C++: how to create thread local/global variable

Question

in this code:

int foo() {
   static int x;
}

is the x global to all threads or local in each thread? Or does that depends on a compiler flag and/or the compiler, so I cannot really know what it is from the code?

Several questions (all of them independent from compiler and compiler flags and OS):

1. How can I create a static variable which is global to all threads?
2. How can I create a static variable which is local to each thread?
3. How can I create a global variable which is global to all threads?
4. How can I create a global variable which is local to each thread?

I guess that this is not in C++ itself. (Is it in C++0x?) Some Boost lib which can do this?

Answer 1

x is global to all threads. Always, independent of compiler and/or its flags. Independent of whether this is in C++11 or C++03. So if you declare a regular global or static local variable, it will be shared between all threads. In C++11 we will have the thread_local keyword. Until then, you can use thread_specific_ptr from Boost.Thread.

Answer 2

Quick partial answers;

(Is it in C++0x?)

Yes. But depends on your compiler's C++0x support too.

Some Boost lib which can do this?

Boost.Threads. See thread local storage therein.

1. How can I create a static variable which is global to all threads?
2. How can I create a static variable which is local to each thread?
3. How can I create a global variable which is global to all threads?
4. How can I create a global variable which is local to each thread?

Note that typically, static refers to duration and global refers to scope.

C++0x thread constructors are variadic: You can pass any number (and type) of arguments. All of these are available to your std::thread object.

#include <thread>

int main()
{
    int foo = 42;
    std::thread t(foo); // copies foo
    std::thread s(&foo); // pass a pointer
    t.join();
}