Manual Pages  — MQ_NOTIFY

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

LIBRARY SYNOPSIS DESCRIPTION RETURN VALUES ERRORS SEE ALSO STANDARDS HISTORY COPYRIGHT

POSIX Real\-time Library (librt, -lrt)

#include <mqueue.h>

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

If the argument notification is not NULL, this system call will register 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 will fail.

The notification argument points to a sigevent structure that defines how the calling process will be notified. If notification->sigev_notify is SIGEV_NONE, then no signal will be posted, but the error status and the return status for the operation will be set appropriately. For SIGEV_SIGNO and SIGEV_THREAD_ID notifications, the signal specified in notification->sigev_signo will be sent to the calling process ( SIGEV_SIGNO) or to the thread whose LWP ID is notification->sigev_notify_thread_id ( SIGEV_THREAD_ID). The information for the queued signal will include:

Member	Value
si_code	SI_MESGQ
si_value	the value stored in notification->sigev_value
si_mqd	mqdes

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

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

Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

The mq_notify() system call will fail if:

[EBADF]
	The mqdes argument is not a valid message queue descriptor.
[EBUSY]
	Process is already registered for notification by the message queue.
[EINVAL]
	The asynchronous notification method in notification->sigev_notify is invalid or not supported.

The mq_notify() system call conforms to IEEE Std 1003.1-2004 ("POSIX.1").

Support for POSIX message queues first appeared in FreeBSD 7.0 .