Path: utzoo!attcan!uunet!uport!plocher
From: plocher@uport.UUCP (John Plocher)
Newsgroups: comp.sources.d
Subject: Re: C Pointers and Integers
Message-ID: <376@uport.UUCP>
Date: 23 Jul 88 22:06:06 GMT
References: <54@libove.UUCP: <701@nod2sco: <3222@bigtex.uucp: <1033@ficc.UUCP: <19789@watmath.waterloo.edu: <1305@ucsfcca.ucsf.edu: <19829@watmath.waterloo.edu: <3027@geac.UUCP> <9857@reed.UUCP>
Reply-To: plocher@uport.UUCP (John Plocher)
Distribution: comp
Organization: Microport Systems, Scotts Valley, CA
Lines: 52

>[ *lots* of stuff about C pointers and integers ]
>
>Yes, I know I'm stupid to get involved in this religious war.  However, K&R2
>contains a detailed discussion on the subject of pointers and integers --

I shouldn't do this, but...

Thanks for the overview - I think that most (if not all) the compilers on
intel 286 machines (one of the more prolific 32/16 bastards out there :-)
handle this without problems.  One *can* do stuff like:

	double *dp;
	...
	if (dp == 0) ...

But because C does no checking of function arguments (K&R C, not ANSI C, for
this example) the following will fail on a machine where sizeof(int) !=
sizeof(pointer):

	void foofunction( i, p, ii )
	int i, ii;
	double *p;
	{
	    if (p)
		printf("%d %f %d\n", i, *p, ii);
	    else
		printf("%d %d\n", i, ii);
	}

	main() 
	{
		foofinction( 1, 0, 2 );
	}

The problem (which is addressed by function prototypes in ANSI C) is that
the compiler doesn't have type info for the 2nd parameter to foofunction().
i.e., what size should the 0 be converted to when it is pushed on the stack
(or how many registers should it use...)

It is in this case that you *MUST* cast 0 to (char *) 0 or (double *) 0.
Since NULL is defined to be 0, you can also type this as (char *) NULL.

Since K&R define NULL as 0, implementations which define it to anything
else are wrong:
	#ifdef LARGE_MODEL
	# define NULL	0L	/* or (char *)0 */
	#else
	# define NULL	0
	#endif
This is the most common (bad) example I've seen in the intel CPU world.

	-John Plocher