\"      $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 .

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