Question

%load_ext cythonmagic

The cythonmagic extension is already loaded. To reload it, use:
  %reload_ext cythonmagic

%%file cy_fib.pxd
cdef extern from "c_fib.h":
    double c_fib(int n)

Overwriting cy_fib.pxd

%%file cy_fib.pyx
cimport cy_fib

cpdef cython_fib(n):
    return cy_fib.c_fib(n)

Overwriting cy_fib.pyx

%%file setup.py
from distutils.core import setup, Extension
from Cython.Build import cythonize

ext = Extension("cy_fib",
              sources=["cy_fib.pyx", "c_fib.c"])

setup(name = "cython_fib",
      ext_modules = cythonize(ext))

Overwriting setup.py

! python setup.py build_ext -i &> /dev/null