c++ - std::condition_variable和std::mutex到底如何工作？

Question

我正在阅读this std::condition_variable示例:

#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();
}

让我们专注于开始:

std::thread worker(worker_thread);

在这里，我们开始工作，它将立即锁定互斥锁:

std::unique_lock<std::mutex> lk(m);
cv.wait(lk, []{return ready;});

然后，将互斥锁锁定在main上以更改ready的值

{
    std::lock_guard<std::mutex> lk(m);
    ready = true;
    std::cout << "main() signals data ready for processing\n";
}

但是我们怎么可能到达ready=true行呢？ mutext m从工作线程中被阻止，因此std::lock_guard<std::mutex> lk(m);行将等待直到互斥体m解锁。据我了解，当互斥锁被锁定并且我们尝试锁定它时，我们将等待直到其被解锁。但是，它永远不会被解锁，因为工作线程正在等待，因此不会释放它。

Answer 1

在您附加的链接中，请注意以下内容

The wait operations atomically release the mutex and suspend the execution of the thread.

因此线

cv.wait(lk, []{return ready;});

释放互斥锁并中止线程的执行，直到通知了条件变量，超时到期(使用wit_for())或发生虚假唤醒，因此唤醒了线程，并且原子性地重新获取了互斥锁