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       mq_notify - register for notification when a message is available


       #include <mqueue.h>

       int 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.  For
       the definition and general details of this structure, see sigevent(7).

       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 structure to which notification 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
              notification 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
       message queue.

       If  notification  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 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
       notification  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
       notification.  This should be done before emptying all unread  messages
       from  the  queue.  (Placing the queue in nonblocking mode is useful for
       emptying 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
              notification->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
       message from the queue and then terminates the process.

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

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

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

           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, &not) == -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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