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基础知识

[FreeRTOS 基础知识] 信号量 概念

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创建信号量

#define queueQUEUE_TYPE_BINARY_SEMAPHORE    ( ( uint8_t ) 3U )
#define semSEMAPHORE_QUEUE_ITEM_LENGTH    ( ( uint8_t ) 0U )
#define xSemaphoreCreateBinary() xQueueGenericCreate( ( UBaseType_t ) 1, semSEMAPHORE_QUEUE_ITEM_LENGTH, queueQUEUE_TYPE_BINARY_SEMAPHORE )

根据上面的定义，可知创建信号量本质就是创建队列。长度（第一个参数）为1，大小（第二个参数）为0

QueueHandle_t xQueueGenericCreate( const UBaseType_t uxQueueLength, const UBaseType_t uxItemSize, const uint8_t ucQueueType )->     if( uxItemSize == ( UBaseType_t ) 0 ) xQueueSizeInBytes = ( size_t ) 0;->     pxNewQueue = ( Queue_t * ) pvPortMalloc( sizeof( Queue_t ) + xQueueSizeInBytes );  // 动态分配大小，只有一个Queue_t结构体大小

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获取信号量

通过队列获取信号量

#define xSemaphoreTake( xSemaphore, xBlockTime )        xQueueSemaphoreTake( ( xSemaphore ), ( xBlockTime ) )

BaseType_t xQueueSemaphoreTake( QueueHandle_t xQueue, TickType_t xTicksToWait )for( ;; )
{taskENTER_CRITICAL();        // 关中断{const UBaseType_t uxSemaphoreCount = pxQueue->uxMessagesWaiting;    // 获取当前信号量值if( uxSemaphoreCount > ( UBaseType_t ) 0 )                            // 信号量值大于0{traceQUEUE_RECEIVE( pxQueue );pxQueue->uxMessagesWaiting = uxSemaphoreCount - ( UBaseType_t ) 1;    // 信号量-1// 检查是否有其他任务阻塞等待信号量。if( listLIST_IS_EMPTY( &( pxQueue->xTasksWaitingToSend ) ) == pdFALSE ){// 如果是，解除阻塞最高优先级的任务。// 1、将最高阻塞任务从xTasksWaitingToSend链表中移除；// 2、将最高阻塞任务从从DelayList移动到ReadyList链表if( xTaskRemoveFromEventList( &( pxQueue->xTasksWaitingToSend ) ) != pdFALSE ){queueYIELD_IF_USING_PREEMPTION();   // 让出CPU使用权}else{mtCOVERAGE_TEST_MARKER();}}else{mtCOVERAGE_TEST_MARKER();}taskEXIT_CRITICAL();   //开中断return pdPASS;        // 返回成功}else{if( xTicksToWait == ( TickType_t ) 0 ) // 是否愿意等待{// 不愿意等待taskEXIT_CRITICAL();   // 打开中断traceQUEUE_RECEIVE_FAILED( pxQueue );return errQUEUE_EMPTY;   // 返回队列空失败}else if( xEntryTimeSet == pdFALSE ){// 信号量计数为0，阻塞时间被指定，所以配置超时结构准备阻塞。vTaskInternalSetTimeOutState( &xTimeOut );xEntryTimeSet = pdTRUE;}else{mtCOVERAGE_TEST_MARKER();}}}taskEXIT_CRITICAL();vTaskSuspendAll();prvLockQueue( pxQueue );if( xTaskCheckForTimeOut( &xTimeOut, &xTicksToWait ) == pdFALSE ){if( prvIsQueueEmpty( pxQueue ) != pdFALSE ){traceBLOCKING_ON_QUEUE_RECEIVE( pxQueue );// 1、当前的任务加入到队列的xTasksWaitingToReceive链表中；// 2、当前的任务从ReadyList移动到DelayListvTaskPlaceOnEventList( &( pxQueue->xTasksWaitingToReceive ), xTicksToWait );prvUnlockQueue( pxQueue );if( xTaskResumeAll() == pdFALSE ){portYIELD_WITHIN_API();}else{mtCOVERAGE_TEST_MARKER();}}        else{prvUnlockQueue( pxQueue );( void ) xTaskResumeAll();}}else{...}

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释放信号量

#define semGIVE_BLOCK_TIME                    ( ( TickType_t ) 0U )
#define xSemaphoreGive( xSemaphore )        xQueueGenericSend( ( QueueHandle_t ) ( xSemaphore ), NULL, semGIVE_BLOCK_TIME, queueSEND_TO_BACK )

BaseType_t xQueueGenericSend( QueueHandle_t xQueue, const void * const pvItemToQueue, TickType_t xTicksToWait, const BaseType_t xCopyPosition )
-> for( ;; ){taskENTER_CRITICAL();    // 关中断 portDISABLE_INTERRUPTS();{if( ( pxQueue->uxMessagesWaiting < pxQueue->uxLength ) || ( xCopyPosition == queueOVERWRITE ) )    // 判断当前信号量是否超过队列信号量长度{        traceQUEUE_SEND( pxQueue );xYieldRequired = prvCopyDataToQueue( pxQueue, pvItemToQueue, xCopyPosition );// 不会写数据，但是会将 uxMessagesWaiting +1if( listLIST_IS_EMPTY( &( pxQueue->xTasksWaitingToReceive ) ) == pdFALSE )          // 判断xTasksWaitingToReceive队列里是否有等待的任务{if( xTaskRemoveFromEventList( &( pxQueue->xTasksWaitingToReceive ) ) != pdFALSE ) // 1、将要写的任务从xTasksWaitingToReceive移除；2、将要写的任务从DelayList移动到ReadyList{queueYIELD_IF_USING_PREEMPTION();    //让出CPU使用权}else{mtCOVERAGE_TEST_MARKER();}}                                else if( xYieldRequired != pdFALSE ){queueYIELD_IF_USING_PREEMPTION();}else{mtCOVERAGE_TEST_MARKER();}}taskEXIT_CRITICAL();    // 开中断return pdPASS;          // 返回成功}else{if( xTicksToWait == ( TickType_t ) 0 ){taskEXIT_CRITICAL();traceQUEUE_SEND_FAILED( pxQueue );return errQUEUE_FULL;         // 返回队列已满}else if( xEntryTimeSet == pdFALSE ){vTaskInternalSetTimeOutState( &xTimeOut ); // 阻塞时间被指定，所以配置超时结构。 xEntryTimeSet = pdTRUE;}else{mtCOVERAGE_TEST_MARKER();}}
}
taskEXIT_CRITICAL();    // 开中断vTaskSuspendAll();
prvLockQueue( pxQueue );if( xTaskCheckForTimeOut( &xTimeOut, &xTicksToWait ) == pdFALSE )
{if( prvIsQueueFull( pxQueue ) != pdFALSE ){traceBLOCKING_ON_QUEUE_SEND( pxQueue );// 1、当前的任务加入到队列的xTasksWaitingToSend链表中；// 2、当前的任务从ReadyList移动到DelayList                vTaskPlaceOnEventList( &( pxQueue->xTasksWaitingToSend ), xTicksToWait );prvUnlockQueue( pxQueue );if( xTaskResumeAll() == pdFALSE ){portYIELD_WITHIN_API();}}else{prvUnlockQueue( pxQueue );( void ) xTaskResumeAll();}
}
else
{prvUnlockQueue( pxQueue );( void ) xTaskResumeAll();traceQUEUE_SEND_FAILED( pxQueue );return errQUEUE_FULL;
}
}

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信号量 内部实现框图