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| From | Tim Rentsch <txr@alumni.caltech.edu> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Casts on lvalues |
| Date | 2012-12-17 02:58 -0800 |
| Organization | A noiseless patient Spider |
| Message-ID | <kfntxrlhrmr.fsf@x-alumni2.alumni.caltech.edu> (permalink) |
| References | (9 earlier) <ln1uf4qbws.fsf@nuthaus.mib.org> <4ld2c8d9mskkhqrfab8d9n9k8jv2b4p7i6@4ax.com> <lnboe6psew.fsf@nuthaus.mib.org> <7216db87-28bd-41a0-9fd9-6a0fe23158d5@googlegroups.com> <ln7gotptyq.fsf@nuthaus.mib.org> |
Keith Thompson <kst-u@mib.org> writes:
> @C3{96}@C3{B6} Tiib <ootiib@hot.ee> writes:
>> On Friday, 7 December 2012 03:22:47 UTC+2, Keith Thompson wrote:
>>> Philipp Thomas <kthomas@linux01.gwdg.de> writes:
>>> > On Wed, 05 Dec 2012 16:09:23 -0800, Keith Thompson <kst-u@mib.org>
>>> > wrote:
>>> >>Or you can use a union; this avoids alignment issues, but the other
>>> >>problems still apply.
>>> >
>>> > When used for type-punning that isn't guaranteed to work but depends
>>> > on the compiler.
>>>
>>> A footnote in the standard (N1370 6.5.2.3p3, footnote 95) says:
>>>
>>> If the member used to read the contents of a union object is
>>> not the same as the member last used to store a value in the
>>> object, the appropriate part of the object representation of
>>> the value is reinterpreted as an object representation in the
>>> new type as described in 6.2.6 (a process sometimes called
>>> "type punning"). This might be a trap representation.
>>>
>>> I'm not sure why that's stated only in a non-normative footnote.
>>> I suppose the implication is that it's already stated normatively,
>>> but it's not clear to me that it is.
>>
>> C99 has such texts:
>>
>> 6.2.5p20, union has an overlapping set of member objects
>> 6.7.2.1p14, the value of at most one of the members can be stored
>> in a union object at any time
>> Annex J.1 the value of a union member other than the last one
>> stored into is unspecified.
>>
>> I do not feel it safe to use union for type punning.
>
> Note that C99 doesn't have that footnote, but N1256 does. It was added
> by one of Technical Corrigenda 3 -- but N1256 (which incorporates TC3)
> still has that clause in Annex J.1. I suspect this is just an
> oversight.
>
> N1570 (essentially C2011) changed the wording of the clause in J.1
> (which is a list of unspecified behaviors) from:
> The value of a union member other than the last one stored into
> (6.2.6.1)
> to:
> The values of bytes that correspond to union members other than the
> one last stored into (6.2.6.1).
>
> I can't find the list of C99 DRs (the link I had,
> <http://www.open-std.org/jtc1/sc22/wg14/www/docs/summary.htm>
> now points to the list of C2011 DRs), so I don't know which DR
> introduced this change.
>
> But the implication is that the committee felt that the existing C99
> standard already implied what the footnote says. [snip]
In fact, the footnote was added precisely because someone was
concerned it wasn't obvious (or obvious enough) that C99's
rules produced the same behavior (ie, type punning) for union
access as had been true before C99.
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Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-02 13:23 +0000
Re: Casts on lvalues SG <sgesemann@gmail.invalid> - 2012-12-02 14:40 +0100
Re: Casts on lvalues Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-12-02 13:53 +0000
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-06 12:26 +0000
Re: Casts on lvalues Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-12-06 15:15 +0000
Re: Casts on lvalues Keith Thompson <kst-u@mib.org> - 2012-12-06 12:05 -0800
Re: Casts on lvalues Keith Thompson <kst-u@mib.org> - 2012-12-02 12:34 -0800
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-02 21:18 +0000
Re: Casts on lvalues Willem <willem@turtle.stack.nl> - 2012-12-05 22:36 +0000
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-05 23:46 +0000
Re: Casts on lvalues Willem <willem@turtle.stack.nl> - 2012-12-06 18:35 +0000
Re: Casts on lvalues pete <pfiland@mindspring.com> - 2012-12-02 16:21 -0500
Re: Casts on lvalues Eric Sosman <esosman@comcast-dot-net.invalid> - 2012-12-02 17:10 -0500
Re: Casts on lvalues pete <pfiland@mindspring.com> - 2012-12-02 18:50 -0500
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-02 19:14 -0500
Re: Casts on lvalues falk@rahul.net (Edward A. Falk) - 2012-12-04 23:57 +0000
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-05 01:35 +0000
Re: Casts on lvalues Piotr Kalinowski <pitkali@gmail.com> - 2012-12-05 16:07 +0100
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-05 16:47 +0000
Re: Casts on lvalues Eric Sosman <esosman@comcast-dot-net.invalid> - 2012-12-05 13:04 -0500
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-05 18:39 +0000
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-05 14:23 -0500
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-05 22:11 +0000
Re: Casts on lvalues Ian Collins <ian-news@hotmail.com> - 2012-12-06 11:51 +1300
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-05 23:58 +0000
Re: Casts on lvalues Piotr Kalinowski <pitkali@gmail.com> - 2012-12-06 22:00 +0100
Re: Casts on lvalues glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-12-06 03:05 +0000
Re: Casts on lvalues Eric Sosman <esosman@comcast-dot-net.invalid> - 2012-12-05 14:43 -0500
Re: Casts on lvalues Greg Martin <greg@softsprocket.com> - 2012-12-05 12:25 -0800
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-05 21:57 +0000
Re: Casts on lvalues Keith Thompson <kst-u@mib.org> - 2012-12-05 16:09 -0800
Re: Casts on lvalues Philipp Thomas <kthomas@linux01.gwdg.de> - 2012-12-07 01:24 +0100
Re: Casts on lvalues Keith Thompson <kst-u@mib.org> - 2012-12-06 17:22 -0800
Re: Casts on lvalues Öö Tiib <ootiib@hot.ee> - 2012-12-07 00:31 -0800
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-07 08:18 -0500
Re: Casts on lvalues Öö Tiib <ootiib@hot.ee> - 2012-12-07 09:07 -0800
Re: Casts on lvalues Keith Thompson <kst-u@mib.org> - 2012-12-07 11:01 -0800
Re: Casts on lvalues glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-12-07 21:57 +0000
Re: Casts on lvalues Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-12-07 22:29 +0000
Re: Casts on lvalues glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-12-08 00:04 +0000
Re: Casts on lvalues lawrence.jones@siemens.com - 2012-12-15 16:51 -0500
Re: Casts on lvalues Keith Thompson <kst-u@mib.org> - 2012-12-15 15:48 -0800
Re: Casts on lvalues lawrence.jones@siemens.com - 2012-12-17 16:37 -0500
Re: Casts on lvalues Tim Rentsch <txr@alumni.caltech.edu> - 2012-12-17 02:58 -0800
Re: Casts on lvalues Tim Rentsch <txr@alumni.caltech.edu> - 2012-12-17 02:53 -0800
Re: Casts on lvalues Tim Rentsch <txr@alumni.caltech.edu> - 2012-12-17 02:43 -0800
Re: Casts on lvalues Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-12-05 22:07 +0000
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-05 22:20 +0000
Re: Casts on lvalues Keith Thompson <kst-u@mib.org> - 2012-12-05 16:10 -0800
Re: Casts on lvalues Philip Lantz <prl@canterey.us> - 2012-12-06 22:49 -0800
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-07 12:54 +0000
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-07 08:22 -0500
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-07 13:48 +0000
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-07 11:17 -0500
Re: Casts on lvalues glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-12-07 17:33 +0000
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-07 18:28 +0000
Re: Casts on lvalues glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-12-07 22:18 +0000
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-07 18:27 +0000
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-07 14:14 -0500
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-07 20:16 +0000
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-07 15:41 -0500
Re: Casts on lvalues Tim Rentsch <txr@alumni.caltech.edu> - 2012-12-17 03:03 -0800
Re: Casts on lvalues Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-12-09 17:54 +0200
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-09 12:38 -0500
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-09 18:50 +0000
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-09 14:31 -0500
Re: Casts on lvalues Keith Thompson <kst-u@mib.org> - 2012-12-09 20:12 -0800
Re: Casts on lvalues Heikki Kallasjoki <fis+usenet@zem.fi> - 2012-12-10 07:38 +0000
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-10 08:12 -0500
Re: Casts on lvalues Heikki Kallasjoki <fis+usenet@zem.fi> - 2012-12-10 17:46 +0000
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-10 12:59 -0500
Re: Casts on lvalues Keith Thompson <kst-u@mib.org> - 2012-12-10 12:07 -0800
Re: Casts on lvalues Tim Rentsch <txr@alumni.caltech.edu> - 2012-12-17 03:11 -0800
Re: Casts on lvalues Ian Collins <ian-news@hotmail.com> - 2012-12-10 20:50 +1300
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-10 07:49 -0500
Re: Casts on lvalues Rui Maciel <rui.maciel@gmail.com> - 2012-12-07 15:19 +0000
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-07 16:54 +0000
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