The stack head pointers can be swapped and flushed without knowing how much data is in it. However, if you need to know the count/size... Then in general you

The stack head pointers can be swapped and flushed without knowing how much data is in it. However, if you need to know the count/size... Then in general you must accept that the count/size of the write stack is dynamic, so even if you use an integer to track the value (and keep updated with atomic exchange or increment), at the point that you read its value on one thread it may have already changed on another. So it doesn't really matter if you reset the value to 0 using an interlocked exchange (this cant be done as an atomic unit with respect to the head pointers unless your platform supports a double CAS). Some loss of count/size information will occur. Alternatively you can safely iterate through the stack to calculate the count/size anytime you need it (provided the next pointers are also treated atomically). Neither option will give you the exact size since as discussed above its always dynamic, but this i no way impedes the functioning of the write/flush stacks as described. In my solution i just set the value of the count/size to 0, and disregard any loss of information since it is not critical.