DaemonNews.org has a story on the the FreeBSD sysctl mechanism, which is based on the so-called linker set technology. "It lets us gather information of a running kernel and
configure it to some extent without rebuilding a new kernel. It can also be very useful for kernel debugging. For example, you may want
to use a sysctl command to modify your TCP keepalive parameters, test new VFS write behind features, or simply to know when the
machine was booted up."
April 14, 2001