In C programming, we might allocate memory dynamically for various tasks but what happens when those pieces of memory are no longer needed? If not managed properly, they can lead to memory leaks, wasting valuable resources, and slowing down our program. Therefore, we need to manage memory in C by properly deallocating it when it's no longer in use to ensure that our programs run efficiently without unnecessary memory consumption.

In this article, we will discuss everything in detail about deleting memory in C, why it's important to free up memory, the common pitfalls we should avoid, and how to deallocate memory correctly with the help of examples.

Types of Memory Allocation in C

Before learning about memory deletion, it’s important to understand the basic difference between static and dynamic memory:

• Static Memory Allocation: It is allocated at compile-time and managed by the system. The system automatically handles the allocation and deallocation of this memory to make sure that it is reclaimed when no longer needed. For example, memory allocated to local variables is automatically reclaimed when a function ends, and for global variables memory is deallocated at program termination automatically.
• Dynamic Memory Allocation: It is allocated at runtime using functions like malloc(), calloc(), and realloc() and should be managed by us manually, how? That we will discuss here.

Memory Deallocation in C

Dynamic memory allocation in C allows us to allocate memory at runtime by using the functions like malloc(), calloc(), or realloc() that provide flexibility in memory management. However, it also requires manual deallocation that can be done using the free() function or realloc function when it's no longer in use to ensure that our programs run efficiently that is used to free or deallocate the dynamically allocated memory and helps in reducing memory wastage and avoiding memory leaks.

Syntax to Use free() Function in C

void free(void *ptr);

Here, ptr is the pointer to the memory block that we want to free or deallocate.

Examples of Dynamic Memory Deallocation in C

The following examples illustrates how we can delete the memory allocated dynamically using the free() function.

Example 1: Deleting Dynamically Allocated Memory Using free()

The below C program illustrate how we can use the malloc() function to allocate memory dynamically and free() function to release that memory.

// C program to demonstrate use of
// free() function using malloc()

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

int main()
{
    // Dynamic memory allocated by using malloc
    int* ptr = (int*)malloc(sizeof(int));

    // Freeing the memory
    free(ptr);

    printf("ptr freed successfully\n");

    return 0;
}

ptr freed successfully

Example 2: Deleting Dynamically Allocated Memory Using realloc()

The below C program illustrates how we can use the calloc() function to allocate memory dynamically and realloc() function to deallocate that memory.

// C program to demonstrate use of
// realloc() function to deallocate memory

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

int main()
{
    // Allocate memory
    int* ptr = (int*)calloc(10, sizeof(int));
    if (ptr == NULL) {
        printf("Memory allocation failed\n");
        return 1;
    }

    // Use the allocated memory
    for (int i = 0; i < 10; i++) {
        ptr[i] = i * 2;
    }

    // Deallocate memory using realloc to shrink size to
    // zero
    ptr = (int*)realloc(ptr, 0);
    if (ptr == NULL) {
        printf("Memory deallocated successfully using "
               "realloc\n");
    }

    return 0;
}

Memory deallocated successfully using realloc

Example 3: Deleting Dynamically Allocated Memory for a 2 Array

The below C program illustrates how we can use the free() function to deallocate the memory of a dynamically allocated 2D array.

// C program to demonstrate use of
// free() function to deallocate the memory of a dynamically
// allocated 2D array

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

int main()
{
    // Define the number of rows and columns
    int rows = 5;
    int cols = 5;

    // Allocate memory for a 2D array
    int** array = (int**)malloc(rows * sizeof(int*));
    if (array == NULL) {
        // Check if memory allocation failed
        printf("Memory allocation failed\n");
        return 1;
    }

    // Allocate memory for each row
    for (int i = 0; i < rows; i++) {
        array[i] = (int*)malloc(cols * sizeof(int));
        if (array[i] == NULL) {
            // Check if memory allocation failed
            printf("Memory allocation failed\n");

            // Free previously allocated memory
            for (int j = 0; j < i; j++) {
                free(array[j]);
            }
            free(array);
            return 1;
        }
    }

    // Use the allocated memory to store values
    for (int i = 0; i < rows; i++) {
        for (int j = 0; j < cols; j++) {
            array[i][j] = i * j;
        }
    }

    // Deallocate memory for each row
    for (int i = 0; i < rows; i++) {
        free(array[i]);
    }

    // Deallocate memory for the array of pointers
    free(array);

    // Print message indicating successful deallocation
    printf("2D array freed successfully\n");

    return 0;
}

2D array freed successfully

Points to Remember

The following are the important points and common mistakes that we must keep in mind while deallocating the dynamic memory in C.

1. Double Freeing

Freeing a pointer twice can lead to undefined behavior. Ensure that a pointer is not freed more than once.

int *ptr = (int *)malloc(10 * sizeof(int));
free(ptr); 
free(ptr); // Undefined behavior

2. Freeing NULL Pointer

It is safe to call free() with a NULL pointer. It has no effect.

int *ptr = NULL;
free(ptr);   // Safe, no effect

3. Dangling Pointers

After freeing a pointer, it's a good practice to set it to NULL to avoid dangling pointers, which point to freed memory.

int *ptr = (int *)malloc(10 * sizeof(int));
free(ptr);
ptr = NULL;  // Good practice

4. Freeing Pointers to Local Variables

Do not ever use the free() function on pointers to stack-allocated (local) variables. This will lead to undefined behavior.

int x;
int *ptr = &x;
free(ptr);   // Undefined behavior

5. Exceptions

• Attempting to free a non-pointer object results in an error.
• Attempting to free a pointer to a stack-allocated variable leads to undefined behavior.

Related Articles

• Dynamic Memory Allocation in C
• Static and Dynamic Memory Allocation in C
• Common Memory/Pointer Related bug in C Programs