Index: Lib/test/test_fileio.py
===================================================================
--- Lib/test/test_fileio.py	(revision 77333)
+++ Lib/test/test_fileio.py	(working copy)
@@ -197,6 +197,17 @@
             f.close()
             self.fail("no error for invalid mode: %s" % bad_mode)
 
+    def testTruncate(self):
+        f = _fileio._FileIO(TESTFN, 'w')
+        f.write(bytes(bytearray(range(10))))
+        self.assertEqual(f.tell(), 10)
+        f.truncate(5)
+        self.assertEqual(f.tell(), 10)
+        self.assertEqual(f.seek(0, os.SEEK_END), 5)
+        f.truncate(15)
+        self.assertEqual(f.tell(), 5)
+        self.assertEqual(f.seek(0, os.SEEK_END), 15)
+        
     def testTruncateOnWindows(self):
         def bug801631():
             # SF bug <http://www.python.org/sf/801631>
Index: Modules/_fileio.c
===================================================================
--- Modules/_fileio.c	(revision 77333)
+++ Modules/_fileio.c	(working copy)
@@ -647,6 +647,7 @@
 fileio_truncate(PyFileIOObject *self, PyObject *args)
 {
 	PyObject *posobj = NULL;
+	PyObject *oldposobj = NULL;
 	Py_off_t pos;
 	int ret;
 	int fd;
@@ -666,25 +667,21 @@
                 if (posobj == NULL)
 			return NULL;
         }
-        else {
-		/* Move to the position to be truncated. */
-                posobj = portable_lseek(fd, posobj, 0);
-        }
 
-#if defined(HAVE_LARGEFILE_SUPPORT)
-	pos = PyLong_AsLongLong(posobj);
-#else
-	pos = PyLong_AsLong(posobj);
-#endif
-	if (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;
@@ -696,12 +693,26 @@
 				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) {