RTOS Basics : Queue

Amruta (talk) 15:58, 14 April 2015 (IST)

Intro

A queue is a FIFO (First In First Out) type buffer where data is written to the end (tail) of the queue and removed from the front (head) of the queue. It is also possible to write to the front of a queue.

A queue can either hold the data or pointer to the data. In FreeRTOS, the data items are directly copied to the queue. Each data item is of fixed size. The size of data item and maximum number of data items are fixed when queue is created.

Queue can also be used as semaphore, mutex, event flag, etc. FreeRTOS does the same. It reduces memory usage in case of using multiple RTOS services e.g. semaphore and queue in same application.

Operations on queue

Create
Read

In read operation, the data item is returned. If queue is empty the requesting task will wait for specified time.If multiple tasks are waiting, then data item will be returned to the either highest priority task or the one which made the request first depending upon RTOS implementation.For the second case waiting list is maintained with each queue. In FreeRTOS, queue is implemented in FIRST way.

Write(Pend)

In write operation, the data item is directly copied to queue. If queue is full the requesting task will wait for specified time. For multiple tasks in waite state, process is same as read operation.

API Details

Creating a queue

Defination	xQueueHandle xQueueCreate ( unsigned portBASE_TYPE uxQueueLength, unsigned portBASE_TYPE uxItemSize)
Input Arguments	unsigned portBASE_TYPE : The maximum number of items that the queue can hold at a time. unsigned portBASE_TYPE : The size in bytes of data item to be stored in the queue.
Return Value	xQueueHandle : NULL when queue is not created Non NULL when queue is created.
Description	It creates queue.
Usage	xQueueHandle MyQueueHandleId; MyQueueHandleId = xQueueCreate(3,20);

Writting data to the queue

Defination	portBASE_TYPE xQueueSend ( xQueueHandle xQueue, const void * pvItemToQueue, portTickType xTicksToWait)
Description	It is equivalent to xQueueSendToBack().
Usage	char TxBuffer[MaxElementsPerQueue]={"Hello world"}; xQueueSend(MyQueueHandleId,TxBuffer,100)

Writting data at the front of queue

Defination	portBASE_TYPE xQueueSendToFront ( xQueueHandle xQueue, const void * pvItemToQueue, portTickType xTicksToWait)
Input Arguments	xQueueHandle : handle to the queue in which data is to be posted const void * : pointer to the item to be posted portTickType : the max amount of time for which task will wait for a space to become available on the queue if queue is full
Return Value	returns TRUE on successfully posting the item else FALSE
Description	It posts item at the back of the queue.
Usage	char TxBuffer[MaxElementsPerQueue]={"Hello world"}; xQueueSendToFront(MyQueueHandleId,TxBuffer,100)

Writting data at the back of queue

Defination	portBASE_TYPE xQueueSendToBack ( xQueueHandle xQueue, const void * pvItemToQueue, portTickType xTicksToWait)
Input Arguments	xQueueHandle : handle to the queue in which data is to be posted const void * : pointer to the item to be posted portTickType : the max amount of time for which task will wait for a space to become available on the queue if queue is full
Return Value	returns TRUE on successfully posting the item else FALSE
Description	It posts item at the front of the queue.
Usage	char TxBuffer[MaxElementsPerQueue]={"Hello world"}; xQueueSendToBack(MyQueueHandleId,TxBuffer,100)

Receiving data from the queue

Defination	portBASE_TYPE xQueueReceive ( xQueueHandle xQueue, void *pvBuffer, portTickType xTicksToWait)
Input Arguments	xQueueHandle : handle to the queue in which data write is desired. void * : pointer to the buffer where received item will be copied portTickType : the max amount of time for which task will wait for a item to become available on the queue if queue is empty
Return Value	returns TRUE on successfully posting the item else FALSE
Description	It reads data from the front of the queue.
Usage	char RxBuffer[MaxElementsPerQueue]; xQueueReceive(MyQueueHandleId,RxBuffer,100);