This is an internal datastructure which can be directly passed to epoll_wait to reduce the overhead. typedef union epoll_data { void *ptr; int fd; uint32_t u32; uint64_t u64; } epoll_data_t; struct epoll_event { uint32_t events; // Epoll events epoll_data_t data; // User data variable }; We use fd if the epoll_data union to store the actual file descriptor of an AbstractEpollChannel and so be able to map it later.