Union initialization semantics

Union initialization semantics

[Alexander Monakov]

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.

If someone knows what semantics GCC implements, that also would be welcome.

Thank you.
Alexander

Re: Union initialization semantics

[Jonathan Wakely]

On Wed, 19 Jun 2024 at 11:57, Alexander Monakov <amonakov@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.

Re: Union initialization semantics

[Martin Uecker]

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 <amonakov@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