Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!iuvax!rutgers!bellcore!texbell!merch!sneaky!gordon
From: gordon@sneaky.TANDY.COM (Gordon Burditt)
Newsgroups: comp.lang.c
Subject: Re: Contiguous Arrays
Message-ID: <7799@sneaky.TANDY.COM>
Date: 23 Feb 89 09:13:26 GMT
References: <2508@ssc-vax.UUCP> <8943@alice.UUCP> <1828@valhalla.ee.rochester.edu>
Reply-To: gordon@sneaky.UUCP (Gordon Burditt)
Organization: Gordon Burditt
Lines: 46

>On a similar note, just yesterday a friend and I were discussing
>tricks to access arrays with different ranges.
>For instance, if the array was to be x[101] to x[110] instead of
>x[0] to x[9], what is the easiest way to do it?
>For automatic arrays one could do
>int space[10], *x; 
>x = &space[0] - 101;

UNPORTABILITY ALERT!

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]

It could happen that (&space[0] - 101) == (int *) NULL.  Even computing
&space[0] - 101 could cause core dumps (MSDOS equivalent:  hang system
forcing use of RESET, optionally trashing FAT on hard disk)
On *86 machines, consider what happens when space starts within 100
bytes of physical address 0.  Having it within 100 bytes of the data
segment origin may cause problems also.  These problems may vary with
model, load address, and the phase of the moon.

Given the declaration:
	int	space[10];
then
	&space[i]
has guaranteed meaning when i = 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10, and 
no other values of i.  You can't use space[10] because it's out of range, 
but &space[10] is guaranteed to have a representation by the ANSI standard.

	struct foobar space[10], *x;
	/* Right */
	for (x = &space[0]; x < &space[10]; x++)
		x->data = 0;

	/* Wrong - this loop may never execute */
	for (x = &space[9]; x > &space[-1]; x--)
		x->data = 0;
I got bit on a loop like the above on a 68000.  As it happened, &space[0]
was less than sizeof(struct foobar), so &space[-1] was positive and very
large.  Since &space[9] < &space[-1], the loop never executed.


					Gordon L. Burditt
					...!texbell!sneaky!gordon