how to cancel a `boost::asio::read` operation while it's waiting

Question

I am using boost::asio to transfer data to & fro from client to server. I have a reader thread on client side to read data received on the socket on client side. Please note that I am using boost::asio::read on client side & boost::asio::writeon server side. Not using async_read or async_write. Everything works great.

However when I close my application, 2 out 10 times the app does not cleanly tear down or close properly. It gets hung while closing down The issue is the following:

My closing function gets called when destructors get called during my app's close down. Following is the code of the Close function:

socket.cancel();
socket.close();
boost::system::error_code ec;
socket.shutdown(boost::asio::ip::tcp::socket::shutdown_both, ec);

The problem is that the boost::asio::read call does not return when it does not get any data & keeps waiting on it. This should be fine as long as I can cancel it. I am trying to do a socket.cancel on it to cancel all read operations while exiting. However, it doesn't seems to work. I read in some forums that socket.cancel only cancels async_read operations. Is it so ? Then what is the way to cancel a boost::asio::read` operation when my app needs to exit ?

Answer 1

That's the nature of blocking IO.

Indeed socket.cancel() (or even io_service::stop()) will not work on synchronous operations.

The only way to interrupt this is to use socket-level timeouts (but Asio doesn't expose that) or to use asynchronous signals (e.g. pressing Ctrl-C in a terminal sends the child process a SIGINT).

I've previously created a poor-man's wrapper if you insist on running single operations with a timeout:

• boost::asio + std::future - Access violation after closing socket