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
       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.

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), errorCode(3tcl), errorInfo(3tcl), interp(3tcl)

KEYWORDS

       background error, reporting