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 message queue.  The name argument points to a string
     naming a message queue.  The name argument should conform to the construction rules for a pathname.  The name should begin with a slash char-
     acter.  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 following 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 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 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 speci-
		 fied, 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 creating 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 descriptor 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.

     Please see the mqueuefs(5) man page for instructions on loading the module or compiling the service into the kernel.

RETURN VALUES

     Upon successful completion, the function returns a message queue descriptor; otherwise, the function returns (mqd_t)-1 and sets the global
     variable 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 per-
			mission 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.

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), mqueuefs(5)

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 Specifications Issue 6, Copyright (C) 2001-2004 by the Institute of Elec-
     trical 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.

BSD
								September 15, 2014							       BSD