source: trunk/src/corelib/thread/qwaitcondition.qdoc@ 651

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41
42/*!
43 \class QWaitCondition
44 \brief The QWaitCondition class provides a condition variable for
45 synchronizing threads.
46
47 \threadsafe
48
49 \ingroup thread
50
51 QWaitCondition allows a thread to tell other threads that some
52 sort of condition has been met. One or many threads can block
53 waiting for a QWaitCondition to set a condition with wakeOne() or
54 wakeAll(). Use wakeOne() to wake one randomly selected condition or
55 wakeAll() to wake them all.
56
57 For example, let's suppose that we have three tasks that should
58 be performed whenever the user presses a key. Each task could be
59 split into a thread, each of which would have a
60 \l{QThread::run()}{run()} body like this:
61
62 \snippet doc/src/snippets/code/src_corelib_thread_qwaitcondition_unix.cpp 0
63
64 Here, the \c keyPressed variable is a global variable of type
65 QWaitCondition.
66
67 A fourth thread would read key presses and wake the other three
68 threads up every time it receives one, like this:
69
70 \snippet doc/src/snippets/code/src_corelib_thread_qwaitcondition_unix.cpp 1
71
72 The order in which the three threads are woken up is undefined.
73 Also, if some of the threads are still in \c do_something() when
74 the key is pressed, they won't be woken up (since they're not
75 waiting on the condition variable) and so the task will not be
76 performed for that key press. This issue can be solved using a
77 counter and a QMutex to guard it. For example, here's the new
78 code for the worker threads:
79
80 \snippet doc/src/snippets/code/src_corelib_thread_qwaitcondition_unix.cpp 2
81
82 Here's the code for the fourth thread:
83
84 \snippet doc/src/snippets/code/src_corelib_thread_qwaitcondition_unix.cpp 3
85
86 The mutex is necessary because the results of two threads
87 attempting to change the value of the same variable
88 simultaneously are unpredictable.
89
90 Wait conditions are a powerful thread synchronization primitive.
91 The \l{threads/waitconditions}{Wait Conditions} example shows how
92 to use QWaitCondition as an alternative to QSemaphore for
93 controlling access to a circular buffer shared by a producer
94 thread and a consumer thread.
95
96 \sa QMutex, QSemaphore, QThread, {Wait Conditions Example}
97*/
98
99/*!
100 \fn QWaitCondition::QWaitCondition()
101
102 Constructs a new wait condition object.
103*/
104
105/*!
106 \fn QWaitCondition::~QWaitCondition()
107
108 Destroys the wait condition object.
109*/
110
111/*!
112 \fn void QWaitCondition::wakeOne()
113
114 Wakes one thread waiting on the wait condition. The thread that
115 is woken up depends on the operating system's scheduling
116 policies, and cannot be controlled or predicted.
117
118 If you want to wake up a specific thread, the solution is
119 typically to use different wait conditions and have different
120 threads wait on different conditions.
121
122 \sa wakeAll()
123*/
124
125/*!
126 \fn void QWaitCondition::wakeAll()
127
128 Wakes all threads waiting on the wait condition. The order in
129 which the threads are woken up depends on the operating system's
130 scheduling policies and cannot be controlled or predicted.
131
132 \sa wakeOne()
133*/
134
135/*!
136 \fn bool QWaitCondition::wait(QMutex *mutex, unsigned long time)
137
138 Releases the locked \a mutex and waits on the wait condition. The
139 \a mutex must be initially locked by the calling thread. If \a
140 mutex is not in a locked state, this function returns
141 immediately. If \a mutex is a recursive mutex, this function
142 returns immediately. The \a mutex will be unlocked, and the
143 calling thread will block until either of these conditions is met:
144
145 \list
146 \o Another thread signals it using wakeOne() or wakeAll(). This
147 function will return true in this case.
148 \o \a time milliseconds has elapsed. If \a time is \c ULONG_MAX
149 (the default), then the wait will never timeout (the event
150 must be signalled). This function will return false if the
151 wait timed out.
152 \endlist
153
154 The mutex will be returned to the same locked state. This
155 function is provided to allow the atomic transition from the
156 locked state to the wait state.
157
158 \sa wakeOne(), wakeAll()
159*/
160
161/*!
162 \fn bool QWaitCondition::wait(QReadWriteLock *readWriteLock, unsigned long time)
163 \since 4.4
164
165 Releases the locked \a readWriteLock and waits on the wait
166 condition. The \a readWriteLock must be initially locked by the
167 calling thread. If \a readWriteLock is not in a locked state, this
168 function returns immediately. The \a readWriteLock must not be
169 locked recursively, otherwise this function will not release the
170 lock properly. The \a readWriteLock will be unlocked, and the
171 calling thread will block until either of these conditions is met:
172
173 \list
174 \o Another thread signals it using wakeOne() or wakeAll(). This
175 function will return true in this case.
176 \o \a time milliseconds has elapsed. If \a time is \c ULONG_MAX
177 (the default), then the wait will never timeout (the event
178 must be signalled). This function will return false if the
179 wait timed out.
180 \endlist
181
182 The \a readWriteLock will be returned to the same locked
183 state. This function is provided to allow the atomic transition
184 from the locked state to the wait state.
185
186 \sa wakeOne(), wakeAll()
187*/
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