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 descriptor mqdes.

The sevp argument is a pointer to a sigevent structure. For the definition 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:

Only one process can be registered to receive notification from a message queue.

If sevp is NULL, and the calling process is currently registered to receive notifications for this message queue, then the registration is removed; another process can then register to receive a message notification for this queue.

Message notification only occurs 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 only occur 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 effect.

Notification occurs once: after a notification is delivered, the notification 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 notification. This should be done before emptying all unread messages from the queue. (Placing the queue in nonblocking mode is useful for emptying 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

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.

CONFORMING TO

POSIX.1-2001.

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 message from the queue and then terminates the process.

#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 %ld bytes from MQ\n", (long) 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)