5. 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++.
This chapter mentions a number of filenames that include an encoded Python
version number. These filenames are represented with the version number shown
XY; in practice,
'X' will be the major version number and
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
5.1. 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 (Legacy version). If you find you really need to do
things manually, it may be instructive to study the project file for the
winsound standard library module.
5.2. 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
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
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
5.3. Using DLLs in Practice¶
Windows Python is built in Microsoft Visual C++; using other compilers may or may not work. 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.dll does not contain any Python functions (such
PyArg_ParseTuple()), but it does know how to find the Python code
The second command created
which knows how to find the necessary functions from spam, and also from the
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
msvcrtxx.lib to the list of libraries.