I have a little micro-framework called fff.h for generating fake functions (mocks) in C. I have blogged about it in the past, and there have been some exciting changes over the last few weeks that I’d like to share.
The basic premise is that testing a C source file is difficult in idiomatic C because of all the external function calls that are hardwired into the production code. The way fff.h helps is to make it a one-liner to create fake implementations of these for the purposes of testing.
Now, the basic formula on my last project for testing a C module is like this:
This was crude but effective. What really grated with me though was the amount of repetitive code that had to be written to answer very basic questions. Was my fake function called? How many times? What parameters were passed in on each occasion? Was the unlock function called before the write function? To answer these questions we ended up writing endless amount of (not very pretty) code like this:
It seemed to me that it should be possible to write a macro to generate this code. This would tidy up the test files in terms of readability and would make it easier for my team to make tests. And that was the genesis of the Fake Function Framework.
The best introduction to the Fake Function Framework is on the fff github site so I won’t rehash that here.
I have since moved on to a new project and haven’t thought about fff.h in a wee while. Through a variety of coincidences it happened that Tore, the architect on my previous project, met James Grenning during a training course and introduced him to fff.h. James played around with fff.h and sent me some great suggestions for cleanup, and how to improve fff.h to produce globally linkable fakes. At first I thought that this was an unneeded complication for an otherwise simple framework, but James convinced me that reusable fakes had a lot of value.
I set to work on making the generator able to generate the new style fakes. I don’t know I would have attempted this without having a full regression test suite for the framework.
The way it works is instead of using the usual syntax to create a fake using the FAKE_xxx_FUNCn macros, you create a header file to hold the declarations:
And then create an implementation file for the function definitions:
Then you can link many test suites against this single object without any problems. Simple!
There have been some breaking changes to fff.h to enable these new features, and I have also taken that opportunity to clean up some weaknesses in the previous design. But since fff.h is just a header file, both will happily exist in your codebase. All you have to do is name the new version of fff.h fff2.h and start using fff2.h in your new tests.
So theses are the basic changes you should know about if you are already familiar with the fake function framework. Somewhere in the test executable you must define the globals for fff. Why not put it beside the main function?
In the old version of fff.h there was a shortcut for resetting all the fakes in one step. This is now gone. The reason is that it only worked in C++ using static initializers and the introduction of global fakes were incompatible with this.
There has also been some cleanup. All the fake function framework variables have been moved into a struct to avoid polluting the global namespace.
And the variables for individual fakes are now defined in a struct.
The fake function framework would not exist as it does today without the support of key folks. Tore Martin Hagen (and his whiteboard), my partner-in-crime in Oslo, was instrumental during the genesis of fff. Jon Jagger, who during ACCU 2011 helped me teach the preprocessor to count. And James Grenning, who convinced me the value of global fakes, sent me a prototype Implementation, and showed me how expressive a DSL can be. Thanks to you all!
Request for comments
I hope you like the new changes, and if you have any feedback or suggestions for further improvements don’t hesitate to leave a comment or contact me via twitter.
- The Fake Function Framework on GitHub
- Previous article on faking memory using the Fake Function Framework
- Article from James Grenning on using the Fake Function Framework to fake an RTOS