PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

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

PROLOG         top

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

NAME         top

       mq_notify — notify process that a message is available (REALTIME)

SYNOPSIS         top

       #include <mqueue.h>

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

DESCRIPTION         top

       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         top

       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         top

       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         top

       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         top

       None.

RATIONALE         top

       None.

FUTURE DIRECTIONS         top

       None.

SEE ALSO         top

       mq_open(3p), mq_send(3p), mq_receive(3p), msgctl(3p), msgget(3p),
       msgrcv(3p), msgsnd(3p)

       The Base Definitions volume of POSIX.1‐2008, mqueue.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) 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.unix.org/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                 2013                       MQ_NOTIFY(3P)