Re: null pointer constant, address constant

From	Szabolcs Nagy <nsz@port70.net>
Newsgroups	comp.std.c
Subject	Re: null pointer constant, address constant
Date	2012-12-18 20:06 +0000
Organization	Aioe.org NNTP Server
Message-ID	<kaqibr$gj3$1@speranza.aioe.org> (permalink)
References	<kapqt2$es4$1@speranza.aioe.org> <kaq01g$6jp$1@dont-email.me>

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James Kuyper <jameskuyper@verizon.net> wrote:
> On 12/18/2012 08:25 AM, Szabolcs Nagy wrote:
>> it seems the definition of 'null pointer constant'
>> relies on integer constant expressions (6.3.2.3p3)
>> http://port70.net/~nsz/c/c11/n1570.html#6.3.2.3p3
>> 
>> so eg.
>> 
>>  (void*)((char*)1 - 1)
>> 
>> is not a null pointer constant because pointer
>> arithmetics cannot give an integer constant expression
>> 
>> but at least this could be used in pointer initializers
>> since address constants are allowed there (6.6p7)
> 
> Two things to keep in mind:
> 
> When converting an arbitrary integer to a pointer type "the result is
> implementation-defined, might not be correctly aligned, might not point
> to an entity of the referenced type, and might be a trap
> representation." (6.3.2.3p1)

does not matter, your concerns only matter if the expression is
evaluated, but it is not:

 sizeof( *(0?(double*)0:(void*)((char*)1 - 1)) )

is this equivalent to sizeof(double) or sizeof(void)?
(now that the expression after : is not a null pointer constant
we know the result is sizeof(void) and thus a constraint violation)

you don't need to evaluate the expression to use the "constness"
of it so any argument about trap representations and alignment is
moot

> However, I get the impression that you were expecting
> 
>    (char*)1-1 == (char*)0.
> 
> The standard says nothing of the kind - in fact, if that expression has
> defined behavior, it's guaranteed to be false. Here's why: if the
> implementation does define (char*)1 as pointing at an element, other
> than the first, of an array, then (char*)1 - 1 will point at the
> immediately preceding element of the array, and therefore, points at an
> object. Now (char*)0 is a null pointer, and a null pointer can never
> compare equal to a pointer to an object (6.3.2.3p3). Therefore, if the

this is a valid argument

> I may be missing something, but as far as I can tell, the rules given in
> 6.5.15p6 give the same result whether the second or third operand is a
> null pointer constant, or merely a constant expression with the type
> void* and a null value. Could you provide a counter-example?

yes

 (0 ? (double*)0 : (void*)0)

has type double*, while

 (0 ? (double*)0 : (void*)1)

has type void*

to exploit this difference you can use sizeof as above


>> but (for me at least) it's more surprising that this is
>> not even a constant expression that can be used in
>> initializers, ie.
>> 
>>  void *p = 0?(void*)0:(void*)0;
>> 
>> is undefined behaviour if i read section 6.6 correctly
>> (not even a constraint violation that requires a diagnostic)
> 
> That does seem a bit surprising, but I believe you're correct. Note,

ok i was wrong about this, an address constant can be a
null pointer (not just null pointer constant)

> This imposes a bit of a strain on the implementor, who would otherwise
> be allowed to delay evaluation of such expressions until run-time. The
> rules for integer constant expressions were intended to keep them
> simple, to make them easy to identify and evaluate.

my problem is that they are not easy to identify

 !(int)1.0
 (void*)(1-1)

are integer constant expressions and null pointer constants

 (int)!1.0
 (void*)(char*)0
 (char*)(void*)0
 (void*)(void*)0
 (void*)((char*)0-(char*)0)

are neither

Thread

null pointer constant, address constant Szabolcs Nagy <nsz@port70.net> - 2012-12-18 13:25 +0000
  Re: null pointer constant, address constant James Kuyper <jameskuyper@verizon.net> - 2012-12-18 09:53 -0500
    Re: null pointer constant, address constant Szabolcs Nagy <nsz@port70.net> - 2012-12-18 20:06 +0000
      Re: null pointer constant, address constant James Kuyper <jameskuyper@verizon.net> - 2012-12-18 15:39 -0500
        Re: null pointer constant, address constant Szabolcs Nagy <nsz@port70.net> - 2012-12-19 00:44 +0000
          Re: null pointer constant, address constant James Kuyper <jameskuyper@verizon.net> - 2012-12-18 20:24 -0500
            Re: null pointer constant, address constant Szabolcs Nagy <nsz@port70.net> - 2012-12-19 10:27 +0000
              Re: null pointer constant, address constant James Kuyper <jameskuyper@verizon.net> - 2012-12-19 07:15 -0500
              Re: null pointer constant, address constant Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-12-21 03:03 +0200
                Re: null pointer constant, address constant James Kuyper <jameskuyper@verizon.net> - 2012-12-21 12:13 -0500
                Re: null pointer constant, address constant Szabolcs Nagy <nsz@port70.net> - 2013-01-09 00:44 +0000
                Re: null pointer constant, address constant James Kuyper <jameskuyper@verizon.net> - 2013-01-08 20:21 -0500
                Re: null pointer constant, address constant Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-01-08 20:46 -0500
          Re: null pointer constant, address constant Shao Miller <sha0.miller@gmail.com> - 2012-12-18 20:37 -0500
      Re: null pointer constant, address constant Shao Miller <sha0.miller@gmail.com> - 2012-12-18 16:15 -0500
        Re: null pointer constant, address constant Keith Thompson <kst-u@mib.org> - 2012-12-18 15:52 -0800
          Re: null pointer constant, address constant Shao Miller <sha0.miller@gmail.com> - 2012-12-18 19:12 -0500
            Re: null pointer constant, address constant Shao Miller <sha0.miller@gmail.com> - 2012-12-18 19:51 -0500
          Re: null pointer constant, address constant Keith Thompson <kst-u@mib.org> - 2012-12-18 17:02 -0800
          Re: null pointer constant, address constant "christian.bau" <christian.bau@cbau.wanadoo.co.uk> - 2012-12-20 05:34 -0800
            Re: null pointer constant, address constant James Kuyper <jameskuyper@verizon.net> - 2012-12-20 10:33 -0500
              Re: null pointer constant, address constant James Kuyper <jameskuyper@verizon.net> - 2012-12-20 12:28 -0500
                Re: null pointer constant, address constant Tim Rentsch <txr@alumni.caltech.edu> - 2012-12-20 21:13 -0800
              Re: null pointer constant, address constant Tim Rentsch <txr@alumni.caltech.edu> - 2012-12-20 21:05 -0800
            Re: null pointer constant, address constant Shao Miller <sha0.miller@gmail.com> - 2012-12-20 12:17 -0500
            Re: null pointer constant, address constant Keith Thompson <kst-u@mib.org> - 2012-12-20 12:10 -0800
              Re: null pointer constant, address constant Tim Rentsch <txr@alumni.caltech.edu> - 2012-12-20 21:25 -0800
  Re: null pointer constant, address constant Shao Miller <sha0.miller@gmail.com> - 2012-12-18 14:02 -0500
    Re: null pointer constant, address constant Szabolcs Nagy <nsz@port70.net> - 2012-12-18 20:09 +0000
      Re: null pointer constant, address constant James Kuyper <jameskuyper@verizon.net> - 2012-12-18 15:54 -0500
        Re: null pointer constant, address constant Shao Miller <sha0.miller@gmail.com> - 2012-12-18 16:18 -0500

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