mq_notify - register/cancel a notification request with a message
int mq_notify(mqd_t mqdes, const struct sigevent *notification);
If the argument notification is not NULL, the mq_notify() system call
registers the calling process to be notified of message arrival at an
empty message queue associated with the message queue descriptor
mqdes. The notification specified by the notification argument will
be sent to the process when the message queue transitions from the
empty state to the non-empty state. At any time, only one process can
be registered for notification with a message queue. If the calling
process, or any other process has already registered for notification
with the specified message queue, subsequent attempts to register with
that queue will fail.
If notification is NULL and the process is currently registered for
notification with the specified message queue, the existing
registration is canceled.
When the notification is sent to the registered process, its
registration is removed. The message queue is then available for
If there is some process blocked in mq_receive() waiting to receive a
message from a message queue, the arrival of a message on the queue
will satisfy the mq_receive(), even if the queue has a registered
notification request. The resulting behavior is as if the message
queue remains empty, and no notification is sent.
mq_notify() returns the following values:
0 Successful completion.
-1 Failure. errno is set to indicate the error.
If mq_notify() fails, errno is set to one of the following values:
[EAGAIN] The system lacks sufficient signal queuing
resources to honor the request.
[EBADF] The mqdes argument is not a valid message queue
Hewlett-Packard Company - 1 - HP-UX Release 11i: November 2000
[EBUSY] A process is already registered for notification
with the message queue.
[EINVAL] An attempt was made to cancel a non-existent
notification request, or notification points to an
[ENOSYS] mq_notify() is not supported by the
mq_notify(): POSIX 1003.1b
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