mq_notify - register for notification when a message is available

#include <mqueue.h>

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

Link with -lrt.

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 message queue 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:

SIGEV_NONE

A "null" notification: the calling process is registered as the target for notification, but when a message arrives, no notification is sent.

SIGEV_SIGNAL

Notify the process by sending the signal specified in sigev_signo. See sigevent(7) for general details. The si_code field of the siginfo_t structure will be set to SI_MESGQ. In addition, si_pid will be set to the PID of the process that sent the message, and si_uid will be set to the real user ID of the sending process.

SIGEV_THREAD

Upon message delivery, invoke sigev_notify_function as if it were the start function of a new thread. See sigevent(7) for details.

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 occurs only 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 occur only 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.)

On success mq_notify() returns 0; on error, -1 is returned, with errno set to indicate the error.

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.

Program source

#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 %zd bytes from MQ\n", 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 */
}

mq_close(3), mq_getattr(3), mq_open(3), mq_receive(3), mq_send(3), mq_unlink(3), mq_overview(7), sigevent(7)