Documentation/dev-tools/kunit/tips.rst

.. 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. */
		KUNIT_ASSERT_NOT_ERR_OR_NULL(test, object);
		do_something_with_object(object);
	}

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``.

Example:

.. code-block:: c

	void example_test_allocation(struct kunit *test)
	{
		char *buffer = kunit_kzalloc(test, 16, GFP_KERNEL);
		/* Ensure allocation succeeded. */
		KUNIT_ASSERT_NOT_ERR_OR_NULL(test, buffer);

		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();

	#ifdef CONFIG_MY_KUNIT_TEST
	#include "my_kunit_test.c"
	#endif

Injecting test-only code
------------------------

Similarly to the above, it can be useful to add test-specific logic.

.. code-block:: c

	/* In my_file.h */

	#ifdef CONFIG_MY_KUNIT_TEST
	/* Defined in my_kunit_test.c */
	void test_only_hook(void);
	#else
	void test_only_hook(void) { }
	#endif

TODO(dlatypov@google.com): add an example of using ``current->kunit_test`` in
such a hook when it's not only updated for ``CONFIG_KASAN=y``.

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];
	generate_sha1_hex_string(some_str);

	/* 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.