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 <div class="section" id="building-c-and-c-extensions-on-windows">
<h1>4. Building C and C++ Extensions on Windows<a class="headerlink" href="#building-c-and-c-extensions-on-windows" title="Permalink to this headline">¶</a></h1>
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++.
<div class="admonition note">
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 <tt class="docutils literal">XY</tt>; in practice, <tt class="docutils literal">'X'</tt> will be the major version number and <tt class="docutils literal">'Y'</tt>
will be the minor version number of the Python release you&#8217;re working with. For
example, if you are using Python 2.2.1, <tt class="docutils literal">XY</tt> will actually be <tt class="docutils literal">22</tt>.
</div>
<div class="section" id="a-cookbook-approach">
<h2>4.1. A Cookbook Approach<a class="headerlink" href="#a-cookbook-approach" title="Permalink to this headline">¶</a></h2>
There are two approaches to building extension modules on Windows, just as there
are on Unix: use the <a class="reference internal" href="../library/distutils.html#module-distutils" title="distutils: Support for building and installing Python modules into an existing Python installation."><tt class="xref py py-mod docutils literal">distutils</tt></a> package to control the build process, or
do things manually. The distutils approach works well for most extensions;
documentation on using <a class="reference internal" href="../library/distutils.html#module-distutils" title="distutils: Support for building and installing Python modules into an existing Python installation."><tt class="xref py py-mod docutils literal">distutils</tt></a> to build and package extension modules
is available in <a class="reference internal" href="../distutils/index.html#distutils-index">Distributing Python Modules</a>. 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++ &#8220;Developer Studio&#8221;; 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
<tt class="file docutils literal">PC\example_nt\</tt> directory.
<ol class="arabic">
<li>Copy the example files &#8212; The <tt class="file docutils literal">example_nt</tt> directory is a
subdirectory of the <tt class="file docutils literal">PC</tt> directory, in order to keep all the PC-specific
files under the same directory in the source distribution. However, the
<tt class="file docutils literal">example_nt</tt> directory can&#8217;t actually be used from this location. You
first need to copy or move it up one level, so that <tt class="file docutils literal">example_nt</tt> is a
sibling of the <tt class="file docutils literal">PC</tt> and <tt class="file docutils literal">Include</tt> directories. Do all your work
from within this new location.
</li>
<li>Open the project &#8212; From VC++, use the File ‣ Open
Solution dialog (not File ‣ Open!). Navigate to and select
the file <tt class="file docutils literal">example.sln</tt>, in the copy of the <tt class="file docutils literal">example_nt</tt> directory
you made above. Click Open.
</li>
<li>Build the example DLL &#8212; In order to check that everything is set up
right, try building:
</li>
<li>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.
</li>
<li>Build the DLL. Choose Build ‣ Build Solution. This
creates all intermediate and result files in a subdirectory called either
<tt class="file docutils literal">Debug</tt> or <tt class="file docutils literal">Release</tt>, depending on which configuration you selected
in the preceding step.
</li>
<li>Testing the debug-mode DLL &#8212; Once the Debug build has succeeded, bring
up a DOS box, and change to the <tt class="file docutils literal">example_nt\Debug</tt> directory. You should
now be able to repeat the following session (<tt class="docutils literal">C&gt;</tt> is the DOS prompt, <tt class="docutils literal">&gt;&gt;&gt;</tt>
is the Python prompt; note that build information and various debug output from
Python may not match this screen dump exactly):
<div class="highlight-c"><pre>C&gt;..\..\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.
&gt;&gt;&gt; import example
[4897 refs]
&gt;&gt;&gt; example.foo()
Hello, world
[4903 refs]
&gt;&gt;&gt;</pre>
</div>
Congratulations! You&#8217;ve successfully built your first Python extension module.
</li>
<li>Creating your own project &#8212; 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 &#8212; you can only import a
module <tt class="xref py py-mod docutils literal">spam</tt> if its initialization function is called <tt class="xref c c-func docutils literal">initspam()</tt>,
and it should call <a class="reference internal" href="../c-api/allocation.html#Py_InitModule" title="Py_InitModule"><tt class="xref c c-func docutils literal">Py_InitModule()</tt></a> with the string <tt class="docutils literal">&quot;spam&quot;</tt> as its
first argument (use the minimal <tt class="file docutils literal">example.c</tt> in this directory as a guide).
By convention, it lives in a file called <tt class="file docutils literal">spam.c</tt> or <tt class="file docutils literal">spammodule.c</tt>.
The output file should be called <tt class="file docutils literal">spam.pyd</tt> (in Release mode) or
<tt class="file docutils literal">spam_d.pyd</tt> (in Debug mode). The extension <tt class="file docutils literal">.pyd</tt> was chosen
to avoid confusion with a system library <tt class="file docutils literal">spam.dll</tt> to which your module
could be a Python interface.

Changed in version 2.5: Previously, file names like <tt class="file docutils literal">spam.dll</tt> (in release mode) or
<tt class="file docutils literal">spam_d.dll</tt> (in debug mode) were also recognized.
Now your options are:
</li>
<li>Copy <tt class="file docutils literal">example.sln</tt> and <tt class="file docutils literal">example.vcproj</tt>, rename them to
<tt class="file docutils literal">spam.*</tt>, and edit them by hand, or
</li>
<li>Create a brand new project; instructions are below.
In either case, copy <tt class="file docutils literal">example_nt\example.def</tt> to <tt class="file docutils literal">spam\spam.def</tt>,
and edit the new <tt class="file docutils literal">spam.def</tt> so its second line contains the string
&#8216;<tt class="docutils literal">initspam</tt>&#8216;. If you created a new project yourself, add the file
<tt class="file docutils literal">spam.def</tt> to the project now. (This is an annoying little file with only
two lines. An alternative approach is to forget about the <tt class="file docutils literal">.def</tt> file,
and add the option /export:initspam somewhere to the Link settings, by
manually editing the setting in Project Properties dialog).
</li>
<li>Creating a brand new project &#8212; Use the File ‣ New
‣ Project dialog to create a new Project Workspace. Select Visual
C++ Projects/Win32/ Win32 Project, enter the name (<tt class="docutils literal">spam</tt>), and make sure the
Location is set to parent of the <tt class="file docutils literal">spam</tt> directory you have created (which
should be a direct subdirectory of the Python build tree, a sibling of
<tt class="file docutils literal">Include</tt> and <tt class="file docutils literal">PC</tt>). 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 <tt class="file docutils literal">spam.def</tt> as instructed in the previous
section. Add the source files to the project, using Project ‣
Add Existing Item. Set the pattern to <tt class="docutils literal">*.*</tt> and select both <tt class="file docutils literal">spam.c</tt>
and <tt class="file docutils literal">spam.def</tt> 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:
<div class="highlight-c"><pre>..\Include,..\PC</pre>
</div>
Then, choose the General category in the Linker tab, and enter
<div class="highlight-c"><pre>..\PCbuild</pre>
</div>
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 <tt class="docutils literal">pythonXY.lib</tt> to the list in the Additional Dependencies
box.
Select Debug in the Configuration dropdown list, and
append <tt class="docutils literal">pythonXY_d.lib</tt> 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.
</li>
</ol>
If your module creates a new type, you may have trouble with this line:
<div class="highlight-c"><div class="highlight"><pre>PyObject_HEAD_INIT(&amp;PyType_Type)
</pre></div>
</div>
Static type object initializers in extension modules may cause
compiles to fail with an error message like &#8220;initializer not a
constant&#8221;. This shows up when building DLL under MSVC. Change it to:
<div class="highlight-c"><div class="highlight"><pre>PyObject_HEAD_INIT(NULL)
</pre></div>
</div>
and add the following to the module initialization function:
<div class="highlight-c"><div class="highlight"><pre>if (PyType_Ready(&amp;MyObject_Type) &lt; 0)
 return NULL;
</pre></div>
</div>
</div>
<div class="section" id="differences-between-unix-and-windows">
<h2>4.2. Differences Between Unix and Windows<a class="headerlink" href="#differences-between-unix-and-windows" title="Permalink to this headline">¶</a></h2>
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 (<tt class="file docutils literal">.so</tt>) 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&#8217;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 (<tt class="file docutils literal">.dll</tt>) 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&#8217;s memory; instead, the code already uses the DLL&#8217;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 (<tt class="file docutils literal">.a</tt>) which contains code
from several object files (<tt class="file docutils literal">.o</tt>). During the link step to create a shared
object file (<tt class="file docutils literal">.so</tt>), the linker may find that it doesn&#8217;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 <tt class="file docutils literal">.lib</tt>). A static library is like a Unix <tt class="file docutils literal">.a</tt>
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 <tt class="file docutils literal">A.a</tt> to the
linker for <tt class="file docutils literal">B.so</tt> and <tt class="file docutils literal">C.so</tt>; that would cause it to be included
twice, so that B and C would each have their own copy. In Windows, building
<tt class="file docutils literal">A.dll</tt> will also build <tt class="file docutils literal">A.lib</tt>. You do pass <tt class="file docutils literal">A.lib</tt> to the
linker for B and C. <tt class="file docutils literal">A.lib</tt> does not contain code; it just contains
information which will be used at runtime to access A&#8217;s code.
In Windows, using an import library is sort of like using <tt class="docutils literal">import spam</tt>; it
gives you access to spam&#8217;s names, but does not create a separate copy. On Unix,
linking with a library is more like <tt class="docutils literal">from spam import *</tt>; it does create a
separate copy.
</div>
<div class="section" id="using-dlls-in-practice">
<h2>4.3. Using DLLs in Practice<a class="headerlink" href="#using-dlls-in-practice" title="Permalink to this headline">¶</a></h2>
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 <tt class="file docutils literal">pythonXY.lib</tt> to the linker.
To build two DLLs, spam and ni (which uses C functions found in spam), you could
use these commands:
<div class="highlight-c"><div class="highlight"><pre>cl /LD /I/python/include spam.c ../libs/pythonXY.lib
cl /LD /I/python/include ni.c spam.lib ../libs/pythonXY.lib
</pre></div>
</div>
The first command created three files: <tt class="file docutils literal">spam.obj</tt>, <tt class="file docutils literal">spam.dll</tt> and
<tt class="file docutils literal">spam.lib</tt>. <tt class="file docutils literal">Spam.dll</tt> does not contain any Python functions (such
as <a class="reference internal" href="../c-api/arg.html#PyArg_ParseTuple" title="PyArg_ParseTuple"><tt class="xref c c-func docutils literal">PyArg_ParseTuple()</tt></a>), but it does know how to find the Python code
thanks to <tt class="file docutils literal">pythonXY.lib</tt>.
The second command created <tt class="file docutils literal">ni.dll</tt> (and <tt class="file docutils literal">.obj</tt> and <tt class="file docutils literal">.lib</tt>),
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
<tt class="docutils literal">_declspec(dllexport)</tt>, as in <tt class="docutils literal">void _declspec(dllexport) initspam(void)</tt> or
<tt class="docutils literal">PyObject _declspec(dllexport) *NiGetSpamData(void)</tt>.
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 <tt class="file docutils literal">msvcrtxx.lib</tt> to the list of libraries.
</div>
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 <ul>
<li><a class="reference internal" href="#">4. Building C and C++ Extensions on Windows</a><ul>
<li><a class="reference internal" href="#a-cookbook-approach">4.1. A Cookbook Approach</a></li>
<li><a class="reference internal" href="#differences-between-unix-and-windows">4.2. Differences Between Unix and Windows</a></li>
<li><a class="reference internal" href="#using-dlls-in-practice">4.3. Using DLLs in Practice</a></li>
</ul>
</li>
</ul>

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