Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!seismo!husc6!hao!ames!ucbcad!ucbvax!jade!eris!mwm
From: mwm@eris.BERKELEY.EDU (Mike (My watch has windows) Meyer)
Newsgroups: comp.sys.amiga
Subject: Re: Manx C and ptr comparisons
Message-ID: <4613@jade.BERKELEY.EDU>
Date: Thu, 6-Aug-87 03:02:57 EDT
Article-I.D.: jade.4613
Posted: Thu Aug  6 03:02:57 1987
Date-Received: Sat, 8-Aug-87 09:43:49 EDT
References: <4540@jade.BERKELEY.EDU> <1836@vax135.UUCP> <4595@jade.BERKELEY.EDU> <3199@zen.berkeley.edu>
Sender: usenet@jade.BERKELEY.EDU
Reply-To: mwm@eris.BERKELEY.EDU (Mike (My watch has windows) Meyer)
Distribution: comp
Organization: Missionaria Phonibalonica
Lines: 65
Keywords: look at the assembly.

[I'm very tempted to cross-post this to comp.lang.c, but I won't. They
see it every few weeks, and would quite rightly flame me for bringing
it up again.]

In article <3199@zen.berkeley.edu> waterman@cory.Berkeley.EDU.UUCP (T.S. Alan Waterman) writes:
<Mike, the deal with this problem is this:
<
<  if (ptr == 0)  compares the pointer with an INT, which is gonna be 16 bits.

In that case, the compiler is doing the wrong thing. Quoting K&R, page
190, section 7.7:

	A pointer to which 0 has been assigned is guaranteed not to
	point to any object, and will appear to be equal to 0 ...

They don't say so, but anyone reasonable would assume that a pionter
which had not been assigned the value 0 will *not* compare equal to 0.
Note, that's "0", not "0L" or "(my_favorite_type *) 0".

From dpANS, page 30, section 3.2.2.3:

	An integral constant with the value 0, or such an expression
	cast to type void *, is called a null pointer constant. If a
	null pointer constant is assigned to or COMPARED FOR EQUALITY
	[emphasis mine - mwm] to a pointer, the constant is converted
	to a pointer of that type.

Once again, that's "0", not "0L".

<In short, the error message is correct. You are being rightly warned the the
<compiler is about to do exactly what you told it to, and not really what
<you wanted to do.

Yes, the warning is correct. It is not doing what I wanted it to do.
But it is not doing what I told it to do. I gave instructions that a
compiler for C will interpret to mean "decide if this pointer is a
pointer to zero."

It's not clear what Manx C is doing. It could be

	1) comparing the pointer to a 32-bit zero.
	2) comparing the low order 16 bits of the pointer to zero.
	3) compare the pointer to the zero + whatever happens to be in
		memory adjacent to it.

Well, it could also be doing what GNU C does with #pragma, or
something really strange. But those three are the only rational
actions. And three isn't very rational.  Since one is the right thing
to do, I assume that it's doing two.

This seems to be something that C compiler writers don't seem to be
able to get right - especially on the eighty-eightysux family. So
not getting it right doesn't qualify as a major flaw.

Continuing to defend it as correct in the face of clear evidence to
the contrary is a major flaw. I'll follow the same path that
Guy Harris, Chris Torek and Henry Spencer followed under those
conditions.

	<mike
--
The weather is here, I wish you were beautiful.		Mike Meyer
My thoughts aren't to clear, but don't run away.	mwm@berkeley.edu
My girlfriend's a bore, my job is to dutiful.		ucbvax!mwm
Hell nobodies perfect, would you like to play?		mwm@ucbjade.BITNET