[fpc-devel] ConsoleIO and flushing buffered output

[Tomas Hajny]

On 2020-06-08 11:39, Michael Van Canneyt wrote:
> On Mon, 8 Jun 2020, Christo Crause via fpc-devel wrote:
  .
  .
>> 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.

The question is whether the potential change would make any difference. 
IMHO, the important points are:

1) The point of flushing is making sure the I/O is really performed (the 
data are sent to the operating system _and_ flushed from internal 
buffers of the underlying operating system / platform). Doing the latter 
without the former makes no sense.

2) From semantic point of view, all data should be sent to the final 
place (block device / console / ...) before the call to Flush is 
finished (otherwise the following actions may have incorrect results - 
the program might finish before all data are transmitted, other I/O may 
be invoked, etc.).

 From this point of view, the proposed change would only lead to code 
duplication between FlushFunc and InOutFunc and / or to increased code - 
e.g. checking FlushFunc being nil before calling InOutFunc just to call 
InOutFunc (or to perform functionality currently included in InOutFunc) 
from within FlushFunc anyway.

Tomas