Linux 管道通信实战：匿名进程池与命名管道服务端模型

3. 命名管道

如果需要在无亲缘关系的进程间通信，可以使用命名管道（FIFO）。它本质上是一个特殊类型的文件，存在于文件系统中，双方通过路径即可访问。

创建命令：mkfifo 文件名 创建函数：mkfifo(const char *pathname, mode_t mode) 删除文件：unlink("文件名")

匿名管道由 pipe 创建并打开，命名管道由 mkfifo 创建后用 open 打开，两者在语义上基本一致。

三、实例：匿名管道实现进程池

进程池是指提前创建好多个子进程，需要任务时直接分配，避免频繁创建销毁的开销。

核心思路：父进程维护多个通道（管道），每个通道对应一个子进程。父进程随机选择通道发送任务，子进程监听通道接收任务执行。

#include <cassert>
#include <cstdio>
#include <cstdlib>
#include <ctime>
#include <iostream>
#include <string>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <vector>

// 状态定义
enum { OK = 0, PIPE_ERR, FORK_ERR, READ_ERR, WRITE_ERR, WAIT_ERR };

const int gprocessnum = 7;

void task1() { std::cout << "这是下载数据任务" << std::endl; }
void task2() { std::cout << "这是打印日志任务" << std::endl; }
void task3() { std::cout << "这是刷新磁盘任务" << std::endl; }
void task4() { std::cout << "这是更新用户状态任务" << std::endl; }

typedef void (*task_t)();
const int gtasknum = 4;
task_t tasks[gtasknum] = {task1, task2, task3, task4};

class ProcessPool {
private:
    class Channel {
    private:
        int _wfd;
        pid_t _id;
        std::string channel_name;
    public:
        Channel(int wfd, pid_t id) : _wfd(wfd), _id(id) {
            channel_name = "channel-" + std::to_string(id);
        }
        void ClosePipe() { close(_wfd); }
        void PrintInfo() {
            printf("管道 wfd: %d, 通道名：%s\n", _wfd, channel_name.c_str());
        }
        int getfd() { return _wfd; }
        const char* getname() { return channel_name.c_str(); }
        void Wait() {
            pid_t rid = waitpid(_id, nullptr, 0);
            if (rid < 0) exit(WAIT_ERR);
            std::cout << "回收子进程：" << _id << std::endl;
        }
    };

public:
    ProcessPool() { srand(time(NULL)); }

    void Init() { CreateProcessChannels(); }
    void Debug() {
        for (auto& channel : channels) channel.PrintInfo();
    }
    void Run() {
        int cnt = 10;
        while (cnt--) {
            int itask = SelectTask();
            int ichannel = SelectChannel();
            printf("向通道%s发送任务 task%d...\n", channels[ichannel].getname(), itask + 1);
            SendTask2Channel(itask, ichannel);
            sleep(1);
        }
    }
    void Quit() {
        for (auto& channel : channels) {
            channel.ClosePipe();
            channel.Wait();
        }
    }

private:
    void CreateProcessChannels() {
        for (int i = 0; i < gprocessnum; i++) {
            int pipefd[2] = {0};
            int n = pipe(pipefd);
            if (n < 0) exit(PIPE_ERR);
            pid_t id = fork();
            if (id < 0) exit(FORK_ERR);
            else if (id == 0) {
                // 子进程逻辑
                if (!channels.empty()) {
                    for (auto& channel : channels) channel.ClosePipe();
                }
                close(pipefd[1]);
                DoTask(pipefd[0]);
                exit(OK);
            } else {
                // 父进程逻辑
                close(pipefd[0]);
                channels.emplace_back(pipefd[1], id);
                printf("创建子进程%d成功\n", id);
            }
        }
    }

    void DoTask(int fd) {
        while (1) {
            int task_code;
            ssize_t n = read(fd, &task_code, sizeof(task_code));
            if (n == sizeof(task_code)) {
                if (task_code >= 0 && task_code < gtasknum) {
                    tasks[task_code]();
                }
            } else if (n == 0) {
                printf("%d任务退出\n", getpid());
                break;
            } else {
                exit(READ_ERR);
            }
        }
    }

    int SelectTask() { return rand() % gtasknum; }
    int SelectChannel() {
        static int i = 0;
        int selected = i;
        i++;
        i %= gprocessnum;
        return selected;
    }

    void SendTask2Channel(int itask, int ichannel) {
        assert(0 <= itask && itask < gtasknum && ichannel >= 0 && ichannel < gprocessnum);
        ssize_t n = write(channels[ichannel].getfd(), &itask, sizeof(itask));
        if (n < 0) exit(WRITE_ERR);
    }

    std::vector<Channel> channels;
};

int main() {
    ProcessPool pp;
    pp.Init();
    pp.Debug();
    pp.Run();
    pp.Quit();
    return 0;
}

四、实例：命名管道实现服务端客户端通信模型

命名管道适合非亲缘进程间的通信，典型应用如服务端与客户端模型。

1. 管道封装类

// Fifo.hpp
#pragma once
#include <cstdio>
#include <cstring>
#include <fcntl.h>
#include <iostream>
#include <string>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>

#define FOR_READ 1
#define FOR_WRITE 2

const std::string myfifo = "./fifo";

class Fifo {
public:
    Fifo(const std::string& filename = myfifo) : _filename(filename), _mode(0666), _fd(-1) {}

    void Build() {
        if (IsExist()) return;
        int n = mkfifo(_filename.c_str(), _mode);
        if (n < 0) {
            std::cerr << "mkfifo error: " << strerror(errno) << std::endl;
            exit(1);
        }
        std::cout << "mkfifo success" << std::endl;
    }

    void Open(int mode) {
        if (mode == FOR_READ) _fd = open(_filename.c_str(), O_RDONLY);
        else if (mode == FOR_WRITE) _fd = open(_filename.c_str(), O_WRONLY);
        if (_fd < 0) {
            std::cerr << "open error: " << strerror(errno) << std::endl;
            exit(2);
        }
        std::cout << "open success" << std::endl;
    }

    void Delete() {
        if (!IsExist()) return;
        int n = unlink(_filename.c_str());
        if (n < 0) {
            std::cerr << "delete error: " << strerror(errno) << std::endl;
            exit(3);
        }
        std::cout << "delete success" << std::endl;
    }

    void Send(std::string& msgin) {
        ssize_t n = write(_fd, msgin.c_str(), msgin.size());
    }

    int Receive(std::string& msgout) {
        char buffer[128];
        ssize_t n = read(_fd, buffer, sizeof(buffer) - 1);
        if (n > 0) {
            buffer[n] = '\0';
            msgout = buffer;
            return n;
        } else if (n == 0) return 0;
        else return -1;
    }

private:
    bool IsExist() {
        struct stat st;
        int n = stat(_filename.c_str(), &st);
        if (n == 0) return true;
        else {
            errno = 0;
            return false;
        }
    }

private:
    std::string _filename;
    mode_t _mode;
    int _fd;
};

2. 服务端代码

// Server.cc
#include "Fifo.hpp"

int main() {
    Fifo pipefile;
    pipefile.Build();
    pipefile.Open(FOR_READ);
    std::string msg;
    while (1) {
        int n = pipefile.Receive(msg);
        if (n > 0) {
            std::cout << "客户端说：" << msg << std::endl;
        } else {
            break;
        }
    }
    pipefile.Delete();
    return 0;
}

3. 客户端代码

// Client.cc
#include "Fifo.hpp"

int main() {
    Fifo fileclient;
    fileclient.Open(FOR_WRITE);
    while (1) {
        std::cout << "请输入：" << std::endl;
        std::string msg;
        std::getline(std::cin, msg);
        fileclient.Send(msg);
    }
    return 0;
}

运行上述代码，即可看到命名管道在服务端和客户端之间的实时通信效果。