How to know when threads have died

[Sanai]

i am using multithreading in my program but im unable to know when all the threads have finished their tasks ... and when they return to the main program so i can display that the program is now ready to exit or task completed ... and also how can i synchronise the threads so that i may not get any write memory exceptions

[dimm_coder]

After U 've run a thread (with CreateThread(), etc) U have a thread handle . U can use WaitForSingleObject()/WaitForMultipleObjects() api functions to determine when one of the thread handle states is changed (in this context it means that thread is ended).
So U need run all of your threads, then put all thread handles to the WaitForMultipleObjects() function and wait until all handles will be in signaled state. Thus will means that all are ended.
Then U can write a message that work is done.

[Andreas Masur]

Originally posted by dimm_coder
After U 've run a thread (with CreateThread(), etc) U have a thread handle . U can use WaitForSingleObject()/WaitForMultipleObjects() api functions to determine when one of the thread handle states is changed (in this context it means that thread is ended).
So U need run all of your threads, then put all thread handles to the WaitForMultipleObjects() function and wait until all handles will be in signaled state. Thus will means that all are ended.
Then U can write a message that work is done.

Note, that this will block your application and also the responsiveness of the GUI (if any)...therefore I would suggest to take a look at 'MsgWaitForSingleObject()'/'MsgWaitForMultipleObjects()'...

[Andreas Masur]

Originally posted by Sanai
and also how can i synchronise the threads so that i may not get any write memory exceptions

Well...the question would be what you are trying to synchronize...the threads itselves or a resource which is accessed by multiple threads...

1. Synchronize threads

class CFoo
{
public:
  ~CFoo() { CloseHandle(m_hEvent); }

  void CreateThreads()
  {
    // Create event
    m_hEvent = CreateEvent(0, TRUE, FALSE, 0);

    CreateThread(0, 0, ThreadOne, &m_hEvent, 0, 0);
    CreateThread(0, 0, ThreadTwo, &m_hEvent, 0, 0);
  }


private:
  HANDLE m_hEvent;

  static UINT ThreadOne(LPVOID Param)
  {
    HANDLE *pEvent = static_cast<HANDLE *>(Param);

    // Wait for event
    if(WaitForSingleObject(*pEvent, INFINITE) == WAIT_OBJECT_0)
    {
      // Thread two is done...start working

      // Reset event
      ResetEvent(*pEvent);
    }
  }

  static UINT ThreadTwo(LPVOID Param)
  {
    HANDLE *pEvent = static_cast<HANDLE *>(Param);

    // Do processing

    // Set event
    SetEvent(*pEvent);
  }
};

Note that the above sample does not handle any error situations though....


2. Synchronize access to resource

class CSync
{
public:
  CSync() { InitializeCriticalSection(&m_CriticalSection); }
  ~CSync() { DeleteCriticalSection(&m_CriticalSection); }
  void Acquire() { EnterCriticalSection(&m_CriticalSection); }
  void Release() { LeaveCriticalSection(&m_CriticalSection); }


private:
  CRITICAL_SECTION m_CriticalSection;              // Synchronization object
};

class CLockGuard
{
public:
  CLockGuard(CSync &refSync) : m_refSync(refSync) { Lock(); }
  ~CLockGuard() { Unlock(); }


private:
  CSync &m_refSync;                     // Synchronization object

  CLockGuard(const CLockGuard &refcSource);
  CLockGuard &operator=(const CLockGuard &refcSource);
  void Lock() { m_refSync.Acquire(); }
  void Unlock() { m_refSync.Release(); }
};

The above mechanism can be used as follows...

int   iGlobalInt = 10;
CSync GlobalSync;

// Thread 1
CLockGuard Gate(GlobalSync);
iGlobalInt = 12;

// Thread 2
CLockGuard Gate(GlobalSync);
iGlobalInt = 22;