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.

>>23,24
you're right, I don't need GC or structures just for parsing but

UR JUST PARSING DA DATA, Y WUD U NEED GC?

see >3. basically, the parser is the first part of what is supposed to be a library for lispy DSLs. besides cons cells and integers, I'll probably need floats, strings and symbols.