RTEMS POSIX API User's Guide
Every message queue has the ability to notify one (and only one) process whenever the queue's state changes from empty (0 messages) to nonempty. This means that the process does not have to block or constantly poll while it waits for a message. By calling mq_notify, you can attach a notification request to a message queue. When a message is received by an empty queue, if there are no processes blocked and waiting for the message, then the queue notifies the requesting process of a message arrival. There is only one signal sent by the message queue, after that the notification request is de-registered and another process can attach its notification request. After receipt of a notification, a process must re-register if it wishes to be notified again.
If there is a process blocked and waiting for the message, that process gets the message, and notification is not sent. It is also possible for another process to receive the message after the notification is sent but before the notified process has sent its receive request.
Only one process can have a notification request attached to a message queue at any one time. If another process attempts to register a notification request, it fails. You can de-register for a message queue by passing a NULL to mq_notify, this removes any notification request attached to the queue. Whenever the message queue is closed, all notification attachments are removed.
RTEMS POSIX API User's Guide
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