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.
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
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 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.
is essential that application code never calls XSetEr
rorHandler directly; applications should use only Tk_Cre
ateErrorHandler.
KEYWORDS
callback, error, event, handler
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