type information in a lisp: DA STANDARD way

so I'm making a toy s-expression parser in C which may or may not become a minimal toy lisp interpreter and I'm trying to figure out the best way to do types. I know the classic way is to use the end of the pointer as tag bits but it doesn't really look standard-conforming or portable.

is there a way of doing it that doesn't rely on undefined behavior? the only thing I can think of is making a struct that holds value/pointer and type information but this seems very wasteful.

>>2
I think this is what I'll end up doing, mostly because I want to make as little assumptions about target architecture as possible. my ultimate goal would be to make an stb-like one-file library which allows people to easily add Lisp-like scripting to C projects, and stb libs are usually more about simplicity/minimalism than raw performance.