Seek sets the offset for the next read() or write() to offset, interpreted according to whence: SeekStart means relative to the start of the file, SeekCurrent means relative to the current offset, and SeekEnd means relative to the end. Seek returns the new offset relative to the start of the file and an error, if any.

Writes p.byteLength bytes from p to the underlying data stream. It resolves to the number of bytes written from p (0 <= n <= p.byteLength) and any error encountered that caused the write to stop early. write() must return a non-null error if it returns n < p.byteLength. write() must not modify the slice data, even temporarily.