NULL pointer

[Niket K. Patwardhan]

>Date:     Sun, 17 Nov 85 3:59:50 EST
>From:     Doug Gwyn (VLD/VMB) <gwyn at BRL.ARPA>
>To:       Davidsen <davidsen%steinmetz.uucp at BRL.ARPA>
>cc:       info-c at brl-tgr.arpa
>Subject:  Re:  NULL pointer
>
>I'll see if I can beat Guy Harris to this one:
>
>The whole topic of the correct #definition for NULL
>has been beat to death more than once on this mailing list.
>
>The answer is that NULL must be defined as simply 0
>and that code that misuses NULL cannot be fixed in
>general by any change in its #definition.
>
>Consider
>
>	hi_func()
>	{
>		...
>		lo_func( ..., NULL, ... );
>		...
>	}
>
>	lo_func( ..., p, ... )
>		int *p;
>	{
>		...
>	}
>
>This code is INCORRECT, whether NULL is #defined as 0
>or as (char *)0.  The only correct way to do this
>(not counting the new X3J11 parameter type coercion
>mechanism) is to change the invocation of lo_func to:
>
>		lo_func( ..., (int *)NULL, ... );
>
>By trying to #define NULL as (char *)0, you are trying
>to keep the programmer from having to understand this
>and deal with it properly.  You also cause problems
>with statements such as
>
>	if ( p == NULL )
>		do_stuff;
>
>when the type of `p' is not (char *).  The only
>universal #definition for NULL that works in all such
>cases is 0.  (Of course, some machines allow sloppier
>usage.  This discussion assumes universality.)
>
>#define	NULL	0	/* don't muck around with it */

The last line: ---- I agree with it whole-heartedly.

But..... for a machine with non-zero null pointer representations

>		lo_func( ..., (int *)NULL, ... );

still doesn't work. What the callee routine gets is a pointer with an all-zero
representation.... which isn't a null pointer. The ONLY thing that does work is

		lo_func( ..., p=0, ....);

where p is a pointer of the requisite type! Of course, with parameter type
coercion (or should I say conversion.. it isn't the same thing), this
shouldn't be necessary!