Re: [boost] New Book: Introduction to the C++ Boost Libraries, Volume I - Foundations

Subject: Re: [boost] New Book: Introduction to the C++ Boost Libraries, Volume I - Foundations
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2010-09-22 08:03:08

Daniel J. Duffy wrote:

> > Part: 18.13 Volatile Variables
> >
> > >  In order to rectify this situation we declare variables to be volatile
> > >  in order to disable register optimisations:
> > >
> > >   // Shared variable
> > >   volatile int i2;
> > >   // The same functions as above but use a volatile variable.
> > >   // Not optimised anymore so results are always correct.
> > >   void FV1() { while (true) i2++; }
> > >   void FV2() { while (true) cout<<i2<<endl; }
> >
> > This is so wrong!
>
> Please explain why this is wrong. Is the code wrong? Is the
> example wrong? Please note that this is a snippet from the full
> test program where two different threads are started to execute
> FV1() and FV2().

volatile affects optimizations but not synchronization.  You need
true synchronization for atomicity.  C++0x does not reuse
"volatile" to mean atomic because there are still
non-synchronization reasons to declare objects volatile (I/O
ports, for example).  Indeed, one can declare a volatile and
atomic object.

> On my dual-core machine the version without volatile produces
> wrong results while the version with volatile produces correct
> results.

That the code happens to work on a particular platform with a
particular toy program does not mean it is correct.

_____
Rob Stewart                           robert.stewart_at_[hidden]
Software Engineer, Core Software      using std::disclaimer;
Susquehanna International Group, LLP  http://www.sig.com

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