我正在尝试在之前的 worker_thread
中的工作开始 后立即启动新线程,但可能已经结束也可能没有。我用时间延迟替换了开始和结束的工作。我的代码是:
#include <iostream>
#include <string>
#include <mutex>
#include <condition_variable>
#include <future>
#include <atomic>
#include <chrono>
#include <thread>
std::mutex m;
std::condition_variable cv;
bool started = false;
void worker_thread()
{
std::unique_lock<std::mutex> lk(m);
static std::atomic<int> count(1);
std::this_thread::sleep_for(std::chrono::milliseconds{(count % 5) * 100});
std::cerr << "Start Worker thread: " << count << "\n";
started = true;
lk.unlock();
cv.notify_one();
std::this_thread::sleep_for(std::chrono::milliseconds{3000});
std::cerr << "Exit Worker thread: " << count << "\n";
++count;
}
int main()
{
while(1) {
std::async(std::launch::async, worker_thread);
std::unique_lock<std::mutex> lk(m);
cv.wait(lk, []{return started;});
started = false;
}
}
输出看起来像这样:
Start Worker thread: 1
Exit Worker thread: 1
Start Worker thread: 2
Exit Worker thread: 2
Start Worker thread: 3
Exit Worker thread: 3
Start Worker thread: 4
Exit Worker thread: 4
Start Worker thread: 5
Exit Worker thread: 5
这不是我想要的行为。我想要的是(不完全是)这样的东西:
Start Worker thread: 1
Start Worker thread: 2
Start Worker thread: 3
Start Worker thread: 4
Exit Worker thread: 1
Exit Worker thread: 3
Exit Worker thread: 4
Exit Worker thread: 2
Start Worker thread: 5
Exit Worker thread: 5
目前下一个线程只有在前一个线程的工作完成后才会启动。但是我想在上一个线程开始工作后立即启动下一个线程,而不是等待它结束,只等待开始。
最佳答案
std::async
返回一个 std::future
保存函数执行的结果。在您的情况下,它是一个被立即销毁的临时对象。 std::future
的文档说:
✔ the shared state was created by a call to std::async
✔ the shared state is not yet ready
✔ this was the last reference to the shared state
所有这些都是正确的,因此对 future
的破坏将阻塞,直到 worker 函数完成执行。
你可以创建分离线程来避免这个问题:
std::thread(worker_thread).detach();
关于c++条件变量通知未按预期工作,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/43156908/