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mq_notify — notify process that a message is available (REALTIME)
POSIX Real-time Library (librt, -lrt)
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. 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
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
mq_open(2), mq_send(2), mq_timedsend(2), 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