Why std::async Blocked?

There is an only kind of std::future as resultant from creation from Async/Task STL function calls that is blocked. It is the one created by std::async. This post will illustrate why it behaves just like that.

The reason why std::future created from std::async will block in its destructor is because of __future_base::_S_make_async_state() function call which in turn calls to create _Async_state_impl in which its destructor has the following definition (notice its call std::thread::join) if it’s joinable then it will wait until the task is done to properly destroy itself.

      // Must not destroy _M_result and _M_fn until the thread finishes.
      // Call join() directly rather than through _M_join() because no other
      // thread can be referring to this state if it is being destroyed.
      ~_Async_state_impl() { if (_M_thread.joinable()) _M_thread.join(); }

Pay a good attention at _Async_state_impl which is mentioned to work with std::async with std::launch::async launching policy only, and not with other where else.

Above code can be seen in future header file bundled with gcc 9.1 on Linux, or similar on other platforms.

Update

Oct, 14, 2019

• Fixed typos, and wording to be clearer.

First published on Oct, 8, 2019

Written by Wasin Thonkaew
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