Question

在下面的代码中，我将创建一个producer thread和n consumer threads，分别从其专用的queue中读取并打印到stdout。该代码有时会在语句consumerQueues[id]->empty()上崩溃。通过调试器，我发现consumerQueues[id]在崩溃时是0x0。现在，在init()函数中，我在创建ith工作者queue之前创建了ith使用者thread。我不确定为什么consumerQueues[id]将保留为0x0。请帮我弄清楚发生了什么。

#include <thread>
#include <queue>
#include <memory>
#include <iostream>
#include <mutex>
#include <condition_variable>

class Test
{
private:
    void producer()
    {
        while(true)
        {
            std::string s = "abc";
            for(const auto& q : consumerQueues)
            {
                std::unique_lock<std::mutex> lock(mutex);
                q->push(s);
                condition_variable.notify_all();
            }
        }
    }

    void consumer(int id)
    {
        while (true)
        {
            std::string job;
            {
                std::unique_lock<std::mutex> lock(mutex);
                while(consumerQueues[id]->empty())
                {
                    condition_variable.wait(lock);
                }
                job = consumerQueues[id]->front();
                consumerQueues[id]->pop();
            }
            std::cout << "ID "<< id << " job " << job << std::endl;
        }
    }

    std::mutex mutex;
    std::condition_variable condition_variable;
    std::vector<std::thread> workers;
    std::vector<std::shared_ptr<std::queue<std::string>>> consumerQueues;
    std::thread producerThread;

public:

    Test(const unsigned n_threads):
    workers(std::vector<std::thread>(n_threads))
    {}

    Test(const Test &) = delete;
    Test(Test &&) = delete;

    Test & operator=(const Test &) = delete;
    Test & operator=(Test &&) = delete;

    void init()
    {
        for (unsigned i = 0; i < workers.size(); ++i)
        {
            consumerQueues.push_back(std::make_shared<std::queue<std::string>>());
            workers[i] = std::thread(&Test::consumer, this, i);
        }
       producerThread  = std::thread(&Test::producer, this);
    }

    ~Test()
    {
        producerThread.join();
        for (unsigned i = 0; i < workers.size(); ++i)
        {
            if(workers[i].joinable())
            {
                workers[i].join();
            }
        }
    }
};


int main()
{
    Test t(1000);
    t.init();
    return 0;
}

Answer 1

您的init函数正在修改没有互斥量的std :: vector。这将在线程逐个启动的同时修改向量。

要使其正常工作，您的init函数必须是这样的：

 void init() {
     for (unsigned i = 0; i < workers.size(); ++i) {
            std::unique_lock<std::mutex> lock(mutex);
            consumerQueues.push_back(std::make_shared<std::queue<std::string>>());
            workers[i] = std::thread(&Test::consumer, this, i);
     }
     producerThread  = std::thread(&Test::producer, this);
 }

发件人：http://www.cplusplus.com/reference/vector/vector/push_back/

数据竞赛

容器已修改。如果发生重新分配，则所有包含   元素被修改。否则，将不访问任何现有元素，并且   同时访问或修改它们是安全的。

重新分配通常从0个元素开始到1000个时发生。因此，您还可以保留向量的大小，以确保没有发生重新分配：

 void init() {
     consumerQueues.reserve(workers.size());
     for (unsigned i = 0; i < workers.size(); ++i) {
            consumerQueues.push_back(std::make_shared<std::queue<std::string>>());
            workers[i] = std::thread(&Test::consumer, this, i);
     }
     producerThread  = std::thread(&Test::producer, this);
 }

C ++单生产者多个消费者程序偶尔崩溃

1 个答案: