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I am working on understanding Core Audio, or rather: Extended Audio File Services

Here, I want to use ExtAudioFileRead() to read some audio data from a file.
This works fine as long as I use one single huge buffer to store my audio data (that is, one AudioBuffer ). As soon as I use more than one AudioBuffer , ExtAudioFileRead() returns the error code -50 ("error in parameter list"). As far as I can figure out, this means that one of the arguments of ExtAudioFileRead() is wrong. Probably the audioBufferList .

I can not use one huge buffer because then, dataByteSize would overflow its UInt32 -integer range with huge files.

Here is the code to create the audioBufferList :

AudioBufferList *audioBufferList;
audioBufferList = malloc(sizeof(AudioBufferList) + (numBuffers-1)*sizeof(AudioBuffer));
audioBufferList->mNumberBuffers = numBuffers;
for (int bufferIdx = 0; bufferIdx<numBuffers; bufferIdx++ ) {
    audioBufferList->mBuffers[bufferIdx].mNumberChannels = numChannels;
    audioBufferList->mBuffers[bufferIdx].mDataByteSize = dataByteSize;
    audioBufferList->mBuffers[bufferIdx].mData = malloc(dataByteSize);
And here is the working, but overflowing code:
UInt32 dataByteSize = fileLengthInFrames * bytesPerFrame; // this will overflow
AudioBufferList *audioBufferList = malloc(sizeof(audioBufferList));
audioBufferList->mNumberBuffers = 1;
audioBufferList->mBuffers[0].mNumberChannels = numChannels;
audioBufferList->mBuffers[0].mDataByteSize = dataByteSize;
audioBufferList->mBuffers[0].mData = malloc(dataByteSize);
And finally, the call of ExtAudioFileRead() (should work with both versions):
UInt32 numFrames = fileLengthInFrames;
error = ExtAudioFileRead(extAudioFileRef,
                         &numFrames,
                         audioBufferList);
Do you know what I am doing wrong here?
I think you're misunderstanding the purpose of the mNumberBuffers field. It's typically 1 for mono and interleaved stereo data. The only reason you would set it to something else is for multi-track data where each channel is in a separate data buffer.
If you want to read a part of a file, you would set dataByteSize of the buffer to a reasonable size, and when you read the file, tell the API only to give you that many bytes, and loop over it.
                I am not sure if this argument is correct since in the aurioTouch2 sample, mNumberBuffers is set to be 2, I am wondering if someone know the reason.
– user454083
                Aug 31, 2013 at 21:40
                And by the way if I want to simultaneously either encode or decode buffer, how I could do this work in separate thread
– user454083
                Aug 31, 2013 at 21:44
I also had the same problem with mNumberBuffers > 1,,...
My work around involved creating my own internal buffer:
something like:
char buffer1[byteSize];
char buffer2[byteSize];
......
you could also use pointers to make things easier,..like: 
buffer[index][byteSize];
then you would have to iterate through them manually and fill them up on the main thread to avoid glitches in audio.
ExtAudioFileRead will only fill buffer[0] in the AudioBufferList, you could then have that pointing to different manually allocated buffers as the audio plays.
hope this helps.
                That was not exactly what I wanted to do, but it certainly helps if you want to fill several different buffers with different data. (My initial thought was that I could fill several buffers with consecutive chunks of data from the same file).
– bastibe
                Apr 6, 2010 at 14:36
                That's not true. I believe mNumberBuffers>1 is used for multi-channel audio. Just that the rest of the AudioBufferList structure has to make sense.
– notthetup
                Sep 17, 2012 at 8:11
        
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