Function wcrtomb 

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

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

widechar to multibyte.