Path: utzoo!attcan!uunet!zaphod.mps.ohio-state.edu!ncar!noao!arizona!gudeman
From: gudeman@cs.arizona.edu (David Gudeman)
Newsgroups: comp.lang.misc
Subject: Re: Answers, Chapter 1: TeX (was C's sins... and others)
Message-ID: <26740@megaron.cs.arizona.edu>
Date: 24 Oct 90 09:00:30 GMT
Organization: U of Arizona CS Dept, Tucson
Lines: 31

In article  <3681@lanl.gov> jlg@lanl.gov (Jim Giles) writes:
][I argue that pointers are bounded just like arrays]
]Really?  I don't know _any_ language that has pointers that places
]bounds on where it can point.

We are comparing pointers to indexes here.  The only time these two
are comparable is when the pointer is one that was calculated with
integer addition on a pointer to an array.  An out-of-bounds pointer
--in correspondence to an out-of-bounds index-- is a pointer whose
value is not in the same array as the starting pointer.

]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.

No, I'm talking about C in general.  And of course I'm talking about
pointer arithmetic, otherwise there is no correspondence between
indexes and pointers.  I don't believe any definition of C has ever
defined the effect of dereferencing a pointer that was miscalculated
so that it points out of the region the starting array.  However, C
_could_ define this as producing an error, and then pointer bounds
checking would be just as safe and reliable as array index bounds
checking.

This is all starting to sound very familiar...
-- 
					David Gudeman
Department of Computer Science
The University of Arizona        gudeman@cs.arizona.edu
Tucson, AZ 85721                 noao!arizona!gudeman