"shm_open failed" and error 3 from AudioUnitInitialize with RemoteIO

Question

I am trying to get microphone input from Core Audio (the Remote I/O audio unit) with the following code:

- (void)audioAU
{
    enum : AudioUnitElement {
        kOutputElement = 0,
        kInputElement = 1
    };
    const UInt32 disableFlag = 0;
    const UInt32 enableFlag = 1;

    OSStatus err = noErr;
    NSError *error = nil;


    // Configure & activate audio session

    AVAudioSession *session = [AVAudioSession sharedInstance];

    if (![session setCategory:AVAudioSessionCategoryRecord error:&error]) NSLog(@"Error configuring session category: %@", error);
    if (![session setMode:AVAudioSessionModeMeasurement error:&error]) NSLog(@"Error configuring session mode: %@", error);
    if (![session setActive:YES error:&error]) NSLog(@"Error activating audio session: %@", error);

    NSLog(@"Session activated. sample rate %f", session.sampleRate);
    NSLog(@"Number of channels %d", session.inputNumberOfChannels);


    // Set up Remote I/O audio unit for audio capture

    AudioComponent component = AudioComponentFindNext(NULL, &(const AudioComponentDescription){
        .componentType = kAudioUnitType_Output,
        .componentSubType = kAudioUnitSubType_RemoteIO,
        .componentManufacturer = kAudioUnitManufacturer_Apple,
        .componentFlags = 0,
        .componentFlagsMask = 0
    });

    AudioComponentInstance unit;

    // Create audio component
    err = AudioComponentInstanceNew(component, &unit);
    if (err != noErr) NSLog(@"Error instantiating audio unit: %ld", err);

    // Enable input
    err = AudioUnitSetProperty(unit, kAudioOutputUnitProperty_EnableIO, kAudioUnitScope_Input, kInputElement, &enableFlag, sizeof(enableFlag));
    if (err != noErr) NSLog(@"Error enabling input for audio unit: %ld", err);

    AudioStreamBasicDescription streamDesc = {
        .mSampleRate = session.sampleRate,
        .mFormatID = kAudioFormatLinearPCM,
        .mFormatFlags = kAudioFormatFlagsAudioUnitCanonical /*matches AudioUnitSampleType*/ | kAudioFormatFlagIsNonInterleaved,
        .mBytesPerPacket = sizeof(AudioUnitSampleType),
        .mFramesPerPacket = 1,
        .mBytesPerFrame = sizeof(AudioUnitSampleType) * session.inputNumberOfChannels,
        .mChannelsPerFrame = session.inputNumberOfChannels,
        .mBitsPerChannel = 8 * sizeof(AudioUnitSampleType),
        .mReserved = 0,
    };
    err = AudioUnitSetProperty(unit, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Input, kOutputElement, &streamDesc, sizeof(streamDesc));
    if (err != noErr) NSLog(@"Error configuring input stream format for audio unit: %ld", err);

    AURenderCallbackStruct callbacks = {
        .inputProc = renderCallback,
        .inputProcRefCon = unit
    };
    err = AudioUnitSetProperty(unit, kAudioOutputUnitProperty_SetInputCallback, kAudioUnitScope_Input, kOutputElement, &callbacks, sizeof(callbacks));
    if (err != noErr) NSLog(@"Error configuring input callbacks for audio unit: %ld", err);

    err = AudioUnitInitialize(unit);
    if (err != noErr) NSLog(@"Error initializing audio unit: %ld", err);

    err = AudioOutputUnitStart(unit);
    if (err != noErr) NSLog(@"Error starting audio unit: %ld", err);

//  err = AudioComponentInstanceDispose(unit);
//  if (err != noErr) NSLog(@"Error disposing audio unit: %ld", err);
}

It works (or at least doesn't crash) in the simulator, but I get this output when running on an iPhone 5:

Session activated. sample rate 44100.000000
Number of channels 1
15:48:45.170 <com.apple.main-thread> shm_open failed: "AppleAURemoteIO.o.262da" (23) flags=0x2 errno=2
15:48:45.172 <com.apple.main-thread> AURemoteIO::ChangeHardwareFormats: error 3
15:48:45.333 <com.apple.main-thread> shm_open failed: "AppleAURemoteIO.o.262da" (23) flags=0x2 errno=2
15:48:45.336 <com.apple.main-thread> AURemoteIO::ChangeHardwareFormats: error 3
15:48:45.497 <com.apple.main-thread> shm_open failed: "AppleAURemoteIO.o.262da" (23) flags=0x2 errno=2
15:48:45.499 <com.apple.main-thread> AURemoteIO::ChangeHardwareFormats: error 3
15:48:45.669 <com.apple.main-thread> shm_open failed: "AppleAURemoteIO.o.262da" (23) flags=0x2 errno=2
15:48:45.671 <com.apple.main-thread> AURemoteIO::ChangeHardwareFormats: error 3
15:48:45.839 <com.apple.main-thread> shm_open failed: "AppleAURemoteIO.o.262da" (23) flags=0x2 errno=2
15:48:45.841 <com.apple.main-thread> AURemoteIO::ChangeHardwareFormats: error 3
Error initializing audio unit: 3

Answer 1

It turns out that when using AVAudioSessionCategoryRecord (or equivalently, kAudioSessionCategory_RecordAudio from the C APIs), you must also disable output on the audio unit:

    ...
    // Disable output
    err = AudioUnitSetProperty(unit, kAudioOutputUnitProperty_EnableIO, kAudioUnitScope_Output, kOutputElement, &disableFlag, sizeof(disableFlag));
    if (err != noErr) NSLog(@"Error disabling output for audio unit: %ld", err);