Tk_CreateErrorHandler(3) Tk Library Procedures Tk_CreateErrorHandler(3)______________________________________________________________________________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_CreateEr-
rorHandler).
_________________________________________________________________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 incor-
rectly, 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:
[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 informa-
tion).
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. Typi-
cally, 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 han-
dler 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 restric-
tion 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_DeleteEr-
rorHandler 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.
For the Tk error handling mechanism to work properly, it is essential
that application code never calls XSetErrorHandler directly; applica-
tions should use only Tk_CreateErrorHandler.
KEYWORDS
callback, error, event, handler
TkTk_CreateErrorHandler(3)
