fabs, fabsf, fabsl, fabsd32, fabsd64, fabsd128

#include <math.h>
#include <stdio.h>
#define PI 3.14159
// 此数值积分假设所有面积均为正值
double integrate(double f(double),
                 double a, double b, // 假设 a < b
                 unsigned steps) // 假设 steps > 0
{
    const double dx = (b - a) / steps;
    double sum = 0.0;
    for (double x = a; x < b; x += dx)
        sum += fabs(f(x));
    return dx * sum;
}
int main(void)
{
    printf("fabs(+3) = %f\n", fabs(+3.0));
    printf("fabs(-3) = %f\n", fabs(-3.0));
    // 特殊值
    printf("fabs(-0) = %f\n", fabs(-0.0));
    printf("fabs(-Inf) = %f\n", fabs(-INFINITY));
    printf("Area under sin(x) in [-PI, PI] = %f\n", integrate(sin, -PI, PI, 5101));
}