routine dir_link (
    dir: file_t;
    file: file_t;
    name: string_t;
    excl: int);

Create a hard link.

If dir and file are not implemented by the same filesystem, EXDEV should be returned. If the two filesystems, however can inter-operate and guarantee the appropriate POSIX semantics, they can communicate by a private protocol and allow hard links between them. If excl is set, then fail if name already exists in dir.