mq_open(3c)							   mq_open(3c)

NAME    [Toc]    [Back]

     mq_open - open/create a message queue

C SYNOPSIS    [Toc]    [Back]

     #include <mqueue.h>

     mqd_t mq_open (const char *mq_name<b>, int oflag<b>, ...	/* mode_t mode<b>,	struct
     mq_attr *mq_attr <b>*/);

DESCRIPTION    [Toc]    [Back]

     mq_open is	used to	open or	to create and open a message queue. It returns
     a message queue descriptor	that is	used as	the handle for other
     operations	on the queue. mq_name points to	a path name that is
     constructed using the standard filename conventions.

     The oflag argument	is used	to specify send, receive or send/receive
     access to the queue.  It is also used to specify creation of the queue.
     The following oflags may be selected (the first three flags are mutually
     exclusive):

     O_RDONLY	  Open the queue for receiving messages, but not for sending
		  messages.

     O_WRONLY	  Open the queue for sending messages, but not for receiving
		  messages.

     O_RDWR	  Open the queue for both sending and receiving	messages.

     O_NONBLOCK	  Do not block on mq_send when sending messages	to a full
		  queue	(or if required	resources are unavailable) or on
		  mq_receive when receiving messages from a empty queue	(or if
		  required resources are unavailable).	[see mq_send(3c) and
		  mq_receive(3c)].

     O_CREAT	  Create the message queue.  If	the message queue exists, this
		  flag has no effect, except as	noted under O_EXCL below.
		  Otherwise, the third argument, mode and the fourth argument
		  mq_attr are used to create the message queue.	The uid	and
		  gid of the queue are set to the effective uid	and gid,
		  respectively,	of the process.	The mode for the queue is set
		  to the value of the mode argument, similar to	that for file
		  creation. The	mode of	queue specifies	the read and write
		  (corresponding to receive and	send) permission for user,
		  group	and the	world. The queue is created according to the
		  specifications in the	fourth argument, mq_attr. If this
		  argument is non-NULL,	the mq_msgsize field of	the mq_attr
		  structure specifies the maximum size of a single message in
		  the queue. Messages larger than this size sent to the	queue
		  will generate	an error. The mq_maxmsg	field specifies	the
		  maximum number of messages on	the queue at any one time.  If
		  the mq_attr argument is NULL,	the queue is created using the
		  default values of 32 and 64 for mq_maxmsg and	mq_msgsize,



									Page 1






mq_open(3c)							   mq_open(3c)



		  respectively.

     O_EXCL	  If O_EXCL and	O_CREAT	are set, mq_open will fail if the
		  queue	exists.	The check for the existence of the file	and
		  the creation of the queue if it does not exist is atomic
		  with respect to other	processes executing mq_open with the
		  same name in the same	directory with O_EXCL and O_CREAT set.

     When a new	message	queue is created, the queue is void of messages.
     Opening an	existing message queue has no effect on	its contents.

     Message queue state is preserved across application termination, but the
     contents of a queue after system reboot are undefined.

     POSIX message queues are compatible across	all MIPS ABIs: o32, n32	and
     64.

     mq_open will fail if one or more of the following conditions are true:

     [EACCES]	    The	message	queue does not exist and permission to create
		    the	queue is denied.

     [EACCES]	    The	message	queue exists, but the permissions in oflag are
		    denied.

     [EACCES]	    A component	of mq_name has no search permission or does
		    not	exist.

     [EEXIST]	    O_CREAT and	O_EXCL are set,	and the	message	queue exists.

     [EFAULT]	    mq_name points outside the allocated address space of the
		    process.

     [EINTR]	    A signal interrupted the mq_open function call.

     [EINVAL]	    The	values of mq_msgsize and mq_maxmsg fields in the
		    mq_attr structure are outside the valid range. Invalid
		    flags specified in the oflag argument.

     [EMFILE]	    The	process	has too	many open files	or message queues[see
		    getrlimit(2)].

     [ENAMETOOLONG] The	length of the mq_name argument exceeds {PATH_MAX}, or
		    the	length of a mq_name component exceeds {NAME_MAX} while
		    {_POSIX_NO_TRUNC} is in effect.

     [ENFILE]	    There are too many files and/or message queues in the
		    system.

     [ENOENT]	    O_CREAT is not set and the named queue does	not exist.





									Page 2






mq_open(3c)							   mq_open(3c)



     [ENOENT]	    O_CREAT is set and a component of the mq_name does not
		    exist or is	not directory.

     [ENOMEM]	    The	system is unable to allocate a message queue
		    descriptor.

     [ENOSPC]	    O_CREAT is set, the	message	queue does not exist and the
		    system cannot create the queue due to lack of resources.

SEE ALSO    [Toc]    [Back]

     mq_close(3c), open(3c), getdtablesize(2), getrlimit(2), intro(3c),
     mq_receive(3c), mq_send(3c), mq_getattr(3c), mq_setattr(3c),
     mq_notify(3c), umask(2)

DIAGNOSTICS    [Toc]    [Back]

     Upon successful completion, the message queue descriptor is returned.
     Otherwise,	a value	of -1 is returned and errno is set to indicate the
     error.


									PPPPaaaaggggeeee 3333