How to get Relation tuples in C function

Greetings!

I would like to know if there is a better way to pass a relation or if the relation name (CString) as a parameter in a C function and thus be able to manipulate its tuples. The documentation is available here: https://www.postgresql.org/docs/13/xfunc-c.html#id-1.8.3.13.11. But it is not quite clear enough on how to retrieve tuples. The handling of these is quite clear. The only function I'm currently using (but not working) is the TupleDesc TypeGetTupleDesc (Oid typeoid, List * colaliases) function. Do we have a function like TupleDesc RelationNameGetTupleDesc (const char * relname) (Old and deprecated)?

regards,