Function fchmodat 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn fchmodat(
    dirfd: c_int,
    path: *const c_char,
    mode: mode_t,
    flags: c_int,
) -> c_int

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