ARC Weak Reference Zeroing Limits in Multithreaded Contexts

Automatic Reference Counting (ARC) in Objective-C manages object lifecycles by automatically zeroing out __weak references when the referenced object is deallocated. This behavior prevents dangling pointers by ensuring the pointer becomes nil before it is accessed.

In complex multithreaded environments, there is a potential race condition between the runtime's process of zeroing the weak pointer and a concurrent read operation on another thread. While the runtime handles the synchronization of the reference count, the exact atomicity of the zeroing process relative to pointer access is a critical design consideration for thread safety.

• Does the Objective-C runtime guarantee that a __weak reference access is atomic relative to the deallocation and zeroing process?
• Under what conditions might a thread read a non-nil value for a weak reference while the object is simultaneously being reclaimed?