Function login_cap_sys::login_getcapbool

pub unsafe extern "C" fn login_getcapbool(
    _lc: *mut login_cap_t, 
    _cap: *mut c_char, 
    _def: c_uint
) -> c_int

From login_getclass(3):

The login_getcapbool() function returns def if no capabilities were found for
this class (typically meaning that the default class was used and the /etc/login.conf file is missing).
It returns a non-zero value if cap, with no value, was found, zero otherwise.