Memory allocation flow - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of malloc and free usage

What is the output of this C program that allocates and frees memory?

C

#include <stdio.h>
#include <stdlib.h>

int main() {
    int *ptr = (int *)malloc(sizeof(int));
    if (ptr == NULL) {
        printf("Allocation failed\n");
        return 1;
    }
    *ptr = 42;
    printf("Value: %d\n", *ptr);
    free(ptr);
    return 0;
}

A Value: 42

B Value: 0

C Segmentation fault

D Compilation error

💡 Hint

Remember malloc returns a pointer to allocated memory which you can use before freeing it.

The program allocates memory for an int, assigns 42, prints it, then frees the memory. So output is 'Value: 42'.

🧠 Conceptual

intermediate

Understanding realloc behavior

What happens when you call realloc with a NULL pointer in C?

A It frees the memory and returns NULL.

B It causes a segmentation fault.

C It behaves like malloc and allocates new memory.

D It returns the original pointer without changes.

💡 Hint

Think about realloc's behavior when given a NULL pointer.

When realloc is called with NULL, it acts like malloc and allocates new memory of the given size.

🔧 Debug

advanced

Identify the cause of memory leak

What causes the memory leak in this code snippet?

C

#include <stdlib.h>

void func() {
    int *arr = malloc(10 * sizeof(int));
    arr = malloc(20 * sizeof(int));
    free(arr);
}

A There is no memory leak; code is correct.

B The first malloc'd memory is overwritten without free, causing a leak.

C free is called twice on the same pointer.

D The second malloc is invalid and causes a crash.

💡 Hint

Look at what happens to the pointer after the first malloc.

The pointer 'arr' is assigned new memory without freeing the old one, so the first allocated memory is lost and leaked.

📝 Syntax

advanced

Identify the syntax error in memory allocation

Which option contains a syntax error in memory allocation?

C

int *ptr = malloc(sizeof(int) * 5);

A int *ptr = malloc(sizeof(int) * 5);

B int *ptr = (int *)malloc(sizeof(int) * 5);

C int *ptr = malloc(sizeof(int) * 5);;

D int *ptr = malloc(sizeof(int) * 5)

💡 Hint

Check for missing semicolons or extra characters.

Option D is missing a semicolon at the end, causing a syntax error.

🚀 Application

expert

Predict the number of allocated blocks

After running this code, how many distinct memory blocks are allocated and not freed?

C

#include <stdlib.h>

int main() {
    int *a = malloc(10 * sizeof(int));
    int *b = malloc(5 * sizeof(int));
    free(a);
    a = malloc(20 * sizeof(int));
    int *c = realloc(b, 15 * sizeof(int));
    return 0;
}

A 2

B 3

C 1

D 0

💡 Hint

Count allocations minus frees carefully, consider realloc behavior.

Initially two blocks allocated (a and b). a is freed then reallocated (still 2 blocks). realloc on b resizes it, not new block. So 2 blocks remain allocated.

Practice

1. What does the malloc function do in C programming?

easy

A. Frees previously allocated memory

B. Allocates a block of memory on the heap

C. Declares a variable

D. Copies data from one memory location to another

Solution

1. Step 1: Understand the purpose of malloc

   malloc is used to allocate memory dynamically on the heap during program execution.

2. Step 2: Differentiate from other memory functions

   free releases memory, declaration creates variables, and copying data is done by functions like memcpy.

3. Final Answer:

   Allocates a block of memory on the heap -> Option B

4. Quick Check:

   malloc = Allocate memory [OK]

Hint: Remember: malloc = get memory, free = release memory [OK]

Common Mistakes:

• Confusing malloc with free
• Thinking malloc declares variables
• Assuming malloc copies data

2. Which of the following is the correct syntax to allocate memory for 10 integers using malloc?

easy

A. int *ptr = malloc(sizeof(int));

B. int *ptr = malloc(10);

C. int ptr = malloc(10 * sizeof(int));

D. int *ptr = malloc(10 * sizeof(int));

Solution

1. Step 1: Calculate total memory needed

   To allocate memory for 10 integers, multiply 10 by the size of an int: 10 * sizeof(int).

2. Step 2: Check pointer type and assignment

   The pointer must be of type int * and assigned the result of malloc. int *ptr = malloc(10 * sizeof(int)); matches this.

3. Final Answer:

   int *ptr = malloc(10 * sizeof(int)); -> Option D

4. Quick Check:

   Correct malloc size and pointer type = int *ptr = malloc(10 * sizeof(int)); [OK]

Hint: Multiply count by sizeof(type) for malloc size [OK]

Common Mistakes:

• Not multiplying by sizeof(int)
• Using wrong pointer type
• Allocating too little memory

3. What will be the output of this C code snippet?

int *ptr = malloc(4 * sizeof(int));
if (ptr == NULL) {
    printf("Allocation failed\n");
} else {
    ptr[0] = 10;
    ptr[1] = 20;
    printf("%d %d\n", ptr[0], ptr[1]);
    free(ptr);
}

medium

A. 10 20

B. 0 0

C. Allocation failed

D. Compilation error

Solution

1. Step 1: Check memory allocation and NULL condition

   The code allocates memory for 4 integers. If allocation fails, it prints "Allocation failed". Otherwise, it proceeds.

2. Step 2: Assign and print values

   Since allocation likely succeeds, ptr[0] = 10 and ptr[1] = 20 are set, then printed as "10 20".

3. Final Answer:

   10 20 -> Option A

4. Quick Check:

   Successful malloc prints assigned values [OK]

Hint: Check if malloc returns NULL before using pointer [OK]

Common Mistakes:

• Ignoring NULL check
• Assuming uninitialized values print zero
• Forgetting to free memory

4. Identify the error in this code snippet related to memory allocation:

char *str = malloc(5);
strcpy(str, "Hello World");
free(str);

medium

A. Memory allocated is too small for the string

B. Missing check if malloc returned NULL

C. Using strcpy without including

D. No error, code is correct

Solution

1. Step 1: Check allocated memory size

   Memory allocated is 5 bytes, but the string "Hello World" needs 12 bytes (11 chars + 1 null terminator).

2. Step 2: Understand consequences of insufficient memory

   Copying a larger string into smaller allocated space causes buffer overflow and undefined behavior.

3. Final Answer:

   Memory allocated is too small for the string -> Option A

4. Quick Check:

   Allocated size < string length = Memory allocated is too small for the string [OK]

Hint: Allocate enough space including null terminator [OK]

Common Mistakes:

• Ignoring string length vs allocated size
• Not checking malloc return value
• Assuming strcpy auto-expands memory

5. You want to allocate memory for an array of 5 integers, initialize them to zero, and then safely release the memory. Which code snippet correctly implements this flow?

hard

A. int *arr = malloc(5 * sizeof(int));
for(int i=0; i<5; i++) arr[i] = 0;

B. int arr[5];
for(int i=0; i<5; i++) arr[i] = 0;

C. int *arr = calloc(5, sizeof(int));
free(arr);

D. int *arr = malloc(5);
memset(arr, 0, 5 * sizeof(int));
free(arr);

Solution

1. Step 1: Understand memory allocation and initialization

   calloc allocates and initializes memory to zero automatically, perfect for this task.

2. Step 2: Check safe memory release

   free(arr); correctly releases the allocated memory after use.

3. Final Answer:

   int *arr = calloc(5, sizeof(int)); free(arr); -> Option C

4. Quick Check:

   calloc zero-initializes and malloc does not [OK]

Hint: Use calloc for zero-initialized memory allocation [OK]

Common Mistakes:

• Using malloc without initializing memory
• Allocating too little memory
• Not freeing dynamically allocated memory