Confused on boost::asio::io_context::run() and boost::thread::join()

Question

As the boost document stated here, boost::asio::io_context::run() function blocks until all work has finished and there are no more handlers to be dispatched, or until the io_context has been stopped.

Now I am confused why my code located before one more handler dispatching (t.join()) was not executed until all the handlers completed their works.

I have tried the boost asio tutorial Timer.5 - Synchronising handlers in multithreaded programs

And I added one line code following the io.run():

int main()
{
    boost::asio::io_context io;
    printer p(io);
    boost::thread t(boost::bind(&boost::asio::io_context::run, &io));
    io.run();
    std::cout << "between run and join" << std::endl;
    t.join();

    return 0;
}

Here are the console outputs:

Timer 2: 0
Timer 1: 1
Timer 2: 2
Timer 1: 3
Timer 1: 4
Timer 2: 5
Timer 2: 6
Timer 1: 7
Timer 2: 8
Timer 1: 9
between run and join
Final count is 10

I wonder why t.join() was executed before my std::cout << "between run and join" << std::endl; code? I expect the output "between run and join" to be before all the 2nd thread's outputs.

I think I must be lost at something.

Luka Rahne Luka Rahne · Accepted Answer · 2019-05-31T09:51:35

line with "Final count" is executed in destructor of printer class, that is executed when main reaches return();

run terminates when all timers expires (there is no more work to wait/do)
Once run returns/unblocks "cout << between run and join" is executed.

Confused on boost::asio::io_context::run() and boost::thread::join()

1 Answers