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Groups > comp.lang.c > #30337

Re: [union] Pointers to inherited structs are valid ?

From Shao Miller <sha0.miller@gmail.com>
Newsgroups comp.lang.c
Subject Re: [union] Pointers to inherited structs are valid ?
Date 2013-01-18 23:02 -0500
Organization A noiseless patient Spider
Message-ID <kdd5sn$ens$1@dont-email.me> (permalink)
References (14 earlier) <ln4nifby8p.fsf@nuthaus.mib.org> <kd9ria$pqs$1@dont-email.me> <lnip6ua1f4.fsf@nuthaus.mib.org> <kdccnu$b7s$1@dont-email.me> <ln622u9clc.fsf@nuthaus.mib.org>

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On 1/18/2013 20:38, Keith Thompson wrote:
> Shao Miller <sha0.miller@gmail.com> writes:
>> On 1/18/2013 11:41, Keith Thompson wrote:
>>> Shao Miller <sha0.miller@gmail.com> writes:
> [...]
>>>> I think all-bits-zero is one null pointer value representation, but I
>>>> was talking about "trap representations" in practice (as opposed to a
>>>> discussion of those that depend on padding bits).  In Windows NT
>>>> kernel-land, more often than not I see that when a null pointer is
>>>> trapped, it's actually _not_ all-bits-zero; differing in the LSB.  The
>>>> debugger still calls it a null pointer.
>>>
>>> A null pointer is not a trap representation; it's a perfectly valid
>>> pointer value (that can't legally be dereferenced).
>>>
>>
>> Agreed.  And that's kind of what I was getting at...  That this
>> representation is:
>>
>> 1. A valid null pointer representation (behaves as a null pointer in C
>> code; no undefined behaviour from using this pointer value unless
>> dereferencing)
>>
>> 2. Very consistently _not_ all-bits-zero, suggesting a practical use for
>> a debugger to trap (as opposed to a same-sized integer/object with
>> all-zeroes)
>
> When you say it "behaves as a null pointer in C code", what exactly do
> you mean by that?
>
> Does it compare equal to NULL?  If not, then it *doesn't* behave as a
> null pointer in C code.
>

Yeah, come to think of it, you're right: I guess it wouldn't've behaved 
_exactly_ the same.

>>> What exactly do you mean when you say "The debugger still calls it a
>>> null pointer"?  Does the debugger actually use the phrase "null pointer"
>>> to refer to a pointer value that compares unequal to NULL?
>>
>> It absolutely does.  (I see it more often than I'd like, some days. ;) )
>>    I suspect that it's an instance of trap representations in practice,
>> as opposed to "trap representations" in C theory.
>
> A lot of technical terms have specific meanings in C that may not apply
> in other contexts.  If a debugger refers to something other than a C
> null pointer as a "null pointer", then (a) it's a pity that a tool that
> deals with C source code uses a term in a manner that's inconsistent
> with the C meaning of the term, but (b) it doesn't have much direct
> bearing on C (i.e., on anything relevant to this newsgroup).
>

Well I have to offer partial disagreement with (a).  I think that the 
practical usage of some terms finds Standard C in a good position to 
consider adjusting its definitions. :)  WinDbg can handle C++ and 
assembly too, so the terms it uses seem more likely to be "as practiced" 
rather than "as preached".

> Can you point to some documentation that discusses this use of the term
> "null pointer"?
>

On 1/16/2013 20:57, Shao Miller wrote:
> I thought null pointers having 0x0C as the least-significant byte was "a thing," too, but now I can't remember having seen that documented anywhere.

Sorry for the confusion.  I was talking about a recollection, as shown 
in the sentence, above.  What I'm recalling is associated with 
"NULL_CLASS_PTR_DEREFERENCE", but please see the sentence above, 
regarding documentation.

I thought I remembered the LSB 0x0C pattern was used as "trap 
representation" for "null pointers", but where neither of these terms 
are quite what they are in C.  If I ever come across such documentation, 
I'll share it.  Maybe Geoff has a clue?

-- 
- Shao Miller
--
"Thank you for the kind words; those are the kind of words I like to hear.

Cheerily,"  -- Richard Harter

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[union] Pointers to inherited structs are valid ? Maciej Labanowicz <m.labanowicz@gmail.com> - 2013-01-01 03:45 -0800
  Re: [union] Pointers to inherited structs are valid ? Barry Schwarz <schwarzb@dqel.com> - 2013-01-01 17:59 -0800
    Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-02 10:18 -0800
  Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-01 21:06 -0500
    Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-02 10:45 -0800
      Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-02 19:56 -0500
        Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-03 08:28 -0500
        Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-03 15:31 -0800
          Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-03 20:55 -0500
            Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-03 21:00 -0500
              Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-04 12:56 -0500
            Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-04 12:59 -0500
            Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-06 20:00 -0800
              Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-07 00:47 -0500
                Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-06 23:06 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-07 04:03 -0500
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-07 08:25 -0500
                Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-13 10:34 -0800
                Re: Off-Topic: Irritating Posting Patterns Shao Miller <sha0.miller@gmail.com> - 2013-01-13 16:00 -0500
                Re: Off-Topic: Irritating Posting Patterns Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-14 12:27 -0800
              Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-07 07:41 -0500
                Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-12 14:52 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-13 15:19 -0500
                Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-14 12:04 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-14 18:52 -0500
                Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-16 00:01 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-16 05:43 -0500
                Re: [union] Pointers to inherited structs are valid ? Geoff <geoff@invalid.invalid> - 2013-01-16 09:01 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-16 18:32 -0500
                Re: [union] Pointers to inherited structs are valid ? Geoff <geoff@invalid.invalid> - 2013-01-16 17:32 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-16 20:57 -0500
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-17 07:55 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-17 16:47 -0500
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-18 08:41 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-18 15:53 -0500
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-18 17:38 -0800
                Re: [union] Pointers to inherited structs are valid ? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-19 02:31 +0000
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-18 23:15 -0500
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-19 20:03 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-20 02:19 -0500
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-20 07:57 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-20 15:25 -0500
                Re: [union] Pointers to inherited structs are valid ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-20 23:58 +0000
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-20 21:30 -0500
                Re: [union] Pointers to inherited structs are valid ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-21 02:53 +0000
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-21 00:05 -0500
                Re: [union] Pointers to inherited structs are valid ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-21 16:31 +0000
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-21 14:07 -0500
                Re: [union] Pointers to inherited structs are valid ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-21 21:05 +0000
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-21 17:13 -0500
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-21 16:11 -0800
                Re: [union] Pointers to inherited structs are valid ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-22 04:59 +0000
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-22 01:05 -0500
                Re: [union] Pointers to inherited structs are valid ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-22 14:53 +0000
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-22 08:38 -0800
                Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-21 20:53 -0800
                Re: [union] Pointers to inherited structs are valid ? Philip Lantz <prl@canterey.us> - 2013-01-22 00:58 -0800
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-22 08:40 -0800
                Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-22 23:32 -0800
                Re: [union] Pointers to inherited structs are valid ? Philip Lantz <prl@canterey.us> - 2013-01-23 23:21 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-24 04:53 -0500
                Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-25 10:49 -0800
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-22 08:25 -0800
                Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-22 23:41 -0800
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-21 13:32 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-21 17:46 -0500
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-22 07:43 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-22 16:07 -0500
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-22 14:15 -0800
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-22 14:31 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-22 21:32 -0500
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-23 17:17 -0500
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-23 19:49 -0500
                Re: [union] Pointers to inherited structs are valid ? Geoff <geoff@invalid.invalid> - 2013-01-23 18:55 -0800
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-24 11:01 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-24 14:11 -0500
                Re: [union] Pointers to inherited structs are valid ? Geoff <geoff@invalid.invalid> - 2013-01-22 17:51 -0800
                Re: [union] Pointers to inherited structs are valid ? Philip Lantz <prl@canterey.us> - 2013-01-20 16:48 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-20 21:33 -0500
                Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-21 09:23 -0800
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-20 17:07 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-20 22:20 -0500
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-20 22:02 -0800
                Re: [union] Pointers to inherited structs are valid ? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-21 07:00 +0000
                Re: [union] Pointers to inherited structs are valid ? James Kuyper <jameskuyper@verizon.net> - 2013-01-21 08:17 -0500
                Re: [union] Pointers to inherited structs are valid ? Dr Nick <nospam-4@temporary-address.org.uk> - 2013-01-21 14:13 +0000
                Re: [union] Pointers to inherited structs are valid ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-21 14:12 +0000
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-21 08:20 -0800
                Re: [union] Pointers to inherited structs are valid ? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-21 20:41 +0000
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-21 12:37 -0500
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-21 12:17 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-21 16:33 -0500
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-21 15:50 -0800
                Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-21 19:45 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-22 00:18 -0500
                Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-22 08:01 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-18 23:02 -0500
                Re: [union] Pointers to inherited structs are valid ? Philip Lantz <prl@canterey.us> - 2013-01-19 01:28 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-19 12:08 -0500
                Re: [union] Pointers to inherited structs are valid ? Geoff <geoff@invalid.invalid> - 2013-01-21 09:27 -0800
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-21 12:46 -0500
                Re: [union] Pointers to inherited structs are valid ? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-21 20:48 +0000
                Re: [union] Pointers to inherited structs are valid ? Geoff <geoff@invalid.invalid> - 2013-01-21 18:52 -0800
                Re: [union] Pointers to inherited structs are valid ? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-22 04:07 +0000
                Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-21 23:15 -0500
                Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-16 10:59 -0800
  Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-02 10:34 -0800

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