Re: C ABI: struct/union inconsistency

[Mark Mitchell <mark at codesourcery dot com>]

--On Friday, September 27, 2002 10:06:16 AM -0700 Richard Henderson 
<rth@redhat.com> wrote:

> On Wed, Sep 25, 2002 at 04:58:11PM -0700, Mark Mitchell wrote:
> > I also do not know if some piece of code in the Linux
> > kernel is going to depend on the current behavior; we might want to
> > have a switch for the old behavior.
> I think that we should leave structures with no members whatsoever
> alone.  I think that as shown by the linux kernel code, this is a
> useful feature.
We should at least document this behavior.  It is, I believe, different
from what any other compiler does.

I must say that I'm concerned about the fact that things like:

 struct S s[3];

 ... sizeof (s) / sizeof (S) ...

won't work, and that s[0] and s[1] have the same address.

At least in C++, that's a major violation of the object model; the
standard basically defines object identity as the pair of the object's
type and the object's address.  (I don't know if the C standard really
defines this concept.)

Finally, the C++ ABI requires that PODs have the same layout as the
equivalent C structures, but, on the other hand, the C++ standard
requires that empty structures have size one.

That means we lose C/C++ compatibility even for a type which is in
the common subset of both languages.

Perhaps we should offer a switch and/or attribute to control this
behavior?

> I don't care what we do with structures with no named members, but
> some unnamed members.  As Joseph points out, this is undefined in C.
> We might as well match whatever the C++ language requires, simply
> for consistency.
That sounds, then, like an agreement to change the alignment of the
union example to match the struct example?

--
Mark Mitchell                mark@codesourcery.com
CodeSourcery, LLC            http://www.codesourcery.com