XS(XS_Net__LibnetRaw_libnet_do_checksum)
[...]
        u_char  buf = (u_char)SvUV(ST(0));
[...]
        RETVAL = libnet_do_checksum(&buf, protocol, len);
        sv_setuv(ST(0), (UV)buf);
        SvSETMAGIC(ST(0));