mq_open(3p) — Linux manual page


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

PROLOG         top

       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         top

       mq_open — open a message queue (REALTIME)

SYNOPSIS         top

       #include <mqueue.h>

       mqd_t mq_open(const char *name, int oflag, ...);

DESCRIPTION         top

       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

       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

RETURN VALUE         top

       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         top

       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

       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

       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.

              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

       The following sections are informative.

EXAMPLES         top




RATIONALE         top



       A future version might require the mq_open() and mq_unlink()
       functions to have semantics similar to normal file system operations.

SEE ALSO         top

       mq_close(3p), mq_getattr(3p), mq_receive(3p), mq_send(3p),
       mq_setattr(3p), mq_unlink(3p), msgctl(3p), msgget(3p), msgrcv(3p),

       The Base Definitions volume of POSIX.1‐2008, mqueue.h(0p)

COPYRIGHT         top

       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 .

       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 .

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)