Data file detection problem

Hi.

There is a function geoip_detect_get_abs_db_filename() in geoip-detect.php file.
But it always returns an empty string (the file GeoLiteCity.dat is in the geoip-detect folder).

As I see the trouble is on the line
$data_filename = apply_filters('geoip_detect_get_abs_db_filename', $data_filename);

Could you explain me what this line should do?

Thanks

http://ww.wp.xz.cn/plugins/geoip-detect/