Testing in mbed OS 5

The way tests are run and compiled in mbed OS 5 is substantially different from previous versions of mbed. Previously, tests were located in one known location and a python file (tools/tests.py) kept track of their dependencies, capabilities, and configurations. mbed OS 5 has adopted a more distributed approach to testing. Test code lives alongside the application code, and which is dynamically discovered by the test tools.

Table of Contents

Using tests

Test code structure

Tests can exist throughout mbed OS and your project’s code. They are located under a special directory called TESTS (case is important!).

Placing code under this directory means it will be ignored when building applications and libraries. This code is only ever used when building tests. This is important since all tests require a main() function, and building it with your application would cause multiple main() functions to be defined.

In addition to being placed under a TESTS directory, test sources must exist under two other directories: a test group directory and a test case directory. The following are an examples of this structure:


In this example, myproject is the project root and all the source files under the test_case_1 directory will be included in the test. Any other source files from the OS, libraries, and your project that apply to your target’s configuration will also be included in the build of your test.

Note: Both the test group and test case directory can be named anything you like. However, the TESTS directory must be named TESTS for the tools to detect the test cases correctly.

Test discovery

Since test cases can exist throughout a project, the tools must find them in your project’s file structure before building them. This is done by searching for paths that match the pattern detailed above in the Test code structure section.

Test discovery also obeys the same rules that are used when building your project. This means that tests that are placed under a directory with a prefix like TARGET_, TOOLCHAIN_, or FEATURE_ will only be discovered, built, and run if your current configuration matches this prefix.

For example, if you place a test under the directory FEATURE_BLE with the following path:


This test case will only be discovered if the target being testing supports the BLE feature. Otherwise, the test will be ignored.

Generally, a test should not be placed under a TARGET_ or TOOLCHAIN_ directory, since most tests should be designed to work for all target and toolchain configurations.

Tests can also be completely ignored by using the .mbedignore file described here

Test names

A test case is named from its position in your project’s file structure. For instance, in the above example, a test case with the path myproject/TESTS/test_group/test_case_1 would be named tests-test_group-test_case_1. You will notice that the name is created by joining the directories that make up the path to the test case with a “dash” (-) character. This will be a unique name to identify the test case. You will see this name used throughout the build and testing process.

Building tests

Tests can be built easily through mbed CLI. For information on using mbed CLI, please see its documentation.

When tests are built for a target and a given toolchain, the available tests are first discovered, then built in series. You can also create a “test specification” file, which can be used by our testing tools to run automated hardware tests. For more information on the test specification file, please see the documentation here.

Building process

The process for building tests is handled by the test.py script (not to be confused with tests.py) located under the tools directory. This handles the discovery and building of all test cases for a given target and toolchain.

The full build process is:

  1. Build the non-test code (all code not under a TESTS folder), but do not link it. The resulting object files are placed in the build directory.
  2. Find all tests that match the given target and toolchain.
  3. For each discovered test, build all of its source files and link it with the non-test code that was built in step 1.
  4. If specified, create a test specification file and place it in the given directory for use by testing tools. This is placed in the build directory by default when using mbed CLI.

App config

When building an mbed application, the presence of a mbed_app.json file allows you to set or override different config settings from libraries and targets. However, because the tests share a common build, this can cause issues when tests have different configurations that affect the OS.

If you need to use app config, this must be set via the --app-config option when calling mbed test. If this option is not specified, the build system will ignore all mbed_app.json files and use the default config values.

Running tests

Automated tests can be run easily through mbed CLI. For information on using mbed CLI, please see its documentation.

The testing process requires that the tests are built and that a test specification JSON file exists that describes these available tests. The test specification format is detailed here.

The actual testing process is handled by the Greentea tool. To read more about this tool, please visit its GitHub repository.

Writing tests

You can write tests for your own project, or add more tests to mbed OS. Tests are written using the Greentea client, UNITY, and utest frameworks, located in /features/frameworks. Below is an example test that uses all of these frameworks:

#include "mbed.h"
#include "greentea-client/test_env.h"
#include "unity.h"
#include "utest.h"
#include "rtos.h"

using namespace utest::v1;

// A test that returns successfully is considered successful
void test_success() {

// Tests that assert are considered failing
void test_failure() {

utest::v1::status_t test_setup(const size_t number_of_cases) {
    // Setup Greentea using a reasonable timeout in seconds
    GREENTEA_SETUP(40, "default_auto");
    return verbose_test_setup_handler(number_of_cases);

// Test cases
Case cases[] = {
    Case("Testing success test", test_success),
    Case("Testing failure test", test_failure),

Specification specification(test_setup, cases);

// Entry point into the tests
int main() {
    return !Harness::run(specification);

This test will first run a case that succeeds, then a case that fails. This is a good template to use when creating tests. For more complex testing examples, please see the documentation for utest.

Debugging tests

Debugging tests is a crucial part of the development and porting process. This section will cover exporting the test, then driving the test with the test tools while the target is attached to a debugger.

Exporting tests

Currently, the easiest way to export a test is to copy the test’s source code from its test directory to your project’s root. This way it will be treated like a normal application by the tools.

You can find the path to the test you wish to export by running the following command:

mbed test --compile-list -n <test name>

Once you’ve copied all of the test’s source files to your project root, go ahead and export your project:

mbed export -i <IDE name>

Your exported project should now be in projectfiles/<IDE>_<target>. Go ahead and open this project in your IDE.

Running a test while debugging

Assuming your test was exported correctly to your IDE, go ahead and build the project and load it onto your target via your debugger.

Bring the target out of reset and run the program. Your target will now be waiting for a synchronizing character string to be sent from the test tools over the serial port. Do not run the mbed test commands, because that will attempt to flash the device, which you’ve already done with your IDE.

Instead, the underlying test tools can be used to drive the test. htrun is the tool that needs to be used in this case. This is installed when you install the requirements for mbed OS. However, if you do not have it installed you can do this by running pip install mbed-host-tests.

First, find your target’s serial port by running the following command:

$ mbed detect

[mbed] Detected KL46Z, port COM270, mounted D:


From the output, take note of your target’s serial port (in this case, it’s COM270).

Run the following command when your device is running the test in your debugger:

mbedhtrun --skip-flashing --skip-reset -p <serial port>:9600

Replace <serial port> with the serial port you found by running mbed detect above.

So for the example above, the command would be:

mbedhtrun --skip-flashing --skip-reset -p COM270:9600

This detects your attached target and drives the test. At this point the test will proceed and allow you to debug it. If you need to rerun the test, simply reset the device with your debugger, run the program, and run the same command.

For an explanation of the arguments used in this command, please run mbedhtrun --help.

Known issues

  • There cannot be a main() function outside of a TESTS directory when building and running tests. This is because this function will be included in the non-test code build as described in the Building process section. When the test code is compiled and linked with the non-test code build, a linker error will occur due to their being multiple main() functions defined. For this reason, please either rename your main application file if you need to build and run tests or use a different project.
  • NOTE: This does not affect building projects or applications, just building and running tests.