Re: [boost] Threadsafe singleton - questions

From: Tobias Schwinger (tschwinger_at_[hidden])
Date: 2007-09-19 14:56:22

Tobias Schwinger wrote:
> JOAQUIN LOPEZ MU?Z wrote:
>> ----- Mensaje original -----
>> De: Tobias Schwinger <tschwinger_at_[hidden]>
>> Fecha: Sábado, Agosto 25, 2007 1:06 am
>> Asunto: Re: [boost] Threadsafe singleton - questions
>> Para: boost_at_[hidden]
>>
>>> Robert Ramey wrote:
>> [...]
>>>> g) what happens if same interface used by different DLLS.
>>>> Is  one instance still created?
>>> Yes.
>> Are you sure? For instance, in Windows globals aren't merged
>> on DLL loading, so if you've got something like:

Currently, not even symbol merging can keep us from potential breakage. 
It can even make things worse, at cases (see below).

<code>

>>
>> you'll get different t1 and t2 values.
> 
> Right.

...which (after some more thinking about how Singletons should work with 
across dynamic library boundaries) is in fact a good thing since a 
dynamic library shouldn't make any assumptions about when it gets unloaded.

> 
>> I don't see how
>> you're able to prevent this behavior for the local static data
>> of singleton_manager.

The data is initialized (and inserted into the manager's list) by the 
Singleton itself, while the manager just handles destruction. Still 
problematic, however.

> 
> Not at all: As client Singletons are user classes it's the users 
> responsibility to eventually add declspec stuff for windows.
> 
> It should probably be mentioned in the docs. Thanks for the heads up.

Unfortunately, it's not that simple...

A Singleton that is exposed in the header will currently get inlined 
into the client (including the manager) which can cause serious trouble:

A Singleton might end up to exist multiple times and a destructor 
located in already-unloaded code.

This situation can even occur if a dynamic library uses a Singleton 
internally and symbols get merged (unless merging is #pragmaTically 
disabled, which might not be possible for all ABIs).

The good news: The next version will introduce means for full control 
over what happens in all sorts of shared library scenarios.

Thanks again.

Regards,
Tobias