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[Queue Set API]

 QueueSetHandle_t xQueueCreateSet
                   const UBaseType_t uxEventQueueLength

configUSE_QUEUE_SETS must be set to 1 in FreeRTOSConfig.h for the xQueueCreateSet() API function to be available.

Queue sets provide a mechanism to allow an RTOS task to block (pend) on a read operation from multiple RTOS queues or semaphores simultaneously. Note that there are simpler alternatives to using queue sets. See the Blocking on Multiple Objects page for more information.

A queue set must be explicitly created using a call to xQueueCreateSet() before it can be used. Once created, standard FreeRTOS queues and semaphores can be added to the set using calls to xQueueAddToSet(). xQueueSelectFromSet() is then used to determine which, if any, of the queues or semaphores contained in the set is in a state where a queue read or semaphore take operation would be successful.


  • Queues and semaphores must be empty when they are added to a queue set. Take particular care when adding objects such as binary semaphores which are created with the semaphore already available [this is the case if the semaphore is created using the vSemaphoreCreateBinary() macro, but not the case if the semaphore is created using the preferred xSemaphoreCreateBinary() function].

  • Blocking on a queue set that contains a mutex will not cause the mutex holder to inherit the priority of the blocked task.

  • An additional 4 bytes of RAM are required for each space in every queue added to a queue set. Therefore a counting semaphore that has a high maximum count value should not be added to a queue set.

  • A receive (in the case of a queue) or take (in the case of a semaphore) operation must not be performed on a member of a queue set unless a call to xQueueSelectFromSet() has first returned a handle to that set member.

uxEventQueueLength   Queue sets store events that occur on the queues and semaphores contained in the set. uxEventQueueLength specifies the maximum number of events that can be queued at once.

To be absolutely certain that events are not lost uxEventQueueLength must be set to the sum of the lengths of the queues added to the set, where binary semaphores and mutexes have a length of 1, and counting semaphores have a length set by their maximum count value. For example:

  • If a queue set is to hold a queue of length 5, another queue of length 12, and a binary semaphore, then uxEventQueueLength should be set to (5 + 12 + 1), or 18.

  • If a queue set is to hold three binary semaphores then uxEventQueueLength should be set to (1 + 1 + 1 ), or 3.

  • If a queue set is to hold a counting semaphore that has a maximum count of 5, and a counting semaphore that has a maximum count of 3, then uxEventQueueLength should be set to (5 + 3), or 8.

If the queue set is created successfully then a handle to the created queue set is returned. Otherwise NULL is returned.
Example usage:
/* Define the lengths of the queues that will be added to the queue set. */
#define QUEUE_LENGTH_1		10
#define QUEUE_LENGTH_2		10

/* Binary semaphores have an effective length of 1. */

/* Define the size of the item to be held by queue 1 and queue 2 respectively.
The values used here are just for demonstration purposes. */
#define ITEM_SIZE_QUEUE_1	sizeof( uint32_t )
#define ITEM_SIZE_QUEUE_2	sizeof( something_else_t )

/* The combined length of the two queues and binary semaphore that will be
added to the queue set. */
                          QUEUE_LENGTH_2 +
                          BINARY_SEMAPHORE_LENGTH )

void vAFunction( void )
static QueueSetHandle_t xQueueSet;
QueueHandle_t xQueue1, xQueue2, xSemaphore;
QueueSetMemberHandle_t xActivatedMember;
uint32_t xReceivedFromQueue1;
something_else_t xReceivedFromQueue2;

    /* Create the queue set large enough to hold an event for every space in
    every queue and semaphore that is to be added to the set. */
    xQueueSet = xQueueCreateSet( COMBINED_LENGTH );

    /* Create the queues and semaphores that will be contained in the set. */
    xQueue1 = xQueueCreate( QUEUE_LENGTH_1, ITEM_SIZE_QUEUE_1 );
    xQueue2 = xQueueCreate( QUEUE_LENGTH_2, ITEM_SIZE_QUEUE_2 );

    /* Create the semaphore that is being added to the set. */
    xSemaphore = xSemaphoreCreateBinary();

    /* Check everything was created. */
    configASSERT( xQueueSet );
    configASSERT( xQueue1 );
    configASSERT( xQueue2 );
    configASSERT( xSemaphore );

    /* Add the queues and semaphores to the set.  Reading from these queues and
    semaphore can only be performed after a call to xQueueSelectFromSet() has
    returned the queue or semaphore handle from this point on. */
    xQueueAddToSet( xQueue1, xQueueSet );
    xQueueAddToSet( xQueue2, xQueueSet );
    xQueueAddToSet( xSemaphore, xQueueSet );

    for( ;; )
        /* Block to wait for something to be available from the queues or
        semaphore that have been added to the set.  Don't block longer than
        200ms. */
        xActivatedMember = xQueueSelectFromSet( xQueueSet,
                                                200 / portTICK_PERIOD_MS );

        /* Which set member was selected?  Receives/takes can use a block time
        of zero as they are guaranteed to pass because xQueueSelectFromSet()
        would not have returned the handle unless something was available. */
        if( xActivatedMember == xQueue1 )
            xQueueReceive( xActivatedMember, &xReceivedFromQueue1, 0 );
            vProcessValueFromQueue1( xReceivedFromQueue1 );
        else if( xActivatedMember == xQueue2 )
            xQueueReceive( xActivatedMember, &xReceivedFromQueue2, 0 );
            vProcessValueFromQueue2( &xReceivedFromQueue2 );
        else if( xActivatedMember == xSemaphore )
            /* Take the semaphore to make sure it can be "given" again. */
            xSemaphoreTake( xActivatedMember, 0 );
            /* The 200ms block time expired without an RTOS queue or semaphore
            being ready to process. */

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