apr22-pi2.c

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <omp.h>

#define CIRCLE(step, i) sqrt(4.0 - (double) (i) * (step) * (double) (i) * (step))

int main() {
    omp_set_num_threads(omp_get_num_procs());

    int N;
    FILE* datp = fopen("apr22-pi2.dat", "r");
    if (datp == NULL) {
        printf("Error. Couldn't open the data file.");
        exit(1);
    }
    if (fscanf(datp, "%d", &N) != 1) {
        printf("Error. Couldn't read the number from the data file.");
        exit(2);
    }
    fclose(datp);

    int i;
    double step, S;
    step = 2.0 / (double) N;
    S = 0.0;

#pragma omp parallel shared(N, step) private(i) reduction(+:S)
    {
#pragma omp for
        for (i = 0; i < N; i++) {
            S += (CIRCLE(step, i) + CIRCLE(step, i + 1)) * step / 2.0;
        }
    }

    printf("Pi = %f\n", S);

    return 0;
}