c ++应该在锁定下通知条件变量

Question

我在www.cppreference.com上找到了以下关于条件变量的示例,http: //en.cppreference.com/w/cpp/thread/condition_variable .对cv.notify_one()的调用放在锁外.我的问题是,如果在保持锁定的同时进行调用以保证等待线程实际上处于等待状态并且将接收通知信号.

#include <iostream>
#include <string>
#include <thread>
#include <mutex>
#include <condition_variable>

std::mutex m;
std::condition_variable cv;
std::string data;
bool ready = false;
bool processed = false;

void worker_thread()
{
    // Wait until main() sends data
    std::unique_lock<std::mutex> lk(m);
    cv.wait(lk, []{return ready;});

    // after the wait, we own the lock.
    std::cout << "Worker thread is processing data\n";
    data += " after processing";

    // Send data back to main()
    processed = true;
    std::cout << "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();
    cv.notify_one();
}

int main()
{
    std::thread worker(worker_thread);

    data = "Example data";
    // send data to the worker thread
    {
        std::lock_guard<std::mutex> lk(m);
        ready = true;
        std::cout << "main() signals data ready for processing\n";
    }
    cv.notify_one();

    // wait for the worker
    {
        std::unique_lock<std::mutex> lk(m);
        cv.wait(lk, []{return processed;});
    }
    std::cout << "Back in main(), data = " << data << '\n';

    worker.join();
}

应该在锁内移动notify_one()调用以保证等待线程接收通知信号,

// send data to the worker thread
{
    std::lock_guard<std::mutex> lk(m);
    ready = true;
    cv.notify_one();
    std::cout << "main() signals data ready for processing\n";
}

Answer 1

您无需在锁定下通知.但是,由于在实际值发生更改时逻辑上发生了通知(否则,为什么要通知？)并且该更改必须在锁定下进行,通常在锁定内完成.

没有实际可观察​​到的差异.

Answer 2

如果我正确理解你的问题,它就等于"如果通知程序线程在尝试通知其他线程中的某些CV时锁定互斥锁"

不,它不是强制性的,甚至会产生一些反作用.
当condition_variable从另一个线程通知它时,它试图重新锁定它被置于睡眠状态的互斥锁.从其他线程锁定该互斥锁将阻止另一个试图锁定它的其他线程,直到该锁定包装器超出范围.

PS
如果你确实从发送数据到工作线程的函数中删除锁定,ready并且processed至少应该是原子.目前它们是通过锁同步的,但是当你移除锁时它们就不再是线程安全的了