An annoying thing about C code is that there are plenty of functions that cannot be unit-tested by some external framework - specifically anything declared as static. Any larger code-base will end up with hundreds of those functions, many of which are short and reasonably self-contained but complex enough to not trust them by looks only. But since they're static I can't access them from the outside (and "outside" is defined as "not in the same file" here).
The approach I've chosen in the past is to move the more hairy ones into separate files or at least declare them normally. That works but is annoying for some cases, especially those that really only get called once. In case you're wondering whether you have at least one such function in your source tree: yes, the bit that parses your commandline arguments is almost certainly complicated and not tested.
Anyway, this week I've finally found the right combination of hacks to make testing static functions easy, and it's:
- #include the source file in your test code.
- Mock any helper functions you'd need to trick the called functions
- Instruct the linker to ignore unresolved symbols
A more detailed writeup is available in this github repo.
For the impatient, the meson snippet for a fictional source file example.c would look like this:
test('test-example', executable('test-example', 'example.c', 'test-example.c', dependencies: [dep_ext_library], link_args: ['-Wl,--unresolved-symbols=ignore-all', '-Wl,-zmuldefs', '-no-pie'], install: false), )
There is no restriction on which test suite you can use. I've started adding a few of test cases based on this approach to libinput and so far it's working well. If you have a better approach or improvements, I'm all ears.