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



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NAME
       bgerror - Command invoked to process background errors

SYNOPSIS
       bgerror message
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DESCRIPTION
       Release  8.5  of Tcl supports the interp bgerror command, which allows applications to register in an |
       interpreter the command that will handle background errors in that interpreter.  In older releases of |
       Tcl,  this  level  of control was not available, and applications could control the handling of back- |
       ground errors only by creating a command with the particular  command  name  bgerror  in  the  global |
       namespace of an interpreter.  The following documentation describes the interface requirements of the |
       bgerror command an application might define to retain compatibility with  pre-8.5  releases  of  Tcl. |
       Applications  intending  to  support only Tcl releases 8.5 and later should simply make use of interp |
       bgerror.

       The bgerror command does not 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 did  not  origi-nate originate
       nate  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-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 a handler  com-mand command
       mand  registered  by  interp  bgerror later as an idle event handler.  The default handler command in
       turn calls the bgerror command .  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 mes-sage message
       sage 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 bgerror returns  with  a  break
       exception, then any remaining errors are skipped without calling bgerror.

       If  you  are  writing code that will be used by others as part of a package or other kind of library,
       consider avoiding bgerror.  The reason for this is that the application programmer may also  want  to
       define a bgerror, or use other code that does and thus will have trouble integrating your code.

EXAMPLE
       This bgerror procedure appends errors to a file, with a timestamp.
              proc bgerror {message} {
                  set timestamp [clock format [clock seconds]]
                  set fl [open mylog.txt {WRONLY CREAT APPEND}]
                  puts $fl "$timestamp: bgerror in $::argv '$message'"
                  close $fl
              }


SEE ALSO
       after(n), interp(n), tclvars(n)


KEYWORDS
       background error, reporting



Tcl                                                  7.5                                          bgerror(n)