From: Gabriel Dos Reis <gdr@cs.tamu.edu>
To: "Giovanni Bajo" <giovannibajo@libero.it>
Cc: "Joseph S. Myers" <jsm28@cam.ac.uk>, <gcc@gcc.gnu.org>
Subject: Re: attribute data structure rewrite
Date: Fri, 24 Sep 2004 04:18:00 -0000 [thread overview]
Message-ID: <m3u0tonznu.fsf@merlin.cs.tamu.edu> (raw)
In-Reply-To: <00cf01c4a1e0$bf25faa0$dc4e2a97@bagio>
"Giovanni Bajo" <giovannibajo@libero.it> writes:
| Gabriel Dos Reis wrote:
|
| > We used to accept it because we did not implement offsetof correctly.
| > For example, the following variation should be rejected (in C89 an C++
| > mode)
| >
| > struct A { char foo[10]; };
| > void bar(void) {
| > int i;
| > for (int i = 0; i < 10; ++i)
| > {
| > char ary[offsetof(struct A, foo[i] + 1);
| > }
| > }
|
| I don't understand your example because I don't think the +1 there is valid.
| Unless you meant:
|
| char ary[offsetof(struct A, foo[i])+1];
Yes, this is what I meant.
Sorry for the typo, and sending a testcase without compiling :-)
| Anyway, what about this in C99?
Even in strict C99 mode, it will not be valid. One could think of GNU
extensions that compute offsetof at run-time, but that is something
we've debated much in the past. I see the move to the new
implementation as a desire to keep away from that.
RTH might want to weight in.
| We don't need a constant expression in this
| context, and even if offsetof is always a constant expression we could still
| accept it as an extension.
I agree that in C99, one does not need a constant expression to make
it work, but the offsetof-subexpression still will not be valid.
| In other words, we could transform it to
| offsetof(struct A, foo)+i*sizeof(foo[0]) in contexts that don't require a
| constant expression.
That is a possibility. I'm not sure how serious the issue is.
-- Gaby
next prev parent reply other threads:[~2004-09-24 2:55 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2004-09-24 0:15 Geoffrey Keating
2004-09-24 0:34 ` Andrew Pinski
2004-09-24 1:30 ` Joseph S. Myers
2004-09-24 1:47 ` Zack Weinberg
2004-09-24 6:52 ` Geoffrey Keating
2004-09-24 1:40 ` Gabriel Dos Reis
2004-09-24 2:34 ` Giovanni Bajo
2004-09-24 2:59 ` Gabriel Dos Reis
2004-09-24 4:17 ` Giovanni Bajo
2004-09-24 4:18 ` Gabriel Dos Reis [this message]
2004-09-24 8:40 ` Andreas Schwab
2004-09-24 8:40 ` Andreas Schwab
2004-09-24 9:17 ` Joseph S. Myers
2004-09-24 14:45 ` Giovanni Bajo
2004-09-24 8:36 ` Geoffrey Keating
2004-09-24 14:03 ` Joseph S. Myers
2004-09-24 2:35 ` Giovanni Bajo
2004-09-24 8:33 ` Geoffrey Keating
2004-09-24 12:24 ` Nathan Sidwell
2004-09-24 22:29 ` Mark Mitchell
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