Am Mittwoch, dem 19.06.2024 um 13:59 +0100 schrieb Jonathan Wakely via Gcc:
> On Wed, 19 Jun 2024 at 11:57, Alexander Monakov <amona...@ispras.ru> wrote:
> >
> > Hello,
> >
> > I vaguely remember there was a recent, maybe within last two months,
> > discussion
> > about semantics of union initialization where sizeof(first member) is less
> > than
> > sizeof(union). The question was whether it's okay to initialize just that
> > first
> > member and leave garbage bits in the other, larger, members of the union,
> > like
> > in this example:
> >
> > union A {
> > char a;
> > long : 0;
> > };
> >
> > void fn(void *);
> >
> > void my(void)
> > {
> > union A a = { 0 };
> > fn(&a);
> > }
> >
> > (except in my example there's no other named member, but I think the example
> > in that discussion was less contrived)
> >
> > Perhaps somebody remembers where it was (I'm thinking Bugzilla) and could
> > point
> > me to it? My attempts to search for it aren't turning anything up so far.
>
> Somebody asked about this internally at Red Hat recently, and I
> responded with this quote from C17 6.2.6.1 p7:
> "When a value is stored in a member of an object of union type, the
> bytes of the object representation that do not correspond to that
> member but do correspond to other members take unspecified values. "
>
> This looks related too:
> https://discourse.llvm.org/t/union-initialization-and-aliasing-clang-18-seems-to-miscompile-musl/77724/3
> They don't seem to have found the quote above though.
>
> I think it got reported to GCC's bugzilla too, I'll see if I can find it
> again.
>
> > If someone knows what semantics GCC implements, that also would be welcome.
>
> GCC seems to initialize the trailing bits, unnecessarily.
Note that C23 will require the padding bits to be initialized with zero
for default initialization {}.
Martin
