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Unit testing

This document describes how to write and test unit tests for Mbed OS. To prevent and solve problems, please see the troubleshooting section.


Unit tests test code in small sections on a host machine. Unlike other testing tools, unit testing doesn't require embedded hardware, and it doesn't need to build the full operating system. Because of this, unit testing can result in faster tests than other testing tools. Unit testing takes place in a build environment where we test each C or C++ class or module in isolation. This means we build test suites into separate test binaries and stub all access outside to remove dependencies on any specific embedded hardware or software combination. This allows us to complete the testing using native compilers on the build machine.


Please install the following dependencies to use Mbed OS unit testing.

  • GNU toolchains.
    • GCC 6 or later. We recommend you use MinGW-W64 on Windows, but any Windows port of the above GCC versions works.
  • CMake 3.0 or newer.
  • Python 2.7.x, 3.5 or newer.
  • Pip 10.0 or newer.
  • Gcovr 4.1 or newer.
  • Mbed CLI 1.8.0 or newer.

Detailed instructions for supported operating systems are below.

Installing dependencies on Debian or Ubuntu

  1. sudo apt-get -y install build-essential cmake

  2. Install Python and Pip with:

    sudo apt-get -y install python python-setuptools
    sudo easy_install pip
  3. Install Gcovr and Mbed CLI with pip install "gcovr>=4.1" mbed-cli.

Installing dependencies on macOS

In a terminal window:

  1. Install Homebrew.

  2. Install Xcode command-line tools with xcode-select --install.

  3. Install CMake with: brew install cmake.

  4. Install Python and Pip:

    brew install python
    sudo easy_install pip
  5. Install Gcovr and Mbed CLI with pip install "gcovr>=4.1" mbed-cli.

  6. (Optional) Install GCC with brew install gcc.

Installing dependencies on Windows

In a terminal window:

  1. Download and install MinGW-W64 from SourceForge.
  2. Download CMake binaries from https://cmake.org/download/, and run the installer.
  3. Download Python 2.7 or Python 3 from https://www.python.org/getit/, and run the installer.
  4. Add MinGW, CMake and Python into system PATH.
  5. Install Gcovr and Mbed CLI with pip install "gcovr>=4.1" mbed-cli.

Test code structure

Find unit tests in the Mbed OS repository under the UNITTESTS folder of each library. We recommend unit test files use an identical directory path as the file under test. This makes it easier to find unit tests for a particular class or a module. For example, if the file you're testing is some/example/path/ClassName.cpp, then all the test files are in the UNITTESTS/some/example/path/ClassName directory. Each test suite needs to have its own unittest.cmake file for test configuration.

All the class stubs should be located in the Mbed OS root directory UNITTESTS/stubs. Multiple test suites can use a single stub class, which should follow the naming convention ClassName_stub.cpp for the source file and ClassName_stub.h for the header file. Use the actual header files for the unit tests, and don't stub headers if possible. The stubbed headers reside in the UNITTESTS/target_h directory.

Test discovery

Registering unit tests to run happens automatically, and the test runner handles registration. However, test files do not automatically build. Build unit tests with a separate system that searches for unit tests under the UNITTESTS directory.

For the build system to find and build any test suite automatically, include a unit test configuration file named unittest.cmake for each unit test suite. This configuration file lists all the source files required for the test build.

Test names

The build system automatically generates names of test suites. The name is constructed by taking a relative file path from the UNITTESTS directory to the test directory and replacing path separators with dashes. For example, the test suite name for some/example/path/ClassName.cpp is some-example-path-ClassName. Suite names are used when deciding which test suites to run.

Unit testing with Mbed CLI

Mbed CLI supports unit tests through the mbed test --unittests command. To learn how to use unit tests with Mbed CLI, please see the unit testing documentation section. For other information on using Mbed CLI, please see the CLI documentation in handbook.

Writing unit tests

A unit tests suite consists of one or more test cases. The test cases should cover all the functions in a class under test. All the external dependencies are stubbed including the other classes in the same module. Avoid stubbing header files. Finally, analyze code coverage to ensure all code is tested, and no dead code is found.

Please see the documentation for Google Test to learn how to write unit tests using its framework. See the documentation for Google Mock if you want to write and use C++ mock classes instead of stubs.

Test suite configuration

Create two files in the test directory for each test suite:

  • Unit test source file (test_ClassName.cpp).
  • Unit test configuration file (unittest.cmake).

List all the files required for the build in the unittest.cmake file. We recommend you list the file paths relative to the UNITTESTS folder. Use the following variables to list the source files and include paths:

  • unittest-includes - List of header include paths. You can use this to extend or overwrite default paths listed in CMakeLists.txt.
  • unittest-sources - List of files under test.
  • unittest-test-sources - List of test sources and stubs.

You can also set custom compiler flags and other configurations supported by CMake in unittest.cmake.


With the following steps, you can write a unit test. This example creates dummy classes to be tested, creates and configures unit tests for a class and stubs all external dependencies.

  1. Create the following dummy classes in mbed-os/example:


    #ifndef MYCLASS_H_
    #define MYCLASS_H_
    namespace example {
    class MyClass {
        int myFunction();


    #include "MyClass.h"
    #include "OtherClass.h"
    namespace example {
    int MyClass::myFunction() {
        OtherClass o = OtherClass();
        return o.otherFunction();


    #ifndef OTHERCLASS_H_
    #define OTHERCLASS_H_
    namespace example {
    class OtherClass {
        int otherFunction();


    #include "OtherClass.h"
    namespace example {
    int OtherClass::otherFunction() {
        return 1;
  2. Create a directory for MyClass unit tests in UNITTESTS/example/MyClass.

  3. Create a configuration file and a source file for MyClass unit tests in UNITTESTS/example/MyClass:


    ## Add here additional test specific include paths
    set(unittest-includes ${unittest-includes}
    ## Add here classes under test
    ## Add here test classes and stubs


    #include "gtest/gtest.h"
    #include "example/MyClass.h"
    class TestMyClass : public testing::Test {
        example::MyClass *obj;
        virtual void SetUp()
            obj = new example::MyClass();
        virtual void TearDown()
            delete obj;
    TEST_F(TestMyClass, constructor)
    TEST_F(TestMyClass, myfunction)
        EXPECT_EQ(obj->myFunction(), 0);
  4. Stub all external dependencies. Create the following stub in the Mbed OS root directory UNITTESTS/stubs:


    #include "example/OtherClass.h"
    namespace example {
    int OtherClass::otherFunction() {
        return 0;

This example does not use any Mbed OS code, but if your unit tests do, then remember to update header stubs in UNITTESTS/target_h and source stubs in UNITTESTS/stubs with any missing type or function declarations.

Building and running unit tests

Use Mbed CLI to build and run unit tests. For advanced use, you can run CMake and a Make program directly.

Build tests directly with CMake

  1. Create a build directory: mkdir UNITTESTS/build.

  2. Move to the build directory: cd UNITTESTS/build.

  3. Run CMake using a relative path to the UNITTESTS folder as the argument. So from UNITTESTS/build use cmake ..:

    • Add -g [generator] if generating files other than Unix Makefiles. For example, for MinGW, use -g "MinGW Makefiles".
    • Add -DCMAKE_MAKE_PROGRAM=<value>, -DCMAKE_CXX_COMPILER=<value> and -DCMAKE_C_COMPILER=<value> to use a specific Make program and compilers.
    • Add -DCMAKE_BUILD_TYPE=Debug for a debug build.
    • Add -DCOVERAGE=True to add coverage compiler flags.
    • Add -Dgtest_disable_pthreads=ON to run in a single thread.
    • See the CMake manual for more information.
  4. Run a Make program to build tests.

Run tests directly with CTest

Run a test binary in the build directory to run a unit test suite. To run multiple test suites at once, use the CTest test runner. Run CTest with ctest. Add -v to get results for each test case. See the CTest manual for more information.

Run tests with GUI test runner

  1. Install gtest-runner according to the documentation.
  2. Run gtest-runner.
  3. Add test executables into the list and run.


  1. Use Mbed CLI to build a debug build. For advanced use, run CMake directly with -DCMAKE_BUILD_TYPE=Debug, and then run a Make program.
  2. Run GDB with a test executable as an argument to debug unit tests.
  3. Run tests with Valgrind to analyze the test memory profile.

Get code coverage

Use Mbed CLI to generate code coverage reports. For advanced use, follow these steps:

  1. Run CMake with both -DCMAKE_BUILD_TYPE=Debug and -DCOVERAGE=True.
  2. Run a Make program to build the tests.
  3. Run the tests.
  4. Run Gcovr or any other code coverage tool directly in the build directory.


Problem: Generic problems with CMake or with the build process.

  • Solution: Delete the build directory. Make sure that CMake, g++, GCC and a Make program can be found in the path and are correct versions.

Problem: (Windows) Virus protection identifies files generated by CMake as malicious and quarantines the files.

  • Solution: Restore false-positive files from the quarantine.

Problem: (Windows) Git with shell installation adds sh.exe to the path and then CMake throws an error: sh.exe was found in your PATH. For MinGW make to work correctly, sh.exe must NOT be in your path.

  • Solution: Remove sh.exe from the system path.

Problem: (Mac OS) CMake compiler check fails on Mac OS Mojave when using GCC-8.

  • Solution: Make sure gnm (binutils) is not installed. Uninstall binutils with brew uninstall binutils.
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