Re: [PATCH] test-stddef: test offsetof compliance

[Eric Blake]

On 08/16/2010 02:21 PM, Eric Blake wrote:
> * tests/test-stddef.c: Add tests for offsetof.
> 
> Signed-off-by: Eric Blake <address@hidden>
> ---
> 
> I was about to push this, so we can detect any gotchas with using
> offsetof according to POSIX.  Then I decided to test first.
> 
> And the first loser is Solaris' /usr/bin/cc:
> "../../gltests/test-stddef.c", line 44: syntax error before or at: &
> "../../gltests/test-stddef.c", line 44: warning: syntax requires ";" after 
> last struct/union member
> "../../gltests/test-stddef.c", line 44: zero-sized struct/union
> "../../gltests/test-stddef.c", line 44: cannot recover from previous errors
> cc: acomp failed for ../../gltests/test-stddef.c
> 
> Adding "redundant" parens to the sizeof(offsetof()) call fixed the
> it for that compiler.  Sure enough, /usr/include/iso/stddef_iso.h has:
> 
> #define offsetof(s, m)  (std::size_t)(&(((s *)0)->m))

The problem is how do we provide a working offsetof replacement, given
that there is no way using standard C89 or C++ constructs to do it?  All
implementations either use something like this which abuses undefined
behavior within the standard but works without warning for that compiler:
#define offsetof(__a,__b) ((size_t)(&(((__a*)0)->__b)))

or do something like gcc in providing a hidden builtin that does the
right thing:
#define offsetof(__a,__b) __builtin_offsetof(__a,__b)

Maybe the thing is to do a case-by-case implementation that provides
implementations that work with the compilers we know about, but issues
#error for remaining compilers; the fix should be to use whatever the
platform's <stddef.h> does.

For that matter, we could compute the correct offsetof expression at
configure time, for any given compiler, by pre-processing a dummy file
and running that output through the Makefile.am sed snippet that we
stick into our replacement stddef.h.  I guess I'll play with that approach.

-- 
Eric Blake   address@hidden    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

signature.asc
Description: OpenPGP digital signature