Function getresgid 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn getresgid(
    rgid: *mut gid_t,
    egid: *mut gid_t,
    sgid: *mut gid_t,
) -> c_int

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