NAME
       bgerror - Command invoked to process background errors

SYNOPSIS
       bgerror message


DESCRIPTION
       The bgerror command doesn't exist as built-in part of Tcl.
       Instead, individual applications or  users  can  define  a
       bgerror  command (e.g. as a Tcl procedure) if they wish to
       handle 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  exe-
       cuting a command specified with the after command, then it
       is a background error.  For a  non-background  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 application 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
       stderr.

       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 bger-
       ror returns with a break  exception,  then  any  remaining
       errors are skipped without calling bgerror.

       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 addition to allowing the
       user to view the stack trace, the dialog provides an addi-
       tional  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.func-
       tion.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.


SEE ALSO
       after(n), tclvars(n)


KEYWORDS
