Xref: utzoo comp.sys.ibm.pc:22292 comp.sys.intel:605
Path: utzoo!utgpu!attcan!uunet!iscuva!carlp
From: carlp@iscuva.ISCS.COM (Carl Paukstis)
Newsgroups: comp.sys.ibm.pc,comp.sys.intel
Subject: Re: correct code for pointer subtraction
Message-ID: <2237@iscuva.ISCS.COM>
Date: 15 Dec 88 21:38:11 GMT
References: <597@mks.UUCP> <8377@bloom-beacon.MIT.EDU> <8455@sequent.UUCP> <600@mks.UUCP>
Organization: ISC Systems Corporation, Spokane, WA
Lines: 37

In article <600@mks.UUCP> egisin@mks.UUCP (Eric Gisin) writes:
.>In article <8455@sequent.UUCP>, sch@sequent.UUCP (Steve Hemminger) writes:
.>> I believe the ANSI std and K&R say pointer subtraction is only allowed inside
.>> one data structure/array.  Since the small/medium/large model only allows
.>> arrays <64K, only a 16bit result needs to be computed. Any code that
.>> depends on subtracting pointers into two totally seperate arrays is
.>> non-portable.
.>
.>Please read my code. I have two pointers to "s", a single object.
.>Declaring "s" with 1 or 10000 makes no difference.
.>And the pointers are seperated by 60000, which is less than 64K.
.>
.>struct six {		/* HINT: this struct is 6 bytes long */
.>	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 */
.>}

Of course, the code you posted is NOT legal, since the two pointers in the
example *do not* point inside the same object.  You have verified that the
incorrect code is generated when you IN FACT declare "struct six s[10000]"?
If so, it's a bona-fide bug.  But if it won't work with your example, the
worst conclusion you can directly draw is that your example is "not 
conformant".
-- 
Carl Paukstis    +1 509 927 5600 x5321  |"The right to be heard does not
                                        | automatically include the right
UUCP:     carlp@iscuvc.ISCS.COM         | to be taken seriously."
          ...uunet!iscuva!carlp         |                  - H. H. Humphrey