Xref: utzoo comp.sys.ibm.pc:22368 comp.sys.intel:610
Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!ames!killer!chasm
From: chasm@killer.DALLAS.TX.US (Charles Marslett)
Newsgroups: comp.sys.ibm.pc,comp.sys.intel
Subject: Re: correct code for pointer subtraction
Summary: Longs are not the same as pointers though!
Message-ID: <6473@killer.DALLAS.TX.US>
Date: 17 Dec 88 21:16:38 GMT
References: <15813@iuvax.cs.indiana.edu> <11477@dartvax.Dartmouth.EDU>
Organization: The Unix(R) Connection, Dallas, Texas
Lines: 31

In article <11477@dartvax.Dartmouth.EDU>, earleh@eleazar.dartmouth.edu (Earle R. Horton) writes:
> In article <15813@iuvax.cs.indiana.edu> bobmon@iuvax.UUCP (RAMontante) writes:
> ...
> >So I agree with Bill Davidsen that the rigorous way to do this is to
> >cast the pointers to huge, and subtract the casts.  Viz., 
> ...
> 
> I don't think anyone has suggested this, but perhaps casting to long
> might be a better way to do this, and portable, too!  I don't do much
> 8086 stuff, but a long int is the same size as a long pointer on just
> about every system I have run across.  Subtraction of longs is never a
> questionable operation, either, whereas pointer subtraction sometimes
> is...

Of course, pointers are not portably convertable to any kind of scalar
(even longs) so the difference of two longs converted from two pointers
is not necessarily going to give you the number of bytes seperating the
two addresses -- this is true of almost any segmented address machine, and
specifically it is true of the Microsoft and Borland C compilers.

The point is, subtraction of longs is valid, conversion of pointers to
longs (even if they are the same size) is not.  And subtraction of
pointers OUGHT TO BE portable (to reiterate my original statement, the
Microsoft and Borland compilers are broken since they do not work "right").

I might add that both of these companies are in business to sell software
-- tell them you do not like this overemphasis on fast over correct results
and we may see improvement next time around!

Charles Marslett
chasm@killer.dallas.tx.us