What does “Allocated Prior to Attach” mean in Xcode instruments?

Question

I am new to Xcode and the profiling tool, instruments. I tried to find leaks of my c program. Everything is fine. It shows no leaks, but I don’t understand some terms used in the program. Does mean I still have memory leaks somewhere else in the program?

Answer 1

Applications like the debugger and Instruments may hook into your program after the OS has already started it running. What the message is saying is that it doesn't know how a given piece of memory was allocated because it was allocated before Instruments hooked into your program. So it has no way to track where it came from. It may or may not mean that there are more actual leaks in your program.

I believe that you can start your program from Instruments in most cases, which I think would eliminate the issue. Once you've run it once, you should be able to press the Record button (the red circle) again and it should run the app once more. I think in that situation, it should have access to all of the allocations your app makes.

For what it's worth, I have seen this message before and had it go away during the same run. It was as if it suddenly found the data that explained where the leak came from. I've also seen it happen on one run and be gone on a subsequent run with the same data. In that case it seemed like a timing issue of Instruments and not a legitimate leak.