Re: AC_CHECK_ALIGNOF cross-compile detection fails with newer gcc and offsetof fallback

[Eric Blake]

On 09/06/2012 01:00 PM, Mike Frysinger wrote:
> if a package doesn't happen to include stddef.h before trying to do
> AC_CHECK_ALIGNOF, the current cross-compile fallback fails with newer
> gcc.
> 
> for example, see the attached generated conftest.c (taken from recent
> glib versions) where it does:
> AC_CHECK_TYPE([guint32],,,[typedef unsigned $gint32 guint32;])
> AC_CHECK_ALIGNOF([guint32], [AC_INCLUDES_DEFAULT
> typedef unsigned $gint32 guint32;])
> 
> compiling this gcc-4.5+ fails (checked 4.4.7, 4.5.3, 4.6.3, and 4.7.1):
> $ gcc -O2 conftest.c
> conftest.c: In function ‘main’:
> conftest.c:216:12: error: storage size of ‘test_array’ isn’t constant

Thanks for the analysis.

> 
> if, however, we happen to pull in the stddef.h from gcc which now
> includes an offsetof definition itself (which uses a builtin),
> everything works nicely:
> $ gcc -O2 conftest.c -DSTDC_HEADERS
> 
> perhaps one fix would be for AC_CHECK_ALIGNOF to require AC_STDC_HEADERS ?

Yes, that sounds reasonable.  Would you like to take the first shot at
writing the patch?

-- 
Eric Blake   address@hidden    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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