C Pointer to Pointer

A pointer to pointer (double pointer) stores the address of another pointer. It is written with two stars, like int **pp.

How it looks

Variable	Stores
num	The value (for example 50)
p	Address of num
pp	Address of p

Example

#include <stdio.h>

int main(void) {
    int num = 50;
    int *p = &num;
    int **pp = &p;

    printf("num = %d\n", num);
    printf("*p  = %d\n", *p);
    printf("**pp = %d\n", **pp);

    **pp = 99;                  // change num
    printf("num = %d\n", num);
    return 0;
}

Output:

One star *pp gives the pointer p. Two stars **pp go one step more and give the value of num.

Where is it used?

• Changing a pointer inside a function (the function needs the address of the pointer).
• Array of strings, like char **argv in command line arguments.
• Dynamic 2D arrays.

Change a pointer in a function

#include <stdio.h>

void point(int **pp, int *target) {
    *pp = target;
}

int main(void) {
    int a = 1, b = 2;
    int *p = &a;

    point(&p, &b);
    printf("%d\n", *p);
    return 0;
}

Output:

Next: C Pointers and Arrays.

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