Re: Interaction of os calls and \r

[Andres Perera <andres.p@...>]

On Tue, May 14, 2013 at 5:13 PM, Sean Conner <sean@conman.org> wrote:
> It was thus said that the Great Jose Torre-Bueno once stated:
>> Can anyone explain why this code produces a line on the console that ticks
>> up every second:
>
>   You are running this on a Unix system, right?
>
>> function testw()
>>       while true do
>>               t = os.time()
>>               io.write(t, '\r')
>>       end
>> end
>>
>> As does this (with less cursor movement)
>>
>> function testw()
>>       while true do
>>               t = os.time()
>>               io.write(t, '\r')
>>               while os.time() == t do
>>                       io.write('\r')
>>               end
>>       end
>> end
>>
>> But this code produces nothing:
>>
>> function testw()
>>       while true do
>>               t = os.time()
>>               io.write(t, '\r')
>>               while os.time() == t do
>>                       io.write('')
>>               end
>>       end
>> end
>
>   Using the C standard IO (which is what Lua uses), you have three modes of
> buffering, none, line and full.  No buffering causes the output to be
> written immediately.  Line buffering waits until an entire line is present
> before output (or the internal buffer is full).  Full buffering waits until
> the internal buffer is full before writing it.
>
>   Now, on Unix, output to a TTY (like the command line) defaults to line
> buffering; everything else defaults to full buffering.  Also, the default
> buffer size is BUFSIZ (an implementation dependent value) but is typically
> around 8,192 bytes in size.

The criteria for default buffering differs depending on more than the
associated file. stderr, for example, defaults to no buffering
regardless of  whether there's a TTY on the receiving end or not.

>   '\r' on Unix causes the TTY cursor to move to the start of the current
> line (it's CARRIAGE RETURN) whereas '\n' causes the cursror to move to the
> start of the next line (LINE FEED) [1].

Actually, that facet of '\n' depends on terminal state. After `stty
-onlcr`, the ternimal will no longer automatically output a carriage
return after a newline, and consequently no longer reposition the
cursor to the start of the next line.