source: CIVL/examples/omp/simple/pi.c.s

/*
  OpenMP example program that computes the value of Pi using the trapeziod rule.
  Compile with gcc -fopenmp -O3 omp_pi.c -o omp_pi
*/
// Online source: http://users.abo.fi/mats/PP2012/examples/OpenMP/omp_pi.c
// // permission obtained
#include <omp.h>
#include <stdio.h>
#include <stdlib.h>
#include <math.h>

void print_usage(char *s) {
  printf("Usage: %s -i <nr of intervals>\n", s);
  exit(0);
}

/* This is the function to integrate */
double f(double x) {
  return (4.0 / (1.0 + x*x));
}

int main (int argc, char *argv[]) {

  const double PI24 = 3.141592653589793238462643;  
  int nthreads, tid;
  double d, x, sum=0.0, pi;
  double starttime, stoptime;
  int n=1000, i;
  char c;

  /* Check if we have at least one argument */
//  if (argc <=1 ) {
//    print_usage(argv[0]);
//  }
//  else {
//    /* Parse the arguments for a -h or -i flag */
//    while ((c=getopt(argc, argv, "hi:")) != EOF) {
//      switch (c) {
//      case 'h':
//        print_usage(argv[0]);
//      case 'i':
//        n = atoi(optarg);    /* Get number of intervals  */
//        break;
 //     default:
//        print_usage(argv[0]);
//      }
//    }
//  }


  /* Compute the size of intervals */
  d = 1.0/(double) n;

  /* Start the threads */
#pragma omp parallel default(shared) private(nthreads, tid, x)
  {
    /* Get the thread number */
    tid = omp_get_thread_num();

    /* The master thread checks how many there are */
#pragma omp master
    {
      nthreads = omp_get_num_threads();
      printf("Number of threads = %d\n", nthreads);
      starttime = omp_get_wtime();  /* Measure the execution time */
    }

    /* This loop is executed in parallel by the threads */
#pragma omp for reduction(+:sum) 
    for (i=0; i<n; i++) {
      x = d*(double)i;
      sum += f(x) + f(x+d);
    }
  }  /* The parallel section ends here */

  /* The multiplication by d and division by 2 is moved out of the loop */  
  pi = d*sum*0.5;

  stoptime = omp_get_wtime();
  printf("The computed value of Pi is %2.24f\n", pi);
  printf("The  \"exact\" value of Pi is %2.24f\n", PI24);
  printf("The difference is %e\n", fabs(PI24-pi));
  printf("Time: %2.4f seconds \n", stoptime-starttime);

  exit(0);
}