When using std::jthread in C++, we have a convenient way to manage threads and their lifetime. However, there may be scenarios where we need to cancel or terminate the execution of a std::jthread before it completes. In this blog post, we will explore how to cancel a std::jthread and handle the necessary cleanup.

Understanding std::jthread

std::jthread is a new addition to the C++ threading library introduced with C++20. It is a wrapper around a native thread handle and provides a mechanism to safely manage the lifetime of the associated thread. It automatically joins the thread on destruction, ensuring proper cleanup.

The problem with canceling a thread

Unlike std::jthread’s counterpart, std::thread, there is no direct method or interface to cancel or terminate a std::jthread forcefully. This design choice is intentional as it discourages abrupt thread termination, which can lead to resource leaks and undefined behavior.

Graceful cancellation

A recommended approach to cancel a std::jthread is to use a cooperative cancellation mechanism. Instead of forcefully terminating the thread, we can use a cancellation flag that is checked periodically within the thread’s execution loop. If the flag is set, the thread can gracefully exit its execution loop, allowing for proper cleanup and resource release.

Example code

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

std::atomic_bool cancelFlag{ false };

void myThreadFunction()
{
    while (!cancelFlag)
    {
        // Thread execution work
    }
}

int main()
{
    std::jthread myThread(myThreadFunction);

    // Do some work

    // Set the cancel flag to request thread cancellation
    cancelFlag = true;

    // Wait for the thread to gracefully exit
    myThread.join();

    return 0;
}

In the above example, we create a std::atomic_bool variable called cancelFlag to serve as the cancellation flag. Inside the myThreadFunction(), the cancellation flag is checked periodically to determine whether the thread should continue execution.

In the main() function, we initialize a std::jthread called myThread and start the thread’s execution. When we want to cancel the thread, we set the cancellation flag to true. The thread’s execution loop will notice this and gracefully exit the loop, allowing for the thread to be joined and cleaned up.

Conclusion

Canceling a std::jthread requires a cooperative approach by using a cancellation flag and checking it periodically within the thread’s execution loop. This approach ensures that proper cleanup is done and avoids resource leaks or undefined behavior. By following this recommendation, we can safely manage the lifetime of threads even in the absence of a direct cancellation functionality in std::jthread.

#C++ #Threading