Here are examples illustrating how to use malloc() and realloc() in C programming:

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C Programming: An explanation of how to use malloc() and realloc() in C programming examples

Example 1: Using malloc() to Allocate Memory

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

int main() {
    int n, *arr;

    printf("Enter the number of elements: ");
    scanf("%d", &n);

    // Allocate memory for n integers
    arr = (int *)malloc(n * sizeof(int));
    if (arr == NULL) {
        printf("Memory allocation failed.\n");
        return 1;
    }

    // Initialize and print the array
    for (int i = 0; i < n; i++) {
        arr[i] = i + 1; // Assigning values
    }

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

    // Free the allocated memory
    free(arr);
    return 0;
}

Explanation:

• For an array of n integers, malloc() dynamically allocates memory.
• Sizeof(int) is used to determine each integer's size.
• Malloc() returns NULL if memory allocation is unsuccessful.
  

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Example 2: Using realloc() to Resize Allocated Memory

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

int main() {
    int n, new_n, *arr;

    printf("Enter the initial number of elements: ");
    scanf("%d", &n);

    // Allocate memory for n integers
    arr = (int *)malloc(n * sizeof(int));
    if (arr == NULL) {
        printf("Memory allocation failed.\n");
        return 1;
    }

    // Initialize the array
    for (int i = 0; i < n; i++) {
        arr[i] = i + 1; // Assigning values
    }

    printf("Enter the new size of the array: ");
    scanf("%d", &new_n);

    // Resize the memory block using realloc
    arr = (int *)realloc(arr, new_n * sizeof(int));
    if (arr == NULL) {
        printf("Memory reallocation failed.\n");
        return 1;
    }

    // Initialize new elements if the array was expanded
    for (int i = n; i < new_n; i++) {
        arr[i] = i + 1;
    }

    // Print the resized array
    printf("Resized array elements: ");
    for (int i = 0; i < new_n; i++) {
        printf("%d ", arr[i]);
    }
    printf("\n");

    // Free the allocated memory
    free(arr);
    return 0;
}

Explanation:

• Memory is first allocated for n integers by malloc().
• The memory block is resized using realloc() to make room for new_n integers.
• New elements are initialized following resizing if the array size grows.
  

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Key Points:

1. malloc():

   • used to dynamically allocate a block of memory.
   • Uninitialized memory is present.
   • To make sure allocation was successful, always verify if malloc() returns NULL.
     

2. realloc():

   • used to resize a block of memory that has already been allocated.
   • The first portion of the data is kept while shrinking, and the remaining portion is truncated.
   • If expanded, unless populated manually, the new memory space may contain junk values.
     

3. free():

   • To prevent memory leaks, always use free() to release dynamically created memory.
     

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Example Output:

For malloc() example:

Enter the number of elements: 5 Array elements: 1 2 3 4 5

For realloc() example:

Enter the initial number of elements: 3 Enter the new size of the array: 5 Resized array elements: 1 2 3 4 5