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From: peter@thirdi.UUCP (Peter Rowell)
Newsgroups: comp.lang.c
Subject: Re: pointer debate raging on...
Message-ID: <31@thirdi.UUCP>
Date: Mon, 27-Apr-87 16:03:26 EDT
Article-I.D.: thirdi.31
Posted: Mon Apr 27 16:03:26 1987
Date-Received: Tue, 28-Apr-87 03:38:12 EDT
References: <149@sds.UUCP>
Reply-To: peter@thirdi.UUCP (Peter Rowell)
Followup-To: comp.lang.c
Organization: Third Eye Software, Palo Alto, CA
Lines: 43
Keywords: pointers, religion, sin, Ritchie, razor blades, etc.

Distribution:


Having watched this for a while, I feel the need to comment.  The
following should be read in the context of the *real* world of *real* C
compilers.  Although ANSI may someday save us from our sins, it ain't
doing it yet.  Anyone who is trying to create code that will work on a
broad spectrum of machine/os/compiler combinations must necessarily use
coding paranoia sufficient for the worst case.

Although it is correct that the comparison (charptr == 0) is guaranteed
to work by the Holy Writ (aka K&R), all other bets are off as you stray
from the homogenized, pasteurized world of the VAXen.

As pointed out, environments where sizeof(int) != sizeof(char *)
require NULL to be define'd as ((char *)0) (or some variation on that
theme).  Unfortunately, this is not the end of this train of thought.
There is NO guarantee I am aware of that sizeof(char *) ==
sizeof(struct foo *)!

For this reason (and to adhere to a local naming convention), we use a
specifically typed nil pointer for each and every type.  For example,

#define fooNil ((struct FOO *)0)
#define sbNil  ((char *)0)		/* string of bytes (aka ASCII) */
#define pbNil  ((unsigned char *)0)	/* pointer to arbitrary bytes */
    etc.

Since we always have a "correct" nil for everything, we always pass
exactly the correct value.  True, this *almost* always means that the
same 32 bit (16 bit/48 bit/64 bit) "0" is being passed, but it saves
you should some compiler person become excessively clever.

It also means that your comparisons are of the form:

    if (foo != fooNil) ...  Which is clearly correct,
       vs.
    if (foo != NULL) ...    Which just doesn't feel safe to me.

This method has stood us in good stead (except when we have foolishly
violated it) in porting a source-level debugger to over 70 different
systems - not all of which were done by followers of the True Faith.

Peter Rowell
Third Eye Software
(415) 321-0967