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Groups > comp.lang.c > #43841

Re: free a pointer and have it point to NULL

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On 04/29/2014 02:50 PM, Keith Thompson wrote:
...
> or more representations have the same value.  I wrote above that the
> bytes making up the representation of an object are themselves objects,
> which are required by 6.2.4p2 to "retain[] its last-stored value
> throughout its lifetime".
> 
> I'm no longer quite as sure of that, given the way 6.2.6.1 defines
> *object representation*:
> 
>     Values stored in non-bit-field objects of any other object type
>     consist of n × CHAR_BIT bits, where n is the size of an object
>     of that type, in bytes. The value may be copied into an object
>     of type unsigned char [n] (e.g., by memcpy); the resulting set
>     of bytes is called the *object representation* of the value.
> 
> which talks about the bytes *after copying them from the object*,
> so at least that paragraph doesn't imply that the bytes within the
> object itself are objects.
> 
> On the other hand, an object is defined as a "region of data storage in
> the execution environment, the contents of which can represent values";
> a byte within a pointer object, for example, would seem to qualify.
> 
> On the other other hand, a program that depends on a byte within a freed
> pointer retaining its previous value would be perverse, and I don't feel
> *too* bad about the possibility of breaking it.

At that level, I would agree. However, if 6.2.4p2 is not to be
interpreted as prohibiting such changes, it renders essentially all uses
of memcmp() useless, at least for any type that is allowed to have
multiple representations of the same value (which could includes any
integer type with padding bits, any signed type with anything other 2's
complement representation, and all floating point and pointer types).

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free a pointer and have it point to NULL G G <gdotone@gmail.com> - 2014-04-22 14:22 -0700
  Re: free a pointer and have it point to NULL James Kuyper <jameskuyper@verizon.net> - 2014-04-22 17:32 -0400
  Re: free a pointer and have it point to NULL G G <gdotone@gmail.com> - 2014-04-22 14:36 -0700
  Re: free a pointer and have it point to NULL Keith Thompson <kst-u@mib.org> - 2014-04-22 14:37 -0700
    Re: free a pointer and have it point to NULL Johannes Bauer <dfnsonfsduifb@gmx.de> - 2014-04-29 13:10 +0200
      Re: free a pointer and have it point to NULL Richard <rgrdev_@gmail.com> - 2014-04-29 13:14 +0200
        Re: free a pointer and have it point to NULL Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-04-29 13:19 +0100
          Re: free a pointer and have it point to NULL Kaz Kylheku <kaz@kylheku.com> - 2014-04-29 13:57 +0000
      Re: free a pointer and have it point to NULL Kaz Kylheku <kaz@kylheku.com> - 2014-04-29 13:43 +0000
      Re: free a pointer and have it point to NULL James Kuyper <jameskuyper@verizon.net> - 2014-04-29 09:59 -0400
        Re: free a pointer and have it point to NULL Keith Thompson <kst-u@mib.org> - 2014-04-29 07:58 -0700
          Re: free a pointer and have it point to NULL Keith Thompson <kst-u@mib.org> - 2014-04-29 11:50 -0700
            Re: free a pointer and have it point to NULL James Kuyper <jameskuyper@verizon.net> - 2014-04-29 15:47 -0400
        Re: free a pointer and have it point to NULL James Kuyper <jameskuyper@verizon.net> - 2014-04-29 12:41 -0400
          Re: free a pointer and have it point to NULL Robert Wessel <robertwessel2@yahoo.com> - 2014-04-29 13:15 -0500
            Re: free a pointer and have it point to NULL glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-04-29 21:29 +0000
      Re: free a pointer and have it point to NULL glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-04-29 15:26 +0000
  Re: free a pointer and have it point to NULL jt@toerring.de (Jens Thoms Toerring) - 2014-04-22 21:43 +0000
    Re: free a pointer and have it point to NULL Keith Thompson <kst-u@mib.org> - 2014-04-22 14:53 -0700
      Re: free a pointer and have it point to NULL Öö Tiib <ootiib@hot.ee> - 2014-04-24 00:38 -0700
        Re: free a pointer and have it point to NULL James Kuyper <jameskuyper@verizon.net> - 2014-04-24 07:52 -0400
          Re: free a pointer and have it point to NULL <william@wilbur.25thandClement.com> - 2014-04-24 16:13 -0700
            Re: free a pointer and have it point to NULL Ian Collins <ian-news@hotmail.com> - 2014-04-25 12:02 +1200
              Re: free a pointer and have it point to NULL <william@wilbur.25thandClement.com> - 2014-04-24 22:03 -0700
                Re: free a pointer and have it point to NULL Ian Collins <ian-news@hotmail.com> - 2014-04-25 17:22 +1200
                Re: free a pointer and have it point to NULL <william@wilbur.25thandClement.com> - 2014-04-24 23:00 -0700
          Re: free a pointer and have it point to NULL Öö Tiib <ootiib@hot.ee> - 2014-04-27 08:32 -0700
    Re: free a pointer and have it point to NULL Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2014-04-22 16:19 -0600
      Re: free a pointer and have it point to NULL Kaz Kylheku <kaz@kylheku.com> - 2014-04-22 22:41 +0000
        Re: free a pointer and have it point to NULL Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2014-04-22 18:17 -0600
          Re: free a pointer and have it point to NULL Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2014-04-24 08:48 -0600
    non-NULL pointers which have the value 0 (Was: Re: free a pointer and have it point to NULL) Andrew Cooper <root@127.0.0.1> - 2014-04-23 03:28 +0100
      Re: non-NULL pointers which have the value 0 (Was: Re: free a pointer and have it point to NULL) Kaz Kylheku <kaz@kylheku.com> - 2014-04-23 04:37 +0000
      Re: non-NULL pointers which have the value 0 glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-04-23 06:14 +0000
  Re: free a pointer and have it point to NULL Kaz Kylheku <kaz@kylheku.com> - 2014-04-22 22:18 +0000
  Re: free a pointer and have it point to NULL Michael Angelo Ravera <maravera@prodigy.net> - 2014-05-03 01:00 -0700

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