Dereferencing

Following the arrow

Dereferencing a pointer means going to the address it holds and using the object there:

int x = 42;
int *p = &x;

printf("%d\n", *p);   // 42 — the object p points AT
*p = 99;              // writes through p: x is now 99
printf("%d\n", x);    // 99

*p and x are the same object. There is one int; two names for it.

Declaration vs dereference: same star, different jobs

In int *p = &x; the star is part of the type. In *p = 99; the star is the dereference operator. Same symbol, different contexts.

Reading and writing through pointers

int a = 1, b = 2;
int *p = &a;
int *q = &b;

*p = *q;        // a = b  (copies the VALUE b holds into a)
p = q;          // p now points at b (copies the ADDRESS)

The first copies what the pointers point to; the second re-aims the pointer. Draw the arrows before you write the code.

Uninitialized pointers

int *p;         // holds a GARBAGE address
*p = 5;         // undefined behavior — writes to a random place

A pointer must be given an address before you dereference it — either &x or NULL.