mq_open(3)						     Library Functions Manual							mq_open(3)

NAME

       mq_open - Establishes the connection between a message queue and a message queue descriptor (P1003.1b)

LIBRARY

       Realtime Library (librt.so, librt.a)

SYNOPSIS

       #include <mqueue.h>

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

PARAMETERS

       *name   Specifies a pointer to the name of a string naming the message queue.  The string can be a pathname.

       oflag   Specifies  the  request	for  receive or send access to the message queue, as well as the blocking attributes of the resulting open
	       descriptor for the calling process.

       This value of this argument is the bitwise inclusive OR of the values from the following  list  of  flags.   An	application  must  specify
       exactly one of the first three flags:

       O_RDONLY       Opens  the message queue for receiving messages.	This enables calls to the mq_receive function but not calls to the mq_send
		      function for the calling process.  Note that each process has its own read/write access to a queue.

       O_WRONLY       Opens the queue for sending messages.  This enables calls to the mq_send function but not calls to the  mq_receive  function
		      for the calling process.

       O_RDWR	      Opens the queue for receiving and sending messages for the calling process.

       Any combination of the following flags is valid:

       O_CREAT	      Creates  a  message  queue.   Use  of  this flag requires specification of two additional arguments:  mode, which is of type
		      mode_t, and attr, which points to a mq_attr structure.  If name has already been used to create  a  message  queue  that	is
		      still  open,  this  flag has no effect, except as noted under O_EXCL.  Otherwise, a message queue is created.  The ID of the
		      message queue becomes the effective user ID, and the group ID of the message queue becomes the effective	group  ID  of  the
		      process.	 The  mode argument sets the file permission bits.  If the attr argument is NULL, the function creates the message
		      queue with default attributes.  If the attr argument is not NULL, and the calling process has appropriate privilege or  per-
		      mission,	the  message  queue attributes are set as if the mq_setattr function were called with the attribute argument attr.
		      If the attr argument is not NULL, but the calling process does not have the appropriate privilege or permission, the mq_open
		      argument fails and returns an error.

       O_EXCL	      Prevents	the opening of a message queue if O_CREAT is set and the message queue already exists.	Use this flag only in com-
		      bination with O_CREAT.

       O_NONBLOCK     Allows calls to the mq_send and mq_receive functions to continue execution while waiting for resources  or  messages.   When
		      this  flag  is  set  and	resources are not available, the mq_send and mq_receive functions fail and errno is set to EAGAIN.
		      This flag is a per-open descriptor flag, and applies to the calling  process  only.   Each  process  has	its  own  blocking
		      attributes for a queue.

DESCRIPTION

       The  mq_open  function establishes the connection between a message queue and a message queue descriptor.  This function creates a new open
       message queue description that refers to a specified message queue and a message queue descriptor that refers to that  open  message  queue
       description.  The message queue descriptor is then available for use by other functions for reference to the message queue from the calling
       process.

RETURN VALUES

       On successful completion, this function returns a message queue descriptor.

       Otherwise, the function returns -1 and sets errno to indicate the error.

ERRORS

       The mq_open function fails under the following conditions:

       [EACCES]       The message queue exists and the permissions specified by oflag are denied, or the message queue does not exist and  permis-
		      sion to create the message queue is denied.

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

       [EFAULT]       An internal virtual memory error occurred.

       [EINTR]	      The mq_open operation was interrupted by a signal.

       [EINVAL]       The  mq_open  operation  is not supported for the given name. 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 string exceeds {PATH_MAX}, or a pathname component is longer than {NAME_MAX} while  {_POSIX_NO_TRUNC}
		      is in effect.

       [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.

RELATED INFORMATION

       Functions: mq_close(3), mq_getattr(3), mq_receive(3), mq_send(3), mq_setattr(3), mq_unlink(3)

       Guide to Realtime Programming delim off

																	mq_open(3)