Sieve of erathostenes to file in C / Criba de eratóstenes en C a archivo de texto

criba.c

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



int main()
{
    FILE * fp;
    char output[] = "result.txt"; // filename
    char s[15];
    fp = fopen(output, "w+");
    int num = 0;
    printf("CALCULADORA DE PRIMOS\n");
    printf("Ingrese un valor limite\n"); // Enter the limit value
    fgets(s, 15, stdin);
    num = strtol(s, NULL, 10);
    bool *flags;

    flags = malloc(sizeof(int)*num); // allocates space for array

    for(int i = 0; i < num; i++)
    {
        flags[i] = true; // fills up array with "true" values
    }
    int upperLimit = sqrt(num); // gets the square number

    for(int i = 2; i <= upperLimit; i++)
    {
        if(flags[i])
        {
            for(int j = i * i; j < num; j += i)
            {
                flags[j] = false; // discards multiples from array
            }
        }
    }



    for(int i = 2; i < num; i++)
    {
        if(flags[i]) // if number is marked as true
        {
            fprintf(fp,"%d,", i); // copy "survivors" into txt file
        }
    }
    fclose(fp); // closes file
    printf("Los primos hasta el %d en: %s \n", num, output); // Primes up to the limit number go to file 
    return 0;
}