Addresses, borrowing, and lifetime

The idea

Bring array bounds and function knowledge. The address operator obtains a pointer to an existing object, and dereferencing accesses that object through the pointer. Passing a pointer still passes a value: the function gets a copy of the address, but both addresses can identify the same caller-owned object. This supports functions that intentionally update caller data without pretending an ordinary value parameter can do so.

A usable pointer must refer to a live object of an appropriate type, with valid alignment and a permitted access. A non-null pointer is not automatically valid. A local object ordinarily ends its lifetime when its block finishes, so returning its address does not preserve it. Array pointer arithmetic must remain within the array or its one-past position; the one-past pointer must never be dereferenced.

#include <stdio.h>
void addTwo(int *value){*value+=2;}
int main(void){int scores[]={4,7,9}; addTwo(&scores[1]); printf("%d %d\n",scores[0],scores[1]);}

4 9

The copied address still identifies scores[1]. Dereferencing updates that element while the first element remains unchanged.