Index: Lib/_pyio.py
===================================================================
--- Lib/_pyio.py	(revision 77317)
+++ Lib/_pyio.py	(working copy)
@@ -867,7 +867,7 @@
         elif pos < 0:
             raise ValueError("negative truncate position %r" % (pos,))
         del self._buffer[pos:]
-        return self.seek(pos)
+        return pos # we do NOT move the file pointer
 
     def readable(self):
         return True
@@ -1226,8 +1226,8 @@
         if pos is None:
             pos = self.tell()
         # Use seek to flush the read buffer.
-        self.seek(pos)
-        return BufferedWriter.truncate(self)
+        self.seek(0, os.SEEK_CUR)
+        return BufferedWriter.truncate(self, pos)
 
     def read(self, n=None):
         if n is None:
Index: Lib/test/test_io.py
===================================================================
--- Lib/test/test_io.py	(revision 77317)
+++ Lib/test/test_io.py	(working copy)
@@ -233,7 +233,13 @@
         support.unlink(support.TESTFN)
 
     def write_ops(self, f):
+    
         self.assertEqual(f.write(b"blah."), 5)
+        f.truncate(0)
+        self.assertEqual(f.tell(), 5)
+        f.seek(0)
+      
+        self.assertEqual(f.write(b"blah."), 5)
         self.assertEqual(f.seek(0), 0)
         self.assertEqual(f.write(b"Hello."), 6)
         self.assertEqual(f.tell(), 6)
@@ -244,8 +250,9 @@
         self.assertEqual(f.write(b"h"), 1)
         self.assertEqual(f.seek(-1, 2), 13)
         self.assertEqual(f.tell(), 13)
+
         self.assertEqual(f.truncate(12), 12)
-        self.assertEqual(f.tell(), 12)
+        self.assertEqual(f.tell(), 13)
         self.assertRaises(TypeError, f.seek, 0.0)
 
     def read_ops(self, f, buffered=False):
@@ -988,7 +995,7 @@
             bufio = self.tp(raw, 8)
             bufio.write(b"abcdef")
             self.assertEqual(bufio.truncate(3), 3)
-            self.assertEqual(bufio.tell(), 3)
+            self.assertEqual(bufio.tell(), 6)
         with self.open(support.TESTFN, "rb", buffering=0) as f:
             self.assertEqual(f.read(), b"abc")
 
Index: Modules/_io/bytesio.c
===================================================================
--- Modules/_io/bytesio.c	(revision 77317)
+++ Modules/_io/bytesio.c	(working copy)
@@ -453,7 +453,6 @@
         if (resize_buffer(self, size) < 0)
             return NULL;
     }
-    self->pos = size;
 
     return PyLong_FromSsize_t(size);
 }
Index: Modules/_io/fileio.c
===================================================================
--- Modules/_io/fileio.c	(revision 77317)
+++ Modules/_io/fileio.c	(working copy)
@@ -759,6 +759,7 @@
 fileio_truncate(fileio *self, PyObject *args)
 {
 	PyObject *posobj = NULL;
+	PyObject *oldposobj = NULL;
 	Py_off_t pos;
 	int ret;
 	int fd;
@@ -778,27 +779,21 @@
                 if (posobj == NULL)
 			return NULL;
         }
-        else {
-		/* Move to the position to be truncated. */
-                posobj = portable_lseek(fd, posobj, 0);
-        }
-	if (posobj == NULL)
-		return NULL;
 
-#if defined(HAVE_LARGEFILE_SUPPORT)
-	pos = PyLong_AsLongLong(posobj);
-#else
-	pos = PyLong_AsLong(posobj);
-#endif
-	if (pos == -1 && PyErr_Occurred())
-		return NULL;
+#ifdef MS_WINDOWS
 
-#ifdef MS_WINDOWS
 	/* MS _chsize doesn't work if newsize doesn't fit in 32 bits,
 	   so don't even try using it. */
 	{
 		HANDLE hFile;
 
+		oldposobj = portable_lseek(fd, NULL, 1); // we save the file pointer position
+		if (oldposobj == NULL)
+			return NULL;
+		posobj = portable_lseek(fd, posobj, 0); // we must move to the truncation position
+		if (posobj == NULL)
+			return NULL;
+
 		/* Truncate.  Note that this may grow the file! */
 		Py_BEGIN_ALLOW_THREADS
 		errno = 0;
@@ -810,12 +805,26 @@
 				errno = EACCES;
 		}
 		Py_END_ALLOW_THREADS
+
+		oldposobj = portable_lseek(fd, oldposobj, 0); // we restore the file pointer position
+		if (oldposobj == NULL)
+			return NULL;
 	}
 #else
+
+#if defined(HAVE_LARGEFILE_SUPPORT)
+	pos = PyLong_AsLongLong(posobj);
+#else
+	pos = PyLong_AsLong(posobj);
+#endif
+	if (PyErr_Occurred())
+		return NULL;
+
 	Py_BEGIN_ALLOW_THREADS
 	errno = 0;
 	ret = ftruncate(fd, pos);
 	Py_END_ALLOW_THREADS
+
 #endif /* !MS_WINDOWS */
 
 	if (ret != 0) {