Desafio Geek - C v2

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

#define BUFFER_SIZE 1024

int main(int argc, char **argv){
    if(argc != 2){
        printf("Usage: %s <size of palindrome>\n", argv[0]);
        exit(0);
    }

    unsigned size = atoi(argv[1]);
    unsigned halfsize = size/2; //to reduce operations
    if(size < 2){
        printf("Size of palindrome must be greater than 1\n");
        exit(0);
    }

    char buffer[BUFFER_SIZE];
    char read[BUFFER_SIZE];
    char *aux = (char *) malloc(sizeof(char) * size);
    unsigned index;
    unsigned length;
    unsigned counter;
    unsigned first, last;
    unsigned long long mult;
    unsigned long long num;
    unsigned long long max;

    while(fgets(read, BUFFER_SIZE-size, stdin) != NULL){
        //fread(&buffer[size], BUFFER_SIZE-size, stdin) ?
        snprintf(buffer, BUFFER_SIZE, "%s%s", &buffer[strlen(buffer)-size+1], read);

        index = 0;
        length = strlen(buffer)-size;
        while(index < length){
            for(counter = 0; counter < halfsize; counter++){
                first = index+counter;
                last = index+ (size-1) -counter;
                if(buffer[first] != buffer[last]){
                    break;
                }
            }
            if(counter == halfsize){
                //for(counter = 0; counter < size; counter++){
                                //        aux[counter] = buffer[index+counter];
                                //}
                                //aux[counter] = '\0';
                //num = atol(aux);
                for(counter = 0, mult = 1, num = 0; counter < size; counter++){
                    //printf("%u - %u - %llu\n", counter, (buffer[index+(size-counter-1)]-48), mult *(buffer[index+(size-counter-1)]-48));
                    num += mult * (buffer[index+(size-counter-1)]-48);
                    mult *= 10;
                }

                //printf("%s <-> %llu\n",aux, num);


                if(num % 2 && num % 3){
                    max = sqrt(num);
                    for (counter = 1; 6*counter+1 < max; counter++){
                        //printf("%u, %u\n", 6*counter+1, 6*counter-1);
                        if(num % (6*counter+1) == 0)break;
                        if(num % (6*counter-1) == 0)break;
                    }
                    if(6*counter+1 > max){
                        printf("%llu\n", num);
                    }
                }

            }
            index++;
        }
    }
    free(aux);
    return 0;

}