How to wake a std::thread while it is sleeping

Question

I am using C++11 and I have a std::thread which is a class member, and it sends information to listeners every 2 minutes. Other that that it just sleeps. So, I have made it sleep for 2 minutes, then send the required info, and then sleep for 2 minutes again.

// MyClass.hpp
class MyClass {

    ~MyClass();
    RunMyThread();

private:
    std::thread my_thread;
    std::atomic<bool> m_running;
}


MyClass::RunMyThread() {

    my_thread = std::thread { [this, m_running] {
    m_running = true;
    while(m_running) {
        std::this_thread::sleep_for(std::chrono::minutes(2));
        SendStatusInfo(some_info);
    }
}};
}

// Destructor
~MyClass::MyClass() {
    m_running = false; // this wont work as the thread is sleeping. How to exit thread here?
}

Issue:
The issue with this approach is that I cannot exit the thread while it is sleeping. I understand from reading that I can wake it using a std::condition_variable and exit gracefully? But I am struggling to find a simple example which does the bare minimum as required in above scenario. All the condition_variable examples I've found look too complex for what I am trying to do here.

Question:
How can I use a std::condition_variable to wake the thread and exit gracefully while it is sleeping? Or are there any other ways of achieving the same without the condition_variable technique?

Additionally, I see that I need to use a std::mutex in conjunction with std::condition_variable? Is that really necessary? Is it not possible to achieve the goal by adding the std::condition_variable logic only to required places in the code here?

Environment:
Linux and Unix with compilers gcc and clang.

Answer 1

Or are there any other ways of achieving the same without the condition_variable technique?

You can use std::promise/std::future as a simpler alternative to a bool/condition_variable/mutex in this case. A future is not susceptible to spurious wakes and doesn't require a mutex for synchronisation.

Basic example:

std::promise<void> pr;
std::thread thr{[fut = pr.get_future()]{
    while(true)
    {
        if(fut.wait_for(std::chrono::minutes(2)) != std::future_status::timeout)
            return;
    }
}};
//When ready to stop
pr.set_value();
thr.join();

Answer 2

You could also use promise/future so that you don't need to bother with conditionnal and/or threads:

#include <future>
#include <iostream>

struct MyClass {

    ~MyClass() {
        _stop.set_value();
    }

    MyClass() {
        auto future = std::shared_future<void>(_stop.get_future());
        _thread_handle = std::async(std::launch::async, [future] () {
            std::future_status status;
            do {
                status = future.wait_for(std::chrono::seconds(2));
                if (status == std::future_status::timeout) {
                    std::cout << "do periodic things\n";
                } else if (status == std::future_status::ready) {
                    std::cout << "exiting\n";
                }
            } while (status != std::future_status::ready);
        });
    }


private:
    std::promise<void> _stop;
    std::future<void> _thread_handle;
};


// Destructor
int main() {
    MyClass c;
    std::this_thread::sleep_for(std::chrono::seconds(9));
}

Answer 3

A working example for you using std::condition_variable:

struct MyClass {
    MyClass()
        : my_thread([this]() { this->thread(); })
    {}

    ~MyClass() {
        {
            std::lock_guard<std::mutex> l(m_);
            stop_ = true;
        }
        c_.notify_one();
        my_thread.join();
    }

    void thread() {
        while(this->wait_for(std::chrono::minutes(2)))
            SendStatusInfo(some_info);
    }

    // Returns false if stop_ == true.
    template<class Duration>
    bool wait_for(Duration duration) {
        std::unique_lock<std::mutex> l(m_);
        return !c_.wait_for(l, duration, [this]() { return stop_; });
    }

    std::condition_variable c_;
    std::mutex m_;
    bool stop_ = false;
    std::thread my_thread;
};

Answer 4

Or are there any other ways of achieving the same without condition_variable technique?

One alternative to a condition variable is you can wake your thread up at much more regular intervals to check the "running" flag and go back to sleep if it is not set and the allotted time has not yet expired:

void periodically_call(std::atomic_bool& running, std::chrono::milliseconds wait_time)
{
    auto wake_up = std::chrono::steady_clock::now();

    while(running)
    {
        wake_up += wait_time; // next signal send time

        while(std::chrono::steady_clock::now() < wake_up)
        {
            if(!running)
                break;

            // sleep for just 1/10 sec (maximum)
            auto pre_wake_up = std::chrono::steady_clock::now() + std::chrono::milliseconds(100);

            pre_wake_up = std::min(wake_up, pre_wake_up); // don't overshoot

            // keep going to sleep here until full time
            // has expired
            std::this_thread::sleep_until(pre_wake_up);
        }

        SendStatusInfo(some_info); // do the regular call
    }
}

Note: You can make the actual wait time anything you want. In this example I made it 100ms std::chrono::milliseconds(100). It depends how responsive you want your thread to be to a signal to stop.

For example in one application I made that one whole second because I was happy for my application to wait a full second for all the threads to stop before it closed down on exit.

How responsive you need it to be is up to your application. The shorter the wake up times the more CPU it consumes. However even very short intervals of a few milliseconds will probably not register much in terms of CPU time.

Answer 5

Additionally, I see that I need to use a std::mutex in conjunction with std::condition_variable? Is that really necessary? Is it not possible to achieve the goal by adding the std::condition_variable logic only to required places in the code piece here?

std::condition_variable is a low level primitive. Actually using it requires fiddling with other low level primitives as well.

struct timed_waiter {
  void interrupt() {
    auto l = lock();
    interrupted = true;
    cv.notify_all();
  }
  // returns false if interrupted
  template<class Rep, class Period>
  bool wait_for( std::chrono::duration<Rep, Period> how_long ) const {
    auto l = lock();
    return !cv.wait_until( l,
      std::chrono::steady_clock::now() + how_long,
      [&]{
        return !interrupted;
      }
    );
  }
private:
  std::unique_lock<std::mutex> lock() const {
    return std::unique_lock<std::mutex>(m);
  }
  mutable std::mutex m;
  mutable std::condition_variable cv;
  bool interrupted = false;
};

simply create a timed_waiter somewhere both the thread(s) that wants to wait, and the code that wants to interrupt, can see it.

The waiting threads do

while(m_timer.wait_for(std::chrono::minutes(2))) {
    SendStatusInfo(some_info);
}

to interrupt do m_timer.interrupt() (say in the dtor) then my_thread.join() to let it finish.

Live example:

struct MyClass {
    ~MyClass();
    void RunMyThread();
private:
    std::thread my_thread;
    timed_waiter m_timer;
};


void MyClass::RunMyThread() {

    my_thread = std::thread {
      [this] {
      while(m_timer.wait_for(std::chrono::seconds(2))) {
        std::cout << "SendStatusInfo(some_info)\n";
      }
    }};
}

// Destructor
MyClass::~MyClass() {
    std::cout << "~MyClass::MyClass\n";
    m_timer.interrupt();
    my_thread.join();
    std::cout << "~MyClass::MyClass done\n";
}

int main() {
    std::cout << "start of main\n";
    {
        MyClass x;
        x.RunMyThread();
        using namespace std::literals;
        std::this_thread::sleep_for(11s);
    }
    std::cout << "end of main\n";
}

Answer 6

How can I use an std::condition_variable to wake the thread and exit gracefully while it was sleeping?

You use std::condition_variable::wait_for() instead of std::this_thread::sleep_for() and first one can be interrupted by std::condition_variable::notify_one() or std::condition_variable::notify_all()

Additionally, I see that I need to use a std::mutex in conjunction with std::condition_variable? Is that really necessary? Is it not possible to achieve the goal by adding the std::condition_variable logic only to required places in the code piece here?

Yes it is necessary to use std::mutex with std::condition_variable and you should use it instead of making your flag std::atomic as despite atomicity of flag itself you would have race condition in your code and you will notice that sometimes your sleeping thread would miss notification if you would not use mutex here.

Answer 7

How can I use an std::condition_variable to wake the thread and exit gracefully while it was sleeping? Or are there any other ways of achieving the same without condition_variable technique?

No, not in standard C++ as of C++17 (there are of course non-standard, platform-specific ways to do it, and it's likely some kind of semaphore will be added to C++2a).

Additionally, I see that I need to use a std::mutex in conjunction with std::condition_variable? Is that really necessary?

Yes.

Is it not possible to achieve the goal by adding the std::condition_variable logic only to required places in the code piece here?

No. For a start, you can't wait on a condition_variable without locking a mutex (and passing the lock object to the wait function) so you need to have a mutex present anyway. Since you have to have a mutex anyway, requiring both the waiter and the notifier to use that mutex isn't such a big deal.

Condition variables are subject to "spurious wake ups" which means they can stop waiting for no reason. In order to tell if it woke because it was notified, or woke spuriously, you need some state variable that is set by the notifying thread and read by the waiting thread. Because that variable is shared by multiple threads it needs to be accessed safely, which the mutex ensures.

Even if you use an atomic variable for the share variable, you still typically need a mutex to avoid missed notifications.

This is all explained in more detail in https://github.com/isocpp/CppCoreGuidelines/issues/554

Answer 8

There is a sad, but true fact - what you are looking for is a signal, and Posix threads do not have a true signalling mechanism.

Also, the only Posix threading primitive associated with any sort of timing is conditional variable, this is why your online search lead you to it, and since C++ threading model is heavily built on Posix API, in standard C++ Posix-compatible primitives is all you get.

Unless you are willing to go outside of Posix (you do not indicate platform, but there are native platform ways to work with events which are free from those limitations, notably eventfd in Linux) you will have to stick with condition variables and yes, working with condition variable requires a mutex, since it is built into API.

Your question doesn't specifically ask for code sample, so I am not providing any. Let me know if you'd like some.