ctypes chi2 example by @jrwren

chi2.c

double chi2(double m, double b, double *x, double *y, double *yerr, int N) {
    int n;
    double result = 0.0, diff;

    for (n = 0; n < N; n++) {
        diff = (y[n] - (m * x[n] + b)) / yerr[n];
        result += diff * diff;
    }

    return result;
}

example.py

#!/usr/bin/env python

from ctypes import *

libchi2 = cdll.LoadLibrary("libchi2.so.1.0.0")

ThreeDoubles = c_double * 3
m = c_double(2.0)
b = c_double(1.0)
x = ThreeDoubles( -1.0, 4.2, 30.6)
y = ThreeDoubles( -1.5, 8.0, 63.0)
y_err = ThreeDoubles( 1.0, 1.5, 0.6)
libchi2.chi2.restype = c_double
result = libchi2.chi2(m, b, x, y, y_err, 3)
print result

Makefile

libchi2.so: chi2.o
	gcc -shared -Wl,-soname,libchi2.so.1 -o libchi2.so.1.0.0 chi2.o -lc

chi2.o: chi2.c
	gcc -fPIC -g -c -Wall chi2.c

Thanks man, this is great. I loved your post and it made me look into ctypes which I'd been meaning to do.

I made a version which uses the numpy.ctypes extensions which will work with arbitrarily long input/output vectors:
https://gist.github.com/3313315

https://gist.github.com/lud4ik/3403220 ?