Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!uunet!philmtl!ray
From: ray@philmtl.philips.ca (Raymond Dunn)
Newsgroups: comp.std.c
Subject: Re: references to dereferenced null pointers
Message-ID: <1136@philmtl.philips.ca>
Date: 15 Mar 90 20:47:58 GMT
References: <51083@microsoft.UUCP> <25EB8EE8.8462@paris.ics.uci.edu> <1990Mar12.175613.12082@utzoo.uucp> <1623@argus.UUCP> <12347@smoke.BRL.MIL>
Organization: Philips Electronics Ltd.  Product Group PC.  Montreal.
Lines: 27

>In article <1623@argus.UUCP> ken@argus.UUCP (Kenneth Ng) writes:
>>But, I was told that NULL is defined as always being the value zero.

All the things that have been said about NULL and 0 and the *internal*
hidden actual value perhaps being different are of course true.

Perhaps Kenneth was actually looking for a simpler answer though.  In any
program, you are free to use *any* value you like to represent *your* value
of the illegal pointer.

So long as you don't define NULL to be this, and so long as you don't pass it
to library routines and expect it to be equivalent to passing NULL, or
expect library routines to return that value, you will have no problems.

As an example you could define:

#define NOPTR(type) ((type *)-1)

and use for example NOPTR(char) within your own code with the restrictions
stated.  It is still of course legal to pass such a value to standard
routines, but if they process NULL gently, they probably wont process your
NOPTR gently, but will indeed generate an exception.
-- 
Ray Dunn.                    | UUCP: ray@philmtl.philips.ca
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