bgerror(n) Tcl Built-In Commands bgerror(n) 







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(n), interp(n), tclvars(n)










background error, reporting







  

    7.5
    Tcl