Re: offset 1 list indexing

From	Nobody <nobody@nowhere.com>
Subject	Re: offset 1 list indexing
Date	2011-06-15 05:58 +0100
Message-Id	<pan.2011.06.15.04.58.15.203000@nowhere.com>
Newsgroups	comp.lang.c
References	<rubrum-E97163.21022514062011@news.albasani.net>
Organization	Zen Internet

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On Tue, 14 Jun 2011 21:02:25 -0700, Michael Press wrote:

> I want to index into a list using offset one indexing.
> The obvious thing to do is make a pointer that points
> one element before the list.

> Is this likely to fail?

No.

> Possible to fail?

Yes.

> Very unlikely to fail?

Depends upon the platform. On a typical flat-memory architecture, it's
unlikely to fail, although I suppose that it could fail if the function is
inlined and the compiler is being especially clever.

> Not guaranteed by the standard?

AFAICT, it's okay by the standard so long as offset0 actually points at
least one element into an array. If it points to the first element of an
array, the behaviour is undefined.

Whether or not it works practice comes down to whether the architecture
cares about referencing invalid memory locations rather than actually
accessing them, e.g. segmented memory, trap representations, etc.

Thread

offset 1 list indexing Michael Press <rubrum@pacbell.net> - 2011-06-14 21:02 -0700
  Re: offset 1 list indexing Malcolm McLean <malcolm.mclean5@btinternet.com> - 2011-06-14 21:29 -0700
  Re: offset 1 list indexing Nobody <nobody@nowhere.com> - 2011-06-15 05:58 +0100
  Re: offset 1 list indexing Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-15 02:51 -0700
  Re: offset 1 list indexing jacob navia <jacob@spamsink.net> - 2011-06-15 12:01 +0200
    Re: offset 1 list indexing gwowen <gwowen@gmail.com> - 2011-06-15 03:55 -0700
  Re: offset 1 list indexing Mark Bluemel <mark_bluemel@pobox.com> - 2011-06-15 11:07 +0100
    Re: offset 1 list indexing Malcolm McLean <malcolm.mclean5@btinternet.com> - 2011-06-15 07:30 -0700
      Re: offset 1 list indexing Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2011-06-15 11:19 -0600
        Re: offset 1 list indexing James Kuyper <jameskuyper@verizon.net> - 2011-06-15 22:57 -0400
        Re: offset 1 list indexing Joseph Huber <joseph.huber@NOREPLY.web.de> - 2011-06-16 09:28 +0200
          Re: offset 1 list indexing Angel <angel+news@spamcop.net> - 2011-06-16 15:05 +0000
          Re: offset 1 list indexing jacob navia <jacob@spamsink.net> - 2011-06-24 01:14 +0200
      Re: offset 1 list indexing Michael Press <rubrum@pacbell.net> - 2011-06-15 15:33 -0700
    Re: offset 1 list indexing Michael Press <rubrum@pacbell.net> - 2011-06-15 15:28 -0700
      Re: offset 1 list indexing Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-15 23:34 -0400
        Re: offset 1 list indexing Michael Press <rubrum@pacbell.net> - 2011-06-15 21:42 -0700
          Re: offset 1 list indexing Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-16 08:18 -0400
            Re: offset 1 list indexing James Kuyper <jameskuyper@verizon.net> - 2011-06-16 08:32 -0400
              Re: offset 1 list indexing Malcolm McLean <malcolm.mclean5@btinternet.com> - 2011-06-16 06:29 -0700
                Re: offset 1 list indexing James Kuyper <jameskuyper@verizon.net> - 2011-06-16 09:43 -0400
              Re: offset 1 list indexing Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-16 20:50 -0400
                Re: offset 1 list indexing James Kuyper <jameskuyper@verizon.net> - 2011-06-17 08:56 -0400
                Re: offset 1 list indexing cri@tiac.net (Richard Harter) - 2011-06-17 15:11 +0000
            Re: offset 1 list indexing Michael Press <rubrum@pacbell.net> - 2011-06-16 23:54 -0700
              Re: offset 1 list indexing Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-06-23 21:45 +0300
                Re: offset 1 list indexing Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-25 09:30 -0700
                Re: offset 1 list indexing pete <pfiland@mindspring.com> - 2011-06-25 13:10 -0400
                Re: offset 1 list indexing cri@tiac.net (Richard Harter) - 2011-06-25 18:18 +0000
                Re: offset 1 list indexing Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-25 15:56 -0400
                Re: offset 1 list indexing James Kuyper <jameskuyper@verizon.net> - 2011-06-25 17:42 -0400
                Re: offset 1 list indexing cri@tiac.net (Richard Harter) - 2011-06-26 04:15 +0000
                Re: offset 1 list indexing pete <pfiland@mindspring.com> - 2011-06-26 01:51 -0400
                Re: offset 1 list indexing cri@tiac.net (Richard Harter) - 2011-06-26 15:48 +0000
                Re: offset 1 list indexing pete <pfiland@mindspring.com> - 2011-06-27 00:30 -0400
                Re: offset 1 list indexing Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-06-28 21:26 +0300
                Re: offset 1 list indexing pete <pfiland@mindspring.com> - 2011-06-28 23:36 -0400
                Re: offset 1 list indexing cri@tiac.net (Richard Harter) - 2011-06-29 15:28 +0000
                Re: offset 1 list indexing pete <pfiland@mindspring.com> - 2011-06-30 15:02 -0400
                Re: offset 1 list indexing Ike Naar <ike@sverige.freeshell.org> - 2011-06-26 07:00 +0000
                Re: offset 1 list indexing cri@tiac.net (Richard Harter) - 2011-06-26 15:20 +0000
                Re: offset 1 list indexing cri@tiac.net (Richard Harter) - 2011-06-26 03:41 +0000
                Re: offset 1 list indexing Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-26 08:51 -0400
                Re: offset 1 list indexing cri@tiac.net (Richard Harter) - 2011-06-26 15:32 +0000
                Re: offset 1 list indexing Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-26 11:48 -0400
                Re: offset 1 list indexing cri@tiac.net (Richard Harter) - 2011-06-26 16:43 +0000
                Re: offset 1 list indexing Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-06-28 21:30 +0300
                Re: offset 1 list indexing Karl Malbrain <malbrain@yahoo.com> - 2011-06-28 14:13 -0700
                Re: offset 1 list indexing Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-06-28 21:14 +0300
                Re: offset 1 list indexing Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-29 01:14 -0700
  Re: offset 1 list indexing Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-15 07:21 -0400
  Re: offset 1 list indexing Shao Miller <sha0.miller@gmail.com> - 2011-06-15 10:09 -0400
    Re: offset 1 list indexing Michael Press <rubrum@pacbell.net> - 2011-06-15 15:38 -0700
  Re: offset 1 list indexing China Blue Dolls <chine.bleu@yahoo.com> - 2011-06-25 21:04 -0700
    Re: offset 1 list indexing Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-26 09:01 -0400
    Re: offset 1 list indexing Michael Press <rubrum@pacbell.net> - 2011-06-26 13:15 -0700

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