Tk_CreateErrorHandler (3)
handle X protocol errors
SYNOPSIS
#include <tk.h>
Tk_ErrorHandler
Tk_CreateErrorHandler(display, error, request, minor, proc, clientData)
Tk_DeleteErrorHandler(handler)
ARGUMENTS
"Tk_ErrorHandler" clientData
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 particular error, five things must occur:
The error must pertain to display.
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.
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.
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.
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
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