This is pmsg/mq_notify_via_signal.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.


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/* mq_notify_via_signal.c

   Usage: mq_notify_via_signal /mq-name

   Demonstrate message notification via signals (catching the signals with
   a signal handler) on a POSIX message queue.

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


static volatile sig_atomic_t gotSig = 1;        /* See comment in main() */
/* Handler for message notification signal */

static void
handler(int sig)
    gotSig = 1;
main(int argc, char *argv[])
    if (argc != 2 || strcmp(argv[1], "--help") == 0)
        usageErr("%s /mq-name\n", argv[0]);

    /* Open the (existing) queue in nonblocking mode so that we can drain
       messages from it without blocking once the queue has been emptied */

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

    /* Establish handler for notification signal */

    struct sigaction sa;
    sa.sa_flags = 0;
    sa.sa_handler = handler;
    if (sigaction(NOTIFY_SIG, &sa, NULL) == -1)

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

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

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

    /* Possibly, a message had already been queued by the time we enter
       the loop below. By initializing 'gotSig' to 1 above, we trigger the
       program to make the initial registration for notification and force
       the queue to be drained of any messages on the first loop iteration. */

    for (int j = 0; ; j++) {
        if (gotSig) {
            gotSig = 0;

            /* Register for message notification */

            struct sigevent sev;
            sev.sigev_notify = SIGEV_SIGNAL;
            sev.sigev_signo = NOTIFY_SIG;
            if (mq_notify(mqd, &sev) == -1)

            /* Drain all messages from the queue */

            ssize_t numRead;
            while ((numRead = mq_receive(mqd, msg,
                                attr.mq_msgsize, NULL)) >= 0) {
                /* Do whatever processing is required for each message */

                printf("Read %zd bytes\n", numRead);
            if (errno != EAGAIN)        /* Unexpected error */

        printf("j = %d\n", j);
        sleep(5);               /* Do some "work" */


Download pmsg/mq_notify_via_signal.c

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