au-review

name

au-review

description

Reviews AudioUnit v2/v3 plugin implementations for spec compliance, thread safety, and correctness. Use when the user asks to review an AudioUnit plugin, check an AUv3 app extension, audit property or parameter handling, or asks "is my AUComponent thread-safe?" or "why does my AU crash in Logic?". Trigger on phrases like "review my AudioUnit", "check my AUv3", "is my render block safe?", or when you see AURenderCallback, internalRenderBlock, kAudioUnitProperty_*, or AUAudioUnit in the code.

AudioUnit (AUv2 / AUv3) Review

AUv2 runs in-process inside the host; AUv3 runs in a sandboxed XPC extension. Both share the same realtime-callback constraints, but AUv3 adds ObjC/Swift runtime exposure and app-extension lifecycle rules that introduce a distinct class of violations.

Step 0 — Run universal checks first

Invoke audio-dsp-review and audio-numerics-review before this skill. This skill adds AU-specific checks on top; it does not replace realtime-safety or numerics reviews.

Step 1 — Identify AU version and structure

Determine which AU variant is under review:

Variant	Entry point	Render surface
AUv2 (Component Manager)	AudioComponentRegister / AUMIDIEffectFactory	AURenderCallback
AUv3 (App Extension)	AUAudioUnit subclass, NSExtensionPrincipalClass	internalRenderBlock

Locate: the render callback / render block, allocateRenderResourcesAndReturnError:, deallocateRenderResources, supportedViewConfigurations, and all kAudioUnitProperty_* / AUParameter access sites.

Step 2 — Scan for AU-specific violations

Violation	Where to look	Risk
ObjC message send ([obj msg] / .property) in render callback	AURenderCallback body, internalRenderBlock capture body	ObjC runtime uses locks and may allocate; not realtime-safe
Render block calls through self, weak references, Swift closures, or ObjC properties	AUv3 render block	Runtime calls and weak-reference loads are not realtime-safe
kAudioUnitProperty_* read/written from render thread	Any render-path property access	AudioUnitGetProperty / SetProperty are not thread-safe
Parameter ramps not applied sample-accurately	AUParameterEvent handling, AUv2 renderProc	Parameter jumps produce zipper noise on DAW automation
Tail time not reported via kAudioUnitProperty_TailTime	Reverb, delay, convolution AUs	Host silences the AU before tail has decayed
AUv3 extension missing com.apple.security.app-sandbox entitlement	Xcode target entitlements	App Store / Notarisation rejection; Logic refuses to load
NSFileManager, NSUserDefaults, or UserDefaults accessed from render block	AUv3 internalRenderBlock	Main-thread-only / file-I/O APIs; realtime violation and potential deadlock
AUAudioUnit subclass not calling super in allocateRenderResources	allocateRenderResourcesAndReturnError: override	Resources for format negotiation never initialised; silent misrender
Format negotiation not honouring outputBusses[n].format before allocating	allocateRenderResources	Mismatch between agreed and actual format; distortion or crash
auval failures not addressed	Overall AU implementation	Plugin rejected or silently disabled by Logic, GarageBand, and MainStage

Step 3 — Write the review

Same format as audio-dsp-review / audio-numerics-review: Verdict · Critical Violations (category, file:line, Why, Fix) · Warnings · What's Done Well · Recommended Fixes (priority order) Prefix the header with AudioUnit Review: [file / class].

auval checklist

Run auval -a to list registered AUs; validate with type/subtype/manufacturer codes: auval -v aufx MYFX MFGR (effect) · auval -v aumu MYSY MFGR (instrument)

Common auval failure	Cause	Fix
AU does not support kAudioUnitProperty_CocoaUI	Missing NSView-based UI factory	Implement AUCocoaViewInfo + property handler
Render returned error	Non-zero OSStatus from render callback	Validate stream format; guard every render call
TailTime not set	Reverb/delay returns 0 tail time	Return duration from kAudioUnitProperty_TailTime
Latency reported as 0	Plugin adds latency but reports none	Return correct samples from kAudioUnitProperty_Latency
Parameter not in range	Default value outside AUParameterInfo min/max	Clamp default inside declared range

Quick fix table

Violation	Fix
ObjC call in render block	Cache plain C/C++ DSP objects, buffers, and atomics before returning the block; call ObjC only outside callback
self / weak-self use in render block	Capture raw C++ kernel pointers or __unsafe_unretained ivars once; do not load weak refs or call properties per render
Property access from render thread	Use std::atomic<> or lock-free FIFO; read/write properties only from main/audio-graph thread
No sample-accurate parameter ramps	Process AUParameterEvent list per-sample; use a one-pole smoother for missing events
Missing tail time	AUv2: implement kAudioUnitProperty_TailTime; AUv3: override tailTime in seconds
Sandbox entitlement missing	Enable the AUv3 app-extension sandbox; add file/network entitlements only when the extension actually needs them
NSUserDefaults in render block	Store params via AUParameter tree; load defaults only in init / allocateRenderResources