Pointers to a well-defined type, but one which does not reveal the underlying datatype so a user is unable to interact with the actual data without going throug

Pointers to a well-defined type, but one which does not reveal the underlying datatype so a user is unable to interact with the actual data without going through the library’s exposed API (or at least strongly disincentivised from doing it). IIRC the normal way to do that in C is to use an incomplete type, aka just have a struct foo; In the header file and define the actual struct contents in the implementation file.