CMSampleBufferRef上的CFRelease - 为什么我需要调用它？

Question

CMSampleBufferRef sampleBuffer = [assetOutput copyNextSampleBuffer];
CMBlockBufferRef buffer = CMSampleBufferGetDataBuffer(sampleBuffer);
CMBlockBufferAppendBufferReference(_sem == 0 ? _buffer0 : _buffer1, buffer, 0, 0, 0);
//if(sampleBuffer)
//    CFRelease(sampleBuffer);

Why does this cause a memory leak at the first line (at least that's where Leaks suggests)? I have my assetOutput.shouldAlwaysCopySampleOutput = NO. Here's my understanding of the situation:

CMSampleBufferRef sampleBuffer = [assetOutput copyNextSampleBuffer];

This line will create a reference to the sample buffer from the assetOutput.

CMBlockBufferRef buffer = CMSampleBufferGetDataBuffer(sampleBuffer);

This line will get the CMBlockBuffer from the CMSampleBuffer but will not allocate a new buffer, and the Get method in this case means it is a temporary (autoreleased) buffer

CMBlockBufferAppendBufferReference(_sem == 0 ? _buffer0 : _buffer1, buffer, 0, 0, 0);

This line will append the reference of the CMBlockBuffer created above, to the selected global-scope buffer. It will not copy any memory blocks.

So in none of these three lines do I allocate any memory nor do I copy any memory, it's all references. I don't understand where the leak is coming from. I tried adding the commented out lines and it still seems to leak (although fewer times)

Answer 1

alwaysCopiesSampleData is not about memory management. It is only about whether you are scribbling on the original sample buffer or a clone of the original. It is somewhat unfortunately named.

copyNextSampleBuffer follows the create rule and as such, should be released when you are done with it. It creates a reference with a retain count of at least 1.

The Create Rule:

https://developer.apple.com/library/ios/DOCUMENTATION/CoreFoundation/Conceptual/CFMemoryMgmt/Concepts/Ownership.html#//apple_ref/doc/uid/20001148-103029

Apple's doc links tend to change, but if the above link dies, just google "The Create Rule"

Answer 2

Core Foundation data structures follows the same ownership rules as Foundation objects.

The rules are pretty simple - whatewer you create (or get the ownership by some other way), you have to destroy. If some other method wants to work with the same structure/object, it has to ask for the ownership and thus preventing the destruction.

Taking ownership = "create" / "retain"
Releasing ownership ("destruction") = "release"

In your sample code, you have created a structure using copyNextSampleBuffer . That means, you have to also destroy it using CFRelease .

(Note that with ARC you don't actually see the retain and release calls but with Core Foundation, you have to use them explicitely).