How do I call CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer?

Question

I\'m trying to figure out how to call this AVFoundation function in Swift. I\'ve spent a ton of time fiddling with declarations and syntax, and got this far.

Answer 1

The answers posted here make assumptions about the size of the necessary AudioBufferList -- which may have allowed them to have work in their particular circumstance, but didn't work for me when receiving audio from a AVCaptureSession. (Apple's own sample code didn't work either.)

The documentation on CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer is not obvious, but it turns out you can ask the function it how big AudioListBuffer item should be first, and then call it a second time with an AudioBufferList allocated to the size it wants.

Below is a C++ example (sorry, don't know Swift) that shows a more general solution that worked for me.

// ask the function how big the audio buffer list should be for this
// sample buffer ref
size_t requiredABLSize = 0;
err = CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer(sampleBuffer,
                      &requiredABLSize,
                      NULL,
                      NULL,
                      kCFAllocatorSystemDefault,
                      kCFAllocatorSystemDefault,
                      kCMSampleBufferFlag_AudioBufferList_Assure16ByteAlignment,
                      NULL);

// allocate an audio buffer list of the required size
AudioBufferList* audioBufferList = (AudioBufferList*) malloc(requiredABLSize);
// ensure that blockBuffer is NULL in case the function fails
CMBlockBufferRef blockBuffer = NULL;

// now let the function allocate fill in the ABL for you
err = CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer(sampleBuffer,
                      NULL,
                      audioBufferList,
                      requiredABLSize,
                      kCFAllocatorSystemDefault,
                      kCFAllocatorSystemDefault,
                      kCMSampleBufferFlag_AudioBufferList_Assure16ByteAlignment,
                      &blockBuffer);

// if we succeeded...
if (err == noErr) {
   // la la la... read your samples...
}

// release the allocated block buffer
if (blockBuffer != NULL) {
    CFRelease(blockBuffer);
    blockBuffer = NULL;
}

// release the allocated ABL
if (audioBufferList != NULL) {
    free(audioBufferList);
    audioBufferList = NULL;
}

I'll leave it up to the Swift experts to offer an implementation in that language.