Proc should have arguments and result that match the type Tk_EventProc:
typedef void Tk_EventProc(
ClientData clientData,
XEvent *eventPtr);
The clientData parameter to proc is a copy of the clientData
argument given to Tk_CreateEventHandler when the callback
was created. Typically, clientData points to a data
structure containing application-specific information about
the window in which the event occurred. EventPtr is
a pointer to the X event, which will be one of the ones
specified in the mask argument to Tk_CreateEventHandler.
Tk_DeleteEventHandler may be called to delete a previously-created event handler: it deletes the first handler it finds that is associated with tkwin and matches the mask, proc, and clientData arguments. If no such handler exists, then Tk_HandleEvent returns without doing anything. Although Tk supports it, it's probably a bad idea to have more than one callback with the same mask, proc, and clientData arguments. When a window is deleted all of its handlers will be deleted automatically; in this case there is no need to call Tk_DeleteEventHandler.
If multiple handlers are declared for the same type of X event on the same window, then the handlers will be invoked in the order they were created.