In this blog post, we are going to explore how to use barriers with std::jthread to synchronize multiple threads and coordinate their execution.

Understanding std::barrier

A barrier is a synchronization mechanism that allows multiple threads to wait for each other at a specified point in their execution before continuing. It provides a way to ensure that a specific set of operations is completed before any thread proceeds further.

In C++20, the std::barrier class is introduced, which encapsulates this functionality. It is typically created with a count representing the number of threads that need to reach the barrier before it is lifted. Once all the threads have reached the barrier, they can continue executing.

Using std::barrier with std::jthread

To use std::barrier with std::jthread, we first need to create an instance of std::barrier with the desired count. Then, we can launch multiple std::jthread instances, passing the barrier to each of them.

Here’s an example that demonstrates the usage:

#include <iostream>
#include <barrier>
#include <jthread>

std::barrier barrier(3); // Create a barrier with a count of 3

void threadFunction() {
    std::cout << "Thread started\n";

    // Do some work

    barrier.arrive_and_wait(); // Wait at the barrier

    // Continue with the rest of the work

    std::cout << "Thread finished\n";
}

int main() {
    std::jthread thread1(threadFunction);
    std::jthread thread2(threadFunction);
    std::jthread thread3(threadFunction);

    // The main thread will also participate in the barrier
    barrier.arrive_and_wait();

    // Now all threads have reached the barrier, continue with the rest of the work

    thread1.join();
    thread2.join();
    thread3.join();

    return 0;
}

In this example, we create a std::barrier with a count of 3, indicating that we want to synchronize three threads. We then launch three std::jthread instances, each executing the threadFunction. Inside the function, we perform some work, wait at the barrier using barrier.arrive_and_wait(), and then continue with the remaining work.

The main thread also participates in the barrier by calling barrier.arrive_and_wait() and waits until all the threads have reached the barrier. Finally, we join all the threads to ensure proper cleanup.

Conclusion

Using barriers with std::jthread provides an elegant and straightforward way to synchronize multiple threads in C++20. By leveraging the power of std::barrier, we can ensure that the desired set of operations is completed before any thread proceeds further. This not only simplifies thread synchronization but also improves the overall efficiency and readability of the code.

#cpp #c++20 #threading #stdjthread #stdbarrier