From: Martin Str|mberg <ams AT ludd DOT luth DOT se>
Message-Id: <200001161111.MAA17304@father.ludd.luth.se>
Subject: Re: _lleek
To: djgpp-workers AT delorie DOT com
Date: Sun, 16 Jan 100 12:11:32 +0100 (MET)
Cc: eliz AT is DOT elta DOT co DOT il (Eli Zaretskii)
In-Reply-To: <Pine.SUN.3.91.1000116104913.11666Y-100000@is> from Eli Zaretskii at "Jan 16, 0 10:49:39 am"
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According to Eli Zaretskii:
> 
> On Thu, 13 Jan 100, Martin Stromberg wrote:
> 
> > The problem is that the INT21 call registers used for offset (input
> > and output) is totally only 32 bits, so we have this mapping: 
> > register contents -> offset
> > 0			0
> > .			.
> > .			.
> > .			.
> > 2^32-2			2^32-2
> > 2^32-1			-1 (failure)
> > 
> > So my tests showed calling lseek(fd, -2, SEEK_SET) would result in the
> > file growing to maxsize (as if lseek(fd, 2^32-2 , SEEK_SET) was
> > called). Please try this and verify my findings, if possible.
> 
> Unfortunately, I don't have any access to a system with FAT32 volumes
> (FAT32 considerably slows down disk I/O, so I routinely refuse
> Windows's suggestion to convert).
> 
> So, if I understand correctly, you are suggesting to implement lseek
> of -2 as lseek of -3 followed by lseek of +1, is that right?

Eeeh... I don't understand what that would give us.

Consider this program:
#include <unistd.h>
#include <stdio.h>
#include <limits.h>
#include <fcntl.h>
#include <sys/stat.h>

int main(int argc, char *argv[])
{
  char ch;
  char new_ch;
  int fd;
  long long ret;

  if( argc != 2
   || sscanf(argv[1], "%c", &new_ch) != 1
      )
  {
    fprintf(stderr, "%s: run like this '%s <char_to_write>'.\n",
            argv[0], argv[0]
            );
    exit(1);
  }

  fd = open("g:/ggg.grr", O_RDWR|O_CREAT, S_IWUSR);
  if( 0 <= fd )
  {
    printf("fd = %d.\n", fd);

    lseek(fd, -3, SEEK_SET);
    ret = read(fd, &ch, 1);
    printf("1: ret = %lld, ch = %d.\n", ret, ch);

    ret = write(fd, &new_ch, 1);
    printf("1: ret = %lld.\n", ret);
  }

  return(0);
}

After running this, I'll have a file g:/ggg.grr that is 2^32-3 big!

Substitute -3 with any big value in [2^31, 2^32-3] and you'll get a
big file (I've tried a couple different ones).

If you use -2 instead of -3, the program behavs erratically. Every two
runs it will create a file of max size and the other runs it will
create a file of size 2!


Glass, Satyagraha,

							MartinS