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From: derstad@CIM-VAX.HONEYWELL.COM ("DAVE ERSTAD")
Newsgroups: comp.sys.apollo
Subject: Re:  The wonderful Pascal compiler...
Message-ID: <9011141659.AA08276@umix.cc.umich.edu>
Date: 14 Nov 90 16:45:00 GMT
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To elaborate on Jim's comments (in hopefully a friendlier fashion :-))
on the null pointer de-referencing, it is important to recognize that
Pascal, unlike C, does not specify order of expression evaluation (unless
this changed in the ISO standard, which I doubt).

For example, the Pascal code
   if (exprA) or (exprB)
may have either exprA or exprB evaluated first.  A compiler may choose
to do one or the other consistently; do the more efficient one; or
choose one based on the phase of the moon and be "correct".

Likewise, with
   if (exprA) and (exprB)
a compiler may choose to evaluate both expressions even if the first one
it evaluates is false.   In fact, it MUST evaluate both when there
is the potential for side effects.

C, by contrast, guarantees both order of evaluation and short
circuiting.  C supporters argue this is good, pointing to examples
like your code.  Detractors would argue this makes code more
difficult to maintain. 

So, in short, Apollo'
s compilers are not to blame (this time).

BTW, there are Domain pascal extensions to specify order
of evaluation (or else; and then), but I wouldn't 
recommend them as they are non-standard.

Dave Erstad
DERSTAD@cim-vax.honeywell.com
Honeywell SSEC