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From: john@viper.UUCP (John Stanley)
Newsgroups: news.software.b,comp.lang.c
Subject: Re: News for Xenix on PC AT ?
Message-ID: <888@viper.UUCP>
Date: Fri, 24-Apr-87 15:42:21 EST
Article-I.D.: viper.888
Posted: Fri Apr 24 15:42:21 1987
Date-Received: Sun, 26-Apr-87 01:12:17 EST
References: <18346@ucbvax.BERKELEY.EDU> <145@sds.UUCP> <17005@sun.uucp> <146@sds.UUCP> <5787@brl-smoke.ARPA>
Reply-To: john@viper.UUCP (John Stanley)
Organization: DynaSoft Systems
Lines: 58
Xref: mnetor news.software.b:539 comp.lang.c:1884

In article <5787@brl-smoke.ARPA> 
gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>) writes:
 >
 >He, I, and others have explained more than once before in the C newsgroup
 >that the only fully correct definition of the macro NULL, as C now stands,
 >is as 0.  (Under the proposed ANSI standard you could probably get away
 >with defining it as (void *)0.)
 >

Actualy, you -must- use (void *)0 -OR- 0 and be consistant one way or
the other.  It's not an option...  If I had to choose, I'd say use

	#define NULL	((void*)0)

because it's used as a pointer in 98% of the code I've seen and for
the following reason...

This isn't important on machines where sizeof(pointer) == sizeof(int),
but if you're using (as a simple example) a machine where pointers are
32 bit and ints are 16 bits then code containing the following function
may have problems if you change how NULL is defined:

	char *myfunc(ind, str, ind2)
	int ind, ind2;
	char *str;
	{
	char mystr[80];

	if (str == NULL)
		str = mystr;


	 ...... some code .....


	return (str);
	}

now, later in my program if I have NULL defined as 0, then the following
line will cause the wrong number of bytes to be pushed on the stack:

	myfunc( i1, NULL, i2 );

If you use NULL as a pointer, it -must- always actualy be a pointer or
your code will break any time you try to pass it as a constant to a 
function.

You may claim, if you wish, that passing NULL as a constant is illegal,
but I've seen too many examples of code where it's been done to accept
that as a valid argument.  If there already exists a large body of code
using a particular construct, then that construct may be "illegal" in
your opinion, but anyone discussing "the only fully correct definition"
must take it into account...

--- 
John Stanley (john@viper.UUCP)
Software Consultant - DynaSoft Systems
UUCP: ...{amdahl,ihnp4,rutgers}!{meccts,dayton}!viper!john