c++主线程通知线程通知主线程

我需要帮助。我想做一些具体的事情，但我缺乏多线程技能，这让我很难受。

基本上我的主程序/线程需要管理一些必须运行多次的"通道"。由于这些运行是独立的，因此每个通道都包含一个执行它们的线程。

因此主线程必须等待所有通道(线程)完成它们的运行才能启动下一个通道。所有的通道都必须等待主线程通知它们可以运行。

下面是我怎么做的-抱歉它有点长!

#include <thread>
#include <mutex>
#include <condition_variable>
#include <iostream>
#include <atomic>
std::mutex              g_lockprint;
std::mutex              g_lockbatch;
std::condition_variable g_nextbatch;
std::mutex              g_lockready;
std::condition_variable g_ready;
int global_id = 0;
int nbChannels = 5;
std::atomic<int> nbChannelsLeftToEnd;
class Channel {
private:
    int _id;
    std::thread _th;
    std::atomic<bool> next_batch;
    std::atomic<bool> stop_th;
public:
    Channel() : _id(global_id++), _th(), next_batch(false), stop_th(false) {}
    void go_for_next_batch() { next_batch = true; }
    void start(int& start, int &end){
        _th = std::thread(&Channel::run, this, std::ref(start), std::ref(end));
    }
    void stop(){
        stop_th = true;
        _th.join();
    }
    void run(int& start, int& end){
        while (!stop_th){
            {
                std::unique_lock<std::mutex> locker(g_lockbatch);
                g_nextbatch.wait(locker, [&](){return (next_batch==true); });
            }
            // print a starting message
            {
                std::unique_lock<std::mutex> locker(g_lockprint);
                std::cout << "[channel " << _id << "]trunning in [" << start << "," << end << "]" << std::endl;
            }
            // simulate work
            std::this_thread::sleep_for(std::chrono::seconds(1));
            // update the number of channels left to run
            nbChannelsLeftToEnd--;
            g_ready.notify_one();
            next_batch = false;
        }
    }
};
int main()
{
    int end = 100;
    int batch = 10;
    int startBatch = 0;
    int endBatch = startBatch + batch;
    // declare some channels (threads)
    std::vector<Channel> channels(nbChannels);
    // start the threads
    for (auto& ch : channels) ch.start(startBatch, endBatch);
    while (endBatch<=end){
        {
            std::unique_lock<std::mutex> locker(g_lockprint);
            std::cout << "[main]trunning in [" << startBatch << "," << endBatch << "]" << std::endl;
        }
        nbChannelsLeftToEnd = nbChannels;
        for (auto& ch : channels) ch.go_for_next_batch();
        g_nextbatch.notify_all();
        std::unique_lock<std::mutex> locker(g_lockready);
        g_ready.wait(locker, [&](){return (nbChannelsLeftToEnd == 0); });
        startBatch += batch;
        endBatch += batch;
    }
    for (auto& ch : channels) ch.stop();
    return 0;
}

但有时程序阻塞，可能线程等待彼此，但我不知道为什么。在任何情况下，连接线程("stop"方法在main的末尾)使我的程序无限期地运行，也看不出为什么。

EDIT:感谢您的评论和一些研究，我设法使用同步屏障获得一个工作程序，因此主线程可以等待所有其他线程完成当前批处理，然后告诉它们开始下一个。我重用了这里有人引用Anthony williams的书中的屏障代码——这里是屏障:

class barrier
{
    unsigned const count;
    std::atomic<unsigned> spaces;
    std::atomic<unsigned> generation;
public:
    explicit barrier(unsigned count_) :
        count(count_), spaces(count), generation(0) {}
    void wait()
    {
        unsigned const my_generation = generation;
        if (!--spaces)
        {
            spaces = count;
            ++generation;
        }
        else
        {
            while (generation == my_generation)
                std::this_thread::yield();
        }
    }
};

下面是使用barrier的Channel类的新的run方法—注意在"stop_th"标志上的附加测试。当线程在上一个批处理之后和加入之前被解除阻塞时，它不应该运行另一个批处理，因此需要进行测试。

void run(int& start, int& end, barrier& b)
{
    while (!stop_th){
        // wait for next batch notification - use the next_batch flag to avoid
        // spurious wake-ups
        {
            std::unique_lock<std::mutex> locker(g_lockbatch);
            g_nextbatch.wait(locker, [&](){return (next_batch==true); });
        }
        if (stop_th) return;
        // simulate work
        std::this_thread::sleep_for(std::chrono::seconds(1));
        // wait for everyone to meet
        next_batch = false;
        b.wait();
    }
}

最后是main:

int main()
{
    int end = 100;
    int batch = 10;
    int startBatch = 0;
    int endBatch = startBatch + batch;
    // declare a barrier where all threads will meet
    barrier b(nbChannels+1);
    // declare some channels (threads)
    std::vector<Channel> channels(nbChannels);
    // start the threads
    for (auto& ch : channels) ch.start(startBatch, endBatch, b);
    while (endBatch<=end){
        // notify the channels they can process one batch
        for (auto& ch : channels) ch.go_for_next_batch();
        g_nextbatch.notify_all();
        // wait until all threads have finished their batch
        b.wait();
        // prepare the next one
        startBatch += batch;
        endBatch += batch;
    }
    // all channels are blocked by the next_batch condition
    // so notify a next batch and join them
    for (auto& ch : channels) ch.stop();
    for (auto& ch : channels) ch.go_for_next_batch();
    g_nextbatch.notify_all();
    for (auto& ch : channels) ch.wait_until_stopped();
    return 0;
}

再次感谢你的评论/回答!!

for (auto& ch : channels) ch.stop();

for (auto& ch : channels) ch.stop();
for (auto& ch : channels) ch.go_for_next_batch();
g_nextbatch.notify_all();
for (auto& ch : channels) ch.wait_until_stopped();