I saw something puzzling today while using zcov – a function I didn’t write that had no coverage. In my parsing code I have a small struct, something like this:

This post is based on an interesting case NASA brought me; a small test program with a phantom condition. In the example, timed_out() is always true, which means checkout() cannot satisfy MC/DC, but let’s pretend it does something interesting. The function has one decision with two conditions, and four outcomes. Let’s measure MC/DC and confirm:

GCC 15.1 was released today, and it comes with prime path coverage support! Try it out and build your programs with -fpath-coverage, it truly is a brilliant metric.

I wrote another paper detailing the implementation of the prime path coverage support. You can get a copy here or from arxiv.