malloc function - Mini Project: Build & Apply

Using malloc to Allocate Memory in C

📖 Scenario: Imagine you want to store a list of numbers, but you don't know how many numbers you will need to store until the program runs. In C, you can use malloc to ask the computer to give you memory space while the program is running.

🎯 Goal: You will create a program that uses malloc to allocate memory for an array of integers, fill it with values, and then print those values.

📋 What You'll Learn

Create a pointer to an integer array

Use malloc to allocate memory for 5 integers

Assign values to the allocated memory

Print the values stored in the allocated memory

💡 Why This Matters

🌍 Real World

Dynamic memory allocation is useful when you don't know the amount of data in advance, like reading user input or processing files.

💼 Career

Understanding <code>malloc</code> is essential for C programmers working on system software, embedded systems, or performance-critical applications.

1

Create an integer pointer

Declare an integer pointer called numbers.

C

# Declare an integer pointer called numbers
# Your code here

Hint

Use int *numbers; to declare a pointer to integers.

2

Allocate memory for 5 integers

Use malloc to allocate memory for 5 integers and assign it to numbers. Include #include <stdlib.h> at the top.

C

#include 

int *numbers;
# Use malloc to allocate memory for 5 integers and assign to numbers
# Your code here

Hint

Use malloc(5 * sizeof(int)) to allocate memory for 5 integers.

3

Assign values to allocated memory

Use a for loop with variable i from 0 to 4 to assign i * 10 to numbers[i].

C

#include 

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

// Assign values to numbers array
# Your code here

Hint

Use a for loop to assign values like 0, 10, 20, 30, 40 to the array.

4

Print the values and free memory

Use a for loop with variable i from 0 to 4 to print numbers[i] with printf. Then use free(numbers); to release the memory.

C

#include 
#include 

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

for (int i = 0; i < 5; i++) {
    numbers[i] = i * 10;
}

// Print values and free memory
# Your code here

Hint

Use printf("%d\n", numbers[i]); inside a loop to print each number. Don't forget to call free(numbers); after printing.

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