null pointer constant, address constant

From	Szabolcs Nagy <nsz@port70.net>
Newsgroups	comp.std.c
Subject	null pointer constant, address constant
Date	2012-12-18 13:25 +0000
Organization	Aioe.org NNTP Server
Message-ID	<kapqt2$es4$1@speranza.aioe.org> (permalink)

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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)
http://port70.net/~nsz/c/c11/n1570.html#6.6p7

but it seems

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

is not a null pointer constant, nor an address constant

it is mildly unexpected that this cannot be used as a
null pointer constant
(this matters when determining the type of ?: (6.5.15)
expressions, assigning it to function pointers (6.5.16.1)
or comparing it to function pointers (6.5.9))

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)

another similar surprising case is

 void *q = (void*)((char*)1 - (char*)1);

the initializer is not an address constant, not a null pointer
constant nor an address constant +- an integer constant
expression.

i wonder if my interpretation is right and if there is
some rational behind this

(my personal intuition is that integer constant expressions
should be defined more generously and may be the result of
pointer arithmetics if the pointers are casted integer
constants or the difference of address constants, then
these surprises would go away)

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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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