条件变量condition_variable用法学习

转自：https://www.cnblogs.com/fenghualong/p/13855360.html

1.介绍

 condition_variable类似于信号量机制，实现了线程的等待和唤醒。

• wait() :阻塞等待的同时释放锁（原子操作），还可以添加阻塞判断函数，详见代码
• notify_all() : 唤醒所有阻塞等待的线程
• notify_one(): 唤醒某一个等待的线程

2.例子

锁+条件变量 实现等待唤醒。

#include <condition_variable>
#include <mutex>
#include <thread>std::mutex mylock;
std::condition_variable condVar;bool dataReady{false};void waitingForWork() {std::cout << "Waiting ..." << std::endl;std::unique_lock<std::mutex> l(mylock);  // 加了unique_lock锁condVar.wait(l, []{return dataReady;});           // (4)std::cout << "Running ..." << std::endl;
}void setDataReady() {{std::lock_guard<std::mutex> l{mylock};  // 加了lock_guard锁dataReady = true;}std::cout << "Data prepared, notify one" << std::endl;condVar.notify_one();                             // (3)
}int main(int argc, char const *argv[])
{std::cout << "==========Begin==========" << std::endl;std::thread t1(waitingForWork);                    // (1)std::thread t2(setDataReady);                      // (2)
t1.join();t2.join();std::cout << "===========End===========" << std::endl;cout<<endl;return 0;
}
// 运行结果
-> % ./main
==========Begin==========
Waiting ...
Data prepared, notify one
Running ...
===========End===========

如果不用锁和等待条件控制，可能会导致虚假唤醒和唤醒丢失问题。

3.用条件变量实现信号量

https://blog.csdn.net/qq_24447809/article/details/115906017

#pragma once
#include<mutex>
#include<condition_variable>
class semaphore {
public:semaphore(long count = 0) :count(count) {}void wait() {std::unique_lock<std::mutex>lock(mx);cond.wait(lock, [&]() {return count > 0; });--count;}void signal() {std::unique_lock<std::mutex>lock(mx);++count;cond.notify_one();}private:std::mutex mx;std::condition_variable cond;long count;
};