This is pmsg/mq_notify_via_signal.c, an example program file from 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 2010, 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[])
    struct sigevent sev;
    mqd_t mqd;
    struct sigaction sa;
    int j;
    char *msg;
    ssize_t numRead;
    struct mq_attr attr;

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

    /* Establish handler for notification signal */

    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 */

    if (mq_getattr(mqd, &attr) == -1)

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

            /* Register for message notification */

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

            /* Drain all messages from the queue */

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

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

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


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