Chapter 15. C Extensions

This chapter looks at the problem of accessing C code from Python. Many of Python’s built-in libraries are written in C, and accessing C is an important part of making Python talk to existing libraries. It’s also an area that might require the most study if you’re faced with the problem of porting extension code from Python 2 to 3.

Although Python provides an extensive C programming API, there are actually many different approaches for dealing with C. Rather than trying to give an exhaustive reference for every possible tool or technique, the approach is to focus on a small fragment of C code along with some representative examples of how to work with the code. The goal is to provide a series of programming templates that experienced programmers can expand upon for their own use.

Here is the C code we will work with in most of the recipes:

/* sample.c */
#include <math.h>

/* Compute the greatest common divisor */
int gcd(int x, int y) {
    int g = y;
    while (x > 0) {
        g = x;
        x = y % x;
        y = g;
    return g;

/* Test if (x0,y0) is in the Mandelbrot set or not */
int in_mandel(double x0, double y0, int n) {
  double x=0,y=0,xtemp;
  while (n > 0) {
    xtemp = x*x - y*y + x0;
    y = 2*x*y + y0;
    x = xtemp;
    n -= 1;
    if (x*x + y*y > 4) return 0;
  return 1;

/* Divide two numbers */
int divide(int a, int b, int *remainder) {
  int quot = a / b;
  *remainder = a % b;
  return quot;

/* Average values in an array */
double avg(double *a, int n) {
  int i;
  double total = 0.0;
  for (i = 0;

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