Freepascal flushes stdout on every output under Windows?

Question

Look at the following four programs. Build them with Freepascal under Windows and run, redirecting output to any file, and noticing the time it would take.

My results are: all the programs run approximately the same time (about 6 seconds), although the fourth does 100 times more bytes of output. This means that the fourth program runs much faster per byte of output than the tree others.

For the second program the reason of slowness is obvious: the flush call. For the third program the reason is not so obvious, but it may be reasonable to assume that each call to writeln to stdout flushes the output buffer implicitly.

However, it is not clear why the first program is so slower than the fourth. However, the fact that adding flush(output); (see program 2) does not change the timing much seems to mean that FPC flushes the output buffer even after every write, this would explain all the behaviour. This happens only when output is to stdout, even redirected; if I explicitly output to a particular file using assign/rewrite, then the program without flush runs much faster than the program with flush — as should be expected.

Under Linux the running times are 0.01s, 0.65s, 0.01s, 0.30s (output being 100 times bigger). Here clearly flush() slows program down, so under Linux FPC seems not to flush stdout each time.

I have tried to google whether FPC really flushes stdout buffer on every output (be it write or writeln), but have found no information except the comment in the example program from flush function documentation at http://www.freepascal.org/docs-html/rtl/system/flush.html, the comment mentions that a writeln to 'output' always causes a flush [as opposed to write]. However, the example there does not produce the intended output neither under Windows, neither under Linux. In fact, the output seems to be flushed after every write and writeln under Windows, redirected or not, and under Linux too when the output is not redirected. Under Linux with redirected output it seems there is no implicit flushing at all.

So, my questions are:

1. Is it true that FPC flushes the output buffer after every output, be it write or writeln, on Windows whether the output is redirected to file or not?
2. If yes, then is there any way to turn this off (some compiler directive, or a workaround)? I still need to keep the output to stdout, so that if I start the program without any redirections, it will output text to the console. (I understand that I might see it appearing at strange times as a result of buffering, it is not a problem.)
3. If no, then why the first program runs much slower then the fourth?

My system is Windows XP with FPC 2.6.4 under VirtualBox under Kubuntu 14.04, and Kubuntu 14.04 itself with FPC 2.6.2. I had no chance trying to run it on a real Windows machine, but I have some reasons to believe that the situation is the same there.

---

The programs:

var i,j:integer;
    s:string;
begin
for j:=1 to 1000 do begin
   for i:=1 to 10 do 
      write('!');
end;
end.

---

var i,j:integer;
    s:string;
begin
for j:=1 to 1000 do begin
   for i:=1 to 10 do begin
      write('!');
      flush(output);
   end;
end;
end.

---

var i,j:integer;
    s:string;
begin
for j:=1 to 1000 do begin
   for i:=1 to 10 do 
      writeln('!');
end;
end.

---

var i,j:integer;
    s:string;
begin
for j:=1 to 10000 do begin
   s:='';
   for i:=1 to 100 do 
      s:=s+'!';
   write(s);
end;
end.

Answer 1

To prevent flushing of Stdout, insert the following code snippets into your program:

// Textrec is defined in sysutils
uses
  sysutils;

// ...

// disabled flushing after each write(ln) on Output, do this at the start of the program
Textrec(Output).FlushFunc:=nil;

But be aware that this means that writelns might not be completed before the program ends.

You can even accelerate the program further by increasing the output buffer of stdout:

// define buffer
var
  buf : array[1..100000] of byte;

  // ...

  // install buffer, do this at the start of the program
  SetTextBuf(Output,buf,sizeof(buf));