前面八章介绍了 C++11 并发编程的基础(抱歉哈，第五章-第八章还在草稿中)，本文将综合运用 C++11 中的新的基础设施(主要是多线程、锁、条件变量)来阐述一个经典问题——生产者消费者模型，并给出完整的解决方案。
生产者消费者问题是多线程并发中一个非常经典的问题，相信学过操作系统课程的同学都清楚这个问题的根源。本文将就四种情况分析并介绍生产者和消费者问题，它们分别是：单生产者-单消费者模型，单生产者-多消费者模型，多生产者-单消费者模型，多生产者-多消费者模型，我会给出四种情况下的 C++11 并发解决方案，如果文中出现了错误或者你对代码有异议，欢迎交流 ;-)。
单生产者-单消费者模型
顾名思义，单生产者-单消费者模型中只有一个生产者和一个消费者，生产者不停地往产品库中放入产品，消费者则从产品库中取走产品，产品库容积有限制，只能容纳一定数目的产品，如果生产者生产产品的速度过快，则需要等待消费者取走产品之后，产品库不为空才能继续往产品库中放置新的产品，相反，如果消费者取走产品的速度过快，则可能面临产品库中没有产品可使用的情况，此时需要等待生产者放入一个产品后，消费者才能继续工作。C++11实现单生产者单消费者模型的代码如下：
#include <unistd.h> #include <cstdlib>
#include <condition_variable>
#include <iostream>
#include <mutex>
#include <thread> static const int kItemRepositorySize = ; // Item buffer size.
static const int kItemsToProduce = ; // How many items we plan to produce. struct ItemRepository {
int item_buffer[kItemRepositorySize]; // 产品缓冲区, 配合 read_position 和 write_position 模型环形队列.
size_t read_position; // 消费者读取产品位置.
size_t write_position; // 生产者写入产品位置.
std::mutex mtx; // 互斥量,保护产品缓冲区
std::condition_variable repo_not_full; // 条件变量, 指示产品缓冲区不为满.
std::condition_variable repo_not_empty; // 条件变量, 指示产品缓冲区不为空.
} gItemRepository; // 产品库全局变量, 生产者和消费者操作该变量. typedef struct ItemRepository ItemRepository; void ProduceItem(ItemRepository *ir, int item)
{
std::unique_lock<std::mutex> lock(ir->mtx);
while(((ir->write_position + ) % kItemRepositorySize)
== ir->read_position) { // item buffer is full, just wait here.
std::cout << "Producer is waiting for an empty slot...\n";
(ir->repo_not_full).wait(lock); // 生产者等待"产品库缓冲区不为满"这一条件发生.
} (ir->item_buffer)[ir->write_position] = item; // 写入产品.
(ir->write_position)++; // 写入位置后移. if (ir->write_position == kItemRepositorySize) // 写入位置若是在队列最后则重新设置为初始位置.
ir->write_position = ; (ir->repo_not_empty).notify_all(); // 通知消费者产品库不为空.
lock.unlock(); // 解锁.
} int ConsumeItem(ItemRepository *ir)
{
int data;
std::unique_lock<std::mutex> lock(ir->mtx);
// item buffer is empty, just wait here.
while(ir->write_position == ir->read_position) {
std::cout << "Consumer is waiting for items...\n";
(ir->repo_not_empty).wait(lock); // 消费者等待"产品库缓冲区不为空"这一条件发生.
} data = (ir->item_buffer)[ir->read_position]; // 读取某一产品
(ir->read_position)++; // 读取位置后移 if (ir->read_position >= kItemRepositorySize) // 读取位置若移到最后，则重新置位.
ir->read_position = ; (ir->repo_not_full).notify_all(); // 通知消费者产品库不为满.
lock.unlock(); // 解锁. return data; // 返回产品.
} void ProducerTask() // 生产者任务
{
for (int i = ; i <= kItemsToProduce; ++i) {
// sleep(1);
std::cout << "Produce the " << i << "^th item..." << std::endl;
ProduceItem(&gItemRepository, i); // 循环生产 kItemsToProduce 个产品.
}
} void ConsumerTask() // 消费者任务
{
static int cnt = ;
while() {
sleep();
int item = ConsumeItem(&gItemRepository); // 消费一个产品.
std::cout << "Consume the " << item << "^th item" << std::endl;
if (++cnt == kItemsToProduce) break; // 如果产品消费个数为 kItemsToProduce, 则退出.
}
} void InitItemRepository(ItemRepository *ir)
{
ir->write_position = ; // 初始化产品写入位置.
ir->read_position = ; // 初始化产品读取位置.
} int main()
{
InitItemRepository(&gItemRepository);
std::thread producer(ProducerTask); // 创建生产者线程.
std::thread consumer(ConsumerTask); // 创建消费之线程.
producer.join();
consumer.join();
}
单生产者-多消费者模型
与单生产者和单消费者模型不同的是，单生产者-多消费者模型中可以允许多个消费者同时从产品库中取走产品。所以除了保护产品库在多个读写线程下互斥之外，还需要维护消费者取走产品的计数器，代码如下:
#include <unistd.h> #include <cstdlib>
#include <condition_variable>
#include <iostream>
#include <mutex>
#include <thread> static const int kItemRepositorySize = ; // Item buffer size.
static const int kItemsToProduce = ; // How many items we plan to produce. struct ItemRepository {
int item_buffer[kItemRepositorySize];
size_t read_position;
size_t write_position;
size_t item_counter;
std::mutex mtx;
std::mutex item_counter_mtx;
std::condition_variable repo_not_full;
std::condition_variable repo_not_empty;
} gItemRepository; typedef struct ItemRepository ItemRepository; void ProduceItem(ItemRepository *ir, int item)
{
std::unique_lock<std::mutex> lock(ir->mtx);
while(((ir->write_position + ) % kItemRepositorySize)
== ir->read_position) { // item buffer is full, just wait here.
std::cout << "Producer is waiting for an empty slot...\n";
(ir->repo_not_full).wait(lock);
} (ir->item_buffer)[ir->write_position] = item;
(ir->write_position)++; if (ir->write_position == kItemRepositorySize)
ir->write_position = ; (ir->repo_not_empty).notify_all();
lock.unlock();
} int ConsumeItem(ItemRepository *ir)
{
int data;
std::unique_lock<std::mutex> lock(ir->mtx);
// item buffer is empty, just wait here.
while(ir->write_position == ir->read_position) {
std::cout << "Consumer is waiting for items...\n";
(ir->repo_not_empty).wait(lock);
} data = (ir->item_buffer)[ir->read_position];
(ir->read_position)++; if (ir->read_position >= kItemRepositorySize)
ir->read_position = ; (ir->repo_not_full).notify_all();
lock.unlock(); return data;
} void ProducerTask()
{
for (int i = ; i <= kItemsToProduce; ++i) {
// sleep(1);
std::cout << "Producer thread " << std::this_thread::get_id()
<< " producing the " << i << "^th item..." << std::endl;
ProduceItem(&gItemRepository, i);
}
std::cout << "Producer thread " << std::this_thread::get_id()
<< " is exiting..." << std::endl;
} void ConsumerTask()
{
bool ready_to_exit = false;
while() {
sleep();
std::unique_lock<std::mutex> lock(gItemRepository.item_counter_mtx);
if (gItemRepository.item_counter < kItemsToProduce) {
int item = ConsumeItem(&gItemRepository);
++(gItemRepository.item_counter);
std::cout << "Consumer thread " << std::this_thread::get_id()
<< " is consuming the " << item << "^th item" << std::endl;
} else ready_to_exit = true;
lock.unlock();
if (ready_to_exit == true) break;
}
std::cout << "Consumer thread " << std::this_thread::get_id()
<< " is exiting..." << std::endl;
} void InitItemRepository(ItemRepository *ir)
{
ir->write_position = ;
ir->read_position = ;
ir->item_counter = ;
} int main()
{
InitItemRepository(&gItemRepository);
std::thread producer(ProducerTask);
std::thread consumer1(ConsumerTask);
std::thread consumer2(ConsumerTask);
std::thread consumer3(ConsumerTask);
std::thread consumer4(ConsumerTask); producer.join();
consumer1.join();
consumer2.join();
consumer3.join();
consumer4.join();
}
多生产者-单消费者模型
与单生产者和单消费者模型不同的是，多生产者-单消费者模型中可以允许多个生产者同时向产品库中放入产品。所以除了保护产品库在多个读写线程下互斥之外，还需要维护生产者放入产品的计数器，代码如下:
#include <unistd.h> #include <cstdlib>
#include <condition_variable>
#include <iostream>
#include <mutex>
#include <thread> static const int kItemRepositorySize = ; // Item buffer size.
static const int kItemsToProduce = ; // How many items we plan to produce. struct ItemRepository {
int item_buffer[kItemRepositorySize];
size_t read_position;
size_t write_position;
size_t item_counter;
std::mutex mtx;
std::mutex item_counter_mtx;
std::condition_variable repo_not_full;
std::condition_variable repo_not_empty;
} gItemRepository; typedef struct ItemRepository ItemRepository; void ProduceItem(ItemRepository *ir, int item)
{
std::unique_lock<std::mutex> lock(ir->mtx);
while(((ir->write_position + ) % kItemRepositorySize)
== ir->read_position) { // item buffer is full, just wait here.
std::cout << "Producer is waiting for an empty slot...\n";
(ir->repo_not_full).wait(lock);
} (ir->item_buffer)[ir->write_position] = item;
(ir->write_position)++; if (ir->write_position == kItemRepositorySize)
ir->write_position = ; (ir->repo_not_empty).notify_all();
lock.unlock();
} int ConsumeItem(ItemRepository *ir)
{
int data;
std::unique_lock<std::mutex> lock(ir->mtx);
// item buffer is empty, just wait here.
while(ir->write_position == ir->read_position) {
std::cout << "Consumer is waiting for items...\n";
(ir->repo_not_empty).wait(lock);
} data = (ir->item_buffer)[ir->read_position];
(ir->read_position)++; if (ir->read_position >= kItemRepositorySize)
ir->read_position = ; (ir->repo_not_full).notify_all();
lock.unlock(); return data;
} void ProducerTask()
{
bool ready_to_exit = false;
while() {
sleep();
std::unique_lock<std::mutex> lock(gItemRepository.item_counter_mtx);
if (gItemRepository.item_counter < kItemsToProduce) {
++(gItemRepository.item_counter);
ProduceItem(&gItemRepository, gItemRepository.item_counter);
std::cout << "Producer thread " << std::this_thread::get_id()
<< " is producing the " << gItemRepository.item_counter
<< "^th item" << std::endl;
} else ready_to_exit = true;
lock.unlock();
if (ready_to_exit == true) break;
}
std::cout << "Producer thread " << std::this_thread::get_id()
<< " is exiting..." << std::endl;
} void ConsumerTask()
{
static int item_consumed = ;
while() {
sleep();
++item_consumed;
if (item_consumed <= kItemsToProduce) {
int item = ConsumeItem(&gItemRepository);
std::cout << "Consumer thread " << std::this_thread::get_id()
<< " is consuming the " << item << "^th item" << std::endl;
} else break;
}
std::cout << "Consumer thread " << std::this_thread::get_id()
<< " is exiting..." << std::endl;
} void InitItemRepository(ItemRepository *ir)
{
ir->write_position = ;
ir->read_position = ;
ir->item_counter = ;
} int main()
{
InitItemRepository(&gItemRepository);
std::thread producer1(ProducerTask);
std::thread producer2(ProducerTask);
std::thread producer3(ProducerTask);
std::thread producer4(ProducerTask);
std::thread consumer(ConsumerTask); producer1.join();
producer2.join();
producer3.join();
producer4.join();
consumer.join();
}
多生产者-多消费者模型
该模型可以说是前面两种模型的综合，程序需要维护两个计数器，分别是生产者已生产产品的数目和消费者已取走产品的数目。另外也需要保护产品库在多个生产者和多个消费者互斥地访问。
代码如下：
#include <unistd.h> #include <cstdlib>
#include <condition_variable>
#include <iostream>
#include <mutex>
#include <thread> static const int kItemRepositorySize = ; // Item buffer size.
static const int kItemsToProduce = ; // How many items we plan to produce. struct ItemRepository {
int item_buffer[kItemRepositorySize];
size_t read_position;
size_t write_position;
size_t produced_item_counter;
size_t consumed_item_counter;
std::mutex mtx;
std::mutex produced_item_counter_mtx;
std::mutex consumed_item_counter_mtx;
std::condition_variable repo_not_full;
std::condition_variable repo_not_empty;
} gItemRepository; typedef struct ItemRepository ItemRepository; void ProduceItem(ItemRepository *ir, int item)
{
std::unique_lock<std::mutex> lock(ir->mtx);
while(((ir->write_position + ) % kItemRepositorySize)
== ir->read_position) { // item buffer is full, just wait here.
std::cout << "Producer is waiting for an empty slot...\n";
(ir->repo_not_full).wait(lock);
} (ir->item_buffer)[ir->write_position] = item;
(ir->write_position)++; if (ir->write_position == kItemRepositorySize)
ir->write_position = ; (ir->repo_not_empty).notify_all();
lock.unlock();
} int ConsumeItem(ItemRepository *ir)
{
int data;
std::unique_lock<std::mutex> lock(ir->mtx);
// item buffer is empty, just wait here.
while(ir->write_position == ir->read_position) {
std::cout << "Consumer is waiting for items...\n";
(ir->repo_not_empty).wait(lock);
} data = (ir->item_buffer)[ir->read_position];
(ir->read_position)++; if (ir->read_position >= kItemRepositorySize)
ir->read_position = ; (ir->repo_not_full).notify_all();
lock.unlock(); return data;
} void ProducerTask()
{
bool ready_to_exit = false;
while() {
sleep();
std::unique_lock<std::mutex> lock(gItemRepository.produced_item_counter_mtx);
if (gItemRepository.produced_item_counter < kItemsToProduce) {
++(gItemRepository.produced_item_counter);
ProduceItem(&gItemRepository, gItemRepository.produced_item_counter);
std::cout << "Producer thread " << std::this_thread::get_id()
<< " is producing the " << gItemRepository.produced_item_counter
<< "^th item" << std::endl;
} else ready_to_exit = true;
lock.unlock();
if (ready_to_exit == true) break;
}
std::cout << "Producer thread " << std::this_thread::get_id()
<< " is exiting..." << std::endl;
} void ConsumerTask()
{
bool ready_to_exit = false;
while() {
sleep();
std::unique_lock<std::mutex> lock(gItemRepository.consumed_item_counter_mtx);
if (gItemRepository.consumed_item_counter < kItemsToProduce) {
int item = ConsumeItem(&gItemRepository);
++(gItemRepository.consumed_item_counter);
std::cout << "Consumer thread " << std::this_thread::get_id()
<< " is consuming the " << item << "^th item" << std::endl;
} else ready_to_exit = true;
lock.unlock();
if (ready_to_exit == true) break;
}
std::cout << "Consumer thread " << std::this_thread::get_id()
<< " is exiting..." << std::endl;
} void InitItemRepository(ItemRepository *ir)
{
ir->write_position = ;
ir->read_position = ;
ir->produced_item_counter = ;
ir->consumed_item_counter = ;
} int main()
{
InitItemRepository(&gItemRepository);
std::thread producer1(ProducerTask);
std::thread producer2(ProducerTask);
std::thread producer3(ProducerTask);
std::thread producer4(ProducerTask); std::thread consumer1(ConsumerTask);
std::thread consumer2(ConsumerTask);
std::thread consumer3(ConsumerTask);
std::thread consumer4(ConsumerTask); producer1.join();
producer2.join();
producer3.join();
producer4.join(); consumer1.join();
consumer2.join();
consumer3.join();
consumer4.join();
}
另外，所有例子的代码(包括前面一些指南的代码均放在github上)，希望对大家学习 C++11 多线程并发有所帮助。