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.

>>16
so the best way of storing type information is not storing type information? I like the simplicity of this solution but I'm not sure how could I do GC with something like this. also, wouldn't there be a noticeable overhead if every operation needed to infer types?

>>17
I guess you're going to be pedantic and argue that it's not a syscall but a function that is a wrapper around it but it doesn't change the fact that it doesn't work on windows.