Versions Compared


  • This line was added.
  • This line was removed.
  • Formatting was changed.


  1. Every mutex has two members, _WaitEvHandle, which is a automatic reset Win32 event handle used for blocking, and _ThreadId, the ID of the current thread
  2. _ThreadId is initialized to 0
  3. To lock, mtx_lock calls InterlockedCompareExchange on _ThreadId. If the return value is 0, we successfully locked the mutex. If it is not 0, then we must block, so go ahead and block on the event handle
  4. To unlock, mtx_unlock sets _ThreadId to 0 and invokes SetEvent to unblock a thread
  5. Note that at no point once someone has blocked on the mutex will the _State member return to -1 until all threads blocked on the mutex 

Waits with timeouts

Timed waits must be careful about lowering the _State variable after their wait times out. In particular, if after the timeout _State == 0, then they actually just got granted the mutex - and, more importantly - the event object is set. When this occurs, they must do a wait with a timeout of zero on the event object and repeat this every time the wait times out until _State no longer equals 0 (which indicates somebody else took the mutex) or until the wait completes (indicating that they got the mutex)