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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 par- |
       ticular 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 com-
       mand,  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	appli-
       cation can report the error in whatever way it wishes.	When  a	 back-
       ground 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 er-
       ror 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	 bger-
       ror 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	 pack-
       age  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.

       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

Tcl				      7.5			    bgerror(n)


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