Index: Python/pythonrun.c
===================================================================
--- Python/pythonrun.c	(revision 59809)
+++ Python/pythonrun.c	(working copy)
@@ -251,11 +251,11 @@
 		initsigs(); /* Signal handling stuff, including initintr() */
 
 	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
Index: setup.py
===================================================================
--- setup.py	(revision 59809)
+++ setup.py	(working copy)
@@ -409,6 +409,8 @@
         exts.append( Extension('operator', ['operator.c']) )
         # _functools
         exts.append( Extension("_functools", ["_functoolsmodule.c"]) )
+        # BytesIO accelerator module
+        exts.append( Extension("_bytesio", ["_bytesio.c"]) )
         # atexit
         exts.append( Extension("atexit", ["atexitmodule.c"]) )
         # Python C API test module
Index: Lib/io.py
===================================================================
--- Lib/io.py	(revision 59809)
+++ Lib/io.py	(working copy)
@@ -394,6 +394,7 @@
 
     def readline(self, limit: int = -1) -> bytes:
         """For backwards compatibility, a (slowish) readline()."""
+        self._checkClosed()
         if hasattr(self, "peek"):
             def nreadahead():
                 readahead = self.peek(1, unsafe=True)
@@ -429,7 +430,7 @@
         return line
 
     def readlines(self, hint=None):
-        if hint is None:
+        if hint is None or hint <= 0:
             return list(self)
         n = 0
         lines = []
@@ -626,6 +627,8 @@
 
         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 ###
@@ -665,7 +668,7 @@
         return self.raw.isatty()
 
 
-class BytesIO(BufferedIOBase):
+class _BytesIO(BufferedIOBase):
 
     """Buffered I/O implementation using an in-memory bytes buffer."""
 
@@ -679,13 +682,19 @@
         self._pos = 0
 
     def getvalue(self):
+        if self.closed:
+            raise ValueError("getvalue of 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
@@ -700,6 +709,8 @@
         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
@@ -711,28 +722,38 @@
         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
@@ -743,7 +764,17 @@
     def seekable(self):
         return True
 
+# Use the faster implementation of BytesIO if available
+try:
+    from _bytesio import _BytesIO
+    
+    class BytesIO(_BytesIO, BufferedIOBase):
+        __doc__ = _BytesIO.__doc__
 
+except ImportError:
+    BytesIO = _BytesIO
+
+
 class BufferedReader(_BufferedIOMixin):
 
     """Buffer for a readable sequential RawIO object."""
@@ -871,6 +902,12 @@
                     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")
@@ -980,6 +1017,13 @@
         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
@@ -1026,14 +1070,6 @@
         """write(s: str) -> int.  Write string s to stream."""
         self._unsupported("write")
 
-    def truncate(self, pos: int = None) -> int:
-        """truncate(pos: int = None) -> int.  Truncate size to pos."""
-        self.flush()
-        if pos is None:
-            pos = self.tell()
-        self.seek(pos)
-        return self.buffer.truncate()
-
     def readline(self) -> str:
         """readline() -> str.  Read until newline or EOF.
 
@@ -1206,9 +1242,15 @@
     # were rendered by the decoder after feeding it those bytes.  We
     # use this to reconstruct intermediate decoder states in tell().
 
-    def _seekable(self):
+    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
@@ -1323,7 +1365,17 @@
         finally:
             decoder.setstate(saved_state)
 
+    def truncate(self, pos: int=None) -> int:
+        """truncate(pos: int=None) -> int.  Truncate size to pos."""
+        self.flush()
+        if pos is None:
+            pos = self.tell()
+        self.seek(pos)
+        return self.buffer.truncate()
+
     def seek(self, pos, 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:
@@ -1393,6 +1445,8 @@
         return line
 
     def readline(self, limit=None):
+        if self.closed:
+            raise ValueError("read from closed file")
         if limit is None:
             limit = -1
         if limit >= 0:
Index: Lib/test/test_io.py
===================================================================
--- Lib/test/test_io.py	(revision 59809)
+++ Lib/test/test_io.py	(working copy)
@@ -94,7 +94,7 @@
         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):
@@ -139,7 +139,7 @@
         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")
@@ -580,6 +580,7 @@
             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):
@@ -660,7 +661,8 @@
                 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
 
Index: Lib/test/test_StringIO.py
===================================================================
--- Lib/test/test_StringIO.py	(revision 59809)
+++ Lib/test/test_StringIO.py	(working copy)
@@ -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()
Index: Lib/test/test_memoryio.py
===================================================================
--- Lib/test/test_memoryio.py	(revision 0)
+++ Lib/test/test_memoryio.py	(revision 0)
@@ -0,0 +1,371 @@
+"""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 _stringio, _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)
+        memio.close()
+        self.assertRaises(ValueError, memio.write, self.buftype(""))
+
+    def test_writelines(self):
+        buf = self.buftype("1234567890")
+        memio = self.ioclass()
+
+        memio.writelines([buf] * 100)
+        self.assertEqual(memio.getvalue(), buf * 100)
+        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)
+        memio.close()
+        self.assertRaises(ValueError, memio.truncate, 0)
+
+    def test_init(self):
+        buf = self.buftype("1234567890")
+        memio = self.ioclass(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.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)
+        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.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, next(memio))
+
+    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)
+
+    def test_none_arg(self):
+        memio = self.ioclass(None)
+
+        self.assertEqual(memio.read(None), self.EOF)
+        self.assertEqual(memio.readline(None), self.EOF)
+        self.assertEqual(memio.readlines(None), [])
+        self.assertEqual(memio.truncate(None), 0)
+
+
+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"")
+        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)
+
+    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.getvalue(), buf)
+        self.assertEqual(memio.write(buf), len(buf))
+        self.assertEqual(memio.getvalue(), buf + buf)
+
+
+if has_c_implementation:
+    class CBytesIOTest(PyBytesIOTest):
+        ioclass = _bytesio._BytesIO
+
+
+def test_main():
+    tests = [PyBytesIOTest, PyStringIOTest]
+    if has_c_implementation:
+        tests.extend([CBytesIOTest])
+    test_support.run_unittest(*tests)
+
+if __name__ == '__main__':
+    test_main()
Index: Lib/test/test_largefile.py
===================================================================
--- Lib/test/test_largefile.py	(revision 59809)
+++ Lib/test/test_largefile.py	(working copy)
@@ -152,7 +152,7 @@
         newsize -= 1
         f.seek(42)
         f.truncate(newsize)
-        expect(f.tell(), 42)        # else pointer moved
+        expect(f.tell(), newsize)   # pointer should be moved
         f.seek(0, 2)
         expect(f.tell(), newsize)   # else wasn't truncated
 
@@ -161,8 +161,8 @@
         # cut it waaaaay back
         f.seek(0)
         f.truncate(1)
-        expect(f.tell(), 0)         # else pointer moved
-        expect(len(f.read()), 1)    # else wasn't truncated
+        expect(f.tell(), 1)         # pointer should be moved
+        expect(len(f.read()), 0)
 
     finally:
         f.close()
Index: Modules/_fileio.c
===================================================================
--- Modules/_fileio.c	(revision 59809)
+++ Modules/_fileio.c	(working copy)
@@ -560,11 +560,10 @@
 			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;
@@ -580,10 +579,10 @@
 	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
 }
 
@@ -630,49 +629,30 @@
 		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
 		errno = 0;
@@ -684,18 +664,6 @@
 				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
@@ -705,7 +673,6 @@
 #endif /* !MS_WINDOWS */
 
 	if (ret != 0) {
-		Py_DECREF(posobj);
 		PyErr_SetFromErrno(PyExc_IOError);
 		return NULL;
 	}
@@ -799,7 +766,8 @@
 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,