https://invisible-island.net/xterm/xtoolkit/
XtDisplayToApplicationContext - obtain an application context
#include <X11/Intrinsic.h>
XtAppContext XtDisplayToApplicationContext(Display* dpy);
dpy Specifies the display that you want the application context
for.
The XtDisplayToApplicationContext function returns the application con-
text for the specified display.
X Toolkit Intrinsics - C Language Interface
Xlib - C Language X Interface
X Version 11 libXt 1.3.1 XtDisplayToApplicationContext(3)