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From: mason@gargoyle.UChicago.EDU (Tony Mason)
Newsgroups: comp.lang.c
Subject: Re: pointer debate raging on...
Message-ID: <636@gargoyle.UChicago.EDU>
Date: Tue, 28-Apr-87 12:28:39 EDT
Article-I.D.: gargoyle.636
Posted: Tue Apr 28 12:28:39 1987
Date-Received: Thu, 30-Apr-87 01:04:58 EDT
References: <149@sds.UUCP> <31@thirdi.UUCP> <6479@mimsy.UUCP>
Reply-To: mason@gargoyle.uchicago.edu.UUCP (Tony Mason)
Organization: U. of Chicago, Computer Science Dept.
Lines: 43
Keywords: pointers, religion, sin, Ritchie, razor blades, etc.

In article <6479@mimsy.UUCP> chris@mimsy.UUCP (Chris Torek) writes:
>In article <31@thirdi.UUCP> peter@thirdi.UUCP (Peter Rowell) writes:
>>As pointed out, environments where sizeof(int) != sizeof(char *)
>>require NULL to be define'd as ((char *)0) (or some variation on that
>>theme).
>
>This is not true.  NULL can be, and indeed, in K&R C, NULL *must*
>be, defined as 0.

Perhaps someone is confusing 0 the int, to 0 the NULL pointer symbol?  K & R
seem to make a distinction here:

(p.192, K&R)

"The compilers currently allow a pointer to be assigned to an integer, an
integer to a pointer, and a pointer to a pointer of another type.  The
assignment is a pure copy operation, with no conversion.  This usage is
nonportable, and may produce pointers which cause addressing exceptions when
used.  However, it is guaranteed that assignment of the constant 0 to a
pointer will produce a null pointer distinguishable from a pointer to any
object."

They DON'T say the "integer 0" but rather the "constant 0".  This must be
considered a context sensitive problem that a compiler designer must take
into account.

Equally, though, K & R don't explicitly say NULL *must* be defined as 0.
Indeed, I have had problems with this area before and don't use NULL much
anymore, as I have seen systems where NULL came DEFINED (from the
distribution) as (char *)0.  It did make code break.  Thus, the safest thing
to do is use 0.  Not the integer, but the constant.  For those who don't
trust compiler writers to stay up to spec, you can use (char *)0, (int *)0,
etc.

There has been a traditional attitude of laxness in the UNIX environment.  I
have cursed at VAX writers who assume that a long was an int (case statements
break when used with ANYTHING besides ints.)  It is only the largess of
compiler writers that allowed this type of behavior.

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