Date: Sat, 15 Jan 2000 20:50:22 +0100 (CET) From: Oliver Fromme <olli@dorifer.heim3.tu-clausthal.de> To: freebsd-questions@FreeBSD.ORG Subject: Re: Volatile variables Message-ID: <200001151950.UAA86770@dorifer.heim3.tu-clausthal.de> In-Reply-To: <85kinv$2ffa$1@atlantis.rz.tu-clausthal.de>
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Mikhail Evstiounin <evstiounin@adelphia.net> wrote in list.freebsd-questions:
> From: Oliver Fromme <olli@dorifer.heim3.tu-clausthal.de>
>>You _do_ need "volatile" in that case. It is necessary for
>>all variables whose contents can change asynchronously, i.e.
>>outside of the normal program flow. For example, this is true
>>for variables which are mapped to hardware registers, and for
>>variables which are located in a shared memory reagion (which
>>is shared with other processes), and for variables which are
>
> that is - in ways not specified by the language.
>
>>accessed from within signal handlers.
>
> This is, in my mind, slightly different.
No, it's _exactly_ the same, as far as the compiler is
concerned. The meaning of "volatile" is to prevent the
compiler's optimizer stage from assuming that the content
of a variable cannot change _asynchronously_ between
C statements. The important word is ``asynchronously''.
It means changes to variables outside of the control flow
which is known to the compiler at compile-time. This
includes signal handlers just as well as hardware registers,
shared memory etc.
I think the explanation which Giorgos gave is pretty good.
> Could you explain me how it helps in your example? I pointed, that
> you can get signal between two assembler commands and it
> does destroys all your assumptions.
The assembler commands do not matter. Only C statements
matter, because this is the smallest separable execution
unit of a C program.
Without "volatile", the programmer would be unable to write
his program in such a way that asynchronous changes work
with register optimizations. This is why "volatile" exists,
and this is why a compiler _must_ obey it.
All of the above is (formally) specified in the standard,
I'd suggest that you read it carefully.
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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