the #GAsyncResult that was provided to the callback.
a #gsize to get the length of the data read in.
a string with the line that was read in (without the newlines). Set @length to a #gsize to get the length of the read line. On an error, it will return %NULL and @error will be set. For UTF-8 conversion errors, the set error domain is %G_CONVERT_ERROR. If there's no content to read, it will still return %NULL, but @error won't be set.
GException on failure.
2.30
Finish an asynchronous call started by g_data_input_stream_read_line_async().