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     mq_notify — notify process that a message is available (REALTIME)


     POSIX Real-time Library (librt, -lrt)


     #include <mqueue.h>

     mq_notify(mqd_t mqdes, const struct sigevent *notification);


     If the argument notification is not NULL, this system call 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 will fail.

     The notification argument points to a sigevent structure that defines how the calling
     process will be notified.  If notification->sigev_notify is SIGEV_NONE, then no signal will
     be posted, but the error status and the return status for the operation will be set
     appropriately.  For SIGEV_SIGNO and SIGEV_THREAD_ID notifications, the signal specified in
     notification->sigev_signo will be sent to the calling process (SIGEV_SIGNO) or to the thread
     whose LWP ID is notification->sigev_notify_thread_id (SIGEV_THREAD_ID).  The information for
     the queued signal will include:

     Member      Value
     si_code     SI_MESGQ
     si_value    the value stored in notification->sigev_value
     si_mqd      mqdes

     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 is removed.  The
     message queue then is 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.


     Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and
     the global variable errno is set to indicate the error.


     The mq_notify() system call will fail if:

     [EBADF]            The mqdes argument is not a valid message queue descriptor.

     [EBUSY]            Process is already registered for notification by the message queue.

     [EINVAL]           The asynchronous notification method in notification->sigev_notify is
                        invalid or not supported.


     mq_open(2), mq_send(2), mq_timedsend(2), sigevent(3), siginfo(3)


     The mq_notify() system call conforms to IEEE Std 1003.1-2004 (“POSIX.1”).


     Support for POSIX message queues first appeared in FreeBSD 7.0.


     Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1,
     2004 Edition, Standard for Information Technology -- Portable Operating System Interface
     (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2004 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