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From: guy%gorodish@Sun.COM (Guy Harris)
Newsgroups: comp.lang.c
Subject: Re: Query: Implementation with non-zero NULL
Message-ID: <20130@sun.uucp>
Date: Mon, 1-Jun-87 17:49:58 EDT
Article-I.D.: sun.20130
Posted: Mon Jun  1 17:49:58 1987
Date-Received: Wed, 3-Jun-87 02:26:49 EDT
References: <158@delftcc.UUCP> <3673@gitpyr.gatech.EDU>
Sender: news@sun.uucp
Lines: 57
Keywords: C, NULL, portability
Summary: If that's true, then PRIME didn't implement C

> On the PRIME system under PRIMIX, there is a special "undefined pointer"
> value.  This value is used by the procedure call mechanism when fewer
> arguments are passed to a subroutine by reference (ie FORTRAN) than
> are expected.  Because of this, constructs such as the
> 
>        if(p)
> 
> Do not yield correct results.

Translation: PRIME has no implementation of C on their machines.  If
the construct

	if (p)

where "p" is a pointer does not compare "p" with the undefined
pointer value, then the language being implemented isn't C.
Furthermore, the clauses

	if (p)

	if (p != NULL)

	if (p != 0)

	if (p != (whatever_p_points_to *)0)

	if (p != (whatever_p_points_to *)NULL)

all have the same meaning in C, so they must all compile to
equivalent code.

It may seem counter-intuitive that the statement

	if (p)

compare the bit pattern stored in "p" with a bit pattern other than
all-zeroes, but that's just too bad - that's the way C works.  One
would hope any C implementor would be sufficiently aware of the way C
works to avoid naively generating a test against 0 for this
construct; if, in fact, the PRIME C compiler does generate such a
test, I guess there are C implementors out there too naive to
properly implement C.

Are you certain that that construct doesn't yield a correct result
(e.g., compares "p" with an all-zero bit pattern), or are you just
assuming this?

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

If you mean that things like <stdio.h> contain

	#define	NULL	0x60000000

then this confirms that they didn't implement C.
	Guy Harris
	{ihnp4, decvax, seismo, decwrl, ...}!sun!guy
	guy@sun.com