C++: 多线程同步机制

C++生产者消费者

  • 基于锁和条件变量的同步机制
  • 基于pipe的同步机制

基于变量的同步机制，在面对select/poll等场景时，无法做到loop线程的唤醒。因此，面对读写事件时，需要使用pipe来进行同步。

  1. 基于锁和条件变量的同步机制
#include <iostream>
#include <mutex>
#include <thread>
#include <memory>
#include <vector>
#include <string>

class context{

public:
    virtual int run()=0;
};

class TaskManager{
private:
    std::mutex mtx;
    std::condition_variable cond;

    std::thread m_handle;

    std::thread::id thread_id;
    bool stopped;
    std::string thread_name;

private:
    virtual void entry();
    std::vector<std::shared_ptr<context>> task;

    void _process(std::vector<std::shared_ptr<context>>&);

public:
    void create(const std::string& name){
        thread_name = name;
        m_handle = std::thread(&TaskManager::entry,this);
        thread_id = m_handle.get_id();
    }

    void join(){
        if (m_handle.joinable())
            m_handle.join();
    }
    void detach(){
        m_handle.detach();
    }
    int add_task(std::shared_ptr<context>);
    void stop(){
        std::lock_guard<std::mutex> lock{mtx};
        stopped = true;
        cond.notify_all();
    }
};

void TaskManager::entry(){
    std::unique_lock<std::mutex> lock{mtx};
    while (!stopped || !task.empty()){
        cond.wait(lock,[this](){return (!task.empty() || stopped);});
        std::vector<std::shared_ptr<context>> tmp;
        tmp.swap(task);
        lock.unlock();
        _process(tmp);
        lock.lock();
    }
}

void TaskManager::_process(std::vector<std::shared_ptr<context>>& tsk){
    for (const auto& tk:tsk){
        tk->run();
    }
}

int TaskManager::add_task(std::shared_ptr<context> ts){
    if (stopped)
        return -1;
    {
        std::lock_guard<std::mutex> lock{mtx};
        task.push_back(ts);
        cond.notify_all();
    }
    return 0;
}


class test: public context{
private:
    std::string name;
public:
    test(const std::string& name):name(name){}
    int run() override {
        std::cout<<"Task Run: "<<name<<std::endl;
        return 0;
    }
 };

int main(int argc,char* argv[]){
    std::shared_ptr<TaskManager> tm = std::make_shared<TaskManager>();
    tm->create("TaskManager");
    tm->add_task(std::make_shared<test>("TaskA"));
    tm->add_task(std::make_shared<test>("TaskB"));
    tm->add_task(std::make_shared<test>(std::string("TaskC")));
    tm->stop();
    tm->join();
}
  1. 基于pipe的同步机制
    基于锁和条件变量的同步机制，无法适用于读写事件的同步，如select/poll/epoll。主线程在select中等待时，则需要其它唤醒方式。
  • 可以使用pipe进行同步
  • 将pipe的读端加入select监听
  • 需要唤醒时，对pipe写端进行写入，此时唤醒等待线程

实现如下，注册监听一组fd，利用管道实现对监听fd的添加和删除：

#include <iostream>
#include <mutex>
#include <thread>
#include <memory>
#include <map>
#include <string>
#include <functional>
#include <array>
#include <string_view>
#include <algorithm>

#include <fcntl.h>
#include <unistd.h>
#include <sys/select.h>

int pipe_ocloexc(int pipefd[2]){
     /*
     ** pipe init
     */
    if (pipe2(pipefd, O_CLOEXEC) < 0){
        return -1;
    }   
    if (::fcntl(pipefd[0], F_SETFL, O_NONBLOCK) < 0){
         ::close(pipefd[1]);
         ::close(pipefd[0]);
         return -1;
    }
    return 0;
}

struct io_handler{
    io_handler(const std::string user):user(user){
        int r = pipe_ocloexc(pipefd);
        if (r < 0){
            exit(-1);
        }
    }
    std::string user;
    int pipefd[2];
    std::function<int(void*)> callback;
};

int read(int fd){
    std::array<char, 256> buf;
    int ret = ::read(fd, (void *)buf.data(), buf.size());
    if (ret < 0) { 
        return -1;
    }
    std::cout<<"Read Data: "<<std::string_view(buf.data(),ret)<<std::endl;
    return 0;
}

int process(void* arg){
    io_handler* ptr = (io_handler*)arg;
    //std::cout<<"IO: "<<ptr->user<<std::endl;
    int ret = read(ptr->pipefd[0]);
    if (ret < 0 )
        return -1;
    return 0;
}

class TaskManager{
private:
    std::mutex mtx;
    int thread_pipe[2];

    std::thread m_handle;

    std::thread::id thread_id;
    std::string thread_name;

    bool stopped;

private:
    virtual void entry();
    using mio = std::map<int,std::shared_ptr<io_handler>>;
    mio io_callback;

    int signal_thread();
    int clear_signal(int);

public:
    int create(const std::string& name);

    void join();
    void detach();
    int register_fd(std::shared_ptr<io_handler>);
    void unregister_fd(std::shared_ptr<io_handler>);
    void stop();
    virtual ~TaskManager(){
        ::close(thread_pipe[1]);
        ::close(thread_pipe[0]);
    }
};

int TaskManager::create(const std::string& name){
    thread_name = name;

    if (pipe_ocloexc(thread_pipe)<0)
        return -1;

    m_handle = std::thread(&TaskManager::entry,this);
    thread_id = m_handle.get_id();
    return 0;
}

int TaskManager::clear_signal(int fd)
{
    std::array<char, 256> buf;
    int ret = ::read(fd, (void *)buf.data(), buf.size());
    if (ret < 0) { 
        return -1;
    }
    return 0;
}

void TaskManager::join(){
    if (m_handle.joinable())
        m_handle.join();
}

void TaskManager::detach(){
    m_handle.detach();
}

void TaskManager::stop(){
    if (stopped)
        return;
    stopped = true;
    signal_thread();
}

void TaskManager::entry(){
    fd_set fdread;
    fd_set fdwrite;
    fd_set fdexcep;
    while (!stopped || !io_callback.empty()){
        int maxfd = -1;
        FD_ZERO(&fdread);
        FD_ZERO(&fdwrite);
        FD_ZERO(&fdexcep);

        FD_SET(thread_pipe[0],&fdread);
        maxfd = thread_pipe[0]+1;

        std::unique_lock<std::mutex> lock{mtx};
        for (const auto& io:io_callback){
            FD_SET(io.first,&fdread);
            if (io.first >= maxfd){
                maxfd = io.first+1;
            }
        }
        lock.unlock();
        int ret = select(maxfd,&fdread,&fdwrite,&fdexcep,NULL);
        if ( ret < 0)
            return;
        lock.lock();
        for (const auto& io:io_callback){
            if (FD_ISSET(io.first,&fdread)){
                //clear_signal(io.first);
                (io.second)->callback(io.second.get());
            }
        }
        if (FD_ISSET(thread_pipe[0],&fdread)){
            clear_signal(thread_pipe[0]);
        }
        lock.unlock();
    }
}

int TaskManager::signal_thread(){
    uint32_t buf = 0;
    int ret = write(thread_pipe[1], (void *)&buf, sizeof(buf));
    if (ret < 0) {
        return -1;
    }
    return 0;
}

int TaskManager::register_fd(std::shared_ptr<io_handler> handler){
    if (stopped)
        return -1;
    {
        std::lock_guard<std::mutex> lock{mtx};
        io_callback.insert(std::pair<int,std::shared_ptr<io_handler>>(handler->pipefd[0],handler));
        signal_thread();
    }
    return 0;
}

void TaskManager::unregister_fd(std::shared_ptr<io_handler> io){
    {
        std::lock_guard<std::mutex> lock{mtx};
        io_callback.erase(io->pipefd[0]);
        signal_thread();
    }
}

int test(std::shared_ptr<io_handler> io){
    std::array<char,256> buf;
    std::copy(io->user.cbegin(),io->user.cend(),buf.data());
    int ret = write(io->pipefd[1], (void *)&buf, io->user.size());
    if (ret < 0) {
        return -1;
    }
    return 0;
}

int main(int argc,char* argv[]){
    std::shared_ptr<TaskManager> tm = std::make_shared<TaskManager>();
    tm->create("TaskManager");
    std::shared_ptr<io_handler> io1 = std::make_shared<io_handler>("io_1");
    io1->callback = process;
    std::shared_ptr<io_handler> io2 = std::make_shared<io_handler>("io_2");
    io2->callback = process;
    std::shared_ptr<io_handler> io3 = std::make_shared<io_handler>("io_3");
    io3->callback = process;
    tm->register_fd(io1);
    tm->register_fd(io2);
    tm->register_fd(io3);

    test(io1);
    test(io3);
    test(io2);

    sleep(3);
    
    tm->unregister_fd(io1);
    tm->unregister_fd(io2);
    tm->unregister_fd(io3);
    tm->stop();
    tm->join();
}
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