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   mq_notify.3p    ( 3 )

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Пролог (Prolog)

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.

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Имя (Name)

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

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Синопсис (Synopsis)

#include <mqueue.h>

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

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Описание (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.

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Возвращаемое значение (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.

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Ошибки (Error)

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.

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Примеры (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 */
           }

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Использование в приложениях (Application usage)

None.

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Обоснование (Rationale)

None.

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Будущие направления (Future directions)

None.

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Смотри также (See also)

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)

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