std::atomic and lambda

Question

Does anybody know why this program goes into an infinite loop, instead of stopping after 5s or so?

This happens with both the latest gcc and clang compiler; does atomic_bool suffer from hte same issues as a vector of bool?

If I use atomic<int> this works fine.

#include <algorithm>
#include <memory>
#include <utility>
#include <iostream>
#include <vector>
#include <functional>
#include <future>
#include <chrono>


using namespace std;
using namespace chrono_literals;

void send_heart_beat()
{
    cout << "sending heartbeat" << endl;
}

std::future<void> f;
int main()
{
   std::atomic<bool> stop(false);
   f = std::async(std::launch::async,[&stop]() { while(!stop) { send_heart_beat(); std::this_thread::sleep_for(1s); } });
   std::this_thread::sleep_for(5s);
   stop = true;
}

Once main exits, stop ceases to exist. Any attempt to access it is UB. — n. m. could be an AI, Jan 04 '18 at 13:20
Your program exhibits undefined behavior, by way of access through a dangling reference. `stop` goes out of scope and is destroyed while the lambda is still running and checking it. — Igor Tandetnik, Jan 04 '18 at 13:20

score 3 · Accepted Answer · answered Jan 04 '18 at 16:37

 std::atomic<bool> stop(false);
 std::future<void> f;

These two variables are in different scopes, and f's scope is longer lived than stop's scope.

f = std::async(std::launch::async,[&stop]() { while(!stop) { send_heart_beat(); std::this_thread::sleep_for(1s); } });

here we bind a reference to stop into a lambda, and then store a (copy) of that lambda into an async object which is managed by f.

When f goes out of scope, its destructor waits for the async task to finish. But because f's scope is longer lasting than stop's, we leave the scope of stop before f waits for the thread to finish.

So our thread mindlessly continues accessing stop after stop no longer exists through a dangling reference.

This results in undefined behavior; any behavior by your program is acceptable to the standard.

std::atomic and lambda

1 Answers1