mq_notify(2) System Calls Manual mq_notify(2)
mq_notify - register/cancel a notification request with a message queue
If the argument notification is not NULL, the system call registers the calling process to be notified of message arrival at an empty mes-
sage 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.
If the type of notification specified in notification->sigev_notify is then the signal specified in notification->sigev_signo will be sent
to the process. If the flag is set for that signal number, then the value specified in notification->sigev_value will be the si_value com-
ponent 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 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 registra-
tion is canceled.
When the notification is sent to the registered process, its registration is removed. The message queue is then available for registra-
If there is some process blocked in waiting to receive a message from a message queue, the arrival of a message on the queue will satisfy
the even if the queue has a registered notification request. The resulting behavior is as if the message queue remains empty, and no noti-
fication is sent.
To use this function, link in the realtime library by specifying on the compiler or linker command line.
returns the following values:
is set to indicate the error.
If fails, 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] is not supported by the implementation.
mq_open(2), mq_send(2), sigaction(2). signal(2), signal(5).