Question

My app (C++, Windows) is communicating with an external device. If the device doesn’t answer after certain period of time, I want to reset a status variable.

My initial approach would be

auto timer = boost::asio::deadline_timer(io_svc);

timer.expires_from_now(boost::posix_time::seconds(10));
timer.async_wait(boost::bind(&Class::CurrRequestTimeout, this, boost::asio::placeholders::error));

io_svc.poll();

and the timeout function

void Class::CurrRequestTimeout(const boost::system::error_code & ec)
{
    if (ec)
    {
        // this timeout was canceled
        return;
    }
    ResetStatusVariable();
}

This should be non-blocking, that’s why I selected poll() instead of run() (seen here). However, with poll() the timeout method is never called. With run() it works just fine, but this blocks the execution.

Answer 1

The poll function only runs handlers that are ready to run at that instant. Since the timer hasn’t timed out yet, it can’t run that handler now. And you asked it not to block. So what would you expect it to do?

If your code is thread safe, create another thread that can block in run. If not, then this thread has to come back and call poll later to give handlers a chance to run.

One caveat: If you do create one or more run threads, you need to make sure there’s always at least one event to wait for, or the thread will be unable to wait for a handler. boost::asio::io_service::work is there for just this purpose. See this question.