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



NAME
       mq_notify - register for notification when a message is available

SYNOPSIS
       #include <mqueue.h>

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

       Link with -lrt.

DESCRIPTION
       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 notification is to be performed.  This
       field has one of the following values:

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

       SIGEV_SIGNAL
              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 infor-
              mation is available if the signal is accepted using sigwaitinfo(2).

       SIGEV_THREAD
              Deliver notification by invoking notification->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
              pthread_attr_init(3)).

       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 mes-
       sage arrives.

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

       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.)

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

ERRORS
       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 cur-
       rently registered to receive notifications for the queue mqdes.

CONFORMING TO
       POSIX.1-2001.

EXAMPLE
       The following program registers a notification request for the message queue named in its command-line argument.  Notifi-
       cation 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)
               handle_error("mq_getattr");
           buf = malloc(attr.mq_msgsize);
           if (buf == NULL)
               handle_error("malloc");

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

           printf("Read %ld bytes from MQ\n", (long) nr);
           free(buf);
           exit(EXIT_SUCCESS);         /* Terminate the process */
       }

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

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

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

           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)
               handle_error("mq_notify");

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

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

COLOPHON
       This page is part of release 3.25 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                                                      2009-09-15                                               MQ_NOTIFY(3)