Session destroy problem

Hi Viewers,
  I am trying to destroy the session by using this $this->session->sess_destroy(); command, but its not destrying properly, if i use if condition, it shows failure only, so please let me know, what is wrong in this.
function logout()
      {
      $logout=$this->session->sess_destroy();
      if(isset($logout))
      {
      redirect(‘login’);
      }
      else
      {
      echo “not destroyed”;
      }
      }