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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 after(1tcl) 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 the bgerror command later as an idle event handler. 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 calling bgerror.
Tcl has no default implementation for bgerror. However, in applications using Tk there will be a default bgerror containing the error message and offers the user a chance to see a stack trace showing where the error occurred.