RE: [boost] problem with global mutex object on windows

From: Darryl Green (Darryl.Green_at_[hidden])
Date: 2004-04-01 22:39:47

> -----Original Message-----
> From: boost-bounces_at_[hidden] 
> [mailto:boost-bounces_at_[hidden]] On Behalf Of Stefan Seefeld
> Sent: Friday, 2 April 2004 4:59 AM
> To: boost_at_[hidden]
> Subject: [boost] problem with global mutex object on windows
> 
> 
> hi there,
> 
> we'v run into a problem with the boost_thread mutex class,
> when used as a global object.
> 
> Imagine a singleton class as
> 
> class Singleton
> {
> public:
>     static Singleton *GetInstance();
>     static void Destroy();
> private:
> 
>     struct Guard
>     {
>       ~Guard() { Singleton::Destroy();}
>     };
>     static Singleton   *instance;
>     static Guard        guard;
>     static boost::mutex mutex;
> };
> 
> where the internal access to 'instance' is serialized by means of the
> mutex (inside methods GetInstance() and Destroy()).
> 
> It is the Singleton::Guard destructor that is responsible to free
> the instance (if it isn't destructed already). It, too, uses the
> mutex to protect access.
> 
> In a specific context on windows xp it happened that the mutex
> variable was destructed before the guard, leading to a global

3.6.2 Objects with static storage duration .. shall be initialized in
the order in which their definition appears in the translation unit.

3.6.3 ... These objects are destroyed in the reverse order of the
completion of their constructor or of the completion of their dynamic
initialization.

So as long as the Singleton::Guard definition is before the definition
of the mutex it should work. I assume that despite having the
definitions in that order it doesn't work? Does that make it a compiler
bug or am I missing something?

In any case, I'm not convinced that this is sufficient to make your
singleton useful. Do you know that GetInstance isn't called by the
constructor of a static object? Do you know that no other threads (which
might call GetInstance) are started prior to the mutex being
constructed?

I haven't tried to get a truly thread-safe singleton working - I
remember several discussions about it here - but I'm not sure that any
of them resulted in an efficient working solution?

Sorry - perhaps not an altogether helpful response.

Darryl.

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