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MQ_NOTIFY(3)		   Linux Programmer's Manual		  MQ_NOTIFY(3)

       mq_notify - register for notification when a message is available

       #include <mqueue.h>

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

       Link with -lrt.

       mq_notify()  allows  the	 calling process to register or unregister for
       delivery of an asynchronous notification when a new message arrives  on
       the  empty  message  queue  referred to by the message queue descriptor

       The sevp argument is a pointer to a sigevent structure.	For the	 defi-
       nition and general details of this structure, see sigevent(7).

       If  sevp	 is a non-null pointer, then mq_notify() registers the calling
       process to receive message notification.	 The sigev_notify field of the
       sigevent	 structure  to which sevp points specifies how notification is
       to be performed.	 This field has one of the following values:

	      A "null" notification: the calling process is registered as  the
	      target  for notification, but when a message arrives, no notifi-
	      cation is sent.

	      Notify  the  process  by	sending	 the   signal	specified   in
	      sigev_signo.   See sigevent(7) for general details.  The si_code
	      field of the siginfo_t structure will be set  to	SI_MESGQ.   In
	      addition, si_pid will be set to the PID of the process that sent
	      the message, and si_uid will be set to the real user ID  of  the
	      sending process.

	      Upon  message  delivery,	invoke	sigev_notify_function as if it
	      were the start function of a new thread.	 See  sigevent(7)  for

       Only  one process can be registered to receive notification from a mes-
       sage queue.

       If sevp is NULL, and the calling process	 is  currently	registered  to
       receive	notifications for this message queue, then the registration is
       removed; another process can then register to receive a message notifi-
       cation for this queue.

       Message	notification  occurs  only  when a new message arrives and the
       queue was previously empty.  If the queue was not  empty	 at  the  time
       mq_notify()  was	 called, then a notification will occur only after the
       queue is emptied and a new message arrives.

       If another process or thread is waiting to read a message from an empty
       queue  using  mq_receive(3), then any message notification registration
       is ignored: the message is delivered to the process or  thread  calling
       mq_receive(3),  and  the	 message  notification registration remains in

       Notification occurs once: after a notification is delivered, the	 noti-
       fication	 registration is removed, and another process can register for
       message notification.  If the notified process wishes  to  receive  the
       next  notification, it can use mq_notify() to request a further notifi-
       cation.	This should be done before emptying all unread	messages  from
       the queue.  (Placing the queue in nonblocking mode is useful for empty-
       ing the queue of messages without blocking once it is empty.)

       On success mq_notify() returns 0; on error, -1 is returned, with	 errno
       set to indicate the error.

       EBADF  The message queue descriptor specified in mqdes is invalid.

       EBUSY  Another  process	has already registered to receive notification
	      for this message queue.

       EINVAL sevp->sigev_notify is  not  one  of  the	permitted  values;  or
	      sevp->sigev_notify  is SIGEV_SIGNAL and sevp->sigev_signo is not
	      a valid signal number.

       ENOMEM Insufficient memory.

       POSIX.1-2008 says that an implementation may generate an	 EINVAL	 error
       if  sevp is NULL, and the caller is not currently registered to receive
       notifications for the queue mqdes.

       For  an	explanation  of	 the  terms  used   in	 this	section,   see

       |Interface   | Attribute	    | Value   |
       |mq_notify() | Thread safety | MT-Safe |


   C library/kernel differences
       In the glibc implementation, the mq_notify() library function is imple-
       mented on top of the system call of the same name.  When sevp is	 NULL,
       or  specifies  a	 notification  mechanism  other than SIGEV_THREAD, the
       library function directly invokes the system call.   For	 SIGEV_THREAD,
       much  of the implementation resides within the library, rather than the
       kernel.	(This is necessarily so, since the thread involved in handling
       the  notification  is  one  that must be managed by the C library POSIX
       threads implementation.)	 The implementation involves the use of a  raw
       netlink(7)  socket  and creates a new thread for each notification that
       is delivered to the process.

       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.

   Program source
       #include <pthread.h>
       #include <mqueue.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>

       #define handle_error(msg) \
	   do { perror(msg); exit(EXIT_FAILURE); } while (0)

       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 max. msg size; allocate buffer to receive msg */

	   if (mq_getattr(mqdes, &attr) == -1)
	   buf = malloc(attr.mq_msgsize);
	   if (buf == NULL)

	   nr = mq_receive(mqdes, buf, attr.mq_msgsize, NULL);
	   if (nr == -1)

	   printf("Read %zd bytes from MQ\n", nr);
	   exit(EXIT_SUCCESS);	       /* Terminate the process */

       main(int argc, char *argv[])
	   mqd_t mqdes;
	   struct sigevent sev;

	   if (argc != 2) {
	       fprintf(stderr, "Usage: %s <mq-name>\n", argv[0]);

	   mqdes = mq_open(argv[1], O_RDONLY);
	   if (mqdes == (mqd_t) -1)

	   sev.sigev_notify = SIGEV_THREAD;
	   sev.sigev_notify_function = tfunc;
	   sev.sigev_notify_attributes = NULL;
	   sev.sigev_value.sival_ptr = &mqdes;	 /* Arg. to thread func. */
	   if (mq_notify(mqdes, &sev) == -1)

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

       mq_close(3),   mq_getattr(3),  mq_open(3),  mq_receive(3),  mq_send(3),
       mq_unlink(3), mq_overview(7), sigevent(7)

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Linux				  2016-03-15			  MQ_NOTIFY(3)