diff -r eccd94d4ee77 Include/sysmodule.h
--- a/Include/sysmodule.h	Thu Sep 20 20:51:14 2012 +0100
+++ b/Include/sysmodule.h	Fri Sep 21 09:15:43 2012 +0300
@@ -22,6 +22,7 @@
 
 #ifndef Py_LIMITED_API
 PyAPI_DATA(PyObject *) _PySys_TraceFunc, *_PySys_ProfileFunc;
+PyAPI_DATA(size_t) _PySys_GetSizeOf(PyObject *);
 #endif
 
 PyAPI_FUNC(void) PySys_ResetWarnOptions(void);
diff -r eccd94d4ee77 Lib/test/test_memoryio.py
--- a/Lib/test/test_memoryio.py	Thu Sep 20 20:51:14 2012 +0100
+++ b/Lib/test/test_memoryio.py	Fri Sep 21 09:15:43 2012 +0300
@@ -9,6 +9,8 @@
 import io
 import _pyio as pyio
 import pickle
+import sys
+import os
 
 class MemorySeekTestMixin:
 
@@ -711,6 +713,25 @@
         memio.close()
         self.assertRaises(ValueError, memio.__setstate__, ("closed", "", 0, None))
 
+    check_sizeof = support.check_sizeof
+
+    @support.cpython_only
+    def test_sizeof(self):
+        basesize = support.calcobjsize('P2PP4c5P')
+        usize = len('\0'.encode('unicode-internal'))
+        stdsize = basesize + 2 * usize + sys.getsizeof(os.linesep)
+        check = self.check_sizeof
+        self.assertEqual(object.__sizeof__(io.StringIO()), basesize)
+        check(io.StringIO(), stdsize)
+        # newline
+        for newline in '', '\n', '\r', '\r\n':
+            check(io.StringIO(newline=newline),
+                basesize + 2 * usize + sys.getsizeof(newline))
+        # UCS4 buffer
+        for c in 'a', '\x80', '\u0100', '\U00010000':
+            check(io.StringIO(c * 1000),
+                  stdsize + len((c * 1000).encode('unicode-internal')))
+
 
 class CStringIOPickleTest(PyStringIOPickleTest):
     UnsupportedOperation = io.UnsupportedOperation
diff -r eccd94d4ee77 Modules/_io/stringio.c
--- a/Modules/_io/stringio.c	Thu Sep 20 20:51:14 2012 +0100
+++ b/Modules/_io/stringio.c	Fri Sep 21 09:15:43 2012 +0300
@@ -500,6 +500,7 @@
 static int
 stringio_clear(stringio *self)
 {
+    Py_CLEAR(self->decoder);
     Py_CLEAR(self->dict);
     return 0;
 }
@@ -649,6 +650,22 @@
     return 0;
 }
 
+static PyObject *
+stringio_sizeof(stringio *self, void *unused)
+{
+    size_t res = sizeof(stringio);
+
+    if (self->buf)
+        res += self->buf_size * sizeof(Py_UNICODE);
+    if (self->readnl) {
+        size_t sz = _PySys_GetSizeOf(self->readnl);
+        if (sz == (size_t)-1 && PyErr_Occurred())
+            return NULL;
+        res += sz;
+    }
+    return PyLong_FromSize_t(res);
+}
+
 /* Properties and pseudo-properties */
 
 PyDoc_STRVAR(stringio_readable_doc,
@@ -854,6 +871,7 @@
 
     {"__getstate__", (PyCFunction)stringio_getstate, METH_NOARGS},
     {"__setstate__", (PyCFunction)stringio_setstate, METH_O},
+    {"__sizeof__", (PyCFunction)stringio_sizeof, METH_NOARGS},
     {NULL, NULL}        /* sentinel */
 };
 
diff -r eccd94d4ee77 Python/sysmodule.c
--- a/Python/sysmodule.c	Thu Sep 20 20:51:14 2012 +0100
+++ b/Python/sysmodule.c	Fri Sep 21 09:15:43 2012 +0300
@@ -807,29 +807,17 @@
 }
 #endif /* USE_MALLOPT */
 
-static PyObject *
-sys_getsizeof(PyObject *self, PyObject *args, PyObject *kwds)
+size_t
+_PySys_GetSizeOf(PyObject *o)
 {
     PyObject *res = NULL;
-    static PyObject *str__sizeof__ = NULL, *gc_head_size = NULL;
-    static char *kwlist[] = {"object", "default", 0};
-    PyObject *o, *dflt = NULL;
     PyObject *method;
-
-    if (!PyArg_ParseTupleAndKeywords(args, kwds, "O|O:getsizeof",
-                                     kwlist, &o, &dflt))
-        return NULL;
-
-    /* Initialize static variable for GC head size */
-    if (gc_head_size == NULL) {
-        gc_head_size = PyLong_FromSsize_t(sizeof(PyGC_Head));
-        if (gc_head_size == NULL)
-            return NULL;
-    }
+    size_t size;
+    static PyObject *str__sizeof__ = NULL;
 
     /* Make sure the type is initialized. float gets initialized late */
     if (PyType_Ready(Py_TYPE(o)) < 0)
-        return NULL;
+        return (size_t)-1;
 
     method = _PyObject_LookupSpecial(o, "__sizeof__",
                                      &str__sizeof__);
@@ -844,24 +832,45 @@
         Py_DECREF(method);
     }
 
-    /* Has a default value been given */
-    if ((res == NULL) && (dflt != NULL) &&
-        PyErr_ExceptionMatches(PyExc_TypeError))
-    {
-        PyErr_Clear();
-        Py_INCREF(dflt);
-        return dflt;
-    }
-    else if (res == NULL)
-        return res;
+    if (res == NULL)
+        return (size_t)-1;
+
+    size = PyLong_AsSize_t(res);
+    Py_DECREF(res);
+    if (size == (size_t)-1 && PyErr_Occurred())
+        return (size_t)-1;
 
     /* add gc_head size */
-    if (PyObject_IS_GC(o)) {
-        PyObject *tmp = res;
-        res = PyNumber_Add(tmp, gc_head_size);
-        Py_DECREF(tmp);
+    if (PyObject_IS_GC(o))
+        size += sizeof(PyGC_Head);
+    return size;
+}
+
+static PyObject *
+sys_getsizeof(PyObject *self, PyObject *args, PyObject *kwds)
+{
+    static char *kwlist[] = {"object", "default", 0};
+    size_t size;
+    PyObject *o, *dflt = NULL;
+
+    if (!PyArg_ParseTupleAndKeywords(args, kwds, "O|O:getsizeof",
+                                     kwlist, &o, &dflt))
+        return NULL;
+
+    size = _PySys_GetSizeOf(o);
+
+    if (size == (size_t)-1 && PyErr_Occurred()) {
+        /* Has a default value been given */
+        if (dflt != NULL && PyErr_ExceptionMatches(PyExc_TypeError)) {
+            PyErr_Clear();
+            Py_INCREF(dflt);
+            return dflt;
+        }
+        else
+            return NULL;
     }
-    return res;
+
+    return PyLong_FromSize_t(size);
 }
 
 PyDoc_STRVAR(getsizeof_doc,