NAME
mq_open - open a message queue (REALTIME)
SYNOPSIS
#include <mqueue.h>
mqd_t mq_open(const char *name, int oflag, ...);
DESCRIPTION
The mq_open() function shall establish the connection between a process
and a message queue with a message queue descriptor. It shall create 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. It is unspecified whether the name appears in
the file system and is visible to other functions that take pathnames
as arguments. The name argument shall conform to the construction
rules for a pathname. If name begins with the slash character, then
processes calling mq_open() with the same value of name shall refer to
the same message queue object, as long as that name has not been
removed. If 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 name is implementation-defined. If
the name argument is not the name of an existing message queue and
creation is not requested, mq_open() shall fail and return an error.
A message queue descriptor may be implemented using a file descriptor,
in which case applications can open up to at least {OPEN_MAX} file and
message queues.
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 shall be 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 shall 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(), but
not mq_send(). A message queue may be open multiple times in the
same or different processes for receiving messages.
O_WRONLY
Open the queue for sending messages. The process can use the
returned message queue descriptor with mq_send() but not
mq_receive(). A message queue may be open multiple times in the
same or different processes for sending messages.
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. A message queue may be open multiple times in the same
or different processes for sending 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, which shall be of type mode_t, and attr, which shall be a
pointer to an mq_attr structure. If the pathname name has
already been used to create a message queue that still exists,
then this flag shall have no effect, except as noted under
O_EXCL. Otherwise, a message queue shall be created without any
messages in it. The user ID of the message queue shall be set to
the effective user ID of the process, and the group ID of the
message queue shall be set to the effective group ID of the
process. The file permission bits shall be set to the value of
mode. When bits in mode other than file permission bits are set,
the effect is implementation-defined. If attr is NULL, the
message queue shall 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 shall 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 shall fail and return an error without
creating the message queue.
O_EXCL If O_EXCL and O_CREAT are set, mq_open() shall fail 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 shall 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() or mq_receive() waits for
resources or messages that are not currently available, or fails
with errno set to [EAGAIN]; see mq_send() and mq_receive() for
details.
The mq_open() function does not add or remove messages from the queue.
RETURN VALUE
Upon successful completion, the function shall return a message queue
descriptor; otherwise, the function shall return (mqd_t)-1 and set
errno to indicate the error.
ERRORS
The mq_open() function shall fail 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.
EINVAL 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.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
None.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
mq_close() , mq_getattr() , mq_receive() , mq_send() , mq_setattr() ,
mq_timedreceive() , mq_timedsend() , mq_unlink() , msgctl() , msgget()
, msgrcv() , msgsnd() , the Base Definitions volume of
IEEE Std 1003.1-2001, <mqueue.h>
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 .