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From: guy@sun.UUCP
Newsgroups: net.unix,net.lang.c
Subject: Re: if (pointer) command
Message-ID: <6989@sun.uucp>
Date: Sun, 7-Sep-86 18:24:00 EDT
Article-I.D.: sun.6989
Posted: Sun Sep  7 18:24:00 1986
Date-Received: Mon, 8-Sep-86 20:54:46 EDT
References: <3325@brl-smoke.ARPA> <241@bsdpkh.UUCP> <1170@ttrdc.UUCP> <1427@ncoast.UUCP>
Followup-To: net.lang.c
Organization: Sun Microsystems, Inc.
Lines: 46
Xref: mnetor net.unix:5409 net.lang.c:5820

(This discussion has ceased to be related to UNIX; it is now a discussion of
C, so it is being moved to net.lang.c.)

> > It DOESN'T???  Gee, then maybe you have a busted malloc().  If malloc()
> > fails, it should return the null pointer.  That is what is being tested in
> > the clause 'if(p=malloc(strlen(s)+1))'.  As so many people have expounded
> > in net.lang.c:  a null pointer is semantically the same as zero in the C
> > language (except for sizeof(), should pointers exist of sizes other than
> > sizeof(int)).

> But if a pointer is 4 bits and an int is 2 bits (some 68000
> implementations), you'll get spurious failures if the pointer returned by
> malloc is a multiple of 0x10000!

Not if you've declared "malloc" to return a "char *", you won't.  All C
compilers will generate equivalent code for

	extern char *malloc();
	char *p;
	unsigned int i;

	if (p = malloc(i))
		foo();

and

	extern char *malloc();
	char *p;
	unsigned int i;

	if ((p = malloc(i)) != 0)
		foo();

Any compiler that does *not* generate equivalent code in these two cases is
not a C compiler; it compiles some other language that may look like C in
some ways, but is not C.  If it was intended to be a C compiler, it has a
bug.  (Any compiler that does not generate equally efficient code for these
two constructs is *also* broken, since they both mean the same thing in C.)

If you haven't declared "malloc" to return a "char *", this code will rarely
if ever work on a machine where "char *" and "int" have different sizes,
regardless of whether "malloc" succeeds or fails.
-- 
	Guy Harris
	{ihnp4, decvax, seismo, decwrl, ...}!sun!guy
	guy@sun.com (or guy@sun.arpa)