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Path: utzoo!mnetor!seismo!rutgers!dayton!rosevax!sds!dave
From: dave@sds.UUCP (dave schmidt x194)
Newsgroups: news.software.b,comp.lang.c
Subject: Re: News for Xenix on PC AT ?
Message-ID: <146@sds.UUCP>
Date: Wed, 22-Apr-87 14:57:40 EST
Article-I.D.: sds.146
Posted: Wed Apr 22 14:57:40 1987
Date-Received: Sat, 25-Apr-87 00:59:54 EST
References: <18346@ucbvax.BERKELEY.EDU> <145@sds.UUCP> <17005@sun.uucp>
Organization: SciCom Data Services, Minnetonka, MN
Lines: 63
Xref: mnetor news.software.b:528 comp.lang.c:1860

In article <17005@sun.uucp>, guy%gorodish@Sun.COM (Guy Harris) writes:
> >   [... commentary on problems w/ news s/w ...]  also present
> >   are things like:
> >	charptr = malloc(1024);
> >	if ( !charptr ) ...	/* this should be (charptr != (char *)NULL) */
> 
> This last one falls under the heading of "stupid problems from
> [censored] compiler writers who didn't understand C before inflicting
> something that they claim is a C compiler on Xenix users."
> 
> 	if (!charptr) ...
> 
> is IDENTICAL in meaning to
> 
> 	if (charptr == 0) ...
> 
> which, in turn, is IDENTICAL in meaning to
> 
> 	if (charptr == (char *)0) ...
> 
> if "charptr" is of type "char *".


Why is "if (charptr == 0)" NOT identical to "if ((int)charptr == 0)" ???
This would not work on a machine where sizeof(char *) > sizeof(int).
In fact, since we are comparing apples (char *'s) to oranges (int's),
a cast is called for and a good, conscientious programmer would perform
the cast themself.  Furthermore, "if (!charptr)" is not correct on all 
architectures: I've heard tell of an implementation of C where NULL was
0xffffffff -- the simple "if (!charptr)" fails in this case.

Coding the above as "if (charptr == (char *)NULL)" saves you from the
"queer machine"; it is also more correct in that it explicitly states what
you desire and is more portable.  Actually, if NULL is *correctly* defined
in stdio.h as "(char *)0" (NULL being the value that malloc() returns
on error, NULL *should* have type char *), this could be written as
"if (charptr == NULL)".  Unfortunately for me, XENIX correctly defines
NULL, leaving me to deal with statements such as:

	int cnt;
	char ch, *ptr;

	ptr = NULL;		/* this is ok			*/
	...
	ch = NULL;		/* expands to: ch = (char *)0;	*/
				/* ch = '\0'; is desired	*/
	...
	if (cnt == NULL)	/* expands to: if (cnt == (char *)0) */
				/* i'd settle for if (!cnt)	*/
	...

Given my #define for NULL, statements of this kind make the compiler
complain (as it should).  That point aside, it makes little sense to
me to say that a pointer to a character and a character may be
assigned the same value without a cast someplace.  This implicitly
assumes that NULL is #defined as 0, which is an assumption that
good programmers (or should I say "programmers that I regard as good"?)
don't make.


Dave Schmidt

(A firm believer in defensive programming.)