Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!seismo!uwvax!oddjob!mimsy!chris
From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.lang.c
Subject: Re: Query: Implementation with non-zero NULL
Message-ID: <6877@mimsy.UUCP>
Date: Mon, 1-Jun-87 23:33:31 EDT
Article-I.D.: mimsy.6877
Posted: Mon Jun  1 23:33:31 1987
Date-Received: Wed, 3-Jun-87 04:20:06 EDT
References: <158@delftcc.UUCP> <3673@gitpyr.gatech.EDU>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 63
Keywords: C, NULL, portability

In article <3673@gitpyr.gatech.EDU> allen@gitpyr.gatech.EDU (P. Allen
Jensen) writes:
>On the PRIME system under PRIMIX, there is a special "undefined pointer"
>value. ... Because of this, constructs such as the
>
>	if(p)
>
>do not yield correct results.

Then the PRIMIX compiler is broken.  See K&R, p. 201.  (This does not,
of course, help those who must use the broken compiler.)

>The value of NULL is 0x60000000 on a prime.

If NULL is `#define'd as 0x60000000, the compiler is even more
broken.  Given this special nil pointer format, and assuming that
the union below gets at the proper portion of a pointer, the program
below should print only `right's:

	#include <stdio.h>
	#define	magic	0x60000000

	/*ARGSUSED*/
	main(argc, argv)
		int argc;
		char **argv;
	{
		union {
			long	l;
			int	*p;
		} u;
		static char r[] = "right\n";
		static char w[] = "wrong\n";

		u.p = 0;
		printf(u.l == magic ? r : w);
		printf(u.p == NULL ? r : w);
		printf(u.p ? w : r);
		exit(0);
	}

The compiler is required to transform any integer constant zero in
any pointer context into the appropriately typed nil pointer.  In
the expressions `if (p) s1; else s2;' and `p ? e1 : e2', where p
is a pointer, p is being implicitly compared to a constant zero;
if p is not equal to the nil pointer resulting from this transformation,
the compiler should evaluate the `true' branch or expression, s1
or e1; otherwise, it should evaluate the `false' branch or expression,
s2 or e2.  If the compiler does something else, it is broken.

Given these conversion rules, the only correct ways to define NULL
are

	#define NULL 0

and

	#define NULL 0L		/* or 0l, or ((long)0) */

The latter is unnecessary and potentially troublesome.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690)
Domain:	chris@mimsy.umd.edu	Path:	seismo!mimsy!chris