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Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!mhuxj!houxm!ihnp4!inuxc!pur-ee!uiucdcsb!davies
From: davies@uiucdcsb.UUCP
Newsgroups: net.lang
Subject: Re: Orphaned Response
Message-ID: <8900022@uiucdcsb.UUCP>
Date: Sat, 26-Jan-85 01:39:00 EST
Article-I.D.: uiucdcsb.8900022
Posted: Sat Jan 26 01:39:00 1985
Date-Received: Sun, 27-Jan-85 07:31:23 EST
References: <2673@dartvax.UUCP>
Lines: 30
Nf-ID: #R:dartvax:-267300:uiucdcsb:8900022:000:737
Nf-From: uiucdcsb!davies    Jan 26 00:39:00 1985

boring!steven states that it is possible to statically insure that nil
pointers are never dereferenced.  The following program provides a simple
counterexample to this claim:

program noway(input,output);
type
    pointer = ^rec;
    rec = record
              data : integer;
              next : pointer;
              end;
var
   p : pointer;
   q : pointer;
begin
    p := nil;
    while not eof(input) do begin
        new(q);
        read(q^.data);
        q^.next := p;
        p := q;
        end;
    writeln(p^.next^.next^.next^.data);
    end.

In this program, whether the "writeln" dereferences a nil pointer is
determined by the number of integers in the input data, and thus cannot be
determined at compile time.
					Jim