2 FreeRTOS.org V5.0.0 - Copyright (C) 2003-2008 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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18 Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
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26 ***************************************************************************
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27 ***************************************************************************
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32 * expedite your project. *
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34 ***************************************************************************
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35 ***************************************************************************
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37 Please ensure to read the configuration and relevant port sections of the
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38 online documentation.
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40 http://www.FreeRTOS.org - Documentation, latest information, license and
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43 http://www.SafeRTOS.com - A version that is certified for use in safety
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46 http://www.OpenRTOS.com - Commercial support, development, porting,
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47 licensing and training services.
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55 typedef xQueueHandle xSemaphoreHandle;
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57 #define semBINARY_SEMAPHORE_QUEUE_LENGTH ( ( unsigned portCHAR ) 1 )
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58 #define semSEMAPHORE_QUEUE_ITEM_LENGTH ( ( unsigned portCHAR ) 0 )
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59 #define semGIVE_BLOCK_TIME ( ( portTickType ) 0 )
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64 * <pre>vSemaphoreCreateBinary( xSemaphoreHandle xSemaphore )</pre>
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66 * <i>Macro</i> that implements a semaphore by using the existing queue mechanism.
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67 * The queue length is 1 as this is a binary semaphore. The data size is 0
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68 * as we don't want to actually store any data - we just want to know if the
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69 * queue is empty or full.
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71 * This type of semaphore can be used for pure synchronisation between tasks or
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72 * between an interrupt and a task. The semaphore need not be given back once
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73 * obtained, so one task/interrupt can continuously 'give' the semaphore while
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74 * another continuously 'takes' the semaphore. For this reason this type of
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75 * semaphore does not use a priority inheritance mechanism. For an alternative
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76 * that does use priority inheritance see xSemaphoreCreateMutex().
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78 * @param xSemaphore Handle to the created semaphore. Should be of type xSemaphoreHandle.
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82 xSemaphoreHandle xSemaphore;
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84 void vATask( void * pvParameters )
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86 // Semaphore cannot be used before a call to vSemaphoreCreateBinary ().
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87 // This is a macro so pass the variable in directly.
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88 vSemaphoreCreateBinary( xSemaphore );
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90 if( xSemaphore != NULL )
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92 // The semaphore was created successfully.
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93 // The semaphore can now be used.
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97 * \defgroup vSemaphoreCreateBinary vSemaphoreCreateBinary
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98 * \ingroup Semaphores
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100 #define vSemaphoreCreateBinary( xSemaphore ) { \
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101 xSemaphore = xQueueCreate( ( unsigned portBASE_TYPE ) 1, semSEMAPHORE_QUEUE_ITEM_LENGTH ); \
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102 if( xSemaphore != NULL ) \
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104 xSemaphoreGive( xSemaphore ); \
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111 * xSemaphoreHandle xSemaphore,
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112 * portTickType xBlockTime
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115 * <i>Macro</i> to obtain a semaphore. The semaphore must have previously been
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116 * created with a call to vSemaphoreCreateBinary(), xSemaphoreCreateMutex() or
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117 * xSemaphoreCreateCounting().
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119 * @param xSemaphore A handle to the semaphore being taken - obtained when
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120 * the semaphore was created.
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122 * @param xBlockTime The time in ticks to wait for the semaphore to become
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123 * available. The macro portTICK_RATE_MS can be used to convert this to a
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124 * real time. A block time of zero can be used to poll the semaphore. A block
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125 * time of portMAX_DELAY can be used to block indefinitely (provided
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126 * INCLUDE_vTaskSuspend is set to 1 in FreeRTOSConfig.h).
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128 * @return pdTRUE if the semaphore was obtained. pdFALSE
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129 * if xBlockTime expired without the semaphore becoming available.
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133 xSemaphoreHandle xSemaphore = NULL;
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135 // A task that creates a semaphore.
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136 void vATask( void * pvParameters )
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138 // Create the semaphore to guard a shared resource.
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139 vSemaphoreCreateBinary( xSemaphore );
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142 // A task that uses the semaphore.
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143 void vAnotherTask( void * pvParameters )
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145 // ... Do other things.
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147 if( xSemaphore != NULL )
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149 // See if we can obtain the semaphore. If the semaphore is not available
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150 // wait 10 ticks to see if it becomes free.
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151 if( xSemaphoreTake( xSemaphore, ( portTickType ) 10 ) == pdTRUE )
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153 // We were able to obtain the semaphore and can now access the
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154 // shared resource.
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158 // We have finished accessing the shared resource. Release the
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160 xSemaphoreGive( xSemaphore );
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164 // We could not obtain the semaphore and can therefore not access
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165 // the shared resource safely.
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170 * \defgroup xSemaphoreTake xSemaphoreTake
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171 * \ingroup Semaphores
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173 #define xSemaphoreTake( xSemaphore, xBlockTime ) xQueueGenericReceive( ( xQueueHandle ) xSemaphore, NULL, xBlockTime, pdFALSE )
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177 * xSemaphoreTakeRecursive(
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178 * xSemaphoreHandle xMutex,
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179 * portTickType xBlockTime
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182 * <i>Macro</i> to recursively obtain, or 'take', a mutex type semaphore.
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183 * The mutex must have previously been created using a call to
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184 * xSemaphoreCreateRecursiveMutex();
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186 * configUSE_RECURSIVE_MUTEXES must be set to 1 in FreeRTOSConfig.h for this
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187 * macro to be available.
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189 * This macro must not be used on mutexes created using xSemaphoreCreateMutex().
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191 * A mutex used recursively can be 'taken' repeatedly by the owner. The mutex
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192 * doesn't become available again until the owner has called
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193 * xSemaphoreGiveRecursive() for each successful 'take' request. For example,
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194 * if a task successfully 'takes' the same mutex 5 times then the mutex will
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195 * not be available to any other task until it has also 'given' the mutex back
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196 * exactly five times.
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198 * @param xMutex A handle to the mutex being obtained. This is the
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199 * handle returned by xSemaphoreCreateRecursiveMutex();
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201 * @param xBlockTime The time in ticks to wait for the semaphore to become
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202 * available. The macro portTICK_RATE_MS can be used to convert this to a
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203 * real time. A block time of zero can be used to poll the semaphore. If
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204 * the task already owns the semaphore then xSemaphoreTakeRecursive() will
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205 * return immediately no matter what the value of xBlockTime.
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207 * @return pdTRUE if the semaphore was obtained. pdFALSE if xBlockTime
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208 * expired without the semaphore becoming available.
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212 xSemaphoreHandle xMutex = NULL;
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214 // A task that creates a mutex.
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215 void vATask( void * pvParameters )
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217 // Create the mutex to guard a shared resource.
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218 xMutex = xSemaphoreCreateRecursiveMutex();
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221 // A task that uses the mutex.
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222 void vAnotherTask( void * pvParameters )
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224 // ... Do other things.
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226 if( xMutex != NULL )
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228 // See if we can obtain the mutex. If the mutex is not available
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229 // wait 10 ticks to see if it becomes free.
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230 if( xSemaphoreTakeRecursive( xSemaphore, ( portTickType ) 10 ) == pdTRUE )
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232 // We were able to obtain the mutex and can now access the
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233 // shared resource.
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236 // For some reason due to the nature of the code further calls to
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237 // xSemaphoreTakeRecursive() are made on the same mutex. In real
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238 // code these would not be just sequential calls as this would make
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239 // no sense. Instead the calls are likely to be buried inside
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240 // a more complex call structure.
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241 xSemaphoreTakeRecursive( xMutex, ( portTickType ) 10 );
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242 xSemaphoreTakeRecursive( xMutex, ( portTickType ) 10 );
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244 // The mutex has now been 'taken' three times, so will not be
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245 // available to another task until it has also been given back
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246 // three times. Again it is unlikely that real code would have
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247 // these calls sequentially, but instead buried in a more complex
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248 // call structure. This is just for illustrative purposes.
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249 xSemaphoreGiveRecursive( xMutex );
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250 xSemaphoreGiveRecursive( xMutex );
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251 xSemaphoreGiveRecursive( xMutex );
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253 // Now the mutex can be taken by other tasks.
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257 // We could not obtain the mutex and can therefore not access
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258 // the shared resource safely.
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263 * \defgroup xSemaphoreTakeRecursive xSemaphoreTakeRecursive
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264 * \ingroup Semaphores
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266 #define xSemaphoreTakeRecursive( xMutex, xBlockTime ) xQueueTakeMutexRecursive( xMutex, xBlockTime )
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270 * xSemaphoreAltTake() is an alternative version of xSemaphoreTake().
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272 * The source code that implements the alternative (Alt) API is much
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273 * simpler because it executes everything from within a critical section.
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274 * This is the approach taken by many other RTOSes, but FreeRTOS.org has the
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275 * preferred fully featured API too. The fully featured API has more
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276 * complex code that takes longer to execute, but makes much less use of
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277 * critical sections. Therefore the alternative API sacrifices interrupt
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278 * responsiveness to gain execution speed, whereas the fully featured API
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279 * sacrifices execution speed to ensure better interrupt responsiveness.
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281 #define xSemaphoreAltTake( xSemaphore, xBlockTime ) xQueueAltGenericReceive( ( xQueueHandle ) xSemaphore, NULL, xBlockTime, pdFALSE )
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285 * <pre>xSemaphoreGive( xSemaphoreHandle xSemaphore )</pre>
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287 * <i>Macro</i> to release a semaphore. The semaphore must have previously been
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288 * created with a call to vSemaphoreCreateBinary(), xSemaphoreCreateMutex() or
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289 * xSemaphoreCreateCounting(). and obtained using sSemaphoreTake().
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291 * This macro must not be used from an ISR. See xSemaphoreGiveFromISR () for
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292 * an alternative which can be used from an ISR.
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294 * This macro must also not be used on semaphores created using
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295 * xSemaphoreCreateRecursiveMutex().
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297 * @param xSemaphore A handle to the semaphore being released. This is the
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298 * handle returned when the semaphore was created.
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300 * @return pdTRUE if the semaphore was released. pdFALSE if an error occurred.
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301 * Semaphores are implemented using queues. An error can occur if there is
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302 * no space on the queue to post a message - indicating that the
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303 * semaphore was not first obtained correctly.
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307 xSemaphoreHandle xSemaphore = NULL;
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309 void vATask( void * pvParameters )
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311 // Create the semaphore to guard a shared resource.
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312 vSemaphoreCreateBinary( xSemaphore );
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314 if( xSemaphore != NULL )
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316 if( xSemaphoreGive( xSemaphore ) != pdTRUE )
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318 // We would expect this call to fail because we cannot give
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319 // a semaphore without first "taking" it!
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322 // Obtain the semaphore - don't block if the semaphore is not
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323 // immediately available.
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324 if( xSemaphoreTake( xSemaphore, ( portTickType ) 0 ) )
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326 // We now have the semaphore and can access the shared resource.
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330 // We have finished accessing the shared resource so can free the
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332 if( xSemaphoreGive( xSemaphore ) != pdTRUE )
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334 // We would not expect this call to fail because we must have
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335 // obtained the semaphore to get here.
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341 * \defgroup xSemaphoreGive xSemaphoreGive
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342 * \ingroup Semaphores
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344 #define xSemaphoreGive( xSemaphore ) xQueueGenericSend( ( xQueueHandle ) xSemaphore, NULL, semGIVE_BLOCK_TIME, queueSEND_TO_BACK )
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348 * <pre>xSemaphoreGiveRecursive( xSemaphoreHandle xMutex )</pre>
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350 * <i>Macro</i> to recursively release, or 'give', a mutex type semaphore.
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351 * The mutex must have previously been created using a call to
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352 * xSemaphoreCreateRecursiveMutex();
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354 * configUSE_RECURSIVE_MUTEXES must be set to 1 in FreeRTOSConfig.h for this
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355 * macro to be available.
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357 * This macro must not be used on mutexes created using xSemaphoreCreateMutex().
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359 * A mutex used recursively can be 'taken' repeatedly by the owner. The mutex
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360 * doesn't become available again until the owner has called
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361 * xSemaphoreGiveRecursive() for each successful 'take' request. For example,
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362 * if a task successfully 'takes' the same mutex 5 times then the mutex will
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363 * not be available to any other task until it has also 'given' the mutex back
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364 * exactly five times.
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366 * @param xMutex A handle to the mutex being released, or 'given'. This is the
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367 * handle returned by xSemaphoreCreateMutex();
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369 * @return pdTRUE if the semaphore was given.
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373 xSemaphoreHandle xMutex = NULL;
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375 // A task that creates a mutex.
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376 void vATask( void * pvParameters )
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378 // Create the mutex to guard a shared resource.
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379 xMutex = xSemaphoreCreateRecursiveMutex();
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382 // A task that uses the mutex.
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383 void vAnotherTask( void * pvParameters )
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385 // ... Do other things.
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387 if( xMutex != NULL )
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389 // See if we can obtain the mutex. If the mutex is not available
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390 // wait 10 ticks to see if it becomes free.
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391 if( xSemaphoreTakeRecursive( xMutex, ( portTickType ) 10 ) == pdTRUE )
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393 // We were able to obtain the mutex and can now access the
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394 // shared resource.
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397 // For some reason due to the nature of the code further calls to
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398 // xSemaphoreTakeRecursive() are made on the same mutex. In real
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399 // code these would not be just sequential calls as this would make
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400 // no sense. Instead the calls are likely to be buried inside
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401 // a more complex call structure.
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402 xSemaphoreTakeRecursive( xMutex, ( portTickType ) 10 );
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403 xSemaphoreTakeRecursive( xMutex, ( portTickType ) 10 );
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405 // The mutex has now been 'taken' three times, so will not be
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406 // available to another task until it has also been given back
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407 // three times. Again it is unlikely that real code would have
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408 // these calls sequentially, it would be more likely that the calls
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409 // to xSemaphoreGiveRecursive() would be called as a call stack
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410 // unwound. This is just for demonstrative purposes.
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411 xSemaphoreGiveRecursive( xMutex );
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412 xSemaphoreGiveRecursive( xMutex );
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413 xSemaphoreGiveRecursive( xMutex );
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415 // Now the mutex can be taken by other tasks.
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419 // We could not obtain the mutex and can therefore not access
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420 // the shared resource safely.
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425 * \defgroup xSemaphoreGiveRecursive xSemaphoreGiveRecursive
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426 * \ingroup Semaphores
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428 #define xSemaphoreGiveRecursive( xMutex ) xQueueGiveMutexRecursive( xMutex )
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431 * xSemaphoreAltGive() is an alternative version of xSemaphoreGive().
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433 * The source code that implements the alternative (Alt) API is much
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434 * simpler because it executes everything from within a critical section.
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435 * This is the approach taken by many other RTOSes, but FreeRTOS.org has the
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436 * preferred fully featured API too. The fully featured API has more
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437 * complex code that takes longer to execute, but makes much less use of
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438 * critical sections. Therefore the alternative API sacrifices interrupt
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439 * responsiveness to gain execution speed, whereas the fully featured API
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440 * sacrifices execution speed to ensure better interrupt responsiveness.
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442 #define xSemaphoreAltGive( xSemaphore ) xQueueAltGenericSend( ( xQueueHandle ) xSemaphore, NULL, semGIVE_BLOCK_TIME, queueSEND_TO_BACK )
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447 xSemaphoreGiveFromISR(
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448 xSemaphoreHandle xSemaphore,
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449 portBASE_TYPE *pxHigherPriorityTaskWoken
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452 * <i>Macro</i> to release a semaphore. The semaphore must have previously been
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453 * created with a call to vSemaphoreCreateBinary() or xSemaphoreCreateCounting().
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455 * Mutex type semaphores (those created using a call to xSemaphoreCreateMutex())
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456 * must not be used with this macro.
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458 * This macro can be used from an ISR.
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460 * @param xSemaphore A handle to the semaphore being released. This is the
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461 * handle returned when the semaphore was created.
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463 * @param pxHigherPriorityTaskWoken xSemaphoreGiveFromISR() will set
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464 * *pxHigherPriorityTaskWoken to pdTRUE if giving the semaphore caused a task
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465 * to unblock, and the unblocked task has a priority higher than the currently
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466 * running task. If xSemaphoreGiveFromISR() sets this value to pdTRUE then
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467 * a context switch should be requested before the interrupt is exited.
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469 * @return pdTRUE if the semaphore was successfully given, otherwise errQUEUE_FULL.
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473 #define LONG_TIME 0xffff
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474 #define TICKS_TO_WAIT 10
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475 xSemaphoreHandle xSemaphore = NULL;
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477 // Repetitive task.
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478 void vATask( void * pvParameters )
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482 // We want this task to run every 10 ticks of a timer. The semaphore
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483 // was created before this task was started.
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485 // Block waiting for the semaphore to become available.
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486 if( xSemaphoreTake( xSemaphore, LONG_TIME ) == pdTRUE )
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488 // It is time to execute.
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492 // We have finished our task. Return to the top of the loop where
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493 // we will block on the semaphore until it is time to execute
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494 // again. Note when using the semaphore for synchronisation with an
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495 // ISR in this manner there is no need to 'give' the semaphore back.
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501 void vTimerISR( void * pvParameters )
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503 static unsigned portCHAR ucLocalTickCount = 0;
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504 static portBASE_TYPE xHigherPriorityTaskWoken;
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506 // A timer tick has occurred.
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508 // ... Do other time functions.
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510 // Is it time for vATask () to run?
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511 xHigherPriorityTaskWoken = pdFALSE;
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512 ucLocalTickCount++;
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513 if( ucLocalTickCount >= TICKS_TO_WAIT )
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515 // Unblock the task by releasing the semaphore.
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516 xSemaphoreGiveFromISR( xSemaphore, &xHigherPriorityTaskWoken );
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518 // Reset the count so we release the semaphore again in 10 ticks time.
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519 ucLocalTickCount = 0;
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522 if( xHigherPriorityTaskWoken != pdFALSE )
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524 // We can force a context switch here. Context switching from an
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525 // ISR uses port specific syntax. Check the demo task for your port
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526 // to find the syntax required.
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530 * \defgroup xSemaphoreGiveFromISR xSemaphoreGiveFromISR
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531 * \ingroup Semaphores
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533 #define xSemaphoreGiveFromISR( xSemaphore, pxHigherPriorityTaskWoken ) xQueueGenericSendFromISR( ( xQueueHandle ) xSemaphore, NULL, pxHigherPriorityTaskWoken, queueSEND_TO_BACK )
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537 * <pre>xSemaphoreHandle xSemaphoreCreateMutex( void )</pre>
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539 * <i>Macro</i> that implements a mutex semaphore by using the existing queue
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542 * Mutexes created using this macro can be accessed using the xSemaphoreTake()
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543 * and xSemaphoreGive() macros. The xSemaphoreTakeRecursive() and
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544 * xSemaphoreGiveRecursive() macros should not be used.
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546 * This type of semaphore uses a priority inheritance mechanism so a task
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547 * 'taking' a semaphore MUST ALWAYS 'give' the semaphore back once the
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548 * semaphore it is no longer required.
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550 * Mutex type semaphores cannot be used from within interrupt service routines.
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552 * See xSemaphoreCreateBinary() for an alternative implementation that can be
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553 * used for pure synchronisation (where one task or interrupt always 'gives' the
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554 * semaphore and another always 'takes' the semaphore) and from within interrupt
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555 * service routines.
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557 * @return xSemaphore Handle to the created mutex semaphore. Should be of type
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558 * xSemaphoreHandle.
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562 xSemaphoreHandle xSemaphore;
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564 void vATask( void * pvParameters )
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566 // Semaphore cannot be used before a call to xSemaphoreCreateMutex().
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567 // This is a macro so pass the variable in directly.
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568 xSemaphore = xSemaphoreCreateMutex();
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570 if( xSemaphore != NULL )
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572 // The semaphore was created successfully.
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573 // The semaphore can now be used.
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577 * \defgroup vSemaphoreCreateMutex vSemaphoreCreateMutex
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578 * \ingroup Semaphores
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580 #define xSemaphoreCreateMutex() xQueueCreateMutex()
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585 * <pre>xSemaphoreHandle xSemaphoreCreateRecursiveMutex( void )</pre>
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587 * <i>Macro</i> that implements a recursive mutex by using the existing queue
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590 * Mutexes created using this macro can be accessed using the
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591 * xSemaphoreTakeRecursive() and xSemaphoreGiveRecursive() macros. The
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592 * xSemaphoreTake() and xSemaphoreGive() macros should not be used.
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594 * A mutex used recursively can be 'taken' repeatedly by the owner. The mutex
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595 * doesn't become available again until the owner has called
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596 * xSemaphoreGiveRecursive() for each successful 'take' request. For example,
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597 * if a task successfully 'takes' the same mutex 5 times then the mutex will
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598 * not be available to any other task until it has also 'given' the mutex back
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599 * exactly five times.
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601 * This type of semaphore uses a priority inheritance mechanism so a task
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602 * 'taking' a semaphore MUST ALWAYS 'give' the semaphore back once the
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603 * semaphore it is no longer required.
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605 * Mutex type semaphores cannot be used from within interrupt service routines.
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607 * See xSemaphoreCreateBinary() for an alternative implementation that can be
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608 * used for pure synchronisation (where one task or interrupt always 'gives' the
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609 * semaphore and another always 'takes' the semaphore) and from within interrupt
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610 * service routines.
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612 * @return xSemaphore Handle to the created mutex semaphore. Should be of type
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613 * xSemaphoreHandle.
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617 xSemaphoreHandle xSemaphore;
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619 void vATask( void * pvParameters )
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621 // Semaphore cannot be used before a call to xSemaphoreCreateMutex().
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622 // This is a macro so pass the variable in directly.
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623 xSemaphore = xSemaphoreCreateRecursiveMutex();
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625 if( xSemaphore != NULL )
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627 // The semaphore was created successfully.
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628 // The semaphore can now be used.
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632 * \defgroup vSemaphoreCreateMutex vSemaphoreCreateMutex
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633 * \ingroup Semaphores
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635 #define xSemaphoreCreateRecursiveMutex() xQueueCreateMutex()
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639 * <pre>xSemaphoreHandle xSemaphoreCreateCounting( unsigned portBASE_TYPE uxMaxCount, unsigned portBASE_TYPE uxInitialCount )</pre>
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641 * <i>Macro</i> that creates a counting semaphore by using the existing
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642 * queue mechanism.
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644 * Counting semaphores are typically used for two things:
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646 * 1) Counting events.
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648 * In this usage scenario an event handler will 'give' a semaphore each time
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649 * an event occurs (incrementing the semaphore count value), and a handler
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650 * task will 'take' a semaphore each time it processes an event
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651 * (decrementing the semaphore count value). The count value is therefore
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652 * the difference between the number of events that have occurred and the
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653 * number that have been processed. In this case it is desirable for the
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654 * initial count value to be zero.
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656 * 2) Resource management.
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658 * In this usage scenario the count value indicates the number of resources
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659 * available. To obtain control of a resource a task must first obtain a
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660 * semaphore - decrementing the semaphore count value. When the count value
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661 * reaches zero there are no free resources. When a task finishes with the
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662 * resource it 'gives' the semaphore back - incrementing the semaphore count
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663 * value. In this case it is desirable for the initial count value to be
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664 * equal to the maximum count value, indicating that all resources are free.
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666 * @param uxMaxCount The maximum count value that can be reached. When the
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667 * semaphore reaches this value it can no longer be 'given'.
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669 * @param uxInitialCount The count value assigned to the semaphore when it is
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672 * @return Handle to the created semaphore. Null if the semaphore could not be
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677 xSemaphoreHandle xSemaphore;
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679 void vATask( void * pvParameters )
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681 xSemaphoreHandle xSemaphore = NULL;
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683 // Semaphore cannot be used before a call to xSemaphoreCreateCounting().
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684 // The max value to which the semaphore can count should be 10, and the
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685 // initial value assigned to the count should be 0.
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686 xSemaphore = xSemaphoreCreateCounting( 10, 0 );
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688 if( xSemaphore != NULL )
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690 // The semaphore was created successfully.
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691 // The semaphore can now be used.
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695 * \defgroup xSemaphoreCreateCounting xSemaphoreCreateCounting
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696 * \ingroup Semaphores
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698 #define xSemaphoreCreateCounting( uxMaxCount, uxInitialCount ) xQueueCreateCountingSemaphore( uxMaxCount, uxInitialCount )
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701 #endif /* SEMAPHORE_H */
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