Pointers to pointers in C - Step-by-Step Execution

Concept Flow - Pointers to pointers

Declare int variable

↓

Declare pointer to int

↓

Declare pointer to pointer to int

↓

Assign int address to pointer

↓

Assign pointer address to pointer to pointer

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Access value via pointer

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Access value via pointer to pointer

This flow shows how a variable's address is stored in a pointer, and that pointer's address is stored in a pointer to pointer, allowing access to the original value through two levels of indirection.

Execution Sample

C

int x = 10;
int *p = &x;
int **pp = &p;
printf("%d\n", **pp);

This code declares an int, a pointer to it, and a pointer to that pointer, then prints the int value via the pointer to pointer.

Execution Table

Step	Variable	Value	Action	Output
1	x	10	Declare int x with value 10
2	p	address of x	Assign p = &x
3	pp	address of p	Assign pp = &p
4	**pp	10	Dereference pp twice to get x's value	10
5			Program ends

💡 Program ends after printing the value 10 accessed via pointer to pointer

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
x	undefined	10	10	10	10
p	undefined	undefined	address of x	address of x	address of x
pp	undefined	undefined	undefined	address of p	address of p

Key Moments - 2 Insights

Why do we need a pointer to a pointer instead of just a pointer?

A pointer to a pointer stores the address of a pointer variable, allowing us to indirectly access or modify the original pointer. See step 3 and 4 in the execution_table where pp holds p's address, and **pp accesses x's value.

What does **pp mean in the code?

**pp means dereferencing twice: first to get the pointer p stored at pp, then to get the value x pointed to by p. This is shown in step 4 of the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of p after step 2?

A 10

B address of x

C address of p

D undefined

💡 Hint

Check the 'Value' column for variable p at step 2 in the execution_table

At which step does **pp give the value 10?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look at the 'Variable' and 'Output' columns in the execution_table for **pp

If we change x to 20 after step 1, what will **pp output at step 4?

A 10

B address of x

C 20

D address of p

💡 Hint

Refer to variable_tracker to see how x's value affects **pp output

Concept Snapshot

Pointers to pointers in C:
- int *p stores address of int variable
- int **pp stores address of pointer p
- Access value via **pp (double dereference)
- Useful for indirect pointer manipulation
- Syntax: int **pp = &p;

Full Transcript

This example shows how pointers to pointers work in C. First, an integer variable x is declared with value 10. Then, a pointer p stores the address of x. Next, a pointer to pointer pp stores the address of p. When we use **pp, we dereference twice: first to get p, then to get x's value. The program prints 10 by accessing x through pp. This helps understand how multiple levels of pointers allow indirect access to variables.