Mailing-List: contact cygwin-help AT cygwin DOT com; run by ezmlm
List-Subscribe: <mailto:cygwin-subscribe AT cygwin DOT com>
List-Archive: <http://sources.redhat.com/ml/cygwin/>
List-Post: <mailto:cygwin AT cygwin DOT com>
List-Help: <mailto:cygwin-help AT cygwin DOT com>, <http://sources.redhat.com/ml/#faqs>
Sender: cygwin-owner AT cygwin DOT com
Mail-Followup-To: cygwin AT cygwin DOT com
Delivered-To: mailing list cygwin AT cygwin DOT com
Date: Thu, 5 Jun 2003 13:58:04 +0200
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: Sparse file criteria malfunction - binutils produces sparse .exe & .dll files
Message-ID: <20030605115804.GL875@cygbert.vinschen.de>
Reply-To: cygwin AT cygwin DOT com
Mail-Followup-To: cygwin AT cygwin DOT com
References: <004201c32902$3e92b020$78d96f83 AT pomello> <20030602133202 DOT GC30498 AT redhat DOT com> <bbl3cu$ph4$1 AT main DOT gmane DOT org>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <bbl3cu$ph4$1@main.gmane.org>
User-Agent: Mutt/1.4.1i

On Wed, Jun 04, 2003 at 05:33:46PM +0200, Markus Mauhart wrote:
> Now it has been proven that each cygwin installation manipulating 1000s of
> files only suffers from this "feature"; not to mention that it breaks
> winfile.exe :-(((.

The next Cygwin version will produce sparse files only if the application
decides to write 64K or more beyond EOF.

> If this (my) interpretation of SetFileValidData() is correct, wouldnt
> then SetFileValidData() do exactly what people can expect from
> 'traditional unix sparse files' (except XP beeing secure) ?

No, the function just tells the OS not to overwrite the hole with 0.
But that's a security problem, in contrast of not having allocated
blocks at all for that hole.  Cygwin has special code which does exactly
that, writing 0 into all blocks in the hole on 9x/Me.  Funny, but in
9x/Me this is a bug.  The SetFileValidData() function converts this
into a performance gain.

> Otherwise, has anyone of the cygwin-is-sparse proponents the information
> whether 'traditional unix sparse files' support unallocated holes inside
> files ? If yes, since when ?

I don't know since when but a simple application helps to show that Linux
supports sparseness "just so":

  #include <stdio.h>
  #include <sys/types.h>
  #include <sys/fcntl.h>
  #include <errno.h>
  #include <unistd.h>
  #include <sys/stat.h>

  int
  main(int argc, char **argv)
  {
    off_t seek = 184;
    int buf[4096];
    char *map;
    off_t len;
    struct stat st;

    if (argc > 1 && atol (argv[1]) > 0)
      seek = atol (argv[1]);
    printf ("Creating file of size %luK\n", seek + 8);

    int fd = open ("sparse.test", O_WRONLY | O_CREAT | O_TRUNC, 0644);
    if (fd < 0)
      {
	printf ("open failed: %d \"%s\"\n", errno, strerror(errno));
	return 1;
      }
    // Write first block
    memset (buf, 1, 4096);
    write (fd, buf, 4096);

    // Seek
    lseek (fd, seek * 1024, SEEK_CUR);

    // Write second block
    memset (buf, 2, 4096);
    write (fd, buf, 4096);

    // Print size values
    if (!fstat (fd, &st))
      {
	printf ("st_size  : %10lu\n", st.st_size);
	printf ("st_blocks: %10lu\n", st.st_blocks);
      }

    close (fd);
    return 0;
  }

$ gcc -o sp sp.c
$ ./sp 200
Creating file of size 208K
st_size  :     212992
st_blocks:         24
$ ./sp 2000
Creating file of size 2008K
st_size  :    2056192
st_blocks:         24
$ ls -sl sparse.test 
12 -rw-r--r--    1 corinna  users     2056192 Jun  5 13:54 sparse.test

> If no, how is the security of reusing other
> peoples not-overwritten clusters handled ?

Security isn't affected.  Unused blocks are never written or allocated.
As soon as a block is coming into use, it's overwritten, that's it.

Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Developer                                mailto:cygwin AT cygwin DOT com
Red Hat, Inc.

--
Unsubscribe info:      http://cygwin.com/ml/#unsubscribe-simple
Problem reports:       http://cygwin.com/problems.html
Documentation:         http://cygwin.com/docs.html
FAQ:                   http://cygwin.com/faq/