Tk_CreateErrorHandler

Prepared by Ready-to-Run Software, Inc. 

Tk_CreateErrorHandler(Tk Library Procedures

_________________________________________________________________

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.



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Prepared by Ready-to-Run Software, Inc. 

Tk_CreateErrorHandler(Tk Library Procedures

     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



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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
     call.





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Prepared by Ready-to-Run Software, Inc. 

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
     Tk_CreateErrorHandler.


KEYWORDS
     callback, error, event, handler















































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