Clarifying the difference between BOOST_ASIO_CONCURRENCY_HINT_UNSAFE and BOOST_ASIO_CONCURRENCY_HINT_UNSAFE_IO

[redboltz]

I've read https://www.boost.org/doc/html/boost_asio/overview/core/concurrency_hint.html

I want to clarify the following two descriptions:

• BOOST_ASIO_CONCURRENCY_HINT_UNSAFE
  • Care must be taken to ensure that all operations on the io_context and any of its associated I/O objects (such as sockets and timers) occur in only one thread at a time.
• BOOST_ASIO_CONCURRENCY_HINT_UNSAFE_IO
  • Care must be taken to ensure that the run functions on the io_context (i.e. run, run_for, run_until, run_one, run_one_for, run_one_until, poll, and poll_one), and all operations on the context's associated I/O objects (such as sockets and timers), occur in only one thread at a time.

---

I understand as follows. Am I understand correctly ?

Operation	UNSAFE	UNSAFE_IO	Examples
run operations	single	single	run, run_for, run_until, run_one, run_one_for, run_one_until, poll, and poll_one
the context's associated I/O objects	single	single	socket, timer
other operations on the io_context	single	multi	post, dispatch, defer

• single: access from single thread only
• multie: can access from multiple threads
• UNSAFE: BOOST_ASIO_CONCURRENCY_HINT_UNSAFE
• UNSAFE_IO: BOOST_ASIO_CONCURRENCY_HINT_UNSAFE_IO

---

Especially I want to confirm what is "other operations on the io_context". My examples are post, dispatch, and defer. I think that I can switch the executor using these function.

// on thread 1
boost::asio::dispatch( // dispatch is called from multiple threads. It is safe when I use UNSAFE_IO but unsafe if I use UNSAFE
  boost::asio::bind_executor(
    io_context_with_unsafe_io.get_executor(), // run() is called on thread 2
    [...] {
       // on thread 2
       socket.async_read_some(...); // io_context_with_unsafe_io associated socket but it is safe 
    }
  )
);

Is that right ?