From: Alex Colomar <alx.manpages@gmail.com>
To: Richard Biener <richard.guenther@gmail.com>
Cc: "Stefan Puiu" <stefan.puiu@gmail.com>,
"GNU C Library" <libc-alpha@sourceware.org>,
linux-man <linux-man@vger.kernel.org>,
gcc@gcc.gnu.org, "Igor Sysoev" <igor@sysoev.ru>,
"Bastien Roucariès" <rouca@debian.org>
Subject: Re: struct sockaddr_storage
Date: Tue, 24 Jan 2023 17:38:35 +0100 [thread overview]
Message-ID: <5dbf06be-e083-4b39-343d-2507802b9881@gmail.com> (raw)
In-Reply-To: <CAFiYyc29oHbDqP8fdrT8cSr0Vnunu24sGQuQb65vK0P37rfnmg@mail.gmail.com>
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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/>
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next prev parent reply other threads:[~2023-01-24 16:38 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-19 14:11 Alejandro Colomar
2023-01-20 10:06 ` Stefan Puiu
2023-01-20 12:39 ` Alejandro Colomar
2023-01-23 7:40 ` Stefan Puiu
2023-01-23 16:03 ` Alejandro Colomar
2023-01-23 16:28 ` Richard Biener
2023-01-24 16:38 ` Alex Colomar [this message]
2023-01-23 16:37 ` Jakub Jelinek
2023-01-24 16:40 ` Alex Colomar
2023-01-24 18:00 ` Alex Colomar
2023-01-24 11:16 ` Rich Felker
2023-01-24 16:53 ` Alex Colomar
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