Wrap C++ lib with Cython

Question

I'm new to Cython and I'm tring to use Cython to wrap a C/C++ static library. I made a simply example as following.

Test.h:

#ifndef TEST_H
#define TEST_H

int add(int a, int b);
int multipy(int a, int b);

#endif

Test.cpp

#include "test.h"
int add(int a, int b)
{
    return a+b;

}

int multipy(int a, int b)
{
    return a*b;
} 

Then I used g++ to compile and build it.

g++ -c test.cpp -o libtest.o
ar rcs libtest.a libtest.o

So now I got a static library called libtest.a.

Test.pyx:

cdef extern from "test.h":
        int add(int a,int b)
        int multipy(int a,int b)

print add(2,3)

Setup.py:

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

ext_modules = [Extension("test",
                     ["test.pyx"],
                     language='c++',
                     include_dirs=[r'.'],
                     library_dirs=[r'.'],
                     libraries=['libtest']
                     )]

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

The I called:

python setup.py build_ext --compiler=mingw32 --inplace

The output was:

running build_ext
cythoning test.pyx to test.cpp
building 'test' extension
creating build
creating build\temp.win32-2.6
creating build\temp.win32-2.6\Release
C:\Program Files\pythonxy\mingw\bin\gcc.exe -mno-cygwin -mdll -O -Wall -I. -IC:\
Python26\include -IC:\Python26\PC -c test.cpp -o build\temp.win32-2.6\Release\test.o
writing build\temp.win32-2.6\Release\test.def
C:\Program Files\pythonxy\mingw\bin\g++.exe -mno-cygwin -mdll -static --entry _D
llMain@12 --output-lib build\temp.win32-2.6\Release\libtest.a --def build\temp.w
in32-2.6\Release\test.def -s build\temp.win32-2.6\Release\test.o -L. -LC:\Python
26\libs -LC:\Python26\PCbuild -ltest -lpython26 -lmsvcr90 -o test.pyd
g++: build\temp.win32-2.6\Release\libtest.a: No such file or directory
error: command 'g++' failed with exit status 1

I also tried to use libraries=['test'] instead of libraries=['libtest']. It gave me the same errors.

Any clue about those?

Thanks!

Answer 1

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 2

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
)