realloc function

Introduction

The realloc function helps you change the size of a memory block you already reserved. It lets you grow or shrink your memory without losing the data you stored.

When you start with a small array but later need to store more items.

When you want to reduce memory usage by shrinking a previously allocated block.

When you don't know the exact amount of memory needed at the start.

When you want to resize a buffer while reading data from a file or user input.

When you want to adjust the size of a dynamic data structure like a list or stack.

Syntax

C

void *realloc(void *ptr, size_t new_size);

If ptr is NULL, realloc works like malloc.

If new_size is zero, behavior depends on the system; it may free the memory and return NULL or return a unique pointer that can be freed.

Examples

This allocates memory for 5 integers, similar to malloc.

C

int *arr = realloc(NULL, 5 * sizeof(int));

This resizes the previously allocated array to hold 10 integers.

C

arr = realloc(arr, 10 * sizeof(int));

This shrinks the array to hold only 3 integers.

C

arr = realloc(arr, 3 * sizeof(int));

Sample Program

This program starts with an array of 3 integers, then uses realloc to increase its size to 5. It adds two more numbers and prints all 5.

C

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

int main() {
    int *numbers = malloc(3 * sizeof(int));
    if (numbers == NULL) {
        printf("Memory allocation failed\n");
        return 1;
    }

    // Initialize array
    for (int i = 0; i < 3; i++) {
        numbers[i] = i + 1;
    }

    // Resize array to hold 5 integers
    int *temp = realloc(numbers, 5 * sizeof(int));
    if (temp == NULL) {
        printf("Reallocation failed\n");
        free(numbers);
        return 1;
    }
    numbers = temp;

    // Add new values
    numbers[3] = 4;
    numbers[4] = 5;

    // Print all values
    for (int i = 0; i < 5; i++) {
        printf("%d ", numbers[i]);
    }
    printf("\n");

    free(numbers);
    return 0;
}

Important Notes

Always check if realloc returns NULL to avoid losing your original memory pointer.

If realloc fails, the original memory block remains valid and must be freed later.

Using realloc can move the memory block to a new location, so always assign its return value back to your pointer.

Summary

realloc changes the size of a memory block while keeping its data.

Use it to grow or shrink dynamic arrays or buffers.

Always check for NULL to handle memory errors safely.

Practice

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

easy

A. It allocates a new memory block without preserving old data.

B. It frees the allocated memory block immediately.

C. It changes the size of a previously allocated memory block, preserving its content.

D. It copies data from one memory block to another.

Solution

1. Step 1: Understand the purpose of realloc

   The realloc function is used to resize an existing memory block allocated by malloc or calloc.

2. Step 2: Check data preservation behavior

   realloc preserves the existing data up to the minimum of old and new sizes.

3. Final Answer:

   It changes the size of a previously allocated memory block, preserving its content. -> Option C

4. Quick Check:

   realloc resizes memory and keeps data [OK]

Hint: Remember: realloc resizes and keeps old data safe [OK]

Common Mistakes:

• Thinking realloc frees memory without resizing
• Assuming realloc always allocates new memory without preserving data
• Confusing realloc with memcpy or free

2. Which of the following is the correct syntax to resize a memory block pointed by ptr to hold 50 integers using realloc?

easy

A. ptr = realloc(ptr, 50 * sizeof(int));

B. ptr = realloc(sizeof(int), 50 * ptr);

C. ptr = realloc(50, ptr * sizeof(int));

D. ptr = realloc(ptr, 50);

Solution

1. Step 1: Understand realloc parameters

   realloc takes a pointer and the new size in bytes. To hold 50 integers, size must be 50 times size of int.

2. Step 2: Check each option's correctness

   ptr = realloc(ptr, 50 * sizeof(int)); correctly multiplies 50 by sizeof(int). Others misuse parameters or sizes.

3. Final Answer:

   ptr = realloc(ptr, 50 * sizeof(int)); -> Option A

4. Quick Check:

   Size in bytes = count * sizeof(type) [OK]

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

Common Mistakes:

• Passing count instead of bytes as size
• Swapping parameters in realloc
• Forgetting sizeof(int) when resizing

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

int *arr = malloc(3 * sizeof(int));
arr[0] = 1; arr[1] = 2; arr[2] = 3;
arr = realloc(arr, 5 * sizeof(int));
arr[3] = 4; arr[4] = 5;
for(int i = 0; i < 5; i++) printf("%d ", arr[i]);
free(arr);

medium

A. 1 2 3 0 0

B. 1 2 3 4 5

C. Garbage values after 3 elements

D. Runtime error due to invalid access

Solution

1. Step 1: Analyze memory allocation and initialization

   Initially, 3 integers allocated and set to 1, 2, 3. Then resized to hold 5 integers.

2. Step 2: Assign new values and print

   After realloc, elements 3 and 4 are assigned 4 and 5. Printing all 5 elements outputs 1 2 3 4 5.

3. Final Answer:

   1 2 3 4 5 -> Option B

4. Quick Check:

   realloc preserves old data and new elements assigned [OK]

Hint: realloc keeps old data; new elements must be assigned [OK]

Common Mistakes:

• Assuming new elements are zeroed automatically
• Expecting garbage values without assignment
• Thinking realloc frees old data immediately

4. Identify the error in this code snippet:

int *ptr = malloc(10 * sizeof(int));
ptr = realloc(ptr, 20 * sizeof(int));
if (ptr == NULL) {
    printf("Memory allocation failed\n");
    free(ptr);
}

medium

A. Calling free(ptr) after ptr is NULL does not free the original memory block.

B. Missing check for malloc failure before realloc.

C. Incorrect size calculation in realloc.

D. No error; code is correct.

Solution

1. Step 1: Understand realloc return and NULL check

   If realloc returns NULL, original pointer is lost if not saved separately, and ptr becomes NULL.

2. Step 2: Analyze free(ptr) after NULL assignment

   Calling free(ptr) after ptr is NULL does not free the original memory block, leading to memory leak.

3. Final Answer:

   Calling free(ptr) after ptr is NULL does not free the original memory block. -> Option A

4. Quick Check:

   Check realloc result before overwriting pointer [OK]

Hint: Save realloc result before overwriting pointer [OK]

Common Mistakes:

• Not checking realloc return before assignment
• Freeing NULL pointer thinking it frees old memory
• Ignoring malloc failure before realloc

5. You have a dynamic array of integers allocated with malloc. You want to safely double its size using realloc without losing data or causing memory leaks. Which approach is correct?

hard

A. ptr = malloc(2 * current_size); memcpy(ptr, old_ptr, current_size); free(old_ptr);

B. ptr = realloc(ptr, 2 * current_size); // directly assign realloc result

C. free(ptr); ptr = malloc(2 * current_size); // allocate new memory after freeing

D. int *temp = realloc(ptr, 2 * current_size); if (temp) ptr = temp; else handle error;

Solution

1. Step 1: Understand safe realloc usage

   Directly assigning realloc result to original pointer risks losing original memory if realloc fails.

2. Step 2: Use temporary pointer to check realloc success

   Assign realloc to temp pointer, check if not NULL, then update original pointer. Else handle error safely.

3. Final Answer:

   int *temp = realloc(ptr, 2 * current_size); if (temp) ptr = temp; else handle error; -> Option D

4. Quick Check:

   Use temp pointer to avoid memory leaks on realloc failure [OK]

Hint: Use temp pointer for realloc to avoid losing original memory [OK]

Common Mistakes:

• Overwriting pointer before checking realloc result
• Freeing pointer before realloc causing data loss
• Not handling realloc failure properly