flock.c

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

char *lock_path = NULL;

int try_lock(const char *path)
{
	int fd;
	int status; 
	struct flock f;

	f.l_type = F_WRLCK;
	f.l_whence = SEEK_SET;
	f.l_start = 0;
	f.l_len = 10;

	fd = open(path, O_RDWR);
	if (fd < 0) {
		printf("open %s failed\n", path);
		return fd;
	} 
	status = fcntl(fd, F_GETLK, &f);
	if (status < 0) {
		printf("fcntl failed\n");
		return status;
	} 
	if (f.l_type != F_UNLCK) {
		printf("pid %d is holding the lock\n", f.l_pid); 
		return status;
	} 
	printf("access ok\n");
	return 0;
}

int acquire_hold_lock(const char *path)
{ 
	int fd;
	int status;
	struct flock f;

	f.l_type = F_WRLCK;
	f.l_whence = SEEK_SET;
	f.l_start = 0;
	f.l_len = 10;

	fd = open(path, O_CREAT|O_TRUNC|O_RDWR, S_IWUSR|S_IRUSR); 
	if (fd < 0) {
		printf("create %s failed\n", path);
		return -1;
	} 
	status = fcntl(fd, F_SETLK, &f);
	if (status < 0) {
		printf("lock failed\n");
		return -1;
	} 
	printf("lock file %s\n", path);
	printf("my pid: %d\n", getpid());
	while(1) {
		sleep(1);
	}
	return 0;
}


int flock_strcmp(const char *s1, const char *s2)
{ 
	while(*s1 == *s2 && *s1 != '\0') { 
		s1 ++;
		s2 ++; 
	}
	return (unsigned char)*s1 - (unsigned char)*s2;
}


int flock_strlen(const char *s)
{
	const char *s1 = s;
	while(*s1 != '\0') {
		s ++;
	}
	return s1 - s;
} 


void print_usage(void)
{
	printf("-l lock name, create a lock and keep it, press CTRL-C to quit\n"
			"-t lock name, access the lock file\n");
} 


void sigint_handler(int signum)
{ 
	int status; 
	status = unlink(lock_path);
	if (status < 0) {
		printf("unlink failed\n");
	} 
	exit(0);
}


int main(int argc, char **argv) 
{ 
	if (argc < 3) {
		print_usage();
		exit(0);
	}
	if (flock_strcmp(argv[1], "-l") == 0) {
		lock_path = argv[2]; 
		signal(SIGINT, sigint_handler); 
		acquire_hold_lock(argv[2]);	
	} else if (flock_strcmp(argv[1], "-t") == 0) {
		try_lock(argv[2]);
	} else {
		print_usage();
	}

}