Xref: utzoo comp.lang.c++:6809 comp.std.c:2611
Newsgroups: comp.lang.c++,comp.std.c
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: references to dereferenced null pointers
Message-ID: <1990Mar14.164539.23685@utzoo.uucp>
Organization: U of Toronto Zoology
References: <51083@microsoft.UUCP> <25EB8EE8.8462@paris.ics.uci.edu> <1990Mar12.175613.12082@utzoo.uucp> <1623@argus.UUCP>
Date: Wed, 14 Mar 90 16:45:39 GMT

In article <1623@argus.UUCP> ken@argus.UUCP (Kenneth Ng) writes:
>I'm confused, is a non zero NULL pointer valid or not?  I'm not asking if
>it will break 90% of the programs out there that use 0 instead of NULL.
>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.

NULL is just a convenient way of writing the constant 0, for all practical
purposes.  It is *not*, by itself, a null pointer, because there is no
"generic null pointer" type.  NULL has to be converted to a specific pointer
type to become a null pointer.  That (compile-time) conversion may well
change the representation in some strange way.  `NULL' must remain 0 (or a
close equivalent), but `(foo *)NULL' can be a different story.

There is absolutely nothing wrong with having a pointer representation in
which the bit pattern for a null pointer is not all zeros... except that
there are a lot of old, badly-written programs which will break.  Thus my
earlier comment that it is valid but unwise.
-- 
MSDOS, abbrev:  Maybe SomeDay |     Henry Spencer at U of Toronto Zoology
an Operating System.          | uunet!attcan!utzoo!henry henry@zoo.toronto.edu