MQ_OPEN(3P) POSIX Programmer's Manual MQ_OPEN(3P)
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This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the corresponding
Linux manual page for details of Linux behavior), or the interface may
not be implemented on Linux.
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 nam-
ing 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 conforms to the construction rules for a
pathname, except that the interpretation of <slash> characters other
than the leading <slash> character in name is implementation-defined,
and that the length limits for the name argument are implementation-
defined and need not be the same as the pathname limits {PATH_MAX} and
{NAME_MAX}. 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 implemen-
tation-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 pro-
tected file.
The value of oflag is the bitwise-inclusive OR of values from the fol-
lowing 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 mes-
sages.
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 argu-
ments: 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 mes-
sage queue shall be set to the effective user ID of the
process. The group ID of the message queue shall be set to
the effective group ID of the process; however, if the name
argument is visible in the file system, the group ID may be
set to the group ID of the containing directory. When bits
in mode other than the file permission bits are specified,
the effect is unspecified. If attr is NULL, the message
queue shall be created with implementation-defined default
message queue attributes. If attr is non-NULL and the call-
ing process has appropriate privileges 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. The values of the mq_flags
and mq_curmsgs members of the mq_attr structure shall be
ignored. If attr is non-NULL, but the calling process does
not have appropriate privileges 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 cur-
rently in use by this process.
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.
If any of the following conditions occur, the mq_open() function may
return (mqd_t)-1 and set errno to the corresponding value.
ENAMETOOLONG
The length of the name argument exceeds {_POSIX_PATH_MAX} on
systems that do not support the XSI option or exceeds
{_XOPEN_PATH_MAX} on XSI systems, or has a pathname component
that is longer than {_POSIX_NAME_MAX} on systems that do not
support the XSI option or longer than {_XOPEN_NAME_MAX} on XSI
systems.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
None.
RATIONALE
None.
FUTURE DIRECTIONS
A future version might require the mq_open() and mq_unlink() functions
to have semantics similar to normal file system operations.
SEE ALSO
mq_close(), mq_getattr(), mq_receive(), mq_send(), mq_setattr(),
mq_unlink(), msgctl(), msgget(), msgrcv(), msgsnd()
The Base Definitions volume of POSIX.1-2008, <mqueue.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the source
files to man page format. To report such errors, see https://www.ker-
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 MQ_OPEN(3P)
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