prime.c

/* compile: gcc prime.c -o prime -fopenmp */

#include <stdio.h>
#include <time.h>
#include <omp.h>

int main(int argc, char *argv[]) {
  double start, end, runTime;
  int num = 1, primes = 0, limit;

  start = omp_get_wtime();
  limit = argv[1] ? atoi(argv[1]) : 100000;

#pragma omp parallel for schedule(dynamic) reduction(+ : primes)
  for (num = 1; num <= limit; num++) {
    int i = 2;
    while(i <= num) {
      if(num % i == 0)
        break;
      i++;
    }
    if(i == num)
      primes++;
  }

  end = omp_get_wtime();
  runTime = end - start;

  /* if(argv[1]) */
    /* printf("%g\n", runTime); */
  /* else */
    printf("This machine calculated all %d prime numbers under %d in %g seconds\n", primes, limit, runTime);

  return 0;
}