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.Dd November 29, 2005
.Dt MQ_NOTIFY 2
.Os
.Sh NAME
.Nm mq_notify
.Nd "notify process that a message is available (REALTIME)"
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In mqueue.h
.Ft int
.Fn mq_notify "mqd_t mqdes" "const struct sigevent *notification"
.Sh DESCRIPTION
If the argument notification is not
.Dv 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, 
.Fa 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.
.Pp
If notification is
.Dv NULL
and the process is currently registered for notification by the specified
message queue, the existing registration will be removed.
.Pp
When the notification is sent to the registered process, its registration
is removed. The message queue then is available for registration.
.Pp
If a process has registered for notification of message arrival at a
message queue and some thread is blocked in
.Fn mq_receive
waiting to receive a message when a message arrives at the queue, the
arriving message will satisfy the appropriate
.Fn mq_receive .
The resulting behavior is as if the message queue remains empty, and no
notification will be sent.
.Sh RETURN VALUES
Upon successful completion, the
.Fn mq_notify
system call returns a value of zero; otherwise, the function returns a value
of -1 and set
.Va errno
to indicate the error.
.Sh ERRORS
The
.Fn mq_notify
system call
will fail if:
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa mqdes
argument is not a valid message queue descriptor.
.It Bq Er EBUSY
process is already registered for notification by the message queue.
.El
.Sh SEE ALSO
.Xr mq_open 2 ,
.Xr mq_send 2 ,
.Xr mq_timedsend 2
.Sh STANDARDS
The
.Fn mq_notify
system call conform to
.St -p1003.1-2004 .
.Sh HISTORY
Support for POSIX message queue first appeared in
.Fx 7.0 .