bgerror - Command invoked to process background errors
bgerror message
Release 8.5 of Tcl supports the
interp bgerror command, which allows
applications to register in an interpreter the command that will handle
background errors in that interpreter. In older releases of Tcl, this level of
control was not available, and applications could control the handling of
background errors only by creating a command with the particular command name
bgerror in the global namespace of an interpreter. The following
documentation describes the interface requirements of the
bgerror
command an application might define to retain compatibility with pre-8.5
releases of Tcl. Applications intending to support only Tcl releases 8.5 and
later should simply make use of
interp bgerror.
The
bgerror command does not 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 did not 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 a handler command registered by
interp bgerror later as an idle
event handler. The default handler command in turn calls the
bgerror
command . 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.
If you are writing code that will be used by others as part of a package or
other kind of library, consider avoiding
bgerror. The reason for this
is that the application programmer may also want to define a
bgerror,
or use other code that does and thus will have trouble integrating your code.
This
bgerror procedure appends errors to a file, with a timestamp.
proc bgerror {message} {
set timestamp [clock format [clock seconds]]
set fl [open mylog.txt {WRONLY CREAT APPEND}]
puts $fl "$timestamp: bgerror in $::argv '$message'"
close $fl
}
after(3tcl),
errorCode(3tcl),
errorInfo(3tcl),
interp(3tcl)
background error, reporting