bgerror(n) Tcl Built-In Commands bgerror(n)
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NAME
bgerror - Command invoked to process background errors
SYNOPSIS
bgerror message
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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 executing 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 appli-cation; application;
cation; 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 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.func-tion.text, *ErrorDialog.function.text,
tion.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 command 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 dialog.
SEE ALSO
after(n), tclvars(n)
KEYWORDS
background error, reporting
Tcl 7.5 bgerror(n)
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