// ipToSockaddrInet6 has special handling here for zero length slices.
	// We do not, because netip has no concept of a generic zero IP address.
	//
	// addr is allowed to be an IPv4 address, because As16 will convert it
	// to an IPv4-mapped IPv6 address.
	// The error message is kept consistent with ipToSockaddrInet6.
	addr := ap.Addr()
	if !addr.IsValid() {

	case syscall.AF_INET:
		sa, err := ipToSockaddrInet4(addr.IP, addr.Port)
		if err != nil {
			return 0, err
		}
		return c.fd.writeToInet4(b, &sa)
	case syscall.AF_INET6:
		sa, err := ipToSockaddrInet6(addr.IP, addr.Port, addr.Zone)
		if err != nil {
			return 0, err
		}
		return c.fd.writeToInet6(b, &sa)
	default:
		return 0, &AddrError{Err: "invalid address family", Addr: addr.IP.String()}
	}
}