C++ condition_variable用法

article/2025/10/28 16:18:36

概述

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

函数接口：
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

代码

#include<iostream>  
#include<thread>  
#include<mutex>  
#include<condition_variable>  
#include<chrono>
using namespace std;
mutex m;
condition_variable cond;
int LOOP = 10;
int flag = 0;void fun(int id) {for (int i = 0; i < LOOP; i++) {unique_lock<mutex> lk(m);	//加锁//写法1，while循环比较，多次唤醒时，只要不满足条件就阻塞，if只判断一次会出错/*while (id != flag)cond.wait(lk);*///写法2，实现原理和上面一样 ，id != flag时会阻塞，唤醒时继续判断，id == flag才会唤醒成功cond.wait(lk, [=]() {return id == flag;});cout << (char)('A' + id) << " ";flag = (flag + 1) % 3;cond.notify_all();}
}
int main() {thread A(fun, 0);thread B(fun, 1);thread C(fun, 2);A.join();B.join();C.join();cout << endl;cout << "main end" << endl;return 0;
}

测试结果：
在这里插入图片描述

semaphore源码

#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;
};