On Wed, 13 Sep 2017, Bernd Edlinger wrote:
> On 09/13/17 19:06, Joseph Myers wrote:
> > What does this warning do in cases where a type has different alignments
> > inside and outside structs? I'm thinking of something like
> >
> > struct s { long long x; } *p;
> > /* ... */
> > (long long *)p
> >
> > on 32-bit x86 - where long long's preferred alignment is 8 bytes, but in
> > structures it's 4 bytes. (Likewise for double in place of long long.) I
> > think a warning for a (long long *)p cast might be surprising in that
> > case.
> >
>
> Well, yes this does get a warning. But doesn't that cast then violate
> the underlying alignment requirement of long long* ?
That's the difference between preferred alignment (__alignof__) and
alignment required in all contexts (C11 _Alignof). The above seems valid,
just like it's valid to take the address of a long long struct element.
That is, the alignment for the target of a pointer to long long is really
4 bytes here, even though the alignment for a standalone long long object
is 8 bytes. And there's a case for the warning to look at the required
alignment in all contexts, not TYPE_ALIGN.
--
Joseph S. Myers
jos...@codesourcery.com
