While historically you are correct, having a single file descriptor for reading and writing to distinct streams is confusing and can break code. Bidirectionally

While historically you are correct, having a single file descriptor for reading and writing to distinct streams is confusing and can break code. Bidirectionally buffered wrappers over file descriptors (FILE, std::fstream for example) share the buffer between the read and write side, so you have to use distinct buffered wrappers with dup'd file descriptor anyway. Also the tty is typically connected to stdin, stdout and stderr, that are separate file descriptor (again, a leaky abstraction when they refer to the same description).