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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 *sevp);

       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  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 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  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  register  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

       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 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.


       For   an   explanation   of   the  terms  used  in  this  section,  see

       │InterfaceAttributeValue   │
       │mq_notify() │ Thread safety │ MT-Safe │




   C library/kernel differences
       In the  glibc  implementation,  the  mq_notify()  library  function  is
       implemented  on  top of the system call of the same name.  When sevp is
       NULL, or specifies a notification mechanism  other  than  SIGEV_THREAD,
       the   library   function   directly   invokes  the  system  call.   For
       SIGEV_THREAD, much of the implementation resides  within  the  library,
       rather  than  the  kernel.   (This  is necessarily so, since the thread
       involved in handling the notification is one that must  be  managed  by
       the  C  library  POSIX  threads  implementation.)   The  implementation
       involves the use of a raw netlink(7) socket and creates  a  new  thread
       for each notification that is delivered to the process.


       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.

   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_overview(7), sigevent(7)


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