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To: To: ;
Cc: Linus Torvalds <torvalds@transmeta.com>
Subject: Unexpected lseek(2) non-feature
Date: Mon, 29 Jun 1998 14:41:10 -0700
Message-ID: <4526.899156470@monkeys.com>
From: "Ronald F. Guilmette" <rfg@monkeys.com>
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Is there some sort of an obscure rule somewhere that says that you cannot
use lseek to pre-extend a file that you have just now opened/created via
a call to open(2) with the O_CREAT option?

The following trivial program fails in exactly the same way on both Linux
(2.0.34) and FreeBSD (2.2.6) although I will be damned if I can understand
why.

It appears that if I open/create the file, and then close it again and
then reopen it, that the lseek will work then, although why I should have
to go through all of that extra work before I am allowed to pre-extend the
file (using lseek) is beyond me.

-- rfg

P.S.  Looking at POSIX 1003.1b-1993, I don't see anything which mandates
this very odd behavior.  Quite the contrary, POSIX 1003.1b-1993 says ``The
lseek(2) function shall allow the file offset to be set beyond the end of
existing data in the file...''

============================================================================
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>

static char const *filename = "datafile.dat";

int
main (void)
{
  register int fd;

  unlink (filename);  /* Just in case.  */

  if ((fd = open (filename, O_RDWR | O_CREAT, 0600)) == -1)
    {
      fprintf (stderr, "Unable to create file `%s': %s\n",
               filename, strerror (errno));
      return 1; 
    }

  if (lseek (fd, SEEK_SET, 16384) == -1)
    {
      fprintf (stderr, "Unable to lseek file `%s': %s\n",
               filename, strerror (errno));
      return 1; 
    }

  return 0;
}

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