Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!husc6!cmcl2!brl-adm!brl-smoke!gwyn
From: gwyn@brl-smoke.ARPA (Doug Gwyn )
Newsgroups: comp.lang.c
Subject: Re: char (*a)[]  (was: Style [++i vs i++])
Message-ID: <6448@brl-smoke.ARPA>
Date: Sun, 20-Sep-87 22:02:31 EDT
Article-I.D.: brl-smok.6448
Posted: Sun Sep 20 22:02:31 1987
Date-Received: Mon, 21-Sep-87 04:14:49 EDT
References: <8298@brl-adm.ARPA> <587@cblpe.ATT.COM>
Reply-To: gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>)
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 28

In article <2474@xanth.UUCP> kent@xanth.UUCP (Kent Paul Dolan) writes:
>OK, X3J11, the ball's in your court.  Do we teach every C programmer
>on every architecture in the world that decrementing pointer loops are
>a no-no, and break half the code in existance, or do we finally bite
>the bullet and decide that compiler writers for brain dead
>architectures, and not the whole C community, pay the penalty for bad
>hardware designs?  Especially since these folks are often the
>perpetrators of the bad hardware design.

A pointer to the [-1]st element of an array may well have an address
CONSIDERABLY smaller than that of the [0]th element, if the array
elements are large.  This is not a problem for the [N+1]st element.
Therefore, there is only a small penalty in requiring implementations
to ensure that [N+1] pointers have valid addresses (generally only
wastes at most one word of "slop" space per segment), but there would
be an unacceptably large penalty with requiring that a pointer to the
[-1]st element of an array have a valid address.

This was in fact discussed by X3J11 and general agreement reached to
require [N+1] pointer validity, but not [-1] pointer validity.  This
permits one common form of somewhat sloppy coding, but not the other.
Please note that the outlawed form was NEVER safe; I've seen it break
in a PDP-11 implementation of bsearch() (due to data space address
wrap-around), for example.  It is not within X3J11's power to "somehow"
make the unworkable work.

And yes, please teach every C programmer in the world how to write
reliable code.  Thanks in advance.