Re: Pointers as error values

Newsgroups	comp.lang.c
From	John Temples <usenet@xargs-spam.com>
Subject	Re: Pointers as error values
References	<bb26d1c2-246b-45cd-8434-583530881c88@googlegroups.com> <lnlhayzw6g.fsf@kst-u.example.com> <n0781l$lg2$1@speranza.aioe.org>
Message-ID	<slrnn2fjpm.dkf.usenet@xargs-spam.com> (permalink)
Date	2015-10-21 17:45 +0000
Organization	Unlimited download news at news.astraweb.com

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On 2015-10-21, Morten W. Petersen <morphex@gmail.com> wrote:
> On 19.10.2015 17:59, Keith Thompson wrote:
>> "Morten W. Petersen" <morphex@gmail.com> writes:
>>> That got me wondering, is it possible to use a low number range as
>>> valid error indicators?

>>> That is, could for example numbers from 1 to 7 be safely used as error
>>> indicators from a function that should return a pointer?
>>
>> Not 100% portably.  A small non-zero integer converted to a pointer
>> type is unlikely to match a valid pointer value, but there's no
>> guarantee.  Even accessing an invalid pointer (without dereferencing
>> it) can have undefined behavior.
>
> Hmm, OK.  That dereferencing wouldn't work was expected, but why would
> working with an address value not work?
>
> What is a real-life example where using 1-7 as pointer return values
> would not work?

There are many 8- and 16-bit platforms that have linear memory
starting at address 0.  The compiler could place an object at address
7, so (void*)7 would be a valid pointer to an object.  You would have
to reserve the memory at addresses 1-7 to prevent that from happening.

Thread

Pointers as error values "Morten W. Petersen" <morphex@gmail.com> - 2015-10-19 00:53 -0700
  Re: Pointers as error values gazelle@shell.xmission.com (Kenny McCormack) - 2015-10-19 08:26 +0000
    Re: Pointers as error values "Morten W. Petersen" <morphex@gmail.com> - 2015-10-19 02:27 -0700
    Re: Pointers as error values Richard Damon <Richard@Damon-Family.org> - 2015-10-19 23:38 -0400
  Re: Pointers as error values jacobnavia <jacob@jacob.remcomp.fr> - 2015-10-19 10:34 +0200
    Re: Pointers as error values "Morten W. Petersen" <morphex@gmail.com> - 2015-10-19 02:29 -0700
    Re: Pointers as error values mark.bluemel@gmail.com - 2015-10-19 05:53 -0700
  Re: Pointers as error values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-19 10:00 +0000
  Re: Pointers as error values Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-10-19 14:00 +0000
  Re: Pointers as error values gwowen <gwowen@gmail.com> - 2015-10-19 08:02 -0700
    Re: Pointers as error values James Kuyper <jameskuyper@verizon.net> - 2015-10-19 11:53 -0400
  Re: Pointers as error values Ken Brody <kenbrody@spamcop.net> - 2015-10-19 11:25 -0400
  Re: Pointers as error values supercat@casperkitty.com - 2015-10-19 08:41 -0700
  Re: Pointers as error values Keith Thompson <kst-u@mib.org> - 2015-10-19 08:59 -0700
    Re: Pointers as error values "Morten W. Petersen" <morphex@gmail.com> - 2015-10-21 07:32 +0200
      Re: Pointers as error values Ian Collins <ian-news@hotmail.com> - 2015-10-21 19:17 +1300
        Re: Pointers as error values "Morten W. Petersen" <morphex@gmail.com> - 2015-10-21 08:31 +0200
      Re: Pointers as error values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-21 12:46 +0100
      Re: Pointers as error values Robert Wessel <robertwessel2@yahoo.com> - 2015-10-21 09:35 -0500
        Re: Pointers as error values Ken Brody <kenbrody@spamcop.net> - 2015-10-21 15:24 -0400
          Re: Pointers as error values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-22 05:44 +0000
      Re: Pointers as error values Keith Thompson <kst-u@mib.org> - 2015-10-21 10:41 -0700
        Re: Pointers as error values supercat@casperkitty.com - 2015-10-21 11:28 -0700
      Re: Pointers as error values John Temples <usenet@xargs-spam.com> - 2015-10-21 17:45 +0000
        Re: Pointers as error values James Kuyper <jameskuyper@verizon.net> - 2015-10-21 14:15 -0400
          Re: Pointers as error values "Morten W. Petersen" <morphex@gmail.com> - 2015-10-29 16:59 +0100
            Re: Pointers as error values supercat@casperkitty.com - 2015-10-29 12:35 -0700
              Re: Pointers as error values raltbos@xs4all.nl (Richard Bos) - 2015-10-29 21:45 +0000
                Re: Pointers as error values "Morten W. Petersen" <morphex@gmail.com> - 2015-10-30 06:09 +0100
                Re: Pointers as error values Rosario19 <Ros@invalid.invalid> - 2015-10-30 08:59 +0100
                Re: Pointers as error values Rosario19 <Ros@invalid.invalid> - 2015-10-30 09:07 +0100
                Re: Pointers as error values Rosario19 <Ros@invalid.invalid> - 2015-10-30 09:38 +0100
                Re: Pointers as error values Rosario19 <Ros@invalid.invalid> - 2015-10-30 09:40 +0100
                Re: Pointers as error values Robert Wessel <robertwessel2@yahoo.com> - 2015-10-30 04:56 -0500
                Re: Pointers as error values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-30 11:00 +0000
            Re: Pointers as error values Robert Wessel <robertwessel2@yahoo.com> - 2015-10-29 23:48 -0500
              Re: Pointers as error values "Morten W. Petersen" <morphex@gmail.com> - 2015-10-30 11:05 +0100
                Re: Pointers as error values Richard Damon <Richard@Damon-Family.org> - 2015-10-30 07:45 -0400
                Re: Pointers as error values Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-10-30 05:04 -0700
                Re: Pointers as error values "Morten W. Petersen" <morphex@gmail.com> - 2015-10-30 16:09 +0100
                Re: Pointers as error values Keith Thompson <kst-u@mib.org> - 2015-10-30 11:16 -0700
                Re: Pointers as error values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-30 20:58 +0000
                Re: Pointers as error values Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-10-30 17:12 -0400
                Re: Pointers as error values supercat@casperkitty.com - 2015-10-30 14:54 -0700
                Re: Pointers as error values Robert Wessel <robertwessel2@yahoo.com> - 2015-10-30 18:48 -0500
                Re: Pointers as error values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-11-01 04:00 +0000
                Re: Pointers as error values Robert Wessel <robertwessel2@yahoo.com> - 2015-11-01 01:24 -0500
                Re: Pointers as error values "Morten W. Petersen" <morphex@gmail.com> - 2015-10-31 20:35 +0100
                Re: Pointers as error values supercat@casperkitty.com - 2015-10-31 13:33 -0700
                Re: Pointers as error values Richard Damon <Richard@Damon-Family.org> - 2015-10-30 22:19 -0400
                Re: Pointers as error values supercat@casperkitty.com - 2015-10-30 08:22 -0700
                Re: Pointers as error values Geoff <geoff@invalid.invalid> - 2015-10-30 13:11 -0700
                Re: Pointers as error values Geoff <geoff@invalid.invalid> - 2015-10-30 13:23 -0700
      Re: Pointers as error values Geoff <geoff@invalid.invalid> - 2015-10-21 11:13 -0700
        Re: Pointers as error values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-21 20:31 +0100
          Re: Pointers as error values Geoff <geoff@invalid.invalid> - 2015-10-22 07:13 -0700
        Re: Pointers as error values "Morten W. Petersen" <morphex@gmail.com> - 2015-10-26 07:16 +0100
          Re: Pointers as error values Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-10-26 09:37 +0000
      Re: Pointers as error values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-22 05:38 +0000
        Re: Pointers as error values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-22 13:51 +0100
        Re: Pointers as error values Keith Thompson <kst-u@mib.org> - 2015-10-22 08:29 -0700
          Re: Pointers as error values supercat@casperkitty.com - 2015-10-22 09:21 -0700
            Re: Pointers as error values Keith Thompson <kst-u@mib.org> - 2015-10-22 10:52 -0700
              Re: Pointers as error values "Morten W. Petersen" <morphex@gmail.com> - 2015-10-26 07:20 +0100
                Re: Pointers as error values Keith Thompson <kst-u@mib.org> - 2015-10-26 08:09 -0700
                Re: Pointers as error values supercat@casperkitty.com - 2015-10-26 08:40 -0700
                Re: Pointers as error values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-26 20:02 +0000
                Re: Pointers as error values supercat@casperkitty.com - 2015-10-26 13:56 -0700
          Re: Pointers as error values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-22 21:58 +0000
            Re: Pointers as error values James Kuyper <jameskuyper@verizon.net> - 2015-10-22 19:12 -0400
            Re: Pointers as error values asetofsymbols@gmail.com - 2015-10-22 18:36 -0700
              Re: Pointers as error values asetofsymbols@gmail.com - 2015-10-29 12:47 -0700
  Re: Pointers as error values Lőrinczy Zsigmond <nospam@for.me> - 2015-10-20 17:06 +0200
    Re: Pointers as error values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-20 19:45 +0100
      Re: Pointers as error values supercat@casperkitty.com - 2015-10-20 14:19 -0700
        Re: Pointers as error values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-21 05:28 +0000
          Re: Pointers as error values supercat@casperkitty.com - 2015-10-21 05:48 -0700
            Re: Pointers as error values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-22 05:52 +0000
          Re: Pointers as error values Ken Brody <kenbrody@spamcop.net> - 2015-10-21 15:19 -0400
            Re: Pointers as error values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-22 06:06 +0000

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