Tk_CreateErrorHandler(3) Tk Library Procedures Tk_CreateErrorHandler(3)
Tk_CreateErrorHandler, Tk_DeleteErrorHandler - handle X protocol errors
Tk_CreateErrorHandler(display, error, request, minor, proc, clientData)
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
int request (in) Match only error events with this value in the
request_code field. If -1, then match any
int minor (in) Match only error events with this value in the
minor_code field. If -1, then match any
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
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).
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 does not
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:
 The error must pertain to display.
 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.
 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.
 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.
 The protocol request to which the error pertains must have been made when the han-
dler was active (see below for more information).
Proc should have arguments and result that match the following type:
typedef int Tk_ErrorProc(
The clientData parameter to proc is a copy of the clientData argument given to Tcl_Cre-
ateErrorHandler 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 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's 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 previ-
ous 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 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_CreateEr-
callback, error, event, handler