oracular (2) mq_notify.2freebsd.gz

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


LIBRARY
     POSIX Real-time Library (librt, -lrt)


SYNOPSIS
     #include <mqueue.h>

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


DESCRIPTION
     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.


RETURN VALUES
     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.


ERRORS
     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.


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


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


HISTORY
     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 http://www.opengroup.org/unix/online.html.