diff --git a/Doc/library/select.rst b/Doc/library/select.rst
index 469fbac71e3..7ca9fde4f8f 100644
--- a/Doc/library/select.rst
+++ b/Doc/library/select.rst
@@ -105,7 +105,7 @@ The module defines the following:
    Files reported as ready for writing by :func:`select`, :func:`poll` or
    similar interfaces in this module are guaranteed to not block on a write
    of up to :const:`PIPE_BUF` bytes.
-   This value is guaranteed by POSIX to be at least 512.
+   This value is guaranteed by POSIX to be at least 512.  Availability: Unix.
 
    .. versionadded:: 2.7
 
diff --git a/Lib/subprocess.py b/Lib/subprocess.py
index c8dcb56e21e..a5bb65065a6 100644
--- a/Lib/subprocess.py
+++ b/Lib/subprocess.py
@@ -418,6 +418,12 @@ else:
     import fcntl
     import pickle
 
+    # When select or poll has indicated that the file is writable,
+    # we can write up to _PIPE_BUF bytes without risk of blocking.
+    # POSIX defines PIPE_BUF as >= 512.
+    _PIPE_BUF = getattr(select, 'PIPE_BUF', 512)
+
+
 __all__ = ["Popen", "PIPE", "STDOUT", "call", "check_call",
            "check_output", "CalledProcessError"]
 
@@ -426,11 +432,6 @@ try:
 except:
     MAXFD = 256
 
-# When select or poll has indicated that the file is writable,
-# we can write up to _PIPE_BUF bytes without risk of blocking.
-# POSIX defines PIPE_BUF as >= 512.
-_PIPE_BUF = getattr(select, 'PIPE_BUF', 512)
-
 _active = []
 
 def _cleanup():
diff --git a/Lib/test/test_subprocess.py b/Lib/test/test_subprocess.py
index 5add023944e..1693c6b4453 100644
--- a/Lib/test/test_subprocess.py
+++ b/Lib/test/test_subprocess.py
@@ -769,7 +769,7 @@ class ProcessTestCase(unittest.TestCase):
 
 unit_tests = [ProcessTestCase]
 
-if subprocess._has_poll:
+if getattr(subprocess, '_has_poll', False):
     class ProcessTestCaseNoPoll(ProcessTestCase):
         def setUp(self):
             subprocess._has_poll = False
diff --git a/Modules/selectmodule.c b/Modules/selectmodule.c
index a4058127017..d8a68c570a1 100644
--- a/Modules/selectmodule.c
+++ b/Modules/selectmodule.c
@@ -1746,7 +1746,9 @@ initselect(void)
 	Py_INCREF(SelectError);
 	PyModule_AddObject(m, "error", SelectError);
 
+#ifdef PIPE_BUF
 	PyModule_AddIntConstant(m, "PIPE_BUF", PIPE_BUF);
+#endif
 
 #if defined(HAVE_POLL)
 #ifdef __APPLE__