no session??

onegun

i try to get logged in id for backend admin, so i did a check when creating simple module.

session_start();
print_r($_SESSION);

empty array returned, if that is the case, where did the system store the logged in info?

admin

It uses the CI Session library which by default stores it in a cookie:
https://ellislab.com/codeigniter/user-guide/libraries/sessions.html