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.. SPDX-License-Identifier: GPL-2.0
Tips For Writing KUnit Tests
Exiting early on failed expectations
``KUNIT_EXPECT_EQ`` and friends will mark the test as failed and continue
execution. In some cases, it's unsafe to continue and you can use the
``KUNIT_ASSERT`` variant to exit on failure.
.. code-block:: c
void example_test_user_alloc_function(struct kunit *test)
void *object = alloc_some_object_for_me();
/* Make sure we got a valid pointer back. */
Allocating memory
Where you would use ``kzalloc``, you should prefer ``kunit_kzalloc`` instead.
KUnit will ensure the memory is freed once the test completes.
This is particularly useful since it lets you use the ``KUNIT_ASSERT_EQ``
macros to exit early from a test without having to worry about remembering to
call ``kfree``.
.. code-block:: c
void example_test_allocation(struct kunit *test)
char *buffer = kunit_kzalloc(test, 16, GFP_KERNEL);
/* Ensure allocation succeeded. */
KUNIT_ASSERT_STREQ(test, buffer, "");
Testing static functions
If you don't want to expose functions or variables just for testing, one option
is to conditionally ``#include`` the test file at the end of your .c file, e.g.
.. code-block:: c
/* In my_file.c */
static int do_interesting_thing();
#include "my_kunit_test.c"
Injecting test-only code
Similarly to the above, it can be useful to add test-specific logic.
.. code-block:: c
/* In my_file.h */
/* Defined in my_kunit_test.c */
void test_only_hook(void);
void test_only_hook(void) { }
This test-only code can be made more useful by accessing the current kunit
test, see below.
Accessing the current test
In some cases, you need to call test-only code from outside the test file, e.g.
like in the example above or if you're providing a fake implementation of an
ops struct.
There is a ``kunit_test`` field in ``task_struct``, so you can access it via
Here's a slightly in-depth example of how one could implement "mocking":
.. code-block:: c
#include <linux/sched.h> /* for current */
struct test_data {
int foo_result;
int want_foo_called_with;
static int fake_foo(int arg)
struct kunit *test = current->kunit_test;
struct test_data *test_data = test->priv;
KUNIT_EXPECT_EQ(test, test_data->want_foo_called_with, arg);
return test_data->foo_result;
static void example_simple_test(struct kunit *test)
/* Assume priv is allocated in the suite's .init */
struct test_data *test_data = test->priv;
test_data->foo_result = 42;
test_data->want_foo_called_with = 1;
/* In a real test, we'd probably pass a pointer to fake_foo somewhere
* like an ops struct, etc. instead of calling it directly. */
KUNIT_EXPECT_EQ(test, fake_foo(1), 42);
Note: here we're able to get away with using ``test->priv``, but if you wanted
something more flexible you could use a named ``kunit_resource``, see :doc:`api/test`.
Failing the current test
But sometimes, you might just want to fail the current test. In that case, we
have ``kunit_fail_current_test(fmt, args...)`` which is defined in ``<kunit/test-bug.h>`` and
doesn't require pulling in ``<kunit/test.h>``.
E.g. say we had an option to enable some extra debug checks on some data structure:
.. code-block:: c
#include <kunit/test-bug.h>
static void validate_my_data(struct data *data)
if (is_valid(data))
kunit_fail_current_test("data %p is invalid", data);
/* Normal, non-KUnit, error reporting code here. */
static void my_debug_function(void) { }
Customizing error messages
Each of the ``KUNIT_EXPECT`` and ``KUNIT_ASSERT`` macros have a ``_MSG`` variant.
These take a format string and arguments to provide additional context to the automatically generated error messages.
.. code-block:: c
char some_str[41];
/* Before. Not easy to tell why the test failed. */
KUNIT_EXPECT_EQ(test, strlen(some_str), 40);
/* After. Now we see the offending string. */
KUNIT_EXPECT_EQ_MSG(test, strlen(some_str), 40, "some_str='%s'", some_str);
Alternatively, one can take full control over the error message by using ``KUNIT_FAIL()``, e.g.
.. code-block:: c
/* Before */
KUNIT_EXPECT_EQ(test, some_setup_function(), 0);
/* After: full control over the failure message. */
if (some_setup_function())
KUNIT_FAIL(test, "Failed to setup thing for testing");
Next Steps
* Optional: see the :doc:`usage` page for a more
in-depth explanation of KUnit.