Index

1. Python and other languages

• Python is a really useful scripting language
  • plenty of modules
• sometimes it is necessary to interface Python to C, C++ ...
  • for games the obvious reason is for speed enhancement
  • however it maybe to link against legacy libraries
  • consider in the future it might be possible to employ Python scripted AI bots in one of the Free Software Quake3 projects

2. Writing C code to be used by Python

• writing a C module for Python, used to be viewed as complex
  • now the swig project seems to have changed that!

3. Advantages of SWIG

• taken from the excellent SWIG documentation
• writing a user interface is painful in a compiled language
• testing is time consuming (the compile/debug cycle)
• not easy to reconfigure or customise without recompilation
• modularisation can be tricky
• security concerns (buffer overflow for instance)
• until recently the prevailing wisdom was for different languages for different tasks
  • the complete task
  • meant that edges of certain tasks were difficult to achieve
  • but the complexity was a price worth paying for the overall goodness of the rest of the project
  • very few projects ever mix languages
• example, quake3 and its scripting
  • the scripting compiles into C and is compiled at game start up and dynamically linked at start up
• example GCC uses a lisp set of rules which are converted into C during compilation
• in both cases the projects have ventured into another language - yet convert the lisp or quake scripts into C
  • most other projects use a single language
• SWIG offers to remove these barriers
• a lot of people use SWIG because they want to break out of the traditional monolithic C programming model

4. Advantages of mixing languages via SWIG

• incorporating C/C++ into a scripting language often results in a more modular design, less code, better flexibility, and increased programmer productivity
• SWIG tries to make the problem of C/C++ integration as painless as possible
• allows you to focus on the underlying C program and using the high-level language interface, but not the tedious and complex chore of making the two languages talk to each other

5. Simple C example

• /* File : example.c */
  
  double  My_variable  = 3.0;
  
  /* Compute factorial of n */
  int  fact(int n) {
    if (n <= 1) return 1;
    else return n*fact(n-1);
  }
  
  
  /* Compute n mod m */
  int my_mod(int n, int m) {
    return(n % m);
  }

• you can download this file here
  

6. SWIG interface file

• /* File : example.i taken from the swig documentation */
  %module example
  %{
  /* Put headers and other declarations here */
  extern double My_variable;
  extern int    fact(int);
  extern int    my_mod(int n, int m);
  %}
  
  
  extern double My_variable;
  extern int    fact(int);
  extern int    my_mod(int n, int m);

• you can download this file here
  

7. Building the module

• firstly check the Python version you are using

• $ python
  Python 2.4.4 (#2, Oct 19 2006, 23:03:48)
  [GCC 4.1.2 20061007 (prerelease) (Debian 4.1.1-16)] on linux2
  Type "help", "copyright", "credits" or "license" for more information.
  >>>

• now use the following command line actions to generate the shared library: _example.so

• $ swig -python example.i
  $ gcc -c -fpic example.c example_wrap.c -I/usr/include/python2.4
  $ gcc -shared example.o example_wrap.o -o _example.so

• obviously you may need to modify /usr/include/python2.4 to another value depending upon which version of Python you have installed
  

8. Testing the module

• $ python
  Python 2.4.4 (#2, Oct 19 2006, 23:03:48)
  [GCC 4.1.2 20061007 (prerelease) (Debian 4.1.1-16)] on linux2
  Type "help", "copyright", "credits" or "license" for more information.
  >>> import example
  >>> print example.fact(4)
  24
  >>> print example.my_mod(30,7)
  2
  >>>

9. SWIG Disadvantages

• converting char ** into a list of strings is possible - but you have to be very careful and the interface file becomes much more complex
• further reading

Index

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