On Mon, Feb 18, 2013 at 1:18 PM, Vicente J. Botet Escriba <vicente.botet@wanadoo.fr> wrote:

I have added some comments in https://svn.boost.org/trac/boost/ticket/8094#comment:1.

Could you please explain what you meant with your comment regarding BOOST_THREAD_LOCAL for non-C++11 compilers Vicente?

As far as I can see, more or less "all" compilers (of relevance to boost) have supported either __declspec(thread) or __thread for quite some years [1]. I therefore supect that BOOST_THREAD_LOCAL can be implemented with a small preprocessor block that only has to switch between thread_local, __declspec(thread) or __thread "backends" (the _MSC_VER check in my patch should probably be replaced with _WIN32 to also support Intel on windows). Please correct me if this view is naive.


"Thread-Local Storage

Thread-local storage (TLS) is a mechanism by which variables are allocated such that there is one instance of the variable per extant thread. The run-time model GCC uses to implement this originates in the IA-64 processor-specific ABI, but has since been migrated to other processors as well. It requires significant support from the linker (ld), dynamic linker (ld.so), and system libraries (libc.so and libpthread.so), so it is not available everywhere."

"On Windows operating systems before Windows Vista, __declspec( thread ) has some limitations. If a DLL declares any data or object as __declspec( thread ), it can cause a protection fault if dynamically loaded. After the DLL is loaded with LoadLibrary, it causes system failure whenever the code references the __declspec( thread ) data. Because the global variable space for a thread is allocated at run time, the size of this space is based on a calculation of the requirements of the application plus the requirements of all the DLLs that are statically linked. When you use LoadLibrary, you cannot extend this space to allow for the thread local variables declared with __declspec( thread ). Use the TLS APIs, such as TlsAlloc, in your DLL to allocate TLS if the DLL might be loaded with LoadLibrary."


There are for sure limitations regarding thread-local variables, such as MSVC not allowing dynamic initialization [2,3], but these limitations can be documented. Furthermore, we can recommend client code to only use the functionality subset supported by most/all compilers.