TCP socket心跳包示例程序

在做游戏开发时，经常需要在应用层实现自己的心跳机制，即定时发送一个自定义的结构体（心跳包），让对方知道自己还活着，以确保连接的有效性。

在TCP socket心跳机制中，心跳包可以由服务器发送给客户端，也可以由客户端发送给服务器，不过比较起来，前者开销可能更大。—— 这里实现的是由客户端给服务器发送心跳包，基本思路是：

1） 服务器为每个客户端保存了IP和计数器count，即map<fd, pair<ip, count>>。服务端主线程采用 select 实现多路IO复用，监听新连接以及接受数据包（心跳包），子线程用于检测心跳：

• 如果主线程接收到的是心跳包，将该客户端对应的计数器 count 清零；
• 在子线程中，每隔3秒遍历一次所有客户端的计数器 count：
  
  • 若 count 小于 5，将 count 计数器加 1；
  • 若 count 等于 5，说明已经15秒未收到该用户心跳包，判定该用户已经掉线；

2） 客户端则只是开辟子线程，定时给服务器发送心跳包（本示例中定时时间为3秒）。

下面是Linux下一个socket心跳包的简单实现：

/*************************************************************************
    > File Name: Server.cpp
    > Author: SongLee
    > E-mail: lisong.shine@qq.com
    > Created Time: 2016年05月05日 星期四 22时50分23秒
    > Personal Blog: http://songlee24.github.io/
 ************************************************************************/
#include<netinet/in.h>   // sockaddr_in
#include<sys/types.h>    // socket
#include<sys/socket.h>   // socket
#include<arpa/inet.h>
#include<unistd.h>
#include<sys/select.h>   // select
#include<sys/ioctl.h>
#include<sys/time.h>
#include<iostream>
#include<vector>
#include<map>
#include<string>
#include<cstdlib>
#include<cstdio>
#include<cstring>
using namespace std;
#define BUFFER_SIZE 1024

enum Type {HEART, OTHER};

struct PACKET_HEAD
{
    Type type;
    int length;
};

void* heart_handler(void* arg);

class Server
{
private:
    struct sockaddr_in server_addr;
    socklen_t server_addr_len;
    int listen_fd;    // 监听的fd
    int max_fd;       // 最大的fd
    fd_set master_set;   // 所有fd集合，包括监听fd和客户端fd
    fd_set working_set;  // 工作集合
    struct timeval timeout;
    map<int, pair<string, int> > mmap;   // 记录连接的客户端fd--><ip, count>
public:
    Server(int port);
    ~Server();
    void Bind();
    void Listen(int queue_len = 20);
    void Accept();
    void Run();
    void Recv(int nums);
    friend void* heart_handler(void* arg);
};

Server::Server(int port)
{
    bzero(&server_addr, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = htons(INADDR_ANY);
    server_addr.sin_port = htons(port);
    // create socket to listen
    listen_fd = socket(PF_INET, SOCK_STREAM, 0);
    if(listen_fd < 0)
    {
        cout << "Create Socket Failed!";
        exit(1);
    }
    int opt = 1;
    setsockopt(listen_fd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));
}

Server::~Server()
{
    for(int fd=0; fd<=max_fd; ++fd)
    {
        if(FD_ISSET(fd, &master_set))
        {
            close(fd);
        }
    }
}

void Server::Bind()
{
    if(-1 == (bind(listen_fd, (struct sockaddr*)&server_addr, sizeof(server_addr))))
    {
        cout << "Server Bind Failed!";
        exit(1);
    }
    cout << "Bind Successfully.\n";
}

void Server::Listen(int queue_len)
{
    if(-1 == listen(listen_fd, queue_len))
    {
        cout << "Server Listen Failed!";
        exit(1);
    }
    cout << "Listen Successfully.\n";
}

void Server::Accept()
{
    struct sockaddr_in client_addr;
    socklen_t client_addr_len = sizeof(client_addr);

    int new_fd = accept(listen_fd, (struct sockaddr*)&client_addr, &client_addr_len);
    if(new_fd < 0)
    {
        cout << "Server Accept Failed!";
        exit(1);
    }

    string ip(inet_ntoa(client_addr.sin_addr));    // 获取客户端IP

    cout << ip << " new connection was accepted.\n";

    mmap.insert(make_pair(new_fd, make_pair(ip, 0)));

    // 将新建立的连接的fd加入master_set
    FD_SET(new_fd, &master_set);
    if(new_fd > max_fd)
    {
        max_fd = new_fd;
    }
}   

void Server::Recv(int nums)
{
    for(int fd=0; fd<=max_fd; ++fd)
    {
        if(FD_ISSET(fd, &working_set))
        {
            bool close_conn = false;  // 标记当前连接是否断开了

            PACKET_HEAD head;
            recv(fd, &head, sizeof(head), 0);   // 先接受包头

            if(head.type == HEART)
            {
                mmap[fd].second = 0;        // 每次收到心跳包，count置0
                cout << "Received heart-beat from client.\n";
            }
            else
            {
                // 数据包，通过head.length确认数据包长度
            }   

            if(close_conn)  // 当前这个连接有问题，关闭它
            {
                close(fd);
                FD_CLR(fd, &master_set);
                if(fd == max_fd)  // 需要更新max_fd;
                {
                    while(FD_ISSET(max_fd, &master_set) == false)
                        --max_fd;
                }
            }
        }
    }
}

void Server::Run()
{
    pthread_t id;     // 创建心跳检测线程
    int ret = pthread_create(&id, NULL, heart_handler, (void*)this);
    if(ret != 0)
    {
        cout << "Can not create heart-beat checking thread.\n";
    }

    max_fd = listen_fd;   // 初始化max_fd
    FD_ZERO(&master_set);
    FD_SET(listen_fd, &master_set);  // 添加监听fd

    while(1)
    {
        FD_ZERO(&working_set);
        memcpy(&working_set, &master_set, sizeof(master_set));

        timeout.tv_sec = 30;
        timeout.tv_usec = 0;

        int nums = select(max_fd+1, &working_set, NULL, NULL, &timeout);
        if(nums < 0)
        {
            cout << "select() error!";
            exit(1);
        }

        if(nums == 0)
        {
            //cout << "select() is timeout!";
            continue;
        }

        if(FD_ISSET(listen_fd, &working_set))
            Accept();   // 有新的客户端请求
        else
            Recv(nums); // 接收客户端的消息
    }
}

// thread function
void* heart_handler(void* arg)
{
    cout << "The heartbeat checking thread started.\n";
    Server* s = (Server*)arg;
    while(1)
    {
        map<int, pair<string, int> >::iterator it = s->mmap.begin();
        for( ; it!=s->mmap.end(); )
        {
            if(it->second.second == 5)   // 3s*5没有收到心跳包，判定客户端掉线
            {
                cout << "The client " << it->second.first << " has be offline.\n";

                int fd = it->first;
                close(fd);            // 关闭该连接
                FD_CLR(fd, &s->master_set);
                if(fd == s->max_fd)      // 需要更新max_fd;
                {
                    while(FD_ISSET(s->max_fd, &s->master_set) == false)
                        s->max_fd--;
                }

                s->mmap.erase(it++);  // 从map中移除该记录
            }
            else if(it->second.second < 5 && it->second.second >= 0)
            {
                it->second.second += 1;
                ++it;
            }
            else
            {
                ++it;
            }
        }
        sleep(3);   // 定时三秒
    }
}

int main()
{
    Server server(15000);
    server.Bind();
    server.Listen();
    server.Run();
    return 0;
}

/*************************************************************************
    > File Name: Client.cpp
    > Author: SongLee
    > E-mail: lisong.shine@qq.com
    > Created Time: 2016年05月05日 星期四 23时41分56秒
    > Personal Blog: http://songlee24.github.io/
 ************************************************************************/
#include<netinet/in.h>   // sockaddr_in
#include<sys/types.h>    // socket
#include<sys/socket.h>   // socket
#include<arpa/inet.h>
#include<sys/ioctl.h>
#include<unistd.h>
#include<iostream>
#include<string>
#include<cstdlib>
#include<cstdio>
#include<cstring>
using namespace std;
#define BUFFER_SIZE 1024

enum Type {HEART, OTHER};

struct PACKET_HEAD
{
    Type type;
    int length;
};

void* send_heart(void* arg); 

class Client
{
private:
    struct sockaddr_in server_addr;
    socklen_t server_addr_len;
    int fd;
public:
    Client(string ip, int port);
    ~Client();
    void Connect();
    void Run();
    friend void* send_heart(void* arg);
};

Client::Client(string ip, int port)
{
    bzero(&server_addr, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    if(inet_pton(AF_INET, ip.c_str(), &server_addr.sin_addr) == 0)
    {
        cout << "Server IP Address Error!";
        exit(1);
    }
    server_addr.sin_port = htons(port);
    server_addr_len = sizeof(server_addr);
    // create socket
    fd = socket(AF_INET, SOCK_STREAM, 0);
    if(fd < 0)
    {
        cout << "Create Socket Failed!";
        exit(1);
    }
}

Client::~Client()
{
    close(fd);
}

void Client::Connect()
{
    cout << "Connecting......" << endl;
    if(connect(fd, (struct sockaddr*)&server_addr, server_addr_len) < 0)
    {
        cout << "Can not Connect to Server IP!";
        exit(1);
    }
    cout << "Connect to Server successfully." << endl;
}

void Client::Run()
{
    pthread_t id;
    int ret = pthread_create(&id, NULL, send_heart, (void*)this);
    if(ret != 0)
    {
        cout << "Can not create thread!";
        exit(1);
    }
}

// thread function
void* send_heart(void* arg)
{
    cout << "The heartbeat sending thread started.\n";
    Client* c = (Client*)arg;
    int count = 0;  // 测试
    while(1)
    {
        PACKET_HEAD head;
        head.type = HEART;
        head.length = 0;
        send(c->fd, &head, sizeof(head), 0);
        sleep(3);     // 定时3秒

        ++count;      // 测试：发送15次心跳包就停止发送
        if(count > 15)
            break;
    }
}

int main()
{
    Client client("127.0.0.1", 15000);
    client.Connect();
    client.Run();
    while(1)
    {
        string msg;
        getline(cin, msg);
        if(msg == "exit")
            break;
        cout << "msg\n";
    }
    return 0;
}

可以看出，客户端启动以后发送了15次心跳包，然后停止发送心跳包。在经过一段时间后（3s*5），服务器就判断该客户端掉线，并断开了连接。