1 files changed, 96 insertions, 20 deletions

diff --git a/libio/wfileops.c b/libio/wfileops.c
index d3f46b2ed1..1087e8df8d 100644
--- a/libio/wfileops.c
+++ b/libio/wfileops.c
@@ -613,6 +613,10 @@ _IO_wfile_seekoff (fp, offset, dir, mode)
 		       && (fp->_wide_data->_IO_write_base
 			   == fp->_wide_data->_IO_write_ptr));
 
+  bool was_writing = ((fp->_wide_data->_IO_write_ptr
+		       > fp->_wide_data->_IO_write_base)
+		      || _IO_in_put_mode (fp));
+
   if (mode == 0)
     {
       /* XXX For wide stream with backup store it is not very
@@ -644,11 +648,8 @@ _IO_wfile_seekoff (fp, offset, dir, mode)
      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-mapped files. */
-
-  if (fp->_wide_data->_IO_write_ptr > fp->_wide_data->_IO_write_base
-      || _IO_in_put_mode (fp))
-    if (_IO_switch_to_wget_mode (fp))
-      return WEOF;
+  else if (was_writing && _IO_switch_to_wget_mode (fp))
+    return WEOF;
 
   if (fp->_wide_data->_IO_buf_base == NULL)
     {
@@ -679,29 +680,104 @@ _IO_wfile_seekoff (fp, offset, dir, mode)
       cv = fp->_codecvt;
       clen = (*cv->__codecvt_do_encoding) (cv);
 
-      if (clen > 0)
+      if (mode != 0 || !was_writing)
 	{
-	  offset -= (fp->_wide_data->_IO_read_end
-		     - fp->_wide_data->_IO_read_ptr) * clen;
-	  /* Adjust by readahead in external buffer.  */
-	  offset -= fp->_IO_read_end - fp->_IO_read_ptr;
+	  if (clen > 0)
+	    {
+	      offset -= (fp->_wide_data->_IO_read_end
+			 - fp->_wide_data->_IO_read_ptr) * clen;
+	      /* Adjust by readahead in external buffer.  */
+	      offset -= fp->_IO_read_end - fp->_IO_read_ptr;
+	    }
+	  else
+	    {
+	      int nread;
+
+	    flushed:
+	      delta = (fp->_wide_data->_IO_read_ptr
+		       - fp->_wide_data->_IO_read_base);
+	      fp->_wide_data->_IO_state = fp->_wide_data->_IO_last_state;
+	      nread = (*cv->__codecvt_do_length) (cv,
+						  &fp->_wide_data->_IO_state,
+						  fp->_IO_read_base,
+						  fp->_IO_read_end, delta);
+	      fp->_IO_read_ptr = fp->_IO_read_base + nread;
+	      fp->_wide_data->_IO_read_end = fp->_wide_data->_IO_read_ptr;
+	      offset -= fp->_IO_read_end - fp->_IO_read_base - nread;
+	    }
 	}
       else
 	{
-	  int nread;
+	  char *new_write_ptr = fp->_IO_write_ptr;
 
-	  delta = fp->_wide_data->_IO_read_ptr - fp->_wide_data->_IO_read_base;
-	  fp->_wide_data->_IO_state = fp->_wide_data->_IO_last_state;
-	  nread = (*cv->__codecvt_do_length) (cv, &fp->_wide_data->_IO_state,
-					      fp->_IO_read_base,
-					      fp->_IO_read_end, delta);
-	  fp->_IO_read_ptr = fp->_IO_read_base + nread;
-	  fp->_wide_data->_IO_read_end = fp->_wide_data->_IO_read_ptr;
-	  offset -= fp->_IO_read_end - fp->_IO_read_base - nread;
+	  if (clen > 0)
+	    offset += (fp->_wide_data->_IO_write_ptr
+		       - fp->_wide_data->_IO_write_base) / clen;
+	  else
+	    {
+	      enum __codecvt_result status;
+	      delta = (fp->_wide_data->_IO_write_ptr
+		       - fp->_wide_data->_IO_write_base);
+	      const wchar_t *write_base = fp->_wide_data->_IO_write_base;
+
+	      /* FIXME: This actually ends up in two iterations of conversion,
+		 one here and the next when the buffer actually gets flushed.
+		 It may be possible to optimize this in future so that
+		 wdo_write identifies already converted content and does not
+		 redo it.  In any case, this is much better than having to
+		 flush buffers for every ftell.  */
+	      do
+		{
+		  /* Ugh, no point trying to avoid the flush.  Just do it
+		     and go back to how it was with the read mode.  */
+		  if (delta > 0 && new_write_ptr == fp->_IO_buf_end)
+		    {
+		      if (_IO_switch_to_wget_mode (fp))
+			return WEOF;
+		      goto flushed;
+		    }
+
+		  const wchar_t *new_wbase = fp->_wide_data->_IO_write_base;
+		  fp->_wide_data->_IO_state = fp->_wide_data->_IO_last_state;
+		  status = (*cv->__codecvt_do_out) (cv,
+						    &fp->_wide_data->_IO_state,
+						    write_base,
+						    write_base + delta,
+						    &new_wbase,
+						    new_write_ptr,
+						    fp->_IO_buf_end,
+						    &new_write_ptr);
+
+		  delta -= new_wbase - write_base;
+
+		  /* If there was an error, then return WEOF.
+		     TODO: set buffer state.  */
+		  if (__builtin_expect (status == __codecvt_error, 0))
+		      return WEOF;
+		}
+	      while (delta > 0);
+	    }
+
+	  /* _IO_read_end coincides with fp._offset, so the actual file position
+	     is fp._offset - (_IO_read_end - new_write_ptr).  This is fine
+	     even if fp._offset is not set, since fp->_IO_read_end is then at
+	     _IO_buf_base and this adjustment is for unbuffered output.  */
+	  offset -= fp->_IO_read_end - new_write_ptr;
 	}
 
       if (fp->_offset == _IO_pos_BAD)
-	goto dumb;
+	{
+	  if (mode != 0)
+	    goto dumb;
+	  else
+	    {
+	      result = _IO_SYSSEEK (fp, 0, dir);
+	      if (result == EOF)
+		return result;
+	      fp->_offset = result;
+	    }
+	}
+
       /* Make offset absolute, assuming current pointer is file_ptr(). */
       offset += fp->_offset;