This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

       None.

       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)

       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.

       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.

       Thefollowingsectionsareinformative.

       None.

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

       mq_open — open a message queue (REALTIME)

       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.

       None.

       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.

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>

       #include <mqueue.h>

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