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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 Qt3Support module 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 #include "q3semaphore.h" |
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43 |
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44 #include "qmutex.h" |
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45 #include "qwaitcondition.h" |
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46 |
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47 QT_BEGIN_NAMESPACE |
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48 |
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49 /*! |
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50 \class Q3Semaphore |
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51 \threadsafe |
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52 \brief The Q3Semaphore class provides a robust integer semaphore. |
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53 |
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54 \compat |
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55 |
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56 A Q3Semaphore can be used to serialize thread execution, in a |
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57 similar way to a QMutex. A semaphore differs from a mutex, in |
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58 that a semaphore can be accessed by more than one thread at a |
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59 time. |
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60 |
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61 For example, suppose we have an application that stores data in a |
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62 large tree structure. The application creates 10 threads |
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63 (commonly called a thread pool) to perform searches on the tree. |
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64 When the application searches the tree for some piece of data, it |
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65 uses one thread per base node to do the searching. A semaphore |
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66 could be used to make sure that two threads don't try to search |
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67 the same branch of the tree at the same time. |
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68 |
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69 A non-computing example of a semaphore would be dining at a |
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70 restaurant. A semaphore is initialized to have a maximum count |
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71 equal to the number of chairs in the restaurant. As people |
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72 arrive, they want a seat. As seats are filled, the semaphore is |
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73 accessed, once per person. As people leave, the access is |
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74 released, allowing more people to enter. If a party of 10 people |
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75 want to be seated, but there are only 9 seats, those 10 people |
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76 will wait, but a party of 4 people would be seated (taking the |
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77 available seats to 5, making the party of 10 people wait longer). |
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78 |
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79 When a semaphore is created it is given a number which is the |
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80 maximum number of concurrent accesses it will permit. This amount |
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81 may be changed using operator++(), operator--(), operator+=() and |
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82 operator-=(). The number of accesses allowed is retrieved with |
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83 available(), and the total number with total(). Note that the |
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84 incrementing functions will block if there aren't enough available |
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85 accesses. Use tryAccess() if you want to acquire accesses without |
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86 blocking. |
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87 */ |
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88 |
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89 #ifdef max |
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90 #undef max |
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91 #endif |
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92 |
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93 class Q3SemaphorePrivate { |
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94 public: |
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95 Q3SemaphorePrivate(int); |
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96 |
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97 QMutex mutex; |
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98 QWaitCondition cond; |
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99 |
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100 int value, max; |
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101 }; |
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102 |
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103 |
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104 Q3SemaphorePrivate::Q3SemaphorePrivate(int m) |
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105 : value(0), max(m) |
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106 { |
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107 } |
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108 |
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109 |
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110 /*! |
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111 Creates a new semaphore. The semaphore can be concurrently |
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112 accessed at most \a maxcount times. |
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113 */ |
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114 Q3Semaphore::Q3Semaphore(int maxcount) |
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115 { |
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116 d = new Q3SemaphorePrivate(maxcount); |
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117 } |
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118 |
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119 |
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120 /*! |
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121 Destroys the semaphore. |
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122 |
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123 \warning If you destroy a semaphore that has accesses in use the |
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124 resultant behavior is undefined. |
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125 */ |
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126 Q3Semaphore::~Q3Semaphore() |
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127 { |
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128 delete d; |
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129 } |
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130 |
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131 |
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132 /*! |
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133 Postfix ++ operator. |
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134 |
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135 Try to get access to the semaphore. If \l available() == 0, this |
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136 call will block until it can get access, i.e. until available() \> |
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137 0. |
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138 */ |
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139 int Q3Semaphore::operator++(int) |
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140 { |
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141 QMutexLocker locker(&d->mutex); |
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142 while (d->value >= d->max) |
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143 d->cond.wait(locker.mutex()); |
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144 |
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145 ++d->value; |
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146 if (d->value > d->max) |
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147 d->value = d->max; |
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148 |
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149 return d->value; |
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150 } |
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151 |
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152 |
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153 /*! |
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154 Postfix -- operator. |
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155 |
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156 Release access of the semaphore. This wakes all threads waiting |
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157 for access to the semaphore. |
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158 */ |
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159 int Q3Semaphore::operator--(int) |
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160 { |
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161 QMutexLocker locker(&d->mutex); |
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162 |
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163 --d->value; |
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164 if (d->value < 0) |
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165 d->value = 0; |
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166 |
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167 d->cond.wakeAll(); |
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168 |
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169 return d->value; |
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170 } |
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171 |
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172 |
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173 /*! |
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174 Try to get access to the semaphore. If \l available() \< \a n, this |
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175 call will block until it can get all the accesses it wants, i.e. |
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176 until available() \>= \a n. |
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177 */ |
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178 int Q3Semaphore::operator+=(int n) |
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179 { |
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180 QMutexLocker locker(&d->mutex); |
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181 |
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182 if (n < 0 || n > d->max) { |
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183 qWarning("Q3Semaphore::operator+=: parameter %d out of range", n); |
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184 n = n < 0 ? 0 : d->max; |
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185 } |
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186 |
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187 while (d->value + n > d->max) |
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188 d->cond.wait(locker.mutex()); |
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189 |
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190 d->value += n; |
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191 |
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192 return d->value; |
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193 } |
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194 |
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195 |
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196 /*! |
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197 Release \a n accesses to the semaphore. |
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198 */ |
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199 int Q3Semaphore::operator-=(int n) |
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200 { |
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201 QMutexLocker locker(&d->mutex); |
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202 |
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203 if (n < 0 || n > d->value) { |
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204 qWarning("Q3Semaphore::operator-=: parameter %d out of range", n); |
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205 n = n < 0 ? 0 : d->value; |
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206 } |
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207 |
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208 d->value -= n; |
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209 d->cond.wakeAll(); |
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210 |
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211 return d->value; |
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212 } |
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213 |
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214 |
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215 /*! |
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216 Returns the number of accesses currently available to the |
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217 semaphore. |
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218 */ |
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219 int Q3Semaphore::available() const |
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220 { |
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221 QMutexLocker locker(&d->mutex); |
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222 return d->max - d->value; |
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223 } |
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224 |
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225 |
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226 /*! |
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227 Returns the total number of accesses to the semaphore. |
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228 */ |
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229 int Q3Semaphore::total() const |
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230 { |
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231 QMutexLocker locker(&d->mutex); |
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232 return d->max; |
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233 } |
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234 |
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235 |
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236 /*! |
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237 Try to get access to the semaphore. If \l available() \< \a n, this |
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238 function will return false immediately. If \l available() \>= \a n, |
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239 this function will take \a n accesses and return true. This |
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240 function does \e not block. |
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241 */ |
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242 bool Q3Semaphore::tryAccess(int n) |
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243 { |
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244 QMutexLocker locker(&d->mutex); |
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245 |
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246 if (d->value + n > d->max) |
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247 return false; |
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248 |
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249 d->value += n; |
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250 |
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251 return true; |
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252 } |
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253 |
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254 QT_END_NAMESPACE |