```/* 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];
}
}

/* 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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