Don't flush write buffer for ftell

[BZ #5298]
Use write pointer state along with the file offset and/or the read
pointers to get the current file position.

1 files changed, 16 insertions, 5 deletions

diff --git a/libio/fileops.c b/libio/fileops.c
index e22efdec13..173091cc6c 100644
--- a/libio/fileops.c
+++ b/libio/fileops.c
@@ -978,6 +978,9 @@ _IO_new_file_seekoff (fp, offset, dir, mode)
   int must_be_exact = (fp->_IO_read_base == fp->_IO_read_end
 		       && fp->_IO_write_base == fp->_IO_write_ptr);
 
+  bool was_writing = (fp->_IO_write_ptr > fp->_IO_write_base
+		      || _IO_in_put_mode (fp));
+
   if (mode == 0)
     dir = _IO_seek_cur, offset = 0; /* Don't move any pointers. */
 
@@ -988,10 +991,8 @@ _IO_new_file_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->_IO_write_ptr > fp->_IO_write_base || _IO_in_put_mode (fp))
-    if (_IO_switch_to_get_mode (fp))
-      return EOF;
+  else if (was_writing && _IO_switch_to_get_mode (fp))
+    return EOF;
 
   if (fp->_IO_buf_base == NULL)
     {
@@ -1010,7 +1011,17 @@ _IO_new_file_seekoff (fp, offset, dir, mode)
     {
     case _IO_seek_cur:
       /* Adjust for read-ahead (bytes is buffer). */
-      offset -= fp->_IO_read_end - fp->_IO_read_ptr;
+      if (mode != 0 || !was_writing)
+	offset -= fp->_IO_read_end - fp->_IO_read_ptr;
+      else
+	{
+	  /* _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 - fp->_IO_write_ptr;
+	}
+
       if (fp->_offset == _IO_pos_BAD)
 	{
 	  if (mode != 0)