http://www.fltk.org/doc-2.0/html/classWidget.html#a117

#include <stdlib.h>

#include <fltk/Window.h>
#include <fltk/run.h>
#include <fltk/Color.h>
#include <fltk/Group.h>
#include <fltk/TabGroup.h>
#include <fltk/Button.h>
#include <fltk/SharedImage.h>

using namespace fltk;

void wyjscie_fun(Widget *, void *);

int main(int argc, char **argv)
{
	Window *window = new Window(640, 480, "Control - Centrum sterowania Cdlinux.pl");
	window->color(WHITE);
	window->image(SharedImage::get("images/control.gif"));
	window->begin();
//...

      			Widget *tab4_text1 = new Widget(15, 20, 200, 20);
			tab4_text1->box(FLAT_BOX);
			tab4_text1->labelsize(14);

//...

	window->end();
	window->show(argc, argv);
	return run();
}

void wyjscie_fun(Widget *, void *)
{
//now haw can i change tab4_text1 label
}