[英]How to use a std::condition_variable correctly?
I'm confused about conditions_variables
and how to use them (safely). 我对
conditions_variables
以及如何使用它们(安全)感到困惑。 In my application I've a class that makes a gui-thread but while the gui is constructed by the gui-thread, the main thread needs to wait. 在我的应用程序中,我有一个创建gui线程的类,但是当gui由gui-thread构造时,主线程需要等待。
The situation is the same as for the function below. 情况与下面的功能相同。 The main thread makes a mutex, lock and
condition_variable
. 主线程产生互斥锁,锁和
condition_variable
。 It then makes the thread. 然后它成为线程。 While this
worker
thread
has not passed a certain point (here printing the numbers), the main thread is not allowed to continue (ie has to wait
for all numbers being printed). 虽然此
worker
thread
尚未通过某一点(此处打印数字),但主线程不允许继续(即必须wait
所有正在打印的数字)。
How do I use condition_variables
correctly in this context? 如何在此上下文中正确使用
condition_variables
? Also, I've read that spontaneous wake-ups are an issue. 此外,我读过自发的唤醒是一个问题。 How can I handle them?
我怎么处理它们?
int main()
{
std::mutex mtx;
std::unique_lock<std::mutex> lck(mtx);
std::condition_variable convar;
auto worker = std::thread([&]{
/* Do some work. Main-thread can not continue. */
for(int i=0; i<100; ++i) std::cout<<i<<" ";
convar.notify_all(); // let main thread continue
std::cout<<"\nworker done"<<std::endl;
});
// The main thread can do some work but then must wait until the worker has done it's calculations.
/* do some stuff */
convar.wait(lck);
std::cout<<"\nmain can continue"<<std::endl; // allowed before worker is entirely finished
worker.join();
}
Typically you'd have some observable shared state on whose change you block: 通常,您有一些可观察的共享状态,您可以阻止其更改:
bool done = false;
std::mutex done_mx;
std::condition_variable done_cv;
{
std::unique_lock<std::mutex> lock(done_mx);
std::thread worker([&]() {
// ...
std::lock_guard<std::mutex> lock(done_mx);
done = true;
done_cv.notify_one();
});
while (true) { done_cv.wait(lock); if (done) break; }
// ready, do other work
worker.join();
}
Note that you wait in a loop until the actual condition is met. 请注意,您在循环中等待,直到满足实际条件。 Note also that access to the actual shared state (
done
) is serialized via the mutex done_mx
, which is locked whenever done
is accessed. 另请注意,通过互斥锁
done_mx
序列化对实际共享状态( done
)的访问,该互斥锁在访问done
时被锁定。
There's a helper member function that performs the condition check for you so you don't need the loop: 有一个帮助成员函数,它为您执行条件检查,因此您不需要循环:
done_cv.wait(lock, [&]() { return done; });
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