Path: utzoo!attcan!uunet!husc6!mailrus!ames!haven!adm!smoke!gwyn
From: gwyn@smoke.BRL.MIL (Doug Gwyn )
Newsgroups: comp.lang.c
Subject: Re: pointers, tests, casts
Message-ID: <8961@smoke.BRL.MIL>
Date: 23 Nov 88 06:09:34 GMT
References: <11130@dartvax.Dartmouth.EDU>
Reply-To: gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>)
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 17

In article <11130@dartvax.Dartmouth.EDU> Eric.J.Bivona@Dartmouth.EDU writes:
>I have a question about tests on pointers, ...

	if ( !ptr )
and
	if ( ptr == 0 )
are both perfectly valid ways to test for a null pointer.  You can
explicitly cast the 0 to the proper type, but it's not necessary.

Explicit casting of 0 (or NULL, defined in several headers) is required
only when the compiler could mistake the 0 for an int constant rather
than the intended null pointer constant.  The main place this can occur
is in the arguments to a function when no prototype is in scope, e.g.
	execl( "/bin/sh", "-sh", "-i", 0 );
(This one is even worse since it's a variable-argument function, so not
even a prototype would help.)  The bug typified by this is seen in a
LOT of code ("well it worked on MY machine!").