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PROLOG

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of this interface
       may differ (consult the corresponding Linux manual page for details of Linux behavior), or the  interface
       may not be implemented on Linux.

NAME

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

SYNOPSIS

       #include <mqueue.h>

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

DESCRIPTION

       If the argument notification is not NULL, this function shall 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 shall 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 shall fail.

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

       When the notification is sent to the registered process, its registration shall be removed.  The  message
       queue shall then be 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() or mq_timedreceive() waiting to receive a message when a message arrives  at  the
       queue,   the   arriving   message  shall  satisfy  the  appropriate  mq_receive()  or  mq_timedreceive(),
       respectively. The resulting behavior is as if the message queue remains empty, and no notification  shall
       be sent.

RETURN VALUE

       Upon  successful  completion,  the  mq_notify()  function  shall  return  a value of zero; otherwise, the
       function shall return a value of −1 and set errno to indicate the error.

ERRORS

       The mq_notify() function shall fail if:

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

       EBUSY  A process is already registered for notification by the message queue.

       The mq_notify() function may fail if:

       EINVAL The notification argument is NULL and the process is currently not registered.

       The following sections are informative.

EXAMPLES

       The following program registers a notification request for the message queue named  in  its  command-line
       argument.  Notification is performed by creating a thread. The thread executes a function which reads one
       message from the queue and then terminates the process.

           #include <pthread.h>
           #include <mqueue.h>
           #include <assert.h>
           #include <stdio.h>
           #include <stdlib.h>
           #include <unistd.h>

           static void                     /* Thread start function */
           tfunc(union sigval sv)
           {
               struct mq_attr attr;
               ssize_t nr;
               void *buf;
               mqd_t mqdes = *((mqd_t *) sv.sival_ptr);

               /* Determine maximum msg size; allocate buffer to receive msg */

               if (mq_getattr(mqdes, &attr) == -1) {
                   perror("mq_getattr");
                   exit(EXIT_FAILURE);
               }
               buf = malloc(attr.mq_msgsize);

               if (buf == NULL) {
                   perror("malloc");
                   exit(EXIT_FAILURE);
               }

               nr = mq_receive(mqdes, buf, attr.mq_msgsize, NULL);
               if (nr == -1) {
                   perror("mq_receive");
                   exit(EXIT_FAILURE);
               }

               printf("Read %ld bytes from message queue\n", (long) nr);
               free(buf);
               exit(EXIT_SUCCESS);         /* Terminate the process */
           }

           int
           main(int argc, char *argv[])
           {
               mqd_t mqdes;
               struct sigevent not;

               assert(argc == 2);

               mqdes = mq_open(argv[1], O_RDONLY);
               if (mqdes == (mqd_t) -1) {
                   perror("mq_open");
                   exit(EXIT_FAILURE);
               }

               not.sigev_notify = SIGEV_THREAD;
               not.sigev_notify_function = tfunc;
               not.sigev_notify_attributes = NULL;
               not.sigev_value.sival_ptr = &mqdes;   /* Arg. to thread func. */
               if (mq_notify(mqdes, &not) == -1) {
                   perror("mq_notify");
                   exit(EXIT_FAILURE);
               }

               pause();    /* Process will be terminated by thread function */
           }

APPLICATION USAGE

       None.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

       mq_open(), mq_send(), mq_receive(), msgctl(), msgget(), msgrcv(), msgsnd()

       The Base Definitions volume of POSIX.1‐2008, <mqueue.h>

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition,
       Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open  Group  Base
       Specifications  Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc
       and The Open Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .

       Any typographical or formatting errors that appear in this page are most likely to have  been  introduced
       during   the   conversion  of  the  source  files  to  man  page  format.  To  report  such  errors,  see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                                   2013                                     MQ_NOTIFY(3POSIX)