Question

Right now in Qt I am faced with the problem where I have 2 threads that have 2 different objects. These objects are not QObjects, thus they are not able to communicate using Signals/Slots. The first thread is the primary thread, and the second thread is in a infinite loop, which process command objects using a queue.

The main thread must wait for the processing thread to finish the request.

How would I go about synchronizing the two different threads without using global mutex, and wait conditions?

Answer 1

You could use mutexes. Lock everytime you pull “request” from queue, and lock everytime you want to append to queue. This way you may have

something like this:

#include <QMutex>
#include <QWaitCondition>

class processingThread
{
public:
    void appendToQueue(Request req)
    {
       sync.lock();
       queue.append(req);
       sync.unlock();
       cond.wakeAll();            
    }

protected:
    void run()
    {
        while(1)
        {
           sync.lock();
           QWaitCondition wait(&sync);
           Request current = queue.takeFirst();

           // process request

           sync.unlock()
        }
    }

private:
    QList<Request> queue;
    QMutex sync;
    QWaitCondition cond;
};

You can now call processingThread::appendToQueue from any thread and get data synchronized. You can use this pattern to sync any data within thread. Just remember to lock any access to data you want sync. Note that QWaitCondition is only to make your thread work only when needed