kqueue

kqueue is an event notification interface used in BSD-derived operating systems such as FreeBSD, NetBSD, OpenBSD, and Apple's macOS. It provides scalable monitoring of a variety of event sources through a single kernel queue. An application creates a kqueue object with kqueue(), registers interests via kevent(), and retrieves events by calling kevent() to poll or wait for them. The API separates changes from event retrieval: you pass a changelist to kevent() to add, modify, or remove interests, and you pass a separate array to collect events that have occurred.

Events are described by kevent structures, which include ident (an identifier for the source, typically a file descriptor or other handle), filter (the type of event), flags (operation modifiers such as EV_ADD, EV_DELETE, EV_ENABLE, EV_DISABLE), fflags (event-specific flags), data (an integer with additional information), and udata (a user-supplied pointer carried through to the user).

Filters cover different categories: EVFILT_READ and EVFILT_WRITE monitor I/O readiness on file descriptors; EVFILT_VNODE monitors file system changes to files and directories; EVFILT_TIMER implements timers; EVFILT_SIGNAL watches POSIX signals; EVFILT_PROC tracks process-related events; EVFILT_AIO supports asynchronous I/O; other filters exist on some systems.

Usage notes and behavior: kqueue supports varied delivery semantics depending on the flags used; it allows batching and targeted notifications, and can be configured for efficient delivery of a large number of events. It scales well for servers and GUI applications that monitor many event sources.

Platform availability: kqueue is standard on macOS and BSD variants. It is not part of the Linux kernel, which relies on epoll, inotify, and related interfaces; some Linux environments provide compatibility layers but not native kqueue.