int mq_notify(mqd_t mqdes, const struct sigevent *notification);
If the argument notification is not NULL, this function shall 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 shall 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 shall fail.
If notification is NULL and the process is currently registered for notification by the specified message queue, the existing registration shall be removed.
When the notification is sent to the registered process, its registration shall be removed. The message queue shall 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 shall satisfy the appropriate mq_receive(). The resulting behavior is as if the message queue remains empty, and no notification shall be sent.
Upon successful completion, the mq_notify() function shall return a value of zero; otherwise, the function shall return a value of -1 and set errno to indicate the error.
The mq_notify() function shall fail if:
- The mqdes argument is not a valid message queue descriptor.
A process is already registered for notification by the message queue.
The following sections are informative.
COPYRIGHTPortions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .