Pointer to pointer

Learn the double pointer (**) concept.

Double pointer

A pointer to pointer holds the address of another pointer. In C it is declared with two stars (**).

Example: int x = 5; int *p = &x; int **pp = &p;. Here p holds the address of x; pp holds the address of p. *pp gives you p (the address of x); **pp gives you x itself. Two levels of indirection are thus achieved.

Modifying values via a double pointer

The most powerful use of a double pointer is changing the original variable from inside another function. Writing **pp = 42; changes the value of x directly.

This is like a single pointer modifying a variable; you are one level further out. *pp gives you p; **pp accesses x itself. That is why double pointers appear often in dynamic array allocation and tree data structures.

Exercise Change a value through a double pointer

Declare a variable int x = 5;. Set up a double pointer with int *p = &x; and int **pp = &p;. Through **pp set x to 42 and print x with printf.

Show example solution

#include <stdio.h>

int main(void) {
    int x = 5;
    int *p = &x;
    int **pp = &p;
    **pp = 42;
    printf("%d\n", x);
    return 0;
}

Test yourself

Which declares a pointer to pointer to int?
- int (*pp)*;
- int *pp[];
- int pp**;
- int **pp;
Show answer

int **pp; — int **pp; says pp is a pointer to int*.

Access x through double indirection

int x = 5;
int *p = &x;
int **pp = &p;
printf("%d\n", ____);

Show answer

int x = 5;
int *p = &x;
int **pp = &p;
printf("%d\n", **pp);

Match the double pointer parts

int *p

Single pointer
int **pp

Double pointer
*pp

value of p
**pp

value of x

Next lesson: Array of pointers

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