malloc function - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this malloc usage?

Consider the following C code snippet. What will be printed when it runs?

C

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

int main() {
    int *ptr = (int *)malloc(3 * sizeof(int));
    if (ptr == NULL) {
        printf("Memory allocation failed\n");
        return 1;
    }
    ptr[0] = 10;
    ptr[1] = 20;
    ptr[2] = 30;
    printf("%d %d %d\n", ptr[0], ptr[1], ptr[2]);
    free(ptr);
    return 0;
}

A Garbage values or segmentation fault

B 0 0 0

C Memory allocation failed

D 10 20 30

💡 Hint

Remember that malloc allocates memory but does not initialize it. Here, values are assigned before printing.

The code allocates memory for 3 integers, assigns values 10, 20, and 30 to them, then prints these values. Since the values are explicitly set, the output is '10 20 30'.

❓ Predict Output

intermediate

What error does this code raise?

What error will this C code produce when run?

C

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

int main() {
    int *ptr = malloc(-5 * sizeof(int));
    if (ptr == NULL) {
        printf("Allocation failed\n");
        return 1;
    }
    ptr[0] = 1;
    printf("%d\n", ptr[0]);
    free(ptr);
    return 0;
}

A Prints 1 without error

B Compilation error due to negative size

C Runtime error or undefined behavior due to negative size in malloc

D Segmentation fault immediately

💡 Hint

malloc expects a size_t argument, which is unsigned. Negative values convert to large positive numbers.

The negative value is converted to a very large unsigned number, causing malloc to try to allocate a huge amount of memory. This usually fails or causes undefined behavior at runtime.

🔧 Debug

advanced

Why does this malloc code cause a memory leak?

Examine the code below. Why does it cause a memory leak?

C

#include <stdlib.h>

int main() {
    int *ptr = malloc(10 * sizeof(int));
    ptr = malloc(20 * sizeof(int));
    free(ptr);
    return 0;
}

A The first allocated memory is lost because ptr is overwritten before free, causing a leak

B No memory leak occurs because free is called

C free(ptr) is called twice causing a leak

D The second malloc fails and causes a leak

💡 Hint

Think about what happens to the pointer after the first malloc before the second malloc.

The pointer ptr is assigned memory twice without freeing the first allocation. The first allocated memory address is lost and never freed, causing a memory leak.

📝 Syntax

advanced

Which option causes a compilation error?

Which of the following malloc usages will cause a compilation error?

A int *p = malloc(5 * sizeof(int)); if (p == NULL) return 1;

B int *p = malloc(5 * int);

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

D int *p = (int *)malloc(5 * sizeof(int));

💡 Hint

Check the syntax of the sizeof operator in each option.

Option B uses '5 * int' which is invalid syntax because 'int' is a type, not a value. sizeof requires a type or expression inside parentheses.

🚀 Application

expert

How many integers can be safely stored?

Given this code, how many integers can be safely stored in the allocated memory?

C

#include <stdlib.h>

int main() {
    int *arr = malloc(50 * sizeof(*arr));
    if (!arr) return 1;
    // How many integers can arr hold?
    free(arr);
    return 0;
}

A 50 integers

B sizeof(*arr) integers

C 1 integer

D Depends on system architecture

💡 Hint

malloc allocates memory for 50 times the size of one integer.

The malloc call allocates memory for 50 integers because it multiplies 50 by the size of the type pointed to by arr, which is int.

Practice

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

easy

A. Declares a variable

B. Frees previously allocated memory

C. Allocates a block of memory dynamically during program execution

D. Prints output to the screen

Solution

1. Step 1: Understand the purpose of malloc

   The malloc function is used to allocate memory dynamically while the program is running, not at compile time.

2. Step 2: Differentiate malloc from other functions

   Unlike free which releases memory, or printf which outputs text, malloc only allocates memory.

3. Final Answer:

   Allocates a block of memory dynamically during program execution -> Option C

4. Quick Check:

   malloc = allocate memory [OK]

Hint: malloc is for getting memory, free is for releasing it [OK]

Common Mistakes:

• Confusing malloc with free
• Thinking malloc prints output
• Assuming malloc declares variables

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

easy

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

B. int *ptr = malloc(10);

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

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

Solution

1. Step 1: Calculate correct size for allocation

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

2. Step 2: Use correct pointer type and syntax

   The pointer must be declared as int * and assigned the result of malloc. int *ptr = malloc(10 * sizeof(int)); correctly does this.

3. Final Answer:

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

4. Quick Check:

   malloc size = count * sizeof(type) [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 the following code?

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

medium

A. Compilation error

B. Allocation failed

C. 0 0

D. 10 20

Solution

1. Step 1: Check if malloc succeeded

   The code checks if p is NULL. If not NULL, allocation succeeded and memory can be used.

2. Step 2: Assign and print values

   Values 10 and 20 are assigned to p[0] and p[1]. Then they are printed, so output is "10 20".

3. Final Answer:

   10 20 -> Option D

4. Quick Check:

   malloc success prints assigned values [OK]

Hint: Check for NULL before using malloc memory [OK]

Common Mistakes:

• Ignoring NULL check
• Printing uninitialized memory
• Forgetting to free memory

4. Identify the error in this code snippet:

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

medium

A. strcpy is not allowed with malloc

B. malloc size is too small for the string

C. free should be called before strcpy

D. printf format specifier is wrong

Solution

1. Step 1: Check allocated memory size

   The string "Hello World" needs 12 bytes (11 chars + 1 for '\0'), but only 5 bytes are allocated.

2. Step 2: Understand consequences of small allocation

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

3. Final Answer:

   malloc size is too small for the string -> Option B

4. Quick Check:

   Allocate enough memory for string + '\0' [OK]

Hint: Allocate string length + 1 for null terminator [OK]

Common Mistakes:

• Allocating less memory than string length
• Calling free before using memory
• Wrong printf format for strings

5. You want to create a dynamic array of integers that doubles its size when full. Which approach correctly uses malloc and realloc to achieve this?

hard

A. Start with malloc for initial size, then use realloc to double size when needed

B. Use malloc once with maximum size needed upfront

C. Use realloc first, then malloc to allocate memory

D. Use malloc repeatedly without freeing old memory

Solution

1. Step 1: Allocate initial memory with malloc

   Start by allocating memory for the initial array size using malloc.

2. Step 2: Use realloc to resize when array is full

   When the array is full, use realloc to double the memory size safely without losing data.

3. Step 3: Avoid incorrect usage

   Using malloc repeatedly without freeing old memory causes leaks, allocating maximum size upfront wastes memory, and using realloc before malloc is incorrect.

4. Final Answer:

   Start with malloc for initial size, then use realloc to double size when needed -> Option A

5. Quick Check:

   malloc first, then realloc to grow [OK]

Hint: malloc once, then realloc to grow array safely [OK]

Common Mistakes:

• Calling realloc before malloc
• Allocating max size upfront wasting memory
• Not freeing old memory causing leaks