[fpc-pascal] Unbuffering I/O

[Henry Vermaak]

I think I've seen this question asked before, my apologies if this was
recently.
I've got two programs intended to be functionally identical, one in Perl and
the other in FPC. They read a unix-domain datagram, decode the message, and
emit output; if this goes to a file then it's reasonable to monitor it using
tail -f
Perl has a variable that you can set to force output to be unbuffered, with
the result that as soon as a message is output it's in the file in its
entirety.
Is there an equivalent for Pascal, or should I be using something like
fpSync(stdout) at opportune times?

Does SetTextBuf() with a buffer of size 1 work? I don't think there is
anything equivalent to setvbuf(). Otherwise you'll have to Flush() them
manually, which is a pain.

Henry
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[Sven Barth via fpc-pascal]

On Wed, Aug 29, 2018 at 03:01:54PM +0000, Mark Morgan Lloyd wrote:> I
think I've seen this question asked before, my apologies if this was>
recently.> > I've got two programs intended to be functionally
identical, one in Perl and> the other in FPC. They read a unix-domain
datagram, decode the message, and> emit output; if this goes to a
file then it's reasonable to monitor it using> tail -f> > Perl has a
variable that you can set to force output to be unbuffered, with> the
result that as soon as a message is output it's in the file in its>
entirety.> > Is there an equivalent for Pascal, or should I be using
something like> fpSync(stdout) at opportune times?
Does SetTextBuf() with a buffer of size 1 work?  I don't think there
isanything equivalent to setvbuf().  Otherwise you'll have to Flush()
themmanually, which is a pain.

I'm a bit wary of SetTextBuf() with a preopened handle. Flush() is
actually no big deal since the program has a well-defined processing
loop... but unfortunately is less than effective.
It's something I can live with, since this is really only a test
program I knocked together to look at how clock_gettime() behaves on
different platforms (the answer being "badly" :-)

SetTextBuf() does not influence the handle itself, only the buffer of
the file/text record, so it should be safe to use.

Regards,
Sven
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[w***@windstream.net]

Is there an equivalent for Pascal, or should I be using something like
fpSync(stdout) at opportune times?

flush();


i use it all the time on my programs that write to the logs... i hate having a
crash and be missing some log output because it was buffered and lost with the
crash... flush() everything...

[w***@windstream.net]

Otherwise you'll have to Flush() them manually, which is a pain.

really? i wrote a wrapper for write and writeln that simply calls them and then
does a flush()... nothing painful other than using mywrite() and mywriteln() or
similar ;)

[Martin Schreiber]

I think I've seen this question asked before, my apologies if this was
recently.
I've got two programs intended to be functionally identical, one in Perl
and the other in FPC. They read a unix-domain datagram, decode the
message, and emit output; if this goes to a file then it's reasonable to
monitor it using tail -f
Perl has a variable that you can set to force output to be unbuffered,
with the result that as soon as a message is output it's in the file in
its entirety.
Is there an equivalent for Pascal, or should I be using something like
fpSync(stdout) at opportune times?

In order to flush textfiles automatically I use
"
ttextrec(<thefile>).flushfunc:= ttextrec(<thefile>).inoutfunc;
"
after it is opened.

Martin

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[Henry Vermaak]

Post by Martin Schreiber

I think I've seen this question asked before, my apologies if this was
recently.
I've got two programs intended to be functionally identical, one in Perl
and the other in FPC. They read a unix-domain datagram, decode the
message, and emit output; if this goes to a file then it's reasonable to
monitor it using tail -f
Perl has a variable that you can set to force output to be unbuffered,
with the result that as soon as a message is output it's in the file in
its entirety.
Is there an equivalent for Pascal, or should I be using something like
fpSync(stdout) at opportune times?

In order to flush textfiles automatically I use
"
ttextrec(<thefile>).flushfunc:= ttextrec(<thefile>).inoutfunc;
"
after it is opened.

Reading text.inc this morning lead me to believe this is the correct
way. This is what the RTL does when opening serial devices in
FileOpenFunc(), for example. There's a comment inside Flush() that says
that InOutFunc() should be used to flush, since the FlushFunc() may not
be assigned.

Thanks for this, Martin.

Henry
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[Henry Vermaak]

In order to flush textfiles automatically I use> "> ttextrec(<thefile>).flushfunc:= ttextrec(<thefile>).inoutfunc;> "> after it is opened.
Reading text.inc this morning lead me to believe this is the correctway. This is what the RTL does when opening serial devices inFileOpenFunc(), for example. There's a comment inside Flush() that saysthat InOutFunc() should be used to flush, since the FlushFunc() may notbe assigned.

I've just checked this and unfortunately it doesn't do very much for the
standard device (?) used by WriteLn() etc., i.e. as would be used for a
quick-and-dirty program.

It definitely works for me. I made a program flush.pas that looks like this:

begin
ttextrec(output).flushfunc := ttextrec(output).inoutfunc;
writeln('hi there');
sleep(1000);
end.

Running `./flush > out.txt` and `tail -F out.txt` shows the output
immediately, while removing the flushfunc assignment causes a delay of a
second before the output appeared in the file.

Henry
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[Henry Vermaak]

Ah yes, that's it, thanks very much.
WriteLn(StdErr, Format('# Socket %s, clock resolution %8.6f uSec',
[socketName, ts.tv_nsec / 1000]));
ttextrec(StdErr).flushfunc:= ttextrec(StdErr).inoutfunc;
while true do begin
ttextrec(Output).flushfunc:= ttextrec(Output).inoutfunc;

You only need to set flushfunc once at startup (for standard handles) or
just after opening a file. The whole output vs stdout thing has
confused me in the past, too.

Henry
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[Marc Santhoff]

I've got two programs intended to be functionally identical, one in Perl
and the other in FPC. They read a unix-domain datagram, decode the
message, and emit output; if this goes to a file then it's reasonable to
monitor it using tail -f
Perl has a variable that you can set to force output to be unbuffered,
with the result that as soon as a message is output it's in the file in
its entirety.
Is there an equivalent for Pascal, or should I be using something like
fpSync(stdout) at opportune times?

I don't know if it really helps, but since I fiddled with a similar topic:

FreeBSD has stdio channel non blocking by default. I had some problems with
that and did this:


procedure SwitchFdBlocking(var channel: Text);
var
res: cint;
begin
res := fpfcntl(TextRec(channel).Handle, F_GETFL);
if ((O_NONBLOCK AND res)>0) then
begin
res := res AND (NOT O_NONBLOCK);
res := fpfcntl(TextRec(channel).Handle, F_SETFL, res);
if (res=-1) then writeln(stderr, 'ERROR on SETFL');
end;
end;

According to the man page of fcntl using the flag "O_DIRECT" instead of
"O_NONBLOCK" for switching of caching as far as possible. At least on FreeBSD
it is like this, check on Linux yourself...