TIP:		337
Title:		Make TclBackgroundException() Public
Version:	$Revision: 1.6 $
Author:		Don Porter <dgp@users.sf.net>
State:		Final
Type:		Project
Vote:		Done
Created:	21-Oct-2008
Post-History:	
Tcl-Version:	8.6

~ Abstract

This TIP proposes to make the ''TclBackgroundException'' routine available in
the public interface so that extensions may notify Tcl about all return codes
that occur during background operations.

~ Background

When background operations in Tcl result in a non-'''TCL_OK''' return code, it
has been recommended practice to call ''Tcl_BackgroundError()'' so that Tcl
can arrange for background error handlers to react to the exceptional
situation. The prototype of that routine is:

 > void '''Tcl_BackgroundError'''(Tcl_Interp *''interp'')

In Tcl 8.5, the new '''interp bgerror''' command permits registration of
handlers that receive the full return options dictionary as an argument [221].
This means that such handlers can respond differently to different exceptions.
At the same time a new internal routine '''TclBackgroundException''' with
prototype:

 > void '''TclBackgroundException'''(Tcl_Interp *''interp'', int ''code'')

and the implementation of '''Tcl_BackgroundError''' became:

|void Tcl_BackgroundError(Tcl_Interp *interp) {
|    TclBackgroundException(interp, TCL_ERROR);
|}

The new '''TclBackgroundException''' routine is a more general routine
suitable for announcing any exceptional code returned by background
operations, not limited to '''TCL_ERROR''' like '''Tcl_BackgroundError''' has
been. The new '''TclBackgroundException''' routine is used throughout the
internal portions of Tcl so that full exception information gets reliably
passed through to background exception handlers, when the background
processing is governed by things which are built in to Tcl, such as
'''after''' and '''chan event'''.

The oversight of [221] was not proposing to make '''TclBackgroundException'''
public at the same time, so that extensions which govern background
operations, notably Tk, have access to the same full expressive power.

~ Proposal

Rename the internal routine '''TclBackgroundException''' to
'''Tcl_BackgroundException''' and place it in the public stubs table.

Apply Tk Patch 1789752
[https://sourceforge.net/support/tracker.php?aid=1789752]
to make use of the new facility.

~ Compatibility

No compatibility concerns from a new public routine.

~ Copyright

This document has been placed in the public domain.