function _lock($mode = LOCK_EX)
    {
        if (stristr(php_uname(), 'Windows 9')) {
            return true;
        }

        if ($mode != LOCK_UN && is_resource($this->_lockFp)) {
            // XXX does not check type of lock (LOCK_SH/LOCK_EX)
            return true;
        }

        $open_mode = 'w';
        // XXX People reported problems with LOCK_SH and 'w'
        if ($mode === LOCK_SH) {
            if (!file_exists($this->_lockfile)) {
                touch($this->_lockfile);
            } elseif (!is_file($this->_lockfile)) {
                return PEAR::raiseError('could not create Dependency lock file, ' .
                    'it exists and is not a regular file');
            }
            $open_mode = 'r';
        }

        if (!is_resource($this->_lockFp)) {
            $this->_lockFp = @fopen($this->_lockfile, $open_mode);
        }

        if (!is_resource($this->_lockFp)) {
            return PEAR::raiseError("could not create Dependency lock file" .
                                     (isset($php_errormsg) ? ": " . $php_errormsg : ""));
        }

        if (!(int)flock($this->_lockFp, $mode)) {
            switch ($mode) {
                case LOCK_SH: $str = 'shared';    break;
                case LOCK_EX: $str = 'exclusive'; break;
                case LOCK_UN: $str = 'unlock';    break;
                default:      $str = 'unknown';   break;
            }

            return PEAR::raiseError("could not acquire $str lock ($this->_lockfile)");
        }

        return true;
    }