Exploding Link Stubs in C

Exploding Link Stubs in C 55

Posted by Michael Feathers Sat, 08 Nov 2008 01:33:00 GMT

When you’re working in a batch of legacy C code, it’s often hard to build and test just a part of it. You may want to build some set of functions, but then you discover that they call other functions, and those functions call yet other functions. When link dependencies are very bad, you may discover that you’re linking in the entire system.

Fortunately, there is a way out of this. Pick a set of files that appear to be a “component” and attempt to build them into an executable along with a simple main function.

You will get link errors.

Create a file called <componentname>_stubs.c and then go through each of the link errors. Look up the full declaration of the variable or function the error describes. Create a stub for each of them in the stub file.

Here’s an example of a function stub:


int irc_send_ppr(struct ppr *pprn, int nSize)
{
}

The only problem with this is that you have to place something in the stub function. For functions which return values you need to, at the very least, provide a return value. But what should it be?


int irc_send_ppr(struct ppr *pprn, int nSize)
{
  /* is negative -1 okay?   If it isn’t, how will we know? */
  return -1;
}

There is alternative ++:


int irc_send_ppr(struct ppr *pprn, int nSize)
{
  assert(!”irc_send_ppr boom!!”);
}

This is an exploding link stub. When you have it in place, you will know when a function you’re testing calls a stub. When you see the boom you go to the code and replace the assert with a better stub implementation.

When you use link stubs, you have to be able to build your component two different ways. Production builds link to the rest of the code. Test builds link to the stubs and a testing main. There’s no need to stub out all of the external functions that a component uses. Many low level functions can remain direct calls, but stubbing out calls to other high level components can give you a decent amount of leverage as you try to get an area under test.

Fortunately, for any given component you only have to go through the massive grunt work once. Once you do, you can reap the reward of easier testing in that component forever.

++ Note: If you are using C99, you can genericize this code by using the __func__ predefined macro. In any function, __func__ expands to the name of the enclosing function.

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Anonymous about 2 hours later:

If you are using any modern C or C++ compiler, you can genericize this code by using the non-standard but universally supported keyword FUNCTION. In any function, FUNCTION expands to the name of the enclosing function, just like func, but in the form of a string literal ready for string concatenation. (C99’s func keyword is not a macro, and does not expand to a string literal so it cannot be concatenated with adjacent strings.)

#define EXPLODE assert(!”Boom in ” FUNCTION);
Sam about 9 hours later:
Actually, to make this really easy, you don’t need to get the right types. Your C compiler/linker will give you a list of undefined dependencies. You can then use some sed/awk/perl/whatever magic to transform this into a C file that will solve these. All you need to do is generate a file like this:
```
#define STUB(x) void x(void) { assert(0 && "Undefined function " __func__); }

STUB(func1);

STUB(func2);
```
It doesn’t matter that their signatures aren’t right; the linker only sees them as symbols. I’ve been using this method successfully in my own project where I need to pull code out of a large ecosystem (the kernel) and link it into another.
Michael Feathers about 17 hours later:

@Anonymous: Thanks. It’s a shame that __func__ is defined the way it is.

@Sam: Thanks for bringing that up. I’ve done that in the past (and since forgotten about it). Great way to get the initial link. The only thing is, I want to get the real signature in whenever I write a test that explodes a stub. Fair call, though, that you don’t have to do it all at once.
Pete 2 days later:

The dependency breaking techniques in your book have been great for getting me ‘unstuck’. Thanks!
owbear@gmail.com 3 days later:

This is Interface Discovery. It can be a very useful way of reasoning about a system—http://connextra.com/aboutUs/mockobjects.pdf
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Exploding Link Stubs in C 55

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