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_SEND 2
+.Os
+.Sh NAME
+.Nm mq_send , mq_timedsend
+.Nd "send a message to message queue (REALTIME)"
+.Sh LIBRARY
+.Lb libc
+.Sh SYNOPSIS
+.In mqueue.h
+.Ft int
+.Fo mq_send
+.Fa "mqd_t mqdes"
+.Fa "const char *msg_ptr"
+.Fa "size_t msg_len"
+.Fa "unsigned msg_prio"
+.Fc
+.Ft int
+.Fo mq_timesend
+.Fa "mqd_t mqdes"
+.Fa "const char *msg_ptr"
+.Fa "size_t msg_len"
+.Fa "unsigned msg_prio"
+.Fa "const struct timespec *abs_timeout"
+.Fc
+.Sh DESCRIPTION
+The
+.Fn mq_send
+syscall adds the message pointed to by the argument
+.Fa msg_ptr
+to the message queue specified by
+.Fa mqdes .
+The 
+.Fa msg_len
+argument specifies the length of the message, in bytes, pointed to by
+.Fa msg_ptr .
+The value of
+.Fa msg_len
+should be less than or equal to the
+.Fa mq_msgsize
+attribute of the message queue, or
+.Fn mq_send
+will fail.
+.Pp
+If the specified message queue is not full,
+.Fn mq_send
+will behave as if the message is inserted into the message queue at
+the position indicated by the
+.Fa msg_prio
+argument. A message with a larger numeric value of
+.Fa msg_prio
+will be inserted before messages with lower values of
+.Fa msg_prio .
+A message will be inserted after other messages in the queue, if any,
+with equal
+.Fa msg_prio .
+The value of
+.Fa msg_prio
+should be less than
+.Brq Dv MQ_PRIO_MAX .
+.Pp
+If the specified message queue is full and
+.Dv O_NONBLOCK
+is not set in the message queue description associated with
+.Fa mqdes ,
+.Fn mq_send
+will block until space becomes available to enqueue the
+message, or until
+.Fn mq_send
+is interrupted by a signal. If more than one thread is
+waiting to send when space becomes available in the message queue and
+the Priority Scheduling option is supported, then the thread of the
+highest priority that has been waiting the longest will be unblocked
+to send its message. Otherwise, it is unspecified which waiting thread
+is unblocked. If the specified message queue is full and
+.Dv O_NONBLOCK
+is set in the message queue description associated with
+.Fa mqdes ,
+the message will not be queued and
+.Fn mq_send
+will return an error.
+.Pp
+The
+.Fn mq_timedsend
+syscall will add a message to the message queue specified by
+.Fa mqdes
+in the manner defined for the
+.Fn mq_send
+syscall. However, if the specified message queue is full and
+.Dv O_NONBLOCK
+is not set in the message queue description associated with
+.Fa mqdes ,
+the wait for sufficient room in the queue will be terminated when
+the specified timeout expires. If
+.Dv O_NONBLOCK
+is set in the message queue description, this syscall is
+equivalent to
+.Fn mq_send .
+.Pp
+The timeout will expire when the absolute time specified by
+.Fa abs_timeout
+passes, as measured by the clock on which timeouts are based (that is,
+when the value of that clock equals or exceeds 
+.Fa abs_timeout ) ,
+or if the absolute time specified by
+.Fa abs_timeout
+has already been passed at the time of the call.
+.Pp
+The timeout is based on the CLOCK_REALTIME clock.
+.Sh RETURN VALUES
+Upon successful completion, the
+.Fn mq_send
+and
+.Fn mq_timedsend
+syscalls return a value of zero. Otherwise, no message will be
+enqueued, the syscalls returns -1, and
+.Va errno
+is set to indicate the error.
+.Sh ERRORS
+The
+.Fn mq_send
+and
+.Fn mq_timedsend
+system calls
+will fail if:
+.Bl -tag -width Er
+.It Bq Er EAGAIN
+The
+.Dv O_NONBLOCK
+flag is set in the message queue description associated with
+.Fa mqdes ,
+and the specified message queue is full.
+.It Bq Er EBADF
+The
+.Fa mqdes
+argument is not a valid message queue descriptor open for writing.
+.It Bq Er EINTR
+A signal interrupted the call to
+.Fn mq_send
+or
+.Fn mq_timedsend. 
+.It Bq Er EINVAL
+The value of
+.Fa msg_prio
+was outside the valid range.
+.It Bq Er EINVAL
+The process or thread would have blocked, and the
+.Fa abs_timeout
+parameter specified a nanoseconds field value less than zero or greater
+than or equal to 1000 million.
+.It Bq Er EMSGSIZE
+The specified message length,
+.Fa msg_len ,
+exceeds the message size attribute of the message queue.
+.It Bq Er ETIMEDOUT
+The
+.Dv O_NONBLOCK
+flag was not set when the message queue was opened, but the timeout
+expired before the message could be added to the queue.
+.El
+.Sh SEE ALSO
+.Xr mq_open 2 ,
+.Xr mq_receive 2 ,
+.Xr mq_setattr 2 ,
+.Xr mq_timereceive 2
+.Sh STANDARDS
+The
+.Fn mq_send
+and
+.Fn mq_timesend
+system calls conform to
+.St -p1003.1-2004 .
+.Sh HISTORY
+Support for POSIX message queue first appeared in
+.Fx 7.0 .