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Date: Fri, 20 Jan 2006 15:13:00 +0100
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: New platform independent problem
Message-ID: <20060120141300.GA8318@calimero.vinschen.de>
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On Jan 20 06:59, Eric Blake wrote:
> According to Corinna Vinschen on 1/20/2006 5:25 AM:
> > I'm also having a problem right now building rcp and scp due to the
> > missing d_ino.  OTOH, the d_ino member is not required by POSIX, but
> > only in X/Open compliant OSes, see
> > 
> >   http://www.opengroup.org/onlinepubs/009695399/basedefs/dirent.h.html
> >   http://www.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap02.html#tag_02_01_04
> > 
> > So, portable applications shouldn't rely on d_ino.
> 
> Is there a compromise here?  Many applications that look at d_ino also
> check whether the d_ino member is 0, in which case they fall back on
> st_ino.  For example, coreutils's pwd.c contains:
> 
> #define NOT_AN_INODE_NUMBER 0
> #ifdef D_INO_IN_DIRENT
> # define D_INO(dp)((dp)->d_ino)
> #else
> /* Some systems don't have inodes, so fake them to avoid lots of ifdefs.  */
> # define D_INO(dp) NOT_AN_INODE_NUMBER
> #endif
> 
> Then later on, it can blindly do:
> 
> ino_t ino = D_INO(dp);
> if (ino == NOT_AN_INODE_NUMBER)
>   {
>     if (lstat (dp->d_name, &ent_sb) < 0)
>       continue; /* Skip any entry we can't stat.  */
>     ino = ent_sb.st_ino;
>   }

Well, other applications like rcp and scp just use d_ino to check for 0
and to ignore it entirely then, like this:

  while (readdir)
    {
      if (d_ino == 0)
        continue;
      [...action...]
    }

This means, if d_ino is 0, it breaks scp and rcp and some other
applications.  If it's always some arbitrary non-zero value, it breaks
pwd from coreutils and some other applications.  If it's available and
doesn't match st_ino, it breaks another set of applications.  If it's
not available at all, it breaks all applications asking for d_ino in any
way. 

> Cygwin would then set it to 0 on WinNT and 2k (where it is
> prohibitively expensive to determine a real value), set it to the hash on
> Win9x and ME (since st_ino is also the hash on those platforms), and set
> it to the actual st_ino value on WinXP and beyond (since the API exists).

That would be a better solution, though it would still not be ok to set
it to 0 on NT4 and 2K.  Maybe we should think about another way to
generate a hash value and use it on all platforms.  I'm just at a loss
right now, to come up with another hash generation method, which would
allow to get the same inode number for hardlinks :-(


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat

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