Re: strange behavior of random numbers with gtkmm

[José Alburquerque <jaalburquerque cox net>]

On Thu, 2010-06-17 at 09:32 +0200, jody wrote:
> Hi
> (i accidentally sent my previous post only to jose, sohere it is again)
> 
> @michi:
> I do some simulations where "randomness" plays a major role.
> Sometimes some special events occur, and then it is advantageous
> to be able to repeat the simulation the exact same way up to
> this occurrence to understand how it happened.
> Of course i could do  a srand just before starting the simulation
> (instead of inside the WIndow constructor) to ensure this reproducibility.
> But still i would like to understand what exactly is going on.
> 
> @jose:
> I built an equivalent C version of my test application (see below), and this one
> does not show this behavior
> 
> Just a thought: if i understand things correctly, there are several active
> threads in a gtkmm application. Is it possible that the thread in which
> the constructor is called is not the same one which the user events are handled?
> If this is the case, it could be that in the user event thread the
> state of the random engine is different from the its state in the main thread
> (the one with the constructor)?
> Or is the state of rand global to the entire process?
> But then again why does this not happen with the gtk-version?
> 
> Jody
> 
> Here's the code of the gtk-version
> 
> #include <stdio.h>
> #include <stdlib.h>
> #include <gtk/gtk.h>
> 
> /*
>  * reset_action
>  *   callback for reset action
>  */
> static void reset_action( GtkWidget *widget,
>                           gpointer   data ) {
>     // one step
>     srand(12343271);
>     printf("Reset\n");
> }
> 
> /*
>  * step_action
>  *   callback for reset action
>  */
> static void step_action( GtkWidget *widget,
>                          gpointer   data ) {
>     // one step
>     printf("One step: %d\n", rand());
> }
> 
> 
> 
> static gboolean delete_event( GtkWidget *widget,
>                               GdkEvent  *event,
>                               gpointer   data )
> {
> 
>     g_print ("delete event occurred\n");
> 
>     /* Change TRUE to FALSE and the main window will be destroyed with
>      * a "delete-event". */
> 
>     return FALSE;
> }
> 
> /* Another callback */
> static void destroy( GtkWidget *widget,
>                      gpointer   data )
> {
>     gtk_main_quit ();
> }
> 
> int main( int   argc,
>           char *argv[] )
> {
>     /* GtkWidget is the storage type for widgets */
>     GtkWidget *window;
>     GtkWidget *box1;
>     GtkWidget *buttonStep;
>     GtkWidget *buttonReset;
> 
>     gtk_init (&argc, &argv);
> 
> 
>     window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
> 
>     g_signal_connect (window, "delete-event",
> 		      G_CALLBACK (delete_event), NULL);
> 
>     g_signal_connect (window, "destroy",
> 		      G_CALLBACK (destroy), NULL);
> 
> 
>     gtk_container_set_border_width (GTK_CONTAINER (window), 10);
> 
> 
>     /* create and insert the box */
>     box1 = gtk_hbox_new (FALSE, 0);
>     gtk_container_add (GTK_CONTAINER (window), box1);
> 
> 
>     /* create and insert "reset" button */
>     buttonReset = gtk_button_new_with_label ("Reset");
> 
>     g_signal_connect (buttonReset, "clicked",
> 		      G_CALLBACK (reset_action), NULL);
> 
>     gtk_box_pack_start (GTK_BOX(box1), buttonReset, TRUE, TRUE, 0);
>     gtk_widget_show (buttonReset);
> 
> 
> 
>     /* create and insert "step" button */
>     buttonStep = gtk_button_new_with_label ("Step");
> 
>     g_signal_connect (buttonStep, "clicked",
> 		      G_CALLBACK (step_action), NULL);
> 
>     gtk_box_pack_start (GTK_BOX(box1), buttonStep, TRUE, TRUE, 0);
>     gtk_widget_show (buttonStep);
> 
> 
>     /* display box */
>     gtk_widget_show (box1);
> 
>     /* display the window */
>     gtk_widget_show (window);
> 
> 
>     reset_action(NULL, NULL);

In your C++ constructor, this line was called before showing your
widgets.  If you do the same in C (ie., move the reset call to before
displaying your box, you'll see that the same thing happens (at least it
does for me).
> 
>     /* run it */
>     gtk_main ();
> 
>     return 0;
> }
> _______________________________________________
> gtkmm-list mailing list
> gtkmm-list gnome org
> http://mail.gnome.org/mailman/listinfo/gtkmm-list

-- 
José