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MQ_OPEN(2)		    BSD	System Calls Manual		    MQ_OPEN(2)

     mq_open --	open a message queue (REALTIME)

     POSIX Real-time Library (librt, -lrt)

     #include <mqueue.h>

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

     The mq_open() system call establishes the connection between a process
     and a message queue with a	message	queue descriptor.  It creates 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 mes-
     sage queue.  The name argument points to a	string naming a	message	queue.
     The name argument should conform to the construction rules	for a path-
     name.  The	name should begin with a slash character.  Processes calling
     mq_open() with the	same value of name refers to the same message queue
     object, as	long as	that name has not been removed.	 If the	name argument
     is	not the	name of	an existing message queue and creation is not re-
     quested, mq_open()	will fail and return an	error.

     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 would 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 follow-
     ing list.	Applications should 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 receiv-
		 ing 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

     O_CREAT	 Create	a message queue.  It requires two additional argu-
		 ments:	mode, which is of type mode_t, and attr, which is a
		 pointer to an mq_attr structure.  If the pathname name	has
		 already been used to create a message queue that still	ex-
		 ists, then this flag has no effect, except as noted under
		 O_EXCL.  Otherwise, a message queue will be created without
		 any messages in it.  The user ID of the message queue will be
		 set to	the effective user ID of the process, and the group ID
		 of the	message	queue will be set to the effective group ID of
		 the process.  The permission bits of the message queue	will
		 be set	to the value of	the mode argument, except those	set in
		 the file mode creation	mask of	the process.  When bits	in
		 mode other than the file permission bits are specified, the
		 effect	is unspecified.	 If attr is NULL, the message queue is
		 created with implementation-defined default message queue at-
		 tributes.  If attr is non-NULL	and the	calling	process	has
		 the appropriate privilege on name, the	message	queue
		 mq_maxmsg and mq_msgsize attributes will be set to the	values
		 of the	corresponding members in the mq_attr structure re-
		 ferred	to by attr.  If	attr is	non-NULL, but the calling
		 process does not have the appropriate privilege on name, the
		 mq_open() function will fail and return an error without cre-
		 ating the message queue.

     O_EXCL	 If O_EXCL and O_CREAT are set,	mq_open() will fail if the
		 message queue name exists.

     O_NONBLOCK	 Determines whether an mq_send() or mq_receive() waits for re-
		 sources or messages that are not currently available, or
		 fails with errno set to EAGAIN; see mq_send(2)	and
		 mq_receive(2) for details.

     The mq_open() system call does not	add or remove messages from the	queue.

     FreeBSD implements	message	queue based on file descriptor.	 The descrip-
     tor is inherited by child after fork(2).  The descriptor is closed	in a
     new image after exec(3).  The select(2) and kevent(2) system calls	are
     supported for message queue descriptor.

     Upon successful completion, the function returns a	message	queue descrip-
     tor; otherwise, the function returns (mqd_t)-1 and	sets the global	vari-
     able errno	to indicate the	error.

     The mq_open() system call will fail if:

     [EACCES]		The message queue exists and the permissions specified
			by oflag are denied, or	the message queue does not ex-
			ist and	permission to create the message queue is de-

     [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.

     [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 sys-

     [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.

     mq_close(2), mq_getattr(2), mq_receive(2),	mq_send(2), mq_setattr(2),
     mq_timedreceive(3), mq_timedsend(3), mq_unlink(3)

     The mq_open() system call conforms	to IEEE	Std 1003.1-2004	("POSIX.1").

     Support for POSIX message queues first appeared in	FreeBSD	7.0.

     This implementation places	strict requirements on the value of name: it
     must begin	with a slash (`/') and contain no other	slash characters.

     Portions of this text are reprinted and reproduced	in electronic form
     from IEEE Std 1003.1, 2004	Edition, Standard for Information Technology
     --	Portable Operating System Interface (POSIX), The Open Group Base Spec-
     ifications	Issue 6, Copyright (C) 2001-2004 by the	Institute of Electri-
     cal 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 ref-
     eree document.  The original Standard can be obtained online at

BSD			       November	29, 2005			   BSD


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