ci框架上传文件有关问题

 function do_upload()
 {
  $config['upload_path'] = base_url().'img/';
  $config['allowed_types'] = 'gif|jpg|png';
  $config['max_size'] = '100';
  $config['max_width']  = '1024';
  $config['max_height']  = '768';
  
  $this->load->library('upload', $config);
 
  if ( ! $this->upload->do_upload())
  {
   $error = array('error' => $this->upload->display_errors());
   
   $this->load->view('admin/upload_form', $error);
  } 
  else
  {
   $data = array('upload_data' => $this->upload->data());
   
  print_r($data);
  }