OOP only disallows inheritance with unimplemented functions when it's a contract violation.
So that is to say, if the base class has a certain function which must be implemented and must provide certain behaviors, then the derived class must implement that function and provide all those behaviors.
The POSIX functions like read and write do not have a contract which says that all implementations of them must successfully transfer data. Being unimplemented (e.g returning -1 with errno EOPNOTSUPP or whatever) is allowed in the contract.
OOP just wants a derived thing to obey the contract of the abtraction it is inheriting, so if you want certain liberties, you have to push them into the contract.
