How to launch an “event” when my Boost::asio tcp server just start running ( AKA io_service.run() )?

Question

Based on an boost::asio client/server relationship, I have to launch the client program from the server program only when the server thread is in a \"waiting to be c

Answer 1

In short, s11n_example::server is in a state where incoming connections will be queued immediately after the constructor call has completed.

---

It may be easier to understand this by defining a distinction between state and operations. A state determines what the OS can do with the object; an application initiates operations that perform actions and may depend on state. For example, when a socket in an open state, the OS will queue data; a read operation retrieves the queued data. The same applies to the acceptors. When an acceptor is in a listen state, the OS will queue connections; an accept operation will complete the connection, removing it from the queue.

An acceptor's [states] and transitions() are as follows:

     .----> [closed] ------.     [closed]:    socket not open
     |                     |     [opened]:    socket open but not listening for
     |                     V                  connections
  close() <------.      open()   [listening]: incoming connections will be
     ^           |         |                  queued until accepted(), causing
     |           |         V                  the connection to be established
[listening]      '---- [opened]
     ^                     |
     |                     |
     '------ listen() <----'

The various overloaded constructors will result in the acceptor starting its lifetime in a closed, opened, or listening state. In the case of s11n_example::server, the acceptor is constructed with an endpoint, so this overload will result in the acceptor being in a listening state post-construction. It is the equivalent of doing:

using boost::asio::ip::tcp;
tcp::endpoint endpoint_(tcp::v4(), 123);
tcp::acceptor acceptor_(io_service); // closed state
acceptor.open(endpoint_.protocol()); // opened state
acceptor.bind(endpoint);
acceptor.listen();                   // listening state

Therefore, the promise can be set after server is constructed and before io_service.run():

void server_thread(std::promise<bool>& run)
{ 
    boost::asio::io_service io_service;
    s11n_example::server server(io_service, 123);
    // The server's acceptor is in a listening state, so connection attempts
    // will be queued even without the io_service event loop running.  The
    // server also has an outstanding asynchronous accept operation.
    run.set_value(true);
    // Run the service, this will start an asynchronous loop that accepts 
    // connections.
    io_service.run();
}

One subtlety to note is that Boost.Asio's acceptors do not provide:

• Reactor based operations for accepting connections. Thus, it is not possible to detect when a connection is ready to be accepted (i.e. a connection is queued and waiting to be accepted).
• An higher-level way to detect if the acceptor is in a listening state. Nevertheless, this may be accomplished by querying the acceptor's native_handle. For example, using getsockopt() to get the value of SOL_SOCKET/SO_ACCEPTCONN.