Bug in pread/pwrite ?

[Christophe GRENIER]

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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