https://invisible-island.net/xterm/xtoolkit/
XtToolkitThreadInitialize - initialize the toolkit for multiple threads
#include <X11/Intrinsic.h>
Boolean XtToolkitThreadInitialize(void);
If XtToolkitThreadInitialize was previously called, it returns. The
application programmer must ensure that two or more threads do not si-
multaneously attempt to call XtToolkitThreadInitialize; the effect of
this is undefined. XtToolkitThreadInitialize returns True if the host
operating system has threads and the Intrinsics are thread safe.
X Toolkit Intrinsics - C Language Interface
Xlib - C Language X Interface
X Version 11 libXt 1.3.1 XtToolkitThreadInitialize(3)