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bgerror(n)			      Tcl Built-In Commands			       bgerror(n)


       bgerror - Command invoked to process background errors

       bgerror message

       The  bgerror  command doesn't exist as built-in part of Tcl.  Instead, individual applica-
       tions or users can define a bgerror command (e.g. as a Tcl procedure) if they wish to han-
       dle background errors.

       A  background  error  is  one  that  occurs in an event handler or some other command that
       didn't originate with the application.  For example, if an error occurs while executing	a
       command	specified with the after command, then it is a background error.  For a non-back-
       ground error, the error can simply be returned up through nested Tcl  command  evaluations
       until  it  reaches  the top-level code in the application; then the application can report
       the error in whatever way it wishes.  When a background error occurs, the  unwinding  ends
       in the Tcl library and there is no obvious way for Tcl to report the error.

       When  Tcl detects a background error, it saves information about the error and invokes the
       bgerror command later as an idle event handler. Before invoking bgerror, Tcl restores  the
       errorInfo  and errorCode variables to their values at the time the error occurred, then it
       invokes bgerror with the error message as its only argument.  Tcl assumes that the  appli-
       cation  has implemented the bgerror command, and that the command will report the error in
       a way that makes sense for the application.  Tcl will ignore any result	returned  by  the
       bgerror command as long as no error is generated.

       If  another  Tcl  error occurs within the bgerror command (for example, because no bgerror
       command has been defined) then Tcl reports the  error  itself  by  writing  a  message  to

       If several background errors accumulate before bgerror is invoked to process them, bgerror
       will be invoked once for each error, in the order  they	occurred.   However,  if  bgerror
       returns	with  a  break	exception,  then any remaining errors are skipped without calling

       Tcl has no default implementation for bgerror. However, in applications using Tk there  is
       a  default  bgerror  procedure  which  posts a dialog box containing the error message and
       offers the user a chance to see a stack trace showing where the error occurred.	In  addi-
       tion  to  allowing  the	user  to  view the stack trace, the dialog provides an additional
       application configurable button which may be used, for example, to save the stack trace to
       a  file.  By default, this is the behavior associated with that button.	This behavior can
       be redefined by setting the option database values *ErrorDialog.function.text, to  specify
       the  caption  for  the  function button, and *ErrorDialog.function.command, to specify the
       command to be run.  The text of the stack trace is appended to  the  command  when  it  is
       evaluated.   If	either	of  these options is set to the empty string, then the additional
       button will not be displayed in the dialog.

       after(n), tclvars(n)

       background error, reporting

Tcl					       7.5				       bgerror(n)
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