Segmentation fault core dumped : Function that returns the next prime number

user_15380286 0
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I get segmentation fault core dumped. My function is supposed to return the prime number following the one put in parameters, or to return the number if it is a prime.

When I compile, I have no errors, I also compiled with -Wall, it worked. But when I run the program I get segmentation fault, core dumped. I can't see the issue.

#include <stdio.h>

int ft_next_prime(int nb)
{
        int i = 2;
        int notPrime = 0;

        while (i++ < 9 || notPrime != 1) {
                if (nb == i)
                        i++;
                else if (nb % i == 0)
                        notPrime = 1;
        }
        return ((notPrime) ? ft_next_prime(nb++) : nb);
}

int main()
{
        printf("%d", ft_next_prime(15));
}

why?

Also, can I write this?

while (i++ < 9 || !notPrime)
    if (nb % (i != nb) == 0)
        notPrime = 1;
primes recursion coredump segmentation-fault c
3 Answers

Use the following line in place of your return statement:

return ((notPrime)? ft_next_prime(nb+1) : nb);

There are a few different causes of a segmentation fault. In your case, you are running out of space on the stack.

The line ft_next_prime(nb++) executes and returns ft_next_prime(nb) — with the exact same parameter — and then increments nb — pointlessly but that's exactly what you told it to do.

user_147591640

"Segmentation fault" means that you tried to access memory that you do not have access to.

user_35439320

The other answers have failed to point out that:

The problem is that you have TWO sources of infinite loop

1) an infinite loop in:

while(i++ < 9 || notPrime != 1)

by having || notPrime != 1 you will iterate until i++ < 9 is false and notPrime is actual 1. In another words you will be iterating until if(nb % i == 0) evaluates to true and variable notPrime is set to 1;

Therefore, in the call

return ((notPrime)? ft_next_prime(nb++) : nb);

it will always call recursively ft_next_prime again, and again. Hence, the infinite loop which ends up causing the Segmentation fault.

You actually meant while(i++ < 9 && notPrime != 1)

2) Furthermore, ft_next_prime(nb++) should be replaced by ft_next_prime(nb+1) otherwise you are calling the recursive function with always the same value.

The other problems that you have is the logic of find the prime number. A number is prime :

A prime number (or a prime) is a natural number greater than 1 that is not a product of two smaller natural numbers.

You stop checking at i++ < 9 which makes no sense because a 143 = 11 * 13 is not prime since it is divisible by 11 and 13. You need to check at least until nb / 2 (actually there is even a better lower boundary that I will leave for you to find).

A possible solution :

#include <stdio.h>

int ft_next_prime(int nb){
        int notPrime = 0;
        for(int i = 2; i < nb / 2 && !notPrime; i++)
            notPrime = nb % i == 0;
        return notPrime ? ft_next_prime(nb + 1) : nb;
}

int main() {
     printf("%d", ft_next_prime(15));
}
user_13668710
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