Kevent

Kevent is part of the kqueue event notification subsystem used in BSD-derived operating systems, notably FreeBSD and macOS. It provides a scalable, event-driven model for monitoring a variety of sources, including file descriptors, file system changes, timers, signals, processes, and user-defined events. A program typically creates a kqueue instance and then registers interest in certain event sources, after which it receives notifications when those events become ready.

The core API consists of two parts: kqueue() and kevent(). The kqueue() function creates a new kernel event queue and returns a descriptor used in subsequent calls. The kevent() function serves both to register or modify event interests and to retrieve events that have occurred. It accepts arrays of struct kevent for changes and for returned events, enabling batch registration and single-shot or continuous monitoring. A timeout parameter allows blocking, polling, or non-blocking behavior.

A kevent is described by a struct containing several fields: ident identifies the event source (such as a file descriptor or a vnode); filter selects the event type (for example EVFILT_READ, EVFILT_WRITE, EVFILT_TIMER, EVFILT_VNODE, EVFILT_SIGNAL, EVFILT_PROC, EVFILT_USER); flags control the operation (EV_ADD, EV_DELETE, EV_ENABLE, EV_DISABLE, EV_ONESHOT, EV_CLEAR); fflags carry filter-specific options; data provides auxiliary information; and udata is a user-defined pointer carried through to the event.

Common use cases include asynchronous I/O, scalable servers, filesystem monitoring, timers, and signal handling. The system is designed for high performance and low overhead in event-driven applications, and remains a standard facility on BSD-based platforms.