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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
641
The ThreadLocal class is used to create thread local variables which can only be read and written by the same thread. For example, if two threads are accessing code having reference to same threadLocal variable then each thread will not see any modification to threadLocal variable done by other thread.
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 runtime model GCC uses to implement this originates in the IA-64 processor-specific ABI, but has since been migrated to other processors as well.
The local variables of a function are unique to each thread that runs the function. However, the static and global variables are shared by all threads in the process. With thread local storage (TLS), you can provide unique data for each thread that the process can access using a global index.
It enables you to create variables that can only be read and write by the same thread. If two threads are executing the same code and that code has a reference to a ThreadLocal variable then the two threads can't see the local variable of each other.
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
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