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MQ_NOTIFY(3P)		   POSIX Programmer's Manual		 MQ_NOTIFY(3P)

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 mes‐
       sage 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 notifi‐
       cation by a message queue. If the calling process or any other  process
       has already registered for notification of message arrival at the spec‐
       ified 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 registra‐
       tion 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_time‐
       dreceive()  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  mes‐
       sage 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, ¬) == -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 Electri‐
       cal 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.ker‐
       nel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group		     2013			 MQ_NOTIFY(3P)
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