Unit Testing for C

Unit Testing

Each line of code you design is testable, and the most efficient way to test it is through unit-tests. In fact, we should test code before the inception of the code itself. Sounds strange? Not really, this is called Test-Driven-Development, or TDD. Without unit-testing, we may be really shit developing rather than software developing. Unit-Tests provide an important barrier very, very close to the developer to ensure quality at inception of the code.

Without unit-tests:

  • You cannot assess how well the code will work (if at all)

  • Someone could alter the code and easily break it (software regression)

  • Refactoring and improving the code is much harder, and time consuming


  • Typically automated; GDB is not a replacement

  • Testing one code module at a time, such as file.c

  • Write code to test code (same programming language)

    • Write a test case for each line of production code

    • Write a code module and test it, such as memory_buffer.c

  • Unit Testing (UT) is not necessarily testing it on your board

    • Typically, UT means you test it on the machine that is compiling the code Hence, no need to load the code to the target, or the embedded processor

    • Using a printf to manually inspect whether your code works is not unit-testing

  • In some industries such as Aerospace, it is required to run UTs on your target

    • This validates that the compiler for the target is free of bugs

    • For example, if you run UT on x86 machine, and your code is targeted on Cortex-M4, then that proves that the code will work on x86, but not necessarily on the M4


  • Develop without hardware

  • Refactor code with confidence

  • Reduce the costly debugging sessions

  • Significantly Accelerate development

  • Establish a strong barrier against bug leaks

  • Serves as a double check during development

  • Reduce the discovery, investigation, and patches for software bugs

  • Get immediate feedback if the code is working as the developer intended


The cost of writing unit-tests is negligible compared to the cost of finding and patching a bug at a later time. This cost is not just the developer time alone, but also the time of many other parties involved, including the customer itself which suffers from lost productivity. It is easy to test weird scenarios your code could get into during unit-tests, and it is difficult to force your code to go into complex scenarios during product's integration tests.

Clean C++


The cost of writing a UI test is often high, and hard to automate. In terms of raw code, it is easy to write an automated test and focus towards a very precise functionality and it is usually difficult to inject and create certain test scenarios towards the higher end of the pyramid. The cost of creating the anti-pattern below is prohibitively high.

Clean C++


Unit-Tests take too much time and slows me down

This is the most popular fictional statement. It is like saying that you do not have time to fix your bicycle when you have miles ahead to travel.

Unit-Tests are lot of maintenance

This should signal code smell, possibly in the unit-test code, production code, or both. Fundamentally, code should be modularized, and designed for simplicity to reduce maintenance. If unit-tests are a lot of maintenance, then they can probably be improved, and this may indicate potential problems with the production code.

We will hire an intern to do unit-testing later

This is an lol statement. Usually, later will never happen anyway. Further, if your code was not designed with testing in mind, then it is probably not testable. Without tests in the first place, the code likely suffers design and quality issues, and trying to perform unit-testing after the fact is not that useful. If the thought of UTs comes after the code is written, some of the benefits are already stolen from you.

It is the developers responsibility to deliver code that is believed to be bug-free, unit-tests provide a good way to ensure high quality before another un-biased party tests your code as a product black-box.

Test Frameworks

Unfortunately C and C++ do not have unit-testing built into the language, and therefore there are many test frameworks that came to existence.

  • None

    • Simply use <assert.h>

  • C & C++ Frameworks

    • Check (C)

    • CGreen (C and C++)

    • Boost (C++)

    • Google Test & Google Mocks (C++)

  • Unity & CMock – C Test framework

    • This README is focused on this

    • Small foot-print to run on the microcontroller itself

    • Combined with Cmock, provides huge value to UT and “mock” APIs


Unity & CMock

The Unity & CMock framework is one of the best frameworks we have found for C language. There has not been anything fundamental this framework lacks, and it reduces a lot of coding effort because it generates the Mock functions for you. You can read about these two frameworks here but it is advisable to read this article first.


There are two basics things to understand about a unit-test framework. The first is the unit-test framework itself; this only provides the ability to perform assertions and write tests in a way that the framework understands how to run. Some frameworks allow you to “register” for the tests, and then they will invoke all of the registered tests. Other frameworks may either use macros that register themselves, or use scripts at compile time to register and run your test methods. Unity provides the ability to run tests in a structured way.

#include "unity.h" // Single Unity Test Framework include void setUp(void) { } void tearDown(void) { } void test_something(void) { TEST_ASSERT_EQUAL(1, 1); }


CMock creates mocks and stubs for C functions. It's useful for interaction-based unit testing, where you want to test how one module interacts with other modules. Instead of trying to compile all those real units together, CMock helps you by creating fake versions of all the "other" modules. You can then use those fake versions to verify that your module is working properly!


A secondary artifact for a test framework is the "mock" functionality. The mocks provide the ability to mock-out an API and hijack the functions calls outside of your code module. For instance, you can mock an API that is going to delete a database and perform assertions. The objective would be that you are testing a code module that is interfacing to a database, but you don’t want to use the database code module itself and instead want to use a dummy mock. This will make a whole lot of sense during our unit-test examples.

CMock is useful when you wish to test a piece of code without inheriting another code module. In the following example, our focus is to test my_app() but we want to Mock database_connect() function and make it return either NULL or a pointer to the database to test further code. CMock in this scenario would allow you to "stub" the database.h API, inject, verify parameters, and make functions return whatever values you wish.

#include "database.h" void my_app(void) { db_s *db = database_connect("google"); if (NULL != db) { // Test this code } }

In your unit-test, you can then do this:

#include "Mockdatabase.h" void test_my_app(void) { // This 'Expect' API is auto-generated by CMock database_connect_ExpectAndReturn("google", NULL); my_app(); }

CMock Plugins

CMock has many plugins that are built when you compile the library. You can simply enable all of them because it is up to your test code to use it or not. Including more plugins than necessary should not cause any side effects.

  • ignore

  • ignore_arg

  • expect_any_args

  • array

  • cexception

  • callback

  • return_thru_ptr

Test Driven Development

This design technique should be used when the requirements of the code are clearly stated. It is meant to simplify code, and produce minimal code necessary to solve a problem.

One of the goals to keep in mind is that It is about how little code solves a problem, not how much and TDD helps you with this.

  1. Create empty tests

  2. Write failing test

  3. Write just enough code to pass

  4. Refactor the code, cleanup and optimize

  5. Repeat until all tests are passing


  • No dead code

  • Sign off with stakeholder

  • Unit Tests shape your production code

In TDD, after you write a test, and just enough code for the test to pass, it reaches a critical point of success:

Now we have reached a remarkable point in the process. If the tests pass now, we always have 100% unit test coverage at this step. Always! Not only 100% in the sense of a technical test coverage metric, such as function coverage, branch coverage, or statement coverage. No, much more important is, that we have 100% unit test coverage regarding the requirements that were already implemented at this point!

Stephan Roth. “Clean C++.”


Let's design a buffer module with TDD.

TODO Screencast with example


The most popular argument against writing unit-tests is that they slow down development. Absolutely nothing could be farther away from reality for this statement. If I could let me nerves respond to this comment, I would respond back by saying that whoever has made such claims is either an incompetent developer, or is simply not experienced enough. This sentiment may exist because the developer has simply not practiced unit-testing to reveal the benefits to debunk this assumption.

Clean C++ shares more or less the same sentiment on the benefits of unit-tests:

  • Fixing bugs after software has shipped is more expensive than having unit tests in place

  • Unit-tests give an immediate feedback about your entire code base. Provided that test coverage is sufficiently high (approx. 100%), developers know in just a few seconds if the code works correctly.

  • Unit tests give developers the confidence to refactor their code without fear of doing something wrong that breaks the code. In fact, a structural change in a code base without a safety net of unit tests is dangerous and should not be called Refactoring.

  • A high coverage with unit tests can prevent time-consuming and frustrating debugging sessions.

  • Unit tests are a kind of executable documentation because they show exactly how the code is designed to be used. They are, so to speak, something of a usage example.

  • Unit tests can easily detect regressions, that is, they can immediately show things that used to work, but have unexpectedly stopped working after a change in the code was made.

  • Unit testing fosters the creation of clean and well-formed interfaces. It can help to avoid unwanted dependencies between units. A Design for Testability is also a good Design for Usability...

Refactoring without tests isn’t refactoring, it is just moving shit around

— Corey Haines

Aim for 100% Test Coverage

Setting a high bar yields high quality software. Anything less than 100% coverage is an arbitrary number, and therefore, the only acceptable measure should be exactly 100% code coverage. As developers write a line of code, it should be immediately tested. This discipline pays off well because the code is testable to begin with, and often times, the developers would be motivated to write less code to solve a problem. Remember that It is not how much code you write, it is how little said one of my past co-workers, and I still remember that statement today.

Emphasize the tests that matter

There may be code that is way too trivial to test. For example, if an RTOS task code, or the code of a main() function is kept simple (and branchless), then that may be once place you can skip the unit-test effort. Generally, creating rules, and then creating exceptions is not the way to go. However, there are certain situations where this particular logic makes sense.

The case we are setting forth is that top level “glue code” may be exempt from 100% code coverage. Experience suggested that when the code is modularized, it was always the modules that were at fault, but not the code that glued different pieces together. This glue code should have the following properties:

  • No branches

  • Uses dependency injection to connect objects

  • Runs a periodic loop or spawns a task

We encourage this rule because even if we were to test this branchless code, the only thing we would test is that certain code is called in the right order with the appropriate parameters. Let’s demonstrate this by example using a top level RTOS task.

In the code above, we have these things going on:

  • Setup a buffer of 512 bytes

  • TCP connection being setup on port 1200

  • Loop to service the TCP connection

The unit-test for this code would look like this:

You should be able to now reason with this approach, but we have to proceed with a discipline that rightfully justifies this exception. In the test code above, all we are really doing is testing if certain functions are getting called, but there is no branch logic to test. The 100% code coverage of buffer, tcp_connection is actually responsible to make sure that the code works well, and there is little that could go wrong in this top-level RTOS task. Sure, you could go ahead and test that the tcp connection is not getting passed a NULL pointer for the buffer, but it is trivial enough to ensure this by running perhaps the simplest test on your target platform.

The task level code should just be the glue code that connects the TCP connection to our buffer, and then the connection utilizes the buffer to perform I/O. Statistically speaking, there is little that can go wrong in this code as it is just few jigsaw pieces that we need to connect. The actual bugs are likely to occur inside of the buffer or the TCP connection code modules, and the suggestion is to maximize the testing to 100% in these modules, rather than focusing on top level glue code that simply pieces things together.

Positive and Negative Testing

Positive and negative testing is a fundamental mindset when testing any unit. It is the idea of testing against both valid inputs and invalid inputs. Testing against invalid inputs ensure the unit under test is sufficiently robust enough to handle irrational inputs.

In this scenario, the valid input domain for resistance R is (0.0, FLOAT_MAX]. Voltage can be anything in this case.

Positive test cases (test rational resistances)

  1. Expect calculate_current(0.0, 1.0) == 0.0

  2. Expect calculate_current(0.0, FLOAT_MAX) == 0.0

Negative test cases (test irrational resistances)

  1. Expect calculate_current(0.0, 0.0) == 0.0 (No runtime divide by 0 exception should occur)

  2. Expect calculate_current(0.0, -1.0) == 0.0 (No runtime divide by 0 exception should occur)

Basics of Unity & CMock

The Unity and CMock unit-test infrastructure relies on developer discipline to create files with consistent names:

  • your_module.h Example: gps_string_parser.h

  • your_module.c Example: gps_string_parser.c

  • test_your_module.c Example: test_gps_string_parser.c

The Test framework automatically picks up source code that starts with test_ and then begins to create an executable specifically to perform the unit-test of one file at a time.

How it works

Unity and CMock framework uses Ruby and Rake to turn your test_your_module.c into a standalone executable What this means is that each test_* is actually a separate executable and is compiled by resolving the header files you included in this test_your_module.c source file.

Run Tests

Running the Unity test framework with rake is very simple.

  • Go to the folder that contains rakefile

  • Type rake on the command-prompt

    • To run a single test, type rake unit single_file=code_test\test_simple.c

  • The rake build system will run all unit-tests as separate executables

  • Ensure that gcc.yml contains the paths to your source code

Files of the Test Infrastructure

There are a few ruby files that glue things together.

  • rakefile

    • This is the entry point when you type rake to run the tests

    • Additional logic can be added here to customize the unit-test framework

  • rakefile_helper.rb

    • rakefile uses this code to compile and run tests

    • We built this from the Cmock example, and customized it

  • gcc.yml

    • rakefile.rb uses this configuration to compile your code

How it Builds

  • The ruby script forms a list of all tests that begin with test_ in your code_test folder

  • Script compiles a separate executable for each test

    • This means each unit-test file, such as test_buffer.c is a standalone program

    • All dependencies need to be #included in your test_ file

  • As part of the compilation of the unit-test executable, files are mocked

    • Each #include that begin with #include "Mock" will not build the real code, but instead it will build Mocked code

    • For foo.h, it will be Mockfoo.h and Mockfoo.c at ut_build/mocks

  • Be careful of nested dependencies, such as your code_under_test.c depending on buffer.c

    • If there is a dependency you #include which has another dependency, you will also need to #include those dependencies in your test_code_under_test.c as well. So if you do not mock buffer.c, then you will also need to build the real sources that the buffer.c depends on.

    • The other option is to mock the header file to avoid picking up nested dependencies. So if you #include "Mockbuffer.h" then you do not need to worry about the nested dependencies of buffer.c

Behind the scenes, the script will include all files you included in your test_buffer.c and use that to build the executable. The trick is that the files that are #included as Mockfoo.h are mocked, meaning that the header file is used to initially compile, but the code is linked to the CMock, instead of the real foo.c file.

Available APIs

In a lot of the sample code below, we will see some "magical" APIs, so let us first unravel the mysteries and point out what kind of function you get with Unity and CMock.


Unity provides you with:

  • Assertion APIs

  • setUp(), tearDown() invocations before and after each test method


The idea behind the Mocks is that sometimes you want to test a module and you do not want to inherit the functionality of another object that you have little or no control over. Consider a naive piece of code that you wish to test.

In the naive example above, you wish to perform two tests:

  • When database_connect() returns NULL

  • When database_connect() returns a valid database pointer

CMock provides the Mock functionality. It extends the Unity assertions, but fundamentally provides you with a make system to stub out an external module's functionality with a fake one.

The APIs are dynamic and auto-generated based on the file/module being mocked. What really happens is that when your test_app.c performs a #include "Mockdatabase.h", then the build system purposely omits the real database.c and replaces the implementation with Mock replacements.

The test code for app_test() would look like this:

To Mock or not to Mock?

This is an important decision. A general guideline is that:

  • Do not Mock APIs that are very trivial

    • Example: bit_count.h

    • Let the bit counting happen the way its meant to be

  • Mock code modules when

    • Code modules are more complex; example: database_connect()

    • Code modules may create a distraction from the object under test

You have two options; the first option is to #include "bit_count.h" at your unit-test file. This way, you do not mock this file but inherit the real functionality of this file.

Sometimes, it is easier to build the real code, such as a simple utility called bit_count.h and it is better to not mock this file. At other times, it is better to be able to hijack an API and make it do what you want for the sake of ease of unit-testing. The idea is that you are unit-testing your module, and you don't want to depend on a behavior of another module you inherited. The testing of the other module is the other module's responsibility, and not yours.

The second option is to mock this header file such that you can hijack its function calls, inject and return data for the sake of unit-testing. Which option you choose depends on your test.

When you include Mockbit_count.h, the UT framework will not build and include the real file bit_count.h. Instead, it will redirect the API as given in your header file with the mock framework's implementation which will impose Expect requirements in your unit-tests. The Expect API has a few flavors as listed in the next section.

Mock Variations

Mocks always provide StubWithCallback which means you can install your own Mock function. Typically, other variations provided should be able to do the job, but the callback can be used to do perhaps a fancy operation inside of the Mock.

The Ignore() should rarely be used. It means that always ignore whenever a function invocation occurs. The ExpectAnyArgs() is better because at least you are stating that a function is expected. Once you do Ignore() in a test function, then whether a function is called zero times, or a million times, it really is ignored.

  • Expect for functions that do not return a value

  • ExpectAndReturn for functions that return a value

  • ExpectAnyArgsAndReturn to ignore input arguments but return a value

  • Ignore dangerous method of just ignoring the expectation

  • IgnoreAndReturn Ignore but always return something for functions that return a value

  • StubWithCallback Use a custom callback that can have a small test driver of your own

What this means is that:

  • For a function with no return values: void foo(void), you will have:

    • foo_Expect()- Expect a function call to occur

    • foo_Ignore()- Ignore the function call (if any)

    • foo_StubWithCallback(your_func)- Go to your your_func() when foo() is called

  • For a function with a return value: int foo(void), you will have the word AndReturn:

    • foo_ExpectAndReturn(#)

    • foo_IgnoreAndReturn(#)

    • foo_StubWithCallback(func_ptr)

Mocks for functions that have arguments are covered in the next section, but here is a brief summary to get the idea of the generated API.

  • For a function with arguments: void foo(int arg_name), you will have:

    • foo_Expect() - Expect a function call with specific argument values

    • foo_ExpectAnyArgs() - Expect the function with no checks on any of the arguments

    • foo_IgnoreArg_arg_name() - After expecting a function call, ignore a particular argument

    • foo_StubWithCallback(func_ptr)


For each function parameter, you would have another API available per parameter. The Ignore_arg_name() API should be called after setting up the Expect().

For example, if a function to mock is void foo(int a, int b), then the following APIs are generated:

  • foo_Expect(#, #) - Expect a function call with specific args

  • foo_ExpectAnyArgs() - Expect the function call with any args

  • foo_Ignore()- Ignore the function call (if any)

  • foo_IgnoreArg_a() - Invoke after foo_Expect(a, #) to ignore the first parameter value

  • foo_IgnoreArg_b() - Invoke after foo_Expect(#, b) to ignore the second parameter value

Pointer Parameters

Functions that use pointers as parameters have more variations. For each pointer parameter, you have the following options available:

  • ReturnMemThruPtr Modify something inside of a parameter that is a pointer

    • For example, if the function parameter was named ptr then:

    • ReturnThruPtr_ptr() will be available

    • Good candidate if your pointers are void* and the length of the data is not known

  • ReturnThruPtr

    • Use when your type is known, such as void foo(int *)

    • You can then ask the Mock to return a type: int value = 2; foo_ReturnThruPtr(&value);

  • ReturnArrayThruPtr

    • Similar to ReturnThruPtr but you can return an array of integers

    • int values[] = {11, 22}; ReturnArrayThruPtr(&value, 2);

Add a Unit-Test file

If you don't want to get your hands dirty, and you want to simply leverage from the test infrastructure that is setup for you, then there is little to do:

  • Create a new C module

    • You may have to edit gcc.yml to include the new folder path of your new module

  • Create a new file that begins with test_ and put it in your test folder

    • Example: test_buffer.c

  • Go to your test folder in a command-shell

    • Type rake to run all unit-tests

    • All test files at <your_test_dir>/tests will execute their unit-tests

Unit Test Best Practices

  • Exclude third party code

  • Unit-test should run extremely quickly (in second)

  • Keep tests focused; one test per functional

  • Code quality for tests should be equal to the production code

  • Avoid hacks and blocks of #ifdef UNIT_TESTING in your production code

  • Test should be independent from each other

    • Clean-up after each test

Unit Test Examples

Test Template

The template demonstrates the basic Unity Test Framework

  • Framework picks up all functions that start with test_

  • Each test_ is surrounded by function invocations of setUp() and tearDown()

  • Since there are no include files other than C library, no further files are included while building the executable

Test Results

The unit-test above is very simple and in fact it does not test anything at all because we have no code module we are including and we are not invoking any functions. In any case, here is the code output that you will see when you run the test. Note that automatic function invocations that the unit-test infrastructure is performing for you.

Test Hello World

The simplest test is a code module that you feed input, and it spits out an output. This should be the preferred software design to make modular code and improve testability. Sometimes, it may be surprisingly simple to change a code module from one that calls another code module, to one that is independent of another code module.

The bit_counter example given below is very naive, but demonstrates a code module that does not depend on any external function call other than function invocations within itself or the standard C library.

  • bit_counter.h

  • bit_counter.c

  • test_bit_counter.c

An Example without a Mock

In this example, we walk through a typical way a software developer would write the code, and demonstrate how we can do dependency injection and improve the code's testability aspect while improving its quality, and make it more reusable.

Typical Code

Here is the typical code that one may write for a hypothetical persistency module.

Improved Code

We made a very subtle but significant change. We simply feed a boolean as the input rather than crudely calling a concrete GPIO API.

  • Our module is no longer dependent on a particular GPIO

  • We can reuse it for another purpose

  • While testing, we do not have to work with gpio.h dependency

Since we improved the code, our code module is now independent of other code modules, and we will not need to Mock anything. As a result, our testing is simplified because this is a simple input/output module.

Unit Test Simplified Persistency Module

Since our module was simplified and no longer invokes an external function such as gpio_get(), we can test it more easily because we can feed the input to our function, rather than the function collecting it from a concrete API.

An Example with a Mock

Do not worry if your code module invokes external code modules. Of course, this will happen in your software because code modules are meant to connect with each other. This is where CMock comes into the picture, and provides you with a way to hijack real APIs and mock them out to facilitate unit-tests.

Let us go back to this piece of code that we previously improved; in other words, let us go back to the code which depends on a gpio function call internally and assume this cannot be changed for whatever reason.

The unit-tests for this would look like this:

Mock with a Callback

Callbacks are designed by adding int call_count at the end of a function. What you are trying to do is that when the gpio_get() is invoked, you want to replace the same function with your own.

  • bool gpio_get(void) becomes: bool gpio_get(int call_count) and then you can call gpio_get_StubWithCallback()

  • And void foobar(int a, void *b) becomes: void foobar(int a, void *b, int call_count)

Expects stack up

The Expect() API can stack up, and you can put them inside loops too. If the number of calls to a Mock function do not match, then the test will fail. What this means is that if you do not setup an Expect() and it is actually called by the code under test, then the test will fail.

Unit-test code

Test Parameters

This example provides the sample code in which you can verify if your module under test is calling other code modules with the correct parameters.

More ways to test Parameters

Stack Parameters

When a code module calls another code module with parameters that are on the stack memory, then that is harder to test the exact values of. In fact, the stack memory values are not deterministic at all. There is a way to ignore certain parameters in this case. Let us work with a similar example as above.

Verify struct data

If parameters into a function are an explicit type (not void*), then they can be compared just like comparing standard C types such as int, char etc.

Verify struct * pointers

What is cool is that the Expect() API also works great with pointers too. As long as your pointers are of a typed data structure (not void*), then the Expect() API can compare data structures by de-referencing the pointers. For instance, this would work equally well.

The CMock ReturnThruPtr Plugin


The ReturnThruPointer add convenience when you want a function to write memory at pointers passed into functions. You can achieve the same effect by writing a custom mock, and then using StubWithCallback() but this method avoid some manual boilerplate effort.


This Mock API is the same as ReturnThruPointer except that it allows you to write an array of memory blocks.


This Mock API is more generic than the other two flavors above. You should avoid writing code that forces you to use this, however, there are cases when you must. Case in point, when the input to a Mocked function is a void* parameter, then the other two ReturnThru variants will not know how much memory to write, and hence this is where ReturnMemThruPtr comes to the rescue.

Test static

static functions and variables can be accessed by your unit-tests using a small trick. The Coding Standards article discussed the Include Order which is necessary to build code without compiler errors, while facilitating unit-testing easily.

There are a couple of ways to test static methods and access static variables. The first solution is to use the Include Order to your advantage, and then totally hijack static keyword by telling the compiler to not have the keyword take effect at all. This also means that you cannot have local variables that are static, but that is part of our Coding Standards

The second solution is to consistently use capital STATIC which is defined to static for production code, and <blank> during unit-test build. We deliberated about these two solutions and ultimately decided to use the first approach above and here is the summary of our discussion:

  • Code looks normal is not compromised in appearance due to unit-testing

    • We will not need to explain and justify a new STATIC keyword

  • STATIC is hard to enforce and will create inconsistencies

    • Developers will mix static and STATIC and slow down code-reviews

    • Developers will need training on the guideline

  • Even with STATIC, we still are dependent on an include order

The reason for this debate is that due to #define static /* blank */, if the Include Order is not followed, it can have un-intended consequencies in your unit-test build. For example, if <stdio.h> uses the static keyword, and we include sl_unit_test_facilitator.h before we include <stdio.h> then the compilation may fail. Same is true if other code modules are included that should not inherit this static hack.


static functions

It is expected that you may have code and you wish to test a static function directly. Let's assume you wish to test the following code:

You can then test by accessing the static functions directly because when your application code included sl_unit_test_facilitator.h, it re-defined static to <blank>, and thereby taking away the private meaning to the use case of static in a source file.

Although the code under test now will not use the true static keyword, we will need to still find function declarations that we can use to test code. There are two solutions here:

  • The static functions that are inside of your_code_module.c can be defined as extern at your unit-test file

    • Example below illustrates that

  • The static functions that are inside of your_code_module.c can be declared in a new file, such as your_code_module_private.h

    • This header file is included by your_code_module.c

    • This header file is also included by test_your_code_module.c

static variables

In general, global variables should be avoided when possible because you should build your code modules that operate on an instance, just like a C++ class functions operate on the instance of the class.

In the case above, the code module itself does not contain any global variables, and they operate on an instance the user provides, and this helps the testability aspect too.

However, since we do not live in a perfect world, let us assume that there are some private variables you need to access in your_module.c for testability purpose. We can access them as long as we #include "sl_unit_test_facilitator.h" at your source code.

If your module includes sl_unit_test_facilitator.h, your unit-test code can then access the static data members. Note that that we are not altering the meaning of static in production code, and this "hack" is only for unit-tests.


Unable to use static inside of a function

The one caveat of #define static to <blank> is that it may affect you when this keyword is used inside of a function, like so:

This use case should not exist and your coding standards should ban this kind of use of a static variable. In any case, this will only affect your unit-tests, and the enormous benefits that #define static provides offsets the cost of not being able to use a static variable inside of a function. Not doing so also improves your unit-testing because it is very difficult to reset a function's static variables to their initial value.

CMock compiler error

The second caveat is that we cannot globally re-define static to <blank> for the entire unit-test build. While this may work for some unit-tests, it will particularly fail for unit-tests that use Mocks. For example, we cannot create a common_header.h that conveniently re-defines static keyword because every includer of that file will inherit this hack. When CMock inherits this and creates your Mock files, there will be a link time failure because there may be duplicate symbols with the same name. The Common Headers section also discusses this issues and proposes a workaround.

Test a large blob

But why do you have a large blob? You should first consider breaking up this module into smaller modules. In the code below, for us to get as far as grid_is_on(), we have to setup a large chain of ExpectAndReturn() functions before we finally reach the grid_is_on() function.

Improvement A

In this approach, we can refactor (inverter_is_on() && inverter_is_not_faulted() && grid_is_on()) into a separate code module that handles multiple conditions of an inverter. Then, the state_machine() is easier to test and the tests of multiple && conditions is moved to another module.

Improvement B

In this approach, we refactor code such that all of the inputs are collected at once with minimal short-circuit logic, and then the inputs are passed into another function. The advantage of this approach is that you have used sort of divide and conquer. You can test state_machine_input_generator() and state_machine_process_inputs() separately and this provides ease of unit-testing.

Test forever loops

Test the main Function

The main() function may be tricky to test because when the unit-tests are run, they have their own main() and you cannot have another main() because that would lead to multiple symbols defined with the same name (linker error).

The approach to fix this problem would be to use a macro.

Test the 'Untestable'

Consider the following code:

It would be really difficult on a POSIX OS to be able to fail writing "file.txt", and hence it would be difficult to unit-test the if (NULL != file_descriptor) failure case. There are two ways to solve this issue:

  • Refactor fopen() and similar File I/O into a "shim" layer

    • This will allow you to "mock" the shim layer and simulate failure cases

  • Refactor the logic inside of the if (NULL != file_descriptor) branch statement to a separate function

    • This will allow you to hit the "private" function directly.

    • See sample code below

Tips & Tricks


Code can be refactored to improve testing. The suggestion here is to first write unit-tests, and then refactor the real code. For example, if you built a bit-counting module, write unit-tests to test it fully. After doing so, you can then refactor and optimize code and your unit-tests would instantly validate if your change is free of bugs.

The NOOP trick

The NOOP trick is meant to spot code coverage issues.

In the example below, if you miss testing the else part of the branch, you can spot it very easily using the standard gcov and lcov code coverage tools.

Common Headers

Here are the consolidated and common header files which you may use in your code. Note that you should avoid defining these in a makefile or similar because your IDE or code indexer may not be able to pick up these code #defines. Further, hiding macros degrades code readability.


Only include this file for your_module_under_test.c such as app.c. The purpose is to alter the definition for this file only, and not globally for everything. Case in point, if you put this in a header file, and your_module.h includes it, then any user that includes this header will inherit the new definition of static. This may sound okay, however, when the CMock framework comes in to Mock a header file, it will also inherit the new definition of static and cause compile time errors.


Code Labs

Lab 1

Let us practice unit-testing, with a little bit of TDD thrown into the mix.

steering.h: This is just a header file and we will Mock out this file and therefore you do not need to write this file's implementation.

steer_processor.h: You will write the implementation of this file yourself at steer_processor.c

test_steer_processor.c You will write the test code, before you write the implementation of steer_processor() function.

Do the following:

  • Put the steering.h in your source code

  • Put the steer_processor.h in your source code

  • Put the test_steer_processor.c in your test code folder

  • Write the implementation of test_steer_processor.c and run the tests to confirm failing tests

  • Write the implementation of steer_processor.c

Lab 2a

Write the unit-tests first, and then the implementation for the following header file:

Lab 2b

Write the unit-tests first, and then the implementation for the following header file. This is a slight variation of the previous lab and it provides you with the static memory based programming pattern popular in Embedded Systems where we deliberately avoid allocating memory on the heap.

Lab 3

In this lab, the objectives are:

  • Practice StubWithCallback or ReturnThruPtr

  • Ignore particular arguments

message.h: This is just an interface, and we will Mock this out.

message_processor.c: This code module processes messages arriving from message__read() function call. There is a lot of nested logic that is testing if the third message contains $ or # at the first byte. To get to this level of the code, it is difficult because you would have to setup your test code to return two dummy messages, and a third message with particular bytes.

To improve testability, you should refactor the } else { logic into a separate static function that you can hit with your unit-tests directly.

test_message_processor.c: Add more unit-tests to this file as needed.

Code Coverage

When the unit-test code is compiled with the -coverage compiler option, it will produce *.gcov files for each source code file. The gcov tool can then b run on *.gocv files to produce coverage artifacts that are useful to identify code coverage. But unfortunately the artifacts are *.gcda which are not human readable.

The *.gcda can then be absorbed by lcov tool that can turn those into HTML reports which can help the developers spot code coverage and improve their unit-testing.