Path: utzoo!attcan!uunet!lll-winken!ames!oliveb!apple!bloom-beacon!mit-eddie!rutgers!att!ulysses!andante!alice!ark
From: ark@alice.UUCP (Andrew Koenig)
Newsgroups: comp.lang.c
Subject: Re: Contiguous Arrays
Message-ID: <8966@alice.UUCP>
Date: 25 Feb 89 17:26:00 GMT
References: <2508@ssc-vax.UUCP> <8943@alice.UUCP> <1828@valhalla.ee.rochester.edu> <7309@killer.DALLAS.TX.US>
Organization: AT&T Bell Laboratories, Liberty Corner NJ
Lines: 25

In article <7309@killer.DALLAS.TX.US>, chasm@killer.DALLAS.TX.US (Charles Marslett) writes:

>  > There is no guarantee that there is a way to represent &space[0] - 101
>  > at all.  Further, computing it may cause overflows such that
>  > 
>  > 	(&space[0] - 101) + 101 != &space[0]
> 
> Please explain?  What machine does this, or what architecture would allow
> the machine or programming language to do it?

The easiest case is if I give you a strictly checking C implementation
in which evaluating &space[0]-101 causes your program to terminate
with an error diagnostic.

For such reasons, C is explicit about what addresses can validly
be generated.  If A is an array with n elements, then &a[0],
&a[1], ... &a[n-1] are all valid addresses and can be used
freely.  Moreover, &a[n] can be generated and used for arithmetic
and comparisons, provided that no attempt is ever made to examine
or change the value of a[n].  A program that attempts to generate
the address of any other `element' of A is an invalid C program
and the implementation is permitted to do as it pleases with it.
-- 
				--Andrew Koenig
				  ark@europa.att.com