Add manuals for POSIX message queue.

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+.\" Copyright (c) 2005 David Xu <davidxu@FreeBSD.org>
+.\" All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice(s), this list of conditions and the following disclaimer as
+.\"    the first lines of this file unmodified other than the possible
+.\"    addition of one or more copyright notices.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice(s), this list of conditions and the following disclaimer in
+.\"    the documentation and/or other materials provided with the
+.\"    distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER(S) ``AS IS'' AND ANY
+.\" EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+.\" PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE
+.\" LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
+.\" BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
+.\" WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE
+.\" OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE,
+.\" EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+.\"
+.\" $FreeBSD$
+.\"
+.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 syscall 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
+syscall 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
+syscall 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 .