[fpc-devel] ConsoleIO and flushing buffered output

[Michael Van Canneyt]

On Mon, 8 Jun 2020, Christo Crause via fpc-devel wrote:

> On Sun, Jun 7, 2020 at 11:54 PM Michael Van Canneyt <michael at freepascal.org>
> wrote:
>
>>
>> On Sun, 7 Jun 2020, Christo Crause via fpc-devel wrote:
>>
>>> I want to change the _haltproc for freertos ESP controllers to use normal
>>> write/writeln functionality to send information to standard output.  On
>>> these targets the output is typically buffered serial provided by the OS.
>>> Text output written just prior to setting the cpu to sleep doesn't get
>>> sent, hence a flush is needed to complete data transmission before
>> putting
>>> the cpu to sleep.  Looking at consoleio.OpenIO there doesn't seem to be a
>>> convenient way to provide a flush helper function. Also the RTL Flush
>>> procedure calls InOutFunc and not FlushFunc.
>>>
>>> What is the proper way to hook a flush helper function into the existing
>>> RTL infrastructure?
>>
>> Simply do this in InoutFunc ?
>>
>> Michael.
>>
>
> Thanks for your response Michael.  Using InOutFunc to also flush the output
> buffer will work, but that seems inefficient, since the flush needs to wait
> until the transmit buffer is empty (at slow UART speeds this could
> potentially take several ms to complete).   Is there a specific reason why
> the RTL Flush procedure does not call the FlushFunc method?

I checked; That code is so old, no idea.

>From what I can see the flush code could be changed to

if Assigned((TextRec(t).FlushFunc) then
   FileFunc(TextRec(t).FlushFunc)(TextRec(t))
else
   FileFunc(TextRec(t).InOutFunc)(TextRec(t));

But keep in mind that the InoutFunc() is only called when actually writing
data, meaning: when the internal text buffer is full or on a terminal with
every writeln(), so I don't think it is inefficient. Writeln() will do a
flush. Whether this happens in .InOutFunc or .FlushFunc is largely irrelevant.

Michael.