pmsg/mq_notify_sig.c

This is pmsg/mq_notify_sig.c (Listing 52-6, page 1080), 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_sig.c

   Usage: mq_notify_sig mq-name

   Demonstrate message notification via signals (catching the signals with
   a signal handler) on a POSIX message queue.
*/
#include <signal.h>
#include <mqueue.h>
#include <fcntl.h>              /* For definition of O_NONBLOCK */
#include "tlpi_hdr.h"

#define NOTIFY_SIG SIGUSR1
static void
handler(int sig)
{
    /* Just interrupt sigsuspend() */
}
/* 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_signal.c for an example of how to deal with that case. */

int
main(int argc, char *argv[])
{
    struct sigevent sev;
    mqd_t mqd;
    struct mq_attr attr;
    void *buffer;
    ssize_t numRead;
    sigset_t blockMask, emptyMask;
    struct sigaction sa;

    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)
        errExit("mq_open");

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

    if (mq_getattr(mqd, &attr) == -1)
        errExit("mq_getattr");

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

    /* Block the notification signal and establish a handler for it */

    sigemptyset(&blockMask);
    sigaddset(&blockMask, NOTIFY_SIG);
    if (sigprocmask(SIG_BLOCK, &blockMask, NULL) == -1)
        errExit("sigprocmask");

    sigemptyset(&sa.sa_mask);
    sa.sa_flags = 0;
    sa.sa_handler = handler;
    if (sigaction(NOTIFY_SIG, &sa, NULL) == -1)
        errExit("sigaction");

    /* Register for message notification via a signal */

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

    sigemptyset(&emptyMask);

    for (;;) {
        sigsuspend(&emptyMask);         /* Wait for notification signal */

        /* Reregister for message notification */

        if (mq_notify(mqd, &sev) == -1)
            errExit("mq_notify");

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

        if (errno != EAGAIN)            /* Unexpected error */
            errExit("mq_receive");
    }
}

 

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