## Tcl8.6.2/Tk8.6.2 Documentation > Tk C API, version 8.6.2 > EventHndlr

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### NAME

Tk_CreateEventHandler, Tk_DeleteEventHandler — associate procedure callback with an X event

### SYNOPSIS

#include <tk.h>
Tk_CreateEventHandler(tkwin, mask, proc, clientData)
Tk_DeleteEventHandler(tkwin, mask, proc, clientData)

### ARGUMENTS

Tk_Window tkwin (in)
  Token for window in which events may occur.

unsigned long mask (in)
  Bit-mask of events (such as ButtonPressMask) for which proc should be called.

Tk_EventProc *proc (in)
  Procedure to invoke whenever an event in mask occurs in the window given by tkwin.

ClientData clientData (in)
  Arbitrary one-word value to pass to proc.

### DESCRIPTION

Tk_CreateEventHandler arranges for proc to be invoked in the future whenever one of the event types specified by mask occurs in the window specified by tkwin. 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 Tk_DoOneEvent or Tk_MainLoop).

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.

### KEYWORDS

bind, callback, event, handler
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