Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!lll-winken!uunet!auspex!guy
From: guy@auspex.auspex.com (Guy Harris)
Newsgroups: comp.lang.c
Subject: Re: calloc (actually NULL =?= 0)
Message-ID: <1433@auspex.auspex.com>
Date: 14 Apr 89 18:16:23 GMT
References: <22842@shemp.CS.UCLA.EDU> <1428@auspex.auspex.com> <987@atanasoff.cs.iastate.edu>
Reply-To: guy@auspex.auspex.com (Guy Harris)
Distribution: na
Organization: Auspex Systems, Santa Clara
Lines: 31


 >>>I always thought a pointer consisting of zero bits is NULL. 
 > 
 >>Nope.  No such guarantee was ever made by any C language spec.
 >
 >   What about the following taken from K&R, Appendix A, section 7.14,
 >   "Assignment operator":
 >
 >       However, it is guaranteed that assignment of the
 >       constant 0 to a pointer will produce a null pointer...

Sigh, time for yet another explanation of what this really means....

This is in no way, shape, or form a guarantee that a null pointer
consists solely of zero bits!  All it guarantees is that assignments
such as

	register char *p;

	p = 0;

cause a null pointer value to be assigned to "p".  If the representation
of a null pointer on some hypothetical implementation is 0xFF000000, the
code generated for the aforementioned statement might be something such
as:

	mov	#FF000000,%r3

if, say, "p" were assigned to register "r3".  The compiler knows, in
this context, that the "0" must be converted to a null pointer of type
"char *", and does precisely that conversion.