bgerror(3tcl)						       Tcl Built-In Commands						     bgerror(3tcl)

__________________________________________________________________________________________________________________________________________________

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 han- |
       dle background errors in that interpreter.  In older releases of Tcl, this level of control was not available, and applications could  con- |
       trol  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 inte-
       grating 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

Tcl									7.5							     bgerror(3tcl)