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Flight Software & Embedded Systems Framework

Unit Testing in F´

Testing is an important part of flight software (FSW) development. Testing is divided into two phases: i) unit testing, and ii) integration testing. Unit testing tests the individual units, such as F′ components, while integration testing tests the integrated system. Test framework classes include the auto-generated TesterBase, the auto-generated GTestBase, and the developer-written Tester. The testing phases and test framework classes are discussed in further detail below.

Unit Testing

Thorough unit testing is critical. It provides unit-level regression tests, and makes integration easier since localized errors are caught early and system-level issues only appear during integration.

F′ provides the support for unit testing at the component level. The overall framework for unit testing is shown in Figure 1.


Figure 1. Unit testing framework overview.

The goal of unit testing is to cover all component-level requirements and achieve close to 100% code coverage with a reasonable amount of system state and path coverage. These requirements should drive the tests. A record of how the tests cover the requirements should be maintained. Mapping the tests to requirements can be recorded on a spreadsheet, or in the actual test using comments or in the console output mechanism.

To start, generate the test classes and add public test methods to the tester. The TesterBase and the GTestBase are auto-generated, while the Tester is developer-written from a generated template.

TesterBase is the base class for testing a component and provides a harness for unit tests. The TesterBase interface is the mirror image of the component (C) under test. For each output port in C there is an input port called a “from port,” and for each input port in C there is an output port called a “to port.” For each “from port” there is a history (H) of data received through a virtual input handler that stores its argument into H. The TesterBase provides utility methods for writing tests for the component. These include sending commands, sending invocations onto ports, and getting and setting parameters and time.

GTestBase is derived from the TesterBase and includes headers for the Google Test framework with F′ specific macros. It supports test assertions, such as ASSERT_EQ(3, x) to check that two values are equal when writing tests. The F′ specific macros check the telemetry received from the ports, the events received from the ports, and the data received (user-defined) from the ports. The GTestBase is factored into a separate class so its use is optional on systems that do not support it.

Tester is derived from the GTestBase and contains the component under test as a member. The autocoder provides a template where the user then adds tests as a public method, and also writes tests in a derived class of the Tester.

Once the test class is generated, the user can begin to send commands, check events and telemetry, check user-defined output ports, set parameters and the time, build and run the unit tests from the component directory, and finally analyze the code coverage from the component directory after building and running the test. However, be sure to review the analysis from the test/ut directory.

A standard approach to writing unit tests is to write a complete test that covers the requirement. If there is overlap, refactoring into functions is the preferred approach to avoid code duplication. A more disciplined approach would be to write functions that test individual behaviors. When writing test code treat unit testing as a programming problem by applying similar style guidelines as the flight code. This approach avoids less code duplication and provides more readable, maintainable, and modifiable tests.

When writing unit tests for a component be sure to test against the interface, such as the send commands and the send data on the output ports. Read the internal component state to verify it is good, and only modify the state through the interface; do not update the state of the component. This approach leads to a more structured test. If there is a requirement to test a function in a component implementation that has a complex algorithm, then write a test against the function interface.

When unit testing a component, model the external behavior to receive commands and send responses by writing a test harness. This approach supports modularity testing that can be used for many tests.

To check event and telemetry histories the user first sends a command, and then checks events and telemetry by writing the following code.

Sending Commands:

// Send command
    cmdSeq, // Command sequence number
    arg1, // Argument 1
    arg2 // Argument 2

// Assert command response
    0, // Index in the history
    Component::OPCODE\_COMMAND\_NAME, // Expected command opcode
    cmdSeq, // Expected command sequence number
    Fw::CmdResponse::OK // Expected command response

Checking Events:

// Send command and check response

// Assert total number of events in history

// Assert number of a particular event

// Assert arguments for a particular event
    0, // Index in history
    arg1, // Expected value of argument 1
    arg2 // Expected value of argument 2

Checking Telemetry:

// Send command and check response

// Assert total number of telemetry entries in history

// Assert number of entries on a particular channel

// Assert value for a particular entry
    0, // Index in history
    value // Expected value

To check the user-defined output ports write the following code.

// Send command and check response

// Assert total number of entries on from ports

// Assert number of entries on a particular from port

// Assert value for a particular entry
    0, // Index in history
    arg1, // Expected value of argument 1
    arg2 // Expected value of argument 2

To set the parameters in a test of component C, write the following code. This call stores the argument in member variables of TesterBase, so when C invokes the ParamGet port it receives the argument.

    value, // Parameter value
    Fw::PARAM_VALID // Parameter status

Next, to set the time in a text of component C, write the following code. Time is an Fw::Time object, so when C invokes the TimeGet port it receives the value time.


The F′ Prime build system provides targets for building and running component unit tests.

To build unit tests, go to the component directory (not the test/ut directory) and run fprime-util generate --ut.

To run unit tests, go to the component directory (not the test/ut directory) and run fprime-util check [parameter flags].

Unit test check parameter Description
--all Run all unit tests, combinable with coverage
--coverage Check for code coverage in unit tests

For example, to run all unit tests and check for code coverage, run fprime-util check --all --coverage.

Choosing a test library

Components that call into libraries have two ways to write tests:

  • Link against the library in the test
  • Link against a mock or stub library

If you link against the library in the test, avoid linking against the mock or stub library. Linking against only the test library proves that the component code works with the actual library.

Linking against a mock or stub library makes it easier to induce behaviors for testing, like injecting faults. This approach may be the only option on some platforms.

Code coverage

Code coverage checks which lines were run at least once during a test. Tools like gcov perform code coverage analysis by compiling and running the tests, then producing a report.

Generally, code coverage checks close to 80% of lines. The remaining lines are usually off-nominal behaviors that may require additional effort to check by reverse reasoning from the desired behavior to synthesize the inputs, or by injecting faults into the library behaviors.

Note that 100% code coverage does not check which system states were tested, nor which paths through the code were tested.

To review code coverage analysis, go to the component directory and review the summary output _gcov.txt files. Next, go to the component directory to review the coverage annotation .hpp.gcov and .cpp.gcov source files.