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 whose errors are to be handled. Match only error events with this value in the error_code field. If -1, then match any error_code value. Match only error events with this value in the request_code field. If -1, then match any request_code value. Match only error events with this value in the minor_code field. If -1, then match any minor_code value. Procedure to invoke whenever an error event is received for display and matches error, request, and minor. NULL means ignore any matching errors. Arbitrary one-word value to pass to proc. 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 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.

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