ioevent

An ioevent is an abstraction used in asynchronous input/output systems to represent a notification about the status of an I/O resource. It is used to implement event-driven I/O where a program reacts to readiness or completion events rather than blocking on I/O operations. An ioevent is typically associated with a handle such as a file descriptor or I/O object and a set of events of interest (for example, readability, writability, error, or hang-up).

Registration occurs by submitting the ioevent to an event loop or kernel I/O subsystem. When one of the registered conditions occurs, the runtime delivers a notification through a callback, a future/promise, or a polling interface. This enables the application to initiate or complete I/O asynchronously and to multiplex many I/O sources with a single thread or process.

Common implementations and contexts include Linux epoll, kqueue on BSD/macOS, Windows IOCP, io_uring, and cross-platform libraries such as libuv and libevent. Some systems distinguish between readiness notifications (readable/writable) and completion notifications (a write has finished, an asynchronous read/write has completed). The semantics can be edge-triggered versus level-triggered, and one-shot versus persistent, affecting how notifications must be re-armed after they fire.

Notes: The exact terminology and APIs vary by platform; "ioevent" is a generic term used in documentation or higher-level libraries to refer to the input/output event handle. It is not a universal kernel object in all systems, and specific implementations may use different names for descriptors or handles.