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Date: Wed, 10 Jul 2002 19:04:59 +0200
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: [BUG]  open(): Opening with flags O_RDONLY | O_APPEND positions the file pointer at the end of the file
Message-ID: <20020710190459.L24137@cygbert.vinschen.de>
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References: <823876622 DOT 20020710153943 AT syntrex DOT com> <20020710163613 DOT GD10966 AT redhat DOT com> <20020710184830 DOT J24137 AT cygbert DOT vinschen DOT de> <20020710165014 DOT GB11381 AT redhat DOT com>
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On Wed, Jul 10, 2002 at 12:50:14PM -0400, Chris Faylor wrote:
> On Wed, Jul 10, 2002 at 06:48:30PM +0200, Corinna Vinschen wrote:
> >On Wed, Jul 10, 2002 at 12:36:13PM -0400, Chris Faylor wrote:
> >> On Wed, Jul 10, 2002 at 03:39:43PM +0200, Pavel Tsekov wrote:
> >> >Hello, there :)
> >> >
> >> >Attached is a testcase which displays the problem. On Linux it will
> >> >properly return 13, while on cygwin it returns 0.
> >> >
> >> >I found this while trying to understand why MC doesn't extract
> >> >properly files from say .zip files.
> >> >
> >> >I don't know exactly why they call O_RDONLY combined with O_APPEND but
> >> >I will mail the mc-devel list ASAP.
> >> >
> >> >Still according to the Linux man page and SUSv2, O_APPEND should be
> >> >taken into account only when writing to the file.
> >> >
> >> >Having in mind that fhandler_base::write() calls SetFilePointer
> >> >before each write, I wonder why fhandler_disk_base::open calls
> >> >SetFilePointer when it detects O_APPEND ?
> >> 
> >> Good question.
> >
> >What if
> >
> > 	fd = open(O_APPEND);
> >	pos = lseek(fd, 0, SEEK_CUR);
> >
> >?  If open() doesn't move the pointer, `pos' is incorrectly set to 0.
> 
> Actually, I'd argue that, in that case, lseek should have some O_APPEND
> logic of its own.

Just checked the following on Linux:

    #include <stdio.h>
    #include <fcntl.h>

    int
    main()
    {
      int fd;
      off_t pos;

      fd = open ("x.c", O_APPEND);
      if (fd >= 0)
	{
	  pos = lseek (fd, 0, SEEK_CUR);
	  printf ("pos1: %ld\n", pos);
	  pos = lseek (fd, 0, SEEK_END);
	  printf ("pos2: %ld\n", pos);
	  pos = lseek (fd, 0, SEEK_SET);
	  printf ("pos3: %ld\n", pos);
	  close (fd);
	}
      return 0;
    }

Output:

pos1: 0
pos2: 382
pos3: 0

So open() doesn't set the pointer.  And lseek() doesn't take O_APPEND
into account either.


Corinna

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

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