请用c++写一段服务端和客户端通信的简单代码并解释

服务端代码：

#include <iostream>
#include <sys/socket.h>
#include <netinet/in.h>
#include <unistd.h>
#include <cstring>

int main() {
    // 创建 socket
    int server_fd = socket(AF_INET, SOCK_STREAM, 0);

    if (server_fd == -1) {
        std::cerr << "Failed to create socket\n";
        return 1;
    }

    // 设置地址重用
    int opt = 1;
    if (setsockopt(server_fd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt)) == -1) {
        std::cerr << "Failed to set socket option\n";
        return 1;
    }

    // 绑定地址和端口
    struct sockaddr_in server_addr{};
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = INADDR_ANY;
    server_addr.sin_port = htons(8888);

    if (bind(server_fd, (struct sockaddr *) &server_addr, sizeof(server_addr)) == -1) {
        std::cerr << "Failed to bind socket\n";
        return 1;
    }

    // 开始监听
    if (listen(server_fd, 5) == -1) {
        std::cerr << "Failed to listen on socket\n";
        return 1;
    }

    std::cout << "Server started, listening on port 8888...\n";

    // 接受客户端连接并发送欢迎消息
    int client_fd = accept(server_fd, nullptr, nullptr);
    const char *welcome_message = "Welcome to the server!";
    send(client_fd, welcome_message, strlen(welcome_message), 0);

    // 接收客户端消息并回复
    char buffer[1024]{};
    while (true) {
        int bytes_received = recv(client_fd, buffer, sizeof(buffer), 0);

        if (bytes_received == -1) {
            std::cerr << "Failed to receive message\n";
            break;
        }

        if (bytes_received == 0) {
            std::cout << "Client disconnected\n";
            break;
        }

        std::cout << "Received message from client: " << buffer << std::endl;

        const char *reply_message = "Message received!";
        send(client_fd, reply_message, strlen(reply_message), 0);
    }

    close(client_fd);
    close(server_fd);

    return 0;
}

客户端代码：

#include <iostream>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <cstring>

int main() {
    // 创建 socket
    int client_fd = socket(AF_INET, SOCK_STREAM, 0);

    if (client_fd == -1) {
        std::cerr << "Failed to create socket\n";
        return 1;
    }

    // 连接到服务端
    struct sockaddr_in server_addr{};
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(8888);
    inet_pton(AF_INET, "127.0.0.1", &server_addr.sin_addr);

    if (connect(client_fd, (struct sockaddr *) &server_addr, sizeof(server_addr)) == -1) {
        std::cerr << "Failed to connect to server\n";
        return 1;
    }

    std::cout << "Connected to server\n";

    // 接收欢迎消息
    char buffer[1024]{};
    int bytes_received = recv(client_fd, buffer, sizeof(buffer), 0);

    if (bytes_received == -1) {
        std::cerr << "Failed to receive message\n";
        return 1;
    }

    std::cout << "Received welcome message from server: " << buffer << std::endl;

    // 发送消息并接收回复
    const char *message = "Hello, server!";
    send(client_fd, message, strlen(message), 0);

    bytes_received = recv(client_fd, buffer, sizeof(buffer), 0);

    if (bytes_received == -1) {
        std::cerr << "Failed to receive message\n";
        return 1;
    }

    std::cout << "Received reply message from server: " << buffer << std::endl;

    close(client_fd);

    return 0;
}

这段代码实现了一个简单的服务端和客户端通信，服务端监听端口 8888，并在客户端连接成功后发送欢迎消息。客户端连接到服务端后接收欢迎消息，并发送一条消息给服务端，服务端接收到消息后回复一条消息给客户端。连接结束后关闭 socket。