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1 /**************************************************************************** |
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2 ** |
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3 ** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies). |
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4 ** All rights reserved. |
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5 ** Contact: Nokia Corporation (qt-info@nokia.com) |
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6 ** |
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7 ** This file is part of the documentation of the Qt Toolkit. |
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8 ** |
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9 ** $QT_BEGIN_LICENSE:LGPL$ |
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10 ** No Commercial Usage |
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11 ** This file contains pre-release code and may not be distributed. |
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12 ** You may use this file in accordance with the terms and conditions |
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13 ** contained in the Technology Preview License Agreement accompanying |
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14 ** this package. |
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15 ** |
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16 ** GNU Lesser General Public License Usage |
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17 ** Alternatively, this file may be used under the terms of the GNU Lesser |
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18 ** General Public License version 2.1 as published by the Free Software |
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19 ** Foundation and appearing in the file LICENSE.LGPL included in the |
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20 ** packaging of this file. Please review the following information to |
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21 ** ensure the GNU Lesser General Public License version 2.1 requirements |
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22 ** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html. |
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23 ** |
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24 ** In addition, as a special exception, Nokia gives you certain additional |
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25 ** rights. These rights are described in the Nokia Qt LGPL Exception |
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26 ** version 1.1, included in the file LGPL_EXCEPTION.txt in this package. |
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27 ** |
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28 ** If you have questions regarding the use of this file, please contact |
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29 ** Nokia at qt-info@nokia.com. |
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30 ** |
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31 ** |
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32 ** |
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33 ** |
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34 ** |
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35 ** |
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36 ** |
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37 ** |
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38 ** $QT_END_LICENSE$ |
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39 ** |
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40 ****************************************************************************/ |
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41 |
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42 /*! |
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43 \class QWaitCondition |
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44 \brief The QWaitCondition class provides a condition variable for |
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45 synchronizing threads. |
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46 |
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47 \threadsafe |
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48 |
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49 \ingroup thread |
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50 |
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51 QWaitCondition allows a thread to tell other threads that some |
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52 sort of condition has been met. One or many threads can block |
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53 waiting for a QWaitCondition to set a condition with wakeOne() or |
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54 wakeAll(). Use wakeOne() to wake one randomly selected condition or |
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55 wakeAll() to wake them all. |
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56 |
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57 For example, let's suppose that we have three tasks that should |
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58 be performed whenever the user presses a key. Each task could be |
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59 split into a thread, each of which would have a |
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60 \l{QThread::run()}{run()} body like this: |
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61 |
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62 \snippet doc/src/snippets/code/src_corelib_thread_qwaitcondition_unix.cpp 0 |
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63 |
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64 Here, the \c keyPressed variable is a global variable of type |
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65 QWaitCondition. |
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66 |
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67 A fourth thread would read key presses and wake the other three |
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68 threads up every time it receives one, like this: |
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69 |
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70 \snippet doc/src/snippets/code/src_corelib_thread_qwaitcondition_unix.cpp 1 |
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71 |
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72 The order in which the three threads are woken up is undefined. |
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73 Also, if some of the threads are still in \c do_something() when |
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74 the key is pressed, they won't be woken up (since they're not |
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75 waiting on the condition variable) and so the task will not be |
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76 performed for that key press. This issue can be solved using a |
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77 counter and a QMutex to guard it. For example, here's the new |
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78 code for the worker threads: |
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79 |
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80 \snippet doc/src/snippets/code/src_corelib_thread_qwaitcondition_unix.cpp 2 |
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81 |
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82 Here's the code for the fourth thread: |
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83 |
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84 \snippet doc/src/snippets/code/src_corelib_thread_qwaitcondition_unix.cpp 3 |
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85 |
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86 The mutex is necessary because the results of two threads |
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87 attempting to change the value of the same variable |
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88 simultaneously are unpredictable. |
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89 |
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90 Wait conditions are a powerful thread synchronization primitive. |
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91 The \l{threads/waitconditions}{Wait Conditions} example shows how |
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92 to use QWaitCondition as an alternative to QSemaphore for |
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93 controlling access to a circular buffer shared by a producer |
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94 thread and a consumer thread. |
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95 |
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96 \sa QMutex, QSemaphore, QThread, {Wait Conditions Example} |
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97 */ |
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98 |
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99 /*! |
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100 \fn QWaitCondition::QWaitCondition() |
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101 |
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102 Constructs a new wait condition object. |
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103 */ |
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104 |
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105 /*! |
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106 \fn QWaitCondition::~QWaitCondition() |
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107 |
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108 Destroys the wait condition object. |
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109 */ |
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110 |
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111 /*! |
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112 \fn void QWaitCondition::wakeOne() |
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113 |
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114 Wakes one thread waiting on the wait condition. The thread that |
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115 is woken up depends on the operating system's scheduling |
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116 policies, and cannot be controlled or predicted. |
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117 |
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118 If you want to wake up a specific thread, the solution is |
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119 typically to use different wait conditions and have different |
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120 threads wait on different conditions. |
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121 |
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122 \sa wakeAll() |
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123 */ |
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124 |
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125 /*! |
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126 \fn void QWaitCondition::wakeAll() |
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127 |
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128 Wakes all threads waiting on the wait condition. The order in |
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129 which the threads are woken up depends on the operating system's |
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130 scheduling policies and cannot be controlled or predicted. |
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131 |
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132 \sa wakeOne() |
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133 */ |
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134 |
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135 /*! |
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136 \fn bool QWaitCondition::wait(QMutex *mutex, unsigned long time) |
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137 |
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138 Releases the locked \a mutex and waits on the wait condition. The |
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139 \a mutex must be initially locked by the calling thread. If \a |
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140 mutex is not in a locked state, this function returns |
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141 immediately. If \a mutex is a recursive mutex, this function |
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142 returns immediately. The \a mutex will be unlocked, and the |
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143 calling thread will block until either of these conditions is met: |
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144 |
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145 \list |
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146 \o Another thread signals it using wakeOne() or wakeAll(). This |
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147 function will return true in this case. |
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148 \o \a time milliseconds has elapsed. If \a time is \c ULONG_MAX |
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149 (the default), then the wait will never timeout (the event |
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150 must be signalled). This function will return false if the |
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151 wait timed out. |
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152 \endlist |
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153 |
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154 The mutex will be returned to the same locked state. This |
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155 function is provided to allow the atomic transition from the |
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156 locked state to the wait state. |
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157 |
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158 \sa wakeOne(), wakeAll() |
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159 */ |
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160 |
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161 /*! |
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162 \fn bool QWaitCondition::wait(QReadWriteLock *readWriteLock, unsigned long time) |
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163 \since 4.4 |
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164 |
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165 Releases the locked \a readWriteLock and waits on the wait |
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166 condition. The \a readWriteLock must be initially locked by the |
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167 calling thread. If \a readWriteLock is not in a locked state, this |
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168 function returns immediately. The \a readWriteLock must not be |
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169 locked recursively, otherwise this function will not release the |
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170 lock properly. The \a readWriteLock will be unlocked, and the |
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171 calling thread will block until either of these conditions is met: |
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172 |
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173 \list |
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174 \o Another thread signals it using wakeOne() or wakeAll(). This |
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175 function will return true in this case. |
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176 \o \a time milliseconds has elapsed. If \a time is \c ULONG_MAX |
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177 (the default), then the wait will never timeout (the event |
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178 must be signalled). This function will return false if the |
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179 wait timed out. |
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180 \endlist |
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181 |
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182 The \a readWriteLock will be returned to the same locked |
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183 state. This function is provided to allow the atomic transition |
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184 from the locked state to the wait state. |
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185 |
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186 \sa wakeOne(), wakeAll() |
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187 */ |