tk_createerrorhandler(3)

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 dis_
play 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_Cre
ateErrorHandler).

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 attempt
ing to map a window that doesn't exist. See the Xlib doc
umentation 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_CreateErrorHan

dler 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 fol
lowing type:

typedef int Tk_ErrorProc(

ClientData clientData,
XErrorEvent *errEventPtr);

The clientData parameter to proc is a copy of the client_ Data 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 nonzero 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 han
dler, which prints an error message and aborts the pro
gram. 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 han
dlers declared by Tk_CreateErrorHandler; disobey it at your own risk.

Tk_DeleteErrorHandler may be called to delete a previ ously-created error handler. The handler argument identi fies 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_DeleteErrorHan dler. However, the actual callback to proc may not occur until after the Tk_DeleteErrorHandler call, due to buffer ing in the client and server. If an error event pertains
to a protocol request made just before calling Tk_Dele teErrorHandler, 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_Dele teErrorHandler 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 guaran
teed 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 XSetEr
rorHandler directly; applications should use only Tk_Cre ateErrorHandler.

KEYWORDS

callback, error, event, handler

Tk Tk_CreateErrorHandler(3)