trap.c

/* gcc -std=c99 -g -Wall -fopenmp -o trap trap.c */
#include <stdio.h>
#include <math.h>
#include <omp.h>

double f(double x){ return (x*x + 2*sin(x)); }

void main(){
    double integral, step;
    double a = 0.0; /*left*/
    double b = 1.0; /*right*/
    int N = 100000; /*subdivisions*/

    step = (b-a)/N;
    integral = (f(b)+f(a))/2;
    #pragma omp parallel for reduction(+: integral)
    for(int i=0; i<N; i++) {
        double x = a+i*step;
        integral += f(x);
    }
    integral *= step;

    printf("%s%d%s%f\n", "WITH N=", N, " TRAPEZOIDS, INTEGRAL=", integral);
}