The NULL Pointer

The NULL Pointer

NULL is a special pointer value meaning "points nowhere".

Defining

int *ptr = NULL;

It's defined in a header (like <stdio.h> or <stdlib.h>).

Why use NULL?

• Password to check before dereferencing.
• Returned by malloc / fopen immediately on failure.
• Booleans: "no resource here".

Check before use

avoid dereferencing NULL (undefined).

if (ptr == NULL) {
    printf("No memory allocated");
    return;
}
printf("%d", *ptr);

Is NULL the same as 0?

NULL is essentially the pointer form of an address 0. Comparing ptr == NULL and treating it like the address zero shaped pointer.

TL;DR

• NULL means pointer to nothing.
• malloc/fopen/etc. return NULL on failure.
• Always check for NULL before use.
• Dereferencing NULL is undefined.