Segmentation Fault with Pointer

Question

I am having a truly bizarre problem with my code here. It works (seemingly) when I use manual print statements to output the values of int *primesArr, but if I try to do so with a for loop it fails. I ran it through gdb and find that it crashes right around where I set the next cell in the array to value 'k', which only occurs when a number is prime. The first iteration is successful (i.e. 2 is set to primesArr[0]) and then the program Segfaults when trying to increment the array. But this only happens when using a for-loop. When I create individual print statements, my program works as expected. I'm not sure how/why I am accessing memory that hasnt been appropriated when using a for-loop. I'm sure I've performed some amateur mistake somewhere, and it probably has something to do with how I'm passing my pointer... but I cannot determine its exact root. I'd appreciate any help and thank you in advance.

#include<stdio.h>

int genPrimes(int seed, int *test){
    int inNumPrimes=0;
    for(int k=0; k<=seed; k++){//k is number being checked for primeness
        int cnt=0;
        for(int i=1; i<=k; i++){//'i' is num 'k' is divided by
            if(k%i == 0){
                cnt++;
                if(cnt > 2){
                    break;
                }
                }else{
            }

        }
        if(cnt == 2){
            printf("%i IS PRIME\n",k);
            *test=k;
            test++;//according to gdb, the problem is somewhere between here
            inNumPrimes++;//and here. I'd wager I messed up my pointer somehow
        }
        //printf("%i\n",k);
    }
    return inNumPrimes;
}

int main(){
    int outNumPrimes=0;
    int *primesArr;
    int n = 0;
    n=20;

    outNumPrimes=genPrimes(n, primesArr);
    printf("Congratulations!  There are %i Prime Numbers.\n",outNumPrimes);

    //If you print using this for loop, the SEGFAULT occurs.  Note that it does not matter how high the limit is; its been set to many values other than 5. It will eventually be set to 'outNumPrimes'
    //for(int a=0; a<5; a++){
    //printf("%i\n",primesArr[a]);
    //}

    //If you print the array elements individually, the correct value--a prime number--is printed.  No SEGFAULT.
    printf("%i\n",primesArr[0]);
    printf("%i\n",primesArr[1]);
    printf("%i\n",primesArr[2]);
    printf("%i\n",primesArr[3]);
    printf("%i\n",primesArr[4]);
    printf("%i\n",primesArr[5]);
    printf("%i\n",primesArr[6]);
    printf("%i\n",primesArr[7]);
    //
    return 0;
}

Output with manual statements:

$ ./a.out 
2 IS PRIME
3 IS PRIME
5 IS PRIME
7 IS PRIME
11 IS PRIME
13 IS PRIME
17 IS PRIME
19 IS PRIME
Congratulations!  There are 8 Prime Numbers.
2
3
5
7
11
13
17
19

Now with the for loop:

$ ./a.out 
2 IS PRIME
Segmentation fault

Answer 1

you are passing an uninitialized pointer into your primes function. the behavior you get is undefined, which is why this seems so mysterious. the variable primesArr could be pointing to anywhere.

for a simple case like this, it'd probably be better to use a std::vector<int>