.. 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`.  This section describes the manual

approach to building Python extensions written in C or C++.

To build extensions using these instructions, you need to have a copy of the

Python sources of the same version as your installed Python. You will need

Microsoft Visual C++ "Developer Studio"; project files are supplied for VC++

version 7.1, but you can use older versions of VC++.  Notice that you should use

the same version of VC++that was used to build Python itself. The example files

described here are distributed with the Python sources in the

:file:`PC\\example_nt\\` directory.

#. **Copy the example files** ---  The :file:`example_nt` directory is a

   subdirectory of the :file:`PC` directory, in order to keep all the PC-specific

   files under the same directory in the source distribution.  However, the

   :file:`example_nt` directory can't actually be used from this location.  You

   first need to copy or move it up one level, so that :file:`example_nt` is a

   sibling of the :file:`PC` and :file:`Include` directories.  Do all your work

   from within this new location.

#. **Open the project** ---  From VC++, use the :menuselection:`File --> Open

   Solution` dialog (not :menuselection:`File --> Open`!).  Navigate to and select

   the file :file:`example.sln`, in the *copy* of the :file:`example_nt` directory

   you made above.  Click Open.

#. **Build the example DLL** ---  In order to check that everything is set up

   right, try building:

#. Select a configuration.  This step is optional.  Choose

   :menuselection:`Build --> Configuration Manager --> Active Solution Configuration`

   and select either :guilabel:`Release`  or :guilabel:`Debug`.  If you skip this

   step, VC++ will use the Debug configuration by default.

#. Build the DLL.  Choose :menuselection:`Build --> Build Solution`.  This

   creates all intermediate and result files in a subdirectory called either

   :file:`Debug` or :file:`Release`, depending on which configuration you selected

   in the preceding step.

#. **Testing the debug-mode DLL** ---  Once the Debug build has succeeded, bring

   up a DOS box, and change to the :file:`example_nt\\Debug` directory.  You should

   now be able to repeat the following session (``C>`` is the DOS prompt, ``>>>``

   is the Python prompt; note that build information and various debug output from

   Python may not match this screen dump exactly)::

      C>..\..\PCbuild\python_d

      Adding parser accelerators ...

      Done.

      Python 2.2 (#28, Dec 19 2001, 23:26:37) [MSC 32 bit (Intel)] on win32

      Type "copyright", "credits" or "license" for more information.

      >>> import example

      [4897 refs]

      >>> example.foo()

      Hello, world

      [4903 refs]

      >>>

   Congratulations!  You've successfully built your first Python extension module.

#. **Creating your own project** ---  Choose a name and create a directory for

   it.  Copy your C sources into it.  Note that the module source file name does

   not necessarily have to match the module name, but the name of the

   initialization function should match the module name --- you can only import a

   module :mod:`spam` if its initialization function is called :cfunc:`initspam`,

   and it should call :cfunc:`Py_InitModule` with the string ``"spam"`` as its

   first argument (use the minimal :file:`example.c` in this directory as a guide).

   By convention, it lives in a file called :file:`spam.c` or :file:`spammodule.c`.

   The output file should be called :file:`spam.pyd` (in Release mode) or  

   :file:`spam_d.pyd` (in Debug mode). The extension :file:`.pyd` was chosen

   to avoid confusion with a system library :file:`spam.dll` to which your module

   could be a Python interface.

   .. versionchanged:: 2.5

      Previously, file names like :file:`spam.dll` (in release mode) or

      :file:`spam_d.dll` (in debug mode) were also recognized.

   Now your options are:

#. Copy :file:`example.sln` and :file:`example.vcproj`, rename them to

      :file:`spam.\*`, and edit them by hand, or

#. Create a brand new project; instructions are below.

   In either case, copy :file:`example_nt\\example.def` to :file:`spam\\spam.def`,

   and edit the new :file:`spam.def` so its second line contains the string

   '``initspam``'.  If you created a new project yourself, add the file

   :file:`spam.def` to the project now.  (This is an annoying little file with only

   two lines.  An alternative approach is to forget about the :file:`.def` file,

   and add the option :option:`/export:initspam` somewhere to the Link settings, by

   manually editing the setting in Project Properties dialog).

#. **Creating a brand new project** ---  Use the :menuselection:`File --> New

   --> Project` dialog to create a new Project Workspace.  Select :guilabel:`Visual

   C++ Projects/Win32/ Win32 Project`, enter the name (``spam``), and make sure the

   Location is set to parent of the :file:`spam` directory you have created (which

   should be a direct subdirectory of the Python build tree, a sibling of

   :file:`Include` and :file:`PC`).  Select Win32 as the platform (in my version,

   this is the only choice).  Make sure the Create new workspace radio button is

   selected.  Click OK.

   You should now create the file :file:`spam.def` as instructed in the previous

   section. Add the source files to the project, using :menuselection:`Project -->

   Add Existing Item`. Set the pattern to ``*.*`` and select both :file:`spam.c`

   and :file:`spam.def` and click OK.  (Inserting them one by one is fine too.)

   Now open the :menuselection:`Project --> spam properties` dialog. You only need

   to change a few settings.  Make sure :guilabel:`All Configurations` is selected

   from the :guilabel:`Settings for:` dropdown list.  Select the C/C++ tab.  Choose

   the General category in the popup menu at the top.  Type the following text in

   the entry box labeled :guilabel:`Additional Include Directories`::

      ..\Include,..\PC

   Then, choose the General category in the Linker tab, and enter ::

      ..\PCbuild

   in the text box labelled :guilabel:`Additional library Directories`.

   Now you need to add some mode-specific settings:

   Select :guilabel:`Release` in the :guilabel:`Configuration` dropdown list.

   Choose the :guilabel:`Link` tab, choose the :guilabel:`Input` category, and

   append ``pythonXY.lib`` to the list in the :guilabel:`Additional Dependencies`

   box.

   Select :guilabel:`Debug` in the :guilabel:`Configuration` dropdown list, and

   append ``pythonXY_d.lib`` to the list in the :guilabel:`Additional Dependencies`

   box.  Then click the C/C++ tab, select :guilabel:`Code Generation`, and select

   :guilabel:`Multi-threaded Debug DLL` from the :guilabel:`Runtime library`

   dropdown list.

   Select :guilabel:`Release` again from the :guilabel:`Configuration` dropdown

   list.  Select :guilabel:`Multi-threaded DLL` from the :guilabel:`Runtime

   library` dropdown list.

If your module creates a new type, you may have trouble with this line::

   PyObject_HEAD_INIT(&PyType_Type)

Change it to::

   PyObject_HEAD_INIT(NULL)

and add the following to the module initialization function::

   MyObject_Type.ob_type = &PyType_Type;

Refer to section 3 of the `Python FAQ <http://www.python.org/doc/faq>`_ for

details on why you must do this.

.. _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 :cfunc:`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.