mq_notify man page on FreeBSD

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MQ_NOTIFY(2)		    BSD System Calls Manual		  MQ_NOTIFY(2)

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 mes‐
     sage queue.  If the calling process or any other process has already reg‐
     istered 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.	 If
     notification->sigev_notify is SIGEV_SIGNAL, then the signal specified in
     notification->sigev_signo will be sent to the process.  The signal will
     be queued to the process and the value specified in
     notification->sigev_value will be the si_value component of the generated
     signal.

     If notification is NULL and the process is currently registered for noti‐
     fication 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 mes‐
     sage 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.

SEE ALSO
     mq_open(2), mq_send(2), mq_timedsend(2), 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.

COPYRIGHT
     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 Spec‐
     ifications Issue 6, Copyright (C) 2001-2004 by the Institute of Electri‐
     cal 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 ref‐
     eree document.  The original Standard can be obtained online at
	  http://www.opengroup.org/unix/online.html.

BSD			       November 29, 2005			   BSD
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