bar.c

#include "bar.h"

void blah(int sz, double *x) {
    for (int i=0; i<sz; i++) {
        x[i] = (double) 2*i;
    }
}

bar.h

void blah(int sz, double *x);

foo.pyx

# distutils: extra_compile_args = -O3 -w
# distutils: sources = bar.c
# cython: boundscheck = False
# cython: initializedcheck=False
# cython: nonecheck = False
# cython: wraparound = False

import numpy as np
cimport numpy as np

### interfacing external c code
# add any c files to the distutils sources directive at the top

cdef extern from "bar.h":
    void blah(int sz, double *x)

def blargh(double[:] x):
    blah(x.shape[0], &x[0])

### writing cython

def foo(double[:,::1] x):
    cdef int i, j
    cdef double[:,::1] y = np.arange(100.).reshape((10,10))

    z = [a**2 for a in range(10)]

    for i in range(x.shape[0]):
        for j in range(x.shape[1]):
            x[i,j] = i*j

    return np.asarray(x), np.asarray(y), z

setup.py

from distutils.core import setup
from Cython.Build import cythonize
import numpy

setup(
    ext_modules=cythonize('**/*.pyx'),
    include_dirs=[numpy.get_include(),],
)