Quick ?s
Cheat Sheets
Man Pages
The Lynx
MQ_NOTIFY(3)		   Linux Programmers Manual		 MQ_NOTIFY(3)

       mq_notify - register for notification when a message is available


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

       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 descriptor mqdes.

       The  notification argument is a pointer to a sigevent structure that is
       defined something like the following:

	   union sigval {	   /* Data passed with notification */
	       int     sival_int;	  /* Integer value */
	       void   *sival_ptr;	  /* Pointer value */

	   struct sigevent {
	       int	    sigev_notify; /* Notification method */
	       int	    sigev_signo;  /* Notification signal */
	       union sigval sigev_value;  /* Data passed with
					     notification */
	       void	  (*sigev_notify_function) (union sigval);
					  /* Function for thread
					     notification */
	       void	   *sigev_notify_attributes;
					  /* Thread function attributes */

       If notification is a non-NULL pointer, then mq_notify()	registers  the
       calling	process  to  receive  message  notification.  The sigev_notify
       field of the sigevent to which notification points specifies how  noti
       fication  is to be performed.  This field has one of the following val

	      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.  If the signal is caught with a signal handler that
	      was registered using the sigaction(2) SA_SIGINFO flag, then  the
	      following  fields  are  set  in  the siginfo_t structure that is
	      passed as the second argument of the handler: si_code is set  to
	      SI_MESGQ;  si_signo is set to the signal number; si_value is set
	      to the value specified in notification->sigev_value;  si_pid  is
	      set  to the PID of the process that sent the message; and si_uid
	      is set to the real user ID of the  sending  process.   The  same
	      information  is  available  if the signal is accepted using sig

	      Deliver	   notification      by       invoking	     notifica
	      tion->sigev_thread_function  as  the  start  function  of  a new
	      thread.  The function is invoked with  notification->sigev_value
	      as  its sole argument.  If notification->sigev_notify_attributes
	      is not NULL, then it should point to a pthread_attr_t  structure
	      that defines attributes for the thread.

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

       If notification is NULL, and the calling process  is  currently	regis
       tered  to receive notifications for this message queue, then the regis
       tration is removed; another process can then register to receive a mes
       sage notification for this queue.

       Message	notification  only  occurs  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 only occur 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 non-blocking mode is useful for emp
       tying 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 descriptor specified in mqdes is invalid.

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

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

       ENOMEM Insufficient memory.


       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.


       #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 %ld bytes from MQ\n", (long) nr);
	   exit(EXIT_SUCCESS);	       /* Terminate the process */

       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)

	   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)

	   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)

       This page is part of release 3.05 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2006-02-25			  MQ_NOTIFY(3)

Yals.net is © 1999-2009 Crescendo Communications
Sharing tech info on the web for more than a decade!
This page was generated Thu Apr 30 17:05:24 2009