source: trunk/src/corelib/thread/qwaitcondition.qdoc@ 846

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1/****************************************************************************
2**
3** Copyright (C) 2011 Nokia Corporation and/or its subsidiary(-ies).
4** All rights reserved.
5** Contact: Nokia Corporation ([email protected])
6**
7** This file is part of the documentation of the Qt Toolkit.
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10** Commercial Usage
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16** GNU Free Documentation License
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19** Foundation and appearing in the file included in the packaging of this
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27
28/*!
29 \class QWaitCondition
30 \brief The QWaitCondition class provides a condition variable for
31 synchronizing threads.
32
33 \threadsafe
34
35 \ingroup thread
36
37 QWaitCondition allows a thread to tell other threads that some
38 sort of condition has been met. One or many threads can block
39 waiting for a QWaitCondition to set a condition with wakeOne() or
40 wakeAll(). Use wakeOne() to wake one randomly selected condition or
41 wakeAll() to wake them all.
42
43 For example, let's suppose that we have three tasks that should
44 be performed whenever the user presses a key. Each task could be
45 split into a thread, each of which would have a
46 \l{QThread::run()}{run()} body like this:
47
48 \snippet doc/src/snippets/code/src_corelib_thread_qwaitcondition_unix.cpp 0
49
50 Here, the \c keyPressed variable is a global variable of type
51 QWaitCondition.
52
53 A fourth thread would read key presses and wake the other three
54 threads up every time it receives one, like this:
55
56 \snippet doc/src/snippets/code/src_corelib_thread_qwaitcondition_unix.cpp 1
57
58 The order in which the three threads are woken up is undefined.
59 Also, if some of the threads are still in \c do_something() when
60 the key is pressed, they won't be woken up (since they're not
61 waiting on the condition variable) and so the task will not be
62 performed for that key press. This issue can be solved using a
63 counter and a QMutex to guard it. For example, here's the new
64 code for the worker threads:
65
66 \snippet doc/src/snippets/code/src_corelib_thread_qwaitcondition_unix.cpp 2
67
68 Here's the code for the fourth thread:
69
70 \snippet doc/src/snippets/code/src_corelib_thread_qwaitcondition_unix.cpp 3
71
72 The mutex is necessary because the results of two threads
73 attempting to change the value of the same variable
74 simultaneously are unpredictable.
75
76 Wait conditions are a powerful thread synchronization primitive.
77 The \l{threads/waitconditions}{Wait Conditions} example shows how
78 to use QWaitCondition as an alternative to QSemaphore for
79 controlling access to a circular buffer shared by a producer
80 thread and a consumer thread.
81
82 \sa QMutex, QSemaphore, QThread, {Wait Conditions Example}
83*/
84
85/*!
86 \fn QWaitCondition::QWaitCondition()
87
88 Constructs a new wait condition object.
89*/
90
91/*!
92 \fn QWaitCondition::~QWaitCondition()
93
94 Destroys the wait condition object.
95*/
96
97/*!
98 \fn void QWaitCondition::wakeOne()
99
100 Wakes one thread waiting on the wait condition. The thread that
101 is woken up depends on the operating system's scheduling
102 policies, and cannot be controlled or predicted.
103
104 If you want to wake up a specific thread, the solution is
105 typically to use different wait conditions and have different
106 threads wait on different conditions.
107
108 \sa wakeAll()
109*/
110
111/*!
112 \fn void QWaitCondition::wakeAll()
113
114 Wakes all threads waiting on the wait condition. The order in
115 which the threads are woken up depends on the operating system's
116 scheduling policies and cannot be controlled or predicted.
117
118 \sa wakeOne()
119*/
120
121/*!
122 \fn bool QWaitCondition::wait(QMutex *mutex, unsigned long time)
123
124 Releases the locked \a mutex and waits on the wait condition. The
125 \a mutex must be initially locked by the calling thread. If \a
126 mutex is not in a locked state, this function returns
127 immediately. If \a mutex is a recursive mutex, this function
128 returns immediately. The \a mutex will be unlocked, and the
129 calling thread will block until either of these conditions is met:
130
131 \list
132 \o Another thread signals it using wakeOne() or wakeAll(). This
133 function will return true in this case.
134 \o \a time milliseconds has elapsed. If \a time is \c ULONG_MAX
135 (the default), then the wait will never timeout (the event
136 must be signalled). This function will return false if the
137 wait timed out.
138 \endlist
139
140 The mutex will be returned to the same locked state. This
141 function is provided to allow the atomic transition from the
142 locked state to the wait state.
143
144 \sa wakeOne(), wakeAll()
145*/
146
147/*!
148 \fn bool QWaitCondition::wait(QReadWriteLock *readWriteLock, unsigned long time)
149 \since 4.4
150
151 Releases the locked \a readWriteLock and waits on the wait
152 condition. The \a readWriteLock must be initially locked by the
153 calling thread. If \a readWriteLock is not in a locked state, this
154 function returns immediately. The \a readWriteLock must not be
155 locked recursively, otherwise this function will not release the
156 lock properly. The \a readWriteLock will be unlocked, and the
157 calling thread will block until either of these conditions is met:
158
159 \list
160 \o Another thread signals it using wakeOne() or wakeAll(). This
161 function will return true in this case.
162 \o \a time milliseconds has elapsed. If \a time is \c ULONG_MAX
163 (the default), then the wait will never timeout (the event
164 must be signalled). This function will return false if the
165 wait timed out.
166 \endlist
167
168 The \a readWriteLock will be returned to the same locked
169 state. This function is provided to allow the atomic transition
170 from the locked state to the wait state.
171
172 \sa wakeOne(), wakeAll()
173*/
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