Why NULL is 0

[Mike Scott]

In article <10576 at mimsy.UUCP> chris at mimsy.UUCP (Chris Torek) writes:
....
>of type (char *), are 48 bits wide.  All other pointers, such as
>(int *) and (double *), are 32 bits wide.
>
>Now suppose we have the following C code:
>
> 	main()
>	{
> 		f1(NULL);	/* wrong */
> 		f2(NULL);	/* wrong */
> 		exit(0);
> 	}
> 
> 	f1(cp) char *cp; { if (cp != NULL) *cp = 'a'; }
> 	f2(dp) double *dp; { if (dp != NULL) *dp = 2.2; }
>
>There are two lines marked `wrong'.  Now suppose we were to define NULL
>as 0.  Clearly both calls are then wrong: both pass `(int)0', when the
>first should be a 48 bit (char *) nil pointer and the second a 32 bit
>(double *) nil pointer.
>

He suggests the form:
>       main()
>       {
>		f1((char *)NULL);
>		f2((double *)NULL);
>		exit(0);
>	}
>
>The preprocessor changes both NULLs to 0s, and the code remains
>correct.
>

If I could add my 2 pennyworth.  From K&R p192, we read " it is
guaranteed that assignment of the constant 0 to a pointer will
produce a null pointer distinguishable from a pointer to any object".

>From page 71: "Within a function, each argument is in effect a local
variable initialized to the value with which the function was called."

It follows at once from these that the integer 0 may be supplied in
any function call where a pointer is expected, and the compiler must
make sure that the proper translation to the correct sort of
null pointer is performed.

[I suppose one might argue about differences between 'assignment' and
'initialisation'. I think it's clear that initialisation of a pointer
to NULL (whatever NULL may be defined as) and assignment of NULL to
that same pointer have to give the same result!]

The real problem is that functions don't have to be declared until
used. Given an ANSII-type declaration, the compiler can sort out
the mess. Until then, I suppose the only way to get portable code is
indeed
to use expicit casts as in the "correct" code above. But until now, I
had no idea that machines with different pointer lengths existed [I'm
rather blinkered by my PDP8/PDP11/VAX spectacles]. I don't really see
how one can guess the needs of various 'strange' machine architectures
that may exists :-(

(Usual disclaimers apply.)
-- 
Regards. Mike Scott (scott at stl.stc.co.uk <or> ...uunet!mcvax!ukc!stl!scott)
phone +44-279-29531 xtn 3133.