thanks igor.
I should really install VM Maker.

Stef

On Aug 7, 2008, at 6:36 PM, Igor Stasenko wrote:

> 2008/8/7 stephane ducasse <stephane.ducasse at free.fr>:
>> Lukas
>>
>> This was not the exact question. We were discussing to know who is  
>> really
>> calling defaultAction (I know how the debugger
>> is raised). The question is who is calling defaultAction when the  
>> stack is
>> empty.
>> Is it the VM or something else.
>>
>> Now in the image the only sender is
>>
>> UndefinedObject>>handleSignal: exception
>>      "When no more handler (on:do:) context left in sender chain  
>> this gets
>> called.  Return from signal with default action."
>>
>>      ^ exception resumeUnchecked: exception defaultAction
>>
>> Who is calling handleSignal:
>>
>> Exception>>pass
>>      "Yield control to the enclosing exception action for the  
>> receiver."
>>
>>      handlerContext nextHandlerContext handleSignal: self
>>
>> Exception>>signal
>>      "Ask ContextHandlers in the sender chain to handle this  
>> signal.  The
>> default is to execute and return my defaultAction."
>>
>>      signalContext := thisContext contextTag.
>>      ^ thisContext nextHandlerContext handleSignal: self
>>
>> ContextPart>>handleSignal: exception
>>      "Sent to handler (on:do:) contexts only.  If my exception class
>> (first arg) handles exception then execute my handle block (second  
>> arg),
>> otherwise forward this message to the next handler context.  If  
>> none left,
>> execute exception's defaultAction (see nil>>handleSignal:)."
>>
>>      | val |
>>      (((self tempAt: 1) handles: exception) and: [self tempAt: 3])
>> ifFalse: [
>>              ^ self nextHandlerContext handleSignal: exception].
>>
>>      exception privHandlerContext: self contextTag.
>>      self tempAt: 3 put: false.  "disable self while executing handle
>> block"
>>      val := [(self tempAt: 2) valueWithPossibleArgs: {exception}]
>>              ensure: [self tempAt: 3 put: true].
>>      self return: val.  "return from self if not otherwise directed  
>> in
>> handle block"
>>
>> So it seems that this is signal and when handleSignal: arrives on  
>> nil then
>> the default action is fetch.
>>
>> Can somebody with a VM knowledge validate my hypothesis?
>>
>
> The primitive, which sits in #findNextHandlerContextStarting which is
> get called from #nextHandlerContext
> Does following:
>
> primitiveFindHandlerContext
> 	"Primitive. Search up the context stack for the next method context
> marked for exception handling starting at the receiver. Return nil if
> none found"
> 	| thisCntx nilOop |
> 	thisCntx := self popStack.
> 	nilOop := nilObj.
>
> 	[(self isHandlerMarked: thisCntx) ifTrue:[
> 			self push: thisCntx.
> 			^nil].
> 		thisCntx := self fetchPointer: SenderIndex ofObject: thisCntx.
> 		thisCntx = nilOop] whileFalse.
>
> 	^self push: nilObj
> ----
>
> This primitive is done just for speed reasons. You can see, that if
> primitive fails (not exists in VM), the method's code does exactly
> same:
>
> findNextHandlerContextStarting
> 	"Return the next handler marked context, returning nil if there is
> none.  Search starts with self and proceeds up to nil."
>
> 	| ctx |
> 	<primitive: 197>
> 	ctx := self.
> 		[ctx isHandlerContext ifTrue:[^ctx].
> 		(ctx := ctx sender) == nil ] whileFalse.
> 	^nil
>
>
>> Setf
>>
>>
>>
>>
>>
>> On Aug 6, 2008, at 2:55 PM, Lukas Renggli wrote:
>>
>>>> This is not easy to figure out since when a debugger gets opened,  
>>>> the
>>>> part
>>>> of the stack used to open it is discarded (how?).
>>>
>>> Error>>#defaultAction raises a new exception, the UnhandledError.
>>> UnhandledError has an instance-variable to remember the original
>>> exception.
>>>
>>> UnhandledError opens the debugger as part of the #defaultAction. The
>>> debugger uses the stack-frame of the original exception to display  
>>> at
>>> the top level. That's why you don't see the handling code itself.
>>>
>>> Lukas
>>>
>>> --
>>> Lukas Renggli
>>> http://www.lukas-renggli.ch
>>>
>>>
>>
>>
>>
>
>
>
> -- 
> Best regards,
> Igor Stasenko AKA sig.
>
>