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Date: Sat, 15 Feb 1997 01:48:58 +0100
To: Giles Lean <giles@nemeton.com.au>
From: Eivind Eklund <eivind@dimaga.com>
Subject: Re: NULL as ((void*)0) (was Re: strlen() question) 
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At 10:17 AM 2/15/97 +1100, Giles Lean wrote:
>On Fri, 14 Feb 1997 17:36:53 +0100  Eivind Eklund wrote:
>
>> I hereby propose changing the default declaration of NULL under FreeBSD
from
>> #define NULL 0
>>  to
>> #define NULL ((void*)0)
>
>This is more of a kludge than a good idea, and I offer the following
>sage advice that I filed away many years ago.

Which I throughly agree with; it also come from people I defineatly respect.
However, I don't feel that it disagree with the above proposition.

[From Henry Spencer, of "10 commandments for C programmers" and regexp fame]
>In any case, this stuff is a concession to badly-written code.  No
>properly-written code under ANSI compilers will notice the difference
>between the different forms of NULL.  In contexts where the desired
>type is not known from context -- basically, function arguments in
>the absence of a prototype or the presence of varargs -- NULL *must*
>be cast to the proper pointer type.

Having NULL ((void*)0) is a combination of a concession to badly written
code (it makes it work on more machines, though not all) as well as a way
of forcing people to avoid using NULL in non-pointer expressions.  IMHO, a
good thing.  (If nobody else feel the same way I won't make any cahnges, of
course.)

BTW: I just got another idea - if we can turn the definition of NULL
between ((void*)0) and 0 we can detect if NULL is abused if compiling on a
machine with different sizeof(void*) and sizeof(int).  If used correctly,
code will be equal no matter what the definition - if used incorrectly,
different code should result.  This can provide fairly automatic detection
of errors, provided we have two different builds.



Eivind Eklund  perhaps@yes.no  http://maybe.yes.no/perhaps/
eivind@freebsd.org