C++0x thread interruption

Question

According to the C++0x final draft, there's no way to request a thread to terminate. That said, if required we need to implement a do-it-yourself solution.

On the other hand boost::thread provides a mechanism to interrupt a thread in a safe manner.

In your opinion, what's the best solution? Designing your own cooperative 'interruption mechanism' or going native?

Answer 1

All the language specification says that the support isn't built into the language. boost::thread::interrupt needs some support from the thread function, too:

When the interrupted thread next executes one of the specified interruption points (or if it is currently blocked whilst executing one)

i.e. when the thread function doesn't give the caller a chance to interrupt, you are still stuck.

I'm not sure what you mean with "going native" - there is no native support, unless you are spellbound to boost:threads.

Still, I'd use an explicit mechanism. You have to think about having enough interruption points anyway, why not make them explicit? The extra code is usually marginal in my experience, though you may need to change some waits from single-object to multiple-objects, which - depending on your library - may look uglier.

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One could also pull the "don't use exceptions for control flow", but compared to messing around with threads, this is just a guideline.

Answer 2

Using native handle to cancel a thread is a bad option in C++ as you need to destroy all the stack allocated objects. This was the main reason they don't included a cancel operation.

Boost.Thread provides an interrupt mechanism, that needs to pool on any waiting primitive. As this can be expensive as a general mechanism, the standard has not included it.

You will need to implement it by yourself. See my answer here to a similar question on how to implement this by yourself. To complete the solution an interruption should be throw when interrupted is true and the thread should catch this interruption and finish.

Answer 3

Here is my humble implementation of a thread canceller (for C++0x). I hope it will be useful.

// Class cancellation_point
#include <mutex>
#include <condition_variable>

struct cancelled_error {};

class cancellation_point
{
public:
    cancellation_point(): stop_(false) {}

    void cancel() {
        std::unique_lock<std::mutex> lock(mutex_);
        stop_ = true;
        cond_.notify_all();
    }

    template <typename P>
    void wait(const P& period) {
        std::unique_lock<std::mutex> lock(mutex_);
        if (stop_ || cond_.wait_for(lock, period) == std::cv_status::no_timeout) {
            stop_ = false;
            throw cancelled_error();
        }
    }
private:
    bool stop_;
    std::mutex mutex_;
    std::condition_variable cond_;
};


// Usage example
#include <thread>
#include <iostream>

class ThreadExample
{
public:
    void start() {
        thread_ = std::unique_ptr<std::thread>(
            new std::thread(std::bind(&ThreadExample::run, this)));
    }
    void stop() {
        cpoint_.cancel();
        thread_->join();
    }
private:
    void run() {
        std::cout << "thread started\n";
        try {
            while (true) {
                cpoint_.wait(std::chrono::seconds(1));
            }
        } catch (const cancelled_error&) {
            std::cout << "thread cancelled\n";
        }
    }
    std::unique_ptr<std::thread> thread_;
    cancellation_point cpoint_;
};

int main() {
    ThreadExample ex;
    ex.start();
    ex.stop();
    return 0;
}

Answer 4

It is unsafe to terminate a thread preemptively because the state of the entire process becomes indeterminate after that point. The thread might have acquired a critical section prior to being terminated. That critical section will now never be released. The heap could become permanently locked, and so on.

The boost::thread::interrupt solution works by asking nicely. It will only interrupt a thread doing something thats interruptible, like waiting on a Boost.Thread condition variable, or if the thread does one of these things after interrupt is called. Even then, the thread isn't unceremoniously put through the meat grinder as, say, Win32's TerminateThread function does, it simply induces an exception, which, if you've been a well-behaved coder and used RAII everywhere, will clean up after itself and gracefully exit the thread.

Answer 5

Implementing a do-it-yourself solution makes the most sense, and it really should not be that hard to do. You will need a shared variable that you read/write synchronously, indicating whether the thread is being asked to terminate, and your thread periodically reads from this variable when it is in a state where it can safely be interrupted. When you want to interrupt a thread, you simply write synchronously to this variable, and then you join the thread. Assuming it cooperates appropriately, it should notice that that the variable has been written and shut down, resulting in the join function no longer blocking.

If you were to go native, you would not gain anything by it; you would simply throw out all the benefits of a standard and cross-platform OOP threading mechanism. In order for your code to be correct, the thread would need to shut down cooperatively, which implies the communication described above.

Answer 6

Its unsafe to terminate a thread, since you would have no control over the state of any data-structures is was working on at that moment.

If you want to interrupt a running thread, you have to implement your own mechanism. IMHO if you need that, your design is not prepared for multiple threads.

If you just want to wait for a thread to finish, use join() or a future.

Answer 7

My implementation of threads uses the pimpl idiom, and in the Impl class I have one version for each OS I support and also one that uses boost, so I can decide which one to use when building the project.

I decided to make two classes: one is Thread, which has only the basic, OS-provided, services; and the other is SafeThread, which inherits from Thread and has method for collaborative interruption.

Thread has a terminate() method that does an intrusive termination. It is a virtual method which is overloaded in SafeThread, where it signals an event object. There's a (static) yeld() method which the running thread should call from time to time; this methods checks if the event object is signaled and, if yes, throws an exception caught at the caller of the thread entry point, thereby terminating the thread. When it does so it signals a second event object so the caller of terminate() can know that the thread was safely stopped.

For cases in which there's a risk of deadlock, SafeThread::terminate() can accept a timeout parameter. If the timeout expires, it calls Thread::terminate(), thus killing intrusively the thread. This is a last-resource when you have something you can't control (like a third-party API) or in situations in which a deadlock does more damage than resource leaks and the like.

Hope this'll be useful for your decision and will give you a clear enough picture about my design choices. If not, I can post code fragments to clarify if you want.

Answer 8

I agree with this decision. For example, .NET allows to abort any worker thread, and I never use this feature and don't recommend to do this to any professional programmer. I want to decide myself, when a worker thread may be interrupted, and what is the way to do this. It is different for hardware, I/O, UI and other threads. If thread may be stopped at any place, this may cause undefined program behavior with resource management, transactions etc.