It does not matter whether the OS is involved. Consider a spinlock; if it is spinning, waiting on the lock to be released, then it is blocking. What matters i

It does not matter whether the OS is involved. Consider a spinlock; if it is spinning, waiting on the lock to be released, then it is blocking. What matters is whether control returns to the process before the operation is complete. If the process waits, it is blocking (aka synchronous); if the process does not wait, it is non-blocking (and possibly also asynchronous if it checks later to see if the operation succeeded).