bgerror man page on MacOSX

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

______________________________________________________________________________

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 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
       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 error‐
       Info 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	 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.

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), errorCode(n), errorInfo(n), interp(n)

KEYWORDS
       background error, reporting

Tcl				      7.5			    bgerror(n)
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