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Date: Wed, 05 Sep 2012 14:40:38 -0400
From: Liviu Nicoara <nikkoara@hates.ms>
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Subject: Re: STDCXX-1056 [was: Re: STDCXX forks]
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On 09/05/12 14:09, Stefan Teleman wrote:
> On Wed, Sep 5, 2012 at 10:52 AM, Martin Sebor <msebor@gmail.com> wrote:
> [...]
> OK so I did a little bit of testing, after looking at the *right*
> __rw_guard class. :-)
>
> I changed the std::numpunct class thusly:
> [...]
> And then:
>
> template <class _CharT>
> inline string numpunct<_CharT>::grouping () const
> {
>      if (!(_C_flags & _RW::__rw_gr)) {
>
>          _RWSTD_MT_GUARD (_C_object_mutex);
>
>          // double-test here to avoid re-writing an already written string
>          if (!(_C_flags & _RW::__rw_gr)) {
>
>              numpunct* const __self = _RWSTD_CONST_CAST (numpunct*, this);
>
>              // [try to] get the grouping first (may throw)
>              // then set a flag to avoid future initializations
>              __self->_C_grouping  = do_grouping ();
>              __self->_C_flags    |= _RW::__rw_gr;
>
>          }
>      }
>
>      return _C_grouping;
> }

I am afraid this would be unsafe, too (if I said otherwise earlier I was wrong). The compiler might re-arrange the protected assignments, such that another thread sees a partially updated object, where the flags are updated and the string not. I don't think we're going to get away with this here without either a simpler and more inefficient top-level locking, or doing away completely with the lazy initialization. Thoughts?

Liviu