[reSIProcate] unsafe static Mutex object initialization

[Aron Rosenberg]

That code in ThreadIf is Win32 specific since it is replicating a feature of
pthreads that is not available in Win32. I referenced that code because it
is written in the way you want. The reason that technique is used is not to
guarantee that it is called before main(), but to help other static global
objects which need to use ThreadIf or in this case, ThreadLocalStorage. A
similiar technique is used for the ContentType mappings I believe.

Aron Rosenberg
Sr. Director, Engineering,
LifeSize, a division of Logitech




On Thu, Jul 21, 2011 at 1:53 PM, Daniel Pocock <daniel at readytechnology.co.uk
> wrote:

>
>
> On 21/07/11 21:03, Aron Rosenberg wrote:
> > The best way to handle this is actually to make the mutex be a static
> > pointer which gets allocated during a class constructor. See ThreadIf
> class
> > and mTlsDestructorsMutex and friends for the best way to implement this.
>
> Thanks for the fast reply about that - I've copied that code just below
>
> I notice that:
>
> a) this code is only for Windows - any reason it is not used on UNIX too?
>
> b) should this be boilerplated into each of those classes I identified,
> or maybe generalised in some way?
>
> c) C++ should guarantee that the object _staticTlsInit will be
> initialised before main() is called (i.e. unlikely that other threads
> are running, unless some other object initialised in this way starts a
> thread)
>
> d) however, other classes may want to perform initialization before
> main() is called too, and they may want to use Mutexes because they
> don't really know for sure that they are being instantiated early or in
> a multi-threaded scenario
>
> e) therefore, while there is not a risk of a race-condition (as only one
> thread is running), there is still a risk of using an object that is not
> initialized?
>
> If I understand it correctly, the only way to guarantee initialization
> is to use a POD style mutex (for POD types, C++ should guarantee that
> the state of the variable is valid before the first instruction is ever
> executed)
>
>
>
> class ThreadIf
> {
>
> .
> .
> .
>
> #ifdef WIN32
> /// Class to initialize TLS destructors array under Windows on program
> startup.
> class TlsDestructorInitializer {
> public:
>   TlsDestructorInitializer();
>   ~TlsDestructorInitializer();
> protected:
>   static unsigned int mInstanceCounter;
> };
> static TlsDestructorInitializer _staticTlsInit;
> #endif
>
> }
>
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