我正在使用以下代码从Internet下载文件：

size_t write_data(void *ptr, size_t size, size_t nmemb, FILE *stream)
{
    size_t written;
    written = fwrite(ptr, size, nmemb, stream);
    return written;
}

int main(int argc, char** argv)
{
 FILE *downloaded_file;
 if ( (downloaded_file = fopen (download_path , "w" ) ) != NULL )
 {
  CURL *curl;
  CURLcode res;
  curl = curl_easy_init();
  if(curl)
  {
   curl_easy_setopt(curl, CURLOPT_URL, "www.asd.com/files/file_to_download.rar");
   curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_data);
   curl_easy_setopt(curl, CURLOPT_WRITEDATA, downloaded_file);
   res = curl_easy_perform(curl);
   curl_easy_cleanup(curl);

   if (res == CURLE_OK)
   {
    printf("Download complete!\n");
   }
  }
  fclose(downloaded_file);
 }
}

如何测量当前下载速度（例如每秒）和完成下载所需的剩余时间？