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window with widgets and adjustable transparency

Dear Sirs/Madams,

I am new in C programming and in GTK, I made a window with a timer, some buttons and a scalebar,
the scalebar is for adjusting the transaparency of the whole window but does not work and at this moment I am stuck, can you please have a look at the code and if you have any good idea how to do it, I will be very happy, thanks a lot,
Best regards: Pablo

#include <gtk/gtk.h>
 int seconds = 0;
 guint timerId = 0;
 GtkWidget *timer;
 static gdouble val = 0.5;
 gboolean timerTick(__attribute__((unused)) gpointer userData)
     static GString *time = NULL;
     if(time == NULL)
         time = g_string_sized_new(10);
     seconds += 1;
     printf("Timer Tick %d \n",seconds);
     gtk_entry_set_text (GTK_ENTRY(timer),time->str);
     return TRUE;
 void stopButtonClicked(__attribute__((unused)) GtkWidget *widget,
                        __attribute__((unused)) gpointer   data)
     g_print("Stop Clicked\n");
     if(timerId != 0)
         timerId = 0;
         g_print("Not Running\n");
 void startButtonClicked(__attribute__((unused)) GtkWidget *widget,
                         __attribute__((unused)) gpointer   data)
     g_print("Start Clicked\n");
     if(timerId == 0)
         timerId = g_timeout_add(1000,timerTick,NULL);
         g_print("Already Running\n");
 void clearButtonClicked(__attribute__((unused)) GtkWidget *widget,
                         __attribute__((unused)) gpointer   data)
     g_print("Clear Clicked\n");
     if(timerId == 0)
         seconds = 0;
         gtk_entry_set_text (GTK_ENTRY(timer),"00:00");
         g_print("Ignored, Running\n");

 void quitButtonClicked(__attribute__((unused)) GtkWidget *widget,
                        __attribute__((unused)) gpointer   data)
     g_print("Quit Clicked\n");
// Handle the user trying to close the window
 gboolean windowDelete(__attribute__((unused)) GtkWidget *widget,
                       __attribute__((unused)) GdkEvent  *event,
                       __attribute__((unused)) gpointer   data)
     g_print("%s called.\n",__FUNCTION__);
     return TRUE;    // Returning TRUE stops the window being deleted.
                     // Returning FALSE allows deletion.  
 void value_changed(GtkRange *range, gpointer window) {
   val = gtk_range_get_value(range);
   gtk_widget_set_opacity (GTK_WIDGET (window), val);

 int main ( int argc, char **argv) {
     GtkWidget *window;
     GtkWidget *box;
     GtkWidget *hscale;
     GtkWidget *startButton;
     GtkWidget *stopButton;
     GtkWidget *clearButton;
     GtkWidget *quitButton;
     gtk_init(&argc , &argv);
     window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
     gtk_window_set_position(GTK_WINDOW(window), GTK_WIN_POS_CENTER);
     gtk_window_set_default_size(GTK_WINDOW(window), 300, 250);
     gtk_container_set_border_width(GTK_CONTAINER(window), 10);
     gtk_window_set_title(GTK_WINDOW(window), "Timer");
     box = gtk_button_box_new(GTK_ORIENTATION_VERTICAL);
     gtk_container_add(GTK_CONTAINER (window), box);
     timer = gtk_entry_new();
     gtk_container_add(GTK_CONTAINER (box), timer);
     hscale = gtk_hscale_new_with_range(0, 1, 0.1);
     gtk_container_add(GTK_CONTAINER (box), hscale);
     startButton = gtk_button_new_with_label("START");
     gtk_container_add(GTK_CONTAINER (box), startButton);
     g_signal_connect(startButton, "clicked", G_CALLBACK (startButtonClicked), NULL);
     stopButton = gtk_button_new_with_label("STOP");
     gtk_container_add(GTK_CONTAINER (box), stopButton);
     g_signal_connect(stopButton, "clicked", G_CALLBACK (stopButtonClicked), NULL);
     clearButton = gtk_button_new_with_label("CLEAR");
     gtk_container_add(GTK_CONTAINER (box), clearButton);
     g_signal_connect(clearButton, "clicked", G_CALLBACK (clearButtonClicked), NULL);
     quitButton = gtk_button_new_with_label("QUIT");
     gtk_container_add(GTK_CONTAINER (box), quitButton);
     g_signal_connect(quitButton, "clicked", G_CALLBACK (quitButtonClicked), NULL);
     g_signal_connect(window, "delete_event", G_CALLBACK(windowDelete), NULL);
     g_signal_connect(hscale, "value-changed", G_CALLBACK(value_changed), NULL);
     gtk_widget_show_all (window);
     gtk_main ();
     return 0;
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