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<HTML><HEAD><TITLE>Tcl Built-In Commands - bgerror manual page</TITLE></HEAD><BODY>
<H3><A NAME="M2">NAME</A></H3>
bgerror - Command invoked to process background errors
<H3><A NAME="M3">SYNOPSIS</A></H3>
<B>bgerror </B><I>message</I><BR>
<H3><A NAME="M4">DESCRIPTION</A></H3>
The <B>bgerror</B> command doesn't exist as built-in part of Tcl.  Instead,
individual applications or users can define a <B>bgerror</B>
command (e.g. as a Tcl procedure) if they wish to handle background
errors.
<P>
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 <B><A HREF="../TclCmd/after.htm">after</A></B> 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.
<P>
When Tcl detects a background error, it saves information about the
error and invokes the <B>bgerror</B> command later as an idle event handler.
Before invoking <B>bgerror</B>, Tcl restores the <B>errorInfo</B>
and <B>errorCode</B> variables to their values at the time the
error occurred, then it invokes <B>bgerror</B> with
the error message as its only argument.
Tcl assumes that the application has implemented the <B>bgerror</B>
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 <B>bgerror</B> command as long as no error is generated.
<P>
If another Tcl error occurs within the <B>bgerror</B> command
(for example, because no <B>bgerror</B> command has been defined)
then Tcl reports the error itself by writing a message to stderr.
<P>
If several background errors accumulate before <B>bgerror</B>
is invoked to process them, <B>bgerror</B> will be invoked once
for each error, in the order they occurred.
However, if <B>bgerror</B> returns with a break exception, then
any remaining errors are skipped without calling <B>bgerror</B>.
<P>
Tcl has no default implementation for <B>bgerror</B>.
However, in applications using Tk there is a default
<B>bgerror</B> 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.

<H3><A NAME="M5">KEYWORDS</A></H3>
<A href="../Keywords/B.htm#background error">background error</A>, <A href="../Keywords/R.htm#reporting">reporting</A>
<HR><PRE>
<A HREF="../copyright.htm">Copyright</A> &#169; 1990-1994 The Regents of the University of California.
<A HREF="../copyright.htm">Copyright</A> &#169; 1994-1996 Sun Microsystems, Inc.
<A HREF="../copyright.htm">Copyright</A> &#169; 1995-1997 Roger E. Critchlow Jr.</PRE>
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