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
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.
Portions 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