Question

I am absolutely new to C and I tried to initialize a array in a function.

But it doesn’t work, because if I want to print the values in the main method I always get a Segmentation fault.

 static void array(int *i)
{
    int j = 0;
    i = (int *) malloc(5 * sizeof (int));
    for (j = 0; j < 5; j++) {
        i[j] = j;
    }

    for (j = 0; j < 5; j++) {
        printf("Hello: %d\n", i[j]);
    }
}

/* Main entry point */
int main(int argc, char *argv[])
{
    int j;
    int *i = NULL;

    array(i);
    for (j = 0; j < 5; j++) {
        printf("Hello: %d\n", i[j]);
    }
    return 0;
}

Would be nice if someone could fix the code and could explain how it works.

Answer 1

static void array(int **i)
{
    int j = 0;
    *i = malloc(5 * sizeof (int));
    for (j = 0; j < 5; j++) {
        (*i)[j] = j;
    }

    for (j = 0; j < 5; j++) {
        printf("Hello: %d\n", (*i)[j]);
    }
}

/* Main entry point */
int main(int argc, char *argv[])
{
    int j;
    int *i = NULL;

    array(&i);
    for (j = 0; j < 5; j++) {
        printf("Hello: %d\n", i[j]);
    }
    return 0;
}

You are passing a pointer by value into array, so what you need to do is pass a pointer to your pointer instead, then set/use that.

---

As for why you shouldn’t cast the result of a malloc, see: Do I cast the result of malloc? and
Specifically, what's dangerous about casting the result of malloc?