Adds clang missing Init function solution.

Author: paulross

doc/sphinx/source/index.rst

@@ -22,6 +22,7 @@ This describes reliable patterns of coding Python Extensions in C. It covers the
    compiler_flags
    debugging/debug
    thread_safety
+   miscellaneous
 
 
 Indices and tables

doc/sphinx/source/miscellaneous.rst

@@ -0,0 +1,41 @@
+.. highlight:: c
+    :linenothreshold: 10
+
+.. toctree::
+    :maxdepth: 2
+
+====================================
+Miscellaneous
+====================================
+
+------------------------------------
+No ``PyInit`` Function Found  
+------------------------------------
+
+This is probably Mac OS X and Clang specific but when you import your extension and you get an error like:
+
+``ImportError: dynamic module does not define module export function (PyInit_Foo)``
+
+Have a look at the binary.
+
+.. code-block:: sh
+
+    $ nm -m Foo.cpython-36m-darwin.so | grep Init
+    00000000000010d0 (__TEXT,__text) non-external (was a private external) _PyInit_Foo
+
+Sometimes (why?) clang does not make the symbol external. I have found that adding ``__attribute__((visibility("default")))`` to the module initialisation function can fix this:
+
+.. code-block:: sh
+
+    __attribute__((visibility("default")))
+    PyMODINIT_FUNC
+    PyInit_Foo(void) {
+        /* ... */
+    }
+
+And the binary now looks like this:
+
+.. code-block:: sh
+
+    $ nm -m cXmlWrite.cpython-36m-darwin.so | grep Init
+    00000000000010d0 (__TEXT,__text) external _PyInit_cXmlWrite