How to Check if a std::thread is Still Running in C ?

How to Determine the Status of a Running std::thread

In this discussion, we will explore an important question in multithreading: how to check if a std::thread is still executing. This knowledge is crucial for effective coordination and monitoring of threads in diverse programming scenarios.

The Challenge

The std::thread class in C lacks a convenient method like timed_join() or joinable() explicitly designed for determining the running status of a thread. The joinable() method is primarily intended for a different purpose, leaving professionals seeking alternative approaches.

Solution 1: Leveraging std::async and std::future

Harnessing the power of std::async and std::future, we can effortlessly monitor a thread's status. std::async runs a task on a separate thread, while std::future provides a handle to its results. By employing std::future's wait_for function with a zero timeout, we can ascertain the thread's running status neatly:

auto future = std::async(std::launch::async, [] {
    std::this_thread::sleep_for(3s);
    return 8;
});

//...
auto status = future.wait_for(0ms);
if (status == std::future_status::ready) {
  std::cout << "Thread finished" << std::endl;
} else {
  std::cout << "Thread still running" << std::endl;
}

Solution 2: Utilizing std::promise and std::future

If using std::async and std::future is not feasible, an alternative involves std::promise and std::future. This approach allows us to obtain a future object directly from the thread:

std::promise<bool> p;
auto future = p.get_future();

//...
p.set_value(true);

Once again, employing wait_for on the future object provides the desired status information.

Solution 3: Straightforward Approach with std::thread

For simpler requirements, the following approach using an atomic flag may suffice:

std::atomic<bool> done(false);

//...
done = true;

By accessing the atomic flag, we can determine if the thread has completed its task.

Additional Solutions

Other solutions include leveraging std::packaged_task, which provides a cleaner integration with std::thread, or utilizing platform-specific mechanisms like Windows APIs on Windows systems.

Conclusion

Choosing the best solution for checking the status of a std::thread depends on the specific requirements and constraints of the application. By understanding these methods, developers can effectively manage multithreaded programs and ensure their desired behavior.

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