The epoll API

How

Central feature of API is the epoll file descriptor (epollfd), the userspace handle to kernel data structures.

Two kernel data structures are associated with an epollfd:

Interest list (input to kernel): file descriptors that process has registered an interest in monitoring; and
Ready list (output from kernel): file descriptors that are ready for I/O.

When an I/O event occurs that changes the "readiness" of a file descriptor in interest list, the kernel adds that file descriptor to ready list.