A way to destroy "thread" class

Question

Here is a skeleton of my thread class:

class MyThread {
public:
   virutal ~MyThread();

   // will start thread with svc() as thread entry point
   void start() = 0;        

   // derive class will specialize what the thread should do
   virtual void svc() = 0;                
};

Somewhere in code I create an instance of MyThread and later I want to destroy it. In this case MyThread~MyThread() is called. MyThread:svc() is still running and using the object's data members. So I need a way politely inform MyThread:svc() to stop spinning, before proceeding with the destructor.

What is the acceptable way to destroy the thread object?

Note: I'm looking for platform agnostic solution.

UPD: It's clear that the root of problem is that there's no relationship between C++ object representing thread and OS thread. So the question is: in context of object destuction, is there an acceptable way to make thread object behave like an ordinary C++ object or should it be treated as an unusual one (e.g. should we call join() before destoying it?

Answer 1

You could havee somthing like this in your svc method

while (alive){ //loops}
//free resources after while.

In your destructor, you could set the alive member to false. Or, you could have a pleaseDie() method, that sets the alive member to false, and can be called from the outside requesting the Thread instance to stop processing.

void
Thread::pleaseDie()
{

 this->alive = false;
}

Answer 2

Usually any OS-specific threads API will allow you to "join" a thread. That is, to block indefinitely on a thread handle until the thread functions returns.

So,

1. Signal the thread function to return (e.g. by setting a flag in its loop to false).
2. Join the thread, to make sure the actual thread terminates before you try to delete the thread object.
3. Then you can proceed with destruction of the thread object (you may also join in the destructor, though some people object to blocking destructors.).

I've had a project before with a similar "thread worker" class and a corresponding "work item" class (a-la Java's Thread and Runnable, except thread does not terminate but waits for a new Runnable object to be executed).

In the end, there was no difference if you join in a separate "shutdown" function or in the destructor, except a separate function is a bit more clear.

1. If you join in a destructor and a thread blocks, you will wait indefinitely.
2. If you join in a separate function and a thread blocks, you will wait indefinitely.
3. If you detach the thread and let it finish on its own, it will usually block application from exiting, so you will wait indefinitely.

So there is no straightforward way to make a thread behave like a regular C++ object and ignore its OS thread semantics, unless you can guarantee that your thread code can terminate almost immediately when notified to do so.

Answer 3

You should add dedicated thread management class (i.e. MyThreadMngr), that handles this and other tasks, like book keeping, owning the thread handles etc. The Thread itself should somehow signal to the thread manager that its going to terminate and MyThreadMngr should i.e. have a loop like Tom proposed.

There will probably be more actions that suite into such a thread manager class.

Answer 4

You first need a way to communicate with the thread to tell it to shut down. The best mechanism to do this depends on what svc() is doing. If, for example, it is looping on a message queue, you could insert a "please stop" message in that queue. Otherwise, you could simply add a member bool variable (and synchronize access to it) that is periodically checked by the svc(), and set by the thread wanting to destroy the object. Your could add a pure virtual stop() function to your base class, giving the implementor a clear signal that it has to implement svc() to make its class "runnable", and to implement stop() to make it "stoppable".

After asking the thread to stop, you must wait for it to exit before destroying the object. Again, there are several ways to do this. One is to make the stop() function blocking. It could wait, for example, for a "ok, I'm really stopped now" condition variable to be set by the thread running svc(). Alternatively, the caller could "wait" on the thread running svc(). The way to "wait" is platform dependent.

Answer 5

Considering your additional requirements posted as comment to Checkers' reply (which is the most straightforward way to do that):

I agree that join in DTor is problematic for various reasons. But from that the lifetime of your thread object is unrelated to the lifetime of the OS thread object.

---

First, you need to separate the data the thread uses from the thread object itself. They are distinct entities with distinct lifetime requirements.

One approach is to make the data refcounted, and have any thread that wants to access it hold a strong reference to the data. This way, no thread will suddenly grab into the void, but the data will be destroyed as soon as noone touches it anymore.

---

Second, about the thread object being destroyed when the thread joins:
I am not sure if this is a good idea. The thread object is normally a way to query the state of a thread - but with a thread object that dies as soon as the thread finishes, noone can tell you wether the thread finished.

Generally, I'd completely decouple the lifetime of the thread object from the lifetime of the OS thread: Destroying your thread object should not affect the thread itself. I see two basic approaches to this:

1. Thread Handle Object - reference counted again, returned by thread creator, can be released as early as one likes without affecting the OS thread. It would expose methods such as Join, IsFinished, and can give access to the thread shared data.

(If the thread object holds relevant execution state, the threafFunc itself could hold a reference to it, thereby ensuring the instance won't be released before the thread ends)

1. Thin Wrapper - You simply create a temporary around an OS thread handle. You could not hold additional state for the thread easily, but it might be just enough to make it work: At any place, you can turn an OS thread handle into an thread object. The majority of communication - e.g. telling the thread to terminate - would be via the shared data.

---

For your code example, this means: separate the start() from the svc()

You'd roughly work with this API (XxxxPtr could be e.g. boost::shared_ptr):

class Thread
{
   public:
     bool IsFinished();
     void Join();
     bool TryJoin(long timeout); 

     WorkerPtr GetWorker();

     static ThreadPtr Start(WorkerPtr worker); // creates the thread
};


class Worker
{
private:
   virtual void Svc() = 0;

   friend class Thread; // so thread can run Svc()
}

Worker could contain a ThreadPtr itself, giving you a guarantee that the thread object exists during execution of Svc(). If multiple threads are allowed to work on the same data, this would have to be a thread list. Otherwise, Thread::Start would have to reject Workers that are already associated with a thread.

---

Motivation: What to do with rogue threads that block?
Assuming a thread fails to terminate within time for one reason or another, even though you told it to. You simply have three choices:

• Deadlock, your applicaiton hangs. That usually happens if you join in the destructor.
• Violently terminate the thread. That's potentially a violent termination of the app.
• Let the thread run to completion on it's own data - you can notify the user, who can safely save & exit. Or you simply let the rogue thread dance on it's own copy of the data (not reference by the main thread anymore) until it completes.

Answer 6

Most thread systems allow you to send a signal to a thead.

Example: pthreads

pthread_kill(pthread_t thread, int sig);

This will send a signall to a thread. You can use this to kill the thread. Though this can leave a few of the resources hanging in an undefined state.

A solution to the resource problem is to install a signall handler.
So that when the signal handler is called it throws an exception. This will cause the thread stack to unwind to the entry point where you can then get the thread to check a variable about weather it is sill alive.

NOTE: You should never allow an exception to propogate out of a thread (this is so undefined my eyes bleed thinking about it). Basically catch the exception at the thread entry point then check some state variable to see if the thread should really exit.

Meanwhile the thread that sends the signal should wait for the thread to die by doing a join.

The only issues are that when you throw out of signal handler function you need to be careful. You should not use a signal that is asynchronus (ie one that could have been generated by a signal in another thread). A good one to use is SIGSEGV. If this happens normally then you have accessed invalid memory any you thread should think about exiting anyway!

You may also need to specify an extra flag on some systems to cope.
See This article

A working example using pthreads:

#include <pthread.h>
#include <iostream>

extern "C" void* startThread(void*);
extern "C" void  shouldIexit(int sig);

class Thread
{
    public:
        Thread();
        virtual ~Thread();
    private:
        friend void* startThread(void*);

        void start();
        virtual void run() = 0;

        bool        running;
        pthread_t   thread;
};


// I have seen a lot of implementations use a static class method to do this.
// DON'T. It is not portable. This is because the C++ ABI is not defined.
//
// It currently works on several compilers but will break if these compilers
// change the ABI they use. To gurantee this to work you should use a
// function that is declared as extern "C" this guarantees that the ABI is 
// correct for the callback. (Note this is true for all C callback functions)
void* startThread(void* data)
{
    Thread* thread  = reinterpret_cast<Thread*>(data);
    thread->start();
}
void shouldIexit(int sig)
{
    // You should not use std::cout in signal handler.
    // This is for Demo purposes only.
    std::cout << "Signal" << std::endl;

    signal(sig,shouldIexit);
    // The default handler would kill the thread.
    // But by returning you can continue your code where you left off.
    // Or by throwing you can cause the stack to unwind (if the exception is caught).
    // If you do not catch the exception it is implementation defined weather the
    // stack is unwound.
    throw int(3);  // use int for simplicity in demo
}


Thread::Thread()
    :running(true)
{
    // Note starting the thread in the constructor means that the thread may
    // start before the derived classes constructor finishes. This may potentially
    // be a problem. It is started here to make the code succinct and the derived
    // class used has no constructor so it does not matter.
    if (pthread_create(&thread,NULL,startThread,this) != 0)
    {
        throw int(5); // use int for simplicity in demo.
    }
}

Thread::~Thread()
{
    void*   ignore;

    running = false;
    pthread_kill(thread,SIGSEGV); // Tell thread it may want to exit.
    pthread_join(thread,&ignore); // Wait for it to finish.

    // Do NOT leave before thread has exited.

    std::cout << "Thread Object Destroyed" << std::endl;
}

void Thread::start()
{
    while(running)
    {
        try
        {
            this->run();
        }
        catch(...)
        {}
    }
    std::cout << "Thread exiting" << std::endl;
}
class MyTestThread:public Thread
{
    public:
        virtual void run()
        {
            // Unless the signal causes an exception
            // this loop will never exit.
            while(true)
            {
                sleep(5);
            }
        }

};

struct Info
{
     Info() {std::cout << "Info" << std::endl;}
    ~Info() {std::cout << "Done: The thread Should have exited before this" << std::endl;}
};

int main()
{
    signal(SIGSEGV,shouldIexit);

    Info                info;
    MyTestThread        test;

    sleep(4);
    std::cout << "Exiting About to Exit" << std::endl;

}


> ./a.exe
Info
Exiting About to Exit
Signal
Thread exiting
Thread Object Destroyed
Done: The thread Should have exited before this
>

Answer 7

I reckon the easiest way to do this is to wrap the thread execution code in a loop

while(isRunning())
{
     ... thread implementation ...
}

You can also stop your thread by doing specific calls, for instance when you're using a WIN32 thread you can call TerminateThread on the thread handle in the destructor.

Answer 8

i give a simple and clean design, no signal, no sync, no kill needed.

per your MyThread, i suggest renaming and adding as below:

class MyThread { 
public: 
   virutal ~MyThread(); 

   // will be called when starting a thread, 
   // could do some initial operations 
   virtual bool OnStart() = 0;  

   // will be called when stopping a thread, say calling join().
   virtual bool OnStop() = 0;

   // derive class will specialize what the thread should do, 
   // say the thread loop such as 
   // while (bRunning) {
   //    do the job.
   // } 
   virtual int OnRun() = 0;                 
};

the thread interface user will control the lifetime of MyThread.

and actually the real thread object is as below:

    class IThread
    {
    public:
        virtual API ~IThread() {}

        /* The real destructor. */
        virtual void Destroy(void) = 0;

        /* Starts this thread, it will call MyThread::OnStart() 
             * and then call MyThread::OnRun() just after created 
         *   the thread. */
        virtual bool Start(void) = 0;

        /* Stops a thread. will call MyThread::OnStop(). */
        virtual void Stop(void) = 0;

        /* If Wait() called, thread won't call MyThread::OnStop().
         * If could, it returns the value of MyThread::OnRun()
         *   returned */
        virtual int Wait(void) = 0;

        /* your staff */
        virtual MyThread * Command(void) = 0;

    };

/* The interface to create a thread */
extern IThread * ThrdCreate(MyThread *p);

See the complete interfaces

http://effoaddon.googlecode.com/svn/trunk/devel/effo/codebase/addons/thrd/include/thrd_i.h

Coding Examples

Case 1. Controlled thread loop

class ThreadLoop : public MyThread
{
private:
   bool m_bRunning;
public:
   virtual bool OnStart() { m_bRunning = true; }  

   virtual bool OnStop() { m_bRunning = false; }

   virtual int OnRun() 
   {
         while (m_bRunning) {
              do your job;
         }
   }                 
};

int main(int argc, char **argv)
{
      ThreadLoop oLoop;

      IThread *pThread = ThrdCreate(&oLoop);
      // Start the thread, it will call Loop::OnStart() 
      //and then call Loop::OnRun() internally.
      pThread->Start();
      do your things here. when it is time to stop the thread, call stop().
      // Stop the thread, it will call Loop::OnStop(), 
      // so Loop::OnRun() will go to the end
      pThread->Stop();
      // done, destroy the thread
      pThread->Destroy();
}

Case 2. Don't know when the thread will stop

class ThreadLoop : public MyThread
{
public:
   virtual bool OnStart() {  }  

   virtual bool OnStop() { }

   virtual int OnRun() 
   {
         do your job until finish.
   }                 
};

int main(int argc, char **argv)
{
      ThreadLoop oLoop;

      IThread *pThread = ThrdCreate(&oLoop);
      // Start the thread, it will call Loop::OnStart() 
      //and then call Loop::OnRun() internally.
      pThread->Start();
      do your things here. Since you don't know when the job will 
      finish in the thread loop. call wait().
      // Wait the thread, it doesn't call Loop::OnStop()
      pThread->Wait();
      // done, destroy the thread
      pThread->Destroy();
}

A complete IThread implementation:

see

http://effoaddon.googlecode.com/svn/trunk/devel/effo/codebase/addons/thrd/src/thrd/thrd.cpp