Doc/extending/windows.rst

.. highlightlang:: c


.. _building-on-windows:

****************************************
Building C and C++ Extensions on Windows
****************************************

This chapter briefly explains how to create a Windows extension module for
Python using Microsoft Visual C++, and follows with more detailed background
information on how it works.  The explanatory material is useful for both the
Windows programmer learning to build Python extensions and the Unix programmer
interested in producing software which can be successfully built on both Unix
and Windows.

Module authors are encouraged to use the distutils approach for building
extension modules, instead of the one described in this section. You will still
need the C compiler that was used to build Python; typically Microsoft Visual
C++.

.. note::

   This chapter mentions a number of filenames that include an encoded Python
   version number.  These filenames are represented with the version number shown
   as ``XY``; in practice, ``'X'`` will be the major version number and ``'Y'``
   will be the minor version number of the Python release you're working with.  For
   example, if you are using Python 2.2.1, ``XY`` will actually be ``22``.


.. _win-cookbook:

A Cookbook Approach
===================

There are two approaches to building extension modules on Windows, just as there
are on Unix: use the :mod:`distutils` package to control the build process, or
do things manually.  The distutils approach works well for most extensions;
documentation on using :mod:`distutils` to build and package extension modules
is available in :ref:`distutils-index`.  If you find you really need to do
things manually, it may be instructive to study the project file for the
:source:`winsound <PCbuild/winsound.vcxproj>` standard library module.


.. _dynamic-linking:

Differences Between Unix and Windows
====================================

.. sectionauthor:: Chris Phoenix <cphoenix@best.com>


Unix and Windows use completely different paradigms for run-time loading of
code.  Before you try to build a module that can be dynamically loaded, be aware
of how your system works.

In Unix, a shared object (:file:`.so`) file contains code to be used by the
program, and also the names of functions and data that it expects to find in the
program.  When the file is joined to the program, all references to those
functions and data in the file's code are changed to point to the actual
locations in the program where the functions and data are placed in memory.
This is basically a link operation.

In Windows, a dynamic-link library (:file:`.dll`) file has no dangling
references.  Instead, an access to functions or data goes through a lookup
table.  So the DLL code does not have to be fixed up at runtime to refer to the
program's memory; instead, the code already uses the DLL's lookup table, and the
lookup table is modified at runtime to point to the functions and data.

In Unix, there is only one type of library file (:file:`.a`) which contains code
from several object files (:file:`.o`).  During the link step to create a shared
object file (:file:`.so`), the linker may find that it doesn't know where an
identifier is defined.  The linker will look for it in the object files in the
libraries; if it finds it, it will include all the code from that object file.

In Windows, there are two types of library, a static library and an import
library (both called :file:`.lib`).  A static library is like a Unix :file:`.a`
file; it contains code to be included as necessary. An import library is
basically used only to reassure the linker that a certain identifier is legal,
and will be present in the program when the DLL is loaded.  So the linker uses
the information from the import library to build the lookup table for using
identifiers that are not included in the DLL.  When an application or a DLL is
linked, an import library may be generated, which will need to be used for all
future DLLs that depend on the symbols in the application or DLL.

Suppose you are building two dynamic-load modules, B and C, which should share
another block of code A.  On Unix, you would *not* pass :file:`A.a` to the
linker for :file:`B.so` and :file:`C.so`; that would cause it to be included
twice, so that B and C would each have their own copy.  In Windows, building
:file:`A.dll` will also build :file:`A.lib`.  You *do* pass :file:`A.lib` to the
linker for B and C.  :file:`A.lib` does not contain code; it just contains
information which will be used at runtime to access A's code.

In Windows, using an import library is sort of like using ``import spam``; it
gives you access to spam's names, but does not create a separate copy.  On Unix,
linking with a library is more like ``from spam import *``; it does create a
separate copy.


.. _win-dlls:

Using DLLs in Practice
======================

.. sectionauthor:: Chris Phoenix <cphoenix@best.com>


Windows Python is built in Microsoft Visual C++; using other compilers may or
may not work (though Borland seems to).  The rest of this section is MSVC++
specific.

When creating DLLs in Windows, you must pass :file:`pythonXY.lib` to the linker.
To build two DLLs, spam and ni (which uses C functions found in spam), you could
use these commands::

   cl /LD /I/python/include spam.c ../libs/pythonXY.lib
   cl /LD /I/python/include ni.c spam.lib ../libs/pythonXY.lib

The first command created three files: :file:`spam.obj`, :file:`spam.dll` and
:file:`spam.lib`.  :file:`Spam.dll` does not contain any Python functions (such
as :c:func:`PyArg_ParseTuple`), but it does know how to find the Python code
thanks to :file:`pythonXY.lib`.

The second command created :file:`ni.dll` (and :file:`.obj` and :file:`.lib`),
which knows how to find the necessary functions from spam, and also from the
Python executable.

Not every identifier is exported to the lookup table.  If you want any other
modules (including Python) to be able to see your identifiers, you have to say
``_declspec(dllexport)``, as in ``void _declspec(dllexport) initspam(void)`` or
``PyObject _declspec(dllexport) *NiGetSpamData(void)``.

Developer Studio will throw in a lot of import libraries that you do not really
need, adding about 100K to your executable.  To get rid of them, use the Project
Settings dialog, Link tab, to specify *ignore default libraries*.  Add the
correct :file:`msvcrtxx.lib` to the list of libraries.