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C++11 condition_variable条件变量的用法说明

2024-04-02 19:55

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1 什么是条件变量

condition_variable是一个类，常和mutex搭配使用。

condition_variable类是一个同步原语，可用于阻塞一个线程或同时阻止多个线程，直到另一个线程修改共享变量并通知condition_variable。

防止多线程场景下，共享变量混乱。

理解条件变量要先理解三个概念：

锁（锁住共享变量，线程独占）
wait 等待 (等待通知条件变量，变化的共享变量是否满足条件)
notify 通知（通知等待的条件变量，共享变量发送变化）

2 condition_variable类定义

2.1 wait函数

void wait( std::unique_lockstd::mutex& lock );
//Predicate是lambda表达式。
template< class Predicate >
void wait( std::unique_lockstd::mutex& lock, Predicate pred );
//以上二者都被notify_one())或notify_broadcast()唤醒，但是
//第二种方式是唤醒后也要满足Predicate的条件。
//如果不满足条件，继续解锁互斥量，然后让线程处于阻塞或等待状态。
//第二种等价于
while (!pred())
{
wait(lock);
}

3 condition_variable用法

condition_variable必定至少有两方，一方是资源修改线程，一方是资源等待线程。就跟打篮球一样，同时篮球只会在一个人手中，投篮后就释放了篮球所有权，其他方就会抢夺篮球所有权。

3.1 资源修改线程步骤

获取一个mutex使用 std::unique_lock< std::mutex >
保持锁定状态，修改共享变量
condition_variable对象执行notify_one或者notify_all（notify_one/notify_all执行前可以释放锁）

3.2 资源等待线程步骤

获取一个mutex使用 std::unique_lock< std::mutex > unlock用于保护要修改的共享变量
检查条件变量，

(1)条件变量满足，线程继续执行

(2)条件变量不满足，wait会释放unlock锁，并挂起线程。

当notify通知条件变量、超时过期或发生虚假唤醒时，线程被唤醒，互斥锁unlock被原子地重新获取。然后，线程应该检查条件，如果唤醒是假的，则继续等待

4 代码示例

4.1 无需notify场景

当wait第一次执行是，条件已经满足，则程序不会阻塞（即无需notify），会直接向下执行。(仅为说明3.2 中第2点（1）的情况)

#include <iostream>
#include <string>
#include <thread>
#include <mutex>
#include <condition_variable>
using namespace std;
std::mutex m;
std::condition_variable cv;
std::string data;
bool ready = false;
bool processed = false;
 
void worker_thread()
{
    std::cout << "3、worker_thread子线程开始执行"  << endl;
    // Wait until main() sends data
    std::unique_lock<std::mutex> lk(m);
    std::cout << "4、worker_thread子线程获取到锁,条件满足无需notify，不阻塞向下执行"  << endl;
    cv.wait(lk, []{return ready;});
 
    // after the wait, we own the lock.
    data += " after processing";
    // Send data back to main()
    processed = true;
    std::cout << "5、Worker thread signals data processing completed\n";
 
    // Manual unlocking is done before notifying, to avoid waking up
    // the waiting thread only to block again (see notify_one for details)
    lk.unlock();
    std::cout << "6、worker_thread子线程交出执行权限，主线程执行"  << endl;
    std::this_thread::sleep_for(std::chrono::milliseconds(2000));
    
    cv.notify_one();
    std::cout << "9、worker_thread调用 notify_one"  << endl;
}
int main()
{
    std::thread worker(worker_thread);
    std::cout << "1、主线程开始执行"  << std::endl;
    data = "Example data";
    // send data to the worker thread
    {
        //std::this_thread::sleep_for(std::chrono::milliseconds(1000));
        std::lock_guard<std::mutex> lk(m);
        ready = true;
    }
    std::cout << "2、锁已经释放了，主线程休眠，子线程执行"  << std::endl;
    std::this_thread::sleep_for(std::chrono::milliseconds(1000));
    //cv.notify_one();
    {
        std::cout << "7、主线程data：" << data << endl;
        std::unique_lock<std::mutex> lk(m);
        std::cout << "8、主线程条件满足无需notify" << endl;
        cv.wait(lk, []{return processed;});
    }
    
    worker.join();
     std::cout << "10、主线程结束" << endl;
}

执行结果：

4.2 正常应用场景1

#include <iostream>
#include <string>
#include <thread>
#include <mutex>
#include <condition_variable>
using namespace std;
std::mutex m;
std::condition_variable cv;
std::string data;
bool ready = false;
bool processed = false;
 
void worker_thread()
{
    std::cout << "3、worker_thread子线程开始执行"  << endl;
    // Wait until main() sends data
    std::unique_lock<std::mutex> lk(m);
    std::cout << "4、worker_thread子线程获取到锁,条件不满足，释放lk锁，子线程阻塞"  << endl;
    cv.wait(lk, []{return ready;});
    std::cout << "8、worker_thread子线程获取到锁,子线程继续执行"  << endl;
    // after the wait, we own the lock.
    data += " after processing";
    // Send data back to main()
    processed = true;
    std::cout << "9、Worker thread signals data processing completed\n";
 
    // Manual unlocking is done before notifying, to avoid waking up
    // the waiting thread only to block again (see notify_one for details)
    lk.unlock();
    std::this_thread::sleep_for(std::chrono::milliseconds(5000));
    std::cout << "10、worker_thread调用 notify_one通知主线程执行"  << endl;
    cv.notify_one();
}
int main()
{
    std::thread worker(worker_thread);
    std::cout << "1、主线程开始执行"  << std::endl;
    data = "Example data";
    // send data to the worker thread
    {
        std::cout << "2、主线程休眠，子线程进入执行"  << std::endl;
        std::this_thread::sleep_for(std::chrono::milliseconds(1000));
        std::cout << "5、主线程结束休眠，主线程获取lk锁，进入执行"  << std::endl;
        std::lock_guard<std::mutex> lk(m);
        ready = true;
        
    }
    std::cout << "6、主线程释放lk,调用notify通知子线程"  << std::endl;
    cv.notify_one();
    {
        std::cout << "7、由于主线程的执行时钟周期未结束，继续执行主线程获取lk, wait检查条件不满足,释放锁" << endl;
        std::unique_lock<std::mutex> lk(m);
        cv.wait(lk, []{return processed;});
    }
   
    worker.join();
     std::cout << "11、主线程结束" << endl;
}

执行结果：

这里notify执行后不一定立即执行子线程，如果cpu执行时钟周期未结束，则主线程会继续执行. 所以7,8，9,10顺序可能变化参见4.3

同时4.1也会因为cpu时钟周期，执行顺序有所变动。

4.3 正常应用场景2

#include <iostream>
#include <string>
#include <thread>
#include <mutex>
#include <condition_variable>
using namespace std;
std::mutex m;
std::condition_variable cv;
std::string data;
bool ready = false;
bool processed = false;
 
void worker_thread()
{
    std::cout << "3、worker_thread子线程开始执行"  << endl;
    // Wait until main() sends data
    std::unique_lock<std::mutex> lk(m);
    std::cout << "4、worker_thread子线程获取到锁,条件不满足，释放lk锁，子线程阻塞"  << endl;
    cv.wait(lk, []{return ready;});
    std::cout << "8、worker_thread子线程获取到锁,子线程继续执行"  << endl;
    // after the wait, we own the lock.
    data += " after processing";
    // Send data back to main()
    processed = true;
    std::cout << "9、Worker thread signals data processing completed\n";
 
    // Manual unlocking is done before notifying, to avoid waking up
    // the waiting thread only to block again (see notify_one for details)
    lk.unlock();
    std::cout << "10、worker_thread调用 notify_one通知主线程执行"  << endl;
    cv.notify_one();
}
int main()
{
    std::thread worker(worker_thread);
    std::cout << "1、主线程开始执行"  << std::endl;
    data = "Example data";
    // send data to the worker thread
    {
        std::cout << "2、主线程休眠，子线程进入执行"  << std::endl;
        std::this_thread::sleep_for(std::chrono::milliseconds(1000));
        std::cout << "5、主线程结束休眠，主线程获取lk锁，进入执行"  << std::endl;
        std::lock_guard<std::mutex> lk(m);
        ready = true;
        
    }
    std::cout << "6、主线程释放lk,调用notify通知子线程"  << std::endl;
    cv.notify_one();
    {
        for(int i = 0; i< 10000000; i++)
        {
            int j = i;
        }
        std::cout << "7、由于主线程的执行时钟周期未结束，继续执行主线程获取lk, wait检查条件不满足,释放锁" << endl;
        std::unique_lock<std::mutex> lk(m);
        cv.wait(lk, []{return processed;});
    }
    
    worker.join();
    std::cout << "11、主线程结束" << endl;
}

执行结果：