kunitoki 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 |