.. SPDX-License-Identifier: GPL-2.0

=========================================
KUnit - Unit Testing for the Linux Kernel
=========================================

.. toctree::
	:maxdepth: 2

	start
	usage
	api/index
	faq

What is KUnit?
==============

KUnit is a lightweight unit testing and mocking framework for the Linux kernel.
These tests are able to be run locally on a developer’s workstation without a VM
or special hardware.

KUnit is heavily inspired by JUnit, Python's ``unittest.mock``, and
Googletest/Googlemock for C++. They have the same structure for defining test
suites and test cases. KUnit defines a way to mock out C style classes and
functions and create expectations on methods called within the code under test.

Get started now: :doc:`start`

Why KUnit?
==========

Aside from KUnit there is no true unit testing framework for the Linux kernel.
Autotest and kselftest are sometimes cited as unit testing frameworks; however,
they are not by most reasonable definitions of unit tests.

A unit test is supposed to test a single unit of code in isolation, hence the
name. A unit test should be the finest granularity of testing and as such should
allow all possible code paths to be tested in the code under test; this is only
possible if the code under test is very small and does not have any external
dependencies outside of the test's control like hardware.

Outside of KUnit, there are no testing frameworks currently
available for the kernel that do not require installing the kernel on a test
machine or in a VM and all require tests to be written in userspace running on
the kernel; this is true for Autotest, and kselftest, disqualifying
any of them from being considered unit testing frameworks.

KUnit addresses the problem of being able to run tests without needing a virtual
machine or actual hardware with User Mode Linux. User Mode Linux is a Linux
architecture, like ARM or x86; however, unlike other architectures it compiles
to a standalone program that can be run like any other program directly inside
of a host operating system; to be clear, it does not require any virtualization
support; it is just a regular program.

KUnit is fast. Excluding build time, from invocation to completion KUnit can run
several dozen tests in only 10 to 20 seconds; this might not sound like a big
deal to some people, but having such fast and easy to run tests fundamentally
changes the way you go about testing and even writing code in the first place.
Linus himself said in his `git talk at Google
<https://gist.github.com/lorn/1272686/revisions#diff-53c65572127855f1b003db4064a94573R874>`_:

	"... a lot of people seem to think that performance is about doing the
	same thing, just doing it faster, and that is not true. That is not what
	performance is all about. If you can do something really fast, really
	well, people will start using it differently."

In this context Linus was talking about branching and merging,
but this point also applies to testing. If your tests are slow, unreliable, are
difficult to write, and require a special setup or special hardware to run,
then you wait a lot longer to write tests, and you wait a lot longer to run
tests; this means that tests are likely to break, unlikely to test a lot of
things, and are unlikely to be rerun once they pass. If your tests are really
fast, you run them all the time, everytime you make a change, and everytime
someone sends you some code. Why trust that someone ran all their tests
correctly on every change when you can just run them yourself in less time than
it takes to read his / her test log?

How do I use it?
===================

*   :doc:`start` - for new users of KUnit
*   :doc:`usage` - for a more detailed explanation of KUnit features
*   :doc:`api/index` - for the list of KUnit APIs used for testing