Re: Bug in pread/pwrite ?

[Christophe GRENIER <grenier at cgsecurity dot org>]

On Thu, 8 Mar 2007, Christopher Faylor wrote:

> The pread() that Cygwin does uses is in fhandler_disk_file.cc.

Thanks, now i know where the culprit is and understand why
I had a problem with my proposed patch ;-)

> All of that aside, I don't see how ignoring an lseek() failure
> could be considered to be a good thing.

I have done more research since, have a look to this glibc
pread implementation:

ssize_t
__libc_pread (int fd, void *buf, size_t nbyte, off_t offset)
{
  /* Since we must not change the file pointer preserve the value so that
     we can restore it later.  */
  int save_errno;
  ssize_t result;
  off_t old_offset = __libc_lseek (fd, 0, SEEK_CUR);
  if (old_offset == (off_t) -1)
    return -1;

  /* Set to wanted position.  */
  if (__libc_lseek (fd, offset, SEEK_SET) == (off_t) -1)
    return -1;

  /* Write out the data.  */
  result = __libc_read (fd, buf, nbyte);

  /* Now we have to restore the position.  If this fails we have to
     return this as an error.  But if the writing also failed we
     return this error.  */
  save_errno = errno;
  if (__libc_lseek (fd, old_offset, SEEK_SET) == (off_t) -1)
    {
      if (result == -1)
        __set_errno (save_errno);
      return -1;
    }
  __set_errno (save_errno);

  return result;
}

For the full file:
http://sources.redhat.com/cgi-bin/cvsweb.cgi/libc/sysdeps/posix/pread.c?rev=1.6&content-type=text/x-cvsweb-markup&cvsroot=glibc

glibc implementation seems correct in ignoring lseek
failure if read has been successfull.

Regards,
	Christophe

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