QWaitCondition Class Reference
[QtCore module]

Detailed Description

The QWaitCondition class provides a condition variable for synchronizing threads.

QWaitCondition allows a thread to tell other threads that some sort of condition has been met. One or many threads can block waiting for a QWaitCondition to set a condition with wakeOne() or wakeAll(). Use wakeOne() to wake one randomly selected condition or wakeAll() to wake them all.

For example, let's suppose that we have three tasks that should be performed whenever the user presses a key. Each task could be split into a thread, each of which would have a run() body like this:

 forever {
     mutex.lock();
     keyPressed.wait(&mutex);
     do_something();
     mutex.unlock();
 }

Here, the keyPressed variable is a global variable of type QWaitCondition.

A fourth thread would read key presses and wake the other three threads up every time it receives one, like this:

 forever {
     getchar();
     keyPressed.wakeAll();
 }

The order in which the three threads are woken up is undefined. Also, if some of the threads are still in do_something() when the key is pressed, they won't be woken up (since they're not waiting on the condition variable) and so the task will not be performed for that key press. This issue can be solved using a counter and a QMutex to guard it. For example, here's the new code for the worker threads:

 forever {
     mutex.lock();
     keyPressed.wait(&mutex);
     ++count;
     mutex.unlock();

     do_something();

     mutex.lock();
     --count;
     mutex.unlock();
 }

Here's the code for the fourth thread:

 forever {
     getchar();

     mutex.lock();
     // Sleep until there are no busy worker threads
     while (count > 0) {
         mutex.unlock();
         sleep(1);
         mutex.lock();
     }
     keyPressed.wakeAll();
     mutex.unlock();
 }

The mutex is necessary because the results of two threads attempting to change the value of the same variable simultaneously are unpredictable.

Wait conditions are a powerful thread synchronization primitive. The Wait Conditions example shows how to use QWaitCondition as an alternative to QSemaphore for controlling access to a circular buffer shared by a producer thread and a consumer thread.

See also QMutex, QSemaphore, QThread, and Wait Conditions Example.

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