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.