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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

              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 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
       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 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.)


       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.


       For an explanation of the terms used in this section, see attributes(7).

       │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

   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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