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annotate src/thread/generic/SDL_syscond.c @ 5112:0846f18eb625

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Improved condition variable documentation
author Sam Lantinga <slouken@libsdl.org>
date Thu, 27 Jan 2011 10:40:17 -0800
parents f7b03b6838cb
children b530ef003506
rev   line source
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1 /*
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2 SDL - Simple DirectMedia Layer
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3 Copyright (C) 1997-2010 Sam Lantinga
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4
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5 This library is free software; you can redistribute it and/or
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6 modify it under the terms of the GNU Lesser General Public
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7 License as published by the Free Software Foundation; either
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8 version 2.1 of the License, or (at your option) any later version.
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9
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10 This library is distributed in the hope that it will be useful,
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11 but WITHOUT ANY WARRANTY; without even the implied warranty of
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12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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13 Lesser General Public License for more details.
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14
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15 You should have received a copy of the GNU Lesser General Public
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16 License along with this library; if not, write to the Free Software
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17 Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
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18
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19 Sam Lantinga
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20 slouken@libsdl.org
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21 */
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22 #include "SDL_config.h"
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23
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24 /* An implementation of condition variables using semaphores and mutexes */
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25 /*
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26 This implementation borrows heavily from the BeOS condition variable
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27 implementation, written by Christopher Tate and Owen Smith. Thanks!
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28 */
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29
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30 #include "SDL_thread.h"
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31
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32 struct SDL_cond
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33 {
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34 SDL_mutex *lock;
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35 int waiting;
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36 int signals;
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37 SDL_sem *wait_sem;
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38 SDL_sem *wait_done;
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39 };
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40
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41 /* Create a condition variable */
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42 SDL_cond *
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43 SDL_CreateCond(void)
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44 {
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45 SDL_cond *cond;
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46
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47 cond = (SDL_cond *) SDL_malloc(sizeof(SDL_cond));
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48 if (cond) {
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49 cond->lock = SDL_CreateMutex();
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50 cond->wait_sem = SDL_CreateSemaphore(0);
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51 cond->wait_done = SDL_CreateSemaphore(0);
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52 cond->waiting = cond->signals = 0;
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53 if (!cond->lock || !cond->wait_sem || !cond->wait_done) {
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54 SDL_DestroyCond(cond);
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55 cond = NULL;
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56 }
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57 } else {
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58 SDL_OutOfMemory();
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59 }
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60 return (cond);
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61 }
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62
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63 /* Destroy a condition variable */
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64 void
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65 SDL_DestroyCond(SDL_cond * cond)
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66 {
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67 if (cond) {
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68 if (cond->wait_sem) {
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69 SDL_DestroySemaphore(cond->wait_sem);
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70 }
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71 if (cond->wait_done) {
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72 SDL_DestroySemaphore(cond->wait_done);
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73 }
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74 if (cond->lock) {
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75 SDL_DestroyMutex(cond->lock);
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76 }
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77 SDL_free(cond);
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78 }
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79 }
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80
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81 /* Restart one of the threads that are waiting on the condition variable */
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82 int
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83 SDL_CondSignal(SDL_cond * cond)
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84 {
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85 if (!cond) {
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86 SDL_SetError("Passed a NULL condition variable");
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87 return -1;
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88 }
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89
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90 /* If there are waiting threads not already signalled, then
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91 signal the condition and wait for the thread to respond.
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92 */
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93 SDL_LockMutex(cond->lock);
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94 if (cond->waiting > cond->signals) {
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95 ++cond->signals;
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96 SDL_SemPost(cond->wait_sem);
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97 SDL_UnlockMutex(cond->lock);
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98 SDL_SemWait(cond->wait_done);
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99 } else {
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100 SDL_UnlockMutex(cond->lock);
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101 }
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102
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103 return 0;
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104 }
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105
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106 /* Restart all threads that are waiting on the condition variable */
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107 int
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108 SDL_CondBroadcast(SDL_cond * cond)
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109 {
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110 if (!cond) {
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111 SDL_SetError("Passed a NULL condition variable");
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112 return -1;
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113 }
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114
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115 /* If there are waiting threads not already signalled, then
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116 signal the condition and wait for the thread to respond.
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117 */
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118 SDL_LockMutex(cond->lock);
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119 if (cond->waiting > cond->signals) {
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120 int i, num_waiting;
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121
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122 num_waiting = (cond->waiting - cond->signals);
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123 cond->signals = cond->waiting;
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124 for (i = 0; i < num_waiting; ++i) {
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125 SDL_SemPost(cond->wait_sem);
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126 }
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127 /* Now all released threads are blocked here, waiting for us.
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128 Collect them all (and win fabulous prizes!) :-)
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129 */
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130 SDL_UnlockMutex(cond->lock);
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131 for (i = 0; i < num_waiting; ++i) {
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132 SDL_SemWait(cond->wait_done);
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133 }
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134 } else {
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135 SDL_UnlockMutex(cond->lock);
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136 }
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137
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138 return 0;
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139 }
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140
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141 /* Wait on the condition variable for at most 'ms' milliseconds.
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142 The mutex must be locked before entering this function!
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143 The mutex is unlocked during the wait, and locked again after the wait.
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144
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145 Typical use:
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146
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147 Thread A:
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148 SDL_LockMutex(lock);
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149 while ( ! condition ) {
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150 SDL_CondWait(cond, lock);
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151 }
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152 SDL_UnlockMutex(lock);
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153
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154 Thread B:
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155 SDL_LockMutex(lock);
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156 ...
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157 condition = true;
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158 ...
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159 SDL_CondSignal(cond);
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160 SDL_UnlockMutex(lock);
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161 */
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162 int
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163 SDL_CondWaitTimeout(SDL_cond * cond, SDL_mutex * mutex, Uint32 ms)
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164 {
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165 int retval;
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166
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167 if (!cond) {
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168 SDL_SetError("Passed a NULL condition variable");
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169 return -1;
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170 }
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171
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172 /* Obtain the protection mutex, and increment the number of waiters.
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173 This allows the signal mechanism to only perform a signal if there
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174 are waiting threads.
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175 */
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176 SDL_LockMutex(cond->lock);
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177 ++cond->waiting;
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178 SDL_UnlockMutex(cond->lock);
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179
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180 /* Unlock the mutex, as is required by condition variable semantics */
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181 SDL_UnlockMutex(mutex);
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182
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183 /* Wait for a signal */
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184 if (ms == SDL_MUTEX_MAXWAIT) {
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185 retval = SDL_SemWait(cond->wait_sem);
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186 } else {
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187 retval = SDL_SemWaitTimeout(cond->wait_sem, ms);
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188 }
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189
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190 /* Let the signaler know we have completed the wait, otherwise
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191 the signaler can race ahead and get the condition semaphore
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192 if we are stopped between the mutex unlock and semaphore wait,
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193 giving a deadlock. See the following URL for details:
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194 http://www-classic.be.com/aboutbe/benewsletter/volume_III/Issue40.html
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195 */
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196 SDL_LockMutex(cond->lock);
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197 if (cond->signals > 0) {
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198 /* If we timed out, we need to eat a condition signal */
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199 if (retval > 0) {
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200 SDL_SemWait(cond->wait_sem);
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201 }
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202 /* We always notify the signal thread that we are done */
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203 SDL_SemPost(cond->wait_done);
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204
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205 /* Signal handshake complete */
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206 --cond->signals;
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207 }
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208 --cond->waiting;
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209 SDL_UnlockMutex(cond->lock);
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210
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211 /* Lock the mutex, as is required by condition variable semantics */
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212 SDL_LockMutex(mutex);
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213
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214 return retval;
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215 }
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216
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217 /* Wait on the condition variable forever */
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218 int
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219 SDL_CondWait(SDL_cond * cond, SDL_mutex * mutex)
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220 {
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221 return SDL_CondWaitTimeout(cond, mutex, SDL_MUTEX_MAXWAIT);
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222 }
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223
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224 /* vi: set ts=4 sw=4 expandtab: */