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From: bob+@andrew.cmu.edu (Bob Sidebotham)
Newsgroups: comp.arch
Subject: Re: negative addresses
Message-ID: <0Wi7Dwy00VsMI4Hkcy@andrew.cmu.edu>
Date: 17 Jun 88 01:21:00 GMT
References: <2393@uvacs.CS.VIRGINIA.EDU> <21541@amdcad.AMD.COM> <1988May12.162906.16901@utzoo.uucp> <7881@brl-smoke.ARPA> <10001@tekecs.TEK.COM>,
	<21670@amdcad.AMD.COM>
Organization: Carnegie Mellon
Lines: 31
In-Reply-To: <21670@amdcad.AMD.COM>

> *Excerpts from magazines.software.z: 18-May-88 Re: negative addresses Tim*
> *Olson@amdcad.AMD.COM (1494)*

> As has been stated in comp.lang.c numerous times: in C, nil can be any
> bit pattern, as long as it is guaranteed not to ever point to valid
> data.  NULL must be 0 (or perhaps (void *)0 under ANSI).  The compiler
> takes care of the appropriate conversions between NULL and nil.  The
> above code is correct C.

>       -- Tim Olson
>       Advanced Mic

I haven't been following this discussion, but I'll add my two-bits worth
anyway:  my current practice, which is apparently not legal C, is to zero data
structures after allocating them, to guarantee the structure is in a reasonable
state.  This works well for most data types, and, I thought, for pointers.

For the moment, I will still consider this a reasonable practice, despite the
fact that it may not work on some obscure machines:  on the machines I work
with, it provides me with a safe way to initialize a data structure which is_
__immune to changes in the definition of the structure._  If I explicitly
initialize all of the fields of a structure, someone will later add a field
without remembering to add the the corresponding initializing code.

It would be preferable to have a C built-in that could be used to intitialize
all of a structure's components to "zero" values, and even more preferable if
hardware manufacturers, operating system builders, compiler writers, and, of
course, language specification writers, all recognized that 0 really is a
_very_ special value.

Bob Sidebotham
P.S. The formatting of this note for the net is beyond my control...