Function getsubopt 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn getsubopt(
    optionp: *mut *mut c_char,
    keylistp: *const *mut c_char,
    valuep: *mut *mut c_char,
) -> c_int

See https://pubs.opengroup.org/onlinepubs/9799919799/functions/getsubopt.html.