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// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
/*!
\class QWaitCondition
\inmodule QtCore
\brief The QWaitCondition class provides a condition variable for
synchronizing threads.
\threadsafe
\ingroup thread
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
\l{QThread::run()}{run()} body like this:
\snippet code/src_corelib_thread_qwaitcondition_unix.cpp 0
Here, the \c 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:
\snippet code/src_corelib_thread_qwaitcondition_unix.cpp 1
The order in which the three threads are woken up is undefined.
Also, if some of the threads are still in \c 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:
\snippet code/src_corelib_thread_qwaitcondition_unix.cpp 2
Here's the code for the fourth thread:
\snippet code/src_corelib_thread_qwaitcondition_unix.cpp 3
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 \l{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.
\sa QMutex, QSemaphore, QThread, {Wait Conditions Example}
*/
/*!
\fn QWaitCondition::QWaitCondition()
Constructs a new wait condition object.
*/
/*!
\fn QWaitCondition::~QWaitCondition()
Destroys the wait condition object.
*/
/*!
\fn void 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.
\sa wakeAll()
*/
/*!
\fn void 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.
\sa wakeOne()
*/
/*!
\fn bool QWaitCondition::wait(QMutex *lockedMutex, unsigned long time)
\overload
Releases the \a lockedMutex and waits on the wait condition for \a time
milliseconds.
*/
/*!
\fn bool QWaitCondition::wait(QReadWriteLock *lockedReadWriteLock, unsigned long time)
\overload
Releases the \a lockedReadWriteLock and waits on the wait condition for \a
time milliseconds.
*/
/*!
\fn bool QWaitCondition::wait(QMutex *lockedMutex, QDeadlineTimer deadline)
\since 5.12
Releases the \a lockedMutex and waits on the wait condition. The
\a lockedMutex must be initially locked by the calling thread. If \a
lockedMutex is not in a locked state, the behavior is undefined. If
\a lockedMutex is a recursive mutex, this function
returns immediately. The \a lockedMutex will be unlocked, and the
calling thread will block until either of these conditions is met:
\list
\li Another thread signals it using wakeOne() or wakeAll(). This
function will return true in this case.
\li the deadline given by \a deadline is reached. If \a deadline is
\c QDeadlineTimer::Forever (the default), then the wait will never
timeout (the event must be signalled). This function will return
false if the wait timed out.
\endlist
The \a 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.
\sa wakeOne(), wakeAll()
*/
/*!
\fn bool QWaitCondition::wait(QReadWriteLock *lockedReadWriteLock, QDeadlineTimer deadline)
\since 5.12
Releases the \a lockedReadWriteLock and waits on the wait
condition. The \a lockedReadWriteLock must be initially locked by the
calling thread. If \a lockedReadWriteLock is not in a locked state, this
function returns immediately. The \a lockedReadWriteLock must not be
locked recursively, otherwise this function will not release the
lock properly. The \a lockedReadWriteLock will be unlocked, and the
calling thread will block until either of these conditions is met:
\list
\li Another thread signals it using wakeOne() or wakeAll(). This
function will return true in this case.
\li the deadline given by \a deadline is reached. If \a deadline is
\c QDeadlineTimer::Forever (the default), then the wait will never
timeout (the event must be signalled). This function will return
false if the wait timed out.
\endlist
The \a 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.
\sa wakeOne(), wakeAll()
*/
/*! \fn void QWaitCondition::notify_one()
\since 5.8
This function is provided for STL compatibility. It is equivalent
to wakeOne().
*/
/*! \fn void QWaitCondition::notify_all()
\since 5.8
This function is provided for STL compatibility. It is equivalent
to wakeAll().
*/
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