MQ_NOTIFY(3)               POSIX Programmer's Manual              MQ_NOTIFY(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

NAME
       mq_notify -- notify process that a message is available (REALTIME)

SYNOPSIS
       #include <mqueue.h>

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

DESCRIPTION
       If the argument notification is not NULL, this function shall register
       the calling process to be notified of message arrival at an empty
       message queue associated with the specified message queue descriptor,
       mqdes.  The notification specified by the notification argument shall
       be sent to the process when the message queue transitions from empty to
       non-empty. At any time, only one process may be registered for
       notification by a message queue. If the calling process or any other
       process has already registered for notification of message arrival at
       the specified message queue, subsequent attempts to register for that
       message queue shall fail.

       If notification is NULL and the process is currently registered for
       notification by the specified message queue, the existing registration
       shall be removed.

       When the notification is sent to the registered process, its
       registration shall be removed. The message queue shall then be
       available for registration.

       If a process has registered for notification of message arrival at a
       message queue and some thread is blocked in mq__receive() or
       mq__timedreceive() waiting to receive a message when a message arrives
       at the queue, the arriving message shall satisfy the appropriate
       mq__receive() or mq__timedreceive(), respectively. The resulting behavior
       is as if the message queue remains empty, and no notification shall be
       sent.

RETURN VALUE
       Upon successful completion, the mq__notify() function shall return a
       value of zero; otherwise, the function shall return a value of -1 and
       set errno to indicate the error.

ERRORS
       The mq__notify() function shall fail if:

       EBADF  The mqdes argument is not a valid message queue descriptor.

       EBUSY  A process is already registered for notification by the message
              queue.

       The mq__notify() function may fail if:

       EINVAL The notification argument is NULL and the process is currently
              not registered.

       The following sections are informative.

EXAMPLES
       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 <assert.h>
           #include <stdio.h>
           #include <stdlib.h>
           #include <unistd.h>

           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 maximum msg size; allocate buffer to receive msg */

               if (mq_getattr(mqdes, &attr) == -1) {
                   perror("mq_getattr");
                   exit(EXIT_FAILURE);
               }
               buf = malloc(attr.mq_msgsize);

               if (buf == NULL) {
                   perror("malloc");
                   exit(EXIT_FAILURE);
               }

               nr = mq_receive(mqdes, buf, attr.mq_msgsize, NULL);
               if (nr == -1) {
                   perror("mq_receive");
                   exit(EXIT_FAILURE);
               }

               printf("Read %ld bytes from message queue\n", (long) nr);
               free(buf);
               exit(EXIT_SUCCESS);         /* Terminate the process */
           }

           int
           main(int argc, char *argv[])
           {
               mqd_t mqdes;
               struct sigevent not;

               assert(argc == 2);

               mqdes = mq_open(argv[1], O_RDONLY);
               if (mqdes == (mqd_t) -1) {
                   perror("mq_open");
                   exit(EXIT_FAILURE);
               }

               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) {
                   perror("mq_notify");
                   exit(EXIT_FAILURE);
               }

               pause();    /* Process will be terminated by thread function */
           }

APPLICATION USAGE
       None.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       mq__open(), mq__send(), mq__receive(), msgctl(), msgget(), msgrcv(),
       msgsnd()

       The Base Definitions volume of POSIX.1-2017, <mqueue.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  In the event of any discrepancy between this version and the
       original IEEE and The Open Group Standard, the original IEEE and The
       Open Group Standard is the referee document. The original Standard can
       be obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                         MQ_NOTIFY(3)
