Calling external C functions

It is perfectly OK do from math import sin to use Python’s sin() function. However, calling C’s own sin() function is substantially faster, especially in tight loops. It can be declared and used in Cython as follows:

cdef extern from "math.h":
    double sin(double)

cdef double f(double x):
    return sin(x*x)

At this point there are no longer any Python wrapper objects around our values inside of the main for loop, and so we get an impressive speedup to 219 times the speed of Python.

Note that the above code re-declares the function from math.h to make it available to Cython code. The C compiler will see the original declaration in math.h at compile time, but Cython does not parse “math.h” and requires a separate definition.

When calling C functions, one must take care to link in the appropriate libraries. This can be platform-specific; the below example works on Linux and Mac OS X:

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

ext_modules=[
    Extension("demo",
              ["demo.pyx"],
              libraries=["m"]) # Unix-like specific
]

setup(
  name = "Demos",
  cmdclass = {"build_ext": build_ext},
  ext_modules = ext_modules
)

If one uses the Sage notebook to compile Cython code, one can use a special comment to tell Sage to link in libraries:

#clib: m

Just like the sin() function from the math library, it is possible to declare and call into any C library as long as the module that Cython generates is properly linked against the shared or static library.