Defining C Preprocessor Symbols

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

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

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

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

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

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

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

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

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

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

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

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

or this:

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

instead of this:

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

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