An API retains a callback and void* context until unregister returns. Which context is sa…

An API retains a callback and void* context until unregister returns. Which context is safe to register?

1. The address of a local variable in a helper that returns immediately
2. Storage whose lifetime extends through completed unregistration and any in-progress calls
3. Any pointer after casting it to void*
4. A pointer to a temporary object used by the registration expression