Dynamic memory allocation in C allows you to manage memory at runtime, rather than at compile-time. This is particularly useful when you don't know the exact amount of memory you need until the program is running. The functions used for dynamic memory allocation are malloc(), calloc(), realloc(), and free(), but we'll focus on malloc() and free() in this example.

Dynamic Memory Allocation with malloc and free

1. malloc(): Memory Allocation

malloc() stands for "memory allocation." It is used to allocate a specified amount of memory during program execution. The syntax is:

void* malloc(size_t size);

• size: The number of bytes to allocate.
• Returns a pointer to the allocated memory. If the allocation fails, it returns NULL.

2. free(): Deallocating Memory

free() is used to deallocate memory that was previously allocated with malloc(). The syntax is:

void free(void* ptr);

• ptr: A pointer to the memory to be freed.

Example: Using malloc() and free()

#include <stdio.h>
#include <stdlib.h> // For malloc and free

int main() {
    // Allocate memory for an array of 5 integers
    int *arr = (int*)malloc(5 * sizeof(int));
    
    // Check if memory allocation was successful
    if (arr == NULL) {
        perror("Error allocating memory");
        return 1;
    }
    
    // Initialize and print the array
    for (int i = 0; i < 5; i++) {
        arr[i] = i * 10; // Assigning values to the array
    }
    
    printf("Array elements:\n");
    for (int i = 0; i < 5; i++) {
        printf("%d ", arr[i]); // Printing values
    }
    printf("\n");
    
    // Deallocate the memory
    free(arr);
    
    return 0;
}
        

Explanation:

Memory Allocation with malloc():

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

• Allocates memory for an array of 5 integers.
• sizeof(int) determines the size of an int in bytes.
• malloc() returns a pointer to the allocated memory, which is cast to int* and assigned to arr.

Error Checking:

if (arr == NULL) {
    perror("Error allocating memory");
    return 1;
}

• Checks if malloc() failed by verifying if arr is NULL.
• perror() prints an error message if memory allocation fails.

Initializing and Using the Array:

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

• Initializes the array with values and prints them.
• arr[i] accesses each element of the dynamically allocated array.

Memory Deallocation with free():

free(arr);

• Frees the memory previously allocated with malloc().
• Important to prevent memory leaks by releasing unused memory.