When mode is zero, the fwide function determines the current orientation of stream. It returns a value stream is wide-character oriented, i.e. if wide character I/O is permitted but char I/O is disallowed. It returns a value stream is byte oriented, i.e. if char I/O is permitted but wide character I/O is disallowed. It returns zero if stream has no orientation yet; in this case the next I/O operation might change the orientation (to byte oriented if it is a char I/O operation, or to wide-character oriented if it is a wide character I/O operation).
Once a stream has an orientation, it cannot be changed and persists until the stream is closed.
Wide-character output to a byte oriented stream can be performed through the fprintf function with the %lc and %ls directives.