Pointers to pointers in C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a pointer to a pointer in C?

A pointer to a pointer is a variable that stores the address of another pointer variable. It allows indirect access to the value pointed to by the first pointer.

beginner

How do you declare a pointer to a pointer for an int variable?

You declare it as int **ptr; where ptr is a pointer to a pointer to an int.

intermediate

Given int x = 10; and int *p = &x;, how do you declare and assign a pointer to pointer pp?

Declare int **pp = &p;. Now pp points to p, which points to x.

intermediate

How do you access the value of x using the pointer to pointer pp?

Use **pp to get the value of x. The first * gets the pointer p, the second * gets the value x.

advanced

Why are pointers to pointers useful in C programming?

They are useful for dynamic memory management, passing pointers by reference to functions, and handling multi-dimensional arrays or complex data structures.

What does int **pp; declare?

A A pointer to an int

B A pointer to a pointer to an int

C An int variable

D A pointer to a char

int **pp; means pp is a pointer to a pointer to an int.

If int x = 5; and int *p = &x;, what is the type of &p?

A int **

B int &

C int

D int *

&p is the address of p, which is a pointer to int, so its type is int **.

How do you access the value of x using a pointer to pointer pp?

A *pp

B &pp

C pp

D **pp

Using **pp dereferences twice to get the value of x.

Which of the following is a valid assignment for a pointer to pointer?

A int **pp = p;

B int **pp = &x;

C int **pp = &p;

D int **pp = x;

pp must store the address of a pointer variable like p, so int **pp = &p; is correct.

Why might you pass a pointer to pointer to a function?

A To modify the pointer itself inside the function

B To pass an integer value

C To avoid using pointers

D To declare a new variable

Passing a pointer to pointer lets the function change the original pointer's value (e.g., allocate memory).

Explain what a pointer to pointer is and how you declare one in C.

Think about a pointer that points to another pointer.

Describe how to access the value of an int variable using a pointer to pointer.

Remember each * removes one level of indirection.

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