Sender: Jorrit DOT Tyberghein AT uz DOT kuleuven DOT ac DOT be
Message-Id: <31B29BA0.2F06@uz.kuleuven.ac.be>
Date: Mon, 03 Jun 1996 10:00:32 +0200
From: Jorrit Tyberghein <Jorrit DOT Tyberghein AT uz DOT kuleuven DOT ac DOT be>
Organization: Universitaire Ziekenhuizen Leuven
Mime-Version: 1.0
To: DJGPP AT delorie DOT com
Subject: fseek slow in DJGPP 2.0?
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Hi,

Recently I upgraded from DJGPP 2.0beta3 to the non-beta version
of DJGPP. When I recompiled my program with the new DJGPP it
worked perfectly except for one thing. Some parts of the program
appeared to be unbearably slow.

I traced the problem down to the 'fseek' function. There are several
places in my program where I scan a file while I only need several
scattered bytes from the file. For example: the 'x' represent bytes
that I need to read, the dots represents bytes that I want to skip:

File: xxxx.......xxx.........xxxxxxxx.......xx.......xxxxx.............

These files are rather large (about 700K) and there are about 10000
skips to be made.

The code to read this file is something like this:

   while (busy)
   {
     fread (&struct, size, 1, file);
     fseek (file, struct.offset, SEEK_CUR);
   }

The beta3 version of DJGPP apparantly had no problem with this code.
The 'fseek' function in that version of DJGPP seemed to be efficient.
But it appears that the non-beta version of DJGPP clears the buffer
everytime an fseek happens.

If I rewrite the code to something like this (dummy is a buffer pointing
to some allocated memory):

   while (busy)
   {
     fread (&struct, size, 1, file);
     fread (dummy, struct.offset, 1, file);
   }

I almost achieve the same speed as the before. But this is not elegant
and only works if the file is read sequentially. This is not always
the case. Sometimes I need to access the file randomly. This is now
much slower with the new DJGPP 2.0.

What's happening here? Why is fseek rewritten in such a way? Can I
get the old behaviour back?

My system:
   Pentium with 16M
   DOS 7.0 (Window 95)
   LFN=n (in DJGPP.ENV)
   DJGPP 2.0 (latest version, only downloaded one week ago)
   GCC 2.7.2 included with DJGPP.

Greetings and many thanks in advance,

-- 
==============================================================================
Jorrit DOT Tyberghein AT uz DOT kuleuven DOT ac DOT be, University Hospitals KU Leuven BELGIUM

People who used magic without knowing what they were doing usually came to
a sticky end. All over the entire room, sometimes.
        -- (Terry Pratchett, Moving Pictures)
==============================================================================