Wrap C++ lib with Cython

Question

I\'m new to Cython and I\'m trying to use Cython to wrap a C/C++ static library. I made a simple example as follow.

Test.h:

#ifndef          


        

Answer 1

Your Test.pyx file isn't doing what you expect. The print add(2,3) line will not call the add() C++ function; you have to explicitly create a wrapper function to do that. Cython doesn't create wrappers for you automatically.

Something like this is probably what you want:

cdef extern from "test.h":
        int _add "add"(int a,int b)
        int _multiply "multiply"(int a,int b)

def add(a, b):
    return _add(a, b)

def multiply(a, b):
    return _multiply(a, b)

print add(2, 3)

You can look at Cython's documentation for more details.

Answer 2

I think you can fix this specific problem by specifying the right library_dirs (where you actually put libtest.a -- apparently it's not getting found), but I think then you'll have another problem -- your entry points are not properly declared as extern "C", so the function's names will have been "mangled" by the C++ compiler (look at the names exported from your libtest.a and you'll see!), so any other language except C++ (including C, Cython, etc) will have problems getting at them. The fix is to declare them as extern "C".

Answer 3

If your C++ code is only used by the wrapper, another option is to let the setup compile your .cpp file, like this:

from distutils.core import setup
from distutils.extension import Extension
from Cython.Distutils import build_ext

ext_modules = [Extension("test",
                     ["test.pyx", "test.cpp"],
                     language='c++',
                     )]

setup(
  name = 'test',
  cmdclass = {'build_ext': build_ext},
  ext_modules = ext_modules
)

For linking to a static library you have to use the extra_objects argument in your Extension:

from distutils.core import setup
from distutils.extension import Extension
from Cython.Distutils import build_ext

ext_modules = [Extension("test",
                     ["test.pyx"],
                     language='c++',
                     extra_objects=["libtest.a"],
                     )]

setup(
  name = 'test',
  cmdclass = {'build_ext': build_ext},
  ext_modules = ext_modules
)