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The programmer runs his new function through the simplest of situations, totally ignoring timing issues or boundary conditions.
So of course the function passes, only to inflict the team with agony when integrated into the entire system. XP's idea of writing the tests in parallel with the code is quite brilliant; only then do we see all possible conditions that can occur. Beck argues for building tests first and deriving the code from these tests. It's sort of like building a comprehensive final exam and then designing a class from that. An intriguing idea. But ' what if the test is wrong?
Test driven development TDD then guarantees the code will be wrong as well. Worse, TDD calls for building even the smallest project by implementing the minimal functionality needed to do anything. Need a function to compute the Fibonacci series? Create the simplest possible test first ' in this case, check to see that fibonnaci 0 is 0. Then write a function that passes that test. Try fibonnaci 1 ; that breaks the test, so recode both the test and the function.
Iterate till correct. The book shows how to build a factorial program, which results in 91 lines of code and 89 of test'- after compilations!
Klaxons sound, common sense alarms trip. Agile proponents love the interactive and fast action of this sort of method. But programming isn't about playing Doom and Quake.
If you're looking for an adrenaline rush try bungee jumping. Oddly, because I suspect Kent Beck is a superprogrammer, the test examples given in the book are exactly like the ones we see dysfunctional programmers creating for their unit tests. None check boundary conditions, like the overflow that any factorial routine is bound to produce for even rather small inputs. When a problem occurs he says: 'Rather than apply minutes of suspect reasoning, we can just ask the computer by making a change and running the tests.
What if the change seems to work, but in actuality masks some deeper problem? He seems to advocate removing thinking from programming, a very dangerous proposition.
This is a fast world: fast food, drive-through liquor stores, ATMs on every corner, instantaneous email. But software needs thought. Slow down when you find a problem. Think deeply. And understand the implications of proposed changes. The alternative is making almost random changes. Glorified hacking. Click on picture to order from Amazon. He's has been writing a book on the subject for some time, and it should be available shortly after this column goes to press.
He generously sent me a PDF of the nearly final version. Test Driven Development for Embedded C I think there should have been a hyphen in the adjectival phrase is hands down the best book on the subject. This is an amiable, readable book with an easy style that is fairly code-centric, taking the reader from the essence of TDD through mastery using detailed examples. It's a welcome addition to the genre as the book is completely C-focused, unlike so many others, and is specifically for those of us writing firmware.
James skips no steps and leads one through the gritty details, but always keeps the discussion grounded so one is not left confused by the particulars. The discussion is laced with homey advice and great insight. He's not reluctant to draw on the wisdom of others which gives the book a sense of completeness. The early phases of a TDD project are mundane to the point of seeming pointlessness.
One writes tests to insure that the most elemental of things work correctly.
Why bother checking to see that what is essentially a simple write works correctly? I've tossed a couple of books on the floor in disgust at this seeming waste of time, but James warns the gentle reader to adopt patience, with a promise, later fulfilled, that he'll show how the process is a gestalt that yields great code. TDD does mean one is buried in the details of a particular method, a particular test, and the path ahead can be obscured by the tests at hand.
If you're a TDD cynic or novice be sure to read the entire book before forming any judgments so you can see how the details morph into a complete system accompanied by a stable of tests. Better than any book I've read on the subject, Test Driven Development for Embedded C lays out the essential contrast between TDD and conventional write-a-lot-of-code-and-start-debugging. With the latter technique we're feeding chili dogs to our ulcers as the bugs stem from work we did long ago, and are correspondingly hard-to-find.
TDD, on the other hand, means today's bug is a result of work one did ten minutes ago. They're exposed, like ecdysiast Gypsy Rose Lee's, uh, assets. A test fails? Well, the bug must be in the last thing you did. One of TDD's core strengths is the testing of boundary conditions. My file of embedded disasters reeks of expensive failures caused by code that failed due to overflows, off-by-one errors and the like.
TDD - or, at least James' approach to it - means getting the "happy" path his word - the basic functionality working and tested, and then writing tests to insure each and every boundary condition is also tested.
Conventional unit testing is rarely so extensive and effective. The problem with early testing is that the target system will not be available. Examples show, for instance, stubbed-LEDs and other components, but one would imagine a complex device could require quite a bit of simulation code. The book is less clear about how one eventually merges the target hardware with the stubbed-out code.
The stubs are a hugely valuable asset in terms of starting testing very early, but of course they are merely facsimiles of reality. Embedded TDD revolves around creating a test harness, which is a software package that allows a programmer to express how production code should behave. James delves into both Unity and CppUTest in detail.
Each test invokes creation and teardown routines to setup and remove the proper environment, like, for instance, initializing a buffer and then checking for buffer overflows. I found that very cool. The book goes much further than showing how to evolve code in concert with the tests.
It would have been interesting to see more on design, which feels relegated to a minor role. And I wished for some comments on TDD in a regulated environment, like avionics.
But Test Driven Development for Embedded C is and active-voice work packed with practical advice and useful aphorisms, like "refactor on green" i. I was also fascinated to find the book had been in beta for some time, and a wide range of developers had made critiques and suggestions for improvement. Rather agile-like. Above all the book stresses having fun while doing development.
And that's why most of us got into this field in the first place. An entire book just about pointers? That was my first reaction when looking at this volume. But Richard Reese's Understanding and Using C Pointers is actually a pretty good source of information about this critical subject. At times it is a bit repetitive; in other cases I think a more detailed explanation about an example would have been helpful. C pointers are a sore subject for a lot of people. Much C code is obviously written to avoid them; those developers probably don't have a deep understanding of the subject.
Competent assembly programmers know that a pointer is akin to assembly's indirection and so figure that the C version is no big deal. But in C they are far more nuanced than when working at the mnemonic level. The book is generally well-written and accurate, though a couple of flagrant mistakes were odd, like a comment that you can make a number negative by setting the MSB.
Well, yeah, that will be negative but flipping the MSB will probably not yield the desired result.
The subject is completely covered, and not just in isolation. For example, there's plenty about dynamic memory use, which is, of course, a place where pointers are required. An experienced C developer will consider the issues identified boneheaded mistakes. These include things like dangling pointers and the like. Perhaps a novice might find this information other than obvious. The last chapter talks about memory-mapped peripherals, aliasing, threads, and has a very good description of callbacks.
The author also goes into some OO concepts in detail, showing how to implement polymorphism in C. Experts will get little to nothing from the book.