omp_Irules1.c

#include <stdio.h>
#include <omp.h>
#define MAX_THREADS 4

static long num_steps = 100000000;
double step;
double f(double x){
    return(4.0/(1.0+x*x));
}
int main ()
{
    int j;
    double pi, full_sum = 0.0;
    double start_time, run_time;
    double sum[MAX_THREADS];
    step = 1.0/(double) num_steps;
    for (j=1;j<=MAX_THREADS ;j++) {
        omp_set_num_threads(j);
        full_sum = 0.0;
        start_time = omp_get_wtime();
#pragma omp parallel
        {
            int i;
            int id = omp_get_thread_num();
            int numthreads = omp_get_num_threads();
            int steps_per_thread=num_steps/numthreads;
            double x;
            double partial_sum = 0;
            double start_position=id*(1.0/numthreads);
            for (i=0;i< steps_per_thread; i+=1) {
                /*Block 1*/
                partial_sum = partial_sum+(f((i+0.5)*step+start_position));
                /*Block 2
                x=(i+0.5)*step+start_position;
                partial_sum = partial_sum+f(x); */
            }

#pragma omp critical   /*Only be executed on one thread at a time.*/
            full_sum += partial_sum;
        }
        pi = step * full_sum;
        run_time = omp_get_wtime() - start_time;
        printf("\n pi is %f in %f seconds %d threads \n ",pi,run_time,j);
    }
}