[andrepd]

No, that's not the "fundamental problem". The fundamental problem is a type system that lies. A "pointer to string" is not actually a pointer to a string, it's a pointer to a string or to nothing. If your api returns a pointer of the latter type, it should signal this by making the return type "maybe-pointer to string" (although it has the same memory representation as "pointer to string"). Then, if the user tries to dereference a maybe-pointer (that is, to use a maybe-pointer as a pointer), the type system can statically catch this and make it a simple type check failure compilation error. The user must first check if it's null through a function that casts a maybe-pointer to a pointer.

Nothing about this precludes the usage of sentinel values.