Xref: utzoo comp.sys.ibm.pc:22652 comp.sys.intel:628
Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!purdue!haven!vrdxhq!bms-at!stuart
From: stuart@bms-at.UUCP (Stuart Gathman)
Newsgroups: comp.sys.ibm.pc,comp.sys.intel
Subject: Re: correct code for pointer subtraction
Summary: Do people read their manual's, much less K&R?
Keywords: C pointer math *not* a bug.
Message-ID: <142@bms-at.UUCP>
Date: 29 Dec 88 22:24:51 GMT
References: <597@mks.UUCP> <3845@pt.cs.cmu.edu> <18123@santra.UUCP>
Organization: Business Management Systems, Inc., Fairfax, VA
Lines: 30

In article <18123@santra.UUCP>, tarvaine@tukki.jyu.fi (Tapani Tarvainen) writes:

> The same error occurs in the following program 
> (with Turbo C 2.0 as well as MSC 5.0):

> main()
> {
>         static int a[30000];
>         printf("%d\n",&a[30000]-a);
> }

> output:  -2768

This is entirely correct.  The difference of two pointers is an *int*.
If you want an unsigned difference, you need to cast to unsigned
(and/or use %u in the printf).  If the difference were defined as
unsigned, how would you indicate negative differences?  If you
make the difference long, all the related arithmetic gets promoted
also for a big performance hit.  The solution is simple, if you
want an unsigned ptrdiff, cast or assign to unsigned.

This is described in the Turbo C manual.

Don't flame the 8086 either.  The same thing happens in 32-bit machines
(just much less often).  16 bits is 16 bits, and segments are not
the problem.  The VAX restricts user programs to 31-bit address space
to avoid this.
-- 
Stuart D. Gathman	<stuart@bms-at.uucp>
			<..!{vrdxhq|daitc}!bms-at!stuart>