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 bgerror Tcl command to report the error in an application-specific fashion. If no bgerror command exists, or if it returns with an error condition, then Tcl_BackgroundError reports the error itself by printing a message on the standard error file.
Tcl_BackgroundError does not invoke bgerror immediately because this could potentially interfere with scripts that are in process at the time the error occurred. Instead, it invokes bgerror later as an idle callback. Tcl_BackgroundError saves the values of the errorInfo and errorCode variables and restores these values just before invoking bgerror.
It is possible for many background errors to accumulate before bgerror is invoked. When this happens, each of the errors is processed in order. However, if bgerror returns a break exception, then all remaining error reports for the interpreter are skipped.