9.3 Defining C Preprocessor Symbols

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

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

AC_CHECK_FUNC([vprintf], [AC_DEFINE([HAVE_VPRINTF], [1],
                          [Define if vprintf exists.])])
if test "x$ac_cv_func_vprintf" != xyes; then
  AC_CHECK_FUNC([_doprnt], [AC_DEFINE([HAVE_DOPRNT], [1],
                            [Define if _doprnt exists.])])
fi

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

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

Macro: AC_DEFINE (variable)

Define variable to value (verbatim), by defining a C preprocessor macro for variable. variable should be a C identifier, optionally suffixed by a parenthesized argument list to define a C preprocessor macro with arguments. The macro argument list, if present, should be a comma-separated list of C identifiers, possibly terminated by an ellipsis ‘...’ if C99-or-later syntax is employed. variable should not contain comments, white space, trigraphs, backslash-newlines, universal character names, or non-ASCII characters.

value may contain backslash-escaped newlines, which will be preserved if you use AC_CONFIG_HEADERS but flattened if passed via @DEFS@ (with no effect on the compilation, since the preprocessor sees only one line in the first place). value should not contain raw newlines. If you are not using AC_CONFIG_HEADERS, value should not contain any ‘#’ characters, as make tends to eat them. To use a shell variable, 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"],
  [Equation string.])

If neither value nor description are given, then value defaults to 1 instead of to the empty string. This is for backwards compatibility with older versions of Autoconf, but this usage is obsolescent and may be withdrawn in future versions of Autoconf.

If the variable is a literal string, it is passed to m4_pattern_allow (see Forbidden Patterns).

If multiple AC_DEFINE statements are executed for the same variable name (not counting any parenthesized argument list), the last one wins.

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

Macro: AC_DEFINE_UNQUOTED (variable)

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

AC_DEFINE_UNQUOTED([config_machfile], ["$machfile"],
  [Configuration machine file.])
AC_DEFINE_UNQUOTED([GETGROUPS_T], [$ac_cv_type_getgroups],
  [getgroups return type.])
AC_DEFINE_UNQUOTED([$ac_tr_hdr], [1],
  [Translated header name.])

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

AC_CHECK_HEADER([elf.h],
  [AC_DEFINE([SVR4], [1], [System V Release 4]) LIBS="-lelf $LIBS"])

or this:

AC_CHECK_HEADER([elf.h],
  [AC_DEFINE([SVR4], [1], [System V Release 4])
   LIBS="-lelf $LIBS"])

instead of this:

AC_CHECK_HEADER([elf.h],
  [AC_DEFINE([SVR4], [1], [System V Release 4]); LIBS="-lelf $LIBS"])

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

The package should ‘#include’ the configuration header file before any other header files, to prevent inconsistencies in declarations (for example, if it redefines const).

To provide for VPATH builds, remember to pass the C compiler a -I. option (or -I..; whichever directory contains config.h). Even if you use ‘#include "config.h"’, the preprocessor searches only the directory of the currently read file, i.e., the source directory, not the build directory.

With the appropriate -I option, you can use ‘#include <config.h>’. Actually, it’s a good habit to use it, because in the rare case when the source directory contains another config.h, the build directory should be searched first.

Macro: AC_CONFIG_HEADERS (header …, [cmds], [init-cmds])

This macro is one of the instantiating macros; see Configuration Actions. Make AC_OUTPUT create the file(s) in the blank-or-newline-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. For example, you might need to make the input file name acceptable to DOS variants:

AC_CONFIG_HEADERS([config.h:config.hin])

Macro: AH_HEADER

This macro is defined as the name of the first declared config header and undefined if no config headers have been declared up to this point. A third-party macro may, for example, require use of a config header without invoking AC_CONFIG_HEADERS twice, like this:

AC_CONFIG_COMMANDS_PRE(
        [m4_ifndef([AH_HEADER], [AC_CONFIG_HEADERS([config.h])])])

See Configuration Actions for more details on header.