[fpc-devel] ConsoleIO and flushing buffered output

[Christo Crause]

On Mon, Jun 8, 2020 at 12:15 PM Tomas Hajny <XHajT03 at hajny.biz> wrote:

> 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.
> <https://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-devel>


Tomas and Michael, thank you for this discussion.  I'm starting to think
the use case I'm considering is perhaps not covered by the current design.
At the moment I directly call the OS flush function before putting the cpu
to sleep and that works; I was just wondering if there is a Pascal way to
accomplish this so that a user doesn't have to remember to call a
nonstandard function when a Flush function already exist in the RTL.
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