NULL and Classic Pointer Mistakes

NULL: the pointer that points at nothing — on purpose

#include <stdio.h>      // NULL lives in stdio.h and friends

int *p = NULL;          // "points at nothing", a defined, testable value
if (p == NULL) { /* not aiming at any object */ }

Dereferencing NULL is undefined behavior — typically a crash (on this course's sandbox, a segmentation fault). That crash is a gift: it is loud and immediate, unlike garbage pointers that silently corrupt.

Initialize pointers to NULL when you have nothing to point at yet.

Mistake 1: dangling pointer

int *p;
{
    int local = 7;
    p = &local;
}                        // local dies here
printf("%d\n", *p);      // p still holds the dead address — undefined behavior

The variable local lived in a stack frame that no longer exists. Pointers to dead locals are dangling.

Mistake 2: dereferencing before assigning

int *p;
*p = 3;          // p was never given an address — undefined behavior

Mistake 3: returning the address of a local

int *make(void) {
    int x = 5;
    return &x;   // x dies when the function returns — dangling on arrival
}

Mistake 4: confusing address with value

int x = 1;
int *p = &x;
if (p == &x) { }      // comparing addresses — true
if (*p == x) { }      // comparing values — true
if (p == x)  { }      // comparing address to int — nonsense (and a warning)

The compiler's warnings are your first defense: read them.