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From: mason@gargoyle.UChicago.EDU (Tony Mason)
Newsgroups: comp.lang.c
Subject: Re: NULL pointers as arguments
Message-ID: <641@gargoyle.UChicago.EDU>
Date: Thu, 30-Apr-87 10:21:09 EDT
Article-I.D.: gargoyle.641
Posted: Thu Apr 30 10:21:09 1987
Date-Received: Sat, 2-May-87 09:03:46 EDT
References: <1130@ius2.cs.cmu.edu> <148@nbisos.NBI.COM>
Reply-To: mason@gargoyle.uchicago.edu.UUCP (Tony Mason)
Distribution: na
Organization: U. of Chicago, Computer Science Dept.
Lines: 45

>All of these problems would go away if people would cast NULL to the
>proper value.
>
>"What about existing code which assumes .... ?"
>
>Tough !  It's wrong !
>
>We don't need to change the language definition to support junior programmers.
>If you can't use a simple tool like lint, then step aside and let someone
>who really cares do the task.  I'm so sick of all of the exceptions being
>made to support and promote mediocrity !

It is NOT wrong.  K & R EXPLICITY state that NULL IS the correct manner in
which to represent this stuff. (K & R p. 97-98):

"We write NULL instead of zero, however, to indicate more clearly that this
is a special value for a pointer.  In general, integers cannot meaningfully
be assigned to pointers; zero is a special case."
                         ^^^^^^^^^^^^^^^^^^^^^^

It would be rather difficult to defend the position that code using THIS
assumption was written by "junior programmers" and that to conform to K & R
is to "support and promote mediocrity."  What we are arguing about are not
problems with using the special symbol '0' but rather scoping rules in C.
With the new ANSI standards, function prototyping should get rid of the
problem.  

Point is, we have to live with what C is now and has been in the past.  It
doesn't suffice to say "it's wrong."  K & R in the famous Appendix A sum it
up (p. 192):

"...it is guaranteed that assignment of the constant 0 to a pointer will
produce a null pointer distinguishable from a pointer to any object."

To say code using this fact is "wrong" isn't correct, OR productive.  Useful
suggestions (such as common coersion) may be acceptable FOR THE NULL POINTER
case.  From K & R, you could assume there was a pointer type "NULL" which was
distinguishable from other pointer types.  Thus, a NULL char * could be the
same as a NULL struct x *.  Coersion of NULL pointers only may be difficult,
but it would conform, and solve the problem.

Tony Mason
Univ. of Chicago, Dept. of C.S.
ARPA: mason@gargoyle.uchicago.edu
UUCP: ...ihnp4!gargoyle!mason