Make os.confstr() return a bytes object, attributing the change to Python 3.2

diff --git a/Doc/library/os.rst b/Doc/library/os.rst
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -2101,7 +2101,8 @@ Miscellaneous System Information
 
 .. function:: confstr(name)
 
-   Return string-valued system configuration values. *name* specifies the
+   Return a string-valued system configuration value as a :class:`bytes`
+   object. *name* specifies the
    configuration value to retrieve; it may be a string which is the name of a
    defined system value; these names are specified in a number of standards (POSIX,
    Unix 95, Unix 98, and others).  Some platforms define additional names as well.
@@ -2119,6 +2120,11 @@ Miscellaneous System Information
 
    Availability: Unix
 
+   .. versionchanged:: 3.2
+      Return type changed to :class:`bytes`; previously, the function
+      attempted to decode the value as UTF-8 and return it as a
+      string.
+
 
 .. data:: confstr_names
 
diff --git a/Doc/whatsnew/3.2.rst b/Doc/whatsnew/3.2.rst
--- a/Doc/whatsnew/3.2.rst
+++ b/Doc/whatsnew/3.2.rst
@@ -123,6 +123,9 @@ New, Improved, and Deprecated Modules
 
   (Contributed by Tarek Ziadé.)
 
+* The :mod:`os` module's :func:`~os.confstr` function now returns the
+  configuration value as a bytes object (:issue:`9580`).
+
 * Socket objects now have a :meth:`~socket.socket.detach()` method which
   puts the socket into closed state without actually closing the underlying
   file descriptor.  The latter can then be reused for other purposes.
@@ -254,6 +257,8 @@ that may require changes to your code:
 
 * bytearray objects cannot be used anymore as filenames: convert them to bytes
 
+* :func:`os.confstr` now returns a bytes object instead of a string.
+
 * PyArg_Parse*() functions:
 
   * "t#" format has been removed: use "s#" or "s*" instead
diff --git a/Lib/test/test_os.py b/Lib/test/test_os.py
--- a/Lib/test/test_os.py
+++ b/Lib/test/test_os.py
@@ -21,7 +21,7 @@ import contextlib
 if (hasattr(os, "confstr_names") and
     "CS_GNU_LIBPTHREAD_VERSION" in os.confstr_names):
     libpthread = os.confstr("CS_GNU_LIBPTHREAD_VERSION")
-    USING_LINUXTHREADS= libpthread.startswith("linuxthreads")
+    USING_LINUXTHREADS= libpthread.startswith(b"linuxthreads")
 else:
     USING_LINUXTHREADS= False
 
diff --git a/Lib/test/test_posix.py b/Lib/test/test_posix.py
--- a/Lib/test/test_posix.py
+++ b/Lib/test/test_posix.py
@@ -154,7 +154,9 @@ class PosixTester(unittest.TestCase):
     def test_confstr(self):
         if hasattr(posix, 'confstr'):
             self.assertRaises(ValueError, posix.confstr, "CS_garbage")
-            self.assertEqual(len(posix.confstr("CS_PATH")) > 0, True)
+            path = posix.confstr("CS_PATH")
+            self.assertIsInstance(path, bytes)
+            self.assertGreater(len(path), 0)
 
     def test_dup2(self):
         if hasattr(posix, 'dup2'):
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -6633,8 +6633,8 @@ conv_confstr_confname(PyObject *arg, int
 }
 
 PyDoc_STRVAR(posix_confstr__doc__,
-"confstr(name) -> string\n\n\
-Return a string-valued system configuration variable.");
+"confstr(name) -> bytes\n\n\
+Return a string-valued system configuration variable as a bytes object.");
 
 static PyObject *
 posix_confstr(PyObject *self, PyObject *args)
@@ -6659,12 +6659,12 @@ posix_confstr(PyObject *self, PyObject *
         }
         else {
             if ((unsigned int)len >= sizeof(buffer)) {
-                result = PyUnicode_FromStringAndSize(NULL, len-1);
+                result = PyBytes_FromStringAndSize(NULL, len-1);
                 if (result != NULL)
-                    confstr(name, _PyUnicode_AsString(result), len);
+                    confstr(name, PyBytes_AS_STRING(result), len);
             }
             else
-                result = PyUnicode_FromStringAndSize(buffer, len-1);
+                result = PyBytes_FromStringAndSize(buffer, len-1);
         }
     }
     return result;