Path: utzoo!attcan!uunet!seismo!sundc!pitstop!sun!amdcad!ames!mailrus!wasatch!utah-gr!uplherc!esunix!bpendlet
From: bpendlet@esunix.UUCP (Bob Pendleton)
Newsgroups: comp.arch
Subject: Re: Software Distribution
Message-ID: <993@esunix.UUCP>
Date: 4 Oct 88 14:26:55 GMT
References: <634@eiger.iis.UUCP>
Organization: Evans & Sutherland, Salt Lake City, Utah
Lines: 49

From article <634@eiger.iis.UUCP>, by prl@iis.UUCP (Peter Lamb):
> In article <978@esunix.UUCP> bpendlet@esunix.UUCP (Bob Pendleton) writes:
>>
>>Try this scenario:
>>
>>So, the compiler that runs on brand X machines must, at least, put a
>>flag in the UIF stating that dereferencing NULL is allowed. ...
> 
> *HOW* are you going to manage this?

Run time checks. How else do you check for illegal operations at run
time? They can be implemented in hardware or software, I don't care
which.

> There is, as far as I can see no general solution to this problem.
> I seem to remember that K&R say (roughly, I don't have it to hand)
> that 0 does not correspond to *ANY* valid data.

Run time checks aren't a general solution? They aren't even very
expensive. At least not when compared to the alternative of buggy
nonportable code.

> Just *what* are you saying when you dereference NULL?

Don't ask me, ask the language definition. If it isn't defined then
you've found a flaw in the language definition. That applies to any
language. If the language defines it (any feature), you have to
implement so that it conforms to thae language specification. If the
language leaves it undefined, then you have to deal with the
fact that it will be used, and missued, in every possible way.

I've used dialects of LISP in which (car NIL) was eq NIL and (cdr NIL)
was eq NIL, and NIL, as a bit pattern, was not 0.

Is it possible that you think I'm in favor of defining *NULL to be
equal to NULL and are responding to that? I'm in favor of defining the
behavior of every operator in a language on all of its operand set.
Since NULL can be stored in a pointer, the actions of all pointer
operators when applied to NULL should, in my opinion, be defined.

> 
>>Bob Pendleton @ Evans & Sutherland
> 
> 
> -- 
> Peter Lamb
> uucp:  seismo!mcvax!ethz!prl	eunet: prl@ethz.uucp	Tel:   +411 256 5241
> Institute for Integrated Systems
> ETH-Zentrum, 8092 Zurich