Passing an array from a thread

Question

For school I have a multi-threaded assignment. Each thread is supposed to take a number from the console input, find all of its prime numbers, and return all those prime numbers as an array. I'm struggling with returning the array in such a way that I can display it. Here is some of my code:

    int *number;

    for (int i =1; i < argc; i++) {
        pthread_create(&thread[i], NULL, prime_nums, (void*)argv[i]);
    }
    for (int i =1; i < argc; i++) {
        pthread_join(thread[i],(void**) &number);
        printf("%d\n", *(number + 0)); //This is just to test if the variable is correct
    }

...

void *prime_nums(void*m) {
    int n = atoi(m);
    int count = 0;
    int f = 2;
    int primeNumbers[MAX_PRIME];
    /*printf("%d: ", n);*/
    while (n > 1 && count < MAX_PRIME) {
        if (n % f == 0) {
            primeNumbers[count++] = f;
            n /= f;
        }
        else {
            f += 1;
        }
    }
    /*printf("\n");*/
    int test[2];
    test[0] = 0;
    test[1] = 1;
    test[2] = 2;
    pthread_exit(test);
}

I've already checked the array primeNumbers in my thread and the data within the array is correct, but I can't figure out how to save that array to another array in my main function.

Answer 1

The array test in your thread function is a local variable that goes out of scope when the function returns (and the thread exits). Returning the address of a local variable and subsequently dereferencing that address invokes undefined behavior.

You need to dynamically allocate memory for the array and return that pointer:

void *prime_nums(void*m) {
    ...
    int *test = malloc(sizeof(int) * 3);
    test[0] = 0;
    test[1] = 1;
    test[2] = 2;
    pthread_exit(test);
}

Also, the second parameter to pthread_exit should be the address of a void *:

void *rval;
int *number
pthread_join(thread[i], &rval);
number = rval;