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From: guy@sun.uucp (Guy Harris)
Newsgroups: net.lang.c
Subject: Re: if (p) ...
Message-ID: <2910@sun.uucp>
Date: Sun, 20-Oct-85 18:11:54 EDT
Article-I.D.: sun.2910
Posted: Sun Oct 20 18:11:54 1985
Date-Received: Tue, 22-Oct-85 06:33:14 EDT
References: <2098@brl-tgr.ARPA> <916@celtics.UUCP>
Organization: Sun Microsystems, Inc.
Lines: 46

> In fact, I've worked in an environment (PRIMOS and other Multics-style
> systems) with segmented addressing schemes (and demand-paged virtual
> memory) wherein  the definition of a NULL pointer was any address in
> the designated-highest-possible segment (defined as octal 7777).  Thus,
> for C to see a pointer as null required conversion from 7777/0 to 0 (or
> seg 0 loc 0, which is a valid address) and back again!

No, it didn't.  The author of the C compiler may have thought so, but if
he/she did then somebody else should have been chosen to write the compiler.
On such a machine,

1) NULL should have been #defined as 0 (just like everywhere else)

and

2) the statement

	if (p)
		foo();

should have compiled into something like

	compare_immediate	p, #07777/0	# seg 7777, loc 0
	beq			around
	subroutine_call		foo
around:

> YEAH, a portable test is necessary that does not depend on NULL being ZERO.

The definition of the C language requires that NULL be ZERO (how many times
do people have to be told this?).  The test

	if (p)

is already portable enough, as is the test

	if (p != 0)

and the test

	if (p != NULL)

In the aforementioned class of machines, all three test should compile into
code like the code listed above.

	Guy Harris