| Operators |
dequeue_message — Receive one or more messages from the message queue.
dequeue_message( : : QueueHandle, GenParamName, GenParamValue : MessageHandle)
dequeue_message dequeues a message from the message queue denoted by the QueueHandle parameter. The message must have been enqueued by any thread using enqueue_message.
The messages are delivered in FIFO (first-in first-out) order, every message is delivered only once. If the queue is not empty, dequeue_message immediately delivers the oldest message from the queue. This message is removed from the queue and a handle to the message is returned in the MessageHandle output parameter. The message data ownership is transferred (no copy) from the message queue to the newly created message handle.
If the queue is empty, dequeue_message blocks until a message becomes available for delivery (being queued from another thread using enqueue_message).
The data stored in the message can be queried using get_message_tuple, get_message_obj, or get_message_param.
The message handles obtained through dequeue_message can be further reused (modified and/or enqueued to another message queue).
If multiple messages were enqueued using a single enqueue_message call, all those messages will be also retrieved together through a single dequeue_message call, delivering multiple mesage handles via the MessageHandle tuple.
The queue access is internally fully synchronized, no external locking is required.
During application reconfiguration or cleanup, it might be necessary to wake threads waiting for messages in dequeue_message. This can be achieved using the operator set_message_queue_param with the parameter 'abort_dequeuing'. In such case the currently blocked dequeue_message calls return immediately with H_ERR_MQCNCL.
Finally, it is possible to adjust dequeue_message behavior through generic parameters within GenParamName and GenParamValue. Currently, just a single generic parameter is supported:
Timeout controlling how long the operator will block while waiting for a message if the queue is empty. When expired, the operator returns with H_ERR_TIMEOUT. The timeout can be specified as an integer or double value in seconds or as the string 'infinite'. When no timeout is specified, infinite timeout is used as default, meaning that the operator would block until a new message is enqueued or until the operation is aborted.
This operator returns a handle. Note that the state of an instance of this handle type may be changed by specific operators even though the handle is used as an input parameter by those operators.
Message queue handle.
Number of elements: QueueHandle == 1
Restriction: QueueHandle != 0
Names of optional generic parameters
Number of elements: GenParamName == GenParamValue
Default value: 'timeout'
List of values: 'timeout'
Values of optional generic parameters
Number of elements: GenParamName == GenParamValue
Default value: 'infinite'
List of values: 'infinite'
Handle(s) of the dequeued message(s).
Number of elements: MessageHandle > 0
Assertion: MessageHandle != 0
create_message_queue (Queue) * ... dequeue_message (Queue, [], [], Message) get_message_obj (Image, Message, 'my_image')
If the operation succeeds, dequeue_message returns 2 (H_MSG_TRUE). Otherwise an exception is raised. Possible error conditions include invalid parameters, message dequeue wait timeout (H_ERR_TIMEOUT) or message dequeue wait abortion (H_ERR_MQCNCL). Note that in some rare cases, when an error occurs when finishing the operator call (after the message has already been removed from the queue), the operator will attempt to requeue the message back to the front of the queue to not loose it. This might lead to a different message delivery order if another thread removed another message from the queue in the meanwhile.
get_message_tuple, get_message_obj, get_message_param
create_message_queue, clear_message_queue, enqueue_message, set_message_queue_param, get_message_queue_param, create_message, clear_message, set_message_tuple, get_message_tuple, set_message_obj, get_message_obj
Foundation
| Operators |