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?
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
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 __hpux || \
defined __xlC__
# define thread_local __thread
# else
# error "Cannot define thread_local"
# endif
#endif