.\"	$NetBSD: mq_open.3,v 1.6 2012/03/15 19:04:47 njoly Exp $
.\"
.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved
.\"
.Dd June 7, 2010
.Dt MQ_OPEN 3
.Os
.Sh NAME
.Nm mq_open
.Nd open a message queue (REALTIME)
.Sh LIBRARY
.Lb librt
.Sh SYNOPSIS
.In mqueue.h
.Ft mqd_t
.Fn mq_open "const char *name" "int oflag"
.Ft mqd_t
.Fn mq_open "const char *name" "int oflag" "mode_t mode" "struct mq_attr *attr"
.Sh DESCRIPTION
The
.Fn mq_open
function establishes 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,
which should conform to the construction rules for a pathname.
The
.Fa name
should begin with a slash character.
The processes calling
.Fn mq_open
with the same value of
.Fa name
will refer to the same message queue object,
as long as that name has not been removed.
If the
.Fa name
argument is not
the name of an existing message queue and creation is not requested,
.Fn mq_open
fails and returns an error.
.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 are
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 must specify exactly one of the first three values
(access modes) below in the value of
.Fa oflag :
.Bl -tag -width ".Dv O_RDONLY"
.It Dv O_RDONLY
Open the message queue for receiving messages.
The process can use the returned message queue descriptor with
.Xr mq_receive 3 ,
but not
.Xr mq_send 3 .
.It Dv O_WRONLY
Open the queue for sending messages.
The process can use the returned message queue descriptor with
.Xr mq_send 3
but not
.Xr mq_receive 3 .
.It Dv 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 .
.El
.Pp
In all cases, a message queue may be open multiple times in the same
or different processes for sending/receiving messages.
.Pp
Any combination of the remaining flags may be specified in the value of
.Fa oflag :
.Bl -tag -width ".Dv O_NONBLOCK"
.It Dv O_CREAT
Create a message queue.
It requires two additional arguments:
.Fa mode
and
.Fa attr .
If the pathname
.Fa name
has already been used to create a message queue that still exists,
then this flag will have 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 will be created with implementation-defined default
message queue attributes.
If
.Fa attr
is
.No non- Ns Dv NULL
and the calling process has the appropriate privilege on
.Fa name ,
the message queue
.Va mq_maxmsg
and
.Va 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
.No non- Ns Dv NULL ,
but the calling process does not have the
appropriate privilege on
.Fa name ,
the
.Fn mq_open
function will fail and return an error without creating the message queue.
.It Dv O_EXCL
If
.Dv O_EXCL
and
.Dv O_CREAT
are set,
.Fn mq_open
fails if the message queue
.Fa name
exists.
The check for the existence of the message queue and the creation of the
message queue if it does not exist will be atomic with respect to other
threads executing
.Fn mq_open
naming the same
.Fa name
with
.Dv O_EXCL
and
.Dv O_CREAT
set.
If
.Dv O_EXCL
is set and
.Dv O_CREAT
is not set, the result is undefined.
.It Dv O_NONBLOCK
Determines whether an
.Xr mq_send 3
or
.Xr mq_receive 3
waits for resources or messages that are not currently available,
or fails with errno set to
.Er EAGAIN .
.El
.Pp
The
.Fn mq_open
function does not add or remove messages from the queue.
.Sh IMPLEMENTATION NOTES
The
.Xr select 2
and
.Xr poll 2
system calls to the message queue descriptor are supported by
.Nx ,
however, it is not portable.
.Sh RETURN VALUES
Upon successful completion,
.Fn mq_open
returns a message queue descriptor.
Otherwise, the function returns
.Pq Dv mqd_t
\-1 and sets the global variable
.Va errno
to indicate the error.
.Sh ERRORS
The
.Fn mq_open
function fails 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, or
.Dv O_CREAT
was specified in
.Fa oflag ,
the value of
.Fa 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
.Fa 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 3 ,
.Xr mq_close 3 ,
.Xr mq_unlink 3
.Sh STANDARDS
This function conforms to the
.St -p1003.1-2001
standard.
.Sh HISTORY
This function first appeared in
.Nx 5.0 .
.Sh COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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
.Lk http://www.opengroup.org/unix/online.html .