Re: [Boost-users] [thread] safe portable static mutex initialization

Subject: Re: [Boost-users] [thread] safe portable static mutex initialization
From: Anthony Williams (anthony.ajw_at_[hidden])
Date: 2012-04-19 02:58:47

On 19/04/12 07:42, Serge Skorokhodov wrote:
> Say, I need a threadsafe static factory method. It may be implemented
> in two ways:
> 
> 1.
> class F
> {
> ...
> public:
>   static PTR_TO_SOMETHING* create_something()
>   {
>      static boost::mutex m;
>      boost::scope_lock(m);
>      ....
>      retrunt something_created;
>   }
> };
> 
> 2.
> H-file:
> class F
> {
> private:
>   static boost::mutex m_;
> ...
> public:
>   static PTR_TO_SOMETHING* create_something();
> };
> CPP-file:
>   boost::mutex F:m_;
> 
>   PTR_TO_SOMETHING* F::create_something()
>   {
>      boost::scope_lock(m_);
>      ....
>      retrunt something_created;
>   }
> 
> create_something is to be called from different threads but after main
> is entered. Are both of the above implementation threadsafe?

That depends on your compiler. In case 1, the mutex is constructed when
"create_something" is first called. With gcc there is a command-line
option (-fthreadsafe-statics) which ensures that this is done in a
thread-safe manner, so you don't have 2 threads thinking they are
"first". On many builds this is enabled by default, but the user can
still disable it. I don't think MSVC has thread-safe statics before
VC11, but I'm not sure. I don't know about other compilers.

In case 2, if you call create_something() after main() has started then
it should be fine, as the class static will be fully constructed before
main.

Anthony
-- 
Author of C++ Concurrency in Action     http://www.stdthread.co.uk/book/
just::thread C++11 thread library             http://www.stdthread.co.uk
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