[erlang-questions] Flush "stdout" buffer

Per Hedeland <>
Sun Sep 3 06:22:13 CEST 2017


On 2017-09-02 19:15, Michael Truog wrote:
> On 09/02/2017 04:08 AM, Per Hedeland wrote:
>> On 2017-09-02 10:07, Michael Truog wrote:
>>> On 09/01/2017 04:58 AM, Frank Muller wrote:
>>>> Can someone help on this please?
>>>>
>>> If you use stdout as an Erlang port like:
>>>
>>> STDOUT =  erlang:open_port({fd, 0, 1}, [out, {line, 256}]),
>>> erlang:port_command(STDOUT, "unbuffered").
>>
>> Can you actually observe a difference *in buffering* between doing that
>> and just
>>
>> io:format("unbuffered").
> 
> Yes, it really does matter.

Unfortunately you didn't answer my question.

> You will notice that the io module doesn't have any flush function,

Yes, of course it doesn't have any flush function, since it doesn't do
any buffering.

> so you are unable to force any buffering in the Erlang VM to flush.

This gets us into the realm of philosophy - is it possible to be unable
to flush buffering that doesn't exist?

> However, if you really care about 
> stdout in unbuffered mode, you need to use the Erlang port as
> described above.

Please provide *some* justification for that claim. At least a way to
observe that buffering is actually happening - ideally a pointer to the
specific piece of code that implements it.

I don't care much what you do in your code, but I strongly oppose that
you lead "newbies" to believe that they have to resort to ugly low-level
stuff like the above in order to achieve functionality that is actually
the default.

> That is similar to using fcntl to set file descriptor 1 with O_NONBLOCK.

No, neither that code nor making C stdout unbuffered is even remotely
similar to making file descriptor 1 non-blocking.

> Programming languages normally have a way of 
> setting stdout (and stderr, though stderr is really suppose to always be in unbuffered mode) in an unbuffered mode so all the output is available as quickly as possible.

Yes, programming languages that have C stdio or a corresponding
buffering mechanism normally have a way of turning it off. Erlang
doesn't have such a mechanism (for non-disk I/O).

[snip irrelevant elaboration on C/C++ programming]

> In Erlang source code we can't really put the io server into an unbuffered mode, and there is no reason to really, because it is simpler to bypass the io server by making an Erlang port, as 
> shown above.

No - there is no reason to do it, because it isn't needed.

Btw, this doesn't mean that Erlang "standard_io" is somehow "higher
level" than C stdio, it's rather the opposite - but it's simpler to use,
since you don't need to care about tweaking the buffering with setvbuf()
and/or fflush() like you do in C.

And finally, yet another demonstration of the lack of buffering in
Erlang "standard_io" - a progress bar!:-)

Eshell V8.3.5.1  (abort with ^G)
1> F = fun (0,_) -> ok; (N,G) -> io:format("*"),timer:sleep(1000),G(N-1,G) end.
#Fun<erl_eval.12.118419387>
2> F(20,F).

--Per


More information about the erlang-questions mailing list