mq_notify(2)							System Calls Manual						      mq_notify(2)

NAME

       mq_notify - register/cancel a notification request with a message queue

SYNOPSIS

DESCRIPTION

       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-
       tion.

       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.

RETURN VALUE

       returns the following values:

	      Successful completion.

	      Failure.
		   is set to indicate the error.

ERRORS

       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.

SEE ALSO

       mq_open(2), mq_send(2), sigaction(2).  signal(2), signal(5).

STANDARDS CONFORMANCE

																      mq_notify(2)