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

↓

Access value via pointer

↓

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.

Practice

1. What does a pointer to a pointer store in C?

easy

A. The size of a variable

B. The value of an integer

C. The address of another pointer

D. The address of a function

Solution

1. Step 1: Understand pointer basics

   A pointer stores the address of a variable.

2. Step 2: Understand pointer to pointer

   A pointer to pointer stores the address of another pointer, not a direct value.

3. Final Answer:

   The address of another pointer -> Option C

4. Quick Check:

   Pointer to pointer = address of pointer [OK]

Hint: Pointer to pointer holds address of pointer, not value [OK]

Common Mistakes:

• Confusing pointer to pointer with pointer to value
• Thinking it stores the value directly
• Mixing it with function pointers

2. Which of the following is the correct way to declare a pointer to a pointer to an int in C?

easy

A. int **ptr;

B. int ptr*;

C. int *ptr*;

D. int &ptr*;

Solution

1. Step 1: Recall pointer declaration syntax

   Single pointer to int is declared as int *ptr;.

2. Step 2: Extend to pointer to pointer

   Pointer to pointer uses two asterisks: int **ptr;.

3. Final Answer:

   int **ptr; -> Option A

4. Quick Check:

   Pointer to pointer declaration uses two * [OK]

Hint: Use two * for pointer to pointer declaration [OK]

Common Mistakes:

• Placing * incorrectly in declaration
• Using & instead of *
• Confusing pointer syntax with references

3. What is the output of the following C code?

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

medium

A. Address of x

B. Compilation error

C. Address of p

D. 10

Solution

1. Step 1: Understand variable assignments

   x = 10, p points to x, pp points to p.

2. Step 2: Evaluate **pp

   *pp gives p (pointer to x), **pp gives value at x, which is 10.

3. Final Answer:

   10 -> Option D

4. Quick Check:

   **pp dereferences twice to get x's value [OK]

Hint: Double dereference **pp gets original value [OK]

Common Mistakes:

• Printing pointer address instead of value
• Using single * instead of **
• Confusing *p and **pp

4. Identify the error in the following code snippet:

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

medium

A. Syntax error in printf

B. pp is not initialized before dereferencing

C. x is not initialized

D. p is not assigned correctly

Solution

1. Step 1: Check pointer initialization

   pp is declared but not initialized to point anywhere.

2. Step 2: Analyze dereferencing *pp = p;

   Dereferencing uninitialized pp causes undefined behavior or crash.

3. Final Answer:

   pp is not initialized before dereferencing -> Option B

4. Quick Check:

   Dereferencing uninitialized pointer causes error [OK]

Hint: Always initialize pointers before dereferencing [OK]

Common Mistakes:

• Assuming pointer auto-initializes
• Ignoring uninitialized pointer usage
• Confusing pointer assignment with value assignment

5. Given the code below, what will be the output?

int a = 3, b = 7;
int *p1 = &a;
int *p2 = &b;
int **pp = &p1;
*pp = p2;
printf("%d %d", *p1, *p2);

hard

A. 7 7

B. 3 7

C. 3 3

D. Compilation error

Solution

1. Step 1: Initial pointer assignments

   p1 points to a (3), p2 points to b (7), pp points to p1.

2. Step 2: Change p1 via pointer to pointer

   *pp = p2 changes p1 to point to b (7).

3. Step 3: Print values

   *p1 now points to b (7), *p2 is 7, so output is "7 7".

4. Final Answer:

   7 7 -> Option A

5. Quick Check:

   Pointer to pointer changed p1 to point to b [OK]

Hint: Pointer to pointer can change pointer target [OK]

Common Mistakes:

• Assuming p1 still points to a
• Confusing *pp and **pp
• Ignoring pointer reassignment effect