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From: john@viper.UUCP (John Stanley)
Newsgroups: comp.lang.c
Subject: Re: pointer debate raging on...  (LONG)
Message-ID: <917@viper.UUCP>
Date: Fri, 1-May-87 16:16:48 EDT
Article-I.D.: viper.917
Posted: Fri May  1 16:16:48 1987
Date-Received: Sun, 3-May-87 01:53:31 EDT
References: <149@sds.UUCP> <17446@sun.uucp>
Reply-To: john@viper.UUCP (John Stanley)
Organization: DynaSoft Systems
Lines: 52

In article <17446@sun.uucp> guy@sun.UUCP (Guy Harris) writes:
 >
 >>Applaud, applaud, applaud.  John has correctly pointed out that
 >>sizeof(pointer) == sizeof(int)  is not the universal constant that 
 >>many people believe it to be.  It is for this exact reason that 
 >>NULL should ***NOT*** be defined as 0.
 >
 >OK, now given that
 >
 >	sizeof (char *) == sizeof (int *)
 >
 >is also not a tautology, what should NULL be defined as?  (char *)0
 >won't cut it, nor will (int *)0.

The answer to this was given in my original reply.  The only pointer
type which is data nonspecific is void*, thus NULL should be ((void*)0).
I have used this on enough projects to know it works and have only
had problems with porting a bug infested program where the author
used NULL every place you or I would have used a 0.  (It turned out
that the program was originaly written by a college sophmore who had
only had one class in C...  He wrote it on a machine where sizeof
a pointer and an int were the same...)

I've looked into this quite a bit in the last 2 weeks and have come
to the conclusion that K&R made a mistake. (gasp :)  Dispite the fact
that they constantly and consistantly refer to NULL as a "pointer",
they fail to ever pass NULL as a parameter in the examples (something 
that would have brought this problem to light much earlier) and 
they consistantly use 0 to #define NULL rather than a pointer.  This 
combined with their only having access to machines where pointers 
were the same size as ints has caused the current state of confusion.

The clearest example of this is K&R page 192 in the first paragraph.  
"The compilers currently allow a pointer to be assigned to an integer,
an integer to be assigned to a pointer, and a pointer to a pointer
of another type.  The assignment is a pure copy operation with no
conversions."  They go on to point out that this assumption is non-
portable...unfortunately, they failed to say why or to deal with the
ambiguity of the integer constant 0 being used as if it were really
a pointer.  While this doesn't effect a majority of applications or
systems, the few where it does require a re-examination of NULL and
a redefinition of the "pointer constant" to something that is in
-all- respects a -true- pointer constant....


  (Please, no jokes about my reply being 
   about a subject that's "NULL and void*"...  :-)

--- 
John Stanley (john@viper.UUCP)
Software Consultant - DynaSoft Systems
UUCP: ...{amdahl,ihnp4,rutgers}!{meccts,dayton}!viper!john