Eric Blake wrote:
> Rather than hard-code the check based on platform-specific defines, it
> would be more in the autoconf spirit to instead write a probe whether
> <stdlib.h> declares alloca(), and if so, define a witness macro.
Unfortunately, the code is also used in bison/data/yacc.c [1], that is, in the
skeleton that bison uses for a certain class of parsers. Therefore we have
to continue to find a code template that does not use information from a
configure script.
And the code is also in the Autoconf manual.
Thomas Klausner wrote:
> Do not fall through to the implicit prototype as it conflicts with
> stdlib.h.
More details:
- The declaration in the code snippet from Autoconf uses 'char *alloca();'.
- On FreeBSD, the declaration of alloca is only conditionally visible,
namely if __BSD_VISIBLE. Its return type is 'void *', and the declaration
is present since 1994.
- On OpenBSD, the declaration of alloca is only conditionally visible,
namely if __BSD_VISIBLE since 2005; another condition was used before.
Its return type is 'void *', and the declaration is present since 1995.
- On NetBSD, the declaration of alloca is only conditionally visible,
namely if _NETBSD_SOURCE. Its return type is usually 'void *'.
- On all three systems, the double-inclusion guard of <stdlib.h> is
_STDLIB_H_.
So, I don't understand how you see a "conflict". The test code in
AC_FUNC_ALLOCA does not include <stdlib.h>, only <stddef.h>; how can that
clash? And after configure has run, the Autoconf manual recommends to
declare alloca as follows:
#ifdef HAVE_ALLOCA_H
# include <alloca.h>
#elif defined __GNUC__
# define alloca __builtin_alloca
#elif defined _AIX
# define alloca __alloca
#elif defined _MSC_VER
# include <malloc.h>
# define alloca _alloca
#else
# include <stddef.h>
# ifdef __cplusplus
extern "C"
# endif
void *alloca (size_t);
#endif
This also cannot conflict with <stdlib.h>.
But seeing that no platform nowadays assumes a return type of 'char *' for
alloca, it would be possible to change the prototype anyway:
2010-08-05 Bruno Haible <br...@clisp.org>
Modernize AC_FUNC_ALLOCA.
* lib/autoconf/functions.m4 (AC_FUNC_ALLOCA): Assume that alloca's
return type is 'void *', not 'char *'.
--- autoconf/lib/autoconf/functions.m4.orig Thu Aug 5 13:19:01 2010
+++ autoconf/lib/autoconf/functions.m4 Thu Aug 5 13:16:25 2010
@@ -388,7 +388,7 @@
#pragma alloca
# else
# ifndef alloca /* predefined by HP cc +Olibcalls */
-char *alloca ();
+void *alloca ();
# endif
# endif
# endif
Bruno
[1] http://git.savannah.gnu.org/gitweb/?p=bison.git;a=blob;f=data/yacc.c;hb=HEAD
