Newsgroups: comp.lang.c
Path: utzoo!utgpu!jarvis.csri.toronto.edu!dgp.toronto.edu!flaps
From: flaps@dgp.toronto.edu (Alan J Rosenthal)
Subject: Re: Header problems
Message-ID: <8803091720.AA09301@explorer.dgp.toronto.edu>
Organization: University of Toronto
References: <2550049@hpisod2.HP.COM> <7412@brl-smoke.ARPA> <3351@chinet.UUCP>
Date: Wed, 9 Mar 88 12:20:17 EST


Doug Gwyn <gwyn@brl.arpa> wrote about NULL, and said that the proper
definition of it is ``#define NULL 0''.

dag@chinet.UUCP (Daniel A. Glasser) wrote:
>Your use of NULL === 0 promotes unportable code ... [moralizing deleted]
>
>On machines were sizeof int != sizeof(void *), the above definition will
>not work on older style function calls (without prototypes) or in var-arg
>situations.  Requiring sizeof int == sizeof(void *) is not a viable
>solution...
>
>	#define	NULL	((void *)0)
>On older compilers, replace 'void' with 'char'.  This allows the use of
>a short NULL representation when the compiler is smart enough to use it
>but forces passing of NULL in argument lists as a pointer sized object.

This doesn't require sizeof(int) == sizeof(void *).  The assumption is
that you always cast NULL before passing it.  Casting is still required
if NULL is defined as ((void *)0); you can't assume that all pointers
have the same size or representation.  Passing a pointer to void to a
routine that expects a pointer to struct gosh is as big a mistake as
passing an int to a routine that expects a pointer to struct gosh.

By the way, if you call f(0) where f expects a char *, the requirement
for it to work is not simply that sizeof(int) == sizeof(char *), but
also that (int)0 and (char *)0 have the same bit representation.

With respect to your phrase "a pointer sized object" - there is no such
thing in C.  There may happen to be such a thing in a particular
implementation of C due to hardware considerations.  (Obviously, it is
usually advantageous to represent all pointers in the same way (but not
always); therefore this is a common occurrence.)

ajr
-- 
If you had eternal life, would you be able to say all the integers?