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)