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From: ralf@b.gp.cs.cmu.edu (Ralf Brown)
Newsgroups: comp.sys.ibm.pc,comp.sys.intel
Subject: Re: correct code for pointer subtraction
Message-ID: <3845@pt.cs.cmu.edu>
Date: 10 Dec 88 02:20:02 GMT
References: <597@mks.UUCP>
Organization: Carnegie-Mellon University, CS/RI
Lines: 32

In article <597@mks.UUCP> egisin@mks.UUCP (Eric Gisin) writes:
-How come I can't find a compiler that generates correct
-code for pointer subtraction in C on 8086s?
-Neither Turbo, Microsoft, or Watcom do it right.
-Here's an example:
-
-struct six {
-	int i[3];
-};
-int diff(struct six far* p, struct six far* q) {
-  	return p - q;
-}
-main(void) {
-	struct six s[1];
-	printf("%d\n", diff(s+10000, s));	/* 10000 */
-	printf("%d\n", diff(s, s+100));		/* -100 */
-}
-
-All of the compilers I tried computed a 16 bit difference,
-then sign extended it before dividing.
-This does not work if the pointers differ by more than 32K.

In addition to HUGE model, which someone already pointed out, I would like to
mention that K&R only guarantees valid results for pointer subtractions 
between pointers to the SAME array.  In non-HUGE models, the largest object
is 64K, so for multi-byte data types, the result is correct (since they can't
differ by more than 32K objects).  For char arrays, you would have an 
ambiguity as to which pointer points lower in the array.
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