14.3. Operations

14.3.1. Creating a Message Queue

The rtems_message_queue_create directive creates a message queue with the user-defined name. The user specifies the maximum message size and maximum number of messages which can be placed in the message queue at one time. The user may select FIFO or task priority as the method for placing waiting tasks in the task wait queue. RTEMS allocates a Queue Control Block (QCB) from the QCB free list to maintain the newly created queue as well as memory for the message buffer pool associated with this message queue. RTEMS also generates a message queue ID which is returned to the calling task.

For GLOBAL message queues, the maximum message size is effectively limited to the longest message which the MPCI is capable of transmitting.

14.3.2. Obtaining Message Queue IDs

When a message queue is created, RTEMS generates a unique message queue ID. The message queue ID may be obtained by either of two methods. First, as the result of an invocation of the rtems_message_queue_create directive, the queue ID is stored in a user provided location. Second, the queue ID may be obtained later using the rtems_message_queue_ident directive. The queue ID is used by other message manager directives to access this message queue.

14.3.3. Receiving a Message

The rtems_message_queue_receive directive attempts to retrieve a message from the specified message queue. If at least one message is in the queue, then the message is removed from the queue, copied to the caller’s message buffer, and returned immediately along with the length of the message. When messages are unavailable, one of the following situations applies:

  • By default, the calling task will wait forever for the message to arrive.

  • Specifying the RTEMS_NO_WAIT option forces an immediate return with an error status code.

  • Specifying a timeout limits the period the task will wait before returning with an error status.

If the task waits for a message, then it is placed in the message queue’s task wait queue in either FIFO or task priority order. All tasks waiting on a message queue are returned an error code when the message queue is deleted.

14.3.4. Sending a Message

Messages can be sent to a queue with the rtems_message_queue_send and rtems_message_queue_urgent directives. These directives work identically when tasks are waiting to receive a message. A task is removed from the task waiting queue, unblocked, and the message is copied to a waiting task’s message buffer.

When no tasks are waiting at the queue, rtems_message_queue_send places the message at the rear of the message queue, while rtems_message_queue_urgent places the message at the front of the queue. The message is copied to a message buffer from this message queue’s buffer pool and then placed in the message queue. Neither directive can successfully send a message to a message queue which has a full queue of pending messages.

14.3.5. Broadcasting a Message

The rtems_message_queue_broadcast directive sends the same message to every task waiting on the specified message queue as an atomic operation. The message is copied to each waiting task’s message buffer and each task is unblocked. The number of tasks which were unblocked is returned to the caller.

14.3.6. Deleting a Message Queue

The rtems_message_queue_delete directive removes a message queue from the system and frees its control block as well as the memory associated with this message queue’s message buffer pool. A message queue can be deleted by any local task that knows the message queue’s ID. As a result of this directive, all tasks blocked waiting to receive a message from the message queue will be readied and returned a status code which indicates that the message queue was deleted. Any subsequent references to the message queue’s name and ID are invalid. Any messages waiting at the message queue are also deleted and deallocated.