Boost asio TCP async server not async?

Question

I am using the code provided in the Boost example.

The server only accepts 1 connection at a time. This means, no new connections until the current one is closed.

Answer 1

Both the original code and the modified code are asynchronous and accept multiple connections. As can be seen in the following snippet, the async_accept operation's AcceptHandler initiates another async_accept operation, forming an asynchronous loop:

        .-----------------------------------.
        V                                   |
void server::do_accept()                    |
{                                           |
  acceptor_.async_accept(...,               |
      [this](boost::system::error_code ec)  |
      {                                     |
        // ...                              |
        do_accept();  ----------------------'
      });
}

The sleep() within the session's ReadHandler causes the one thread running the io_service to block until the sleep completes. Hence, the program will be doing nothing. However, this does not cause any outstanding operations to be cancelled. For a better understanding of asynchronous operations and io_service, consider reading this answer.

---

Here is an example demonstrating the server handling multiple connections. It spawns off a thread that creates 5 client sockets and connects them to the server.

#include 
#include 
#include 
#include 
#include 
#include 
#include 

using boost::asio::ip::tcp;

class session
  : public std::enable_shared_from_this
{
public:
  session(tcp::socket socket)
    : socket_(std::move(socket))
  {
  }

  ~session()
  {
    std::cout << "session ended" << std::endl;
  }

  void start()
  {
    std::cout << "session started" << std::endl;
    do_read();
  }

private:
  void do_read()
  {
    auto self(shared_from_this());
    socket_.async_read_some(boost::asio::buffer(data_, max_length),
        [this, self](boost::system::error_code ec, std::size_t length)
        {
          if (!ec)
          {
            do_write(length);
          }
        });
  }

  void do_write(std::size_t length)
  {
    auto self(shared_from_this());
    boost::asio::async_write(socket_, boost::asio::buffer(data_, length),
        [this, self](boost::system::error_code ec, std::size_t /*length*/)
        {
          if (!ec)
          {
            do_read();
          }
        });
  }

  tcp::socket socket_;
  enum { max_length = 1024 };
  char data_[max_length];
};

class server
{
public:
  server(boost::asio::io_service& io_service, short port)
    : acceptor_(io_service, tcp::endpoint(tcp::v4(), port)),
      socket_(io_service)
  {
    do_accept();
  }

private:
  void do_accept()
  {
    acceptor_.async_accept(socket_,
        [this](boost::system::error_code ec)
        {
          if (!ec)
          {
            std::make_shared(std::move(socket_))->start();
          }

          do_accept();
        });
  }

  tcp::acceptor acceptor_;
  tcp::socket socket_;
};

int main(int argc, char* argv[])
{
  try
  {
    if (argc != 2)
    {
      std::cerr << "Usage: async_tcp_echo_server \n";
      return 1;
    }

    boost::asio::io_service io_service;

    auto port = std::atoi(argv[1]);
    server s(io_service, port);

    boost::thread client_main(
        [&io_service, port]
        {
          tcp::endpoint server_endpoint(
              boost::asio::ip::address_v4::loopback(), port);

          // Create and connect 5 clients to the server.
          std::vector> clients;
          for (auto i = 0; i < 5; ++i)
          {
              auto client = std::make_shared(
                  std::ref(io_service));
              client->connect(server_endpoint);
              clients.push_back(client);
          }

          // Wait 2 seconds before destroying all clients.
          boost::this_thread::sleep(boost::posix_time::seconds(2));
        });

   io_service.run();
   client_main.join();
  }
  catch (std::exception& e)
  {
    std::cerr << "Exception: " << e.what() << "\n";
  }

  return 0;
}

The output:

session started
session started
session started
session started
session started
session ended
session ended
session ended
session ended
session ended