Man Linux: Main Page and Category List


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

              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.  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

              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 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)


       This  page  is  part of release 3.24 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at