Xref: utzoo comp.sys.ibm.pc:22152 comp.sys.intel:595
Path: utzoo!attcan!uunet!sdrc!scjones
From: scjones@sdrc.UUCP (Larry Jones)
Newsgroups: comp.sys.ibm.pc,comp.sys.intel
Subject: Re: correct code for pointer subtraction
Summary: But the code's still wrong!
Message-ID: <470@sdrc.UUCP>
Date: 12 Dec 88 22:22:08 GMT
References: <597@mks.UUCP> <8377@bloom-beacon.MIT.EDU> <8455@sequent.UUCP>
Organization: Structural Dynamics Research Corp., Cincinnati
Lines: 24

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.

But even then you need a **17** bit (intermediate) result since
intermediate results between -65534 and +65534 are possible
(assuming the worst case - an array of 2-byte objects).  Since
dpANS treats the element just past the end of the array as valid
for pointer arithmetic, 17 bits really only allows objects to be
64k - 1 bytes long.

Huge model is the only one that supports 64k objects completely.
All the others only support 32767.  Compiler vendors who tell you
otherwise are lying (although not necessarily intentionally).

----
Larry Jones                         UUCP: uunet!sdrc!scjones
SDRC                                      scjones@sdrc.uucp
2000 Eastman Dr.                    BIX:  ltl
Milford, OH  45150                  AT&T: (513) 576-2070
"Save the Quayles" - Mark Russell