double fork

2fork.c

int main(void){
	pid_t = pid;
	if((pid = fork()) < 0){
		err;
	}
	else if(pid == 0){
		if((pid = fork()) < 0) err;
		else if(pid > 0) exit(0);
	
		sleep(2);
		printf("second child");
		exit(0);
	}
	
	if (waitpid(pid, NULL, 0) != pid)
		err;
	exit(0);
}

prevent zombie process