Provided by: manpages-posix-dev_2013a-2_all 
PROLOG
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 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() 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.
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 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 .