https://invisible-island.net/xterm/xtoolkit/
XtAppGetSelectionTimeout, XtAppSetSelectionTimeout - set and obtain
selection timeout values
#include <X11/Intrinsic.h>
unsigned long XtAppGetSelectionTimeout(XtAppContext app_context);
void XtAppSetSelectionTimeout(XtAppContext app_context, unsigned long
timeout);
app_context
Specifies the application context.
timeout Specifies the selection timeout in milliseconds.
The XtAppGetSelectionTimeout function returns the current selection
timeout value, in milliseconds. The selection timeout is the time
within which the two communicating applications must respond to one
another. The initial timeout value is set by the selectionTimeout
application resource, or, if selectionTimeout is not specified, it
defaults to five seconds.
The XtAppSetSelectionTimeout function sets the Intrinsics's selection
timeout mechanism. Note that most applications should not set the
selection timeout.
XtOwnSelection(3)
X Toolkit Intrinsics – C Language Interface
Xlib - C Language X Interface
X Version 11 libXt 1.1.5.2 XtAppGetSelectionTimeout(3)