Xref: utzoo comp.lang.c++:6811 comp.std.c:2612
Path: utzoo!attcan!uunet!auspex!guy
From: guy@auspex.auspex.com (Guy Harris)
Newsgroups: comp.lang.c++,comp.std.c
Subject: Re: references to dereferenced null pointers
Message-ID: <3036@auspex.auspex.com>
Date: 14 Mar 90 17:48:02 GMT
References: <25EB8EE8.8462@paris.ics.uci.edu> <1990Mar12.175613.12082@utzoo.uucp> <1623@argus.UUCP>
Followup-To: comp.lang.c++
Organization: Auspex Systems, Santa Clara
Lines: 66

>I'm confused, is a non zero NULL pointer valid or not?

The confusion stems from confusion over the meaning of "non-zero".

4.1.5:

	...

	The macros are

	     NULL

	which expands to an implementation-defined null pointer
	constant; ...

3.2.2.3:

	...

	   An integral constant expression with the value 0, or such an
	expression cast to type "void *", is called a *null pointer
	constant*.

So if some implementation defines NULL as something other than 0 or
(void *)0 or (17 - 17) or ((void *)(17 - 17)) or..., the implementation
is not a valid C implementation. 

*However*, this does *not* mean that the implementation must *represent*
a null pointer of any type as a bit string of all zero bits.

>I'm not asking if it will break 90% of the programs out there that
>use 0 instead of NULL.

Defining NULL as something other than the values listed (or implied...)
and having that be the only null pointer constant will break all of the
programs out there at use 0 instead of NULL.  Thus, doing so would be
invalid.

Representing a null pointer constant as something other than a bit
string of all zero bits will not break *any* valid ANSI C program.  It
will break some *in*valid programs, which is why Henry described it as
"unwise" even though it's completely valid.

>On a 370 here I'd love to define NULL as -1 because it will cause an
>immediate addressing exception if it is referenced.  But, I was told that
>NULL is defined as always being the value zero.

If by "define NULL as -1" you mean putting in something like

	#define	NULL	-1

that would, indeed, be invalid.

If you mean changing your 3*0 C implementation such that null pointers
are represented by 32 1 bits, *including making sure that the statements

	char *p;

	p = 0;

assigns a value of 32 1 bits to "p"* (this is a bit that confuses a lot
of people who think that the "p = 0;" is obliged to assign an
all-zero-bits value to "p"), then it would be valid as long as
you make sure this representation convention is followed consistently;
you also have to make sure that non-C code that calls C code or is
called by C code obeys this convention.