Re: Unspecified behavior in C

From	James Kuyper <jameskuyper@verizon.net>
Newsgroups	comp.lang.c
Subject	Re: Unspecified behavior in C
Date	2015-08-04 21:27 -0400
Organization	A noiseless patient Spider
Message-ID	<mpromb$m9t$1@dont-email.me> (permalink)
References	(16 earlier) <mpev80$mfg$1@dont-email.me> <8d3d7a31-90d4-4e81-bdd3-0e82fa5f970a@googlegroups.com> <mphe9c$abl$1@dont-email.me> <kfnd1z38g7g.fsf@x-alumni2.alumni.caltech.edu> <5ce66777-e0c9-47d2-ac53-33182dd079b0@googlegroups.com>

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On 08/04/2015 09:02 PM, David Kleinecke wrote:
> On Tuesday, August 4, 2015 at 4:07:40 AM UTC-7, Tim Rentsch wrote:
>> James Kuyper <jameskuyper@verizon.net> writes:
>>
>>> For example, the expression --p->mem[a]++
>>
>> The expression --p->mem[a]++ contains a constraint violation.
>>
>>> contains all of the the following 5 sub-expressions:
>>>     --p->mem[a]    subscript expression
>>>     --p->mem       member selection expression
>>>     a	             primary expression
>>>     --p            pre-fix decrement expression
>>>     p              primary expression
>>
>> And this isn't how it is parsed.  The subexpressions are
>>
>>     p
>>     a
>>     p->mem
>>     p->mem[a]
>>     p->mem[a]++
>>
>> which are proper subexpressions, and the entire original
>> expression, which is a non-proper subexpression.

Tim is correct, I was insufficiently careful putting together my example.

>      p
>      p->mem
>      a
>      p->mem[a]
>      p->mem[a]++
> 
> ?

It's a rather important point that the C standard says that the
evalation of a is sequenced before it's use as a subscript, but that it
is unsequenced relative to the evaluations of p and p->mem. Therefore,
your order is just as acceptable as Tim's. The order matters only if the
relevant expressions have side effects, which would apply to 'p' and 'a'
only if they were volatile-qualified. If so, those side-effects are
unsequenced relative to any of the other evaluations in the [corrected
version of] that expression, because it contains not a single sequence
point. In a slightly more complicated expression, this is easier to see:

	(--p)->mem[++a]++

That expression has three different side-effects: --p, ++a, and the
final ++. Because there are no sequence points in that expression, those
side-effects are unsequenced relative to each other.
-- 
James Kuyper

Thread

Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-07 14:44 -0700
  Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-07-07 15:34 -0700
    Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-08 08:47 -0700
      Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-07-08 09:11 -0700
        Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-08 10:24 -0700
          Re: Unspecified behavior in C John Bode <jfbode1029@gmail.com> - 2015-07-08 10:46 -0700
          Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-08 13:54 -0400
          Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-07-08 11:12 -0700
            Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-09 09:15 -0700
              Re: Unspecified behavior in C Richard Heathfield <rjh@cpax.org.uk> - 2015-07-09 17:33 +0100
                Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-10 09:24 -0700
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-10 12:54 -0400
              Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-07-09 10:58 -0700
                Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-10 09:33 -0700
                Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-07-10 12:11 -0700
                Re: Unspecified behavior in C glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-07-10 21:05 +0000
                Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-07-10 17:14 -0700
                Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-11 09:32 -0700
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-11 12:59 -0400
                Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-07-11 17:21 -0700
                Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-11 09:23 -0700
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-11 13:03 -0400
                Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-07-11 17:15 -0700
                Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-12 09:52 -0700
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-12 14:13 -0400
                Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-13 09:02 -0700
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-13 13:18 -0400
                Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-13 11:09 -0700
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-13 15:06 -0400
                Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-14 08:14 -0700
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-14 12:02 -0400
                Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-07-13 12:03 -0700
                Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-14 07:54 -0700
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-14 11:16 -0400
                Re: Unspecified behavior in C Richard Heathfield <rjh@cpax.org.uk> - 2015-07-14 16:17 +0100
                Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-14 09:16 -0700
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-14 12:29 -0400
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-14 14:57 -0400
                Re: Unspecified behavior in C Öö Tiib <ootiib@hot.ee> - 2015-07-14 13:12 -0700
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-14 16:30 -0400
                Re: Unspecified behavior in C Öö Tiib <ootiib@hot.ee> - 2015-07-14 15:54 -0700
                Re: Unspecified behavior in C Morris Dovey <mrdovey@iedu.com> - 2015-07-14 21:41 -0500
                Re: Unspecified behavior in C Morris Dovey <mrdovey@iedu.com> - 2015-07-14 21:51 -0500
                Re: Unspecified behavior in C Öö Tiib <ootiib@hot.ee> - 2015-07-15 12:42 -0700
                Re: Unspecified behavior in C Morris Dovey <mrdovey@iedu.com> - 2015-07-15 16:13 -0500
                Re: Unspecified behavior in C Morris Dovey <mrdovey@iedu.com> - 2015-07-16 08:49 -0500
                Re: Unspecified behavior in C Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-20 10:10 -0700
                Re: Unspecified behavior in C Morris Dovey <mrdovey@iedu.com> - 2015-07-20 13:24 -0500
                Re: Unspecified behavior in C Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-23 02:56 -0700
                Re: Unspecified behavior in C Morris Dovey <mrdovey@iedu.com> - 2015-07-23 11:26 -0500
                Re: Unspecified behavior in C Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-26 11:24 -0700
                Re: Unspecified behavior in C Morris Dovey <mrdovey@iedu.com> - 2015-07-23 19:03 -0500
                Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-15 21:19 -0700
                Re: Unspecified behavior in C Martin Shobe <martin.shobe@yahoo.com> - 2015-07-15 23:45 -0500
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-16 01:37 -0400
                Re: Unspecified behavior in C Richard Heathfield <rjh@cpax.org.uk> - 2015-07-16 09:42 +0100
                Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-16 17:05 -0700
                Re: Unspecified behavior in C Richard Heathfield <rjh@cpax.org.uk> - 2015-07-17 01:32 +0100
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-16 21:07 -0400
                We're good - don't you worry 'bout a thing, babe (Was: Unspecified behavior in C) gazelle@shell.xmission.com (Kenny McCormack) - 2015-07-17 07:21 +0000
                Re: Unspecified behavior in C raltbos@xs4all.nl (Richard Bos) - 2015-07-19 20:59 +0000
                Re: Unspecified behavior in C Richard Heathfield <rjh@cpax.org.uk> - 2015-07-19 22:02 +0100
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-16 17:59 -0400
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-16 23:09 -0400
                Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-17 18:48 -0700
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-17 23:09 -0400
                Re: Unspecified behavior in C raltbos@xs4all.nl (Richard Bos) - 2015-07-19 20:57 +0000
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-19 17:22 -0400
                Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-19 18:57 -0700
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-19 22:16 -0400
                Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-19 19:30 -0700
                Re: Unspecified behavior in C John Bode <jfbode1029@gmail.com> - 2015-07-30 14:20 -0700
                Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-07-30 15:30 -0700
                Re: Unspecified behavior in C John Bode <jfbode1029@gmail.com> - 2015-07-30 15:53 -0700
                Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-30 16:46 -0700
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-31 00:59 -0400
                Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-31 17:15 -0700
                Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-07-31 18:28 -0700
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-31 23:28 -0400
                Re: Unspecified behavior in C Tim Rentsch <txr@alumni.caltech.edu> - 2015-08-04 04:07 -0700
                Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-08-04 18:02 -0700
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-08-04 21:27 -0400
                Re: Unspecified behavior in C Tim Rentsch <txr@alumni.caltech.edu> - 2015-08-24 09:33 -0700
                Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-08-24 17:11 -0700
                Re: Unspecified behavior in C Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-06 12:44 -0700
                Re: Unspecified behavior in C Ken Brody <kenbrody@spamcop.net> - 2015-08-03 12:09 -0400
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-08-03 13:26 -0400
                Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-08-03 17:53 -0700
                Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-08-03 18:44 -0700
                Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-08-04 17:59 -0700
                Re: Unspecified behavior in C Richard Heathfield <rjh@cpax.org.uk> - 2015-08-05 02:23 +0100
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-08-04 21:39 -0400
                Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-08-04 19:27 -0700
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-08-04 00:25 -0400
                Re: Unspecified behavior in C Ken Brody <kenbrody@spamcop.net> - 2015-08-04 11:55 -0400
                Re: Unspecified behavior in C Philip Lantz <prl@canterey.us> - 2015-08-04 10:31 -0700
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-08-04 13:32 -0400
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-08-04 22:46 -0400
                Re: Unspecified behavior in C Ken Brody <kenbrody@spamcop.net> - 2015-08-10 12:16 -0400
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-08-10 13:26 -0400
                Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-08-03 17:46 -0700
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-08-05 12:34 -0400
                Re: Unspecified behavior in C Richard Heathfield <rjh@cpax.org.uk> - 2015-07-14 17:57 +0100
                Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-14 10:31 -0700
                Re: Unspecified behavior in C Richard Heathfield <rjh@cpax.org.uk> - 2015-07-14 18:50 +0100
                Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-15 20:59 -0700
                Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-07-14 11:34 -0700
                Re: Unspecified behavior in C raltbos@xs4all.nl (Richard Bos) - 2015-07-19 20:46 +0000
                Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-07-12 12:07 -0700
                Re: Unspecified behavior in C glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-07-12 19:53 +0000
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-12 16:12 -0400
              Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-09 14:20 -0400
                Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-07-09 12:18 -0700
                Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-10 09:47 -0700
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-10 13:36 -0400
                Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-11 09:19 -0700
                Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-11 12:56 -0400
                Re: Unspecified behavior in C gordonb.j19di@burditt.org (Gordon Burditt) - 2015-07-11 20:36 -0500
                Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-12 10:10 -0700
              Re: Unspecified behavior in C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-07-09 20:13 +0100
          Re: Unspecified behavior in C raltbos@xs4all.nl (Richard Bos) - 2015-07-09 11:17 +0000
        Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-08 10:37 -0700
          Re: Unspecified behavior in C John Bode <jfbode1029@gmail.com> - 2015-07-08 10:51 -0700
          Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-08 14:14 -0400
          Re: Unspecified behavior in C Keith Thompson <kst-u@mib.org> - 2015-07-08 12:19 -0700
      Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-08 12:14 -0400
        Re: Unspecified behavior in C David Kleinecke <dkleinecke@gmail.com> - 2015-07-08 10:40 -0700
          Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-08 14:29 -0400
            Re: Unspecified behavior in C raltbos@xs4all.nl (Richard Bos) - 2015-07-09 11:20 +0000
      Re: Unspecified behavior in C Robert Wessel <robertwessel2@yahoo.com> - 2015-07-09 02:30 -0500
  Re: Unspecified behavior in C James Kuyper <jameskuyper@verizon.net> - 2015-07-07 22:30 -0400

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