mq_notify man page


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.

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


#include <mqueue.h>

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


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

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.


The mq_notify() function shall fail if:

The mqdes argument is not a valid message queue descriptor.
A process is already registered for notification by the message queue.

The mq_notify() function may fail if:

The notification argument is NULL and the process is currently not registered.

The following sections are informative.


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) {
    buf = malloc(attr.mq_msgsize);

    if (buf == NULL) {

    nr = mq_receive(mqdes, buf, attr.mq_msgsize, NULL);
    if (nr == -1) {

    printf("Read %ld bytes from message queue\n", (long) nr);
    exit(EXIT_SUCCESS);         /* Terminate the process */

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) {

    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) {

    pause();    /* Process will be terminated by thread function */

See Also

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

The Base Definitions volume of POSIX.1‐2008, <mqueue.h>