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.\" Portions of this text are reprinted and reproduced in electronic form
.\" from IEEE Std 1003.1, 2004 Edition, Standard for Information Technology --
.\" Portable Operating System Interface (POSIX), The Open Group Base
.\" Specifications Issue 6, Copyright (C) 2001-2004 by the Institute of
.\" Electrical and Electronics Engineers, Inc and The Open Group.  In the
.\" event of any discrepancy between this version and the original IEEE and
.\" The Open Group Standard, the original IEEE and The Open Group Standard is
.\" the referee document.  The original Standard can be obtained online at
.\"	http://www.opengroup.org/unix/online.html.
.\"
.\" $FreeBSD$
.\"
.Dd July 15, 2016
.Dt MQ_NOTIFY 2
.Os
.Sh NAME
.Nm mq_notify
.Nd "notify process that a message is available (REALTIME)"
.Sh LIBRARY
.Lb librt
.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
.Fa 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
The
.Fa notification
argument points to a
.Vt sigevent
structure that defines how the calling process will be notified.
If
.Fa notification->sigev_notify
is
.Dv SIGEV_NONE ,
then no signal will be posted, but the error status and the return status
for the operation will be set appropriately.
For
.Dv SIGEV_SIGNO
and
.Dv SIGEV_THREAD_ID
notifications,
the signal specified in
.Fa notification->sigev_signo
will be sent to the calling process
.Pq Dv SIGEV_SIGNO
or to the thread whose LWP ID is
.Fa notification->sigev_notify_thread_id
.Pq Dv SIGEV_THREAD_ID .
The information for the queued signal will include:
.Bl -column ".Va si_value"
.It Sy Member Ta Sy Value
.It Va si_code Ta Dv SI_MESGQ
.It Va si_value Ta
the value stored in
.Fa notification->sigev_value
.It Va si_mqd Ta Fa mqdes
.El
.Pp
If
.Fa 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
.Rv -std
.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.
.It Bq Er EINVAL
The asynchronous notification method in
.Fa notification->sigev_notify
is invalid or not supported.
.El
.Sh SEE ALSO
.Xr mq_open 2 ,
.Xr mq_send 2 ,
.Xr mq_timedsend 2 ,
.Xr sigevent 3 ,
.Xr siginfo 3
.Sh STANDARDS
The
.Fn mq_notify
system call conforms to
.St -p1003.1-2004 .
.Sh HISTORY
Support for
.Tn POSIX
message queues first appeared in
.Fx 7.0 .
.Sh COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2004 Edition, Standard for Information Technology --
Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2004 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.  In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard is
the referee document.  The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html.