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Groups > comp.lang.c > #76839

Re: Objective C

From Stephen Sprunk <stephen@sprunk.org>
Newsgroups comp.lang.c
Subject Re: Objective C
Date 2015-11-22 01:55 -0600
Organization A noiseless patient Spider
Message-ID <n2rs8j$mpf$1@dont-email.me> (permalink)
References (10 earlier) <8nj05bha7qo58upepjqipdds0fgjkl8umb@4ax.com> <m4k05bdossoe7criehdedlhgfpv1lml9rp@4ax.com> <ln7flb8u7t.fsf@kst-u.example.com> <n2qvdu$gqi$1@dont-email.me> <56510709.8000908@verizon.net>

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On 21-Nov-15 18:06, James Kuyper wrote:
> On 11/21/2015 06:43 PM, Stephen Sprunk wrote:
>> On 21-Nov-15 05:31, Keith Thompson wrote:
>>> Robert Wessel <robertwessel2@yahoo.com> writes:
>>>> And, of course, writing to one union member and reading from the
>>>> other is undefined.
>>>
>>> Incorrect.
>>>
>>> N1570 6.5.2.3p3, footnote:
>>>
>>>     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.
>>
>> That says the representation in the new type could be either a valid
>> value or a trap representation, ...
> 
> True.
> 
>> ... which is the very definition of an
>> indeterminate value. ...
> 
> No, it is not. When you have an indeterminate value, which value you 
> have (including whether or not it's a trap representation) is
> completely unspecified.

That's what I see above: the result is either a valid value or a trap
representation, and that is the definition of indeterminate value.

> When you read the value of one member of a union after having set
> the value of a different member, the key point is that "the
> appropriate part of the object representation of the value is
> reinterpreted as an object representation in the new type". That is
> defined behavior - though exactly what the behavior is that it
> defines depends upon the representation of the two types.

... and, assuming the read member's type _has_ trap representations,
that could result in either a valid value or a trap representation.

If not, then that leaves only valid values, and there would seem to be
no problem--but see below.

>> ... Reading an indeterminate value invokes undefined behavior,
>> even if it happens to _not_ be a trap representation.
> 
> Citation, please?

N1570 J.2
"1 The behavior is undefined in the following circumstances:
...
-- The value of an object with automatic storage duration is used while
its value is indeterminate."

That doesn't apply to non-auto objects, which I wasn't aware of, but it
seems clear that if an auto object with indeterminate value happens to
have a valid value rather than a trap representation (and even if its
type doesn't _have_ trap representations), using it still invokes
undefined behavior.

> The only argument I know of for undefined behavior from the use of
> indeterminate values depends upon the fact that a trap representation
> is a possibility.

To be fair, that's the only case where I'd expect an actual problem.

> That argument therefore breaks down if it's a type that is guaranteed
> to have no trap representations (such as unsigned char) or a type that
> happens to have no trap representations on the current platform.

That's what I assume the intent was--and what I assume every compiler
actually implements, but I'm not sure that a strict reading of the
wording actually promises that.

S

-- 
Stephen Sprunk         "God does not play dice."  --Albert Einstein
CCIE #3723         "God is an inveterate gambler, and He throws the
K5SSS        dice at every possible opportunity." --Stephen Hawking

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Objective C Alla _ <modelling.data@gmail.com> - 2015-11-10 09:10 -0800
  Re: Objective C "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-10 09:16 -0800
  Re: Objective C Richard Heathfield <rjh@cpax.org.uk> - 2015-11-10 17:17 +0000
  Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-11-10 09:32 -0800
    Re: Objective C Mark Storkamp <mstorkamp@yahoo.com> - 2015-11-10 11:38 -0600
  Re: Objective C James Kuyper <jameskuyper@verizon.net> - 2015-11-10 12:36 -0500
    Re: Objective C Alla _ <modelling.data@gmail.com> - 2015-11-10 11:15 -0800
      Re: Objective C Richard Heathfield <rjh@cpax.org.uk> - 2015-11-10 21:51 +0000
        Re: Objective C Alla _ <modelling.data@gmail.com> - 2015-11-10 23:13 -0800
      Re: Objective C supercat@casperkitty.com - 2015-11-13 16:01 -0800
        Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-14 01:07 +0000
          Re: Objective C supercat@casperkitty.com - 2015-11-14 06:26 -0800
            Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-14 22:22 +0000
              Re: Objective C supercat@casperkitty.com - 2015-11-14 15:46 -0800
                Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-15 01:47 +0000
                Re: Objective C BartC <bc@freeuk.com> - 2015-11-15 10:43 +0000
                Re: Objective C supercat@casperkitty.com - 2015-11-15 13:19 -0800
            Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-11-14 15:43 -0800
            Re: Objective C David Brown <david.brown@hesbynett.no> - 2015-11-15 23:50 +0100
              Re: Objective C gazelle@shell.xmission.com (Kenny McCormack) - 2015-11-16 00:05 +0000
              Re: Objective C supercat@casperkitty.com - 2015-11-15 19:10 -0800
                Re: Objective C David Brown <david.brown@hesbynett.no> - 2015-11-16 09:11 +0100
                Re: Objective C supercat@casperkitty.com - 2015-11-16 11:57 -0800
                Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-16 20:38 +0000
                Re: Objective C supercat@casperkitty.com - 2015-11-16 13:20 -0800
                Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-16 22:28 +0000
                Re: Objective C supercat@casperkitty.com - 2015-11-16 15:33 -0800
                Re: Objective C supercat@casperkitty.com - 2015-11-16 15:56 -0800
                Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-17 01:43 +0000
                Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-17 01:21 +0000
                Re: Objective C supercat@casperkitty.com - 2015-11-16 18:11 -0800
                Re: Objective C supercat@casperkitty.com - 2015-11-16 20:12 -0800
                Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-19 00:59 +0000
                Re: Objective C supercat@casperkitty.com - 2015-11-18 22:46 -0800
                Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-19 14:25 +0000
                Re: Objective C supercat@casperkitty.com - 2015-11-19 07:58 -0800
                Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-19 22:13 +0000
                Re: Objective C supercat@casperkitty.com - 2015-11-19 15:29 -0800
                Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-27 08:44 -0800
                Re: Objective C glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-11-17 01:26 +0000
                Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-16 21:48 -0600
                Re: Objective C supercat@casperkitty.com - 2015-11-16 20:27 -0800
                Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-17 13:58 -0600
                Re: Objective C supercat@casperkitty.com - 2015-11-17 12:28 -0800
                Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-18 13:00 -0600
                Re: Objective C supercat@casperkitty.com - 2015-11-18 12:31 -0800
                Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-18 16:43 -0600
                Re: Objective C supercat@casperkitty.com - 2015-11-18 15:57 -0800
                Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-19 19:09 -0600
                Re: Objective C supercat@casperkitty.com - 2015-11-19 19:05 -0800
                Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-20 15:00 -0600
                Re: Objective C supercat@casperkitty.com - 2015-11-20 15:30 -0800
                Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-22 17:11 -0600
                Re: Objective C supercat@casperkitty.com - 2015-11-22 15:53 -0800
                Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-23 13:51 -0600
                Re: Objective C raltbos@xs4all.nl (Richard Bos) - 2015-11-24 12:00 +0000
                Re: Objective C David Brown <david.brown@hesbynett.no> - 2015-11-17 10:26 +0100
                Re: Objective C supercat@casperkitty.com - 2015-11-17 06:20 -0800
                Re: Objective C David Brown <david.brown@hesbynett.no> - 2015-11-17 16:41 +0100
                Re: Objective C supercat@casperkitty.com - 2015-11-17 12:08 -0800
                Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-17 16:27 -0600
                Re: Objective C supercat@casperkitty.com - 2015-11-17 14:55 -0800
                Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-18 13:59 -0600
                Re: Objective C supercat@casperkitty.com - 2015-11-18 13:06 -0800
                Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-24 14:24 -0600
                Re: Objective C supercat@casperkitty.com - 2015-11-24 15:11 -0800
                Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-26 17:02 -0600
                Re: Objective C supercat@casperkitty.com - 2015-11-27 07:26 -0800
                Re: Objective C supercat@casperkitty.com - 2015-11-18 21:56 -0800
                Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-22 02:17 -0600
                Re: Objective C supercat@casperkitty.com - 2015-11-22 07:16 -0800
                Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-26 17:11 -0600
                Re: Objective C Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-11-22 19:09 +0000
                Re: Objective C Robert Wessel <robertwessel2@yahoo.com> - 2015-11-17 23:28 -0600
                Re: Objective C David Brown <david.brown@hesbynett.no> - 2015-11-18 15:33 +0100
                Re: Objective C supercat@casperkitty.com - 2015-11-18 23:06 -0800
                Re: Objective C David Brown <david.brown@hesbynett.no> - 2015-11-19 09:12 +0100
                Re: Objective C supercat@casperkitty.com - 2015-11-19 09:25 -0800
                Re: Objective C David Brown <david.brown@hesbynett.no> - 2015-11-19 21:06 +0100
                Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-20 13:49 -0800
                Re: Objective C Robert Wessel <robertwessel2@yahoo.com> - 2015-11-21 05:01 -0600
                Re: Objective C Robert Wessel <robertwessel2@yahoo.com> - 2015-11-21 05:09 -0600
                Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-11-21 03:31 -0800
                Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-21 17:43 -0600
                Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-11-21 16:00 -0800
                Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-22 01:12 -0600
                Re: Objective C raltbos@xs4all.nl (Richard Bos) - 2015-11-27 16:42 +0000
                Re: Objective C James Kuyper <jameskuyper@verizon.net> - 2015-11-27 12:38 -0500
                Re: Objective C supercat@casperkitty.com - 2015-11-27 11:09 -0800
                Re: Objective C James Kuyper <jameskuyper@verizon.net> - 2015-11-21 19:06 -0500
                Re: Objective C supercat@casperkitty.com - 2015-11-21 17:00 -0800
                Re: Objective C glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-11-22 14:44 +0000
                Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-22 01:55 -0600
                Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-11-22 00:03 -0800
                Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-22 10:18 +0000
                Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-23 03:21 -0800
                Re: Objective C James Kuyper <jameskuyper@verizon.net> - 2015-11-22 10:53 -0500
                Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-22 20:28 -0600
                Re: Objective C supercat@casperkitty.com - 2015-11-22 18:52 -0800
                Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-23 11:58 -0600
                Re: Objective C supercat@casperkitty.com - 2015-11-23 15:24 -0800
                Re: Objective C James Kuyper <jameskuyper@verizon.net> - 2015-11-23 09:46 -0500
                Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-11-23 08:13 -0800
                Re: Objective C James Kuyper <jameskuyper@verizon.net> - 2015-11-23 12:04 -0500
                Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-23 11:52 -0600
                Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-23 16:05 -0800
                Re: Objective C supercat@casperkitty.com - 2015-11-24 09:09 -0800
                Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-12-08 08:48 -0800
                Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-12-08 10:40 -0800
                Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-12-09 12:34 -0800
                Re: Objective C supercat@casperkitty.com - 2015-12-08 10:50 -0800
                Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-12-09 12:42 -0800
                Re: Objective C supercat@casperkitty.com - 2015-12-09 13:38 -0800
                Re: Objective C raltbos@xs4all.nl (Richard Bos) - 2015-12-10 19:54 +0000
                Re: Objective C David Thompson <dave.thompson2@verizon.net> - 2015-12-06 08:02 -0500
                Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-22 14:36 -0800
                Re: Objective C Robert Wessel <robertwessel2@yahoo.com> - 2015-11-23 00:41 -0600
                Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-23 07:39 -0800
                Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-22 14:23 -0800
                Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-17 10:24 -0800
                Re: Objective C David Brown <david.brown@hesbynett.no> - 2015-11-18 15:40 +0100
                Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-20 16:02 -0800
                Re: Objective C David Brown <david.brown@hesbynett.no> - 2015-11-21 15:53 +0100
                Re: Objective C supercat@casperkitty.com - 2015-11-21 08:58 -0800
                Re: Objective C David Brown <david.brown@hesbynett.no> - 2015-11-22 22:14 +0100
                Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-23 03:39 -0800
                Re: Objective C raltbos@xs4all.nl (Richard Bos) - 2015-11-23 13:24 +0000
                Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-26 09:39 -0800
                Re: Objective C Robert Wessel <robertwessel2@yahoo.com> - 2015-11-26 13:16 -0600
                Re: Objective C supercat@casperkitty.com - 2015-11-26 12:25 -0800
                Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-27 14:34 -0800
                Re: Objective C Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-26 12:39 -0800
                Re: Objective C raltbos@xs4all.nl (Richard Bos) - 2015-11-27 16:43 +0000
                Re: Objective C Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-27 09:29 -0800
                Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-27 14:18 -0800
                Re: Objective C glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-11-23 22:35 +0000
                Re: Objective C James Kuyper <jameskuyper@verizon.net> - 2015-11-23 17:40 -0500
                Re: Objective C glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-11-24 05:35 +0000
                Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-25 12:15 -0800
                Re: Objective C glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-11-25 22:49 +0000
                Re: Objective C raltbos@xs4all.nl (Richard Bos) - 2015-11-27 16:44 +0000
                Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-27 14:03 -0800
                Re: Objective C Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-11-29 09:29 +0000
                Re: Objective C Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-11-22 19:01 +0000
                Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-23 07:31 -0800
                Re: Objective C Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-23 07:43 -0800
                Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-11-23 08:17 -0800
                Re: Objective C David Brown <david.brown@hesbynett.no> - 2015-11-23 17:34 +0100
                Re: Objective C Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-11-23 22:02 +0000
                Re: Objective C David Brown <david.brown@hesbynett.no> - 2015-11-24 10:12 +0100
                Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-27 08:39 -0800
          Re: Objective C Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-11-14 18:24 +0000
        Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-11-14 12:23 -0800
          Re: Objective C Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-14 12:36 -0800
            Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-11-14 13:13 -0800
              Re: Objective C Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-14 14:04 -0800
                Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-11-14 14:15 -0800
                Re: Objective C Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-14 14:29 -0800
                Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-11-14 15:39 -0800
                Re: Objective C Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-14 16:37 -0800
                Re: Objective C Ian Collins <ian-news@hotmail.com> - 2015-11-15 13:46 +1300
                Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-15 02:06 +0000
                Re: Objective C Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-15 03:25 -0800
                Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-15 12:56 +0000
                Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-15 07:53 -0600
                Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-15 07:19 -0600
                Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-15 13:29 +0000
                Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-11-15 10:56 -0800
                Re: Objective C supercat@casperkitty.com - 2015-11-15 14:35 -0800
                Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-11-15 16:52 -0800
                Re: Objective C supercat@casperkitty.com - 2015-11-15 19:49 -0800
                Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-16 11:02 +0000
                Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-11-16 10:36 -0800
                Re: Objective C David Thompson <dave.thompson2@verizon.net> - 2015-12-06 08:03 -0500
                Re: Objective C Melzzzzz <mel@zzzzz.com> - 2015-11-16 02:01 +0100
                Re: Objective C supercat@casperkitty.com - 2015-11-15 19:34 -0800
                Re: Objective C Chad <cdalten@gmail.com> - 2015-11-15 19:54 -0800
                Re: Objective C raltbos@xs4all.nl (Richard Bos) - 2015-11-17 22:36 +0000
                Re: Objective C supercat@casperkitty.com - 2015-11-17 15:27 -0800
                Re: Objective C Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-17 21:15 -0800
          Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-14 22:08 +0000
      Re: Objective C fir <profesor.fir@gmail.com> - 2015-11-27 14:36 -0800
  Re: Objective C David Brown <david.brown@hesbynett.no> - 2015-11-10 19:03 +0100
    Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-11-10 10:50 -0800

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