epoll

epoll is a scalable I/O event notification facility built into the Linux kernel, designed to efficiently monitor large numbers of file descriptors for readiness to perform I/O. It was introduced with Linux 2.6 to address the limited scalability of traditional polling mechanisms like poll and select when dealing with many connections.

The API centers on an epoll instance created with epoll_create or epoll_create1. File descriptors are registered with epoll_ctl using operation codes EPOLL_CTL_ADD, EPOLL_CTL_MOD, or EPOLL_CTL_DEL, and are associated with an event mask that can include flags such as EPOLLIN, EPOLLOUT, EPOLLERR, EPOLLRDHUP, and others. Each registration also carries user data via the epoll_event structure. The epoll_wait call blocks (or with a timeout) until one or more registered fds become ready, returning an array of epoll_event structures describing the ready FDs and their events.

epoll supports both level-triggered and edge-triggered modes. By default, epoll uses level-triggered notifications, where an FD remains reported as ready as long as it is ready. In edge-triggered mode (EPOLLET), events are delivered only when the state changes, which can reduce event flood but requires the programmer to drain the socket or consume all available data to avoid missing subsequent events.

Internally, epoll maintains a red-black tree of registered FDs and a ready list of FDs with pending events, allowing efficient event retrieval even with many descriptors. This design provides near-constant-time event handling for large sets of fds, with memory usage proportional to the number of registered descriptors.

epoll is Linux-specific and not available on other operating systems. It is commonly used in high-performance network servers and event-driven applications that manage many concurrent connections, often in combination with non-blocking I/O and edge-triggered processing.