“内存清理器”应用程序如何确定iOS上的已用内存量？

Every now and then "memory cleaner" apps bubble to the top of the download charts on the iOS AppStore, yet I am always puzzled: How can these apps figure out how much memory is used when they are sandboxed and can't access any memory outside of their process? I'm not an expert in memory allocation on C, so maybe the solution is obvious and I just don't know it but I am curious as to how this works/could work.Every now and then "memory cleaner" apps bubble