Using __thread in c++0x

Question

I read that there was a new keyword in C++: it's __thread from what I've read.

All I know is that it's a keyword to be used like the static keyword but I know nothing else. Does this keyword just mean that, for instance, if a variable were declared like so:

__thread int foo;

then anything to do with that variable will be executed with a new thread?

Answer 1

It's thread_local, not __thread. It's used to define variables which has storage duration of the thread.

thread_local is a new storage duration specifier added in C++0x. There are other storage duration : static, automatic and dynamic.

From this link:

thread local storage duration (C++11 feature). The variable is allocated when the thread begins and deallocated when the thread ends. Each thread has its own instance of the variable. Only variables declared thread_local have this storage duration.

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I think the introduction of this keyword was made possible by introducing a standardized memory model in C++0x:

• C++11 introduced a standardized memory model. What does it mean? And how is it going to affect C++ programming?

Answer 2

From the Wikipedia article on "Thread-local storage":

Thread-local storage (TLS) is a computer programming method that uses static or global memory local to a thread.

This is sometimes needed because normally all threads in a process share the same address space, which is sometimes undesirable.

And:

C++0x introduces the thread_local keyword. Aside that, various C++ compiler implementations provide specific ways to declare thread-local variables:

Sun Studio C/C++, IBM XL C/C++, GNU C and Intel C/C++ (Linux systems) use the syntax:

    __thread int number;

Visual C++, Intel C/C++ (Windows systems), Borland C++ Builder and Digital Mars C++ use the syntax:

    __declspec(thread) int number;

Borland C++ Builder also supports the syntax:

    int __thread number;

So, whilst __thread does exist in practice and on some systems, thread_local is the new, official, C++0x keyword that does the same thing.

Prefer it to non-standard __thread whenever you have access to C++0x.