Provided by: tcl8.5-doc_8.5.19-1_all 
NAME
bgerror - Command invoked to process background errors
SYNOPSIS
bgerror message ________________________________________________________________________________________________________________
DESCRIPTION
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.
EXAMPLE
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
}
SEE ALSO
after(3tcl), interp(3tcl), tclvars(3tcl)
KEYWORDS
background error, reporting