Why is ATOMIC_FLAG_INIT false?

Question

In C++11 there is the std::atomic_flag that is useful for thread loops:

static std::atomic_flag s_done(ATOMIC_FLAG_INIT);

void ThreadMain() {
    while (s_done.test_and_set()) {  // returns current value of s_done and sets to true
        // do some stuff in a thread
    }
}

// Later:
  s_done.clear();  // Sets s_done to false so the thread loop will drop out

The ATOMIC_FLAG_INIT sets the flag to false which means that the thread never gets in the loop. A (bad) solution is possibly to do this:

void ThreadMain() {
    // Sets the flag to true but erases a possible false
    // which is bad as we may get into a deadlock
    s_done.test_and_set();
    while (s_done.test_and_set()) {
        // do some stuff in a thread
    }
}

The default constructor for std::atomic_flag specifies that the flag will be in an unspecified state.

Can I initialize the atomic_flag to true? Is this the correct use of the atomic_flag?

Answer 1

You could always call test_and_set before starting the thread.