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annotate src/thread/dc/SDL_syscond.c @ 1650:ee4f2d77206f

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author Sam Lantinga <slouken@libsdl.org>
date Sat, 22 Apr 2006 19:50:23 +0000
parents d910939febfa
children 782fd950bd46 c121d94672cb a1b03ba2fcd0
rev   line source
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1 /*
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2 SDL - Simple DirectMedia Layer
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3 Copyright (C) 1997-2006 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 * SDL_CreateCond(void)
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43 {
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44 SDL_cond *cond;
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45
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46 cond = (SDL_cond *) SDL_malloc(sizeof(SDL_cond));
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47 if ( cond ) {
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48 cond->lock = SDL_CreateMutex();
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49 cond->wait_sem = SDL_CreateSemaphore(0);
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50 cond->wait_done = SDL_CreateSemaphore(0);
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51 cond->waiting = cond->signals = 0;
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52 if ( ! cond->lock || ! cond->wait_sem || ! cond->wait_done ) {
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53 SDL_DestroyCond(cond);
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54 cond = NULL;
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55 }
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56 } else {
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57 SDL_OutOfMemory();
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58 }
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59 return(cond);
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60 }
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61
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62 /* Destroy a condition variable */
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63 void SDL_DestroyCond(SDL_cond *cond)
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64 {
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65 if ( cond ) {
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66 if ( cond->wait_sem ) {
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67 SDL_DestroySemaphore(cond->wait_sem);
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68 }
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69 if ( cond->wait_done ) {
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70 SDL_DestroySemaphore(cond->wait_done);
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71 }
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72 if ( cond->lock ) {
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73 SDL_DestroyMutex(cond->lock);
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74 }
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75 SDL_free(cond);
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76 }
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77 }
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78
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79 /* Restart one of the threads that are waiting on the condition variable */
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80 int SDL_CondSignal(SDL_cond *cond)
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81 {
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82 if ( ! cond ) {
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83 SDL_SetError("Passed a NULL condition variable");
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84 return -1;
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85 }
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86
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87 /* If there are waiting threads not already signalled, then
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88 signal the condition and wait for the thread to respond.
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89 */
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90 SDL_LockMutex(cond->lock);
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91 if ( cond->waiting > cond->signals ) {
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92 ++cond->signals;
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93 SDL_SemPost(cond->wait_sem);
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94 SDL_UnlockMutex(cond->lock);
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95 SDL_SemWait(cond->wait_done);
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96 } else {
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97 SDL_UnlockMutex(cond->lock);
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98 }
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99
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100 return 0;
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101 }
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102
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103 /* Restart all threads that are waiting on the condition variable */
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104 int SDL_CondBroadcast(SDL_cond *cond)
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105 {
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106 if ( ! cond ) {
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107 SDL_SetError("Passed a NULL condition variable");
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108 return -1;
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109 }
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110
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111 /* If there are waiting threads not already signalled, then
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112 signal the condition and wait for the thread to respond.
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113 */
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114 SDL_LockMutex(cond->lock);
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115 if ( cond->waiting > cond->signals ) {
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116 int i, num_waiting;
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117
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118 num_waiting = (cond->waiting - cond->signals);
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119 cond->signals = cond->waiting;
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120 for ( i=0; i<num_waiting; ++i ) {
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121 SDL_SemPost(cond->wait_sem);
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122 }
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123 /* Now all released threads are blocked here, waiting for us.
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124 Collect them all (and win fabulous prizes!) :-)
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125 */
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126 SDL_UnlockMutex(cond->lock);
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127 for ( i=0; i<num_waiting; ++i ) {
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128 SDL_SemWait(cond->wait_done);
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129 }
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130 } else {
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131 SDL_UnlockMutex(cond->lock);
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132 }
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133
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134 return 0;
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135 }
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136
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137 /* Wait on the condition variable for at most 'ms' milliseconds.
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138 The mutex must be locked before entering this function!
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139 The mutex is unlocked during the wait, and locked again after the wait.
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140
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141 Typical use:
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142
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143 Thread A:
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144 SDL_LockMutex(lock);
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145 while ( ! condition ) {
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146 SDL_CondWait(cond);
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147 }
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148 SDL_UnlockMutex(lock);
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149
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150 Thread B:
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151 SDL_LockMutex(lock);
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152 ...
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153 condition = true;
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154 ...
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155 SDL_UnlockMutex(lock);
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156 */
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157 int SDL_CondWaitTimeout(SDL_cond *cond, SDL_mutex *mutex, Uint32 ms)
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158 {
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159 int retval;
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160
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161 if ( ! cond ) {
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162 SDL_SetError("Passed a NULL condition variable");
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163 return -1;
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164 }
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165
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166 /* Obtain the protection mutex, and increment the number of waiters.
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167 This allows the signal mechanism to only perform a signal if there
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168 are waiting threads.
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169 */
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170 SDL_LockMutex(cond->lock);
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171 ++cond->waiting;
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172 SDL_UnlockMutex(cond->lock);
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173
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174 /* Unlock the mutex, as is required by condition variable semantics */
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175 SDL_UnlockMutex(mutex);
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176
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177 /* Wait for a signal */
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178 if ( ms == SDL_MUTEX_MAXWAIT ) {
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179 retval = SDL_SemWait(cond->wait_sem);
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180 } else {
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181 retval = SDL_SemWaitTimeout(cond->wait_sem, ms);
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182 }
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183
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184 /* Let the signaler know we have completed the wait, otherwise
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185 the signaler can race ahead and get the condition semaphore
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186 if we are stopped between the mutex unlock and semaphore wait,
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187 giving a deadlock. See the following URL for details:
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188 http://www-classic.be.com/aboutbe/benewsletter/volume_III/Issue40.html
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189 */
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190 SDL_LockMutex(cond->lock);
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191 if ( cond->signals > 0 ) {
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192 /* If we timed out, we need to eat a condition signal */
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193 if ( retval > 0 ) {
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194 SDL_SemWait(cond->wait_sem);
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195 }
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196 /* We always notify the signal thread that we are done */
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197 SDL_SemPost(cond->wait_done);
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198
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199 /* Signal handshake complete */
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200 --cond->signals;
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201 }
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202 --cond->waiting;
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203 SDL_UnlockMutex(cond->lock);
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204
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205 /* Lock the mutex, as is required by condition variable semantics */
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206 SDL_LockMutex(mutex);
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207
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208 return retval;
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209 }
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210
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211 /* Wait on the condition variable forever */
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212 int SDL_CondWait(SDL_cond *cond, SDL_mutex *mutex)
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213 {
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214 return SDL_CondWaitTimeout(cond, mutex, SDL_MUTEX_MAXWAIT);
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215 }