第十五章：C语言扩展

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

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

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

/* sample.c */_method
#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; i < n; i++) {
        total += a[i];
    }
    return total / n;
}

/* A C data structure */
typedef struct Point {
    double x,y;
} Point;

/* Function involving a C data structure */
double distance(Point *p1, Point *p2) {
    return hypot(p1->x - p2->x, p1->y - p2->y);
}

This code contains a number of different C programming features. First, there are a fewsimple functions such as gcd() and is_mandel(). The divide() function is an exampleof a C function returning multiple values, one through a pointer argument. The avg()function performs a data reduction across a C array. The Point and distance() functioninvolve C structures.

For all of the recipes that follow, assume that the preceding code is found in a file namedsample.c, that definitions are found in a file named sample.h and that it has been compiledinto a library libsample that can be linked to other C code. The exact details ofcompilation and linking vary from system to system, but that is not the primary focus.It is assumed that if you’re working with C code, you’ve already figured that out.