https://invisible-island.net/xterm/xtoolkit/
XtAppSetExitFlag, XtAppGetExitFlag - thread support functions
#include <X11/Intrinsic.h>
void XtAppSetExitFlag(XtAppContext app_context);
Boolean XtAppGetExitFlag(XtAppContext app_context);
app_context
Specifies the application context.
To indicate that an application context should exit, use XtAppSetExit-
Flag.
To test the exit status of an application context, use XtAppGetExit-
Flag.
X Toolkit Intrinsics - C Language Interface
Xlib - C Language X Interface
X Version 11 libXt 1.3.1 XtAppSetExitFlag(3)