mq_notify(3p) — Linux manual page

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.1‐2017, mqueue.h(0p)

COPYRIGHT         top

       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(3P)

Pages that refer to this page: mqueue.h(0p)mq_getattr(3p)msgctl(3p)msgget(3p)msgrcv(3p)msgsnd(3p)