file:setvbuf (mode [, size])

Sets the buffering mode for an output file. There are three available modes:

"no": no buffering; the result of any output operation appears immediately.
"full": full buffering; output operation is performed only when the buffer is full (or when you explicitly flush the file (see io.flush ())).
"line": line buffering; output is buffered until a newline is output or there is any input from some special files (such as a terminal device).

For the last two cases, sizes specifies the size of the buffer, in bytes. The default is an appropriate size.