tcl_backgrounderror(3tcl) [debian man page]

Tcl_BackgroundError(3tcl)				      Tcl Library Procedures					 Tcl_BackgroundError(3tcl)

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NAME

       Tcl_BackgroundError - report Tcl error that occurred in background processing

SYNOPSIS

       #include <tcl.h>

       Tcl_BackgroundError(interp)

ARGUMENTS

       Tcl_Interp *interp (in)		Interpreter in which the error occurred.
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DESCRIPTION

       This  procedure	is typically invoked when a Tcl error occurs during "background processing" such as executing an event handler.  When such
       an error occurs, the error condition is reported to Tcl or to a widget or some other C code, and there is not usually any obvious  way  for
       that  code  to  report  the  error  to the user.  In these cases the code calls Tcl_BackgroundError with an interp argument identifying the
       interpreter in which the error occurred.  At the time Tcl_BackgroundError is invoked, the interpreter's result is expected  to  contain	an
       error  message.	Tcl_BackgroundError will invoke the command registered in that interpreter to handle background errors by the interp bger-
       ror command.  The registered handler command is meant to report the error in an application-specific fashion.  The handler command receives
       two arguments, the result of the interp, and the return options of the interp at the time the error occurred.  If the application registers
       no handler command, the default handler command will attempt to call bgerror to report the error.   If  an  error  condition  arises  while
       invoking the handler command, then Tcl_BackgroundError reports the error itself by printing a message on the standard error file.

       Tcl_BackgroundError  does  not  invoke  the  handler  command immediately because this could potentially interfere with scripts that are in
       process at the time the error occurred.	Instead, it invokes the handler command later as an idle callback.

       It is possible for many background errors to accumulate before the handler command is invoked.  When this happens, each of  the	errors	is
       processed  in  order.   However,  if the handle command returns a break exception, then all remaining error reports for the interpreter are
       skipped.

KEYWORDS

       background, bgerror, error, interp

Tcl									7.5						 Tcl_BackgroundError(3tcl)

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

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

KEYWORDS

       background error, reporting

Tcl									7.5								bgerror(n)