Re: drivers/block/ub.c

["David S. Miller" <davem@redhat.com>]

On Mon, 28 Jun 2004 17:18:57 -0400
Scott Wood <scott@timesys.com> wrote:

> On Mon, Jun 28, 2004 at 02:03:43PM -0700, David S. Miller wrote:
> > You have not considered what is supposed to happen when this
> > structure is embedded within another one.  What kind of alignment
> > rules apply in that case?  For example:
> > 
> > struct foo {
> > 	u32	x;
> > 	u8	y;
> > 	u16	z;
> > } __attribute__((__packed__));
> > 
> > struct bar {
> > 	u8		a;
> > 	struct foo 	b;
> > };
> 
> As long as bar is not packed, why shouldn't the beginning of bar.b be
> aligned?

No!  bar.b starts at offset 1 byte.  That's how this stuff works.

This is exactly why you cannot assume the alignment of any structure
which is given attribute __packed__.  The example above shows that
quite clearly.

Try it out if you don't believe someone who has maintained the
Linux networking for 7 years and sits on the GCC Steering Committee.
:-)