NAME

       Tcl_BackgroundException,  Tcl_BackgroundError  -  report  Tcl  exception  that occurred in
       background processing

SYNOPSIS

       #include <tcl.h>

       Tcl_BackgroundException(interp, code)

       Tcl_BackgroundError(interp)

ARGUMENTS

       Tcl_Interp *interp (in)          Interpreter in which the exception occurred.

       int code (in)                    The exceptional return code to be reported.
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DESCRIPTION

       This procedure is typically invoked when a Tcl  exception  (any  return  code  other  than
       TCL_OK)  occurs  during  “background processing” such as executing an event handler.  When
       such an exception occurs, the condition is reported to Tcl or to a widget or some other  C
       code,  and  there  is not usually any obvious way for that code to report the exception to
       the user.  In these cases the code calls Tcl_BackgroundException with an  interp  argument
       identifying  the  interpreter in which the exception occurred, and a code argument holding
       the return code value of the exception.  The state of the interpreter, including any error
       message  in  the  interpreter  result, and the values of any entries in the return options
       dictionary, is captured and saved.  Tcl_BackgroundException then arranges  for  the  event
       loop  to  invoke  at  some later time the command registered in that interpreter to handle
       background errors by the interp bgerror command, passing the captured values as arguments.
       The registered handler command is meant to report the exception in an application-specific
       fashion.  The handler command receives two arguments, the result of the  interp,  and  the
       return options of the interp at the time the error occurred.  If the application registers
       no handler command, the default handler command will attempt to call bgerror to report the
       error.    If   an  error  condition  arises  while  invoking  the  handler  command,  then
       Tcl_BackgroundException reports the error itself by printing a  message  on  the  standard
       error file.

       Tcl_BackgroundException does not invoke the handler command immediately because this could
       potentially interfere with scripts that are in process at the  time  the  error  occurred.
       Instead, it invokes the handler command later as an idle callback.

       It  is possible for many background exceptions to accumulate before the handler command is
       invoked.  When this happens, each of the exceptions is processed in  order.   However,  if
       the  handler  command  returns a break exception, then all remaining error reports for the
       interpreter are skipped.

       The Tcl_BackgroundError routine  is  an  older  and  simpler  interface  useful  when  the
       exception code reported is TCL_ERROR.  It is equivalent to:

              Tcl_BackgroundException(interp, TCL_ERROR);

KEYWORDS

       background, bgerror, error, interp