watchdog가.. 뭐예요..

아래에서 watchdog에 대한 언급이 있었는데요..
구체적으로 어떤건지 알고 싶습니다..
인터넷에서 간단한 코드를 한번 찾아보았는데요..
간단하게 코드 해석도 부탁드립니다..

#define _FILE_OFFSET_BITS 64
#define _LARGEFILE_SOURCE 1
#define _LARGEFILE64_SOURCE 1

#include <stdlib.h>
#include <stdio.h>
#include <fcntl.h>
#include <time.h>
#include <unistd.h>
#include <sys/mman.h>
#include <signal.h>

#define MAGIC 0x0badf00dL

typedef int (*function_ptr)(void);

struct {
	int fd;
	time_t last_pat;
} watchdog;

/* We don't like malloc */
void *xmalloc(size_t size) {
	return mmap((void *)0,size, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS,-1,0);
}

void xfree(void *mapping, size_t size) {
	munmap(mapping,size);
}

void init_dog(void) {
	watchdog.fd = 0;
	watchdog.last_pat = 0;
}

void open_dog(const char *device) {
	watchdog.fd = open(device,O_WRONLY);
	if(watchdog.fd == -1) {
		fprintf(stderr,"Error Opening Device\n");
		exit(1);
	}
}

int pat_dog(void) {
	size_t ret = 0;
	ret = write(watchdog.fd,"\0",1);
	fsync(watchdog.fd);
	watchdog.last_pat = time(NULL);
	if(ret != 1) {
		return 1;
	} else {
		return 0;
	}
}

void sleep_remain(int seconds) {
	seconds -= time(NULL) - watchdog.last_pat;
	sleep(seconds);
}

int file_test(void) {
	char buffer[4096];
	char filename[] = "/tmp/watchdog-file_test-XXXXXX";
	int fd,i;
	size_t done = 0;
	fd = mkstemp(filename);
	if(fd != -1) {
		memset(buffer,0xff,4096);
		for(i = 0; done < 4096 && i < 10; ++i) {
			done += write(fd,buffer,4096);
			fsync(fd);
		}
		close(fd);
		unlink(filename);
	}
	if(done == 4096) {
		return 0;
	} else {
		return 1;
	}
}

int fork_test(void) {
	char filename[] = "/tmp/watchdog-fork_test-XXXXXX";
	int fd;
	int ret_val = 1;
	int i;
	pid_t child = 0;
	long *map = NULL;
	
	/* Make a Temp File */
	fd = mkstemp(filename);
	if(fd == -1) {
		return 1;
	}
	for(i = 0; i < sizeof(long); ++i) {
		write(fd,"\0",1);
	}
	fsync(fd);
	
	/* Map it */
	map = mmap((void *)0, sizeof(long), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
	if(map == NULL) {
		close(fd);
		unlink(filename);
		return 1;
	}
	/* Zero the Int */
	(*map) = 0;
	msync(map,sizeof(long),MS_SYNC);
	
	/* Fork */
	child = fork();
	if(child == 0) {
		/* We are the Child */
		(*map) = MAGIC;
		msync(map,sizeof(long),MS_INVALIDATE);
		exit(0);
	} else {
		wait(NULL);
		if((*map) == MAGIC) {
			ret_val = 0;
		}
		munmap(map, sizeof(long));
		close(fd);
		unlink(filename);
	}
	return ret_val;
}

int malloc_test(void) {
	char *buffer = NULL; 
	size_t page_size = getpagesize();
	size_t size;
	for(size = 1; size < 32; ++size) {
		buffer = xmalloc(size*page_size);
		if(buffer == NULL) {
			return 1;
		}
		memset(buffer,0x00,size*page_size);
		xfree(buffer,size*page_size);
	}
	return 0;
}

void pat() {
	pat_dog();
	sleep_remain(10);
	fprintf(stderr,"Pat() at %ld\n",time(NULL));
}

const int num_tests = 3;
function_ptr tests[3];

void set_tests(void) {
	tests[0] = &malloc_test;
	tests[1] = &file_test;
	tests[2] = &fork_test;
}

void set_signals(void) {
	signal(SIGHUP,SIG_IGN);
	//signal(SIGTERM,SIG_IGN);
}

int main(int argc, const char *argv[]) {
	int ret;
	int i;
	if(argc < 2) {
		fprintf(stderr, "Please specify a watchdog device\n");
		exit(1);
	}
	set_tests();
	set_signals();
	init_dog();
	open_dog(argv[1]);
	pat_dog();
	for(;;) {
		for(i = 0; i < num_tests; ++i) {
			ret = tests[i]();
			if(ret == 0) {
				/* No More Tests for 60 Seconds */
				for(ret = 0; ret < 6; ++ret) {
					pat();
				}
			}
		}
	}
	return 0;
}