Date: Wed, 6 Nov 2002 10:59:29 -0800 From: Jordan K Hubbard <jkh@queasyweasel.com> To: Marc Olzheim <marcolz@stack.nl> Cc: hackers@FreeBSD.ORG Subject: Re: /usr/src/ed/bin/re.c:99 Message-ID: <E0C3C3FE-F1B9-11D6-A916-000393BB9222@queasyweasel.com> In-Reply-To: <20021106164653.GA95733@stack.nl>
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It's legal, though one would have to know what the author was thinking
(or at least read the surrounding code) before stating that it's also
"correct".
It's legal because, unlike the example given in that FAQ entry you
referenced, there's an implicit ordering in the expression that even
the most aggressive compiler optimizer couldn't change. In order for
the value of parse_char_class() to be returned, its arguments must
obviously be evaluated first and that means that ++nd will always occur
before the assignment operator.
Stylistically, of course, it's ugly as hell and would be dinged by any
CS professor grading this as homework. Is nd a local variable? If
so, why didn't the author simply pass "nd + 1" as the argument since
the extra assignment from the unary operator is essentially wasted
cycles? Or is nd a global variable also referenced from within
parse_char_class(), thus requiring the use of the ++ operator and if
so, then why didn't parse_char_class() simply side-effect the global
rather than forcing a re-assignment from within the parent function?
Indeed, why is nd a global at all? These and other questions are left
as an exercise for the reader. :-)
- Jordan
On Wednesday, November 6, 2002, at 08:46 AM, Marc Olzheim wrote:
> ..
> if ((nd = parse_char_class(++nd)) == NULL) {
> ..
>
> Hmmm... is this legal ?
>
> http://www.eskimo.com/~scs/C-faq/q3.1.html seems to tell otherwise...
>
> Zlo
>
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--
Jordan K. Hubbard
Engineering Manager, BSD technology group
Apple Computer
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