用SWIG包装C代码

Swig通过解析C头文件并自动创建扩展代码来操作。 要使用它，你先要有一个C头文件。例如，我们示例的头文件如下：

/* sample.h */

#include <math.h>
extern int gcd(int, int);
extern int in_mandel(double x0, double y0, int n);
extern int divide(int a, int b, int *remainder);
extern double avg(double *a, int n);

typedef struct Point {
    double x,y;
} Point;

extern double distance(Point *p1, Point *p2);
一旦你有了这个头文件，下一步就是编写一个Swig”接口”文件。 按照约定，这些文件以”.i”后缀并且类似下面这样：

// sample.i - Swig interface
%module sample
%{
#include "sample.h"
%}

/* Customizations */
%extend Point {
    /* Constructor for Point objects */
    Point(double x, double y) {
        Point *p = (Point *) malloc(sizeof(Point));
        p->x = x;
        p->y = y;
        return p;
   };
};

/* Map int *remainder as an output argument */
%include typemaps.i
%apply int *OUTPUT { int * remainder };

/* Map the argument pattern (double *a, int n) to arrays */
%typemap(in) (double *a, int n)(Py_buffer view) {
  view.obj = NULL;
  if (PyObject_GetBuffer($input, &view, PyBUF_ANY_CONTIGUOUS | PyBUF_FORMAT) == -1) {
    SWIG_fail;
  }
  if (strcmp(view.format,"d") != 0) {
    PyErr_SetString(PyExc_TypeError, "Expected an array of doubles");
    SWIG_fail;
  }
  $1 = (double *) view.buf;
  $2 = view.len / sizeof(double);
}

%typemap(freearg) (double *a, int n) {
  if (view$argnum.obj) {
    PyBuffer_Release(&view$argnum);
  }
}

/* C declarations to be included in the extension module */

extern int gcd(int, int);
extern int in_mandel(double x0, double y0, int n);
extern int divide(int a, int b, int *remainder);
extern double avg(double *a, int n);

typedef struct Point {
    double x,y;
} Point;

extern double distance(Point *p1, Point *p2);
一旦你写好了接口文件，就可以在命令行工具中调用Swig了：

bash % swig -python -py3 sample.i
bash %
swig的输出就是两个文件，sample_wrap.c和sample.py。 后面的文件就是用户需要导入的。 而sample_wrap.c文件是需要被编译到名叫 _sample 的支持模块的C代码。 这个可以通过跟普通扩展模块一样的技术来完成。 例如，你创建了一个如下所示的 setup.py 文件：

# setup.py
from distutils.core import setup, Extension

setup(name='sample',
      py_modules=['sample.py'],
      ext_modules=[
        Extension('_sample',
                  ['sample_wrap.c'],
                  include_dirs = [],
                  define_macros = [],

                  undef_macros = [],
                  library_dirs = [],
                  libraries = ['sample']
                  )
        ]
)
要编译和测试，在setup.py上执行python3，如下：

bash % python3 setup.py build_ext --inplace
running build_ext
building '_sample' extension
gcc -fno-strict-aliasing -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes
-I/usr/local/include/python3.3m -c sample_wrap.c
 -o build/temp.macosx-10.6-x86_64-3.3/sample_wrap.o
sample_wrap.c: In function ‘SWIG_InitializeModule’:
sample_wrap.c:3589: warning: statement with no effect
gcc -bundle -undefined dynamic_lookup build/temp.macosx-10.6-x86_64-3.3/sample.o
 build/temp.macosx-10.6-x86_64-3.3/sample_wrap.o -o _sample.so -lsample
bash %
如果一切正常的话，你会发现你就可以很方便的使用生成的C扩展模块了。例如：

>>> import sample
>>> sample.gcd(42,8)
2
>>> sample.divide(42,8)
[5, 2]
>>> p1 = sample.Point(2,3)
>>> p2 = sample.Point(4,5)
>>> sample.distance(p1,p2)
2.8284271247461903
>>> p1.x
2.0
>>> p1.y
3.0
>>> import array
>>> a = array.array('d',[1,2,3])
>>> sample.avg(a)
2.0
>>>