Using KUnit

The purpose of this document is to describe what KUnit is, how it works, how it is intended to be used, and all the concepts and terminology that are needed to understand it. This guide assumes a working knowledge of the Linux kernel and some basic knowledge of testing.

For a high level introduction to KUnit, including setting up KUnit for your project, see Getting Started.

Organization of this document

This document is organized into two main sections: Testing and Mocking. The first covers what a unit test is and how to use KUnit to write them. The second covers how to use KUnit to mock out dependencies and make it possible to unit test code that was otherwise un-unit-testable.


What is KUnit?

“K” is short for “kernel” so “KUnit” is the “(Linux) Kernel Unit Testing Framework.” KUnit is intended first and foremost for writing unit tests; it is general enough that it can be used to write integration tests; however, this is a secondary goal. KUnit has no ambition of being the only testing framework for the kernel; for example, it does not intend to be an end-to-end testing framework.

What is Unit Testing?

A unit test is a test that tests code at the smallest possible scope, a unit of code. For the C programming language, defining a unit is easy: it is the compilation unit; however, that does not mean compilation units cannot be too large and should not be broken down to get more coverage or just because they are hard to reason about.

A unit test should test all the publicly exposed functions in a unit; so that is all the functions that are exported in either a class (defined below) or all functions which are not static.

Writing Tests

Test Cases

The fundamental unit in KUnit is the test case. A test case is a function with the signature void (*)(struct test *test). It calls a function to be tested and then sets expectations for what should happen. For example:

void example_test_success(struct test *test)

void example_test_failure(struct test *test)
        FAIL(test, "This test never passes.");

In the above example example_test_success always passes because it does nothing; no expectations are set, so all expectations pass. On the other hand example_test_failure always fails because it calls FAIL, which is a special expectation that logs a message and causes the test case to fail.

Preferably, a single test case will be pretty short, it should be pretty easy to understand, and it should really only try to test at most a handful of very closely relating things.


An expectation is a way to make an assertion about what a piece of code should do in a test. An expectation is called like a function. A test is made by setting expectations about the behavior of a piece of code under test; when one or more of the expectations fail, the test case fails and information about the failure is logged. For example:

void add_test_basic(struct test *test)
        EXPECT_EQ(test, 1, add(1, 0));
        EXPECT_EQ(test, 2, add(1, 1));
        EXPECT_EQ(test, 0, add(-1, 1));
        EXPECT_EQ(test, INT_MAX, add(0, INT_MAX));
        EXPECT_EQ(test, -1, add(INT_MAX, INT_MIN));

In the above example add_test_basic makes a number of assertions about the behavior of a function called add; the first parameter is always of type struct test *, which contains information about the current test context; the second parameter, in this case, is what the value is expected to be; the last value is what the value actually is. If add passes all of these expectations, the test case, add_test_basic will pass; if any one of these expectations fail, the test case will fail.

To learn about more expectations supported by KUnit, see Test API.


KUnit also has the concept of an assertion. An assertion is just like an expectation except the assertion immediately terminates the test case if it is not satisfied.

For example:

static void mock_test_do_expect_default_return(struct test *test)
        struct mock_test_context *ctx = test->priv;
        struct mock *mock = ctx->mock;
        int param0 = 5, param1 = -5;
        const char *two_param_types[] = {"int", "int"};
        const void *two_params[] = {&param0, &param1};
        const void *ret;

        ret = mock->do_expect(mock,
                              "test_printk", test_printk,
                              two_param_types, two_params,
        ASSERT_NOT_ERR_OR_NULL(test, ret);
        EXPECT_EQ(test, -4, *((int *) ret));

In this example, the method under test should return a pointer to a value, so if the pointer returned by the method is null or an errno, we don’t want to bother continuing the test since the following expectation could crash the test case if the pointer is null. ASSERT_NOT_ERR_OR_NULL(…) allows us to bail out of the test case if the appropriate conditions have not been satisfied to complete the test.

Modules / Test Suites

Because test cases are supposed to test just a single function and then only test a set of closely related concepts about that function, multiple test cases are usually needed to fully test a unit. This is where the concept of a test suite or a module comes in; it is just a collection of test cases for a unit of code. In addition to specifying a set of test cases, a test suite also supports specifying setup and tear down functions which allow functions to be specified to run before and after each test case, respectively; this is useful when the procedure to setup a unit for testing requires several steps and needs to be done in every test case.


static struct test_case example_test_cases[] = {

static struct test_module example_test_module[] = {
        .name = "example",
        .init = example_test_init,
        .exit = example_test_exit,
        .test_cases = example_test_cases,

In the above example the test suite, example_test_module, would run the test cases example_test_foo, example_test_bar, and example_test_baz, each would have example_test_init called immediately before it and would have example_test_exit called immediately after it. module_test(example_test_module) registers the test suite with the KUnit test framework.


A test case will only be run if it is associated with a test suite.

For a more information on these types of things see the Test API.


The most important aspect of unit testing that other forms of testing do not provide is the ability to limit the amount of code under test to a single unit. In practice, this is only possible by being able to control what code gets run when the unit under test calls a function and this is usually accomplished through some sort of indirection where a function is exposed as part of an API such that the definition of that function can be changed without affecting the rest of the code base. In the kernel this primarily comes from two constructs, classes, structs that contain function pointers that are provided by the implementer, and architecture specific functions which have definitions selected at compile time.


Classes are not a construct that is built into the C programming language; however, it is an easily derived concept. Accordingly, pretty much every project that does not use a standardized object oriented library (like GNOME’s GObject) has their own slightly different way of doing object oriented programming; the Linux kernel is no exception.

The central concept in the kernel object oriented programming is the class. In the kernel, a class is a struct that contains function pointers. This creates a contract between implementers and users since it forces them to use the same function signature without having to call the function directly. In order for it to truly be a class, the function pointers must specify that a pointer to the class, known as a class handle, be one of the parameters; this makes it possible for the member functions (also known as methods) to have access to member variables (more commonly known as fields) allowing the same implementation to have multiple instances.

Typically a class can be overridden by child classes by embedding the parent class in the child class. Then when a method provided by the child class is called, the child implementation knows that the pointer passed to it is of a parent contained within the child; because of this, the child can compute the pointer to itself because the pointer to the parent is always a fixed offset from the pointer to the child; this offset is the offset of the parent contained in the child struct. For example:

struct shape {
        int (*area)(struct shape *this);

struct rectangle {
        struct shape parent;
        int length;
        int width;

int rectangle_area(struct shape *this)
        struct rectangle *self = container_of(this, struct shape, parent);

        return self->length * self->width;

void rectangle_new(struct rectangle *self, int length, int width)
        self->parent.area = rectangle_area;
        self->length = length;
        self->width = width;

In this example (as in most kernel code) the operation of computing the pointer to the child from the pointer to the parent is done by container_of.

Mocking Classes

In order to unit test a piece of code that calls a method in a class, the behavior of the method must be controllable, otherwise the test ceases to be a unit test and becomes an integration test. KUnit allows classes to be mocked which means that it generates subclasses whose behavior can be specified in a test case. KUnit accomplishes this with two sets of macros: the mock generation macros and the EXPECT_CALL macro.

For example, let’s say you have a file named drivers/foo/example.c and it contains the following code you would like to test:

int example_bar(struct example *example)
        if (example->foo(example, 5))
                return -EIO;
                return 0;

For the purposes of this example, assume struct example is defined as such:

struct example {
        int (*foo)(struct example *, int);

You would create a test file named drivers/foo/example-test.c and it would contain the following code:

/* Define the mock. */


                         PARAMS(struct example *, int));

static int example_init(struct MOCK(example) *mock_example)
        struct example *example = mock_get_trgt(mock_example);

        example->foo = foo;
        return 0;

DEFINE_STRUCT_CLASS_MOCK_INIT(example, example_init);

/* Define the test cases. */

static void foo_example_test_success(struct test *test)
        struct MOCK(example) *mock_example = test->priv;
        struct example *example = mock_get_trgt(mock_example);
        struct mock_expectation *handle;

        handle = EXPECT_CALL(foo(mock_get_ctrl(mock_example), int_eq(test, 5)));
        handle->action = int_return(test, 0);

        EXPECT_EQ(test, 0, example_bar(example));

static void foo_example_test_failure(struct test *test)
        struct MOCK(example) *mock_example = test->priv;
        struct example *example = mock_get_trgt(mock_example);
        struct mock_expectation *handle;

        handle = EXPECT_CALL(foo(mock_get_ctrl(mock_example), int_eq(test, 5)));
        handle->action = int_return(test, -EINVAL);
        EXPECT_EQ(test, -EINVAL, example_bar(example));

static int example_test_init(struct test *test)
        test->priv = CONSTRUCT_MOCK(example, test);
        if (!test->priv)
                return -EINVAL;

        return 0;

static void example_test_exit(struct test *test)

static struct test_case foo_example_test_cases[] = {

static struct test_module foo_example_test_module = {
        .name = "example",
        .init = example_test_init,
        .exit = example_test_exit,
        .test_cases = foo_example_test_cases,

foo_example_test_success uses the mock allocated in init. It asserts that will get called with 5 as a parameter with the int_eq parameter matcher. EXPECT_CALL the returns a handle that a user can use to specify additional behavior on the mock; it must always specify a return value using an action. Finally, it calls the function under test.

For more information on class mocking see Class and Function Mocking.

Mocking Arbitrary Functions


Always prefer class mocking over arbitrary function mocking where possible. Class mocking has a much more limited scope and provides more control.

Sometimes it is necessary to mock a function that does not use any class style indirection. First and foremost, if you encounter this in your own code, please rewrite it so that uses class style indirection discussed above, but if this is in some code that is outside of your control you may use KUnit’s function mocking features.

KUnit provides macros to allow arbitrary functions to be overridden so that the original definition is replaced with a mock stub. For most functions, all you have to do is label the function __mockable:

int __mockable example(int arg) {...}

If a function is __mockable and a mock is defined:


When the function is called, the mock stub will actually be called.


There is no performance penalty or potential side effects from doing this. When not compiling for testing, __mockable compiles away.


__mockable does not work on inlined functions.


Sometimes it is desirable to have a mock function that delegates to the original definition in some or all circumstances. This is possible by making the function redirect-mockable:

DEFINE_REDIRECT_MOCKABLE(i2c_add_adapter, RETURNS(int), PARAMS(struct i2c_adapter *));
int REAL_ID(i2c_add_adapter)(struct i2c_adapter *adapter)

This allows the function to be overridden by a mock as with __mockable; however, it associates the original definition of the function with alternate symbol that KUnit can still reference. This makes it possible to mock the function and then have the mock delegate to the original function definition with the INVOKE_REAL(...) action:

static int aspeed_i2c_test_init(struct test *test)
        struct mock_param_capturer *adap_capturer;
        struct mock_expectation *handle;
        struct aspeed_i2c_test *ctx;
        int ret;

        ctx = test_kzalloc(test, sizeof(*ctx), GFP_KERNEL);
        if (!ctx)
                return -ENOMEM;
        test->priv = ctx;

        handle = EXPECT_CALL(
        handle->action = INVOKE_REAL(test, i2c_add_adapter);
        ret = of_fake_probe_platform_by_name(test,
        if (ret < 0)
                return ret;

        ASSERT_PARAM_CAPTURED(test, adap_capturer);
        ctx->adap = mock_capturer_get(adap_capturer, struct i2c_adapter *);

        return 0;

For more information on function mocking see Class and Function Mocking.

Platform Mocking

The Linux kernel generally forbids normal code from accessing architecture specific features. Instead, low level hardware features are usually abstracted so that architecture specific code can live in the arch/ directory and all other code relies on APIs exposed by it.

KUnit provides a mock architecture that currently allows mocking basic IO memory accessors and in the future will provide even more. A major use case for platform mocking is unit testing platform drivers, so KUnit also provides helpers for this as well.

In order to use platform mocking, CONFIG_PLATFORM_MOCK must be enabled in your kunitconfig.

For more information on platform mocking see Platform Mocking.

Method Call Expectations

Once we have classes and methods mocked, we can place more advanced expectations. Previously, we could only place expectations on simple return values. With the EXPECT_CALL() macro, which allows you to make assertions that a certain mocked function is called with specific arguments given some code to be run.

Basic Usage

Imagine we had some kind of dependency like this:

struct Printer {
        void (*print)(int arg);

// Printer's print
void printer_print(int arg)

struct Foo {
        struct Printer *internal_printer;
        void (*print_add_two)(struct Foo*, int);

// Foo's print_add_two:
void foo_print_add_two(struct Foo *this, int arg)
        internal_printer->print(arg + 2);

and we wanted to test struct Foo’s behaviour, that foo->print_add_two actually adds 2 to the argument passed. To properly unit test this, we create mocks for all of struct Foo’s dependencies, like struct Printer. We first setup stubs for MOCK(Printer) and its print function.

In the real code, we’d assign a real struct Printer to the internal_printer variable in our struct Foo object, but in the test, we’d construct a struct Foo with our MOCK(Printer).

Finally, we can place expectations on the MOCK(Printer).

For example:

static int test_foo_add_two(struct test *test)
        struct MOCK(Printer) *mock_printer = get_mocked_printer();
        struct Foo *foo = initialize_foo(mock_printer);

        // print() is a mocked method stub
        EXPECT_CALL(print(any(test), int_eq(12)));

        foo->print_add_two(foo, 10);

Here, we expect that the printer’s print function will be called (by default, once), and that it will be called with the argument 12. Once we’ve placed expectations, we can call the function we want to test to see that it behaves as we expected.


Above, we see any and int_eq, which are matchers. A matcher simply asserts that the argument passed to that function call fulfills some condition. In this case, any() matches any argument, and int_eq(12) asserts that the argument passed to that function must equal 12. If we had called: foo->print_add_two(foo, 9) instead, the expectation would not have been fulfilled. There are a variety of built-in matchers: Class and Function Mocking has a section about these matchers.


EXPECT_CALL() only works with mocked functions and methods. Matchers may only be used within the function inside the EXPECT_CALL().

Additional EXPECT_CALL() Properties

The return value of EXPECT_CALL() is a struct mock_expectation(). We can capture the value and add extra properties to it as defined by the struct mock_expectation() interface.

Times Called

In the previous example, if we wanted assert that the method is never called, we could write:

struct mock_expectation* handle = EXPECT_CALL(...);
handle->min_calls_expected = 0;
handle->max_calls_expected = 0;

Both those fields are set to 1 by default and can be changed to assert a range of times that the method or function is called.

Mocked Actions

Because mock_printer is a mock, it doesn’t actually perform any task. If the function had some side effect that struct Foo requires to have been done, such as modifying some state, we could mock that as well.

Each expectation has an associated struct mock_action() which can be set with handle->action. By default, there are two actions that mock return values. Those can also be found in Class and Function Mocking.

Custom actions can be defined by simply creating a struct mock_action() and assigning the appropriate function to do_action. Mocked actions have access to the parameters passed to the mocked function, as well as have the ability to change / set the return value.

The Nice, the Strict, and the Naggy

KUnit has three different mock types that can be set on a mocked class: nice mocks, strict mocks, and naggy mocks. These are set via the corresponding macros NICE_MOCK(), STRICT_MOCK(), and NAGGY_MOCK(), with naggy mocks being the default.

The type of mock simply dictates the behaviour the mock exhibits when expectations are placed on it.


Naggy (default)


Method called with no expectations on it

Do nothing

Prints warning for uninteresting call

Fails test, prints warning uninteresting call

Method called with no matching expectations on it

Fails test, prints warnings, prints tried expectations

Test ends with an unfulfilled expectation

Fail test, print warning

These macros take a MOCK(struct_name) and so should be used when retrieving the mocked object. Following the example in Getting Started, there was this test case:

static void misc_example_bar_test_success(struct test *test)
        struct MOCK(misc_example) *mock_example = test->priv;
        struct misc_example *example = mock_get_trgt(mock_example);
        struct mock_expectation *handle;

        handle = EXPECT_CALL(misc_example_foo(mock_get_ctrl(mock_example),
                                              int_eq(test, 5)));
        handle->action = int_return(test, 0);

        EXPECT_EQ(test, 0, misc_example_bar(example));

If we wanted mock_example to be a nice mock instead, we would simply write:

struct MOCK(misc_example) *mock_example = NICE_MOCK(test->priv);