Why only std::atomic_flag is guaranteed to be lock-free?

Question

From C++ Concurrency in Action:

difference between std::atomic and std::atomic_flag is that std::atomic may not be lock-free; the implementation may h

Answer 1

The standard does not garantee atomic objects are lock-free. On a platform that doesn't provide lock-free atomic operations for a type T, std::atomic objects may be implemented using a mutex, which wouldn't be lock-free. In that case, any containers using these objects in their implementation would not be lock-free either.

The standard provide an opportunity to check if an std::atomic variable is lock-free: you can use var.is_lock_free() or atomic_is_lock_free(&var). For basic types such as int, there is also macros provided (e.g. ATOMIC_INT_LOCK_FREE) which specify if lock-free atomic access to that type is available.

std::atomic_flag is an atomic boolean type. Almost always for boolean type it's not needed to use mutex or another way for synchronization.