malloc function

Introduction

The malloc function helps you get memory space while your program is running. It lets you ask for memory when you need it, instead of before the program starts.

When you don't know how many items you will need to store before the program runs.

When you want to create a flexible list or array that can grow or shrink.

When you want to save memory by only using what you need at the moment.

When you want to manage memory yourself for better control.

When you want to create complex data structures like linked lists or trees.

Syntax

C

void *malloc(size_t size);

malloc returns a pointer to a block of memory of the requested size in bytes.

If malloc fails to allocate memory, it returns NULL.

Examples

This asks for memory enough to store one integer and saves the address in ptr.

C

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

This allocates memory for 100 characters and stores the pointer in buffer. Casting is optional in C.

C

char *buffer = malloc(100 * sizeof(char));

This reserves space for 10 floats in memory.

C

float *arr = malloc(10 * sizeof(float));

Sample Program

This program uses malloc to create an array of 5 integers. It fills the array with multiples of 10, prints them, and then frees the memory.

C

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

int main() {
    int n = 5;
    int *arr = malloc(n * sizeof(int));
    if (arr == NULL) {
        printf("Memory allocation failed\n");
        return 1;
    }

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

    printf("Array elements:\n");
    for (int i = 0; i < n; i++) {
        printf("%d ", arr[i]);
    }
    printf("\n");

    free(arr); // Always free memory when done
    return 0;
}

Important Notes

Always check if malloc returns NULL to avoid using invalid memory.

Remember to use free to release memory when you no longer need it, or your program may waste memory.

malloc does not initialize the memory; it may contain garbage values.

Summary

malloc lets you get memory while your program runs.

You must ask for the right size in bytes and check if it worked.

Always free the memory after use to keep your program healthy.

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