Hi Eric, On 4/14/23 18:08, Zack Weinberg via Libc-alpha wrote: > On 2023-04-06 3:37 PM, Eric Blake wrote: >> Since Issue 7 is tied to C99, and Issue 8 will be tied to C17, both of >> which use the same section number despite being a different edition of >> the C standard, being that specific may work. Or, we might try >> something focusing more on wording instead of document location, as >> in: >> >> Additionally, the structures shall be defined in such a way that the >> compiler treats an access to the stored value of the sa_family_t >> member of any of these structures, via an lvalue expression whose type >> involves any other one of these structures, as permissible even if the >> types involved would not otherwise be deemed compatible with the >> effective type of the object ultimately being accessed.
The wording I see in <https://austingroupbugs.net/view.php?id=1641#c6262> doesn't seem to cover the case of aliasing a sockaddr_storage as a protocol-specific address for setting other members. Aliasing rules don't allow one to declare an object of type sockaddr_storage and then fill the structure as if it were another structure, even if alignment and size are correct. We would need some wording that says something like: When a pointer to a sockaddr_storage structure is first aliased as a pointer to a protocol-specific address structure, the effective type of the object will be set to the protocol-specific structure. This is similar to what happens when malloc(3) is assigned to a non-character type. That's a big hammer, but it does the job. Maybe we would need some looser language? I CCd GCC, in case they have concerns about this wording. Cheers, Alex > > I quite like this way of putting it. It subsumes both what I wrote and > the related potential headache with deciding whether the sa_family_t > field is considered an object or just a range of bytes within a larger > object. > > zw -- <http://www.alejandro-colomar.es/> GPG key fingerprint: A9348594CE31283A826FBDD8D57633D441E25BB5
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