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

22 Nov 2007

      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@mac.com>
Littleton, MA 01460