This procedure is typically invoked when a Tcl error occurs during
“background processing” such as executing an event
handler. When such an error occurs, the error condition is reported
to Tcl or to a widget or some other C code, and there is not
usually any obvious way for that code to report the error to the
user. In these cases the code calls Tcl_BackgroundError with
an interp argument identifying the interpreter in which the
error occurred. At the time Tcl_BackgroundError is invoked,
the interpreter's result is expected to contain an error message.
Tcl_BackgroundError will invoke the command registered in
that interpreter to handle background errors by the interp bgerror command. The
registered handler command is meant to report the error in an
application-specific fashion. The handler command receives two
arguments, the result of the interp, and the return options of the
interp at the time the error occurred. If the application registers
no handler command, the default handler command will attempt to
call bgerror to report
the error. If an error condition arises while invoking the handler
command, then Tcl_BackgroundError reports the error itself
by printing a message on the standard error file.
Tcl_BackgroundError does not invoke the handler command
immediately because this could potentially interfere with scripts
that are in process at the time the error occurred. Instead, it
invokes the handler command later as an idle callback.
It is possible for many background errors to accumulate before
the handler command is invoked. When this happens, each of the
errors is processed in order. However, if the handle command
returns a break exception, then all remaining error reports for the
interpreter are skipped.