#include <gtk/gtk.h>
 #ifdef GDK_WINDOWING_X11
 #include <gdk/gdkx.h>
 #endif
 #ifdef GDK_WINDOWING_WAYLAND
 #include <gdk/gdkwayland.h>
 #endif

 int main (int argc, char *argv[])
 {
   GdkDisplay *display;

   gtk_init (&argc, &argv);

   display = gdk_display_get_default ();

 #ifdef GDK_WINDOWING_X11
   if (GDK_IS_X11_DISPLAY (display))
     {
       g_print ("This code is running under X11");
     }
 #endif
 #ifdef GDK_WINDOWING_WAYLAND
   if (GDK_IS_WAYLAND_DISPLAY (display))
     {
       g_print ("This code is running under Wayland");
     }
 #endif

   return 0;
 }