perfect_fast.c

#include <stdio.h>
#include <math.h>
#include <stdlib.h>

int main(void)
{
    int end = 35000000;
    int *sums = malloc(end * sizeof(int));
    // Limit where we start counting divisors twice
    int max_div = sqrtf(end);

    // Initialize all sums with 1
    for (int i = 0; i < end; i++) {
        sums[i] = 1;
    }

    for (int div = 2; div <= max_div; div++) {
        // Start at the cube to not count divisors twice
        int mult = div * div;
        int counter = div;
        while (mult < end) {
            sums[mult] += div;
            sums[mult] += counter;
            mult += div;
            counter++;
        }
    }

    // skip 1
    for (int i = 2; i < end; i++) {
        if (sums[i] == i) {
            printf("%d\n", i);
        }
    }

    free(sums);
    return 0;
}