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



PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (con-
       sult 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 descrip-
       tor. 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 shall
       conform to the construction rules for a pathname. 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.   The  interpretation  of
       slash  characters  other than the leading slash character in name 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 mes-
              sages.

       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, and the group ID of the message queue shall be set to the effective group ID  of  the  process.
              The  file permission bits shall be set to the value of mode. When bits in mode other than file permission bits are
              set, the effect is implementation-defined. If attr is NULL, the message queue shall be  created  with  implementa-
              tion-defined  default  message  queue  attributes. If attr is non-NULL and the calling process has the appropriate
              privilege on name, the message queue mq_maxmsg and mq_msgsize attributes shall be set to the values of the  corre-
              sponding  members  in the mq_attr structure referred to by attr. If attr is non-NULL, but the calling process does
              not have the appropriate privilege 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.

       ENAMETOOLONG
              The length of the name argument exceeds {PATH_MAX} or a pathname component is longer than {NAME_MAX}.

       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.


       The following sections are informative.

EXAMPLES
       None.

APPLICATION USAGE
       None.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       mq_close(),  mq_getattr(),  mq_receive(),  mq_send(),  mq_setattr(),  mq_timedreceive(),   mq_timedsend(),   mq_unlink(),
       msgctl(), msgget(), msgrcv(), msgsnd(), the Base Definitions volume of IEEE Std 1003.1-2001, <mqueue.h>

COPYRIGHT
       Portions  of  this  text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for
       Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6,  Copy-
       right (C) 2001-2003 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.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                   MQ_OPEN(3P)