@bun-win32/avrt

Zero-dependency, zero-overhead Win32 AVRT bindings for Bun on Windows.

Overview

@bun-win32/avrt exposes the avrt.dll exports using Bun's FFI. It provides a single class, Avrt, which lazily binds native symbols on first use. You can optionally preload a subset or all symbols up-front via Preload().

avrt.dll is the Multimedia Class Scheduler Service (MMCSS) client library: it lets a thread register with a multimedia task ("Audio", "Pro Audio", "Games", …), raise its scheduling priority, query the system-responsiveness reservation, and coordinate work across threads via thread-ordering groups — the same primitives low-latency audio and capture engines use.

The bindings are strongly typed for a smooth DX in TypeScript.

Features

• Bun-first ergonomics on Windows 10/11.
• Direct FFI to avrt.dll (MMCSS thread characteristics, AVRT priorities, system responsiveness, thread-ordering groups).
• In-source docs in structs/Avrt.ts with links to Microsoft Docs.
• Lazy binding on first call; optional eager preload (Avrt.Preload()).
• No wrapper overhead; calls map 1:1 to native APIs.
• Strongly-typed Win32 aliases (see types/Avrt.ts).

Requirements

• Bun runtime
• Windows 10 or later

Installation

bun add @bun-win32/avrt

Quick Start

import Avrt, { AVRT_PRIORITY } from '@bun-win32/avrt';

// Join the "Pro Audio" MMCSS task; TaskIndex must be 0 on the first call.
const taskIndex = Buffer.alloc(4);
const handle = Avrt.AvSetMmThreadCharacteristicsW(Buffer.from('Pro Audio\0', 'utf16le').ptr, taskIndex.ptr);

if (handle !== 0n) {
  Avrt.AvSetMmThreadPriority(handle, AVRT_PRIORITY.AVRT_PRIORITY_CRITICAL);

  const responsiveness = Buffer.alloc(4);
  Avrt.AvQuerySystemResponsiveness(handle, responsiveness.ptr);
  console.log(`${responsiveness.readUInt32LE(0)}% reserved for non-MMCSS threads`);

  // ... run latency-sensitive work ...

  Avrt.AvRevertMmThreadCharacteristics(handle); // always pair with the join
}

[!NOTE] AI agents: see AI.md for the package binding contract and source-navigation guidance. It explains how to use the package without scanning the entire implementation.

Examples

Run the included examples:

bun run example/mmcss-jitter-scope.ts
bun run example/mmcss-profile-report.ts

Notes

• Either rely on lazy binding or call Avrt.Preload().
• Always pair AvSetMm*ThreadCharacteristics with AvRevertMmThreadCharacteristics.
• Thread-ordering-group APIs require the thread-ordering server; it is disabled on some SKUs (AvRtCreateThreadOrderingGroup then fails with ERROR_SERVICE_DISABLED).
• Windows only. Bun runtime required.
• SAL types & naming: nullability is in the type — Optional<T> (formally optional, SAL _*opt_) and Nullable<T> (plain [in]/[out] the docs say can be NULL), the null sentinel derived from T (null for pointers LP*/P*, 0n for handles/by-value addresses); direction is in the parameter name — _out (_Out_), _in_out (_Inout_), _In_ bare. See AI.md and the repo AGENTS.md.