PYTHON setup.py build_ext --inplace
PYTHON -c "from pkg import a; assert a.test() == 43"

######## setup.py ########

from Cython.Build import cythonize

from distutils.core import setup, Extension

extensions = [
    Extension('pkg.a', sources=['pkg/a.pyx', 'pkg/alib.c'],
              include_dirs=['pkg'])
]

setup(
  ext_modules = cythonize(extensions)
)

######## pkg/__init__.py ########

######## pkg/a.pyx ########

cdef extern from "alib.h":
    int c_function(int x)

def test():
    return c_function(42)


######## pkg/alib.c ########

int c_function(int x) {
    return x + 1;
}

######## pkg/alib.h ########

int c_function(int x);