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Here's an example watchdog petting module:

/*
 * Linux watchdog demo for LPC313x
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 *
 */
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/ioctl.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <getopt.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>

#include <linux/watchdog.h>

#define WATCHDOGDEV "/dev/watchdog0"
static const char *const short_options = "hd:i:";
static const struct option long_options[] = {
   {"help", 0, NULL, 'h'},
   {"dev", 1, NULL, 'd'},
   {"interval", 1, NULL, 'i'},
   {NULL, 0, NULL, 0},
};

static void print_usage(FILE * stream, char *app_name, int exit_code)
{
   fprintf(stream, "Usage: %s [options]\n", app_name);
   fprintf(stream,
      " -h  --help                Display this usage information.\n"
      " -d  --dev <device_file>   Use <device_file> as watchdog device file.\n"
      "                           The default device file is '/dev/watchdog'\n"
      " -i  --interval <interval> Change the watchdog interval time\n");

   exit(exit_code);
}

int main(int argc, char **argv)
{
   int fd;         /* File handler for watchdog */
   int interval;      /* Watchdog timeout interval (in secs) */
   int bootstatus;      /* Wathdog last boot status */
   char *dev;      /* Watchdog default device file */

   int next_option;   /* getopt iteration var */
   char kick_watchdog;   /* kick_watchdog options */

   /* Init variables */
   dev = WATCHDOGDEV;
   interval = 0;
   kick_watchdog = 0;

   printf("WDT\n");

   /* Parse options if any */
   do {
      next_option = getopt_long(argc, argv, short_options,
                 long_options, NULL);
      switch (next_option) {
      case 'h':
         print_usage(stdout, argv[0], EXIT_SUCCESS);
      case 'd':
         dev = optarg;
         break;
      case 'i':
         interval = atoi(optarg);
         break;
      case '?':   /* Invalid options */
         print_usage(stderr, argv[0], EXIT_FAILURE);
      case -1:   /* Done with options */
         break;
      default:   /* Unexpected stuffs */
         abort();
      }
   } while (next_option != -1);

   /* Once the watchdog device file is open, the watchdog will be activated by
      the driver */
   fd = open(dev, O_RDWR);
   if (-1 == fd) {
      fprintf(stderr, "Error: %s\n", strerror(errno));
      exit(EXIT_FAILURE);
   }

   /* If user wants to change the watchdog interval */
   if (interval != 0) {
      fprintf(stdout, "Set watchdog interval to %d\n", interval);
      if (ioctl(fd, WDIOC_SETTIMEOUT, &interval) != 0) {
         fprintf(stderr,
            "Error: Set watchdog interval failed\n");
         exit(EXIT_FAILURE);
      }
   }

   /* Display current watchdog interval */
   if (ioctl(fd, WDIOC_GETTIMEOUT, &interval) == 0) {
      fprintf(stdout, "Current watchdog interval is %d\n", interval);
   } else {
      fprintf(stderr, "Error: Cannot read watchdog interval\n");
      exit(EXIT_FAILURE);
   }

   /* Check if last boot is caused by watchdog */
   if (ioctl(fd, WDIOC_GETBOOTSTATUS, &bootstatus) == 0) {
      fprintf(stdout, "Last boot is caused by : %s\n",
         (bootstatus != 0) ? "Watchdog" : "Power-On-Reset");
   } else {
      fprintf(stderr, "Error: Cannot read watchdog status\n");
      exit(EXIT_FAILURE);
   }

   fprintf(stdout, "Start service watchdog\n");
   do {
         ioctl(fd, WDIOC_KEEPALIVE, NULL);
	 sleep(interval);
   } while (1);


   /* There are two ways to kick the watchdog:
      - by writing any dummy value into watchdog device file, or
      - by using IOCTL WDIOC_KEEPALIVE
    */
   fprintf(stdout,
      "Use:\n"
      " <w> to kick through writing over device file\n"
      " <i> to kick through IOCTL\n" " <x> to exit the program\n");
   do {
      kick_watchdog = getchar();
      switch (kick_watchdog) {
      case 'w':
         write(fd, "w", 1);
         fprintf(stdout,
            "Kick watchdog through writing over device file\n");
         break;
      case 'i':
         ioctl(fd, WDIOC_KEEPALIVE, NULL);
         fprintf(stdout, "Kick watchdog through IOCTL\n");
         break;
      case 'x':
         fprintf(stdout, "Goodbye !\n");
         break;
      default:
         fprintf(stdout, "Unknown command\n");
         break;
      }
   } while (kick_watchdog != 'x');

   /* The 'V' value needs to be written into watchdog device file to indicate
      that we intend to close/stop the watchdog. Otherwise, debug message
      'Watchdog timer closed unexpectedly' will be printed
    */
   write(fd, "V", 1);
   /* Closing the watchdog device will deactivate the watchdog. */
   close(fd);
}