///
  /// Plugins:
  ///   * Must set `status` to `TF_OK` and return current position if no error.
  ///   * Must set `status` to any other value and return -1 in case of error.
  int64_t (*tell)(const TF_WritableFile* file, TF_Status* status);

  /// Flushes `*file` and syncs contents to filesystem.
  ///
  /// This call might not block, and when it returns the contents might not have
  /// been fully persisted.