https://invisible-island.net/xterm/xtoolkit/
XtAppAddActionHook, XtRemoveActionHook - register an action hook proce-
dure
#include <X11/Intrinsic.h>
XtActionHookId XtAppAddActionHook(XtAppContext app_context, XtAction-
HookProc proc, XtPointer client_data);
void XtRemoveActionHook(XtActionHookId id);
app_context
Specifies the application context.
proc Specifies the action hook procedure.
num_args Specifies the application-specific data to be passed to the
action hook.
XtAppAddActionHook adds the specified procedure to the front of a list
maintained in the application context. In the future, when an action
routine is about to be invoked for any widget in this application con-
text, either through the translation manager or via XtCallActionProc,
the action hohok procedures will be called in reverse order of regis-
tration jut prior to invoking the action routine.
Action hook procedures are removed automatically and the XtActionHookId
s destroyed when the application context in which they were added is
destroyed.
XtRemoveActionHook removes the specified action hook procedure from the
list in which it was registered.
X Toolkit Intrinsics – C Language Interface
Xlib - C Language X Interface
X Version 11 libXt 1.1.5.2 XtAppAddActionHook(3)