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From: john@physiol.OZ (John Mackin)
Newsgroups: net.lang.c
Subject: ints vs. pointers
Message-ID: <80@physiol.OZ>
Date: Thu, 15-Nov-84 00:20:29 EST
Article-I.D.: physiol.80
Posted: Thu Nov 15 00:20:29 1984
Date-Received: Sat, 10-Nov-84 21:28:46 EST
Organization: Physiology, Uni of Sydney, Australia
Lines: 16

I can remain silent no longer.  In the debate about NULL vs non-NULL,
people seem to be overlooking the fact that the number of actual "data"
bits (as opposed to "noise" bits such as object size, etc., used on
some CPUs) in a pointer MUST BE no bigger than the number of bits in an int!
[Before the flames start I KNOW K&R's not a standard but it's the closest
we've got.]  I refer you to Appendix A, para 7.4 (page 189 in my edition):

	If two pointers of the same type are subtracted, the result is con-
	verted (by division by the length of the object) to an int representing
	the number of objects separating the pointed-to objects.

This is clearly not necessarily possible if you have (say) 32-bit pointers
(with ALL 32 bits being address) and 16-bit ints.

John Mackin, Physiology Department, University of Sydney, Sydney, Australia
...!decvax!mulga!john.physiol.su