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?
One of the biggest challenges for someone experiencing TDD the first time is working in the small steps. The small steps of TDD appear to be an independent discovery of John Gall’s observation described in the Systems Bible:
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.
TDD ignores design; that is a frequently stated misconception. Many people get this idea from the code focus of TDD. TDD does not call for the creation of any non-executable design documentation. So the questioning developer gets the idea that there is no design. But I say, “yes there is”.
A recent TDD for Embedded C attendee asks me “TDD does not help reduce the time I spend in the lab during system integration testing”
(This is asked in the context of embedded development. But the answer is half applicable to any development where there is integration.)
TDD should definitely help reduce the time you spend in integration. How does it do that? It helps you eliminate the non-integration problems before you get to the integration lab. An honest appraisal of what you do in integration may reveal that during integration you are finding problems you could have discovered before integration. It would help reduced integration time if you find fewer problems during integration. Don’t you think?
This cause/effect diagram helps to visualize the relationship of TDD to time spend in integration.
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.
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
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.