
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``.


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

There are two approaches to building extension modules on Windows,
just as there are on Unix: use the ``distutils`` package to control
the build process, or do things manually.  The distutils approach
works well for most extensions; documentation on using ``distutils``
to build and package extension modules is available in *Distributing
Python Modules*.  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 ``PC\example_nt\``
directory.

1. **Copy the example files** ---  The ``example_nt`` directory is a
   subdirectory of the ``PC`` directory, in order to keep all the PC-
   specific files under the same directory in the source distribution.
   However, the ``example_nt`` directory can't actually be used from
   this location.  You first need to copy or move it up one level, so
   that ``example_nt`` is a sibling of the ``PC`` and ``Include``
   directories.  Do all your work from within this new location.

2. **Open the project** ---  From VC++, use the *File ‣ Open Solution*
   dialog (not *File ‣ Open*!).  Navigate to and select the file
   ``example.sln``, in the *copy* of the ``example_nt`` directory you
   made above.  Click Open.

3. **Build the example DLL** ---  In order to check that everything is
   set up right, try building:

4. Select a configuration.  This step is optional.  Choose *Build ‣
   Configuration Manager ‣ Active Solution Configuration* and select
   either *Release*  or *Debug*.  If you skip this step, VC++ will use
   the Debug configuration by default.

5. Build the DLL.  Choose *Build ‣ Build Solution*.  This creates all
   intermediate and result files in a subdirectory called either
   ``Debug`` or ``Release``, depending on which configuration you
   selected in the preceding step.

6. **Testing the debug-mode DLL** ---  Once the Debug build has
   succeeded, bring up a DOS box, and change to the
   ``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.

7. **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 ``spam`` if
   its initialization function is called ``initspam()``, and it should
   call ``Py_InitModule()`` with the string ``"spam"`` as its first
   argument (use the minimal ``example.c`` in this directory as a
   guide). By convention, it lives in a file called ``spam.c`` or
   ``spammodule.c``. The output file should be called ``spam.pyd`` (in
   Release mode) or ``spam_d.pyd`` (in Debug mode). The extension
   ``.pyd`` was chosen to avoid confusion with a system library
   ``spam.dll`` to which your module could be a Python interface.

   Now your options are:

8. Copy ``example.sln`` and ``example.vcproj``, rename them to
   ``spam.*``, and edit them by hand, or

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

   In either case, copy ``example_nt\example.def`` to
   ``spam\spam.def``, and edit the new ``spam.def`` so its second line
   contains the string '``initspam``'.  If you created a new project
   yourself, add the 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 ``.def`` file, and add the option
   */export:initspam* somewhere to the Link settings, by manually
   editing the setting in Project Properties dialog).

10. **Creating a brand new project** ---  Use the *File ‣ New ‣
    Project* dialog to create a new Project Workspace.  Select *Visual
    C++ Projects/Win32/ Win32 Project*, enter the name (``spam``), and
    make sure the Location is set to parent of the ``spam`` directory
    you have created (which should be a direct subdirectory of the
    Python build tree, a sibling of ``Include`` and ``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 ``spam.def`` as instructed in the
    previous section. Add the source files to the project, using
    *Project ‣ Add Existing Item*. Set the pattern to ``*.*`` and
    select both ``spam.c`` and ``spam.def`` and click OK.  (Inserting
    them one by one is fine too.)

    Now open the *Project ‣ spam properties* dialog. You only need to
    change a few settings.  Make sure *All Configurations* is selected
    from the *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 *Additional Include
    Directories*:

       ..\Include,..\PC

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

       ..\PCbuild

    in the text box labelled *Additional library Directories*.

    Now you need to add some mode-specific settings:

    Select *Release* in the *Configuration* dropdown list. Choose the
    *Link* tab, choose the *Input* category, and append
    ``pythonXY.lib`` to the list in the *Additional Dependencies* box.

    Select *Debug* in the *Configuration* dropdown list, and append
    ``pythonXY_d.lib`` to the list in the *Additional Dependencies*
    box.  Then click the C/C++ tab, select *Code Generation*, and
    select *Multi-threaded Debug DLL* from the *Runtime library*
    dropdown list.

    Select *Release* again from the *Configuration* dropdown list.
    Select *Multi-threaded DLL* from the *Runtime library* dropdown
    list.

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

   PyVarObject_HEAD_INIT(&PyType_Type, 0)

Static type object initializers in extension modules may cause
compiles to fail with an error message like "initializer not a
constant".  This shows up when building DLL under MSVC.  Change it to:

   PyVarObject_HEAD_INIT(NULL, 0)

and add the following to the module initialization function:

   if (PyType_Ready(&MyObject_Type) < 0)
        return NULL;


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

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 (``.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 (``.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 (``.a``) which
contains code from several object files (``.o``).  During the link
step to create a shared object 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 ``.lib``).  A static library is like a
Unix ``.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
``A.a`` to the linker for ``B.so`` and ``C.so``; that would cause it
to be included twice, so that B and C would each have their own copy.
In Windows, building ``A.dll`` will also build ``A.lib``.  You *do*
pass ``A.lib`` to the linker for B and C.  ``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.


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

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 ``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: ``spam.obj``, ``spam.dll`` and
``spam.lib``.  ``Spam.dll`` does not contain any Python functions
(such as ``PyArg_ParseTuple()``), but it does know how to find the
Python code thanks to ``pythonXY.lib``.

The second command created ``ni.dll`` (and ``.obj`` and ``.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 ``msvcrtxx.lib`` to the list of
libraries.
