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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], [1], [Define if vprintf exists.])]) if test "x$ac_cv_func_vprintf" != xyes; then AC_CHECK_FUNC([_doprnt], [AC_DEFINE([HAVE_DOPRNT], [1], [Define if _doprnt exists.])]) fi
If AC_CONFIG_HEADERS
has been called, then instead of creating
DEFS
, AC_OUTPUT
creates a header file by substituting the
correct values into #define
statements in a template file.
See Configuration Headers, for more information about this kind of
output.
Define variable to value (verbatim), by defining a C preprocessor macro for variable. variable should be a C identifier, optionally suffixed by a parenthesized argument list to define a C preprocessor macro with arguments. The macro argument list, if present, should be a comma-separated list of C identifiers, possibly terminated by an ellipsis ‘...’ if C99-or-later syntax is employed. variable should not contain comments, white space, trigraphs, backslash-newlines, universal character names, or non-ASCII characters.
value may contain backslash-escaped newlines, which will be
preserved if you use AC_CONFIG_HEADERS
but flattened if passed
via @DEFS@
(with no effect on the compilation, since the
preprocessor sees only one line in the first place). value should
not contain raw newlines. If you are not using
AC_CONFIG_HEADERS
, value should not contain any ‘#’
characters, as make
tends to eat them. To use a shell
variable, use AC_DEFINE_UNQUOTED
instead.
description is only useful if you are using
AC_CONFIG_HEADERS
. In this case, description is put into
the generated config.h.in as the comment before the macro define.
The following example defines the C preprocessor variable
EQUATION
to be the string constant ‘"$a > $b"’:
AC_DEFINE([EQUATION], ["$a > $b"], [Equation string.])
If neither value nor description are given, then value defaults to 1 instead of to the empty string. This is for backwards compatibility with older versions of Autoconf, but this usage is obsolescent and may be withdrawn in future versions of Autoconf.
If the variable is a literal string, it is passed to
m4_pattern_allow
(see Forbidden Patterns).
If multiple AC_DEFINE
statements are executed for the same
variable name (not counting any parenthesized argument list),
the last one wins.
Like AC_DEFINE
, but three shell expansions are
performed—once—on variable and value: variable expansion
(‘$’), 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 oddity 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"])
Next: Setting Output Variables, Up: Results [Contents][Index]