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.