网络编程的基本流程

• 服务端

1）socket----创建socket对象。

2）bind----绑定本机ip+port。

3）listen----监听来电，若在监听到来电，则建立起连接。

4）accept----再创建一个socket对象给其收发消息。原因是现实中服务端都是面对多个客户端，那么为了区分各个客户端，则每个客户端都需再分配一个socket对象进行收发消息。

5）read、write----收发消息

• 客户端

1）socket----创建socket对象。

2）connect----根据服务端ip+port，发起连接请求。

3）write、read----建立连接后，就可发收消息了

创建 socket

创建socket分为4步，创建上下文iocontext，选择协议，生成socket，打开socket。

int create_tcp_socket() {
    // Step 1. An instance of 'io_service' class is required by
            // socket constructor. 
    asio::io_context  ios;

    // Step 2. Creating an object of 'tcp' class representing
    // a TCP protocol with IPv4 as underlying protocol.
    asio::ip::tcp protocol = asio::ip::tcp::v4();

    // Step 3. Instantiating an active TCP socket object.
    asio::ip::tcp::socket sock(ios);

    // Used to store information about error that happens
    // while opening the socket.
    boost::system::error_code ec;

    // Step 4. Opening the socket.
    sock.open(protocol, ec);

    if (ec.value() != 0) {
        // Failed to open the socket.
        std::cout
            << "Failed to open the socket! Error code = "
            << ec.value() << ". Message: " << ec.message();
        return ec.value();
    }

    return 0;
}

上述socket只是通信的socket，如果是服务端，我们还需要生成一个acceptor的socket，用来接收新的连接。

int  create_acceptor_socket() {
    // Step 1. An instance of 'io_service' class is required by
        // socket constructor. 
    asio::io_context ios;

    // Step 2. Creating an object of 'tcp' class representing
            // a TCP protocol with IPv6 as underlying protocol.
    asio::ip::tcp protocol = asio::ip::tcp::v6();

    // Step 3. Instantiating an acceptor socket object.
    asio::ip::tcp::acceptor acceptor(ios);

    // Used to store information about error that happens
    // while opening the acceptor socket.
    boost::system::error_code ec;

    // Step 4. Opening the acceptor socket.
    acceptor.open(protocol, ec);

    if (ec.value() != 0) {
        // Failed to open the socket.
        std::cout
            << "Failed to open the acceptor socket!"
            << "Error code = "
            << ec.value() << ". Message: " << ec.message();
        return ec.value();
    }

    return 0;
}

绑定acceptor

对于acceptor类型的socket，服务器要将其绑定到指定的端口,所有连接这个端口的连接都可以被接收到。

int  bind_acceptor_socket() {

    // Step 1. Here we assume that the server application has
        // already obtained the protocol port number.
    unsigned short port_num = 3333;

    // Step 2. Creating an endpoint.
    asio::ip::tcp::endpoint ep(asio::ip::address_v4::any(),
        port_num);

    // Used by 'acceptor' class constructor.
    asio::io_context  ios;

    // Step 3. Creating and opening an acceptor socket.
    asio::ip::tcp::acceptor acceptor(ios, ep.protocol());

    boost::system::error_code ec;

    // Step 4. Binding the acceptor socket.
    acceptor.bind(ep, ec);

    // Handling errors if any.
    if (ec.value() != 0) {
        // Failed to bind the acceptor socket. Breaking
        // execution.
        std::cout << "Failed to bind the acceptor socket."
            << "Error code = " << ec.value() << ". Message: "
            << ec.message();

        return ec.value();
    }

    return 0;
}

连接指定的端点

作为客户端可以连接服务器指定的端点进行连接

int  connect_to_end() {
    // Step 1. Assume that the client application has already
            // obtained the IP address and protocol port number of the
            // target server.
    std::string raw_ip_address = "127.0.0.1";
    unsigned short port_num = 3333;

    try {
        // Step 2. Creating an endpoint designating 
        // a target server application.
        asio::ip::tcp::endpoint
            ep(asio::ip::address::from_string(raw_ip_address),
                port_num);

        asio::io_context ios;

        // Step 3. Creating and opening a socket.
        asio::ip::tcp::socket sock(ios, ep.protocol());

        // Step 4. Connecting a socket.
        sock.connect(ep);

        // At this point socket 'sock' is connected to 
        // the server application and can be used
        // to send data to or receive data from it.
    }
    // Overloads of asio::ip::address::from_string() and 
    // asio::ip::tcp::socket::connect() used here throw
    // exceptions in case of error condition.
    catch (system::system_error& e) {
        std::cout << "Error occured! Error code = " << e.code()
            << ". Message: " << e.what();

        return e.code().value();
    }
}

服务器接受连接

int accept_new_connection(){
    // The size of the queue containing the pending connection
            // requests.
    const int BACKLOG_SIZE = 30;

    // Step 1. Here we assume that the server application has
    // already obtained the protocol port number.
    unsigned short port_num = 3333;

    // Step 2. Creating a server endpoint.
    asio::ip::tcp::endpoint ep(asio::ip::address_v4::any(),
        port_num);

    asio::io_context  ios;

    try {
        // Step 3. Instantiating and opening an acceptor socket.
        asio::ip::tcp::acceptor acceptor(ios, ep.protocol());

        // Step 4. Binding the acceptor socket to the 
        // server endpint.
        acceptor.bind(ep);

        // Step 5. Starting to listen for incoming connection
        // requests.
        acceptor.listen(BACKLOG_SIZE);

        // Step 6. Creating an active socket.
        asio::ip::tcp::socket sock(ios);

        // Step 7. Processing the next connection request and 
        // connecting the active socket to the client.
        acceptor.accept(sock);

        // At this point 'sock' socket is connected to 
        //the client application and can be used to send data to
        // or receive data from it.
    }
    catch (system::system_error& e) {
        std::cout << "Error occured! Error code = " << e.code()
            << ". Message: " << e.what();

        return e.code().value();
    }
}