NAME

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

SYNOPSIS

[MSG] #include <mqueue.h>

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

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.

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.

ERRORS

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.

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 <signal.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

Since the <mqueue.h> header is only required to declare the sigevent structure tag as naming an incomplete structure type, in order to use mq_notify() and pass it a pointer to a sigevent structure, applications need to include <signal.h> so that sigevent will be fully defined.

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

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

XBD <mqueue.h>

CHANGE HISTORY

First released in Issue 5. Included for alignment with the POSIX Realtime Extension.

Issue 6

The mq_notify() function is marked as part of the Message Passing option.

The [ENOSYS] error condition has been removed as stubs need not be provided if an implementation does not support the Message Passing option.

The mq_timedsend() function is added to the SEE ALSO section for alignment with IEEE Std 1003.1d-1999.

Issue 7

SD5-XSH-ERN-38 is applied, adding the mq_timedreceive() function to the DESCRIPTION.

Austin Group Interpretation 1003.1-2001 #032 is applied, adding the [EINVAL] error.

An example is added.

Issue 8

Austin Group Defect 1282 is applied, changing the EXAMPLES and APPLICATION USAGE sections.