https://invisible-island.net/xterm/xtoolkit/
       XtGetDisplays - retrieve a list of displays associated with an applica-
       tion context
       #include <X11/Intrinsic.h>
       void XtGetDisplays(XtAppContext app_context, Display ***dpy_return,
              Cardinal* num_dpy_return);
       app_context
                 Specifies the application context.
       dpy_return
                 Returns a list of open X display connections in the specified
                 application context.
       num_dpy_return
                 Returns the count of open X display connections in dpy_re-
                 turn.
       To free the list of displays, use XtFree.
       X Toolkit Intrinsics - C Language Interface
       Xlib - C Language X Interface
X Version 11                      libXt 1.3.1                 XtGetDisplays(3)