DOKK / manpages / debian 11 / tk8.6-doc / Tk_CreateErrorHandler.3tk.en
Tk_CreateErrorHandler(3tk) Tk Library Procedures Tk_CreateErrorHandler(3tk) 







Tk_CreateErrorHandler, Tk_DeleteErrorHandler - handle X protocol
    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.

  
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:



  
[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
    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 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 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_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_CreateErrorHandler.








callback, error, event, handler







  

    
    Tk