创建守护进程的时候为啥要exit(0)第一子进程

    protected static function daemonize()
    {
        if (!self::$daemonize) {
            return;
        }
        umask(0);
        $pid = pcntl_fork();
        if (-1 === $pid) {
            throw new Exception('fork fail');
        } elseif ($pid > 0) {
            exit(0);
        }
        if (-1 === posix_setsid()) {
            throw new Exception("setsid fail");
        }
        // Fork again avoid SVR4 system regain the control of terminal.
        $pid = pcntl_fork();
        if (-1 === $pid) {
            throw new Exception("fork fail");
        } elseif (0 !== $pid) {
            exit(0);
        }
    }