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mq_notify.2 (2010 09)

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mq_notify(2) mq_notify(2)
NAME
mq_notify - register/cancel a notification request with a message queue
SYNOPSIS
#include <sys/mqueue.h>
int mq_notify(mqd_t mqdes, const struct sigevent *notification);
DESCRIPTION
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 mes-
sage queue transitions from the empty state to the non-empty state.
If the type of notification specified in notification->sigev_notify is
SIGEV_SIGNAL, then the signal
specified in notification->sigev_signo will be sent to the process. If the
SA_SIGINFO flag is set for that
signal number, then the value specified in notification->sigev_value will be the si_value component of the
siginfo_t structure passed to the signal catching function at the time of signal delivery.
At any time, only one process can be registered for notification with a message queue. If the calling pro-
cess, or any other process has already registered for notification with the specified message queue, subse-
quent 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 registration.
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.
To use this function, link in the realtime library by specifying
-lrt on the compiler or linker command
line.
RETURN VALUE
mq_notify() returns the following values:
0 Successful completion.
-1 Failure. errno is set to indicate the error.
ERRORS
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 descriptor.
[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 invalid address.
[ENOSYS]
mq_notify() is not supported by the implementation.
SEE ALSO
mq_open(2), mq_send(2), sigaction(2). signal(2), signal(5).
STANDARDS CONFORMANCE
mq_notify(): POSIX 1003.1b
HP-UX 11i Version 3: September 2010 − 1 − Hewlett-Packard Company 1

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