diff -r 89873f3b18fd Include/sysmodule.h
--- a/Include/sysmodule.h	Wed Nov 06 20:25:50 2013 -0800
+++ b/Include/sysmodule.h	Thu Nov 07 19:28:33 2013 +0200
@@ -31,6 +31,10 @@
 PyAPI_FUNC(void) PySys_AddXOption(const wchar_t *);
 PyAPI_FUNC(PyObject *) PySys_GetXOptions(void);
 
+#ifndef Py_LIMITED_API
+PyAPI_DATA(size_t) _PySys_GetSizeOf(PyObject *);
+#endif
+
 #ifdef __cplusplus
 }
 #endif
diff -r 89873f3b18fd Lib/test/test_memoryio.py
--- a/Lib/test/test_memoryio.py	Wed Nov 06 20:25:50 2013 -0800
+++ b/Lib/test/test_memoryio.py	Thu Nov 07 19:28:33 2013 +0200
@@ -8,6 +8,7 @@
 
 import io
 import _pyio as pyio
+import sys
 import pickle
 
 class MemorySeekTestMixin:
@@ -658,12 +659,11 @@
 
     @support.cpython_only
     def test_sizeof(self):
-        basesize = support.calcobjsize('P2nN2Pn')
+        basesize = support.calcobjsize('P2n2Pn')
         check = self.check_sizeof
         self.assertEqual(object.__sizeof__(io.BytesIO()), basesize)
-        check(io.BytesIO(), basesize )
-        check(io.BytesIO(b'a'), basesize + 1 + 1 )
-        check(io.BytesIO(b'a' * 1000), basesize + 1000 + 1 )
+        check(io.BytesIO(), basesize)
+        check(io.BytesIO(b'a' * 1000), basesize + sys.getsizeof(b'a' * 1000))
 
 class CStringIOTest(PyStringIOTest):
     ioclass = io.StringIO
diff -r 89873f3b18fd Modules/_io/bytesio.c
--- a/Modules/_io/bytesio.c	Wed Nov 06 20:25:50 2013 -0800
+++ b/Modules/_io/bytesio.c	Thu Nov 07 19:28:33 2013 +0200
@@ -4,10 +4,9 @@
 
 typedef struct {
     PyObject_HEAD
-    char *buf;
+    PyObject *buf;
     Py_ssize_t pos;
     Py_ssize_t string_size;
-    size_t buf_size;
     PyObject *dict;
     PyObject *weakreflist;
     Py_ssize_t exports;
@@ -18,6 +17,12 @@
     bytesio *source;
 } bytesiobuf;
 
+/* The bytesio object can be in three states:
+  * Py_REFCNT(buf) == 1, exports == 0.
+  * Py_REFCNT(buf) > 1.  exports == 0, string_size == PyBytes_GET_SIZE(buf),
+    first modification or export causes the internal buffer copying.
+  * exports > 0.  Py_REFCNT(buf) == 1, any modifications are forbidden.
+*/
 
 #define CHECK_CLOSED(self)                                  \
     if ((self)->buf == NULL) {                              \
@@ -33,39 +38,58 @@
         return NULL; \
     }
 
+#define SHARED_BUF(self) (Py_REFCNT(self->buf) > 1)
+
 
 /* Internal routine to get a line from the buffer of a BytesIO
    object. Returns the length between the current position to the
    next newline character. */
 static Py_ssize_t
-get_line(bytesio *self, char **output)
+scan_eol(bytesio *self, Py_ssize_t len)
 {
-    char *n;
-    const char *str_end;
-    Py_ssize_t len;
+    const char *start, *n;
+    Py_ssize_t maxlen;
 
     assert(self->buf != NULL);
 
     /* Move to the end of the line, up to the end of the string, s. */
-    str_end = self->buf + self->string_size;
-    for (n = self->buf + self->pos;
-         n < str_end && *n != '\n';
-         n++);
+    start = PyBytes_AS_STRING(self->buf) + self->pos;
+    maxlen = self->string_size - self->pos;
+    if (len < 0 || len > maxlen)
+        len = maxlen;
 
-    /* Skip the newline character */
-    if (n < str_end)
-        n++;
-
-    /* Get the length from the current position to the end of the line. */
-    len = n - (self->buf + self->pos);
-    *output = self->buf + self->pos;
-
+    if (len) {
+        n = memchr(start, '\n', len);
+        if (n)
+            /* Get the length from the current position to the end of
+               the line. */
+            len = n - start + 1;
+    }
     assert(len >= 0);
     assert(self->pos < PY_SSIZE_T_MAX - len);
-    self->pos += len;
 
     return len;
 }
+/* Internal routine for detaching the shared buffer of BytesIO objects.
+   The caller should ensure that the 'size' argument is non-negative and
+   not lesser than self->string_size.  Returns 0 on success, -1 otherwise. */
+static int
+unshare_buffer(bytesio *self, size_t size)
+{
+    PyObject *new_buf, *old_buf;
+    assert(SHARED_BUF(self));
+    assert(self->exports == 0);
+    assert(size >= self->string_size);
+    new_buf = PyBytes_FromStringAndSize(NULL, size);
+    if (new_buf == NULL)
+        return -1;
+    memcpy(PyBytes_AS_STRING(new_buf), PyBytes_AS_STRING(self->buf),
+           self->string_size);
+    old_buf = self->buf;
+    self->buf = new_buf;
+    Py_DECREF(old_buf);
+    return 0;
+}
 
 /* Internal routine for changing the size of the buffer of BytesIO objects.
    The caller should ensure that the 'size' argument is non-negative.  Returns
@@ -75,8 +99,7 @@
 {
     /* Here, unsigned types are used to avoid dealing with signed integer
        overflow, which is undefined in C. */
-    size_t alloc = self->buf_size;
-    char *new_buf = NULL;
+    size_t alloc = PyBytes_GET_SIZE(self->buf);
 
     assert(self->buf != NULL);
 
@@ -87,30 +110,32 @@
 
     if (size < alloc / 2) {
         /* Major downsize; resize down to exact size. */
-        alloc = size + 1;
+        alloc = size;
     }
     else if (size < alloc) {
         /* Within allocated size; quick exit */
         return 0;
     }
-    else if (size <= alloc * 1.125) {
+    else if (size <= alloc + (alloc / 2)) {
         /* Moderate upsize; overallocate similar to list_resize() */
-        alloc = size + (size >> 3) + (size < 9 ? 3 : 6);
+        alloc = size + (size / 2) + (size < 9 ? 3 : 6);
     }
     else {
         /* Major upsize; resize up to exact size */
-        alloc = size + 1;
+        alloc = size;
     }
 
     if (alloc > ((size_t)-1) / sizeof(char))
         goto overflow;
-    new_buf = (char *)PyMem_Realloc(self->buf, alloc * sizeof(char));
-    if (new_buf == NULL) {
-        PyErr_NoMemory();
-        return -1;
+
+    if (SHARED_BUF(self)) {
+        if (unshare_buffer(self, alloc) < 0)
+            return -1;
     }
-    self->buf_size = alloc;
-    self->buf = new_buf;
+    else {
+        if (_PyBytes_Resize(&self->buf, alloc) < 0)
+            return -1;
+    }
 
     return 0;
 
@@ -129,10 +154,14 @@
     assert(self->pos >= 0);
     assert(len >= 0);
 
-    if ((size_t)self->pos + len > self->buf_size) {
+    if ((size_t)self->pos + len > PyBytes_GET_SIZE(self->buf)) {
         if (resize_buffer(self, (size_t)self->pos + len) < 0)
             return -1;
     }
+    else if (SHARED_BUF(self)) {
+        if (unshare_buffer(self, self->string_size) < 0)
+            return -1;
+    }
 
     if (self->pos > self->string_size) {
         /* In case of overseek, pad with null bytes the buffer region between
@@ -143,13 +172,13 @@
           |   |            <--to pad-->|<---to write--->    |
           0   buf                   position
         */
-        memset(self->buf + self->string_size, '\0',
+        memset(PyBytes_AS_STRING(self->buf) + self->string_size, '\0',
                (self->pos - self->string_size) * sizeof(char));
     }
 
     /* Copy the data to the internal buffer, overwriting some of the existing
        data if self->pos < self->string_size. */
-    memcpy(self->buf + self->pos, bytes, len);
+    memcpy(PyBytes_AS_STRING(self->buf) + self->pos, bytes, len);
     self->pos += len;
 
     /* Set the new length of the internal string if it has changed. */
@@ -231,7 +260,22 @@
 bytesio_getvalue(bytesio *self)
 {
     CHECK_CLOSED(self);
-    return PyBytes_FromStringAndSize(self->buf, self->string_size);
+    if (self->string_size <= 1 || self->exports > 0)
+        return PyBytes_FromStringAndSize(PyBytes_AS_STRING(self->buf),
+                                         self->string_size);
+
+    if (self->string_size != PyBytes_GET_SIZE(self->buf)) {
+        if (SHARED_BUF(self)) {
+            if (unshare_buffer(self, self->string_size) < 0)
+                return NULL;
+        }
+        else {
+            if (_PyBytes_Resize(&self->buf, self->string_size) < 0)
+                return NULL;
+        }
+    }
+    Py_INCREF(self->buf);
+    return self->buf;
 }
 
 PyDoc_STRVAR(isatty_doc,
@@ -299,7 +343,15 @@
     }
 
     assert(self->buf != NULL);
-    output = self->buf + self->pos;
+    if (size > 1 &&
+        self->pos == 0 && size == PyBytes_GET_SIZE(self->buf) &&
+        self->exports == 0) {
+        self->pos += size;
+        Py_INCREF(self->buf);
+        return self->buf;
+    }
+
+    output = PyBytes_AS_STRING(self->buf) + self->pos;
     self->pos += size;
 
     return PyBytes_FromStringAndSize(output, size);
@@ -359,14 +411,18 @@
         return NULL;
     }
 
-    n = get_line(self, &output);
+    n = scan_eol(self, size);
 
-    if (size >= 0 && size < n) {
-        size = n - size;
-        n -= size;
-        self->pos -= size;
+    if (n > 1 &&
+        self->pos == n && n == PyBytes_GET_SIZE(self->buf) &&
+        self->exports == 0) {
+        self->pos += n;
+        Py_INCREF(self->buf);
+        return self->buf;
     }
 
+    output = PyBytes_AS_STRING(self->buf) + self->pos;
+    self->pos += n;
     return PyBytes_FromStringAndSize(output, n);
 }
 
@@ -410,7 +466,9 @@
     if (!result)
         return NULL;
 
-    while ((n = get_line(self, &output)) != 0) {
+    output = PyBytes_AS_STRING(self->buf) + self->pos;
+    while ((n = scan_eol(self, -1)) != 0) {
+        self->pos += n;
         line = PyBytes_FromStringAndSize(output, n);
         if (!line)
             goto on_error;
@@ -422,6 +480,7 @@
         size += n;
         if (maxsize > 0 && size >= maxsize)
             break;
+        output += n;
     }
     return result;
 
@@ -455,7 +514,7 @@
             len = 0;
     }
 
-    memcpy(raw_buffer, self->buf + self->pos, len);
+    memcpy(raw_buffer, PyBytes_AS_STRING(self->buf) + self->pos, len);
     assert(self->pos + len < PY_SSIZE_T_MAX);
     assert(len >= 0);
     self->pos += len;
@@ -514,16 +573,26 @@
 static PyObject *
 bytesio_iternext(bytesio *self)
 {
-    char *next;
+    const char *next;
     Py_ssize_t n;
 
     CHECK_CLOSED(self);
 
-    n = get_line(self, &next);
+    n = scan_eol(self, -1);
 
-    if (!next || n == 0)
+    if (n == 0)
         return NULL;
 
+    if (n > 1 &&
+        self->pos == 0 && n == PyBytes_GET_SIZE(self->buf) &&
+        self->exports == 0) {
+        self->pos += n;
+        Py_INCREF(self->buf);
+        return self->buf;
+    }
+
+    next = PyBytes_AS_STRING(self->buf) + self->pos;
+    self->pos += n;
     return PyBytes_FromStringAndSize(next, n);
 }
 
@@ -655,10 +724,7 @@
 static PyObject *
 bytesio_close(bytesio *self)
 {
-    if (self->buf != NULL) {
-        PyMem_Free(self->buf);
-        self->buf = NULL;
-    }
+    Py_CLEAR(self->buf);
     Py_RETURN_NONE;
 }
 
@@ -788,10 +854,7 @@
                         "deallocated BytesIO object has exported buffers");
         PyErr_Print();
     }
-    if (self->buf != NULL) {
-        PyMem_Free(self->buf);
-        self->buf = NULL;
-    }
+    Py_CLEAR(self->buf);
     Py_CLEAR(self->dict);
     if (self->weakreflist != NULL)
         PyObject_ClearWeakRefs((PyObject *) self);
@@ -811,7 +874,7 @@
     /* tp_alloc initializes all the fields to zero. So we don't have to
        initialize them here. */
 
-    self->buf = (char *)PyMem_Malloc(0);
+    self->buf = PyBytes_FromStringAndSize(NULL, 0);
     if (self->buf == NULL) {
         Py_DECREF(self);
         return PyErr_NoMemory();
@@ -834,13 +897,26 @@
     self->string_size = 0;
     self->pos = 0;
 
+    if (self->exports > 0) {
+        PyErr_SetString(PyExc_BufferError,
+                        "Existing exports of data: object cannot be re-sized");
+        return -1;
+    }
     if (initvalue && initvalue != Py_None) {
-        PyObject *res;
-        res = bytesio_write(self, initvalue);
-        if (res == NULL)
-            return -1;
-        Py_DECREF(res);
-        self->pos = 0;
+        if (PyBytes_CheckExact(initvalue)) {
+            Py_INCREF(initvalue);
+            Py_XDECREF(self->buf);
+            self->buf = initvalue;
+            self->string_size = PyBytes_GET_SIZE(initvalue);
+        }
+        else {
+            PyObject *res;
+            res = bytesio_write(self, initvalue);
+            if (res == NULL)
+                return -1;
+            Py_DECREF(res);
+            self->pos = 0;
+        }
     }
 
     return 0;
@@ -852,8 +928,8 @@
     Py_ssize_t res;
 
     res = sizeof(bytesio);
-    if (self->buf)
-        res += self->buf_size;
+    if (self->buf && !SHARED_BUF(self))
+        res += _PySys_GetSizeOf(self->buf);
     return PyLong_FromSsize_t(res);
 }
 
@@ -963,11 +1039,16 @@
 {
     int ret;
     bytesio *b = (bytesio *) obj->source;
+    if (SHARED_BUF(b)) {
+        if (unshare_buffer(b, b->string_size) < 0)
+            return -1;
+    }
     if (view == NULL) {
         b->exports++;
         return 0;
     }
-    ret = PyBuffer_FillInfo(view, (PyObject*)obj, b->buf, b->string_size,
+    ret = PyBuffer_FillInfo(view, (PyObject*)obj,
+                            PyBytes_AS_STRING(b->buf), b->string_size,
                             0, flags);
     if (ret >= 0) {
         b->exports++;
diff -r 89873f3b18fd Python/sysmodule.c
--- a/Python/sysmodule.c	Wed Nov 06 20:25:50 2013 -0800
+++ b/Python/sysmodule.c	Thu Nov 07 19:28:33 2013 +0200
@@ -846,30 +846,17 @@
 }
 #endif /* USE_MALLOPT */
 
-static PyObject *
-sys_getsizeof(PyObject *self, PyObject *args, PyObject *kwds)
+size_t
+_PySys_GetSizeOf(PyObject *o)
 {
     PyObject *res = NULL;
-    static PyObject *gc_head_size = NULL;
-    static char *kwlist[] = {"object", "default", 0};
-    PyObject *o, *dflt = NULL;
     PyObject *method;
+    size_t size;
     _Py_IDENTIFIER(__sizeof__);
 
-    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;
-    }
-
     /* 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, &PyId___sizeof__);
     if (method == NULL) {
@@ -883,24 +870,45 @@
         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,