Re: getline buffering
On Feb 19, 8:49 pm, "P.J. Plauger" <p...@dinkumware.com> wrote:
"Jacek Dziedzic" <jacek.dziedzic.n.o.s.p....@gmail.com> wrote in message
On Feb 19, 5:44 pm, "Erik Wikstr?m" <eri...@student.chalmers.se>
On Feb 19, 12:44 pm, "toton" <abirba...@gmail.com> 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
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
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++.
I got the problem. It has nothing to do with Qt or other
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
///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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