Re: References in C

From	James Kuyper <jameskuyper@verizon.net>
Newsgroups	comp.lang.c
Subject	Re: References in C
Date	2011-07-01 07:58 -0400
Organization	A noiseless patient Spider
Message-ID	<iukcp6$50a$1@dont-email.me> (permalink)
References	(3 earlier) <kfnboxj7yn3.fsf@x-alumni2.alumni.caltech.edu> <96srjcF2dgU10@mid.individual.net> <rubrum-B04D1B.13411029062011@news.albasani.net> <slrnj0n59l.hni.usenet-nospam@guild.seebs.net> <rubrum-0CD7A1.09264430062011@news.albasani.net>

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On 06/30/2011 12:26 PM, Michael Press wrote:
> In article <slrnj0n59l.hni.usenet-nospam@guild.seebs.net>,
>  Seebs <usenet-nospam@seebs.net> wrote:
> 
>> On 2011-06-29, Michael Press <rubrum@pacbell.net> wrote:
>>> In article <96srjcF2dgU10@mid.individual.net>,
>>>  Ian Collins <ian-news@hotmail.com> wrote:
>>>> Look at the proportion of posts here that relate to pointers.
>>
>>> Anyone who knows how a stored a stored program,
>>> digital computer works has no difficulty with pointers.
>>
>> *No* difficulty?
>>
>> I'm moderately optimistic that I have a pretty good basic understanding
>> of how pointers work at this point, but I still trip on 'em sometimes.
> 
> There is a list of storage registers holding a number.
> 
> Each storage register has an address, itself a number.
> 
> The addresses are sequential: e.g. 0, 1, 2, 3, ..., N.
> 
> The contents of a storage register can be read, and its
> contents interpreted as a machine instruction and
> executed.
> 
> The contents of a storage register can be interpreted 
> as an address, A, to fetch the contents of the storage
> register at address A.
> 
> That's it. That is the only difficulty. The rest is
> details and thinking straight---a chronic difficulty
> for some, but not an inherent difficulty of pointers. 

Pointer values are more than just the addresses of things; they are a
higher level construct than that, and most of the problems associated
with them occur at the higher level. The most important high-level
feature of pointers, which is not explicitly covered by your
description, is the fact that they can be invalid. They can become
invalid for some purposes while remaining valid for others:

1. comparison of two pointer values for equality
2. comparison of two pointer values for relative order
3. reading through the pointer value
4. writing through the pointer value

Your description of hardware addresses incorrectly implies that
comparison of addresses for equality should always be safe, and that
comparison of addresses for relative order should always produce
consistent results; there are real-world fully-conforming
implementations of C for which that's not the case for C pointer values.

Uninitialized pointer objects, and pointer objects whose representation
has been manipulated by means other than assignment through an lvalue of
a compatible type, may have trap representations. The non-existent value
of such pointer objects, and pointer values that used to point at
objects whose lifetime has since ended, have undefined behavior for all
four uses.

Null pointer values can only be compared for equality with other pointer
values; the other three uses all have undefined behavior.
Comparing pointer values for relative order that do not point within (or
one past the end of) the same array object (with single objects being
treated as 1-element arrays) has undefined behavior.

Attempting to write through a pointer value that points at an object
defined as 'const' has undefined behavior; the same is true of any
pointer value that points into the array corresponding to a string literal.

> Likely we are thinking of different notions when using 
> the word "difficulty."

Possibly; in my experience, most of the difficulty with C pointers comes
from the necessity of avoiding the undefined behavior referred to above,
an issue not even hinted at by your description of hardware addresses.
With care, it's possible to avoid most such problems; but care is indeed
required, and it's not always easy. To say it's "no difficulty" is to
imply that such care is unnecessary.
-- 
James Kuyper

Thread

References in C jacob navia <jacob@spamsink.net> - 2011-06-24 10:42 +0200
  Re: References in C BGB <cr88192@hotmail.com> - 2011-06-24 02:53 -0700
    Re: References in C Ian Collins <ian-news@hotmail.com> - 2011-06-24 22:10 +1200
  Re: References in C "BartC" <bc@freeuk.com> - 2011-06-24 11:14 +0100
    Re: References in C jacob navia <jacob@spamsink.net> - 2011-06-24 16:59 +0200
  Re: References in C Shao Miller <sha0.miller@gmail.com> - 2011-06-24 09:26 -0400
    Re: References in C jacob navia <jacob@spamsink.net> - 2011-06-24 20:08 +0200
      Re: References in C Keith Thompson <kst-u@mib.org> - 2011-06-24 11:40 -0700
      Re: References in C Shao Miller <sha0.miller@gmail.com> - 2011-06-24 14:53 -0400
      Re: References in C jt@toerring.de (Jens Thoms Toerring) - 2011-06-24 20:44 +0000
        Re: References in C jacob navia <jacob@spamsink.net> - 2011-06-24 23:09 +0200
          Re: References in C Joe Wright <joewwright@comcast.net> - 2011-06-24 17:38 -0400
            Re: References in C "BartC" <bc@freeuk.com> - 2011-06-24 22:58 +0100
            Re: References in C Ian Collins <ian-news@hotmail.com> - 2011-06-25 10:11 +1200
              Re: References in C Shao Miller <sha0.miller@gmail.com> - 2011-06-26 17:00 -0500
                Re: References in C "BartC" <bc@freeuk.com> - 2011-06-26 22:55 +0100
                Re: References in C Shao Miller <sha0.miller@gmail.com> - 2011-06-26 18:25 -0500
                Re: References in C "BartC" <bc@freeuk.com> - 2011-06-26 23:38 +0100
                Re: References in C Shao Miller <sha0.miller@gmail.com> - 2011-06-26 19:19 -0500
                Re: References in C jacob navia <jacob@spamsink.net> - 2011-06-27 01:27 +0200
                Re: References in C Keith Thompson <kst-u@mib.org> - 2011-06-26 19:13 -0700
                Re: References in C Ian Collins <ian-news@hotmail.com> - 2011-06-27 16:50 +1200
                Re: References in C jacob navia <jacob@spamsink.net> - 2011-06-27 06:55 +0200
                Re: References in C Ian Collins <ian-news@hotmail.com> - 2011-06-27 17:10 +1200
                Re: References in C lawrence.jones@siemens.com - 2011-06-27 10:04 -0400
                Re: References in C jt@toerring.de (Jens Thoms Toerring) - 2011-06-26 22:54 +0000
                Re: References in C jacob navia <jacob@spamsink.net> - 2011-06-27 01:29 +0200
                Re: References in C Shao Miller <sha0.miller@gmail.com> - 2011-06-26 19:39 -0500
                Re: References in C pacman@kosh.dhis.org (Alan Curry) - 2011-06-27 07:16 +0000
                Re: References in C jacob navia <jacob@spamsink.net> - 2011-06-27 09:46 +0200
                Re: References in C Shao Miller <sha0.miller@gmail.com> - 2011-06-27 12:03 -0400
                Re: References in C Stephen Sprunk <stephen@sprunk.org> - 2011-06-27 12:09 -0500
                Re: References in C pacman@kosh.dhis.org (Alan Curry) - 2011-06-27 22:16 +0000
                Re: References in C Shao Miller <sha0.miller@gmail.com> - 2011-06-27 11:57 -0400
                Re: References in C pacman@kosh.dhis.org (Alan Curry) - 2011-06-27 22:21 +0000
                Re: References in C Ian Collins <ian-news@hotmail.com> - 2011-06-28 13:59 +1200
                Re: References in C Ike Naar <ike@iceland.freeshell.org> - 2011-06-27 07:05 +0000
                Re: References in C jt@toerring.de (Jens Thoms Toerring) - 2011-06-27 20:10 +0000
                Re: References in C Stephen Sprunk <stephen@sprunk.org> - 2011-06-27 16:39 -0500
                Re: References in C Keith Thompson <kst-u@mib.org> - 2011-06-26 15:23 -0700
                Re: References in C Shao Miller <sha0.miller@gmail.com> - 2011-06-26 18:57 -0500
                Re: References in C Keith Thompson <kst-u@mib.org> - 2011-06-26 19:21 -0700
                Re: References in C Shao Miller <sha0.miller@gmail.com> - 2011-06-27 09:40 -0400
                Re: References in C Keith Thompson <kst-u@mib.org> - 2011-06-27 09:00 -0700
                Re: References in C pacman@kosh.dhis.org (Alan Curry) - 2011-06-27 23:15 +0000
                Re: References in C Keith Thompson <kst-u@mib.org> - 2011-06-28 22:50 -0700
                Re: References in C Seebs <usenet-nospam@seebs.net> - 2011-06-29 17:46 +0000
                Re: References in C Dr Nick <3-nospam@temporary-address.org.uk> - 2011-06-29 20:23 +0100
                Re: References in C pacman@kosh.dhis.org (Alan Curry) - 2011-06-29 22:35 +0000
                Re: References in C Keith Thompson <kst-u@mib.org> - 2011-06-29 16:52 -0700
                Re: References in C Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-07-03 01:31 +0300
                Re: References in C Chad <cdalten@gmail.com> - 2011-07-02 17:20 -0700
                Re: References in C Shao Miller <sha0.miller@gmail.com> - 2011-06-29 18:54 -0500
                Re: References in C Michael Press <rubrum@pacbell.net> - 2011-06-30 10:32 -0700
                Re: References in C "io_x" <a@b.c.invalid> - 2011-07-01 10:19 +0200
                Re: References in C pacman@kosh.dhis.org (Alan Curry) - 2011-07-01 19:21 +0000
                Re: References in C Keith Thompson <kst-u@mib.org> - 2011-07-01 12:31 -0700
                Re: References in C pacman@kosh.dhis.org (Alan Curry) - 2011-07-01 19:32 +0000
                Re: References in C Keith Thompson <kst-u@mib.org> - 2011-07-01 13:05 -0700
                Re: References in C Seebs <usenet-nospam@seebs.net> - 2011-07-01 23:58 +0000
                Re: References in C Shao Miller <sha0.miller@gmail.com> - 2011-07-01 20:13 -0500
                Re: References in C Michael Press <rubrum@pacbell.net> - 2011-07-01 12:44 -0700
                Re: References in C Shao Miller <sha0.miller@gmail.com> - 2011-07-01 15:57 -0500
                Re: References in C Tim Rentsch <txr@alumni.caltech.edu> - 2011-07-03 14:54 -0700
                Re: References in C Michael Press <rubrum@pacbell.net> - 2011-07-03 20:29 -0700
                Re: References in C Tim Rentsch <txr@alumni.caltech.edu> - 2011-07-04 16:05 -0700
                Re: References in C Michael Press <rubrum@pacbell.net> - 2011-07-05 08:32 -0700
                Re: References in C Lowell Gilbert <lgusenet@be-well.ilk.org> - 2011-07-05 15:34 -0400
                Re: References in C Tim Rentsch <txr@alumni.caltech.edu> - 2011-07-06 15:30 -0700
                Re: References in C Ian Collins <ian-news@hotmail.com> - 2011-06-27 13:27 +1200
                Re: References in C Shao Miller <sha0.miller@gmail.com> - 2011-06-27 09:02 -0400
              Re: References in C "Joel C. Salomon" <joelcsalomon@gmail.com> - 2011-06-26 18:35 -0400
                Re: References in C James Kuyper <jameskuyper@verizon.net> - 2011-06-26 19:11 -0400
                Re: References in C "Joel C. Salomon" <joelcsalomon@gmail.com> - 2011-06-28 00:52 -0400
                Re: References in C James Kuyper <jameskuyper@verizon.net> - 2011-06-28 07:56 -0400
          Re: References in C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-06-24 22:52 +0100
          Re: References in C jt@toerring.de (Jens Thoms Toerring) - 2011-06-24 23:58 +0000
  Re: References in C Tom St Denis <tom@iahu.ca> - 2011-06-24 06:48 -0700
    Re: References in C David Resnick <lndresnick@gmail.com> - 2011-06-24 07:14 -0700
      Re: References in C jacob navia <jacob@spamsink.net> - 2011-06-24 16:58 +0200
        Re: References in C Tom St Denis <tom@iahu.ca> - 2011-06-24 09:08 -0700
          Re: References in C Stephen Sprunk <stephen@sprunk.org> - 2011-06-24 11:33 -0500
          Re: References in C gazelle@shell.xmission.com (Kenny McCormack) - 2011-06-24 17:47 +0000
            Re: References in C JG <jgrove24@hotmail.com> - 2011-07-01 14:24 -0700
          Re: References in C Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-06-25 04:39 -0700
            Re: References in C BGB <cr88192@hotmail.com> - 2011-06-25 16:15 -0700
              Re: References in C Ian Collins <ian-news@hotmail.com> - 2011-06-26 11:27 +1200
                Re: References in C BGB <cr88192@hotmail.com> - 2011-06-25 19:13 -0700
                Re: References in C cri@tiac.net (Richard Harter) - 2011-06-26 04:22 +0000
                Re: References in C Robert Wessel <robertwessel2@yahoo.com> - 2011-06-26 01:31 -0500
                Re: References in C BGB <cr88192@hotmail.com> - 2011-06-26 00:03 -0700
                Re: References in C Shao Miller <sha0.miller@gmail.com> - 2011-06-26 13:54 -0500
                Re: References in C luser- -droog <mijoryx@yahoo.com> - 2011-07-07 11:19 -0700
            Re: References in C Dr Nick <3-nospam@temporary-address.org.uk> - 2011-06-26 08:15 +0100
    Re: References in C Ike Naar <ike@sverige.freeshell.org> - 2011-06-24 16:08 +0000
      Re: References in C Tom St Denis <tom@iahu.ca> - 2011-06-24 11:12 -0700
        Re: References in C Ike Naar <ike@iceland.freeshell.org> - 2011-06-24 21:42 +0000
  Re: References in C Stephen Sprunk <stephen@sprunk.org> - 2011-06-24 11:49 -0500
  Re: References in C Keith Thompson <kst-u@mib.org> - 2011-06-24 13:17 -0700
    Re: References in C Shao Miller <sha0.miller@gmail.com> - 2011-06-24 16:46 -0400
      Re: References in C Keith Thompson <kst-u@mib.org> - 2011-06-24 14:32 -0700
        Re: References in C Gareth Owen <gwowen@gmail.com> - 2011-06-25 09:39 +0100
          Re: References in C Shao Miller <sha0.miller@gmail.com> - 2011-07-04 17:38 -0400
  Re: References in C Shao Miller <sha0.miller@gmail.com> - 2011-06-24 16:33 -0400
    Re: References in C jacob navia <jacob@spamsink.net> - 2011-06-24 23:04 +0200
  Re: References in C Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-26 17:53 -0700
    Re: References in C Ian Collins <ian-news@hotmail.com> - 2011-06-27 17:04 +1200
      Re: References in C Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-27 14:28 -0700
        Re: References in C Ian Collins <ian-news@hotmail.com> - 2011-06-28 14:11 +1200
          Re: References in C Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-28 16:27 -0700
          Re: References in C Michael Press <rubrum@pacbell.net> - 2011-06-29 13:41 -0700
            Re: References in C Seebs <usenet-nospam@seebs.net> - 2011-06-29 21:15 +0000
              Re: References in C Michael Press <rubrum@pacbell.net> - 2011-06-30 09:26 -0700
                Re: References in C Seebs <usenet-nospam@seebs.net> - 2011-06-30 20:55 +0000
                Re: References in C Michael Press <rubrum@pacbell.net> - 2011-06-30 16:20 -0700
                Re: References in C Seebs <usenet-nospam@seebs.net> - 2011-06-30 23:45 +0000
                Re: References in C James Kuyper <jameskuyper@verizon.net> - 2011-07-01 07:58 -0400
    Re: References in C jacob navia <jacob@spamsink.net> - 2011-06-27 10:43 +0200
      Re: References in C Keith Thompson <kst-u@mib.org> - 2011-06-27 08:32 -0700
      Re: References in C Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-28 11:01 -0700
        Re: References in C jacob navia <jacob@spamsink.net> - 2011-06-28 22:17 +0200
          Re: References in C Ian Collins <ian-news@hotmail.com> - 2011-06-29 09:05 +1200
          Re: References in C Keith Thompson <kst-u@mib.org> - 2011-06-28 22:24 -0700
          Re: References in C Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-29 01:07 -0700
            Re: References in C Ian Collins <ian-news@hotmail.com> - 2011-06-29 21:16 +1200
              Re: References in C Seebs <usenet-nospam@seebs.net> - 2011-06-29 17:46 +0000
              Re: References in C Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-29 12:19 -0700
          Re: References in C sandeep <nospam@nospam.com> - 2011-07-03 20:53 +0000
            Re: References in C Chad <cdalten@gmail.com> - 2011-07-03 14:02 -0700
            Re: References in C Keith Thompson <kst-u@mib.org> - 2011-07-03 14:33 -0700
              Re: References in C jacob navia <jacob@spamsink.net> - 2011-07-04 00:22 +0200
                Re: References in C Keith Thompson <kst-u@mib.org> - 2011-07-04 13:41 -0700
              Re: References in C gazelle@shell.xmission.com (Kenny McCormack) - 2011-07-04 00:00 +0000
              Re: References in C Seebs <usenet-nospam@seebs.net> - 2011-07-04 00:20 +0000
      Re: References in C Shao Miller <sha0.miller@gmail.com> - 2011-07-04 17:43 -0400

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