c++实现网络通讯，支持 tcp 协议，支持异步

在C++中实现网络通讯，可以使用操作系统提供的socket库。下面是一个简单的示例代码，支持TCP协议和异步通信：

#include <iostream>
#include <string>
#include <thread>
#include <chrono>
#include <cstring>
#include <cstdlib>
#include <cstdio>
#include <cerrno>

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

const int BUFFER_SIZE = 1024; // 缓冲区大小

void error_handling(const char* message) {
    perror(message);
    exit(1);
}

int create_socket() {
    int sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (sock == -1) {
        error_handling("socket() error");
    }
    return sock;
}

void set_nonblock(int sock) {
    int flag = fcntl(sock, F_GETFL, 0);
    if (flag == -1) {
        error_handling("fcntl(F_GETFL) error");
    }
    if (fcntl(sock, F_SETFL, flag | O_NONBLOCK) == -1) {
        error_handling("fcntl(F_SETFL, O_NONBLOCK) error");
    }
}

int connect_server(int sock, const char* ip, int port) {
    struct sockaddr_in server_addr;
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = inet_addr(ip);
    server_addr.sin_port = htons(port);

    int ret = connect(sock, (struct sockaddr*)&server_addr, sizeof(server_addr));
    if (ret == -1) {
        if (errno == EINPROGRESS) { // 连接正在进行
            std::cout << "connecting..." << std::endl;
            fd_set write_fds;
            FD_ZERO(&write_fds);
            FD_SET(sock, &write_fds);
            int max_fd = sock + 1;
            struct timeval timeout;
            timeout.tv_sec = 5; // 超时时间为5秒
            timeout.tv_usec = 0;

            if (select(max_fd, nullptr, &write_fds, nullptr, &timeout) == -1) {
                error_handling("select() error");
            }
            if (!FD_ISSET(sock, &write_fds)) {
                error_handling("connection timeout");
            }
        } else {
            error_handling("connect() error");
        }
    }

    std::cout << "connect success" << std::endl;
    return 0;
}

void recv_data(int sock) {
    char buffer[BUFFER_SIZE];
    while (true) {
        memset(buffer, 0, BUFFER_SIZE);
        int ret = recv(sock, buffer, BUFFER_SIZE - 1, 0);
        if (ret == -1) {
            if (errno == EAGAIN || errno == EWOULDBLOCK) { // 没有数据可读
                std::this_thread::sleep_for(std::chrono::milliseconds(100));
                continue;
            } else {
                error_handling("recv() error");
                break;
            }
        } else if (ret == 0) { // 连接被断开
            std::cout << "connection closed by server" << std::endl;
            break;
        } else {
            std::cout << "recv: " << buffer << std::endl;
        }
    }
}

void send_data(int sock) {
    std::string message;
    char buffer[BUFFER_SIZE];
    while (true) {
        std::getline(std::cin, message);
        if (message == "exit") {
            break;
        }
        memset(buffer, 0, BUFFER_SIZE);
        strncpy(buffer, message.c_str(), message.size());
        int ret = send(sock, buffer, message.size(), 0);
        if (ret == -1) {
            error_handling("send() error");
            break;
        }
    }
}

int main(int argc, char* argv[]) {
    if (argc != 3) {
        std::cerr << "Usage: " << argv[0] << " <IP> <port>" << std::endl;
        return 1;
    }

    int sock = create_socket();
    set_nonblock(sock);
    connect_server(sock, argv[1], std::atoi(argv[2]));

    std::thread t1(recv_data, sock);
    std::thread t2(send_data, sock);

    t1.join();
    t2.join();

    close(sock);
    return 0;
}

上述代码中，我们使用socket函数创建了一个TCP协议的套接字，然后使用fcntl函数将该套接字设置成非阻塞模式。在connect时，如果连接正在进行中，则使用select函数等待连接完成，超时时间为5秒。接着，我们使用两个线程分别处理接收数据和发送数据的任务，实现了异步通信。