primes.c

#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[])
{
  int x=1000000; //default 1 million primes
  if (argc > 1) x = (int)(strtol(argv[1], NULL, 10));
  printf ("Program will calculate %i prime",x); if(x!=1) printf("s"); printf(". Size of int is %lu bytes. Tap enter to start allocating memory.\n",sizeof(int));
  getchar();
  int howmanyprimes=x-1;
  unsigned int* primes=calloc(howmanyprimes, sizeof(int));
  int iscomposite = 0;
  primes[0] = 3;
  unsigned int index = 0;
  if (x==0)
  {
    return 0;
  }
  if (x==1)
  {
    printf ("2 \nTap enter key to exit.");
    getchar();
    return 0;
  }
  printf ("2 ");
  unsigned int n = 0;
  for (n=3;;n+=2)
  {
    iscomposite=0;
    unsigned int n2 = 0;
    for (n2=0;primes[n2]*primes[n2]<=n;n2++)
    {
      if ((n%primes[n2])==0)
      {
        iscomposite = 1;
        break;
      }
    }
    if (iscomposite==0)
    {
      index++;
      primes[index]=n;
      printf ("%i ",n);
    }
    if (index>howmanyprimes-1) break;
  }
  printf("\nTap enter key to exit.");
  getchar();
  return 0;
}