mq_notify(3)



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);

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  noti-
       fication  is to be performed.  This field has one of the following val-
       ues:

       SIGEV_NONE
              A "null" notification: the calling process is registered as  the
              target  for notification, but when a message arrives, no notifi-
              cation 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
              information  is  available  if the signal is accepted using sig-
              waitinfo(2).

       SIGEV_THREAD
              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(), then any message notification registration is
       ignored: the message is delivered to  the  process  or  thread  calling
       mq_receive(),  and  the  message  notification  registration remains in
       effect.

       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.)

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  notifica-
              tion->sigev_signo is not a a valid signal number.

       ENOMEM Insufficient memory.

CONFORMING TO
       POSIX.1-2001.

EXAMPLE
       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 die(msg) { perror(msg); exit(EXIT_FAILURE); }

       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) die("mq_getattr");
           buf = malloc(attr.mq_msgsize);
           if (buf == NULL) die("malloc");

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

           printf("Read %ld bytes from MQ0, (long) nr);
           free(buf);
           exit(EXIT_SUCCESS);         /* Terminate the process */
       }

       int
       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) die("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) die("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)

Linux 2.6.16                      2006-02-25                      MQ_NOTIFY(3)

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