void button_draw_red_circle_clicked_event(...)
    {
        mode = DRAW_RED_CIRCLE;
        gtk_widget_draw_area(darea);

    }

    void button_draw_blue_square_clicked_event(...)
    {
        mode = DRAW_BLUE_SQUARE;
        gtk_widget_draw_area(darea);

    }

static gboolean on_expose_event(GtkWidget *widget,GdkEventExpose *event,gpointer data)
    {
        if (mode == DRAW_RED_CIRCLE) draw_a_red_circle(darea);
        if (mode == DRAW_BLUE_SQUARE) draw_a_blue_square(darea);
    }