由于指针导致的C分段错误

Question

I have recently started coding in C, and am doing some stuff on project Euler. This is my code for challenge three so far. The only problem is when I run the compiled code it throws a segmentation fault. I think it may be due to a pointer I called, the suspect pointer is underneath my comment. I did some research into the subject but I cant seem to be able to fix the error. Any advice?

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

bool is_prime(int k);
int * factors(int num);

int main(){
    int input;
    while (true){
        printf("Enter a number to get the prime factorization of: ");
        scanf("%d", &input);
        if (is_prime(input) == true){
            printf("That number is already prime!");

        }else{
            break;
        }
    }

    //This is the pointer I think is causing the problem
    int * var = factors(input);
    int k;
    for (k = 0; k < 12; k++){
        printf("%d", var[k]);
    }
}

bool is_prime(int k){
    int i;
    double half = ceil(k / 2);
    for (i = 2; i <= half; i++){
        if (((int)(k) % i) == 0){
            return false;
            break;
        }
    }
    return true;
}

int * factors(int num){
    int xi;
    static int array[1000];
    int increment = 0;
    for (xi = 1;xi < ceil(num / 2); xi++){
        if (num % xi == 0){
            array[increment] = xi;
            increment++;
        }
    }
}     

Answer 1

The factors function has no return statement. It's supposed to return a pointer but it doesn't return anything.

Side note: Enable your compiler's warnings (eg, with gcc -Wall -Wextra ). If they're already enabled don't ignore them!

Answer 2

Your function is declared as

int * factors(int num);

but it's definition doesn't return anything and yet you are using it's return value in assignment. This triggers undefined behavior . It will compile if compiled without rigorous warnings and the return value will most likely be whatever random value happened to be left in the return register (eg EAX on x86).

---

C-99 Standard § 6.9.1/12 Function definitions

If the } that terminates a function is reached, and the value of the function call is used by the caller, the behavior is undefined.