void my_callback( Fl_Widget* old, void*  ) {
   Fl_Button* new=(Fl_Button*)old;
   new ->label("Kliknąłeś mnie!");
   new ->redraw();
 }

int main(int argc, char **argv) {
  Fl_Window *win= new Fl_Window(500,400);
  Fl_Button *button = new Fl_Button(15,15,150,150,"Możesz mnie kliknąć");
  button->callback( my_callback );

  win->end();
  win->show(argc, argv);
  return Fl::run();
}