using pam_sm_auth

Hi:

I'm createing a pam_sm_authenticate module for the gnome login windows, in this I
need the userName and the password, I need to validate this information against a
database, I have already obtained the username, and use pam_get_item(pamh,
PAM_AUTHTOK, (const void **) pass);  to obtain the pass, where pass is a (char *),
everything works fine, but when the user logs on, the variable pass is NULL, and I
can not get the pass that the user put in the gnome login windows.

How can I get the password of a user usign pam_get_item, using the item_type
(PAM_AUTHTOK), can somebody give an example.

greetings