Multi-Level Pointers

Five pieces on int ** — a pointer to a pointer — which sounds abstract right up until it's the only way to let a function hand back a freshly allocated pointer to its caller.

int x = 10;
int *p = &x;
int **pp = &p;

printf("%d\n", **pp);   /* 10 — dereference twice to get back to x */
*pp = NULL;              /* this changes p itself, not x — p now points at nothing */

When ** is actually needed

void allocate(int **out) {
    *out = malloc(sizeof(int));
    **out = 42;
}

int main(void) {
    int *p;
    allocate(&p);     /* p itself gets set inside allocate — needs int **, not int * */
    printf("%d\n", *p);
    free(p);
    return 0;
}

A plain int *out parameter would only let allocate change what the pointer points to, not the pointer variable itself back in main. To modify the caller's pointer — make it point somewhere new — the function needs a pointer to that pointer. This exact shape shows up constantly in kernel code that allocates a resource and hands it back through an out-parameter.

Triple pointers (int ***) exist, but the only place I've actually seen them is arrays of pointers-to-pointers — genuinely rare outside of specific data structures.