C++解决方法：多线程同步经典案例之生产者消费者问题

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生产者消费者问题（英语：Producer-consumer problem），也称有限缓冲问题（英语：Bounded-buffer problem），是一个多线程同步问题的经典案例。该问题描述了共享固定大小缓冲区的两个线程——即所谓的“生产者”和“消费者”——在实际运行时会发生的问题。生产者的主要作用是生成一定量的数据放到缓冲区中，然后重复此过程。与此同时，消费者也在缓冲区消耗这些数据。该问题的关键就是要保证生产者不会在缓冲区满时加入数据，消费者也不会在缓冲区中空时消耗数据。
要解决该问题，就必须让生产者在缓冲区满时休眠（要么干脆就放弃数据），等到下次消费者消耗缓冲区中的数据的时候，生产者才能被唤醒，开始往缓冲区添加数据。同样，也可以让消费者在缓冲区空时进入休眠，等到生产者往缓冲区添加数据之后，再唤醒消费者。

本文用一个ItemRepository类表示产品仓库，其中包含一个数组和两个坐标表示的环形队列、一个std::mutex成员、用来保证每次只被一个线程读写操作（为了保证打印出来的消息是一行一行的，在它空闲的时候也借用的这个互斥量╮(╯▽╰)╭）、两个std::condition_variable表示队列不满和不空的状态，进而保证生产的时候不满，消耗的时候不空。

#pragma once#include <chrono>//std::chrono#include <mutex>//std::mutex,std::unique_lock,std::lock_guard#include <thread>//std::thread#include <condition_variable>//std::condition_variable#include <iostream>//std::cout,std::endl#include <map>//std::mapnamespace MyProducerToConsumer {    static const int gRepositorySize = 10;//total size of the repository    static const int gItemNum = 97;//number of products to produce    std::mutex produce_mtx, consume_mtx;//mutex for all the producer thread or consumer thread    std::map<std::thread::id, int> threadPerformance;//records of every thread's producing/consuming number    struct ItemRepository {//repository class        int m_ItemBuffer[gRepositorySize];//Repository itself (as a circular queue)        int m_ProducePos;//rear position of circular queue        int m_ConsumePos;//head position of circular queue        std::mutex m_mtx;//mutex for operating the repository        std::condition_variable m_RepoUnfull;//indicating that this repository is unfull(then producers can produce items)        std::condition_variable m_RepoUnempty;//indicating that this repository is unempty(then consumers can produce items)    }gItemRepo;    void ProduceItem(ItemRepository *ir, int item) {        std::unique_lock <std::mutex>ulk(ir->m_mtx);        while ((ir->m_ProducePos + 1) % gRepositorySize == ir->m_ConsumePos) {//full(spare one slot for indicating)            std::cout << "Reposity is full. Waiting for consumers..." << std::endl;            ir->m_RepoUnfull.wait(ulk);//unlocking ulk and waiting for unfull condition        }        //when unfull        ir->m_ItemBuffer[ir->m_ProducePos++] = item;//procude and shift        std::cout << "Item No." << item << " produced successfully by "            <<std::this_thread::get_id()<<"!" << std::endl;        threadPerformance[std::this_thread::get_id()]++;        if (ir->m_ProducePos == gRepositorySize)//loop            ir->m_ProducePos = 0;        ir->m_RepoUnempty.notify_all();//item produced, so it's unempty; notify all consumers    }    int ConsumeItem(ItemRepository *ir) {        std::unique_lock<std::mutex>ulk(ir->m_mtx);        while (ir->m_ConsumePos == ir->m_ProducePos) {//empty            std::cout << "Repository is empty.Waiting for producing..." << std::endl;            ir->m_RepoUnempty.wait(ulk);        }        int item = ir->m_ItemBuffer[ir->m_ConsumePos++];        std::cout << "Item No." << item << " consumed successfully by "            <<std::this_thread::get_id()<<"!" << std::endl;        threadPerformance[std::this_thread::get_id()]++;        if (ir->m_ConsumePos == gRepositorySize)            ir->m_ConsumePos = 0;        ir->m_RepoUnfull.notify_all();//item consumed, so it's unempty; notify all consumers        return item;    }    void ProducerThread() {        static int produced = 0;//static variable to indicate the number of produced items        while (1) {            std::this_thread::sleep_for(std::chrono::milliseconds(10));//sleep long enough in case it runs too fast for other threads to procude            std::lock_guard<std::mutex>lck(produce_mtx);//auto unlock when break            produced++;            if (produced > gItemNum)break;            gItemRepo.m_mtx.lock();            std::cout << "Producing item No." << produced << "..." << std::endl;            gItemRepo.m_mtx.unlock();            ProduceItem(&gItemRepo, produced);        }        gItemRepo.m_mtx.lock();        std::cout << "Producer thread " << std::this_thread::get_id()            << " exited." << std::endl;        gItemRepo.m_mtx.unlock();    }    void ConsumerThread() {        static int consumed = 0;        while (1) {            std::this_thread::sleep_for(std::chrono::milliseconds(10));            std::lock_guard<std::mutex>lck(consume_mtx);            consumed++;            if (consumed > gItemNum)break;            gItemRepo.m_mtx.lock();            std::cout << "Consuming item available..." << std::endl;            gItemRepo.m_mtx.unlock();            ConsumeItem(&gItemRepo);        }        gItemRepo.m_mtx.lock();        std::cout << "Consumer thread " << std::this_thread::get_id()            << " exited." << std::endl;        gItemRepo.m_mtx.unlock();    }    void InitItemRepository(ItemRepository* ir) {        ir->m_ConsumePos = 0;        ir->m_ProducePos = 0;    }    void Run() {        InitItemRepository(&gItemRepo);        std::thread thdConsume[11];        std::thread thdProduce[11];        for (auto& t : thdConsume)t = std::thread(ConsumerThread);        for (auto& t : thdProduce)t = std::thread(ProducerThread);        for (auto& t : thdConsume)t.join();        for (auto& t : thdProduce)t.join();        for (auto& iter : threadPerformance)cout << iter.first << ":" << iter.second << endl;    }}

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