Re: getline buffering

Ismo Salonen <nobody@another.invalid>
Tue, 20 Feb 2007 06:10:07 GMT
toton wrote:

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

"Jacek Dziedzic" <> wrote in message


toton wrote:

On Feb 19, 5:44 pm, "Erik Wikstr?m" <>

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

  I am reading some large text files and parsing it. typical file size
I am using is 3 MB. It takes around 20 sec just to use std::getline (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 opens 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 large
buffer, as it changes the file position. While I need the visible 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 read
the whole file at once so it's faster.
Each ifstream has a buffer associated with it, you can get a pointer
to it with the rdbuf()-method and you can specify an array to use as
buffer with the pubsetbuf()-method. See the following link for a short
Erik Wikstr?m

  I had checked it in a separate console project (multi threaded ) it
is running perfectly, and reads within .8 sec. However the same code
takes 12 sec when running inside my Qt app.
I fear Qt lib is interacting with c++ runtime is some way to cause the
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.


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