Linker Substitution in C – Limitations and Workarounds

Let’s say you have code that in some test cases you want the real code and other test cases you want a test double. In C, using link-time binding, you only get to have a single function with a specific name.

I’d suggest that most the time this comes up you want to directly test some module (or function) and you want a substitute when testing code that depends on the module’s interface.

What options do you have?

  • Create multiple test runners, where one test runner would use the real function and the other test runner would use the test double.
  • Function pointer substitution.
  • #include the .c file in a namespace in the test file.
  • Linker wrapping.
  • Weak linking.

Multiple test runners

This might be a decent solution. Though it can lead to needing many test runners.

Function pointer substitution

Changing a direct function call to an indirect function call is a small change, and the compiler and linker will make sure you get it right. There are a couple examples in my book, in chapter nine. Here’s one of them.

// RandomMinute.h -- original
int RandomMinute_Get(void);
// RandomMinute.c -- original

int RandomMinute_Get(void)
{
    return bound - rand() % (bound * 2 + 1);
}
// RandomMinute.h - revised
extern int (*RandomMinute_Get)(void);
// RandomMinute.c - revised

static int __randomMinute_Get(void)
{
    return bound - rand() % (bound * 2 + 1);
}

int (*RandomMinute_Get)(void) = __randomMinute_Get;

That refactor is pretty easy, but you must make sure all users of RandomMinute_Get() include the header file. If you happen to be lazy about calling functions without having them advertised in a header file, the caller will think this is a direct function call. Bad things will happen.

In the test group file, write FakeRandomMinute_Get().

//Test file for SomethingThatUses_RandomMinute_Get

extern "C"
{
    static randomMinuteValue;
    static randomMinuteIncrement;
    static void FakeRandomMinute_Init(int first, int increment)
    {
        randomMinuteValue = first;
        randomMinuteIncrement = increment;
    }

    static int FakeRandomMinute_Get(void)
    {
        int value = randomMinuteValue;
        randomMinuteValue += randomMinuteIncrement;
        return value
    }
}

in setup() override RandomMinute_Get() with FakeRandomMinute_Get().

//Test file for SomethingThatUses_RandomMinute_Get - Continued

TEST_GROUP(SomethingThatUses_RandomMinute_Get)
{
    void setup()
    {
        //other setup
        FakeRandomMinute_Init(-30, 3);
        UT_PTR_SET(RandomMinute_Get, FakeRandomMinute_Get);
    }
}
    void teardown()
    {
    }
}

UT_PTR_SET overrides the function pointer and then restores its original value after teardown(). Your code under test now has predcitable random minutes.

In another test group, the production RamdomMinute_Get() function can be tested without overriding the function pointer.

// RandomMinuteTest.cpp

TEST_GROUP(RandomMinute) {}

TEST(RandomMinute, GetIsInRange)
{
    for (int i = 0; i < 100; i++)
    {
        minute = RandomMinute_Get();
        AssertMinuteIsInRange();
    }
}

But maybe you don’t want to use a function pointer. You might prefer this next solution.

#include the .c file in a C++ namespace

In this solution, the test-double’s function name in FakeRandomMinute.c is the same as the function being faked, business as usual for link-time substitution.

// FakeRandomMinute.h
#include "RandomMinute.h"
void FakeRandomMinute_Init(int first, int increment);
// FakeRandomMinute.c
#include "FakeRandomMinute.h"

void FakeRandomMinute_Init(int first, int increment)
{
    randomMinuteValue = first;
    randomMinuteIncrement = increment;
}

int RandomMinute_Get(void)
{
    int value = randomMinuteValue;
    randomMinuteValue += randomMinuteIncrement;
    return value
}

To test the production implementation, the the RandomMinute.c file can be #included in the test file. In addition, the #include needs to be wrapped in a C++ namespace to keep the name out of the global namespace.

// in your test file

namespace Test
{
    #include "RandomMinute.c"
}

In the test cases, make calls to Test::RandomMinute_Get().

TEST(RandomMinute, GetIsInRange)
{
    for (int i = 0; i < 100; i++)
    {
        minute = Test::RandomMinute_Get();
        AssertMinuteIsInRange();
    }
}

Linker Wrapping

The linker tricks are not generally available except from gcc. I’ve experimented with wrapping in gcc., but could not find it in visual studio or IAR (at the time I looked).

In gcc, wrap RamdomMinute_Get() with these command line options:

gcc -Wl,--wrap=RamdomMinute_Get ...

The linker will redirect calls to RamdomMinute_Get() to __wrap_RamdomMinute_Get(). You can get at there production implementation by calling __real_RamdomMinute_Get(). You need to provide __wrap_RamdomMinute_Get() to hold your fake implimentation and to keep the linker happy.

int __wrap_RamdomMinute_Get()
{
    // some fake implementation
}

Maybe some test cases want the real RamdomMinute_Get(), you could make the wrapped function default to the real with function.

static int (*customRamdomMinute_Get)() = 0;
int __wrap_RamdomMinute_Get()
{
    if (customRamdomMinute_Get)
            return customRamdomMinute_Get();
    return __real_RamdomMinute_Get();
}

I chose to use a function pointer for test specific behavior. This lets you have a generic fake implemention that is customized close to your test case.

There are some limitations so you may want to look at this Stack Overflow article for linker wrapping

Weak Linking

I’d say stay away from weak linking as it means adding non-standard stuff to your producion C code.

Finally

You can mix and match these techniques. Let me know how this works for you or if you know some other solutions to this problem.

TDD Guided by ZOMBIES

Have you had a hard time figuring out where to start with Test-Driven Development. What if ZOMBIES could help you build code that does exactly what you think it is supposed to do? What if ZOMBIES at the same time help you to build a test harness that can help you keep your code clean and behaving properly for a long and useful life? What if ZOMBIES could help!

I’m not talking about those zombies! ZOMBIES is an acronym.
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Accessing static Data and Functions in Legacy C — Part 2

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.
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Accessing static Data and Functions in Legacy C — Part 1

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?”
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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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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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Boy Scout Rule Applied to Every Day Coding

The Boy Scouts have a rule: leave the camp cleaner than you found it. This does not mean that all the trash has to be cleaned up now, but you can’t let it get worse, and it must get at least a little better. In Bob Martin’s book, Clean Code, he asks, “What if code got a little better every time you change it?” I’ll answer it: the industry would not find itself in the mess it’s in. The industry norm is for code to incrementally worsen with each change.

Much of the time, following the Boy Scout Rule won’t be hard. It’s an incremental strategy. It’s easy to start and easy to sustain. Here are some typical challenges and ideas on how to be a Boy Scout.

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