Tk_CreateErrorHandler(3) Tk Library Procedures Tk_CreateErrorHandler(3)

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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).
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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 does not 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's 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

Tk Tk_CreateErrorHandler(3)