Desafio Geek - solução

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

#define BUFFER_SIZE 1024

int checkPrime(char *);

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]);
    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;

    while(fgets(read, BUFFER_SIZE-size, stdin) != NULL){
        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 < size/2; counter++){
                first = index+counter;
                last = index+ (size-1) -counter;
                if(buffer[first] != buffer[last]){
                    break;
                }
            }
            if(counter == size/2){
                for(counter = 0; counter < size; counter++){
                    aux[counter] = buffer[index+counter];
                }
                aux[counter] = '\0';
                printf("%s - palindrome!", aux);

                if(checkPrime(aux) == 0){
                    printf(" IS PRIME!");
                }else{
                    printf(" ...");
                }
                printf("\n");
            }
            index++;
        }
    }
    free(aux);
    return 0;

}

int checkPrime(char *number){
    unsigned long long num = atol(number);
    unsigned long long max = num/2;
    unsigned long long counter = 0;

    if(num % 2 == 0){
        return 2;
    }
    for (counter = 3; counter < max;counter+=2){
        if(num % counter == 0){
            return counter;
        }
    }
    return 0;
}