pthread 实现线程同步功能的 Queue

这几天需要使用 pthread 实现了一个线程同步功能的 Queue，过程中踩了两个坑：

• pthread_cond_wait() 需要放在 pthread_mutex_lock() 和 pthread_mutex_unlock() 之中，而不是之外，否则将发生死锁，这里的概念有点绕，需要理解 pthread_cond_wait() 是会释放当前的 lock，以便其他线程进入临界区，当其他线程 pthread_cond_signal() 的时候，wait 线程被唤醒，又重新获得锁；

• pthread_cond_wait() 唤醒后需要再次条件判断，并且条件判断形式必须是 while 而不能是 if，之所以必须这样做的原因是pthread_cond_signal() 可能唤醒多个正处于 wait 状态的线程（多cpu情况），所以被唤醒的线程需要再次检测是否真有数据需要处理，如不需要处理应当继续进入 wait 以等待下次唤醒。

我将 Queue 实现成可支持一对一、一对多、多对一、多对多的线程同步机制，并写了一个简单的生产者消费者模型用以测试。完整程序如下，测试环境是 ubuntu 20.04：

/********************************************
* Queue with thread synchronization
* Copyright (C) i@foxzzz.com
* 
* Using pthread implementation.
* Can be used in the producer-consumer model
* of one-to-one, one-to-many, many-to-one,
* many-to-many patterns.
*********************************************/

#include <iostream>
#include <sstream>
#include <vector>
#include <cstdio>
#include <sys/time.h>
#include <pthread.h>
#include <unistd.h>

/*!
* @brief queue with thread synchronization
*/
template<typename T>
class Queue {
public:
    Queue(int capacity) :
        front_(0),
        back_(0),
        size_(0),
        capacity_(capacity),
        cond_send_(PTHREAD_COND_INITIALIZER),
        cond_receive_(PTHREAD_COND_INITIALIZER),
        mutex_(PTHREAD_MUTEX_INITIALIZER) {
        arr_ = new T[capacity_];
    }

    ~Queue() {
        delete[] arr_;
    }

public:
    /*!
    * @brief data entry queue
    * @param[in] data needs to be put into the queue
    */
    void send(const T& data) {
        pthread_mutex_lock(&mutex_);
        while (full()) {
            pthread_cond_wait(&cond_receive_, &mutex_);
        }
        enqueue(data);
        pthread_mutex_unlock(&mutex_);
        pthread_cond_signal(&cond_send_);
    }

    /*!
    * @brief retrieve data from the queue
    * @param[out] data retrieved from the queue
    */
    void receive(T& data) {
        pthread_mutex_lock(&mutex_);
        while (empty()) {
            pthread_cond_wait(&cond_send_, &mutex_);
        }
        dequeue(data);
        pthread_mutex_unlock(&mutex_);
        pthread_cond_signal(&cond_receive_);
    }

private:
    void enqueue(const T& data) {
        arr_[back_] = data;
        back_ = (back_ + 1) % capacity_;
        ++size_;
    }

    void dequeue(T& data) {
        data = arr_[front_];
        front_ = (front_ + 1) % capacity_;
        --size_;
    }

    bool full()const {
        return (size_ == capacity_);
    }

    bool empty()const {
        return (size_ == 0);
    }

private:
    T* arr_;
    int front_;
    int back_;
    int size_;
    int capacity_;
    pthread_cond_t cond_send_;
    pthread_cond_t cond_receive_;
    pthread_mutex_t mutex_;
};

/*!
* @brief a demonstration of queue operations
*/
template<typename T, typename Make>
class Demo {
public:
    Demo(int capacity) :
        queue_(capacity) {
        start();
    }

public:
    /*!
    * @brief generate the data and queue it(for producer thread)
    * @param[in] origin The starting value of the data
    * @param[in] count The amount of data to be generated
    * @param[in] interval the time interval(ms) to enter the queue
    */
    void send(int origin, int count, int interval) {
        Make make;
        while (count--) {
            T data = make(origin);
            queue_.send(data);
            print("send", data);
            usleep(interval * 1000);
        }
    }

    /*!
    * @brief retrieve data from the queue(for consumer thread)
    * @param[in] interval the time interval(ms) to enter the queue
    */
    void receive(int interval) {
        while (true) {
            T data;
            queue_.receive(data);
            print("receive", data);
            usleep(interval * 1000);
        }
    }

private:
    void print(const char* name, const T& data) {
        char buffer[256] = { 0 };
        sprintf(buffer, "[%-4lu ms][pid %lu][%-10s] ", elapsedMS(), pthread_self(), name);
        std::stringstream ss;
        ss << buffer;
        ss << data;
        ss << "\n";
        std::cout << ss.str();
    }

private:
    void start() {
        gettimeofday(&start_time_, nullptr);
    }

    long elapsedMS() const {
        struct timeval current;
        gettimeofday(&current, nullptr);
        return diffMS(start_time_, current);
    }

    static long diffMS(const timeval& start, const timeval& end) {
        long seconds = end.tv_sec - start.tv_sec;
        long useconds = end.tv_usec - start.tv_usec;
        return (long)(((double)(seconds) * 1000 + (double)(useconds) / 1000.0) + 0.5);
    }

private:
    timeval start_time_;
    Queue<T> queue_;
};

/*!
* @brief generates integer data
*/
class IntMake {
public:
    IntMake() : count_(0) {

    }

public:
    int operator() (int origin) {
        return (origin + count_++);
    }

private:
    int count_;
};

/*!
* @brief thread type
*/
enum {
    TYPE_THREAD_SEND,
    TYPE_THREAD_RECEIVE
};

/*!
* @brief thread arguments
*/
struct Args {
    Args(Demo<int, IntMake>& demo, int type, int interval) :
        demo(demo),
        type(type),
        interval(interval),
        origin(0),
        count(0) {

    }

    Args(Demo<int, IntMake>& demo, int type, int interval, int origin, int count) :
        demo(demo),
        type(type),
        interval(interval),
        origin(origin),
        count(count) {

    }

    Demo<int, IntMake>& demo;
    int type;
    int interval;
    int origin;
    int count;
};

/*!
* @brief thread info
*/
struct ThreadInfo {
    ThreadInfo(const Args& args) :
        tid(0),
        args(args) {

    }
    pthread_t tid;
    Args args;
};

/*!
* @brief producer thread function
*/
void* thread_func_send(void* arg) {
    Args* args = (Args*)arg;
    args->demo.send(args->origin, args->count, args->interval);
    return nullptr;
}

/*!
* @brief consumer thread function
*/
void* thread_func_receive(void* arg) {
    Args* args = (Args*)arg;
    args->demo.receive(args->interval);
    return nullptr;
}

/*!
* @brief start to work
*/
void work(std::vector<ThreadInfo>& list) {
    for (auto& it : list) {
        switch (it.args.type) {
        case TYPE_THREAD_SEND:
            pthread_create(&it.tid, nullptr, thread_func_send, &it.args);
            break;
        case TYPE_THREAD_RECEIVE:
            pthread_create(&it.tid, nullptr, thread_func_receive, &it.args);
            break;
        }
    }

    for (auto& it : list) {
        pthread_join(it.tid, nullptr);
    }
}

int main() {
    Demo<int, IntMake> demo(10);

    //configuration of the threads
    std::vector<ThreadInfo> list = {
        ThreadInfo(Args(demo, TYPE_THREAD_SEND, 2, 1, 50)),
        ThreadInfo(Args(demo, TYPE_THREAD_RECEIVE, 2)),
    };

    work(list);
    return 0;
}

程序配置了一个生产端线程，一个消费端线程，生产端和消费端效率都设置成2ms：

Demo<int, IntMake> demo(10);

//configuration of the threads
std::vector<ThreadInfo> list = {
    ThreadInfo(Args(demo, TYPE_THREAD_SEND, 2, 1, 50)),
    ThreadInfo(Args(demo, TYPE_THREAD_RECEIVE, 2)),
};

从程序打印输出结果看，生产端和消费端类似于回合制：

修改配置，降低消费端效率，由原来的2ms修改为20ms：

Demo<int, IntMake> demo(10);

//configuration of the threads
std::vector<ThreadInfo> list = {
    ThreadInfo(Args(demo, TYPE_THREAD_SEND, 2, 1, 50)),
    ThreadInfo(Args(demo, TYPE_THREAD_RECEIVE, 20)),
};

从程序打印输出结果看，当队列满后（队列容量设置为10），生产端需要等待消费端从队列中拿走数据后方可再生产：

修改配置，降低生产端效率，由原来的2ms修改为20ms：

Demo<int, IntMake> demo(10);

//configuration of the threads
std::vector<ThreadInfo> list = {
    ThreadInfo(Args(demo, TYPE_THREAD_SEND, 20, 1, 50)),
    ThreadInfo(Args(demo, TYPE_THREAD_RECEIVE, 2)),
};

从程序打印输出结果看，生产端和消费端又类似于回合制，这是因为消费端效率高，它得等到生产端生产：

修改配置，生产端数量增加到3，消费端数量不变，生产端效率是20ms，消费端效率是2ms：

Demo<int, IntMake> demo(10);

//configuration of the threads
std::vector<ThreadInfo> list = {
    ThreadInfo(Args(demo, TYPE_THREAD_SEND, 20, 1, 50)),
    ThreadInfo(Args(demo, TYPE_THREAD_SEND, 20, 51, 50)),
    ThreadInfo(Args(demo, TYPE_THREAD_SEND, 20, 101, 50)),
    ThreadInfo(Args(demo, TYPE_THREAD_RECEIVE, 2)),
};

从程序打印输出结果看，消费端效率还是远高于生产端，即便生产端有3个，但无法填满队列：

修改配置，生产端数量依然是3，消费端数量变为2，生产端效率是10ms，消费端效率是2ms：

Demo<int, IntMake> demo(10);

//configuration of the threads
std::vector<ThreadInfo> list = {
    ThreadInfo(Args(demo, TYPE_THREAD_SEND, 10, 1, 50)),
    ThreadInfo(Args(demo, TYPE_THREAD_SEND, 10, 51, 50)),
    ThreadInfo(Args(demo, TYPE_THREAD_SEND, 10, 101, 50)),
    ThreadInfo(Args(demo, TYPE_THREAD_RECEIVE, 2)),
    ThreadInfo(Args(demo, TYPE_THREAD_RECEIVE, 2)),
};

程序打印输出呈现多对多模式：

以上是几个示例的演示，配置比较简单，可按自己意思设计（创建生产端另外的两个参数一个是起始数值，一个是生产数量）。