Re: volatile keyword for C++ member functions

James Kanze <>
Fri, 30 Nov 2007 13:24:06 -0800 (PST)
On Nov 30, 6:39 pm, gpderetta <> wrote:

James Kanze wrote:

More to the point, since you started out by talking about
threading: what is the significance of volatile with regards to
threading? Posix makes no guarantees concerning volatile and
threading; I don't know what Windows "guarantees", but the
current VC++ compiler doesn't generate anything that could
possibly be useful for threading.

Actually I think that VC++ generates proper memory barriers
for accesses to atomic volatile objects.

I've looked at the generated code from VC++ 8, and they weren't
there. There may be some option to cause the compiler to create
them; I've not studied the issue further.

Microsoft did tell the standards committee once that this was
part of its (then future) specifications, so perhaps future
versions of the the compiler will use it.

I've never used this feature, but IIRC it is documented.

As I said, I looked at the generated code.

(Andrei once wrote an article
in which he used volatile to ensure correctly locked accesses,
but his code worked not because it used any of volatiles
semantics, but because it exploited the way volatile works in
the type system.)

More correctly, it happened to work in the specific compiler
he was using (and IIRC he explicitly noted it). There is no
(portable) guarantee that volatile has any meaning for
multithreaded code or is anyway useful for multithreading.

There is a guarantee that it is taken into account by the type
system, and that's all his code (or at least the parts I looked
at) depended on. Memory synchronization was still done by mutex

BTW, C++0x will likely have a detailed memory model for
threading, but it gives no new meanings to volatile.

I know. Microsoft did vaguely propose defining volatile more
strictly, but there was absolutely no resonance, and as far as I
know, they dropped it. (As far as I know, it was never more
than an informal suggestion made in an oral presentation

James Kanze (GABI Software)
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