/*
 * call-seq:
 *  read_io(io, url, encoding, options)
 *
 * Create a new document from an IO object
 */
static VALUE read_io( VALUE klass,
                      VALUE io,
                      VALUE url,
                      VALUE encoding,
                      VALUE options )
{
  const char * c_url    = (url == Qnil) ? NULL : StringValuePtr(url);
  const char * c_enc    = (encoding == Qnil) ? NULL : StringValuePtr(encoding);

  xmlInitParser();

  xmlDocPtr doc = xmlReadIO(
      (xmlInputReadCallback)io_read_callback,
      (xmlInputCloseCallback)io_close_callback,
      (void *)io,
      c_url,
      c_enc,
      NUM2INT(options)
  );

  if(doc == NULL) {
    xmlFreeDoc(doc);
    rb_raise(rb_eRuntimeError, "Couldn't create a document");
    return Qnil;
  }

  return Nokogiri_wrap_xml_document(klass, doc);
}