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| From | Keith Thompson <kst-u@mib.org> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Pointers as error values |
| Date | 2015-10-26 08:09 -0700 |
| Organization | None to speak of |
| Message-ID | <lnsi4xfz03.fsf@kst-u.example.com> (permalink) |
| References | (3 earlier) <n09sod$e2a$1@speranza.aioe.org> <lnlhaux6oi.fsf@kst-u.example.com> <fd50cbf6-4bc6-43e4-9c38-35836dab9bbb@googlegroups.com> <lnh9lix03a.fsf@kst-u.example.com> <n0kgnd$6t9$1@speranza.aioe.org> |
"Morten W. Petersen" <morphex@gmail.com> writes:
> On 22.10.2015 19:52, Keith Thompson wrote:
>> supercat@casperkitty.com writes:
> [...]
>>> I recall having read that
>>> casts or comparisons to a constant literal zero are special, and
>>> that even when a platform defines conversions between pointers and
>>> numbers they need not be consistent with that.
>>
>> N1570 6.3.2.3p3:
>>
>> An integer constant expression with the value 0, or such
>> an expression cast to type void *, is called a null pointer
>> constant. If a null pointer constant is converted to a pointer
>> type, the resulting pointer, called a null pointer, is guaranteed
>> to compare unequal to a pointer to any object or function.
>>
>> Integer-to-pointer conversions are otherwise implementation-defined
>> (same section, paragraph 5).
>>
>> Which means that given:
>>
>> int zero = 0;
>> void *p0 = (void*)0;
>> void *p1 = (void*)zero;
>>
>> the value of p0 is guaranteed to be a null pointer, but it's not
>> guaranteed that p0 == p1.
>
> This can be tested, that the different pointers and pointer < 8 works;
> the only thing I don't see a way to test, is whether it is possible that
> a pointer that is <8 could point to a valid address.
It can't be tested *reliably*. Even if (ptr != (void*)7) works reliably
when you test it on your system, that's no guarantee that it will work
when your code is recompiled for some other system, or even when it's
run on a differently configured system without recompilation.
--
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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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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