Re: [PATCH.HURD] fix fdatasync/fsync if file_sync is not supported

[Roland McGrath]

> AIUI, POSIX says there can be other, implementation-defined errors.

That's true.  But when POSIX specifies a particular error code for a
particular condition, then you must yield the specified error code for that
situation.  If this is arising for something like using a file-oriented
call on an fd that is a socket or pipe (i.e. something that any program
could do on any POSIX system) then it really should result in the expected
POSIX error code.