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MQ_OPEN(3P) POSIX Programmer's Manual MQ_OPEN(3P)
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.
mq_open — open a message queue (REALTIME)
#include <mqueue.h>
mqd_t mq_open(const char *name, int oflag, ...);
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 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 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. 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 calling 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(3p) and
mq_receive(3p) for details.
The mq_open() function does not add or remove messages from the
queue.
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.
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.
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.
None.
None.
None.
A future version might require the mq_open() and mq_unlink()
functions to have semantics similar to normal file system operations.
mq_close(3p), mq_getattr(3p), mq_receive(3p), mq_send(3p),
mq_setattr(3p), mq_unlink(3p), msgctl(3p), msgget(3p), msgrcv(3p),
msgsnd(3p)
The Base Definitions volume of POSIX.1‐2008, mqueue.h(0p)
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 Electrical 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.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 MQ_OPEN(3P)
Pages that refer to this page: mqueue.h(0p), mq_close(3p), mq_getattr(3p), mq_notify(3p), mq_receive(3p), mq_send(3p), mq_setattr(3p), mq_unlink(3p), msgctl(3p), msgget(3p), msgrcv(3p), msgsnd(3p), umask(3p)