与队列的发送对应，一般的队列接收函数也是由宏来实现，底层用一个函数来实现，队列接收函数有下面两个：
#define xQueuePeek( xQueue, pvBuffer, xTicksToWait ) xQueueGenericReceive( xQueue, pvBuffer, xTicksToWait, pdTRUE )
#define xQueueReceive( xQueue, pvBuffer, xTicksToWait ) xQueueGenericReceive( xQueue, pvBuffer, xTicksToWait, pdFALSE )
xQueuePeek与xQueueReceive的区别在于前者只接收而不删除条目，后者在成功接收后会删除原来的条目，两者均通过xQueueGenericReceive函数来实现，不能在中断中调用。
下面分析xQueueGenericReceive的实现方式：
signed portBASE_TYPE
xQueueGenericReceive( xQueueHandle pxQueue, void * const pvBuffer,
portTickType xTicksToWait, portBASE_TYPE xJustPeeking )
{
signed portBASE_TYPE xEntryTimeSet = pdFALSE;
xTimeOutType xTimeOut;
signed portCHAR *pcOriginalReadPosition;
/* This function relaxes the coding standard somewhat to allow return
statements within the function itself. This is done in the interest
of execution time efficiency. */
for( ;; )
{
taskENTER_CRITICAL();
{
/* Is there space on the queue now? To be running we must be
the highest priority task wanting to access the queue. */
if( pxQueue->uxMessagesWaiting > ( unsigned portBASE_TYPE ) 0 ) /*队列非空*/
{
/* Remember our read position in case we are just peeking. */
pcOriginalReadPosition = pxQueue->pcReadFrom;
prvCopyDataFromQueue( pxQueue, pvBuffer );
if( xJustPeeking == pdFALSE ) /*复制完成后删除条目*/
{
traceQUEUE_RECEIVE( pxQueue );
/* We are actually removing data. */
--( pxQueue->uxMessagesWaiting );
#if ( configUSE_MUTEXES == 1 )
{
if( pxQueue->uxQueueType == queueQUEUE_IS_MUTEX ) /*与互斥锁相关的操作，改变互斥锁的拥有者为当前任务*/
{
/* Record the information required to implement
priority inheritance should it become necessary. */
pxQueue->pxMutexHolder = xTaskGetCurrentTaskHandle();
}
}
#endif
if( listLIST_IS_EMPTY( &( pxQueue->xTasksWaitingToSend ) ) == pdFALSE ) /*如果过有条目在等待发送，则移出等待队列并进行切换*/
{
if( xTaskRemoveFromEventList( &( pxQueue->xTasksWaitingToSend ) ) == pdTRUE )
{
taskYIELD();
}
}
}
else /*不删除条目*/
{
traceQUEUE_PEEK( pxQueue );
/* We are not removing the data, so reset our read
pointer. */
pxQueue->pcReadFrom = pcOriginalReadPosition;
/* The data is being left in the queue, so see if there are
any other tasks waiting for the data. */
if( !listLIST_IS_EMPTY( &( pxQueue->xTasksWaitingToReceive ) ) )
{
/* Tasks that are removed from the event list will get added to
the pending ready list as the scheduler is still suspended. */
if( xTaskRemoveFromEventList( &( pxQueue->xTasksWaitingToReceive ) ) != pdFALSE )
{
/* The task waiting has a higher priority than this task. */
taskYIELD();
}
}
}
taskEXIT_CRITICAL();
return pdPASS;
}
else /*条目为空，如果xTicksToWait>0则插入到等待队列，否则直接返回，下面的操作跟队列发送时类似*/
{
if( xTicksToWait == ( portTickType ) 0 )
{
/* The queue was empty and no block time is specified (or
the block time has expired) so leave now. */
taskEXIT_CRITICAL();
traceQUEUE_RECEIVE_FAILED( pxQueue );
return errQUEUE_EMPTY;
}
else if( xEntryTimeSet == pdFALSE )
{
/* The queue was empty and a block time was specified so
configure the timeout structure. */
vTaskSetTimeOutState( &xTimeOut );
xEntryTimeSet = pdTRUE;
}
}
}
taskEXIT_CRITICAL();
/* Interrupts and other tasks can send to and receive from the queue
now the critical section has been exited. */
vTaskSuspendAll();
prvLockQueue( pxQueue );
/* Update the timeout state to see if it has expired yet. */
if( xTaskCheckForTimeOut( &xTimeOut, &xTicksToWait ) == pdFALSE )
{
if( prvIsQueueEmpty( pxQueue ) )
{
traceBLOCKING_ON_QUEUE_RECEIVE( pxQueue );
#if ( configUSE_MUTEXES == 1 )
{
if( pxQueue->uxQueueType == queueQUEUE_IS_MUTEX )
{
portENTER_CRITICAL();
{
vTaskPriorityInherit( ( void * ) pxQueue->pxMutexHolder );
}
portEXIT_CRITICAL();
}
}
#endif
vTaskPlaceOnEventList( &( pxQueue->xTasksWaitingToReceive ), xTicksToWait );
prvUnlockQueue( pxQueue );
if( !xTaskResumeAll() )
{
taskYIELD();
}
}
else
{
/* Try again. */
prvUnlockQueue( pxQueue );
( void ) xTaskResumeAll();
}
}
else
{
prvUnlockQueue( pxQueue );
( void ) xTaskResumeAll();
traceQUEUE_RECEIVE_FAILED( pxQueue );
return errQUEUE_EMPTY;
}
}
}
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用户394669 2009-6-20 22:39
用户1361860 2009-6-20 18:27