The callback to proc will be made by Tk_HandleEvent; this mechanism only works in programs that dispatch events through Tk_HandleEvent (or through other Tk procedures that call Tk_HandleEvent, such as Tcl_DoOneEvent or Tk_MainLoop).
Proc should have arguments and result that match the type Tk_GenericProc:
typedef int Tk_GenericProc(
ClientData clientData,
XEvent *eventPtr);
The clientData parameter to proc is a copy of the
clientData argument given to Tk_CreateGenericHandler
when the callback was created. Typically, clientData points
to a data structure containing application-specific information
about how to handle events. EventPtr is a pointer to the X
event.
Whenever an X event is processed by Tk_HandleEvent, proc is called. The return value from proc is normally 0. A non-zero return value indicates that the event is not to be handled further; that is, proc has done all processing that is to be allowed for the event.
If there are multiple generic event handlers, each one is called for each event, in the order in which they were established.
Tk_DeleteGenericHandler may be called to delete a previously-created generic event handler: it deletes each handler it finds that matches the proc and clientData arguments. If no such handler exists, then Tk_DeleteGenericHandler returns without doing anything. Although Tk supports it, it's probably a bad idea to have more than one callback with the same proc and clientData arguments.
Establishing a generic event handler does nothing to ensure that the process will actually receive the X events that the handler wants to process. For example, it is the caller's responsibility to invoke XSelectInput to select the desired events, if that is necessary.