Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!husc6!cmcl2!rutgers!mcnc!rti!xyzzy!meissner
From: meissner@xyzzy.UUCP (Michael Meissner)
Newsgroups: comp.lang.c,comp.unix.wizards
Subject: Re: pointer alignment when int != char *
Message-ID: <226@xyzzy.UUCP>
Date: Wed, 26-Aug-87 09:55:10 EDT
Article-I.D.: xyzzy.226
Posted: Wed Aug 26 09:55:10 1987
Date-Received: Fri, 28-Aug-87 05:03:36 EDT
References: <493@its63b.ed.ac.uk> <6061@brl-smoke.ARPA>
Reply-To: meissner@nightmare.UUCP (Michael Meissner)
Organization: Data General (Languages @ Research Triangle Park, NC.)
Lines: 21
Xref: mnetor comp.lang.c:3927 comp.unix.wizards:3899

In article <934@bsu-cs.UUCP> dhesi@bsu-cs.UUCP (Rahul Dhesi) writes:
# 
# To subtract two independent large-model pointers of the type
# segment:offset, I tried this:
# 
#      (unsigned long) p2 - (unsigned long) p1
# 
# I was hoping that the cast to unsigned long would convert each pointer
# to a sort of absolute memory address in bytes, and the subtraction
# would yield the difference in bytes.  Under Borland's Turbo C at least,
# such a cast is a no-op, so the resulting unsigned long does not
# necessarily increase monotonically with increasing memory address to
# which the original pointer points.

This is bad practice.  I know of machines that have different formats for
pointers to words and pointers to bytes, and other machines that use things
like bit pointers.  In none of these cases, or a segmented machine like the
80*86 will subtraction give you what you want.  This is yet another symptom
of the world is not a VAX syndrome.
-- 
Michael Meissner, Data General.		Uucp: ...!mcnc!rti!xyzzy!meissner
					Arpa/Csnet:  meissner@dg-rtp.DG.COM