Function login_cap_sys::login_getcapstr

pub unsafe extern "C" fn login_getcapstr(
    _lc: *mut login_cap_t, 
    _cap: *mut c_char, 
    _def: *mut c_char, 
    _err: *mut c_char
) -> *mut c_char

From login_getclass(3):

The login_getcapstr() function queries the database entry for a field
named cap.  If the field is found, its value is returned.  If the field
is not found, the value specified by def is returned.  If an error is
encountered while trying to find the field, err is returned.  See
login.conf(5) for a discussion of the various textual forms the value may
take.