bgerror(n) Tcl (7.5) 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
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 is a default bgerror procedure
which posts a dialog box containing the error message and
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bgerror(n) Tcl (7.5) bgerror(n)
offers the user a chance to see a stack trace showing where
the error occurred.
KEYWORDS
background error, reporting
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