Return TRUE if the cached data was successfully stored (or if there was no data to store), or FALSE if the data could not be stored.
This method is called in response to fflush() calls on the stream. If you have cached data in your stream but not yet stored it into the underlying storage, you should do so now. Return TRUE if the cached data was successfully stored (or if there was no data to store), or FALSE if the data could not be stored.