangle names differently if they contain an underscore.
AC_F77_WRAPPERS). shellvar is
optional; if it is not supplied, the shell variable will be simply
name. The purpose of this macro is to give the caller a way to
access the name-mangling information other than through the C
preprocessor as above, for example, to call Fortran routines from some
language other than C/C++.
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The following macros check for operating system services or capabilities.
xmkmf on a
trivial `Imakefile' and examining the `Makefile' that it
produces. If that fails (such as if xmkmf is not present), look
for the files in several directories where they often reside. If either
method is successful, set the shell variables x_includes and
x_libraries to their locations, unless they are in directories
the compiler searches by default.
If both methods fail, or the user gave the command line option
`--without-x', set the shell variable no_x to `yes';
otherwise set it to the empty string.
AC_PATH_X. It adds the C compiler flags
that X needs to output variable X_CFLAGS, and the X linker flags
to X_LIBS. Define X_DISPLAY_MISSING if X is not
available.
This macro also checks for special libraries that some systems need in
order to compile X programs. It adds any that the system needs to
output variable X_EXTRA_LIBS. And it checks for special X11R6
libraries that need to be linked with before `-lX11', and adds
any found to the output variable X_PRE_LIBS.
interpval; it will be set to `yes'
if the system supports `#!', `no' if not.
CC. Define
_FILE_OFFSET_BITS and _LARGE_FILES if necessary.
Large-file support can be disabled by configuring with the `--disable-largefile' option.
If you use this macro, check that your program works even when
off_t is longer than long, since this is common when
large-file support is enabled. For example, it is not correct to print
an arbitrary off_t value X with printf ("%ld",
(long) X).
HAVE_LONG_FILE_NAMES.
ac_cv_sys_posix_termios to
`yes'. If not, set the variable to `no'.
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The following macros check for certain operating systems that need special treatment for some programs, due to exceptional oddities in their header files or libraries. These macros are warts; they will be replaced by a more systematic approach, based on the functions they make available or the environments they provide.
_ALL_SOURCE. Allows the use of some BSD
functions. Should be called before any macros that run the C compiler.
_GNU_SOURCE.
Allows the use of some GNU functions. Should be called
before any macros that run the C compiler.
LIBS if necessary for POSIX facilities. Call this
after AC_PROG_CC and before any other macros that use POSIX
interfaces. INTERACTIVE UNIX is no longer sold, and Sun says that
they will drop support for it on 2006-07-23, so this macro is becoming
obsolescent.
_MINIX and _POSIX_SOURCE and define
_POSIX_1_SOURCE to be 2. This allows the use of POSIX
facilities. Should be called before any macros that run the C compiler.
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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 section 6.1 Language Choice). They do not cache the results of their tests for future use (see section 7.3 Caching Results), because they don't know enough about the information they are checking for to generate a cache variable name. They also do not print any messages, for the same reason. The checks for particular kinds of features call these macros and do cache their results and print messages about what they're checking for.
When you write a feature test that could be applicable to more than one software package, the best thing to do is encapsulate it in a new macro. See section 9. Writing Autoconf Macros, for how to do that.
6.1 Language Choice Selecting which language to use for testing 6.2 Writing Test Programs Forging source files for compilers 6.3 Running the Preprocessor Detecting preprocessor symbols 6.4 Running the Compiler Detecting language or header features 6.5 Running the Linker Detecting library features 6.6 Checking Run Time Behavior Testing for run-time features 6.7 Systemology A zoology of operating systems 6.8 Multiple Cases Tests for several possible values
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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'.
Supported languages are:
CC and CPP and use extension
`.c' for test programs.
CXX and CXXCPP and use
extension `.C' for test programs.
F77 and use extension `.f' for
test programs.
AC_LANG) on a stack, and
then select the language. Use this macro and AC_LANG_POP
in macros that need to temporarily switch to a particular language.
AC_LANG_PUSH, and remove it from the stack.
If given, language specifies the language we just quit. It is a good idea to specify it when it's known (which should be the case...), since Autoconf will detect inconsistencies.
AC_LANG_PUSH(Fortran 77) # Perform some tests on Fortran 77. # ... AC_LANG_POP(Fortran 77) |
AC_REQUIRE (see section 9.4.1 Prerequisite Macros) with an
argument of either AC_PROG_CPP or AC_PROG_CXXCPP,
depending on which language is current.
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Autoconf tests follow is common scheme: feeding some program with some input, and most of the time, feeding a compiler with some source file. This section is dedicated to these source samples.
6.2.1 Guidelines for Test Programs General rules for writing test programs 6.2.2 Test Functions Avoiding pitfalls in test programs 6.2.3 Generating Sources Source program boilerplate
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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 if a header is
functional might let your configure accept a header which will
cause some compiler error. Do not hesitate checking header with
other headers included before, especially required headers.
Make sure the symbols you use are properly defined, i.e., refrain for simply declaring a function yourself instead of including the proper header.
Test programs should not write anything to the standard output. They
should return 0 if the test succeeds, nonzero otherwise, so that success
can be distinguished easily from a core dump or other failure;
segmentation violations and other failures produce a nonzero exit
status. Test programs should exit, not return, from
main, because on some systems (old Suns, at least) the argument
to return in main is ignored.
Test programs can use #if or #ifdef to check the values of
preprocessor macros defined by tests that have already run. For
example, if you call AC_HEADER_STDC, then later on in
`configure.ac' you can have a test program that includes an
ANSI C header file conditionally:
#if STDC_HEADERS # include <stdlib.h> #endif |
If a test program needs to use or create a data file, give it a name
that starts with `conftest', such as `conftest.data'. The
configure script cleans up by running `rm -rf conftest*'
after running test programs and if the script is interrupted.
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Function declarations in test programs should have a prototype conditionalized for C++. In practice, though, test programs rarely need functions that take arguments.
#ifdef __cplusplus foo (int i) #else foo (i) int i; #endif |
Functions that test programs declare should also be conditionalized for C++, which requires `extern "C"' prototypes. Make sure to not include any header files containing clashing prototypes.
#ifdef __cplusplus extern "C" void *malloc (size_t); #else void *malloc (); #endif |
If a test program calls a function with invalid parameters (just to see
whether it exists), organize the program to ensure that it never invokes
that function. You can do this by calling it in another function that is
never invoked. You can't do it by putting it after a call to
exit, because GCC version 2 knows that exit never returns
and optimizes out any code that follows it in the same block.
If you include any header files, 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).
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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 section 6.1 Language Choice) to "format" the output properly.
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.
AC_DEFINE performed so far.
For instance executing (observe the double quotation!):
AC_INIT(Autoconf Documentation, 2.57, bug-autoconf@gnu.org) AC_DEFINE([HELLO_WORLD], ["Hello, World\n"]) AC_LANG_CONFTEST( [AC_LANG_SOURCE([[const char hw[] = "Hello, World\n";]])]) gcc -E -dD conftest.c -o - |
results in:
# 1 "conftest.c" # 1169 "configure" # 1 "confdefs.h" 1 #define PACKAGE_NAME "Autoconf Documentation" #define PACKAGE_TARNAME "autoconf-documentation" #define PACKAGE_VERSION "2.57" #define PACKAGE_STRING "Autoconf Documentation 2.57" #define PACKAGE_BUGREPORT "bug-autoconf@gnu.org" #define HELLO_WORLD "Hello, World\n" # 1170 "configure" 2 const char hw[] = "Hello, World\n"; |
main in
C). Since it uses AC_LANG_SOURCE, the feature of the latter are
available.
For instance:
AC_INIT(Autoconf Documentation, 2.57, bug-autoconf@gnu.org)
AC_DEFINE([HELLO_WORLD], ["Hello, World\n"])
AC_LANG_CONFTEST(
[AC_LANG_PROGRAM([[const char hw[] = "Hello, World\n";]],
[[fputs (hw, stdout);]])])
gcc -E -dD conftest.c -o -
|
results in:
# 1 "conftest.c"
# 1169 "configure"
# 1 "confdefs.h" 1
#define PACKAGE_NAME "Autoconf Documentation"
#define PACKAGE_TARNAME "autoconf-documentation"
#define PACKAGE_VERSION "2.57"
#define PACKAGE_STRING "Autoconf Documentation 2.57"
#define PACKAGE_BUGREPORT "bug-autoconf@gnu.org"
#define HELLO_WORLD "Hello, World\n"
# 1170 "configure" 2
const char hw[] = "Hello, World\n";
int
main ()
{
fputs (hw, stdout);
;
return 0;
}
|
main in C). Since it uses AC_LANG_PROGRAMS, the feature
of the latter are available.
This function will probably be replaced in the feature by a version which would enable specifying the arguments. The use of this macro is not encouraged, as it violates strongly the typing system.
main in C): a
simple (function pointer) assignment. Since it uses
AC_LANG_PROGRAMS, the feature 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).
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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 to the temptation of following the easiest path.
Nevertheless, if you need to run the preprocessor, then use
AC_PREPROC_IFELSE.
AC_LANG_PROGRAM and friends.
This macro uses CPPFLAGS, but not CFLAGS, because
`-g', `-O', etc. are not valid options to many C
preprocessors.
It is customary to report unexpected failures with
AC_MSG_FAILURE.
For instance:
AC_INIT(Autoconf Documentation, 2.57, bug-autoconf@gnu.org)
AC_DEFINE([HELLO_WORLD], ["Hello, World\n"])
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... 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 ANSI C... none needed checking how to run the C preprocessor... gcc -E OK |
The macro AC_TRY_CPP (see section 15.4 Obsolete Macros) used to play the
role of AC_PREPROC_IFELSE, but double quotes its argument, making
it impossible to use it to ellaborate 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?
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To check for a syntax feature of the (C, C++, or Fortran 77) 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.
AC_LANG_PROGRAM and friends.
This macro uses CFLAGS or CXXFLAGS if either C or C++ is
the currently selected language, as well as CPPFLAGS, when
compiling. If Fortran 77 is the currently selected language then
FFLAGS will be used when compiling.
It is customary to report unexpected failures with
AC_MSG_FAILURE. This macro does not try to link; use
AC_LINK_IFELSE if you need to do that (see section 6.5 Running the Linker).
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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 run-time 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 section 5.4 Library Files), by adding the library being
checked for to LIBS temporarily and trying to link a small
program.
AC_LANG_PROGRAM and friends.
This macro uses CFLAGS or CXXFLAGS if either C or C++ is
the currently selected language, as well as CPPFLAGS, when
compiling. If Fortran 77 is the currently selected language then
FFLAGS will be used when compiling.
It is customary to report unexpected failures with
AC_MSG_FAILURE. This macro does not try to execute the program;
use AC_RUN_IFELSE if you need to do that (see section 6.6 Checking Run Time Behavior).
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Sometimes you need to find out how a system performs at run time, such as whether a given function has a certain capability or bug. If you can, make such checks when your program runs instead of when it is configured. You can check for things like the machine's endianness when your program initializes itself.
If you really need to test for a run-time behavior while configuring,
you can write a test program to determine the result, and compile and
run it using AC_RUN_IFELSE. Avoid running test programs if
possible, because this prevents people from configuring your package for
cross-compiling.
The input can be made by AC_LANG_PROGRAM and friends. This
macro uses CFLAGS or CXXFLAGS, CPPFLAGS,
LDFLAGS, and LIBS
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 exit status of the program is available in the shell variable `$?', but be very careful to limit yourself to positive values smaller than 127; bigger values should be saved into a file by the program. Note also that you have simply no guarantee that this exit status is issued by the program, or by the failure of its compilation. In other words, use this feature if sadist only, it was reestablished because the Autoconf maintainers grew tired of receiving "bug reports".
It is customary to report unexpected failures with
AC_MSG_FAILURE.
Try to provide a pessimistic default value to use when cross-compiling
makes run-time tests impossible. You do this by passing the optional
last argument to AC_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 will not be able to configure your package for
cross-compiling. A few of the macros distributed with Autoconf produce
this warning message.
To configure for cross-compiling you can also choose a value for those parameters based on the canonical system name (see section 11. Manual Configuration). Alternatively, set up a test results cache file with the correct values for the host system (see section 7.3 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.
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This section aims at presenting some systems and pointers to documentation. It may help you addressing particular problems reported by users.
The Rosetta Stone for Unix contains a lot of interesting crossed information on various Unices.
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).
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Some operations are accomplished in several possible ways, depending on the UNIX variant. Checking for them essentially requires a "case statement". Autoconf does not directly provide one; however, it is easy to simulate by using a shell variable to keep track of whether a way to perform the operation has been found yet.
Here is an example that uses the shell variable fstype to keep
track of whether the remaining cases need to be checked.
AC_MSG_CHECKING([how to get file system type])
fstype=no
# The order of these tests is important.
AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <sys/statvfs.h>
#include <sys/fstyp.h>]])],
[AC_DEFINE(FSTYPE_STATVFS) 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) 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) fstype=AIX])
fi
# (more cases omitted here)
AC_MSG_RESULT([$fstype])
|
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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.
7.1 Defining C Preprocessor Symbols Defining C preprocessor symbols 7.2 Setting Output Variables Replacing variables in output files 7.3 Caching Results Speeding up subsequent configureruns7.4 Printing Messages Notifying configureusers
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A common action to take in response to a feature test is to define a C
preprocessor symbol indicating the results of the test. That is done by
calling AC_DEFINE or AC_DEFINE_UNQUOTED.
By default, AC_OUTPUT places the symbols defined by these macros
into the output variable DEFS, which contains an option
`-Dsymbol=value' for each symbol defined. Unlike in
Autoconf version 1, there is no variable DEFS defined while
configure is running. To check whether Autoconf macros have
already defined a certain C preprocessor symbol, test the value of the
appropriate cache variable, as in this example:
AC_CHECK_FUNC(vprintf, [AC_DEFINE(HAVE_VPRINTF)]) if test "$ac_cv_func_vprintf" != yes; then AC_CHECK_FUNC(_doprnt, [AC_DEFINE(HAVE_DOPRNT)]) fi |
If AC_CONFIG_HEADERS has been called, then instead of creating
DEFS, AC_OUTPUT creates a header file by substituting the
correct values into #define statements in a template file.
See section 4.8 Configuration Header Files, for more information about this kind of
output.
AC_CONFIG_HEADERS it should not contain any `#'
characters, as make tends to eat them. To use a shell variable
(which you need to do in order to define a value containing the M4 quote
characters `[' or `]'), use AC_DEFINE_UNQUOTED instead.
description is only useful if you are using
AC_CONFIG_HEADERS. In this case, description is put into
the generated `config.h.in' as the comment before the macro define.
The following example defines the C preprocessor variable
EQUATION to be the string constant `"$a > $b"':
AC_DEFINE(EQUATION, "$a > $b") |
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.
AC_DEFINE, but three shell expansions are
performed--once--on variable and value: variable expansion
(`$'), command substitution (``'), and backslash escaping
(`\'). Single and double quote characters in the value have no
special meaning. Use this macro instead of AC_DEFINE when
variable or value is a shell variable. Examples:
AC_DEFINE_UNQUOTED(config_machfile, "$machfile") AC_DEFINE_UNQUOTED(GETGROUPS_T, $ac_cv_type_getgroups) AC_DEFINE_UNQUOTED($ac_tr_hdr) |
Due to 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 spaces or
newlines. That is, do this:
AC_CHECK_HEADER(elf.h, [AC_DEFINE(SVR4) LIBS="$LIBS -lelf"]) |
or this:
AC_CHECK_HEADER(elf.h, [AC_DEFINE(SVR4) LIBS="$LIBS -lelf"]) |
instead of this:
AC_CHECK_HEADER(elf.h, [AC_DEFINE(SVR4); LIBS="$LIBS -lelf"]) |
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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 section 4.7.1 Preset Output Variables, for a list of output
variables that are always available.
AC_OUTPUT
substitute the variable variable into output files (typically one
or more `Makefile's). This means that AC_OUTPUT will
replace instances of `@variable@' in input files with the
value that the shell variable variable has when AC_OUTPUT
is called. This value of variable should not contain literal
newlines.
If value is given, in addition assign it to variable.
AC_OUTPUT insert (without substitutions) the contents of the file
named by shell variable variable into output files. This means
that AC_OUTPUT will replace instances of
`@variable@' in output files (such as `Makefile.in')
with the contents of the file that the shell variable variable
names when AC_OUTPUT is called. Set the variable to
`/dev/null' for cases that do not have a file to insert.
This macro is useful for inserting `Makefile' fragments containing
special dependencies or other make directives for particular host
or target types into `Makefile's. For example, `configure.ac'
could contain:
AC_SUBST_FILE(host_frag) host_frag=$srcdir/conf/sun4.mh |
and then a `Makefile.in' could contain:
@host_frag@ |
Running configure in 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 will depend upon bizarre-cc being the C compiler. If
for some reason the user runs ./configure again, or if it is
run via `./config.status --recheck', (See section 4.7.4 Automatic Remaking,
and see section 14. Recreating a Configuration), 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.
Being precious means that
AC_SUBST'd.
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 CC in `./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 FOO is foo can be two very
different runs.
configure runs. For instance:
$ ./configure --silent --config-cache $ CC=cc ./configure --silent --config-cache configure: error: `CC' was not set in the previous run configure: error: changes in the environment can compromise \ the build configure: error: run `make distclean' and/or \ `rm config.cache' and start over |
and similarly if the variable is unset, or if its content is changed.
$ CC=/usr/bin/cc ./configure undeclared_var=raboof --silent $ ./config.status --recheck running /bin/sh ./configure undeclared_var=raboof --silent \ CC=/usr/bin/cc --no-create --no-recursion |
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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 section 7.3.2 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.
configure was not given the `--quiet' or
`--silent' option, print a message saying that the result was
cached; otherwise, run the shell commands commands-to-set-it. If
the shell commands are run to determine the value, the value will be
saved in the cache file just before configure creates its output
files. See section 7.3.1 Cache Variable Names, for how to choose the name of the
cache-id variable.
The commands-to-set-it must have no side effects except for setting the variable cache-id, see below.
AC_CACHE_VAL that takes care of printing the
messages. This macro provides a convenient shorthand for the most
common way to use these macros. It calls AC_MSG_CHECKING for
message, then AC_CACHE_VAL with the cache-id and
commands arguments, and AC_MSG_RESULT with cache-id.
The commands-to-set-it must have no side effects except for setting the variable cache-id, see below.
It is very common to find buggy macros using AC_CACHE_VAL or
AC_CACHE_CHECK, because people are tempted to call
AC_DEFINE in the commands-to-set-it. Instead, the code that
follows the call to AC_CACHE_VAL should call
AC_DEFINE, by examining the value of the cache variable. For
instance, the following macro is broken:
AC_DEFUN([AC_SHELL_TRUE],
[AC_CACHE_CHECK([whether true(1) works], [ac_cv_shell_true_works],
[ac_cv_shell_true_works=no
true && ac_cv_shell_true_works=yes
if test $ac_cv_shell_true_works = yes; then
AC_DEFINE([TRUE_WORKS], 1
[Define if `true(1)' works properly.])
fi])
])
|
This fails if the cache is enabled: the second time this macro is run,
TRUE_WORKS will not be defined. The proper implementation
is:
AC_DEFUN([AC_SHELL_TRUE],
[AC_CACHE_CHECK([whether true(1) works], [ac_cv_shell_true_works],
[ac_cv_shell_true_works=no
true && ac_cv_shell_true_works=yes])
if test $ac_cv_shell_true_works = yes; then
AC_DEFINE([TRUE_WORKS], 1
[Define if `true(1)' works properly.])
fi
])
|
Also, commands-to-set-it should not print any messages, for
example with AC_MSG_CHECKING; do that before calling
AC_CACHE_VAL, so the messages are printed regardless of whether
the results of the check are retrieved from the cache or determined by
running the shell commands.
7.3.1 Cache Variable Names Shell variables used in caches 7.3.2 Cache Files Files configureuses for caching7.3.3 Cache Checkpointing Loading and saving the cache file
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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 will be Boolean (`yes' or `no') or the names of files or functions; so this is not an important restriction.
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A cache file is a shell script that caches the results of configure tests run on one system so they can be shared between configure scripts and configure runs. It is not useful on other systems. If its contents are invalid for some reason, the user may delete or edit it.
By default, configure uses no cache file (technically, it uses
`--cache-file=/dev/null'), to avoid problems caused by accidental
use of stale cache files.
To enable caching, configure accepts `--config-cache' (or
`-C') to cache results in the file `config.cache'.
Alternatively, `--cache-file=file' specifies that
file be the cache file. The cache file is created if it does not
exist already. When configure calls configure scripts in
subdirectories, it uses the `--cache-file' argument so that they
share the same cache. See section 4.11 Configuring Other Packages in 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 section 11. Manual Configuration).
The site initialization script can specify a site-wide cache file to
use, instead of the usual per-program cache. In this case, the cache
file will gradually accumulate information whenever someone runs a new
configure script. (Running configure merges the new cache
results with the existing cache file.) This may cause problems,
however, if the system configuration (e.g., the installed libraries or
compilers) changes and the stale cache file is not deleted.
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If your configure script, or a macro called from `configure.ac', happens
to abort the configure process, it may be useful to checkpoint the cache
a few times at key points using AC_CACHE_SAVE. Doing so will
reduce the amount of time it takes to re-run the configure script with
(hopefully) the error that caused the previous abort corrected.
AC_INIT.
AC_OUTPUT, but it can be quite useful to call
AC_CACHE_SAVE at key points in `configure.ac'.
For instance:
... AC_INIT, etc. ... # Checks for programs. AC_PROG_CC AC_PROG_GCC_TRADITIONAL ... more program checks ... AC_CACHE_SAVE # Checks for libraries. AC_CHECK_LIB(nsl, gethostbyname) AC_CHECK_LIB(socket, connect) ... more lib checks ... AC_CACHE_SAVE # Might abort... AM_PATH_GTK(1.0.2,, [AC_MSG_ERROR([GTK not in path])]) AM_PATH_GTKMM(0.9.5,, [AC_MSG_ERROR([GTK not in path])]) ... AC_OUTPUT, etc. ... |
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configure scripts need to give users running them several kinds
of information. The following macros print messages in ways appropriate
for each kind. The arguments to all of them get enclosed in shell
double quotes, so the shell performs variable and back-quote
substitution on them.
These macros are all wrappers around the echo shell command.
configure scripts should rarely need to run echo directly
to print messages for the user. Using these macros makes it easy to
change how and when each kind of message is printed; such changes need
only be made to the macro definitions and all of the callers will change
automatically.
To diagnose static issues, i.e., when autoconf is run, see
9.3 Reporting Messages.
configure is checking for a particular
feature. This macro prints a message that starts with `checking '
and ends with `...' and no newline. It must be followed by a call
to AC_MSG_RESULT to print the result of the check and the
newline. The feature-description should be something like
`whether the Fortran compiler accepts C++ comments' or `for
c89'.
This macro prints nothing if configure is run with the
`--quiet' or `--silent' option.
AC_MSG_CHECKING, and the result-description should be
the completion of the message printed by the call to
AC_MSG_CHECKING.
This macro prints nothing if configure is run with the
`--quiet' or `--silent' option.
AC_MSG_NOTICE([checking if stack overflow is detectable]) |
This macro prints nothing if configure is run with the
`--quiet' or `--silent' option.
configure from
completing. This macro prints an error message to the standard error
output and exits configure with exit-status (1 by default).
error-description should be something like `invalid value
$HOME for \$HOME'.
The error-description should start with a lower-case letter, and "cannot" is preferred to "can't".
AC_MSG_ERROR wrapper 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.
configure user of a possible problem. This macro
prints the message to the standard error output; configure
continues running afterward, so macros that call AC_MSG_WARN should
provide a default (back-up) behavior for the situations they warn about.
problem-description should be something like `ln -s seems to
make hard links'.
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Autoconf is written on top of two layers: M4sugar, which provides convenient macros for pure M4 programming, and M4sh, which provides macros dedicated to shell script generation.
As of this version of Autoconf, these two layers are still experimental, and their interface might change in the future. As a matter of fact, anything that is not documented must not be used.
8.1 M4 Quotation Protecting macros from unwanted expansion 8.2 Using autom4teThe Autoconf executables backbone 8.3 Programming in M4sugar Convenient pure M4 macros 8.4 Programming in M4sh Common shell Constructs
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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.
8.1.1 Active Characters Characters that change the behavior of M4 8.1.2 One Macro Call Quotation and one macro call 8.1.3 Quotation and Nested Macros Macros calling macros 8.1.4 changequoteis EvilWorse than INTERCAL: M4 + changequote 8.1.5 Quadrigraphs Another way to escape special characters 8.1.6 Quotation Rule Of Thumb One parenthesis, one quote
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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 themselves, and finally `(' and `)' (which M4 tries to match by pairs).
In order to understand the delicate case of macro calls, we first have to present some obvious failures. Below they are "obvious-ified", 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 will try hopelessly to use her macro
array:
define([array], [int tab[10];]) array =>int tab10; [array] =>array |
How can you correctly output the intended results(2)?
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Let's proceed on the interaction between active characters and macros with this small macro, which just returns its first argument:
define([car], [$1]) |
The two pairs of quotes above are not part of the arguments of
define; rather, they are understood by the top level when it
tries to find the arguments of define. Therefore, it is
equivalent to write:
define(car, $1) |
But, while it is acceptable for a `configure.ac' to avoid 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([#]) =># |
The reader will easily understand the following examples:
car(foo, bar) =>foo car([foo, bar]) =>foo, bar car((foo, bar)) =>(foo, bar) car([(foo], [bar)]) =>(foo car([], []) => car([[]], [[]]) =>[] |
With this in mind, we can explore the cases where macros invoke macros....
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The examples below use the following macros:
define([car], [$1]) define([active], [ACT, IVE]) define([array], [int tab[10]]) |
Each additional embedded macro call introduces other possible interesting quotations:
car(active) =>ACT car([active]) =>ACT, IVE car([[active]]) =>active |
In the first case, the top level looks for the arguments of car,
and finds `active'. Because M4 evaluates its arguments
before applying the macro, `active' is expanded, which results in:
car(ACT, IVE) =>ACT |
In the second case, the top level gives `active' as first and only
argument of car, which results in:
active =>ACT, IVE |
i.e., the argument is evaluated after the macro that invokes it.
In the third case, car receives `[active]', which results in:
[active] =>active |
exactly as we already saw above.
The example above, applied to a more realistic example, gives:
car(int tab[10];) =>int tab10; car([int tab[10];]) =>int tab10; car([[int tab[10];]]) =>int tab[10]; |
Huh? The first case is easily understood, but why is the second wrong,
and the third right? To understand that, you must know that after
M4 expands a macro, the resulting text is immediately subjected
to macro expansion and quote removal. This means that the quote removal
occurs twice--first before the argument is passed to the car
macro, and second after the car macro expands to the first
argument.
As the author of the Autoconf macro car, you then consider it to
be incorrect that your users have to double-quote the arguments of
car, so you "fix" your macro. Let's call it qar for
quoted car:
define([qar], [[$1]]) |
and check that qar is properly fixed:
qar([int tab[10];]) =>int tab[10]; |
Ahhh! That's much better.
But note what you've done: now that the arguments are literal strings, if the user wants to use the results of expansions as arguments, she has to use an unquoted macro call:
qar(active) =>ACT |
where she wanted to reproduce what she used to do with car:
car([active]) =>ACT, IVE |
Worse yet: she wants to use a macro that produces a set of cpp
macros:
define([my_includes], [#include <stdio.h>]) car([my_includes]) =>#include <stdio.h> qar(my_includes) error-->EOF in argument list |
This macro, qar, because it double quotes its arguments, forces
its users to leave their macro calls unquoted, which is dangerous.
Commas and other active symbols are interpreted by M4 before
they are given to the macro, often not in the way the users expect.
Also, because qar behaves differently from the other macros,
it's an exception that should be avoided in Autoconf.
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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`'), 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 n-th argument, `$' by
default, the parenthesis around arguments etc.).
These primitives are really meant to make M4 more useful for specific
domains: they should be considered like command line options:
`--quotes', `--comments', `--words', and
--syntax. Nevertheless, they are implemented as M4 builtins, as
it makes M4 libraries self contained (no need for additional options).
There lies the problem....
The problem is that it is then tempting to use them in the middle of an M4 script, as opposed to its initialization. This, if not carefully thought 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 will be lost, as the current syntax will probably not be the one they were implemented with.
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When writing an Autoconf macro you may occasionally need to generate special characters that are difficult to express with the standard Autoconf quoting rules. For example, you may need to output the regular expression `[^[]', which matches any character other than `['. This expression contains unbalanced brackets so it cannot be put easily into an M4 macro.
You can work around this problem by using one of the following quadrigraphs:
Quadrigraphs are replaced at a late stage of the translation process,
after m4 is run, so they do not get in the way of M4 quoting.
For example, the string `^@<:@', independently of its quotation,
will appear as `^[' in the output.
The empty quadrigraph can be used:
Trailing spaces are smashed by autom4te. This is a feature.
For instance `@<@&t@:@' produces `@<:@'.
For instance you might want to mention AC_FOO in a comment, while
still being sure that autom4te will still catch unexpanded
`AC_*'. Then write `AC@&t@_FOO'.
The name `@&t@' was suggested by Paul Eggert:
I should give some credit to the `@&t@' pun. The `&' is my own invention, but the `t' came from the source code of the ALGOL68C compiler, written by Steve Bourne (of Bourne shell fame), and which used `mt' to denote the empty string. In C, it would have looked like something like:
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.
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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 will note that these two examples are rigorously equivalent, since M4 swallows both the `changequote(<<, >>)' and `<<' `>>' when it collects the arguments: these quotes are not part of the arguments!
Simplified, the example above is just doing this:
changequote(<<, >>)dnl <<[]>> changequote([, ])dnl |
instead of simply:
[[]] |
With macros that do not double quote their arguments (which is the rule), double-quote the (risky) literals:
AC_LINK_IFELSE([AC_LANG_PROGRAM(
[[#include <time.h>
#ifndef tzname /* For SGI. */
extern char *tzname[]; /* RS6000 and others reject char **tzname. */
#endif]],
[atoi (*tzname);])],
[ac_cv_var_tzname=yes],
[ac_cv_var_tzname=no])
|
See section 8.1.5 Quadrigraphs, for what to do if you run into a hopeless case where quoting does not suffice.
When you create a configure script using newly written macros,
examine it carefully to check whether you need to add more quotes in
your macros. If one or more words have disappeared in the M4
output, you need more quotes. When in doubt, quote.
However, it's also possible to put on too many layers of quotes. If
this happens, the resulting configure script will contain
unexpanded macros. The autoconf program checks for this problem
by doing `grep AC_ configure'.
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autom4te
The Autoconf suite, including M4sugar, M4sh, and Autotest, in addition
to Autoconf per se, heavily rely on M4. All these different uses
revealed common needs factored into a layer over m4:
autom4te(3).
autom4te should basically considered as a replacement of
m4 itself.
8.2.1 Invoking autom4teA GNU M4 wrapper 8.2.2 Customizing autom4teCustomizing the Autoconf package
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autom4te The command line arguments are modeled after M4's:
autom4te options files |
where the files are directly passed to m4. In addition
to the regular expansion, it handles the replacement of the quadrigraphs
(see section 8.1.5 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 will actually
behave as if you had run:
autom4te --warnings=syntax,$WARNINGS,category |
If you want to disable autom4te's defaults and
WARNINGS, but (for example) enable the warnings about obsolete
constructs, you would use `-W none,obsolete'.
autom4te displays a back trace for errors, but not for
warnings; if you want them, just pass `-W error'. For instance,
on this `configure.ac':
AC_DEFUN([INNER], [AC_RUN_IFELSE([AC_LANG_PROGRAM([exit (0)])])]) AC_DEFUN([OUTER], [INNER]) AC_INIT OUTER |
you get:
$ autom4te -l autoconf -Wcross configure.ac:8: warning: AC_RUN_IFELSE called without default \ to allow cross compiling $ autom4te -l autoconf -Wcross,error -f configure.ac:8: error: AC_RUN_IFELSE called without default \ to allow cross compiling acgeneral.m4:3044: AC_RUN_IFELSE is expanded from... configure.ac:2: INNER is expanded from... configure.ac:5: OUTER is expanded from... configure.ac:8: the top level |
file.m4f will be
replaced with file.m4. This helps tracing the macros which
are executed only when the files are frozen, typically
m4_define. For instance, running:
autom4te --melt 1.m4 2.m4f 3.m4 4.m4f input.m4 |
is roughly equivalent to running:
m4 1.m4 2.m4 3.m4 4.m4 input.m4 |
while
autom4te 1.m4 2.m4f 3.m4 4.m4f input.m4 |
is equivalent to:
m4 --reload-state=4.m4f input.m4 |
autom4te freezing is stricter
than M4's: it must produce no warnings, and no output other than empty
lines (a line with whitespace is not empty) and comments
(starting with `#'). Please, note that contrary to m4,
this options takes no argument:
autom4te 1.m4 2.m4 3.m4 --freeze --output=3.m4f |
corresponds to
m4 1.m4 2.m4 3.m4 --freeze-state=3.m4f |
As another additional feature over m4, autom4te
caches its results. GNU M4 is able to produce a regular
output and traces at the same time. Traces are heavily used in the
GNU Build System: autoheader uses them to build
`config.h.in', autoreconf to determine what
GNU Build System components are used, automake to
"parse" `configure.ac' etc. To save the long runs of
m4, traces are cached while performing regular expansion,
and conversely. This cache is (actually, the caches are) stored in
the directory `autom4te.cache'. It can safely be removed
at any moment (especially if for some reason autom4te
considers it is trashed).
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 section 3.4 Using autoconf to Create configure, for examples of trace uses.
autoconf preselects all the macros that
autoheader, automake, autoreconf etc. will
trace, so that running m4 is not needed to trace them: the
cache suffices. This results in a huge speed-up.
Finally, autom4te introduces the concept of Autom4te
libraries. They consists in a powerful yet extremely simple feature:
sets of combined command line arguments:
M4sugar
M4sh
Autotest
Autoconf
As an example, if Autoconf is installed in its default location, `/usr/local', running `autom4te -l m4sugar foo.m4' is strictly equivalent to running `autom4te --prepend-include /usr/local/share/autoconf m4sugar/m4sugar.m4f --warnings syntax foo.m4'. Recursive expansion applies: running `autom4te -l m4sh foo.m4' is the same as `autom4te --language M4sugar m4sugar/m4sh.m4f foo.m4', i.e., `autom4te --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'.
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autom4te
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 section 8.2.1 Invoking autom4te) 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':
@verbatim ## ------------------ ## ## User Preferences. ## ## ------------------ ##
begin-language: "Autoconf" args: --no-cache end-language: "Autoconf"
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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".
8.3.1 Redefined M4 Macros M4 builtins changed in M4sugar 8.3.2 Evaluation Macros More quotation and evaluation control 8.3.3 Forbidden Patterns Catching unexpanded macros
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With a few exceptions, all the M4 native macros are moved in the
`m4_' pseudo-namespace, e.g., M4sugar renames define as
m4_define etc.
Some M4 macros are redefined, and are slightly incompatible with their native equivalent.
m4_dnl is defined.
m4_undefine.
m4exit.
ifelse.
m4_ifdef([macro], [m4_undefine([macro])]) |
to recover the behavior of the builtin.
patsubst. The name m4_patsubst
is kept for future versions of M4sh, on top of GNU M4 which will
provide extended regular expression syntax via epatsubst.
m4_undefine.
regexp. The name m4_regexp
is kept for future versions of M4sh, on top of GNU M4 which will
provide extended regular expression syntax via eregexp.
m4wrap.
You are encouraged to end text with `[]', so that there are
no risks that two consecutive invocations of m4_wrap result in an
unexpected pasting of tokens, as in
m4_define([foo], [Foo]) m4_define([bar], [Bar]) m4_define([foobar], [FOOBAR]) m4_wrap([bar]) m4_wrap([foo]) =>FOOBAR |
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The following macros give some control over the order of the evaluation by adding or removing levels of quotes. They are meant for hard-core M4 programmers.
The following example aims at emphasizing the difference between (i), not
using these macros, (ii), using m4_quote, and (iii), using
m4_dquote.
$ cat example.m4 # Overquote, so that quotes are visible. m4_define([show], [$[]1 = [$1], $[]@ = [$@]]) m4_divert(0)dnl show(a, b) show(m4_quote(a, b)) show(m4_dquote(a, b)) $ autom4te -l m4sugar example.m4 $1 = a, $@ = [a],[b] $1 = a,b, $@ = [a,b] $1 = [a],[b], $@ = [[a],[b]] |
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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$'.
Of course, you might encounter exceptions to these generic rules, for instance you might have to refer to `$m4_flags'.
m4_pattern_forbid pattern.
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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.
For the time being, it is not mature enough to be widely used.
M4sh provides portable alternatives for some common shell constructs that unfortunately are not portable in practice.
dirname command.
mkdir that
lack support for the `-p' option.
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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.
9.1 Macro Definitions Basic format of an Autoconf macro 9.2 Macro Names What to call your new macros 9.3 Reporting Messages Notifying autoconfusers9.4 Dependencies Between Macros What to do when macros depend on other macros 9.5 Obsoleting Macros Warning about old ways of doing things 9.6 Coding Style Writing Autoconf macros à la Autoconf
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Autoconf macros are defined using the AC_DEFUN macro, which is
similar to the M4 builtin m4_define macro. In addition to
defining a macro, AC_DEFUN adds to it some code that is used to
constrain the order in which macros are called (see section 9.4.1 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 section `How to define new macros' in GNU m4, for more complete information on writing M4 macros.
Be sure to properly quote both the macro-body and the macro-name to avoid any problems if the macro happens to have been previously defined.
Each macro should have a header comment that gives its prototype, and a brief description. When arguments have default values, display them in the prototype. For example:
# AC_MSG_ERROR(ERROR, [EXIT-STATUS = 1])
# --------------------------------------
m4_define([AC_MSG_ERROR],
[{ _AC_ECHO([configure: error: $1], 2); exit m4_default([$2], 1); }])
|
Comments about the macro should be left in the header comment. Most other comments will make their way into `configure', so just keep using `#' to introduce comments.
If you have some very special comments about pure M4 code, comments
that make no sense in `configure' and in the header comment, then
use the builtin dnl: it causes M4 to discard the text
through the next newline.
Keep in mind that dnl is rarely needed to introduce comments;
dnl is more useful to get rid of the newlines following macros
that produce no output, such as AC_REQUIRE.
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All of the Autoconf macros have all-uppercase names starting with `AC_' to prevent them from accidentally conflicting with other text. All shell variables that they use for internal purposes have mostly-lowercase names starting with `ac_'. To ensure that your macros don't conflict with present or future Autoconf macros, you should prefix your own macro names and any shell variables they use with some other sequence. Possibilities include your initials, or an abbreviation for the name of your organization or software package.
Most of the Autoconf macros' names follow a structured naming convention that indicates the kind of feature check by the name. The macro names consist of several words, separated by underscores, going from most general to most specific. The names of their cache variables use the same convention (see section 7.3.1 Cache Variable Names, for more information on them).
The first word of the name after `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.
C
DECL
FUNC
GROUP
HEADER
LIB
PATH
PROG
MEMBER
SYS
TYPE
VAR
After the category comes the name of the particular feature being
tested. Any further words in the macro name indicate particular aspects
of the feature. For example, AC_FUNC_UTIME_NULL checks the
behavior of the utime function when called with a NULL
pointer.
An internal macro should have a name that starts with an underscore;
Autoconf internals should therefore start with `_AC_'.
Additionally, a macro that is an internal subroutine of another macro
should have a name that starts with an underscore and the name of that
other macro, followed by one or more words saying what the internal
macro does. For example, AC_PATH_X has internal macros
_AC_PATH_X_XMKMF and _AC_PATH_X_DIRECT.
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When macros statically diagnose abnormal situations, benign or fatal,
they should report them using these macros. For dynamic issues, i.e.,
when configure is run, see 7.4 Printing Messages.
autoconf die.
When the user runs `autoconf -W error', warnings from
AC_DIAGNOSE and AC_WARNING are reported as error, see
3.4 Using autoconf to Create configure.
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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.
9.4.1 Prerequisite Macros Ensuring required information 9.4.2 Suggested Ordering Warning about possible ordering problems
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A macro that you write might need to use values that have previously
been computed by other macros. For example, AC_DECL_YYTEXT
examines the output of flex or lex, so it depends on
AC_PROG_LEX having been called first to set the shell variable
LEX.
Rather than forcing the user of the macros to keep track of the
dependencies between them, you can use the AC_REQUIRE macro to do
it automatically. AC_REQUIRE can ensure that a macro is only
called if it is needed, and only called once.
AC_DEFUN or else contain a call to AC_PROVIDE to indicate
that it has been called.
AC_REQUIRE must be used inside an AC_DEFUN'd macro; it
must not be called from the top level.
AC_REQUIRE is often misunderstood. It really implements
dependencies between macros in the sense that if one macro depends upon
another, the latter will be expanded before the body of the
former. In particular, `AC_REQUIRE(FOO)' is not replaced with the
body of FOO. For instance, this definition of macros:
AC_DEFUN([TRAVOLTA], [test "$body_temperature_in_celsius" -gt "38" && dance_floor=occupied]) AC_DEFUN([NEWTON_JOHN], [test "$hair_style" = "curly" && dance_floor=occupied]) AC_DEFUN([RESERVE_DANCE_FLOOR], [if date | grep '^Sat.*pm' >/dev/null 2>&1; then AC_REQUIRE([TRAVOLTA]) AC_REQUIRE([NEWTON_JOHN]) fi]) |
with this `configure.ac'
AC_INIT RESERVE_DANCE_FLOOR if test "$dance_floor" = occupied; then AC_MSG_ERROR([cannot pick up here, let's move]) fi |
will not leave you with a better chance to meet a kindred soul at other times than Saturday night since it expands into:
test "$body_temperature_in_Celsius" -gt "38" && dance_floor=occupied test "$hair_style" = "curly" && dance_floor=occupied fi if date | grep '^Sat.*pm' >/dev/null 2>&1; then fi |
This behavior was chosen on purpose: (i) it prevents messages in required macros from interrupting the messages in the requiring macros; (ii) it avoids bad surprises when shell conditionals are used, as in:
if ...; then AC_REQUIRE([SOME_CHECK]) fi ... SOME_CHECK |
You are encouraged to put all AC_REQUIREs at the beginning of a
macro. You can use dnl to avoid the empty lines they leave.
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Some macros should be run before another macro if both are called, but neither requires that the other be called. For example, a macro that changes the behavior of the C compiler should be called before any macros that run the C compiler. Many of these dependencies are noted in the documentation.
Autoconf provides the AC_BEFORE macro to warn users when macros
with this kind of dependency appear out of order in a
`configure.ac' file. The warning occurs when creating
configure from `configure.ac', not when running
configure.
For example, AC_PROG_CPP checks whether the C compiler
can run the C preprocessor when given the `-E' option. It should
therefore be called after any macros that change which C compiler is
being used, such as AC_PROG_CC. So AC_PROG_CC contains:
AC_BEFORE([$0], [AC_PROG_CPP])dnl |
This warns the user if a call to AC_PROG_CPP has already occurred
when AC_PROG_CC is called.
AC_BEFORE. The
macro called-macro-name must have been defined using
AC_DEFUN or else contain a call to AC_PROVIDE to indicate
that it has been called.
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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.
AC_DEFUN is that the user will be warned that
old-macro is now obsolete.
If she then uses autoupdate, the call to old-macro will be
replaced by the modern implementation. The additional
message is then printed.
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The Autoconf macros follow a strict coding style. You are encouraged to follow this style, especially if you intend to distribute your macro, either by contributing it to Autoconf itself, or via other means.
The first requirement is to pay great attention to the quotation. For more details, see 3.1.2 The Autoconf Language, and 8.1 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 section 9.2 Macro Names),
you are unlikely to generate conflicts. Nevertheless, when you need to
set a special value, avoid using a regular macro name; rather,
use an "impossible" name. For instance, up to version 2.13, the macro
AC_SUBST used to remember what symbols were already defined
by setting AC_SUBST_symbol, which is a regular macro name.
But since there is a macro named AC_SUBST_FILE, it was just
impossible to `AC_SUBST(FILE)'! In this case,
AC_SUBST(symbol) or _AC_SUBST(symbol) should
have been used (yes, with the parentheses)...or better yet, high-level
macros such as AC_EXPAND_ONCE.
No Autoconf macro should ever enter the user-variable name space; i.e.,
except for the variables that are the actual result of running the
macro, all shell variables should start with ac_. In
addition, small macros or any macro that is likely to be embedded in
other macros should be careful not to use obvious names.
Do not use dnl to introduce comments: most of the comments you
are likely to write are either header comments which are not output
anyway, or comments that should make their way into `configure'.
There are exceptional cases where you do want to comment special M4
constructs, in which case dnl is right, but keep in mind that it
is unlikely.
M4 ignores the leading spaces before each argument, use this feature to indent in such a way that arguments are (more or less) aligned with the opening parenthesis of the macro being called. For instance, instead of
AC_CACHE_CHECK(for EMX OS/2 environment, ac_cv_emxos2, [AC_COMPILE_IFELSE([AC_LANG_PROGRAM(, [return __EMX__;])], [ac_cv_emxos2=yes], [ac_cv_emxos2=no])]) |
write
AC_CACHE_CHECK([for EMX OS/2 environment], [ac_cv_emxos2],
[AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [return __EMX__;])],
[ac_cv_emxos2=yes],
[ac_cv_emxos2=no])])
|
or even
AC_CACHE_CHECK([for EMX OS/2 environment],
[ac_cv_emxos2],
[AC_COMPILE_IFELSE([AC_LANG_PROGRAM([],
[return __EMX__;])],
[ac_cv_emxos2=yes],
[ac_cv_emxos2=no])])
|
When using AC_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
purpose. With the s command, the preferred separator is `/'
unless `/' itself is used in the command, in which case you should
use `,'.
See section 9.1 Macro Definitions, for details on how to define a macro. If a
macro doesn't use AC_REQUIRE and it is expected to never be the
object of an AC_REQUIRE directive, then use m4_define. In
case of doubt, use AC_DEFUN. All the AC_REQUIRE
statements should be at the beginning of the macro, dnl'ed.
You should not rely on the number of arguments: instead of checking whether an argument is missing, test that it is not empty. It provides both a simpler and a more predictable interface to the user, and saves room for further arguments.
Unless the macro is short, try to leave the closing `])' at the
beginning of a line, followed by a comment that repeats the name of the
macro being defined. This introduces an additional newline in
configure; normally, that is not a problem, but if you want to
remove it you can use `[]dnl' on the last line. You can similarly
use `[]dnl' after a macro call to remove its newline. `[]dnl'
is recommended instead of `dnl' to ensure that M4 does not
interpret the `dnl' as being attached to the preceding text or
macro output. For example, instead of:
AC_DEFUN([AC_PATH_X], [AC_MSG_CHECKING([for X]) AC_REQUIRE_CPP() # ...omitted... AC_MSG_RESULT([libraries $x_libraries, headers $x_includes]) fi]) |
you would write:
AC_DEFUN([AC_PATH_X], [AC_REQUIRE_CPP()[]dnl AC_MSG_CHECKING([for X]) # ...omitted... AC_MSG_RESULT([libraries $x_libraries, headers $x_includes]) fi[]dnl ])# AC_PATH_X |
If the macro is long, try to split it into logical chunks. Typically,
macros that check for a bug in a function and prepare its
AC_LIBOBJ replacement should have an auxiliary macro to perform
this setup. Do not hesitate to introduce auxiliary macros to factor
your code.
In order to highlight the recommended coding style, here is a macro written the old way:
dnl Check for EMX on OS/2. dnl _AC_EMXOS2 AC_DEFUN(_AC_EMXOS2, [AC_CACHE_CHECK(for EMX OS/2 environment, ac_cv_emxos2, [AC_COMPILE_IFELSE([AC_LANG_PROGRAM(, return __EMX__;)], ac_cv_emxos2=yes, ac_cv_emxos2=no)]) test "$ac_cv_emxos2" = yes && EMXOS2=yes]) |
and the new way:
# _AC_EMXOS2
# ----------
# Check for EMX on OS/2.
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 "$ac_cv_emxos2" = yes && EMXOS2=yes[]dnl
])# _AC_EMXOS2
|
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When writing your own checks, there are some shell-script programming techniques you should avoid in order to make your code portable. The Bourne shell and upward-compatible shells like the Korn shell and Bash have evolved over the years, but to prevent trouble, do not take advantage of features that were added after UNIX version 7, circa 1977 (see section 6.7 Systemology).
You should not use shell functions, aliases, negated character
classes, or other features that are not found in all Bourne-compatible
shells; restrict yourself to the lowest common denominator. Even
unset is not supported by all shells! Also, include a space
after the exclamation point in interpreter specifications, like this:
#! /usr/bin/perl |
If you omit the space before the path, then 4.2BSD based systems (such as DYNIX) will ignore the line, because they interpret `#! /' as a 4-byte magic number. Some old systems have quite small limits on the length of the `#!' line too, for instance 32 bytes (not including the newline) on SunOS 4.
The set of external programs you should run in a configure script
is fairly small. See section `Utilities in Makefiles' in GNU Coding Standards, for the list. This
restriction allows users to start out with a fairly small set of
programs and build the rest, avoiding too many interdependencies between
packages.
Some of these external utilities have a portable subset of features; see 10.9 Limitations of Usual Tools.
There are other sources of documentation about shells. See for instance the Shell FAQs.
10.1 Shellology A zoology of shells 10.2 Here-Documents Quirks and tricks 10.3 File Descriptors FDs and redirections 10.4 File System Conventions File- and pathnames 10.5 Shell Substitutions Variable and command expansions 10.6 Assignments Varying side effects of assignments 10.7 Special Shell Variables Variables you should not change 10.8 Limitations of Shell Builtins Portable use of not so portable /bin/sh 10.9 Limitations of Usual Tools Portable use of portable tools 10.10 Limitations of Make Portable Makefiles
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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 \title\{http://www.faqs.org/faqs/unix-faq/shell/shell-differences/, the Shell difference FAQ} includes a small history of Unix shells, and a comparison between several of them.
Below we describe some of the members of the Bourne shell family.
ash is often used on GNU/Linux and BSD
systems as a light-weight Bourne-compatible shell. Ash 0.2 has some
bugs that are fixed in the 0.3.x series, but portable shell scripts
should work around them, since version 0.2 is still shipped with many
GNU/Linux distributions.
To be compatible with Ash 0.2:
foo= false $foo echo "Don't use it: $?" |
cat ${FOO=`bar`}
|
bash, test if
BASH_VERSION is set. To disable its extensions and require
POSIX compatibility, run `set -o posix'. See section `Bash POSIX Mode' in The GNU Bash Reference Manual, for details.
bash use a different format for the
output of the set builtin, designed to make evaluating its
output easier. However, this output is not compatible with earlier
versions of bash (or with many other shells, probably). So if
you use bash 2.05 or higher to execute configure,
you'll need to use bash 2.05 for all other build tasks as well.
/usr/xpg4/bin/sh on Solaris
/usr/xpg4/bin/sh and is part of an extra optional package.
There is no extra charge for this package, but it is also not part of a
minimal OS install and therefore some folks may not have it.
zsh, test if
ZSH_VERSION is set. By default zsh is not
compatible with the Bourne shell: you have to run `emulate sh' and
set NULLCMD to `:'. See section `Compatibility' in The Z Shell Manual, for details.
Zsh 3.0.8 is the native /bin/sh on Mac OS X 10.0.3.
The following discussion between Russ Allbery and Robert Lipe is worth reading:
Russ Allbery:
The GNU assumption that/bin/shis 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/shprograms 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.
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Don't rely on `\' being preserved just because it has no special
meaning together with the next symbol. In the native /bin/sh
on OpenBSD 2.7 `\"' expands to `"' in here-documents with
unquoted delimiter. As a general rule, if `\\' expands to `\'