C ++-多线程-线程之间的通信 [英] C++ - Multi-threading - Communication between threads

问题描述

#include <iostream>
#include <thread>
#include <condition_variable>
#include <queue>
#include <cstdlib>
#include <chrono>
#include <ctime>
#include <random>

using namespace std;

//counts every number that is added to the queue
static long long producer_count = 0;
//counts every number that is taken out of the queue
static long long consumer_count = 0;

void generateNumbers(queue<int> & numbers, condition_variable & cv, mutex & m, bool & workdone){
    while(!workdone) {
        unique_lock<std::mutex> lk(m);
        int rndNum = rand() % 100;
        numbers.push(rndNum);
        producer_count++;
        cv.notify_one();
     }
}

void work(queue<int> & numbers, condition_variable & cv, mutex & m, bool & workdone) {
    while(!workdone) {
        unique_lock<std::mutex> lk(m);
        cv.wait(lk);
        cout << numbers.front() << endl;
        numbers.pop();
        consumer_count++;

     }
}

int main() {
    condition_variable cv;
    mutex m;
    bool workdone = false;
    queue<int> numbers;

    //start threads
    thread producer(generateNumbers, ref(numbers), ref(cv), ref(m),     ref(workdone));
    thread consumer(work, ref(numbers), ref(cv), ref(m), ref(workdone));

    //wait for 3 seconds, then join the threads
    this_thread::sleep_for(std::chrono::seconds(3));
    workdone = true;

    producer.join();
    consumer.join();

    //output the counters
    cout << producer_count << endl;
    cout << consumer_count << endl;

    return 0;
}

大家好， 我试图用C ++实现Producer-Consumer-Pattern. 生产者线程会生成随机整数，将其添加到队列中，然后通知消费者线程已添加了新数字.

使用者线程等待通知，然后将队列的第一个元素打印到控制台并将其删除.

我为添加到队列中的每个数字增加了一个计数器，为从队列中取出的每个数字增加了一个计数器.

我希望程序完成后，两个计数器保持相同的值，但是相差很大. 代表队列增加的计数器始终在百万范围内(在我的上次测试中为3871876)，代表从队列中取出数字的消费者的计数器始终在100k以下(在我的上次测试中为89993).

有人可以向我解释为什么会有如此巨大的差异吗? 我是否必须添加另一个条件变量，以便生产者线程也等待使用者线程? 谢谢！

解决方案

不需要第二个std::condition_variable，只需重复使用您已有的那个即可.正如其他人所提到的，您应该考虑使用std::atomic<bool>而不是普通的bool.但是我必须承认，带有-O3的g ++并不能对其进行优化.

#include <iostream>
#include <thread>
#include <condition_variable>
#include <queue>
#include <cstdlib>
#include <chrono>
#include <ctime>
#include <random>
#include <atomic>

//counts every number that is added to the queue
static long long producer_count = 0;
//counts every number that is taken out of the queue
static long long consumer_count = 0;

void generateNumbers(std::queue<int> & numbers, std::condition_variable & cv, std::mutex & m, std::atomic<bool> & workdone)
{
    while(!workdone.load())
    {
        std::unique_lock<std::mutex> lk(m);
        int rndNum = rand() % 100;
        numbers.push(rndNum);
        producer_count++;
        cv.notify_one(); // Notify worker
        cv.wait(lk); // Wait for worker to complete
     }
}

void work(std::queue<int> & numbers, std::condition_variable & cv, std::mutex & m, std::atomic<bool> & workdone)
{
    while(!workdone.load())
    {
        std::unique_lock<std::mutex> lk(m);
        cv.notify_one(); // Notify generator (placed here to avoid waiting for the lock)
        cv.wait(lk); // Wait for the generator to complete
        std::cout << numbers.front() << std::endl;
        numbers.pop();
        consumer_count++;
     }
}

int main() {
    std::condition_variable cv;
    std::mutex m;
    std::atomic<bool> workdone(false);
    std::queue<int> numbers;

    //start threads
    std::thread producer(generateNumbers, std::ref(numbers), std::ref(cv), std::ref(m), std::ref(workdone));
    std::thread consumer(work, std::ref(numbers), std::ref(cv), std::ref(m), std::ref(workdone));


    //wait for 3 seconds, then join the threads
    std::this_thread::sleep_for(std::chrono::seconds(3));
    workdone = true;
    cv.notify_all(); // To prevent dead-lock

    producer.join();
    consumer.join();

    //output the counters
    std::cout << producer_count << std::endl;
    std::cout << consumer_count << std::endl;

    return 0;
}

为避免偶发的一字不漏错误，您可以使用以下代码:

#include <iostream>
#include <thread>
#include <condition_variable>
#include <queue>
#include <cstdlib>
#include <chrono>
#include <ctime>
#include <random>
#include <atomic>

//counts every number that is added to the queue
static long long producer_count = 0;
//counts every number that is taken out of the queue
static long long consumer_count = 0;

void generateNumbers(std::queue<int> & numbers, std::condition_variable & cv, std::mutex & m, std::atomic<bool> & workdone)
{
    while(!workdone.load())
    {
        std::unique_lock<std::mutex> lk(m);
        int rndNum = rand() % 100;
        numbers.push(rndNum);
        producer_count++;
        cv.notify_one(); // Notify worker
        cv.wait(lk); // Wait for worker to complete
     }
}

void work(std::queue<int> & numbers, std::condition_variable & cv, std::mutex & m, std::atomic<bool> & workdone)
{
    while(!workdone.load() or !numbers.empty())
    {
        std::unique_lock<std::mutex> lk(m);
        cv.notify_one(); // Notify generator (placed here to avoid waiting for the lock)
        if (numbers.empty())
            cv.wait(lk); // Wait for the generator to complete
        if (numbers.empty())
            continue;
        std::cout << numbers.front() << std::endl;
        numbers.pop();
        consumer_count++;
     }
}

int main() {
    std::condition_variable cv;
    std::mutex m;
    std::atomic<bool> workdone(false);
    std::queue<int> numbers;

    //start threads
    std::thread producer(generateNumbers, std::ref(numbers), std::ref(cv), std::ref(m), std::ref(workdone));
    std::thread consumer(work, std::ref(numbers), std::ref(cv), std::ref(m), std::ref(workdone));


    //wait for 3 seconds, then join the threads
    std::this_thread::sleep_for(std::chrono::seconds(1));
    workdone = true;
    cv.notify_all(); // To prevent dead-lock

    producer.join();
    consumer.join();

    //output the counters
    std::cout << producer_count << std::endl;
    std::cout << consumer_count << std::endl;

    return 0;
}

#include <iostream>
#include <thread>
#include <condition_variable>
#include <queue>
#include <cstdlib>
#include <chrono>
#include <ctime>
#include <random>

using namespace std;

//counts every number that is added to the queue
static long long producer_count = 0;
//counts every number that is taken out of the queue
static long long consumer_count = 0;

void generateNumbers(queue<int> & numbers, condition_variable & cv, mutex & m, bool & workdone){
    while(!workdone) {
        unique_lock<std::mutex> lk(m);
        int rndNum = rand() % 100;
        numbers.push(rndNum);
        producer_count++;
        cv.notify_one();
     }
}

void work(queue<int> & numbers, condition_variable & cv, mutex & m, bool & workdone) {
    while(!workdone) {
        unique_lock<std::mutex> lk(m);
        cv.wait(lk);
        cout << numbers.front() << endl;
        numbers.pop();
        consumer_count++;

     }
}

int main() {
    condition_variable cv;
    mutex m;
    bool workdone = false;
    queue<int> numbers;

    //start threads
    thread producer(generateNumbers, ref(numbers), ref(cv), ref(m),     ref(workdone));
    thread consumer(work, ref(numbers), ref(cv), ref(m), ref(workdone));

    //wait for 3 seconds, then join the threads
    this_thread::sleep_for(std::chrono::seconds(3));
    workdone = true;

    producer.join();
    consumer.join();

    //output the counters
    cout << producer_count << endl;
    cout << consumer_count << endl;

    return 0;
}

Hello Everyone, I tried to implement the Producer-Consumer-Pattern with C++. The producer thread generates random integers, adds them to a queue and then notifies the consumer thread that a new number was added.

The consumer thread waits for the notification and then prints the first element of the queue to the console and deletes it.

I incremented a counter for every number that is added to the queue and another counter for every number that is taken out of the queue.

I expected the two counters to hold the same value after the program is finished, however the difference is huge. The counter that represents the addition to the queue is always in the million range (3871876 in my last test) and the counter that represents the consumer which takes numbers out of the queue is always below 100k (89993 in my last test).

Can someone explain to me why there is such a huge difference? Do I have to add another condition variable so that the producer threads waits for the consumer thread as well? Thanks!

解决方案

No need for a second std::condition_variable, just reuse the one you have. As mentioned by other you should consider using std::atomic<bool> instead of plain bool. But I must admit that g++ with -O3 does not optimize it away.

#include <iostream>
#include <thread>
#include <condition_variable>
#include <queue>
#include <cstdlib>
#include <chrono>
#include <ctime>
#include <random>
#include <atomic>

//counts every number that is added to the queue
static long long producer_count = 0;
//counts every number that is taken out of the queue
static long long consumer_count = 0;

void generateNumbers(std::queue<int> & numbers, std::condition_variable & cv, std::mutex & m, std::atomic<bool> & workdone)
{
    while(!workdone.load())
    {
        std::unique_lock<std::mutex> lk(m);
        int rndNum = rand() % 100;
        numbers.push(rndNum);
        producer_count++;
        cv.notify_one(); // Notify worker
        cv.wait(lk); // Wait for worker to complete
     }
}

void work(std::queue<int> & numbers, std::condition_variable & cv, std::mutex & m, std::atomic<bool> & workdone)
{
    while(!workdone.load())
    {
        std::unique_lock<std::mutex> lk(m);
        cv.notify_one(); // Notify generator (placed here to avoid waiting for the lock)
        cv.wait(lk); // Wait for the generator to complete
        std::cout << numbers.front() << std::endl;
        numbers.pop();
        consumer_count++;
     }
}

int main() {
    std::condition_variable cv;
    std::mutex m;
    std::atomic<bool> workdone(false);
    std::queue<int> numbers;

    //start threads
    std::thread producer(generateNumbers, std::ref(numbers), std::ref(cv), std::ref(m), std::ref(workdone));
    std::thread consumer(work, std::ref(numbers), std::ref(cv), std::ref(m), std::ref(workdone));


    //wait for 3 seconds, then join the threads
    std::this_thread::sleep_for(std::chrono::seconds(3));
    workdone = true;
    cv.notify_all(); // To prevent dead-lock

    producer.join();
    consumer.join();

    //output the counters
    std::cout << producer_count << std::endl;
    std::cout << consumer_count << std::endl;

    return 0;
}

EDIT:

To avoid the sporadic off-by-one error you could use this:

#include <iostream>
#include <thread>
#include <condition_variable>
#include <queue>
#include <cstdlib>
#include <chrono>
#include <ctime>
#include <random>
#include <atomic>

//counts every number that is added to the queue
static long long producer_count = 0;
//counts every number that is taken out of the queue
static long long consumer_count = 0;

void generateNumbers(std::queue<int> & numbers, std::condition_variable & cv, std::mutex & m, std::atomic<bool> & workdone)
{
    while(!workdone.load())
    {
        std::unique_lock<std::mutex> lk(m);
        int rndNum = rand() % 100;
        numbers.push(rndNum);
        producer_count++;
        cv.notify_one(); // Notify worker
        cv.wait(lk); // Wait for worker to complete
     }
}

void work(std::queue<int> & numbers, std::condition_variable & cv, std::mutex & m, std::atomic<bool> & workdone)
{
    while(!workdone.load() or !numbers.empty())
    {
        std::unique_lock<std::mutex> lk(m);
        cv.notify_one(); // Notify generator (placed here to avoid waiting for the lock)
        if (numbers.empty())
            cv.wait(lk); // Wait for the generator to complete
        if (numbers.empty())
            continue;
        std::cout << numbers.front() << std::endl;
        numbers.pop();
        consumer_count++;
     }
}

int main() {
    std::condition_variable cv;
    std::mutex m;
    std::atomic<bool> workdone(false);
    std::queue<int> numbers;

    //start threads
    std::thread producer(generateNumbers, std::ref(numbers), std::ref(cv), std::ref(m), std::ref(workdone));
    std::thread consumer(work, std::ref(numbers), std::ref(cv), std::ref(m), std::ref(workdone));


    //wait for 3 seconds, then join the threads
    std::this_thread::sleep_for(std::chrono::seconds(1));
    workdone = true;
    cv.notify_all(); // To prevent dead-lock

    producer.join();
    consumer.join();

    //output the counters
    std::cout << producer_count << std::endl;
    std::cout << consumer_count << std::endl;

    return 0;
}

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