Comment by the_why_of_y

You're right that just "volatile" isn't enough; typically you'd declare the variable sig_atomic_t to be portable, which makes the necessary guarantees since C89 so predates C++11. (It does not guarantee anything regarding access from multiple threads, of course.)

The problem with std::atomic<T> is that it may be implemented with a mutex, in which case it can deadlock in a signal handler. But as you say, you can check for that with is_lock_free.