Path: utzoo!mnetor!uunet!lll-winken!lll-tis!ames!umd5!mimsy!chris
From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.lang.c
Subject: Re: Is &a[NTHINGS] legal
Message-ID: <11289@mimsy.UUCP>
Date: 30 Apr 88 16:09:21 GMT
References: <12074@tut.cis.ohio-state.edu>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 31

In article <12074@tut.cis.ohio-state.edu> lvc@tut.cis.ohio-state.edu
(Lawrence V. Cipriani) writes:
>Is it legal to apply the & (address of) operator to an array
>element that is non-existent?

Depends:

>Given:
>	sometype a[NTHINGS], *p;
>Should:
>	for (p = a; p < &a[NTHINGS]; p++)	/* 1 */
>be written as:
>	for (p = a; p <= &a[NTHINGS-1]; p++)	/* 2 */

This is not necessary.

>Will 1 be guaranteed to work in ANSI-C?

Yes.  If necessary, the compiler will put a one-byte (or word or
whatever) shim after the array in that array's address space, so
that &a[NTHINGS] will be meaningful.

Note, however, that the corresponding count-down loop

	for (p = &a[NTHINGS]; --p >= a;)

is not.  In particular, this sort of code fails if &a[-1] `wraps
around' the address space of the array.  It can even fail on a flat
address space machine like the Vax if sizeof(a[0]) is large enough.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris@mimsy.umd.edu	Path:	uunet!mimsy!chris