Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!sdd.hp.com!apollo!beierl_c
From: beierl_c@apollo.HP.COM (Christopher Beierl)
Newsgroups: comp.sys.apollo
Subject: Re: The wonderful Pascal compilier...
Message-ID: <4e01079e.20b6d@apollo.HP.COM>
Date: 14 Nov 90 15:56:00 GMT
References: <43023@cci632.UUCP>
Sender: root@apollo.HP.COM
Reply-To: beierl_c@apollo.HP.COM (Christopher Beierl)
Organization: Apollo Computer, Chelmsford, MA
Lines: 27

In article <43023@cci632.UUCP> mth@cci.com (Michael Hickman) writes:
>                                            ...  The program compiled fine,
>but when I ran it, it barfed with an 'reference to illegal address (OS/MST 
>manager)' on this:
>
>    while not((this_net = nil) or 
>              ((this_net^.plusminus = net.plusminus) and
>               (this_net^.net_name  = net.net_name))) do
>
>Looks like it tried evaluating the expression after the 'or' when the pointer 
>was nil.  I changed it to this:
>
>    while ((this_net <> nil) and 
>       not((this_net^.plusminus = net.plusminus) and 
>           (this_net^.net_name  = net.net_name))) do
>

Pascal does NOT guarantee short-circuit evaluation of Boolean expression
in an IF statement as C does.  You MUST use the second form at all times
to avoid the possibility of attempted to dereference a nil pointer.

See "Domain Pascal Language Reference", Chapter 4 "Code", p4-93 which
talks about the IF statement.
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 Christopher T. Beierl  Internet: beierl_c@apollo.HP.COM;beierl_c@apollo.com
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