Messages in this thread | | | Date | Thu, 10 May 2007 00:47:24 +0200 | From | "Jesper Juhl" <> | Subject | Re: [PATCH] "volatile considered harmful" document |
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On 10/05/07, Satyam Sharma <satyam.sharma@gmail.com> wrote: > On 5/10/07, Jesper Juhl <jesper.juhl@gmail.com> wrote: > > On 09/05/07, Jonathan Corbet <corbet@lwn.net> wrote: <snip> > > > +"the new C book") has the following to say about the volatile keyword: > > > + > > > + The purpose of volatile is to force an implementation to suppress > > > + optimization that could otherwise occur. For example, for a > > > + machine with memory-mapped input/output, a pointer to a device > > > + register might be declared as a pointer to volatile, in > > > + order to prevent the compiler from removing apparently redundant > > > + references through the pointer. > > > + > > > +C programmers have often taken volatile to mean that the variable could be > > > +changed outside of the current thread of execution; as a result, they are > > > > you write: "... that the variable could be changed outside of the > > current thread of execution ..." > > > > I suggest: "... that the variable could be changed outside of the > > current thread of execution - a sort of simple atomic variable ..." > > I'm not so sure here. Why would any C programmer (worth his weight in > salt) think that volatile objects are automatically _atomic_? At
I honestly don't really know, but I've encountered that confusion a few times. Both from friends who (for some reason) believed that and from documents on the web that implied it, aparently it's a common confusion - a few examples:
http://lists.freebsd.org/pipermail/freebsd-perl/2004-June/000124.html "... volatile (atomic) fixes the problem. ..."
http://blogs.msdn.com/ricom/archive/2006/04/28/586406.aspx "That's the point of the volatile keyword. It makes sure that the line "dict = d;" is atomic."
http://forum.java.sun.com/thread.jspa?threadID=5126877&start=0 "A volatile variable is also guaranteed to be read or written as an atomic operation ..." (yes, this link talks about Java, which I don't know, but if java volatile means atomic, that might explain why some people assume the same for C).
In any case, it's not an uncommon belief, so I just thought it made sense to also make that little note.
> worst, the mistake someone might make would be to _implement_ locking > primitives using volatile. "that the variable could be changed outside > of the current thread of execution" sounds sufficient to me, and after > all, that is exactly what volatile hints to the compiler. >
-- Jesper Juhl <jesper.juhl@gmail.com> Don't top-post http://www.catb.org/~esr/jargon/html/T/top-post.html Plain text mails only, please http://www.expita.com/nomime.html - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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