MQ_OPEN(3)                 POSIX Programmer's Manual                MQ_OPEN(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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.1-2017, <mqueue.h>

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(3)
