Pointers and Functions

Six pieces, and this is the chapter where pointers stop being an abstract exercise and become the actual reason C functions can do anything to a caller's data at all.

The swap pattern

void swap(int *a, int *b) {
    int temp = *a;
    *a = *b;
    *b = temp;
}

int main(void) {
    int x = 1, y = 2;
    swap(&x, &y);   /* x is now 2, y is now 1 */
    return 0;
}

Pass the addresses, dereference inside the function to read and write the caller's actual variables. Without the & and *, swap would just be shuffling local copies that vanish the moment the function returns.

Returning pointers

Returning a pointer to a local variable is a classic bug — that variable's stack frame is gone the instant the function returns, so the pointer is left dangling. Returning a pointer is only safe if it points at something that outlives the function: something malloc'd, something static, or something the caller passed in.

const parameters

void print_array(const int *arr, int n) {
    for (int i = 0; i < n; i++) {
        printf("%d ", arr[i]);
        /* arr[i] = 0;   compile error — arr is read-only through this pointer */
    }
}

const int *arr is a promise to the caller — and to the compiler — that this function won't modify what the pointer points to, even though it has full access to it. Cheap to add, and it turns a whole category of accidental-write bugs into compile errors instead of runtime surprises.