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

NAME
     mq_open — open a message queue (REALTIME)

LIBRARY
     POSIX Real-time Library (librt, -lrt)

SYNOPSIS
     #include <mqueue.h>

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

DESCRIPTION
     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
     requested, 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
     oflag:

     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
		 exists, 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
		 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 will be set to the values
		 of the corresponding 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 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
		 resources 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.

NOTES
     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.

RETURN VALUES
     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.

ERRORS
     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
			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 sys‐
			tem.

     [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.

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

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

HISTORY
     Support for POSIX message queues first appeared in FreeBSD 7.0.

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

COPYRIGHT
     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
	  http://www.opengroup.org/unix/online.html.

BSD			       November 29, 2005			   BSD
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