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 |