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From: dhesi@bsu-cs.UUCP (Rahul Dhesi)
Newsgroups: comp.lang.c,comp.unix.wizards
Subject: Re: pointer alignment when int != char *
Message-ID: <934@bsu-cs.UUCP>
Date: Fri, 7-Aug-87 09:42:05 EDT
Article-I.D.: bsu-cs.934
Posted: Fri Aug  7 09:42:05 1987
Date-Received: Sun, 9-Aug-87 02:51:40 EDT
References: <493@its63b.ed.ac.uk> <6061@brl-smoke.ARPA>
Reply-To: dhesi@bsu-cs.UUCP (Rahul Dhesi)
Organization: CS Dept, Ball St U, Muncie, Indiana
Lines: 28
Xref: mnetor comp.lang.c:3526 comp.unix.wizards:3604

In article <179@xyzzy.UUCP> meissner@nightmare.UUCP (Michael Meissner) writes:
>All of the standards say that
>pointer subtraction is only defined within an aggregate.  This allows putting
>each top level item into a separate segment on say an 80*86, and only doing
>the subtraction between the two offsets.  Many MSDOS compilers do this
>already.

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.

I understand that the requirement on such casts is that they be
unsurprising and reversible, to the extent that these are possible.  It
would be nice if "unsurprising" were interpreted to mean that the
subtraction I was attempting would work.  The only catch is that
reversibility would be weakened because in the 8086 architecture many
different long pointers can point to the same address, but I could live
with that.
-- 
Rahul Dhesi         UUCP:  {ihnp4,seismo}!{iuvax,pur-ee}!bsu-cs!dhesi