gtk2/docs/reference/gdk/x11.md

Title: X Window System Interaction

X backend-specific functions

The functions in this section are specific to the GDK X11 backend. To use them, you need to include the <gdk/x11/gdkx.h> header and use the X11-specific pkg-config file gtk4-x11 to build your application.

Checking for the X11 backend

GDK can be built with support for multiple backends, which means you will need to perform both compile time and run time checks if you wish to call backend-specific API; for instance, the code sample below will guard the calls to different backends, and error out on unsupported windowing systems:

#ifdef GDK_WINDOWING_X11
#include <gdk/x11/gdkx.h>
#endif
#ifdef GDK_WINDOWING_WAYLAND
#include <gdk/wayland/gdkwayland.h>
#endif

#ifdef GDK_WINDOWING_X11
  if (GDK_IS_X11_DISPLAY (display))
    {
      // make X11-specific calls here
    }
  else
#endif
#ifdef GDK_WINDOWING_WAYLAND
  if (GTK_IS_WAYLAND_DISPLAY (display))
    {
      // make Wayland-specific calls here
    }
  else
#endif
  g_error ("Unsupported GDK backend");

The compile time check is performed by using the GDK_WINDOWING_* pre-processor symbols; there is one defined for each windowing system backend built into GDK. For X11, the symbol is GDK_WINDOWING_X11.

The run time check is performed by looking at the type of the [class@Gdk.Display] object. For X11, the display objects will be of type GdkX11Display.