Function mbrtowc 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn mbrtowc(
    pwc: *mut wchar_t,
    s: *const c_char,
    n: size_t,
    ps: *mut mbstate_t,
) -> size_t

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

Only works for UTF8 at the moment.