[Bug c/51628] __attribute__((packed)) is unsafe in some cases

["rguenther at suse dot de" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51628

--- Comment #6 from rguenther at suse dot de <rguenther at suse dot de> 2011-12-20 11:04:37 UTC ---
On Tue, 20 Dec 2011, ebotcazou at gcc dot gnu.org wrote:

> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51628
> 
> --- Comment #5 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2011-12-20 10:52:19 UTC ---
> > The point is that even if you use sth like
> > 
> > typedef int myint __attribute__((aligned(1)));
> > 
> > to capture the misaligned pointer to the packed structure element:
> > 
> > myint *p = &s->i;
> > 
> > then accesses like '*p' will still assume an _aligned_ int at 'p' for
> > STRICT_ALIGNMENT targets.
> > 
> > That's a long-long-long-standing bug and a cause of major headache for
> > more modern GCCs even ...
> 
> That's a limitation rather than a bug.  Clearly, on strict-alignment targets,
> you must know what you're doing when you start to misalign things.  As for
> 
>  typedef int myint __attribute__((aligned(1)));
> 
> that's an abomination I don't even want to know of ;-)

Huh, it's not.  It's the same as a packed struct or enum type.  Why
can't you strict-align people simply fix this case?

Richard.