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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>

       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_notify_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	  (see

       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.

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


       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>

       #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 3.22 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at

Linux				  2009-03-31			  MQ_NOTIFY(3)

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