It’s day one of adding tests to your legacy C code. You get stopped dead when the compiler announces that the code you are coaxing into the test harness can’t be compiled on this machine. You are stuck on the Make it compile step of Crash to Pass.
Moving your embedded legacy C code (embedded C code without tests) into a test harness can be a challenge. The legacy C code is likely to be tightly bound to the target processor. This might not be a problem for production, but for off-target unit testing, it is a big problem.
For C we have a limited mechanisms for breaking dependencies. In my book, I describe at length link-time and function pointer substitutions, but only touch on preprocessor stubbing.
In this article we’ll look at
#include Test-Double as a way to break dependencies on a problem
My last article featured a hand crafted a spy to monitor
asm directives. Now let’s use CppUMock (the mock support companion CppUTest) to create a mock version of
Sometimes embedded developers have to use inline assembler instructions to get better control of the processor, or to improve performance. How should we deal with those when we’re doing TDD and testing off the target?
What’s the problem? The embedded
asm statements cause compilation errors if the assembler instructions are not part of the off-target test platform instruction set. Also some of the instructions might not be legal in the test environment. This article shows how to insert a test double for the
asm directives with gcc and CppUTest.
Here is a legacy code change policy for a team adopting TDD that has a legacy code base:
- Test-drive new code
- Add tests to legacy code before modification
- Test-drive changes to legacy code
Refactoring without tests is dangerous; with all the details we must keep straight, a mistake is easy to make. How many code reviews have you been in where the recommended design changes are not made because “we already tested it”? You avoid the change because it’s dangerous to change code without tests. So, the Boy Scout adds tests too. For more on Boy Scouts, see previous post.
One important realization on the journey from a BDUF approach to an iterative and agile approach is that design is never done. Designs evolve. The waterfall emphasis has been to unnaturally try to control software physics by imposing requirements freezes and burdensome change control. The process of developing software is part science and part creative. You are applying science toward the invention of something. Design is capturing knowledge both about what the end user need is, and one solution to that need.
Constrained Memory is the reality for many embedded developers. Running tests in the development system won’t suffer the same memory constraints found in the target. Here are a few things to help TDD in constrained memory situations.
Embedded software has all the challenges of “regular” software, like poor quality and unreliable schedules. It is just software with some additional challenges. The additional challenges do not disqualify TDD for embedded. TDD even helps with some of those uniquely embedded challenges.
A unit test harness’ job is to provide:
- A concise common language to express test cases
- A concise common language to express expected results
- A place to collect all the unit test cases for the project, system, or subsystem
- The facilities to run the test cases, either in full or partial batches
- A concise report of the test suite success or failure
- A detailed report of any test failures
Prior to the Deep Agile conference, I received a number of questions about getting people to change, to try new things. Change is hard. People need to be motivated to change. “If it ain’t broke, don’t fix it” they say. But there is always some things that are broken.
First there needs to be awareness/acceptance that there are problems to solve. Do a retrospective of the last release. Find the problems that people are passionate about. Try not have blame session. Build a logic chain from the problem to some solution you think will help. Get people to sign up to try the new approach for a month or two, not the rest of their lives. Iterations give a great opportunity for this kind of experimentation.
You have to try things, rather than just talk about them. I am not sure where this quote is from, but it is profound:
“It’s easier to act your way into thinking differently than to think your way into acting differently”
Read on for some specific questions, and my answers.
During my TDD session at the Embedded Systems Conference yesterday, I did a demo. Before the demo, I make the case for TDD as a way to prevent bugs (see Physics of TDD). For the live demo I usually code on my mac and run the tests there as well. The question always comes up: “You are running tests on your PC, can you run them on the target?” or maybe “Sure you can TDD on a PC, but what about the real hardware?”