Qt5 QWaitCondition example - c++

Question

I am getting myself familiar with QT5 concurrency library. I was looking at the QWaitCondition example (http://qt-project.org/doc/qt-5.0/qtcore/qwaitcondition.html#details).
Here, one thread (Thread B), reads user input, and all other threads (Thread A) process this input.
Thread A:
forever {
mutex.lock();
keyPressed.wait(&mutex);
++count;
mutex.unlock();
do_something();
mutex.lock();
--count;
mutex.unlock();
}
Thread B:
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 reason for using count variable and the extensive mutex synchronization is to prevent symbols loss.
The problem is, I think there is still a chance that a symbol will get lost:
imagine the following scenario:
Thread A processes a symbol, and decreases the countes (--count);
the mutex is released; then Thread A stops
Thread B returns from sleep, aquires the mutex, sees, that count ==
0, and calls keyPressed.wakeAll(), and then unlocks the mutex.
However, the wakeAll() call goes to nowhere, since the Thread A is
not waiting.
Thread A starts again, aquaires the mutex and goes into wait().
Since wakeAll() was already called, the symbol is lost.
Am I right, or am I missing something? If I am right, how to correct the example to really prevent it from skipping the symbols?

Answer 1

you are right
to fix this the loop should be entered with the mutex already acquired:
mutex.lock();
forever {
keyPressed.wait(&mutex);
++count;
mutex.unlock();
do_something();
mutex.lock();
--count;
}
mutex.unlock();