Question

What happens if we forcefully kill a running thread

I have a thread namely RecordThread() which calls some complex and time consuming functions. In these functions I am using try-catch blocks, allocating and deallocation memory and using critical section variables etc.

like

  void RecordThread()
  {
    AddRecord();
    FindRecord();
    DeleteRecord();
    // ...
    ExitThread(0);
   } 

After creating this thread, I am immediately killing it before the thread completes its execution. In this case what happens if the thread is forcefully killed? Do the internal functions (AddRecord, DeleteRecord) complete their execution after we killed the thread?

Answer 1

After creating this thread, I am immediately killing it before the thread completes its execution.

I assume you mean you are using TerminateThread() in the following fashion:

HANDLE thread = CreateThread(...);

// ...
// short pause or other action?
// ...

TerminateThread(thread, 0); // Dangerous source of errors!
CloseHandle(thread);

If that is the case, then no, the thread executing RecordThread() will be stopped exactly where it is at the time that the other thread calls TerminateThread(). As per the notes in the TerminateThread() documentation, this exact point is somewhat random and depends on complex timing issues which are out of your control. This implies that you can’t handle proper cleanup inside a thread and thus, you should rarely, if ever, kill a thread.

The proper way to request the thread to finish is by using WaitForSingleObject() like so:

HANDLE thread = CreateThread(...);

// ...
// some other action?
// ...

// you can pass a short timeout instead and kill the thread if it hasn't
// completed when the timeout expires.
WaitForSingleObject(thread, INFINITE);
CloseHandle(thread);