网络编程基本流程
网络编程的基本流程对于服务端是这样的 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----就是收发消息了。
对于客户端是这样的 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。
图示如下
相关的网络编程技术可以看看我之前写的文章
https://llfc.club/articlepage?id=2LXIKWJtKGblnWtHT7TplLKK6ze
接下来按照上述流程，我们用boost::asio逐步介绍。
终端节点的创建
所谓终端节点就是用来通信的端对端的节点，可以通过ip地址和端口构造，其的节点可以连接这个终端节点做通信. 如果我们是客户端，我们可以通过对端的ip和端口构造一个endpoint，用这个endpoint和其通信。
int client_end_point() {
// Step 1. Assume that the client application has already
// obtained the IP-address and the protocol port number.
std::string raw_ip_address = "127.0.0.1";
unsigned short port_num = 3333;
// Used to store information about error that happens
// while parsing the raw IP-address.
boost::system::error_code ec;
// Step 2. Using IP protocol version independent address
// representation.
asio::ip::address ip_address =
asio::ip::address::from_string(raw_ip_address, ec);
if (ec.value() != 0) {
// Provided IP address is invalid. Breaking execution.
std::cout
<< "Failed to parse the IP address. Error code = "
<< ec.value() << ". Message: " << ec.message();
return ec.value();
}
// Step 3.
asio::ip::tcp::endpoint ep(ip_address, port_num);
// Step 4. The endpoint is ready and can be used to specify a
// particular server in the network the client wants to
// communicate with.
return 0;
}
如果是服务端，则只需根据本地地址绑定就可以生成endpoint
int server_end_point(){
// Step 1. Here we assume that the server application has
//already obtained the protocol port number.
unsigned short port_num = 3333;
// Step 2. Create special object of asio::ip::address class
// that specifies all IP-addresses available on the host. Note
// that here we assume that server works over IPv6 protocol.
asio::ip::address ip_address = asio::ip::address_v6::any();
// Step 3.
asio::ip::tcp::endpoint ep(ip_address, port_num);
// Step 4. The endpoint is created and can be used to
// specify the IP addresses and a port number on which
// the server application wants to listen for incoming
// connections.
return 0;
}
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断点,所有连接这个端点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();
}
}
服务器接收连接
当有客户端连接时，服务器需要接收连接
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();
}
}
关于buffer
任何网络库都有提供buffer的数据结构，所谓buffer就是接收和发送数据时缓存数据的结构。
boost::asio提供了asio::mutable_buffer 和 asio::const_buffer这两个结构，他们是一段连续的空间，首字节存储了后续数据的长度。
asio::mutable_buffer用于写服务，asio::const_buffer用于读服务。但是这两个结构都没有被asio的api直接使用。
对于api的buffer参数，asio提出了MutableBufferSequence和ConstBufferSequence概念，他们是由多个asio::mutable_buffer和asio::const_buffer组成的。也就是说boost::asio为了节省空间，将一部分连续的空间组合起来，作为参数交给api使用。
我们可以理解为MutableBufferSequence的数据结构为std::vector
每隔vector存储的都是mutable_buffer的地址，每个mutable_buffer的第一个字节表示数据的长度，后面跟着数据内容。
这么复杂的结构交给用户使用并不合适，所以asio提出了buffer()函数，该函数接收多种形式的字节流，该函数返回asio::mutable_buffers_1 o或者asio::const_buffers_1结构的对象。
如果传递给buffer()的参数是一个只读类型，则函数返回asio::const_buffers_1 类型对象。
如果传递给buffer()的参数是一个可写类型，则返回asio::mutable_buffers_1 类型对象。
asio::const_buffers_1和asio::mutable_buffers_1是asio::mutable_buffer和asio::const_buffer的适配器，提供了符合MutableBufferSequence和ConstBufferSequence概念的接口，所以他们可以作为boost::asio的api函数的参数使用。
简单概括一下，我们可以用buffer()函数生成我们要用的缓存存储数据。
比如boost的发送接口send要求的参数为ConstBufferSequence类型
template<typename ConstBufferSequence>
std::size_t send(const ConstBufferSequence & buffers);
我们需要将"Hello Word转化为该类型"
void use_const_buffer() {
std::string buf = "hello world!";
asio::const_buffer asio_buf(buf.c_str(), buf.length());
std::vector<asio::const_buffer> buffers_sequence;
buffers_sequence.push_back(asio_buf);
}
最终buffers_sequence就是可以传递给发送接口send的类型。但是这太复杂了，可以直接用buffer函数转化为send需要的参数类型
void use_buffer_str() {
asio::const_buffers_1 output_buf = asio::buffer("hello world");
}
output_buf可以直接传递给该send接口。我们也可以将数组转化为send接受的类型
void use_buffer_array(){
const size_t BUF_SIZE_BYTES = 20;
std::unique_ptr<char[] > buf(new char[BUF_SIZE_BYTES]);
auto input_buf = asio::buffer(static_cast<void*>(buf.get()), BUF_SIZE_BYTES);
}
对于流式操作，我们可以用streambuf，将输入输出流和streambuf绑定，可以实现流式输入和输出。
void use_stream_buffer() {
asio::streambuf buf;
std::ostream output(&buf);
// Writing the message to the stream-based buffer.
output << "Message1\nMessage2";
// Now we want to read all data from a streambuf
// until '\n' delimiter.
// Instantiate an input stream which uses our
// stream buffer.
std::istream input(&buf);
// We'll read data into this string.
std::string message1;
std::getline(input, message1);
// Now message1 string contains 'Message1'.
}