This is pmsg/mq_notify_thread.c (Listing 52-7, page 1082), an example from the book, The Linux Programming Interface.

The source code file is copyright 2024, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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/* mq_notify_thread.c

   Demonstrate message notification via threads on a POSIX message queue.
#include <pthread.h>
#include <mqueue.h>
#include <signal.h>
#include <fcntl.h>              /* For definition of O_NONBLOCK */
#include "tlpi_hdr.h"

/* This program does not handle the case where a message already exists on
   the queue by the time the first attempt is made to register for message
   notification. In that case, the program would never receive a notification.
   See mq_notify_via_thread.c for an example of how to deal with that case. */

static void notifySetup(mqd_t *mqdp);
static void                     /* Thread notification function */
threadFunc(union sigval sv)
    ssize_t numRead;
    mqd_t *mqdp;
    void *buffer;
    struct mq_attr attr;

    mqdp = sv.sival_ptr;

    /* Determine mq_msgsize for message queue, and allocate an input buffer
       of that size */

    if (mq_getattr(*mqdp, &attr) == -1)

    buffer = malloc(attr.mq_msgsize);
    if (buffer == NULL)

    /* Reregister for message notification */


    while ((numRead = mq_receive(*mqdp, buffer, attr.mq_msgsize, NULL)) >= 0)
        printf("Read %ld bytes\n", (long) numRead);

    if (errno != EAGAIN)                        /* Unexpected error */

static void
notifySetup(mqd_t *mqdp)
    struct sigevent sev;

    sev.sigev_notify = SIGEV_THREAD;            /* Notify via thread */
    sev.sigev_notify_function = threadFunc;
    sev.sigev_notify_attributes = NULL;
            /* Could be pointer to pthread_attr_t structure */
    sev.sigev_value.sival_ptr = mqdp;           /* Argument to threadFunc() */

    if (mq_notify(*mqdp, &sev) == -1)
main(int argc, char *argv[])
    mqd_t mqd;

    if (argc != 2 || strcmp(argv[1], "--help") == 0)
        usageErr("%s mq-name\n", argv[0]);

    mqd = mq_open(argv[1], O_RDONLY | O_NONBLOCK);
    if (mqd == (mqd_t) -1)

    pause();                    /* Wait for notifications via thread function */


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