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From: guy@sun.uucp (Guy Harris)
Newsgroups: net.lang.c
Subject: Re: Re: how has C bitten you?  (Really,
Message-ID: <2756@sun.uucp>
Date: Wed, 4-Sep-85 02:03:40 EDT
Article-I.D.: sun.2756
Posted: Wed Sep  4 02:03:40 1985
Date-Received: Thu, 5-Sep-85 10:42:52 EDT
References: <259@watmum.UUCP> <2000002@eel.UUCP>
Organization: Sun Microsystems, Inc.
Lines: 43

> 	One final, subtle, point.  K&R does not guarantee that the *value* 0
> 	is distinguishable from all other pointers, but rather, that the
> 	*constant* 0 is.  That is to say, you may compare against 0 to
> 	determine the validity of a pointer (or assign to guarantee
> 	invalidity), but you may not assume that comparison against (or
> 	assignment of) an int variable whose value is 0 will have the same
> 	result.
> 
> I don't see why the restriction applies to non-pointer variables.  As long
> as type coercions are explicit, this should apply to all values of zero,
> whether encountered as a literal in the program or as the value of a
> variable of integral type.

("Oh no, Mabel!  Here comes another K&R quote!")

7.7 Equality operators

	A pointer may be compared to an integer, but the result is
	machine independent unless the integer is *the constant* 0.
	(Italics mine)

7.13 Conditional operator

	...otherwise, one must be a pointer and the other *the constant*
	0, and the result has the type of the pointer.  (Italics mine)

7.14 Assignment operators

	...However, it is guaranteed that assignment of *the constant*
	0 to a pointer will produce a null pointer distinguishable
	from a pointer to any object.  (Italics mine)

I'd say the intent of K and R was pretty clear here, wouldn't you?

As for "why" - think of a machine where a null pointer *didn't* have the
same bit pattern as the integer 0.  Every time you assigned an integer to a
pointer, you'd have to check whether the integer was zero or not and assign
a null pointer instead (unless the computation you had to do to convert an
integer to a pointer did this anyway).  Why penalize those assignments
solely to make assigning a 0 other than a constant 0 set the pointer to a
null pointer?

	Guy Harris