On Tue, Sep 4, 2012 at 10:49 PM, Martin Sebor <mse...@gmail.com> wrote:
> template <class _CharT> > inline string numpunct<_CharT>::grouping () const > { > if (!(_C_flags & _RW::__rw_gr)) { > > numpunct* const __self = _RWSTD_CONST_CAST (numpunct*, this); > > _RWSTD_MT_GUARD (_C_mutex); > > // [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; > } That's what I wanted to do originally - use a per-object mutext. Unfortunately the _C_mutex member in rw::__rw_synchronized is static: struct __rw_synchronized { // static so that it takes up no space static _RWSTD_EXPORT __rw_mutex _C_mutex; void _C_lock () { } void _C_unlock () { } __rw_guard _C_guard () { return __rw_guard (_C_mutex); } and __rw::rw_guard doesn't have an appropriate constructor. Intel C++ complains about it too: /src/steleman/programming/stdcxx-intel/stdcxx-4.2.1-thread-safe/include/loc/_numpunct.h(181): error: no instance of constructor "__rw::__rw_guard::__rw_guard" matches the argument list argument types are: (const __rw::__rw_mutex) _RWSTD_MT_GUARD (_C_mutex); ^ This works: template <class _CharT> inline string numpunct<_CharT>::grouping () const { if (!(_C_flags & _RW::__rw_gr)) { numpunct* const __self = _RWSTD_CONST_CAST (numpunct*, this); _RWSTD_MT_STATIC_GUARD (_Type); // [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; } Although I'm not sure of the performance implications difference between _RWSTD_MT_STATIC_GUARD and _RWSTD_MT_CLASS_GUARD for this particular problem. I'm going with "nothing in real life". :-) And even so, this is still not thread-safe: Two different threads [ T1 and T2 ], seeking two different locales [en_US.UTF-8 and ja_JP.UTF-8], enter std::numpunct<_CharT>::grouping() at the same time - because they are running on two different cores. They both test for if (!(_C_flags & _RW::__rw_gr)) and then -- assuming the expression above evaluates to true -- one of them wins the mutex [T1], and the other one [T2] blocks on the mutex. When T1 is done and exits the function, the grouping is set to en_US.UTF-8 and the mutex is released. Now T2 acquires the mutex, and proceeds to setting grouping to ja-JP.UTF-8. Woe on the caller running from T1 who now thinks he got en_US.UTF-8, but instead he gets ja_JP.UTF-8, which was duly set so by T2, but T1 had no clue about it (remember, the std::string grouping _charT buffer is shared by the caller from T1 and the caller from T2). So at a minimum, the locking must happen before evaluating the if (!(_C_flags & _RW::__rw_gr)) expression. This still doesn't solve what ends up being returned in grouping. If we lock at the top of the function, then, when T2 acquires the mutex, the test expression will evaluate to false. Therefore T2 will return whatever is in grouping right now, which happens to be en_US.UTF-8 as set by T1, when T2 really wanted ja_JP.UTF-8. I really think the appropriate fix here -- which would address the performance implications -- is more complex than this. I am thinking about creating and using a (non-publicly accessible) internal locale cache: typedef std::map<std::string, std::locale> locale_cache; where all the locales are stored fully initialized, on demand. There is only one locale instantiation and initialization overhead cost per locale. After a locale has been instantiated and placed into the cache, the caller of any specfic locale gets a copy from the cache, fully instantiated and initialized. But this breaks ABI, so I'm thinking it's for stdcxx 5. Thoughts? --Stefan -- Stefan Teleman KDE e.V. stefan.tele...@gmail.com