NAME
mq_open —
open a message queue
(REALTIME)
LIBRARY
POSIX Real-time Library (librt, -lrt)
SYNOPSIS
#include <mqueue.h>
mqd_t
mq_open(
const
char *name,
int
oflag);
mqd_t
mq_open(
const
char *name,
int
oflag,
mode_t mode,
struct mq_attr *attr);
DESCRIPTION
The
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
name argument points to a string naming a
message queue, which should conform to the construction rules for a pathname.
The
name should begin with a slash character. The
processes calling
mq_open() with the same value of
name will refer to the same message queue object, as
long as that name has not been removed. If the
name
argument is not the name of an existing message queue and creation is not
requested,
mq_open() fails and returns an error.
The
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.
The value of
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
oflag:
-
-
O_RDONLY
- Open the message queue for receiving messages. The process
can use the returned message queue descriptor with
mq_receive(3), but not
mq_send(3).
-
-
O_WRONLY
- Open the queue for sending messages. The process can use
the returned message queue descriptor with
mq_send(3) but not
mq_receive(3).
-
-
O_RDWR
- Open the queue for both receiving and sending messages. The
process can use any of the functions allowed for
O_RDONLY
and
O_WRONLY
.
In all cases, a message queue may be open multiple times in the same or
different processes for sending/receiving messages.
Any combination of the remaining flags may be specified in the value of
oflag:
-
-
O_CREAT
- Create a message queue. It requires two additional
arguments: mode and attr. If
the pathname name has already been used to create a
message queue that still exists, then this flag will have no effect,
except as noted under
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 mode argument, except those set in the file
mode creation mask of the process. When bits in mode
other than the file permission bits are specified, the effect is
unspecified. If attr is
NULL
, the message queue will be 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 mq_attr structure
referred to by attr. If attr
is non-NULL
, but the
calling process does not have the appropriate privilege on
name, the mq_open() function will
fail and return an error without creating the message queue.
-
-
O_EXCL
- If
O_EXCL
and
O_CREAT
are set, mq_open() fails
if the message queue 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
mq_open() naming the same name
with O_EXCL
and O_CREAT
set. If O_EXCL
is set and
O_CREAT
is not set, the result is undefined.
-
-
O_NONBLOCK
- Determines whether an
mq_send(3) or
mq_receive(3) waits for
resources or messages that are not currently available, or fails with
errno set to
EAGAIN
.
The
mq_open() function does not add or remove messages from
the queue.
IMPLEMENTATION NOTES
The
select(2) and
poll(2) system calls to the
message queue descriptor are supported by
NetBSD,
however, it is not portable.
RETURN VALUES
Upon successful completion,
mq_open() returns a message queue
descriptor. Otherwise, the function returns (
mqd_t
) -1
and sets the global variable
errno to indicate the
error.
ERRORS
The
mq_open() function fails if:
-
-
- [
EACCES
]
- The message queue exists and the permissions specified by
oflag are denied, or the message queue does not
exist and permission to create the message queue is denied.
-
-
- [
EEXIST
]
O_CREAT
and
O_EXCL
are set and the named message queue already
exists.
-
-
- [
EINTR
]
- The mq_open() function was interrupted by
a signal.
-
-
- [
EINVAL
]
- The mq_open() function is not supported
for the given name, or
O_CREAT
was specified in
oflag, the value of attr is
not NULL
, and either
mq_maxmsg or mq_msgsize was
less than or equal to zero.
-
-
- [
EMFILE
]
- Too many message queue descriptors or file descriptors are
currently in use by this process.
-
-
- [
ENAMETOOLONG
]
- The length of the name argument
exceeds {
PATH_MAX
} or a pathname component is
longer than {NAME_MAX
}.
-
-
- [
ENFILE
]
- Too many message queues are currently open in the
system.
-
-
- [
ENOENT
]
O_CREAT
is not set and the named
message queue does not exist.
-
-
- [
ENOSPC
]
- There is insufficient space for the creation of the new
message queue.
SEE ALSO
mq(3),
mq_close(3),
mq_unlink(3)
STANDARDS
This function conforms to the
IEEE Std 1003.1-2001
(“POSIX.1”) standard.
HISTORY
This function first appeared in
NetBSD 5.0.
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
http://www.opengroup.org/unix/online.html.