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_________________________________________________________________
NAME
Tk_CreateErrorHandler, Tk_DeleteErrorHandler - handle X pro-
tocol 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).
_________________________________________________________________
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 documenta-
tion 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 follow-
ing 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 struc-
ture 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 sub-
sequent (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 pro-
tocol 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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