[Cyph0n]

I work on IOS-XR, a router OS written in C.

I agree: UT is a pain to write precisely because you need to spend quite a bit of effort to stub out your dependencies. And when you do stub things out, they usually end up being “dumb” stubs where the function just returns EOK. Thankfully, there has been a recent effort in XR to leverage the Cmocka test framework to make stub functions a bit smarter.

Even if you have great UT, there is a bigger issue: the UT only tests and validates your code, but does not validate interactions with other components. With a system as complex as IOS-XR, there are non-trivial situations that you simply cannot trigger with UT.

This is where IT shines, imo: you can bring up a full router and test all known interactions at the system level. The test runtime is much longer, of course, but in my experience, it’s worth the wait to avoid hitting the issue down the line.