wn_register_function_c

	status.i4.v = wn_register_function_c(window_id.i4.v, type.i4.v,
					     callb_function.f.v
					     [,callb_data.g.v])

	This routine will register a function to be executed when a
	particular type of event occurs in the indicated window.

	window_id	window ID
	type		type of function
			(WN_DESTRUCTOR_FUNC -> called when window is deleted,
			 WN_EXPOSE_FUNC -> called when the window is popped
					   to the top,
			 WN_OCCLUDE_FUNC -> called when the window is
					    occluded)
	callb_function	address of function to be called as follows:
			callback(wid.i2.v, callb_data.g.v, op_data.s.r)
			("op_data" is a structure which is dependent on the
			 operation generating the callback having the
			 following types:
			 WN_EXPOSE_DATA -> window popped to the top,
			 WN_OCCLUDE_DATA -> window occluded)
	[callb_data]	pointer to user-supplied data to be passed to
			callback function (default is NULL)

	This function returns status values as follows:

	OK			success
	CBS_INVARG		invalid window ID or function type was specified
	CBS_NO_WINDOW		requested window does not exist
	CBS_MEMFAIL		dynamic memory allocation failure

	This function requires the following include files:

	cns_data_structs_h, cbslib_h, acnet_errors_h

	Related functions:

	wn_unregister_function_c, wn_delete(_c)

	C/C++ usage:

	short	window_id;
	int	status;
	int	type = WN_DESTRUCTOR_FUNC;
	void	callb_function(int window_id, void *callback_data);
	void	*callb_data = (void *) NULL;

	status = wn_register_function_c(window_id,type,
					callb_function,callb_data);