Linux 高级 IO：I/O 多路转接之 poll 接口原理与 TCP 服务器实现

2. 核心逻辑实现

• Start 函数：初始化 listensock 到 _event_fds[0]，设置 events 为 POLLIN。调用 poll 循环监听。
• Dispatcher 函数：遍历 _event_fds，检查 revents 是否包含 POLLIN。若是 listensock 则调用 Accepter，否则调用 Recver。
• Accepter 函数：接受新连接，查找空闲位置放入 _event_fds。若满则关闭连接。
• Recver 函数：读取数据。若 read 返回 0 或错误，关闭 fd 并重置位置。

void Dispatcher() {
    for (int i = 0; i < fd_num_max; i++) {
        int fd = _event_fds[i].fd;
        if (fd == defaultfd) continue;
        if (_event_fds[i].revents & POLLIN) {
            if (fd == _listensock.Fd()) {
                Accepter();
            } else {
                Recver(fd, i);
            }
        }
    }
}

void Start() {
    _event_fds[0].fd = _listensock.Fd();
    _event_fds[0].events = POLLIN;
    int timeout = 3000;
    for (;;) {
        int n = poll(_event_fds, fd_num_max, timeout);
        switch (n) {
            case 0: cout << "time out..." << endl; break;
            case -1: cout << "poll error" << endl; break;
            default: cout << "get a new link" << endl; Dispatcher(); break;
        }
    }
}

void Accepter() {
    string clientip;
    uint16_t clientport;
    int sock = _listensock.Accept(&clientip, &clientport);
    if (sock < 0) return;
    lg(Info, "accept success, %s:%d, sock fd: %d", clientip.c_str(), clientport, sock);
    int pos = 1;
    for (; pos < fd_num_max; pos++) {
        if (_event_fds[pos].fd == defaultfd) break;
    }
    if (pos == fd_num_max) {
        lg(Warning, "server is full, close %d now", sock);
        close(sock);
    } else {
        _event_fds[pos].fd = sock;
        _event_fds[pos].events = POLLIN;
        _event_fds[pos].revents = non_event;
        PrintFd();
    }
}

void Recver(int fd, int pos) {
    char buffer[1024];
    ssize_t n = read(fd, buffer, sizeof(buffer) - 1);
    if (n > 0) {
        buffer[n - 1] = 0;
        cout << "get a message: " << buffer << endl;
    } else if (n == 0) {
        lg(Info, "client quit, me too, close fd: %d", fd);
        close(fd);
        _event_fds[pos].fd = defaultfd;
    } else {
        lg(Warning, "recv error, fd: %d", fd);
        close(fd);
        _event_fds[pos].fd = defaultfd;
    }
}

void PrintFd() {
    cout << "online fd list: ";
    for (int i = 0; i < fd_num_max; i++) {
        if (_event_fds[i].fd != defaultfd) {
            cout << _event_fds[i].fd << ' ';
        }
    }
    cout << endl;
}

3. 主程序入口

#include"PollServer.hpp"
#include<memory>

int main() {
    unique_ptr<PollServer> svr(new PollServer());
    svr->Init();
    svr->Start();
    return 0;
}

4. Makefile

pollserver:Main.cc
g++ -o $@ $^ -std=c++11
.PHONY:clean
clean:
 rm -f pollserver

5. 辅助头文件

Log.hpp

#pragma once
#include<iostream>
#include<string>
#include<ctime>
#include<cstdio>
#include<cstdarg>
#include<sys/types.h>
#include<sys/stat.h>
#include<fcntl.h>
#include<unistd.h>

#define SIZE 1024
#define Info 0
#define Debug 1
#define Warning 2
#define Error 3
#define Fatal 4
#define Screen 1
#define Onefile 2
#define Classfile 3
#define LogFile "log.txt"

class Log {
public:
    Log() { printMethod = Screen; path = "./log/"; }
    void Enable(int method) { printMethod = method; }
    ~Log() {}
    std::string levelToString(int level) {
        switch (level) {
            case Info: return "Info";
            case Debug: return "Debug";
            case Warning: return "Warning";
            case Error: return "Error";
            case Fatal: return "Fatal";
            default: return "";
        }
    }
    void operator()(int level, const char* format, ...) {
        time_t t = time(nullptr);
        struct tm* ctime = localtime(&t);
        char leftbuffer[SIZE];
        snprintf(leftbuffer, sizeof(leftbuffer), "[%s][%d-%d-%d %d:%d:%d]",
                 levelToString(level).c_str(), ctime->tm_year + 1900, ctime->tm_mon + 1,
                 ctime->tm_mday, ctime->tm_hour, ctime->tm_min, ctime->tm_sec);
        va_list s;
        va_start(s, format);
        char rightbuffer[SIZE];
        vsnprintf(rightbuffer, sizeof(rightbuffer), format, s);
        va_end(s);
        char logtxt[2 * SIZE];
        snprintf(logtxt, sizeof(logtxt), "%s %s", leftbuffer, rightbuffer);
        printLog(level, logtxt);
    }
    void printLog(int level, const std::string& logtxt) {
        switch (printMethod) {
            case Screen: std::cout << logtxt << std::endl; break;
            case Onefile: printOneFile(LogFile, logtxt); break;
            case Classfile: printClassFile(level, logtxt); break;
            default: break;
        }
    }
    void printOneFile(const std::string& logname, const std::string& logtxt) {
        std::string _logname = path + logname;
        int fd = open(_logname.c_str(), O_WRONLY | O_CREAT | O_APPEND, 0666);
        if (fd < 0) return;
        write(fd, logtxt.c_str(), logtxt.size());
        close(fd);
    }
    void printClassFile(int level, const std::string& logtxt) {
        std::string filename = LogFile;
        filename += ".";
        filename += levelToString(level);
        printOneFile(filename, logtxt);
    }
private:
    int printMethod;
    std::string path;
};
Log lg;

Socket.hpp

#pragma once
#include<iostream>
#include<string>
#include<cstring>
#include<sys/types.h>
#include<sys/socket.h>
#include<netinet/in.h>
#include<arpa/inet.h>
#include"Log.hpp"

const int backlog = 10;
enum { SocketErr = 1, BindErr, ListenErr };

class Sock {
public:
    Sock() {}
    void Socket() {
        sockfd_ = socket(AF_INET, SOCK_STREAM, 0);
        if (sockfd_ < 0) {
            lg(Fatal, "socket error, %s : %d", strerror(errno), errno);
            exit(SocketErr);
        }
        int opt = 1;
        setsockopt(sockfd_, SOL_SOCKET, SO_REUSEPORT, &opt, sizeof(opt));
    }
    void Bind(uint16_t port) {
        struct sockaddr_in local;
        memset(&local, 0, sizeof(local));
        local.sin_family = AF_INET;
        local.sin_port = htons(port);
        local.sin_addr.s_addr = INADDR_ANY;
        socklen_t len = sizeof(local);
        if (bind(sockfd_, (struct sockaddr*)&local, len) < 0) {
            lg(Fatal, "bind error, %s : %d", strerror(errno), errno);
            exit(BindErr);
        }
    }
    void Listen() {
        if (listen(sockfd_, backlog) < 0) {
            lg(Fatal, "listen error, %s : %d", strerror(errno), errno);
            exit(ListenErr);
        }
    }
    int Accept(std::string* clientip, uint16_t* clientport) {
        struct sockaddr_in peer;
        socklen_t len = sizeof(peer);
        int newfd = accept(sockfd_, (struct sockaddr*)&peer, &len);
        if (newfd < 0) {
            lg(Warning, "accept error, %s : %d", strerror(errno), errno);
            return -1;
        }
        char ipstr[128];
        inet_ntop(AF_INET, &(peer.sin_addr), ipstr, sizeof(ipstr));
        *clientip = ipstr;
        *clientport = ntohs(peer.sin_port);
        return newfd;
    }
    bool Connect(const std::string& serverip, uint16_t serverport) {
        struct sockaddr_in peer;
        memset(&peer, 0, sizeof(peer));
        peer.sin_family = AF_INET;
        peer.sin_port = htons(serverport);
        inet_pton(AF_INET, serverip.c_str(), &(peer.sin_addr));
        socklen_t len = sizeof(peer);
        int n = connect(sockfd_, (struct sockaddr*)&peer, len);
        if (n == -1) {
            std::cerr << "connect to " << serverip << ':' << serverport << "error" << std::endl;
            return false;
        }
        return true;
    }
    void Close() {
        if (sockfd_ > 0) {
            close(sockfd_);
        }
    }
    int Fd() { return sockfd_; }
    ~Sock() {}
private:
    int sockfd_;
};

四、poll 的缺点

1. poll 解决了 select 的 fd 上限和重置问题，但效率仍有瓶颈。
2. poll 仍需用户维护数组，大量遍历（O(N)）。在分派器、连接管理器及内核检测中均需遍历。
3. 当 fd 数量极大（如 1 亿）时，遍历开销显著影响性能。
4. 针对此硬伤，后续将引入 epoll 多路转接方案进行优化。

总结

本文详细讲解了 Linux poll 接口的原理及其在 TCP 服务器中的应用。通过对比 select，展示了 poll 在减少数据拷贝和重置方面的优势，并提供了完整的 C++ 实现代码。尽管 poll 有所改进，但在大规模并发场景下仍存在遍历开销，epoll 是进一步的解决方案。