write - Unit Testing C Code

unit testing tools (20)

I worked on an embedded system this summer written in straight C. It was an existing project that the company I work for had taken over. I have become quite accustomed to writing unit tests in Java using JUnit but was at a loss as to the best way to write unit tests for existing code (which needed refactoring) as well as new code added to the system.

Is there any way to make unit testing plain C code as easy as unit testing Java code with, for example, JUnit? Any insight that would apply specifically to embedded development (cross-compiling to arm-linux platform) would be greatly appreciated.

API Sanity Checker — test framework for C/C++ libraries:

An automatic generator of basic unit tests for a shared C/C++ library. It is able to generate reasonable (in most, but unfortunately not all, cases) input data for parameters and compose simple ("sanity" or "shallow"-quality) test cases for every function in the API through the analysis of declarations in header files.

The quality of generated tests allows to check absence of critical errors in simple use cases. The tool is able to build and execute generated tests and detect crashes (segfaults), aborts, all kinds of emitted signals, non-zero program return code and program hanging.


CppUTest - Highly recommended framework for unit testing C code.

The examples in the book that is mentioned in this thread TDD for embedded C are written using CppUTest.

Cmockery is a recently launched project that consists on a very simple to use C library for writing unit tests.

After reading Minunit I thought a better way was base the test in assert macro which I use a lot like defensive program technique. So I used the same idea of Minunit mixed with standard assert. You can see my framework (a good name could be NoMinunit) in k0ga's blog

I don't use a framework, I just use autotools "check" target support. Implement a "main" and use assert(s).

My test dir Makefile.am(s) look like:

check_PROGRAMS = test_oe_amqp

test_oe_amqp_SOURCES = test_oe_amqp.c
test_oe_amqp_LDADD = -L$(top_builddir)/components/common -loecommon
test_oe_amqp_CFLAGS = -I$(top_srcdir)/components/common -static

TESTS = test_oe_amqp

I say almost the same as ratkok but if you have a embedded twist to the unit tests then...

Unity - Highly recommended framework for unit testing C code.

The examples in the book that is mentioned in this thread TDD for embedded C are written using Unity (and CppUTest).

I used RCUNIT to do some unit testing for embedded code on PC before testing on the target. Good hardware interface abstraction is important else endianness and memory mapped registers are going to kill you.

I'm currently using the CuTest unit test framework:


It's ideal for embedded systems as it's very lightweight and simple. I had no problems getting it to work on the target platform as well as on the desktop. In addition to writing the unit tests, all that's required is:

  • a header file included wherever you're calling the CuTest routines
  • a single additional 'C' file to be compiled/linked into the image
  • some simple code added to to main to set up and call the unit tests - I just have this in a special main() function that gets compiled if UNITTEST is defined during the build.

The system needs to support a heap and some stdio functionality (which not all embedded systems have). But the code is simple enough that you could probably work in alternatives to those requirements if your platform doesn't have them.

With some judicious use of extern "C"{} blocks it also supports testing C++ just fine.

If you are familiar with JUnit then I recommend CppUnit. http://cppunit.sourceforge.net/cppunit-wiki

That is assuming you have c++ compiler to do the unit tests. if not then I have to agree with Adam Rosenfield that check is what you want.

If you're still on the hunt for test frameworks, CUnitWin32 is one for the Win32/NT platform.

This solves one fundamental problem that I faced with other testing frameworks. Namely global/static variables are in a deterministic state because each test is executed as a separate process.

LibU (http://koanlogic.com/libu) has an unit test module that allows explicit test suite/case dependencies, test isolation, parallel execution and a customizable report formatter (default formats are xml and txt).

The library is BSD licensed and contains many other useful modules - networking, debugging, commonly used data structures, configuration, etc. - should you need them in your projects ...

Michael Feather's book "Working Effectively with Legacy Code" presents a lot of techniques specific to unit testing during C development.

There are techniques related to dependency injection that are specific to C which I haven't seen anywhere else.

One unit testing framework in C is Check; a list of unit testing frameworks in C can be found here and is reproduced below. Depending on how many standard library functions your runtime has, you may or not be able to use one of those.


AceUnit (Advanced C and Embedded Unit) bills itself as a comfortable C code unit test framework. It tries to mimick JUnit 4.x and includes reflection-like capabilities. AceUnit can be used in resource constraint environments, e.g. embedded software development, and importantly it runs fine in environments where you cannot include a single standard header file and cannot invoke a single standard C function from the ANSI / ISO C libraries. It also has a Windows port. It does not use forks to trap signals, although the authors have expressed interest in adding such a feature. See the AceUnit homepage.

GNU Autounit

Much along the same lines as Check, including forking to run unit tests in a separate address space (in fact, the original author of Check borrowed the idea from GNU Autounit). GNU Autounit uses GLib extensively, which means that linking and such need special options, but this may not be a big problem to you, especially if you are already using GTK or GLib. See the GNU Autounit homepage.


Also uses GLib, but does not fork to protect the address space of unit tests.


Standard C, with plans for a Win32 GUI implementation. Does not currently fork or otherwise protect the address space of unit tests. In early development. See the CUnit homepage.


A simple framework with just one .c and one .h file that you drop into your source tree. See the CuTest homepage.


The premier unit testing framework for C++; you can also use it to test C code. It is stable, actively developed, and has a GUI interface. The primary reasons not to use CppUnit for C are first that it is quite big, and second you have to write your tests in C++, which means you need a C++ compiler. If these don’t sound like concerns, it is definitely worth considering, along with other C++ unit testing frameworks. See the CppUnit homepage.


embUnit (Embedded Unit) is another unit test framework for embedded systems. This one appears to be superseded by AceUnit. Embedded Unit homepage.


A minimal set of macros and that’s it! The point is to show how easy it is to unit test your code. See the MinUnit homepage.

CUnit for Mr. Ando

A CUnit implementation that is fairly new, and apparently still in early development. See the CUnit for Mr. Ando homepage.

This list was last updated in March 2008.



CMocka is a test framework for C with support for mock objects. It's easy to use and setup. CMocka official homepage.


Criterion is a cross-platform C unit testing framework supporting automatic test registration, parameterized tests, theories, and that can output to multiple formats, including TAP and JUnit XML. Each test is run in its own process, so signals and crashes can be reported or tested if needed. See the Criterion homepage for more information.


HWUT is a general Unit Test tool with great support for C. It can help to create Makefiles, generate massive test cases coded in minimal 'iteration tables', walk along state machines, generate C-stubs and more. The general approach is pretty unique: Verdicts are based on 'good stdout/bad stdout'. The comparison function, though, is flexible. Thus, any type of script may be used for checking. It may be applied to any language that can produce standard output. See HWUT homepage.

Wikipedia gives a detailed list of C unit testing frameworks under List of unit testing frameworks: C

Personally I like the Google Test framework.

The real difficulty in testing C code is breaking the dependencies on external modules so you can isolate code in units. This can be especially problematic when you are trying to get tests around legacy code. In this case I often find myself using the linker to use stubs functions in tests.

This is what people are referring to when they talk about "seams". In C your only option really is to use the pre-processor or the linker to mock out your dependencies.

A typical test suite in one of my C projects might look like this:

#include "myimplementationfile.c"
#include <gtest/gtest.h>

// Mock out external dependency on mylogger.o
void Logger_log(...){}

TEST(FactorialTest, Zero) {
    EXPECT_EQ(1, Factorial(0));

Note that you are actually including the C file and not the header file. This gives the advantage of access to all the static data members. Here I mock out my logger (which might be in logger.o and give an empty implementation. This means that the test file compiles and links independently from the rest of the code base and executes in isolation.

As for cross-compiling the code, for this to work you need good facilities on the target. I have done this with googletest cross compiled to Linux on a PowerPC architecture. This makes sense because there you have a full shell and os to gather your results. For less rich environments (which I classify as anything without a full OS) you should just build and run on the host. You should do this anyway so you can run the tests automatically as part of the build.

I find testing C++ code is generally much easier due to the fact that OO code is in general much less coupled than procedural (of course this depends a lot on coding style). Also in C++ you can use tricks like dependency injection and method overriding to get seams into code that is otherwise encapsulated.

Michael Feathers has an excellent book about testing legacy code. In one chapter he covers techniques for dealing with non-OO code which I highly recommend.

Edit: I've written a blog post about unit testing procedural code, with source available on GitHub.

Edit: There is a new book coming out from the Pragmatic Programmers that specifically addresses unit testing C code which I highly recommend.

There is an elegant unit testing framework for C with support for mock objects called cmocka. It only requires the standard C library, works on a range of computing platforms (including embedded) and with different compilers.

It also has support for different message output formats like Subunit, Test Anything Protocol and jUnit XML reports.

cmocka has been created to also work on embedded platforms and also has Windows support.

A simple test looks like this:

#include <stdarg.h>
#include <stddef.h>
#include <setjmp.h>
#include <cmocka.h>

/* A test case that does nothing and succeeds. */
static void null_test_success(void **state) {
    (void) state; /* unused */

int main(void) {
    const struct CMUnitTest tests[] = {
    return cmocka_run_group_tests(tests, NULL, NULL);

The API is fully documented and several examples are part of the source code.

To get started with cmocka you should read the article on LWN.net: Unit testing with mock objects in C

cmocka 1.0 has been released February 2015.

We wrote CHEAT (hosted on GitHub) for easy usability and portability.

It has no dependencies and requires no installation or configuration. Only a header file and a test case is needed.

#include <cheat.h>

    cheat_assert(2 + 2 == 4);
    cheat_assert_not(2 + 2 == 5);

Tests compile into an executable that takes care of running the tests and reporting their outcomes.

$ gcc -I . tests.c
$ ./a.out
2 successful of 2 run

It has pretty colors too.