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Kernel memory leaks may remain hidden, though, until they cause a kernel panic. Actions which are normally instant result in a pause, sometimes long enough to display the spinning beachball. The first indication that there might be a memory leak somewhere is normally a noticeable fall in performance. The only way to free that allocated memory was to quit the app, which returns all the used app memory to the general pool. If you watched the app’s memory in Activity Monitor, you could see that each time it read a file in, the memory used grew, but never fell again. The penultimate call to deallocate theBuffer is crucial: without that, the memory allocated to theBuffer simply grows and grows, every time the code reads file data into theBuffer. Hasher.update(bufferPointer: theBufferPointer) } }

Let theBufferPointer = UnsafeRawBufferPointer(start: theBuffer, count: read) Let read = theFileStream.read(theBuffer, maxLength: theBufferSize) Let theBuffer = UnsafeMutablePointer.allocate(capacity: theBufferSize) Here’s the correct code for calculating a hash on a stream read from a file, in Swift: My integrity checking apps Dintch and Fintch once had a serious leak because I had misunderstood the documentation. User or app memory leaks are far more common, and most often occur because the developer forgets to make a call to free memory when it’s no longer required. Although kernel memory leaks are very rare in release versions of macOS, they could occur in any version, not just major updates. The last major kernel memory leak occurred in the Catalina 10.15.6 update in July 2020, right at the end of its development cycle. While that shouldn’t bring the whole system down, it’s still bad news. However, a memory leak in an app also reaches a point where that app grinds to a halt. User memory is allocated to each app, and in theory is almost limitless for 64-bit apps. When kernel memory leaks, disaster is inevitable, as without the ability to use free memory, the kernel and its extensions simply can’t run, and a kernel panic results. They fall into two main groups: kernel and user memory. There are hundreds, possibly many thousands, of different pools of memory in your Mac. At that point, the leaking app comes to a grinding halt.

Over time, that accumulates, until no more memory can be allocated from that pool. In a memory leak that return doesn’t happen, and instead of the memory being freed it’s left still in use. The data might be read from a file, or created by converting other data, etc., and when that allocated memory is no longer required, it has to be freed and returned for reuse. All code running in apps and elsewhere has to allocate memory to contain data.