segmentation fault with uint8_t double pointer

Question

I have a segmentation fault when affecting a value to a[1][0], i thought my mallocs were correct but maybe there're not..

int main() {
    uint8_t **a;

    a = malloc(sizeof(uint8_t) * 6);
    *a = malloc(sizeof(uint8_t) * 2);

    a[0][0] = 1; // WORKS
    a[1][0] = 1; // DOES NOT WORK
}

Answer 1

Remember that the type of a is "pointer to pointer to uint8_t.

In your first malloc, it looks like you want a to point to an array of 6 uint8_t *. So your sizeof is wrong; you should do a = malloc(sizeof(uint8_t *) * 6).

Now a points to (the first element of) an array of 6 pointers, which are a[0] through a[5], each of which is uninitialized and probably doesn't point to anything useful.

In your second malloc, you allocate enough memory to hold 2 uint8_t, and set *a, which is the same as a[0], to point to that memory. Note that a[1], ..., a[5] still contain uninitialized pointers.

So accessing a[0][0] is fine, since a[0] does in fact point to valid memory.

a[1][0] is not fine, because a[1] doesn't point to valid memory. You never initialized a[1] with anything.