SV* my_xs_function(SV* buf_sv) {
    STRLEN buf_len;
    const char* buf = SvPVutf8(buf_sv, buf_len);

    if (buf_len == 0)
        return newSVpvs("");

    {
        STRLEN hex_len = buf_len * 3 - 1;
        SV* hex_sv = sv_2mortal(newSV(hex_len));
        SvPOK_on(hex_sv);
        SvCUR_set(hex_sv, hex_len);
        SvUTF8_on(hex_sv);
        my_c_function(SvPVX(hex_sv), buf, buf_len);
        return hex_sv;
    }
}