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_ClientMessageProc:
typedef int Tk_ClientMessageProc(
Tk_Window tkwin,
XEvent *eventPtr);
The tkwin parameter to proc is the Tk window which is
associated with this event. EventPtr is a pointer to the X event.
Whenever an X ClientMessage event is processed by Tk_HandleEvent, the proc is called if it was not handled as a WM_PROTOCOL. 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 ClientMessage event handlers, each one is called for each event, in the order in which they were established.
Tk_DeleteClientMessageHandler may be called to delete a previously-created ClientMessage event handler: it deletes each handler it finds that matches the proc argument. If no such handler exists, then Tk_DeleteClientMessageHandler returns without doing anything. Although Tk supports it, it's probably a bad idea to have more than one callback with the same proc argument.