diff --git a/Lib/io.py b/Lib/io.py
index 3172554..3a1546a 100644
--- a/Lib/io.py
+++ b/Lib/io.py
@@ -490,6 +490,7 @@
         terminator(s) recognized.
         """
         # For backwards compatibility, a (slowish) readline().
+        self._checkClosed()
         if hasattr(self, "peek"):
             def nreadahead():
                 readahead = self.peek(1)
@@ -531,7 +532,7 @@
         lines will be read if the total size (in bytes/characters) of all
         lines so far exceeds hint.
         """
-        if hint is None:
+        if hint is None or hint <= 0:
             return list(self)
         n = 0
         lines = []
@@ -726,6 +727,8 @@
 
         if pos is None:
             pos = self.tell()
+        # XXX: Should seek() be used, instead of passing the position
+        # XXX  directly to truncate?
         return self.raw.truncate(pos)
 
     ### Flush and close ###
@@ -765,7 +768,7 @@
         return self.raw.isatty()
 
 
-class BytesIO(BufferedIOBase):
+class _BytesIO(BufferedIOBase):
 
     """Buffered I/O implementation using an in-memory bytes buffer."""
 
@@ -779,13 +782,19 @@
     def getvalue(self):
         """Return the bytes value (contents) of the buffer
         """
+        if self.closed:
+            raise ValueError("getvalue on closed file")
         return bytes(self._buffer)
 
     def read(self, n=None):
+        if self.closed:
+            raise ValueError("read from closed file")
         if n is None:
             n = -1
         if n < 0:
             n = len(self._buffer)
+        if len(self._buffer) <= self._pos:
+            return self._buffer[:0]
         newpos = min(len(self._buffer), self._pos + n)
         b = self._buffer[self._pos : newpos]
         self._pos = newpos
@@ -802,6 +811,8 @@
         if isinstance(b, str):
             raise TypeError("can't write str to binary stream")
         n = len(b)
+        if n == 0:
+            return 0
         newpos = self._pos + n
         if newpos > len(self._buffer):
             # Inserts null bytes between the current end of the file
@@ -813,28 +824,38 @@
         return n
 
     def seek(self, pos, whence=0):
+        if self.closed:
+            raise ValueError("seek on closed file")
         try:
             pos = pos.__index__()
         except AttributeError as err:
             raise TypeError("an integer is required") from err
         if whence == 0:
             self._pos = max(0, pos)
+            if pos < 0:
+                raise ValueError("negative seek position %r" % (pos,))
         elif whence == 1:
             self._pos = max(0, self._pos + pos)
         elif whence == 2:
             self._pos = max(0, len(self._buffer) + pos)
         else:
-            raise IOError("invalid whence value")
+            raise ValueError("invalid whence value")
         return self._pos
 
     def tell(self):
+        if self.closed:
+            raise ValueError("tell on closed file")
         return self._pos
 
     def truncate(self, pos=None):
+        if self.closed:
+            raise ValueError("truncate on closed file")
         if pos is None:
             pos = self._pos
+        elif pos < 0:
+            raise ValueError("negative truncate position %r" % (pos,))
         del self._buffer[pos:]
-        return pos
+        return self.seek(pos)
 
     def readable(self):
         return True
@@ -845,6 +866,16 @@
     def seekable(self):
         return True
 
+# Use the faster implementation of BytesIO if available
+try:
+    import _bytesio
+
+    class BytesIO(_bytesio._BytesIO, BufferedIOBase):
+        __doc__ = _bytesio._BytesIO.__doc__
+
+except ImportError:
+    BytesIO = _BytesIO
+
 
 class BufferedReader(_BufferedIOMixin):
 
@@ -978,6 +1009,12 @@
                     raise BlockingIOError(e.errno, e.strerror, overage)
         return written
 
+    def truncate(self, pos=None):
+        self.flush()
+        if pos is None:
+            pos = self.raw.tell()
+        return self.raw.truncate(pos)
+
     def flush(self):
         if self.closed:
             raise ValueError("flush of closed file")
@@ -1097,6 +1134,13 @@
         else:
             return self.raw.tell() - len(self._read_buf)
 
+    def truncate(self, pos=None):
+        if pos is None:
+            pos = self.tell()
+        # Use seek to flush the read buffer.
+        self.seek(pos)
+        return BufferedWriter.truncate(self)
+
     def read(self, n=None):
         if n is None:
             n = -1
@@ -1145,11 +1189,7 @@
 
     def truncate(self, pos: int = None) -> int:
         """Truncate size to pos."""
-        self.flush()
-        if pos is None:
-            pos = self.tell()
-        self.seek(pos)
-        return self.buffer.truncate()
+        self._unsupported("truncate")
 
     def readline(self) -> str:
         """Read until newline or EOF.
@@ -1346,6 +1386,12 @@
     def seekable(self):
         return self._seekable
 
+    def readable(self):
+        return self.buffer.readable()
+
+    def writable(self):
+        return self.buffer.writable()
+
     def flush(self):
         self.buffer.flush()
         self._telling = self._seekable
@@ -1539,7 +1585,16 @@
         finally:
             decoder.setstate(saved_state)
 
+    def truncate(self, pos=None):
+        self.flush()
+        if pos is None:
+            pos = self.tell()
+        self.seek(pos)
+        return self.buffer.truncate()
+
     def seek(self, cookie, whence=0):
+        if self.closed:
+            raise ValueError("tell on closed file")
         if not self._seekable:
             raise IOError("underlying stream is not seekable")
         if whence == 1: # seek relative to current position
@@ -1626,6 +1681,8 @@
         return line
 
     def readline(self, limit=None):
+        if self.closed:
+            raise ValueError("read from closed file")
         if limit is None:
             limit = -1
 
diff --git a/Lib/test/test_StringIO.py b/Lib/test/test_StringIO.py
deleted file mode 100644
index 4f3ce83..0000000
--- a/Lib/test/test_StringIO.py
+++ /dev/null
@@ -1,127 +0,0 @@
-# Tests StringIO and cStringIO
-
-import sys
-import unittest
-import io
-from test import test_support
-
-
-class TestGenericStringIO:
-    # use a class variable CLASS to define which class is being tested
-    CLASS = None
-
-    # Line of data to test as string
-    _line = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ!'
-
-    # Constructor to use for the test data (._line is passed to this
-    # constructor)
-    constructor = str
-
-    def setUp(self):
-        self._line = self.constructor(self._line)
-        self._lines = self.constructor((self._line + '\n') * 5)
-        self._fp = self.CLASS(self._lines)
-
-    def test_reads(self):
-        eq = self.assertEqual
-        self.assertRaises(TypeError, self._fp.seek)
-        eq(self._fp.read(10), self._line[:10])
-        eq(self._fp.readline(), self._line[10:] + '\n')
-        eq(len(self._fp.readlines(60)), 2)
-
-    def test_writes(self):
-        f = self.CLASS()
-        self.assertRaises(TypeError, f.seek)
-        f.write(self._line[:6])
-        f.seek(3)
-        f.write(self._line[20:26])
-        f.write(self._line[52])
-        self.assertEqual(f.getvalue(), 'abcuvwxyz!')
-
-    def test_writelines(self):
-        f = self.CLASS()
-        f.writelines([self._line[0], self._line[1], self._line[2]])
-        f.seek(0)
-        self.assertEqual(f.getvalue(), 'abc')
-
-    def test_writelines_error(self):
-        def errorGen():
-            yield 'a'
-            raise KeyboardInterrupt()
-        f = self.CLASS()
-        self.assertRaises(KeyboardInterrupt, f.writelines, errorGen())
-
-    def test_truncate(self):
-        eq = self.assertEqual
-        f = self.CLASS()
-        f.write(self._lines)
-        f.seek(10)
-        f.truncate()
-        eq(f.getvalue(), 'abcdefghij')
-        f.truncate(5)
-        eq(f.getvalue(), 'abcde')
-        f.write('xyz')
-        eq(f.getvalue(), 'abcdexyz')
-        self.assertRaises(ValueError, f.truncate, -1)
-        f.close()
-        self.assertRaises(ValueError, f.write, 'frobnitz')
-
-    def test_closed_flag(self):
-        f = self.CLASS()
-        self.assertEqual(f.closed, False)
-        f.close()
-        self.assertEqual(f.closed, True)
-        f = self.CLASS(self.constructor("abc"))
-        self.assertEqual(f.closed, False)
-        f.close()
-        self.assertEqual(f.closed, True)
-
-    def test_isatty(self):
-        f = self.CLASS()
-        self.assertRaises(TypeError, f.isatty, None)
-        self.assertEqual(f.isatty(), False)
-        f.close()
-        self.assertRaises(ValueError, f.isatty)
-
-    def test_iterator(self):
-        eq = self.assertEqual
-        unless = self.failUnless
-        eq(iter(self._fp), self._fp)
-        # Does this object support the iteration protocol?
-        unless(hasattr(self._fp, '__iter__'))
-        unless(hasattr(self._fp, '__next__'))
-        i = 0
-        for line in self._fp:
-            eq(line, self._line + '\n')
-            i += 1
-        eq(i, 5)
-        self._fp.close()
-        self.assertRaises(StopIteration, next, self._fp)
-
-class TestioStringIO(TestGenericStringIO, unittest.TestCase):
-    CLASS = io.StringIO
-
-    def test_unicode(self):
-
-        if not test_support.have_unicode: return
-
-        # The StringIO module also supports concatenating Unicode
-        # snippets to larger Unicode strings. This is tested by this
-        # method. Note that cStringIO does not support this extension.
-
-        f = self.CLASS()
-        f.write(self._line[:6])
-        f.seek(3)
-        f.write(str(self._line[20:26]))
-        f.write(str(self._line[52]))
-        s = f.getvalue()
-        self.assertEqual(s, str('abcuvwxyz!'))
-        self.assertEqual(type(s), str)
-
-
-def test_main():
-    test_support.run_unittest(TestioStringIO)
-
-
-if __name__ == '__main__':
-    test_main()
diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index ba3ceeb..fa3a8bb 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -98,7 +98,7 @@
         self.assertEqual(f.seek(-1, 2), 13)
         self.assertEqual(f.tell(), 13)
         self.assertEqual(f.truncate(12), 12)
-        self.assertEqual(f.tell(), 13)
+        self.assertEqual(f.tell(), 12)
         self.assertRaises(TypeError, f.seek, 0.0)
 
     def read_ops(self, f, buffered=False):
@@ -143,7 +143,7 @@
         self.assertEqual(f.tell(), self.LARGE + 2)
         self.assertEqual(f.seek(0, 2), self.LARGE + 2)
         self.assertEqual(f.truncate(self.LARGE + 1), self.LARGE + 1)
-        self.assertEqual(f.tell(), self.LARGE + 2)
+        self.assertEqual(f.tell(), self.LARGE + 1)
         self.assertEqual(f.seek(0, 2), self.LARGE + 1)
         self.assertEqual(f.seek(-1, 2), self.LARGE)
         self.assertEqual(f.read(2), b"x")
@@ -727,6 +727,7 @@
             txt.write("BB\nCCC\n")
             txt.write("X\rY\r\nZ")
             txt.flush()
+            self.assertEquals(buf.closed, False)
             self.assertEquals(buf.getvalue(), expected)
 
     def testNewlines(self):
@@ -807,7 +808,8 @@
                 txt = io.TextIOWrapper(buf, encoding="ascii", newline=newline)
                 txt.write(data)
                 txt.close()
-                self.assertEquals(buf.getvalue(), expected)
+                self.assertEquals(buf.closed, True)
+                self.assertRaises(ValueError, buf.getvalue)
         finally:
             os.linesep = save_linesep
 
diff --git a/Lib/test/test_largefile.py b/Lib/test/test_largefile.py
index 4cafe33..0856346 100644
--- a/Lib/test/test_largefile.py
+++ b/Lib/test/test_largefile.py
@@ -120,14 +120,15 @@
             newsize -= 1
             f.seek(42)
             f.truncate(newsize)
-            self.assertEqual(f.tell(), 42)       # else pointer moved
-            f.seek(0, 2)
             self.assertEqual(f.tell(), newsize)  # else wasn't truncated
+            f.seek(0, 2)
+            self.assertEqual(f.tell(), newsize)
             # XXX truncate(larger than true size) is ill-defined
             # across platform; cut it waaaaay back
             f.seek(0)
             f.truncate(1)
-            self.assertEqual(f.tell(), 0)       # else pointer moved
+            self.assertEqual(f.tell(), 1)       # else pointer moved
+            f.seek(0)
             self.assertEqual(len(f.read()), 1)  # else wasn't truncated
 
 def test_main():
diff --git a/Lib/test/test_memoryio.py b/Lib/test/test_memoryio.py
new file mode 100644
index 0000000..0b09ca4
--- /dev/null
+++ b/Lib/test/test_memoryio.py
@@ -0,0 +1,405 @@
+"""Unit tests for memory-based file-like objects.
+StringIO -- for unicode strings
+BytesIO -- for bytes
+"""
+
+import unittest
+from test import test_support
+
+import io
+
+try:
+    import _bytesio
+    has_c_implementation = True
+except ImportError:
+    has_c_implementation = False
+
+
+class MemoryTestMixin:
+
+    def write_ops(self, f, t):
+        self.assertEqual(f.write(t("blah.")), 5)
+        self.assertEqual(f.seek(0), 0)
+        self.assertEqual(f.write(t("Hello.")), 6)
+        self.assertEqual(f.tell(), 6)
+        self.assertEqual(f.seek(5), 5)
+        self.assertEqual(f.tell(), 5)
+        self.assertEqual(f.write(t(" world\n\n\n")), 9)
+        self.assertEqual(f.seek(0), 0)
+        self.assertEqual(f.write(t("h")), 1)
+        self.assertEqual(f.truncate(12), 12)
+        self.assertEqual(f.tell(), 12)
+
+    def test_write(self):
+        buf = self.buftype("hello world\n")
+        memio = self.ioclass(buf)
+
+        self.write_ops(memio, self.buftype)
+        self.assertEqual(memio.getvalue(), buf)
+        memio = self.ioclass()
+        self.write_ops(memio, self.buftype)
+        self.assertEqual(memio.getvalue(), buf)
+        self.assertRaises(TypeError, memio.write, None)
+        memio.close()
+        self.assertRaises(ValueError, memio.write, self.buftype(""))
+
+    def test_writelines(self):
+        buf = self.buftype("1234567890")
+        memio = self.ioclass()
+
+        self.assertEqual(memio.writelines([buf] * 100), None)
+        self.assertEqual(memio.getvalue(), buf * 100)
+        memio.writelines([])
+        self.assertEqual(memio.getvalue(), buf * 100)
+        memio = self.ioclass()
+        self.assertRaises(TypeError, memio.writelines, [buf] + [1])
+        self.assertEqual(memio.getvalue(), buf)
+        self.assertRaises(TypeError, memio.writelines, None)
+        memio.close()
+        self.assertRaises(ValueError, memio.writelines, [])
+
+    def test_writelines_error(self):
+        memio = self.ioclass()
+        def error_gen():
+            yield self.buftype('spam')
+            raise KeyboardInterrupt
+
+        self.assertRaises(KeyboardInterrupt, memio.writelines, error_gen())
+
+    def test_truncate(self):
+        buf = self.buftype("1234567890")
+        memio = self.ioclass(buf)
+
+        self.assertRaises(ValueError, memio.truncate, -1)
+        memio.seek(6)
+        self.assertEqual(memio.truncate(), 6)
+        self.assertEqual(memio.getvalue(), buf[:6])
+        self.assertEqual(memio.truncate(4), 4)
+        self.assertEqual(memio.getvalue(), buf[:4])
+        self.assertEqual(memio.tell(), 4)
+        memio.write(buf)
+        self.assertEqual(memio.getvalue(), buf[:4] + buf)
+        pos = memio.tell()
+        self.assertEqual(memio.truncate(None), pos)
+        self.assertEqual(memio.tell(), pos)
+        self.assertRaises(TypeError, memio.truncate, '0')
+        memio.close()
+        self.assertRaises(ValueError, memio.truncate, 0)
+
+    def test_init(self):
+        buf = self.buftype("1234567890")
+        memio = self.ioclass(buf)
+        self.assertEqual(memio.getvalue(), buf)
+        memio = self.ioclass(None)
+        self.assertEqual(memio.getvalue(), self.EOF)
+        memio.__init__(buf * 2)
+        self.assertEqual(memio.getvalue(), buf * 2)
+        memio.__init__(buf)
+        self.assertEqual(memio.getvalue(), buf)
+
+    def test_read(self):
+        buf = self.buftype("1234567890")
+        memio = self.ioclass(buf)
+
+        self.assertEqual(memio.read(0), self.EOF)
+        self.assertEqual(memio.read(1), buf[:1])
+        self.assertEqual(memio.read(4), buf[1:5])
+        self.assertEqual(memio.read(900), buf[5:])
+        self.assertEqual(memio.read(), self.EOF)
+        memio.seek(0)
+        self.assertEqual(memio.read(), buf)
+        self.assertEqual(memio.read(), self.EOF)
+        self.assertEqual(memio.tell(), 10)
+        memio.seek(0)
+        self.assertEqual(memio.read(-1), buf)
+        memio.seek(0)
+        self.assertEqual(memio.read(None), buf)
+        self.assertRaises(TypeError, memio.read, '')
+        memio.close()
+        self.assertRaises(ValueError, memio.read)
+
+    def test_readline(self):
+        buf = self.buftype("1234567890\n")
+        memio = self.ioclass(buf * 2)
+
+        self.assertEqual(memio.readline(0), self.EOF)
+        self.assertEqual(memio.readline(), buf)
+        self.assertEqual(memio.readline(), buf)
+        self.assertEqual(memio.readline(), self.EOF)
+        memio.seek(0)
+        self.assertEqual(memio.readline(5), buf[:5])
+        self.assertEqual(memio.readline(5), buf[5:10])
+        self.assertEqual(memio.readline(5), buf[10:15])
+        memio.seek(0)
+        self.assertEqual(memio.readline(-1), buf)
+        memio.seek(0)
+        self.assertEqual(memio.readline(0), self.EOF)
+
+        buf = self.buftype("1234567890\n")
+        memio = self.ioclass((buf * 3)[:-1])
+        self.assertEqual(memio.readline(), buf)
+        self.assertEqual(memio.readline(), buf)
+        self.assertEqual(memio.readline(), buf[:-1])
+        self.assertEqual(memio.readline(), self.EOF)
+        memio.seek(0)
+        self.assertEqual(memio.readline(None), buf)
+        self.assertRaises(TypeError, memio.readline, '')
+        memio.close()
+        self.assertRaises(ValueError,  memio.readline)
+
+    def test_readlines(self):
+        buf = self.buftype("1234567890\n")
+        memio = self.ioclass(buf * 10)
+
+        self.assertEqual(memio.readlines(), [buf] * 10)
+        memio.seek(5)
+        self.assertEqual(memio.readlines(), [buf[5:]] + [buf] * 9)
+        memio.seek(0)
+        self.assertEqual(memio.readlines(15), [buf] * 2)
+        memio.seek(0)
+        self.assertEqual(memio.readlines(-1), [buf] * 10)
+        memio.seek(0)
+        self.assertEqual(memio.readlines(0), [buf] * 10)
+        memio.seek(0)
+        self.assertEqual(memio.readlines(None), [buf] * 10)
+        self.assertRaises(TypeError, memio.readlines, '')
+        memio.close()
+        self.assertRaises(ValueError, memio.readlines)
+
+    def test_iterator(self):
+        buf = self.buftype("1234567890\n")
+        memio = self.ioclass(buf * 10)
+
+        self.assertEqual(iter(memio), memio)
+        self.failUnless(hasattr(memio, '__iter__'))
+        self.failUnless(hasattr(memio, '__next__'))
+        i = 0
+        for line in memio:
+            self.assertEqual(line, buf)
+            i += 1
+        self.assertEqual(i, 10)
+        memio.seek(0)
+        i = 0
+        for line in memio:
+            self.assertEqual(line, buf)
+            i += 1
+        self.assertEqual(i, 10)
+        memio = self.ioclass(buf * 2)
+        memio.close()
+        self.assertRaises(ValueError, memio.__next__)
+
+    def test_getvalue(self):
+        buf = self.buftype("1234567890")
+        memio = self.ioclass(buf)
+
+        self.assertEqual(memio.getvalue(), buf)
+        memio.read()
+        self.assertEqual(memio.getvalue(), buf)
+        memio = self.ioclass(buf * 1000)
+        self.assertEqual(memio.getvalue()[-3:], self.buftype("890"))
+        memio = self.ioclass(buf)
+        memio.close()
+        self.assertRaises(ValueError, memio.getvalue)
+
+    def test_seek(self):
+        buf = self.buftype("1234567890")
+        memio = self.ioclass(buf)
+
+        memio.read(5)
+        self.assertRaises(ValueError, memio.seek, -1)
+        self.assertRaises(ValueError, memio.seek, 1, -1)
+        self.assertRaises(ValueError, memio.seek, 1, 3)
+        self.assertEqual(memio.seek(0), 0)
+        self.assertEqual(memio.seek(0, 0), 0)
+        self.assertEqual(memio.read(), buf)
+        self.assertEqual(memio.seek(3), 3)
+        self.assertEqual(memio.seek(0, 1), 3)
+        self.assertEqual(memio.read(), buf[3:])
+        self.assertEqual(memio.seek(len(buf)), len(buf))
+        self.assertEqual(memio.read(), self.EOF)
+        memio.seek(len(buf) + 1)
+        self.assertEqual(memio.read(), self.EOF)
+        self.assertEqual(memio.seek(0, 2), len(buf))
+        self.assertEqual(memio.read(), self.EOF)
+        memio.close()
+        self.assertRaises(ValueError, memio.seek, 0)
+
+    def test_overseek(self):
+        buf = self.buftype("1234567890")
+        memio = self.ioclass(buf)
+
+        self.assertEqual(memio.seek(len(buf) + 1), 11)
+        self.assertEqual(memio.read(), self.EOF)
+        self.assertEqual(memio.tell(), 11)
+        self.assertEqual(memio.getvalue(), buf)
+        memio.write(self.EOF)
+        self.assertEqual(memio.getvalue(), buf)
+        memio.write(buf)
+        self.assertEqual(memio.getvalue(), buf + self.buftype('\0') + buf)
+
+    def test_tell(self):
+        buf = self.buftype("1234567890")
+        memio = self.ioclass(buf)
+
+        self.assertEqual(memio.tell(), 0)
+        memio.seek(5)
+        self.assertEqual(memio.tell(), 5)
+        memio.seek(10000)
+        self.assertEqual(memio.tell(), 10000)
+        memio.close()
+        self.assertRaises(ValueError, memio.tell)
+
+    def test_flush(self):
+        buf = self.buftype("1234567890")
+        memio = self.ioclass(buf)
+
+        self.assertEqual(memio.flush(), None)
+
+    def test_flags(self):
+        memio = self.ioclass()
+
+        self.assertEqual(memio.writable(), True)
+        self.assertEqual(memio.readable(), True)
+        self.assertEqual(memio.seekable(), True)
+        self.assertEqual(memio.isatty(), False)
+        self.assertEqual(memio.closed, False)
+        memio.close()
+        self.assertEqual(memio.writable(), True)
+        self.assertEqual(memio.readable(), True)
+        self.assertEqual(memio.seekable(), True)
+        self.assertRaises(ValueError, memio.isatty)
+        self.assertEqual(memio.closed, True)
+
+    def test_subclassing(self):
+        buf = self.buftype("1234567890")
+        def test1():
+            class MemIO(self.ioclass):
+                pass
+            m = MemIO(buf)
+            return m.getvalue()
+        def test2():
+            class MemIO(self.ioclass):
+                def __init__(me, a, b):
+                    self.ioclass.__init__(me, a)
+            m = MemIO(buf, None)
+            return m.getvalue()
+        self.assertEqual(test1(), buf)
+        self.assertEqual(test2(), buf)
+
+
+class PyBytesIOTest(MemoryTestMixin, unittest.TestCase):
+    @staticmethod
+    def buftype(s):
+        return s.encode("ascii")
+    ioclass = io._BytesIO
+    EOF = b""
+
+    def test_read1(self):
+        buf = self.buftype("1234567890")
+        memio = self.ioclass(buf)
+
+        self.assertRaises(TypeError, memio.read1)
+        self.assertEqual(memio.read(), buf)
+
+    def test_readinto(self):
+        buf = self.buftype("1234567890")
+        memio = self.ioclass(buf)
+
+        b = bytearray(b"hello")
+        self.assertEqual(memio.readinto(b), 5)
+        self.assertEqual(b, b"12345")
+        self.assertEqual(memio.readinto(b), 5)
+        self.assertEqual(b, b"67890")
+        self.assertEqual(memio.readinto(b), 0)
+        self.assertEqual(b, b"67890")
+        b = bytearray(b"hello world")
+        memio.seek(0)
+        self.assertEqual(memio.readinto(b), 10)
+        self.assertEqual(b, b"1234567890d")
+        b = bytearray(b"")
+        memio.seek(0)
+        self.assertEqual(memio.readinto(b), 0)
+        self.assertEqual(b, b"")
+        self.assertRaises(TypeError, memio.readinto, '')
+        import array
+        a = array.array('b', b"hello world")
+        memio = self.ioclass(buf)
+        memio.readinto(a)
+        self.assertEqual(a.tostring(), b"1234567890d")
+        memio.close()
+        self.assertRaises(ValueError, memio.readinto, b)
+
+    def test_relative_seek(self):
+        buf = self.buftype("1234567890")
+        memio = self.ioclass(buf)
+
+        self.assertEqual(memio.seek(-1, 1), 0)
+        self.assertEqual(memio.seek(3, 1), 3)
+        self.assertEqual(memio.seek(-4, 1), 0)
+        self.assertEqual(memio.seek(-1, 2), 9)
+        self.assertEqual(memio.seek(1, 1), 10)
+        self.assertEqual(memio.seek(1, 2), 11)
+        memio.seek(-3, 2)
+        self.assertEqual(memio.read(), buf[-3:])
+        memio.seek(0)
+        memio.seek(1, 1)
+        self.assertEqual(memio.read(), buf[1:])
+
+    def test_unicode(self):
+        memio = self.ioclass()
+
+        self.assertRaises(TypeError, self.ioclass, "1234567890")
+        self.assertRaises(TypeError, memio.write, "1234567890")
+        self.assertRaises(TypeError, memio.writelines, ["1234567890"])
+
+    def test_bytes_array(self):
+        buf = b"1234567890"
+        import array
+        a = array.array('b', list(buf))
+        memio = self.ioclass(a)
+        self.assertEqual(memio.getvalue(), buf)
+        self.assertEqual(memio.write(a), 10)
+        self.assertEqual(memio.getvalue(), buf)
+
+
+class PyStringIOTest(MemoryTestMixin, unittest.TestCase):
+    buftype = str
+    ioclass = io.StringIO
+    EOF = ""
+
+    def test_relative_seek(self):
+        memio = self.ioclass()
+
+        self.assertRaises(IOError, memio.seek, -1, 1)
+        self.assertRaises(IOError, memio.seek, 3, 1)
+        self.assertRaises(IOError, memio.seek, -3, 1)
+        self.assertRaises(IOError, memio.seek, -1, 2)
+        self.assertRaises(IOError, memio.seek, 1, 1)
+        self.assertRaises(IOError, memio.seek, 1, 2)
+
+    # XXX: For the Python version of io.StringIO, this is highly
+    # dependent on the encoding used for the underlying buffer.
+    # def test_widechar(self):
+    #     buf = self.buftype("\U0002030a\U00020347")
+    #     memio = self.ioclass(buf)
+    #
+    #     self.assertEqual(memio.getvalue(), buf)
+    #     self.assertEqual(memio.write(buf), len(buf))
+    #     self.assertEqual(memio.tell(), len(buf))
+    #     self.assertEqual(memio.getvalue(), buf)
+    #     self.assertEqual(memio.write(buf), len(buf))
+    #     self.assertEqual(memio.tell(), len(buf) * 2)
+    #     self.assertEqual(memio.getvalue(), buf + buf)
+
+if has_c_implementation:
+    class CBytesIOTest(PyBytesIOTest):
+        ioclass = io.BytesIO
+
+def test_main():
+    tests = [PyBytesIOTest, PyStringIOTest]
+    if has_c_implementation:
+        tests.extend([CBytesIOTest])
+    test_support.run_unittest(*tests)
+
+if __name__ == '__main__':
+    test_main()
diff --git a/Lib/test/test_mimetools.py b/Lib/test/test_mimetools.py
index cf0e191..ef333f5 100644
--- a/Lib/test/test_mimetools.py
+++ b/Lib/test/test_mimetools.py
@@ -58,7 +58,7 @@
             s.add(nb)
 
     def test_message(self):
-        msg = mimetools.Message(io.StringIO(msgtext1))
+        msg = mimetools.Message(io.StringIO(str(msgtext1)))
         self.assertEqual(msg.gettype(), "text/plain")
         self.assertEqual(msg.getmaintype(), "text")
         self.assertEqual(msg.getsubtype(), "plain")
diff --git a/Modules/_bytesio.c b/Modules/_bytesio.c
new file mode 100644
index 0000000..00cb06b
--- /dev/null
+++ b/Modules/_bytesio.c
@@ -0,0 +1,744 @@
+#include "Python.h"
+
+typedef struct {
+    PyObject_HEAD
+    char *buf;
+    Py_ssize_t pos;
+    Py_ssize_t string_size;
+    size_t buf_size;
+} BytesIOObject;
+
+#define CHECK_CLOSED(self)                                  \
+    if ((self)->buf == NULL) {                              \
+        PyErr_SetString(PyExc_ValueError,                   \
+                        "I/O operation on closed file.");   \
+        return NULL;                                        \
+    }
+
+/* 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(BytesIOObject *self, char **output)
+{
+    char *n;
+    const char *str_end;
+    Py_ssize_t len;
+
+    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++);
+
+    /* 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;
+
+    assert(len >= 0);
+    assert(self->pos < PY_SSIZE_T_MAX - len);
+    self->pos += len;
+
+    return len;
+}
+
+/* Internal routine for changing the size of the buffer of BytesIO objects.
+   The caller should ensure that the 'size' argument is non-negative.  Returns
+   0 on success, -1 otherwise. */
+static int
+resize_buffer(BytesIOObject *self, size_t size)
+{
+    /* 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;
+
+    assert(self->buf != NULL);
+
+    /* For simplicity, stay in the range of the signed type. Anyway, Python
+       doesn't allow strings to be longer than this. */
+    if (size > PY_SSIZE_T_MAX)
+        goto overflow;
+
+    if (size < alloc / 2) {
+        /* Major downsize; resize down to exact size. */
+        alloc = size + 1;
+    }
+    else if (size < alloc) {
+        /* Within allocated size; quick exit */
+        return 0;
+    }
+    else if (size <= alloc * 1.125) {
+        /* Moderate upsize; overallocate similar to list_resize() */
+        alloc = size + (size >> 3) + (size < 9 ? 3 : 6);
+    }
+    else {
+        /* Major upsize; resize up to exact size */
+        alloc = size + 1;
+    }
+
+    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;
+    }
+    self->buf_size = alloc;
+    self->buf = new_buf;
+
+    return 0;
+
+  overflow:
+    PyErr_SetString(PyExc_OverflowError,
+                    "new buffer size too large");
+    return -1;
+}
+
+/* Internal routine for writing a string of bytes to the buffer of a BytesIO
+   object. Returns the number of bytes wrote, or -1 on error. */
+static Py_ssize_t
+write_bytes(BytesIOObject *self, const char *bytes, Py_ssize_t len)
+{
+    assert(self->buf != NULL);
+    assert(self->pos >= 0);
+    assert(len >= 0);
+
+    /* This overflow check is not strictly necessary. However, it avoids us to
+       deal with funky things like comparing an unsigned and a signed
+       integer. */
+    if (self->pos > PY_SSIZE_T_MAX - len) {
+        PyErr_SetString(PyExc_OverflowError,
+                        "new position too large");
+        return -1;
+    }
+    if (self->pos + len > self->buf_size) {
+        if (resize_buffer(self, self->pos + len) < 0)
+            return -1;
+    }
+
+    if (self->pos > self->string_size) {
+        /* In case of overseek, pad with null bytes the buffer region between
+           the end of stream and the current position.
+
+          0   lo      string_size                           hi
+          |   |<---used--->|<----------available----------->|
+          |   |            <--to pad-->|<---to write--->    |
+          0   buf                   position
+        */
+        memset(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);
+    self->pos += len;
+
+    /* Set the new length of the internal string if it has changed. */
+    if (self->string_size < self->pos) {
+        self->string_size = self->pos;
+    }
+
+    return len;
+}
+
+static PyObject *
+bytesio_get_closed(BytesIOObject *self)
+{
+    if (self->buf == NULL)
+        Py_RETURN_TRUE;
+    else
+        Py_RETURN_FALSE;
+}
+
+/* Generic getter for the writable, readable and seekable properties */
+static PyObject *
+return_true(BytesIOObject *self)
+{
+    Py_RETURN_TRUE;
+}
+
+PyDoc_STRVAR(flush_doc,
+"flush() -> None.  Does nothing.");
+
+static PyObject *
+bytesio_flush(BytesIOObject *self)
+{
+    Py_RETURN_NONE;
+}
+
+PyDoc_STRVAR(getval_doc,
+"getvalue() -> string.\n"
+"\n"
+"Retrieve the entire contents of the BytesIO object.");
+
+static PyObject *
+bytesio_getvalue(BytesIOObject *self)
+{
+    CHECK_CLOSED(self);
+    return PyString_FromStringAndSize(self->buf, self->string_size);
+}
+
+PyDoc_STRVAR(isatty_doc,
+"isatty() -> False.\n"
+"\n"
+"Always returns False since BytesIO objects are not connected\n"
+"to a tty-like device.");
+
+static PyObject *
+bytesio_isatty(BytesIOObject *self)
+{
+    CHECK_CLOSED(self);
+    Py_RETURN_FALSE;
+}
+
+PyDoc_STRVAR(tell_doc,
+"tell() -> current file position, an integer\n");
+
+static PyObject *
+bytesio_tell(BytesIOObject *self)
+{
+    CHECK_CLOSED(self);
+    return PyLong_FromSsize_t(self->pos);
+}
+
+PyDoc_STRVAR(read_doc,
+"read([size]) -> read at most size bytes, returned as a string.\n"
+"\n"
+"If the size argument is negative, read until EOF is reached.\n"
+"Return an empty string at EOF.");
+
+static PyObject *
+bytesio_read(BytesIOObject *self, PyObject *args)
+{
+    Py_ssize_t size, n;
+    char *output;
+    PyObject *arg = Py_None;
+
+    CHECK_CLOSED(self);
+
+    if (!PyArg_ParseTuple(args, "|O:read", &arg))
+        return NULL;
+
+    if (PyLong_Check(arg)) {
+        size = PyLong_AsSsize_t(arg);
+    }
+    else if (arg == Py_None) {
+        /* Read until EOF is reached, by default. */
+        size = -1;
+    }
+    else {
+        PyErr_Format(PyExc_TypeError, "integer argument expected, got '%s'",
+                     Py_TYPE(arg)->tp_name);
+        return NULL;
+    }
+
+    /* adjust invalid sizes */
+    n = self->string_size - self->pos;
+    if (size < 0 || size > n) {
+        size = n;
+        if (size < 0)
+            size = 0;
+    }
+
+    assert(self->buf != NULL);
+    output = self->buf + self->pos;
+    self->pos += size;
+
+    return PyString_FromStringAndSize(output, size);
+}
+
+
+PyDoc_STRVAR(read1_doc,
+"read1(size) -> read at most size bytes, returned as a string.\n"
+"\n"
+"If the size argument is negative or omitted, read until EOF is reached.\n"
+"Return an empty string at EOF.");
+
+static PyObject *
+bytesio_read1(BytesIOObject *self, PyObject *n)
+{
+    PyObject *arg, *res;
+
+    arg = PyTuple_Pack(1, n);
+    if (arg == NULL)
+        return NULL;
+    res  = bytesio_read(self, arg);
+    Py_DECREF(arg);
+    return res;
+}
+
+PyDoc_STRVAR(readline_doc,
+"readline([size]) -> next line from the file, as a string.\n"
+"\n"
+"Retain newline.  A non-negative size argument limits the maximum\n"
+"number of bytes to return (an incomplete line may be returned then).\n"
+"Return an empty string at EOF.\n");
+
+static PyObject *
+bytesio_readline(BytesIOObject *self, PyObject *args)
+{
+    Py_ssize_t size, n;
+    char *output;
+    PyObject *arg = Py_None;
+
+    CHECK_CLOSED(self);
+
+    if (!PyArg_ParseTuple(args, "|O:readline", &arg))
+        return NULL;
+
+    if (PyLong_Check(arg)) {
+        size = PyLong_AsSsize_t(arg);
+    }
+    else if (arg == Py_None) {
+        /* No size limit, by default. */
+        size = -1;
+    }
+    else {
+        PyErr_Format(PyExc_TypeError, "integer argument expected, got '%s'",
+                     Py_TYPE(arg)->tp_name);
+        return NULL;
+    }
+
+    n = get_line(self, &output);
+
+    if (size >= 0 && size < n) {
+        size = n - size;
+        n -= size;
+        self->pos -= size;
+    }
+
+    return PyString_FromStringAndSize(output, n);
+}
+
+PyDoc_STRVAR(readlines_doc,
+"readlines([size]) -> list of strings, each a line from the file.\n"
+"\n"
+"Call readline() repeatedly and return a list of the lines so read.\n"
+"The optional size argument, if given, is an approximate bound on the\n"
+"total number of bytes in the lines returned.\n");
+
+static PyObject *
+bytesio_readlines(BytesIOObject *self, PyObject *args)
+{
+    Py_ssize_t maxsize, size, n;
+    PyObject *result, *line;
+    char *output;
+    PyObject *arg = Py_None;
+
+    CHECK_CLOSED(self);
+
+    if (!PyArg_ParseTuple(args, "|O:readlines", &arg))
+        return NULL;
+
+    if (PyLong_Check(arg)) {
+        maxsize = PyLong_AsSsize_t(arg);
+    }
+    else if (arg == Py_None) {
+        /* No size limit, by default. */
+        maxsize = -1;
+    }
+    else {
+        PyErr_Format(PyExc_TypeError, "integer argument expected, got '%s'",
+                     Py_TYPE(arg)->tp_name);
+        return NULL;
+    }
+
+    size = 0;
+    result = PyList_New(0);
+    if (!result)
+        return NULL;
+
+    while ((n = get_line(self, &output)) != 0) {
+        line = PyString_FromStringAndSize(output, n);
+        if (!line)
+            goto on_error;
+        if (PyList_Append(result, line) == -1) {
+            Py_DECREF(line);
+            goto on_error;
+        }
+        Py_DECREF(line);
+        size += n;
+        if (maxsize > 0 && size >= maxsize)
+            break;
+    }
+    return result;
+
+  on_error:
+    Py_DECREF(result);
+    return NULL;
+}
+
+PyDoc_STRVAR(readinto_doc,
+"readinto(bytes) -> int.  Read up to len(b) bytes into b.\n"
+"\n"
+"Returns number of bytes read (0 for EOF), or None if the object\n"
+"is set not to block as has no data to read.");
+
+static PyObject *
+bytesio_readinto(BytesIOObject *self, PyObject *buffer)
+{
+    void *raw_buffer;
+    Py_ssize_t len;
+
+    CHECK_CLOSED(self);
+
+    if (PyObject_AsWriteBuffer(buffer, &raw_buffer, &len) == -1)
+        return NULL;
+
+    if (self->pos + len > self->string_size)
+        len = self->string_size - self->pos;
+
+    memcpy(raw_buffer, self->buf + self->pos, len);
+    assert(self->pos + len < PY_SSIZE_T_MAX);
+    assert(len >= 0);
+    self->pos += len;
+
+    return PyLong_FromSsize_t(len);
+}
+
+PyDoc_STRVAR(truncate_doc,
+"truncate([size]) -> int.  Truncate the file to at most size bytes.\n"
+"\n"
+"Size defaults to the current file position, as returned by tell().\n"
+"Returns the new size.  Imply an absolute seek to the position size.");
+
+static PyObject *
+bytesio_truncate(BytesIOObject *self, PyObject *args)
+{
+    Py_ssize_t size;
+    PyObject *arg = Py_None;
+
+    CHECK_CLOSED(self);
+
+    if (!PyArg_ParseTuple(args, "|O:truncate", &arg))
+        return NULL;
+
+    if (PyLong_Check(arg)) {
+        size = PyLong_AsSsize_t(arg);
+    }
+    else if (arg == Py_None) {
+        /* Truncate to current position if no argument is passed. */
+        size = self->pos;
+    }
+    else {
+        PyErr_Format(PyExc_TypeError, "integer argument expected, got '%s'",
+                     Py_TYPE(arg)->tp_name);
+        return NULL;
+    }
+
+    if (size < 0) {
+        PyErr_Format(PyExc_ValueError,
+                     "negative size value %zd", size);
+        return NULL;
+    }
+
+    if (size < self->string_size) {
+        self->string_size = size;
+        if (resize_buffer(self, size) < 0)
+            return NULL;
+    }
+    self->pos = size;
+
+    return PyLong_FromSsize_t(size);
+}
+
+static PyObject *
+bytesio_iternext(BytesIOObject *self)
+{
+    char *next;
+    Py_ssize_t n;
+
+    CHECK_CLOSED(self);
+
+    n = get_line(self, &next);
+
+    if (!next || n == 0)
+        return NULL;
+
+    return PyString_FromStringAndSize(next, n);
+}
+
+PyDoc_STRVAR(seek_doc,
+"seek(pos, whence=0) -> int.  Change stream position.\n"
+"\n"
+"Seek to byte offset pos relative to position indicated by whence:\n"
+"     0  Start of stream (the default).  pos should be >= 0;\n"
+"     1  Current position - pos may be negative;\n"
+"     2  End of stream - pos usually negative.\n"
+"Returns the new absolute position.");
+
+static PyObject *
+bytesio_seek(BytesIOObject *self, PyObject *args)
+{
+    Py_ssize_t pos;
+    int mode = 0;
+
+    CHECK_CLOSED(self);
+
+    if (!PyArg_ParseTuple(args, "n|i:seek", &pos, &mode))
+        return NULL;
+
+    if (pos < 0 && mode == 0) {
+        PyErr_Format(PyExc_ValueError,
+                     "negative seek value %zd", pos);
+        return NULL;
+    }
+
+    /* mode 0: offset relative to beginning of the string.
+       mode 1: offset relative to current position.
+       mode 2: offset relative the end of the string. */
+    if (mode == 1) {
+        if (pos > PY_SSIZE_T_MAX - self->pos) {
+            PyErr_SetString(PyExc_OverflowError,
+                            "new position too large");
+            return NULL;
+        }
+        pos += self->pos;
+    }
+    else if (mode == 2) {
+        if (pos > PY_SSIZE_T_MAX - self->string_size) {
+            PyErr_SetString(PyExc_OverflowError,
+                            "new position too large");
+            return NULL;
+        }
+        pos += self->string_size;
+    }
+    else if (mode != 0) {
+        PyErr_Format(PyExc_ValueError,
+                     "invalid whence (%i, should be 0, 1 or 2)", mode);
+        return NULL;
+    }
+
+    if (pos < 0)
+        pos = 0;
+    self->pos = pos;
+
+    return PyLong_FromSsize_t(self->pos);
+}
+
+PyDoc_STRVAR(write_doc,
+"write(str) -> int.  Write string str to file.\n"
+"\n"
+"Return the number of bytes written.");
+
+static PyObject *
+bytesio_write(BytesIOObject *self, PyObject *obj)
+{
+    const char *bytes;
+    Py_ssize_t size;
+    Py_ssize_t n = 0;
+
+    CHECK_CLOSED(self);
+
+    if (PyObject_AsReadBuffer(obj, (void *)&bytes, &size) < 0)
+        return NULL;
+
+    if (size != 0) {
+        n = write_bytes(self, bytes, size);
+        if (n < 0)
+            return NULL;
+    }
+
+    return PyLong_FromSsize_t(n);
+}
+
+PyDoc_STRVAR(writelines_doc,
+"writelines(sequence_of_strings) -> None.  Write the strings to the file.\n"
+"\n"
+"Note that newlines are not added.  The sequence can be any iterable object\n"
+"producing strings. This is equivalent to calling write() for each string.");
+
+static PyObject *
+bytesio_writelines(BytesIOObject *self, PyObject *v)
+{
+    PyObject *it, *item;
+    PyObject *ret;
+
+    CHECK_CLOSED(self);
+
+    it = PyObject_GetIter(v);
+    if (it == NULL)
+        return NULL;
+
+    while ((item = PyIter_Next(it)) != NULL) {
+        ret = bytesio_write(self, item);
+        Py_DECREF(item);
+        if (ret == NULL) {
+            Py_DECREF(it);
+            return NULL;
+        }
+        Py_DECREF(ret);
+    }
+    Py_DECREF(it);
+
+    /* See if PyIter_Next failed */
+    if (PyErr_Occurred())
+        return NULL;
+
+    Py_RETURN_NONE;
+}
+
+PyDoc_STRVAR(close_doc,
+"close() -> None.  Disable all I/O operations.");
+
+static PyObject *
+bytesio_close(BytesIOObject *self)
+{
+    if (self->buf != NULL) {
+        PyMem_Free(self->buf);
+        self->buf = NULL;
+    }
+    Py_RETURN_NONE;
+}
+
+static void
+bytesio_dealloc(BytesIOObject *self)
+{
+    if (self->buf != NULL) {
+        PyMem_Free(self->buf);
+        self->buf = NULL;
+    }
+    Py_TYPE(self)->tp_free(self);
+}
+
+static PyObject *
+bytesio_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
+{
+    BytesIOObject *self;
+
+    assert(type != NULL && type->tp_alloc != NULL);
+    self = (BytesIOObject *)type->tp_alloc(type, 0);
+    if (self == NULL)
+        return NULL;
+
+    self->string_size = 0;
+    self->pos = 0;
+    self->buf_size = 0;
+    self->buf = (char *)PyMem_Malloc(0);
+    if (self->buf == NULL) {
+        Py_DECREF(self);
+        return PyErr_NoMemory();
+    }
+
+    return (PyObject *)self;
+}
+
+static int
+bytesio_init(BytesIOObject *self, PyObject *args, PyObject *kwds)
+{
+    PyObject *initvalue = NULL;
+
+    if (!PyArg_ParseTuple(args, "|O:BytesIO", &initvalue))
+        return -1;
+
+    /* In case, __init__ is called multiple times. */
+    self->string_size = 0;
+    self->pos = 0;
+
+    if (initvalue && initvalue != Py_None) {
+        PyObject *res;
+        res = bytesio_write(self, initvalue);
+        if (res == NULL)
+            return -1;
+        Py_DECREF(res);
+        self->pos = 0;
+    }
+
+    return 0;
+}
+
+static PyGetSetDef bytesio_getsetlist[] = {
+    {"closed",  (getter)bytesio_get_closed, NULL,
+     "True if the file is closed."},
+    {0},            /* sentinel */
+};
+
+static struct PyMethodDef bytesio_methods[] = {
+    {"readable",   (PyCFunction)return_true,        METH_NOARGS, NULL},
+    {"seekable",   (PyCFunction)return_true,        METH_NOARGS, NULL},
+    {"writable",   (PyCFunction)return_true,        METH_NOARGS, NULL},
+    {"close",      (PyCFunction)bytesio_close,      METH_NOARGS, close_doc},
+    {"flush",      (PyCFunction)bytesio_flush,      METH_NOARGS, flush_doc},
+    {"isatty",     (PyCFunction)bytesio_isatty,     METH_NOARGS, isatty_doc},
+    {"tell",       (PyCFunction)bytesio_tell,       METH_NOARGS, tell_doc},
+    {"write",      (PyCFunction)bytesio_write,      METH_O, write_doc},
+    {"writelines", (PyCFunction)bytesio_writelines, METH_O, writelines_doc},
+    {"read1",      (PyCFunction)bytesio_read1,      METH_O, read1_doc},
+    {"readinto",   (PyCFunction)bytesio_readinto,   METH_O, readinto_doc},
+    {"readline",   (PyCFunction)bytesio_readline,   METH_VARARGS, readline_doc},
+    {"readlines",  (PyCFunction)bytesio_readlines,  METH_VARARGS, readlines_doc},
+    {"read",       (PyCFunction)bytesio_read,       METH_VARARGS, read_doc},
+    {"getvalue",   (PyCFunction)bytesio_getvalue,   METH_VARARGS, getval_doc},
+    {"seek",       (PyCFunction)bytesio_seek,       METH_VARARGS, seek_doc},
+    {"truncate",   (PyCFunction)bytesio_truncate,   METH_VARARGS, truncate_doc},
+    {NULL, NULL}        /* sentinel */
+};
+
+PyDoc_STRVAR(bytesio_doc,
+"BytesIO([buffer]) -> object\n"
+"\n"
+"Create a buffered I/O implementation using an in-memory bytes\n"
+"buffer, ready for reading and writing.");
+
+static PyTypeObject BytesIO_Type = {
+    PyVarObject_HEAD_INIT(NULL, 0)
+    "_bytesio._BytesIO",                       /*tp_name*/
+    sizeof(BytesIOObject),                     /*tp_basicsize*/
+    0,                                         /*tp_itemsize*/
+    (destructor)bytesio_dealloc,               /*tp_dealloc*/
+    0,                                         /*tp_print*/
+    0,                                         /*tp_getattr*/
+    0,                                         /*tp_setattr*/
+    0,                                         /*tp_compare*/
+    0,                                         /*tp_repr*/
+    0,                                         /*tp_as_number*/
+    0,                                         /*tp_as_sequence*/
+    0,                                         /*tp_as_mapping*/
+    0,                                         /*tp_hash*/
+    0,                                         /*tp_call*/
+    0,                                         /*tp_str*/
+    0,                                         /*tp_getattro*/
+    0,                                         /*tp_setattro*/
+    0,                                         /*tp_as_buffer*/
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,  /*tp_flags*/
+    bytesio_doc,                               /*tp_doc*/
+    0,                                         /*tp_traverse*/
+    0,                                         /*tp_clear*/
+    0,                                         /*tp_richcompare*/
+    0,                                         /*tp_weaklistoffset*/
+    PyObject_SelfIter,                         /*tp_iter*/
+    (iternextfunc)bytesio_iternext,            /*tp_iternext*/
+    bytesio_methods,                           /*tp_methods*/
+    0,                                         /*tp_members*/
+    bytesio_getsetlist,                        /*tp_getset*/
+    0,                                         /*tp_base*/
+    0,                                         /*tp_dict*/
+    0,                                         /*tp_descr_get*/
+    0,                                         /*tp_descr_set*/
+    0,                                         /*tp_dictoffset*/
+    (initproc)bytesio_init,                    /*tp_init*/
+    0,                                         /*tp_alloc*/
+    bytesio_new,                               /*tp_new*/
+};
+
+PyMODINIT_FUNC
+init_bytesio(void)
+{
+    PyObject *m;
+
+    if (PyType_Ready(&BytesIO_Type) < 0)
+        return;
+    m = Py_InitModule("_bytesio", NULL);
+    if (m == NULL)
+        return;
+    Py_INCREF(&BytesIO_Type);
+    PyModule_AddObject(m, "_BytesIO", (PyObject *)&BytesIO_Type);
+}
diff --git a/Modules/_fileio.c b/Modules/_fileio.c
index 9868e56..7966878 100644
--- a/Modules/_fileio.c
+++ b/Modules/_fileio.c
@@ -552,11 +552,10 @@
 			PyErr_SetString(PyExc_TypeError, "an integer is required");
 			return NULL;
 		}
-#if !defined(HAVE_LARGEFILE_SUPPORT)
-		pos = PyLong_AsLong(posobj);
+#if defined(HAVE_LARGEFILE_SUPPORT)
+		pos = PyLong_AsLongLong(posobj);
 #else
-		pos = PyLong_Check(posobj) ?
-			PyLong_AsLongLong(posobj) : PyLong_AsLong(posobj);
+		pos = PyLong_AsLong(posobj);
 #endif
 		if (PyErr_Occurred())
 			return NULL;
@@ -572,10 +571,10 @@
 	if (res < 0)
 		return PyErr_SetFromErrno(PyExc_IOError);
 
-#if !defined(HAVE_LARGEFILE_SUPPORT)
-	return PyLong_FromLong(res);
-#else
+#if defined(HAVE_LARGEFILE_SUPPORT)
 	return PyLong_FromLongLong(res);
+#else
+	return PyLong_FromLong(res);
 #endif
 }
 
@@ -622,48 +621,29 @@
 		return NULL;
 
 	if (posobj == Py_None || posobj == NULL) {
+		/* Get the current position. */
                 posobj = portable_lseek(fd, NULL, 1);
                 if (posobj == NULL)
-                  return NULL;
+			return NULL;
         }
         else {
-		Py_INCREF(posobj);
+		/* Move to the position to be truncated. */
+                posobj = portable_lseek(fd, posobj, 0);
         }
 
-#if !defined(HAVE_LARGEFILE_SUPPORT)
-	pos = PyLong_AsLong(posobj);
+#if defined(HAVE_LARGEFILE_SUPPORT)
+	pos = PyLong_AsLongLong(posobj);
 #else
-	pos = PyLong_Check(posobj) ?
-		PyLong_AsLongLong(posobj) : PyLong_AsLong(posobj);
+	pos = PyLong_AsLong(posobj);
 #endif
-	if (PyErr_Occurred()) {
-		Py_DECREF(posobj);
+	if (PyErr_Occurred())
 		return NULL;
-	}
 
 #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;
-		PyObject *pos2, *oldposobj;
-
-		/* store the current position */
-		oldposobj = portable_lseek(self->fd, NULL, 1);
-		if (oldposobj == NULL) {
-			Py_DECREF(posobj);
-			return NULL;
-		}
-
-		/* Have to move current pos to desired endpoint on Windows. */
-		errno = 0;
-		pos2 = portable_lseek(fd, posobj, SEEK_SET);
-		if (pos2 == NULL) {
-			Py_DECREF(posobj);
-			Py_DECREF(oldposobj);
-			return NULL;
-		}
-		Py_DECREF(pos2);
 
 		/* Truncate.  Note that this may grow the file! */
 		Py_BEGIN_ALLOW_THREADS
@@ -676,18 +656,6 @@
 				errno = EACCES;
 		}
 		Py_END_ALLOW_THREADS
-
-		if (ret == 0) {
-			/* Move to the previous position in the file */
-			pos2 = portable_lseek(fd, oldposobj, SEEK_SET);
-			if (pos2 == NULL) {
-				Py_DECREF(posobj);
-				Py_DECREF(oldposobj);
-				return NULL;
-			}
-		}
-		Py_DECREF(pos2);
-		Py_DECREF(oldposobj);
 	}
 #else
 	Py_BEGIN_ALLOW_THREADS
@@ -697,7 +665,6 @@
 #endif /* !MS_WINDOWS */
 
 	if (ret != 0) {
-		Py_DECREF(posobj);
 		PyErr_SetFromErrno(PyExc_IOError);
 		return NULL;
 	}
@@ -791,7 +758,8 @@
 PyDoc_STRVAR(truncate_doc,
 "truncate([size: int]) -> None.	 Truncate the file to at most size bytes.\n"
 "\n"
-"Size defaults to the current file position, as returned by tell().");
+"Size defaults to the current file position, as returned by tell()."
+"The current file position is changed to the value of size.");
 #endif
 
 PyDoc_STRVAR(tell_doc,
diff --git a/Python/pythonrun.c b/Python/pythonrun.c
index a80cb14..e2ba185 100644
--- a/Python/pythonrun.c
+++ b/Python/pythonrun.c
@@ -240,11 +240,11 @@
     }
 
 	initmain(); /* Module __main__ */
+	if (!Py_NoSiteFlag)
+		initsite(); /* Module site */
 	if (initstdio() < 0)
 		Py_FatalError(
 		    "Py_Initialize: can't initialize sys standard streams");
-	if (!Py_NoSiteFlag)
-		initsite(); /* Module site */
 
 	/* auto-thread-state API, if available */
 #ifdef WITH_THREAD
diff --git a/setup.py b/setup.py
index 397c392..ab1ad99 100644
--- a/setup.py
+++ b/setup.py
@@ -426,6 +426,8 @@
         exts.append( Extension('operator', ['operator.c']) )
         # _functools
         exts.append( Extension("_functools", ["_functoolsmodule.c"]) )
+        # Memory-based IO accelerator modules
+        exts.append( Extension("_bytesio", ["_bytesio.c"]) )
         # atexit
         exts.append( Extension("atexit", ["atexitmodule.c"]) )
         # Python C API test module