I am new to test-driven development, but I'm loving it. There is, however, a main problem that prevents me from using it effectively.

I work for embedded medical applications, plain C, with safety issues.

Suppose you have module A that has a function A_function() that I want to test. This function call a function B_function, implemented in module B. I want to decouple the module so, as James Grenning teaches, I create a Mock module B that implements a mock version of B_function.

However the day comes when I have to implement module B with the real version of B_function. Of course the two B_function can not live in the same executable, so I don't know how to have a unique "launcher" to test both modules.

James Grenning way out is to replace, in module A, the call to B_function with a function pointer that can have the value of the mock or the real function according to the need. However I work in a team, and I can not justify this decision that would make no sense if it were not for the test, and no one asked me explicitly to use test-driven approach.

Maybe the only way out is to generate different a executable for each module.

Any smarter solution? Thank you

  • Why do you need a single executable to test both functions?
    – Mat
    Commented Sep 2, 2012 at 8:39
  • The test suite I use (CppUTest) supports many "test suites", but generates only one executable. My fear in having many executables is that I will not run all of them every time, and I will discover that one test is broken too late.Maybe I only need a more powerful IDE that supports multiple project active as one and runs more executables with one command.Which one do you use?
    – Angelo
    Commented Sep 2, 2012 at 10:42
  • 1
    Running all of your tests every time should be a no-brainer. Script it, make sure it works right and run that instead of running the test tool directly.
    – Blrfl
    Commented Sep 2, 2012 at 13:34
  • agree, I'll go that way.
    – Angelo
    Commented Sep 3, 2012 at 6:09
  • I need a single executable to test as downloading to the embedded device takes a minimum of 3 minutes, which includes physically inserting and removing the programming dongle. (Our current test scripts utilize the GetATechnitianToDoIt() function)
    – Philip
    Commented Sep 5, 2012 at 16:48

1 Answer 1


Based upon what you've described, I would suggest that's a horrible reason to use a function pointer. It will jack up your ability to debug, analyze core dumps, and it will complicate future development / maintenance. Function pointers have their place, but this isn't one of them.

I think you're approaching the testing from the wrong point of view. A_func depends upon B_func's presence for it to operate. Therefore, you can't truly test A without having the actual implementation of B in place. Putting a mock B_func in place will only generate a limited set of tests for A_func. By definition, you can't fully exercise A_func without the real B_func in place.

Presuming you're not dealing with a cascading series of module inclusions, the appropriate test suite would have some tests to exercise B_func and then some additional tests for exercising the variations that A_func can go through. It's understood that you may have some redundancy between the tests for B_func and the indirect testing of B_func from A_func, but it's generally not a big deal.

  • I agree that's a bad use for function pointers, but I don't agree that I can't test a function without the implementation of the functions it depends on. In embedded SW, "B_func" might be a function that accesses the HW that maybe is not available, while "A_func" might simply be a business function that manages data, and can be tested with fake values. By mocking B_func I don't limit the test on A_func but, on the contrary, I can let B_func return any value, even the ones the are hardly ever returned.
    – Angelo
    Commented Sep 3, 2012 at 6:03
  • @Angelo - that is true so long as B_func has a contract in place as to what it is guaranteed to produce. If the root issue is missing X (hardware) to drive B_func, and B_func is well specified on its returns, then you could use some pre-compiler conditional flags to stub in the test harness you need. It makes the code a little uglier, but is far more tolerable to the rest of the team.
    – user53019
    Commented Sep 3, 2012 at 12:37
  • 1
    What if you give it a hip name like Dependency Injection? Is it a good idea then?
    – Philip
    Commented Sep 5, 2012 at 16:41
  • @Philip - only if you don't intend to try and debug core dumps.... :-)
    – user53019
    Commented Sep 5, 2012 at 17:54

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