boost::asio server - detect failure to listen to server port

Question

I have a boost::asio based server which works fine, except that I'm trying to add a check that nothing else is accepting connections on the same port. If I create two of the servers, one of them consistently accepts the connections, but the other does not report any error ( which of the two accepts all connections appears to be random ).

The relevant bits of the server class ( it's a template which uses a base class which has Accepted() and typedefs for the connection type to create ) are:

        MessageServer ( boost::asio::io_service &io, unsigned short port_num )
            : BaseServerType ( io ), acceptor_ ( io, boost::asio::ip::tcp::endpoint ( boost::asio::ip::tcp::v4(), port_num ) ) {
            Listen();
        }
        void Listen () {
            boost::system::error_code ec;
            acceptor_.listen ( boost::asio::socket_base::max_connections, ec );

            if ( !ec ) {
                start_accept();
            } else {
                // not reached even if a separate process 
                // is already listening to that port
                connection_pointer new_connection;
                BaseServerType::Accepted ( new_connection );
            }
        }

    private:
        void start_accept() {
            connection_pointer new_connection ( CreateConnection ( acceptor_.io_service() ) );

            acceptor_.async_accept ( new_connection -> Socket(),
                                     boost::bind ( &MessageServer::handle_accept, this, new_connection,
                                                   boost::asio::placeholders::error ) );
        }

        void handle_accept ( connection_pointer new_connection, const boost::system::error_code &error ) {
            if ( !error ) {
                BaseServerType::Accepted ( new_connection );
                new_connection -> Start();
                start_accept();
            } else {
                // never reached
                new_connection.reset();
                BaseServerType::Accepted ( new_connection );
            }
        }

        boost::asio::ip::tcp::acceptor acceptor_;

acceptor.listen() doesn't throw either.

How is a failure to listen to a server port reported in boost::asio?

score 9 · Accepted Answer · edited May 23 '17 at 12:03

9

boost::asio::ip::tcp::acceptor constructor you use has a default argument reuse_address = true. this sets SO_REUSEADDR socket option. disabling this option you'll receive error on acceptor.listen(). more info about SO_REUSEADDR here

Be aware that this option is treated differently on windows and linux

edited May 23 '17 at 12:03

Community

1
1

answered Feb 15 '11 at 13:41

Andriy Tylychko

15,967
6
64
112

If I translate above description, here is code. MessageServer ( boost::asio::io_service &io, unsigned short port_num ) : BaseServerType ( io ) , acceptor_ ( io, boost::asio::ip::tcp::endpoint ( boost::asio::ip::tcp::v4(), port_num, false ) ) { Listen(); } ... try { server = new MessageServer(io_service_, 5000); } catch (std::exception const& ex) { printf("bind error: %s\n", ex.what()); } – Hill Nov 09 '15 at 08:16
From the linked website: "On Windows it lets you bind to the same TCP port multiple times without errors. Verified experimentally by failing test, I never bothered to check out MSDN docs for this one, I just don't use it on Windows." – Arthur Tacca Jul 05 '18 at 12:12

score 0 · Answer 2 · answered Feb 15 '11 at 16:40

0

The error you want would be thrown or returned from boost::asio::ip::tcp::acceptor::bind(). The constructor you are using will exhibit this same behavior, I suggest you boil your example down to something smaller that is reproducible. It is not obvious to me how you handle exceptions thrown from your MessageServer ctor.

answered Feb 15 '11 at 16:40

Sam Miller

23,808
4
67
87

As with any constructor which can throw, whatever creates the servers either wraps the constructor in try/catch if it is recoverable, or let it percolate to the outer scope if it is not. – Pete Kirkham Feb 15 '11 at 16:57

boost::asio server - detect failure to listen to server port

2 Answers2