OpenSSL::PKey::DH#to_der

static VALUE
ossl_dh_to_der(VALUE self)
{
    OSSL_3_const DH *dh;
    unsigned char *p;
    long len;
    VALUE str;

    GetDH(self, dh);
    if((len = i2d_DHparams(dh, NULL)) <= 0)
        ossl_raise(eDHError, NULL);
    str = rb_str_new(0, len);
    p = (unsigned char *)RSTRING_PTR(str);
    if(i2d_DHparams(dh, &p) < 0)
        ossl_raise(eDHError, NULL);
    ossl_str_adjust(str, p);

    return str;
}