prime.c

   1 /*
   2 Generate Prime Number
   3 */
   4
   5 int isprime(unsigned x)
   6 {
   7 unsigned z,y;
   8         z = x%6U;
   9         if (z!=1 && z!=5) return 0;
  10         z = 5;
  11         for (;;)
  12                 {
  13                 y = x / z;
  14                 if (y < z || z == x) return 1;
  15                 if (!(x % z)) return 0;
  16                 z += 2;
  17                 }
  18 }
  19
  20 void calcprim(unsigned c)
  21 {
  22         while (c >= 2)
  23                 {
  24                 if (isprime(c))
  25                         {
  26                         printf("Found prime number %u.\n",c);
  27                         exit(0);
  28                         }
  29                 c--;
  30                 }
  31         printf("Prime number not found!\n");
  32         exit(1);
  33 }
  34
  35 void main(int argc,char **argv)
  36 {
  37 int c;
  38 char *k;
  39         if (argc != 2)
  40                 {
  41                 printf("Usage: prime <number>\n");
  42                 exit(1);
  43                 }
  44         c = strtoul(argv[1], &k, 0);
  45         calcprim(c);
  46 }