LKL (Linux Kernel Library) is aiming to allow reusing the Linux kernel code as extensively as possible with minimal effort and reduced maintenance overhead.
Examples of how LKL can be used are: creating userspace applications (running on Linux and other operating systems) that can read or write Linux filesystems or can use the Linux networking stack, creating kernel drivers for other operating systems that can read Linux filesystems, bootloaders support for reading/writing Linux filesystems, etc.
With LKL, the kernel code is compiled into an object file that can be directly linked by applications. The API offered by LKL is based on the Linux system call interface.
LKL is implemented as an architecture port in arch/lkl. It uses host operations defined by the application or a host library (tools/lkl/lib).
$ pkg install binutils gcc49 gnubc
If you don't have a gcc binary: $ ln -sf /usr/local/bin/gcc49 /usr/local/bin/gcc
Prefer ports binutils and GNU bc(1): $ export PATH=/sbin:/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/usr/lib64/ccache
$ cd tools/lkl $ gmake
% cd tools/lkl % make
will build LKL as a object file, it will install it in tools/lkl/lib together with the headers files in tools/lkl/include then will build the host library, tests and a few of application examples:
-
tests/boot - a simple applications that uses LKL and exercises the basic LKL APIs
-
fs2tar - a tool that converts a filesystem image to a tar archive
-
cptofs/cpfromfs - a tool that copies files to/from a filesystem image
The supported hosts for now are POSIX and Windows userspace applications.
Q: How is LKL different from UML?
A: UML prodivides a full OS environment (e.g. user/kernel separation, user processes) and also has requirements (a filesystem, processes, etc.) that makes it hard to use it for standalone applications. UML also relies heavily on Linux hosts. On the other hand LKL is designed to be linked directly with the application and hence does not have user/kernel separation which makes it easier to use it in standalone applications.
Q: How is LKL different from LibOS?
A: LibOS re-implements high-level kernel APIs for timers, softirqs, scheduling, sysctl, SLAB/SLUB, etc. LKL behaves like any arch port, implementing the arch level operations requested by the Linux kernel. LKL also offers a host interface so that support for multiple hosts can be implemented.