Null revisited (briefly)

[John Woods]

In article <13068 at steinmetz.ge.com>, davidsen at steinmetz.ge.com (William E. Davidsen Jr) writes:
>   Some peopke put a zero in a char using NULL, such as "ffoA[n]=NULL".
> I think this is portable for either type, but I have seen compilers
> which warn about type conversion is NULL is a pointer.

If NULL is #defined as 0, this will work, but if it is #defined as (void *)0
(which I think is the other ANSI-legal #definition; if you don't know, assume
that only 0 works, if you do know, quietly correct me by mail), this is not
guaranteed to work:  on a sufficiently bizarre architecture, the nil-pointer
need not have an all-zero-bits representation, as long as the compiler can
(a) properly recognize that the constant 0 must be coerced to the nil-pointer
pattern, and (b) can coerce the (void *)nil-pointer to the (foo *)nil-pointer
for all types "foo".  The compiler is not obligated to know how to turn
(void *)0 back into 0.

Myself, I usually use #define NUL '\000', named after the ASCII NUL character,
though I often worry about misunderstandings between NUL and NULL.
-- 
John Woods, Charles River Data Systems, Framingham MA, (508) 626-1101
...!decvax!frog!john, john at frog.UUCP, ...!mit-eddie!jfw, jfw at eddie.mit.edu

Presumably this means that it is vital to get the wrong answers quickly.
		Kernighan and Plauger, The Elements of Programming Style