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From: guy@gorodish.UUCP
Newsgroups: comp.lang.c
Subject: Re: Query: Implementation with non-zero NULL
Message-ID: <20163@sun.uucp>
Date: Tue, 2-Jun-87 00:21:59 EDT
Article-I.D.: sun.20163
Posted: Tue Jun  2 00:21:59 1987
Date-Received: Thu, 4-Jun-87 01:48:10 EDT
References: <158@delftcc.UUCP> <3673@gitpyr.gatech.EDU> <3675@gitpyr.gatech.EDU>
Sender: news@sun.uucp
Lines: 40
Keywords: C, NULL, portability
Summary: That's more like it!

> Correction - if(p) yields  false if p = (char *) NULL

In other words, constructs like

	if (p)

*do* yield correct results, and PRIME's C implementation *is*
correct.  (Assuming that "if (!p)" yields *true* if is a null
pointer.)  BIG difference!

> Also, if(p != 0) also yields false.

In other words, "if (p)" and "if (p != 0)" (and "if (p != NULL)")
*are* the same.  This is as it should be; those three constructs mean
the same thing in C, and should all yield true iff "p" is not a null
pointer.

> The 0 is cast to (char *).

Actually, the 0 is *coerced* to (char *); it is only *cast* to "char *"
if you write "(char *)0".  Casting is one way of generating a
coercion, but it isn't the only way.  In this case, the compiler
knows that one operand of the "==" is a pointer, and the other is a
constant 0, so it knows that it must coerce the constant 0 to a null
pointer value of the appropriate type; no cast is necessary.

> The value of 0x60000000 is correct.  If you do a union with a long and
> compare the long with zero (0), they will not be equal.  Also, since in
> some modes a pointer may be 48 bits, you would only get the first 32 bits
> in the union.

I presume "the value of 0x60000000 is correct" means "that is the bit
pattern stuffed into a null pointer", not "NULL is #defined to be
0x60000000".

Both of the behaviors you describe are perfectly legitimate, and
permissible in a C implementation.
	Guy Harris
	{ihnp4, decvax, seismo, decwrl, ...}!sun!guy
	guy@sun.com