# Socket An async non-blocking TCP socket backed by libevent. ``` final class Socket ``` ## Overview `Socket` wraps a POSIX file descriptor configured for non-blocking I/O and drives it through an [`EventLoop`](/documentation/Event/EventLoop) so that [`read(maxBytes:)`](/documentation/Event/Socket/read(maxBytes:)) and [`write(_:)`](/documentation/Event/Socket/write(_:)) resume their callers only when the kernel indicates data readiness. The async surface (`async throws` methods) composes naturally with Swift structured concurrency; the underlying mechanism is libevent callbacks bridging to `CheckedContinuation`s. `Socket` covers three roles: - **TCP client** — acquired via ``doc://Event/documentation/Event/Socket/connect(to:port:loop:)`` or ``doc://Event/documentation/Event/Socket/connect(to:loop:)``. - **Accepted server connection** — delivered by ``doc://Event/documentation/Event/ServerSocket/accept()`` or the ``doc://Event/documentation/Event/ServerSocket/connections`` stream. - **Wrapping an existing descriptor** — not exposed publicly (the initializer is `internal`), but used by the library itself when an external fd must be adopted. Server-listener construction lives on [`listen(port:backlog:loop:)`](/documentation/Event/Socket/listen(port:backlog:loop:)) and [`listen(on:backlog:loop:)`](/documentation/Event/Socket/listen(on:backlog:loop:)), which return a [`ServerSocket`](/documentation/Event/ServerSocket) — not a `Socket`. ### Resource ownership A `Socket` owns its descriptor by default: `deinit` calls `close(2)`. The `ownsDescriptor` initializer flag exists so callers can wrap an externally-owned fd without double-close risk; this path is internal today. The single-ownership invariant is a constitution Principle II concern — see [Production Considerations](/documentation/Event/ProductionConsiderations). ### Concurrency Marked `@unchecked Sendable` to permit handoff of ownership across task boundaries. Concurrent I/O on the same `Socket` from multiple tasks (e.g. `read` from one task while `write` from another) is **undefined behavior**: the libevent callback state is not synchronized, and `fd` is mutated unconditionally in `deinit`. Use structured concurrency scopes to bound ownership. See [Production Considerations](/documentation/Event/ProductionConsiderations) “Concurrency Model” for the honest description.