Question

I am currently writing a multithreaded program where a thread may sometimes be created depending on certain circumstances. If this thread is created it needs to run independently of all other threads and I cannot afford to block any other threads to wait for it to join. The length of time the spawned thread runs for varies; sometimes it can take up to a few hours.

I have tried spawning the thread and putting a join in the destructor of the class which works fine, however if the code within the spawned thread finishes a long time before the destructor is called (which will be around 99% of the time) I would like the thread to kill itself freeing all its resources etc.

I looked into using detach for this, but you can’t rejoin a detached thread and on the off chance the destructor is called before this thread finishes then the spawned thread will not finish and could have disastrous consequences.

Is there any possible solution that ensures the thread finishes before the class is destructed as well as allowing it to join as soon as the thread finishes its work?

I am using boost/c++11 for threading. Any help at all would be greatly appreciated.

Thanks

Answer 1

The thread may detach itself, releasing its resources. If the destructor sees that the thread is joinable, i.e. still running, let it join. If the thread reaches its end, self-detach. Possible race condition: is_joinable() returns true in destructor – thread detaches itself – destructor joins and fails miserably. So use a mutex guarding the thread’s decease:

struct ThreadContainer
{
   std::mutex threadEndMutex;
   std::thread theThread;

   ThreadContainer()
     : theThread([=]()
       {
         /* do stuff */

         // if the mutex is locked, the destructor is just
         // about to join, so we let him.
         if (threadEndMutex.try_lock())
           theThread.detach();
       })
   {}

   ~ThreadContainer()
   {
     // if the mutex is locked, the thread is just about 
     // to detach itself, so no need to join.
     // if we got the mutex but the thread is not joinable, 
     // it has detached itself already.
     if (threadEndMutex.try_lock() && theThread.is_joinable())
       theThread.join();
   }
};

PS:
you might not even need the call to is_joinable, because if the thread detached itself, it never unlocked the mutex and try_lock fails.

PPS:
instead of the mutex, you may use std::atomic_flag:

struct ThreadContainer
{
   std::atmoic_flag threadEnded;
   std::thread theThread;

   ThreadContainer()
     : threadEnded(ATOMIC_FLAG_INIT)
     , theThread([=]()
       {
         /* do stuff */

         if (!threadEnded.test_and_set())
           theThread.detach();
       })
   {}

   ~ThreadContainer()
   {
     if (!threadEnded.test_and_set())
       theThread.join();
   }
};