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From: "qinzhao at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/108896] provide "element_count" attribute to give more context to __builtin_dynamic_object_size() and -fsanitize=bounds
Date: Thu, 02 Mar 2023 19:47:56 +0000	[thread overview]
Message-ID: <bug-108896-4-dyUQ2enABi@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-108896-4@http.gcc.gnu.org/bugzilla/>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108896

--- Comment #11 from qinzhao at gcc dot gnu.org ---
(In reply to Martin Uecker from comment #9)
> 
> https://www.open-std.org/jtc1/sc22/wg14/www/wg14_document_log
thanks for the info. 
> 
> But we made variably modified types mandatory in C23 to
> help with bounds checking and this already works quite
> nicely with GCC / Clang:
> 
> https://godbolt.org/z/ddfsdWPMj
nice!
can you provide a pointer to the section in C23 that made this change?
> 
> > when this variable length concept is extended to global scope, not sure how to
> > implement the size expression? need some study here.
> 
> Here, we want to use a member of the struct as a size 
> expression. This could work equally at function and file scope.
> But the semantics need to be worked out.  I have started to work
> on a patch for GCC a couple of weeks ago using PLACEHOLDER_EXPR,
> but did not get very far.
> 
> The idea is to evaluate the size expression whenever the member
> with the size is accesses. If the size is not set before, this
> would be undefined behavior.
> 
> Other languages such as Ada support this, so in principle this
> should be a piece of cake.
Oh, Ada can support this already?
how does Ada implement this?
then we can just borrow Ada's implementation idea to implement this in C if
this is approved as an GCC extension for C. 

> > this proposal basically is to extend the VLA concept from function scope to
> > global scope. is my understanding correct?
> 
> I would say the idea is to allow size expressions to refer
> to member of a struct instead of only automatic variables.
> 
Okay.

> > a question here is:
> > 
> > for the following nested structure: 
> > 
> > struct object {
> >         ...
> >         char items;
> >         ...
> >         struct inner {
> >                 ...
> >                 int flex[];
> >         };
> > } *ptr;
> > 
> > what kind of syntax is good to represent the upper bound of "flex" in the inner
> > struct with "items" in the outer structure? any suggestion?
> 
> I would disallow it. At least at first. It also raises some
> questions: For example, one could form a pointer to the inner
> struct, and then it is not clear how 'items' could be accessed
> anymore.
> 
Okay.

  parent reply	other threads:[~2023-03-02 19:47 UTC|newest]

Thread overview: 53+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-02-22 21:26 [Bug c/108896] New: " kees at outflux dot net
2023-02-22 21:31 ` [Bug c/108896] " kees at outflux dot net
2023-02-22 21:32 ` pinskia at gcc dot gnu.org
2023-02-23  8:44 ` rguenth at gcc dot gnu.org
2023-02-23  9:10 ` jakub at gcc dot gnu.org
2023-02-24 15:44 ` muecker at gwdg dot de
2023-03-01 22:54 ` qinzhao at gcc dot gnu.org
2023-03-01 23:27 ` kees at outflux dot net
2023-03-02 15:50 ` muecker at gwdg dot de
2023-03-02 17:34 ` qinzhao at gcc dot gnu.org
2023-03-02 18:17 ` muecker at gwdg dot de
2023-03-02 18:34 ` muecker at gwdg dot de
2023-03-02 19:47 ` qinzhao at gcc dot gnu.org [this message]
2023-03-02 19:56 ` qinzhao at gcc dot gnu.org
2023-03-02 20:07 ` muecker at gwdg dot de
2023-03-03 20:27 ` isanbard at gmail dot com
2023-03-03 21:32 ` muecker at gwdg dot de
2023-03-03 23:18 ` isanbard at gmail dot com
2023-03-04  7:52 ` muecker at gwdg dot de
2023-03-06 19:15 ` isanbard at gmail dot com
2023-03-06 19:18 ` jakub at gcc dot gnu.org
2023-03-06 19:38 ` muecker at gwdg dot de
2023-03-06 19:57 ` muecker at gwdg dot de
2023-03-06 20:05 ` siddhesh at gcc dot gnu.org
2023-03-08 16:56 ` qinzhao at gcc dot gnu.org
2023-03-08 17:13 ` qinzhao at gcc dot gnu.org
2023-03-08 17:36 ` qinzhao at gcc dot gnu.org
2023-03-08 17:38 ` qinzhao at gcc dot gnu.org
2023-03-08 17:43 ` qinzhao at gcc dot gnu.org
2023-03-08 17:48 ` muecker at gwdg dot de
2023-03-08 18:37 ` muecker at gwdg dot de
2023-03-08 19:20 ` qinzhao at gcc dot gnu.org
2023-03-08 19:47 ` qinzhao at gcc dot gnu.org
2023-03-08 20:20 ` muecker at gwdg dot de
2023-03-08 20:47 ` qinzhao at gcc dot gnu.org
2023-03-29 16:12 ` muecker at gwdg dot de
2023-04-03 20:29 ` qinzhao at gcc dot gnu.org
2023-04-03 21:53 ` muecker at gwdg dot de
2023-04-04 15:07 ` qinzhao at gcc dot gnu.org
2023-04-04 16:33 ` muecker at gwdg dot de
2023-04-04 20:08 ` qinzhao at gcc dot gnu.org
2023-04-19 16:32 ` qinzhao at gcc dot gnu.org
2023-05-03 13:57 ` qinzhao at gcc dot gnu.org
2023-05-03 15:32 ` kees at outflux dot net
2023-05-04 15:16 ` muecker at gwdg dot de
2023-05-04 15:30 ` qinzhao at gcc dot gnu.org
2023-05-25 18:14 ` qinzhao at gcc dot gnu.org
2023-05-25 18:47 ` ndesaulniers at google dot com
2023-10-05 19:54 ` tg at mirbsd dot org
2023-10-05 20:21 ` muecker at gwdg dot de
2023-12-27  6:31 ` sean@rogue-research.com
2024-03-06 14:40 ` qinzhao at gcc dot gnu.org
2024-05-31 17:50 ` qinzhao at gcc dot gnu.org

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