A pointer is a variable that holds the address of another object. & takes an address, * follows one. Pointers are what make C fast, flexible — and dangerous.

Declare, take an address, dereference

C

#include <stdio.h>

int main(void) {
    int x = 10;
    int *p = &x;          /* p holds the address of x */

    printf("x = %d, *p = %d\n", x, *p);
    *p = 20;              /* write through the pointer */
    printf("x = %d\n", x);
    printf("p == &x ? %d\n", p == &x);
    printf("sizeof p = %zu, sizeof *p = %zu\n", sizeof p, sizeof *p);
    return 0;
}

Output

x = 10, *p = 10
x = 20
p == &x ? 1
sizeof p = 8, sizeof *p = 4

Expression	Meaning	Value here
`x`	The `int` itself	20
`&x`	Address of `x` (type `int *`)	e.g. `0x16fdff1ac`
`p`	The address stored in `p`	same as `&x`
`*p`	The `int` at that address	20
`&p`	Address of the pointer variable (type `int **`)	another address

Printing addresses

C — addresses change on every run (ASLR), so no fixed output

#include <stdio.h>

int main(void) {
    int a = 1;
    double d = 2.0;
    printf("&a = %p\n&d = %p\n", (void *)&a, (void *)&d);
    return 0;
}

NULL and pointer states

C

#include <stdio.h>

int find(const int *a, int n, int key, const int **where) {
    for (int i = 0; i < n; i++)
        if (a[i] == key) { *where = &a[i]; return 1; }
    *where = NULL;
    return 0;
}

int main(void) {
    int nums[] = {4, 8, 15};
    const int *hit;
    if (find(nums, 3, 8, &hit)) printf("found %d at index %td\n", *hit, hit - nums);
    if (!find(nums, 3, 99, &hit) && hit == NULL) printf("99 not found, hit is NULL\n");
    return 0;
}

Output

found 8 at index 1
99 not found, hit is NULL

Dereferencing NULL

C — NULL dereference (UBSan: load of null pointer; without it, a segfault)

#include <stdio.h>

int main(void) {
    int *p = NULL;
    printf("%d\n", *p);      /* segmentation fault */
    return 0;
}

What is the size of a pointer?

What is the difference between NULL, 0 and '\0'?

Why use pointers at all?