Re: getline buffering

"=?iso-8859-1?q?Erik_Wikstr=F6m?=" <>
19 Feb 2007 23:24:04 -0800
On Feb 20, 7:45 am, "toton" <> wrote:

On Feb 20, 11:10 am, Ismo Salonen <nob...@another.invalid> wrote:

toton wrote:

On Feb 19, 8:49 pm, "P.J. Plauger" <> wrote:

"Jacek Dziedzic" <> wrote in mes=



toton wrote:

On Feb 19, 5:44 pm, "Erik Wikstr=F6m" <>

On Feb 19, 12:44 pm, "toton" <> wrote:

  I am reading some large text files and parsing it. typical fil=

e size

I am using is 3 MB. It takes around 20 sec just to use std::getl=

ine (I

need to treat newlines properly ) for whole file in debug , and =

8 sec

while optimization on.
 It is for Visual Studio 7.1 and its std library. While vim open=

s it

in a fraction of sec.
 So, is it that getline is reading the file line by line, instead
reading a chunk at a time in its internal buffer? is there any
function to set how much to read from the stream internally ?
  I am not very comfortable with read and readsome , to load a l=


buffer, as it changes the file position. While I need the visibl=

e file

position to be the position I am actually, while "internally" it
should read some more , may be like 1MB chunk ... ?

I'm not sure, but I think it's the other way around, Vim does not=


the whole file at once so it's faster.
Each ifstream has a buffer associated with it, you can get a poin=


to it with the rdbuf()-method and you can specify an array to use=


buffer with the pubsetbuf()-method. See the following link for a =




Erik Wikstr=F6m

  I had checked it in a separate console project (multi threaded )=


is running perfectly, and reads within .8 sec. However the same co=


takes 12 sec when running inside my Qt app.
I fear Qt lib is interacting with c++ runtime is some way to cause=


problem ....
May be I need to build the Qt lib a fresh to check what is wrong.
Thanks for answering the question ....

  Make sure you decouple stream I/O from stdio, i.e. do

Normally good advice, but unnecessary with VC++.

P.J. Plauger
Dinkumware, Ltd.

I got the problem. It has nothing to do with Qt or other
libraries ....
I was using a tellg() to get the current position. Now my question is
why tellg is such costly ? Won't it just return the current strem
position ?
To explain,
  boost::progress_timer t;
  std::ifstream in("Y:/Data/workspaces/tob4f/tob4f.dat");
  std::string line;
           ///int pos = in.tellg();
This code takes 0.58 sec in my computer while if I uncomment the line
in.tellg(), it takes 120.8 sec !

Could it be that you have opened the file in text mode and the tellg()
seeks to beginning always and rereads characters (counting cr+lf pairs
as one ). Try switching to binary mode and handle cr+lf yourself.


The whole purpose of using getline is that only. I am not sure why
tellg have to behave like that in text mode , it is stored one !
Tested the same with gcc .The program in mingw is not giving any big
here is the program
#include <fstream>
#include <iostream>
#include <ctime>
int main(){
                //boost::progress_timer t;
                time_t start,end;
                std::ifstream in("Y:/Data/workspaces/tob4f/tob4f.dat");
                std::string line;
                        int pos = in.tellg();

With & without comment on the line , it takes 2 sec & 3 sec
respectively (without -o2 flag ) It looks fine to me ...
 Even the visual studio std code looks quite simple one ....
anyone else has tested it with a big file (4-8 MB )and found a huge
difference ?

On a 22.5MB file I get one second running time without tellg, 4
seconds if the file is opened in text mode and 2 seconds if opened in
binary mode. Seems quite reasonable to me.

Erik Wikstr=F6m

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