Question

I’m just working on getting at samples for processing from the microphone. I have an audio unit set up for input and output and both have render callbacks. My question is about the callback for the microphone callback.

I want core-audio to allocate the buffer in the microphone callback for me.

 UInt32 shouldAllocateBuffer = 1;
AudioUnitSetProperty(audioUnit, kAudioUnitProperty_ShouldAllocateBuffer, kAudioUnitScope_Global, 1, &shouldAllocateBuffer, sizeof(shouldAllocateBuffer));

Doing this however always results in a NULL ioData pointer in the callback. Am I stuck allocating my own buffer?

Input

static OSStatus recordingCallback(void *inRefCon, 
                          AudioUnitRenderActionFlags *ioActionFlags, 
                          const AudioTimeStamp *inTimeStamp, 
                          UInt32 inBusNumber, 
                          UInt32 inNumberFrames, 
                          AudioBufferList *ioData) {

OSStatus status;
status = AudioUnitRender(audioUnit, 
                         ioActionFlags, 
                         inTimeStamp, 
                         inBusNumber, 
                         inNumberFrames, 
                        ioData);  // ioData is null here
}

Playback

static OSStatus playbackCallback(void *inRefCon, 
                                  AudioUnitRenderActionFlags *ioActionFlags, 
                                  const AudioTimeStamp *inTimeStamp, 
                                  UInt32 inBusNumber, 
                                  UInt32 inNumberFrames, 
                                  AudioBufferList *ioData) {    
// ioData is not NULL here but I get silence in the headphones.
    return noErr;
}

Answer 1

AudioUnitRender still expects the AudioBufferList parameter (ioData) to be non-NULL.

If ioData is a pointer to AudioBufferList, then the buffer that should be is ioData->mBuffers[0].mData for most cases (see docs). If you are not allocating buffers, the buffer list should probably be a local variable, since it is only valid for the callback.

So, basically use a local variable and not ioData.

Note that you still need to set up your AudioBufferList to match the topology you are describing.

Here’s an example:

AudioBufferList bufferList;
bufferList.mNumberBuffers = 1;
bufferList.mBuffers[0].mData = NULL;
OSStatus status;
status = AudioUnitRender(audioUnit, 
                         ioActionFlags, 
                         inTimeStamp, 
                         inBusNumber, 
                         inNumberFrames, 
                         &bufferList); // bufferList.mBuffers[0].mData is null
// Use the input data in now valid bufferList.mBuffers[0].mData
// note the buffer is only valid for the callback.

Note that if you need more than one buffers (you probably don’t, even for stereo), you can use malloc to extend the size of the mBuffers array.