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_OPEN 2
+.Os
+.Sh NAME
+.Nm mq_open 
+.Nd "open a message queue (REALTIME)"
+.Sh LIBRARY
+.Lb libc
+.Sh SYNOPSIS
+.In mqueue.h
+.Ft mqd_t
+.Fn mq_open "const char *name" "int oflag" "..."
+.Sh DESCRIPTION
+The
+.Fn mq_open
+system call establishs the connection between a process and a message queue
+with a message queue descriptor. It creates an open message queue
+description that refers to the message queue, and a message queue descriptor
+that refers to that open message queue description. The message queue
+descriptor is used by other functions to refer to that message queue. The
+.Fa name
+argument points to a string naming a message queue. It is unspecified
+whether the name appears in the file system and is visible to other functions
+that take pathnames as arguments. The
+.Fa name
+argument conforms to the construction rules for a pathname. If 
+.Fa name
+begins with the slash character, then processes calling
+.Fn mq_open
+with the same value of 
+.Fa name
+refers to the same message queue object, as long as that name has not been
+removed. If
+.Fa name
+does not begin with the slash character, the effect is implementation-defined.
+The interpretation of slash characters other than the leading slash character
+in
+.Fa name
+is implementation-defined. If the
+.Fa name
+argument is not the name of an existing message queue and creation is not
+requested,
+.Fn mq_open
+will fail and returns an error.
+.Pp
+A message queue descriptor may be implemented using a file descriptor, in
+which case applications can open up to at least 
+.Brq Dv OPEN_MAX
+file and message queues.
+.Pp
+The
+.Fa oflag
+argument requests the desired receive and/or send access to the message
+queue. The requested access permission to receive messages or send messages
+would be granted if the calling process would be granted read or write access,
+respectively, to an equivalently protected file.
+.Pp
+The value of
+.Fa oflag
+is the bitwise-inclusive OR of values from the following list.
+Applications should specify exactly one of the first three values (access
+modes) below in the value of
+.Fa oflag :
+.Bl -tag -width Er
+.It Bq Er O_RDONLY
+Open the message queue for receiving messages. The process can use the
+returned message queue descriptor with
+.Fn mq_receive ,
+but not
+.Fn mq_send .
+A message queue may be open multiple times in the same or different processes
+for receiving messages.
+.It Bq Er O_WRONLY
+Open the queue for sending messages. The process can use the returned
+message queue descriptor with
+.Fn mq_send
+but not
+.Fn mq_receive .
+A message queue may be open multiple times in the same or different processes
+for sending messages.
+.It Bq Er O_RDWR
+Open the queue for both receiving and sending messages. The process can use
+any of the functions allowed for
+.Dv O_RDONLY
+and
+.Dv O_WRONLY .
+A message queue may be open multiple times in the same or different processes
+for sending messages.
+.El
+.Pp
+Any combination of the remaining flags may be specified in the value of
+.Fa oflag :
+.Bl -tag -width Er
+.It Bq Er O_CREAT
+Create a message queue. It requires two additional arguments:
+.Fa mode ,
+which is of type
+.Vt mode_t ,
+and
+.Fa attr ,
+which is a pointer to an 
+.Vt mq_attr
+structure. If the pathname
+.Fa name
+has already been used to create a message queue that still exists, then
+this flag has no effect, except as noted under
+.Dv O_EXCL .
+Otherwise, a message queue will be created without any messages
+in it. The user ID of the message queue will be set to the effective user ID
+of the process, and the group ID of the message queue will be set to the
+effective group ID of the process. The permission bits of the message queue
+will be set to the value of the
+.Fa mode
+argument, except those set in the file mode creation mask of the process.
+When bits in
+.Fa mode
+other than the file permission bits are specified, the effect is
+unspecified. If
+.Fa attr
+is
+.Dv NULL ,
+the message queue is created with implementation-defined default message
+queue attributes. If attr is non-NULL and the calling process has the
+appropriate privilege on name, the message queue mq_maxmsg and mq_msgsize
+attributes will be set to the values of the corresponding members in the
+.Vt mq_attr
+structure referred to by
+.Fa attr .
+If
+.Fa attr
+is non-NULL, but the calling process does not have the appropriate privilege
+on name, the
+.Fn mq_open
+function will fail and return an error without creating the message queue.
+.It Bq Er O_EXCL
+If
+.Dv O_EXCL
+and
+.Dv O_CREAT
+are set,
+.Fn mq_open
+will fail if the message queue name exists.
+.It Bq Er O_NONBLOCK
+Determines whether an
+.Fn mq_send
+or
+.Fn mq_receive
+waits for resources or messages that are not currently available, or fails
+with errno set to 
+.Er EAGAIN ;
+see
+.Fn mq_send
+and
+.Fn mq_receive
+for details.
+.Pp
+The
+.Fn mq_open
+function does not add or remove messages from the queue.
+.Sh NOTES
+FreeBSD implements message queue based on file descriptor. The descriptor
+is inherited by child after
+.Fn fork .
+The descriptor is closed in new image after
+.Fn exec .
+.Fn select
+and
+.Fn kevent
+syscalls are supported for message queue descriptor.
+.Sh RETURN VALUES
+Upon successful completion, the function returns a message queue
+descriptor; otherwise, the function returns (mqd_t)-1 and set
+.Va errno
+to indicate the error.
+.Sh ERRORS
+The
+.Fn mq_open
+system call
+will fail if:
+.Bl -tag -width Er
+.It Bq Er EACCES
+The message queue exists and the permissions specified by
+.Fa oflag
+are denied, or the message queue does not exist and permission to create the
+message queue is denied
+.It Bq Er EEXIST
+.Dv O_CREAT
+and
+.Dv O_EXCL
+are set and the named message queue already exists.
+.It Bq Er EINTR
+The
+.Fn mq_open
+function was interrupted by a signal.
+.It Bq Er EINVAL
+The
+.Fn mq_open
+function is not supported for the given name.
+.It Bq Er EINVAL
+.Dv O_CREAT
+was specified in
+.Fa oflag ,
+the value of attr is not
+.Dv NULL ,
+and either
+.Va mq_maxmsg
+or
+.Va mq_msgsize
+was less than or equal to zero.
+.It Bq Er EMFILE
+Too many message queue descriptors or file descriptors are currently in use
+by this process.
+.It Bq Er ENAMETOOLONG
+The length of the name argument exceeds
+.Brq Dv PATH_MAX
+or a pathname component
+is longer than
+.Brq Dv NAME_MAX .
+.It Bq Er ENFILE
+Too many message queues are currently open in the system.
+.It Bq Er ENOENT
+.Dv O_CREAT
+is not set and the named message queue does not exist.
+.It Bq Er ENOSPC
+There is insufficient space for the creation of the new message queue.
+.El
+.Sh SEE ALSO
+.Xr mq_close 2 ,
+.Xr mq_getattr 2 ,
+.Xr mq_receive 2 ,
+.Xr mq_send 2 ,
+.Xr mq_setattr 2 ,
+.Xr mq_timedreceive 3 ,
+.Xr mq_timedsend 3
+.Xr mq_unlink 3
+.Sh STANDARDS
+The
+.Fn mq_open
+system call conform to
+.St -p1003.1-2004 .
+.Sh HISTORY
+Support for POSIX message queue first appeared in
+.Fx 7.0 .
+.Sh BUGS
+This implementation places strict requirements on the value of
+.Fa name :
+it must begin with a slash
+.Pq Ql / ,
+contain no other slash characters.