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mq_open(3)
NAME
  mq_open - Establishes the connection between a message queue and a message
  queue descriptor (P1003.1b)

SYNOPSIS
  #include <mqueue.h>

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

LIBRARY
  Realtime Library (librt.so, librt.a)

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

  ooption
      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 options.  An application must specify exactly one of
  the first three options:

  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 options is valid:

  O_CREAT
      Creates a message queue.	Use of this option 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 option 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 permission, 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 option only in combination 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 option is
      set and resources are not available, the mq_send and mq_receive
      functions fail and errno is set to EAGAIN. This option is a per-open
      descriptor option, 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 ooption 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.

  [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 ooption, the value of attr is not NULL, and either
      mq_maxmsg or mq_msgsize was less than or equal to zero.

      POSIX message queues should be created and modified only through POSIX
      message queue routines. It is possible for this error to be returned if
      the given name refers to a file that cannot be recognized, perhaps
      because it was created or modified by an unsupported call.

  [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.

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

  Guide to Realtime Programming

Index for Section 3

Alphabetical listing for M

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