The base of Linux is the kernel. You could replace each and every library, but as long as the Linux kernel remained, it would still be Linux. The kernel contains device drivers, memory management, process management and communication management. The kernel hacker gurus follow POSIX guidelines which sometimes makes programming easier and sometimes harder. If your program behaves differently on a new Linux kernel release, chances are that a new POSIX guideline has been implemented. For programming information about the Linux kernel, read the Linux Kernel Hacker's Guide.