Tk_CreateErrorHandler(3) Tk Library Procedures Tk_CreateErrorHandler(3)
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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 pre-vious previous
vious call to Tk_CreateErrorHandler).
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DESCRIPTION
Tk_CreateErrorHandler arranges for a particular procedure (proc) to be called whenever certain proto-col protocol
col 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:
[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 argu-ment argument
ment 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 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 applica-tion-specific application-specific
tion-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 indi-rectly) indirectly)
rectly) 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 argu-ment argument
ment identifies the error handler, and should be a value returned by a previous call to Tk_Cre-ateEventHandler. Tk_CreateEventHandler.
ateEventHandler.
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_DeleteErrorHan-dler, Tk_DeleteErrorHandler,
dler, 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 per-formance performance
formance 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
Tk Tk_CreateErrorHandler(3)
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