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Tk_CreateErrorHandler(3Tk)			    Tk_CreateErrorHandler(3Tk)

NAME
     Tk_CreateErrorHandler, Tk_DeleteErrorHandler - handle X protocol errors

SYNOPSIS
     #include <tk.h>

     Tk_ErrorHandler
     Tk_CreateErrorHandler(display, error, request, minor, proc, clientData)

     Tk_DeleteErrorHandler(handler)

ARGUMENTS
     Display	       *display	    (in)      Display whose errors are to be
					      handled.

     int	       error	    (in)      Match only error events with
					      this value in the error_code
					      field.  If -1, then match any
					      error_code value.

     int	       request	    (in)      Match only error events with
					      this value in the request_code
					      field.  If -1, then match any
					      request_code value.

     int	       minor	    (in)      Match only error events with
					      this value in the minor_code
					      field.  If -1, then match any
					      minor_code value.

     Tk_ErrorProc      *proc	    (in)      Procedure to invoke whenever an
					      error event is received for
					      display and matches error,
					      request, and minor.  NULL means
					      ignore any matching errors.

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

     Tk_ErrorHandler   handler	    (in)      Token for error handler to
					      delete (return value from a
					      previous call to
					      Tk_CreateErrorHandler).

DESCRIPTION
     Tk_CreateErrorHandler arranges for a particular procedure (proc) to be
     called whenever certain protocol errors occur on a particular display
     (display).	 Protocol errors occur when the X protocol is used
     incorrectly, such as attempting to map a window that doesn't exist.  See
     the Xlib documentation for XSetErrorHandler for more information on the
     kinds of errors that can occur.  For proc to be invoked to handle a

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Tk_CreateErrorHandler(3Tk)			    Tk_CreateErrorHandler(3Tk)

     particular error, five things must occur:

     [1]  The error must pertain to display.

     [2]  Either the error argument to Tk_CreateErrorHandler must have been
	  -1, or the error argument must match the error_code field from the
	  error event.

     [3]  Either the request argument to Tk_CreateErrorHandler must have been
	  -1, or the request argument must match the request_code field from
	  the error event.

     [4]  Either the minor argument to Tk_CreateErrorHandler must have been
	  -1, or the minor argument must match the minor_code field from the
	  error event.

     [5]  The protocol request to which the error pertains must have been made
	  when the handler was active (see below for more information).

     Proc should have arguments and result that match the following type:
	  typedef int Tk_ErrorProc(
	       ClientData clientData,
	       XErrorEvent *errEventPtr);
     The clientData parameter to proc is a copy of the clientData argument
     given to Tcl_CreateErrorHandler when the callback was created.
     Typically, clientData points to a data structure containing application-
     specific information that is needed to deal with the error.  ErrEventPtr
     is a pointer to the X error event.	 The procedure proc should return an
     integer value.  If it returns 0 it means that proc handled the error
     completely and there is no need to take any other action for the error.
     If it returns non-zero it means proc was unable to handle the error.

     If a value of NULL is specified for proc, all matching errors will be
     ignored:  this will produce the same result as if a procedure had been
     specified that always returns 0.

     If more than more than one handler matches a particular error, then they
     are invoked in turn.  The handlers will be invoked in reverse order of
     creation:	most recently declared handler first.  If any handler returns
     0, then subsequent (older) handlers will not be invoked.  If no handler
     returns 0, then Tk invokes X'es default error handler, which prints an
     error message and aborts the program.  If you wish to have a default
     handler that deals with errors that no other handler can deal with, then
     declare it first.

     The X documentation states that ``the error handler should not call any
     functions (directly or indirectly) on the display that will generate
     protocol requests or that will look for input events.''  This restriction
     applies to handlers declared by Tk_CreateErrorHandler; disobey it at your
     own risk.

									Page 2

Tk_CreateErrorHandler(3Tk)			    Tk_CreateErrorHandler(3Tk)

     Tk_DeleteErrorHandler may be called to delete a previously-created error
     handler.  The handler argument identifies the error handler, and should
     be a value returned by a previous call to Tk_CreateEventHandler.

     A particular error handler applies to errors resulting from protocol
     requests generated between the call to Tk_CreateErrorHandler and the call
     to Tk_DeleteErrorHandler.	However, the actual callback to proc may not
     occur until after the Tk_DeleteErrorHandler call, due to buffering in the
     client and server.	 If an error event pertains to a protocol request made
     just before calling Tk_DeleteErrorHandler, then the error event may not
     have been processed before the Tk_DeleteErrorHandler call.	 When this
     situation arises, Tk will save information about the handler and invoke
     the handler's proc later when the error event finally arrives.  If an
     application wishes to delete an error handler and know for certain that
     all relevant errors have been processed, it should first call
     Tk_DeleteErrorHandler and then call XSync;	 this will flush out any
     buffered requests and errors, but will result in a performance penalty
     because it requires communication to and from the X server.  After the
     XSync call Tk is guaranteed not to call any error handlers deleted before
     the XSync call.

     For the Tk error handling mechanism to work properly, it is essential
     that application code never calls XSetErrorHandler directly; applications
     should use only Tk_CreateErrorHandler.

KEYWORDS
     callback, error, event, handler

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