TDD and the Real-World

One of the attendees of my training objected to TDD stating “TDD does not resolve the real-world (temperature, pressure, timing, noisy signals, etc.) issues that my project is encountering.”

You are right! I’ll add TDD does not resolve anything. TDD is not a magic incantation that solves any problem the embedded developer may encounter. From discussions at your company, I think you realize this. But it does not change the fact that you have to spend a lot of time chasing these kinds of problems. So let’s see how TDD can support this activity.
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Unit testing RTOS dependent code – RTOS Test-Double – Part 3

I’m going to bend the example a little, and look at how to write tests that interact with an interrupt service routine and again with a semaphore. I’ll also get some help with the tedious parts of building fakes by using Mike Long’s Fake Function Framework. (BTW: I am using a beta version, that has changes the released version does not yet have.)

This figure shows the concurrent runtime relationship between the MessageProcessor and its SerialDriver.

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Unit testing RTOS dependent code – RTOS Test-Double – Part 2

In the last article, the OSSemPend() test-double was coded to handle a specific OSSemPend() application and test need. The semaphore was being used to signal when there is a message to process. It was the first need for a OSSemPend() test double and was quickly developed. As more RTOS dependent code is brought under test, a more general solution will be needed.

In this article, we’ll look at a test double that can be customized for each application.
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Unit testing RTOS dependent code – RTOS Test-Double

When you’ve got legacy code that depends on the Real-time Operating System, you have a challenge to get your code off the target for unit testing. If you want to test with the concurrency provided by the RTOS, these are not really unit tests, and you won’t be able to write thorough tests, and you need unit tests to be through.

You’re going to need to make a test-double so you can get your code off the RTOS and into your development system for unit testing. In this article we’ll go through the steps to get started.

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Hiding Non-standard C Keywords for Off-Target Testing

Some silicon vendors extend the C language so the programmers can easily interact with the silicon. Using these extensions tie production code to the silicon vendors compiler and consequently the code can only run on the target system. This is not a problem during production, but is a problem for off-target unit testing.

The good news is that we may be able to get around this problem without having to change production code, one of our goals when adding tests to legacy code.
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#include Test Double

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 #include file.

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Spying on Embedded ‘asm’ directives

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.

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Legacy Code Change – a Boy Scout Adds Tests

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.

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