Object Mentor Blog: Exploding Link Stubs in C

Exploding Link Stubs in C

Michael Feathers — Fri, 07 Nov 2008 19:33:00 -0600

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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