Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c > #77003

Re: Understanding C89/C99 aliasing

From Keith Thompson <kst-u@mib.org>
Newsgroups comp.lang.c
Subject Re: Understanding C89/C99 aliasing
Date 2015-11-23 08:28 -0800
Organization None to speak of
Message-ID <lnoaek3ckd.fsf@kst-u.example.com> (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>

Show all headers | View raw

James Kuyper <jameskuyper@verizon.net> writes:
[...]
> 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.
[...]

The definition of "access" (3.1) is:

    <execution-time action> to read or modify the value of an object

6.5p7 says (emphasis added):

    An object shall have *its stored value* accessed only by an lvalue
    expression that has one of the following types:
    ...

Writing to an object certainly accesses the object, but I wouldn't say
that it accesses the value of the object.  In fact I don't think
"accessing a value" is even defined.

    int i;
    i = 42;

The assignment accesses the object i, but not its value.  The value
before the assignment is indeterminate, and could be a trap
representation, but no undefined behavior occurs.

It's plausible that 6.5p7 is *intended* to refer to accessing the object
rather than its value (i.e., that it's meant to apply to both reads and
writes), but the wording is not as clear as it should be.

Perhaps 6.5p7 should be reworded to:

    An object shall be accessed only by an lvalue expression that has
    one of the following types:
    ...

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

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

csiph-web