第十五章：C语言扩展

本章着眼于从Python访问C代码的问题。许多Python内置库是用C写的，访问C是让Python的对现有库进行交互一个重要的组成部分。这也是一个当你面临从Python 2 到 Python 3扩展代码的问题。虽然Python提供了一个广泛的编程API，实际上有很多方法来处理C的代码。相比试图给出对于每一个可能的工具或技术的详细参考，我么采用的是是集中在一个小片段的C++代码，以及一些有代表性的例子来展示如何与代码交互。这个目标是提供一系列的编程模板，有经验的程序员可以扩展自己的使用。

这里是我们将在大部分秘籍中工作的代码：

/* 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 few simple functions such as gcd() and is_mandel(). The divide() function is an example of 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() function involve C structures.

For all of the recipes that follow, assume that the preceding code is found in a file named sample.c, that definitions are found in a file named sample.h and that it has been compiled into a library libsample that can be linked to other C code. The exact details of compilation 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.

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