QWaitCondition¶
The QWaitCondition
class provides a condition variable for synchronizing threads. More…
Synopsis¶
Functions¶
def
notify_all
()def
notify_one
()def
wait
(lockedMutex, time)def
wait
(lockedMutex[, deadline=QDeadlineTimer(QDeadlineTimer.Forever)])def
wait
(lockedReadWriteLock, time)def
wait
(lockedReadWriteLock[, deadline=QDeadlineTimer(QDeadlineTimer.Forever)])def
wakeAll
()def
wakeOne
()
Detailed Description¶
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 thread 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 = count + 1 mutex.unlock() do_something() mutex.lock() count = count - 1 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 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
- class PySide6.QtCore.QWaitCondition¶
Constructs a new wait condition object.
- PySide6.QtCore.QWaitCondition.notify_all()¶
This function is provided for STL compatibility. It is equivalent to wakeAll()
.
- PySide6.QtCore.QWaitCondition.notify_one()¶
This function is provided for STL compatibility. It is equivalent to wakeOne()
.
- PySide6.QtCore.QWaitCondition.wait(lockedMutex[, deadline=QDeadlineTimer(QDeadlineTimer.Forever)])¶
- Parameters
lockedMutex –
PySide6.QtCore.QMutex
deadline –
PySide6.QtCore.QDeadlineTimer
- Return type
bool
Releases the lockedMutex
and waits on the wait condition. The lockedMutex
must be initially locked by the calling thread. If lockedMutex
is not in a locked state, the behavior is undefined. If lockedMutex
is a recursive mutex, this function returns immediately. The lockedMutex
will be unlocked, and the calling thread will block until either of these conditions is met:
Another thread signals it using
wakeOne()
orwakeAll()
. This function will return true in this case.the deadline given by
deadline
is reached. Ifdeadline
isQDeadlineTimer::Forever
(the default), then the wait will never timeout (the event must be signalled). This function will return false if the wait timed out.
The lockedMutex
will be returned to the same locked state. This function is provided to allow the atomic transition from the locked state to the wait state.
- PySide6.QtCore.QWaitCondition.wait(lockedMutex, time)
- Parameters
lockedMutex –
PySide6.QtCore.QMutex
time – int
- Return type
bool
This is an overloaded function.
- PySide6.QtCore.QWaitCondition.wait(lockedReadWriteLock[, deadline=QDeadlineTimer(QDeadlineTimer.Forever)])
- Parameters
lockedReadWriteLock –
PySide6.QtCore.QReadWriteLock
deadline –
PySide6.QtCore.QDeadlineTimer
- Return type
bool
Releases the lockedReadWriteLock
and waits on the wait condition. The lockedReadWriteLock
must be initially locked by the calling thread. If lockedReadWriteLock
is not in a locked state, this function returns immediately. The lockedReadWriteLock
must not be locked recursively, otherwise this function will not release the lock properly. The lockedReadWriteLock
will be unlocked, and the calling thread will block until either of these conditions is met:
Another thread signals it using
wakeOne()
orwakeAll()
. This function will return true in this case.the deadline given by
deadline
is reached. Ifdeadline
isQDeadlineTimer::Forever
(the default), then the wait will never timeout (the event must be signalled). This function will return false if the wait timed out.
The lockedReadWriteLock
will be returned to the same locked state. This function is provided to allow the atomic transition from the locked state to the wait state.
- PySide6.QtCore.QWaitCondition.wait(lockedReadWriteLock, time)
- Parameters
lockedReadWriteLock –
PySide6.QtCore.QReadWriteLock
time – int
- Return type
bool
This is an overloaded function.
- PySide6.QtCore.QWaitCondition.wakeAll()¶
Wakes all threads waiting on the wait condition. The order in which the threads are woken up depends on the operating system’s scheduling policies and cannot be controlled or predicted.
See also
- PySide6.QtCore.QWaitCondition.wakeOne()¶
Wakes one thread waiting on the wait condition. The thread that is woken up depends on the operating system’s scheduling policies, and cannot be controlled or predicted.
If you want to wake up a specific thread, the solution is typically to use different wait conditions and have different threads wait on different conditions.
See also
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