How to Check If a Std::Thread Is Still Running

How to check if a std::thread is still running?

If you are willing to make use of C++11 std::async and std::future for running your tasks, then you can utilize the wait_for function of std::future to check if the thread is still running in a neat way like this:

#include <future>
#include <thread>
#include <chrono>
#include <iostream>

int main() {
    using namespace std::chrono_literals;

    /* Run some task on new thread. The launch policy std::launch::async
       makes sure that the task is run asynchronously on a new thread. */
    auto future = std::async(std::launch::async, [] {
        std::this_thread::sleep_for(3s);
        return 8;
    });

    // Use wait_for() with zero milliseconds to check thread status.
    auto status = future.wait_for(0ms);

    // Print status.
    if (status == std::future_status::ready) {
        std::cout << "Thread finished" << std::endl;
    } else {
        std::cout << "Thread still running" << std::endl;
    }

    auto result = future.get(); // Get result.
}

If you must use std::thread then you can use std::promise to get a future object:

#include <future>
#include <thread>
#include <chrono>
#include <iostream>

int main() {
    using namespace std::chrono_literals;

    // Create a promise and get its future.
    std::promise<bool> p;
    auto future = p.get_future();

    // Run some task on a new thread.
    std::thread t([&p] {
        std::this_thread::sleep_for(3s);
        p.set_value(true); // Is done atomically.
    });

    // Get thread status using wait_for as before.
    auto status = future.wait_for(0ms);

    // Print status.
    if (status == std::future_status::ready) {
        std::cout << "Thread finished" << std::endl;
    } else {
        std::cout << "Thread still running" << std::endl;
    }

    t.join(); // Join thread.
}

Both of these examples will output:

Thread still running

This is of course because the thread status is checked before the task is finished.

But then again, it might be simpler to just do it like others have already mentioned:

#include <thread>
#include <atomic>
#include <chrono>
#include <iostream>

int main() {
    using namespace std::chrono_literals;

    std::atomic<bool> done(false); // Use an atomic flag.

    /* Run some task on a new thread.
       Make sure to set the done flag to true when finished. */
    std::thread t([&done] {
        std::this_thread::sleep_for(3s);
        done = true;
    });

    // Print status.
    if (done) {
        std::cout << "Thread finished" << std::endl;
    } else {
        std::cout << "Thread still running" << std::endl;
    }

    t.join(); // Join thread.
}

Edit:

There's also the std::packaged_task for use with std::thread for a cleaner solution than using std::promise:

#include <future>
#include <thread>
#include <chrono>
#include <iostream>

int main() {
    using namespace std::chrono_literals;

    // Create a packaged_task using some task and get its future.
    std::packaged_task<void()> task([] {
        std::this_thread::sleep_for(3s);
    });
    auto future = task.get_future();

    // Run task on new thread.
    std::thread t(std::move(task));

    // Get thread status using wait_for as before.
    auto status = future.wait_for(0ms);

    // Print status.
    if (status == std::future_status::ready) {
        // ...
    }

    t.join(); // Join thread.
}

Check if std::thread is still running

The closest you can get is std::thread::joinable(). To cite from the reference:

Checks if the thread object identifies an active thread of execution. Specifically, returns true if get_id() != std::thread::id(). So a default constructed thread is not joinable.

A thread that has finished executing code, but has not yet been joined is still considered an active thread of execution and is therefore joinable.

Is it possible to know if a std::thread finished given its std::thread::id?

I think I found out how:

struct logger {
enum level : int16_t { debug = 0, info = 1, warn = 2, error = 3, fatal = 4 };

~logger() {
    write_all();
}

logger & operator()(level p_level) {
    add(true)
            << "[" << get_level(p_level) << "]"
            << "[" << std::time(NULL) << "]"
            << "[" << std::this_thread::get_id() << "] ";
    return *this;
}

template <typename T>
logger & operator << (const T & p_val) {
    add() << p_val;
    return *this;
}
private:

struct at_thread_end {
  at_thread_end (std::function<void (std::thread::id)> p_callback
                 , std::thread::id p_id)
      : m_callback(p_callback)
      , m_id(p_id){}

  at_thread_end(at_thread_end && p_at_thread_end)
      : m_callback(std::move(p_at_thread_end.m_callback))
      , m_id(std::move(p_at_thread_end.m_id)) {}

  at_thread_end & operator=(at_thread_end && p_at_thread_end) {
      m_callback = std::move(p_at_thread_end.m_callback);
      m_id = std::move(p_at_thread_end.m_id);
      return *this;
  }

  ~at_thread_end() {
      m_callback(m_id);
  }

private:
  std::function<void (std::thread::id)> m_callback;
  std::thread::id m_id;
};

typedef std::map<std::thread::id, std::stringstream> streams;

void on_thread_end(std::thread::id p_id) {
    std::cout << "thread " << p_id << " ended" << std::endl;
    streams::iterator _ite = m_streams.find(p_id);
    if (_ite != m_streams.end()) {
        write(_ite->second.str());
        std::lock_guard<std::mutex> _lock(m_mutex_streams);
        m_streams.erase(_ite);
    }
}

std::stringstream & add(bool p_clean = false) {
    std::thread::id _id  = std::this_thread::get_id();
    streams::iterator _ite = m_streams.find(_id);
    if (_ite != m_streams.end()) {
        if (p_clean) {
            write(_ite->second.str());
            _ite->second.str("");
        }
        return _ite->second;
    }


    using std::placeholders::_1;
    thread_local at_thread_end
            _at_thread_end(std::bind(&logger::on_thread_end, this, _1)
                           ,_id);

    std::lock_guard<std::mutex> _lock(m_mutex_streams);
    m_streams[_id] = std::stringstream();
    return m_streams[_id];
}

std::string get_level(level p_level) {
    switch (p_level) {
    case level::debug:
        return "DEB";
    case level::info:
        return "INF";
    case level::warn:
        return "WAR";
    case level::error:
        return "ERR";
    case level::fatal:
        return "FAT";
    }
    return "LEVEL UNKNOW";
}

void write(const std::string & p_str) {
    std::lock_guard<std::mutex> _lock(m_mutex_write);
    if (p_str.size()) {
        std::cout << p_str << std::endl;
    }
}

void write_all() {
    std::lock_guard<std::mutex> _lock(m_mutex_streams);
    streams::iterator _end = m_streams.end();
    for (streams::iterator _ite = m_streams.begin(); _ite != _end; ++_ite) {
        write(_ite->second.str());
    }
}

streams m_streams;
std::mutex m_mutex_streams;
std::mutex m_mutex_write;

};

You can look at a full example at https://pastebin.com/z2HrF70U

Can C++11 tell if std::thread is active?

No, I don't think that this is possible. I would also try to think about your design and if such a check is really necessary, maybe you are looking for something like the interruptible threads from boost.

However, you can use std::async - which I would do anyway - and then rely on the features std::future provides you.

Namely, you can call std::future::wait_for with something like std::chrono::seconds(0). This gives you a zero-cost check and enables you to compare the std::future_status returned by wait_for.

auto f = std::async(foo);
...
auto status = f.wait_for(std::chrono::seconds(0));
if(status == std::future_status::timeout) {
    // still computing
}
else if(status == std::future_status::ready) {
    // finished computing
}
else {
    // There is still std::future_status::defered
}

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