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Path: utzoo!sq!msb
From: msb@sq.uucp (Mark Brader)
Date: Wed, 25-May-88 20:17:23 EDT
Message-ID: <1988May25.201723.28041@sq.uucp>
Newsgroups: comp.lang.c
Subject: null pointers (was: negative addresses)
Summary: ANSI does guarantee null pointer equality
References: <2393@uvacs.CS.VIRGINIA.EDU> <21541@amdcad.AMD.COM> <10001@tekecs.TEK.COM> <206@proxftl.UUCP>
Reply-To: msb@sq.UUCP (Mark Brader)
Organization: SoftQuad Inc., Toronto

> An interesting point: ANSI does not define (at least not anywhere
> I can find it) the result of `x == y' when x and y are both null
> pointers.

No, this bug (which I, at least, pointed out in the first-round public
comments) has been fixed in the January 1988 draft.  I may as well quote the
paragraph preceding the new one, as well.  From section 3.2.2.3 (on page 38):

# An integral constant expression with the value 0, or such an expression
# converted to type void *, is called a "null pointer constant".  If a null
# pointer constant is assigned to or compared for equality to a pointer,
# the constant is converted to a pointer of that type.  Such a pointer,
# called a "null pointer", is guaranteed to compare unequal to a pointer
# to any object or function.
#
# Two null pointers, converted through possibly different sequences of
# casts to pointer types, shall compare equal.

By the way, section 4.1.5 guarantees on page 99 that the macro NULL,
defined in certain standard #include headers,

# expands to an implementation-defined null pointer constant

thus it could be #defined as 0, 0L, (void*)0, 1-1, etc., but not (char*)0,
which always was wrong.  This has stayed the same through several drafts
and I think it is most unlikely to change before the final Standard.

Mark Brader, SoftQuad Inc., Toronto, utzoo!sq!msb, msb@sq.com
#define	MSB(type)	(~(((unsigned type)-1)>>1))