a #gsize to get the length of the data read in.
optional #GCancellable object, %NULL to ignore.
a NUL terminated UTF-8 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
Reads a UTF-8 encoded line from the data input stream.
If @cancellable is not %NULL, then the operation can be cancelled by triggering the cancellable object from another thread. If the operation was cancelled, the error %G_IO_ERROR_CANCELLED will be returned.