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MQ_NOTIFY(2)              FreeBSD System Calls Manual             MQ_NOTIFY(2)

     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

     The mq_notify() system call will fail if:

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

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

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

     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

     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

FreeBSD 11.1-RELEASE-p4          July 15, 2016         FreeBSD 11.1-RELEASE-p4
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