Under older versions of Linux and some other OSs the full path to the module was provided within the PAM configuration file, such as
/lib/security/pam_stack.so
However, since the advent of multilib systems, which store 64-bit PAM modules within the /lib64/security/ directory, the directory path is omitted because the application is linked to the appropriate version of libpam, which can locate the correct version of the module.