Question

Let’s say I keep DWORD thread_Id in some structure (WinAPI) .
(I am asking about thread_ID not thread_handle in this question. Indeed it is easy to confure the two)

I need to initialize threadId variable to some some null value.

So I am going to define my NULL_THREAD_ID value.

Is it documented anywhere on msdn which DWORD value real thread can never have as thread_ID ? 0 or (DWORD)-1 ?

   HANDLE WINAPI CreateThread(
     __in_opt   LPSECURITY_ATTRIBUTES lpThreadAttributes,
     __in       SIZE_T dwStackSize,
     __in       LPTHREAD_START_ROUTINE lpStartAddress,
     __in_opt   LPVOID lpParameter,
     __in       DWORD dwCreationFlags,
     __out_opt  LPDWORD lpThreadId
    );

Answer 1

GetThreadId, for example, gives you the answer to the puzzle. It returns the thread ID and the documentation states:

If the function fails, the return value is zero.

Therefore, zero can never be a valid thread ID.