The term “test-first” software development comes from the original days of extreme programming (XP). In Kent Beck’s 1999 book, Extreme Programming Explained: Embrace Change (Addison-Wesley), his direction is to create an automated test before making any changes to the code.
Nowadays, test-first development usually means test-driven development (TDD): a well-defined, continuous feedback cycle of code, test, and refactor. You write a test, write some code to make it pass, make improvements, and then repeat. Automation is key though, so you can run the tests easily at any time.
TDD is well regarded as a useful software development technique. The proponents of TDD (including myself) like the way in which the code incrementally evolves from the interface as well as the comprehensive test suite that is created. The test suite is the safety net that allows the code to be refactored freely, without worry of breaking anything. It’s a powerful tool in the battle against code rot.
To date, TDD has had greater adoption in web and application development than with embedded software. Recent advances in unit test tools however are set to make TDD more accessible for embedded development.
In 2011 James Grenning published his book, Test Driven Development for Embedded C (Pragmatic Bookshelf). Six years later, this is still the authoritative reference for embedded test-first development and the entry point to TDD for many embedded software developers. It explains how TDD works in detail for an unfamiliar audience and addresses many of the traditional concerns, like how will this work with custom hardware. Today, the book is still completely relevant, but when it was published, the state-of-the art tools were simple unit test and mocking frameworks. These frameworks require a lot of boilerplate code to run tests, and any mock objects need to be created manually.
In the rest of the software world though, unit test tools are significantly more mature. In most other languages used for web and application development, it’s easy to create and run many unit tests, as well as to create mock objects automatically.
Since 2011, the current state of TDD tools has advanced considerably with the development of the open-source tool Ceedling. It automates running of unit tests and generation of mock objects in C applications, making it a lot easier to do TDD. Today, if you want to test-drive embedded software in C, you don’t need to roll-your-own test build system or mocks.
With better tools making unit testing easier, I suspect that in the future test-first development will be more widely adopted by embedded software developers. While previously relegated to the few early adopters willing to put in the effort, with tools lowering the barrier to entry it will be easier for everyone to do TDD.
Besides the tools to make TDD easier, another driving force behind greater adoption of test-first practices will be the simple need to produce better-quality embedded software. As embedded software continues its infiltration into all kinds of devices that run our lives, we’ll need to be able to deliver software that is more reliable and more secure.
Currently, unit tests for embedded software are most popular in regulated industries—like medical or aviation—where the regulators essentially force you to have unit tests. This is one part of a strategy to prevent you from hurting or killing people with your code. The rest of the “unregulated” embedded software world should take note of this approach.
With the rise of the Internet of things (IoT), our society is increasingly dependent on embedded devices connected to the Internet. In the future, the reliability and security of the software that runs these devices is only going to become more critical. There may not be a compelling business case for it now, but customers—and perhaps new regulators—are going to increasingly demand it. Test-first software can be one strategy to help us deal with this challenge.
This article appears in Circuit Cellar 318.