If the argument
notification is not
NULL, this function will register the calling process to be notified of message arrival at an empty message queue associated with the specified message queue descriptor,
mqdes. The notification specified by the
notification argument will be sent to the process when the message queue transitions from empty to non-empty. At any time, only one process may be registered for notification by a message queue. If the calling process or any other process has already registered for notification of message arrival at the specified message queue, subsequent attempts to register for that message queue fails.
If
notification is
NULL and the process is currently registered for notification by the specified message queue, the existing registration will be removed.
When the notification is sent to the registered process, its registration will be removed. The message queue will then be available for registration.
If a process has registered for notification of message arrival at a message queue and some thread is blocked in
mq_receive() waiting to receive a message when a message arrives at the queue, the arriving message will satisfy the appropriate
mq_receive(). The resulting behavior is as if the message queue remains empty, and no notification will be sent.