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Tk_CreateErrorHandler(Tk Library Procedures



Tk_CreateErrorHandler, Tk_DeleteErrorHandler - handle X pro- tocol errors


#include <tk.h> Tk_ErrorHandler Tk_CreateErrorHandler(display, error, request, minor, proc, clientData) Tk_DeleteErrorHandler(handler)


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. Tk 1 Prepared by Ready-to-Run Software, Inc.

Tk_CreateErrorHandler(Tk Library Procedures

     ClientData        clientData   (in)      Arbitrary  one-word
                                              value  to  pass  to

     Tk_ErrorHandler   handler      (in)      Token   for   error
                                              handler  to  delete
                                              (return value  from
                                              a  previous call to


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 Tk 2 Prepared by Ready-to-Run Software, Inc.

Tk_CreateErrorHandler(Tk Library Procedures

     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

Tk                                                              3

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Tk_CreateErrorHandler(Tk Library Procedures

     For the Tk error handling mechanism to work properly, it  is
     essential that application code never calls XSetErrorHandler
     directly;      applications      should       use       only


callback, error, event, handler Tk 4 Prepared by Ready-to-Run Software, Inc.


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