When to release CMBlockBufferRef?

Question

I'm reading LPCM samples from a track in the iPod library, by means of the export functionality of AV Foundation. Most of my code is borrowed from Chris Adamson's example here.

I'm setting up a new CMBlockBufferRef and retaining it with CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer. According to the apple CFType reference, I need to use CFRelease on any object I explicitly retain.

The problem is that I can't seem to figure out where exactly to release the CMBlockBufferRef object. If I don't release it at all, I get a large memory leak. If I try to release it anywhere inside my loop, the app crashes with a EXC_BAD_ACCESS error in my GrabAudioData method. If I try to initiate and release it outside of the loop, the memory leak is still there. I've tried testing to see if the blockBuffer is NULL, before releasing but that doesn't work. Please help!

while (assetReader.status != AVAssetReaderStatusCompleted)
{
    if (assetReader.status == AVAssetReaderStatusReading)
    {
        CMSampleBufferRef nextBuffer = [readerOutput copyNextSampleBuffer];

        if (nextBuffer)
        {
            AudioBufferList bufferList;
            CMBlockBufferRef blockBuffer;
            CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer(nextBuffer, NULL, &bufferList, sizeof(bufferList), NULL, NULL, kCMSampleBufferFlag_AudioBufferList_Assure16ByteAlignment, &blockBuffer);

            // Using the AudioBufferList
            bufferManager->GrabAudioData(&bufferList); 

            CMSampleBufferInvalidate(nextBuffer);
            CFRelease(nextBuffer);

            // Releasing here causes a crash
            // CFRelease(blockBuffer);
        }
        else
        {
            break;
        }
    }    
}

Does this [post](http://stackoverflow.com/questions/4972677/reading-audio-samples-via-avassetreader) help? — Steve McFarlin, Jun 22 '12 at 21:09
Thanks Steve, I've actually gone over that post over and over again, I just can't seem to get my own specific code to work. Definitely a great resource though. — dpcasady, Jun 23 '12 at 06:59
I'm wondering if the problems might actually lie in what I'm doing with the data and not actually the release of it. — dpcasady, Jun 23 '12 at 07:01

score 2 · Answer 1 · answered Jul 11 '12 at 06:18

2

blockBuffer is retained from nextBuffer, so it should be released BEFORE nextBuffer.

answered Jul 11 '12 at 06:18

sinn246

21
1

score 1 · Accepted Answer · answered Sep 27 '12 at 02:02

In addition to @sinn246's answer, you must also check the return of CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer:

OSStatus err = CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer(nextBuffer, NULL, &bufferList, sizeof(bufferList), NULL, NULL, kCMSampleBufferFlag_AudioBufferList_Assure16ByteAlignment, &blockBuffer);

if (!err && blockBuffer && bufferList.mBuffers[0].mData && (bufferList.mBuffers[0].mDataByteSize > 0))
{
    // Using the AudioBufferList
    bufferManager->GrabAudioData(&bufferList);
}

if (blockBuffer) // Double check that what you are releasing actually exists!
{
    CFRelease(blockBuffer);
}

CMSampleBufferInvalidate(nextBuffer);
CFRelease(nextBuffer);

When to release CMBlockBufferRef?

2 Answers2