Originally published: May 27, 2005

Abstract

In the article Python Extensions In C++ Using SWIG, I describe how to extend Python with C++ code. That article also contains instructions for building SWIG extension DLLs on Windows. This article contains instructions for using GNU Automake and Autoconf to build SWIG extensions as shared objects on GNU/Linux.

Building Manually on the Command Line

Without adding the SWIG steps to your makefiles, you can build a SWIG extension using these commands:


    swig -c++ -python -o your_extension_wrap.cpp your_extension.i
    gcc -fPIC -c your_extension_wrap.cpp -o your_extension_wrap.o -I/usr/include/python
    g++ -shared your_extension_wrap.o <your-other-object-files> -o _your_extension.so

Note: It’s important to prefix the name of the shared object with an underscore, otherwise Python won’t be able to find it when import your_extension is executed.

I also tried with ld instead of g++, but that resulted in the following error in Python:


    >>> import _your_extension
    Traceback (most recent call last):
      File "<stdin>", line 1, in ?
    ImportError: ./_your_extension.so: undefined symbol: __dso_handle

This occurs as soon as iostream is included in the C++ source. Using g++ solves the problem.

Adding SWIG to Autoconf

You need additional Autoconf macros to enable SWIG support. I downloaded the file ac_pkg_swig.m4 from the Autoconf Macro Archive and placed it in the m4 subdirectory of my project tree:


    project/
        m4/
            ac_pkg_swig.m4
        pyext/
            Makefile.am
            swig.i
        src/
            Makefile.am
        Makefile.am
        configure.ac

When you place ac_pkg_swig.m4 in a subdirectory, you need to add this line to your project/Makefile.am:


    ACLOCAL_AMFLAGS = -I m4

You must also specify the option -I m4 whenever you invoke autoreconf.

Now, add these lines to your configure.ac:


    AC_DISABLE_STATIC
    AC_PROG_LIBTOOL
    AM_PATH_PYTHON(2.3)
    AC_PROG_SWIG(1.3.21)
    SWIG_ENABLE_CXX
    SWIG_PYTHON

Adding SWIG to Automake

In your Makefile.am, you need to do the following:

Run swig on your interface definition (.i) files
Build a shared library from your C++ code and the generated SWIG wrapper
Instruct <make install> to place the resulting files in the Python folder

To make this happen, add these lines to the file pyext/Makefile.am:

Note: I assume that your .i files are stored in a subdirectory (pyext/) that is a sibling of the directory that contains your C++ sources (src/). If this is not the case, you will have to change the paths accordingly.


    BUILT_SOURCES = $(srcdir)/swig_wrap.cpp
    SWIG_SOURCES = swig.i <more .i files...>

    pkgpython_PYTHON = your_extension.py
    pkgpyexec_LTLIBRARIES = _your_extension.la
    _your_extension_la_SOURCES = $(srcdir)/swig_wrap.cpp $(SWIG_SOURCES)
    _your_extension_la_CPPFLAGS = $(SWIG_PYTHON_CPPFLAGS) -I$(top_srcdir)/src
    _your_extension_la_LDFLAGS = -module
    _your_extension_la_LIBADD = ../src/libproject.la

    $(srcdir)/swig_wrap.cpp : $(SWIG_SOURCES)
         $(SWIG) $(SWIG_PYTHON_OPT) -I$(top_srcdir)/src -o $@ $<

This code explained:

pkgpython expands to something like /usr/local/lib/python2.4/site-packages/project. This is where a make install will install the SWIG-generated .py wrapper. pkgpyexec is usually the same directory, but, according to the Autoconf manual, that’s where shared libraries should be installed.
Adding the option -module to the LDFLAGS is important. Without it, Libtool requires all libraries to have a “lib” prefix. However, SWIG requires an underscore prefix.
The line _your_extension_la_LIBADD = ../src/libproject.la links the extension library to another shared library that is built from the C++ sources in src/Makefile.am. This is often useful when the C++ part of your project is not only used in the Python extension. A single shared library, libproject.la in the example, can be used to link to your Python extension as well as to your C++ executables.
If you don’t want to do this, you can just list your C++ sources in the line _your_extension_la_SOURCES = ....

An Example Project

These are the relevant files from the mfGraph project:


    mfgraph/
        m4/
            ac_pkg_swig.m4
        pymfg/
            Makefile.am
            mfg_graph.i
            ...
            swig.i
        src/
            Makefile.am
            mfg_graph.cpp
            mfg_graph.h
            ...
        Makefile.am
        autogen.sh
        configure.ac


mfgraph/autogen.sh:
    #!/bin/sh

    autoreconf --force --install -I config -I m4


mfgraph/configure.ac:
    dnl Process this file with autoconf to produce a configure script.
    AC_INIT(mfGraph, 0.3, me@mail.com)
    AC_CONFIG_AUX_DIR(config)
    AC_CONFIG_SRCDIR(src/mfg_graph.cpp)

    AM_INIT_AUTOMAKE

    AC_DISABLE_STATIC
    AC_PROG_LIBTOOL
    AM_PATH_PYTHON(2.3)
    AC_PROG_SWIG(1.3.21)
    SWIG_ENABLE_CXX
    SWIG_PYTHON

    AC_OUTPUT(Makefile src/Makefile pymfg/Makefile)


mfgraph/Makefile.am:
    ACLOCAL_AMFLAGS = -I m4
    SUBDIRS = src pymfg

    EXTRA_DIST = autogen.sh


mfgraph/pymfg/Makefile.am:
    BUILT_SOURCES = $(srcdir)/swig_wrap.cpp
    SWIG_SOURCES = swig.i mfg_graph.i <...>

    pkgpython_PYTHON = pymfg.py
    pkgpyexec_LTLIBRARIES = _pymfg.la
    _pymfg_la_SOURCES = $(srcdir)/swig_wrap.cpp $(SWIG_SOURCES)
    _pymfg_la_CPPFLAGS = $(SWIG_PYTHON_CPPFLAGS) -I$(top_srcdir)/src
    _pymfg_la_LDFLAGS = -module
    _pymfg_la_LIBADD = ../src/libmfgraph.la

    $(srcdir)/swig_wrap.cpp : $(SWIG_SOURCES)
         $(SWIG) $(SWIG_PYTHON_OPT) -I$(top_srcdir)/src -o $@ $<


mfgraph/src/Makefile.am:
lib_LTLIBRARIES = libmfgraph.la
libmfgraph_la_SOURCES = mfg_graph.cpp <...>
pkginclude_HEADERS = mfg_graph.h <...>

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