if (offset < 0) { __set_errno (EINVAL); return EOF; }
_IO_new_file_seekoff (fp, offset, dir, mode)     _IO_FILE *fp;     _IO_off64_t offset;     int dir;     int mode;{  _IO_off64_t result;  _IO_off64_t delta, new_offset;  long count;  /* POSIX.1 8.2.3.7 says that after a call the fflush() the file     offset of the underlying file must be exact.  */  int must_be_exact = (fp->_IO_read_base == fp->_IO_read_end		       && fp->_IO_write_base == fp->_IO_write_ptr);  if (mode == 0)    dir = _IO_seek_cur, offset = 0; /* Don't move any pointers. */  /* Flush unwritten characters.     (This may do an unneeded write if we seek within the buffer.     But to be able to switch to reading, we would need to set     egptr to ptr.  That can't be done in the current design,     which assumes file_ptr() is eGptr.  Anyway, since we probably     end up flushing when we close(), it doesn't make much difference.)     FIXME: simulate mem-papped files. */  if (fp->_IO_write_ptr > fp->_IO_write_base || _IO_in_put_mode (fp))    if (INTUSE(_IO_switch_to_get_mode) (fp))      return EOF;  if (fp->_IO_buf_base == NULL)    {      /* It could be that we already have a pushback buffer.  */      if (fp->_IO_read_base != NULL)	{	  free (fp->_IO_read_base);	  fp->_flags &= ~_IO_IN_BACKUP;	}      INTUSE(_IO_doallocbuf) (fp);      _IO_setp (fp, fp->_IO_buf_base, fp->_IO_buf_base);      _IO_setg (fp, fp->_IO_buf_base, fp->_IO_buf_base, fp->_IO_buf_base);    }  switch (dir)    {    case _IO_seek_cur:      /* Adjust for read-ahead (bytes is buffer). */      offset -= fp->_IO_read_end - fp->_IO_read_ptr;      if (fp->_offset == _IO_pos_BAD)	goto dumb;      /* Make offset absolute, assuming current pointer is file_ptr(). */      offset += fp->_offset;      dir = _IO_seek_set;      break;    case _IO_seek_set:      break;    case _IO_seek_end:      {	struct _G_stat64 st;	if (_IO_SYSSTAT (fp, &st) == 0 && S_ISREG (st.st_mode))	  {	    offset += st.st_size;	    dir = _IO_seek_set;	  }	else	  goto dumb;      }    }  /* At this point, dir==_IO_seek_set. */  /* If we are only interested in the current position we've found it now.  */  if (mode == 0)    return offset;  /* If destination is within current buffer, optimize: */  if (fp->_offset != _IO_pos_BAD && fp->_IO_read_base != NULL      && !_IO_in_backup (fp))    {      /* Offset relative to start of main get area. */      _IO_off64_t rel_offset = (offset - fp->_offset				+ (fp->_IO_read_end - fp->_IO_read_base));      if (rel_offset >= 0)	{#if 0	  if (_IO_in_backup (fp))	    _IO_switch_to_main_get_area (fp);#endif	  if (rel_offset <= fp->_IO_read_end - fp->_IO_read_base)	    {	      _IO_setg (fp, fp->_IO_buf_base, fp->_IO_buf_base + rel_offset,			fp->_IO_read_end);	      _IO_setp (fp, fp->_IO_buf_base, fp->_IO_buf_base);	      {		_IO_mask_flags (fp, 0, _IO_EOF_SEEN);		goto resync;	      }	    }#ifdef TODO	    /* If we have streammarkers, seek forward by reading ahead. */	    if (_IO_have_markers (fp))	      {		int to_skip = rel_offset		  - (fp->_IO_read_ptr - fp->_IO_read_base);		if (ignore (to_skip) != to_skip)		  goto dumb;		_IO_mask_flags (fp, 0, _IO_EOF_SEEN);		goto resync;	      }#endif	}#ifdef TODO      if (rel_offset < 0 && rel_offset >= Bbase () - Bptr ())	{	  if (!_IO_in_backup (fp))	    _IO_switch_to_backup_area (fp);	  gbump (fp->_IO_read_end + rel_offset - fp->_IO_read_ptr);	  _IO_mask_flags (fp, 0, _IO_EOF_SEEN);	  goto resync;	}#endif    }#ifdef TODO  INTUSE(_IO_unsave_markers) (fp);#endif  if (fp->_flags & _IO_NO_READS)    goto dumb;  /* Try to seek to a block boundary, to improve kernel page management. */  new_offset = offset & ~(fp->_IO_buf_end - fp->_IO_buf_base - 1);  delta = offset - new_offset;  if (delta > fp->_IO_buf_end - fp->_IO_buf_base)    {      new_offset = offset;      delta = 0;    }  result = _IO_SYSSEEK (fp, new_offset, 0);  if (result < 0)    return EOF;  if (delta == 0)    count = 0;  else    {      count = _IO_SYSREAD (fp, fp->_IO_buf_base,			   (must_be_exact			    ? delta : fp->_IO_buf_end - fp->_IO_buf_base));      if (count < delta)	{	  /* We weren't allowed to read, but try to seek the remainder. */	  offset = count == EOF ? delta : delta-count;	  dir = _IO_seek_cur;	  goto dumb;	}    }  _IO_setg (fp, fp->_IO_buf_base, fp->_IO_buf_base + delta,	    fp->_IO_buf_base + count);  _IO_setp (fp, fp->_IO_buf_base, fp->_IO_buf_base);  fp->_offset = result + count;  _IO_mask_flags (fp, 0, _IO_EOF_SEEN);  return offset; dumb:  INTUSE(_IO_unsave_markers) (fp);  result = _IO_SYSSEEK (fp, offset, dir);  if (result != EOF)    {      _IO_mask_flags (fp, 0, _IO_EOF_SEEN);      fp->_offset = result;      _IO_setg (fp, fp->_IO_buf_base, fp->_IO_buf_base, fp->_IO_buf_base);      _IO_setp (fp, fp->_IO_buf_base, fp->_IO_buf_base);    }  return result;resync:  /* We need to do it since it is possible that the file offset in     the kernel may be changed behind our back. It may happen when     we fopen a file and then do a fork. One process may access the     the file and the kernel file offset will be changed. */  if (fp->_offset >= 0)    _IO_SYSSEEK (fp, fp->_offset, 0);  return offset;}