Re: AC_CHECK_ALIGNOF fails when cross-compiling with gcc >= 4.5

[Michael Olbrich]

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.

Michael

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