On Thu, Apr 26, 2012 at 09:15:09AM +0200, Michael Olbrich wrote: > On Wed, Apr 25, 2012 at 10:34:18AM -0600, Eric Blake wrote: > > On 04/25/2012 02:56 AM, Michael Olbrich wrote: > > > when cross-compiling with gcc >= 4.5 (I think), AC_CHECK_ALIGNOF fails to > > > detect the alignment. This happens because the compiler fails with "error: > > > storage size of ‘test_array’ isn’t constant" instead of succeeding. > > > > Thanks for the report. Alas, you just missed the release autoconf 2.69, > > so if this turns out to be an autoconf bug, you have the honor of the > > first bug report on the new release (even though the problem itself is > > older). > > > > > Using __builtin_offsetof helps. Patch attached. I'm not quite sure when > > > __builtin_offsetof should be used, but it is used in the Linux kernel for > > > gcc 4.x, so I think this is correct. > > > > Thanks for the attempted patch. However, the real bug is that we are > > trying to use our own offsetof, instead of relying on the system headers > > to do the heavy lifting on our behalf. That is, <stddef.h> should > > already by using __builtin_offsetof, and we should be turning to > > <stddef.h> rather than reinventing the work in autoconf. > > > > > +++ b/lib/autoconf/types.m4 > > > @@ -800,8 +800,12 @@ m4_define([_AC_CHECK_ALIGNOF], > > > _AC_CACHE_CHECK_INT([alignment of $1], [AS_TR_SH([ac_cv_alignof_$3])], > > > [(long int) offsetof (ac__type_alignof_, y)], > > > [AC_INCLUDES_DEFAULT([$2]) > > > > Normally, AC_INCLUDES_DEFAULT should already be including <stddef.h>; > > the only time this is not true is if $2 is non-empty (you were providing > > specific headers to include instead of relying on the defaults). And > > once <stddef.h> is in the picture, > > While AC_INCLUDES_DEFAULT is in $2 this put me on the right track. > The problem ist that STDC_HEADERS is not defined. > > > > -#ifndef offsetof > > > -# define offsetof(type, member) ((char *) &((type *) 0)->member - (char > > > *) 0) > > > > offsetof should already be defined for gcc compilers, at which point, > > this fallback is good enough for non-compliant platforms with older > > compilers that don't warn like gcc does. > > > > So the real question is: how did you trigger this failure in the first > > place, and could it be a bug in your configure.ac that resulted in the > > $2 argument passed to _AC_CHECK_ALIGNOF overriding the > > AC_INCLUDES_DEFAULT in such a manner that <stddef.h> is not being included? > > > > Can you post both a configure.ac that reproduces the problem, and the > > corresponding config.log that shows how what program was actually being > > compiled when you hit the gcc warning? > > http://git.gnome.org/browse/glib/tree/configure.ac#n3451 > > From what I can tell, the code needed to define STDC_HEADERS is generated > for the first AC_CHECK_HEADER. In this case for zlib.h: > http://git.gnome.org/browse/glib/tree/configure.ac#n466 > > However this is only executed for 'test "x$found_zlib" = "xno"', so > STDC_HEADERS is never defined. If I put an unconditional AC_CHECK_HEADER > before the zlib stuff, then STDC_HEADERS is defined and AC_CHECK_ALIGNOF > works as expected.
I've attached a minimalistic configure.ac that demonstrated the problem. That should be easier than the one from glib. Regards, Michael Olbrich -- Pengutronix e.K. | | Industrial Linux Solutions | http://www.pengutronix.de/ | Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-0 | Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 |
AC_PREREQ([2.68])
AC_INIT([foo],[1],[m.olbr...@pengutronix.de])
AC_CONFIG_SRCDIR([configure.ac])
AC_PROG_CC
if false; then
AC_CHECK_HEADER(stdint.h)
fi
AC_CHECK_TYPE([unsigned long])
AC_CHECK_ALIGNOF([unsigned long])
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