func cgoLookupPTR(ctx context.Context, addr string) (names []string, err error) {
	ip, err := netip.ParseAddr(addr)
	if err != nil {
		return nil, &DNSError{Err: "invalid address", Name: addr}
	}
	sa, salen := cgoSockaddr(IP(ip.AsSlice()), ip.Zone())
	if sa == nil {
		return nil, &DNSError{Err: "invalid address " + ip.String(), Name: addr}
	}

	return doBlockingWithCtx(ctx, addr, func() ([]string, error) {