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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>

       int mq_notify(mqd_t mqdes, const struct sigevent *sevp);

       Link with -lrt.

       mq_notify()  allows the calling process to register or unregister for delivery of an asyn‐
       chronous notification when a new message arrives on the empty message queue referred to by
       the descriptor mqdes.

       The  sevp  argument  is a pointer to a sigevent structure.  For the definition and general
       details of this structure, see sigevent(7).

       If sevp 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 sevp
       points specifies how notification is to be performed.  This field has one of the following

              A  "null" notification: the calling process is registered as the target for notifi‐
              cation, 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 func‐
              tion of a new thread.  See sigevent(7) for details.

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

       If sevp 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 regis‐
       ter to receive a message notification for this queue.

       Message notification occurs only 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 occur only 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 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 fur‐
       ther 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 mes‐
       sages without blocking once it is empty.)

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

       EBADF  The descriptor specified in mqdes is invalid.

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

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

       ENOMEM Insufficient memory.

       POSIX.1-2008 says that an implementation may generate an EINVAL error if sevp 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 exe‐
       cutes a function which reads one message from the queue and then terminates the process.

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

       main(int argc, char *argv[])
           mqd_t mqdes;
           struct sigevent sev;

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

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

           sev.sigev_notify = SIGEV_THREAD;
           sev.sigev_notify_function = tfunc;
           sev.sigev_notify_attributes = NULL;
           sev.sigev_value.sival_ptr = &mqdes;   /* Arg. to thread func. */
           if (mq_notify(mqdes, &sev) == -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_over‐
       view(7), sigevent(7)

       This  page  is  part of release 3.74 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at http://www.kernel.org/doc/man-pages/.

Linux                                       2014-04-06                               MQ_NOTIFY(3)

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