I'm investigating an intermittent crash in an iOS audio app when launching it through an Inter-App Audio (IAA) host Audiobus and AUM.
The crash occurs during the host connection/startup process. The app begins initializing its audio engine, successfully creates and registers an ABAudioSenderPort, starts its output audio unit, and then crashes.
Crash details
Exception:
EXC_BAD_ACCESS KERN_INVALID_ADDRESS at 0x50
Backtrace:
#0 _dispatch_sync_f
#1 caulk::concurrent::details::dispatch_receiver::~dispatch_receiver
#2 std::__1::__shared_weak_count::__release_shared
#3 caulk::concurrent::messenger::~messenger
#4 CADeprecated::CAAutoDelete<caulk::concurrent::messenger>::set
#5 AURemoteIO::IPC_Uninitialize
#6 AURemoteIO::IPC_Uninitialize
#7 AUWrappedOutputUnit::AUUninitialize
#8 _XUninitialize
#9 IPCAudioUnit_server
#10 dispatch_mig_server
#11 _dispatch_client_callout
#12 _dispatch_continuation_pop
#13 _dispatch_source_latch_and_call
#14 _dispatch_source_invoke
#15 _dispatch_lane_serial_drain
#16 _dispatch_lane_invoke
#17 _dispatch_root_queue_drain_deferred_wlh
#18 _dispatch_workloop_worker_thread
#19 _pthread_wqthread
Repro:
Quit Audiobus completely.
Relaunch Audiobus.
Launch the app from Audiobus.
The app begins initialization, but crashes during or shortly after the initial host connection.
Interestingly, if Audiobus remains open and I attempt to launch the app again, the crash doesn't appear to occur. Reloading the app via AUM is also reproducing the crash. A number of other apps are able to launch via Audiobus, so I think it must be how we are initialising our engine, but I haven't been able to find where we are going wrong.
The app configures its audio session as follows:
Category: AVAudioSessionCategoryPlayback
Mode: AVAudioSessionModeDefault
Preferred sample rate: 48000 Hz
Preferred I/O buffer duration: approximately 5.3 ms
Reported buffer size: 256 frames
Input channels: 0
Output channels: 2
Audio-unit initialization
The engine creates and initializes its audio unit successfully:
AudioComponentInstanceNew: status = 0
AudioUnitInitialize: status = 0
AudioOutputUnitStart: status = 0
The app then creates an ABAudioSenderPort, configures its client format, and registers it with ABAudiobusController. These operations also complete successfully.
The engine's RemoteIO audio unit and the sender port's audio unit are separate instances. I haven't yet worked out which instance is being uninitialized when the crash occurs.
This seems to be happening across multiple iPhone and iPad models, back to at least iOS 17, and my test environment, Xcode 26.6 and iPadOS 26.5.2.
Questions
- Has anyone encountered this caulk::concurrent::messenger::~messenger / AURemoteIO::IPC_Uninitialize crash when using IAA, Audiobus, AUM?
- Could this indicate a race or invalid lifecycle transition in Apple's RemoteIO/IAA IPC implementation?
- Are there known requirements concerning the order of audio-unit initialization, sender-port registration, host connection, and uninitialization that could explain this behavior?
- What additional diagnostics would help determine which audio-unit instance is being uninitialized and what triggers the failure?
Any suggestions or reports of similar behavior would be greatly appreciated.