Re: File output and buffering

[Georg Bauhaus <rm.dash-bauhaus@futureapps.de>]

Maciej Sobczak schrieb:

>> In particular: how can I *flush* the buffer?
> 
> By calling Ada.Text_IO.Flush.
> 
> Which means that Georg Bauhaus fell into the trap of my confusion. :-)

Sort of, but, as you say, the issue remains.
> Still valid question:
> 
>> 3. Why is buffered Ada.Text_IO as slow as non-buffered C's stdio? Who
>> is eating the 20x factor?


Text_IO is demonstrably slow. There are some speedy
shortcuts in the GNAT implementation of Put (e.g. Write_Buf).
But AFAICS there is (and has to be) a lot of protecting code
around the OS calls.

Using the following stupid programs for comparison,
and using strace, I get 3370 calls to write(2) from C,
but 50_000 from both C++ and Ada. Among other things open
to speculation (or open to inspection).  There are 4622
different lines in the 50_000 lines of output.

I think that if you have a formatted (constrained) string,
system I/O using fputs and flush might be a lot faster
(modulo threading issues).


#include <stdio.h>

int main()
{
    char s[68 + 1] =
"********************************************************************";

    for (int k = 0; k < 50000; ++k)
    {
        s[k % 68] = (char)(33 + k % 67);
        fputs(s, stdout), fputc('\n', stdout);
    }
    return 0;
}



#include <iostream>

int main()
{
    std::string s =
"********************************************************************";

    for (int k = 0; k < 50000; ++k)
    {
        s[k % 68] = static_cast<char>(33 + k % 67);
        std::cout << s << std::endl;
    }
    return 0;
}

with Ada.Text_IO;
procedure Ada_Wrt is
   S: String := (1 .. 68 => '*');
begin
   for K in 0 .. 50_000 - 1 loop
      S(1 + K rem 68) := Character'Val(33 + K rem 67);
      Ada.Text_IO.Put_Line(S);
   end loop;
end Ada_Wrt;

--
Georg Bauhaus
Y A Time Drain  http://www.9toX.de