Eventfd

eventfd Aggregates Notifications in a Kernel Counter

An eventfd can absorb several notification writes before userspace services the descriptor. The kernel stores those writes in a 64-bit counter, so readiness represents pending counter state rather than a queue containing one record per notification. That distinction matters in event loops. A producer can add values while a consumer is occupied, and the next read can collapse accumulated state into one result. With EFD_SEMAPHORE, the same object exposes a different consumption rule without changing its readiness model.