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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

  1. Install Homebrew.

  2. Install GCC compilers and CMake with: brew install gcc cmake.

  3. Install Python and Pip:

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

Installing dependencies on Windows

  1. Download and install MinGW-W64.
  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

Unit tests are located in the Mbed OS repository under the UNITTESTS folder. We recommend unit test files use an identical directory path to the file under test. This makes it easier to find unit tests for a particular class or a module. For example, if the file under test 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.

Test discovery

Registering unit tests for running is automatic, and the test runner handles registration. However, test files are not automatically assigned to be built. We build unit tests by using a separate build system, which searches for unit tests under the UNITTESTS directory.

For the build system to find and build any test suite automatically, you must include a unit test configuration file named unittest.cmake for each unit test suite. This configuration file contains all the source files required for the 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 mbed test --unittests command. To learn how to use unit tests with Mbed CLI, please see the appropriate documentation section. For other information on using Mbed CLI, please see the CLI documentation in handbook.

Writing unit tests

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.

With the following steps, you can write a simple unit test. In this example, rtos/Semaphore.cpp is a class under test.

  1. Create a directory for unit test files in UNITTESTS/rtos/Semaphore.

  2. Create a test configuration file UNITTESTS/rtos/Semaphore/unittest.cmake with the following content:

  3. Create a test source file UNITTESTS/rtos/Semaphore/test_Semaphore.cpp with the following content:

#include  "gtest/gtest.h"
#include  "rtos/Semaphore.h"

static osStatus_t retval = osOK;
static uint32_t count = 0;

// Test stubs
osStatus_t osSemaphoreAcquire(osSemaphoreId_t semaphore_id, uint32_t timeout)
    return retval;
osStatus_t osSemaphoreDelete(osSemaphoreId_t semaphore_id)
    return retval;
osStatus_t osSemaphoreRelease(osSemaphoreId_t semaphore_id)
    return retval;
uint32_t osSemaphoreGetCount(osSemaphoreId_t semaphore_id)
    return count;
osSemaphoreId_t osSemaphoreNew(uint32_t max_count, uint32_t initial_count, const osSemaphoreAttr_t *attr)
    return (void *)&count; // Just a dymmy reference

class  TestSemaphore : public  testing::Test {
    rtos::Semaphore *sem;

    virtual  void  SetUp()
        sem = new rtos::Semaphore();

    virtual  void  TearDown()
        delete sem;

TEST_F(TestSemaphore, constructor)

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 UNITTESTS folder as the argument. So from UNITTESTS/build use cmake ..:
    • Add -g [generator] if generating other than Unix Makefiles such in case of MinGW use -g "MinGW Makefiles".
    • Add -DCOVERAGE:True to add coverage compiler flags.
    • See the CMake manual for more information.
  4. Run a make program (Make, Gmake, Mingw32-make and so on) to build the 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 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 using the documentation.
  2. Run gtest-runner
  3. Add test executables into the list.
  4. Run them.

Get code coverage

Use Mbed CLI to generate code coverage reports. For advanced use, you can 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: Virus protection identifies files generated by CMake as malicious and quarantines the files on Windows.

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