NAME

       mq_notify — notify process that a message is available (REALTIME)

LIBRARY

       POSIX Real-time Library (librt, -lrt)

SYNOPSIS

       #include <mqueue.h>

       int
       mq_notify(mqd_t mqdes, const struct sigevent *notification);

DESCRIPTION

       If  the  argument  notification  is  not  NULL,  this system call will register the calling process to be
       notified of message arrival at an empty  message  queue  associated  with  the  specified  message  queue
       descriptor,  mqdes.   The notification specified by the notification argument will be sent to the process
       when the message queue transitions from empty to non-empty.   At  any  time,  only  one  process  may  be
       registered  for notification by a message queue.  If the calling process or any other process has already
       registered for notification of message arrival at the specified message  queue,  subsequent  attempts  to
       register for that message queue will fail.

       The  notification  argument  points  to a sigevent structure that defines how the calling process will be
       notified.  If notification->sigev_notify is SIGEV_NONE, then no signal will  be  posted,  but  the  error
       status  and the return status for the operation will be set appropriately.  If notification->sigev_notify
       is SIGEV_SIGNAL, then the signal specified in notification->sigev_signo will be sent to the process.  The
       signal will be queued to the process and the value specified in  notification->sigev_value  will  be  the
       si_value component of the generated signal.

       If notification is NULL and the process is currently registered for notification by the specified message
       queue, the existing registration will be removed.

       When  the notification is sent to the registered process, its registration is removed.  The message queue
       then is available for registration.

       If a process has registered for notification of message arrival at a message queue  and  some  thread  is
       blocked  in  mq_receive()  waiting to receive a message when a message arrives at the queue, the arriving
       message will satisfy the appropriate mq_receive().  The resulting behavior is as  if  the  message  queue
       remains empty, and no notification will be sent.

RETURN VALUES

       Upon  successful  completion,  the value 0 is returned; otherwise the value -1 is returned and the global
       variable errno is set to indicate the error.

ERRORS

       The mq_notify() system call will fail if:

       [EBADF]            The mqdes argument is not a valid message queue descriptor.

       [EBUSY]            Process is already registered for notification by the message queue.

SEE ALSO

       mq_open(2), mq_send(2), mq_timedsend(2), siginfo(3)

STANDARDS

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

HISTORY

       Support for POSIX message queues first appeared in FreeBSD 7.0.

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 Electrical 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.

Debian                                          November 29, 2005                                   MQ_NOTIFY(2)