A background error is one that occurs in a command that didn't originate with the application. For example, if an error occurs while executing a command specified with a bind of 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 Tk library and there is no obvious way for Tk to report the error.
When Tk detects a background error, it saves information about the error and invokes the tkerror command later when Tk is idle. Before invoking tkerror, Tk restores the errorInfo and errorCode variables to their values at the time the error occurred, then it invokes tkerror with the error message as its only argument. Tk assumes that the application has implemented the tkerror command, and that the command will report the error in a way that makes sense for the application. Tk will ignore any result returned by the tkerror command.
If another Tcl error occurs within the tkerror command (for example, because no tkerror command has been defined) then Tk reports the error itself by writing a message to stderr.
If several background errors accumulate before tkerror is invoked to process them, tkerror will be invoked once for each error, in the order they occurred. However, if tkerror returns with a break exception, then any remaining errors are skipped without calling tkerror.
The Tk script library includes a default tkerror procedure that posts a dialog box containing the error message and offers the user a chance to see a stack trace showing where the error occurred.
Copyright © 1990-1994 The Regents of the University of California. Copyright © 1994-1996 Sun Microsystems, Inc. Copyright © 1995, 1996 Roger E. Critchlow Jr.