Understanding realloc() function in C

Understanding realloc() function in C

02 May 2024
12 min read
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The realloc() in C: An Overview

The realloc()  function de-allocates the old object pointed to by pointer (ptr) and returns a pointer to a new object that has the size specified by size. The contents of the new object are similar to that of the old object leading to deallocation, up to the lesser of the new and old sizes. 

In this C Tutorial, we will explore more about the realloc() function in C Programming which will include what is the realloc() function in C, the syntax of the realloc() function in C, realloc() function with its example. 

What is the realloc() function in C?

  • The "realloc" is a standard library function in C.
  • It is mainly used to resize dynamically allocated memory blocks.
  • It has two arguments, a pointer to a previously allocated memory block and the new size to which the memory block needs to be resized.

Syntax :

The following is the syntax of realloc() function:

 ptr = realloc (ptr,new_size);  

Parameters of realloc() Function in C

The realloc() method has two inputs. They are as follows:


  • It is nothing but a pointer to the memory block which has to be resized.
  • The ptr parameter is the pointer returned by a previous malloc() function, calloc() function, or realloc() function.
  • If suppose this parameter is null, realloc() acts like a malloc() function.


  • This parameter is an unsigned integer that indicates the new size of the memory block.
  • The size parameter is in bytes structure.
  • The realloc() function frees the memory pointed to by ptr and returns a null pointer if the size is zero.

Parameters of Realloc() Function in C

Code Implementation of realloc in C

#include <stdlib.h>

int main() {
    // Allocate memory for 5 integers
    int *ptr = (int *) malloc(5 * sizeof(int));
    ptr[0] = 1;
    ptr[1] = 2;
    ptr[2] = 3;
    ptr[3] = 4;
    ptr[4] = 5;

    // Reallocate memory for 10 integers
    ptr = (int *) realloc(ptr, 10 * sizeof(int));
    for (int i = 4; i < 10; i++) {
        ptr[i] = i + 1;
    for (int i = 0; i < 10; i++)
        printf("%d ", ptr[i]);
    return 0;


1 2 3 4 5 6 7 8 9 10

realloc() Function with Dynamic stack

Let's elaborate the realloc() function in the C Compiler.
typedef struct {  
    int* arr;  
    int top;  
    int size;  
} Stack;  
Stack* create_stack(int size) {  
    Stack* stack = (Stack*)malloc(sizeof(Stack));  
    if (stack == NULL) {  
printf("Failed to allocate memory\n");  
        return NULL;  
    stack->arr = (int*)malloc(size * sizeof(int));  
    if (stack->arr == NULL) {  
printf("Failed to allocate memory\n");  
        return NULL;  
    stack->top = -1;  
    stack->size = size;  
    return stack;  
void push(Stack* stack, int x) {  
    if (stack->top == stack->size - 1) {  
        // stack is full, resize it  
        int new_size = 2 * stack->size;  
        int* new_arr = (int*)realloc(stack->arr, new_size * sizeof(int));  
        if (new_arr == NULL) {  
printf("Failed to reallocate memory\n");  
        stack->arr = new_arr;  
        stack->size = new_size;  
    stack->arr[stack->top] = x;  
int pop(Stack* stack) {  
    if (stack->top == -1) {  
printf("Stack is empty\n");  
        return -1;  
    int x = stack->arr[stack->top];  
    return x;  
void free_stack(Stack* stack) {  
int main() {  
    Stack* stack = create_stack(3);  
    if (stack == NULL) {  
        return 1;  
push(stack, 4);  
push(stack, 5);  
push(stack, 6);  
push(stack, 7);  
push(stack, 8);  
printf("%d\n", pop(stack));  
printf("%d\n", pop(stack));  
printf("%d\n", pop(stack));  
printf("%d\n", pop(stack));  
printf("%d\n", pop(stack));  
    return 0;  



Uses of realloc() Function in C

  • Array Resizing: It is mainly used to change the size of an array that is previously allocated on the heap and it will be useful in the case when the program needs to store more data than was originally allocated.
  • Dynamic memory: This function is used in conjunction with malloc() to manage memory dynamically.
  • Optimizing Memory allocation: It is used to optimize memory usage by allocating memory only when it is needed.
  • Dynamic data structures: It can be used to create dynamic data structures such as linked lists and trees, where the number of nodes in the structure can change at runtime.

Advantages of realloc() function

  • Efficient Memory usage: This function allows more efficient use of memory by resizing allocated memory blocks. So it reduces fragmentation and improves overall performance.
  • Low Complexity: Using this function to resize memory blocks it can simplify the process of managing dynamic memory allocation because it eliminates the need to keep track of multiple memory blocks and also manually copy data between them.
  • Flexibility: The function allows for more flexibility in the design of a code, and it can be used to dynamically allocate and resize memory.

Disadvantages of realloc() function

  • Fragmentation: If the program frequently calls realloc() function to change the size of memory blocks, it can lead to memory fragmentation.
  • Memory leaks: If the code calls this function and then fails to update the pointers to the new memory block or to free the old memory block, it will cause memory leaks.
  • Slower Process: It allocates new memory and copies the data over if you know the size of the array we are going to use, it’s more efficient to allocate the appropriate amount of memory in the first place and use "memcpy" to copy the data over.
So, we have learned about the realloc() function in C. I hope you enjoyed learning these concepts while programming with C. Feel free to ask any questions from your side. Your valuable feedback or comments about this article are always welcome. To learn C Programming from scratch, enroll in our C Programming Certification Course and get the best guide to learn C step by step with real-time examples.


Q1. What is realloc and free function?

The realloc() function is used to resize the dynamically allocated memory and free() is used to deallocate a memory block. 

Q2. Can we use realloc without malloc?

Yes.malloc is not required in each case , you can use realloc only.

Q3. Why is realloc function used?

It is used to resize the memory block pointed to a pointer that was previously allocated to the variable by the malloc() or calloc() function.
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