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 background 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
A background error is one that occurs in an event handler or some
other command that did not 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-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 command 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
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 bgerror returns
with a break exception, then any remaining errors are skipped without
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.