Prevent to exec second copy of application

[jakonda jakarta <janjakon mail ru>]

Helo, I try to force my application to be only one on display,
I mean if somebody do

$./my-program
OK!
$./my-program
There is another copy. Use it

My algorithm is follow: create atom, check if it selected by somebody, if it so another copy of application 
is 
running, if it not so we are alone, select atom and work.

Code is follows:
--------------------------------------------------------------------------------------------------------------
$ cat main.cpp
#include <gdk/gdk.h>

#include <gtk/gtk.h>
#include <cstdlib>
#include <cstring>


gint OnDelete(GtkWidget *widget, GdkEvent *event, gpointer data);
gint OnDestroy(GtkWidget *widget, GdkEvent *event, gpointer data);

int main(int argc, char *argv[])
{
  gtk_init(&argc, &argv);

  
  GtkWidget *topLevelWindow=gtk_window_new(GTK_WINDOW_TOPLEVEL);
  

  gtk_signal_connect(GTK_OBJECT(topLevelWindow), "delete_event", 
                     GTK_SIGNAL_FUNC(OnDelete), NULL);
  gtk_signal_connect(GTK_OBJECT(topLevelWindow), "destroy", 
                     GTK_SIGNAL_FUNC(OnDestroy), NULL);
  gtk_widget_show(topLevelWindow);
  
  GdkAtom atom=gdk_atom_intern("MY_PRIVATE_ATOM_GTK_EXAMPLE", FALSE);
  if(gdk_selection_owner_get_for_display(gdk_display_get_default(), atom)!=NULL)
        g_print("There is another copy\n");
  gtk_selection_owner_set_for_display(gdk_display_get_default(),
                                                   topLevelWindow, atom, 0);
  
  gtk_main();
  g_print("Done!\n");
  return EXIT_SUCCESS;
}

gint OnDelete(GtkWidget *widget, GdkEvent *event, gpointer data)
{
  return FALSE;
}

gint OnDestroy(GtkWidget *widget, GdkEvent *event, gpointer data)
{
  g_print("Shutting down\n");
  gtk_main_quit();
  return 0;
}
--------------------------------------------------------------------------------------------------------------
but it didn't work, what wrong?
May be there is another algorithm to prevent starting second copy of application in the same dipslay?

With regards, Jan.

PS
  I don't want to use bonobo.