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_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 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_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 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
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_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 pro-
cessed, 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.
is essential that application code never calls XSetEr-
rorHandler directly; applications should use only
Tk_CreateErrorHandler.
KEYWORDS
callback, error, event, handler