Re: [Boost-users] Boost semaphore with condition variable

From: Steve Byan (Steve.Byan_at_[hidden])
Date: 2007-11-22 08:48:17

On Nov 21, 2007, at 5:25 PM, Fabian Sturm wrote:

> My problem is, that I have two threads A and B and thread A needs some
> information that B will calculate at runtime. Therefore I need some  
> kind
> of synchronization.
>
> So my first attempt was to use a mutex and a condition variable:
>
> Thread A:
>
> boost::thread::mutex::scoped_lock lock ( mMutex );
> mConditionVar.wait ( lock );
>
> Thread B:
>
> mConditionVar.notify_one ();
>
> This works nice as long as thread A already started waiting on the
> condition before thread B finished providing it. Otherwise the notify
> will be lost.

You need to create a shared condition variable, set by thread B and  
read by thread A.
Thread A has to wait on the condition variable in a loop, like this:

shared variable:
bool theCondition = false;

Thread A:

{
        boost::thread::mutex::scoped_lock lock ( mMutex );
        while (!theCondition) {
                mConditionVar.wait ( lock );
        }
}

Thread B:

{
        boost::thread::mutex::scoped_lock lock ( mMutex );
        theCondition = true;
        mConditionVar.notify_one ();
}

I suggest you take a look at a tutorial text on POSIX threads  
(pthreads); it should explain the use of mutexes and condition  
variables more thoroughly.

Regards,
-Steve

-- 
Steve Byan <stevebyan_at_[hidden]>
Littleton, MA 01460