code: func (rd *RAMDirectory) FileLength(name string) (length int64, err error) {
  	rd.EnsureOpen()
  	rd.fileMapLock.RLock()
  	defer rd.fileMapLock.RUnlock()
  	if file, ok := rd.fileMap[name]; ok {
  		return file.Length(), nil
  	}
  	return 0, os.ErrNotExist
  }
docs: // Returns the length in bytes of a file in the directory.