Crate pipecrab_audio_cpal 

pipecrab-audio-cpal: the cpal-backed audio backend for pipecrab.

CpalSource captures from an input device and CpalSink plays to an output device, both implementing the platform-neutral AudioSource / AudioSink traits — so a pipeline drives audio I/O without ever naming cpal. Both are opened from one shared CpalConfig (which device per side, plus chunk/buffer sizing); input_device_names / output_device_names enumerate the choices for DeviceSelection::Name.

A native backend (macOS, Windows, Linux, iOS, Android). The browser/wasm audio path is a separate future crate, not cpal: cpal’s wasm backend runs on the main thread and isn’t the intended web path, so this crate is not built for wasm32.

The real-time boundary

cpal’s device callbacks run on a real-time thread that must never block, allocate, or lock. Each callback only moves f32 samples across a lock-free rtrb ring and wakes a futures::task::AtomicWaker; the async side (next_chunk / play) polls the ring and registers that waker. Waking from the callback is a pragmatic simplification — glitch-free at these ~20 ms buffer sizes; a strict wait-free bridge is deferred.

The AudioSource / AudioSink trait is Send, but a cpal::Stream is !Send. So the stream is built and parked on a dedicated owning thread (see stream), and CpalSource / CpalSink hold only the Send ring end — a server can spawn one pump per session.

Format & timing

Capture and playback run at their device’s default sample rate (no resampling — a shared-clock same-device setup keeps the rates matched), mono end to end: multi-channel input is downmixed, mono output is duplicated across the device’s channels. Chunk size and ring depth come from CpalConfig.

Structs

CpalConfig

Configuration shared by the source and the sink.

CpalSink

Plays mono f32 audio to an output device.

CpalSource

Captures mono f32 audio from an input device.

Enums

DeviceSelection

Which device to open.

Functions

input_device_names

Names of the available input (capture) devices, for building a DeviceSelection::Name.

output_device_names

Names of the available output (playback) devices, for building a DeviceSelection::Name.