2 FreeRTOS.org V4.3.0 - Copyright (C) 2003-2007 Richard Barry.
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4 This file is part of the FreeRTOS.org distribution.
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6 FreeRTOS.org is free software; you can redistribute it and/or modify
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7 it under the terms of the GNU General Public License as published by
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8 the Free Software Foundation; either version 2 of the License, or
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9 (at your option) any later version.
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11 FreeRTOS.org is distributed in the hope that it will be useful,
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12 but WITHOUT ANY WARRANTY; without even the implied warranty of
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13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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14 GNU General Public License for more details.
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16 You should have received a copy of the GNU General Public License
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18 Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
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20 A special exception to the GPL can be applied should you wish to distribute
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26 ***************************************************************************
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27 See http://www.FreeRTOS.org for documentation, latest information, license
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28 and contact details. Please ensure to read the configuration and relevant
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29 port sections of the online documentation.
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31 Also see http://www.SafeRTOS.com for an IEC 61508 compliant version along
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32 with commercial development and support options.
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33 ***************************************************************************
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41 typedef xQueueHandle xSemaphoreHandle;
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43 #define semBINARY_SEMAPHORE_QUEUE_LENGTH ( ( unsigned portCHAR ) 1 )
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44 #define semSEMAPHORE_QUEUE_ITEM_LENGTH ( ( unsigned portCHAR ) 0 )
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45 #define semGIVE_BLOCK_TIME ( ( portTickType ) 0 )
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50 * <pre>vSemaphoreCreateBinary( xSemaphoreHandle xSemaphore )</pre>
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52 * <i>Macro</i> that implements a semaphore by using the existing queue mechanism.
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53 * The queue length is 1 as this is a binary semaphore. The data size is 0
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54 * as we don't want to actually store any data - we just want to know if the
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55 * queue is empty or full.
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57 * @param xSemaphore Handle to the created semaphore. Should be of type xSemaphoreHandle.
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61 xSemaphoreHandle xSemaphore;
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63 void vATask( void * pvParameters )
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65 // Semaphore cannot be used before a call to vSemaphoreCreateBinary ().
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66 // This is a macro so pass the variable in directly.
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67 vSemaphoreCreateBinary( xSemaphore );
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69 if( xSemaphore != NULL )
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71 // The semaphore was created successfully.
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72 // The semaphore can now be used.
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76 * \defgroup vSemaphoreCreateBinary vSemaphoreCreateBinary
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77 * \ingroup Semaphores
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79 #define vSemaphoreCreateBinary( xSemaphore ) { \
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80 xSemaphore = xQueueCreate( ( unsigned portCHAR ) 1, semSEMAPHORE_QUEUE_ITEM_LENGTH ); \
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81 if( xSemaphore != NULL ) \
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83 xSemaphoreGive( xSemaphore ); \
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90 * xSemaphoreHandle xSemaphore,
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91 * portTickType xBlockTime
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94 * <i>Macro</i> to obtain a semaphore. The semaphore must of been created using
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95 * vSemaphoreCreateBinary ().
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97 * @param xSemaphore A handle to the semaphore being obtained. This is the
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98 * handle returned by vSemaphoreCreateBinary ();
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100 * @param xBlockTime The time in ticks to wait for the semaphore to become
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101 * available. The macro portTICK_RATE_MS can be used to convert this to a
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102 * real time. A block time of zero can be used to poll the semaphore.
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104 * @return pdTRUE if the semaphore was obtained. pdFALSE if xBlockTime
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105 * expired without the semaphore becoming available.
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109 xSemaphoreHandle xSemaphore = NULL;
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111 // A task that creates a semaphore.
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112 void vATask( void * pvParameters )
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114 // Create the semaphore to guard a shared resource.
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115 vSemaphoreCreateBinary( xSemaphore );
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118 // A task that uses the semaphore.
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119 void vAnotherTask( void * pvParameters )
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121 // ... Do other things.
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123 if( xSemaphore != NULL )
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125 // See if we can obtain the semaphore. If the semaphore is not available
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126 // wait 10 ticks to see if it becomes free.
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127 if( xSemaphoreTake( xSemaphore, ( portTickType ) 10 ) == pdTRUE )
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129 // We were able to obtain the semaphore and can now access the
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130 // shared resource.
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134 // We have finished accessing the shared resource. Release the
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136 xSemaphoreGive( xSemaphore );
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140 // We could not obtain the semaphore and can therefore not access
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141 // the shared resource safely.
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146 * \defgroup xSemaphoreTake xSemaphoreTake
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147 * \ingroup Semaphores
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149 #define xSemaphoreTake( xSemaphore, xBlockTime ) xQueueReceive( ( xQueueHandle ) xSemaphore, NULL, xBlockTime )
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153 * <pre>xSemaphoreGive( xSemaphoreHandle xSemaphore )</pre>
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155 * <i>Macro</i> to release a semaphore. The semaphore must of been created using
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156 * vSemaphoreCreateBinary (), and obtained using sSemaphoreTake ().
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158 * This must not be used from an ISR. See xSemaphoreGiveFromISR () for
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159 * an alternative which can be used from an ISR.
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161 * @param xSemaphore A handle to the semaphore being released. This is the
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162 * handle returned by vSemaphoreCreateBinary ();
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164 * @return pdTRUE if the semaphore was released. pdFALSE if an error occurred.
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165 * Semaphores are implemented using queues. An error can occur if there is
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166 * no space on the queue to post a message - indicating that the
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167 * semaphore was not first obtained correctly.
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171 xSemaphoreHandle xSemaphore = NULL;
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173 void vATask( void * pvParameters )
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175 // Create the semaphore to guard a shared resource.
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176 vSemaphoreCreateBinary( xSemaphore );
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178 if( xSemaphore != NULL )
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180 if( xSemaphoreGive( xSemaphore ) != pdTRUE )
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182 // We would expect this call to fail because we cannot give
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183 // a semaphore without first "taking" it!
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186 // Obtain the semaphore - don't block if the semaphore is not
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187 // immediately available.
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188 if( xSemaphoreTake( xSemaphore, ( portTickType ) 0 ) )
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190 // We now have the semaphore and can access the shared resource.
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194 // We have finished accessing the shared resource so can free the
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196 if( xSemaphoreGive( xSemaphore ) != pdTRUE )
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198 // We would not expect this call to fail because we must have
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199 // obtained the semaphore to get here.
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205 * \defgroup xSemaphoreGive xSemaphoreGive
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206 * \ingroup Semaphores
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208 #define xSemaphoreGive( xSemaphore ) xQueueSend( ( xQueueHandle ) xSemaphore, NULL, semGIVE_BLOCK_TIME )
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213 xSemaphoreGiveFromISR(
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214 xSemaphoreHandle xSemaphore,
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215 portSHORT sTaskPreviouslyWoken
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218 * <i>Macro</i> to release a semaphore. The semaphore must of been created using
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219 * vSemaphoreCreateBinary (), and obtained using xSemaphoreTake ().
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221 * This macro can be used from an ISR.
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223 * @param xSemaphore A handle to the semaphore being released. This is the
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224 * handle returned by vSemaphoreCreateBinary ();
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226 * @param sTaskPreviouslyWoken This is included so an ISR can make multiple calls
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227 * to xSemaphoreGiveFromISR () from a single interrupt. The first call
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228 * should always pass in pdFALSE. Subsequent calls should pass in
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229 * the value returned from the previous call. See the file serial .c in the
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230 * PC port for a good example of using xSemaphoreGiveFromISR ().
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232 * @return pdTRUE if a task was woken by releasing the semaphore. This is
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233 * used by the ISR to determine if a context switch may be required following
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238 #define LONG_TIME 0xffff
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239 #define TICKS_TO_WAIT 10
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240 xSemaphoreHandle xSemaphore = NULL;
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242 // Repetitive task.
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243 void vATask( void * pvParameters )
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247 // We want this task to run every 10 ticks or a timer. The semaphore
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248 // was created before this task was started
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250 // Block waiting for the semaphore to become available.
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251 if( xSemaphoreTake( xSemaphore, LONG_TIME ) == pdTRUE )
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253 // It is time to execute.
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257 // We have finished our task. Return to the top of the loop where
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258 // we will block on the semaphore until it is time to execute
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265 void vTimerISR( void * pvParameters )
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267 static unsigned portCHAR ucLocalTickCount = 0;
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269 // A timer tick has occurred.
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271 // ... Do other time functions.
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273 // Is it time for vATask () to run?
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274 ucLocalTickCount++;
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275 if( ucLocalTickCount >= TICKS_TO_WAIT )
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277 // Unblock the task by releasing the semaphore.
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278 xSemaphoreGive( xSemaphore );
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280 // Reset the count so we release the semaphore again in 10 ticks time.
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281 ucLocalTickCount = 0;
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285 * \defgroup xSemaphoreGiveFromISR xSemaphoreGiveFromISR
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286 * \ingroup Semaphores
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288 #define xSemaphoreGiveFromISR( xSemaphore, xTaskPreviouslyWoken ) xQueueSendFromISR( ( xQueueHandle ) xSemaphore, NULL, xTaskPreviouslyWoken )
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