From: Joseph Myers <joseph@codesourcery.com>
To: Martin Sebor <msebor@gmail.com>
Cc: Bernd Schmidt <bernds_cb1@t-online.de>,
Bernd Schmidt <bschmidt@redhat.com>,
Gcc Patch List <gcc-patches@gcc.gnu.org>
Subject: Re: [PATCH] c/67882 - improve -Warray-bounds for invalid offsetof
Date: Tue, 20 Oct 2015 16:54:00 -0000 [thread overview]
Message-ID: <alpine.DEB.2.10.1510201650350.7944@digraph.polyomino.org.uk> (raw)
In-Reply-To: <56265E51.4070009@gmail.com>
On Tue, 20 Oct 2015, Martin Sebor wrote:
> I think -Warray-bounds should emit consistent diagnostics for invalid
> array references regardless of the contexts. I.e., given
>
> struct S {
> int A [5][7];
> int x;
> } s;
>
> these should both be diagnosed:
>
> int i = offsetof (struct S, A [0][7]);
>
> int *p = &s.A [0][7];
>
> because they are both undefined and both can lead to surprising
> results when used.
But both are valid. &s.A [0][7] means s.A[0] + 7 (as explicitly specified
in C11, neither the & nor the [] is evaluated in this case, but the []
turns into a +), and s.A[0] is an object of type int[7], which decays to a
pointer to the first element of that array, so adding 7 produces a
just-past-end pointer. It's not valid to dereference that pointer, but
the pointer itself is valid (and subtracting 1 from it produces a pointer
you can dereference).
--
Joseph S. Myers
joseph@codesourcery.com
next prev parent reply other threads:[~2015-10-20 16:53 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-10-09 2:55 Martin Sebor
2015-10-15 21:59 ` [PING] " Martin Sebor
2015-10-16 12:28 ` Bernd Schmidt
2015-10-16 17:27 ` Joseph Myers
2015-10-16 19:34 ` Martin Sebor
2015-10-20 13:21 ` Bernd Schmidt
2015-10-20 15:33 ` Martin Sebor
2015-10-20 15:52 ` Bernd Schmidt
2015-10-20 16:57 ` Martin Sebor
2015-10-20 17:11 ` Joseph Myers
2015-10-20 19:10 ` Martin Sebor
2015-10-20 16:54 ` Joseph Myers [this message]
2015-10-20 20:36 ` Bernd Schmidt
2015-10-20 22:19 ` Joseph Myers
2015-10-23 11:17 ` Bernd Schmidt
2015-10-23 15:15 ` Martin Sebor
2015-10-23 16:53 ` Joseph Myers
2015-10-23 17:45 ` Bernd Schmidt
2015-10-23 20:54 ` Martin Sebor
2015-10-26 11:44 ` Bernd Schmidt
2015-10-26 11:51 ` Jakub Jelinek
2015-10-26 12:01 ` Bernd Schmidt
2015-10-26 12:04 ` Jakub Jelinek
2015-10-26 12:32 ` Richard Biener
2015-10-27 11:18 ` Bernd Schmidt
2015-11-03 19:15 ` Martin Sebor
2015-11-07 23:38 ` Segher Boessenkool
2015-11-09 22:46 ` Martin Sebor
2015-11-10 0:02 ` Joseph Myers
-- strict thread matches above, loose matches on Subject: below --
2015-10-09 2:49 Martin Sebor
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