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| From | Ben Bacarisse <ben.usenet@bsb.me.uk> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Understanding C89/C99 aliasing |
| Date | 2015-11-23 17:03 +0000 |
| Organization | A noiseless patient Spider |
| Message-ID | <87k2p8k5qs.fsf@bsb.me.uk> (permalink) |
| References | (16 earlier) <n2r135$luc$1@dont-email.me> <87wptalujc.fsf@bsb.me.uk> <n2t94q$tsd$1@dont-email.me> <874mgdlef2.fsf@bsb.me.uk> <n2uu9e$5vv$1@dont-email.me> |
James Kuyper <jameskuyper@verizon.net> writes:
> On 11/22/2015 07:58 PM, Ben Bacarisse wrote:
>> Stephen Sprunk <stephen@sprunk.org> writes:
>>
>>> On 21-Nov-15 18:58, Ben Bacarisse wrote:
>>>> Stephen Sprunk <stephen@sprunk.org> writes:
>>>>> On 21-Nov-15 04:17, Ben Bacarisse wrote:
>>>>>> It may well be able to re-order the writes but that does not make
>>>>>> the code shown undefined. It might break something else in the
>>>>>> program, but the example is too brief to know.
>>>>>
>>>>> Okay, here's a very simple example:
>>>>>
>>>>> void foo(float *fp, int *ip) {
>>>>> *fp = 42.0;
>>>>> *ip = 42;
>>>>> printf("%d\n", *ip);
>>>>> }
>>>>>
>>>>> If the compiler doesn't reorder the assignments, then the read is
>>>>> well-defined. However, it is allowed to assume fp and ip are not
>>>>> aliases and thus reorder the assignments, but if they actually are
>>>>> aliases, then the read would become undefined. Correct?
>>>>
>>>> I don't think so. If ip and fp alias then they refer to some common
>>>> object. If that has a declared type of int, then the assignment
>>>> through *fp is undefined because it violates a "shall" in the
>>>> standard. Similarly if it's an object declared to be float. The read
>>>> of *ip is not involved in any of that -- the code is already
>>>> undefined before it gets there.
>
> And the undefined behavior might take the form of causing this function
> to malfunction because the writes are re-ordered. The fact that the
> order only matters if they violate 6.5p7 is what allows the reordering.
>
>>> Thanks; that explains why the pointers' origin matters.
>>>
>>>> But there is an other case. If ip and fp both point to the same
>>>> properly sized and aligned allocated storage, then there is nothing
>>>> wrong with the above code. The effective type rules mean that
>>>> allocated storage gets it's effective type from the type of the
>>>> lvalue expression last used to write it.
>
> And 6.5p7 is violated by any attempt to write to that same location
> using a type other than one of the ones listed in 6.5p7 - and if it is
> one of those types, then the effective type remains unchanged. The
> relevant verb is "access", which includes both reads and writes. It's
> therefore only possible to set the effective type allocated memory; once
> set, it cannot be changed.
I disagree. Paragraph 6 tells you *how* to determine the effective
type, then paragraph 7 tells you how to use that information. From
paragraph 6:
"If a value is stored into an object having no declared type through
an lvalue having a type that is not a character type, then the type of
the lvalue becomes the effective type of the object for that access
and for subsequent accesses that do not modify the stored value."
Had your meaning been intended, the exclusion of subsequent accesses
that *do* modify the stored value would not be there. It could also
have simply stated the rule for objects that do not yet have an
effective type rather than for those that have no declared type. There
are several ways to convey your reading, and all of the them seem to me
simpler than the words actually used.
--
Ben.
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Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-11-18 11:25 -0800
Re: Understanding C89/C99 aliasing Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-19 01:39 +0000
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-11-18 19:55 -0800
Re: Understanding C89/C99 aliasing Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-19 14:37 +0000
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-11-19 07:07 -0800
Re: Understanding C89/C99 aliasing Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-19 22:58 +0000
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-11-18 20:00 -0800
Re: Understanding C89/C99 aliasing Nick Bowler <nbowler@draconx.ca> - 2015-11-19 15:44 +0000
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-11-19 09:46 -0800
Re: Understanding C89/C99 aliasing Martin Shobe <martin.shobe@yahoo.com> - 2015-11-19 13:28 -0600
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-11-19 11:54 -0800
Re: Understanding C89/C99 aliasing Martin Shobe <martin.shobe@yahoo.com> - 2015-11-19 14:41 -0600
Re: Understanding C89/C99 aliasing Martin Shobe <martin.shobe@yahoo.com> - 2015-11-19 14:46 -0600
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-11-19 13:16 -0800
Re: Understanding C89/C99 aliasing Nick Bowler <nbowler@draconx.ca> - 2015-11-19 21:11 +0000
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-11-19 13:34 -0800
Re: Understanding C89/C99 aliasing Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-19 23:12 +0000
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-11-19 15:18 -0800
Re: Understanding C89/C99 aliasing Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-20 02:21 +0000
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-11-19 18:40 -0800
Re: Understanding C89/C99 aliasing Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-21 00:32 +0000
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-11-20 16:47 -0800
Re: Understanding C89/C99 aliasing Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-21 01:14 +0000
Re: Understanding C89/C99 aliasing Stephen Sprunk <stephen@sprunk.org> - 2015-11-20 20:25 -0600
Re: Understanding C89/C99 aliasing Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-21 10:17 +0000
Re: Understanding C89/C99 aliasing Stephen Sprunk <stephen@sprunk.org> - 2015-11-21 18:11 -0600
Re: Understanding C89/C99 aliasing Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-22 00:58 +0000
Re: Understanding C89/C99 aliasing Stephen Sprunk <stephen@sprunk.org> - 2015-11-22 14:41 -0600
Re: Understanding C89/C99 aliasing Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-23 00:58 +0000
Re: Understanding C89/C99 aliasing Stephen Sprunk <stephen@sprunk.org> - 2015-11-22 19:33 -0600
Re: Understanding C89/C99 aliasing Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-26 01:16 +0000
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-11-25 21:57 -0800
Re: Understanding C89/C99 aliasing Stephen Sprunk <stephen@sprunk.org> - 2015-11-26 18:01 -0600
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-11-27 10:17 -0800
Re: Understanding C89/C99 aliasing Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-27 23:38 -0800
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-11-28 07:22 -0800
Re: Understanding C89/C99 aliasing Tim Rentsch <txr@alumni.caltech.edu> - 2015-12-08 08:51 -0800
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-11-22 18:34 -0800
Re: Understanding C89/C99 aliasing James Kuyper <jameskuyper@verizon.net> - 2015-11-23 06:48 -0500
Re: Understanding C89/C99 aliasing Keith Thompson <kst-u@mib.org> - 2015-11-23 08:28 -0800
Re: Understanding C89/C99 aliasing Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-27 09:21 -0800
Re: Understanding C89/C99 aliasing raltbos@xs4all.nl (Richard Bos) - 2015-11-27 17:45 +0000
Re: Understanding C89/C99 aliasing Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-27 10:13 -0800
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-11-27 10:50 -0800
Re: Understanding C89/C99 aliasing Keith Thompson <kst-u@mib.org> - 2015-11-27 13:46 -0800
Re: Understanding C89/C99 aliasing Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-28 08:57 -0800
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-11-28 11:00 -0800
Re: Understanding C89/C99 aliasing Tim Rentsch <txr@alumni.caltech.edu> - 2015-12-08 09:04 -0800
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-12-08 10:42 -0800
Re: Understanding C89/C99 aliasing Keith Thompson <kst-u@mib.org> - 2015-12-08 10:54 -0800
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-12-08 11:44 -0800
Re: Understanding C89/C99 aliasing glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-12-09 00:11 +0000
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-12-08 16:39 -0800
Re: Understanding C89/C99 aliasing Keith Thompson <kst-u@mib.org> - 2015-12-08 19:15 -0800
Re: Understanding C89/C99 aliasing glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-12-09 04:21 +0000
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-12-09 09:04 -0800
Re: Understanding C89/C99 aliasing Stephen Sprunk <stephen@sprunk.org> - 2015-12-08 22:15 -0600
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-12-09 07:04 -0800
Re: Understanding C89/C99 aliasing Stephen Sprunk <stephen@sprunk.org> - 2015-12-09 14:29 -0600
Re: Understanding C89/C99 aliasing Keith Thompson <kst-u@mib.org> - 2015-12-09 13:06 -0800
Re: Understanding C89/C99 aliasing glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-12-09 22:58 +0000
Re: Understanding C89/C99 aliasing Stephen Sprunk <stephen@sprunk.org> - 2015-12-10 11:35 -0600
Re: Understanding C89/C99 aliasing James Kuyper <jameskuyper@verizon.net> - 2015-12-10 12:58 -0500
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-12-10 10:12 -0800
Re: Understanding C89/C99 aliasing Stephen Sprunk <stephen@sprunk.org> - 2015-12-10 13:39 -0600
Re: Understanding C89/C99 aliasing raltbos@xs4all.nl (Richard Bos) - 2015-12-10 22:45 +0000
Re: Understanding C89/C99 aliasing Stephen Sprunk <stephen@sprunk.org> - 2015-12-11 12:27 -0600
Re: Understanding C89/C99 aliasing James Kuyper <jameskuyper@verizon.net> - 2015-12-10 20:30 -0500
Re: Understanding C89/C99 aliasing Stephen Sprunk <stephen@sprunk.org> - 2015-12-10 13:54 -0600
Re: Understanding C89/C99 aliasing James Kuyper <jameskuyper@verizon.net> - 2015-12-10 20:57 -0500
Re: Understanding C89/C99 aliasing glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-12-11 03:44 +0000
Re: Understanding C89/C99 aliasing James Kuyper <jameskuyper@verizon.net> - 2015-12-11 07:26 -0500
Re: Understanding C89/C99 aliasing Stephen Sprunk <stephen@sprunk.org> - 2015-12-11 11:58 -0600
Re: Understanding C89/C99 aliasing Richard Damon <Richard@Damon-Family.org> - 2015-12-11 23:17 -0500
Re: Understanding C89/C99 aliasing James Kuyper <jameskuyper@verizon.net> - 2015-12-11 14:00 -0500
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-12-09 13:17 -0800
Re: Understanding C89/C99 aliasing Keith Thompson <kst-u@mib.org> - 2015-12-09 14:22 -0800
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-12-09 15:06 -0800
Re: Understanding C89/C99 aliasing glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-12-09 23:18 +0000
Re: Understanding C89/C99 aliasing James Kuyper <jameskuyper@verizon.net> - 2015-12-09 18:48 -0500
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-12-09 15:49 -0800
Re: Understanding C89/C99 aliasing Stephen Sprunk <stephen@sprunk.org> - 2015-12-10 12:44 -0600
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-12-10 11:12 -0800
Re: Understanding C89/C99 aliasing raltbos@xs4all.nl (Richard Bos) - 2015-12-10 20:18 +0000
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-12-10 12:29 -0800
Re: Understanding C89/C99 aliasing Stephen Sprunk <stephen@sprunk.org> - 2015-12-10 16:19 -0600
Re: Understanding C89/C99 aliasing raltbos@xs4all.nl (Richard Bos) - 2015-12-10 22:49 +0000
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-12-10 14:58 -0800
Re: Understanding C89/C99 aliasing raltbos@xs4all.nl (Richard Bos) - 2015-12-15 18:25 +0000
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-12-15 10:50 -0800
Re: Understanding C89/C99 aliasing Keith Thompson <kst-u@mib.org> - 2015-12-15 11:32 -0800
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-12-15 12:20 -0800
Re: Understanding C89/C99 aliasing Keith Thompson <kst-u@mib.org> - 2015-12-15 13:08 -0800
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-12-15 13:24 -0800
Re: Understanding C89/C99 aliasing Keith Thompson <kst-u@mib.org> - 2015-12-15 14:35 -0800
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-12-15 15:34 -0800
Re: Understanding C89/C99 aliasing David Brown <david.brown@hesbynett.no> - 2015-12-16 00:00 +0100
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-12-15 15:47 -0800
Re: Understanding C89/C99 aliasing David Brown <david.brown@hesbynett.no> - 2015-12-16 11:08 +0100
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-12-16 09:45 -0800
Re: Understanding C89/C99 aliasing David Brown <david.brown@hesbynett.no> - 2015-12-17 13:51 +0100
Re: Understanding C89/C99 aliasing Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-12-17 05:10 -0800
Re: Understanding C89/C99 aliasing Richard Heathfield <rjh@cpax.org.uk> - 2015-12-17 13:46 +0000
Re: Understanding C89/C99 aliasing David Brown <david.brown@hesbynett.no> - 2015-12-17 15:24 +0100
Re: Understanding C89/C99 aliasing Jerry Stuckle <jstucklex@attglobal.net> - 2015-12-17 09:57 -0500
Re: Understanding C89/C99 aliasing Les Cargill <lcargill99@comcast.com> - 2015-12-17 09:39 -0600
Re: Understanding C89/C99 aliasing Jerry Stuckle <jstucklex@attglobal.net> - 2015-12-17 12:12 -0500
Re: Understanding C89/C99 aliasing Les Cargill <lcargill99@comcast.com> - 2015-12-17 12:27 -0600
Re: Understanding C89/C99 aliasing Jerry Stuckle <jstucklex@attglobal.net> - 2015-12-17 14:22 -0500
Re: Understanding C89/C99 aliasing Les Cargill <lcargill99@comcast.com> - 2015-12-17 17:58 -0600
Re: Understanding C89/C99 aliasing Jerry Stuckle <jstucklex@attglobal.net> - 2015-12-17 20:26 -0500
Re: Understanding C89/C99 aliasing Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2015-12-17 20:35 -0500
Re: Understanding C89/C99 aliasing Jerry Stuckle <jstucklex@attglobal.net> - 2015-12-17 21:49 -0500
Re: Understanding C89/C99 aliasing Les Cargill <lcargill99@comcast.com> - 2015-12-17 20:52 -0600
Re: Understanding C89/C99 aliasing Jerry Stuckle <jstucklex@attglobal.net> - 2015-12-17 22:01 -0500
Re: Understanding C89/C99 aliasing Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-12-17 20:55 -0800
Re: Understanding C89/C99 aliasing Jerry Stuckle <jstucklex@attglobal.net> - 2015-12-18 08:40 -0500
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-12-17 09:10 -0800
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-12-15 14:27 -0800
Re: Understanding C89/C99 aliasing Keith Thompson <kst-u@mib.org> - 2015-12-15 16:30 -0800
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-12-16 12:47 -0800
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-12-15 14:32 -0800
Re: Understanding C89/C99 aliasing Öö Tiib <ootiib@hot.ee> - 2015-12-09 08:09 -0800
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-12-09 09:10 -0800
Re: Understanding C89/C99 aliasing Öö Tiib <ootiib@hot.ee> - 2015-12-09 10:38 -0800
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-12-09 11:18 -0800
Re: Understanding C89/C99 aliasing glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-12-09 19:14 +0000
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-12-09 11:27 -0800
Re: Understanding C89/C99 aliasing Stephen Sprunk <stephen@sprunk.org> - 2015-12-09 15:03 -0600
Re: Understanding C89/C99 aliasing Öö Tiib <ootiib@hot.ee> - 2015-12-10 01:23 -0800
Re: Understanding C89/C99 aliasing raltbos@xs4all.nl (Richard Bos) - 2015-12-10 20:21 +0000
Re: Understanding C89/C99 aliasing Tim Rentsch <txr@alumni.caltech.edu> - 2015-12-09 13:06 -0800
Re: Understanding C89/C99 aliasing glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-12-09 00:00 +0000
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-12-08 16:27 -0800
Re: Understanding C89/C99 aliasing Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-12-09 01:18 +0000
Re: Understanding C89/C99 aliasing Keith Thompson <kst-u@mib.org> - 2015-12-08 19:13 -0800
Re: Understanding C89/C99 aliasing Tim Rentsch <txr@alumni.caltech.edu> - 2015-12-09 13:04 -0800
Re: Understanding C89/C99 aliasing raltbos@xs4all.nl (Richard Bos) - 2015-12-10 20:07 +0000
Re: Understanding C89/C99 aliasing Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-23 17:03 +0000
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-11-23 15:39 -0800
Re: Understanding C89/C99 aliasing Stephen Sprunk <stephen@sprunk.org> - 2015-11-23 18:11 -0600
Re: Understanding C89/C99 aliasing Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-24 01:13 +0000
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-11-24 09:29 -0800
Re: Understanding C89/C99 aliasing Stephen Sprunk <stephen@sprunk.org> - 2015-11-20 09:10 -0600
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-11-20 10:08 -0800
Re: Understanding C89/C99 aliasing Stephen Sprunk <stephen@sprunk.org> - 2015-11-21 19:02 -0600
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-11-21 18:17 -0800
Re: Understanding C89/C99 aliasing Stephen Sprunk <stephen@sprunk.org> - 2015-11-21 20:52 -0600
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-11-21 19:28 -0800
Re: Understanding C89/C99 aliasing Stephen Sprunk <stephen@sprunk.org> - 2015-11-22 20:00 -0600
Re: Understanding C89/C99 aliasing Keith Thompson <kst-u@mib.org> - 2015-11-20 19:12 -0800
Re: Understanding C89/C99 aliasing Stephen Sprunk <stephen@sprunk.org> - 2015-11-20 23:10 -0600
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-11-20 23:07 -0800
Re: Understanding C89/C99 aliasing Stephen Sprunk <stephen@sprunk.org> - 2015-11-21 18:37 -0600
Re: Understanding C89/C99 aliasing Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-21 10:29 +0000
Re: Understanding C89/C99 aliasing supercat@casperkitty.com - 2015-11-21 10:04 -0800
Re: Understanding C89/C99 aliasing Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-21 19:35 +0000
Re: Understanding C89/C99 aliasing Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-19 23:07 +0000
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