C++11 condition_variable条件变量的用法说明

目录

1 什么是条件变量

2 condition_variable类定义

2.1 wait函数

3 condition_variable用法

3.1 资源修改线程步骤

3.2 资源等待线程步骤

4 代码示例

4.1 无需notify场景

4.2 正常应用场景1

4.3 正常应用场景2

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:

以上为个人经验,希望能给大家一个参考,也希望大家多多支持易知道(ezd.cc)。 

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