Making a StringBuffer in C, and questioning my sanity

Hm, this implementation seems allergic to passing types by value, which eliminates half of the allocations. It also makes the mistake of being mutable-first, and provides some fundamentally-inefficient operations.

The main mistake that this makes in common with most string implementations make is to only provide a single type, rather than a series of mostly-compatible types that can be used generically in common contexts, but which differ in ways that sometimes matter. Ownership, lifetime, representation, etc.

How would you recommend doing that sort of "subtyping"? _Generic and macros?

Yup. It's a lot saner in C++, but people who refuse to use C++ for political reasons can do it the ugly way using C11 or GNU C.

"political reasons"?

I switched from C++ to C because C++ is too complex and dealing with this complexity was stealing my time. I would not call this a "political reason".