LKL (Linux Kernel Library) is aiming to allow reusing the Linux kernel code as extensively as possible with minimal effort and reduced maintenance overhead. It allows us to link the library with any programs (which wish to call as a function call) containing Linux kernel code. There are many use cases: reading/writing files without general system calls, putting experimental protocol implementation without neither of host kernel update nor kernel module installation, using customized kernel in container instance, building a unikernel based on existing rumprun framework, or testing/fuzzing kernel implementation in userspace execution, etc.