I started pulling the common code I use in my different projects, and it's slowly becoming my own codebase, as I see is usual to do here. The problem is, I don't know how to manage it. At first I copied and pasted the code and changed it only on the project I was working on, then copied the new version to a newer project and so on, but for example some times I needed to fix bugs in the codebase for many projects, so I needed to duplicate the work for all of the affected ones. Now I use a monorepo as an umbrella, where all my projects live and share the same codebase. I generally include one header cb.h, compile the full cb.c and that's it. But then, when I change some API, I need to change it for all affected projects, otherwise they do not compile anymore.

What is your way of managing your codebase?

But then, when I change some API, I need to change it for all affected projects, otherwise they do not compile anymore.

I'm doing just that, updating the different projects to use the new API, and only when I actually need to do a modification to a project (I'm not updating every project every time I modify the API).

I never found that to be an issue, but I suppose it might depend on what your projects are and how often you need to modify them. If I really need to recompile a project with old APIs, I use SVN to get the version of the API that the project was build against.