Hi Richard,

On 1/23/23 17:28, Richard Biener wrote:
>> The common initial sequence of structures is only allowed if the structures form
>> part of a union (which is why to avoid UB you need a union; and still, you need
>> to make sure you don't invoke UB in a different way).
>> <https://port70.net/%7Ensz/c/c11/n1570.html#6.5.2.3p6>
> 
> GCC only allows it if the union is visible as part of the access, that
> is, it allows it
> under its rule of allowing punning for union accesses and not specially because
> of the initial sequence rule.  So
> 
>   u.a.x = 1;
>   ... = u.b.x;
> 
> is allowed but
> 
>    struct A *p = &u.a;
>    p->x = 1;
>    struct B *q = &u.b;
>    ... = q->x;
> 
> is UB with GCC if struct A and B are the union members with a common
> initial sequence.

Yep.  That's why we need a union that is defined in libc, so that it can 
be used both in and out of glibc.  sockaddr_storage can be reconverted 
to that purpose.

Cheers,

Alex

-- 
<http://www.alejandro-colomar.es/>