Xref: utzoo comp.arch:4914 comp.lang.c:10251
Path: utzoo!utgpu!water!watmath!clyde!att!alberta!calgary!radford
From: radford@calgary.UUCP (Radford Neal)
Newsgroups: comp.arch,comp.lang.c
Subject: Re: negative addresses (real C null pointers)
Message-ID: <1618@vaxb.calgary.UUCP>
Date: 19 May 88 21:45:10 GMT
References: <2393@uvacs.CS.VIRGINIA.EDU> <21541@amdcad.AMD.COM> <10001@tekecs.TEK.COM>
Organization: U. of Calgary, Calgary, Ab.
Lines: 39

In article <10001@tekecs.TEK.COM>, andrew@frip.gwd.tek.com (Andrew Klossner) writes:

> >> Unfortunately, it is a real problem, because there are zillions of
> >> programs that implicitly assume that [null] pointers are all-zeros.
> 
> > I don't think this is true.  How about an example?
> 
> Sure Doug, from the system V kernel... In routine ttout:
> 
> 		if (tbuf->c_ptr)

My understanding is that this is standards-conforming and portable.
I assume that c_ptr is declared to be of pointer type. The above 
statement is equivalent to

		if (tbuf->c_ptr!=0)

which is equivalent to

		if (tbuf->c_ptr!=(char*)0)

(or (int*)0 or whatever). The expression (char*)0 is _defined_ to be
the null pointer, whatever its bit pattern may be. Note that "NULL"
has nothing to do with anything, being merely a macro found in the
<stdio.h> include file.

An example of a non-portable program is the following:

    char *p;
    int i;
    ...
    i = (int)p;
    if (i!=0)
       *p = ...; /* no guarantee that p is not null... */

Only occurences of 0 in the explicit or implicit context (...*) 0
are magically converted to null pointers.

    Radford Neal