#include <sys/types.h>
#include <sys/wait.h>
#include <sys/file.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>



int lock_file(int fd, int l_options) {

	int ret;
	
	ret = flock(fd, l_options);

	if (ret) 
		perror("flock error");
	return ret;
}

int main() {

	int fd;
	char *filename = "/tmp/lockf1";
	pid_t pid;
	int syncpipe[2];
	char c;

	pipe(syncpipe);
	
	fd = open(filename, O_CREAT | O_RDWR, 0666);
	if(fd < 0) {
		perror("parent could not open file");
		exit(1);
	}

	pid = fork();

	if(pid < 0) {
		perror("fork failed");
		exit(1);
	}
	
	if (pid) {
		lock_file(fd, LOCK_EX);
		if (close(fd))
			perror("parent: error closing file");
		write(syncpipe[1], &c, 1);
	} else {
		/* Wait until the parent has taken the lock */
		read(syncpipe[0], &c, 1);
		
		lock_file(fd, LOCK_UN);
                lock_file(fd, LOCK_EX);
                if(close(fd))
			perror("child: error closing file");
		exit(0);
	}

	wait(NULL);
	return 0;
	
}