bgerror(n) Tcl Built-In Commands bgerror(n)_________________________________________________________________NAMEbgerror - Command invoked to process background errors
SYNOPSISbgerror message
_________________________________________________________________DESCRIPTION
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 exe-
cuting a command specified with the 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 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 bger-
ror 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
procedure that posts a dialog box containing the error
Tcl 7.5 1
bgerror(n) Tcl Built-In Commands bgerror(n)
message and offers the user a chance to see a stack trace
showing where the error occurred.
KEYWORDS
background error, reporting
Tcl 7.5 2
