a gsize to get the length of the data read in.
optional gio.Cancellable 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.