Find all perfect primes in range.

perfect_primes.c

#include <stdio.h>
#include <string.h>

#define N 1000000000000000000ULL

short is_prime(unsigned long long n)
{
    unsigned long long i;
    if (n < 2) return 0;
    if (n == 2) return 1;
    for (i = 3; i*i <= n; i += 2) {
        if (n % i == 0) return 0;
    }
    return 1;
}

int main()
{
    unsigned long long p, k;
    for (p = 2; (k = ((1ULL << p) - 1) * (1ULL << (p-1))) <= N; ++p) {
        if (is_prime(k)) {
            // número perfecto ya que (2^p - 1) * 2^(p-1) es perfecto si (2^p - 1) es primo
            printf("%llu\n", k);
        }
    }   
}