#include <mqueue.h> int mq_notify(mqd_t mqdes, const struct sigevent *notification);
The mq_notify() function provides an asynchronous mechanism for processes to receive notice that messages are available in a message queue, rather than synchronously blocking (waiting) in mq_receive(3C).
If notification is not NULL, this function registers the calling process to be notified of message arrival at an empty message queue associated with the message queue descriptor, mqdes. The notification specified by notification will be sent to the process when the message queue transitions from empty to non-empty. See signal.h(3HEAD). At any time, only one process may be registered for notification by a specific 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 will fail.
If notification is NULL and the process is currently registered for notification by the specified message queue, the existing registration is removed. The message queue is then available for future registration.
When the notification is sent to the registered process, its registration is removed. The message queue is then available for registration.
If a process has registered for notification of message arrival at a message queue and some processes is blocked in mq_receive(3C) waiting to receive a message when a message arrives at the queue, the arriving message will be received by the appropriate mq_receive(3C), and no notification will be sent to the registered process. The resulting behavior is as if the message queue remains empty, and this notification will not be sent until the next arrival of a message at this queue.
Any notification registration is removed if the calling process either closes the message queue or exits.
Upon successful completion, mq_notify() returns 0; otherwise, it returns −1 and sets errno to indicate the error.
The mq_notify() function will fail if:
The mqdes argument is not a valid message queue descriptor.
A process is already registered for notification by the message queue.
See attributes(5) for descriptions of the following attributes: