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

[ebotcazou at gcc dot gnu.org]

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 ;-)