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.so man.macros

.TH Tk_CreateErrorHandler 3 "" Tk "Tk Library Procedures"

.BS

.SH NAME

Tk_CreateErrorHandler, Tk_DeleteErrorHandler \- handle X protocol errors

.SH SYNOPSIS

.nf

\fB#include \fR

.sp

Tk_ErrorHandler

\fBTk_CreateErrorHandler\fR(\fIdisplay, error, request, minor, proc, clientData\fR)

.sp

\fBTk_DeleteErrorHandler\fR(\fIhandler\fR)

.SH ARGUMENTS

.AS "Tk_ErrorHandler" clientData

.AP Display *display in

Display whose errors are to be handled.

.AP int error in

Match only error events with this value in the \fIerror_code\fR

field.  If -1, then match any \fIerror_code\fR value.

.AP int request in

Match only error events with this value in the \fIrequest_code\fR

field.  If -1, then match any \fIrequest_code\fR value.

.AP int minor in

Match only error events with this value in the \fIminor_code\fR

field.  If -1, then match any \fIminor_code\fR value.

.AP Tk_ErrorProc *proc in

Procedure to invoke whenever an error event is received for

\fIdisplay\fR and matches \fIerror\fR, \fIrequest\fR, and \fIminor\fR.

NULL means ignore any matching errors.

.AP ClientData clientData in

Arbitrary one-word value to pass to \fIproc\fR.

.AP Tk_ErrorHandler handler in

Token for error handler to delete (return value from a previous

call to \fBTk_CreateErrorHandler\fR).

.BE

.SH DESCRIPTION

.PP

\fBTk_CreateErrorHandler\fR arranges for a particular procedure

(\fIproc\fR) to be called whenever certain protocol errors occur on a

particular display (\fIdisplay\fR).  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 \fBXSetErrorHandler\fR

for more information on the kinds of errors that can occur.

For \fIproc\fR to be invoked

to handle a particular error, five things must occur:

.IP [1]

The error must pertain to \fIdisplay\fR.

.IP [2]

Either the \fIerror\fR argument to \fBTk_CreateErrorHandler\fR

must have been -1, or the \fIerror\fR argument must match

the \fIerror_code\fR field from the error event.

.IP [3]

Either the \fIrequest\fR argument to \fBTk_CreateErrorHandler\fR

must have been -1, or the \fIrequest\fR argument must match

the \fIrequest_code\fR field from the error event.

.IP [4]

Either the \fIminor\fR argument to \fBTk_CreateErrorHandler\fR

must have been -1, or the \fIminor\fR argument must match

the \fIminor_code\fR field from the error event.

.IP [5]

The protocol request to which the error pertains must have been

made when the handler was active (see below for more information).

.PP

\fIProc\fR should have arguments and result that match the

following type:

.CS

typedef int Tk_ErrorProc(

        ClientData \fIclientData\fR,

        XErrorEvent *\fIerrEventPtr\fR);

.CE

The \fIclientData\fR parameter to \fIproc\fR is a copy of the \fIclientData\fR

argument given to \fBTcl_CreateErrorHandler\fR when the callback

was created.  Typically, \fIclientData\fR points to a data

structure containing application-specific information that is

needed to deal with the error.  \fIErrEventPtr\fR is

a pointer to the X error event.

The procedure \fIproc\fR should return an integer value.  If it

returns 0 it means that \fIproc\fR handled the error completely and there

is no need to take any other action for the error.  If it returns

non-zero it means \fIproc\fR was unable to handle the error.

.PP

If a value of NULL is specified for \fIproc\fR, all matching errors

will be ignored:  this will produce the same result as if a procedure

had been specified that always returns 0.

.PP

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.

.PP

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 \fBTk_CreateErrorHandler\fR;

disobey it at your own risk.

.PP

\fBTk_DeleteErrorHandler\fR may be called to delete a

previously-created error handler.  The \fIhandler\fR argument

identifies the error handler, and should be a value returned by

a previous call to \fBTk_CreateEventHandler\fR.

.PP

A particular error handler applies to errors resulting

from protocol requests generated between

the call to \fBTk_CreateErrorHandler\fR and the call to

\fBTk_DeleteErrorHandler\fR.  However, the actual callback

to \fIproc\fR may not occur until after the \fBTk_DeleteErrorHandler\fR

call, due to buffering in the client and server.

If an error event pertains to

a protocol request made just before calling \fBTk_DeleteErrorHandler\fR,

then the error event may not have been processed

before the \fBTk_DeleteErrorHandler\fR

call.  When this situation arises, Tk will save information about

the handler and

invoke the handler's \fIproc\fR 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 \fBTk_DeleteErrorHandler\fR and then

call \fBXSync\fR;  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

\fBXSync\fR call Tk is guaranteed not to call any error

handlers deleted before the \fBXSync\fR call.

.PP

For the Tk error handling mechanism to work properly, it is essential

that application code never calls \fBXSetErrorHandler\fR directly;

applications should use only \fBTk_CreateErrorHandler\fR.

.SH KEYWORDS

callback, error, event, handler