The Single UNIX ® Specification, Version 2
Copyright © 1997 The Open Group


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


#include <mqueue.h>

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


If the argument notification is not NULL, this function registers 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 fail.

If notification is NULL and the process is currently registered for notification by the specified message queue, the existing registration is removed.

When the notification is sent to the registered process, its registration will be removed. The message queue will then be 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 satisfies the appropriate mq_receive(). The resulting behaviour is as if the message queue remains empty, and no notification is sent.


Upon successful completion, the mq_notify() function returns a value of zero; otherwise, the function returns a value of -1 and sets errno to indicate the error.


The mq_notify() function will fail if:
The mqdes argument is not a valid message queue descriptor.
A process is already registered for notification by the message queue.
The function mq_notify() is not supported by this implementation.






mq_open(), mq_send(), <mqueue.h>, msgctl(), msgget(), msgrcv(), msgsnd().


Derived from the POSIX Realtime Extension (1003.1b-1993/1003.1i-1995)

UNIX ® is a registered Trademark of The Open Group.
Copyright © 1997 The Open Group
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