How to declare a variable as thread local portably?

Question

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

in MSVC, it's written as `__declspec( thread )` (see http://msdn.microsoft.com/en-us/library/4ax54352.aspx) — Christoph, Aug 18 '13 at 11:20
@KingsIndian Jupp. That's why I asked what the best way to define this macro is. — fuz, Aug 18 '13 at 11:32
@KingsIndian Generic may include a bunch of special cases for stupid compilers. — fuz, Aug 18 '13 at 11:56

score 13 · Accepted Answer · answered Aug 18 '13 at 12:15

Combining information from Wikipedia with this list of compiler macros, I came up with the following (untested) version:

#ifndef thread_local
# if __STDC_VERSION__ >= 201112 && !defined __STDC_NO_THREADS__
#  define thread_local _Thread_local
# elif defined _WIN32 && ( \
       defined _MSC_VER || \
       defined __ICL || \
       defined __DMC__ || \
       defined __BORLANDC__ )
#  define thread_local __declspec(thread) 
/* note that ICC (linux) and Clang are covered by __GNUC__ */
# elif defined __GNUC__ || \
       defined __SUNPRO_C || \
       defined __xlC__
#  define thread_local __thread
# else
#  error "Cannot define thread_local"
# endif
#endif

Hey that's nice. Thank you very much! – fuz Aug 18 '13 at 12:18 — fuz, Aug 18 '13 at 12:18

How to declare a variable as thread local portably?

Edit

1 Answers1

Linked