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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
Release 8.5 of Tcl supports the interp bgerror command, which allows applications to reg- |
ister in an interpreter the command that will handle background errors in that inter- |
preter. In older releases of Tcl, this level of control was not available, and applica- |
tions 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 applica-
tions or users can define a bgerror command (e.g. as a Tcl procedure) if they wish to han-
dle 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-back-
ground 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 program-
mer 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(n), interp(n), tclvars(n)
KEYWORDS
background error, reporting
Tcl 7.5 bgerror(n)
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