a #gsize to get the length of the data read in.
optional #GCancellable object, %NULL to ignore.
a NUL terminated byte array 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. If there's no content to read, it will still return %NULL, but @error won't be set.
GException on failure.
Reads a line from the data input stream. Note that no encoding checks or conversion is performed; the input is not guaranteed to be UTF-8, and may in fact have embedded NUL characters.
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.