queue.c

一个开源的Modbus协议栈

				As detailed above we do not require mutual exclusion on the event
				list as nothing else can modify it or the ready lists while we
				have the scheduler suspended and queue locked.
				
				It is possible that an ISR has removed data from the queue since we
				checked if any was available.  If this is the case then the data
				will have been copied from the queue, and the queue variables
				updated, but the event list will not yet have been checked to see if
				anything is waiting as the queue is locked. */
				vTaskPlaceOnEventList( &( pxQueue->xTasksWaitingToSend ), xTicksToWait );
	
				/* Force a context switch now as we are blocked.  We can do
				this from within a critical section as the task we are
				switching to has its own context.  When we return here (i.e. we
				unblock) we will leave the critical section as normal.
				
				It is possible that an ISR has caused an event on an unrelated and
				unlocked queue.  If this was the case then the event list for that
				queue will have been updated but the ready lists left unchanged -
				instead the readied task will have been added to the pending ready
				list. */
				taskENTER_CRITICAL();
				{
					/* We can safely unlock the queue and scheduler here as
					interrupts are disabled.  We must not yield with anything
					locked, but we can yield from within a critical section.
					
					Tasks that have been placed on the pending ready list cannot
					be tasks that are waiting for events on this queue.  See
					in comment xTaskRemoveFromEventList(). */
					prvUnlockQueue( pxQueue );
	
					/* Resuming the scheduler may cause a yield.  If so then there
					is no point yielding again here. */
					if( !xTaskResumeAll() )
					{
						taskYIELD();
					}

					/* We want to check to see if the queue is still full
					before leaving the critical section.  This is to prevent
					this task placing an item into the queue due to an
					interrupt making space on the queue between critical
					sections (when there might be a higher priority task
					blocked on the queue that cannot run yet because the
					scheduler gets suspended). */
					if( pxQueue->uxMessagesWaiting == pxQueue->uxLength )
					{
						/* We unblocked but there is no space in the queue,
						we probably timed out. */
						xReturn = errQUEUE_FULL;
					}
	
					/* Before leaving the critical section we have to ensure
					exclusive access again. */
					vTaskSuspendAll();
					prvLockQueue( pxQueue );				
				}
				taskEXIT_CRITICAL();
			}
		}
			
		/* If xReturn is errQUEUE_FULL then we unblocked when the queue
		was still full.  Don't check it again now as it is possible that
		an interrupt has removed an item from the queue since we left the
		critical section and we don't want to write to the queue in case
		there is a task of higher priority blocked waiting for space to
		be available on the queue.  If this is the case the higher priority
		task will execute when the scheduler is unsupended. */
		if( xReturn != errQUEUE_FULL )
		{
			/* When we are here it is possible that we unblocked as space became
			available on the queue.  It is also possible that an ISR posted to the
			queue since we left the critical section, so it may be that again there
			is no space.  This would only happen if a task and ISR post onto the
			same queue. */
			taskENTER_CRITICAL();
			{
				if( pxQueue->uxMessagesWaiting < pxQueue->uxLength )
				{
					/* There is room in the queue, copy the data into the queue. */			
					prvCopyQueueData( pxQueue, pvItemToQueue );		
					xReturn = pdPASS;
		
					/* Update the TxLock count so prvUnlockQueue knows to check for
					tasks waiting for data to become available in the queue. */
					++( pxQueue->xTxLock );
				}
				else
				{
					xReturn = errQUEUE_FULL;
				}
			}
			taskEXIT_CRITICAL();
		}

		if( xReturn == errQUEUE_FULL )
		{
			if( xTicksToWait > 0 )
			{
				if( xTaskCheckForTimeOut( &xTimeOut, &xTicksToWait ) == pdFALSE )
				{
					xReturn = queueERRONEOUS_UNBLOCK;
				}
			}
		}
	}
	while( xReturn == queueERRONEOUS_UNBLOCK );

	prvUnlockQueue( pxQueue );
	xTaskResumeAll();

	return xReturn;
}
/*-----------------------------------------------------------*/

signed portBASE_TYPE xQueueSendFromISR( xQueueHandle pxQueue, const void *pvItemToQueue, signed portBASE_TYPE xTaskPreviouslyWoken )
{
	/* Similar to xQueueSend, except we don't block if there is no room in the
	queue.  Also we don't directly wake a task that was blocked on a queue
	read, instead we return a flag to say whether a context switch is required
	or not (i.e. has a task with a higher priority than us been woken by this
	post). */
	if( pxQueue->uxMessagesWaiting < pxQueue->uxLength )
	{
		prvCopyQueueData( pxQueue, pvItemToQueue );

		/* If the queue is locked we do not alter the event list.  This will
		be done when the queue is unlocked later. */
		if( pxQueue->xTxLock == queueUNLOCKED )
		{
			/* We only want to wake one task per ISR, so check that a task has
			not already been woken. */
			if( !xTaskPreviouslyWoken )		
			{
				if( !listLIST_IS_EMPTY( &( pxQueue->xTasksWaitingToReceive ) ) )
				{
					if( xTaskRemoveFromEventList( &( pxQueue->xTasksWaitingToReceive ) ) != pdFALSE )
					{
						/* The task waiting has a higher priority so record that a
						context	switch is required. */
						return pdTRUE;
					}
				}
			}
		}
		else
		{
			/* Increment the lock count so the task that unlocks the queue
			knows that data was posted while it was locked. */
			++( pxQueue->xTxLock );
		}
	}

	return xTaskPreviouslyWoken;
}
/*-----------------------------------------------------------*/

signed portBASE_TYPE xQueueReceive( xQueueHandle pxQueue, void *pvBuffer, portTickType xTicksToWait )
{
signed portBASE_TYPE xReturn = pdTRUE;
xTimeOutType xTimeOut;

	/* This function is very similar to xQueueSend().  See comments within
	xQueueSend() for a more detailed explanation.

	Make sure other tasks do not access the queue. */
	vTaskSuspendAll();

	/* Capture the current time status for future reference. */
	vTaskSetTimeOutState( &xTimeOut );

	/* Make sure interrupts do not access the queue. */
	prvLockQueue( pxQueue );

	do
	{
		/* If there are no messages in the queue we may have to block. */
		if( prvIsQueueEmpty( pxQueue ) )
		{
			/* There are no messages in the queue, do we want to block or just
			leave with nothing? */			
			if( xTicksToWait > ( portTickType ) 0 )
			{
				vTaskPlaceOnEventList( &( pxQueue->xTasksWaitingToReceive ), xTicksToWait );
				taskENTER_CRITICAL();
				{
					prvUnlockQueue( pxQueue );
					if( !xTaskResumeAll() )
					{
						taskYIELD();
					}

					if( pxQueue->uxMessagesWaiting == ( unsigned portBASE_TYPE ) 0 )
					{
						/* We unblocked but the queue is empty.  We probably
						timed out. */
						xReturn = errQUEUE_EMPTY;
					}
	
					vTaskSuspendAll();
					prvLockQueue( pxQueue );
				}
				taskEXIT_CRITICAL();
			}
		}
	
		if( xReturn != errQUEUE_EMPTY )
		{
			taskENTER_CRITICAL();
			{
				if( pxQueue->uxMessagesWaiting > ( unsigned portBASE_TYPE ) 0 )
				{
					pxQueue->pcReadFrom += pxQueue->uxItemSize;
					if( pxQueue->pcReadFrom >= pxQueue->pcTail )
					{
						pxQueue->pcReadFrom = pxQueue->pcHead;
					}
					--( pxQueue->uxMessagesWaiting );
					memcpy( ( void * ) pvBuffer, ( void * ) pxQueue->pcReadFrom, ( unsigned ) pxQueue->uxItemSize );
		
					/* Increment the lock count so prvUnlockQueue knows to check for
					tasks waiting for space to become available on the queue. */
					++( pxQueue->xRxLock );
					xReturn = pdPASS;
				}
				else
				{
					xReturn = errQUEUE_EMPTY;
				}
			}
			taskEXIT_CRITICAL();
		}

		if( xReturn == errQUEUE_EMPTY )
		{
			if( xTicksToWait > 0 )
			{
				if( xTaskCheckForTimeOut( &xTimeOut, &xTicksToWait ) == pdFALSE )
				{
					xReturn = queueERRONEOUS_UNBLOCK;
				}
			}
		}
	} while( xReturn == queueERRONEOUS_UNBLOCK );

	/* We no longer require exclusive access to the queue. */
	prvUnlockQueue( pxQueue );
	xTaskResumeAll();

	return xReturn;
}
/*-----------------------------------------------------------*/

signed portBASE_TYPE xQueueReceiveFromISR( xQueueHandle pxQueue, void *pvBuffer, signed portBASE_TYPE *pxTaskWoken )
{
signed portBASE_TYPE xReturn;

	/* We cannot block from an ISR, so check there is data available. */
	if( pxQueue->uxMessagesWaiting > ( unsigned portBASE_TYPE ) 0 )
	{
		/* Copy the data from the queue. */
		pxQueue->pcReadFrom += pxQueue->uxItemSize;
		if( pxQueue->pcReadFrom >= pxQueue->pcTail )
		{
			pxQueue->pcReadFrom = pxQueue->pcHead;
		}
		--( pxQueue->uxMessagesWaiting );
		memcpy( ( void * ) pvBuffer, ( void * ) pxQueue->pcReadFrom, ( unsigned ) pxQueue->uxItemSize );

		/* If the queue is locked we will not modify the event list.  Instead
		we update the lock count so the task that unlocks the queue will know
		that an ISR has removed data while the queue was locked. */
		if( pxQueue->xRxLock == queueUNLOCKED )
		{
			/* We only want to wake one task per ISR, so check that a task has
			not already been woken. */
			if( !( *pxTaskWoken ) )
			{
				if( !listLIST_IS_EMPTY( &( pxQueue->xTasksWaitingToSend ) ) )
				{
					if( xTaskRemoveFromEventList( &( pxQueue->xTasksWaitingToSend ) ) != pdFALSE )
					{
						/* The task waiting has a higher priority than us so
						force a context switch. */
						*pxTaskWoken = pdTRUE;
					}
				}
			}
		}
		else
		{
			/* Increment the lock count so the task that unlocks the queue
			knows that data was removed while it was locked. */
			++( pxQueue->xRxLock );
		}

		xReturn = pdPASS;
	}
	else
	{
		xReturn = pdFAIL;
	}

	return xReturn;
}
/*-----------------------------------------------------------*/

unsigned portBASE_TYPE uxQueueMessagesWaiting( xQueueHandle pxQueue )
{
unsigned portBASE_TYPE uxReturn;