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7.1 Defining C Preprocessor Symbols

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

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

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

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

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

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

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

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 This is for backwards compatibility with older versions of Autoconf, but this usage is obsolescent and may be withdrawn in future versions of Autoconf.

— 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 (`\'). 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"])