#include <stdio.h>
#include <fcntl.h>
#include <sys/errno.h>

int fd;

void
lock_file()
{
	struct flock lock;
	int ret;

	lock.l_whence = SEEK_SET;
	lock.l_start = 0;
	lock.l_len = 0;
	lock.l_type = F_WRLCK;

	printf ("locking file\n");
	fflush(stdout);

	ret = fcntl(fd, F_SETLKW, &lock);
	if (ret < 0) {
		perror ("lock failed:");
		exit(1);
	}
}

void
unlock_file()
{

	struct flock lock;
	int ret;

	lock.l_whence = SEEK_SET;
	lock.l_start = 0;
	lock.l_len = 0;
	lock.l_type = F_UNLCK;

	printf ("unlocking file\n");
	fflush(stdout);

	ret = fcntl(fd, F_SETLKW, &lock);
	if (ret < 0) {
		perror ("unlock lock failed:");
		exit(1);
	}
}


main(int argc, char *argv[])
{

	char buf[1024];

	fd = open(argv[1], O_RDWR);
	if (fd < 0) {
		perror ("open failed:");
		exit(1);
	}

	while(1) {

		lock_file();

		printf ("File locked\n");
		fflush(stdout);
		fgets(buf, sizeof(buf), stdin);

		unlock_file();
		printf ("File unlocked\n");
		fflush(stdout);
	}
}