curl_multi_perform(CURLM, @num.i)
 *message.CURLMsg = curl_multi_info_read(CURLM, @info.i)

 struct CURLMsg {
    CURLMSG msg;       /* what this message means */
    CURL *easy_handle; /* the handle it concerns */
    union {
      void *whatever;    /* message-specific data */
      CURLcode result;   /* return code for transfer */
    } data;
  };

Structure CURLMsg
  msg
  *easy_handle
  StructureUnion
    *whatever
    result
  EndStructureUnion
EndStructure