Why NULL is 0

[Dave Sill]

In article <800 at zippy.eecs.umich.edu> Jan Wolter <janc at palam.eecs.umich.EDU> writes:
>One other vaguely related question:  Which of the following produce a null
>pointer?
>
>	int zero = 0;
>	char *p1 = 0;             /* this is a null pointer! */

Yes.

>	char *p2 = zero;          /* is this a null pointer? */

Maybe.  K&R say assignments between pointers and ints are nonportable,
as are assignments between different types of pointers.

>	char *p3 = (char *)zero;  /* what's this? */

Exactly the same as p2.  K&R define a cast as performing the
conversions required to assign the operand to a variable of the type
of the cast.

>As I read K&R, a null pointer is only produced when a *constant* 0 is assigned
>to a pointer.

Almost, once generated a null pointer can be produced by assignment.
I.e.,
	char *pc1, *pc2;
	pc1 = 0;
	pc2 = pc1;

>While K&R says assignment is a bitwise copy, they say explicitly typecasting
>an integer to a pointer gives a machine dependent result.  Thus it seems
>possible that the p1, p2, and p3 could be three different pointers.

No, at most two different pointers.  p1 is null.  p2 and p3 will be
the same nonportable pointer.

>(Frankly,
>the more I read on this subject, the more I think K&R didn't have their minds
>entirely clear on this business either.)

Maybe you should read some more.

=========
The opinions expressed above are mine.

"Give me a pointer and I'll dereference the world."
					-- David Keppel