AVFoudation 将视频转为NSData

Question

 - (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection{
    CMBlockBufferRef blockBufferRef = CMSampleBufferGetDataBuffer(sampleBuffer);

    size_t length = CMBlockBufferGetDataLength(blockBufferRef);
    Byte buffer[length];
    CMBlockBufferCopyDataBytes(blockBufferRef, 0, length, buffer);
    NSData *data = [NSData dataWithBytes:buffer length:length];
    NSLog(@"video data %lu",data.length);
}

预览层显示没啥问题，添加了视频输出，但打印那输出的总是0啊，这是为什么呢？