asynchronous cancellation

[Jason Nye <jnye@nbnet.nb.ca>]

Hi, all

I've noticed a lot of you discussing asynchronous cancellation and
mentioning how difficult it is to do under win32. I did some research
and found a bullet-proof way of doing it (from a J. Richter example).
Here is a sample of the code I used in my library:

If thread x wants to cancel thread y asynchronously, thread x should
call cancelThread(y):

---------------------------------------------------------------------------------------------------

void cancelThread(HANDLE hThread)
{
    ::SuspendThread(hThread);
    if (::WaitForSingleObject(hThread, 0) != WAIT_TIMEDOUT) {
            // Ok, thread did not exit before we got to it.
           CONTEXT context;
           context.ContextFlags = CONTEXT_CONTROL;
           ::GetThreadContext(hThread, &context);
           // _x86 only!!!
           context.Eip = (DWORD)AsyncCancelPoint;
           ::SetThreadContext(hThread, &context);
           ::ResumeThread(hThread);
    }
}

// declare AsyncCancelPoint:
void AsyncCancelPoint()
{
    // pSelf is a pointer to a ThreadInfo (each thread has one --
declared as __declspec(thread)).
    popCancelCleanupHandlers(pSelf);
    callDestructors(pSelf);
    _endthreadex(PTHREAD_CANCELLED);
}
---------------------------------------------------------------------------------------------------

That is it. If a thread's cancel state is asynchronous and another
thread requests that it be cancelled, the thread will suddenly find
itself executing AsyncCancelPoint which is exactly what you want.

If you want to see it in action in my C++ library, go to
http://www3.nbnet.nb.ca/jnye , follow the "current software projects"
link and download the latest version of ObjectThread. You'll see how it
fits into a complete library.

Hopefully this is useful,
Jason