bgerror(n) Tcl Built-In Commands bgerror(n)______________________________________________________________________________NAMEbgerror - Command invoked to process background errors
The bgerror command doesn't 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 errors.
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 com‐
mand, 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 appli‐
cation can report the error in whatever way it wishes. When a back‐
ground 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 vari‐
ables 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 com‐
mand will report the error in a way that makes sense for the applica‐
tion. 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 is a default bgerror procedure which posts a dialog box
containing the error message and offers the user a chance to see a
stack trace showing where the error occurred. In addition to allowing
the user to view the stack trace, the dialog provides an additional
application configurable button which may be used, for example, to save
the stack trace to a file. By default, this is the behavior associated
with that button. This behavior can be redefined by setting the option
database values *ErrorDialog.function.text, to specify the caption for
the function button, and *ErrorDialog.function.command, to specify the
command to be run. The text of the stack trace is appended to the com‐
mand when it is evaluated. If either of these options is set to the
empty string, then the additional button will not be displayed in the
SEE ALSOafter(n), tclvars(n)KEYWORDS
background error, reporting
Tcl 7.5 bgerror(n)