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Groups > comp.lang.c > #78216

Re: Understanding C89/C99 aliasing

From Keith Thompson <kst-u@mib.org>
Newsgroups comp.lang.c
Subject Re: Understanding C89/C99 aliasing
Date 2015-12-08 19:13 -0800
Organization None to speak of
Message-ID <lnsi3cmi0k.fsf@kst-u.example.com> (permalink)
References (10 earlier) <kfn8u5ixdsf.fsf@x-alumni2.alumni.caltech.edu> <587e1cad-46d3-41d0-8b5f-defd495d5f98@googlegroups.com> <kfnr3iwx45q.fsf@x-alumni2.alumni.caltech.edu> <n47qul$8k1$1@speranza.aioe.org> <f6e4e7db-d017-482e-a459-2bf7e26b4120@googlegroups.com>

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supercat@casperkitty.com writes:
> On Tuesday, December 8, 2015 at 6:00:35 PM UTC-6, glen herrmannsfeldt wrote:
>> Why is it that C didn't allow for variable length arrays until so
>> recently?  Other languages that allow for dynamic (auto) allocation
>> allow for them, when the size is known at block entry.
>
> Having automatic allocation at block entry is not possible unless either
> (1) a language has a defined means of handling or predicting an inability
> to allocate the necessary space or predict failure, or (2) nobody cares
> about being able to handle nor predict such failures.
>
> Some languages take the first approach.  C99 took the second.  IMHO, the
> second approach is pretty much worthless in a "standard", but other people
> may differ.

C90 automatic objects (arrays or otherwise) have exactly the same
behavior: if allocation fails, the behavior is undefined.

For some usage patterns, VLAs can make the problem less severe.
In C90, you might define an automatic array that's big enough for
any possible usage; in C99, you can define a VLA that's no bigger
than it needs to be.

-- 
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"

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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

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