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

       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

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 exists.

       EINTR  The mq_open() function was interrupted by a signal.

       EINVAL The mq_open() function is not supported for the given

       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

       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 systems.

       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

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), msgsnd(3p)

       The Base Definitions volume of POSIX.1‐2017, mqueue.h(0p)

COPYRIGHT         top

       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 .

       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               2017                       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)