Path: utzoo!yunexus!geac!daveb
From: daveb@geac.UUCP (David Collier-Brown)
Newsgroups: comp.lang.c
Subject: Re: Is &a[NTHINGS] legal
Summary: Well, yes, but it can be hard under some boundary conditions.
Message-ID: <2699@geac.UUCP>
Date: 5 May 88 12:45:20 GMT
Article-I.D.: geac.2699
Posted: Thu May  5 08:45:20 1988
References: <12074@tut.cis.ohio-state.edu> <7806@brl-smoke.ARPA> <2637@ttrdc.UUCP> <1450@iscuva.ISCS.COM>
Reply-To: daveb@geac.UUCP (David Collier-Brown)
Organization: GEAC Computers, Toronto, CANADA
Lines: 29

In article <1450@iscuva.ISCS.COM> carlp@iscuva.ISCS.COM (Carl Paukstis) writes:
>The above notion follows from a quick reading of Mr. Gwyn's original
>followup.  It seems, however, that there need not be an addressable OBJECT
>beyond the array bound, and it is an error to attempt to dereference
>a[NTHINGS+1].  There must be a valid ADDRESS (or at least simulation of
>one), so that size calculations will work, mostly, but also so that
>bound-checking as proposed in the original posting is possible.

  My understanding is that one has to be able to generate the
address of the last-plus-one'th element of an array:
	thing a[NTHINGS];
	...

	if (x < &a[NTHINGS]) ...

  I'm not so sure about NTHINGS+1, which is the last-plus-two'th
(last-plus-tooth?) element.

  As you might expect, this reqirement identifies a problem with
machines like the DPS-6, which will need the shim at &a[NTHINGS] to
keep from trapping when the address is loaded into a register and
checked for legality.

 --dave
-- 
 David Collier-Brown.                 {mnetor yunexus utgpu}!geac!daveb
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