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From: mhcoffin@watmsg.uwaterloo.ca (Michael Coffin)
Newsgroups: comp.lang.misc
Subject: Re: Answers, Chapter 1: TeX (was C's sins... and others)
Message-ID: <1990Oct24.144732.12503@watmath.waterloo.edu>
Date: 24 Oct 90 14:47:32 GMT
References: <26726@megaron.cs.arizona.edu> <3681@lanl.gov>
Sender: daemon@watmath.waterloo.edu (Owner of Many System Processes)
Organization: University of Waterloo
Lines: 24

In article <3681@lanl.gov> jlg@lanl.gov (Jim Giles) writes:
> ...
>What you are talking about here is the ANSI C (which is _very_ new)
>constraint on the validity of pointer _arithmetic_.  This constraint
>merely says that comparing (or subtracting) pointers that currently
>point to within separately allocated objects is undefined.  The same
>status occurs if you add (or subtract) integers to (or from) pointers
>so that the result leaves the bounds of a single allocated object.

These restrictions are not new.  I quote from page 98 of "The C
Programming Language" by Kernighan and Ritchie, 1978:

    "Any pointer can be meaningfully compared for equality or
    inequality with NULL.  But all bets are off if you do arithmetic
    or comparisons with pointers pointing to different arrays.  If
    you're lucky, you'll get obvious nonsense on all machines.  If
    you're unlucky, your code will owrk on one machine but collapse
    mysteriously on another."

---
Michael Coffin				mhcoffin@watmsg.waterloo.edu
Dept. of Computer Science		office: (519) 885-1211
University of Waterloo			home:   (519) 725-5516
Waterloo, Ontario, Canada N2L 3G1