SOLVED threading in a dll where the dll must return before child thread finishes

Question

I am working on writing a wrapper dll to interface a communication dll for a yokogawa WT1600 power meter, to a PC based automation package. I got the communication part to work but I need to thread it so that a 50ms scan time of the automation package can be maintained. (The Extended Function Block (EFB) Call will block the scan until it returns)

These are the steps I need to do.

1. Call EFB
2. EFB creates a thread to perform communication setup (takes about 200ms to do)
3. EFB returns EFB_BUSY while the thread is doing the work 3a. (automation program continues scanning until it comes back to the EFB call)
4. Call EFB passing in that it returned busy on the last call
5. EFB checks if the thread has returned
6. If the thread returned Then the EFB returns success, Else return EFB_BUSY
7. repeat 3a-6 until efb returns success

So my problem is, how do I create a thread that exists past the life of the function that called it? and how do I get that thread return value when I call back into the dll?

EDIT #1

  HeavyFunction::HeavyFunction^ hf;  //HeavyFunction is a class that has a time consuming function in it
  ThreadStart^ efbThreadDelegate;
  Thread^ efbThread;

  if( pEfbData->nBlockingRecall != DOEFB_BUSY ) {
   hf  = gcnew HeavyFunction::HeavyFunction;
   hf->iiStart = (int)(pEfbData->uParams[0].dw);
   hf->iiEnd = (int)(pEfbData->uParams[1].dw);
   efbThreadDelegate = gcnew ThreadStart( hf, &HeavyFunction::HeavyFunction::iGetPrime );
   efbThread = gcnew Thread( efbThreadDelegate );
   efbThread->Start();
   return DOEFB_BUSY;
  }else if ( efbThread->IsAlive ) {
   return DOEFB_BUSY;
  }else {
   uRetValue->dw = hf->iReturn;
            return 0;
  }

Will efbThread still have the same thread handle upon a subsequent call?

EDIT #2

I got it to work by creating a global HANDLE for a Mutex and a thread. Initializing the mutex in the init entry point (done upon dll loading) and creating the thread in the main function when a call is actually made to the dll.

I used the sample code from MSDN: Creating Threads as my model.

Answer 1

Any thread created (whether in a DLL or elsewhere) will not stop spontaneously. In particular, the function that created the thread may return. The new thread would still run even if the creator thread exited. That is, assuming it didn't hit the end of its entry function.

Windows threads return a DWORD when ready. To peek, call WaitForSingleObject on the thread handle with a 0 second timeout, and it that succeeds, call GetExitCodeThread .

I don't understand your whole "EFB" thing, neither what it is nor what it does, though. If it is doing funny things to normal Windows threads, all bets are off.