How to declare a variable as thread local portably?

C11 introduces the _Thread_local storage class specifier that can be used in combination with the static and extern storage class specifiers to declare a variable as thread local. The GNU C compiler suite implements a storage class specifier __thread with the same same semantics.

Unfortunately I did not find any compiler (I tried gcc, clang and SUN studio) that actually implements the _Thread_local keywords. I currently use the following construct to declare a keyword thread_local:

/* gcc doesn't know _Thread_local from C11 yet */#ifdef __GNUC__# define thread_local __thread#elif __STDC_VERSION__ >= 201112L# define thread_local _Thread_local#else# error Don't know how to define thread_local#endif

I know that this probably does not work with MSVC and other compilers. Can anybody suggest me a better method to declare thread_local in a way that it works in as many compilers as possible?

Edit

Christoph suggested that Microsoft Visual C allows __declspec(thread). This is the updated macro definition:

/* gcc doesn't know _Thread_local from C11 yet */#ifdef __GNUC__# define thread_local __thread#elif __STDC_VERSION__ >= 201112L# define thread_local _Thread_local#elif defined(_MSC_VER)# define thread_local __declspec(thread)#else# error Cannot define thread_local#endif