Date: Thu, 13 Jan 2000 03:45:47 +0100 (CET) From: Oliver Fromme <olli@dorifer.heim3.tu-clausthal.de> To: freebsd-questions@FreeBSD.ORG Subject: Re: Volatile variables Message-ID: <200001130245.DAA65229@dorifer.heim3.tu-clausthal.de> In-Reply-To: <85jdi6$1paq$1@atlantis.rz.tu-clausthal.de>
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Mikhail Evstiounin <evstiounin@adelphia.net> wrote in list.freebsd-questions:
> [...]
> Like I told above it should be done in atomic way, but vilotale means -
> avoid
> optimiozations.
"volatile" does not mean that the compiler should not optimize.
It only means that the compiler must not make certain
assumptions about a variable, and that its value may be read or
written asynchronously, i.e. its value might not be consistent
between statements. No more, no less. Of course, this
inhibits certain types of optimizations, naturally.
> And again it's a hint - just a hint. Some aggresive
> optimizers
> could ignore it.
No! A C compiler must not ignore the "volatile" qualifier.
It's the compiler's job to make sure that the program is not
working with a "stale" variable value.
> For example, if commands cli/sti could be used by compiler
> and works like it work in 8086, optimizer could do the following:
Yeah, it could, but this is UNIX, not DOS. :)
Regards
Oliver
--
Oliver Fromme, Leibnizstr. 18/61, 38678 Clausthal, Germany
(Info: finger userinfo:olli@dorifer.heim3.tu-clausthal.de)
"In jedem Stück Kohle wartet ein Diamant auf seine Geburt"
(Terry Pratchett)
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