emphasize that cffi is better than extension modules for portability
diff --git a/Doc/extending/extending.rst b/Doc/extending/extending.rst
index e78fe0c..c10efa9 100644
--- a/Doc/extending/extending.rst
+++ b/Doc/extending/extending.rst
@@ -20,12 +20,17 @@
The compilation of an extension module depends on its intended use as well as on
your system setup; details are given in later chapters.
-Do note that if your use case is calling C library functions or system calls,
-you should consider using the :mod:`ctypes` module rather than writing custom
-C code. Not only does :mod:`ctypes` let you write Python code to interface
-with C code, but it is more portable between implementations of Python than
-writing and compiling an extension module which typically ties you to CPython.
+.. note::
+ The C extension interface is specific to CPython, and extension modules do
+ not work on other Python implementations. In many cases, it is possible to
+ avoid writing C extensions and preserve portability to other implementations.
+ For example, if your use case is calling C library functions or system calls,
+ you should consider using the :mod:`ctypes` module or the `cffi
+ <http://cffi.readthedocs.org>`_ library rather than writing custom C code.
+ These modules let you write Python code to interface with C code and are more
+ portable between implementations of Python than writing and compiling a C
+ extension module.
.. _extending-simpleexample: