mq_notify

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>

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

       Link with -lrt.

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 message queue descriptor
       mqdes.

       The sevp argument is a pointer to a sigevent structure.  For the  defi-
       nition 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:

       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.   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.

       SIGEV_THREAD
              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 mes-
       sage queue.

       If sevp is NULL, and the calling process is currently registered to re-
       ceive  notifications  for  this message queue, then the registration is
       removed; another process can then register to receive a message notifi-
       cation 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 ef-
       fect.

       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 nonblocking mode is useful for empty-
       ing 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 message queue 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.

ATTRIBUTES
       For an  explanation  of  the  terms  used  in  this  section,  see  at-
       tributes(7).

       +------------+---------------+---------+
       |Interface   | Attribute     | Value   |
       +------------+---------------+---------+
       |mq_notify() | Thread safety | MT-Safe |
       +------------+---------------+---------+

CONFORMING TO
       POSIX.1-2001.

NOTES
   C library/kernel differences
       In the glibc implementation, the mq_notify() library function is imple-
       mented on top of the system call of the same name.  When sevp is  NULL,
       or  specifies a notification mechanism other than SIGEV_THREAD, the li-
       brary 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.

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.

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

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

           printf("Read %zd bytes from MQ\n", nr);
           free(buf);
           exit(EXIT_SUCCESS);         /* Terminate the process */
       }

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

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

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

           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)
               handle_error("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), sigevent(7)

COLOPHON
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Linux                             2019-03-06                      MQ_NOTIFY(3)