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 ([email protected])
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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 | ** Commercial Usage
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11 | ** Licensees holding valid Qt Commercial licenses may use this file in
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12 | ** accordance with the Qt Commercial License Agreement provided with the
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13 | ** Software or, alternatively, in accordance with the terms contained in
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14 | ** a written agreement between you and Nokia.
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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 | ** GNU General Public License Usage
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29 | ** Alternatively, this file may be used under the terms of the GNU
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30 | ** General Public License version 3.0 as published by the Free Software
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31 | ** Foundation and appearing in the file LICENSE.GPL included in the
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32 | ** packaging of this file. Please review the following information to
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33 | ** ensure the GNU General Public License version 3.0 requirements will be
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34 | ** met: http://www.gnu.org/copyleft/gpl.html.
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35 | **
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36 | ** If you have questions regarding the use of this file, please contact
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37 | ** Nokia at [email protected].
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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 | \group thread
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44 | \title Threading Classes
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45 | */
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46 |
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47 | /*!
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48 | \page threads.html
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49 | \title Thread Support in Qt
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50 | \brief A detailed discussion of thread handling in Qt.
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51 |
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52 | \ingroup frameworks-technologies
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53 |
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54 | \nextpage Starting Threads with QThread
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55 |
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56 | Qt provides thread support in the form of platform-independent
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57 | threading classes, a thread-safe way of posting events, and
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58 | signal-slot connections across threads. This makes it easy to
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59 | develop portable multithreaded Qt applications and take advantage
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60 | of multiprocessor machines. Multithreaded programming is also a
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61 | useful paradigm for performing time-consuming operations without
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62 | freezing the user interface of an application.
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63 |
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64 | Earlier versions of Qt offered an option to build the library
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65 | without thread support. Since Qt 4.0, threads are always enabled.
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66 |
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67 | \section1 Topics:
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68 |
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69 | \list
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70 | \o \l{Recommended Reading}
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71 | \o \l{The Threading Classes}
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72 | \o \l{Starting Threads with QThread}
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73 | \o \l{Synchronizing Threads}
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74 | \o \l{Reentrancy and Thread-Safety}
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75 | \o \l{Threads and QObjects}
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76 | \o \l{Concurrent Programming}
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77 | \o \l{Thread-Support in Qt Modules}
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78 | \endlist
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79 |
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80 | \section1 Recommended Reading
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81 |
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82 | This document is intended for an audience that has knowledge of,
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83 | and experience with, multithreaded applications. If you are new
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84 | to threading see our Recommended Reading list:
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85 |
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86 | \list
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87 | \o \l{Threads Primer: A Guide to Multithreaded Programming}
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88 | \o \l{Thread Time: The Multithreaded Programming Guide}
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89 | \o \l{Pthreads Programming: A POSIX Standard for Better Multiprocessing}
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90 | \o \l{Win32 Multithreaded Programming}
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91 | \endlist
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92 |
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93 | \section1 The Threading Classes
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94 |
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95 | These classes are relevant to threaded applications.
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96 |
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97 | \annotatedlist thread
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98 |
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99 | \omit
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100 | \list
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101 | \o QThread provides the means to start a new thread.
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102 | \o QThreadStorage provides per-thread data storage.
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103 | \o QThreadPool manages a pool of threads that run QRunnable objects.
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104 | \o QRunnable is an abstract class representing a runnable object.
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105 | \o QMutex provides a mutual exclusion lock, or mutex.
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106 | \o QMutexLocker is a convenience class that automatically locks
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107 | and unlocks a QMutex.
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108 | \o QReadWriteLock provides a lock that allows simultaneous read access.
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109 | \o QReadLocker and QWriteLocker are convenience classes that automatically
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110 | lock and unlock a QReadWriteLock.
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111 | \o QSemaphore provides an integer semaphore (a generalization of a mutex).
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112 | \o QWaitCondition provides a way for threads to go to sleep until
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113 | woken up by another thread.
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114 | \o QAtomicInt provides atomic operations on integers.
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115 | \o QAtomicPointer provides atomic operations on pointers.
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116 | \endlist
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117 | \endomit
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118 |
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119 | \note Qt's threading classes are implemented with native threading APIs;
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120 | e.g., Win32 and pthreads. Therefore, they can be used with threads of the
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121 | same native API.
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122 | */
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123 |
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124 | /*!
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125 | \page threads-starting.html
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126 | \title Starting Threads with QThread
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127 |
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128 | \contentspage Thread Support in Qt
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129 | \nextpage Synchronizing Threads
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130 |
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131 | A QThread instance represents a thread and provides the means to
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132 | \l{QThread::start()}{start()} a thread, which will then execute the
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133 | reimplementation of QThread::run(). The \c run() implementation is for a
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134 | thread what the \c main() entry point is for the application. All code
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135 | executed in a call stack that starts in the \c run() function is executed
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136 | by the new thread, and the thread finishes when the function returns.
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137 | QThread emits signals to indicate that the thread started or finished
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138 | executing.
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139 |
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140 | \section1 Creating a Thread
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141 |
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142 | To create a thread, subclass QThread and reimplement its
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143 | \l{QThread::run()}{run()} function. For example:
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144 |
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145 | \snippet doc/src/snippets/threads/threads.h 0
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146 | \codeline
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147 | \snippet doc/src/snippets/threads/threads.cpp 0
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148 | \snippet doc/src/snippets/threads/threads.cpp 1
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149 | \dots
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150 | \snippet doc/src/snippets/threads/threads.cpp 2
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151 |
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152 | \section1 Starting a Thread
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153 |
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154 | Then, create an instance of the thread object and call
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155 | QThread::start(). Note that you must create the QApplication (or
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156 | QCoreApplication) object before you can create a QThread.
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157 |
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158 | The function will return immediately and the
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159 | main thread will continue. The code that appears in the
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160 | \l{QThread::run()}{run()} reimplementation will then be executed
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161 | in a separate thread.
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162 |
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163 | Creating threads is explained in more detail in the QThread
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164 | documentation.
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165 |
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166 | Note that QCoreApplication::exec() must always be called from the
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167 | main thread (the thread that executes \c{main()}), not from a
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168 | QThread. In GUI applications, the main thread is also called the
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169 | GUI thread because it's the only thread that is allowed to
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170 | perform GUI-related operations.
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171 | */
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172 |
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173 | /*!
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174 | \page threads-synchronizing.html
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175 | \title Synchronizing Threads
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176 |
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177 | \previouspage Starting Threads with QThread
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178 | \contentspage Thread Support in Qt
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179 | \nextpage Reentrancy and Thread-Safety
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180 |
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181 | The QMutex, QReadWriteLock, QSemaphore, and QWaitCondition
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182 | classes provide means to synchronize threads. While the main idea
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183 | with threads is that they should be as concurrent as possible,
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184 | there are points where threads must stop and wait for other
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185 | threads. For example, if two threads try to access the same
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186 | global variable simultaneously, the results are usually
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187 | undefined.
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188 |
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189 | QMutex provides a mutually exclusive lock, or mutex. At most one
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190 | thread can hold the mutex at any time. If a thread tries to
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191 | acquire the mutex while the mutex is already locked, the thread will
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192 | be put to sleep until the thread that currently holds the mutex
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193 | unlocks it. Mutexes are often used to protect accesses to shared
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194 | data (i.e., data that can be accessed from multiple threads
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195 | simultaneously). In the \l{Reentrancy and Thread-Safety} section
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196 | below, we will use it to make a class thread-safe.
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197 |
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198 | QReadWriteLock is similar to QMutex, except that it distinguishes
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199 | between "read" and "write" access to shared data and allows
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200 | multiple readers to access the data simultaneously. Using
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201 | QReadWriteLock instead of QMutex when it is possible can make
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202 | multithreaded programs more concurrent.
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203 |
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204 | QSemaphore is a generalization of QMutex that protects a certain
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205 | number of identical resources. In contrast, a mutex protects
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206 | exactly one resource. The \l{threads/semaphores}{Semaphores}
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207 | example shows a typical application of semaphores: synchronizing
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208 | access to a circular buffer between a producer and a consumer.
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209 |
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210 | QWaitCondition allows a thread to wake up other threads when some
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211 | condition has been met. One or many threads can block waiting for
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212 | a QWaitCondition to set a condition with
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213 | \l{QWaitCondition::wakeOne()}{wakeOne()} or
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214 | \l{QWaitCondition::wakeAll()}{wakeAll()}. Use
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215 | \l{QWaitCondition::wakeOne()}{wakeOne()} to wake one randomly
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216 | selected event or \l{QWaitCondition::wakeAll()}{wakeAll()} to
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217 | wake them all. The \l{threads/waitconditions}{Wait Conditions}
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218 | example shows how to solve the producer-consumer problem using
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219 | QWaitCondition instead of QSemaphore.
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220 |
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221 | Note that Qt's synchronization classes rely on the use of properly
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222 | aligned pointers. For instance, you cannot use packed classes with
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223 | MSVC.
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224 | */
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225 |
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226 | /*!
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227 | \page threads-reentrancy.html
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228 | \title Reentrancy and Thread-Safety
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229 |
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230 | \keyword reentrant
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231 | \keyword thread-safe
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232 |
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233 | \previouspage Synchronizing Threads
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234 | \contentspage Thread Support in Qt
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235 | \nextpage Threads and QObjects
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236 |
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237 | Throughout the documentation, the terms \e{reentrant} and
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238 | \e{thread-safe} are used to mark classes and functions to indicate
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239 | how they can be used in multithread applications:
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240 |
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241 | \list
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242 | \o A \e thread-safe function can be called simultaneously from
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243 | multiple threads, even when the invocations use shared data,
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244 | because all references to the shared data are serialized.
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245 | \o A \e reentrant function can also be called simultaneously from
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246 | multiple threads, but only if each invocation uses its own data.
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247 | \endlist
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248 |
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249 | Hence, a \e{thread-safe} function is always \e{reentrant}, but a
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250 | \e{reentrant} function is not always \e{thread-safe}.
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251 |
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252 | By extension, a class is said to be \e{reentrant} if its member
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253 | functions can be called safely from multiple threads, as long as
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254 | each thread uses a \e{different} instance of the class. The class
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255 | is \e{thread-safe} if its member functions can be called safely
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256 | from multiple threads, even if all the threads use the \e{same}
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257 | instance of the class.
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258 |
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259 | C++ classes are often reentrant, simply because they only access
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260 | their own member data. Any thread can call a member function on an
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261 | instance of a reentrant class, as long as no other thread can call
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262 | a member function on the \e{same} instance of the class at the
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263 | same time. For example, the \c Counter class below is reentrant:
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