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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.

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
       for Information  Technology  --  Portable  Operating  System  Interface  (POSIX),  The  Open  Group  Base
       Specifications  Issue  7, 2018 Edition, Copyright (C) 2018 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 .

       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                                   2017                                       MQ_OPEN(3POSIX)