This is pmsg/mq_notify_via_thread.c, an example to accompany the book, The Linux Programming Interface.

This file is not printed in the book; it is a supplementary file for Chapter 52.

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

In the listing below, the names of Linux system calls and C library functions are hyperlinked to manual pages from the Linux man-pages project, and the names of functions implemented in the book are hyperlinked to the implementations of those functions.


Download pmsg/mq_notify_via_thread.c

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Function list (Bold in this list means a function is not static)

/* mq_notify_via_thread.c

   Demonstrate message notification via threads on a POSIX message queue.

   See also mq_notify_thread.c.
#include <pthread.h>
#include <mqueue.h>
#include <signal.h>
#include <fcntl.h>              /* For definition of O_NONBLOCK */
#include "tlpi_hdr.h"

static void notifySetup(mqd_t *mqdp);
/* Drain all messages from the queue referred to by 'mqd' */

static void
drainQueue(mqd_t mqd)
    /* Determine mq_msgsize for message queue, and allocate
       a buffer of that size */

    struct mq_attr attr;
    if (mq_getattr(mqd, &attr) == -1)

    char *msg = malloc(attr.mq_msgsize);
    if (msg == NULL)

    ssize_t numRead;
    while ((numRead = mq_receive(mqd, msg, attr.mq_msgsize, NULL)) >= 0) {

        /* Do whatever processing is required for message */

        printf("Read %zd bytes\n", numRead);

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

static void                     /* Thread notification function */
threadFunc(union sigval sv)
    mqd_t *mqdp = sv.sival_ptr;

    /* Reregister for message notification */

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

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

    drainQueue(mqd);    /* Handle possibility that messages were already
                           queued before we established notification */

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


Download pmsg/mq_notify_via_thread.c

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