Here is a good question, and my reply, from a recent attendee of my Test-Driven Development for Embedded C training.
As I work more with TDD, one of the concepts I am still struggling to grasp is how to test “leaf” components that touch real hardware. For example, I am trying to write a UART driver. How do I test that using TDD? It seems like to develop/write the tests, I will need to write a fake UART driver that doesn’t touch any hardware. Let’s say I do that. Now I have a really nice TDD test suite for UART drivers. However, I still need to write a real UART driver…and I can’t even run the TDD tests I created for it on the hardware. What value am I getting from taking the TDD approach here?
Maybe you read Part 1 of this article. If you did you’ll know it concerns adding tests to legacy code (legacy code is code without tests). You will also know that the code has file scope functions and data that we want to test directly.
My opinion on accessing private parts of well designed code, is that you do not need to. You can test well design code through its public interface. Take it as a sign that the design is deteriorating when you cannot find a way to fully test a module through its public interface.
Part 1 showed how to
#include the code under test in the test file to gain access to the private parts, a pragmatic thing to do when wrestling untested code into a test harness. This article shows another technique that may have an advantage for you over the technique shown in Part 1. Including the code under test in a test case can only be done once in a test build. What if you need access to the hidden parts in two test cases? You can’t. That causes multiple definition errors at link time.
This article shows how to create a test access adapter to overcome that problem.
And a Happy Leap Year Bug
It’s a new year; last year was a leap year; so the quadrennial reports of leap year bugs are coming in. Apologies are in the press from Apple, TomTom, and Microsoft. Trains we stopped from running in China. Somehow calling them glitches seems to make it someone else’s fault, something out of their control. How long have leap years been around? Julius Caesar introduced Leap Years in the Roman empire over 2000 years ago. The Gregorian calendar has been around since 1682. This is not a new idea, or a new bug.
I’m going to try to take one excuse away from the programmers that create these bugs by answering a question that comes up all the time, “How do I test static functions in my C code?”
This is the second article addressing the misconception that TDD ignores design. In the previous article, I explained how TDD acts as a design rot radar. In this article, I’ll explain why I think TDD also acts as a homing beacon for well structured code.
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.
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.
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.
I’m in agreement with Robert Glass when he says “100% test coverage is insufficient. 35% of the faults are missing logic paths.” It’s not controversial, but I’d like to give my perspective on it.
If you have an automated unit test suite, low code coverage is an indication that you need more tests. Unfortunately, high code coverage does not tell you if you have enough tests or the right tests. Adding to Robert Glass’ observation, executed code is not necessarily tested code. Imagine a test case that runs through many lines of code, but never checks that they are doing the right thing. At best this is the “I don’t have any bad pointers” test.
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.
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